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Ind. Eng. Chem. Res. 2007, 46, 2550-2557
Removal of Trivalent Arsenic (As(III)) from Contaminated Water by Calcium Chloride (CaCl2)-Impregnated Rice Husk Carbon Prasenjit Mondal, Chandrajit Balo Majumder,* and Bikash Mohanty Department of Chemical Engineering, Indian Institute of Technology (IIT) Roorkee, Roorkee-247667, Uttaranchal, India
This paper deals with the arsenic removal ability of activated carbons produced from calcium chloride (CaCl2)impregnated rice husks (RH). The optimum concentration of Ca2+ ions in calcium chloride solution (CCS) for impregnation was determined to be 2%, which produced ARHC(Ca-2.0). The maximum specific uptake (18.2 ( 0.05 µg/g) was obtained using ARHC(Ca-2.0) at an initial arsenic concentration of 1000 ppb. It was observed that the percentage removal and specific uptake of trivalent arsenic (As(III)) by ARHC(Ca-2.0) were ∼480% and ∼550% higher than that of activated rice husk carbon without impregnation (ARHC(Ca-0)), for an arsenic solution with an initial concentration of 100 ppb. However, using ARHC(Ca-2.0) as an adsorbent, when the initial arsenic concentration was increased from 100 ppb to 1000 ppb, the specific uptake was increased by ∼769% and the percentage removal was decreased by ∼13%. The spent adsorbent gave ∼80% desorption of the adsorbed As(III) in 5 N H2SO4. The fitness of the isotherm equations used to explain the adsorption phenomena decreased in the following order: polynomial isotherm > Freundlich isotherm > Langmuir isotherm. 1. Introduction Arsenic, which is the most widely available hazardous chemical in the world,1 is found in the groundwater of many countries (such as Argentina, Bangladesh, India, Mexico, Mongolia, Germany, Thailand, China, Chile, the United States, Canada, Hungary, Romania, Vietnam, Nepal, Myanmar, and Cambodia) above the maximum contaminant limit (MCL) set by the corresponding nations.2-6 Arsenic enters into the groundwater through the dissolution of arsenic compounds associated with the pyrite ores of aquifer sediments into the water via geothermal, geohydrological, and biogeochemical factors.7 It produces various types of cancer in the human body. Recent research efforts toward improvement in the cost effectiveness of an arsenic removal process lies either in the modification of conventional techniques, such as adsorption, or in the introduction of new technology, such as biofiltration and phytoremediation.6 The arsenic adsorption capacities of some adsorbents have been improved recently by their surface modification by a chemical agent.8-16 These adsorbents include copper-impregnated coconut husk carbon, iron oxide-coated polymeric materials, iron oxide-coated cement, bead celluloseloaded with iron oxyhydroxide, iron oxide-coated sand, etc. The percentage removal values in these surface-modified adsorbents have been determined to be >95%. Both activated rice husk carbon (ARHC) and granular activated carbon (GAC) have less arsenic removal capacity. A recent report17 shows that the percentage removal of trivalent arsenic (As(III)) by rice husk (RH) ash and GAC at an adsorbent dose of 10 g/L and a shaking time of 6 h are 5% and 50%, respectively. However, it can be improved considerably by their surface modification via the impregnation of metals such as iron, manganese, and copper.12,18,19 Although impregnation of the carbon surface by calcium chloride (CaCl2) is rarely reported, it is a well-known fact that the presence of CaCl2 improves the arsenic removal by coagulation precipitation methods.20 Hence, * To whom correspondence should be addressed. E-mail: chandfch@ iitr.ernet.in.
by modifying the activated carbon surface with the help of CaCl2, its arsenic removal capacity is expected to improve. A literature review shows that the improvement in the arsenic removal capacity of rice husk carbon by modification of its surface by CaCl2 is rarely reported. Because of the fact that RH is readily available in abundance in countries such as India and Bangladesh, such a method may prove to be economical. The efficiency of an adsorptive removal process is described either by specific uptake or percentage removal. Specific uptake is used to describe the total removal of adsorbate per unit mass of adsorbent, whereas percentage removal gives a comparison between the concentrations of adsorbate in the solution before and after treatment. Adsorption processes are normally modeled by some isotherms. To determine the best-fit isotherms, Marquardt’s percent standard deviation (MPSD) error function may be generated as follows:21
MPSD (%) ) 100 ×
x
1
n
∑
n - p i)1
(
)
qe,exp - qe,calc qe,exp
2
i
The MPSD error function is similar, in some respects, to a geometric mean error distribution modified according to the number of degrees of freedom of the system. In the groundwater of West Bengal, ∼60%-90% of the total amount of arsenic exists as As(III), which is the more toxic form of arsenic.22 The removal of As(III) and pentavelent arsenic (As(V)) via adsorption, using surface-modified adsorbents such as GAC and ARHC, is highly influenced by the pH of the water. It is a well-known fact that the As(III) remains as a neutral species in the neutral pH range and As(V) is removed easily at neutral pH by the same adsorbent, which can remove As(III)14-16 at a higher pH. By increasing the pH of the solution, the As(III) may be converted to a negatively charged moiety and can be removed by the same adsorbent. However, there are few reports on the removal of As(III) using such adsorbents. Under the previously described conditions, the objective of the present study is to investigate the adsorption characteristics of CaCl2-impregnated ARHC. The investigation includes the
10.1021/ie060702i CCC: $37.00 © 2007 American Chemical Society Published on Web 03/21/2007
Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 2551 Table 1. Characteristics of Various Adsorbent Samples
a
adsorbent sample
CaCl2 solution used
moisture content (%)
ash content (%)
bulk density (kg/m3)
surface area (m2/g)
ARHC(Ca-0)a ARHC(Ca-0.25) ARHC(Ca-0.5) ARHC(Ca-0.75) ARHC(Ca-1.0) ARHC(Ca-1.5) ARHC(Ca-2.0) ARHC(Ca-2.5)
CCS(Ca-0) CCS(Ca-0.25) CCS(Ca-0.5) CCS(Ca-0.75) CCS(Ca-1.0) CCS(Ca-1.5) CCS(Ca-2.0) CCS(Ca-2.5)
10 ( 0.02 10 ( 0.01 9.9 ( 0.01 8.5 ( 0.01 8.1 ( 0.01 8.1 ( 0.01 8.0 ( 0.01 8.0 ( 0.01
34.1 ( 0.01 36.3 ( 0.01 36.6 ( 0.01 37.6 ( 0.01 38.6 ( 0.01 39.4 ( 0.01 40.5 ( 0.01 40.8 ( 0.01
485 ( 2 489 ( 2 493 ( 2 495 ( 2 498 ( 2 501 ( 2 504 ( 2 505 ( 2
101 ( 1 109 ( 1 118 ( 1 128 ( 1 141 ( 1 155 ( 1 171 ( 1 173 ( 1
Elemental analysis of ARHC(Ca-0): C, 50.07; H, 0.67; N, 0.66; S, nil; other, 48.6.
Figure 1. Fourier transform infrared (FTIR) spectra of activated rice husk carbons (ARHCs): ARHC(Ca-0) (bottom) and ARHC(Ca-2.0) (top).
study on the effect of Ca2+ ion concentration in the impregnating solution, as well as the initial arsenic concentration in the solution, the pH of the solution, the ash content of ARHCs, and regeneration of the spent adsorbent on the removal of arsenic from contaminated water. Adsorption isotherms have also been developed and compared. 2. Experimentation The preparation of various types of ARHCs (adsorbent samples) and their characterization, arsenic standards, and synthetic samples, and the experimental procedure, are described below. 2.1. Preparation of Activated Carbon from Rice Husk and Its Characterization. RH collected from the state of Uttranchal, India were washed with doubly distilled water, to remove dirt and other contaminants present, and then were oven-dried at 110 °C for 24 h. Seven different CaCl2 solutions (CCSs) containing 0%, 0.25%, 0.50%, 0.75%, 1.0%, 1.5%, 2.0%, and 2.5% Ca+2 ions (w/w) were prepared by dissolving a calculated amount of CaCl2‚2H2O into ultrapure water obtained from a Milli-Q water purification unit (Millipore Corp., Bedford, MA). The pH of these solutions was in the neutral range (pH 6.8 ( 0.2). Hereafter, these solutions are referenced as CCS(Ca-0), CCS(Ca-0.25), CCS(Ca-0.5), CCS(Ca-0.75), CCS(Ca-1.0), CCS(Ca-1.5), CCS(Ca-2.0), and CCS(Ca-2.5), as detailed in Table 1. In 1 L of each CCS, 100 g of RH was added and heated at ∼70 °C in a water bath until the excess water had evaporated for impregna-
tion of the CaCl2 in RH. The samples were then oven-dried at 120 °C for 24 h. The samples of impregnated RH were then placed at the center of a stainless steel tubular reactor, which was 150 mm in length and 68 mm in diameter. This reactor was placed horizontally inside a muffle furnace and was heated at 600 °C for 4 h to complete the process of carbonization.23 The treated adsorbents were then crushed and sieved to 150/ 170 BSS mesh (average particle size of 98 µm). The bulk density was measured using a pycnometer, and elemental analysis of the rice husk carbon without impregnation was conducted using an elemental analyzer system (Elemental Analysensysteme GmbH, model Vario-EL V3.00). Infrared (IR) spectra of the adsorbents have been obtained using a Thermo Fourier transform infrared (FTIR) system (model AVATR 370 csl), coupled with EZOMNIC software (version 6.2). The surface area of the samples was measured with the N2 adsorption isotherm (using a Micromeritics ASAP 2010 instrument) via the BrunauerEmmett-Teller (BET) method, using the software of Micromeritics. Nitrogen was used as a cold bath (77.15K). Characteristics of different adsorbent samples are given in Table 1. The FTIR spectra of ARHC(Ca-0) and ARHC(Ca-2.0) are shown in Figure 1 (bottom and top spectra, respectively). It is evident that, for both spectra, there are broad bands centered at ∼1100 cm-1 (BB1), ∼1650 cm-1 (BB2), and ∼3500 cm-1 (BB3). BB1 indicates the presence of inorganic silicates, aluminosilicates,
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Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007
Figure 2. Scanning electron microscopy (SEM) micrographs of (a) ARHC(Ca-0) and (b) ARHC(Ca-2.0), each shown at a magnification of 1500×.
and clay-type materials. The associated water in such inorganic oxides gives BB2, whereas BB3 is due to the presence of moisture in the samples.24-26 Some additional bands are also visible at the following wavenumbers: 455 cm-1 (AB1), 790 cm-1 (AB2), and 2350 cm-1 (AB3). AB1 and AB2 may be due to the presence of silica,25 and AB3 may be due to the presence of CaO.25-27 Hence, silica (SiO2), alumina (Al2O3), and CaO are present in both ARHC(Ca-0) and ARHC(Ca-2.0). The greater peak area of the spectrum of ARHC(Ca-0), relative to that of ARHC(Ca-2.0) at a wavelength of λ ≈ 3500 cm-1 indicates the association of more water in ARHC(Ca-0) than in ARHC(Ca-2.0),27 which is also evident from Table 1. The peak area and the sharpness of the peaks at wavenumbers of 455 and 700 cm-1 are decreased from ARHC(Ca-0) to ARHC(Ca-2.0), which may be due to a decrease in the relative concentration of silica in the ARHC(Ca-2.0), because the mass of CaO is added during the production of ARHC(Ca-2.0). Again, at wavelengths of 1650 and 2350 cm-1, the peak area and the sharpness of the peaks are increased, which indicates an increase in the relative concentration of CaO in ARHC(Ca-2.0). The increase in ash content and bulk density from ARHC(Ca-0) to ARHC(Ca-2.0), shown in Table 1, also supports the inclusion of more CaO onto ARHC(Ca-2.0). It is important to note that the presence of chloride is not evident in the spectral line of ARHC(Ca-2.0); this observation may be due to the conversion of CaCl2 to CaO during carbonization, as per the following equations:28
CaCl2 + CO2 + H2O ) CaCO3 + 2HCl CaCO3 + C ) CaO + 2CO During carbonization, as the impregnated RH are heated inside the stainless steel reactor in the absence of oxygen, CaCO3 is produced first, which is subsequently converted to CaO by the fixed carbon in the ARHC. This CaO may form a layer on the surface of the ARHCs. The formation of such layer is also evident via scanning electron microscopy (SEM) analysis of ARHC(Ca-0) and ARHC(Ca-2.0), as shown in the comparison of Figures 2a and 2b. 2.2. Preparation of Standard Solution of Arsenic. Stock solution of 1000 mg/L As(III) was prepared by dissolving analytical-grade sodium arsenite (S.D. Fine Chemicals, India) in Milli-Q water.29 Secondary As(III) standards (10 mg/L) were prepared by diluting the stock solutions. 2.3. Procedure. Five synthetic arsenic solutions with initial arsenic concentrations (Aso) of 1000, 750, 500, 250, and 100 ppb were prepared from secondary standard solutions. Two grams of each of the above-stated carbon types were added in a 50-mL synthetic arsenic solution containing 1000 ppb of initial arsenic concentration separately in a 100-mL flask. The initial
pH of the solution was maintained at 10.75 ( 0.25. Similar experiments were performed with an arsenic solution containing 750, 500, 250, and 100 ppb Aso. After the addition of carbon, the solution was mixed well, using a magnetic stirrer for 15 h.13 The pH was checked every 2 h and adjusted via the dropwise addition of (N/10)NaOH, when required. The solutions then were filtered through 0.45-µm filters. The filtrate was analyzed for total arsenic by the Gutzet method, using an ultraviolet (UV) spectrometer at a wavelength of λ ) 535 nm by occasional cross-checking with inductively coupled plasma mass spectroscopy (ICPMS). The temperature of the removal process was 28 °C. The pH was varied from 6 to 12 to study the effect of pH on the removal of As(III). Each experiment was performed three times. 3. Results and Discussion The removal of As(III) by these adsorbents, the effect of pH on the removal, the regeneration of spent adsorbents, the adsorption isotherm, and the advantages of this method are discussed in the subsequent sections. 3.1. Role of Ca2+ Ion Impregnation on Arsenic Removal. The specific uptake (Qe) of As(III) for the adsorption of As(III) by the ARHC at various equilibrium concentrations (Ase), corresponding to the Aso values of 100, 250, 500, 750, and 1000 ppb, are shown in Figure 3. For ARHC(Ca-2.0), these equilibrium values are 14.7, 47.5, 123, 183.75, and 258 µg/L, respectively. From Figure 3, it is evident that, initially, the specific uptake increases as the Ca2+ concentration increases; however, beyond a Ca2+ concentration of 2%, the increase in specific uptake is insignificant for all Ase. Hence, the CCS solution containing 2% Ca2+ gives an optimum specific uptake. The increase in the ability for As(III) removal (specific uptake) of ARHC with the increase in the concentration of impregnated Ca2+ ions can be explained as follows. Arsenic sorption on material surfaces may be performed via the formation of outer sphere (physisorbed) complexes, the formation of inner sphere (chemisorbed) complexes, or surface precipitation.6,30 For physical adsorption, surface area and porosity are more important. The greater the surface area, the greater the micropore and mesopore volumes and, consequently, the greater the removal. From Table 1, it is evident that the surface area of the ARHC increases from 111 ( 1 m2/g to 171 ( 1 m2/g, because of the increase in Ca2+ concentration from 0% to 2%, which also increases the pore volume of ARHC. However, the surface areas of ARHC(Ca-2.0) and ARHC(Ca-2.5) are similar. In 2000, Yalcın and Sevinc also reported the initial increase in the surface area of CaCl2-impregnated rice husk carbon, followed by a decrease at higher concentrations of CaCl2.23
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Figure 3. Effect of the initial concentration of Ca2+ ions in a calcium chloride solution(CCS) on the arsenic removal.
Alternatively, at the experimental pH range, the As(III) exists as a negatively charged moiety. Hence, the greater the positive charge on the ARHC, the greater the adsorption of As(III) will be. The positive charge on the ARHC surface is dependent on the presence of Ca2+ ions on its surface, which are incorporated during impregnation. RH can impregnate Ca2+ ions on its surface, either via the entrapment of this ion in the pore volume of RH or via the chemical attachment of Ca2+ ions with negatively charged active sites on the surface of the RH, which develops positive charges on its surface and increases the net positive charge (NPC). When the Ca2+ concentration in the CCS increase, a situation arises when most of the negative sites of the RH are occupied by the Ca2+ ions and a further increase in the number of Ca2+ ions in the CCS cannot increase the attachment of Ca2+ ions on it. During carbonization, these Ca2+ ions are converted to CaO and develop a positive charge on the ARHC. In this case, ARHC(Ca-2.0), which is obtained from the carbonization of RH, impregnated with 2% Ca2+ ions, develops an optimum NPC on its surface. The change in NPC on the RH surface due to the impregnation of Ca2+ ions onto the surface of RH is schematically shown in Figure 4. In the pH range of 2.2-11, the positive and negative sites on the surface of the RH will be present in such a ratio that the NPC value will be positive. Therefore, the NPC of 2δ+, as shown in Figure 4a, may schematically represent the surface of the RH in the pH range of the CCS (pH 6.8 ( 0.2) and the entire figure can also represent the Ca2+ impregnation by the CCS. In this case, the ARHC(Ca-2.0) produced by the carbonization of impregnated RH with CCS containing 2% Ca2+ ions gives the optimum NPC on the surface of the ARHC that gives optimum As(III) removal. There are very few reports on the removal of arsenic from contaminated water via adsorption on ARHC. A recent report12 on As(III) removal by iron-impregnated GAC has shown that, using GAC treated with 4% iron and a 0.24% manganese solution, a maximum arsenic removal capacity of 2.8 µg/g can be achieved from a synthetic solution containing 120 ( 2 µg/L sodium arsenite. In the same study,12 it is also reported that GAC without any impregnation has low specific uptake for arsenic, equal to 0.81 µg/g. Comparing the results of this study with the present study on the arsenic removal at Aso ) 120 µg/L, it is evident that the ARHC(Ca-0) has an specific uptake of 0.5 µg/g, which is only 68% of the specific uptake obtained in the previous experiment using an impregnated activated carbon. In 2001, Saha et al. have reported the greater arsenic removal capacity of GAC, relative to that of RH ash.17
Figure 4. Schematic diagram for the impregnation of rice husk (RH) by Ca2+ ions: (a) surface charge without impregnation (net positive charge (NPC) ) 2δ+), (b) surface charge with medium impregnation (NPC ) 6δ+), and (c) surface charge with optimum impregnation (NPC ) 8δ+).
In this experiment, it is interesting to note that the inherent ash content of ARHC(Ca-0) is ∼36.67%, whereas for GAC, it is 16.6%; however, As(III) removal is less for ARHC(Ca-0) than that of GAC, which indicates that the inherent ash content has a lesser effect on the percentage removal of the arsenic. Again, the inclusion of Ca2+ ions on the surface of the ARHC increases its ash content, because the percentage removal of As(III) increases from ARHC(Ca-0) to ARHC(Ca-2.0). This ash is not the structural part of the ARHC itself and is termed “free ash”. Therefore, it is evident that the free ash content, rather than inherent ash content, has a major role in the percentage removal of As(III), which is bonded with ARHC either by a monodentate bond or by the formation of an outer sphere complex.31 An outer sphere complex is one in which the ligand is not coordinated directly to the structural cations but instead is bound to the surface OH or OH2, probably by means of a hydrogen bond.31 Pattnaik et al. (2000), have also reported the dominating role of free ash content on the percentage removal of As(III) by GAC.32 The increment of ash content of the ARHC, from 34.11% ARHC(Ca-0) to 40.52% ARHC(Ca-2.0), increases the specific uptake from 0.5 µg/g to 2.10 µg/g at an Aso value of 100 ppb.
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Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007 Table 3. MPSD Values for Various Adsorbents MPSD Value adsorbent sample ARHC(Ca-2.0) ARHC(Ca-0)
Figure 5. Effect of pH on the percent removal of trivalent arsenic (As(III)). Table 2. Ionic Character of Arsenic Species at Various pH Valuesa pH As(III)
0-9 H3AsO3
10-12 H2AsO3-1
13 HAsO3-2
14 AsO3-3
pH As(V)
0-2 H3AsO4
3-6 H2AsO4-1
7-11 HAsO4-2
12-14 AsO4-3
a
Data taken from ref 34.
The greater the free ash content in the ARHC, the greater the arsenic removal. This observation also is consistent with a recent report32 where a similar finding on the minor role of inherent ash content on the removal capacity of GAC has been reported. 3.2. Effect of pH. The percentage removal of As(III) from the sample water within the pH range of 6-12 is shown in Figure 5. ARHC(Ca-2.0) is used as an adsorbent and the solution contains 100 ppb Aso. From this figure, it is evident that the maximum removal (∼85%) of As(III) is obtained when the pH is 10.75 ( 0.25, and, at neutral pH range, the removal is ∼65%. It is also evident that the percentage removal declines sharply after pH 11. The above observation can be explained as follows. The adsorption of arsenic species is guided by the aqueous phase chemistry and the adsorbent surface chemistry.33 ARHC contains oxides of aluminum, calcium, and silicon, these oxides are responsible for the development of charges on the adsorbent surface when the ARHC comes into contact with water. The change in the chemical environment of the adsorbent surface, according to the pH of the solution, is as follows:
M-OH + H+ f M-OH2+
Freundlich isotherm
polynomial isotherm
12.15 6.50
10.58 1.93
0.01 0.92
surface precipitation has been reported recently. The maximum removal of As(III) by copper-impregnated coconut husk carbon has also been reported recently at the pHZPC of CuO (pH 12).26 3.3. Adsorption Isotherms. The concentration dependence of the sorption isotherm often conforms to either the Langmuir or Freundlich isotherm. The Langmuir isotherm theory is based on the assumption that adsorption is a first-order chemical process and a monolayer of adsorbed material is formed onto a series of distinct sites (unisite) on the surface of the solid. The Langmuir isotherm can be represented by the following equation:
qe )
QbCe 1 + bCe
(1)
where qe is the amount of adsorbate adsorbed per unit mass of adsorbent (mg/g), Ce is the equilibrium concentration of adsorbate in aqueous solution (mg/L), and Q (mg/g) and b (L/ mg) are Langmuir constants that are related to the capacity and energy of adsorption, respectively. The Freundlich isotherm, developed based on the formation of a monolayer, which is due to adsorption onto a rough heterogeneous surface (multisites), can be presented by the following equation:35
qe ) KfCe1/n
(2)
where Kf and 1/n are the Freundlich isotherm constants related to the adsorption capacity and degree of favorability of adsorption, respectively. To evaluate the isotherm constants of eqs 1 and 2, these equations are converted to their linear forms, as stated in eqs 3 and 4, respectively.
1 1 1 ) + qe QbCe Q
(3)
and
M-OH + OH- f M-O- + H2O where M ) Al, Ca, or Si. The zero point charge (ZPC) values of SiO2, Al2O3, and CaO are 2.2, 8.3, and 11.0, respectively; hence, an NPC exists on the surface at lower pH, which gradually decreases and attains a negative value at higher pH (∼11.0). Again, the ionic character of the arsenic species also varies with pH, which is shown in Table 2. For As(III), although the negative charge increases as the pH increases, the positive nature of the ARHC surface also is reduced; hence, the As(III) removal is also reduced at higher pH (>11). A considerable amount of negatively charged As(III) is removed at pH 11, when the NPC of the ARHC is zero. Similarly, at neutral pH, the As(III) is also considerable when most of the As(III) is available as a neutral molecule. These observations indicate that As(III) adsorption by ARHC is a complex process and is not controlled by electrostatic attraction alone. The removal of arsenic by the formation of an outer sphere complex, the formation of an inner sphere complex, or
Langmuir isotherm
log qe ) log Kf +
log Ce n
(4)
In these converted linear equations, the error associated with the data is not expressed properly. Hence, the development of the isotherm model in original form (i.e., qe as a function of Ce), by regression, is gradually becoming more popular in recent years as the error associated with the data is properly expressed in original form. Recently, a quadratic polynomial correlation between Qe and Ase (the polynomial isotherm) has been reported.36 The quadratic polynomial isotherm, in the present study, which has been obtained by computer regression for ARHC(Ca-2.0) and ARHC(Ca-0), are mentioned in eqs 5 and 6, respectively.
qe ) 0.0011672 + 0.0710840Ase - 0.0289806Ase2 (5) and
qe ) 0.0003607 + 0.0026887Ase - 0.0007164Ase2 (6)
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Figure 6. Regeneration of spent ARHCs by various chemicals.
Table 4. Comparison between Some Surface-Modified Adsorbents source
adsorbenta
As species used
Aso (ppm)
present study Xu et al.9 Manju et al.10 Katsoyiannis and Zouboulis11 Katsoyiannis and Zouboulis11 Goel et al.12 Kundu and Gupta13 Guo and Chen14 Guo and Chen14 Lenoble et al.15 Lenoble et al.15
ARHC ALSZ CHC-Cu IOCPM IOCPM GAC-Fe, GAC-Mn IOCC BCLIO BCLIO PRMnO PRMnO
As(III) As(V) As(III) As(III) As(V) As(V) As(V) As(III) As(V) As(III) As(V)
0.1-1.0 9.23 50-150 0.050-0.200 0.050-0.200 0.200 0.200 300 300 60 60
adsorbent dose 40 g/L 5 g/L 2 g/L 50 mL/L 50 mL/L 30 g/L 20 mL/L 20 mL/L 1.6 g/L 1.6 g/L
pH
specific uptake (mg/g)
10-11 6-8 12 7 7 7-11 4-5 7 7 7-8.5 7-8.5
0.0021-0.0183 5.11 22.15-55.43 23 93 0.0028 0.0127 33.2 99.6 21.3 49.7
improvement in activity 8.1-fold 5-fold 3.46-fold
a Legend: ALSZ , aluminum-loaded Shirasu-zeolite; CHC-Cu, Cu-impregnated coconut husk carbon; IOCPM, iron-oxide-coated polymeric materials; IOCC, iron-oxide-coated cement; BCLIO, bead cellulose loaded with iron oxyhydroxide; and PRMnO, MnO-loaded polystyrene resin.
From the above equations, the specific uptakes for ARHC(Ca-2.0) and ARHC(Ca-0) at Aso ) 1000 ppb are calculated as 18.247 and 2.214 µg/g, respectively. The corresponding specific uptake values obtained from the Langmuir isotherm are 14.41 and 2.012 µg/g, respectively. However, if the Freundlich isotherm is used, then the corresponding specific uptake values are 14.9 and 2.174 µg/g, respectively. The corresponding experimental specific uptake values for these ARHCs are 18.20 ( 0.05 µg/g and 2.2 0 ( 0.05 µg/g, respectively. From the above observation, it is evident that the polynomial isotherm equations for ARHC(Ca-2.0) and ARHC(Ca-0) give minimum errors (∼0.02% for ARHC(Ca-2.0) and ∼1.6% for ARHC(Ca-0)) in specific uptake values, which are negligible. The errors in specific uptakes are ∼20% for ARHC(Ca-2.0) and ∼9% for ARHC(Ca-0) when the Langmuir isotherm is used. The corresponding errors are ∼17% for ARHC(Ca-2.0) and ∼3% for ARHC(Ca-0) when the Freundlich isotherm is used. The MPSD values on the measurement of the specific uptake by the three isotherm equations for ARHC(Ca-0) and ARHC(Ca-2.0) are shown in Table 3. From this table, it is also evident that the MPSD value for both the ARHC(Ca-0) and ARHC(Ca-2.0) is minimum when the original form of the isotherm is used to calculate the specific uptake. This indicates that the polynomial isotherm gives a more-accurate prediction about the specific uptake value than the Freundlich or Langmuir isotherms. This may be due to the proper expression of errors associated with experimental data in the polynomial isotherm. However, the polynomial form is purely empirical in nature and lacks any physical significance.36 The MPSD values for Freundlich isotherms are less than those for Langmuir isotherms. This indicates that the prediction of the Freundlich isotherm is
a better fit than that of the Langmuir isotherm, relative to the experimental values of specific uptake. It seems that, because of the impregnation, perhaps the heterogeneity and roughness of the surface each increase, which leads to the better fit of the Freundlich isotherm than the Langmuir isotherm to explain the adsorption phenomena. 3.4. Regeneration of Adsorbent. The spent adsorbent has been treated with 1 N NaOH, 5 N H2SO4, and 30% H2O2 in 0.5 M HNO3.17 Figure 6 shows the percentage regeneration of the spent adsorbent observed using 1 N NaOH, 5 N H2SO4, and 30% H2O2 in 0.5 M HNO3. In each case, desorption was performed in three cycles. It is evident that, among these regenerating chemicals, 5 N H2SO4 gives the maximum regeneration. After the first cycle of regeneration, the ARHC(Ca-2.0) has ∼80% of its original arsenic removal capacity. After the end of the third cycle, this value is reduced to ∼74%. The highest desorption of As(III) by 5 N H2SO4 is observed, because of the formation of neutral H3AsO3 (As(III) species), which is not adsorbed onto the positive surface of activated carbon. The use of 30% H2O2 in HNO3 also produces H3AsO3 (As(III) species); however, in this case, the As(III) may be partially converted to As(V), which may be adsorbed on the surface of the ARHC. At pH >13, the surface of the ARHC becomes negative and the As(III) also exists as a negatively charged moiety; therefore, 1 N NaOH can desorb As(III). Desorption of As(III) from the surface of a spent copper-impregnated coconut husk carbon has been performed using distilled water and H2O2 in HNO3,11 where a higher desorption of As(III) by 30% H2O2 in HNO3 was observed. They have also reported >94% recovery of the adsorbed As(III).
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Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007
3.5. Advantages of the New Adsorbent. Comparison among some recently reported surface-modified adsorbents has been shown in Table 4. It is difficult to compare these adsorbents, because they have been used over a wide range of Aso and pH values. However, under optimized conditions, at Aso ) 1000 ppb, the specific uptake for the present material is twice of that of iron oxide-coated sand. It is also evident that the improvement in the arsenic removal capacity of the rice husk carbon, because of impregnation, is ∼8 times larger when Aso ) 1000 ppb. This improvement is the maximum among the reported adsorbents. It is interesting to note that the final concentration of As(III) can be reduced to
