Journal of Environmental Quality 31:1106-1114 (2002)
© 2002 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORTS
Heavy Metals in the Environment
Influence of Quaternary Ammonium on Sorption of Selected Metal Cations onto Clinoptilolite Zeolite
Zhaohui Li*,a,
Daniel Alessia and
Lori Allenb
a Chemistry Dep. and Geology Dep., Univ. of Wisconsin-Parkside, 900 Wood Road, Kenosha, WI 53141
b Chemistry Dep., Univ. of Wisconsin-Parkside, 900 Wood Road, Kenosha, WI 53141
* Corresponding author (li{at}uwp.edu)
Received for publication December 15, 2000.
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ABSTRACT
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Clay minerals and zeolites have large cation exchange capacities, which enable them to be modified by cationic surfactant to enhance their sorption of organic and anionic contaminants. In this study, the influence of quaternary ammonium surfactants on sorption of five metal cations (Cs+, Sr2+, La3+, Pb2+, and Zn2+) onto a clinoptilolite zeolite was investigated. Generally, the metal cation sorption capacity and affinity for the zeolite decreased, indicating that presorbed cationic surfactants blocked sorption sites for metal cations, as the surfactant loading on the zeolite increased. Cesium and Pb2+ sorption was affected to a small extent, indicating that selective sorption for Cs+ and specific sorption for Pb2+ play an important role in addition to cation exchange. Sorption of cationic surfactants on zeolite preloaded with different metal cations showed a strong correlation with the chain length of the surfactant tail group, while the roles of the charges and types of the metal cations were minimal. As the chain length increases, the critical micelle concentration decreases and the surfactant molecules become more hydrophobic, resulting in progressive bilayer coverage. Desorption of presorbed metal cations by cationic surfactants was strongly affected by the surfactant chain length and metal type. More metal cations, particularly Sr2+ and Zn2+, desorbed with an increase in surfactant chain length. The results, in combination with those from organic and oxyanion sorption on surfactant-modified zeolite, may be used for future surfactant modification to target sorption and desorption of a specific type of contaminant or a mixture of different types of contaminants.
Abbreviations: AA, atomic absorption • CEC, cation exchange capacity • CMC, critical micelle concentration • DDTMA, dodecyltrimethylammonium • ECEC, external cation exchange capacity • HDTMA, hexadecyltrimethylammonium • HPLC, high performance liquid chromatography • OTMA, octyltrimethylammonium • SMZ, surfactant-modified zeolite
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INTRODUCTION
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STUDIES of contaminant sorption onto soils and natural sediments and transport of contaminants in surface and ground water have drawn great interest since the 1970s, when several environmental laws and regulations were first introduced. Results showed that the uptake of nonionic organic compounds from water by soils and sediments was primarily due to partitioning of organic molecules into the soil organic phase (Chiou et al., 1983) and the sorption of hydrophobic contaminants by soils and sediments from solution was a function of total organic carbon content of the soils and sediments (Karickhoff et al., 1979). These studies indicated that materials having a high organic carbon content could be used as sorbents to retard the movement of dissolved organic contaminants. Minerals are normally low in organic carbon content, and are not suitable as sorbents even though they are relatively inexpensive. On the other hand, some minerals, such as clays and zeolites, have permanent negative charges on their surfaces, enabling them to be modified by cationic surfactants. Such modification results in a significant increase in total organic carbon content of the minerals and drastically raises their sorption capacity for organic compounds. The use of surfactant-modified clay minerals (often called organoclays) to remove contaminants from water was proposed by Boyd et al. (1988). Since then, more studies have been conducted to determine the effect of chain length on quaternary ammonium uptake by clays and on subsequent contaminant sorption by the organoclays (Zhang et al., 1993) and the influence of surfactant surface configuration on the sorption of different types of organic contaminants (e.g., cationic, anionic, and nonionic) (Li et al., 2000).
The studies on use of surfactant-modified clay minerals and zeolites for environmental remediation were limited to removal of organic contaminants from water until Haggerty and Bowman (1994) showed that surfactant-modified zeolite (SMZ) significantly increased the sorption of chromate. The sorption of chromate was attributed to anion exchange on the outermost surface created by the sorbed surfactant bilayer (Li et al., 1998). Further research has revealed that clay minerals could also be modified by cationic surfactant to increase their uptake of anionic contaminants and that the same mechanism that governs chromate sorption on SMZ dominates chromate sorption by surfactant-modified clay minerals (Li and Bowman, 1998).
Zeolites are hydrated aluminosilicates that have a unique three-dimensional cage-like structure, with channel apertures on the order of a nanometer. These channels are accessible to small inorganic cations, but not the head group of cationic surfactants. The modification of clay minerals and zeolites by cationic surfactant utilizes their high cation exchange capacities and permanent negative charges. Due to sorption of cationic surfactant, desorption of metal cations is inevitable. Bouchard et al. (1988) studied the sorption of ethylhexadecyldimethylammonium, a cationic surfactant, on two aquifer soils and the desorption of metal cations from the aquifer soils due to the sorption of the surfactant. At surfactant sorption of 25 to 50% of the soils' cation exchange capacity (CEC), a stoichiometric relationship was found between the amount of surfactant sorbed and that of metal cation desorbed, indicating that cation exchange was the main mechanism for surfactant sorption. Hayes et al. (1995) studied the effects of quaternary ammonium surfactants on the sorption of trace metals onto quartz, kaolinite, and montmorillonite. Their experiments were performed in such a way that metals (Co2+ and Sr2+) and minerals were mixed for 24 h before surfactants were introduced and then the system was re-shaken for another 24 h. Therefore, their studies dealt merely with desorption of presorbed metal cations by surfactant or the competition of surfactant against Co2+ and Sr2+ for the sorption sites. Brusseau et al. (1997) found that cyclodextrin, a compound that can solubilize small, slightly polar organic compounds and form cationic heavy metal complexes, greatly enhanced removal of Cd, Ni, and Sr from soil. Malakul et al. (1998) studied metal sorption and desorption by surfactant-modified clay complexes composed of anchoring surfactant molecules sorbed on the clay surface by cation exchange and metal-chelating ligands retained by hydrophobic interaction with the sorbed surfactant molecules. Their results showed that the metal sorption capacity increased by an order of magnitude after the clays were modified by these two types of organics sequentially, but that Cd sorption capacity was totally lost when montmorillonite was modified with only the anchoring long chain cationic surfactant (Malakul et al., 1998). Sorption of Cs+ and Sr2+ on untreated and hexadecylpyridinium chloride–treated vermiculites revealed that when small amounts of Cs+ and Sr2+ were present, distribution coefficients for modified vermiculites were significantly reduced compared with untreated clay, but the difference became less when larger amounts of cations were sorbed (Bors et al., 1997). Sorption of Pb was not greatly affected in the presence of 0.5% linear alkylate sulfonate, but Pb sorption decreased at a higher surfactant concentration (Cajuste et al., 1996).
This study aims at assessing the effect of cationic surfactants on the sorption of metal cations onto and desorption of metal cations from a clinoptilolite zeolite as a function of surfactant loading and surfactant chain length. Due to the same mechanism governing the sorption of metal cations and cationic surfactants on clays and zeolites, we hypothesized that the competition between these two types of cations for the same sorption sites would adversely affect the uptake of metal cations, and thus limit usage of surfactant-modified zeolite, particularly if the contaminated waters have a significant amount of heavy metals. On the other hand, if the surfactant molecules have higher affinity than the metal cations for clay and zeolite surfaces, the desorption of metal cations from zeolite or clay mineral surfaces due to the sorption of cationic surfactant may present a new approach for removing heavy metals from clay and soil surfaces. The metals studied were Cs+, Sr2+, La3+, Pb2+, and Zn2+. There are several reasons to choose these five metals. First, the selected metals are representative of the full range of metallic chemical behavior by including Cs, an alkali metal, Sr, an alkaline earth, Pb and Zn, transition metals, and La3+, a lanthanide. Second, the metals are representative of monovalent, divalent, and trivalent metals. Third, Cs+ and Sr2+ can exist as hazardous radionuclides that can be removed from water by cation exchange using zeolite while Pb2+ and Zn2+ are common heavy metals in the environment. Thus, the results, in addition to those from studies using SMZ to remove organic (Li et al., 2000) and inorganic anions (Li et al., 1998), may provide a valuable guide to surfactant modification of zeolite for specific metal contaminant removal needs.
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EXPERIMENTS
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Zeolite
The zeolite was obtained from the St. Cloud Mining Co., Winston, NM, with particle size of aggregates in the range of 0.43 to 0.83 mm. It contains 74% clinoptilolite, 12% feldspar, 12% quartz plus cristobalite, with traces of clay minerals (Sullivan et al., 1997). It has K+ and Ca2+ as the major exchangeable cations with an internal exchange capacity of 23 cmol/kg and an external cation exchange capacity (ECEC) of 11 cmol/kg (Li and Bowman, 1997).
Surfactant Modification of the Zeolite
Previous results showed that the maximum loading level of hexadecyltrimethylammonium (HDTMA) on zeolite was 200 mmol/kg, corresponding to a surfactant bilayer surface coverage of about 200% of the ECEC. At this surface coverage, sorption of chromate by SMZ reached a maximum (Li et al., 1998). To study the influence of HDTMA on metal cation sorption, the zeolite was modified by HDTMA (Aldrich, Milwaukee, WI) to 25, 50, 75, 100, 150, and 200% of its ECEC. To each 250-mL centrifuge bottle, 60 g of raw zeolite and 180 mL of HDTMA (8.5, 17, 25, 33, 50, and 67 mmol/L) were mixed on a shaker table (150 rpm) at room temperature for 8 h. This was a sufficient amount of time to obtain maximum HDTMA sorption (Li and Bowman, 1997). The mixture was centrifuged, washed with two portions of deionized water, and then air-dried. The supernatants, after equilibration with HDTMA, and each wash were analyzed by high performance liquid chromatography (HPLC) (Waters [Mildord, MA] 510 pump, Waters 994 tunable UV-Visible detector, and Waters 717 autoinjector; or Alltech [Deerfield, IL] Model 426 pump and Model 200 tunable UV-Visible detector). The HDTMA concentrations at the lowest surface coverage were below the detection limit of 0.03 mmol/L and at the high surface coverage were less than 1% of the input concentration, resulting in final HDTMA loadings of about 25, 50, 75, 100, 150, and 200 mmol/kg.
Metal Cation Sorption on Surfactant-Modified Zeolite
In each 40-mL polyallomer centrifuge tube, 2.00 g of zeolite or SMZ and 20.0 mL of aqueous solution of CsCl, SrCl2·6H2O, LaCl3·7H2O, ZnBr2 (Fisher Scientific, Pittsburgh, PA), or PbCl2 (Merck, Whitehouse Station, NJ) with initial concentrations ranging from 0 to 4.0 mM were combined and shaken at 150 rpm at room temperature for 24 h. The samples were centrifuged at 5000 rpm for 20 min and the supernatants were analyzed for equilibrium metal concentration by atomic absorption (AA) (Thermo Jarrell Ash, Franklin, MA) or inductively coupled plasma (ICP) (PerkinElmer [Norwalk, CT] Model Plasma 400). For AA analysis, air–acetylene was the fuel for Pb2+, Zn2+, and Cs+ analysis while nitrous oxide was the fuel for Sr2+ analysis. Proper dilution was made to bring the final concentration within the linear range of AA or ICP responses.
Metal Sorption on Zeolite
To further test the influence of different types and charges of metal cations on sorption of cationic surfactants onto zeolite and the desorption behavior of metal cations by cationic surfactants, the zeolite was preloaded with each cation. In each 250-mL bottle, 40 g of raw zeolite and 160 mL of 10 mM metal solution were mixed on a shaker at 150 rpm and room temperature for 24 h. The mixtures were centrifuged, washed with two portions of deionized water, and then air-dried. The metal concentrations in equilibrium with the zeolite and in each wash solution were analyzed by AA. The amounts of metal loading are 39, 25, 37, and 28 mmol/kg for Cs+, Sr2+, Pb2+, and Zn2+, respectively.
Sorption of Surfactant and Desorption of Metal Cations
To each 40-mL polyallomer centrifuge tube, 2.10 g of metal-loaded zeolite and 21.0 mL of HDTMA, dodecyltrimethylammonium (DDTMA) (Acros, Pittsburgh, PA), or octyltrimethylammonium (OTMA) (Fluka, Ronkonkoma, NY) aqueous solutions were combined. The initial concentrations were 4, 8, 12, 16, 20, 24, and 28 mM for HDTMA and DDTMA, and 1, 3, 6, 9, 12, 15, 18, and 21 mM for OTMA. Each mixture was shaken (150 rpm) at room temperature for 24 h and then centrifuged at 5000 rpm for 20 min. The supernatants were analyzed for HDTMA, DDTMA, OTMA, and counterion bromide concentrations by HPLC (Li and Bowman, 1997) and for metal cation concentrations using AA. The equilibrium pH values measured with an Orion (Beverly, MA) Model 710A pH meter were 7.0 to 8.0, 6.5 to 7.3, 5.7 to 6.1, 6.0 to 7.2, and 6.0 to 6.4 for Cs, Sr, La, Pb, and Zn, respectively.
All samples were prepared in duplicate. All calibrations were prepared using four to six standards covering the concentration ranges. Linear coefficients of determination (r2) for the standard curves were no less than 0.999 for HPLC analysis and 0.99 for AA analysis.
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RESULTS AND DISCUSSION
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Previous results showed that sorption of HDTMA on zeolite followed a typical Langmuir-type isotherm (Li and Bowman, 1997; Li et al., 1998):
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where CS is the amount of solute sorbed, CL the equilibrium solution concentration, Cm the solute sorption maximum, and KL the Langmuir sorption coefficient reflecting the affinity of the solute for the surface. The Langmuir isotherm has been used extensively to fit observed data of Sr2+ and Pb2+ sorption on raw and SMZ (Bowman et al., 2000), Zn2+ sorption on natural bentonite and soils (Mellah and Chegrouche, 1997; Prasad et al., 1997; Shuman, 1999), Pb2+ sorption on soils (Cajuste et al., 1996), La3+ sorption on soils (Stokes et al., 1999), and Cs+ sorption on soils (Campbell and Davies, 1995). We think that zeolite has a finite cation exchange capacity fulfilling the requirement of fitting observed data to the Langmuir isotherm. Therefore, the sorption data of the metal cations and the quaternary ammonium surfactants were fitted to Langmuir isotherms in this study.
Metal Sorption on Surfactant-Modified Zeolite and Surfactant Desorption from Surfactant- Modified Zeolite
The results of metal sorption on zeolite modified with different levels of HDTMA are plotted in Fig. 1
, while the fitted parameters of their Langmuir isotherms are listed in Table 1
. Desorption of HDTMA by metal cations at an initial concentration of 4 mM can be seen in Table 2
. It is well known that this zeolite sorbs Cs preferentially over trivalent, divalent, and other monovalent cations due in part to the geometry of the zeolite pores. Based on published research the selectivity of clinoptilolite is predicted to be: Cs >> Pb > Zn > La 
Sr (Ouki and Kavanagh, 1997; Mumpton, 1999). The selectivity of the zeolite for the five metal cations before the surfactant modification followed: Cs > Pb > La > Zn > Sr. At 100 and 200% surface coverages the selectivity was about the same (i.e., Cs > Pb > Zn > La Sr). The same selectivity sequence ensures that simultaneous use of surfactant and zeolite for treatment of wastewater will not alter the water chemistry.
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Fig. 1. Sorption of (a) Cs+, (b) Sr2+, (c) La3+, (d) Zn2+, and (e) Pb2+ on raw zeolite ( ), hexadecyltrimethylammonium (HDTMA)-treated zeolite to 25 ( ), 50 ( ), 75 (•), 100 ( ), 150 ( ), and 200 ( )% surface coverage. The lines are Langmuir fits to the experimental data.
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Table 2. Desorption of hexadecyltrimethylammonium (HDTMA) due to sorption of metal cations with initial concentration of 4 mM.
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As seen for all five metal cations, an increase in the amount of surfactant loading resulted in a reduction in the uptake of metal cations by the zeolite, and thus, a decrease in both Cm and KL. The sorption of cationic surfactant on charged surfaces is due to cation exchange at low surfactant coverage when the surfactant loading is less than the CEC, and hydrophobic bonding as the surfactant loading exceeds the CEC (Xu and Boyd, 1995; Li and Bowman, 1997).
Due to competition for sorption sites between metal cations and cationic surfactant molecules, the metal sorption capacity and Langmuir sorption coefficients are expected to be lower. However, for different types of cations, the degree of influence of surfactant on metal sorption was different. The trends for Cs+ adsorption shown in Table 1 (KL and Cm vs. percent HDTMA surface coverage) and Table 2 (amount of cation sorbed vs. percent HDTMA surface coverage) are substantially different than those exhibited by Sr2+, La3+, Pb2+, and Zn2+. As the percentage of HDTMA surface coverage increases from 0 to 200, values of KL, Cm, and amount of cation sorbed for Sr2+, La3+, Pb2+, and Zn2+ continuously decrease or decrease and then level off. Furthermore, the values of Cm and amount of cation sorbed are somewhat similar given a particular percent HDTMA surface coverage. This is definitely not the situation for Cs+. In regard to the trends shown for Cs+, as HDTMA surface coverage increases from 0 to 200, KL decreases and then increases (22 to 7.8 to 19 L/mmol), Cm does the opposite by increasing and then decreasing (80 to 94 to 54 mmol/kg), while the amount of Cs+ sorbed maintains a fairly constant value between 39.6 and 38.6 mmol/kg. Sorption of Cs+ on minerals was greater when the organic matter content was less, but very selective Cs+ sorption sites are less sensitive to the presence of organic macromolecules (Dumat et al., 1997). Thus, the slight effect of HDTMA on Cs+ sorption may suggest that Cs+ sorption on clinoptilolite is very selective, as it is on illite.
The sorbed surfactants greatly inhibited Sr2+ and La3+ sorption on zeolite (Fig. 1b,c). Before surfactant modification, the Sr2+ and the La3+ sorption capacity was 27 and 30 mmol/kg, respectively. As the surfactant loading increased, both Cm and KL decrease systematically. At surfactant bilayer surface coverage (200% ECEC), the sorption capacities were reduced to 5 mmol/kg for both metals (Table 1). The reduction may be due to the blockage of sorption sites by HDTMA cations. At 200% ECEC, the percentage of HDTMA desorbed to Sr2+ and La3+ sorbed was 77 and 68%, suggesting that significant portions of the sorbed Sr2+ and La3+ were on the external sites due to cation exchange of Sr2+ and La3+ for HDTMA+. On the other hand, the remaining metal sorption capacity indicates that some external sorption sites or internal sorption sites are still available for metal cation sorption.
Studies of Cs+ and Sr2+ sorption on untreated and hexadecylpyridinium chloride–treated vermiculite showed that larger distribution coefficients were found for Cs+ compared with Sr2+ on both untreated and modified samples (Bors et al., 1997). The relative affinity of Cs+ and Sr2+ for the clinoptilolite surface was comparable with their results. However, the influence of HDTMA on Sr2+ sorption was more significant throughout the concentration range compared with their results. The sorption of cations to the organo-vermiculite was attributed to incomplete exchange of interlayer cations by surfactant cations, resulting in a distinct number of sorption sites remaining available for the sorption of inorganic cations (Bors et al., 1997). The sorption of cations to SMZ may be attributed to a combined effect of the available internal cation exchange sites and the blocking of other internal exchange sites by HDTMA+.
The influence of surfactant molecules on La3+ sorption has not been reported in the literature. Surface precipitation of La-hydroxide occurred at pH greater than or equal to 5 and appeared to be the dominant sorption mechanism on Mn oxides, but surface precipitation was not observed on rutile until pH was higher than 6.5, and was not observed on goethite until pH was higher than 8.0 (Fendorf and Fendorf, 1996). Compared with those results, the reduction of La3+ sorption on HDTMA-treated zeolite shows that surface precipitation is probably not occurring. Instead, the high percentage of HDTMA desorbed to La3+ sorbed indicates that cation exchange is the predominant mechanism of La3+ sorption on zeolite.
The influences of preloaded HDTMA on Pb2+ and Zn2+ sorption differed (Fig. 1d,e), even though they are both transition elements and heavy metals. Zinc sorption is similar to Sr2+ sorption on HDTMA-treated zeolite (i.e., decreased metal uptake was accompanied by increased surfactant coverage on the zeolite) (Fig. 1b,e). Thus, even if the former is a transition metal, while the latter is an alkaline earth metal, their sorption on zeolite may still be governed by the same mechanism (i.e., cation exchange). Zinc sorption capacity was found to be greater in smectite-dominated soils compared with illite and kaolinite-dominated soils (Prasad et al., 1997), indicating that cation exchange plays an important role in zinc sorption.
Sorbed surfactants also affect the uptake of Pb2+ from solution, but to a lesser extent. Even at the surfactant bilayer coverage, the Pb2+ sorption maximum was still around 25 to 30 mmol/kg (Table 1). However, the KL decreased as the surfactant loading increased (Table 1). These results indicate that besides the cation exchange mechanism, there might be specific sorption of Pb2+. Since both Zn2+ and Pb2+ are transition metals, they should behave similarly if only cation exchange played a role in their sorption.
Quaternary Ammonium Sorption on Metal-Preloaded Zeolite
The HDTMA, DDTMA, OTMA, and counterion bromide sorption on the clinoptilolite after sorption of metals can be seen in Fig. 2, 3, and 4
, respectively. The solid curves are Langmuir fits to the observed data. The sorption maxima and Langmuir sorption coefficients are tabulated in Table 3
. These results show that the amount of surfactant sorbed is affected significantly by the chain length of the surfactant tail group, but insignificantly by the types of metal cations presorbed. The HDTMA molecule has 16 carbons in the tail group and had a sorption maximum of 200 mmol/kg, though the HDTMA sorption capacity is slightly lower for La3+ preloaded zeolite (Fig. 2). The amount of bromide sorbed is slightly greater than 50% of HDTMA sorbed, indicating admicelle and bilayer formation on zeolite surfaces (Li and Bowman, 1997). The DDTMA molecule has 12 carbons in the tail group and the amount of DDTMA sorbed is about 160 mmol/kg for any surfactant input concentration. The amount of bromide sorbed is about 50% of the DDTMA sorbed, suggesting that admicelle sorption or bilayer formation also occurred for DDTMA, but to a lesser extent (Fig. 3). The OTMA molecule has eight carbons in the tail group and the sorption of OTMA on zeolite only reached 60 mmol/kg. In addition, the amount of counterion sorbed is minimal compared with OTMA sorption (Fig. 4). Since the ECEC of the zeolite is 11 cmol/kg (Li and Bowman, 1997), the latter case may indicate that the sorbed OTMA molecules form only a partial monolayer. The KL values, reflecting the affinity of the solute for the solid surfaces, are two orders of magnitude higher for HDTMA sorption than for DDTMA and OTMA sorption (Table 3). Smaller KL values were observed for HDTMA sorption on Cs+ and Pb2+ preloaded zeolite compared with the other three metals, again showing the greater affinity of Cs+ and Pb2+ for the zeolite. The critical micelle concentrations (CMC) of HDTMA-Br, DDTMA-Br, and OTMA-Br are 0.9, 15, and 140 mM, respectively (Rosen, 1989). Thus, the CMC of the surfactant directly controls whether a surfactant bilayer forms or not, and to what extent a bilayer forms. A similar trend was observed for HDTMA-Cl, DDTMA-Cl, and OTMA-Cl sorption on kaolinite and montmorillonite (Hayes et al., 1995).
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Fig. 2. Sorption of hexadecyltrimethylammonium (HDTMA) and counterion bromide on zeolite preloaded with (a) Cs+, (b) Sr2+, (c) La3+, (d) Pb2+, and (e) Zn2+. The lines are Langmuir fits to the experimental data.
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Fig. 3. Sorption of dodecyltrimethylammonium (DDTMA) and counterion bromide on zeolite preloaded with (a) Cs+, (b) Sr2+, (c) La3+, (d) Pb2+, and (e) Zn2+. The lines are Langmuir fits to the experimental data.
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Fig. 4. Sorption of octyltrimethylammonium (OTMA) and counterion bromide on zeolite preloaded with (a) Cs+, (b) Sr2+, (c) La3+, (d) Pb2+, and (e) Zn2+. The lines are Langmuir fits to the experimental data.
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Desorption of Metal Cations by Quaternary Ammonium
Desorption of metal cations and sorption of OTMA have a linear relationship, suggesting that the uptake of OTMA by the zeolite is due to exchange of OTMA+ for metal cations because only OTMA+ monomers are present in the solution (Fig. 5a–d)
. A linear relationship between metals desorbed and DDTMA sorbed also occurred when the amount of DDTMA sorbed was less than 100 mmol/kg (the ECEC of the zeolite), or when the input DDTMA concentration is less than its CMC. Beyond that point, a further increase in surfactant sorption did not increase the amount of metal cations desorbed. This trend indicates that cation exchange was the dominant mechanism when DDTMA sorption was less than 100 mmol/kg. When DDTMA sorption was above 100 mmol/kg, due to the presence of micelles, hydrophobic bonding between the surfactant admicelles on the zeolite surface became the dominant mechanism (Fig. 5e–h). For HDTMA sorption, all input concentrations were greater than its CMC. However, a linear relationship between the metal cations desorbed and HDTMA sorbed is still present, indicating cation exchange when the HDTMA loading is less than its ECEC. The HDTMA sorption shows a similar trend when Sr2+ and Zn2+ are the preloaded cations (Fig. 5i,j). But an increased desorption of Cs+ and Pb2+ occurs when the HDTMA loading is about 200 mmol/kg (Fig. 5k,l). In addition, the amount of Sr2+ and Zn2+ desorbed is more than an order of magnitude greater than that of Cs+ and Pb2+ desorbed. The difference in desorption behavior between Sr2+ and Zn2+ on one hand and Cs+ and Pb2+ on the other hand may again suggest different sorption mechanisms. As observed by Cajuste et al. (1996), at low surfactant concentration (up to 0.5%) most of the Pb was retained by soils after 15 min of shaking, but at higher surfactant concentrations, substantial amounts of Pb remained in solution, indicating competition for sorption sites. Thus, desorption of Cs and Pb when HDTMA loading is above 200 mmol/kg may result from the higher HDTMA solution concentration.
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Fig. 5. Desorption of metal cations due to sorption of octyltrimethylammonium (OTMA) (a–d), dodecyltrimethylammonium (DDTMA) (e–h), and hexadecyltrimethylammonium (HDTMA) (i–l).
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CONCLUSIONS
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From this research the following conclusions can be reached:
(i) Sorption of cationic surfactant will reduce the metal cation sorption on the zeolite. The reduction in metal cation uptake by the SMZ is controlled by the surfactant loading on the zeolite surface and by the type of metal cations. As the surfactant loading on zeolite increases, the affinity of the zeolite for metal cations decreases. The reduction in metal cation uptake is more pronounced for Zn2+, Sr2+, La3+, and less for Cs+ and Pb2+.
(ii) Sorption of quaternary ammonium surfactant onto zeolite is a function of the chain length and of the CMC of the surfactant. When the surfactant chain length is short and the CMC is high, the sorbed surfactant molecules are present as monomers. As the surfactant chain length increases and the CMC decreases, the sorbed surfactant forms admicelles and bilayers on the zeolite surface.
(iii) The competition between cationic surfactant and metal cations for sorption sites indicates that SMZ may not be a good sorbent for some of the metal cations studied. On the other hand, because of the strong affinity of cationic surfactants for the zeolite surface, they may be used as desorbing agents for soil washing and decontamination.
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ACKNOWLEDGMENTS
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This research was partially sponsored by the U.S. DOE under contract DE-AR21-95-MC32018 from the Federal Energy Technology Center and by the State of Wisconsin Groundwater Coordinating Council. Funding from Creative Research Activity, Professional Opportunity Fund, and Collaborative Undergraduate Research Project of University of Wisconsin-Parkside is greatly appreciated. We thank Robert S. Bowman of New Mexico Tech for reviewing the draft manuscript. We appreciate the constructive suggestions and comments made by the three reviewers.
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ABSTRACT
INTRODUCTION
