Published in J. Environ. Qual. 32:2387-2391 (2003).
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TECHNICAL REPORTS
Waste Management
Sorption of Arsenic from Soil-Washing Leachate by Surfactant-Modified Zeolite
Enid J. Sullivan*,a,
Robert S. Bowmanb and
Irene A. Legiecc
a Los Alamos National Laboratory, RRES Division, MS J599, Los Alamos, NM 87545
b New Mexico Institute of Mining and Technology, Department of Earth and Environmental Science, Socorro, NM 87801
c E.I. DuPont de Nemours & Co., Inc., Jackson Lab, Chambers Works, Deepwater, NJ 08023
* Corresponding author (ejs{at}lanl.gov).
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ABSTRACT
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Post-treatment of leachate from soil-washing remedial actions may be necessary depending on the amounts of dissolved contaminants present. Uptake of arsenic species by surfactant-modified zeolite (SMZ) from a synthetic soil leachate (pH of approximately 12 [NaOH]) was measured as a test of SMZ as a post-treatment sorbent. Batch sorption isotherms were prepared using leachate to SMZ ratios from 40:1 to 4:1, and temperatures of 25 and 15°C. Equilibrium levels of dissolved and total solution arsenic were similar. At each temperature, sorption appeared to reach a plateau or maximum, then decreased at the highest solution concentration, corresponding to the lowest amount of zeolite added (2.5 g). A maximum sorption value of 72.0 mmol of arsenic per kg of SMZ (5400 mg/kg) was observed at 25°C, and 42.1 mmol/kg (3150 mg/kg) at 15°C. Total arsenic recoveries varied from 74 to 125%. Surfactant-modified zeolite removed up to 97% of dissolved organic carbon and decolorized the leachate solutions. Excluding the points for the highest arsenic to SMZ ratio, the sorption isotherms were well described by the linearized form of the Langmuir equation, with coefficients of determination greater than 0.90 at both temperatures. Sorption of arsenic by SMZ is attributed to anion exchange with counterions on the surfactant head groups, and/or partitioning of organic carbon–complexed arsenic into the surfactant bilayer.
Abbreviations: SMZ, surfactant-modified zeolite
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INTRODUCTION
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SURFACTANT-MODIFIED ZEOLITE has been shown to be an effective and economical sorbent for nonpolar organics, inorganic anions, and inorganic cations dissolved in water (Haggerty and Bowman, 1994; Bowman et al., 1995; Li et al., 1998). Due to its ability to sorb this wide range of contaminants and its low unit cost of about $450 per Mg (Bowman et al., 2001), SMZ is an attractive alternative to activated carbon, ion exchange resins, and other common but expensive sorbents. Surfactant-modified zeolite thus has potential use as a sorbent for toxic compounds in subsurface permeable barriers or in ex situ water treatment systems. Oxyanions demonstrated to be sorbed by SMZ include chromate, nitrate, selenate, and sulfate (Bowman et al., 1995; Li et al., 1998).
Soil washing is a one of a class of ex situ remediation treatment technologies that utilize wet classification, mechanical separation, and chemical extraction processes to remove contamination from soil. Ex situ processes can play an important role in remediating contaminant source areas at a site (i.e., media containing high concentrations of contaminants). By-products of soil washing include fluids containing high concentrations of contaminants, fine particulates, and organic carbon. Secondary treatment of the fluid by-products may be necessary (Legiec et al., 1997), and, thus, SMZ was tested as a post-treatment for the soil-washing fluid residues. The sorption efficiency of SMZ was not expected to be compromised by high pH or the presence of organic carbon (Bowman et al., 1995).
Arsenic in small quantities is highly toxic to humans and is an undesirable contaminant in drinking water. Inorganic arsenic speciation is dependent on the redox potential and pH of the environment. The stable oxidation states in natural waters are arsenate, As(V), and arsenite, As(III), with the species H2AsO-4, HAsO2-4, and AsO3-4 becoming progressively dominant as the pH increases under oxidizing conditions (Pourbaix, 1974; Hem, 1992). These anionic forms are highly mobile in many soils and aquifers.
Arsenic sorption to natural zeolites was studied by Elizalde-González et al. (2001a)( b,c, 2002). They used zeolites from a variety of geologic sources to sorb arsenic in both inorganic [As(III) and As(V)] and organic forms. Some zeolites were modified by acid washing or by addition of iron. Sorption of up to 75% of an initial arsenic concentration of 200 µg/L was measured in one study. In another, 98% of H3AsO3 was removed from a 500 µg/L solution at pH 4. Generally, they found that sorption decreased with increasing pH. They proposed an acid-catalyzed sorption mechanism. Manning and Goldberg (1997) studied the adsorption of As(III) onto clay minerals. Clays and zeolites are known to have some similar surface chemical attributes. Up to 90% adsorption of arsenic was found in some cases. Oxidation of As(III) to As(V) was found in solutions at a pH of >9. Enhanced heterogeneous oxidation of As(III) to As(V) at the clay surface was found on illite and kaolinite, resulting in more strongly bound As(V). Inner-sphere surface complexation was proposed as the sorption mechanism. Shevade et al. (2001) studied the removal of arsenic from water using synthetic modified and unmodified zeolites. They used an inorganic modification by addition of Fe2+ or Fe3+ to the zeolite. Up to 100% removal of arsenic from a 5 mg/L solution was found. The arsenic species removed were not specified, however. They proposed surface complexation as the sorption mechanism. No references to surfactant or organic-modified zeolites as sorbents for arsenic were found in the literature.
Using sequential extraction tests on a contaminated soil, Legiec et al. (1997) found that bound arsenic was associated primarily with the soil organic fraction and the iron oxide–manganese fraction. Pierce and Moore (1982) found a similar association. Arsenic can be removed from soils by leaching at either low or high pH; however, the high-pH method has been found to be most efficient (Legiec et al., 1997) for As(V) as HAsO2-4 and AsO3-4. Legiec et al. found that arsenic in high-pH leachate was associated with fine-grained soil solids and organic carbon and thus was present in both soluble and suspended forms. Arsenic associated with organic fractions in the soil would tend to leach under alkaline conditions as humic materials are extracted off of the soils with the strong caustic. In this case, arsenic and humic fractions would be bound together and might present difficulties in recovering arsenic from leachate.
In this study, we used batch tests to determine the ability of SMZ to sorb arsenic from a synthetic alkaline soil leachate. Because much of the arsenic may have been associated with dissolved organic carbon, removal of both the carbon fraction and dissolved arsenate from the leachate was necessary. Surfactant-modified zeolite was tested as a means of accomplishing removal of both inorganic and carbon-associated forms of arsenic in one step.
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MATERIALS AND METHODS
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Materials
The SMZ was prepared from a clinoptilolite zeolite from the St. Cloud mine in Winston, New Mexico. The external cation exchange capacity of the zeolite was determined to be approximately 70 to 90 mmolc/kg using the method of Ming and Dixon (1987). The raw zeolite was treated with hexadecyltrimethylammonium bromide (HDTMA-Br) surfactant to a level of 150 mmolc/kg (42.6 g HDTMA per kg of zeolite). The surfactant forms a stable organophilic coating on the zeolite surface, and at sufficient concentrations forms a bilayer. Further details on the preparation and properties of SMZ may be found in Bowman et al. (2000).
A synthetic soil leachate was prepared using Glenelg channery silt loam (fine-loamy, mixed, semiactive, mesic Typic Hapludults) from Chester County, PA. Five 150-g soil samples were each first leached with 750 mL of 0.1 M NaOH solution (pH > 14) to yield a humate-rich solution. The leachate fractions were combined and diluted to 6 L with deionized water, centrifuged at 8000 x g for 15 min, then spiked with As2O5 to approximately 500 mg/L (6.7 mM) as arsenic. The final leachate was dark brown in color and opaque. After 48 h at 4°C, the pH had decreased to 7.5, and a small amount of brown precipitate had formed. The pH was readjusted to 12.5 with a saturated KOH solution to dissolve the precipitate. The batch studies were begun immediately after the pH adjustment.
Methods
Sorption isotherms were prepared by adding a measured quantity of SMZ (2.5–25 g) to 100 mL of leachate in 500-mL polypropylene centrifuge bottles. Solid (10 g of zeolite in 100 mL of ASTM Type-I water from a Milli-Q system; Millipore, Billerica, MA) and solution (leachate with no zeolite) blanks also were prepared. Each initial condition was prepared in duplicate. Since the goal of the experiments was to evaluate sorption at high arsenic concentrations, a constant volume of solution and constant arsenic concentration were used to maintain a constant ionic strength among the different initial conditions. Varying the SMZ to solution ratio effectively varied the amount of arsenic available for sorption per unit mass of SMZ. Although some investigators have noted a "solids concentration effect" when the solid to solution ratio is varied (e.g., O'Connor and Connolly, 1980; Voice and Weber, 1985), artifacts due to SMZ-derived solutes or colloids were not expected in these high-ionic-strength, alkaline soil extracts.
Samples were shaken at either 15 or 25°C for 24 h (previously shown sufficient to attain sorption equilibrium) and then centrifuged at the same temperature at 19800 x g for 45 min (sufficient to remove colloid-sized particles of >0.1 µm). The supernatant from each sample was decanted and the pH measured. The supernatant and residue in each centrifuge bottle were collected, and both sets of samples were stored at 4°C until analysis.
The supernatants and solids were analyzed using USEPA Method 7060 for dissolved and total arsenic in solution, USEPA Method 7060 with digestion for total arsenic in the solid fraction, and USEPA Method 9060 for total organic carbon in solution (USEPA, 1994). Results were reported as elemental arsenic.
USEPA Method 7060 is a graphite furnace atomic absorption method for arsenic in solutions and solids. The analytical wavelength is 193.7 nm. Both liquid and solid samples are prepared using an acid digestion step to remove the effects of organic compounds before analysis. Typical detection limits are 1 µg/L. Aqueous samples are pre-acidified to a pH of <2, then mixed with 2 mL of 30% H2O2 and sufficient concentrated HNO3 to result in an acid concentration of 1%. The samples are heated at 95°C until digestion is complete and volume is reduced to less than 50 mL. Five milliliters of the digested solution is mixed with 1 mL of 1% nickel nitrate solution, diluted to 10 mL with reagent water, then injected into the furnace. Solid samples are digested from a representative 1 to 2 g wet weight with repeated additions of nitric acid with heat treatment at 95°C, followed by addition of 30% H2O2. Particulates from the digestate are removed by centrifugation or filtration before injection into the furnace.
USEPA Method 9060 for total organic carbon uses a carbonaceous analyzer. This instrument converts the organic carbon in a sample to carbon dioxide by wet chemical oxidation. The CO2 formed is measured by an infrared detector. The amount of CO2 in a sample is directly proportional to the concentration of carbonaceous material in the sample. The samples are pre-acidified with HCl to a pH of <2 to remove inorganic carbonate and to preserve the samples from biological degradation.
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RESULTS AND DISCUSSION
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Final solution pH values of the SMZ-treated leachate samples ranged from 11.8 to 12.6, with the lowest values associated with the samples having the greatest amount of SMZ. The initial solution pH for all samples containing leachate was 12.5. Type-I water equilibrated with SMZ had a final pH of 7.8 to 8.0. Because both the initial and final pH values of the leachate samples were well within the stability range for arsenate (Pourbaix, 1974), it is assumed that all inorganic arsenic measured in the solution and solid phases was in the form of arsenate. All values in the following discussion, table, and figures are expressed as elemental arsenic.
Figure 1
shows a comparison of total versus dissolved arsenic in the leachate samples equilibrated with SMZ. The two sets of analyses show a 1:1 correspondence, suggesting that all of the equilibrated arsenic was in a dissolved form. Therefore, we used our values for dissolved arsenic in determining arsenic sorption. Figure 2
presents sorption isotherms for arsenic in the leachate at 15 and 25°C. The sorbed concentration was calculated from the difference between initial arsenic concentration in solution and the dissolved concentration after treatment with SMZ. At each temperature, sorption appeared to reach a plateau or maximum, and then decreased at the point with the lowest SMZ to solution ratio (corresponding to the lowest amount of zeolite added, 2.5 g). Previous work (Haggerty and Bowman, 1994; Li and Bowman, 1997; Li et al., 1998) has shown that anion sorption by SMZ is well-described by the Langmuir isotherm equation:
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where C* is the equilibrium sorbed concentration, C is the equilibrium solution concentration, 
is a constant related to the binding energy, and ß is the maximum amount of solute that can be sorbed by the solid.
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Fig. 1. Comparison of total versus dissolved arsenic in solution for 25 and 15°C samples. A 1:1 line is shown for reference.
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Fig. 2. Arsenic sorption from leachate by surfactant-modified zeolite (SMZ) at 25 and 15°C. Lines are best-fits of the Langmuir isotherm Eq. [1] to the observed data.
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The linearized form of the Langmuir equation was fitted to the dissolved arsenic data for each temperature, omitting the data for the samples that contained 2.5 g SMZ (see below). The Langmuir fits to the 15 and 25°C data are plotted along with the data in Fig. 2. The data were well described by the Langmuir isotherm, with the coefficient of determination for each set of the linearized data exceeding 0.95 (Table 1).
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Table 1. Langmuir isotherm linear fit coefficients for the 25 and 15°C batch studies. Data from the samples containing 2.5 g of surfactant-modified zeolite (SMZ) have been omitted.
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Assuming that arsenic existed primarily in anionic forms within the pH range of the isotherms, and that exchange sites were initially occupied by Br- and OH-, the exchange reaction between Br- (and/or OH-) and arsenic on SMZ may be written conceptually as:
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The equilibrium constant, K, for this reaction would be:
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where parentheses denote activities. While the conditions of these experiments did not allow determination of activities in the exchanger phase, the differences in sorption between 25 and 15°C do allow conclusions about the enthalpy of the exchange of anionic arsenic species for Br- and/or OH-.
At constant pressure, the simplified van't Hoff equation provides a relationship between equilibrium constants at two different temperatures as long as the temperatures are within 10 to 15°C (Langmuir, 1997):
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where 
H0 is the enthalpy of reaction, R is the gas constant, and T1 and T2 are the temperatures of interest.
Figure 2 and Table 1 show that arsenic was more strongly sorbed and reached a higher sorption maximum at 25°C (T2) than at 15°C (T1). Thus K2 is greater than K1, and, from Eq. [4], H0 of sorption for arsenic is positive under the conditions of the isotherms.
The sorption maxima calculated from the Langmuir plots give estimates of the sorption capacity of SMZ for arsenic from leachate. The calculated maxima are 72 mmol/kg (5400 mg/kg) at 25°C and 42 mmol/kg (3150 mg/kg) at 15°C (Table 1).
The points from the samples containing 2.5 g SMZ yielded anomalously low sorption values (Fig. 2), and were omitted from both sets of data for the Langmuir fits. It is possible that the small ratio of SMZ to solution (2.5 g SMZ to 100 mL leachate) caused errors in the results for these samples. It is also possible that in these SMZ-limited samples the large amount of sorbed organic carbon interfered with arsenic sorption or possibly competed with arsenic sorption sites on the SMZ (see below). One replicate for the 2.5-g SMZ samples in both the 25 and 15°C isotherms actually indicated negative arsenic sorption, at values greater than could be explained by analytical errors. Since neither the blanks in Type-I water nor an elemental analysis of the zeolite showed the presence of arsenic, the reasons for the apparent negative sorption are unclear.
Results of the solid-phase arsenic analysis and the calculated sorbed arsenic amounts from the dissolved solution analyses were generally in agreement (Fig. 3)
. At the two lowest SMZ levels (2.5 and 5 g) at 25°C the apparent amount of arsenic sorbed was significantly less when based on direct solids analysis. This may have been due to incomplete recovery of arsenic from these lower masses of SMZ, with their relatively higher fractions of sorbed organic carbon. Arsenic recoveries at the two temperatures were calculated based on the measured dissolved and solid phase arsenic concentrations. Total recoveries for individual samples varied from 74 to 114% for the 25°C samples and from 88 to 125% for the 15°C samples.
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Fig. 3. Sorbed arsenic from direct solids analysis versus arsenic concentrations calculated from the difference between initial and final solution concentrations.
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Sorption of organic carbon from the leachate by SMZ is plotted in Fig. 4
. Removal of organic carbon was equally effective at 25 and 15°C. Sorption of organic carbon followed a linear sorption isotherm with a partition coefficient (Kd) of 0.027 L/kg for the 25°C data and 0.024 L/kg for the 15°C data. It is known that SMZ is a good sorbent for low molecular weight organic compounds (Neel and Bowman, 1991; Bowman et al., 1995). In addition, it was observed that SMZ effectively decolorized the leachate solutions, with increasing quantities of SMZ providing more color removal.
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Fig. 4. Sorption of dissolved organic carbon by surfactant-modified zeolite (SMZ). The regression line has been forced through zero.
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Analogous to the sorption of other anions such as chromate, nitrate, and sulfate, the predominant mechanism for sorption of arsenate to SMZ is probably anion exchange with surfactant counterions on the external portion of the surfactant bilayer (Bowman et al., 2000; Li et al., 1998). In addition, there may be a small amount of arsenate that can bind near surfactant headgroups on the zeolite surface, as has been noted for chromate (Sullivan et al., 1998). Arsenic that is complexed with soluble or suspended organic carbon may also sorb as part of an organic partitioning process. The removal of color and reduction of solution organic carbon indicates that this partitioning is occurring in proportion to the amount of SMZ, and, thus, the amount of surface-bound surfactant available. This is similar to linear sorption observed for other organic compounds such as PCE (Bowman et al., 1995).
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CONCLUSIONS
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Sorption of arsenic from soil leachate by SMZ was measured and was consistent with anion exchange and partitioning mechanisms. The results of this study illustrate that SMZ can effectively sorb arsenic from high-pH leachate solutions while simultaneously removing the large concentrations of dissolved organic carbon typically generated by alkaline soil washing processes. Sorption was particularly significant in light of the extremely alkaline conditions (pH of approximately 12). In addition to arsenic removal, SMZ would probably remove other anions of concern from soil leachates.
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ACKNOWLEDGMENTS
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This study was supported by U.S. Department of Energy Contract DE-AR21-95-MC32108 through the Federal Energy Technology Center. Brian Campbell, E.I. DuPont de Nemours and Co., Inc., produced the arsenic leachate. The analyses were carried out at Conoco Analytical Laboratories, Ponca City, Oklahoma.
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