Abstract

Microporous activated carbon was prepared from almond shell powder and activated with H3PO4 and was used for the removal of Cr (VI). The characterization of activated carbon was done for Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area measurement and elemental analysis. The batch experiments were conducted to study the effects of contact time, solution pH, adsorbent dose, initial chromium concentration and temperature on removal of Cr (VI). The Cr (VI) removal was found to be 100% at an initial pH of 2. The equilibrium data for the adsorption of Cr (VI) on the adsorbent were fitted with Langmuir, Freundlich, Temkin and Dubinin–Radushkevich adsorption isotherm models. The Langmuir isotherm model fitted better and values of model parameters qm, b, R2 and RL were found to be 195, 0.024, 0.98 and 0.45, respectively. Adsorption kinetics was analyzed for the pseudo first order, pseudo second order and intra-particle diffusion situations. Thermodynamic parameters revealed the spontaneous, endothermic and increased randomness nature of the adsorption process. The values of Δ and Δ were found to be 22.9 KJ/mol and 95.3 J/mol-K.

INTRODUCTION

Chromium is present in industrial effluents produced from electroplating, leather tanning, cement, mining, textile dyeing, dye manufacturing, paper, ink, aluminum conversion coating operations, steel fabrication, plants producing industrial inorganic chemicals, wood treatment units, paints and pigments, metal cleaning, fertilizer and photography industries and cause severe environmental and public health problems (Anupam et al. 2011; Tan et al. 2015). In general, industrial wastes contain both hexavalent and trivalent forms of chromium (Nakajima & Baba 2004; Sonwani et al. 2018). Cr (III) is nearly insoluble and hence aqueous concentrations are usually well below water quality standards (Anderson et al. 1994). The hexavalent species are relatively more soluble and are 500 times more toxic than the trivalent species (Gupta & Babu 2009). It is proven to cause skin irritation, epigastric pain, nausea, vomiting, severe diarrhea, hemorrhage and carcinogenicity in humans (Mohanty et al. 2005; Wang et al. 2014). According to the World Health Organization (WHO), the permissible level in surface water bodies should be lower than 0.05 mg/L. Therefore, it is necessary to reduce Cr (VI) to acceptable levels before discharging effluents into aquatic environments.

Conventional technologies for Cr (VI) removal from aqueous solutions include chemical reduction/precipitation, ion-exchange, membrane separation, and adsorption (Imai & Gloyna 1990; Selvi et al. 2001; Stasinakis et al. 2003; Owlad et al. 2009; Sonwani et al. 2018). Increasing attention has been paid recently to the use of eco-friendly and low-cost biomaterials as adsorbents/bio-sorption for Cr removal from wastewater, such as bacteria, fungi, algae, industrial and agricultural wastes (Mohan et al. 2006; Han et al. 2007; Park et al. 2007; Singh et al. 2017). Most of these methods suffer various drawbacks such as from high operational costs, unreliable nature of operation, etc. Adsorption is a reliable and cost effective method provided that the adsorbent used in the process has good adsorption capacity and is cheaply available. Many studies have been done on adsorptive removal of Cr (VI) from waste by using activated carbons derived from different materials (Table 1). However, there is very limited published information available on the use of almond shell for preparation of activated carbon and its application for the removal of Cr (VI).

Table 1

Comparing of different activated carbon as Cr (VI) adsorbent

Activated carbon sources Initial pH BET surface area (m2 g−1Pore volume (cm3 g−1Maximum adsorption capacity (mg g−1References 
Tamarind wood 1,322 1.042 28 Acharya et al. (2009)  
Apricot stone 1,462 0.6338 262 Ozdemir et al. (2011)  
Rubber wood sawdust 1,673 – 65 Karthikeyan et al. (2005)  
Coconut shell fibers 1,565 0.65 21.75 Mohan et al. (2005)  
Terminallia Arjuna nuts 1,260 – 28.43 Mohanty et al. (2005)  
Bael fruit – – 17.27 Kumar & Mandal (2009)  
Peanut shell 95.51 0.35 16.26 Othman et al. (2012)  
Longan seed 1,511.8 0.742 35.02 Yang & Chen (2015)  
Sterculia guttata shell 498.29 0.232 45.5 Rangabhashiyam & Selvaraju (2015)  
Sal sawdust 3.5 – – 9.55 Baral et al. (2006)  
Almond shell 1,223 0.326 167.5 Present study 
Activated carbon sources Initial pH BET surface area (m2 g−1Pore volume (cm3 g−1Maximum adsorption capacity (mg g−1References 
Tamarind wood 1,322 1.042 28 Acharya et al. (2009)  
Apricot stone 1,462 0.6338 262 Ozdemir et al. (2011)  
Rubber wood sawdust 1,673 – 65 Karthikeyan et al. (2005)  
Coconut shell fibers 1,565 0.65 21.75 Mohan et al. (2005)  
Terminallia Arjuna nuts 1,260 – 28.43 Mohanty et al. (2005)  
Bael fruit – – 17.27 Kumar & Mandal (2009)  
Peanut shell 95.51 0.35 16.26 Othman et al. (2012)  
Longan seed 1,511.8 0.742 35.02 Yang & Chen (2015)  
Sterculia guttata shell 498.29 0.232 45.5 Rangabhashiyam & Selvaraju (2015)  
Sal sawdust 3.5 – – 9.55 Baral et al. (2006)  
Almond shell 1,223 0.326 167.5 Present study 

Almond is a dry fruit which is produced in several countries including India. The shell of almond fruit is a very good agro-waste and excellent adsorbent. Since it is primarily lingo-cellulosic in nature and hard, it can be used as a feed-stock for making activated carbon for adsorption of water pollutants including Cr (VI) to save the woody biomass which is used for preparation of activated carbon at industrial scale. Keeping this view, almond shell was used as a raw material to produce biochar which was finally activated using H3PO4. The prepared activated carbon was used for removal of Cr (VI) in batch adsorption experiments under optimum operating conditions.

MATERIALS AND METHODS

Adsorbate: chromium (VI)

All the solutions were prepared by using deionized water, and stock solution of 1,000 mg/L Cr (VI) was prepared by dissolving potassium dichromate (analytical reagent [AR] grade). The test solutions of Cr (VI) were prepared by diluting the stock solutions to specific concentrations as per the requirements.

Preparation of activated carbon

The almond shells were obtained from the local market. The shells were crushed in the disintegrator and sieved to obtain 14–72 mesh (BSS, British Standard Sieve) fractions. Chemical activation of the powdered shells was performed with 40% H3PO4 (AR grade) using the procedure reported by Girgis et al. (2002). The impregnation ratio of 3:1 (weight of H3PO4:weight of powdered shells, w/w) was used for proper penetration and the mixture was kept for 24 h. The chemically mixed sample was dried in the oven at 70 °C for 10 h. The dried pre-treated almond shell powder was carbonized at 600 °C for 2 hours in a laboratory muffle furnace (NSW India, NSW-101). The cooled carbonized sample was washed thoroughly with distilled water and its pH was adjusted to 7 with 1N HCl and 1N NaOH. The sample was then dried at 70 °C overnight. The carbonized almond shell activated carbon (ASAC) then obtained was kept in airtight bottles for characterization and adsorption experiments.

Physicochemical characterization of activated carbon

The physicochemical characterizations of the samples were carried out in order to know their chemical composition, functional groups, morphology, and surface area. Various functional groups present on the surface of activated carbon were analyzed using a Fourier transform infrared spectroscopy (FTIR) analyzer (Scientific, Nicolet 5700).

The surface morphology was analyzed using scanning electron microscopy (SEM) (FEI™, Quanta 200F) to determine the surface texture and the porosity. The average pore size or diameter was also calculated using SEM results.

The elemental analysis of the almond shell powder and activated carbon were determined by using an elemental analyzer (Euro-EA) and results are presented in Table 2. The surface area, pore volume and pore size of the almond shell powder and activated carbon were determined using the N2 adsorption at −196 °C and N2 desorption at ambient temperature with degassing at 300 °C using a Smart Sorbs 92/93 surface area analyzer (Smart Instruments Co. Pvt. Ltd).

Table 2

Physical properties of almond shell activated carbon

Parameters Value 
BET surface area (m2 g−11,223 
Pore volume (cm3 g−10.326563 
Average pore size (nm) 2.39 
pH at the point of zero surface charge (pHpzc5.5 
Elemental analysis (%)  
75.56 
2.55 
1.45 
15.44 
Parameters Value 
BET surface area (m2 g−11,223 
Pore volume (cm3 g−10.326563 
Average pore size (nm) 2.39 
pH at the point of zero surface charge (pHpzc5.5 
Elemental analysis (%)  
75.56 
2.55 
1.45 
15.44 

The pH value required to give zero net surface charge is pHpzc. The surface is positively charged below this pH and negatively charged above this pH. A plot of the equilibrium pH versus initial pH yielded a curve from where the pHpzc was identified as the point at which the change of pH is zero.

Adsorption experiments

The adsorption experiments were conducted in 250 mL Erlenmeyer flasks containing 100 mL solution of Cr (VI) to investigate the effects of contact time (0–300 min), solution pH (2–10), adsorbate dose (0.25–1.0 g/100 mL), initial concentration (50–1,000 mg/L) and temperature and obtain their optimum values. All adsorption experiments were carried out at a constant agitation speed of 150 rpm.

Equilibrium adsorption data were obtained by conducting the experiments at various temperatures (298, 303, and 308 K), with sample solutions containing varying concentrations of Cr (VI) ranging from 50–1,000 mg/L. An adsorption kinetic study was performed by adding a dose of 0.25 g/100 mL at pH 2 with varying Cr (VI) concentrations of 100, 150, 250 and 300 mg/L at 308 K and analyzing the samples after consistent intervals of time.

The applicability of different absorption isotherm models was studied using the experimental findings. The residual concentration of adsorbate was determined by measuring the absorbance of the purple complex of Cr (VI) with 1,5–diphenyl carbazide at 540 nm using a UV-spectrophotometer (Elico, SL 159). The difference in Cr (VI) concentration before and after the adsorption was used to calculate the percentage adsorption of Cr (VI). The amount of adsorbed Cr (VI) per gram of activated carbon was calculated using Equation (1): 
formula
(1)
where C0 (mg/L) and Ce (mg/L) are the initial and equilibrium concentration of Cr (VI) respectively, V (L) is the volume of solution, W (g) is the mass of activated carbon, and qe (mg/g) is the metal uptake capacity of activated carbon.

Adsorption isotherms

Adsorption isotherms provide useful information, including the adsorption mechanism for the adsorption of Cr (VI) on the adsorbent surface, and are also important in the designing of adsorption systems. The two parameter isotherm models like Langmuir, Freundlich, Temkin and Dubinin–Radushkevich (D–R) were used to analyze the equilibrium data obtained in the present study.

Langmuir isotherm

The Langmuir isotherm model (Langmuir 1918) is valid for the monolayer adsorption which assumes that all active sites of adsorption are homogeneous in nature and the adsorption at one site does not affect an adjacent active site of adsorption. The linear from of the Langmuir isotherm model is expressed by Equation (2): 
formula
(2)
where b is the adsorption equilibrium constant (L/mg) related to the energy of adsorption and qm (mg/g) is the quantity of adsorbate required to make a single layer on a unit mass of adsorbent.
A further analysis of the Langmuir isotherm can be made on the basis of a dimensionless equilibrium parameter, RL, also known as the separation factor and given by Equation (3): 
formula
(3)
where b is the adsorption equilibrium constant (L/mg) related to the energy of adsorption and qm (mg/g) is the quantity of adsorbate required to make a single layer on a unit mass of adsorbent. The value of RL lies between 0 and 1 for a favorable adsorption, while RL>1 represents an unfavorable adsorption, and RL = 1 represents linear adsorption, while the adsorption operation is irreversible if RL = 0.

Freundlich isotherm

The Freundlich model is an empirical equation which assumes that the active sites of adsorption are heterogeneous in nature and adsorption at one site affects the adsorption at the adjacent site. Kanjanarong et al. (2017) reported the linearized form of the Freundlich isotherm model. The linearized form of the Freundlich isotherm model can be expressed as in Equation (4): 
formula
(4)
where Kf and n are the Freundlich isotherm constants. Kf (mg/g (L/mg)1/n) indicates the adsorption capacity and n stands for the intensity of the adsorption.

Temkin isotherm

The Temkin isotherm model contains a factor that describes the interactions between adsorbent and adsorbate (Tempkin & Pyzhev 1940; Dada et al. 2012). This model assumes that the heat of adsorption of all the molecules in the layer decreases linearly due to the coverage of adsorbate molecules, and the adsorption of adsorbate is uniformly distributed. This model also assumes that the decrement in the heat of adsorption is linear rather than logarithmic, as used in the Freundlich isotherm model. The non-linear form of the Temkin model is given by Equation (5): 
formula
(5)
The above equation can be simplified as given by Equation (6): 
formula
(6)
where = (RT)/, R is the universal gas constant (8.314 J mol/K) and T (K) is the absolute temperature. The constant describes the heat of adsorption and explains the equilibrium binding constant (L/min) corresponding to maximum binding energy. A graph of qe versus ln Ce is used to calculate the Temkin isotherm constants, and.

Dubinin–Radushkevich isotherm

The D–R isotherm model is used to estimate the nature of the adsorption process, whether it is physical or chemical (Dubinin & Radushkevich 1947; Dada et al. 2012). The D–R isotherm is more general than the Langmuir isotherm because this model is not based on the assumption of a homogeneous surface or constant adsorption potential, but the apparent energy of adsorption is calculated. The D–R model is represented using the following non-linear Equation (7): 
formula
(7)
where K is a constant related to adsorption energy (mol2/J2). ɛ, the Polanyi potential is calculated from the equation, RT ln(1+ (1/Ce)), where R (J/mol K) is the universal gas constant and T is the temperature. The mean free energy of adsorption (E) is calculated using Equation (8): 
formula
(8)

The E value gives information about the adsorption type; if the value of E < 8 kJ/mol the adsorption process is governed by physisorption, while for E > 16 kJ/mol chemisorption prevails.

Kinetic studies

In order to analyze the kinetics of the adsorption process, the equilibrium data were fitted to pseudo-first-order, pseudo-second-order and intra-particle diffusion models. To investigate the adsorption kinetic, experiments were performed at different initial concentrations of Cr (VI) at pH 2, temperature of 35 °C and dose of 2.5 g/L. The contact time was set to 240 min in each experiment to ensure each adsorption process could reach the equilibrium value.

Pseudo first and second order models

Lagergren proposed a model on pseudo-first-order kinetics (Kanjanarong et al. 2017). The linearized-integral form of the pseudo-first-order model is represented as Equation (9): 
formula
(9)
where qe and qt are the sorption capacities at equilibrium and at time t (mg/g), respectively, and k1 is the rate constant of pseudo-first-order sorption (min−1). The values of k1 and qe can be determined from the slope and intercept of the graph of ln (qe−qt) versus t.
The pseudo-second-order sorption kinetics (Kanjanarong et al. 2017) can be expressed as shown in Equation (10): 
formula
(10)
where qe and qt are the sorption capacity at equilibrium and at time t (mg/g), respectively, and k2 is the rate constant of the pseudo-second-order sorption (g/mg-min) which can be determined by plotting the graph of t/qt versus t.

Intra-particle diffusion model

The intra-particle diffusion model was proposed by Weber and Morris (Weber & Morris 1962) and describes information about mechanisms and rate controlling steps in the adsorption kinetics. This equation provides information about the rate-limiting step in the adsorption process. This may occur due to the diffusion of the adsorbate into the adsorbent layer. The model is represented using the following equation: 
formula
(11)
where kid is the intra-particle diffusion rate constant (mg/g min1/2), and the value of I (mg/g) represents the thickness of the boundary layer. The values of I and kid are obtained from the intercept and slope of the curve, respectively.

Thermodynamic study

In order to find out the nature of the adsorption process, the mechanism, spontaneity and heat change in adsorption of Cr (VI) on ASAC, it is important to determine relevant thermodynamic parameters. The enthalpy change, ΔH°, entropy change, ΔS° and Gibbs free energy changeΔG°, were determined using: 
formula
(12)
 
formula
(13)
where T is the absolute temperature (K), R is the gas constant (8.314 J/mol K) and K0 is the distribution coefficient calculated using the following equation: 
formula
(14)
Thermodynamic properties were calculated by plotting the graph of ln K0 versus 1/T.

Statistical analysis

Maximum experimental findings were analyzed by statistics software tools. These data were calculated and the values of average, standard deviation and relative standard errors (RSE) were found. For the other parts of the experiments, mostly single experiments were carried out in this work.

RESULTS AND DISCUSSION

Characterization of activated carbon

The pHpzc value of ASAC was found to be 5.5, which indicates that at this pH the net surface charge of the activated carbon is zero, whereas at pH <5.5, the adsorbent surface is positively charged and at pH >5.5, it is negatively charged.

FTIR spectra of the adsorbent before and after adsorption were recorded in the range of 400–4,000 cm−1. The following wave numbers appeared in ASAC: 650 cm−1 indicated the C–H vibration in aromatic compounds; the peak at 1,590 and 1,385 cm−1 corresponded to COO− and >C = O− groups, 1,820–1,900 cm−1 corresponded to the >C = O− stretch of saturated carboxylic acids and aliphatic esters (Droussi et al. 2009; Zhang et al. 2011; Chia et al. 2012; Singh et al. 2017; Talha et al. 2018). The peaks around 2,760.5–1,899.5 cm−1 might be assigned to aldehydes, carbonyls, carboxylic acids and esters present on the surface (Pedroza et al. 2014; Kanjanarong et al. 2017; Talha et al. 2018). These functional groups have an affinity toward chromium ions. FTIR spectra of Cr (VI) loaded ASAC show the shifted peak locations and low transmittance intensity at 3,428, 2,934, 1,634 and 1,246 cm−1 due to Cr (VI) adsorption (Figure 1).

Figure 1

FTIR spectra of the almond shell activated carbon (a) before adsorption and (b) after adsorption.

Figure 1

FTIR spectra of the almond shell activated carbon (a) before adsorption and (b) after adsorption.

Differences in SEM images were observed between surface topography of ASAC before and after adsorption. SEM photomicrographs before and after adsorption are shown in Figure 2. From Figure 2, it is clear that ASAC has a highly porous structure with greater homogeneity. The SEM photomicrograph after Cr (VI) adsorption shows that a layer is formed due to Cr (VI) adsorption on the surface of the activated carbon and some Cr (VI) has also filled the pores.

Figure 2

SEM micrographs of ASAC (a) before adsorption and (b) after adsorption of Cr (VI).

Figure 2

SEM micrographs of ASAC (a) before adsorption and (b) after adsorption of Cr (VI).

From Table 2, it is observed, as expected, that the percent carbon content in activated carbon (78.0%) is substantially higher than that of raw almond shell (44.4%). This is nearly comparable to other activated carbon reported for olive pits (64.4%) (Ugurlu & Karaoglu 2008), sugar beet bagasse (70–80%) (Onal et al. 2007) and hazelnut-husk (80.4%) (Karacetin et al. 2014). The Brunauer–Emmett–Teller (BET) surface area of the adsorbent was found to be 1,223.4 m2/g which indicates that ASAC may be a good adsorbent for adsorption of Cr (VI) and other pollutants from wastewater (Table 2). The other important physical properties like pore volume, average pore size and point of zero charge of prepared activated carbon were found to be 0.3266 cm3/g, 2.39 nm and 5.20 (Table 2), respectively, which may play an important role in the adsorption.

Effect of adsorbent dose and contact time

Adsorbent dose and contact time are important parameters for the adsorption, in the study of maximum Cr (VI) adsorption capacity. From Figure 3, it is observed that the removal of Cr (VI) is rapid in the early stages up to 100 minutes and becomes almost constant after 240 minutes. This may be due to the availability of more adsorption sites for the removal of Cr (VI) ions at the initial stage, and after some time the slow rate of Cr (VI) adsorption is due to hindrance or repulsion of absorbed Cr (VI) ions onto the adsorbent surface (Hsu et al. 2009). It is observed that the uptake of Cr (VI) increases as amount of the adsorbent dose decreases. From Figure 3, it is clear that the adsorption capacities decreased from 16 to 4 mg/g as the dosage was increased from 2.5 to 10 g/L. Figure 3 shows that increasing the adsorbent dose above the optimum value did not result in an increase in the Cr (VI) ion uptake. This can be explained by the overlapped and unsaturated sites of the adsorbent (Rai et al. 2016). The optimum bio-sorbent dosage of 2.5 g/L and contact time of 240 min were selected for further adsorption experiments. The value of SD and SD error were found to be 24.3 and 2.40 for removal efficiencies. These values agree with the reported values for various pollutants (Giri et al. 2012). Using statistical analysis the standard deviation for the Qe values were calculated and found to be 4.79, 3.16, 2.24 and 1.89 while relative standard error (RSE) values were 14.4, 14.4, 15.0 and 16.7 for the concentrations of 1, 0.75, 0.5 and 0.25 g/100 mL, respectively.

Figure 3

Effect of activated carbon dose and effect of time on metal uptake (initial con. 50 ppm, pH 6, agitation speed 150 rpm, pH 5.5, temperature 35 °C).

Figure 3

Effect of activated carbon dose and effect of time on metal uptake (initial con. 50 ppm, pH 6, agitation speed 150 rpm, pH 5.5, temperature 35 °C).

Effect of pH

Figure 4 exhibits the influence of pH (2–8) on the adsorption process. As seen from Figure 4, the uptake of Cr (VI) from aqueous solution is dependent on the pH value, and the maximum adsorption capacity is found to be 20 mg/g at pH 2.0. Increasing the pH value from 2.0 to 8.0 results in decreasing Cr (VI) adsorption capacity. This could be explained by the fact that Cr (VI) exists in ionic form and the existence of these ions, such as CrO42−, HCrO4 or Cr2O72−, depend on the pH of the aqueous solution (Suksabye & Thiravetyan 2012). Within the pH range of 2.0–6.0 HCrO4 and Cr2O72− ions are found in equilibrium, and after increasing the pH chromate ions (CrO42−) predominate. HCrO4 or Cr2O72− ions need only one active site, whereas a chromate ion (CrO42−) needs two active sites due to its two negative charges. Hence, an increase in the adsorption capacity of Cr (VI) is observed due to more HCrO4 ions at lower pH values which need a single site for adsorption (Baral et al. 2006). At higher pH the adsorption capacity is decreased due to competitive adsorption between chromate and hydroxyl ions. The surface of the adsorbent was positively charged at a pH lower than the pHpzc and negatively charged at pH values greater than pHpzc. Thus, the initial solution pH of 2.0 was selected as the optimum pH value for Cr (VI) adsorption for all the remaining experiments.

Figure 4

Effect of solution pH on metal uptake by almond shell activated carbon (initial concentration 50 mg/L, time 300 min, agitation speed 150 rpm, temperature 35 °C, dose 2.5 g/L).

Figure 4

Effect of solution pH on metal uptake by almond shell activated carbon (initial concentration 50 mg/L, time 300 min, agitation speed 150 rpm, temperature 35 °C, dose 2.5 g/L).

Effect of initial Cr (VI) concentration

The effect of initial concentration on Cr (VI) removal by ASAC was studied at different initial Cr (VI) concentrations (100–1,000 mg/L) at a contact time of 240 min, pH of 2 and optimal adsorbent dosage of 2.50 g/L. The adsorption capacity increased from 40.0 to 168 mg/g as initial Cr (VI) concentration increased from 100 to 1,000 mg/L (Figure 5). Statistical analyses of findings were carried out using basic statistical software. Standard deviation was found to be 26.2 and relative standard error was 10.6 for removal efficiencies. The increase in adsorption capacity of the adsorbents with an increase in initial Cr (VI) concentration is due to more interaction of Cr (VI) ions with the absorbent surface (Verma et al. 2006; Nemr 2009). The percentage removal of Cr (VI) decreased from 100 to 40.5% with an increase in Cr (VI) concentration from 100 to 1,000 mg/L. The decrease in percentage removal of Cr (VI) is due to less active sites being available for Cr (VI) ions because their sites saturate above a certain concentration. Therefore, Cr (VI) removal was dependent on initial Cr (VI) concentration.

Figure 5

Effect of initial concentration of Cr (VI) on metal uptake and % removal by almond shell activated carbon (pH 2; time 4 h; agitation speed 150 rpm; temperature 35 °C; dose 2.5 g/L).

Figure 5

Effect of initial concentration of Cr (VI) on metal uptake and % removal by almond shell activated carbon (pH 2; time 4 h; agitation speed 150 rpm; temperature 35 °C; dose 2.5 g/L).

Adsorption isotherm

The values of the isotherm parameter and the coefficient of determination (R2) of four equilibrium models at different temperatures are shown in Table 3. The maximum adsorption capacities (qm) determined by the Langmuir model were found to be 202.34, 197.34 and 195.34 mg/g at temperatures of 35, 30 and 25 °C, respectively. The coefficient of determination values (R2) were found to be high for the Langmuir model in comparison to the other three models which confirmed the monolayer adsorption of Cr (VI) on ASAC. The calculated RL values were in the range 0–1 which indicated favorable adsorption of Cr (VI). Freundlich parameters were determined from the linear plot of ln (qe) versus ln (Ce). The obtained n values were greater than 1.0, which indicated a favorable Cr (VI) adsorption process. R2 values for the Freundlich model were very low as compared to the Langmuir model, which suggests that equilibrium data do not follow the Freundlich isotherm model.

Table 3

Isotherm parameters for Cr (VI) adsorption on ASAC

Adsorption isotherms Parameters Temperature
 
25 °C 30 °C 35 °C 
Langmuir isotherm qm 195.34 197.34 202.34 
b 0.024 0.027 0.018 
R2 0.976 0.964 0.986 
RL 0.4545 0.4255 0.5263 
Freundlich isotherm Kf 79.35 80.74 84.34 
n 1.334 1.247 1.124 
R2 0.8945 0.835 0.876 
Tempkin  862.64 701.345 627.63 
 226.67 223.32 220.18 
R2 0.643 0.661 0.712 
Dubinin–Radushkevich  132.55 133.54 134.42 
K × 106 3.02 2.50 
E 0.4068 0.447 0.5 
R2 0.7532 0.7732 0.7854 
Adsorption isotherms Parameters Temperature
 
25 °C 30 °C 35 °C 
Langmuir isotherm qm 195.34 197.34 202.34 
b 0.024 0.027 0.018 
R2 0.976 0.964 0.986 
RL 0.4545 0.4255 0.5263 
Freundlich isotherm Kf 79.35 80.74 84.34 
n 1.334 1.247 1.124 
R2 0.8945 0.835 0.876 
Tempkin  862.64 701.345 627.63 
 226.67 223.32 220.18 
R2 0.643 0.661 0.712 
Dubinin–Radushkevich  132.55 133.54 134.42 
K × 106 3.02 2.50 
E 0.4068 0.447 0.5 
R2 0.7532 0.7732 0.7854 

The constants AT and bT, were obtained for the Temkin isotherm model and tabulated in Table 3. The R2 value for the Temkin isotherm is 0.712 which is less than that for the Langmuir isotherm model. The constants, qm and K obtained for the Dubinin–Radushkevich isotherm model were 134.42 mg/g and 2.0 × 10−6 mol2/J2, respectively at 35 °C. The maximum absorption capacity, qm, obtained using the D–R model was 134.42 mg/g which is less in comparison to the Langmuir model (202.34 mg/g). The mean free energy of adsorption (E) was obtained as 0.5 kJ/mol at 35 °C, i.e. lower than 8.0 kJ/mol. This indicates that the adsorption mechanism is governed by physical adsorption. The value of the coefficient of determination (R2 = 0.784) obtained for the D–R model is significantly lower than that of the Langmuir model (R2= 0.987).

Kinetic study

The calculated values of different kinetics parameters are presented in Table 4. The qe value can be calculated from the pseudo-first-order model (Figure 6) and pseudo-second-order model (Figure 7). The average qe values for the pseudo-first-order model were found to be 0.83, 1.57, 1.77 and 2.73 while standard deviation values were 0.78, 1.25, 1.15 and 0.90 for the concentration ranging from 100 to 250 ppm. R2 values were near to 1 for all concentrations (Figure 6) which is very near to linear and comparable with experimental results. All the data follow the statistical properties and relative standard errors were found to be 46.6, 32.5, 24.5 and 12.4 for the concentration range of 100–250 ppm.

Table 4

Kinetics parameters for adsorption of Cr (VI) on ASAC

Kinetic parameters models Initial concentration (mg/L)
 
100 150 200 250 
qe(exp) (mg/g) 45.12 65.103 84.73 108.072 
Pseudo-first-order model 
 k1 0.019 0.015 0.011 0.008 
 qe(cal) 9.89 32.29 40.23 56.82 
 R2 0.943 0.991 0.948 0.962 
Pseudo-second-order model 
 k2 0.1820 0.0546 0.0588 0.0322 
 qe(cal) 47.6 71.42 90.90 125.2 
 R2 0.999 0.999 0.999 0.998 
Intra-particle diffusion model 
 kid 0.668 2.281 3.045 3.460 
 I 36.53 39.65 43.17 48.94 
 R2 0.928 0.965 0.911 0.939 
Kinetic parameters models Initial concentration (mg/L)
 
100 150 200 250 
qe(exp) (mg/g) 45.12 65.103 84.73 108.072 
Pseudo-first-order model 
 k1 0.019 0.015 0.011 0.008 
 qe(cal) 9.89 32.29 40.23 56.82 
 R2 0.943 0.991 0.948 0.962 
Pseudo-second-order model 
 k2 0.1820 0.0546 0.0588 0.0322 
 qe(cal) 47.6 71.42 90.90 125.2 
 R2 0.999 0.999 0.999 0.998 
Intra-particle diffusion model 
 kid 0.668 2.281 3.045 3.460 
 I 36.53 39.65 43.17 48.94 
 R2 0.928 0.965 0.911 0.939 
Figure 6

Pseudo-first-order kinetic model of Cr (VI) removal by ASAC (solution pH 2; dose 2.5 g/100 ml; agitation speed 150 rpm; temperature 35 °C).

Figure 6

Pseudo-first-order kinetic model of Cr (VI) removal by ASAC (solution pH 2; dose 2.5 g/100 ml; agitation speed 150 rpm; temperature 35 °C).

Figure 7

Pseudo-second-order kinetic model for Cr (VI) removal by ASAC (solution pH 2; dose 2.5 g/L; agitation speed 150 rpm; temperature 35 °C).

Figure 7

Pseudo-second-order kinetic model for Cr (VI) removal by ASAC (solution pH 2; dose 2.5 g/L; agitation speed 150 rpm; temperature 35 °C).

It has been found that the qe values calculated from the pseudo-second-order model are close to experimental values of qe for all Cr (VI) concentrations. The coefficient of correlation (R2) was also compared for all three kinetic models and it was found that the R2 value was more (near to unity) for the pseudo-second-order model. It is seen that the intra-particle diffusion model parameters increase with increasing initial Cr (VI) concentration. The increase in the values of I corresponds to the increase in boundary layer thickness which gives more importance to the surface adsorption in the rate-limiting step. Since the value of I is not zero, this indicates the involvement of some other mechanisms in the adsorption process along with the intra-particle diffusion. The average t/qt values for the second-order kinetic model were calculated as 1.75, 1.99, 1.84 and 1.54 for the concentration range of 100–250 ppm. Statistical analyses of data were carried out for the standard deviation and relative standard error to authenticate the findings and the values of RSE were found to be 24.1, 23.2, 23.0 and 21.5 corresponding to the concentrations of 100, 150, 200, and 250 ppm, respectively. These findings prove the results are statistically correct.

Regeneration study of ASAC

Disposal of Cr (VI)-loaded activated carbon is not safe because of its environmental constraints. Hence, before disposal, it must be regenerated and the activated carbon reused. The adsorption Cr (VI) on ASAC is a physico-sorption process. In the present study, the adsorption of Cr (VI) is highly dependent on pH value and active in the lower pH range. Hence, the regeneration of Cr (VI) is achieved by increasing the solution pH through different concentrations of NaOH (1.0, 2.0 and 2.5 M). The desorption percentages for the Cr (VI) saturated adsorbents are 25.35, 54.35 and 63.45% for ASAC at the three different concentrations of NaOH. The regeneration study results indicated that a higher concentration of NaOH is effective for increasing the desorption efficiency of ASAC.

Thermodynamic study

The thermodynamic parameters were obtained for initial Cr (VI) concentration of 270 mg/L at pH 2. The negative value of Gibbs free energy change indicates the feasibility and spontaneity of the adsorption process on ASAC. The enthalpy change is positive and confirms that the adsorption on ASAC was endothermic in nature. The positive value of ΔS shows that the adsorption process is governed by entropy rather than enthalpy. The values of ΔH° and ΔS° were found to be 22.9 KJ/mol and 95.3 J/mol-K. Rangabhashiyam & Selvaraju (2015) reported the negative ΔG° values showed the thermodynamically feasible and spontaneous nature of the bio-sorption process for an initial Cr (VI) concentration of 100 and 200 mg/L, but the free energy change value was found to be positive at the high Cr (VI) concentrations of 300, 400 and 500 mg/L signifying the non-spontaneous nature.

Practical application of this work

The remediation of Cr (VI) is a challenging problem due to its high level of toxicity even at very low concentration. Presence of more than the permissible level of chromium in water and soil poses severe health issues in many parts of the world. There are various techniques available for removal of chromium, each with their advantages and limitations. Cost effectiveness and affordability of the available techniques are the most important aspects. In the present study an agro-waste is utilized for conversion into biochar/activated carbon which is then used for removal of chromium with good removal efficiency. The investigated material is quite attractive because of its low cost. It will also help to reduce solid agro-waste. Another advantage of biochar is that it improves the fertility of soil. Therefore, it can be effectively used to remove the chromium toxicity of the soil along with improving its fertility.

Future research prospects

The development process can be extended to a pilot-plant-scale investigation for the wastewater treatment system for generation of design parameters for scale-up. Further, the developed process can be employed at commercial scale in existing treatment systems with some retrofitting to the existing operation units. A few important and useful outcomes include the removal of Cr (VI) toxicity to living beings, and also the treated biochar/activated carbon may be useful in agriculture as a bio-fertilizer to enhance the soil quality by increasing the fertility. Further research in the following direction is required to establish the potential of the biochar/activated carbon developed in the present work for real-time applications: (a) the possibility of regeneration/reuse of the biochar/activated carbon should be explored; (b) the work should be planned to use the material for reducing chromium toxicity of soil; and (c) the work should be planned using the biochar/activated carbon along with the microbial population able to convert chromium into non-toxic forms.

CONCLUSIONS

In the present study, ASAC was prepared and used as a base material for the removal of Cr (VI). Characterization results show a very high BET surface area of 1,223.4 m2/g, presence of favorable functional groups for adsorption, and a highly porous structure with greater homogeneity which makes the prepared material a perfect adsorbent. The characterization results were also validated with very high adsorption capacity of 202.34 mg/g under optimum operating conditions which shows the potential of the material for commercial application. The kinetic study of the adsorption process showed that the process followed pseudo-second-order kinetics. The thermodynamic study revealed the endothermic nature of the process.

ACKNOWLEDGEMENTS

The authors would like to gratefully thank the DIH – Project Varanasi, IIT (BHU) for the financial support towards successful completion of this work.

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