Tartaric Acid-Modified <i>Holarrhena antidysenterica</i> and <i>Citrullus colocynthis</i> Biowaste for Efficient Eradication of Crystal Violet Dye from Water

Basharat, Sumaira; Rehman, Rabia; Mahmud, Tariq; Basharat, Sara; Mitu, Liviu

doi:https://doi.org/10.1155/2020/8862167

Journal of Chemistry

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Abstract Introduction Materials and Methods Results and Discussion Conclusion Abbreviations Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2020 | Article ID 8862167 | https://doi.org/10.1155/2020/8862167

Tartaric Acid-Modified Holarrhena antidysenterica and Citrullus colocynthis Biowaste for Efficient Eradication of Crystal Violet Dye from Water

Sumaira Basharat,¹Rabia Rehman,¹Tariq Mahmud,¹Sara Basharat,²and Liviu Mitu³

Academic Editor: Hassan Arida

Received23 Sept 2020

Revised26 Nov 2020

Accepted04 Dec 2020

Published16 Dec 2020

Abstract

Two novel adsorbents Holarrhena antidysenterica (HA) and Citrullus colocynthis (CC) were collected from native Pakistan and treated with tartaric acid. The adsorbents were characterized by Fourier-transform infrared spectroscopy (FT-IR) and scanning electron microscopy, and their adsorptive behavior was studied against model cationic dye crystal violet (CV). Role of biosorbent dose, time of contact, temperature studies, agitation rates, and solution pH was investigated. Optimum conditions obtained for the removal of CV dye for H. antidysenterica-tartaric acid modified (HA-TA) were as follows: 0.8 g adsorbent dose, 35 minutes contact time, 5.0 pH, 40°C temperature, and 150 rpm agitation rates as compared to H. antidysenterica that gave 1.4 g adsorbent dose, 40 minutes time of contact, 6.0 pH, 50°C temperature, and 150 rpm agitation speed. C. colocynthis-tartaric acid modified (CC-TA) removed CV dye at 0.6 g adsorbent dose, 30 minutes contact interval, 4.0 pH, 40°C temperature, and 125 rpm agitation speed in contrast to C. colocynthis which gave 0.8 g adsorbent dose, 40 minutes time of contact, 6.0 pH, 50°C temperature, and 125 rpm agitation speed, respectively. Isothermal studies for both raw and modified biosorbents were compliant with the Langmuir model indicating monolayer, chemisorption. The maximum Langmuir capacities were up to 128.20 mg/g, 136.98 mg/g, 144.92 mg/g, and 166.66 mg/g for HA, CC, HA-TA, and CC-TA. Pseudo-second-order kinetic model well fitted the dye removal data. The rate-determining steps involved both surface and intraparticle diffusion mechanisms. Adsorption of dye molecules on active surfaces was governed by electrostatic attractions and chelating abilities. Thermodynamics research revealed the spontaneous and exothermic nature of the reaction. The adsorbents serve promising candidates for the effective removal of hazardous dyes from aqueous solutions.

1. Introduction

Environment around us is constantly being damaged, due to the incessant release of waste substances, even at minute amounts. Multiple industries such as textile, paints, and plastics contain dyes in their wastewaters. Dyes are chemical compounds that have aromatic rings and high degree of saturation which adhere to any fiber or material and impart characteristic color to it [1–4]. When discharged into water, they are difficult to degrade and biological entities suffer severe damage due to their structure, such as high mitotic index and chromosomal and nuclear aberrations [5–9]. Thus, the treatment of wastewaters has been major concerns for centuries. Traditional approaches such as coagulation, sedimentation, ion-exchange, ﬂocculation, reverse osmosis, electrochemical operation, and biological treatments [10] have the delimitations of high energy consumption, expensive equipment, incomplete ion removal, and toxic sludge production [11], and none of them provide complete color removal from wastewater, so constant efforts are made in continuation of the newest approach [12]. Adsorption therefore is a far superior method to all these methods because of its cheapness and high efficiency. Biowastes comprise many functional groups that encapsulate a wide variety of organic and inorganic pollutants. Moreover, higher capacities, adsorption rates, kinetics, and pollutant selectivity along with nominal pretreatment make them outstanding candidates replacing activated carbon, which is most frequently used and is quite expensive [13].

Crystal violet (CV) having molecular formula C₂₅H₃₀N₃Cl is also known as basic violet 3, gentian violet, and methyl violet 10B. Its IUPAC name is N-[4-[bis[4-dimethylamino-phenyl]-methylene]-2,5-cyclohexadien-1-ylidine]-N-methylmethanaminium chloride. It belongs to triaryl methane dyes having λ_max from 589 to 594 nm, and molecular weight is 407.98. It is utilized as a bacteriostatic agent [14] and is an active ingredient in Gram’s stain and is used for disinfection [15]. It is a purple dye used in ink and paints and cotton and silk textile industries [16]. Its harmful effects include pain provoking sensitivity to sunlight, eye irritation, persistent trauma to the eye, and damage to cells of mammals [17]. In extreme cases, absorption through skin results in skin and digestive tract irritation causing permanent blindness, respiratory, and kidney failure [18]. It has been removed from wastewaters using membrane separation techniques, electrocoagulation, ozonization, photocatalysis, and adsorption [19–22] among which the most promising is adsorption [13]. Various adsorbents such as NaOH-modified rice husk, bottom ash, coir pith, sawdust, sugarcane fiber, zeolites, stalks, clays, stems, wheat straws, and pyrophyllites have been utilized for CV dye removal [6, 23–34].

The adsorbents used in this study are novel Holarrhena antidysenterica and Citrullus colocynthis. H. antidysenterica is commonly addressed as Inderajao and Kurai and belongs to Apocynaceae family. It is favored in many medical areas such as for the treatment of dysentery and skin inflammations and helps in the cure of osteoporosis and rheumatism [35]. It contains a variety of functional groups due to the presence of steroidal alkaloids, flavonoids, triterpenoids, phenolic acids, tannin, resin, coumarins, saponins, and ergosterol [36] along with cellulose, hemicellulose, and lignin. These groups provide oxygen, nitrogen, phenol, and organic carboxylic acid moieties that provide ambient groups for the scavenging of CV from wastewaters.

C. colocynthis is called as Indrayan and Colocynth and belongs to Cucurbitaceae family. It is similar to peanuts, an ordinary plantation residing in temperate wilderness, helps in treatment for breast cancer and diabetes, and stimulates increase in hair length [37]. Many chemical groups such as carbohydrates, proteins, amino acids, tannins, saponins, phenolics, flavonoids, flavone glucosides, terpenoids, alkaloids, anthranol, steroids, cucurbitacins, saponarin, cardiac glycosides, and trace elements are present in it [38].

Both the adsorbents are cheap (5 $ per kg), easily available in Asian countries and highly effective, providing great removal capacities both in raw and modified forms. Acidic treatment reduces the chances in fungal or bacterial growth, improving the efficiency of the biowaste for scavenging dyes from wastewaters on industrial level.

This work aims to (a) explore novel, cheap adsorbents HA and CC, from indigenous agricultural resources in Pakistan, modify with tartaric acid to enhance their sorption capacity, increasing shelf life (b) to characterize the raw and treated adsorbents by different techniques such as FTIR and SEM (c) test for removing CV dye from wastewaters by various adsorption parameters such as dose, agitation speed, time of contact, pH, and temperature studies, along with statistical modelling of isothermal, kinetics, and thermodynamics (d) new theoretical results contributing towards a deeper understanding of the mechanism involved for the adsorption of crystal violet dye on the adsorbents, further enhancing the worth of this work.

2. Materials and Methods

2.1. Preparation of the Biosorbents

Biosorbents HA and CC were acquired from the regional marketplace of Lahore, Pakistan. They were washed first with tap water and then by distilled water. Left over moisture was removed by sun drying. The final adsorbent was grinded into 60 mesh size by using an electric grinder. For chemical modification, the obtained powder was soaked in 10% tartaric acid aqueous solution and then filtered and dried again prior to usage.

2.2. Chemicals

Analytical grade chemicals were utilized. Crystal violet dye, Basic Violet 3, C.I. 42555, molecular weight: 407.98 g·mol⁻¹, and λ_max: 590 nm was obtained from Sigma-Aldrich (Merck, Germany). HCl 37% of 11.6 M, having molecular weight: 36.46 g·mol⁻¹ was obtained from Sigma-Aldrich (Merck, Germany). NaOH having molecular weight: 40 g·mol⁻¹ from Sigma-Aldrich (Merck, Germany) was used. UV/Vis spectrophotometer (Labomed UVD-3500, China) was used for the absorbance calibrations at λ_max: 590 nm. For controlling pH, pH meter (Hanna, USA) was used. Orbital shaker (model OSM-747), Fourier-Transform Infrared (FT-IR) spectrometer (Agilent Cary 630, USA), and scanning electron microscope (Nova Nano Scanning Electron Microscope (SEM) 450 and Thermo Fischer Scientific, USA) were also utilized for this research work.

2.3. Selection of Chemical Treatment Applied for Medication of Biosorbents

Chemical modification enhances biosorption capacity and increases stability of biosorbents by reducing moisture contents, retarding biological decomposition due to fungal or bacterial attack [39]. Functionality of biosorbent is also enhanced due to uniform distribution of ions. Especially acid modifications yield increased protonation [40, 41] and ester linkages [42] while basic treatment provided high micropore percentages [43]. Rapid achievement of equilibrium is attained [44]. To check the best performance, 0.1 M hydrochloric acid, 0.1 M sodium hydroxide, methanol, ethanol, iso-propanol, iso-butanol, acetone, urea [45], thio-urea [46], citric acid [47, 48], tartaric acid [49], and ethylene diamine tetra acetic acid (EDTA) [50] were used. Tartaric acid gave the maximum biosorption capacity, so it was selected for further studies, as shown in Figure 1. 5 g of HA and CC was selected and added in aqueous solutions of 25 mL of abovementioned solvents and kept in glass beakers for 4 hours approximately. The containers were completely covered by aluminum sheets. These solutions were subjected to filtration. After drying, 0.5 g of the resultant biosorbent residue was added to 25 ml of 10 mg/L CV dye solution, for 15 minutes at 100 rpm. Filtration of the resultant solutions helped in determining the final dye concentrations [51].

2.4. Batch Adsorption Studies

Dye standard solutions (5–25 mg/L) were prepared from stock solutions (1000 mg/L). Batch wise experiments comprised the study of time effects, adsorbent dose, pH studies, temperatures, and agitation rates. Dye concentration was monitored in filtrates using a UV/Vis spectrophotometer at λ_max: 590 nm.

For contact time studies, 5 minutes to one hour with a difference of 5 minutes each was selected. 0.4 g of adsorbents was added to 25 mg/L dye solution, revolving at 100 rpm at ambient temperatures.

Adsorbent dose was studied by selecting a range of 0.2 g to 2 g of adsorbents, added to 25 mg/L dye solutions, and agitated at 100 rpm at room temperature for 35 minutes.

For pH effects, range of 1–10 was selected. 0.4 g of adsorbents was added to 25 mg/L dye solution and agitated at 100 rpm at room temperature for half hour.

Temperature studies were done at 20–80°C range with 10°C interval. 0.4 g of adsorbents was added to 25 mg/L dye solution and rotated at 100 rpm at room temperature for half hour.

25 rpm to 200 rpm range with 25 rpm interval was selected for agitation speeds study. 0.4 g of adsorbents was added to 25 mg/L dye solution and rotated for different speeds at ambient temperatures for 30 minutes.

For isothermal studies, 10–40 mg/L concentrations of CV dye along with optimum parameter conditions were used. For kinetic studies, 25 mg/L dye solutions was selected at 25°C with all optimized conditions. Thermodynamic studies were employed utilizing raw and modified forms with a temperature range of 283–323 K.

The experiments were repeated, and percentage adsorption of dye was studied as follows:where C_o (mg/L) is the initial CV dye concentration and C_e (mg/L) is the adsorption at equilibrium. The amount of dye adsorbed per unit mass is given as follows:where q (mg/g) is the quantity of dye adsorbed by the adsorbent, V (L) is volume of dye solution, and m (g) is the mass of biowaste used [52, 53].

3. Results and Discussion

3.1. FTIR Analysis before and after Adsorption

FTIR spectra for the CV dye adsorption on untreated HA (Figure 2(a)) were obtained, and it ensures the dominance of hydroxyl groups along with carboxylic moieties, on the sites of adsorbent. The peaks at 3904.4 cm⁻¹ to 3482.5 cm⁻¹ are responsible for the presence of –OH (bending), physically adsorbed water molecules [54]. Also, 3011.9 cm⁻¹ represents carboxylic acid moieties. Bands at 2921.8 cm⁻¹ and 2852.1 cm⁻¹ represent asymmetric and symmetric C-H groups, and 2366.2 cm⁻¹ shows CO₂ from the atmosphere. 1740.4 cm⁻¹ shows aldehyde stretch. 1636.3 cm⁻¹ peaks depict doubly bonded carbons of alkene moieties, whereas 1457.6 cm⁻¹ shows aromaticity. Also, 1168.3 cm⁻¹ represents C-N groups. When CV dye on the acid-treated HA was seen (Figure 2(b)), bands from 3919.5 cm⁻¹ till 3502.5 cm⁻¹ represent the free hydroxyl moieties. Decreased bands were observed at 2922.5 cm⁻¹ and 2852.3 cm⁻¹ for C-H functional groups, indicating the reaction has been occurred. 2359.9 cm⁻¹ shows single bonded alkanes. Alkyne functional groups were depicted by stretching at 1989 cm⁻¹, while 1789 cm⁻¹ represents alkenes. 1559.2 cm⁻¹ shows aromaticity, while 1718.5 cm⁻¹ represents aldehydic groups. 1363.5 cm⁻¹ represents bending of alkanes.

CC when reacts with CV (Figure 2(c)) displays wide peaks from 3841 cm⁻¹ to 3328 cm⁻¹ that are responsible for the presence of hydroxyl groups indicating the reaction of the dye with the adsorbent. Stretching at 2183 cm⁻¹ represents double bonds of alkynes. 1636 cm⁻¹ indicates the presence of doubly bonded carbons of alkenes, and peaks at 1419 cm⁻¹ represent C=C of aromatic rings, indicting the presence of CV dye within the adsorbent surfaces. 1705 cm⁻¹ displays -C=O stretch of aldehydes, while 1363 cm⁻¹ shows the presence of -N=O groups. Band at 1029 cm⁻¹ contributes to C-N groups of aliphatic amines. Acid-modified CC adsorbent when reacts with CV dye (Figure 2(d)) displays reduced consecutive peaks from 3902 cm⁻¹ to 3447 cm⁻¹, showing the presence of freed OH groups which is due to the reaction between dye and adsorbent [19]. Peak at 2132 cm⁻¹ contributes towards unsaturated -C=C bonds of alkyne moieties. 1653 cm⁻¹ stretching responds towards the doubly bonded carbon groups, and 1314.1 cm⁻¹ contributes towards the nitrile stretching CN of aromatic amines. 1363 cm⁻¹ stretch depicts -N=O groups, and 1028 cm⁻¹ confirms the presence of -CN stretch of aliphatic amines. After dye adsorption, bands intensity decreases and elongated bands of –OH widens, confirming the reaction of CV dye with the adsorbents. The presence of oxygen containing functional moieties provides more adsorption surfaces and thus helps in the achievement of greater adsorption of dyes. The nitrogen groups facilitate the eradication of dyes and organic pollutants.

3.2. SEM Analysis

HA (Figure 3(a)) displayed voids [19] in between compact, agglomerated, spherical structures [10]. Tartaric acid modification resulted in conversion of round structures to flaky, crumpled, chiseled ones containing numerous voids/cages/channels that provide additional surface sites for CV adsorption (Figure 3(b)). CC displayed large flaky structures with channels between them (Figure 3(c)). Acidic modification resulted in numerous spherical, vertical, agglomerated compact structures, offering many potential sites for dye adsorption (Figure 3(d)). The presence of granular structures of acid moieties, clustered well on surface, is evident of the successful preparation of the adsorbent.

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3.3. Factors Affecting Adsorption of Dyes on the Biosorbents

Different factors such as adsorption dosage, pH effect, time of contact, temperature, and agitation rates were employed for the determination of equilibrium parameters for the eradication of CV on to the raw and modified adsorbents.

3.3.1. Impact of Biosorbent Dose

Biosorbent dose is a significant criterion towards the achievement of biosorption efficiency. At low concentration, maximum adsorption is achieved due to availability of many adsorption sites, but at higher concentration, less saturation of surfaces leads towards a lower dye removal. 0.2–2g dose of adsorbents was selected. For HA, 96% dye removal was observed at 1.4 g after which the dye establishes equilibrium with the adsorbent. The modified HA develops equilibrium at 0.8 g removing 97% of the dye. Figure 4(a) displays the % age removal of the CV dye with the raw and tartaric acid-modified adsorbents. Acid treatment incorporates the addition of oxygen containing functional moieties that chelate the dye molecules increasing the rates of adsorption. CC adsorbent removed CV dye up to 97% at 0.8 g, whereas the acid treated yielded 97.5% dye removal at 0.6 g. Decrease in adsorption after equilibrium is due to decline in amount biosorbed by unit mass of the sorbent, leaving vacant adsorption sites [55].

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3.3.2. Impact of Time of Contact

Time of contact helps in rate determining steps and possible mechanism involved. Adsorption raised up to a certain time until equilibrium is achieved [56]. The decrease in adsorption is due to the repulsion among the adsorbed ions towards the unadsorbed ones. The experiment was allowed to run for a 60-minute time period with a difference of 5 minutes (Figure 4(b)). HA removed 97% CV dye at 40 minutes as compared to tartaric acid-modified form which gave 97% dye removal at 35 minutes. A steady decrease in rate of adsorption was observed after reaching plateau (saturation point) due to complete saturated adsorption surfaces. CC observed 86% dye removal at 40 minutes which raised to 98% at 30 min for the modified form. Modification helped in availability of more active sites which increased the biosorption efficiencies. This increase may be attributed to (a) dye molecules crossing the outer boundary layer and (b) diffusion of CV towards the internal exfoliated porous surface (intraparticle diffusion) along with reduction in viscosities and lower complexation [57–59]; however, accumulation of dye fragments over the period of time halts the adsorption at the later stages [37].

3.3.3. Impact of pH

The effect of pH on solution holds prime importance as the molecules are self-ionizable. Point of zero charge pHpzc is the isoelectronic point where the negative and positive surfaces become equivalent [60, 61]. Negative charged surface is formed if the pH is higher than pHpzc due to deprotonation of functional moieties like OH- and COO- groups; thus, cationic dyes adsorption occurs; however, if the pH is less than pHpzc, surface becomes positively charged because of protonation [10]. Hence, anionic dyes adsorption occurs. Initial pH is taken at the x-axis, and difference between initial and final (ΔpH) is taken as the y-axis to find the value of pHpzc, as shown in Figure 4(c). It was determined by an electrochemical method [62], where 50 mL of (0.05 M) sodium chloride was added to a series of beakers, pH was adjusted from 2 to 10 by adding appropriate amounts of hydrochloric acid (0.1 M) and sodium hydroxide (0.1 M), followed by addition of 40 mg adsorbent. The prepared solutions were kept at constant agitation at room temperature for 48 hours [60]. The pHpzc for HA is 6, whereas CC is 5 that infers to a negative surface above this pH and below a positive one.

As seen in Figure 4(d), adsorption capacity was found 97% at pH 6 for HA, whereas CC showed 86% dye removal at pH 6. This is because the interactions between the charged dye ions and adsorbent surfaces control the adsorption capacities. CV⁺ is a cationic dye. As the number of -OH ions increases, solution pH and adsorption capacities of the dye tend to increase. Low eradication of dye in the acidic circumstances was assigned towards the struggle for potential sites, among the cationic part of the dye and the H⁺ ions, therefore contributing an immense feature in the electrostatic mechanism of adsorption method [60, 63]. When surface ionized density is low, it provides repulsion in the dye particles and adsorbent sites, thus offering adsorption in acidic environment [64]. Here, CV and excess H⁺ ions compete for adsorption sites, thus lowering adsorption capacity in the basic medium; also, adsorbent surface hydrolysis creates positive charged sites, thus rendering the favorable adsorption of the dye in an acidic medium. Tartaric acid-modified HA gave 98% dye adsorption at pH 5 as compared to modified CC which gave 97% at pH 4.

3.3.4. Impact of Temperature

Temperature impacts on the biosorption procedure imply whether it is an exothermic or endothermic one [27]. The study was done for the adsorptive removal of CV dye on raw and tartaric acid-treated adsorbents and observed the adsorption decreases along with the increase in temperature, indicating the process might be an exothermic one. This increase is attributed to the increase in movement of the CV dye particles with the rise in kinetic energy and enhanced rates of intraparticle diffusion. Furthermore, increasing temperature brings swelling effect which increases further penetration of the dye molecules. A range of 30°C–80°C was selected for the temperature studies, as seen in Figure 4(e); 91% adsorption at 50°C was observed for HA which increased to 98% at 40°C for treated HA, while CC displayed 94% dye removal at 50°C that raised to 98.7% at 40°C for modified CC.

3.3.5. Impact of Agitation Speed

The distribution of solutes among the bulk and external boundary films formation is determined by rotating speed. CV reduces the resistance of the external boundary sheet resulting in favored correlation of dyes to that of the biosorbent, thus rising movement. Better adsorption of the dye molecules resulted in the increased external mass transfer coefficient [65]. Agitation speed with a range of 20–200 rpm was checked for CV dye removal on raw and tartaric acid-modified forms, and 89% adsorption efficiency was observed at 150 rpm for HA that elevated to 93% for modified HA, whereas CC gave 84% dye removal at 125 rpm that increased to 96% for modified CC, as shown in Figure 4(f).

3.4. Isothermal Studies

A relationship between adsorbate and sorbent was established to fully comprehend the environment of adsorption process and reaction mechanism by utilizing isothermal models. These are Langmuir, Freundlich, Temkin, and the D-R isotherms.

The relationships among the equilibrium bulk concentrations and masses of adsorbate per unit weight of adsorbent is given by the Langmuir model. Also, it says that adsorption takes place homogeneously on the adsorption sites [66, 67]. The linear model of the Langmuir isotherm is given as follows:where C_e is the liquid phased concentrations of the CV dye at the equilibrium stage (mg·L⁻¹), q_max is the monolayer sorption capacity of the adsorbents (mg g⁻¹), q_e is the concentration of dye adsorbed on the biosorbent at equilibrium (mg·g⁻¹), and b represents energy of biosorption (L·g⁻¹). A plot of 1/C_e vs. 1/q_e gives a straight line for this isotherm (Figure 5(a)). The dimensionless equilibrium parameter R_L which is also used for evaluating the possibility of the biosorption process is obtained by employing the underlying equation:where C_o is the highest initially dye concentrations (mg·g⁻¹) and K_L is the Langmuir adsorption constant (L·mg⁻¹). Adsorption is favorable if 0 < R_L < 1, unfavorable if R_L > 1, linear if R_L = 1, or irreversible if R_L = 0 [66].

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As seen in Table 1, Langmuir removal capacities q_max for biosorption of CV dye on the raw and modified biosorbents displayed higher values for the modified form as compared to the raw ones, indicating that the acid-treated biosorbents display better adsorption capacity for CV dye due to the availability of more binding sites of their masses and also increase in functional groups, as depicted by the FTIR spectrum which favors the adsorption of dye. Q_max was found to be highest for CC-TA among the four adsorbents. R² values were highest in Langmuir as compared to Freundlich, Temkin, and D-R isotherms indicating homogeneous, chemisorption with no side reactions [66]. Positive b values are clearly indicative of the appreciable affinities of the biosorbents towards the removal of CV dye. R_L values were less than 1, displaying a favorable adsorption procedure.

Heterogeneous surfaces in multilayer adsorption along with nonuniform distribution of heat of surface adsorption are presumed by Freundlich [68]. Its linear form is represented as follows:where K_F is the Freundlich constant which relates towards biosorption capacity and n is the biosorption strength factor, in which 1–10 value is indicative of favorable biosorption, n > 2, n < 1, and n = 0-1 are representative of better, average, and good adsorptions [69], and 1/n is the constant related to intensity of biosorption or heterogeneity factor. 1/n < 1 displays Freundlich isotherm, whereas 1/n > 1 indicates cooperative biosorption [66]. Graphical plots of log qe vs. log Ce give a straight line from which values of n and K_F are determined from the slope and intercept of the graphical plot (Figure 5(b)). When checked for Freundlich Isotherm as in Table 1, it was dawned upon that R² values were lesser than Langmuir’s; moreover, values of n were found between 1 and 2, favoring good adsorption characteristics. The relatively higher charge to mass ratio of the CV dye particles along with favorable chelating capabilities of modified forms was accountable for the favorable dye eradication.

Temkin isotherms only validate for an intermediate range of ionic concentrations and consider indirect adsorbate/adsorbate interactions. Moreover, it also conveys that an increase in surface coverage decreases heats of biosorption (ΔHads) of all the particles in the films [70]. Linear form of this isotherm is given as where A_T = equilibrium binding constant related to maximum binding energy (L·g⁻¹) and B_T = Temkin isotherm constant relating to adsorption heat (J·mol⁻¹) [71]. Physical or chemical interactions among the dyes and adsorbent are given by B_T values, where <8 shows physical nature while >8 is representative of chemical interactions [72, 73]. Less than 8 values of B_T, as shown in Table 1, depicts the weak and physical forces between the dye molecule and the adsorbents. Graph between q and ln C_e gives the desired results (Figure 5(c)). Tartaric acid offered strong binding forces for the dye molecules exhibiting an increase in the B_T for the modified forms in comparison to raw biosorbents. Unsatisfactory fitting of the Temkin model is observed due to less R² values than the Langmuir and Freundlich ones.

Dubinin–Radushkevich isothermic model [74] represents biosorption mechanism with Gaussian energy distributions onto heterogeneous surfaces. It is only appropriate for intermediate ranges of biosorbate concentration. The model describes pore filling mechanism and consists of semiempirical equation that qualitatively describes the biosorption of gases and vapours on microporous sorbents [75]. It assumes a multilayer character for physical adsorption involving Van der Waal’s forces. Linearly, it is shown as following equations:

q_e is the CV adsorbed on adsorbents, C_e is the concentration of CV at equilibrium, q_m (mg·g⁻¹) is the Dubinin–Radushkevich constant related to degree of adsorption. B_D (mol²·kJ⁻²) is the free energies of CV dye that travel from infinite distance towards the sorbent, and is the Polanyi potential. Mean free energy, E (kJ/mol), is calculated as follows:

Value of E also inferences about adsorption mechanism where E < 8 kJ·mol⁻¹ represents physisorption, while 8 < E < 16 kJ·mol⁻¹ shows ion-exchange and E > 16 kJ·mol⁻¹ displays chemisorption process [76].

In Table 1, R² values were found to be less than those of Langmuir isotherm, and values of q_m were found to be increasing for the modified forms which might be because of the increase in possible spots as a result of acid treatment. E values gave results lesser than 8 inferring the biosorption procedure to be physiosorption. Figure 5(d) displays the comparative adsorption for D-R isotherm.

3.5. Kinetic Study

The rate and mechanism of the reaction can be studied from kinetic modelling to ensure a competent and swift biosorption procedure for commercial purposes. For this purpose, popular kinetic models like Elovich, Lagergren pseudo-first-order kinetics, and Lagergren pseudo-second-order kinetics are employed. Quantitative checking can also be done by using percent relative deviation (P), as follows:where q_e(exp) is the experimented biosorption capability (mg/g), q_e(cal) is the biosorption capacities obtained after calculations by utilizing kinetic models(mg/g), and N is the total number of observations [77].

The Elovich model is based on the principal that adsorption increases exponentially with adsorption sites implying multilayered adsorption [78]. Chemisorption of gas onto solids was first described by this equation [79]. Linear form of the Elovich equation is shown as follows [80]:where qt (mg/g) is the concentration of dye adsorbed at any time t, α (g/mg min) is the initial rate of sorption, and b (g/mg) is linked to number of potential sites covering the surfaces. Higher α values represent chemisorption. For this model, maximum adsorption capacity along with Elovich constants can be calculated from slope and intercept of graph of ln t versus qt. Table 2 represents the Elovich parameters obtained for raw and modified adsorbents, and Figure 6(a) displays graphs obtained after adsorption.

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Lagergren pseudo-first-order kinetic model was established upon the assumption that adsorption is equivalent to the amount of freely bonded surfaces [81, 82].

It is shown as follows:where K₁ (h⁻¹) is the rate constant and q_t (mg·g⁻¹) represents the quantity of CV dye taken at any instant t while q_e (mg g⁻¹) shows the quantity of CV dye taken at the equilibrium stage. A larger variance (P) amidst the experimented and calculated q_e values renders the first-order ineffective. Negative P values show that q_e calculated was higher than q_e experimental. Moreover, lesser values of correlation coefficient R² implement the unsatisfactory fittings of the pseudo-first-order model, as evident in Table 2 and Figure 6(b).

Ho’s model [83, 84] is supported by the presumption that rate of reaction is equivalent to dye concentration, and the square of binding locations present on the biosorbent is shown as in the following equation:where k₂ (g h⁻¹) is the rate constant for pseudo-second-order. Also, q_e (mg/g) is the uptake capacity of CV dye at the equilibrium stage and q_t (mg/g) is the uptake capacity of CV dye at a given instant t.

The high value of R² and lower relative percentage deviation (P%) as compared to first-order kinetics indicates higher fitness of the pseudo-second-order model to the kinetics data. Moreover, q_e(cal) is closest to that of q_e(exp), suggesting adsorption process to be chemisorption [85]. Initial rate of adsorption (h) (mg/g·min) is

Product of uptake capacity and rate constant give the half-life (t^1/2) of the adsorption process, and the time in which process is halved, as given in the following equation:

It was seen that h values of the HA-TA and CC-TA displayed an increment in the initial sorption rate than unmodified ones, while half-life of modified adsorbents was lower than untreated ones [43, 86]. Table 2 represents the desired results along with Figure 6(c).

For understanding of the mechanism of the biosorption process, intraparticle diffusion model was studied [82, 87], which is shown by Weber and Morris, as follows:

The coefficient of intraparticle diffusion is represented by , and·C (mg g⁻¹) is the boundary layer effect. If the plot of q_t vs. t^1/2 is a straight line and pass through the origin, then the intraparticle diffusion is the mechanism; otherwise, some other mechanisms are involved such as film diffusion [88]. Dual nature was observed for the plots for diffusion mechanism with the (a) first to be a curved line contributing towards boundary layer (surface diffusion) (b) and the straight line in the later stages inferring towards intraparticle diffusion (Figures 5(d) and 5(e)). This leads towards complex mechanism of the CV dye adsorption [19], as clear from Table 2. Not even one of the graphical plots passed through the origin; hence, it is concluded intraparticle diffusion model was not the complete mechanism involved [89].

Boyd proposed another mechanism for predicting the mechanism of the biosorption process which is based upon the fractional achievement of equilibrium with variance of time [90]. The following equation showswhere Boyd’s constant is represented by B_b and the fractional achievement of equilibrium q_t/q_e is shown by F at any instant t. This equation is simplified as follows:

If a graph with a straight line passes through the origin, then the mechanism involved will be the Webber–Morris plot; otherwise, boundary layer diffusion will be the mechanism involved (Figures 5(f) and 5(g)). R² was compared in Table 2 and was seen that the graphs exhibited not only intraparticle diffusion behavior but also film or boundary layer as the rate determining step for the removal of CV dye on the raw and tartaric acid-modified biosorbents [10].

3.6. Nonlinear Fashion of Equilibrium and Kinetic Modelling

Nonlinear equations were employed to check the validation of the equilibrium data in the linear mode. Nonlinear equations of Langmuir, Freundlich, Temkin, and Dubnin–Radouskevich [91] are shown in the following equations:

Root mean square error (RMSE) values [92] were calculated using the following formula:

Lesser value of RMSE as evident in Table 2 indicates the fitness of experimental to the calculated data.

From the alterations of linear equations, nonlinear equations of Elovich, pseudo-first-order, and pseudo-second-order forms [93] are obtained (equations (24)–(26)):where q_t (mg/g) is the uptaking capacities of CV at any time t, q_e (mg/g) is the equilibrium binding capacities, α (g/mg min) is the initial rate of adsorption, and b (g/mg) is the number of active sites. k₁ (h⁻¹) and k₂ (g h⁻¹) are the first- and second-order rate constants, respectively.

RMSE results were calculated to investigate the most fitted of the kinetic models, comparing the experimental vs. calculated values, by using the following equation:where q_t(cal) (mg·g⁻¹) is the calculated binding capacity, q_t(exp) (mg·g⁻¹) is the experimental binding capacity at any instant t, and N is the number of observations. The lowest RMSE values are seen for those of second order hence making it most appropriate (Table 2).

3.7. Thermodynamic Studies

Temperature modifications affect the dye removal process as it instantly changes the kinetic energy involved in the procedure; increment in temperature results in raised diffusion in the lignocellulosic structures [94]. Vant Hoff’s equation is given as follows:

A graphical plot was commenced between lnK_D versus 1/T (Figure 6). Negative values of ΔH° and positive ΔS° [95] show exothermic reaction and increase in arbitrariness of the system [96, 97], as clear from Table 3. Moreover, a spontaneous and favorable adsorption procedure at 283 K–323 K temperatures is shown by negative values of Gibbs energy (ΔG°) [98]. The magnitude of K_d representing distribution coefficient [93] rises with temperature implying that temperature raise increases biosorption. More surface area, expansion of sizes of pores, and activated biosorbent surfaces are responsible for the greater sorption efficiency of the CV dye-modified adsorbents, as compared to raw ones [99–101].

3.8. Mechanism of Adsorption

To understand the adsorption mechanism of the CV dye, the surface of adsorbent is considered which consists of cellulosic moieties mainly consisting of hydroxyl and carboxyl groups, as seen in the FT-IR analysis. When the adsorbent interacts with acidic groups of tartaric acid, it interacts with the functional groups, thus leading towards increment in acidity of the adsorbent which further helps to interact with the adsorption process. The cationic dye in solution dissociates into CV⁺ and Cl⁻ groups that adhere on the surface by interacting with hydrogen bonding with the carboxyl and hydroxyl functionalities (Figure 7). Overall, the adsorption of CV on the adsorbents involves a monolayer procedure including spontaneous occupation of available active surfaces (bulk diffusion) followed by penetration of the adsorbate on the microspores from surface films (pore and intraparticle diffusion steps) and attaining equilibrium stage at the last step (Collins and Elijah, 2019 [102]). Although the Langmuir and pseudo-second-order imply this procedure to be a chemisorption but isotherms such as Temkin and D-R pose it to be a physical one, adsorption of CV dye on raw and modified novel Holarrhena antidysenterica and Citrullus colocynthis is concluded to be a physicochemical one and its maximum adsorption capacity is comparable with reported sorbing materials, as clear from Table 4.

4. Conclusion

Thus, utilization of H. antidysenterica and C. colocynthis for effective eradication of CV dye from wastewaters is considered very effective as it provides the maximum removal efficiencies both in raw and modified forms. Tartaric acid provides the availability of more acidic functional groups such as hydroxyl and carbonyl as seen by FT-IR and SEM analyses. Batch adsorption studies confirmed the modified forms to be better adsorbing than the nonmodified ones. Isothermal studies were performed using Langmuir, Freundlich, Temkin, and D-R models indicating the Langmuir isotherm to be most promising due to highest regression coefficients hence proving monolayer adsorption. q_max (mg g⁻¹) for the HA was 128.2051 mg·g⁻¹ that increased up to 144.9275 mg·g⁻¹ for the tartaric acid forms while CC gave 136.9863 mg·g⁻¹ that raised to 166.6667 mg·g⁻¹ for the modified ones. Pseudo-second-order kinetics were followed by all the adsorbents. Thermodynamics proved the adsorption procedure to be an exothermic and spontaneous process due to negative values of ΔH and ΔG°. The mechanism involves both bulk and pore diffusion by the adhering dye molecules to the adsorbent via hydrogen bonding and electrostatic interactions. Thus, it is a physicochemical biosorption procedure.

Abbreviations

FT-IR:	Fourier-transform infrared spectroscopy
CV:	Crystal violet dye
HA:	Holarrhena antidysenterica
CC:	Citrullus colocynthis
HA-TA:	Holarrhena antidysenterica-tartaric acid modified
CC-TA:	Citrullus colocynthis-tartaric acid modified
SEM:	Scanning electron microscopy
PHpzc:	Point of zero charge.

Data Availability

All data related to this work are presented in Results along with references.

Conflicts of Interest

The authors have no conflicts of interest regarding publication of this paper.

Acknowledgments

The authors are grateful to the University of the Punjab, Lahore, Pakistan, for funding this project. The authors are thankful to home institute for funding this work.

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Copyright © 2020 Sumaira Basharat et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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