Abstract
Rice husk was chemically modified for the preparation of activated carbon. Rice husk was treated with nitric acid and carbonized at 700 C. After carbonization, the resulting rice husk char was treated with NaOH at room temperature. The 5 M NaOH-treated rice husk had the highest surface area (750 m2/g). Proximate analysis of activated carbon confirmed that NaOH treatment removed silica completely. Temperature programmed decomposition (TPD) graphs showed that the total gas contents (CO and CO2) liberated by CRH and H2O-treated CRH and CRH5M were 2l5 μmol/g, 390 μmol/g, and 970 μmol/g, respectively. The adsorption studies of the activated carbon during Cr(VI) removal from the aqueous medium indicated that CRH5M showed the highest rate of adsorption. The effect of adsorbent dosage, Cr(VI) concentration, pH, and temperature were studied to determine the best removal efficiency. With a decrease in pH from 4.4 to 2, the adsorption capacity increased from 3 mg/g to 25.2 mg/g. The adsorption of Cr(VI) followed pseudo-second-order behaviour. The changes in Gibbs free energy, enthalpy, and entropy affected by thermodynamic parameters were found to be negative, which confirmed that the adsorption of Cr(VI) on CRH5M is spontaneous, exothermic, and favours low temperatures.
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Alkali-treated Carbonized Rice Husk for the Removal of Aqueous Cr(VI)
Bhaskar Devu Mukri, Killi Krushnamurty, Arif Chowdhury, Duvvuri Suryakala, and Challapalli Subrahmanyam *
Rice husk was chemically modified for the preparation of activated carbon. Rice husk was treated with nitric acid and carbonized at 700 C. After carbonization, the resulting rice husk char was treated with NaOH at room temperature. The 5 M NaOH-treated rice husk had the highest surface area (750 m2/g). Proximate analysis of activated carbon confirmed that NaOH treatment removed silica completely. Temperature programmed decomposition (TPD) graphs showed that the total gas contents (CO and CO2) liberated by CRH and H2O-treated CRH and CRH5M were 2l5 μmol/g, 390 μmol/g, and 970 μmol/g, respectively. The adsorption studies of the activated carbon during Cr(VI) removal from the aqueous medium indicated that CRH5M showed the highest rate of adsorption. The effect of adsorbent dosage, Cr(VI) concentration, pH, and temperature were studied to determine the best removal efficiency. With a decrease in pH from 4.4 to 2, the adsorption capacity increased from 3 mg/g to 25.2 mg/g. The adsorption of Cr(VI) followed pseudo-second-order behaviour. The changes in Gibbs free energy, enthalpy, and entropy affected by thermodynamic parameters were found to be negative, which confirmed that the adsorption of Cr(VI) on CRH5M is spontaneous, exothermic, and favours low temperatures.
Keywords: Rice husk; Surface area; Temperature-programmed decomposition (TPD); Adsorption study; 1,5-Diphenylcarbazide
Contact information: Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, Telangana 502285, India; *Corresponding author: csubbu@iith.ac.in
INTRODUCTION
Chromium and its ionic compounds are toxic aqueous pollutants. Chromium enters water bodies from many industrial processes such as metal finishing, electroplating, wood preservation, paint manufacturing, leather tanning, pigment manufacturing, chemical manufacturing, and agricultural runoff (Gupta et al. 2010). In the aquatic environments, chromium occurs in two predominant oxidation states, Cr(VI) and Cr(III). Hexavalent chromium is more toxic than the trivalent ion and has mutagenic and carcinogenic properties. Its consumption by humans may cause adverse effects such as vomiting, nausea, epigastric problems, diarrhea, etc. (Tiravanti et al. 1997).
There are several tested methods for the removal of chromium from aquatic environments, including precipitation (Zhou et al. 1993), chemical reduction (Seaman et al. 1999), ion exchange (Petruzzelli et al. 1995), electrochemical precipitation (Kongsricharoern and Polprasert 1996), membrane separation (Kozlowski and Walkowiak 2002), adsorption (Srivastava et al. 1997), biosorption (Aravindhan et al. 2004), and photocatalytic reduction (Testa et al. 2004). Most of these techniques have drawbacks such as high costs and/or the incomplete removal of toxic metals (Demirbas et al. 2004). Of these techniques, adsorption appears to be commercially suitable. Activated carbon (AC) is an effective material in removing hazardous compounds such as pesticides, heavy metal ions, etc. (Huang and Wu 1977; Ouki et al. 1997; Demirbas et al. 2004). Even though AC is widely tested, its production costs are high and there is a need to develop low-cost, tailor-made ACs.
Agricultural biowaste materials have been tested for the removal of heavy metal ions from water, including sawdust (Garg et al. 2004), bamboo (Dula et al. 2014), neem leaves (Babu and Gupta 2008), olive cake (Dakiky et al. 2002), green coconut shell (Pino et al. 2006), groundnut husk (Dubey and Gopal 2007), and peanut shell (Romero et al. 2004). Biowaste is a suitable candidate for the preparation of AC because of its underutilization and/or availability in large quantities (Garg et al. 2007). Carbonized rice husk has been tested for the removal of metal ions such as Cr(VI), Ni(II), Zn(II), and Cd(II) ions from aqueous streams (Srivastava et al. 2007), but most reports do not deal with the choice of AC source, role of surface chemistry, etc. There is a need to understand the influence of the surface chemistry of AC on the adsorption capacity. In the present study, AC was prepared from rice husk and it was tested for the removal of Cr(VI) from an aqueous medium. The kinetics of metal ion removal were measured and the thermodynamic parameters were determined.
EXPERIMENTAL METHODS
Preparation of Adsorbents
Rice husk was collected from the Srinivasa Rice Mill (Hyderabad, India), washed with distilled water, and dried overnight at 100 C. Fifty grams of dried rice husk was mixed with 500 mL of 5M HNO3 solution and heated at 60 C for 24 h with continuous stirring. HNO3-treated rice husk was filtered, washed with distilled water, and dried at 100 C for 5 h. HNO3-treated rice wash was carbonized by placing the rice husk in a tubular furnace and heating it at 700 C for 5 h under a N2 flow rate of 20 mL/min. After carbonization, the product was allowed to cool in an N2 atmosphere.
Carbonized acid-treated rice husk (CRH) was then treated with different concentrations of NaOH. For this purpose, 100 mL of 0.5 M NaOH solution was added to 10 g of carbonized rice husk, and this mixture was stirred for 24 h. After the 0.5 M NaOH treatment, the carbonized rice husk was filtered, washed with distilled water until the filtrate was neutral, and dried at 60 C for 10 h. For identification, 0.5M NaOH-treated rice husk was denoted as CRH0.5M. Similarly, 1 M, 3 M, and 5 M NaOH-treated rice husk was denoted as CRH1M, CRH3M, and CRH5M, respectively.
Characterization of Adsorbents
The specific surface area of NaOH-treated samples was measured with the BET-N2 adsorption method using a NOVA 2200e (Quantachrome Instruments, Florida, USA). Moisture content was estimated from the weight loss after heating 1 g of the sample in a ceramic crucible at 120 C, whereas ash content was estimated by burning the sample at 550 C. The proximate analysis was estimated using the following equations,
(1)
(2)
(3)
where Wi is the initial weight of the sample, WM is the weight of the sample after the removal of moisture, and WA is the weight of the final ash.
A temperature programmed decomposition (TPD) study was performed to understand the functional groups formed on the surface of the carbons during chemical treatment. TPD was performed using a Quantachrome gas sorption analyzer (Quantachrome Instruments, Florida, USA), which has both a thermal conductivity detector (TCD) and a quadrapole mass spectrometer (RGA PRIS MA PLUS 200 AMU). For TPD measurements, 0.1 g of the sample was placed in a quartz reactor and purged with helium gas with the appropriate flow rate at room temperature. Later, the sample was heated gradually from room temperature to 900 C at a heating rate of 10 C/min in helium flow with simultaneous quantification of CO and CO2, which indicate the decomposition of acidic and basic groups, respectively (Subrahmanyam et al. 2005; Manoj Kumar Reddy et al. 2013).
Adsorption Study
The adsorption study was performed during the removal of Cr(VI) from the aqueous medium. A 100-mg/L stock solution of Cr(VI) was prepared by dissolving 0.283 g of K2Cr2O7 into 1000 mL of distilled water, and 10 mg/L, 30 mg/L, 50 mg/L, 70 mg/L, and 100 mg/L Cr(VI) test solutions were prepared from the stock solution. A total of 100 mL of each test solution was placed in a 250-mL conical flask containing 0.1 g of NaOH-treated rice husk and stirred at room temperature. The concentration of Cr(VI) was analyzed every 10 min using a UV-Visible spectrophotometer (PG Instruments, Ltd, Leicestershire, UK). The intensity of the Cr(VI)-1,5 diphenylcarbazide complex was measured by estimating the maximum absorbance at 542 nm. For this purpose, 2 mL of 10% H2SO4 solution and 2 mL of 0.02 M 1,5-diphenylcarbazide solution were added to 1 mL of Cr(VI) solution and diluted into a 50-mL volumetric flask. The adsorption percentage of Cr(VI) was determined using Eq. 4
(4)
where Co is the initial concentration of Cr(VI) (mg/L) in the solution and Ct is the concentration of Cr(VI) ions (mg/L) at time t (min). Furthermore, adsorption studies were carried out by varying the adsorbent dosage, temperature, and pH. The pH was adjusted by adding 0.1 M HNO3 or 0.1 M NaOH and read with a digital pH meter (Systronics, Ahmedabad, India).
RESULTS AND DISCUSSION
The physico-chemical characteristics of the CRH, CRH0.5M, CRH1M, CRH3M, and CRH5M samples are summarized in Table 1, which confirmed that surface area increased with NaOH treatment. The surface area of NaOH-treated carbonized rice husk varied from 380 m2/g to 750 m2/g on increasing NaOH concentration from 0.5 M to 5 M, respectively. The proximate analysis of moisture, carbon, and ash contents of carbonized rice husks is shown in Table 2. CRH5M had the highest fixed carbon content and almost no ash content, probably because silica was removed by 5 M NaOH. TPD quantified the surface functional groups that decomposed to give CO2 and CO (Eq. 5):
(COOH, CHOH, C=O) CO2, CO (5)
Functional groups
BET Surface Area Analysis
Table 1 lists the BET surface area and porosity characteristics of ACs prepared at different concentrations of NaOH. The textural properties of AC produced from RHs, especially the surface area and pore volumes, significantly improved with increasing concentration of NaOH up to 5 M. At the low concentration, the pore structure was inadequately developed, indicating that only a mild reaction occurred between the char and activating agent. In contrast, at 5 M, the sample showed remarkably improved porosity when compared with the other activation temperatures, with a maximum Brunauer– Emmett–Teller surface area and total pore volume of 750 m2/g and 0.56 cm3/g, respectively was obtained.
Table 1. Surface Area and Pore Volume of NaOH-treated Carbonized Rice Husks
Table 2. Proximate Analysis of Carbonized Rice Husks
The TPD graphs of CRH, H2O-treated CRH, and CRH5M are shown in Fig. 1(a). For comparison, the H2O-treated CRH sample was prepared by passing H2O vapors along with N2 gas through the tubular furnace containing CRH at 700 C for 1 h. The mass spectra of the CO and CO2 liberated during the TPD of CRH5M are shown in Fig. 1(b). The total gases liberated (quantified by TCD) and individual CO and CO2 gases (measured by mass spectrometer) are summarized in Table 3. The total gas evolution by CRH was 215 μmol/g, which was consistent with the values for individual CO (180 μmol/g) and CO2 (45 μmol/g). Similarly, CRH5M showed the highest gas liberation (970 μmol/g), out of which 615 μmol/g was CO and 410 μmol/g was CO2. The TPD value of H2O-treated CRH was slightly higher than that of CRH, but less than that of CRH5M due to additional functional groups created from the presence of H2O.
Table 3. CO and CO2 Quantification by TPD
Fig. 1. (a) Temperature programmed decomposition of CRH, H2O-treated CRH, and CRH5M; (b) mass spectra of CO and CO2 gases evolved by CRH5M
SEM
Figure 2 shows the scanning electron microscopy (SEM) micrographs of ACs prepared at different concentrations of NaOH. SEM micrographs of (a) carbonized rice husk (CRH), showing the more silica content, (b) a magnified image of Fig. 2(a) showing that silica was present in the CRH, (c) an image showing that 3M NaOH treatment of RH (CRH3M) removes all of the silica present in the CRH, and (d) CRH 5M showing more pores inside the carbon.
Fig. 2. Scanning electron microscopy (SEM) micrographs of (a) carbonized rice husk (CRH), showing the more silica content (shown by EDAX spectrum) (b) magnified image of Fig. 2(a), (c) 3M NaOH treated RH (CRH3M) has removed whole silica present in the CRH (no Si peak in the EDAX spectrum), and (d) CRH 5M showing more pores inside the carbon (indicated by arrows).
FTIR
FT-IR spectra were recorded for qualitative identification of surface functional groups formed during the activation. The spectra of activated carbons indicating considerable amounts of carboxyl groups, as shown in Fig. 3.
Fig. 3. FT-IR spectrum of CRH and CRH 5M
For CRH the peak around 1095 cm-1 is due to Si–O-Si asymmetric stretching vibration. The peak at 967 cm-1 is due to Si–OH stretching vibration. The peaks appearing at 819 cm-1 are due to Si–O–Si symmetric stretching and the band at 1640 cm-1 due to O-H bending vibration (Rahim et al. 2015). FTIR spectra of CRH 5M shows carbonyl C=O (1719 cm-1), aromatic C=C (1620 cm-1), carboxyl O=C-O (1356 cm-1), epoxy C-O (1217 cm-1), and alkoxy C-O (1049 cm-1) stretching vibrational modes.
Cr(VI) adsorption changed as a function of time on SiO2 (CRH ash), CRH, CRH1M, CRH3M, and CRH5M (Fig. 4). Because CRH5M showed the highest adsorption capacity, it was chosen for subsequent experiments varying Cr(VI) concentration, adsorbent dosage, and pH.
Fig. 4. Effect of Cr(VI) adsorption percentage on differently treated carbonized rice husks. Conditions: 1 g carbonized rice husk and 100 mL 10-mg/L Cr(VI) solution
Fig. 5. (a) Efficiency of Cr(VI) adsorption and (b) amount of Cr(VI) adsorbed onto CRH5M with different concentrations of Cr(VI) solution, using 0.1 g of carbonized rice husk and 100 mL of Cr(VI) solution
Cr(VI) adsorption and the adsorption capacity of CRH5M for different Cr(VI) concentrations are shown in Fig. 5. The adsorption of Cr(VI) by CRH5M increased with time reaching equilibrium at 90 min. The removal of Cr(VI) depends on Cr(VI) initial concentration. By changing the initial concentration from 10 mg/L to 100 mg/L, the adsorption percentage decreased from 22.5% to 3%, whereas the adsorption capacity of CRH5M increased from 2.25 mg/g to 3 mg/g after 90 min. The adsorbent dosage also affected Cr(VI) removal efficiency (Fig. 6). When the amount of adsorbent increased from 100 mg to 400 mg, the percentage of Cr(VI) adsorption efficiency also increased from 22.5% to 39% within 90 min.
Fig. 6. Efficiency of Cr(VI) adsorption using different amounts of CRH5M and 100 mL of 10 mg/L Cr(VI) solution
The pH of the test solution is an important factor governing adsorption capacity, as it alters the surface chemistry of the adsorbent. The adsorption behaviour of CRH5M was studied in the pH range 2 to 10 (Fig. 7(a)). Because 100 ppm Cr(VI) solution is acidic (pH 4.4), the pH of pure water was adjusted to 4.4, which increased to 5.4 with the addition of 0.1 g of adsorbent. Under the same experimental conditions, the pH of 100 ppm of the Cr(VI) solution increased only to 4.8 with the addition of CRH5M. A positively charged surface has a higher adsorption capacity for HCrO4− than for Cr2O72− (Dean and Tobin 1999; Rodrigues et al. 2010).
For CRH5M, the decrease in pH was not significant, and the adsorption of Cr(VI) ions was higher. Similarly, the adsorption of Cr(VI) ions at pH 8 was lower compared with the adsorption at pH 4 because of the presence of more hydroxide ions, which block the CrO42− species from adsorbing onto the surface of the adsorbent. The effect of pH during the adsorption of Cr(VI) is shown in Fig. 7(b). As the pH decreased from 4.4 to 2, the adsorption capacity increased from 3 mg/g to 25.2 mg/g; under basic conditions at pH 10, the adsorption of Cr(VI) ions decreased to 1.2 mg/g.
Fig. 7. (a) Change in pH with the addition of CRH5M in water and Cr(VI) solution; (b) amount of Cr(VI) adsorbed with varying pH values of the solution
Table 4. Comparison of Adsorption Capacities of the Adsorbents for the Removal of Cr(VI) with those of other Adsorbents
In order to understand the mechanism of adsorption on CRH5M, the experimental data for various concentrations of Cr(VI) was fit into several kinetic models, including first-order (Hossain et al. 2005), pseudo-first-order (Annadurai et al. 2002), second-order (Sparks 1989), and pseudo-second-order models (Rengaraj et al. 2003) (Fig. 8). The data did not fit well into first, pseudo-first, or second-order rate expressions, but the coefficient of determination (R2) value of the pseudo-second-order model was nearly 1 (Table 5). Hence, the adsorption of Cr(VI) by carbonized NaOH-treated rice husk displayed pseudo-second-order kinetic behavior.
Table 5. Comparison of Adsorption Kinetic Models
Fig. 8. Plots of (a) first-order, (b) pseudo-first-order, (c) second-order, and (d) pseudo-second-order adsorption kinetics of Cr(VI) on CRH5M with different initial concentrations
Thermodynamic parameters determine the nature of Cr(VI) adsorption on the adsorbent. The change in Gibb’s free energy (ΔG), change in enthalpy (ΔH), and change in entropy (ΔS) were calculated using the following expressions (Ramaraju et al. 2014),
where Kd is the distribution constant, qe is the amount adsorbed at equilibrium (mg/g), Ce is the concentration of Cr(VI) in solution at equilibrium (mg/L), R (8.314 J/molK) is the universal gas constant, and T (K) is the absolute temperature.
To determine thermodynamic parameters, the adsorption capacity of Cr(VI) was followed on CRH5M at different temperatures, and the percentage of Cr(VI) adsorption vs. time was plotted (Fig. 9). The pseudo-second-order plot for different temperatures is shown in Fig. 10, and the reciprocal slope of each curve determined the amount adsorbed at equilibrium (qe). The free energy changes at different temperatures are summarized in Table 6. The negative values of ΔG from -2.705 kJ/mol (25 C) to -0.905 kJ/mol (55 C) indicated the spontaneity of the Cr(VI) adsorption on CRH5M. The intercept and slope of the G vs. T plot (Fig. 11; Table 6) provided the values of H and S, respectively (-24.2 kJ/mol and -71.2 J/molK, respectively). These values confirmed that Cr(VI) adsorption on CRH5M is spontaneous, exothermic, and is favorable at low temperatures.
Fig. 9. Effect of Cr(VI) adsorption percentage on CRH5M at different temperatures, using 0.1 g of carbonized rice husks and 100 mL of 10 mg/L Cr(VI) solution
Fig. 10. Pseudo-second-order adsorption kinetics of Cr(VI) on CRH5M at different temperatures
Fig. 11. G versus T-plot of Cr(VI) adsorption on CRH5M with 100 mL of 10-mg/L concentration
Table 5. Thermodynamic Parameters of Cr(VI) Adsorption
Table 6. Change in Enthalpy (ΔH) and Entropy (ΔS) for Cr(VI) Adsorption
CONCLUSIONS
- Rice husk was used to prepare activated carbon with a high surface area and treated sample was found to be effective in treating water contaminated by hexavalent chromium.
- Sodium hydroxide treatment was crucial in order to obtain a high surface area AC. An increase in surface area as a consequence of alkali treatment is due to removal of SiO2 and NaOH-treated carbonized rice husk showed the best performance during the removal of aqueous Cr(VI).
- Increasing concentrations of Cr(VI) increased the adsorption efficiency. In a similar manner, increasing the adsorbent dosage increases Cr(VI) adsorption because of the availability of more surface active sites.
- Acidic pH favors the best adsorption of Cr(VI). The adsorption of Cr(VI) on carbonized rice husk follows pseudo-second-order kinetics, whereas thermodynamic studies indicated the spontaneity and exothermic nature of the adsorption process.
ACKNOWLEDGMENTS
The authors are grateful to MHRD, India for financial support.
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Article submitted: December 4, 2015; Peer review completed: February 13, 2016; Revised version received and accepted: June 16, 2016; Published: September 12, 2016.
DOI: 10.15376/biores.11.4.9175-9189