Environ. Sci. Technol. 2006, 40, 1636-1643
Removal of Arsenite and Arsenate Using Hydrous Ferric Oxide Incorporated into Naturally Occurring Porous Diatomite M I N J A N G , * ,‡ S O O - H O N G M I N , § TAK-HYUN KIM,⊥ AND JAE KWANG PARK† Department of Civil and Environmental Engineering, University of Wisconsin-Madison, 1415 Engineering Drive, Madison, Wisconsin 53706
In this study, a simplified and effective method was tried to immobilize iron oxide onto a naturally occurring porous diatomite. Experimental results for several physicochemical properties and arsenic edges revealed that iron oxide incorporated into diatomite was amorphous hydrous ferric oxide (HFO). Sorption trends of Fe (25%)-diatomite for both arsenite and arsenate were similar to those of HFO, reported by Dixit and Hering (Environ. Sci. Technol. 2003, 37, 4182-4189). The pH at which arsenite and arsenate are equally sorbed was 7.5, which corresponds to the value reported for HFO. Judging from the number of moles of iron incorporated into diatomite, the arsenic sorption capacities of Fe (25%)-diatomite were comparable to or higher than those of the reference HFO. Furthermore, the surface complexation modeling showed that the constants of tSHAsO4- or tSAsO42- species for Fe (25%)-diatomite were larger than those reference values for HFO or goethite. Larger differences in constants of arsenate surface species might be attributed to aluminum hydroxyl (≡Al-OH) groups that can work better for arsenate removal. The pH-controlled differential column batch reactor (DCBR) and small-scale column tests demonstrated that Fe (25%)-diatomite had high sorption speeds and high sorption capacities compared to those of a conventional sorbent (AAFS-50) that is known to be the first preference for arsenic removal performance in Bangladesh. These results could be explained by the fact that Fe (25%)-diatomite contained well-dispersed HFO having a great affinity for arsenic species and well-developed macropores as shown by scanning electron microscopy (SEM) and pore size distribution (PSD) analyses.
Introduction Arsenic contamination in ground- and surface water is creating potentially serious environmental problems for * Corresponding author phone: (814) 865-9425; fax: (814) 8637304; e-mail:
[email protected]. † University of Wisconsin-Madison. ‡ Present address: Department of Civil and Environmental Engineering, The Pennsylvania State University, 205 Sackett, State College, Pennsylvania 16802. § Samsung Corporation, 270-1 Seohyun-dong, Bundang-gu, Sungnam-si, Gyonggi-do, Korea 463-824. ⊥ Korea Atomic Energy Research Institute, 150 Dukjin-dong, Yuseong, Daejeon, Korea 305-353. 1636
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human beings and other living organisms. Because of the recognition of this problem, a new arsenic limit of 10 µg L-1 will become effective in 2006 for drinking water systems in the United States. As a result of this new regulation, amall public water facilities will face heavy financial burdens. Arsenic contamination of groundwater is problematic throughout the world, where most people rely on tube wells as a source of drinking water. About 40-60% of the population (125 million people) of Bangladesh has been estimated to be adversely affected by arsenic-contaminated drinking water (1). Thus, there is an urgent demand for an economical, effective, and reliable technique that is capable of removing arsenic species to this new level. Adsorption is considered to be one of the most promising technologies because it is economical and easy to set up. Among several types of conventional sorbents, activated alumina with a proprietary additive, AAFS-50, was developed by Alcan to enhance arsenic removal (2). Sutherland et al. (3) reviewed different types of adsorption processes for arsenic removal in Bangladesh, evaluating the performance of nine technologies in terms of removal efficiency, cost, biological contamination, flow rate, reagent use, filtration time, and maintenance. Adsorption using AAFS-50 was found to be the best in relative performance among the nine technologies because of its high removal efficiency and flow rate (3, 4). However, it was more expensive for both initial and ongoing operating costs than other reported technologies (4). The methods of preparing amorphous hydrous ferric oxide (HFO) incorporated porous naturally occurring diatomite described in this study are environmentally acceptable, costeffective, and simple. As a porous supporting material, diatomite (diatomaceous earth) is a lightweight sedimentary rock composed mainly of silica microfossils of aquatic unicellular algae, consisting of various pores with up to 8090% voids (5). Diatomite has been approved as a food-grade material by the U.S. Food and Drug Administration (FDA) and is stable in contact with liquids given that it originates and is produced from seas or lakes. Because of its high pore volume and surface area, as well as its low density, diatomite has found several industrial applications not only for the adsorption of inorganic and organic pollutants, but also for several other techniques (6, 7). Yang et al. used two different local diatomites to find the relationship between the amount of urokinase adsorbed and the change in isoelectrical point (IEP) for sorption media (7). Al-degs et al. tried to modify the surface of diatomite with manganese oxide to remove lead ions in solution. Mn-modified diatomite was found to have an adsorption capacity of ∼100 mg of lead ions g-1, because of an increase in surface area and negatively charged surface (8). ADI developed iron-impregnated diatomite (ADI medium G2) for arsenic removal (9), in which calcined diatomite (30 × 60 mesh) and iron chloride were used as the supporting medium and iron precursor, respectively. Their preparation includes a pH increase to at least 9.0 with a highly alkaline solution (10 N sodium hydroxide). The iron solution that remained in excess should be properly treated, and there is a maximum limit of iron impregnation percentage (5-30% based on the mass of diatomite). Hydrous ferric oxide (HFO) has been extensively studied as a promising adsorption material for removing both arsenate and arsenite from aqueous media because of its high isoelectric point (IEP ) 8.1) (10) and selectivity for arsenic species. However, several iron oxides including HFO are made as suspensions in aqueous solution and are available only as fine powders. Therefore, it is not suitable to use these fine powders in column applications because of their low 10.1021/es051501t CCC: $33.50
2006 American Chemical Society Published on Web 01/28/2006
hydraulic conductivity (11). To overcome this disadvantage, HFO granulation techniques have been developed. Granular ferric hydroxide (GFH) was synthesized from ferric chloride solution by neutralization and precipitation with sodium hydroxide, followed by centrifugation and granulation under high pressure (12, 13). GFH shows poor mechanical strength to create fine particles, causing a significant headloss pressure within a short operating time (13). Because of its high diffusion limits, the adsorption capacity for arsenic is reduced by 50% when larger-sized materials (1-2 mm) are used (13). To overcome these disadvantages of ADI medium G2 and GFH, an incipient wetness impregnation method was developed to disperse and incorporate nanoscale HFO homogeneously in the pores of diatomite using a vortex mixer. Compared to the preparation methods of other media, this technique is simple and economical because fewer steps and a smaller volume of precursor solution are needed for medium preparation. In addition, there is no iron solution remaining in excess that should be treated. The objectives of this study were (1) to incorporate active HFO homogeneously into the pores of diatomite through an incipient wetness impregnation technique; (2) to evaluate the arsenic adsorption capacities of the medium produced by examining the adsorption edges and pH-controlled differential column batch reactor (DCBR) and small-scale column tests and comparing the performance with that of a conventional medium, AAFS-50; and finally (3) to understand the adsorption behavior of the medium through physicochemical characterizations and surface complexation modeling.
Methodology Incorporation of Iron Oxide into Diatomite. A precursor of iron oxide, iron nitrate nonahydrate [Fe(NO3)3‚9H2O], was incorporated into the pore surface of granular-size porous diatomite (Harborlite mesh 10 × 12) that mainly consists of aluminum oxide (12.8%, w/w) and silica (76.8%, w/w). The following coating procedure was developed to impregnate iron precursor into pore structures of diatomite as homogeneously as possible: (1) Dissolve iron precursor in deionized water at a given concentration to give a final volume of the metal-dissolved solution of 15 mL. (2) Place 10 g of dried diatomite into a 500-mL Teflon bottle. (3) Disperse the iron precursor solution over the diatomite using a 5-mL micropipet. (4) After closing the cap, place the Teflon bottle on a vortex mixer (Genie) and vigorously mix at ∼2000 rpm for 5 min to obtain a homogeneous mixing of the iron solution and diatomite (homogeneity was judged by observing the change of red color distribution on the medium). (5) Dry the solids at room temperature for 1 day. (6) Calcine the material in an oven with a programmed temperature profile that started from room temperature and increased at a rate of 1.0 °C/min to 90 °C, which was then held for 24 h. After calcination, the solids were stored in a vacuum chamber. The solids were then washed with deionized water until the conductivity of the filtrate was close to that of deionized water. The conductivity tests followed the procedure described by Jang et al. (14). In this study, 25% iron-incorporated diatomite [designated Fe (25%)-diatomite] was prepared and used for subsequent experiments. Characterization. Using a Stoe high-resolution X-ray diffractometer (Microphotonics, Allentown, PA) with Cu KR radiation (40 kV, 25 mA), X-ray diffraction patterns over a wide range of angles (10-70°) were acquired for plain diatomite and Fe (25%)-diatomite. For scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) analysis, several samples were coated with gold using a Denton Vacuum Dest II sputter machine. Then, the sample plug was introduced into the SEM (JEOL JSM-6100) to obtain images at different scales, and energy-dispersive X-ray (EDX) tests were conducted to find the degree of homogeneous disper-
sion of iron in different sizes of particles. N2 gas adsorptiondesorption isotherms were performed at 77 K using a Micromeritics ASAP 2000 analyzer (Norcross, GA). Arsenic Sorption Edges and Surface Complexation Modeling. Sodium arsenate (Na2HAsO4‚7H2O, Sigma) and sodium arsenite (NaAsO2, 1000 mg L-1 As, Fluka) solution were used without any modification as the arsenate and arsenite stock solutions, respectively. A stock solution of 0.01 M NaNO3 in deionized water was prepared. The solution of a predetermined volume was poured into a polyethylene bottle. A small volume of arsenic stock solution was added to achieve the target arsenic concentration. Then, the pH was adjusted to 3-11 for each arsenic solution with an automatic pH titrator, using small volumes of acid (HNO3, 0.1 M) and base (NaOH, 0.1 M) stock solutions. Meanwhile, AAFS-50 (Alcan mesh 28×48) and Fe (25%)-diatomite were homogeneously pulverized to be smaller than 75 µm (sieve no. 200) and suspended at a concentration of 10 g L-1 in NaNO3 (0.01 M) solution. While the suspension was being stirred with a magnetic stirrer, a predetermined volume of suspension was added to the arsenic solution. All samples were mixed in a rotary shaker at 250 rpm and 20 ( 0.5 °C. After 8 h of shaking, the pH of the samples was readjusted to 3-11 with the automatic pH titrator. All samples were then shaken again in the rotary shaker until equilibrium was reached. X-ray absorption fine structure (XAFS) and FTIR spectroscopic studies have recently provided evidence for the formation of inner-sphere adsorption complexes of oxy anions on hydrous ferric oxide surfaces (15, 16). Because of this, arsenic adsorption behaviors of various solids have been investigated using the constant capacitance model (CCM) in the intrinsic surface complexation modeling (17-19). The computer program FITEQL 4.0 (20) was used to optimize the intrinsic surface complexation constants, reactive surface site densities, and surface hydrolysis constant. In this study, the one-site adsorption model was employed. Constants for protonation of aqueous arsenic species were obtained from MINEQL 4.5 (21). The surface reaction and equilibrium expression for intrinsic surface complexation of arsenic sorption on the medium are summarized in the Supporting Information (Table S1). To optimize all parameters, the sequential optimization strategy was applied (22). First, five adjustable parameters (two surface hydrolysis constants, two surface complexation constants, and one hydroxyl density) were optimized to achieve satisfactory fits of the CCM to the experimental data of arsenite sorption edges. However, because FITEQL would not converge when the five adjustable parameters were optimized simultaneously, a sequential optimization scheme was used where surface hydrolysis constants were optimized first. Then, a separate optimization of the surface hydroxyl density and two arsenite surface complexation constants was performed while holding the optimized surface hydrolysis constants fixed. Then, the surface hydrolysis constants and hydroxyl density were selfconsistent for optimizing the three surface complexation constants of arsenate sorption edges. This approach yielded the best fits of the model to experimental data based on values of the weighted sum of squares divided by the degrees of freedom (WSOS/DF) calculated in FITEQL 4.0. pH-Controlled Differential Column Batch Reactor (DCBR) Tests. Because it was not possible to conduct conventional kinetic tests in suspension given that Fe (25%)diatomite floats (its bulk density is 0.25 g cm-3), pH-controlled differential column batch reactor (DCBR) tests were conducted. In previous work, Badruzzaman et al. (23) did not control the pH of arsenic feed solution during DCBR operation. This test might have some errors in arsenic removal speeds and capacities because pH is one of the most significant parameters in affecting arsenic species and active VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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surface sites of metal oxide. Thus, it is important to control the pH of the arsenic feed solution by use of an automatic pH controller. In this study, arsenite or arsenate solution was circulated through a DCBR at a constant pH from a mixed feed container (1.0 L) with 3.3 min of empty bed contact time (EBCT). The schematic and methodology of the DCBR test are presented in the Supporting Information (Figure S1). A pseudo-second-order kinetic equation was found to fit well for much of the chemisorption using heterogeneous materials (24, 25). All of the kinetic data from our experiments were fitted with a pseudo-second-order kinetic model to estimate the rate constants, initial sorption rates, and arsenic sorption capacities on different media. The pseudo-secondorder kinetic rate equation is well described by Jang et al. (14). Along with the pseudo-second-order kinetic model, the shrinking-core model (26) was used to estimate mass-transfer characteristic parameters of arsenic removal, assuming that arsenic adsorption is a fast reaction relative to diffusion. Considering external film diffusion and intraparticle diffusion control, this model had been applied to find the apparent diffusivity of metal ions in various adsorbents by fitting equations to experimental data (26-28). For a process controlled by the diffusion of arsenic through the liquid film (film diffusion control), the extent of the arsenic adsorption as a function of time is given by the expression
X)
∫ C dt
3D δRC0
t
(1)
0
If the process is controlled by diffusion through the reacted shell (particle diffusion control), the model is represented by the expression
F(X) ) 1 -3(1 - X)2/3 + 2(1 - X) )
6D R2C0
∫ C dt t
0
(2)
Consequently, a plot of either X or F(X) vs ∫0tC dt will give a linear relationship. X denotes the fraction of arsenic adsorbed on the adsorbent, C (mol cm-3) is the concentration of arsenic in the solution, and C0 (mol cm-3) is the initial arsenic concentration at the beginning of the sorption. D (cm2 s-1), δ (cm), R (cm), and t (s) are the apparent diffusivity of arsenic, the liquid film thickness, the average radius of the adsorbent particles, and the time, respectively. The apparent diffusivity of the medium can be obtained from the slope of such a plot as follows:
D ) (slope)C0
R2 6
(3)
Small-Scale Column Tests. Each column (1.5-cm internal diameter and 15-cm length) was carefully packed with 1.5 g of Fe (25%)-diatomite (mesh 10 × 12, avg 1.55 mm) or AAFS-50 (mesh 28 × 48, avg 0.45 mm), ensuring that the mixture was homogeneously distributed. The influent, containing 500 µg L-1 of arsenate or arsenite, was at pH 6.5, which is a typical pH for ground- or surface water. For these tests, the EBCT was set at 3.3 min, which is the same value as used in the previous DCBR tests. Bulk densities of each medium were measured experimentally to give different flow rates, but the same EBCT. The columns were initially flushed with several pore volumes of distilled water before the synthetic arsenate or arsenite water was introduced. Arsenic Analysis. Arsenate and arsenite were analyzed with an inductively coupled plasma atomic emission spectrometer (ICP-AES, Jobin Yyon Inc., Edison, NJ) for samples having initial arsenic concentrations greater than 0.133 mmol L-1. An atomic absorption spectrophotometer (AAS, Varian AA-975) and a GTA-95 graphite tube atomizer with a programmable sample dispenser (Varian, Palo Alto, CA) were 1638
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used for samples having arsenic concentrations lower than 0.133 mmol L-1. In this case, a 50 mg L-1 nickel solution was used as the matrix modifier. The detection limits of ICP-AES and AAS-graphite for arsenic are 0.033 and 0.0006 µmol L-1, respectively.
Results and Discussion X-ray Diffraction. Wide-angle X-ray diffraction patterns of plain diatomite and Fe (25%)-diatomite are shown in the Supporting Information (Figure S2). The plain diatomite exhibited the X-ray diffraction (XRD) pattern of amorphous aluminum silicate, in which there is no peak but only a broad curve at 15-30° (29). The XRD results for Fe (25%)-diatomite showed the two-line ferrihydrite (or HFO) pattern with two broad diffraction peaks at 35° and 62°, corresponding to d spacings of 0.250 and 0.148 nm, respectively (30, 31). Thus, the iron oxide impregnated into the pores of diatomite is thought to be amorphous hydrous ferric oxide (HFO). Scanning Electron Microscopy. Figure 1 shows the SEM images of plain diatomite and well- or poorly impregnated Fe (25%)-diatomite. Plain diatomite has large-size pores of about 50-200-µm diameter (Figure 1A). Parts B and D of Figure 1 show a comparison between good and poor iron oxide impregnation results. When iron precursor solution was homogeneously dispersed with the method previously described, predominantly macropores developed, and discrete phases of iron oxide did not form at the outsides of individual granules (Figure 1 B,C). This result shows that diatomite can be impregnated with a large mass of iron oxide without overspreading the surface of diatomite. To find the optimum ratio of the volume of iron precursor solution to the mass of diatomite, several trials were performed. If the ratio was over 1.5, slurry phases of iron precursor were observed (Figure 1D), covering the surface of the diatomite. Therefore, the ratio was thought to be a critical parameter for controlling the quality of impregnation of iron oxide into diatomite. EDX tests were conducted to obtain more specific results of homogeneous dispersion of iron for different sizes of particles. Figure 1E shows the results of EDX tests and the weight percentage (with standard deviation) of each element. Chemical composition revealed that iron had a higher average mass percentage (37.0%) than the impregnated percentage. This is due to the limited information depth (1 µm) of EDX. Compared to the iron mass percentage, the small standard deviation (2.9%) for several trials confirms the homogeneous dispersion of iron. Nitrogen Adsorption-Desorption Isotherms. Figure 2A shows the nitrogen adsorption-desorption isotherms of diatomite and Fe (25%)-diatomite. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, diatomite exhibited a typical type II isotherm (32), suggesting a macroporous material. Compared to diatomite, the volume of N2 adsorbed for Fe (25%)-diatomite increased for a wide range of relative pressures P/P0. The BET surface area (62.3 m2 g-1) and BJH adsorption pore volume (0.061 cm3 g-1) of Fe (25%)-diatomite were about 1.4 and 1.6 times higher than those of plain diatomite, respectively. These increases are thought to have been caused by impregnating HFO that is known to have a high surface area and volume (30). Notably, the volumes adsorbed at 0.9 increased sharply, suggesting that both micropores (type I) and macropores (type II) were well developed for Fe (25%)-diatomite. In addition, Fe (25%)-diatomite had a moderate value of the C constant (107.2) obtained with the BET equation fit and a tiny adsorption-desorption hysteresis for 0.4 < P/P0 < 0.8, indicating a continuous mesopore size distribution (PSD) in the range of 1-5 nm (30). To investigate the change of primary PSD with iron impregnation, the recently developed Kruk, Jaroniec, and Sayari (KJS) approach (33) was used for the nitrogen isotherms
FIGURE 1. SEM images of (A) pure diatomite, (B) well-dispersed HFO impregnated diatomite, (C) Fe (25%)-diatomite at high magnification, (D) poorly impregnated Fe-diatomite, (E) well-dispersed HFO-diatomite including energy-dispersive X-ray (EDX) analysis and weight percent (with standard deviation) of each element for several different sizes of particles. (Figure 2B). The PSD plots were derived from the adsorption branch of the nitrogen hysteresis of the nitrogen isotherms. Fe (25%)-diatomite showed higher pore volumes for pores with diameters 100 Å than diatomite. According to the PSD differences between diatomite and Fe (25%)diatomite, two different sizes of major mesopores (24 and 39 Å) and a broad range of macropore width (509 Å) resulted from iron impregnation based on the frameworks of diatomite. Therefore, it can be suggested that HFO incorporated into diatomite predominantly developed not only in micropores and mesopores below 80 Å, but also in a high portion of macropores. High sorption speeds for Fe (25%)-diatomite in arsenic removal could be expected because well-developed
macropores could reduce the internal diffusion of arsenic species, which can act as a rate-limiting step (23). Arsenic Sorption Edges for pH Effects and Surface Complexation Modeling. Figure 3 shows arsenite and arsenate adsorption edge data for Fe (25%)-diatomite and AAFS-50, as well as surface complexation model fits (sum and individual arsenic complex species) based on parameters in Table 2 for Fe (25%)-diatomite. Although the sorption trends of the two media were different, Fe (25%)-diatomite had higher sorption capacities than AAFS-50 for the whole pH range of both arsenite and arsenate solutions. Specifically, the arsenite and arsenate sorption capacities of Fe (25%)diatomite were 2-7 and 1.2 times greater, respectively, than VOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. (A) Nitrogen adsorption-desorption isotherms and (B) pore size distributions obtained from adsorption branches of pure diatomite and Fe (25%)-diatomite. those of AAFS-50, showing that AAFS-50 had a lower arsenic removal capability. Thus, arsenite must be oxidized to arsenate by an oxidizing agent (e.g., chlorine) to achieve maximum performance with AAFS-50 (2). The trends for arsenic sorption onto Fe (25%)-diatomite are similar to the trends for arsenic sorption onto amorphous iron oxide (HFO) reported by Dixit and Hering (10), who studied the relative affinities of several iron oxide for the arsenic species. The crossover pH at which arsenite and arsenate are equally sorbed was also pH 7.5, corresponding to the value for HFO (10). For arsenite, the sorption capacity of Fe (25%)-diatomite increased as the pH increased up to 8, but significantly decreased at pH >9. The maximum sorption capacity for arsenite of Fe (25%)-diatomite was about 0.4 mmol g-1 at pH 8.2. Judging from the number of moles of iron incorporated into diatomite, the arsenite sorption capacity can be calculated to about 0.124 mol of As (mol of Fe)-1. Thus, this value is comparable to the data for arsenite sorption edges on HFO [about 0.107 mol of As (mol of Fe)-1] at similar As-to-Fe ratios (0.14-0.16). On the other hand, the effect of pH was more pronounced for arsenate sorption onto Fe (25%)-diatomite. The arsenate sorption capacity of Fe (25%)-diatomite was 0.45 and 0.32 mmol g-1, corresponding to 0.143 and 0.098 mol of As (mol of Fe)-1, at pH 4 and 8, respectively. According to the number of moles of iron adsorbed at pH 8, Fe (25%)-diatomite had an arsenate sorption capacity about 1.6 times higher than that of HFO [0.062 mol of As (mol of Fe)-1] (10). The sorption capacities of both arsenite and arsenate on Fe (25%)-diatomite were also higher than those of goethite or magnetite in the edge tests of Dixit and Hering (10) performed at the same solid concentration (0.5 g L-1). Thus, it can be suggested from these results that the iron oxide incorporated into diatomite was homogeneously dispersed amorphous hydrous ferric oxide (HFO) having great affinities for both arsenite and arsenate. These high affinities indicate that HFO in diatomite 1640
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FIGURE 3. Edge tests of Fe (25%)-diatomite and AAFS-50: (A) arsenite and (B) arsenate (arsenic concentration, 0.267 mmol L-1; medium concentration, 0.5 g L-1). Solid and dotted lines represent the total and individual arsenic species, respectively, obtained by surface complexation model fits based on parameters in Table 2. has high-energy sites with some degree of specificity for arsenic (34). In the surface complexation modeling (SCM), five adjustable parameters (site density, surface hydrolysis constants, and intrinsic surface complexation constants) were obtained by fitting the edge data of arsenite on Fe (25%)-diatomite. The fitted value of WSOS/DF (4.76) was much lower than 20, indicating a reasonably good fit (20). Fe (25%)-diatomite had lower surface protonation [log KS+(int) ) 5.18] and deprotonation [log KS-(int) ) -6.38] constants than HFO or goethite, as reported by Dixit and Hering (10). With the surface hydrolysis constants of HFO or goethite, it was not possible to achieve convergence of fits to the edge data. This result could be caused by the diatomite surface having a dominant phase of silicate. The point-of-zero-charge (PZC) values for silicate are typically below pH 3 because of a permanent structural negative charge (22). The constants for ≡SH2AsO3 (where S is a surface metal ion) species were similar for each medium, while the constants for ≡SH2AsO3- species were larger (by 2.2-2.7 log units) for Fe (25%)-diatomite than those of goethite or HFO [data are given in the Supporting Information (Table S2)]. After optimizing three intrinsic surface complexation constants of arsenate edge data using the previously determined self-consistent surface hydrolysis constants and hydroxyl density, the fitted value of WSOS/DF was found to be 15.4, still showing a reasonably good fit. In contrast to the arsenite sorption data, large differences in the constants of arsenate sorption were found between Fe (25%)-diatomite and other reference media. Compared to the constant for tSH2AsO4 species, the constants for tSHAsO4- or tSAsO42- species for Fe (25%)-diatomite were higher than the reference values for HFO or goethite [see Supporting Information (Table S2)]. Therefore, tSHAsO4or tSAsO42- species that existed at high pH were dominant for Fe (25%)-diatomite. Surface chemical analyses in the
FIGURE 4. Sorption kinetics of (A) arsenite and (B) arsenate on Fe (25%)-diatomite, AAFS-50, and plain diatomite using pH-controlled DCBR (pH 6.5; arsenic concentration, 0.133 mmol L-1; medium mass, 1 g; EBCT, 3.3 min). The dashed lines were obtained from the pseudo-second-order kinetic model. Data conversions using models of film diffusion control (FDC) and particle diffusion control (PDC) for (C) arsenite and (D) arsenate removal. SEM were conducted by EDX to find the change in aluminum contents before and after the incorporation of iron oxide. The EDX results showed that the aluminum content in Fe (25%)-diatomite was 5.9 ( 0.3%, lower than that in pure diatomite (11.8%). Thus, aluminum ions could work as another surface metal ion for Fe (25%)-diatomite, whereas the surface metal ions (S) are only iron in the case of HFO or goethite (35). Larger differences in constants for arsenate surface species between Fe (25%)-diatomite and reference media (HFO or goethite) can be attributed to aluminum hydroxyl (tAl-OH) groups that can work better for arsenate removal. Kinetics of Arsenic Sorption Using pH-Controlled DCBR. Figure 4A,B shows the arsenite and arsenate sorption kinetics for AAFS-50, Fe (25%)-diatomite, and plain diatomite, along with the curves obtained from the pseudo-second-order kinetic model. The pseudo-second-order kinetic model fit closely with kinetic data of all media, showing high determination coefficients (R2) of over 0.95. In the results for arsenite sorption kinetics, Fe (25%)-diatomite showed an exponential increase in arsenic sorption capacity and a maximum sorption capacity of 0.145 mmol g-1. This result is about 48 and 3 times higher than those of plain diatomite (0.003 mmol g-1) and AAFS-50 (0.047 mmol g-1), respectively. The initial sorption rate (v0) of Fe (25%)-diatomite (18.4 × 10-3 mmol g-1 h-1) was about 35 and 11 times greater than those of plain diatomite (5.3 × 10-4 mmol g-1 h-1) and AAFS50 (1.7 × 10-3 mmol g-1 h-1), respectively. From the comparison between Fe (25%)-diatomite and plain diatomite, HFO incorporated into the pores of diatomite provided the most dominant sorption site for arsenite removal. For the arsenate solution, Fe (25%)-diatomite also showed the
highest sorption capacity (0.124 mmol g-1) and initial sorption rate (16 × 10-3 mmol g-1 h-1), although the capacity and initial sorption rate are about 10% lower than the values for arsenite solution. AAFS-50 and plain diatomite exhibited better performances in arsenate removal than arsenite removal. In terms of arsenate removal, Fe (25%)-diatomite had about 8.9 (3) and 1.3 (3.1) times greater sorption capacities (initial sorption rates) than plain diatomite and AAFS-50, respectively. Although the sorption capacities in the kinetics results were lower than those in edge tests because of the low As-to-Fe ratio in the experiments, the relative differences among the media were similar to those obtained from the edge data at pH 6.5, suggesting reliable data comparisons. Graphically, 50% of the arsenic sorption capacity at equilibrium for Fe (25%)-diatomite was obtained within 10 h, whereas it occurred within 20 h for AAFS-50 (Figure 4). AAFS-50 (0.094 mmol g-1) had a slightly lower sorption capacity of arsenate than Fe (25%)-diatomite (0.124 mmol g-1). However, it had a low initial sorption rate (5.1 × 10-3 mmol g-1 h-1), which is three times lower than that (16 × 10-3 mmol g-1 h-1) of Fe (25%)-diatomite. This could be due to the fact that diffusion-controlled transport significantly limits the rate of arsenic adsorption for AAFS50. The shrinking-core model was used to fit experimental data, considering external film diffusion and intraparticle diffusion control. Good fits (R2 ) 0.96-0.99) were found by controlling intraparticle diffusion in all experimental trials with Fe (25%)-diatomite and AAFS-50 for both arsenite and arsenate solutions (Figure 4 C,D). This indicates that the ratelimiting step for arsenic adsorption in this study was intraparticle diffusion, which has been explained by fixedVOL. 40, NO. 5, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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arsenic species compared to AAFS-50 in the same column setup because of high sorption capacities and larger effective diffusivities of Fe (25%)-diatomite as shown in other batch tests. More experiments, including large-scale column tests using arsenic-contaminated water, should be conducted to optimize the sorption process and estimate cost benefits of HFO-diatomite. Judging from the preliminary tests in this study, HFO-immobilized diatomite has excellent potential for use as a sorption material for water treatment (e.g., POE/ POU) because of its high removal efficiencies of both arsenite and arsenate species.
Acknowledgments The present research was jointly funded by Industrial and Economic Development Research Program (I&EDR) Grants at the University of Wisconsin-Madison and preliminary research grants from Tyco Inc.
Supporting Information Available
FIGURE 5. Small-scale column tests of Fe (25%)-diatomite and AAFS-50: (A) arsenite and (B) arsenate (pH 6.5; arsenic concentration, 500 µg L-1; medium mass, 1.5 g; EBCT, 3.3 min). bed column tests for various kinds of adsorbates with such as GFH (36, 37), natural manganese oxide (38), and activated alumina (39). Apparent diffusion constants [7.67 × 10-11 cm2 s-1 (arsenite) and 4.26 × 10-11 cm2 s-1 (arsenate)] for Fe (25%)-diatomite are much higher (about 20-50 times) than those for AAFS-50 [0.15 × 10-11 cm2 s-1 (arsenite) and 0.20 × 10-11 cm2 s--1 (arsenate)] for both arsenic species. AAFS-50 is made from granular activated alumina that has amorphous matrixes of aluminum oxide containing bottleneck shapes of micropores or mesopores (40). These shapes of pores can hinder the accessibility of arsenic molecules to the active sites of the medium (40). Small-Scale Column Tests. Column tests were conducted to determine arsenic removal efficiencies for both AAFS-50 and Fe (25%)-diatomite. The bulk densities of the two media were measured to be 0.25 and 1.07 g mL-1 for Fe (25%)diatomite and AAFS-50, respectively. Different flow rates were applied to have the same EBCT (3.3 min) for each medium: 1.8 [Fe (25%)-diatomite] and 0.4 mL min-1 (AAFS-50). Thus, arsenic solution was fed into the column of Fe (25%)diatomite at about a 4.5 times higher flow rate than it was fed into the AAFS-50 column. The arsenic concentrations in the effluents are shown in Figure 5 in terms of bed volumes (BVs) of arsenic solution. For arsenite solution (Figure 5A), Fe (25%)-diatomite treated about 1100 and 2200 BVs at target concentrations of 10 µg L-1 (0.133 µmol L-1) and 50 µg L-1 (0.667 µmol L-1), respectively. These results indicate much better performance than AAFS-50, which treated only about 150 BVs below 50 µg L-1. Figure 5B shows the arsenate removal capacities of both media under the same conditions as the arsenite tests. As revealed in pH-contolled DCBR tests, AAFS50 had a higher removal capacity for arsenate (350 BVs below 50 µg L-1) than arsenite. Fe (25%)-diatomite provided better performances than AAFS-50. About 1100 and 2100 BVs of arsenate solution were treated below 10 and 50 µg L-1, respectively, by Fe (25%)-diatomite. Therefore, Fe (25%)diatomite had similar performances for both arsenite and arsenate in column tests. Although the small-scale column tests were not conducted under typical conditions for drinking water or groundwater in this study, Fe (25%)diatomite showed better performances in removing both 1642
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Surface reaction and equilibrium expression for intrinsic surface complexation of arsenic sorption (Table S1), methodology of pH-controlled DCBR tests (Figure S1), and characterization results [XRD (Figure S2) and SCM (Table S2)]. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review July 30, 2005. Revised manuscript received December 28, 2005. Accepted December 30, 2005. ES051501T
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