Abstract
The use of activated carbon (AC) from lignocellulosic wastes has gained a lot of research interest because of its great economic and environmental value. In this work, AC was prepared from mahogany fruit husk (MFH) via chemical activation with phosphoric acid and heat treatment. The relationships among the activation parameters H3PO4%, heating temperature, and holding time, and their effect on chromium (VI) removal, were investigated using the response surface method (RSM), following a central composite design (CCD). The optimized activation conditions resulted in a 92.3% Cr6+ removal efficiency for a 50 mg/L Cr6+ aqueous solution. The surface properties of the optimized MFHAC were investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared (FTIR) spectroscopy, and nitrogen adsorption/desorption studies. The MFHAC prepared under the optimized conditions had a high surface area (SBET) of 1130 m2/g, with a well-developed porous structure. The equilibrium data of Cr6+ adsorption onto the MFHAC was best fit by the Langmuir isotherm model, while the adsorption kinetic data followed the pseudo-second order kinetic model. Hence, MFHAC proved to be an efficient technology for removing Cr6+ from simulated wastewater.
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Optimization and Response Surface Modelling of Activated Carbon Production from Mahogany Fruit Husk for Removal of Chromium (VI) from Aqueous Solution
Bryan John A. Magoling, and Angelica A. Macalalad *
The use of activated carbon (AC) from lignocellulosic wastes has gained a lot of research interest because of its great economic and environmental value. In this work, AC was prepared from mahogany fruit husk (MFH) via chemical activation with phosphoric acid and heat treatment. The relationships among the activation parameters H3PO4%, heating temperature, and holding time, and their effect on chromium (VI) removal, were investigated using the response surface method (RSM), following a central composite design (CCD). The optimized activation conditions resulted in a 92.3% Cr6+ removal efficiency for a 50 mg/L Cr6+ aqueous solution. The surface properties of the optimized MFHAC were investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared (FTIR) spectroscopy, and nitrogen adsorption/desorption studies. The MFHAC prepared under the optimized conditions had a high surface area (SBET) of 1130 m2/g, with a well-developed porous structure. The equilibrium data of Cr6+ adsorption onto the MFHAC was best fit by the Langmuir isotherm model, while the adsorption kinetic data followed the pseudo-second order kinetic model. Hence, MFHAC proved to be an efficient technology for removing Cr6+ from simulated wastewater.
Keywords: Activated carbon; Chromium (VI); Mahogany fruit husk; Response surface method
Contact information: Department of Chemistry, College of Arts and Sciences, Batangas State University, Rizal Avenue, Batangas City, Philippines; *Corresponding author: aamacalalad@batstate-u.edu.ph
INTRODUCTION
Industrialization remains one of the most prevalent sources of pollutants in the natural environment. Of these pollutants, heavy metals are considered to be the most toxic because of their inability to be immediately degraded in nature (Al-Othman et al. 2011) and their detrimental effect on many living organisms, including humans. Most industrial wastewaters released into the environment consist of undesirable amounts of arsenic, cadmium, chromium, lead, mercury, and nickel. Among these heavy metals, chromium imposes a greater threat to human health because of its solubility in water, which leads to possible contamination of surface and groundwater (Acharya et al. 2009). Chromium occurs in the environment in trivalent and hexavalent oxidation states. Chromium (III) is an essential trace element in humans that assists in carbohydrate and fat metabolism, and in maintaining blood glucose levels through regulation of insulin in the body (Setshedi et al. 2013), while chromium (VI) (Cr6+) is a known carcinogen and mutagen (Brauer and Wetterhahn 1991). Chromium (VI) also causes dermatitis, ulcers, hemorrhages, pulmonary congestion, and liver damage (Gładysz-Płaska et al. 2012; Kushwaha et al. 2012).
Process industries that typically discharge chromium (VI) ions in their effluents include leather tanning, electroplating, textile and dye manufacturing, metal plating, and battery production (Jayakumar et al. 2014). Thus, there is a great need for the efficient removal of Cr6+ from wastewater before it is discharged into the environment. The maximum allowable limit of Cr6+ in surface water set by the U.S. Environmental Protection Agency is 0.05 mg/L (Baral and Engelken 2002), and the maximum permissible level of Cr6+ in drinking water is 0.05 mg/L (World Health Organization 2011).
Several treatment processes have been applied for the removal of chromium (VI) from wastewaters, such as precipitation, ultrafiltration, ion exchange, and electro-deposition. However, these methods include disadvantages, like low selectivity, limited removal efficiency, high-energy requirements, and consumption of large amounts of chemicals (Shuhong et al. 2014). Compared with the conventional technologies mentioned, adsorption seems to be one of the most efficient methods for the removal of different heavy metal ions from industrial effluents because of its wide range of applications and simple operation (Thilagavathy and Santhi 2014).
Adsorption processes that utilize commercially available activated carbon (AC) are generally expensive because of the high production cost of AC. For this reason, a lot of research effort is being devoted to finding new precursors from inexpensive sources, like agricultural waste, which will yield AC that has a superior adsorbing capacity compared to commercially available AC (Krishnan et al. 2011; Saka 2012). Various modifications on the surface of lignocellulosic-based AC are also being explored to further improve their pore structure and properties.
The chemical activation of biomass involves the impregnation of the precursor with a chemical, followed by a heating treatment. Compared with other chemical activating agents such as sulfuric acid, potassium hydroxide, and zinc chloride, the use of phosphoric acid has been reported to promote the development of a mesoporous surface on lignocellulosic precursors. A mesoporous surface is desirable for the adsorption of pollutants such as heavy metals (Li et al. 2010). It has also been reported to activate the lignocellulosic precursor at a lower activation temperature (about 450 °C) than potassium hydroxide (above 700 °C), with less toxic residues compared to zinc chloride (Sun et al. 2016).
The utilization of response surface method (RSM) in optimizing independent variables is particularly advantageous when the parameters being studied, including their levels and responses, are not yet fully understood. A standard RSM design of experiment called a central composite design (CCD) is preferred for generating a quadratic response surface. A CCD also helps to analyze the interaction between the factors and to determine the optimum parameters, with a minimal number of experiments (Alslaibi et al. 2013).
This study focused on the preparation, optimization, and characterization of phosphoric acid activated mahogany fruit husk (MFH) using RSM following a CCD. Batch adsorption experiments were performed to investigate the effect of activation parameters, such as the percent concentration of phosphoric acid (H3PO4%), heating temperature, and holding time, on the removal of Cr6+ from an aqueous solution. The adsorption equilibrium data was analyzed using the Langmuir and Freundlich isotherm models, while the adsorption kinetic studies were performed and fitted using pseudo-first and pseudo-second-order kinetic models.
EXPERIMENTAL
Preparation of Aqueous Solution
A stock solution of hexavalent chromium with a concentration of 1000 mg/L was prepared by dissolving a sufficient amount of potassium dichromate (K2Cr2O7) in distilled water. The standards and solutions for the adsorption experiments were obtained by diluting the stock solution to the desired concentrations. For the pH adjustments, 0.1 M HCl and 0.1 M NaOH were used. All chemicals utilized in this work were of analytical reagent (AR) grade.
Design of Experiment
The independent variables utilized in this study were H3PO4% (A), heating temperature (B), and holding time (C). Each independent variable was varied over three levels between -1 and +1 at specific ranges. Table 1 shows the variables and their corresponding levels.
Table 1. Independent Variables and their Coded Levels for the Central Composite Design (CCD)
For the central composite design (CCD) with three factors, the number of tests required included the standard 2k points with the origin located at the center, 2k points fixed axially at a distance, and replicate tests at the center, where k is the number of factors. The axial points were assigned so that they allowed for readability, which was to ensure that the variance of the model prediction was constant at all points equidistant from the design center (Sahu et al. 2009). Hence, the total number of experiments for this study was 20 (from 2k + 2k + 8), where the number of factors (k) was 3. The performance of the process was evaluated by analyzing the percent hexavalent chromium removal efficiency of the derived AC.
The optimization and response surface modelling of the chromium (VI) removal efficiency of the phosphoric acid activated carbon from MFH were done using the Design-Expert® (Stat-Ease Inc., Minneapolis, MN, USA) software. An analysis of variance (ANOVA) was carried out to test for the validity of the generated model. The goodness-of-fit of the obtained responses with the model was established by using the correlation coefficient (R2). The model’s Fisher variation ratio (F-value), probability value (Prob. > F), and adequate precision (AP) were also evaluated to test for its significance and adequacy.
Preparation of Activated Carbon
The MFH was locally obtained from Batangas City, Philippines. It was rinsed three times with distilled water to remove soluble impurities and then dried in an oven at 105 °C for 24 h. The dried samples were ground and sieved to a particle size of approximately 800 µm. The carbonization was carried out for 1 h by loading a dried precursor into a stainless steel vertical tube reactor under purified nitrogen flow. The activation process of the MFH was performed according to the run order listed in Table 2 for the CCD. The char was chemically activated using H3PO4 as the impregnating agent. The carbonized MFHs were soaked in varying percent concentrations of phosphoric acid (30%, 40%, and 50% v/v) with an impregnation ratio of 1:2 (w/w of char and H3PO4 solution) for 2 h at 80 °C. Afterwards, the samples were filtered and oven dried for 24 h at 105 °C. The thermal treatment of the impregnated chars was carried out using a muffle furnace at different temperature levels (400, 600, and 800 °C) and holding times (30, 60, and 90 min). The derived AC was cooled to room temperature, and washed with hot deionized water and 0.1 M HCl until the filtrate reached a pH of 6 to 7. The MFHAC samples were oven dried for 24 h at 105 °C and stored in air-tight containers.
Characterization of Optimized MFHAC
A volumetric adsorption analyzer (Quantachrome, Boynton Beach, FL, USA) was used to study the surface properties of the optimized MFHAC. The surface area, total pore volume, and average pore diameter of the optimized MFHAC prepared under optimum conditions were obtained through the analysis of the N2 adsorption isotherms at -196 °C. The BET surface area was measured from the adsorption isotherm using the Brunauer–Emmett–Teller (BET) equation. The total pore volume was estimated to be the liquid volume of nitrogen at a relative pressure of 0.99. The surface morphology of the optimized MFHAC and the detection of the adsorbed chromium onto the MFHAC were investigated using a JSM-5310 (JEOL Ltd., Tokyo, Japan) scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (EDS). The surface functional groups of the AC were determined using a Nicolet 6700 (Thermo Nicolet Co., Madison, WI, USA) Fourier-transform infrared (FTIR) spectrophotometer.
Batch Adsorption Studies
The batch adsorption process was performed for the hexavalent chromium adsorption onto the derived AC. Following the CCD of the experiment, 20 adsorption tests were conducted. Each flask contained 50 mL of 50 mg/L Cr6+ aqueous solution. The prepared AC samples (0.10 g) were added to the individual flasks, which were then agitated with an isothermal shaker set at 30 °C and 200 rpm until the equilibrium time of 1 h was reached. After agitation, the samples were filtered immediately using Whatman #1 filter paper (Sigma-Adrich Pte. Ltd, Nucleos, Singapore) to separate the adsorbent from the solution. During filtration, the samples were divided into two parts. The first part was used to saturate the filter paper to avoid the effects of adsorption onto the filter paper and the filtrate was disregarded. The residual Cr6+ concentrations in the filtrate from the second portions were measured using a GENESYS 5 (Spectronic Co., Waltman, MA, USA) double beam UV-Visible spectrophotometer at 540 nm upon compleximetric reaction with 1,5-diphenylcarbazide at a pH of 2.0. The adsorbed Cr6+ concentrations were obtained from the difference between the initial and final concentrations in the solution, while the percentage removal at equilibrium was calculated based on the following equation,
(1)
where Co and Ce are the liquid-phase concentrations of the initial state and at equilibrium (mg/L), respectively. The amount of Cr6+ adsorbed per unit mass of the derived AC at equilibrium time, qe (mg/g), was calculated using Eq. 2,
(2)
where qe (mg/g) is the amount of Cr6+ adsorbed per unit weight of adsorbent, Co and Ce (mg/L) are the liquid-phase concentrations of adsorbate at the initial and equilibrium conditions, respectively, V (L) is the volume of the solution, and m (g) is the mass of the derived AC used.
Adsorption Isotherms
In this study, the Langmuir and Freundlich models were employed to describe the adsorption mechanism of Cr6+ onto the surface of the optimized MFHAC.
Langmuir model
The Langmuir isotherm model is based on the assumption of monolayer adsorption onto a surface with the equivalent and identical number of localized sites (Langmuir 1916). The maximum adsorption is achieved when the surface is covered by a monolayer of the adsorbate. The linear form of the Langmuir isotherm is expressed by the following equation,
(3)
where Ce (mg/L) is the equilibrium liquid-phase concentration of Cr6+, qe (mg/g) is the equilibrium uptake capacity, Q (mg/g) is the Langmuir constant related to the adsorption capacity, and b (L/mg) is the Langmuir constant related to the sorption energy, which quantitatively describes the interaction between the adsorbent and the adsorbate.
Freundlich model
The Freundlich model is valid for multilayer adsorption onto a heterogeneous surface with a varying distribution of active sites (Freundlich 1906). This isotherm model does not describe any adsorbate saturation. Instead, infinite surface coverage is mathematically predicted. The linear form of the Freundlich isotherm is expressed by the following equation,
(4)
where qe (mg/g) is the amount of Cr6+ adsorbed at equilibrium, Ce (mg/L) is the adsorbate concentration, Kf (mg/g)(L/mg)1/n is the Freundlich constant related to the adsorption capacity, and 1/n is the Freundlich constant related to sorption intensity of the sorbent.
Adsorption Kinetics
The adsorption kinetics studies were done in order to describe the rate of Cr6+ uptake onto the optimized MFHAC, and provided major insight into the adsorption mechanism and the possible rate-controlling processes, mass transfer, and chemical reaction. The controlling mechanism of the adsorption process was investigated by fitting the experimental data with pseudo-first-order and pseudo-second-order kinetic models.
Pseudo-first-order model
The adsorption of liquid-solid systems based on the solid capacity follows a pseudo-first-order model (Lagergren 1898). Generally, the pseudo-first-order kinetic model is only applicable for the initial stage of the adsorption process. The linear form of the pseudo-first-order rate equation is illustrated by the following equation,
(5)
where k1 (1/min) is the pseudo-first-order adsorption rate constant, and qe and qt are the amount of adsorbed Cr6+ in mg/g at the equilibrium time and at time t (min), respectively.
Pseudo-second-order model
The pseudo-second-order model predicts the adsorption mechanism over a range of different points in time during the adsorption process. The pseudo-second-order model equation is given as,
(6)
where k2 (g/mg·min) is the equilibrium rate constant of the pseudo-second-order model.
RESULTS AND DISCUSSION
A total of 20 experiments were conducted to develop a response surface model (RSM) for the Cr6+ removal efficiency of the AC prepared from MFH at varying H3PO4%, heating temperatures, and holding times. The experimental factors used in this study and their responses are listed in Table 2. The results from the experiment revealed Cr6+ removal efficiencies that varied from 27.1% to 91.8%.
Table 2. Experimental Factors and Responses
Statistical Analysis
An analysis of variance (ANOVA) was performed to evaluate the acceptability of the model. The results of the second-order response surface model fitting for Cr6+ removal are given in Table 3. The quality of the model developed was evaluated based on the correlation coefficient (R2) and standard deviation. The model was significant at the 5% confidence level.
A model is considered to demonstrate a good fit if the coefficient of determination reaches a value of 0.80 and above (Bashir et al. 2010). The closer R2 is to 1, the more accurate and reliable the response will be predicted by the model. A high R2 value of 0.9954 was obtained in the present study, which indicated that only 0.46% of the total variation was unexplained by the generated model.
The ANOVA results for the quadratic response surface model for Cr6+ removal had an F-value of 370.9363 and a corresponding probability greater than F that was less than 0.05, which indicated that the model was significant. For the model terms, probability factor (Prob. > F) values less than 0.05 are considered significant. In this work, the model terms A, B, C, AC, BC, A2, and C2 were significant, while AB and B2 were insignificant terms. To improve the model’s efficiency, the insignificant model terms were excluded from the study.
Table 3. Analysis of Variance of the Quadratic Model for the Chromium (VI) Removal Efficiency of the Derived MFHAC
The adequate precision (AP) measures the ratio between the signal and noise, and determines whether the predicted model can be used to move along the design space (Bashir et al. 2010). AP values higher than 4 are desirable. A high AP ratio of 51.3 was obtained, and suggested that the model can navigate in the space defined by the CCD utilized in this work. The obtained coefficient of variance (CV) value of 2.47% was below 10%, which meant that the model for Cr6+ removal gave reproducible results. Based on the statistical data obtained, the model presented in this work was efficient and able to predict the Cr6+ removal within the established set of parameters. The final regression model with respect to the variables used is shown by the following equation,
(7)
where A is the H3PO4% (30% to 50% v/v), B is the heating temperature (400 °C to 800 °C), and C is the holding time (30 min to 90 min).
The activation process of MFH using phosphoric acid and heat treatment in a furnace was optimized with the Design-Expert software. The simultaneous evaluation of different combinations of the factors used (H3PO4%, heating temperature, and holding time) at various levels and their responses was performed to determine the optimum activation conditions for MFH that will yield the highest Cr6+ removal efficiency. The optimum conditions and the corresponding predicted and actual Cr6+ removal efficiencies are presented in Table 4. The experiment was performed in triplicate to validate the Cr6+ removal efficiency of the optimized MFHAC. As shown in Table 4, the obtained experimental 92.29% Cr6+ removal from a 50 mg/L Cr6+ solution was in close agreement with the predicted removal efficiency of 94.3022%. The relative percent error between the predicted and obtained experimental values was 2.13%, which validated the reliability of the generated predictive model and the goodness-of-fit with the experimental results.
Table 4. Experimental Confirmation of the Predicted Cr6+ Removal Efficiency of the MFHAC Prepared Under the Optimum Conditions
To further illustrate the effect of the relationships among the variables of H3PO4%, heating temperature, and holding time on the responses predicted by the model, three dimensional (3D) surface response plots were generated. In the plots shown in Fig. 1, one variable was kept constant at the optimum value, while the two remaining parameters were varied within their experimental ranges. Figure 1a shows the 3D response surface of the combined effects of the H3PO4% and heating temperature, while the holding time was kept at the optimum value of 70.58 min. The maximum Cr6+ removal was observed when the char was chemically impregnated with about 40% H3PO4, followed by a heat treatment at around 430 °C. This suggested that pore development was improved when relatively high amounts of H3PO4 was introduced into the char followed by a thermal treatment at temperatures near 430 °C, which led to the formation of improved active sites. Beyond the optimum heating temperature, the pores of the AC may have started to break down, which explained the decreased removal efficiency.
Figure 1b illustrates the 3D response plot of the combined effects of the holding time and H3PO4% at a constant heating temperature of 428.72 °C. The maximum Cr6+ removal efficiency was observed when both parameters were at approximately the middle values of their respective ranges. The H3PO4 promoted the development of pores by initially occupying cavities in the char. When thermally treated, the H3PO4 molecules swelled and eventually volatilized. The porous structure was produced from the voids left by the H3PO4 molecules (Girgis and El-Hendawy 2002). At the optimum holding time of 70.58 min, most of the H3PO4 would have already volatilized, which left only the developed pores. The continued heating beyond the optimum holding time may have caused the newly developed pores to break, and resulted in a reduced Cr6+ removal efficacy.
Figure 1c shows the 3D response surface of the combined effects of the heating temperature and holding time, while the H3PO4% was held constant at 39.55%. The contour plot demonstrated an improvement in the Cr6+ removal at an increased holding time of up to 70 min. However, the increased Cr6+ removal was seen only up to a heating temperature of about 430 °C. This may have been explained by the fact that as the heating temperature increased above the optimum level, the active sites started to disintegrate, which resulted in a decrease in the Cr6+ removal. Holding times above 70 min may have caused the pores to rupture, which explained the reduction in the Cr6+ adsorption.
Fig. 1. 3D response plots of the Cr6+ removal efficiencies of the optimized MFHAC with respect to the effects of (a) heating temperature and H3PO4%, (b) holding time and H3PO4%, and (c) heating temperature and holding time
Surface Characteristics of the Optimized MFHAC
Figure 2 shows the SEM images of the optimized MFHAC before and after Cr6+ adsorption. It can be seen in Fig. 2a that the MFHAC derived under the optimum conditions developed a porosity that consisted of macropores. The presence of the deep macropores and micropores, along with varied cavities on the surface of the MFHAC suggested a well-developed pore structure. The developed pore structures provided a higher probability for Cr6+ entrapment and adsorption onto the surface of the MFHAC.
Fig. 2. SEM images of (a) MFHAC before adsorption (1000x), and Cr6+ loaded MFHAC at (b) 1000x and (c) 3500x magnification
Figures 2b and c show the SEM morphology of MFHAC loaded with Cr6+. As can be seen in Fig. 2c, a new layer was formed on the surface of the MFHAC.
The EDX spectra of the optimized MFHAC, before and after Cr6+ adsorption, are illustrated in Figure 3. Based on the spectrum shown in Figure 3a, the derived MFHAC consisted mainly of C, N, O, and P, along with other elements such as Na, S, and K. The new peaks at around 0.45 and 5.40 keV were detected in the surface of the Cr6+ loaded MFHAC (Fig. 3b). This confirmed the adsorption of Cr on the surface of the optimized MFHAC. A decrease in the intensity of the P peak was also seen in spectrum of the Cr6+ loaded MFHAC (Fig. 3b). The same results were observed in the works of Cheng et al. (2016) and Maneechakr and Karnjanakom (2017). 
Fig. 3. EDX spectra of (a) MFHAC before adsorption and (b) Cr6+ loaded MFHAC
To further understand the mechanisms on how the Cr6+ ions get adsorbed on the surface of the MFHAC, FTIR studies were employed on the optimized MFHAC, before and after Cr6+ adsorption. The FTIR spectra for the optimized MFHAC and the Cr6+ loaded MFHAC are presented in Fig. 4. The intensity at around 3414 cm-1 indicated the presence of H from hydroxyl groups (Cheng et al. 2006). There were also narrow bands present at around 2918 and 2849 cm-1 that related to C–H of alkyl structures (Günzler and Bock 1990; Zhang et al. 2010), which were due to methyl (CH3) and methylene (CH2) asymmetric stretching. The absorption energy near 2284 cm-1 is characteristic of the possible presence of nitriles or isocyanates. An absorbance band was also visible at 1702 cm-1 for C=O stretching (Chiang et al. 2000). The intensity at 1615 cm-1 indicates the presence of aromatic and olefinic C=C and C=O of bonded conjugated ketones, aldehydes, quinines, and aromatic groups (Wang et al. 2009). The peak observed at 1511 cm-1 indicated the presence of N-H stretching from amines and amide groups, and nitro compounds (Simha et al. 2016). The band at 1081 cm-1 in the MFHAC suggests the presence of stretching vibration of C–O functional groups, including alcohols, ethers, acids, and esters (Sharma et al. 2004). The presence of hydroxyl groups, carbonyl groups, and ethers is evidence that the structure of the MFHAC contained lignocellulose. The functional groups mentioned above have been reported to exhibit a high affinity toward Cr6+ ions (Rai et al. 2016). This was confirmed in the FTIR spectrum of MFHAC loaded with Cr6+ illustrated in Fig. 4. The observed decrease in transmittance intensity and shifts in peak locations at different frequencies suggested the interactions between the Cr6+ ions and the functional groups present in the surface of the MFHAC. Similar observations were also seen in the studies of Cheng et al. (2016) and Rai et al. (2016).
Fig. 4. FTIR spectra of (a) MFHAC before adsorption and (b) Cr6+ loaded MFHAC
The MFHAC prepared under the optimum conditions had a BET surface area (SBET) of 1128 m2/g with a total pore volume (VT) of 0.832 cm3/g and a mean pore diameter (D) of 3.41 nm. The developed porous carbon structure of the MFHAC may have been attributed to the employment of phosphoric acid as the activating agent that accompanied heating at around 430 °C. Chemical impregnating agents, like H3PO4, work by dehydrating the char. This results in the lowering of the activation temperature necessary during chemical activation, and thereby, further promoting a well-developed porous structure in the resulting AC (Girgis and El-Hendawy 2002).
Adsorption Isotherms
The equilibrium isotherm parameters and correlation coefficients obtained from linear fitting with the Langmuir and Freundlich isotherm models are shown in Table 6.
Table 6. Langmuir and Freundlich Isotherm Parameters for the Adsorption of Chromium (VI) onto the MFHAC
The equilibrium data from the adsorption isotherm studies of the Cr6+ ions onto the MFHAC were fitted and linearized using the Langmuir and Freundlich isotherm models. The linear form of the Langmuir model (Fig. 5a) was obtained by plotting Ce against Ce/qe, while the linear form of the Freundlich isotherm (Fig. 5b) was achieved from the plot of ln qe against ln Ce. The determined correlation coefficient (R2) values from the line fitting of the equilibrium data using the Langmuir and Freundlich isotherm models were 0.9935 and 0.9785, respectively. The lower R2 of 0.9785 of the linear Freundlich isotherm plot suggested less precise fit between the Freundlich model and the equilibrium data. In contrast, the high R2 of the linear Langmuir isotherm plot revealed good agreement between the Langmuir parameters and the experimental results of the equilibrium studies. This revealed that the adsorption of Cr6+ occurred via monolayer adsorption of the Cr6+ ions onto the internal and external surface of the MFHAC. The favorability of the equilibrium data with the Langmuir model was further supported by the computed separation factor (RL) value of 0.4346. An RL that is between 0 and 1 corresponds to an adsorption that is favorable toward the Langmuir model.
(a)
(d)
(b)
(c)
Fig 5. (a) Langmuir and (b) Freundlich isotherm plots for the adsorption of Cr6+ onto MFHAC; (c) Pseudo-first-order and (d) Pseudo-second-order kinetic model plots of Cr6+ onto MFHAC for different initial concentrations
Adsorption Kinetics
Table 7 presents the kinetic parameters for the pseudo-first-order and pseudo-second-order kinetic models for the adsorption of Cr6+ onto the MFHAC prepared under optimum conditions.
Table 7. Kinetic Parameters for the Adsorption of Chromium (VI) onto the MFHAC at Different Initial Concentration
The maximum adsorption capacity (qe) of 46.71 mg/g was experimentally determined at a constant temperature of 30 °C. As shown in Table 7, low correlation coefficient (R2) values were obtained for the pseudo-first-order model at different Cr6+ initial concentrations, which indicated there was poor agreement of the kinetics data with the pseudo-first-order model of the adsorption of Cr6+ onto the adsorbent. In contrast, high R2 values (Table 7) were obtained for the linear plots of (t/qt) versus t for the pseudo-second-order equation for different Cr6+ initial concentrations. The high R2 values obtained suggested that the adsorption of Cr6+ onto the surface of MFHAC followed pseudo-second-order kinetics, suggesting that the adsorption of the Cr6+ onto the MFHAC may have taken place via diffusion of the Cr6+ ions across the external boundary layer and in the internal micropores of the AC, which was consistent with the findings reported in literature (Hubbe et al. 2012a,b). It was also observed that there was good agreement between the experimental and calculated maximum adsorption capacity (qe) values, which further confirmed the goodness-of-fit of the kinetics data with the model. The obtained maximum Cr6+ adsorption capacity (46.7 mg/g) of the optimized MFHAC was comparable to the reported adsorption capacities of the other adsorbents listed in Table 8. The considerably high Cr6+ adsorption capacity obtained in this work indicated the efficacy of the MFHAC for removing Cr6+ in aqueous solution.
Table 8. Comparison of Cr (VI) Adsorption Capacities of Different Adsorbents
CONCLUSIONS
- The activation of MFH was successfully optimized using RSM through chemical impregnation with 39.6% H3PO4 (v/v), followed by a heat treatment at 428.7 °C for 70.6 min. The optimized MFHAC exhibited a high removal efficiency of 92.3% from a 50 mg/L Cr6+ aqueous solution at 30 °C.
- A quadratic response surface model equation that can be used for the prediction of Cr6+ removal efficiencies of the MFHAC prepared at varying H3PO4%, heating temperatures, and holding times was successfully developed.
- The MFHAC prepared under the optimum conditions had a well-developed porous surface structure with a high surface area (SBET) of 1128 m2/g. It also contained functional groups that exhibited interactions with the Cr6+ ions during adsorption.
- The adsorption equilibrium data demonstrated the best fit with the Langmuir isotherm model, which indicated a monolayer adsorption process. The adsorption kinetic data for different Cr6+ initial concentrations were best explained by the pseudo-second-order kinetic model. The obtained maximum Cr6+ adsorption capacity (46.7 mg/g) of the optimized MFHAC was comparable with the adsorption capacities of other reported adsorbents.
ACKNOWLEDGMENTS
The authors are grateful to Batangas State University, Philippines for providing the facilities to conduct this study, and to Ms. Nora L. Talain of the Bureau of Soils – Region IV, Philippines, and Dr. Christina A. Binag of RCNAS, University of Santo Tomas, Philippines for their technical support.
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Article submitted: November 4, 2016; Peer review completed: January 20, 2017; Revised version received: February 23, 2017; Accepted: February 24, 2017; Published: March 2, 2017.
DOI: 10.15376/biores.12.2.3001-3016