Removal of Patent Blue (V) Dye Using Indian Bael Shell Biochar: Characterization, Application and Kinetic Studies

: The prospective utilization of bael shell ( Aegle marmelos ) as an agro-waste for the production of biochar was investigated along with its characterization and application for the abatement of hazardous aqueous Patent Blue (PB) dye solution. The sorptive removal of PB on bael shell biochar (BSB) was investigated under the following operational conditions: (pH, 2.7–10.4; biochar dosage, 2–12 g/L; and contact time, 0–60 min). The removal efﬁciency of PB by BSB in a batch adsorption experiment was 74% (pH 2.7 and 30 ± 5 ◦ C). In addition, a clear relationship between the adsorption and pH of the solution was noticed and the proposed material recorded a maximum sorption capacity of 3.7 mg/g at a pH of 2.7. The adsorption of PB onto BSB was best explained by the pseudo-second order kinetic model (R 2 = 0.972), thereby asserting the predominant role of chemisorption. The active role of multiple surface-active functionalities present on BSB during PB sorption was elucidated with the help of Freundlich isotherm (R 2 = 0.968). Further, an adsorption mechanism was proposed by utilizing Fourier transform infrared spectroscopy (FTIR).


Introduction
Indian bael (Aegle marmelos) is a readily available common fruit widely accessible throughout the regions of India and mostly concentrated in the eastern Gangetic belt region [1]. Usually, the inner portion of the bael fruit is edible. In contrast, the outer part is hard and considered as agro waste. Bael fruit has several medicinal uses in diseases such as piles, edema, jaundice, obesity, pediatric disorders, gastrointestinal diseases, vomiting, gynecological disorders, urinary complaints and as a rejuvenate [2]. Upon the consumption of fruit, the bael shell (BS) is discarded as a waste product. Hence, in this research, the bael shell has been effectively utilized as a biomass feed for synthesizing bio-char and the physicochemical properties of the resulting bio-char are evaluated and analyzed. It is found that bael initiated. In the absence of oxygen, charcoal is produced as the final product. At about 270 • C, it begins to spontaneously decompose while the heat is evolved simultaneously. Pyrolysis oil also called bio-oil can be obtained by heating dried biomass without oxygen in a reactor at a temperature of about 500 • C with subsequent cooling [29].
First, 10 kg of bael shell was collected from a local juice vendor shop. It was cleaned with deionized water and parched under sunlight for ten days. The dehydrated bael shell was crushed, ground and sieved to a mesh particle size of 60 BSS (250 µm). The bael shell sample was then carbonized at 500 • C for a residence time of 3 h in a Pyrolyzer. The carbonized bael shell sample was cleaned with hot deionized water, dried at 75 • C for 2 h in an oven (Oven Universal NSW-143, New Delhi, India) and then used for further studies.

Elemental Analysis
Scanning electron microscope (SEM) examination was conducted using a JEOL JSM-6400 Scanning Microscope (JEOL, Tokyo, Japan) at The IIT BHU, Varanasi, India. Various magnifications were used to compare changes or modifications in the structural and surface characteristics of the biochar samples during treatment of Patent Blue dye.
Elemental analyses of bael shell (BS) and bael shell biochar (BSB) were performed to compare the carbon, nitrogen and hydrogen content in BS and BSB using an elemental analyzer, various instruments and software from Euro Vector, Germany. Furthermore, the moisture, ash and volatile matter contents were analyzed for both BS and BSB at different temperature ranges (110, 715 and 930 • C).
The presence of diverse functionalities on BSB surface (before and after sorption process) was examined using FTIR spectroscopy. The spectral range fixed for FTIR analysis was in the range of 4000-400 cm −1 using a Thermo-Fisher FTIR analyzer (Nicolet 5700, Tokyo, Japan). Surface morphology of the adsorbent was characterized using an EVO 18 Research Scanning Electron Microscope (SEM), Carl Zeiss, Jena, Germany.

Determination of the Point of Zero Charge of Bael Shell Biochar (BSB)
Point of zero charge (pH z ) is a theory developed to account for the sorption mechanism; it specifies the condition at which the surface charge density of the sorbent reaches zero [19,20]. It is generally evaluated in relation to the pH of an electrolyte so that the pH z value is allocated to a particular colloidal particle or substrate [18]. In this study, the salt addition method was effectively utilized to determine the point of zero charge (pH z ). A 0.1M KNO 3 solution was made with the initial pH between 2.55-11.3 using 0.1 M solutions of NaOH and HNO 3 . Later, the pH values were measured using a pH tutor from Eutech Instruments, Singapore. Then, 50 mL of 0.1 M KNO 3 was placed in a 150 mL flask and 1g of BSB was added to each flask. An incubation period of 24 h was followed by the subsequent measurement of final pH. A graph was plotted between the initial-final pH difference and the initial pH. The point where ∆pH equals zero was denoted as pH z .

Batch Adsorption Studies
The batch sorption experiments were conducted by placing 100 mL of PB(V) dye solution of predetermined concentration (50 mg/L) in five different (250 mL capacity) flasks with five different loading amounts of the BSB (0.5, 1.0, 1.5, 2.0 and 3.0 g). The mixture was placed in an incubator shaker (Caltan made orbital shaking incubator cum B.O.D., India) at 110 rpm until equilibrium was observed to be attained. Then, a double beam spectrophotometer (Elico SL 169 operating at λ max = 635 nm) was used to determine the supernatant PB concentration. The sorption percentage of PB (V) dye and equilibrium sorption capacity, q e (mg/g), was evaluated by: where, C i and C e are the initial and equilibrium concentrations of PB (V) (mg/L), respectively, V is the volume of the dye solution (L) and W is the weight of BSB (g).

Adsorption Isotherms
In the present study, two isotherms (Langmuir and Freundlich models) were applied to understand the sorption equilibrium. The analysis was conducted by placing 100mL of 50 mg/LPB dye in 250 mL flasks with five different adsorbent dosages as aforementioned. A spectrophotometer was utilized to measure the equilibrium absorbance of the solution. The adsorption capacity and equilibrium solution concentrations were then measured and the suitability of the isotherm was investigated.

Langmuir Isotherm
The Langmuir isotherm model presumes that the sorption processes occur in a monolayer manner. It also assumes that the energy of adsorption is uniform throughout the adsorbed layer on the adsorbent surface, at a constant temperature [30,31].
The linear form of the Langmuir equation can be given as: where q e (mg/g) is the amount of dye adsorbed at equilibrium, q m (mg/g) is the amount of dye adsorbed when saturation is attained, C e is the equilibrium dye concentration (mg/L) and K l is the Langmuir constant related to the binding strength of the dye onto the adsorbent.

Freundlich Isotherm
The Freundlich isotherm model empirically relates the distribution of solute molecules between the aqueous and solid phases at equilibrium. This isotherm presumes an exponential disparity in the energy of surface-active sorption sites, while the decrease in heat of adsorption is logarithmic [32]. The linearized form of the Freundlich equation is expressed as: where, K f and n are the Freundlich constants that represent adsorption capacity and intensity (strength) of adsorption, respectively. The value of n describes the process as follows: (1) n = 1, a linear process; (2) n < 1, a chemical process; and (3) n > 1, a physical process [33].

Adsorption Kinetics
To analyze the uptake of PB dye by BSB during sorption, pseudo-first and second order kinetics was applied in this study.

Pseudo-First-Order Kinetic Model
The linear form of the pseudo-first-order kinetic model is represented by ln q e − q t = lnq e − k 1 t (5) where, q e and q t are the values of amount of dye adsorbed per unit mass of adsorbent added at equilibrium and at time t, respectively. In addition, k 1 is the pseudo-first-order adsorption rate constant (min −1 ). The values of k 1 and calculated q e can be determined from the slope and intercept of the linear plot of ln (q e − q t ) versus t, respectively.

Pseudo-Second-Order Kinetic Model
The pseudo-second-order kinetic model can be expressed by Equation (6) below, where k 2 is the pseudo-second-order adsorption rate constant (g/mg.min) and q e is the amount of dye adsorbed (mg/g) on the adsorbent at equilibrium. The plot of t/q t versus t gives a linear relationship, which allows the calculation of k 2 and q e . Here, it should be noted that the model with a higher regression coefficient (R 2 ) and agreement between the experimental and calculated value of q e is utilized as the appropriate model for supporting the adsorption kinetics [34].

Characterization
The carbon content of BSB was observed to be highly elevated as compared to the raw biomass through constitutional analysis which can be ascribed to the high carbonization temperature (500 • C) ( Table 1). The decrease in nitrogen content of the BSB relative to raw BS is ascribable to the loss of nitrogen-containing functional groups, such as amides or amines that may decompose above 400 • C [35]. The reduced hydrogen content may be due to the eviction of hydrogen as water during pyrolysis.

Point of Zero Charge
The functional groups present in BSB have a significant influence on the point of zero charge of BSB (pH z ). From Figure 1, the ∆pH z for BSB is 8.80. Here, it is noteworthy to mention that cationic contaminants can be adsorbed on BSB when pH > pH z , while anionic compounds can be adsorbed at pH < pH z . The specific adsorption of cations would shift the value of pH z lower; whereas, for that of anions, the value of pH z would shift higher.

Effect of pH on Adsorption of Patent Blue (V)
The initial pH of the solution is a key to determine the extent of PB (V) dye adsorption on the BSB surface. In this respect, when the adsorption of PB was studied in the pH range of 2.7-10.4, maximum sorption was perceived at pH 2.7. The percentage adsorption values of PB onto BSB measured at seven pH points (2.7, 4, 6, 7, 8, 9.2 and 10.4) were 74%, 62%, 58%, 55%, 47%, 40% and 39%, respectively. At lower pH values, the surface charge density of BSB is predominantly positive; thus, adsorbing high quantities of anionic PB dye molecules [36]. The maximum adsorption capacity of BSB towards PB (V) was found to be 3.7 mg/g at a pH of 2.7. Similar adsorption trends were observed for the sorption of PB (V) on Ginger waste material by [8]. These authors obtained a maximum adsorption capacity of 9.56 mg/gat pH 2 with an initial waste concentration of 10 mg/L. The decrease in PB (V) removal upon an increase in pH might be due to competitive adsorption between the hydroxyl ions (OH-) present in the solution and the anionic portions of the PB (V) dye. On the other hand, interference by OH-ions was limited at lower pH values, thus facilitating the attraction of PB anions on a positively charged BSB surface [34,37].

Kinetic Studies
The reaction pathways and dye removal rate from water can be better understood by adsorption kinetics. The pseudo-first and second order kinetic plots are given in Figure 2. In addition, the corresponding rate constants and regression coefficients (k1, k2, qe and R 2 ) are outlined in Table 2. The obtained experimental qe value (2.35 mg/g) was near the calculated qe of the pseudo-second order kinetic model (2.49 mg/g). Moreover, the resulting regression coefficient value of pseudo-second-order kinetics was higher than the pseudo-first-order kinetics ( Table 2). The observed results indicate the sorption process is primarily governed by the pseudo-second-order kinetics. As such, this observation indicates the chemical interaction between BSB and PB (V) dye. Interestingly, activated carbon produced from bael shell has been observed to follow pseudo-second order kinetic model for the sorptive removal of anionic pollutants [38][39][40].

Effect of pH on Adsorption of Patent Blue (V)
The initial pH of the solution is a key to determine the extent of PB (V) dye adsorption on the BSB surface. In this respect, when the adsorption of PB was studied in the pH range of 2.7-10.4, maximum sorption was perceived at pH 2.7. The percentage adsorption values of PB onto BSB measured at seven pH points (2.7, 4, 6, 7, 8, 9.2 and 10.4) were 74%, 62%, 58%, 55%, 47%, 40% and 39%, respectively. At lower pH values, the surface charge density of BSB is predominantly positive; thus, adsorbing high quantities of anionic PB dye molecules [36]. The maximum adsorption capacity of BSB towards PB (V) was found to be 3.7 mg/g at a pH of 2.7. Similar adsorption trends were observed for the sorption of PB (V) on Ginger waste material by [8]. These authors obtained a maximum adsorption capacity of 9.56 mg/gat pH 2 with an initial waste concentration of 10 mg/L. The decrease in PB (V) removal upon an increase in pH might be due to competitive adsorption between the hydroxyl ions (OH-) present in the solution and the anionic portions of the PB (V) dye. On the other hand, interference by OH-ions was limited at lower pH values, thus facilitating the attraction of PB anions on a positively charged BSB surface [34,37].

Kinetic Studies
The reaction pathways and dye removal rate from water can be better understood by adsorption kinetics. The pseudo-first and second order kinetic plots are given in Figure 2. In addition, the corresponding rate constants and regression coefficients (k 1 , k 2 , q e and R 2 ) are outlined in Table 2. The obtained experimental q e value (2.35 mg/g) was near the calculated q e of the pseudo-second order kinetic model (2.49 mg/g). Moreover, the resulting regression coefficient value of pseudo-second-order kinetics was higher than the pseudo-first-order kinetics ( Table 2). The observed results indicate the sorption process is primarily governed by the pseudo-second-order kinetics. As such, this observation indicates the chemical interaction between BSB and PB (V) dye. Interestingly, activated carbon produced from bael shell has been observed to follow pseudo-second order kinetic model for the sorptive removal of anionic pollutants [38][39][40].

Adsorption Isotherm
Freundlich and Langmuir isotherms were applied to reveal the interactions between the equilibrium concentration of PB in solution and the quantity of PB sorbed per unit mass of BSB. The Freundlich and Langmuir isotherm plots are elucidated in Figure 3. The results of the adsorption study are tabulated in Tables 2 and 3. In addition, the value of 1/n is used to indicate the strength of process heterogeneity [41]. For instance, if the value of 1/n is less than 1, it implies the normal Langmuir isotherm models [27]. In contrast, the value of 1/n is above 1, it is indicative of cooperative adsorption. When the value of 1/n becomes close to zero, it may indicate a more heterogeneous process [41]. The isotherm data of PB adsorption on BSB fit well with the Freundlich isotherm model. From the obtained R 2 value of Langmuir (0.4224) and Freundlich isotherm (0.968) models, it was concluded that the latter offered the best fit. In addition, the 1/n value was found to be 0.8264, which indicates a favorable multi-layer adsorption of PB (V) dye onto the positively charged BSB surface. Similar experimental findings were also reported for the sorptive removal of PB via Ginger waste material [8].

Reaction Models Parameters Values
Pseudo-first-order model k 1 (L/min) 0.0342 q e (mg/g) 1

Adsorption Isotherm
Freundlich and Langmuir isotherms were applied to reveal the interactions between the equilibrium concentration of PB in solution and the quantity of PB sorbed per unit mass of BSB. The Freundlich and Langmuir isotherm plots are elucidated in Figure 3. The results of the adsorption study are tabulated in Tables 2 and 3. In addition, the value of 1/n is used to indicate the strength of process heterogeneity [41]. For instance, if the value of 1/n is less than 1, it implies the normal Langmuir isotherm models [27]. In contrast, the value of 1/n is above 1, it is indicative of cooperative adsorption. When the value of 1/n becomes close to zero, it may indicate a more heterogeneous process [41]. The isotherm data of PB adsorption on BSB fit well with the Freundlich isotherm model. From the obtained R 2 value of Langmuir (0.4224) and Freundlich isotherm (0.968) models, it was concluded that the latter offered the best fit. In addition, the 1/n value was found to be 0.8264, which indicates a favorable multi-layer adsorption of PB (V) dye onto the positively charged BSB surface. Similar experimental findings were also reported for the sorptive removal of PB via Ginger waste material [8].

Proposed Mechanism for Adsorption of Patent Blue PB (V) Dye on to Bael Shell Biochar (BSB) Surface
The adsorption of PB dye over the BSB surface can be explained by hydrogen bonding, electrostatic interactions and Vander Waal forces. These interrelationships play a pivotal role in the sorption process, which can be explained in the following points:(a) the carboxylic group present on the BSB surface is expected to completely dissociate at higher pH (basic medium); dissociation would create an ionic repulsion amongst the carboxylate ion of BSB and the negatively charged portion of PB dye molecule, resulting in the reduction of PB dye removal from wastewater compared to lower pH values; (b) the formation of hydrogen bonds between oxygen and nitrogen bearing functionalities of BSB and PB (V) dye; and (c) the Vander Waal forces of attraction can play a role due to the presence of hydrophobic BSB regions and a hydrophilic portion of the PB (V) dye and (d) protonation of carboxylic and hydroxyl groups at lower pH range (acidic medium). In general, carboxylic groups have a pKa value in the range of 3-5 [36]. At pH < pKa, carboxylic groups are positively charged and provide a platform for electrostatic attraction via the SO3-functionalities of PB molecules. On the contrary, the BSB provides a positively charged surface for electrostatic interrelationships with the SO3-functionality of the PB dye when pH < pHz. This synergetic effect by BSB for the sorption of PB (V) dye was supported by FTIR analysis. Here, the BSB sample was investigated before and after adsorption at pH 2.3. The peak at 3408.7 cm −1 (Figure 4a,b) indicates the presence of H-bonded O-H stretching vibrations of hydroxyl groups from organic acids, alcohols and phenols in BSB, which diminishes after adsorption with PB (V) dye. This decrease in BSB intensity confirms the hydrogen bond formation between PB dye amine and BSB hydroxyl groups. Protonation of hydroxyl and carboxylic groups occurred at lower pH (2.34), which resulted in the ionic interrelationship amongst the anionic PB dye and protonated groups. Furthermore, the shift in BSB peak value from 2523.7 cm −1 (Figure 4b) to 2463.9 cm −1 (Figure 4b

Proposed Mechanism for Adsorption of Patent Blue PB (V) Dye on to Bael Shell Biochar (BSB) Surface
The adsorption of PB dye over the BSB surface can be explained by hydrogen bonding, electrostatic interactions and Vander Waal forces. These interrelationships play a pivotal role in the sorption process, which can be explained in the following points:(a) the carboxylic group present on the BSB surface is expected to completely dissociate at higher pH (basic medium); dissociation would create an ionic repulsion amongst the carboxylate ion of BSB and the negatively charged portion of PB dye molecule, resulting in the reduction of PB dye removal from wastewater compared to lower pH values; (b) the formation of hydrogen bonds between oxygen and nitrogen bearing functionalities of BSB and PB (V) dye; and (c) the Vander Waal forces of attraction can play a role due to the presence of hydrophobic BSB regions and a hydrophilic portion of the PB (V) dye and (d) protonation of carboxylic and hydroxyl groups at lower pH range (acidic medium). In general, carboxylic groups have a pK a value in the range of 3-5 [36]. At pH < pK a , carboxylic groups are positively charged and provide a platform for electrostatic attraction via the SO 3 -functionalities of PB molecules. On the contrary, the BSB provides a positively charged surface for electrostatic interrelationships with the SO 3 -functionality of the PB dye when pH < pH z . This synergetic effect by BSB for the sorption of PB (V) dye was supported by FTIR analysis. Here, the BSB sample was investigated before and after adsorption at pH 2.3. The peak at 3408.7 cm −1 (Figure 4a,b) indicates the presence of H-bonded O-H stretching vibrations of hydroxyl groups from organic acids, alcohols and phenols in BSB, which diminishes after adsorption with PB (V) dye. This decrease in BSB intensity confirms the hydrogen bond formation between PB dye amine and BSB hydroxyl groups. Protonation of hydroxyl and carboxylic groups occurred at lower pH (2.34), which resulted in the ionic interrelationship amongst the anionic PB dye and protonated groups. Furthermore, the shift in BSB peak value from 2523.7 cm −1 (Figure 4b) to 2463.9 cm −1 (Figure 4b) after adsorption signifies the interaction of an O-H bond of the carboxylic acid group with dye. The emergence of a new peak at 2889.5 cm −1 (Figure 4b) indicates the formation of new -CH stretching vibrations. Moreover, after adsorption, the peak at 1308.1 cm −1 belonging to the -SO 3− functionality was widened and reinforced ( Figure 4) for BSB. This clearly indicates that the -SO 3 − group acts as a main functional group for PB dye interaction on the BSB surface ( Figure 4). A proposed adsorption pathway of PB (V) dye on to the BSB is given in Figure 5.
Sustainability 2018, 10, x FOR PEER REVIEW 9 of 13 emergence of a new peak at 2889.5 cm −1 (Figure 4b) indicates the formation of new -CH stretching vibrations. Moreover, after adsorption, the peak at 1308.1 cm −1 belonging to the -SO 3− functionality was widened and reinforced ( Figure 4) for BSB. This clearly indicates that the -SO3 − group acts as a main functional group for PB dye interaction on the BSB surface ( Figure 4). A proposed adsorption pathway of PB (V) dye on to the BSB is given in Figure 5.      (Figure 4b) indicates the formation of new -CH stretching vibrations. Moreover, after adsorption, the peak at 1308.1 cm −1 belonging to the -SO 3− functionality was widened and reinforced ( Figure 4) for BSB. This clearly indicates that the -SO3 − group acts as a main functional group for PB dye interaction on the BSB surface ( Figure 4). A proposed adsorption pathway of PB (V) dye on to the BSB is given in Figure 5.

Bael Shell Biochar (BSB) as a Low-Cost Adsorbent for Pollutant Treatment
Biochar can be produced from non-usable materials by thermal conversion in sealed containers, which are rich in carbon content and soil nutrients [27]. In this way, one can reduce waste production in developing regions [46]. This kind of process might reduce the operation cost of conventional sewage-treatment infrastructure. Biochar can also be used for the creation of microporous spaces in which lactic-acid bacteria can inoculate and degrade waste matter at a lower pH value. This kind of degraded material can be used as food for earthworms (Vermiculture) [47]. In this respect, compost can be generated; further treatment of this composite, in the presence of air, over three to 12 months using a Terra preta sanitation process can lead to natural fertilizer [47], which can be sold or used as a fertilizing material for diverse types of vegetation.
On the other hand, the prepared biochar from waste material can be used as an adsorbent for the treatment of diverse pollutants. For instance, the biochar produced from bael shell was found to be a good packing media in a packed bed adsorbent reactor for the removal of Congo red [36]. In another study, up to 52% of fluoride content was removed from waste water upon 60 min of contact time with an adsorbent dose of 2 g/L of BSB [40]. Likewise, the removal efficiency of BSB towards iron was found to approach 60% [48]. The adsorption capacity of BSB towards Cr (VI) was found to be 17.3 mg/g [38].Hence, it is evident that biochar generated from BS biomass is an efficient material for pollutant remediation, especially in wastewater technologies.

Life Cycle Assessment (LCA) of Biochar Systems
LCA is a well-known technique that is used to analyze the potential impact of the product on the environment throughout the cycle of its usage. One category of environmental impacts included in the commercial LCA software tools are human health, resource use, eutrophication, acidification and photo-oxidants formation [49].
In this study, the raw material used for the production of biochar was bael shell, which is an agro-waste and commonly available in many parts of the world. In comparison with raw bael shell, carbonized bael shell was used to produce both biochar, which was found to be rich in carbon content and bio-oil with a high calorific value of 20.4 MJ/kg [2].The spent BSB can be directly landfilled, which would act as fertilizer for the growth of plants and crops [47]. Here, the pollutant adsorbed on the BSB may be broken down to simpler components by microbial degradation, which can thus act as nutrients for the growth of plants and crops. Moreover, the presence of higher carbon content in the BSB can help increase soil respiration, as well as fungal and bacterial growth rates [50]. In addition, when BSB was fertilized under the soil for three years, the alkalinity of the BSB would decrease slowly by sequential loss of cations such as K, Na and Ca to the soil. The reduced alkalinity may help enhance the growth of the associated microbial community in the soil [50]. Therefore, from the preparation to decomposition of spent BSB, it was concluded that BSB is an appropriate material for contaminant removal.

Conclusions
This study focused on the abatement of PB (V) dye from aqueous solutions through the effective utilization of BSB. Initially, the prepared BSB was characterized to determine its surface morphology, composition and type of functional groups. Later, the characterized BSB was utilized for the adsorption of PB (V) dye, which was observed to greatly depend upon the pH of the aqueous solution. The highest removal of PB (V) dye was observed to take place in the lower pH range (pH 2.7) and approached 74%. The obtained highest sorption capacity was noted to be 3.7 mg/g via a chemisorption methodology. The analysis of sorption isotherm revealed that the sorption capacity of BSB was linearly correlated with the amount of PB (V) dye. In addition, the suitability of Langmuir isotherm demonstrated that the adsorption of PB (V) dye was mainly based on the monolayer formation on the BSB surface. The experimental observations of the present investigation show that the bael shell which remains largely unused by the consumers can be used to efficiently remove pigments and dyes from water/wastewater. Such innovative usage of indigenously produced waste biomass holds a great potential for sustainable waste management as well as a cost-effective control on pollution processes.