Adsorption of dodecyl trimethylammonium and hexadecyl trimethylammonium onto kaolinite — Competitive adsorption and chain length effect

Adsorption of dodecyl trimethylammonium and hexadecyl trimethylammonium onto kaolinite — Competitive adsorption and chain length effect

Applied Clay Science 35 (2007) 250 – 257 www.elsevier.com/locate/clay Adsorption of dodecyl trimethylammonium and hexadecyl trimethylammonium onto ka...

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Applied Clay Science 35 (2007) 250 – 257 www.elsevier.com/locate/clay

Adsorption of dodecyl trimethylammonium and hexadecyl trimethylammonium onto kaolinite — Competitive adsorption and chain length effect Zhaohui Li ⁎, Linda Gallus Department of Geosciences, University of Wisconsin - Parkside, 900 Wood Road, Kenosha, WI 53141, USA Received 7 June 2005; received in revised form 31 August 2006; accepted 3 September 2006 Available online 24 October 2006

Abstract Adsorption of dodecyl trimethylammonium (DDTMA) and hexadecyl trimethylammonium (HDTMA) from single and mixed surfactant solutions onto kaolinite was investigated in this study. It was found that the surfactant chain length had minimal effects on the amount of cations desorbed. In contrast, the amounts of surfactant and counterion bromide adsorbed were strongly affected by surfactant chain lengths. Compared to DDTMA, HDTMA and counterion bromide adsorption was much higher. This fact further verified that both cation exchange and hydrophobic interaction played an important role in cationic surfactant adsorption on negatively charge mineral surfaces. The results also indicate that chain length has a minimal influence on surfactant adsorption via cation exchange, but a longer chain length promotes stronger hydrophobic interaction and thus, resulted in a higher surfactant adsorption. © 2006 Elsevier B.V. All rights reserved. Keywords: Adsorption; Cation exchange capacity; DDTMA; HDTMA; Kaolinite; FTIR

1. Introduction Clay minerals have high cation exchange capacities (CEC) and large surface areas, thus are ideal as sorbents for surface modification. In recent years, many studies have been focused on using surfactant-modified clay minerals (often called organoclays) to remove hydrophobic organic contaminants from water (Boyd et al., 1988; Zhang et al., 1993). Adsorption of cationic surfactants such as hexadecyl trimethylammonium (HDTMA) by clay minerals was initially attributed to cation ex-

⁎ Corresponding author. Tel.: +1 262 595 2487; fax: +1 262 595 2056. E-mail address: [email protected] (Z. Li). 0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2006.09.004

change. Even when the amount of HDTMA added was greater than the CEC of the clay minerals, the observed HDTMA adsorption plateau was reported only to the CEC or a little above (Lee et al., 1989; Zhang et al., 1993). Thus, in most early studies, the HDTMA loading levels were limited to the CECs of the clay minerals (Boyd et al., 1988; Jaynes and Boyd, 1991). Adsorption of cationic surfactants beyond the CEC of the clays was observed (Jaynes and Boyd, 1991), but little explanation was given until mid 1990's when a series of papers were published by Xu and Boyd (1994, 1995a,b), in which they attributed two mechanisms to the adsorption of cationic surfactants on swelling clay minerals: ion exchange and hydrophobic bonding. The former is indicated by stoichiometric release of inorganic cations

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accompanying HDTMA adsorption, when the HDTMA input was well below the CEC (Xu and Boyd, 1994). As the HDTMA input approached the CEC of the soil, both mechanisms became operative (Xu and Boyd, 1995a,b), and the amount of HDTMA adsorbed exceeded the cation released. A similar study showed stoichiometric release of Ca2+ accompanying HDTMA adsorption on smectitic soils when the HDTMA input was less than 0.7 CEC of the soils, above which HDTMA adsorbed more than Ca2+released, indicating that hydrophobic bonding commenced at 0.6– 0.8 CEC of the smectitic soils (Sheng et al., 1998). One consequence of HDTMA adsorption via hydrophobic bonding is the formation of surfactant admicelles or bilayers balanced by counterion adsorption at the outermost surfaces. However, in the studies mentioned above, counterion adsorption data were not provided. Thus, it is difficult to assess the ratio of HDTMA adsorbed by hydrophobic interaction to that by cation exchange. Parallel to the study of using organoclays to remove dissolved organics from water, it was found that the organoclays developed anion exchange properties when excessive HDTMA was present, which could be used to remove anionic contaminants from water (Li and Bowman, 1998). Kaolinite (KGa-1b) modified by HDTMA to twice of its CEC showed an adsorption capacity of 10 mmol/kg for chromate and 20 mmol/kg for nitrate (Li and Bowman, 2001). Quantitative desorption of counterion bromide due to adsorption of chromate or nitrate indicated that surface anion exchange was responsible for anion adsorption by the organo-kaolinite (Li and Bowman, 2001). Although positively charged surfaces were formed due to HDTMA bilayer formation, which was supported by a 2 to 1 ratio of HDTMA adsorbed to counterion adsorbed and by positive electrophoretic mobility measurement (Li and Bowman, 1997; Wang et al., 1999; Li and Bowman, 2001), no cation desorption data were provided in these studies. Effects of chain lengths on surfactant adsorption on clays and subsequent adsorption of contaminants on organoclays were studied to certain details. Compared to dodecyl trimethylammonium (DDTMA), HDTMA had a higher adsorption capacity on the soils studied (Lee et al., 1989). When the initial surfactant input was equivalent to 200% of a montmorillonite's CEC, the ratio of HDTMA adsorbed to cation desorbed was 2.1 to 2.4, while the ratio of DDTMA adsorbed to cation desorbed was only 1.2 to 1.3 (Zhang et al., 1993). Using zeta-potential measurement and adsorption isotherm, Wang et al. (1998, 1999) concluded that longer chain length promoted both cation exchange and hydrophobic interaction more effectively, thus, resulted in a higher adsorption. Similar observation was noticed for cationic surfactant adsorption onto

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clinoptilolite (Ersoy and Çelik, 2003). In these studies, cation desorption and counterion adsorption data were not available, making it difficult to evaluate the degree of effects of cation exchange or hydrophobic interaction on surfactant adsorption, as well as the surfactant surface configuration of different chain lengths. Fourier Transform Infrared (FTIR) can be used to study surfactant surface configuration. Long chain alkylammonium revealed two absorption bands at 2850 and 2920 cm − 1 , corresponding to symmetric and asymmetric stretching vibrations of C–C in the alkyl chain (Kung and Hayes, 1993). At a higher surface coverage, adsorbed HDTMA molecules resemble more with micelles than monomers as revealed by vibration band frequencies (Li and Gallus, 2005). At monomer surface configuration the vibrations shifted to slightly higher frequencies at 2860 and 2930 cm− 1 (Kung and Hayes, 1993; Li and Gallus, 2005). This study focuses on elucidating the influence of surfactant chain lengths on the adsorption and surface configuration of alkylammonium on kaolinite using adsorption data of alkylammonium and counterion bromide, as well as the desorption data of cations as a function of initial surfactant concentration and contact time. The hypothesis was that cation desorption would be a good indication of surfactant adsorption via cation exchange, while counterion adsorption be a good indication of surfactant adsorption via hydrophobic interaction. By comparing these values together with the amount of surfactant adsorption, the surfactant surface configuration could be determined. 2. Materials and methods 2.1. Materials The kaolinite used (KGa-1b, well crystallized kaolinite, and KGa-2, poorly crystallized kaolinite) was from the Clay Mineral Repository (Purdue University, West Lafayette, IN). Their CECs determined by an ammonia method are 30 and 37 meq/kg for KGa-1b and KGa-2, respectively (Borden and Giese, 2001). The surfactants used were HDTMA bromide (from Aldrich, Milwaukee, WI) and dodecyl trimethylammonium (DDTMA, from Acros, Pittsburgh, PA). 2.2. Alkylammonium adsorption Two grams of kaolinite and 20 mL of DDTMA or mixture of HDTMA and DDTMA solution with initial concentrations of 1, 2, 4, 6, 8, 12, 16, or 20 mM were combined in a 50 mL centrifuge tube. The mixture was shaken at 150 rpm for 24 h, centrifuged at 4000 rpm for 30 min, and the supernatant filtered through a 0.45 μm syringe filter.

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Same amount of kaolinite and DDTMA or mixture of HDTMA and DDTMA solutions with initial concentrations of 6 mM were combined in a 50 mL centrifuge tube for kinetic study. The mixture was shaken at 150 rpm for the flowing time intervals: 1/16, 1/8, 1/4, 1/2, 1, 2, 4, 8, 16, 32, and 64 h, centrifuged at 4000 rpm for 3 min for samples shaken less than 1 h and 30 min for samples shaken more than 1 h and then the supernatant was filtered through a 0.45 μm syringe filter. 2.3. Chemical analysis Alkylammonium was analyzed by a Supelco C-18 column and detected by a Linear 100 UV-Vis detector at a wavelength of 254 nm. Samples were injected using a Shimadzu 9-A autoinjector. The mobile phase was 5 mM p-tolunesulfonate in 45% water and 55% methanol. At a flow rate of 1 mL/min, the retention times were 2.2 and 3.2 min for DDTAM and HDTMA, respectively. A dilution at 1/10 ratio was made for concentrations greater than 2 mM. Bromide analysis was performed by another HPLC system using a different Shimadzu 9-A autoinjector and an Alltech electric conductivity detector. The mobile phase was 2 mM potassium phthalate with pH 6 adjusted by NaOH. The column used was Hamilton PRP-X100 anion chromatographic column. At a flow rate of 2 mL/min, the retention time was 3.1 min. The Ca2+ and Mg2+ concentrations were analyzed using an HPLC method. The mobile was 3.5 mM cupric sulfate, and the column was PRP-X200 cation chromatographic column. At a flow rate of 2.0 mL/min, the retention times for Ca2+ and Mg2+ were 2.9 and 3.9 min, respectively. The detection limit was 0.05 mmol/kg for both cations.

Fig. 2. Desorption of Na+ (×), K+ (+), Ca2+ (⋄), Mg2+(□), and total cations (▵) as a function of DDTMA loading on KGa-1b (a) and KGa2 (b).

Fig. 1. Adsorption of HDTMA (a) and DDTMA (b) (solid symbols) and counterion bromide (open symbols) on KGa-1b (squires) and KGa-2 (diamonds). Lines are Langmuir fit to the observed data. The dash line is the critical micelle concentration.

Fig. 3. Ratios of DDTMA adsorbed to bromide adsorbed (□), DDTMA adsorbed to cations desorbed (⋄), and cations desorbed to bromide adsorbed (▵) as a function of initial DDTMA concentration for KGa-1b (a) and KGa-2 (b).

The K+ and Na+ concentrations were analyzed by atomic emission spectrometry using a Perkin Elmer 560 Atomic Absorption spectrophotometer. The amount of alkyl ammonium and bromide adsorbed was determined by the difference between the initial and equilibrium concentrations.

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Fig. 6. Adsorption of DDTMA (⋄,♦), HDTMA (▵,▴), sum of DDTMA and HDTMA (○,•), and counterion bromide (□,n) on KGa-1b (open symbols) and KGa-2 (closed symbols) as affected by contact time at an initial concentration of 6 mM.

(ATR) accessory. The spectra were obtained by accumulating 256 scans at a resolution of 4 cm− 1.

3. Results and discussion Fig. 4. Adsorption of DDTMA (▴), HDTMA (♦), sum of DDTMA and HDTMA (□), and counterion bromide (○) on KGa-1b (a) and KGa-2 (b) from a mixture of HDTMA and DDTMA solutions of equal initial concentration.

2.4. FTIR spectroscopy FTIR spectra were acquired on a Perkin Elmer Spectra One Spectrometer equipped with Attenuated Total Reflection

Fig. 5. Desorption of metal cations Na+(×), K+(+), Ca2+(⋄), Mg2+(□), and total cations (▵) from KGa-1b (a) and KGa-2 (b) by DDTMA at an initial concentration of 6 mM and varying amounts of contact time. Adsorption of DDTMA and bromide as affected by contact time is plotted as ▴ and ♦, respectively. The dashed line is the average of total metal cations desorbed, while the dotted line is the average of bromide adsorbed.

Adsorption of HDTMA and DDTMA from single surfactant solution by KGa-1b and KGa-2 is plotted in Fig. 1. Similar to a previous study (Li and Bowman, 2001), HDTMA adsorption data were well described by the Langmuir isotherm. The calculated HDTMA adsorption capacities are 58 ± 3 and 85 ± 3 mmol/kg for KGa-1b and KGa-2, respectively. However, for

Fig. 7. Desorption of metal cations Na+(○), Ca2+(⋄), Mg2+(□), and total cations (▵) from KGa-1b (a) and KGa-2 (b) by a mixture of HDTMA and DDTMA each at an initial concentration of 3 mM as affected by contact time. Adsorption of HDTMA + DDTMA and bromide as affected by contact time is plotted as ▴ and ♦, respectively. The dashed line is the average of total metal cations desorbed, while the dotted line is the average of bromide adsorbed.

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DDTMA, the adsorption isotherm follows more closely to Freundlich than Langmuir isotherm with their adsorption plateaus at about 50 and 70 mmol/kg for KGa1b and KGa-2, respectively (Fig. 1b). An increase in alkylammonium adsorption on a kaolinite from 35 mmol/kg to 45 mmol/kg was found when HDTMA, instead of DDTMA, was used (Wang et al., 1998). Accompanying HDTMA adsorption, counterion bromide adsorption reached to 37 ± 3 and 58 ± 6 mmol/kg for KGa-1b and KGa-2, respectively (Fig. 1a). The bromide adsorption is more than half of HDTMA adsorption, indicating an overbalanced HDTMA admicelle formation on kaolinite (Li and Gallus, 2005). In contrast, bromide adsorption accompanying DDTMA is slightly different. No major bromide adsorption occurred when equilibrium bromide concentration was less than 5 mM. Even at higher initial surfactant concentrations, the ratio of bromide to DDTMA adsorbed is less than 1 / 2 (Fig. 1b). The results may indicate that the percentage of surfactant

adsorption via hydrophobic interaction is different for HDTMA and DDTMA. Fig. 1 also shows that adsorption of surfactant and counterion bromide on KGa-2 is higher compared to that on KGa-1b. The release of cations K+, Na+, Ca2+, and Mg2+ due to adsorption of DDTMA is plotted in Fig. 2. The release of cations is less sensitive to the initial DDTMA concentration except at very low concentrations. Major cation released from KGa-1b was Ca2+ (Fig. 2a), in contrast to almost equal amounts of Na+, Ca2+, and Mg2+ released from KGa-2 (Fig. 2b). The amounts of total cations released were 15± 1 and 9 ± 1 meq/kg from KGa-1b and KGa-2, respectively, compared to 18± 2 and 11 ± 2 meq/kg desorbed By HDTMA (Li and Gallus, 2005). Similar to a previous study, less cations desorbed from KGa-2 compared to KGa-1b, even though the former had higher DDTMA and bromide adsorption (Figs. 1 and 2). The amount of cations released remained constant while the amount of DDTMA and bromide adsorbed increases as

Fig. 8. FTIR spectra of DDTMA solid (a), DDTMA adsorbed on KGa-1b (b) and KGa-2 (c) after equilibrated with varying DDTMA concentrations. A shift of vibration to lower frequencies indicates a transition from monomer to micelles and to more ordered molecule arrangement.

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the initial surfactant concentration increased. This trend further confirmed that besides cation exchange hydrophobic interaction was responsible for additional DDTMA and bromide adsorption. As the initial DDTMA concentration increased the ratios of DDTMA adsorbed to bromide adsorbed became smaller and reached constant when the initial DDTMA concentration exceeded 10 mmol/L (Fig. 3). The ratios of cations desorbed to bromide adsorbed showed similar trend (Fig. 3). In contrast, the ratios of DDTMA adsorbed to cations desorbed increase as the initial surfactant concentration increased (Fig. 3), which is caused by consistent increase in DDTMA adsorption via hydrophobic interaction while cation desorption remained constant. Competitive adsorption between HDTMA and DDTMA, as well as counterion bromide adsorption from mixture of equal concentrations of HDTMA and DDTMA solution is plotted in Fig. 4. HDTMA adsorption on KGa-1b and KGa-2 reached plateaus at 45 and 55 mmol/kg, respectively. In contrast, DDTMA adsorption only reached at 5 mmol/kg, and is less sensitive to initial concentration. The amounts of total alkylammonium adsorbed on KGa-1b and KGa-2 were 50, and 60 mmol/kg, less than the HDTMA adsorption of 58 ± 3 and 85 ± 3 mmol/kg from a solution containing HDTMA only (Li and Gallus, 2005). Bromide adsorptions on KGa-1b and KGa-2 were about 27 and 40 mmol/kg, more than 50% of total alkylammonium adsorbed, indicating an overbalanced surfactant bilayer formation (Fig. 4). Kinetic study showed that, at an initial DDTMA concentration of 6 mM, both DDTMA adsorption and cation desorption are instantaneous (Fig. 5). The mean total cations desorbed are 16 ± 1 and 10 ± 1 meq/kg for

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KGa-1b and KGa-2, respectively, similar to 15 ± 1 and 9 ± 1 as determined from adsorption isotherm study (Fig. 2). The average amounts of DDTMA adsorbed are 18 ± 1 and 24 ± 1 meq/kg for KGa-1b and KGa-2, respectively. Counterion bromide adsorption was also instantaneous with adsorption plateaus of 5 ± 1 and 8 ± 1 meq/kg for KGa-1b and KGa-2, respectively (Fig. 5). The amount of bromide adsorbed to that of the total cations desorbed is 1/3 for KGa-1b and 1/1 for KGa-2. The instantaneous adsorption of DDTMA and bromide agrees with a previous result of HDTMA adsorption on kaolinite (Li and Gallus, 2005), further indicating that diffusion played no major roles in surfactant and counterion adsorption and their later rearrangement on kaolinite, because all adsorption sites on kaolinite were immediately accessible. Adsorption of HDTMA, DDTMA, total alkylammonium, and counterion bromide from HDTMA and DDTMA mixture as a function of time showed a similar feature. Again, both minerals showed instantaneous adsorption of alkylammonium and bromide and instantaneous desorption of cations (Fig. 6). For KGa-1b, cations desorbed equaled to that of bromide adsorbed and is about half of total alkylammonium adsorbed (Fig. 7). In contrast, bromide adsorbed is twice the amount of cations desorbed for KGa-2, indicating an overbalanced admicelle formation (Fig. 7). Contrast Figs. 5–7, one could see that changing 50% of input surfactant from DDTMA to HDTMA caused no increase in cation desorption, but increased alkylammonium adsorption from 18 to 34 and 20 to 35 mmol/kg for KGa-1b and KGa-2, respectively. Accompanying the increased alkylammonium adsorption, bromide adsorption increased from 5 to 17 and 8 to 17 mmol/kg for KGa-1b and KGa-2, respectively. The simultaneous increase in alkylammonium and bromide

Table 1 Averages of alkylammonium adsorbed, bromide adsorbed, and cations desorbed at an initial surfactant concentration of 6 mM and a liquid to solid ratio of 10 to 1 Samples

KGa-1b, HDTMA KGa-1b, Mix KGa-1b, DDTMA KGa-2, HDTMA KGa-2, Mix KGa-2, DDTMA

Average SD Average SD Average SD Average SD Average SD Average SD

There are 22 replicates per treatment.

HDTMA adsorbed (meq/kg)

Bromide adsorbed (meq/kg)

Cations desorbed (meq/kg) Ca2+

Mg2+

Na+

K+

Sum cations

47 4 34 0.3 18 1 57 1 35 0.2 24 1

33 4 16 2 5 1 40 2 16 2 8 1

10.1 0.6 9.7 0.7 9.4 0.6 4.5 0.4 4.3 0.4 4.3 0.7

4.4 0.2 5.1 0.4 3.8 0.4 3.2 0.1 3.8 0.2 2.7 0.2

2.6 0.2 1.0 0.2 2.6 0.3 2.5 0.4 1.3 0.3 3.0 0.4

0.26 0.01 0.10 0.01 0.23 0.02 0.39 0.03 0.22 0.05 0.35 0.03

17.2 0.8 15.9 0.9 15.7 0.9 10.6 0.7 9.6 0.6 10.0 0.7

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Table 2 Comparison of CEC determined from different methods Unit in meg/kg

KGa-1b

KGa-2

HDTMA, adsorption study, 14 replicates HDTMA, kinetic study, 22 replicates DDTMA, adsorption, 14 replicates DDTMA, kinetic study, 18 replicates Mixture, kinetic study, 18 replicates Source clay (http://cms.lanl.gov/chem.htm) Ammonia method (Borden and Giese, 2001) Cu complexation method (Ammann, 2003)

18.3 ± 2.1 17.2 ± 0.8 15.6 ± 0.8 15.7 ± 0.9 15.9 ± 0.9 20 30 ± 1 11–17

11.0 ± 2.0 10.6 ± 0.7 9.1 ± 0.4 10.0 ± 0.7 9.6 ± 0.6 33 37 ± 1

adsorption with constant cation desorption when HDTMA was present in solution indicates that the additional adsorption of alkylammonium and counterion bromide is completely caused by hydrophobic interaction. Thus, hydrophobic interaction played more important role as the chain length of the surfactant tail group increased from 12 to 16. Furthermore, increasing chain lengths of the surfactant only increased surfactant adsorption by hydrophobic interaction, contrast to Wang et al. (1998, 1999) observation that longer chain length promoted both cation exchange and hydrophobic interaction. FTIR spectra of DDTMA solid showed absorptions at 2850 and 2917 cm− 1 (Fig. 8a), similar to that of HTDMA (Li and Gallus, 2005). The former was attributed to symmetric and the latter to asymmetric stretching vibrations of C–C in the alkyl chain (Kung and Hayes, 1993). In contrast, HDTMA monomer and micelles had vibrations at 2860 and 2930 cm− 1 and 2853, and 2923 cm− 1, respectively (Kung and Hayes, 1993). When the initial DDTMA concentration was below 10 mmol/L, the asymmetric and symmetric C–C bands were at 2858 and 2931 cm− 1, similar to those of monomer vibrations (Fig. 8). As the surfactant loading on kaolinite increased further, the vibration bands shifted to lower frequencies slightly, indicating more ordered arrangement of adsorbed surfactant molecules. Even at an initial DDTMA concentration of 20 mmol/L, the adsorption bands were at 2856 and 2926 cm− 1, still higher than 2852 and 2920 cm− 1 produced by adsorbed HDTMA admicelles (Fig. 8b–c). Contrast to HDTMA adsorption on kaolinite, the adsorbed DDTMA may never form complete admicelle configuration. The lack of complete surfactant admicelle configuration is responsible for the lower amount of counterion bromide adsorption, which explains why DDTMA modified clay minerals have a much lower adsorption affinity for anionic contaminants (Li et al., 2003). A previous study showed that HDTMA adsorption on kaolinite revealed an admicelle formation with an overbalanced upper layer, i.e. the amount of counterion adsorbed was more than half of HDTMA adsorbed (Li and

Gallus, 2005). Compared to HDTMA adsorption, DDTMA adsorption showed under balanced upper layer (Table 1) while an evenly balanced upper and lower layer was achieved by HDTMA and DDTMA mixture. Another important feature from this and previous studies is the correlation of alkylammonium adsorbed to cations desorbed. Regardless of how much alkylammonium adsorbed at the adsorption plateau, the cations desorbed are 15–18 meq/kg for KGa-1b and 9–11 meq/kg for KGa2, respectively (Table 2). If the desorption of metal cations accompanying surfactant adsorption is due to cation exchange, the CEC of KGa-1b and KGa-2 as determined by alkylammonium adsorption/cation desorption study will be 15–18, and 9–11 meq/kg, respectively, much less than the values determined by an ammonia method (Borden and Giese, 2001) and reference values listed on Clay Mineralogical Society website (http://cms.lanl.gov/ chem.htm). However, the value of cations desorbed from KGa-1b is similar to the CEC values of KGa-1 determined by a complexation method using copper bisethylenediamine and copper triethylenetetramine in the presence and absence of buffer tris(hydroxymethyl)aminomethane (Ammann, 2003). Acknowledgments This research was partially sponsored 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. References Ammann, L., 2003. Cation exchange and adsorption on clays and clay minerals. Dissertation, Christian-Albrechts-Universität, Kiel, 119 pp. Borden, D., Giese, R.F., 2001. Baseline studies of the clay minerals society source clays: cation exchange capacity measurements by the ammonia-electrode method. Clay Clay Miner. 49, 444–445. Boyd, S.A., Mortland, M.M., Chiou, C.T., 1988. Sorption characteristics of organic compounds on hexadecyltrimethylammoniumsmectite. Soil Sci. Soc. Am. J. 52, 652–657. Ersoy, B., Çelik, M.S., 2003. Effect of hydrocarbon chain length on adsorption of cationic surfactants onto clinoptilolite. Clay Clay Miner. 51, 172–180. Jaynes, W.F., Boyd, S.A., 1991. Clay mineral type and organic compound sorption by hexadecyltrimethylammonium clays. Soil Sci. Soc. Am. J. 55, 43–48. Kung, K., Hayes, K.F., 1993. Fourier transform infrared spectroscopic study of the adsorption of cetyltrimethylammonium bromide and cetylpyridinium chloride on silica. Langmuir 9, 263–267. Lee, J., Crum, J.R., Boyd, S.A., 1989. Enhanced retention of organic contaminants by soils exchanges with organic cations. Environ. Sci. Technol. 23, 1365–1372.

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