Abstract

Biochar obtained through the pyrolysis of Pongamia glabra seed cover (PGSC) at 550 °C with a heating rate of 40 °C/min was characterized and its ability to adsorb the dyes Methylene blue (MB) and Rhodamine B (RB) from aqueous solutions was investigated. The effect of pH, temperature and initial concentration of the dyes on adsorption behavior were investigated. The equilibrium sorption data were analyzed by using Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D-R) isotherms. Equilibrium data were well fitted for D-R isotherm in case of MB and Langmuir isotherm in case of RB dyes. The kinetics of dye adsorption on PGSC biochar was well described by applying pseudo-second-order rate equations. The surface of adsorbent before and after the removal of dyes was characterized by using scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analysis. The study suggested that PGSC biochar could be used as a highly efficient adsorbent for the removal of synthetic dyes.

INTRODUCTION

Large amounts of dye effluents are annually discharged by textile, cosmetics, paper, leather, pharmaceutical, food and other industries (Safarikova et al. 2005; Mahvi 2008). A significant amount of these effluents is released into environment untreated, thus causing pollution of surface water (Mahvi et al. 2008; Carneiro et al. 2010), ground water (Dubey et al. 2010) and even soils through irrigation (Topaç et al. 2009; Zhou & Wang 2010). Presence of low concentration (˃1 ppm) of dyes is highly visible and can adversely affect the aquatic environment by preventing light penetration. Some dyes and their degradation products are found to be toxic, mutagenic and carcinogenic in nature (Keharia & Datta 2003). Thus, the removal of dyes from effluents has been given utmost importance.

Synthetic dyes are resistant to natural degradation and pose numerous environmentally-oriented drawbacks when released into natural water bodies. Methylene blue (MB) and Rhodamine B (RB) are basic dyes used extensively in the dyeing of various products including cotton, silk, paper, bamboo, weed, straw and leather (Shakir et al. 2010). Basic dyes are also considered as cationic dyes because they form a colored cationic salt when dissolved in water (Ghaly et al. 2014). Cationic dyes are considered more toxic than the anionic dyes, because they can easily interact with the negatively charged surface of cell membranes, and can enter into the cells (Orfanos et al. 2016). MB can cause eye burns in humans and animals, methemoglobinemia, cyanosis, convulsions, tachycardia, dyspnea, irritation to the skin, and if ingested, irritation to the gastrointestinal tract, nausea, vomiting, and diarrhea (Senthilkumaar et al. 2005). Similarly, RB can cause skin and eye irritation with redness and pain, irritation to the respiratory tract and gastro-intestinal tract (Shakir et al. 2010).

There are several treatment processes for effluent containing dyes including biodegradation, chemical oxidation, foam flotation, electrolysis, photocatalysis, electro-coagulation, and adsorption (Mohammed 2014). Adsorption is one of the most efficient and attractive methods for removing pollutants from wastewater because of its easy process control, low cost and minimal energy requirements (Yang et al. 2014). Various adsorbents, such as activated bituminous coal (El-Qada et al. 2006), clay (Gurses et al. 2006), leaf powder (Bhattacharyya & Sharma 2005), activated carbon from oil palm wood (Ahmad et al. 2007), etc. have been studied for adsorption of dye from aqueous solutions. Compared to activated carbon, biochar may be utilized as a potentially low-cost and effective adsorbent. The production of activated carbon needs higher temperature and additional activation process. In contrast, production of biochar is cheaper with lower energy requirements (Tan et al. 2015). A few studies have been reported on removal of dyes on using biochar as adsorbent.

Biochar, the carbonaceous by-product obtained by pyrolysis of biomass in absence of oxygen, was recently identified as a ‘supersorbent’ for a wide variety of organic and inorganic contaminates due to its porous nature, surface area and abundant functional groups (Qiu et al. 2009; Mohan et al. 2014). Various biowastes such as deoiled cakes, seed covers, microalgae species, aquatic weed species, agro wastes, agro industrial wastes were valorized to produce bio-oil (a potential biofuel) and biochar at different temperatures. Recent biofuel policy of Government of India's aims to achieve the target of 20% biofuels (i.e. biodiesel, bioethanol and bio-oil) blending with petroleum by 2017 (MNRE, Govt. of India 2009). Pongamia glabra is a non-edible oil seed species abundantly available in Northeast India and can be used for the production of biodiesel (Sarma 2006). The seed cover of the oil bearing seeds of P. glabra is a byproduct, which is often discarded as wastes, generated during the process of oil extraction for biodiesel production. Pyrolytic valorization of this wastes biomass can fulfill multiple objectives of waste utilization, improvement in process economics of biodiesel production, biofuel (bio-oil) production and co-production of biochar.

The objective of the present study is to characterize the Pongamia glabra seed cover (PGSC) biochar and investigate its adsorption efficacy for the removal of synthetic dyes MB and RB from aqueous solution. The present study also reports the physicochemical characteristics of adsorbent following dye adsorption, adsorption kinetics and isotherms.

MATERIALS AND METHODS

Materials

The PGSC sample, obtained after dehulling of P. glabra seeds, was collected from the Biofuel Laboratory of Department of Energy, Tezpur University, India. The PGSC sample was ground with a Wiley mill. The ground sample was allowed to pass through 0.2 mm (70 mesh) (as per TAPPI T257 Om-85 methods). Sample was then oven dried and kept in a desiccator for further analysis and pyrolysis experiments.

MB (C.I. 52015) and RB (C.I. 45170) were purchased from Sigma-Aldrich, Co., USA. The absorption maximum of these dyes are 668 and 554 nm respectively, as validated by the spectrophotometric scan from 200–800 nm.

Preparation of biochar

Biochar was produced from the PGSC by thermochemical conversion method – pyrolysis. The pyrolysis experiments were carried out in a fixed bed vertical, tubular reactor (length: 30 cm and internal diameter: 2.47 cm) made of quartz glass in which the temperature was controlled by Ni-Cr thermocouple placed in the center of electrical furnace as reported elsewhere (Bordoloi et al. 2015). The experiments were carried out with a temperature increment of 40 °C/min to the final temperature of 550 °C, under constant flow rate of nitrogen at 100 ml/min. The biochar obtained was ground and passed through 0.2 mm sieve. The biochar was washed with deionized water in order to remove soluble inorganic ions and was dried at 60 °C for further testing.

Characterization of biochar

Proximate and ultimate analyses of the biochar were carried out according to ASTM D3172-07a. The calorific value of biochar was determined using a bomb calorimeter (5E-1AC/ML, auto bomb calorimeter) according to ASTM D2015. pH value was measured by adding biochar to deionized water in a mass ratio of 1:20. The solution was then hand shaken and allowed to stand for 5 min before measuring the pH with a pH meter (EUTECH Instruments pH 700). Electrical conductivity (EC) was measured with conductivity meter (Digital TDS/Conductivity Meter MK509) using the same solution as prepared for pH measurement. Scanning electron microscopy (SEM) images of the obtained char were taken with a JEOL (JSM-6390 LV) microscope with an acceleration voltage of 20 kV. SEM analysis was performed to find out the surface morphology of the biochar before and after the adsorption of dyes. Images of biochars were taken at 3,000× magnifications. Functional groups of pre and post sorption biochars were determined by using Nicolet Impact I-410 model IR spectrometer.

Adsorption kinetics and isotherm study

Various amounts of biomass were incubated individually for adsorption of MB and RB from aqueous solutions (50 ppm). For optimum adsorption of MB, 50 g/L of biochar and for RB, 30 g/L of biochar were required. The tubes were incubated at 28 °C for 24 h in a shaker at 60 rpm. Thereafter, the biomass was segregated by centrifugation and the dye concentration in the supernatant was estimated using a spectrophotometer (MultiScan Go, Thermo Scientific). Effect of pH on adsorption of MB and RB by biochar was studied. The rate of adsorptions of MB and RB were observed at different concentrations of dye (10, 20, 30, 40 and 50 mg/L). Kinetic studies of adsorption were done at various concentrations of MB wherein the extent of adsorption was investigated as a function of time. (mg/g), depicting the amount of adsorption at time, t was calculated by using the Equation (1). The amount of biomass exhibiting maximum adsorption was selected for further analysis. The study involved measuring adsorptions of MB and RB at regular time intervals using the following equation:  
formula
(1)
where (mg/g) is the amount of dye adsorbed at time t, and (mg/L) denotes the un-adsorbed dye initially and at time, t, respectively. W denotes mass of the dry bio-sorbent used and V denotes volume of the solution. Based on the kinetics results, the adsorption isotherms were calculated. Amount of dye adsorbed at equilibrium, i.e. (mg/g) was determined by  
formula
(2)
where (mg/L) is the concentration of the un-adsorbed dye at equilibrium.

RESULTS AND DISCUSSION

Characterization of biochar

The effect of temperature on product yield has been shown in Supplementary Figure S1 (available with the online version of this paper), obtained through pyrolysis of PGSC at 550 °C at heating rate of 40 °C/min under N2 atmosphere. At 550 °C, 28% of oil fraction, 20% of aqueous phase, 28% of solid product (i.e. biochar) and 23% of gaseous product was obtained (gas yield was calculated by difference). The physicochemical properties of biochar obtained at 550 °C along with raw biomass are shown in Supplementary Table S1 (available with the online version of this paper). As already discussed in our previous studies (Bordoloi et al. 2015), high percentage of volatile matter (74.58%) and less percentage of ash content (2.72%) of biomass strongly influences its combustion behavior and thermal decomposition. Dry moisture content of the feedstock (2.72%) was found to be very low which had an obvious effect on the conversion efficiency of pyrolysis process. C, H, N and O contents of PGSC biomass obtained on dry basis were found to be 44.0%, 5.46%, 1.61% and 48.8% respectively. Oxygen content was calculated by difference. C, H, N and O contents of biochar produced at 550 °C were found to be 73.7%, 1.89%, 1.82% and 21.6%, respectively. pH and EC values are important parameters of biochar to determine whether biochar can be used as a soil amendment. Higher ash content in biochar was due to high relative concentration of Ca, Mg, K, P, S and Zn (Bordoloi et al. 2015).

Optimization of adsorption parameters

Optimization of biomass, pH and temperature for the efficient uptake of two basic dyes namely MB and RB by the biochar are shown in Figure 1(a)1(c). It was evident that adsorption of MB was superior to RB by the PGSC biochar. It was observed that for adsorption of MB and RB at a concentration of 50 ppm, the optimum biochar requirement was 50 g/L and 30 g/L, respectively. Effect of pH on adsorption of MB by biochar was found to be minimal from pH 3.0 to 10.0. However, a sharp decline in adsorption was found when pH was raised to 11.0 (Figure 1(b)). For RB adsorption, effect of pH was fairly uniform in the range of 3.0 to 11.0 with a minor drop in adsorption percentage at pH 8.0. In general, with decrease in pH of the system, the number of negatively charged adsorbent sites decreased and positively charged surface sites increased, which might not favor the adsorption of positively charged dye cations. Since the adsorption percentage of the dyes by biochar remained almost constant in a wide range of pH, there might exist another mode of adsorption, e.g. ion exchange (Garg et al. 2003). Adsorption of dyes in a wide range of pH attributes advantage to PGSC biochar for its potential use for the management of various industrial effluents released at different pH. The optimum temperature for both MB and RB adsorption was found to be 28 °C (Figure 1(c)). The dependence on temperature was minimal for adsorption of RB, while adsorption percentage varied significantly with temperature in case of MB.

Figure 1

Plots for optimization of (a) biomass, (b) pH and (c) temperature for the adsorption of Methylene blue (MB) and Rhodamine B (RB) by PGSC biochar.

Figure 1

Plots for optimization of (a) biomass, (b) pH and (c) temperature for the adsorption of Methylene blue (MB) and Rhodamine B (RB) by PGSC biochar.

Effect of initial concentration

For achieving efficient adsorption of synthetic dyes by various adsorbents, the initial concentration of dyes is one of the major parameters for consideration. To study the effect of the initial concentrations of MB and RB on the rate of dye adsorption by PGSC biochar, the experiments were carried out at different initial dye concentrations of MB and RB (10, 20, 30, 40 and 50 mg/L) for different time intervals (10–480 min) at 28 °C as shown in Figure 2(a) and 2(b), respectively. The initial concentration of the dye was varied, keeping the amount of biosorbent constant. It was observed that the initial adsorption of dye increased with rise in concentration of synthetic dyes initially and reached a point of equilibrium, beyond which there was no further increase in the adsorption. It was also witnessed that adsorption of lower concentration of dyes reached equilibrium earlier than those with higher concentration. Further, for adsorption of both dyes, an optimal point for adsorption was reached remarkably rapidly within a range of 20 to 50 min. This might be due to the fact that biochar produced at higher temperature is rendered highly absorptive in nature (Mohan et al. 2014). Such dehydrated biochar takes up water rapidly when subjected to water solutions containing dye for adsorption. This leads to an interaction of the dissolved dye molecules with the surface moieties in the adsorbate, thereby leading to prompt adsorption of synthetic dyes. Similar observation was reported by Mohan et al. (2014), where removal of Malachite green (MG) dye was found to be 95% percent within 40 min at pH 5 and temperature 25 °C. Rapid adsorption of dyes by the biochar may prove advantageous for its probable commercial application for adsorptive removal of dye-containing effluents.

Figure 2

Effect of initial concentration on uptake of (a) Methylene blue (MB) and (b) Rhodamine B (RB) by PGSC biochar.

Figure 2

Effect of initial concentration on uptake of (a) Methylene blue (MB) and (b) Rhodamine B (RB) by PGSC biochar.

ADSORPTION KINETICS

Adsorptions of dyes are known to follow a pseudo-second-order kinetics which can be expressed in linear form as:  
formula
(3)
where the dye adsorbed at equilibrium () and the second order constants (g/mg h) can be determined from the slope and intercept of plot t/ vs t. For the sorption process, highest correlation coefficients have been observed for the pseudo-second-order kinetics system in contrast to the pseudo-first-order kinetics system (Dey et al. 2017). Therefore, a pseudo-second-order kinetic study has been carried out for the adsorption of MB and RB by using PGSC biochar. Supplementary Figure S2(a) and S2(b) (available with the online version of this paper) represent the pseudo-second-order sorption kinetics of MB and RB on biochar. The values for and (both experimental and calculated) are represented in Table 1. The data analysis suggested that the correlation coefficients (R2) for fitting the data to the pseudo-second-order kinetic model close to unity. Values of calculated using pseudo-second-order kinetic model were in agreement with the experimentally determined values of . Thus, the pseudo-second-order model reasonably described the mechanism of the MB and RB adsorption by the biochars. This observation supported the assumption that the adsorption could predominantly be due to chemisorption (Malash & Mi 2010).
Table 1

Pseudo-second-order kinetics parameters for adsorption of MB and RB on PGSC biochar

Dye Initial concentration (ppm) qe experimental (mg/g) Parameters for pseudo-second-order kinetics
 
K2 (g/mg h) qe calculated (mg/g) R2 
Methylene blue (MB) 10 0.28 165.99 0.28 0.99 
20 0.61 31.15 0.61 0.99 
30 0.91 9.07 0.92 0.99 
40 1.18 5.32 1.19 0.99 
50 1.32 3.36 1.36 0.99 
Rhodamine B (RB) 10 0.15 46.54 0.15 0.99 
20 0.31 22.82 0.32 0.99 
30 0.45 11.35 0.46 0.99 
40 0.49 9.21 0.51 0.99 
50 0.57 6.75 0.59 0.99 
Dye Initial concentration (ppm) qe experimental (mg/g) Parameters for pseudo-second-order kinetics
 
K2 (g/mg h) qe calculated (mg/g) R2 
Methylene blue (MB) 10 0.28 165.99 0.28 0.99 
20 0.61 31.15 0.61 0.99 
30 0.91 9.07 0.92 0.99 
40 1.18 5.32 1.19 0.99 
50 1.32 3.36 1.36 0.99 
Rhodamine B (RB) 10 0.15 46.54 0.15 0.99 
20 0.31 22.82 0.32 0.99 
30 0.45 11.35 0.46 0.99 
40 0.49 9.21 0.51 0.99 
50 0.57 6.75 0.59 0.99 

ADSORPTION ISOTHERM

To establish a relation between adsorbate concentration in the solution and the amount adsorbed at the adsorbent interface, adsorption isotherm studies were carried out. Isotherm results were analyzed by using four isotherms, namely, Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms.

The Langmuir isotherm model assumes that maximum adsorption corresponds to a saturated monolayer of adsorbate molecules on adsorbent surface, with no lateral interaction between the adsorbed molecules (Dey & Mukhopadhyay 2015). The linear form of Langmuir equation can be expressed as  
formula
(4)
where (mg/L) is the equilibrium concentration in the solution, (mg/g) is the maximum sorption capacity, b is the constant of Langmuir model (L/mg) related to binding energy for sorption. The Langmuir plots of Ce/qe vs. Ce are shown in Figure 3(a) and 3(b) for MB and RB dyes, respectively, and the isotherm constants are given in Table 2. Adsorption of both dyes with PGSC biochar showed high regression coefficient (R2) value of 0.98. The RL value for MB was 0.04 while the value for RB adsorption was 0.02 which suggests that both the adsorption could follow Langmuir isotherm.
Table 2

Isotherm constants for the adsorption of MB and RB with PGSC biochar

Isotherm parameters Methylene blue Rhodamine B 
Langmuir 
Qo (mg/g) 1.623 0.683 
b (L/mg) 0.462 0.952 
 R2 0.98 0.98 
Freundlich 
Kf 1.727 4.653 
n 2.545 3.059 
 R2 0.82 0.91 
Temkin 
 A 4.018 2.661 
 B 0.374 0.142 
 R2 0.88 0.93 
Dubinin–Radushkevich 
 qD 1.385 1.795 
 BD 6*10−7 2*10−6 
 R2 0.99 0.95 
Isotherm parameters Methylene blue Rhodamine B 
Langmuir 
Qo (mg/g) 1.623 0.683 
b (L/mg) 0.462 0.952 
 R2 0.98 0.98 
Freundlich 
Kf 1.727 4.653 
n 2.545 3.059 
 R2 0.82 0.91 
Temkin 
 A 4.018 2.661 
 B 0.374 0.142 
 R2 0.88 0.93 
Dubinin–Radushkevich 
 qD 1.385 1.795 
 BD 6*10−7 2*10−6 
 R2 0.99 0.95 
Figure 3

Isotherm plots for adsorption of (a) Methylene blue (MB) and (b) Rhodamine B (RB) by PGSC biochar.

Figure 3

Isotherm plots for adsorption of (a) Methylene blue (MB) and (b) Rhodamine B (RB) by PGSC biochar.

The Freundlich isotherm model describes the heterogeneous adsorption process. The linear form of the Freundlich equation is expressed by the equation (Dey & Mukhopadhyay 2015).  
formula
(5)
where KF and n are Freundlich constants with KF (mg/g(L/mg)1/n) being the adsorption capacity of the sorbent and n indicating favorability of adsorption process. Values of KF and n were calculated from the plot of against shown in Figure 3(a) and 3(b) for MB and RB, respectively. Isotherm parameters suggested relatively low regression coefficient (R2) values of 0.82 and 0.91 for adsorption of the MB and RB, respectively (Table 2).
Temkin model denote the changes in heat of adsorption as a result of adsorbent–adsorbate interaction. The Temkin isotherm can be represented in linear form as (Dada et al. 2012).  
formula
(6)
where B = RT/b, A is the Temkin isotherm constant (L/g), R is the gas constant (8.314 J/molK), T is the absolute temperature (K) and b is the Temkin constant related to heat of sorption (J/mol). Isotherm constants are determined by plotting qe vs. for MB and RB, respectively (Figure 3(a) and 3(b)). The regression coefficient (R2) values for Temkin isotherm model were lower than that of Langmuir isotherm model (Table 2).
In order to investigate the mode of dye uptake processes, i.e. whether physical or chemical in nature, the equilibrium uptake data were applied to the Dubinin–Radushkevich (D-R) isotherm model given as (Dada et al. 2012)  
formula
where (mol/g) is the theoretical monolayer saturation capacity of the adsorbent, B is constant and ɛ (known as Polyanyi potential) is given as  
formula
(7)
where R is the gas constant (kJ/mol/K), T is the absolute temperature and Ce is the equilibrium dye concentration.
The constant B (mol2/J2) given by the following equation, the mean free energy E (J/mol) of adsorption per molecule of adsorbate is  
formula
(8)
The linear from of D-R equation is  
formula
(9)

The D-R plots of vs. ɛ2 were presented in Figure 3(a) and 3(b) for MB and RB, respectively, and the isotherm constants are given in Table 2. For the adsorption of MB by PGSC biochar, a very high regression coefficient (R2) value of 0.99 was calculated, which suggested the adsorption fitted best the D-R adsorption isotherm model. This data implied a physical mode of adsorption characterized by porous adsorbent biomass. The adsorption of RB with the biochar showed a high R2 value of 0.95, though the value was less than that of Langmuir isotherm model. From four isotherm analyses, it may be concluded that adsorption of MB followed D-R isotherm model and RB followed Langmuir isotherm model.

POST SORPTION CHARACTERISTICS OF BIOCHAR

SEM analysis

SEM is used for high magnification imaging. SEM images of the biochars before and after the adsorption of MB and RB dyes are shown in Supplementary Figure S3(a), S3(b) and S3(c), respectively (available with the online version of this paper). The SEM images of biochars demonstrated physical changes in surface structures during adsorption. SEM image of biochar showed a smooth surface with distinct pores before adsorption, while biochars after adsorption showed rough texture. After adsorption of MB dyes, the pores on the biochar surface become covered with dye particles and the surface becomes uneven. After adsorption of RB dyes, some bright spots were observed on the surface of biochar covering the pores. Rough surface was observed for the adsorption of RB dyes as compared with adsorption of MB dyes on biochar. The heterogeneous compactness of the biomass surfaces indicates a non-uniform adsorption process.

FTIR analysis

The Fourier transform infrared spectroscopy (FTIR) spectra of biochar before and after adsorption of MB and RB dyes were shown in Figure 4. The absorption peak at 3,200–3,550 cm−1 indicates the presence of O–H group (or N–H) along with moisture (Pütün et al. 1999). The C–H stretching vibrations between 2,830 and 2,928 cm1 and C–H deformation vibrations between 1,410 and 1,515 cm−1 indicate the presence of alkanes. Moreover, the location of bending vibration of C–H groups at 1,378 cm−1 provides additional evidence of the fact that this band is very important for the detection of methyl groups in a given compound (Apaydin-Varol et al. 2014). The region between 700 and 900 cm−1 contains various bands related to the aromatic, out of plane C–H bending. The high intensity of the above mentioned bands indicates that the aromatic hydrogen is located in aromatic rings with high degree of substitution (Ozbay et al. 2006) after adsorption of MB and RB dyes.

Figure 4

FTIR spectrum of PGSC biochar before (black) and after adsorption with MB (blue) and RB (red). Please refer to the online version of this paper to see this figure in color: http://dx.doi.org/10.2166/wst.2017.579.

Figure 4

FTIR spectrum of PGSC biochar before (black) and after adsorption with MB (blue) and RB (red). Please refer to the online version of this paper to see this figure in color: http://dx.doi.org/10.2166/wst.2017.579.

After adsorption of MB on the biochar surface, N–H bending vibration was observed from 1,650–1,590 cm−1 and C–N stretching was observed from 1,090–1,020 cm−1 for primary amine. Intensity of the peak at 3,433 cm−1 increased after adsorption of MB, and indicates the presence of O–H and N–H groups of amine. Adsorption of RB on the biochar surface, the peak appearing at 3,441 cm−1 attributed to O–H stretch from carboxylic group (–COOH) of RB while the weak band at 1,629 cm−1 can be related to stretch from carboxylic group and peaks at 1,403 cm−1 and 1,049 cm−1 might be due to O–H in plane bending of hydroxyl groups of carboxylic acid. N–H bend was observed at 1,650–1,550 cm−1 and C–N stretch was observed at 1,123 cm−1 for secondary amine (Rani et al. 2016). All these changes in chemical groups of the PGSC biochar post dye adsorption suggested the role of these groups in adsorption of the dyes.

CONCLUSION

The present study investigated the efficacy of biochar derived from PGSC produced at 550 °C as an adsorbent for the removal of cationic dyes, MB and RB from aqueous solutions. The raw biomass (PGSC), a discarded waste generated during the process of oil extraction for biodiesel production, was used for the production of biochar (which is again generated as a by-product during thermo-chemical conversion of PGSC). Therefore, the work indicates the probable application of PGSC biochar, a waste generated by product, as a dye-removal strategy for industries generating dye-containing effluents. It was observed that, at 50 ppm concentration, for adsorption of both MB and RB, the optimum biochar requirement was 50 g/L and 30 g/L, respectively. Adsorption was found to be independent of solution pH and optimum at 28 °C. The adsorption of MB by biochar followed D-R adsorption isotherm model with a regression coefficient (R2) of 0.99. Adsorption of RB by biochar, however, followed Langmuir adsorption isotherm model with a regression coefficient (R2) of 0.98 and an RL value of 0.02. This observation suggested that the adsorption of MB by biochar is a heterogeneous adsorption with the pores in the adsorbent and that the adsorption of RB on biochar is homogenous and in a monolayer fashion.

ACKNOWLEDGEMENT

The authors would like to acknowledge UGC for the grant and Tezpur University for providing the laboratory facilities and SAIC of Tezpur University for FTIR and SEM analysis. MDD acknowledges fellowship received from UGS-SAP DRS-II program in MBBT Department, Tezpur University.

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