Abstract
This paper aims at demonstrating the significance of biochar risk evaluation and reviewing risk evaluation from the aspects of pyrolysis process, feedstock, and sources of hazards in biochar and their potential effects and the methods used in risk evaluation. Feedstock properties and the resultant biochar produced at different pyrolysis process influence their chemical, physical, and structural properties, which are vital in understanding the functionality of biochar. Biochar use has been linked to some risks in soil application such as biochar being toxic, facilitating GHGs emission, suppression of the effectiveness of pesticides, and effects on soil microbes. These potential risks originate from feedstock, contaminated feedstock, and pyrolysis conditions that favor the creation of characteristics and functional groups of this nature. These toxic compounds formed pose a threat to human health through the food chain. Determination of toxicity levels is a first step in the risk management of toxic biochar. Various sorption methods of biochar utilized low-cost adsorbents, engineered surface functional groups, and nZVI modified biochars. The mechanisms of organic compound removal was through sorption, enhanced sorption, modified biochar, postpyrolysis thermal air oxidation and that of PFRs degradation was through activation, photoactive functional groups, magnetization, and hydrothermal synthesis. Emissions of GHGs in soils amended with biochar emanated through physical and biotic mediated mechanisms. BCNs have a significance in reducing the health quotient indices for PTEs risk contamination by suppressing cancer risk arising from consumption of contaminated food. The degree of environmental risk assessment of HM pollution in biomass and biochars has been determined by using potential ecological risk index and RAC while organic contaminant degradation by EPFRs was considered when assessing the environmental roles of biochar in regulating the fate of contaminants removal. The magnitude of technologies’ net benefit must be considered in relation to the associated risks.
1. Introduction
The endeavored activities a man is involved in generate unwanted material. Toxic waste management issues are of national concern in many countries because of the increased waste generation and lack of recycling, disposal, and handling capacity and facilities. Toxic waste has been utilized as biomass in pyrolysis processes to generate biochar for soil application and has resulted on spreading of toxic elements in the soil that ends up polluting the environment, abetting GHGs emission and suppression of the effectiveness of pesticides, and in the food chain affecting human health. Such potential risks arise from the feedstock, contaminated feedstock, and favorable pyrolysis environments that influence their creation. These toxic compounds formed pose a threat to the environment and human health as they end up in the food chain through toxic biochar soil application. Determination of toxicity levels of biomass for biochar use is an initial step in managing the risks of toxic biochar. Comparable developed countries have adopted systematic approaches to toxic waste identification and management, including scientific research in reducing, reusing, and recycling related to the economic level, and waste related policy with regulatory institutions, and the construction of facilities [1, 2] that has led to progressive sustainable development changes [3].
An increased number of studies have been paying attention to cost-effective and environmentally friendly solutions on toxic waste management using pyrolysis for biochar production for environmental sustainability. Some of these wastes utilized as biomass (garbage or refuse, sludge from a waste treatment plant, discarded material resulting from industrial, commercial, mining, agricultural, and community activities) contain high toxic levels of contaminants such as heavy metals (HM), polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyl (PCB), inorganic pesticides, dioxins, and persistent organic pollutants (POPs) [4–6]. The composition and volume of waste generated and its pyrolysis treatment and disposal management methods for biochar determine the number of GHGs released into the air and PTE leaching. Without proper management and improvement of the pyrolysis and biochar techniques of waste management, the emissions of GHGs and toxic material are anticipated to rise. There have been some gaps in the production and use of biochar without evaluating the risks posed from the feedstock and pyrolysis process, undermining the sources of hazards and their potential effects to humans and the environment.
The nature of hazardous and solid waste materials for recycling is of great necessity to reduce any contamination to the environment. Many hazardous wastes can be recycled safely and effectively other than treatment and disposal, with benefits of reducing the consumption of raw materials and the volume of waste materials [7]. The materials used in the construction of landfill caps include low-permeability soils and geosynthetic products, which prevent permeability of water through the waste and highly porous soils that drain water [8]. To prevent toxic leaching, highly mobile waste and fluid pesticides cannot be directly landfilled without coagulation [9]. Stabilization method applies binding and filling materials (e.g., cement, lime, pozzolanas, thermoplastics, fly-ash, and silicate by-products) on hazardous waste leachable liquid and semisolid contaminants to physically modify and produce a stabilized solid and reduce the mobility of contaminants [8]. Stabilized wastes for landfill disposal may hinder other use of the environment and it is not effective in immobilizing organic contaminants [10].
Incineration is a high-temperature (870°C to 1200°C) destructive ex situ treatment of contaminated waste (including soil) fed into the incinerator, under controlled conditions of high temperature in the presence of oxygen, volatilizing and combusting the contaminants into innocuous substances [8], with an efficiency of up to 99.99% for PCBs and dioxins [11]. On the contrary, burning POPs (e.g., pesticides and PCBs) in incinerators have been linked with the spread of recent POPs (e.g., dioxins and furans) contaminating the immediate surrounding [12]. Additionally, industrial use standards and regulations of emission of gases have brought about the development of new systems where biomass is heated with controlled or no oxygen for efficient gasification or pyrolysis [13]. Very high temperatures and long residence time in the cement kiln offer an alternative to a high destruction efficiency of hazardous waste. The highly alkaline conditions in a cement kiln are ideal for decomposing chlorinated organic waste [8], with the destruction of compounds being more than 99% with no adverse effect on the quality of the exhaust gas [14]. High calorific value hazardous waste provide high energy resulting in energy saving. Improved kiln processes to pyrolysis, mitigate pollution, and increase the energy efficiency by producing biochar, gases, and liquids for the production of bioenergy and flaring the gases to reduce global warming [13].
Contaminants’ mobility and bioavailability have augmented concerns due to soil contamination, food safety, arising health risks, and GHGs emission. Two major voluntary initiatives offer standardization programs to ensure sustainable biochar production and safety: International Biochar Initiative (IBI) offers standardized product definition and testing guidelines for biochar used in soil [15] and European Biochar Certification (EBC) gives a guideline for biochar production [16], and they have been developed by scientists to guide the public on adherence to high ethical standards of safety and appropriate use. The initiatives have some contaminant parameters where standard methodologies are used to examine and meet the minimum required standards for biochar application [17]. Schimmelpfennig and Glaser [18] established a minimum set of analytical properties and thresholds for biochar identification, suitable for soil amendment, and carbon sequestration. This helps in defining the desired stability properties of biochar for particular use. The application of biochar that immobilizes, stabilizes, and degrades contaminant elements with resultant safe residue offers a reassurance on the reduction of environmental risk. Figure 1 shows the remediation and evaluation of contaminated biochar. This paper aims at reviewing the risks posed on the utilization of biochar derived from multiple wastes and the necessary available mitigation and control measures. The significance of the biochar risk evaluation and reviewing the risk evaluation of biochar is demonstrated from the aspects of (1) pyrolysis process, (2) feedstock, (3) sources of hazards in biochar and their potential effects, and (4) the methods of risk evaluation. The irreversibility of applying biochar to soil necessitates an effective assessment of its stability for use.
2. Biochar
Biochar results from the break down of organic matter undergoing chemical decomposition to a stable form of carbon through pyrolysis in an oxygen-limited environment, usually at temperatures of 350°C–600°C [19]. Pyrolysis is the thermal degradation of biomass under pressure in the absence of reacting gases [8]. The highly recalcitrant aromatic nature of biochar can remain stable in soil for hundreds to thousands of years [20] reducing GHG emission and sequester carbon [21]. Synthetic gas (syngas) and pyrolysis liquor (bio-oil) are additional products of pyrolysis process that are source of renewable energy [22]. The resultant products and their chemical composition depend on the kind of feedstock used and the controlling rate of pyrolysis temperature. Pyrolysis is applicable to solid and organic materials that may undergo chemical decomposition in the presence of high temperature with contaminants. It is linked to thermal desorption through the conversion of energy of waste such as solid hazardous waste, mercury-contaminated soil, hospital waste, and coal [8]. This technology is not applicable for treatment of liquids and explosives materials with highly oxidizing nature under heat treatment and materials that cannot be decomposed by thermal treatment at 600°C [8]. A variety of feedstock and pyrolysis methods used significantly affect the results in biochar properties. Tables 1 and 2 show biochar functional group property changes from varied feedstock and pyrolysis temperature.
2.1. Properties of Biochar
The soil amendment of general properties of biochars irrespective of feedstock used or the pyrolysis method is the improvement in soil water holding capacity (WHC), cation-exchange capacity (CEC), soil carbon, soil nutrient, and crop productivity. These are often recorded in highly degraded and nutrient-poor soils due to ability to absorb and retain moisture longer in the soil [64–68]. Available nutrients are recycled through the critical role played by the biochar amendment, thus increasing crop productivity through improved indirect nutrient availability.
This is by means of pH change, CEC, and soil structure, resulting in improved fertilizer efficiency, decreased nutrient leaching, and some effects on nutrient availability [69]. Depending on pyrolysis conditions, biochar has a porous structure with carboxyl and hydroxyl functional groups effective in the adsorption of contaminants in soil and minimize the risk of contaminants entering the human food chain [70] with the longevity of soil biochar on application. These soil properties make biochar behave in a different way in different soil properties due to their varying adsorption behavior and biological activity [13, 71–74]. Other carbonaceous materials like biochar such as activated carbon have been used and exhibit varied performance for toxic compound absorption by prevention of leaching and secondary contamination [75].
2.2. Advantages and Disadvantages of Biochar
Biochar soil amendment for stressed agricultural soil improves soil structure and properties by increasing absorbency capacity and aeration of the soil [76, 77]. It also mitigates global warming, restores degraded lands, and balances the effect of water pollution by removing organic contaminants such as pesticides and dyes [78, 79]. Through this means, it has an affinity for adsorbing contaminants and keeping them away from plants [64]. Nutrient balancing created through biochar application with carbon sequestration in arable land is a way to alleviate GHG emission as some farmland has exhausted soil organic carbon (SOC) [64]. However, its application to fertile soils does not necessarily increase crop yield [64]. There are, however, some negatives effects of biochar with the use of contaminated feedstock for biochar. Table 1 shows some properties of biochar generated from biomass in the production of biochar with resultant toxic elements.
Since biochar production is from a range of feedstock sources, some contaminants can be present including PAHs, POPs, HMs, and organic compounds; therefore, there is an important consideration in the context of adding biochar to agricultural soils [80] due to negative effects on soil properties and function. These contaminants may also end up in the biochar depending on the pyrolysis conditions or the use of processing conditions that may favor their production [81]. High rates of biochar application in the soil can affect micro-organism survival rates, including plants and animals, thus necessitating testing of biochars effects before the application on agricultural fields to avoid detrimental effects [82]. However, the negative implications and harmful effects on the ecological system due to the continued use of biochar have not been wholly understood [83]. There is need to identify suitable feedstock for the production of biochar from a range of biomass as provided for by European Biochar Certificate (EBC) in the positive list of biomass feedstock approved for use in producing biochar [84].
Production of biochar for soil application is an important means for establishing a long-term carbon sink with low-risk return of CO2 to the atmosphere and the improvement in soil properties. Utilization of biochar for additional soil carbon has additional considerable potential to value addition beyond waste management and prevention of environmental contamination. The benefits improve soil nutrient availability and water holding capacity, hence improving degraded soils and promoting soil health. However, a range of contaminated feedstock sources should be avoided by utilizing suitable sources. These are key components in waste management and the development agricultural sustainability.
3. Effects of Pyrolysis Process and Feedstock on Biochar Properties
3.1. Effect of Pyrolysis on Biochar Properties
Laird et al. defined pyrolysis as a thermochemical process where high temperature transforms waste materials such as wood chips, crop residues, manures, and municipal wastes in the complete or near absence of oxygen into renewable energy products–biochar, bio-oil, and syngas [85]. Biochar is produced as a product or coproduct from several pyrolysis methods that include slow pyrolysis (SP), fast pyrolysis (FP), gasification, and flash pyrolysis [79, 83]. SP feedstock is combusted at temperatures between 350°C and 800°C, with the residence time varying from minutes to several hours, while in FP, the feedstock is combusted at temperatures between 425°C and 550°C, with the residence time being about 2 s [79]. In gasification, the feedstock is combusted in the presence of oxygen, and at times, the addition of steam or CO2, at temperatures more than or equal to 800°C, with the residence time varying from a few seconds to some hours [79]. SP and FP biochars have different characteristics physically and chemically and hence different behavior in soil [71, 86]; therefore, application of partly pyrolyzed biomass heightens the immobilization of soil N needed by soil plants and animals [71]. A study FP left a labile unpyrolyzed biomass fraction that reduced pH and particle size and increased surface area from wheat straw-derived biochar, unlike SP biochar that pyrolyzed completely [87]. The residence time in SP biochars completely pyrolyzes resulting in less volatile C substrate, ultimately reducing N immobilization [87]. Table 2 illustrates functional group changes of biochar arising from varied feedstock and pyrolysis temperature.
Hydrochar is produced by hydrothermal carbonization of biomass treated with hot compressed water without drying [88]. The process of hydrochar production is eco-friendly with no hazardous chemical waste or by-products generation compared to dry pyrolysis [89]. The advantage is that hydrothermal process takes place at low temperatures (150°C to 350°C), and damp feedstock (moist animal manures, sewage sludge, and algae) can be used [90]. Moreover, hydrochar reduces alkali, alkaline Earth, and HM contents over biochar [88]. The disadvantage of hydrochar, however, is that it has generally a low surface area and poor microporosity [91], with a less C stability unlike biochar [64]. Temperature affects the resultant pyrolysis of biomass such that increasing pyrolysis temperature (450°C–700°C) decreases the particle size of biochar [13] and increases particle density and porosity [92]. The increasing temperature (300°C to 700°C) of urban sewage sludge biochar increased the resultant pH and electrical conductivity (EC) of biochar [92]. Low temperatures of 300°C produced biochar with high N and organic carbon (OC) but low carbon-to-nitrogen (C/N) ratio and total Na, K, and P contents [92]. In a poultry manure biochar, increased temperature of pyrolysis (300°C to 600°C) decreased yield, N, OC contents, and CEC but increased pH, ash content, OC stability, and the surface area [93]. The increase of pyrolysis temperature leads to the increase of surface area of biochar, which facilitates higher sorption.
The pyrolysis process with a varied feedstock composition, temperature, pressure, vapor residence time, moisture, and heat rate can be varied resulting in different quantities of syngas, biochar, and bio-oil product as well as their intrinsic properties. This means that maximizing biochar yields comes at the expense of the by-products. Novak et al. [94] opined that biochar production process can be customized to have specific characteristics matching select physicochemical problems of a particular degraded soil for the application of the biochar. Low pyrolysis temperature biochar (350°C) may contain large quantities of favorable nutrients with low sorptive capacities in comparison to high pyrolysis temperature (800°C) [95]. This arises because the C content and aromaticity of a biomass intensifies with the rise in temperature, whilst oxygen, hydrogen, and polarity reduce with an increase in micropores [52]. In a study, biochar produced at a higher temperature of 700°C was alkaline and applicable in neutralization of acidic soil and improved soil fertility and sequester C, while conversely, biochar produced at lower temperatures of 300°C was applicable for alkaline soils to correct the alkalinity problems [96]. To generate agricultural use biochar from poultry litter, pyrolysis at 300°C is suited unlike for C sequestration and other environmental applications [93].
3.2. Effects of Feedstock on Biochar Properties
A variety of feedstock is used in the preparation of biochar by pyrolysis method. Farmers and researchers are becoming more aware of the use of organic wastes and biomass as feedstock sources for pyrolysis; therefore, careful consideration has to be taken owing to the large effects of the feedstock on the resultant physicochemical properties of biochar. Most organic substances (crop and forestry residues, industrial by-products, animal manure, and sewage sludge) can be pyrolyzed [83] for the production of biochar. However, not all organic substances are suitable for producing biochar for agricultural purpose [83] due to the nature of the source and the conditions these biomass might have been exposed to. In addition, some pyrolysis conditions and feedstock source create biochar that cannot hold nutrients and are subject to microbial decay [97].
There are various potential feedstocks that are ideal with certain processing conditions for a desired maximum potential benefit in their end-use applications. Woody types with a high lignin content such as nutshells, residues from sawmills, and forest waste materials suit to capitalize on biochar yields [98] due to high quantities of lignin that is tough to break down in comparison to cellulose and hemicellulose [22]. Switch grass, a bioenergy crop, pyrolyzed in SP at 450°C–550°C, without oxygen, with more biomass, and vapor residence times, and in FP at 450°C–500°C, with little biomass and vapor residence time, generated syngas, bio-oil, and biochar [20, 86, 99]. A higher EC and CEC for crop residue biochar was observed in a study as compared to wood biochar [30], likened to corn cob biochar, wood biochar had higher CEC and greater pH neutralization in another study [100]. Furthermore, the variety of biomass and existing pyrolysis systems results in variable biochar produced [78]. Oxidized functional group, ash, and alkali (Na, K, Mg, and Ca) of feedstock affect biochar pH and EC; hence, their variability with pyrolysis conditions influences the resultant nutrient content accessible to plants on the application of biochar [83].
Biochar is also produced using animal wastes and food remains. Most manure is used in biogas production and the solid residue could be biomass in pyrolysis [22]. Plant nutrients therein, such as P, K, N, Mg, and Ca, maybe at a high level in the biochar. The use of plant product, nutrient-rich manure, and animal product for soil application biochar may decrease nutrient run-off and GHG emissions, such as CH4 and N2O [101]. The efficacy of rice straw char in a study was more than bamboo char in alleviating CH4 and CO2 emissions from paddy soils [21]. Amendment of biochars to soil enhanced the sorption of carbaryl pesticide due to chemical and biological degradation of the pesticide [41]. Pig manure char enhanced hydrolysis of carbaryl and atrazine in biochar at 700°C by decomposing the pesticide by 71.8% and 27.9% in 12 hours [51]. Some agricultural biomass are however not appropriate as biochar for agricultural application [80, 102]. Pyrolysis conditions and feedstock types can result in ineffective biochar in retaining nutrients and are prone to microbial decay [102]. Biochar derived from MSW and animal manure are rich in nutrients; however, they have been in limited use for agricultural soil due to safety concerns from toxic contamination of HM and organic pollutants such as PAHs dependent on their source. On the contrary, some sewage sludge biochars have low HM contents below risk levels and of low bioavailability levels to plants [96].
A variety of biomass is being employed in generating biochar at varied pyrolysis conditions, governing biochar properties. It is hence essential to categorize and rank suitable feedstock for the production of biochar [83]. Varied properties of feedstock materials and the resultant biochar produced due to different production process influence their chemical, physical, and structural properties. Appropriate feedstock is a limitation since most available sources comprise residues from agricultural and forest biomass. The resultant properties of biochar are vital to appreciate functionality of biochar in the soil and potential to control GHG emission.
4. Hazards in Biochar
4.1. Sources of Hazards in Biochar
The use of biochar is linked to certain risks, such as biochar being possibly toxic, retaining HMs, and suppression of the effectiveness of pesticides and effects on soil microbes, in agricultural land [28, 103–105]. The biochar may potentially contain elements that may facilitate the emission of GHGs on their application to plant production. These potential risks are formed from feedstock, contaminated feedstock, and pyrolysis conditions favoring the creation of characteristics and functional groups of this nature. Table 3 shows identified concentration of toxic organic and inorganic compounds in biochars.
Biochar has received increasing attention concerning its ability to immobilize HMs and reduce their bioavailability in soil plants. However, it has also been attributed to being a source or an enhancer of HMs content. Among some of the HMs identified in a variety of biochar include iron (Fe), zinc (Zn), copper (Cu), manganese (Mn), lead (Pb), silver (Ag), cadmium (Cd), chromium (Cr), calcium (Ca), Mercury (Hg), Arsenic (As), and nickel (Ni). Biochar produced from organic waste such as sewage sludge has been associated with carrying numerous HMs [109]. Sludge such as municipal sewage, pulp and paper mill effluent, and slaughterhouse sludge have high potential of toxicity due to high contents of HMs such as Cu, Cr, Pb, Ni, Cd, and Zn [24, 106]. In a study, Cu and arsenic (As) HM contents were enhanced more than 30 times with the application of biochar and at the same time an increase in C and pH in the soil [28]. Organic elements of biochars with high carboxyl contents activate Cu taken by alkaline soil [33]. Similarly, an increase in As and Cu mobility was noted on biochar application contrary to the effect on Cd that was insignificant [110]. These HMs become toxic contaminants to the soil because of their bioaccumulation potential to micro- and macro-organisms. High concentrations of HMs in contaminated soils may cause continued risks to the environment affecting plants, animals, and human health.
Potential sources of hazardous compounds include PAHs, polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) [18], and toxins such as volatile organic compounds, xylenols, cresols, acrolein, and formaldehyde [56, 103, 111, 112]. PAHs have been identified to be detrimental to plants and microbial organisms [113, 114]. Some toxic PAHs formed during biochar synthesis through incomplete incineration are recalcitrant to some extent; nonetheless, an increase in synthesis temperature can affect PAH contents [64]. In a study, PAH levels increased with increasing pyrolysis temperatures in straw-based biochar unlike a reduction in wood-based biochar [30]. Numerous biochars produced from pitch pine wood has exhibited PAHs levels [25]. The toxic elements are frequently catalyzed by Fe and Cu and may be produced by the catalytic combination of dioxin structures from O2, C, and chloride (Cl) at temperatures of 300°C to 325°C including other reactions after combustion [103]. Table 3 shows some studies that have highlighted toxic organic contaminants (PAH) and heavy metals measured in the studied biochar material. These are potentially toxic substances that can result in accumulation of contaminants in the soils amended with biochar.
4.2. Potential Risks of Hazards in Biochar
4.2.1. Effects on Crop Production and Soil Quality
Proper application of biochar in the soil increases soil quality resulting in increased productivity; however, improper application might reduce crop productivity and deteriorate soil quality [64]. Studies conducted revealed the reduction of the grain yield of rice by 10% and 26% on application of biochar at 8 t and 16 t·ha−1, respectively, [115] and a reduction of ryegrass yield by 8% and 30% on application of biochar 100 t and 120 t·ha−1, respectively [116]. Yield reduction on increasing biochar application per hectare can be owed to immobilization of N caused by excessive C/N ratios [117], and hydrophobicity [111]. Moreover, application of biochar on heavy clayey soil can also cause waterlogging including harming acid-loving organisms [118]. The nutrients contained in biochar can increase its levels in the soil on application subject to biochar feedstock. Biochar is not as beneficial in soil with plenty of soil organic matter (SOM) as a result, the application may reduce plant growth [118].
Improper biochar type application to soils such as alkaline soil amended with high pH biochar might adversely affect soil quality [119]. Additionally, other biochars may also have a large quantity of ash containing salts causing salinity [64]. Sorption of pesticides by biochar has been shown to reduce pesticide remains in crop soil. 1% application of biochar to spring onions reduced the bioavailability of two pesticides applied in soil by less than 50% over 35 days’ period [120]. This will, however, have direct implications in controlling pests due to the inefficiency of pesticides in the soils, which, in addition, may result in their increased application in the soil to control pests, hence endangering the crop harvest [121, 122]. This is an indication of the ability of biochar to immobilize plant nutrients and chemicals in the soil affecting economic pesticide application and quality of the harvest.
4.2.2. Effects on GHG Emission
The effect of biochar application into soil has been documented to either reducing or contributing to GHG emission through CO2, CH4, and N2O. Studies have shown that biochar application enhances GHG emission. Biochar application pyrolyzed at 350°C–550°C from wheat straw at 40 t·ha−1, with and without N heightened the CH4 emission by 34% and 41%, respectively [26], CO2 emission by 12% [29] and 44.9% from municipal bio-waste biochar in rice [123]. It was also reported that 24 t and 48 t·ha−1 of biochar applied from pyrolysis (500°C) of wheat straw increased the emission of N2O by 150% and 190%, respectively [124].
Soils amended with biochar at high rate derived from bamboo and rice straw pyrolyzed at 600°C reduced CH4 emissions from the paddy soil by 51.1% and 91.2%, respectively, in comparison to paddy soil without biochar, while more CO2 was dissolved in the water under alkaline condition reducing CO2 emission from the paddy soil at all rates of biochar application evaluated [21]. There was no significant effect on CH4 uptake from forest soil and grassland soils applied with pine sawdust-derived biochar produced at 550°C with and without steam activation, but cumulative N2O emission significantly reduced by 27.5% and 31.5% in forest soil and 14.8% and 11.7% in the grassland soil, respectively, while cumulative CO2 emission from the forest soil by 16.4%, but not from the grassland soil as compared to the control soil without biochar [125]. Increased soil organic matter through biochar application has positive impacts on soil physical properties, indirectly contributing to climate change mitigation by decreasing the quantity of N fertilizer required for crop production.
4.2.3. Effects on Human Health
Biochar in its dust form poses danger to humans during application in agricultural farms. Biochar generated from rice husk at high temperatures above 550°C contains toxic crystalline substance [126] including silica, that poses risk to human health [127]. When inhaled during biochar production, movement, and application process, it affects the respiratory system. There is a need for care during top dressing with biochar, and care should be taken to prevent erosion by wind and water and to manage health risks from biochar dust. Hence, such biochar producers must ensure quality control on the use by employing appropriate health and safety precautions during handling and application to soil [126]. Human health is additionally affected by the use of intoxicated biochar as a soil applicant, as indicated in Figure 2. Toxic elements and organic compounds pose a prominent risk to human health, leading to organ failure, due to their highly toxic carcinogenic substances in their compounds [128]. These toxic compounds become a threat to human health through the consumption of food through plants.
5. Risk Evaluation of Hazards in Biochar
Several evaluation methods have been utilized in various studies on the concentration of HMs, organic compounds, and the alleviation of GHGs. They include sorbent of extractable HMs, immobilization, stabilization of HMs, and metalloids concentration. Several mechanisms have been described occurring on the surface of the biochar among them: electrostatic attraction of metal cations with mineral; cation exchange of metal ions with mineral ions; interaction of metals ions with functional groups of biochars; and precipitation of HMs. The properties of the soil are important factors to consider in the immobilization and bioavailability of HMs, and hence, regulating to determine the right condition using various biochars would be important. There are a number of ways to evaluate the risk parameters of biochar depending on the content of evaluation.
5.1. Toxicity Characteristics of Leaching Procedure (TCLP)
TCPL determines if waste meets the environmental protection toxicity definition levels of hazardous waste. The TCLP is designed to determine the mobility of both organic and inorganic analytes present in liquid, solid, and multiphasic wastes with capability to analyze and test for 40 contaminants of maximum concentration for toxicity characteristic [129]. If a sample of tested waste fails one or more of these contaminant compounds, then it is considered to have a hazardous waste characteristic. It is important to have in mind that a characteristic waste with any of the 40 contaminant material may still be considered as hazardous waste even if there is an exemption that applies [129]. Leaching behavior of HMs and organic and inorganic analytes can be analyzed using the toxicity characteristic leaching procedures defined by the US EPA TCLP procedure [129].
5.2. Heavy Metal Concentration
HMs are a group of metals and metalloids with relatively high densities and are toxic at low-level concentrations occurring naturally or artificially. The release of these metals into the environment both naturally and anthropogenically can cause serious pollution through leaching of HMs into water resources and soil. Table 4 shows identified heavy metals and their reported effect in biochar application. The controlling factor to recycle and dispose contaminated waste is the accumulation and bioavailability of toxic elements. HMs, unlike organic pollutants, are non-biodegradable with a tendency to accumulate in living organisms such as plants and animals. The reuse of waste becomes possible with the removal of contaminants or immobilized/stabilized wastes present in order to allow recycle into raw material. Devi and Saroha [106], in their experiment to determine the risk analysis of bioavailability and eco-toxicity of HMs in biochar, used sequential extraction procedure to determine HM concentration in biomass and biochar using the following equation [106, 135]:
The toxicity of HMs depends on the total and bioavailable concentrations. Copyrolysis of pig manure and rice straw considerably reduced in the biochar the extractable concentrations of bioavailable Cu and considerably reduced the concentration of interchangeable and carbonate-associated Zn as compared to pig manure biochar at the same temperatures [136]. The dilution effect reduced the total and bioavailable Cu and Zn concentration associated with the minerals, surface area, and surface functional groups of biochar that are believed to reduce the release of HMs by chemical extraction reagent [136–139]. Similarly, this was observed in the copyrolysis of sewage sludge with rice straw/husks, with the addition of rice straw [140, 141].
Various sorption methods have been examined on HMs using a variety of biochars, including surface precipitation, functional groups, coprecipitation, and π-π interaction [63, 131, 133, 142]. Sorption of Pb using sludge-derived biochar in the determination of acid mine drainage treatment containing metals efficiently removes Pb2+, the reason being that Pb sorption primarily involved the coordination with organic hydroxyl and carboxyl functional groups, as well as the coprecipitation with the release of Ca2+ and Mg2+ [63]. Adsorption of Cu2+ by three crop straws biochars indicated that adsorption involved carboxyl and hydroxyl groups, with canola straw having more adsorption of Cu2+ compared to the other two biochars [131]. The relationship between Pb2+ adsorption and physicochemical properties of peanut shell and material from Chinese medicine-derived biochars indicated that functional groups complexation, Pb2+-π interaction, and minerals precipitation jointly contributed to Pb2+ adsorption [133]. Two biochars in the sorption of Pb2+, Cu2+, Ni2+, and Cd2+ from aqueous solutions indicated effectiveness in removing the four HMs mainly through surface precipitation mechanism [142].
Broiler litter biochar (350°C and 700°C) enhanced HM immobilization (CdII, CuII, NiII, and PbII), and cation exchange was outweighed by the coordination of π-π electrons of carbon and precipitation [33]. HM anions stabilizers (Al2O3, CaCO3, FeCl3, and NaOH) on biochar with varied pyrolysis temperature have been experimented on. Slow pyrolysis hickory wood (600°C) with the modification of NaOH considerably improved biochar’s surface area, cation-exchange capacity, and thermal stability showed higher sorption of HMs (Pb2+, Cu2+, Ni2+, Cd2+, and Zn2+) but preferentially removed Pb2+ and Cu2+ out of the mixed metal solution adsorption capacity as compared to pristine biochar [40].
Low-cost adsorbents including synthesizing biochar with Fe and Ca has been used in the removal of As and Cr from aqueous solutions. Organic municipal solid wastes, sewage sludge, rice husk and sandy-loam soil biochars (300°C) in the adsorption of As(V) and Cr(III and VI) from aqueous solutions managed to remove more Cr(III) and less As(V) and Cr(VI) due to high Fe2O3 content [143]. Independently, sewage sludge biochar removed 89% of Cr(VI) and 53% of As(V) due to enhanced metal adsorption via precipitation, unlike sandy-loam soil biochar that most effectively removed As(V) but could not retain metal anions unlike biochars [143]. Further investigations using rice husk biochar (300°C) impregnated with Ca and organic municipal solid wastes and rice husk biochars (300°C) impregnated with Fe for the removal of As(V) and Cr(VI), revealed that the enhanced biochars demonstrated low Cr(VI) removal rates. However, there was high As(V) removal capacity compared to the non-impregnated biochars due to metal precipitation and electrostatic interactions [144]. Pyrolyzed magnetic biochar (hematite mineral and pine wood biomass) at 600°C revealed a cheap source and unlike the unmodified biochar, the hematite-modified biochar not only had strong magnetic property but also the superior capability to remove As, due to sorption sites created through electrostatic interactions with Fe [54].
Engineering surface functional groups of biochar has been crucial in the advancement of high-performance adsorbents of HM and organic contaminants and cocontamination. Cd pollution in soil and water resources is a serious threat due to its release in the smelting of iron, lead, and copper ores. Modification of biochars using Bentonite (Bt), Fe and Mn oxides, and nanoscale zero-valent iron have been utilized in the removal of As and Cr. Due to the poor adsorption capacity of pure nanometals caused by agglomeration in aqueous solution, modified nanometals have demonstrated high-capacity adsorption. Bt-coated rosin biochar pyrolyzed at 400°C revealed fast and high Cr(VI) adsorption capacity with removal effectiveness of 95% within a minute under both acidic and basic conditions due to the dispersion of nanoparticles through the biochar network [145]. Fe-impregnated biochar indicated more sorption of aqueous As compared to pristine biochar, and further investigation suggested that As sorption was mainly controlled by the chemisorption mechanism [146]. Pyrolysis of paper mill sludge produced with Fe/Ca-rich engineered biochar showed a decrease in adsorption of As(V) and Cd(II) with an increase in PO43− concentration and Ni(II) ion, respectively, and NaOH or HCl desorption renewed the absorption [134]. Biochar modified with sodium alginate using Ca(II)-impregnated biomass resulted in the removal of high Pb(II) capacity compared to most adsorbents due to anti-interference caused by functional groups and minerals of the biochar [147].
Nanoscale zero-valent iron (nZVI) biochars for the remediation of Cr-contaminated soil exhibited immobilization of Cr(VI) and reduced the phytotoxicity of Cr and the leachable Fe favorable for plant growth [130]. Modified biochar produced with acid, base, and oxidation treatment that supported zero-valent iron nanoparticles improved the removal of Cr(VI) using acid treated biochar, owing to the large surface area, low surface negative charge, and low pH [148]. Magnesium oxide nanoparticles stabilized on N-doped biochar synthesized by fast pyrolysis (400–600°C) resulted in a high Pb adsorption capacity in short equilibrium time (<10 min) and a large material through the system including removal of Cd2+ and tetracycline [149]. Modified biochar through activation has demonstrated great sorption efficiency of HMs.
The degree of environmental risk assessment of HM pollution in biomass and biochars has been determined by using potential ecological risk index and risk assessment code. Potential ecological risk index (RI) is used to assess the degree of potential risk of HM pollution in biomass and resultant biochar. The following ecological risk index equation were proposed and used [106, 150].
Cf, the contaminant factor of a HM, is the sum ratio of the HM concentrations extracted from the sequential extraction to the concentration of the HM in the residual fraction [151]. This value is inversely proportional to the leaching potential of the HM; Ci is the mobile fraction and Cn is stable fraction of the HMs; Er is the potential ecological index for individual HM; Tr is the toxic factor of the individual HM; and RI is the potential ecological risk index and it is obtained by multiplying the contamination factor (Cf) of the HM with the toxic factor (Tr) of the HM. The Tr values for individual metal can be obtained from Hakanson, [150, 152]. The potential ecological risk index (RI) of biomass and resultant biochar is obtained by adding the potential ecological index (Er) of each HM present in the solid [106].
Risk assessment code (RAC) evaluates HM toxicity in the environment including assessment of potential risk of HM in biomass and resultant biochar [24, 46]. RAC is based on the percentage of directly bioavailable exchangeable metal and carbonate-associated fractions of the total HM, and the value is obtained from the total amount divided by the total concentration of available HM multiplied by one hundred percent [46].
5.3. Organic Chemical (POPs) Concentration
Biochar from a variety of biomass and waste products has been utilized and investigated in the elimination of organic substances through various mechanisms. The porosity of biochar develops more with the increase in relative high pyrolysis temperature and the lack of activation process limits pollutant removal efficiency, including other value-added applications [35]. Table 5 shows some reported effects of biochar utilization on detected organic pollutants. The mechanisms of removal by biochar through adsorption include hydrogen-bond, π-π electron donor-acceptor interaction, pore-filling, and hydrophobic effect for organic compounds [50, 51, 57, 159]. Single- and bisolute sorption of organic compounds (1,3-dichlorbenzene (DCB), 1,3-dinitrobenzene (DNB), and 2,4-dichlorophenol (DCP)) on ground tire rubber char (200°C–800°C) in a study showed that the organic compound surface area, aromaticity, and hydrophobicity increase greatly with pyrolytic temperature [57]. The adsorption was attributed to pep electron donor-acceptor interaction, H-bonding, and partition [57]. Soybean stover and peanut shell biochars (300°C and 700°C) removed organic compounds from water through adsorption dependent on the biochar’s properties and its efficiency comparable to that of activated carbon due to increased hydrophobicity, surface area, and decrease in polarity [50]. Pig manure biochar adsorbed pesticide (carbaryl and atrazine) with enhanced hydrolysis due to high pH through hydrophobic effect, pore-filling, and π-π electron donor-acceptor interactions [51]. The sorption behavior of organic pollutants on biochars (300°C and 700°C) determined from orange peel, pine needle, and sugarcane bagasse feedstock resulted in biochar (300°C) displaying high sorption due to high adsorption fraction on the surface and pore-filling mechanisms [159].
Biochar with enhanced sorption capacity or selectivity for pollutant removal through activation, magnetization [60, 154], and hydrothermal synthesis has been researched on. Recently, N-doped porous carbons have demonstrated better performance in adsorption [35, 53, 60], catalysis, and capacitors relative to pure carbons [35]. Lian et al., [35] reported a high adsorption capacity for cationic and anionic organic compound dyes, better than many other reported adsorbents using N-doped biochar (600°C–800°C) from crop straws as raw material. Pyrolysis at 800°C had the highest anionic and cationic adsorption that was attributed to electrostatic attraction, π-π electron donor-accepter interaction, Lewis acid-base interaction, and microporous structure formed in the biochar [35]. Active sites on biochar-graphene and wood biochar (300°C, 500°C, 700°C) in the adsorption of phthalic acid esters (PAEs) organic compounds resulted in biochar-graphene exhibiting higher adsorption capacity for contaminants remediation due to the pore-diffusion mechanism, π-π electron donor-acceptor interaction, and hydrophobicity [60].
Modification of biochar with metal ions has shown sorption capacity of organic pollutants by means of creating active sites. Shan et al., [154] prepared magnetic biochar and activated carbon with Fe3O4 by ball milling for removal of pharmaceutical compounds by adsorption and mechanochemical degradation. The hybrid adsorbents exhibited high removal through degrading and were easily separated magnetically and the sequential quartz sand milling improved the mechanochemical degradation of pharmaceutical compounds on biochar [154]. The formation of magnetic Fe3O4 on pine sawdust biochar (650°C) via oxidative hydrolysis of FeCl2 to remove organic compound sulfamethoxazole solution ended up in favorable adsorption of the organic compound onto biochar through exothermic adsorption and physisorption due to hydrophobic interaction [53]. 20% optimized MgO-impregnated porous biochar from sugarcane harvest residue prepared using adsorption pyrolysis method (550°C) from swine wastewater exhibited maximum adsorption capabilities for phosphate, ammonium, and organic substances from nutrient-rich livestock wastewaters [58].
Postpyrolysis thermal air oxidation of biochar has enhanced the sorption of organic compounds [160] and water-extractable substances that are toxic to aquatic plants and animals [161]. Investigation of the effects of thermal air oxidation on corn cob biochar after pyrolysis (300°C–700°C) showed that well-carbonized biochar was made at 600°C and 700°C with increased surface area, porosity, and adsorption, 120 times that of neutral organic substances [160]. The effects of thermal air oxidation of wood and pecan shell biochar had adsorptive properties towards organic compounds, with up to 100-fold of enhanced adsorption by means of enlarged surface area and nanopores [161].
Modified biochar is also an effective degradation method of organic pollutants through PFRs. Degradation can completely remove organic toxins from the environment as compared with sorption method. Persistent free radicals (PFRs) in biochar has indicated tremendous ability to activate persulfate/hydrogen peroxide/oxygen for the degradation of organic contaminants. The outcomes provide a method of manipulating the transformation of PFRs of contaminants in the biochar for the development of activator persulfate-based towards remediation of contaminated soils. Pine needles, wheat, and maize straw biochars effectively activated H2O2 for PFRs degradation to produce hydroxyl radical that degraded the organic compound [162]. Hydroxyl radical generation from biochar suspensions in the presence of oxygen degraded the organic compound diethyl phthalate (DEP) [163]. Metals (Fe3+, Cu2+, Ni2+, and Zn2+) and phenolic compound loaded on biomass increased the concentrations of PFRs in biochar and changed the type of PFRs formed to persulfate, indicating that the manipulation of the number of metals and phenolic compounds in biomass is an effective method to control PFRs in biochar [164]. Additionally, PCBs contaminants efficiently degraded with the catalytic ability of biochar to persulfate activation [164].
Fallen-leaves and wood chips hydrochar enhanced sulfadimidine organic chemical degradation due to abundant photoactive surface oxygenated functional groups in daylight irradiation than in the dark compared to fallen-leaves and wood chips pyrochar that generated reactive oxygen in the dark due to PFRs present [165]. Photogeneration of reactive oxygen species from pine needles and wheat straws biochar degraded and partially mineralized diethyl phthalate organic pollutant under UV and simulated solar lights [166]. Similarly, environmental persistent free radicals (EPFR) in the presence of different types of biochars promoted degradation of organic compounds (p-nitrophenol and p-aminophenol) and not even coating of biochar with natural organic matter inhibited p-nitrophenol degradation, suggesting the organic compound degradation capability of biochars in soil and natural water [155]. With such promising effects to degrade organic compounds with biochar, it is of importance to take caution on pesticide remediation process and the intended use of pesticide to bring a balance.
5.4. GHGs Emission
GHG effect is the heat-trapping process by GHGs within the surface-troposphere system. N2O, CO2, and CH4 are potent GHGs. Taking CO2 as a reference point with a global warming potential (GWP) of 1, N2O, and CH4 are estimated to have a GWP of 28–36 and 265–298 times that of CO2, respectively [167]. Emission of the main long-standing atmospheric GHGs, N2O, CO2, and CH4, increases global warming and consequently the necessity to mitigate them from the environment [168]. On average, CH4 absorbs more energy than CO2 and CH4 and N2O remain in the atmosphere more than a decade while CO2 remains in the atmosphere for thousands of years [167]. Agricultural activities influence global warming as a result of the considerable discharge of GHGs, notwithstanding it being the major sink of CO2 during photosynthesis [169, 170]. Biochar has a primary key function of carbon sequestration, and its stability in the soil can affect its efficiency. The stability and resistance to microbial degradation of carbon in biochar is the basis as a sequestration technique, due to its steadiness in severe weather conditions and resilience to the effects of chemicals [171]. The priming effect of biochar, however, reduces with the rise in pyrolysis temperature.
Biochar and its application to soil are mostly known for the effect of increasing the soil carbon among other nutrients through the pyrolysis process as carbon is sequestered more than its release in the atmosphere. Despite this, some application of biochar may increase the release of CO2, suggesting signs of decay. In a study, CO2 in soil increased with biochar application rate; however, it diminished within 6 days of the incubation [172]. The use of biochars derived from different types of biomass has enabled the maximum utilization of biochar in the pursuit of global climate change mitigation. Varied biochar application amount and time have enabled the investigation of GHGs, with results indicating the effectiveness to sequester carbon [72, 99] with a low degradation rate and long-term stability in soil [173]. CO2 is the principal GHG and the consistent rise in its release is the main cause of global warming. There are some controversies regarding the role of biochar on CO2 release [174]. Pyrolysis temperature during biochar preparation determines the CO2 emission from soil. Application of biochars to soils in stabilization of soil organic matter is influenced by the soil type, and high-temperature biochar produced is suitable for long-term soil-C sequestration while low-temperature biochar is suitable in the increase of soil fertility due to mineralization [175].
The longer reaction time of over 12 months soil incubation mineralized biochar and the mean residence times for the biochars projected between 44 years and 610 years [38]. The mean residence time may, however, vary under different environmental and soil conditions, an indication that the biochar stabilized by variable charge minerals at high temperatures [38]. A 5-year laboratory experiment on the stability of 11 biochars (400°C and 550°C) observed that 0.5% and 8.9% of the biochar C was mineralized over 5 years with C in manure-based biochars mineralizing faster than that in plant-based at 400°C than at 550°C biochars [176]. The estimated mean residence time of C in the 11 biochars varied between 90 years and 1600 years; however, it is likely to be higher under field conditions with lower moisture and temperatures or nutrient availability constraints [176]. High hydrothermal temperature, longer reaction residence time, and biomass of higher-lignin content with larger particle size produced biochar with higher stability [38, 176–179]. The effect of peak temperature, particle size, and pressure on the potential stability of slow pyrolysis (800°C) vine shoots biochar activated with aluminum oxide observed that particle size under higher peak temperatures conditions was the most influential as large particles lead to an increase in the fixed-carbon yield, percentage of aromatic carbon, and pH, hence a more stable biochar [179].
Designer biochars with enhanced capacity for carbon sequestration and stability using beneficial minerals have been used in copyrolyzing feedstock resulting in biochar enriched with minerals, adding to soil fertility. Pyrolyzed rice straw with kaolin, calcite, and calcium dihydrogen phosphate minerals to biochar enhanced the stability of biochar [178]. Yak dung and attapulgite clay mixed to produce biochars at 50/50 ratio clay to dung in Tibet resulted in low-cost high pasture yields and grass nutrition quality [180]. Higher pyrolysis temperature with the addition of clay proportions resulted in higher concentration of stable carbon, surface area and porosity, surface mineral concentration, and electrochemical capacitance, contrary to the lower temperature that resulted in higher concentration of total C and N, C/O and N functional groups, and magnetic moment [180].
Carbon sequestration potential of chicken manure-derived biochars impregnated with mineral salts (CaCl2, MgCl2, and FeCl3) prior to pyrolysis affected biochar nutrient composition and dynamics and increased C sequestration potential [181]. The bioavailability of enriched Cu and Zn in the biochars significantly reduced, and the biochar treated with Fe mineral salt samples had the least C loss during pyrolysis and chemical oxidation and the greatest chemical and biological stability compared to pristine biochars [181]. Soils having high minerals favor long-term stability of biochar. The interaction between soil minerals (FeCl3, AlCl3, CaCl2, and kaolinite) to investigate biochar stability and the long-term stability for comprehensive assessment of carbon sequestration efficiency demonstrated that the minerals attached tightly to biochar (surface or inner pores) and organometallic complexes (Fe-O-C) were generated with all the 4 minerals, enhancing the oxidation resistance of biochar surface by decreasing the bond of C-O, C=O, and COOH [182]. Through chemical oxidation with kaolinite, the stability of biochar increased by reducing the biodegradable C loss of total biochar, hence beneficial long-term carbon sequestration in the environment [183].
The incorporation of metals (Mg, Al, Fe, Ni, Ca, and Na) enhanced the CO2 adsorption onto the metalized walnut shell biochars and N2 heat treatment, with Mg-biochar and Na-biochar being highest and lowest adsorbers due to physisorption [59]. Additionally, Mg-biochar indicated a great stability of cycles of adsorption-desorption with no loss of capture capacity, easy regeneration, and fast desorption kinetic, an indication of a superior capture performance towards CO2 over N2, O2, and CH4 [59]. Sewage sludge use with the addition of Ca(OH)2 to improve carbon stability in biochar indicated an increase in dissolved organic carbon content, carbon retention, and improved the surface area and alkalinity of the biochar due to the formation of CaCO3 and an increase in carbon-containing functional groups [44]. Cottonwood biochar treated with metal ions (aluminum hydroxide, magnesium hydroxide, and iron oxide), pyrolyzed at 600°C, indicated that at room temperature and atmospheric pressure, biochar optimization with metal ions enhanced CO2 adsorption ability, with aluminum hydroxide-biochar composite capturing more CO2 than other metal composites [184]. This was attributed by surface adsorption mechanisms causing surface bonding from carbon surface and metal oxyhydroxide particles [184].
Methane has more global warming potential than CO2 with paddy fields being among the sources of its global release [26, 169]. The method of biochar application to agricultural soil as a mitigation measure for CH4 and N2O emissions has been studied. N2O emissions had a significant increase with rice plant and rice-straw-derived biochar amendment under ambient CO2 concentration and air temperature, while N2O emissions were suppressed under simultaneous elevated CO2 concentration and air temperature, with and without biochar amendment, thus weakening the biochar [185]. Reduced mineral N concentrations and increased dissolved organic carbon concentrations could inhibit N2O emission at simultaneously elevated CO2 concentration and air temperature [185].
Anaerobic incubation of paddy soil for 14 days with rice straw biochar showed that abundance of denitrifying bacteria was reduced with biochar amendments, contributing to the decreased N2O emissions while increased abundance of iron-reducing bacteria, competed with methanogens to produce CH4, thereby leading to lower increase in CH4 emission [186]. It was concluded in this study that biochar amendments with high pH and surface area were effective in mitigating the emission of N2O and CH4 from paddy soil [186]. N2O, CO2, and CH4 were monitored twice a week for 1.5 months after adding biochar that resulted in high CH4 uptake, but no significant differences were found in CO2 and N2O emissions [187]. Adding rice straw-derived biochar in a paddy soil reduced CH4 emission under ambient and elevated temperature and CO2, attributed to the decreased microbial activity along with the increased CH4 oxidation activity [188]. Soilborne emissions are predominantly the major sources of N2O in the air [32] caused mainly by nitrogen transformation microbes in the soil through nitrification and denitrification. Suppression of N2O becomes an important climate change mitigation, varying with biomass source and pyrolysis environment. The high GWP of N2O makes it an important GHG; hence, progressive reduction of its emission from paddy fields is of importance. The alternating wet and dry conditions of rice paddies make it a major source of N2O emissions [170].
It has been observed that the use of biochar in the mitigation of N2O emission escalates CO2 emission. An incubation study with 4 contrasting soils and oil mallee, wheat chaff, and poultry litter biochars resulted in Tenosol soil having the highest mitigation of N2O in that biochar limited the availability of NO3− with the resultant rise in N2O including liming and increased microbial respiration [48]. Biochar used in an experiment to mitigate CO2 and N2O from agricultural soils suppressed N2O at moderate levels without earthworms, and CO2 and N2O emissions in the presence of earthworms increased, which was influenced by biochar type and application rates [189]. It was concluded that normal agricultural conditions suppress N2O under high biochar application and heightens CO2 emissions [189]. Biochar amendment can affect bacteria composition of N2O-reducing functional microbial traits in soil [39]. Biochar enhancement in the growth of organisms involved in N cycling and flux of N2O in the soil showed that biochar acts as a transitory store of nitrogen in the soil, moderating N cycling dynamics, thereby reducing N losses to leaching and gas fluxes [190]. Additionally, biochar influenced bacterial N cycling by either promoting the denitrification, N2O to N2, or possibly producing NH4+, adsorbed to biochar and alter soil N dynamics [190].
Emission of GHGs (CO2, CH4, and N2O) in soils amended with biochar was through physical and biotic mediated mechanisms and corrected with soils and biochar properties. Table 6 indicates some effects of feedstock on GHG reduction. The use of various biomass for biochar, different soil types, enhancement with beneficial minerals and salts, and increasing the soil incubation time has brought various results suitable in the pursuit of global climate change mitigating with some arising contradiction attributed to the different soils and biochar properties used. Continuous research on the effects of biochar type on soils, microbial community on HM concentration, GHGs, and organic contaminants should continue.
5.5. Health Risk
Soil pollution generates serious effects endangering the natural environment, agricultural sustainability in food safety, and the health of those who consume the food. Biomass intended to be used for the production of biochar may contain contaminants that pose a risk to the environmental and health of humans, plants, and animals. In a soil pollution survey in China cropland, HM contaminants (Zn, Se, and Cd) were identified and reported to be affecting subsistence-diet farmers in rice grain, raising health concerns [194, 195]. Cd health risk through food exposure from consumption of rice has been a concern originating from contaminated acidic rice paddies irrigated with wastewater from municipal sewage and mining tailing as well as chemical fertilization in South China [194, 196, 197]. Biochar brought about a profound implication among those using it as an agricultural field applicant. The effect of biochar amendment on rice in a Cd contaminated paddy field reduced Cd plant uptake in a 2 year monitoring by 16.8%, 37.1%, and 45.0% in 2009 and by 42.7%, 39.9%, and 61.9% in 2010, while the total plant Cd uptake was found to decrease by 28.1%, 45.7%, and 54.2% in 2009 and by 14.4%, 35.9%, and 45.9% in 2010, with biochar amendment at 10t, 20t, and 40 t/ha, respectively [196]. Biochar amendment in combination with low Cd cultivars may offer a basic option to reduce Cd levels in rice as well as to reduce GHGs emissions in rice agriculture in contaminated paddies [196].
PTEs discharge to the soil environment through increased anthropogenic activities is a global threat and plants grown in PAH-contaminated soils or water can become contaminated [153]. PAHs detected in the aqueous extracts are believed to be partly responsible for the reduction in corn seedling growth with repeated leaching of biochars eliminating the negative effects on the seedling growth [153]. These PTEs can have harmful and chronic-persistent health effects on exposed populations through food consumption grown on contaminated soils. Efforts to investigate the transformation mechanism and accumulation behavior of PTEs in soil plant system and their adverse health effects have been focused extensively. However, limited studies address biochar nanosheets (BCNs) as a potential soil amendment to reduced humans’ health risks through dietary intake of food-crop grown on PTE-contaminated soil [55]. BCNs synthesized from pine wood sawdust used as soil amendment to reduce potential risks of PTEs through consumption of food grown in PTE-contaminated (Cd, Cr, Ni, and Pb) soils showed some cutback on health hazards of PTEs through reduced bioavailability and phyto-accumulation and their daily intake via consumption of wheat compared to both conventional organic amendments (COAs) and control [55]. The risk assessment outcomes for the hazard indices (HIs) were <1 for PTEs in all treatments with the BCNs addition significantly () reduced risk level, when compared to control. BCNs addition significantly suppressed cancer risk for Cd, Cr, and Ni over a lifetime of exposure compared to control [55].
Consumption of rice contaminated with PTEs is a major pathway for human exposure to PTEs as revealed in China’s so called “Cancer Villages”; hence, sewage sludge biochar was applied to suppress PTE (As, Cd, Co, Cu, Mn, Pb, and Zn) phyto-availability in soil to reduce PTE levels in rice grown in mining-impacted paddy soils [61]. Risk assessment indicated that 10% biochar () decreased the daily intake, associated with the consumption of rice by 68, 42, 55, 29, 43, 38, and 22% PTEs, respectively [61]. Health quotient (HQ) indices for PTEs (except for As, Cu, and Mn) were <1, indicating a suppression of health risk pointing to the incremental lifetime cancer (ILTR) value for iAs (AsIII + AsV) associated with the consumption of rice significantly reducing () by 66% [61]. Biochar application can enhance phthalic acid ester adsorption in soils, which is a priority pollutant, endocrine-disrupting compounds, and its accumulation in the human body causes potential mutagenic health threats [198]. The immobilizing ability of enhanced biochar is useful to consider when designing phthalic acid ester immobilizer for the reduction of phthalic acid ester bioavailability [51]. The presence of contaminants such as HMs and organic compounds in biochar for soil application presents undesirable agricultural and human health risks with its continuous use. Testing for the presence of toxic content should be a priority component of biochar quality assessment due to the particular concern of the hydrophobic, recalcitrant, persistent, potentially carcinogenic, mutagenic, and phytotoxic properties [153].
6. Environmental Implications
In order to evaluate the environmental implications of biochar application, the heterogeneous properties need to be understood. Application of biochar to soils creates an irreversible condition of its removal; hence, cautious consideration on the use of waste is needed on its production and assessment of effects on the environment and agricultural use. Identified of potential sources of hazards can be prevented at the initial stage of the biochar production process by isolating and prohibiting contaminated feedstock/biomass, including regulating the pyrolysis environment that may favor their production to avoid detrimental resultant biochar. Physical and chemical properties of biochar such as composition and particle and pore size distribution are largely controlled by feedstock type and pyrolysis conditions, hence the necessity to determine its behavior to plants, outcome to the environment, suitability to soil improvement application, and contaminant removal. Other properties of biochar, such as CEC, pH, and functional groups, also vary. This enables the introduction of a necessary control measure to assess and monitor the production quality of biochar suitable for particular uses such as land-use type, climate change, soil type property improvement, and soil contaminants.
There are numerous biomass wastes such as plant and animal residue, sewage sludge, and MSW with beneficial use as optional feedstock sources. Minimization of agricultural waste and reuse of manmade biomass for biochar has brought positive contamination remediation and mitigation effects to the environment. However, some feedstock may contain toxicants (HMs, organic compounds) and biological (pathogens) threats to human health and the environment with the uncontrolled application. The concentration of metal elements, including essential elements, may lead to DNA and cell membrane damage [199], including oxidative stress [200]. The potential to alter these threats through controlled feedstock, pyrolysis conditions, modified biochar, activation materials, metal oxide nanoparticles on N-doped, and zero-valent iron nanoparticles offers substantial environmental advantage through nonavailability of metal ions, immobilization, and stabilization of harmful ions, cations, and compounds. The eco-toxicity of HM in sludge has been shown to decrease significantly after pyrolysis or liquefaction processes resulting in a decrease in the environmental risk of biochar utilization [24, 106]. Regulating the type of feedstock used controls undesired detrimental properties from the initial.
Thermal air oxidation on post-pyrolysis has been shown to affect bioavailability of organic substances. Thermal air oxidation of biochar at high-heat thermal temperatures open pores, introducing hydrogen bonds for ionizable compounds to enhance the adsorption of organic substances affecting bioavailability to plants, soil, and water [160, 161]. Hydrochar as compared to pyrochar had more organic pollutant degradation due to higher photochemical reactive oxygen generation ability, an indication of a viable environmental remediation tool under solar light irradiation [165]. Photo generation of reactive oxygen species in biochar further reveals that biochar particles can partake in the photo-transformation of contaminants in the natural environment for the degradation of organic contaminants in wastewater [166]. Organic contaminant degradation by EPFRs can be a process considered when assessing the environmental roles of biochar and other carbonaceous materials regulating the fate of contaminants removal [155]. Multilayer surface sorption, pore-filling, and thermodynamic of low-temperature biochar are suitable for remediating environmental organic molecules pollutants, hence a basis for designing biochars multifunctional sorbents of pollutants [159]. Manipulation of metal and organic concentrations in biomass significantly changed the concentrations factors of PFRs in biochar which was an alternative activator of persulfate for the degradation of contaminants, a strategy for the reuse of hyperaccumulator biomass in the phytoremediation of HM from soil. Hydrogen peroxide and persulfate oxidants can be replaced with pyrolyzed hyperaccumulator biomass to make catalytic oxidant decomposition for organic pollutant degradation and contaminated soil remediation [164].
Carbon sequestration has been proposed from varied feedstock, with some addition of minerals, applied to diverse soil and plant environment, each with different results; hence, the total environmental recalcitrance is a function of both the properties of the biochar generated and soil environment to which it is applied [201]. The reduction of CH4 production with the use of biochar in paddy soil under the elevated temperature and CO2 condition is relevant to the prediction of global warming environment and points the essence of biochar in assisting with slowing down the greenhouse effect [188]. Biochar has been shown to promote changes in bacterial families; hence, advanced research to measure temporal changes and metabolism of specific bacteria in the soil can be considered beneficial, more importantly in mitigation of GHGs fluctuation [190]. The MgO-impregnated livestock wastewater biochar facilitates resource conservation, nutrient cycling, and sustainable development of the environment through removal of phosphate and ammonium, availing a source of nutrient-rich fertilizer product for agricultural soil application and reduces agricultural waste disposal as it lessens CO2 emission [58].
The effective use of biochar as a carbon sequestration strategy requires quality assessment with regards to environmental recalcitrance during the sourcing of feedstock and biochar preparation and before and after application to soil. There are a number of characterization procedures used involving cost and time of analysis associated resulting in unreasonable to the wide-range application [177]. To this understanding, there is not a well-known current framework conducting quality assurance/quality control checks in pre- and postapplication assessment of biochar carbon sequestration. A framework for assessing biochar carbon sequestration ought to describe the environmental effect on degradation, which includes temperature, moisture, mineralogy, and organic matter of soils [201–205]. The development of R50-based model accounts for variability in appropriate environmental situations and predict intermittent biochar carbon loss and recalcitrance likelihood over time. The model can be incorporated with an economic model to assess the long-term tradeoffs of a specific R50 biochar application to soil compared to other alternatives [177]. The R50 index-based methodological framework assesses the environmental recalcitrance including carbon sequestration potential of biochars applicable to the preapplication screening of modified carbons into Class A (R50 ≥ 0.70), Class B (0.50 ≤ R50 < 0.70), or Class C (R50 < 0.50) recalcitrance/carbon sequestration classes [177]. By coupling R50 with biochar properties, it can be screened to find the “optimum” biochar for practice applications with additional targeted benefits for soil improvement [206].
Biochar for soil amendment can be produced depending on the conditional needs of the soil to enhance soil properties and reduce the threats of contaminants from pollution and overuse of soil nutrients and organic compounds on the environment. The remediation effect of biochar to remove the contaminants and make the soil contaminant free and nutritious will ensure the normal growth of various crops. Usage of biochar in the right way, appropriate biochar, and dose in a particular soil are necessary. Screening of effective biochars as engineered sorbents has been proposed with the use of peanut shell and Chinese medicine material-derived biochars to remove or immobilize Pb2+ in polluted water and soil [132]. Pyrolysis of some biomass can be a source of secondary pollution of HMs. A single chemical extraction procedure does not reveal the total bioavailable HMs in the biochars, and therefore, to assess the potential risk and long-term stability of the biochars, the chemical speciation of HMs in the biochars need to be examined and assessed. Sequential extraction procedures have been successfully used to determine the extractable total and bioavailable HMs in manure [136–139]. This enhances the utilization of suitable engineering production, and risk assessment approaches to ensure potential hazards are eluded. It is necessary to keep abreast with up-to-date research and engage the expertise on the current production technologies to control possible hazards.
Biochar use as an alternative adsorbent to remediate contaminants could be advantageous and cheap compared to activated carbon since less energy is required with low-cost and readily available pre- or postactivation materials. Modified biochar with magnet (hematite) and NaOH are cheap sources to enhance biochar properties and create friendly environmental substitute adsorbent in the removal of As contaminant using external magnet, and HMs (Pb2+, Cd2+, Cu2+, Zn2+, Ni2+) [40, 54]. Magnesium oxide nanoparticles on N-doped biochar is suitable for Pb adsorption and the removal of Cd2+ and tetracycline in various environment obstructions (pH, natural organic matter, and metal ions), thus its suitability in the treatment of wastewater, natural water resources, and drinking water [149]. Similarly, modified and efficient zero-valent iron nanoparticles methods are cost effective. Bt-coated rosin with Fe2O3 can be efficiently used for Cr(VI) removal in water resources in varied pH range or even during leakage [145, 148]. The resultant Cr(III)/Fe(III) hydroxides produced in the biochar are insoluble and can easily be removed to eliminate secondary pollution [148]. The release of scrap tire chars in the environment is also a cost-effective sorbent in the treatment of organic contaminated (DCB, DNB, and DCP) wastewater if properly managed and would exhibit distinct effects on the fate and transport of organic contaminants due to hydrophobicity, polarity, and H-bonding acceptor/donor properties [57].
Toxic compounds that are related with thermal treatment products and feared for soil amendments should be controlled for biochar use. PAH can be produced during the pyrolysis process and is controlled by substrate composition and temperature, which play an important role in its formation, and it is adsorbed onto biochar surfaces [153]. Consideration can be made on the flexibility of minimizing the production of contaminated biochar and maximizing production of syngas and bio-oil, co-products of pyrolysis. The health quotient (HQ) is often used for assessing potential risks and adverse health effects resulting from the ingestion of pollutants, with food chain being a major pathway for human exposure [61]. High-capacity accumulation of PTEs in rice and its high level of consumption are the main source of exposure with dietary consumption exposure being 3–11 times higher than that in vegetables, with rice consumption contributing >75% of the PTE intake for a population of a village near the abandoned mine [61].
An assessment of biochar production suitability for use as a soil additive or production of energy, without environmental and production risks, requires a comprehensive analysis of biomass. The variety of biochar properties, with a range of nutrients and acceptable contaminant levels, suggests soil application suitability for growth of plants for consumption and forestry. Certification schemes operated by governing bodies need to administer a robust set of standards to ensure the sustainable use of biomass resources by identifying suitable biomass and analyze test procedures for industry standard. Minimum risks of measured quantity levels that do not pose a significant risk have been recommended. Standards given by IBI and EBC provide a starting point guidance to consider possible risks that can be tested, measured, monitored, and controlled. A continuous and essential quality assessment and monitoring methodology can demonstrate effective control of hazards in biochar. Effective risk evaluation, management, and sustainability guidelines, with scientific research being a driving factor, will cause biochar to be an indispensable tool for environmental management.
7. Conclusion
It is important to have in mind the characteristics of waste being considered as biomass. The degree of biochar net benefit must supersede the related risks. The toxicity of HMs depends on the total and bioavailable concentrations. Various sorption methods of biochar utilized, among them low-cost adsorbents, engineered surface functional groups, and nZVI modified biochars, have been examined on HMs using a variety of biochars, which involved surface precipitation, functional groups, coprecipitation, and π-π interaction. The chemisorption mechanism involved the coordination with organic hydroxyl and carboxyl functional groups, functional group complexation, minerals precipitation, electrostatic interactions, and the dispersion of nanoparticles through the biochar network. The mechanisms of organic compound removal through sorption and enhanced sorption was through activation, magnetization, and hydrothermal synthesis. These involved hydrogen-bond, π-π electron donor-acceptor interaction, pore-filling, pep electron donor-acceptor interaction, hydrophobicity, H-bonding, partition, polarity, high surface adsorption fraction, electrostatic attraction, Lewis acid-base interaction, and microporous structure and pore-diffusion mechanism. Modified biochar with metal ions involved magnetic separation through exothermic adsorption and physisorption due to hydrophobic interaction. Postpyrolysis thermal air oxidation had adsorptive properties through increased surface area, porosity, and adsorption. Degradation through Persistent free radicals (PFRs) activated persulfate/hydrogen peroxide/oxygen due to abundant photoactive surface oxygenated functional groups. Emissions of GHGs in soils amended with biochar emanate through physical and biotic mediated mechanisms, which are lessened with enhanced soils and biochar properties. High-temperature biochar produced is suitable for long-term soil-C sequestration. High hydrothermal temperature, longer reaction residence time, and biomass of higher-lignin content with larger particle size produced biochar with higher stability. Designer biochar with the addition of beneficial minerals enhanced capacity for carbon sequestration and long-term stability attributed by surface adsorption mechanisms. Biochar and BCNs have a significance in reducing the health quotient indices for PTEs risk contamination by suppressed cancer risk arising from consumption of food contaminated with PTEs, pointing to a reduction in the incremental lifetime cancer (ILTC) value.
The immobilizing ability of enhanced biochar has been successfully used to reduce bioavailability of pollutants with mutagenic health threats. The degree of environmental risk assessment of HM pollution in biomass and biochars has been determined by using potential ecological risk index and RAC in assessing the degree of potential risk of HM pollution in biomass and resultant biochar. Organic contaminant degradation by EPFRs can be a process considered when assessing the environmental roles of biochar and other carbonaceous materials regulating the fate of contaminants removal. Identified potential sources of hazards can be prevented at the initial stage of the biochar production process by isolating and prohibiting contaminated feedstock/biomass, including regulating the pyrolysis environment that may favor their production to avoid detrimental resultant biochar. The potential to alter these threats through controlled feedstock, pyrolysis conditions, biochar, and modified biochar offers substantial environmental advantage through nonavailability of PTE. Hazards posed by the use of biochar necessitate it to be at a manageable level such that the resulting risks are considered acceptable. Challenges and disparity of laboratory pot experiments outcomes under optimum operating conditions and the recommendations given by researchers on biochar applications need to translate into pilot scale and to commercial level under normal environmental conditions. Effective implementation of risk control in biochar production, management, and sustainability mechanisms, driven by up-to-date research and acceptable social standards and policy will advance safe utilization of biochar as a soil applicant, to control contaminants and in GHG management.
Conflicts of Interest
The authors declare that they have no significant competing financial, professional, or personal interests that might have influenced the performance or presentation of the work described in this manuscript.
Acknowledgments
This study was supported by National Natural Science Foundation of China (51504174) and Open Research Foundation of Center of Material Research and Measurement of Wuhan University of Technology (2018KFJJ12).