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Published online 25 May 2007
Published in J Environ Qual 36:1069-1075 (2007)
DOI: 10.2134/jeq2006.0432
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Waste Management

Removal of Perchlorate from Contaminated Waters Using Surfactant-Modified Zeolite

Pengfei Zhanga,*, David M. Avudzegaa and Robert S. Bowmanb

a Dep. of Earth and Atmospheric Sciences, City College of New York, New York, NY 10031
b Dep. of Earth and Environmental Science, New Mexico Inst. of Mining and Technology, Socorro, NM 87801

* Corresponding author (pzhang{at}sci.ccny.cuny.edu)

Received for publication October 6, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
We investigated the potential of using surfactant (hexadecyltrimethylammonium)-modified zeolite (SMZ) as an inexpensive sorbent for removing perchlorate (ClO4–) from contaminated waters in the presence of competing anions. In batch systems, the presence of 10 mM OH– (i.e., pH 12), CO32–, Cl–, or SO42– had little effect on the sorption of ClO4– by SMZ, indicating that the sorption of ClO4– by SMZ was very selective. The presence of 10 mM NO3–, however, reduced the sorption of ClO4– at low initial concentrations. The maximum sorption capacity for ClO4– by the SMZ remained relatively constant (40–47 mmol kg–1), in the absence or presence of the competing ions. In flow-through systems, ClO4– broke through the SMZ columns much later than other anions present in an artificial ground water. The affinity of the anions for SMZ followed the sequence of ClO4– > > NO3– > SO42– > Cl–. Perchlorate loading under dynamic flow-through conditions was 34 mmol kg–1, somewhat less than the maximum loading of 40 to 47 mmol kg–1 determined by the batch method. Less than 1% of previously sorbed ClO4– was leached out by ultra-pure water, by extraction fluid #1 of the standard toxicity characteristic leaching procedure (TCLP), or by a solution of 0.28 M Na2CO3/0.5 M NaOH. About 40% of the previously sorbed ClO4– was leached out from SMZ by a 0.5 M NO3– solution. The exchange of ClO4– with NO3– corroborated results of the batch tests where NO3– was shown to compete with ClO4– sorption.

Abbreviations: HDTMA, hexadecyltrimethylammonium • PV, pore volume • SMZ, surfactant-modified zeolite


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
PERCHLORATE (ClO4–) is an exceedingly mobile and stable anion found in ground and surface waters in at least 35 states in the USA. (USEPA, 2005). The majority of anthropogenic ClO4– contamination originates from historical disposal of perchlorate salts from rocket-fuel manufacturing facilities or from the demilitarization of weaponry (Urbansky, 1998). At high oral doses, ClO4– interferes with the uptake of iodide in the thyroid gland, thereby affecting the production of thyroid hormones (Clark, 2000). The U.S. Environmental Protection Agency (USEPA) recently adopted the findings of the National Research Council and established an official reference dose (RfD) of 0.0007 mg kg–1 d–1 (drinking water equivalent level of 24.5 µg L–1) (USEPA, 2005). Although the USEPA has not yet set a drinking water standard for ClO4–, many states have already established very low advisory levels (e.g., 5 µg L–1 for New York, 6 µg L–1 for California, and 14 µg L–1 for Arizona) (USEPA, 2005).

Numerous biological, chemical, and physical treatment processes have been developed in recent years to remove ClO4– from contaminated waters. Biological reduction processes (e.g., Frankenberger, 1999; Giblin et al., 2000; Herman and Logan et al., 2001; Brown et al., 2005; Cang et al., 2004; Hatzinger, 2005) are fast and cost-effective, but biological treatment systems for drinking water require careful monitoring and are yet to be broadly accepted by U.S. regulatory agencies and the public. Chemical treatment processes (e.g., chemical reduction with iron metal) are typically sluggish (Moore et al., 2003). However, recent studies demonstrate high reduction rates of ClO4– with iron metal at elevated temperatures (200°C) (Oh et al., 2006) or with nano-scale iron (Cao et al., 2005). Nevertheless, it is currently impractical to apply these latest developments to in situ or ex situ drinking water treatment systems.

Anion exchange using resins with quaternary trialkylammonium groups has been shown to selectively remove low concentrations of ClO4– from contaminated waters (Gu et al., 2000, 2005), and this method appears to be the most promising one for treating ClO4– in drinking water. The high affinity of ClO4– for such resins has made it impractical to regenerate spent resins in the conventional manner using brine solutions. However, recent developments in regeneration using tetrachloroferrate (FeCl4–) (Gu et al., 2001, 2003) may significantly lower the cost of using these types of resins. Very recently, granular activated carbon (GAC) tailored with ammonia (Chen et al., 2005) and GAC tailored with cationic surfactants (with quaternary ammonium groups) (Parette and Cannon, 2005; Parette et al., 2005) have also been developed to remove ClO4– from water.

The goal of the study is to evaluate the potential of using surfactant-modified zeolite (SMZ) as an inexpensive (about $460 per metric ton, or $0.21 per pound [Bowman, 2003]) alternative for ClO4– removal from contaminated waters. Natural zeolites (e.g., clinoptilolite) are hydrated aluminosilicate minerals with high internal/external surface areas and high cation exchange capacities. Many natural zeolites form stable aggregates that can be crushed and sieved to achieve desired hydraulic conductivities. Modification of natural zeolites with quaternary amines such as the cationic surfactant hexadecyltrimethylammonium (HDTMA) results in a sorbent material (i.e., SMZ) that has anion exchange properties. The effectiveness of SMZ for removing a variety of inorganic cations, inorganic anions, and neutral organics from water has been demonstrated in laboratory and field experiments (Bowman et al., 1995; Bowman, 2003). SMZ is readily manufactured using standard particle processing technology and is available commercially in multi-ton quantities (Barker et al., 2004). Among other applications, SMZ has been tested for use in permeable reactive barriers (PRB) to treat contaminated ground water (Bowman et al., 2001).

The objectives of this study are: (i) to determine the sorption of ClO4– by SMZ; (ii) to examine the effects of competing anions (e.g., CO32–, Cl–, SO42–, and NO3–) on ClO4– sorption; (iii) to examine the leachability of sorbed ClO4– from SMZ using various extraction solutions; and (iv) to evaluate ClO4– removal efficiencies under dynamic flow-through conditions in the presence of competing anions.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Surfactant-Modified Zeolite Preparation
A natural clinoptilolite-rich zeolite (from the St. Cloud Mine near Winston, New Mexico) with a size of 8–14 mesh (2.4–1.4 mm) was used in this study. Detailed information on the mineral composition, cation exchange capacity, and surface area of the zeolite can be found in previous studies (Li and Bowman, 1997; Sullivan et al., 1997). The maximum sorption of HDTMA-Br by this batch of zeolite (determined using a batch sorption method [Li and Bowman, 1997]) was 219 ± 4 mmol kg–1, similar to the previously published value of 208 mmol kg–1 with zeolite from the same source (Li and Bowman, 1997).

A small batch of SMZ was prepared using a column method. Two glass columns (15 cm long and 2.5 cm in diameter, Kontes, Vineland, NJ) were filled with raw zeolite (71 ± 1 g each) and modified with 2 L of 40 mM HDTMA-Br solution at a flow rate of 3.0 mL min–1 using a peristaltic pump (Masterflex L/S, Cole-Parmer, Vernon Hills, IL) fitted with Tygon tubing. The columns were then rinsed with degassed Milli-Q (Millipore Corp., Billerica, MA) water at 3.0 mL min–1 for 4.5 pore volumes (PVs), and the treated zeolite was air-dried overnight in a fume hood. The influent, effluent, and the rinse solutions were properly diluted and analyzed for HDTMA using a high performance liquid chromatography (HPLC) method, described below. The HDTMA-Br loading for the SMZ prepared by this column method was 201 ± 5 mmol kg–1, close to the loading of 219 mmol kg–1 determined with the batch method.

Batch Sorption Experiments
One liter of 10.06 mM (1000.0 mg L–1) ClO4– stock solution was made with sodium perchlorate (reagent grade, Acros Organics, Morris Plains, NJ) and the following concentrations were prepared from the stock solution: 1.01, 1.26, 1.51, 2.01, 2.51, 3.02, 3.52, 3.77, and 4.02 mM (corresponding to 100.0, 125.0, 150.0, 200.0, 250.0, 300.0, 350.0, 375.0, and 400.0 mg L–1, respectively). One gram of the SMZ was weighed into a 50-mL polypropylene centrifuge tube (Corning, Acton, MA) and 12 mL of ClO4– solution of a particular concentration was added. Preliminary tests showed no sorption of ClO4– by the polypropylene tubes. Duplicates were prepared for each of the nine initial ClO4– concentrations. The samples were shaken on an orbital shaker at 150 rpm and room temperature for 24 h, and then centrifuged at 4000 rpm (1717 g) at 25°C for 30 min. Ten milliliters of supernatant was removed for pH and chemical analyses.

To examine the effects of competing anions (e.g., CO32–, Cl–, SO42–, and NO3–, which often coexist with ClO4– in contaminated waters) on ClO4– sorption by SMZ, batch sorption experiments were repeated in the presence of 10.0 mM solutions of CO32–, Cl–, SO42–, or NO3–. This concentration (10.0 mM) was 2.5 to 10 times higher than the initial ClO4– concentrations (1.0 to 4.0 mM). Batch sorption experiments were also conducted at a higher pH (pH of 12), in the absence of other competing ions, to examine the impact of high pH on ClO4– sorption by SMZ. Again, duplicate samples were prepared for each of the nine initial ClO4– concentrations.

Leaching Tests
Four different types of extraction solutions were used to examine the leachability of sorbed ClO4– from SMZ: (i) ultra-pure Milli-Q water; (ii) extraction fluid #1 of the standard toxicity characteristic leaching procedure (TCLP); (iii) a 0.28 M Na2CO3/0.5-M NaOH solution; and (iv) a 0.5 M NaNO3 solution. The TCLP extraction fluid #1 is a 0.1 M HAc solution adjusted to a pH of 4.9 with NaOH (USEPA, 1992). The 0.28 M Na2CO3/0.5 M NaOH solution has been shown to effectively extract chromate (CrO42–) from soils and from SMZ (James et al., 1995; Li, 1998). Batch tests showed competition of NO3– with ClO4– sorption by SMZ and thus the 0.5 M NO3– solution was used to examine further the leachability of ClO4–. The leaching tests were conducted using the same batch method described above. Briefly, centrifuge tubes each containing 1 g of SMZ and 12.0 mL of 4.0 mM (400 mg L–1) ClO4– solution were shaken for 24 h, centrifuged, and the supernatant was removed for analysis. Twenty milliliters of the leaching solution was then added (to maintain a solution to solid ratio of 20:1 according to the standard TCLP [USEPA, 1992]) and analyzed for ClO4– after overnight shaking. Triplicates were prepared for each leaching solution.

Transport Experiments
Flow-through column experiments were conducted to provide a more realistic simulation of dynamic field conditions and to quantify the sorption of ClO4– in the SMZ columns. The SMZ was prepared by the column method described above, except that the SMZ remained in the columns after modification. The packed SMZ columns had a PV of 44 mL, a porosity of 0.59, and a bulk density of 1.06 g cm–3. Perchlorate solution (0.10 mM, or 10.0 mg L–1) was prepared with simulated ground water (3 mM NaHCO3, 1 mM CaCl2, 0.5 mM MgCl2, 0.5 mM Na2SO4, and 0.5 mM KNO3 [Gu et al., 2000]) and was injected at the bottom of the columns at a linear velocity of 2.0 m d–1 (corresponding to a residence time of 1.8 h) until full breakthrough was observed. Effluent samples were collected periodically using 10-mL plastic syringes and analyzed via ion chromatography (IC), described below.

Chemical Analyses
The HDTMA was analyzed via a Dionex Summit HPLC consisting of a P680 gradient pump and a UVD 170U UV absorbance detector (Dionex, Sunnyvale, CA). A Nucleosil CN analytical column (150 x 4.6 mm) and a Nucleosil CN guard column (10 x 4.6 mm), both from Supelco (Bellefonte, PA), were used for the analysis. The mobile phase consisted of 5 mM p-toluenesulfonate and methanol (45:55, v/v) (Li and Bowman, 1997), the detection wavelength was 254 nm, and the flow rate was 1 mL min–1. With a 20-µL sample loop, the detection limit was 0.05 mM and the linear response range was up to 3.0 mM. The detection limit was 0.01 mM with a 100-µL sample loop.

Perchlorate was analyzed using IC with suppressed conductivity detection according to USEPA Method 314.0 (USEPA, 1999) and Dionex Application Update 145 (Dionex, 2003). The ICS-2500 IC consisted of a model GS50 gradient pump, a CD25 conductivity detector, an LC25 oven, and an AS40 autosampler (Dionex, Sunnyvale, CA). An IonPac AS16 analytical column (2 x 250 mm) and an IonPac AG16 guard column (2 x 50 mm), both from Dionex, were used for the analysis. The flow rate was 0.25 mL min–1, the mobile phase was 35 mM NaOH, and the oven temperature was 30°C. The conductivity of the mobile phase was suppressed via an ASRS-ULTRA (2-mm) anion self-regenerating suppressor (Dionex). A 25-µL sample loop was used for high ClO4– concentrations (100 µg L–1–10 mg L–1), and a 1000-µL sample loop was used for low ClO4– concentrations (1–100 µg L–1). Calibration standards were prepared from a 1000 mg L–1 ClO4– standard solution obtained from Spex CertiPrep (Metuchen, NJ). The correlation coefficients for all the calibration curves were greater than 0.999. All aqueous samples were filtered through 0.2-µm IC filters (Nalgene, Rochester, NY) before injection.

To examine potential interference of HDTMA on ClO4– analysis via IC, aliquots of the ClO4– standards were spiked with HDTMA-Br (final HDTMA concentrations of 0.05 and 0.02 mM) and analyzed.

Other anions (ClO3–, ClO2–, Cl–, SO42–, and NO3–) were analyzed using the same IC with an IonPac AS14 analytical column (3 x 150 mm) and an IonPac AG14 guard column (3 x 50 mm), both from Dionex. The mobile phase was composed of 8.0 mM Na2CO3 and 1.0 mM NaHCO3, the flow rate was 0.5 mL min–1, and the oven temperature was 30°C.

Data Analyses
Batch sorption of ClO4– by SMZ was modeled using the Langmuir isotherm:

Formula 1[1]
where S is the amount sorbed on solid phase at equilibrium (mmol kg–1), C is the equilibrium concentration in aqueous phase (mmol L–1), Sm is the sorption capacity (mmol kg–1), and KL is the Langmuir coefficient (L mmol–1) that reflects the affinity of the sorbate onto the sorbent (Hunter, 1993). At low equilibrium concentrations (i.e., KLC < < 1), Eq. [1] reduces to a linear sorption isotherm:

Formula 2[2]
where Kd is the distribution coefficient (L kg–1). Equation [1] can be rearranged to give the linearized form of the Langmuir sorption isotherm:

Formula 3[3]

Column transport data were modeled using CXTFIT 2.1, which uses the following advection-dispersion equation to describe the one-dimensional transport of a solute subject to equilibrium sorption (Toride et al., 1995):

Formula 4[4]
where C is the flux averaged concentration, D is the dispersion coefficient, v is the average linear velocity, t is the travel time, x is the travel distance, and R is the retardation factor defined as

Formula 5[5]
where {rho}b is the bulk density and {theta} is the porosity. Equation [5] assumes a linear sorption isotherm, which is valid for the ClO4– concentration used in the transport experiment (0.1 mM). Batch sorption experiments indicated that the sorption of ClO4– by SMZ was linear up to an initial concentration of 1.0 mM. Perchlorate breakthrough data were fitted using CXTFIT and R and D values were determined. The Kd value was then calculated using Eq. [5].


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Interference of Hexadecyltrimethylammonium on Perchlorate Analysis
The measured ClO4– concentrations in the presence of HDTMA were plotted against those in the absence of HDTMA (Fig. 1 ). The range of ClO4– concentrations tested here (0.01–0.15 mM, or 1–15 mg L–1) was the calibration range used for most samples. The presence of 0.05 and 0.02 mM HDTMA lowered the apparent ClO4– concentrations by about 15 and 6%, respectively (Fig. 1). The interference is likely due to the complexation of some ClO4– ions with HDTMA molecules and the inability of the conductivity detector to detect the ion pairs. As such, the extent of interference would be less as the ratio of ClO4– to HDTMA increases.


Figure 1
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  		Fig. 1. Effect of hexadecyltrimethylammonium (HDTMA) on apparent perchlorate concentration determined using ion chromatography (IC). The presence of 0.05 and 0.02 mM HDTMA lowered the apparent perchlorate concentration by approximately 15 and 6%, respectively.

 
The presence of anions in solution tends to stabilize the surfactant bilayer on the zeolite surface. In the batch sorption experiments, the HDTMA concentrations were below 0.021 mM when 10 mM of competing anion was present. In test tubes without competing anions (ClO4– only experiment), a maximum HDTMA concentration of 0.137 mM was observed. After sorption, most samples were diluted by at least a factor of four before ClO4– analysis, leading to very low HDTMA concentrations. In the column experiment, the HDTMA concentration decreased from 0.22 mM at 0 PV to 0.05 mM at about 160 PVs, and to 0.02 mM at about 275 PVs. Significant perchlorate breakthrough did not occur until 400 PVs (see section "Transport Experiment" below). Therefore, the potential interference of HDTMA on ClO4– analyses in this study was considered minimal.

Batch Perchlorate Sorption by Surfactant-Modified Zeolite
The sorption isotherms for ClO4– alone as well as those with the various competing ions are presented in Fig. 2 . The sorption of ClO4– by SMZ is an anion exchange process and was well described by the Langmuir isotherm, with coefficients of determination (r2) exceeding 0.99 (Table 1) when the data were plotted using the linearized form of the isotherm (Eq. [3]). The presence of 10 mM OH– (pH 12 solution), CO32–, Cl–, or SO42– appeared to cause little change in the sorption isotherms (i.e., the isotherms were essentially identical within experimental error, Fig. 2A–2D). This suggested that the sorption of ClO4– by the SMZ was very selective, and the presence of these competing ions at concentrations 2.5 to 10 times higher than the ClO4– concentration had little effect on the sorption of ClO4– by the SMZ. In contrast, the presence of 10 mM NO3– changed the ClO4– sorption isotherm significantly, especially at lower initial ClO4– concentrations (i.e., initial NO3– to ClO4– ratio > 3, Fig. 2E), suggesting that high concentrations of NO3– would compete with ClO4– for sorption on SMZ.


Figure 2
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  		Fig. 2. Sorption isotherms of perchlorate on surfactant-modified zeolite (SMZ) in the presence of: (A) 10 mM OH– (pH 12); (B) 10 mM CO32–; (C) 10 mM Cl–, (D) 10 mM SO42–, and (E) 10 mM NO3–. The sorption isotherm of perchlorate in the absence of the competing ion (open symbols and dashed line) is also presented in each panel for comparison purposes.

 

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  		Table 1. Maximum perchlorate sorption capacity (Sm) and Langmuir coefficient (KL) determined using the linearized form of Langmuir isotherm.

 
The sorption maximum for ClO4– in the absence of competing ions was 40.0 mmol kg–1. The maximum sorption of ClO4– by SMZ did not decrease when the competing ions were present (41.2–46.8 mmol kg–1, Fig. 2 and Table 1). The maximum sorption of ClO4– by SMZ (40–47 mmol kg–1) was about an order of magnitude less than the maximum sorption of ClO4– by strong-base anion (SBA) exchange resins (about 550 mmol kg–1) (Gu et al., 2005). However, the cost of the SMZ (about $0.50 kg–1) is about an order of magnitude lower than the cost of typical anion exchange resins (about $5–25 kg–1). Therefore, the use of SMZ to treat ClO4––contaminated water would likely be as cost-effective as the use of resins. It is noted that the difference in the maximum sorption capacity between the SMZ and the SBA resins could be attributed to the difference in anion exchange capacities (AECs). For instance, the AEC for the resins was in the range of 2000 to 4000 meq kg–1 (Gu et al., 2005), whereas the AEC for this batch of the SMZ was about 100 meq kg–1. When the maximum sorption of ClO4– was normalized to AEC, the SMZ (1 ClO4– per 2.5 exchange sites) and the SBA resins (about 1 ClO4– per five exchange sites) yielded similar results.

The maximum sorption of ClO4– by this batch of SMZ was an order of magnitude less than that that of the surfactant-tailored activated carbon developed by Cannon and colleagues (about 377 mmol kg–1, or roughly 1 ClO4– per exchange site) (Parette, 2005). However, the particle size of the HDTMA-tailored activated carbon (200–400 mesh, or 0.074–0.037 mm) was much smaller than the size of the SMZ used here (8–14 mesh, or 2.4–1.4 mm).

Leaching Tests
A very small amount (<1%) of the previously sorbed ClO4– was leached out by ultra-pure water, TCLP extraction fluid #1 (0.1 M acetic acid adjusted to pH of 4.9), or a pH 14, 0.28 M Na2CO3/0.5 M NaOH solution (Table 2), demonstrating the stability of the sorbed ClO4– over a wide pH range. The lack of exchange of ClO4– with high concentrations of CO32– and OH– (two orders of magnitude higher than the ClO4– concentration) is consistent with the batch sorption tests that showed little competition of these two ions with ClO4– (Fig. 2A and 2B).


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  		Table 2. Amount of previously sorbed perchlorate leached from surfactant-modified zeolite (SMZ) by various leaching fluids.

 
About 40% of the previously sorbed ClO4– was leached out from SMZ by the 0.5 M NO3– solution. The exchange of ClO4– with NO3– agreed with the results from the batch sorption tests where NO3– was shown to compete with ClO4– sorption. While the competition of NO3– with ClO4– may be a concern during ClO4– removal by SMZ, this competition may provide a means for regeneration of spent SMZ (saturated with ClO4–) using a high concentration NO3– solution (Zhang and Pathan, 2007). The resulting concentrate of ClO4– and NO3– could potentially be treated by microbes capable of simultaneous degradation of the two anions (Herman and Frankenberger, 1999; Cang et al., 2004).

Transport Experiments
The breakthrough curves (BTCs) of ClO4– and other anions (Cl–, SO42–, and NO3–) in the artificial ground water from the two duplicate SMZ columns are shown in Fig. 3 . Bicarbonate (HCO3–) could not be analyzed by IC and therefore was not monitored. Chloride (Cl–) broke through the columns first, followed by SO42– and NO3–, and the time to reach full breakthrough was about 10, 70, and 130 PVs for Cl–, SO42–, and NO3–, respectively (Fig. 3A). Perchlorate broke through the columns much later (after 400 PVs) than these competing ions, and did not reach full breakthrough until about 700 PVs (Fig. 3B). The order of anion breakthrough clearly demonstrated the preferential sorption of ClO4– by SMZ, i.e., the affinity of these anions to quaternary trimethyl ammonium followed the sequence of ClO4– > > NO3– > SO42– > Cl–. The same affinity sequence of NO3– > SO42– > Cl– was observed by Tao and Zhou (1988) for resins with quaternary ammonium functional groups, while a slightly different affinity sequence of NO3– > Cl– > SO42– was observed by others (Gu et al., 2004) for similar resins.


Figure 3
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  		Fig. 3. Breakthrough curves of perchlorate and other anions present in the (A) artificial ground water and (B) elution profile of hexadecyltrimethylammonium (HDTMA). HDTMA concentration decreased to less than 0.02 mM at about 275 PVs.

 
The retardation factor for ClO4– (determined from inverse modeling of the BTCs) was 676 for Column #1 and 581 for Column #2. The corresponding Kd values were 376 and 323 L kg–1 for the two columns, somewhat less than the Kd value (478 L kg–1) derived from the linear part of the Langmuir isotherm (first three initial concentrations of the ClO4––alone experiment, Fig. 2). The smaller Kd values from the column experiments could be due to the presence of the competing ions, lack of equilibrium in the column system (1.8 h residence time) vs. the batch system (24 h equilibration time), and/or desorption of HDTMA and hence lowered anion exchange capacity. A total of about 2.7 mmol of ClO4– was removed by each SMZ column, corresponding to a loading of about 34 mmol ClO4– per kilogram of SMZ. This loading is less than the maximum ClO4– loading of 40 to 47 mmol kg–1 determined by the batch tests (Table 1). The difference in maximum loading could again be attributed to desorption of HDTMA during the column experiments. As much as 7% of the sorbed HDTMA was removed from SMZ by the artificial ground water at the end of the experiments. The HDTMA is preferentially removed from the outer layer of SMZ bilayer (Li et al., 2003); each mmol of HDTMA removed would be expected to lower the SMZ anion exchange capacity by 1 mmol.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Surfactant-modified zeolite was very effective in selectively removing ClO4– from water in the presence of competing ions. The sorbed ClO4– was stable against leaching by a variety of fluids over a wide pH range (5–14). The affinity of various anions for SMZ determined by this study may be relevant to the sorption and stability of ClO4– and other anions to other sorbent materials with quaternary amine functional groups.

Perchlorate-saturated SMZ potentially could be regenerated by leaching with a concentrated nitrate solution (Zhang and Pathan, 2007) followed by simultaneous biodegradation of the two anions in the leachate. Alternatively, the bound ClO4– could be destroyed by thermal treatment; common perchlorate salts decompose in the temperature range 270 to 500°C (Lide, 1992). Incineration at these temperatures would also pyrolyze the bound HDTMA (Sullivan et al., 1997) and possibly cause structural changes in the zeolite (Bish and Carey, 2001). The spent zeolite material would be nonhazardous and could be landfilled.

In addition to its anion exchange capacity, SMZ also has high sorption capacity for neutral organic contaminants due to the partitioning of organics into the hydrophobic surfactant tails (Bowman et al., 1995; Li and Bowman, 1998; Ranck et al., 2005). The low cost of the SMZ material and the high affinity of SMZ for ClO4– and other organic contaminants makes SMZ an attractive alternative for treating wastes contaminated with products of explosives production and use.


   ACKNOWLEDGMENTS
 
This work was funded in part by grants from the PSC-CUNY, New Mexico WERC, and the K.C. Wong Education Foundation, Hong Kong.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
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 CONCLUSIONS
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