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Sorption of 2,4-dichlorophenoxy propionic acid by organo-clay complexes
Applied Clay Science 16 Ž2000. 147–159 www.elsevier.nlrlocaterapclaysci
Sorption of 2,4-dichlorophenoxy propionic acid by organo-clay complexes Y.H. Hsu, M.K. Wang ) , C.W. Pai, Y.S. Wang Graduate Institute of Agricultural Chemistry, National Taiwan UniÕersity, Taipei, 10764, Taiwan Received 7 October 1998; received in revised form 6 July 1999; accepted 16 July 1999
Abstract Wyoming montmorillonite ŽSWy-1, MON., Charng-Bin ŽSCB. soil montmorillonite, and W.R. Grace vermiculite ŽVER. have been used as adsorbents to prepare organo-clays. Sorption of 2,4-dichlorophenoxy propionic acid Ž2,4-DP. by organo-clays is greater after adding of hexadecyltrimethyl-ammonium ŽHDTMA. to 200% CEC of the clay than after adding the same amount of HDTMA to 100% CEC of clay ŽHDTMA–clay.. The Freundlich adsorption isotherm describes 2,4-DP sorption by HDTMA–clays Ž K f s 1.828–11.474 l kgy1, n f s l.130–1.454, R s 0.919– 0.944. quite well. Low surface charge montmorillonite can allow a high concentration of 2,4-DP to penetrate into the interlayer with low residual charge on the wedge. Thus, with increasing 2,4-DP concentration, the basal d-spacings of HDTMA-montmorillonite complexes increased from 2.15 to 2.71 nm. However, increasing the concentration of 2,4-DP did not result in further swelling of vermiculite owing to the high surface charge. In general, HDTMA–montmorillonite ŽO-MON. sorbed more 2,4-DP than HDTMA–soil montmorillonite ŽO-SCB. and HDTMA– vermiculite ŽO-VER.. Thus, the interlayer sorption of 2,4-DP exhibited a critical level, beyond which further expansion of the clays does not occur. The equilibrated pH in the suspension affected the adsorption by HDTMA–clay complexes. The maximum 2,4-DP adsorption occurs at a pH level of approximately 3 corresponding to p K a s 3.0 of 2,4-DP in the adsorption–pH curve. q 2000 Elsevier Science B.V. All rights reserved. Keywords: hexadecyltrimethyl-ammonium clays ŽHDTMA–clays.; 2,4-dichlorophenoxy propionic acid Ž2,4-DP.; montmorillonite; sorption; vermiculite
1. Introduction Soil is known to function as a chemical as well as a biological filter that lessens the environmental impact of organic chemicals introduced into the )
Corresponding author. Tel.: q886-2-2363-0231 ext. 2491 or 3066; Fax: q886-2-2366-0751; E-mail: [email protected] 0169-1317r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 3 9 - 3
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biosphere by various processes. Soil acts as a first line of defense against the leakage of these compounds into the groundwater. Soil achieves this by absorbing organic chemicals, as well as biological andror chemical degradation. Consequently, to predict the behavior of organic pollutants in soils, and to evaluate the risk that a particular chemical might leak into groundwater, it is necessary to understand the nature and extent of sorption and degradation making it an important subject for research Ž McBride, 1994. . Adsorption and desorption of organic molecules in the soils is primarily controlled by the chemical properties of the molecules and surface properties of the specific soil. The 2:1 layer silicates are important constituents of soil to prepare organoclays for 2,4-dichlorophenoxy propionic acid Ž 2,4-DP. sorption. Vermiculite, with its high surface charge density is mostly substituted in the tetrahedral sheets of the clays; while montmorillonite, with its small surface charge, is substituted mostly in the octahedral sheet Ž Dixon and Weed, 1989. . Inorganic cations in the interlayers can be exchanged with organic or inorganic cations. Hydrophobic clays can be changed to organophilic clays by organic chemicals. Such organophilic clays have various applications in paper, petroleum Žoil-based drilling fluids for deep wells. , catalytic, water treatment, and several other industries, and also act as sorbents for a great variety of organic pollutants, such as phenols ŽJordan et al., 1950; Theng, 1974; Mortland et al., 1986; Boyd et al., 1988a. , and some weakly acidic pesticides which have been detected in surface waters ŽWauchope, 1978; Hermosin and Cornejo, 1992, 1993. . The organo-clays have been shown to act as partition media in adsorption of these organic pollutants ŽMortland, 1970; Boyd et al., 1988a; Janes and Boyd, 1991. . The HDTMA–montmorillonite Ž O-MON. , HDTMA–soil montmorillonite ŽO-SCB., and HDTMA–vermiculite Ž O-VER. were selected to act as sorbents for adsorption of 2,4-DP. 2,4-DP is a metabolic of herbicide and inhibits the growth of broad leaf weeds in soil. Thus, the objectives of this study were to investigate the sorption capacity and mechanism of 2,4-DP by three types of HDTMA–clays. These results may be applicable in the black soils typical of eastern Taiwan since clay fractions in black soils contain large amounts of montmorillonite.
2. Materials and methods 2.1. Materials 2.1.1. Clays, soils and chemicals The reference clay mineral, namely Na-montmorillonite Ž SWy-1. was obtained from Source Clay Repository Ž Clay Minerals Society Ž CMS. , Columbia, MO., and zonalite vermiculite was obtained from W.R. Grace Repository Ž W.R.
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Grace and Co., Cambridge, MA. . The black soil sample was collected from surface horizon of the Charng-Bin ŽSCB. in eastern Taiwan, which is known as an abundant source of montmorillonite Ži.e., 60%. and small amount of vermiculite Ži.e., 3%. in clay fractions Ž Pai et al., 1999. . The CEC of untreated original SWy-1 ŽMON., soil clay ŽSCB. and vermiculite ŽVER. were 80.5, 54.0, and 71.3 cmol kgy1, respectively The soil samples were first treated with H 2 O 2 , and dithionite–citrate–bicarbonate solutions to remove organic matter, free metal oxides and hydroxides ŽMehra and Jackson, 1960. . All the samples were then ultrasonicated and fractionated to less than 2 mm of clays, prepared as Na-saturated clays, and then freeze dried. Cation-exchange capacities of clays ŽŽ CarMg. exchange. were determined as described by Jackson Ž1979. . The clay samples were characterized, saturated with Mg and K, and prepared in the form of slurries on petrographic slides for X-ray diffraction ŽXRD. analysis. For the Mg-saturated SWy-1 and soil clays, the XRD peak shifted from 1.4 to 1.85 nm after glycerol-solvation; whereas with the K-saturated vermiculite heated at 1058C, the d-spacing of the XRD peak shifted from 1.4 to 1.0 nm. The HDTMA and 2,4-DP were obtained from Sigma. 2.2. Methods 2.2.1. Sorption and determinations of HDTMA Bromophenol blue Ž BPB. stock standard solution Ž 5 = 10y4 M. was prepared by dissolving accurately weighed amounts of dye in 5 ml of 0.01 M NaOH solution and diluting it to 200 ml with distilled water. For HDTMA determination, the pH of the filtrate was adjusted to 8.0 with 0.1 M phosphate buffer solution. Appropriate amounts of BPB and HDTMA solutions were mechanically shaken for 20 min with 5 ml chloroform ŽYamamoto, 1995. . After phase separation, the absorption of the organic phase was measured against that of chloroform at wavelength 608 nm. A GBC911 UV–Vis spectrophotometer was used for recording the spectra and determining the absorbance measurements of the solutions placed in quartz cells with 10-mm path length. 2.2.2. Preparation and characterization of HDTMA–clays The clay samples Ž 0.1 g. were first treated three times with HDTMA Ž 30 ml of 60%, 100%, 150%, 200%, 250% and 300% of the CEC to clays. to equilibrate within 8 h. The clay-organic suspensions were then centrifuged, washed with an ethanol–water solution Ž 50:50. five times, and then freeze-dried. Different concentrations of HDTMA were obtained by adding 100% or 200% of the CEC to clays and are referred as O-MON1, O-MON2; O-SCB1, O-SCB2; and O-VER1, O-VER2, respectively. The effect of temperature was investigated in the range 308C, 408C, 608C, 708C and 808C. Methanol, ether, acetone, and chloroform Ž 10 ml. were also used for adsorption of HDTMA and the results
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were compared with those of water. After each treatment, the HDTMA–clays were then prepared as slurries and dropped onto petrographic slides for XRD analysis Ž Jackson, 1979. . The oriented samples were examined by X-ray diffractometer, using a Rigaku Miniflex with CuK a radiation generated at 30 kV and 10 mA. The XRD patterns were recorded in the range of 2 to 3082 u at a scanning speed of 182 u miny1. For FT-IR spectroscopic analysis, the HDTMA–clays were also freeze-dried and prepared as KBr disc Ž Biorad FTs-7 IR.. 2.2.3. 2,4-DP sorption by HDTMA–clays For the 2,4-DP adsorption isotherm studies, 0.02 g of HDTMA–clay was placed in 50 ml centrifuge tube and supplemented with 10 ml of 0, 0.04, 0.20, 0.40, 0.80 and 1.60 mM of 2,4-DP solutions and shaken 24 h. Small amounts of suspension were dropped onto petrographic slide for XRD analysis. The other suspensions were centrifuged at 24,000 = g and 258C for 30 min and had their 2,4-DP concentration determined in supernatants. Successive adsorptions of 2,4-DP by HDTMA–clays were prepared by treatment of 0.1 g of HDTMA–clay with 10 ml of 0.5 mM of 2,4-DP, shaken for 4 h. The HDTMA–clay was first separated from the supernatant by centrifugation and then, the old supernatant was removed and replaced by fresh 10 ml of 2,4-DP solution. After eight successive treatments, the complexes were prepared as slurries and dropped onto petrographic slides for XRD analysis. Sorption of 2,4-DP at different pH levels was measured using 0.02 g of HDTMA–clay and 10 ml of 1 mM 2,4-DP solution in 0.01 M CaCl 2 . The pH of the suspensions was initially adjusted to pH levels of 1, 2, 3, 4, 5, 6, 7 and 8 with 0.1 M HCl or NaOH. These suspensions were then shaken for 24 h and centrifuged under the same conditions as the adsorption isotherm experiments. The equilibrium pH and 2,4-DP concentrations were then determined in the supernatants. The amount of adsorbed herbicide was calculated from the difference between the initial and equilibrium 2,4-DP concentrations, as determined by UV spectroscopy at 230 nm ŽBogus et al., 1990; Hermosin and Cornejo, 1993. . All the experimental determinations were done in duplicate. A blank sample Ž 0.02 g of sample with 5 ml of 0.01 M CaC1 2 . was used for background correction.
3. Results 3.1. Preparation of HDTMA–clays The basal spacing Ž d oo1 . and interlayer d-spacings obtained from d-spacing of the XRD analysis of Na-montmorillonite ŽSWy-1. treated with HDTMA ŽMON. are summarized in Fig. 1. Interlayer d-spacings of HDTMA–clays were
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Fig. 1. Basal Ž d oo1 . and interlayer d-spacings of HDTMA–clay ŽSWy-1..
calculated from the corresponding d oo1 minus basal spacing of 2:1 clays Ž i.e., 0.96 nm. ŽDixon and Weed, 1989. . For addition of 150% CEC of montmorillonite ŽSWy-1. , basal d-spacing as well as interlayer d-spacing exhibit maximums at 2.25 and 1.31 nm, respectively. The basal d-spacing then reaches a constant value when more HDTMA is added to the clays. Sorption characterization of HDTMA by soil montmorillonite Ž SCB. is similar to that of MON, with the only difference that HDTMA was observed to be unable to penetrate into the interlayer completely. The XRD data of the SCB clay showed that aluminum hydroxy-interlayered compounds were present in the interlayer. The XRD peak in the Mg-saturated SCB-clay shifted to 1.7 nm when glycerol solvation after the sample was pre-treated with 0.3 M Na-citrate Ž Pai et al., 1999. . Although the properties of soil clay were characterized as montmorillonite, its clay intercalated with HDTMA molecules is not expandable as that of SWy-1. Infrared spectra of HDTMA–clay complexes show that C–H stretching at 2929 and 2856 cmy1 for the 60% CEC clay sample gradually shifted to 2917 and 2849 cmy1 for the 300% CEC clay sample ŽFig. 2A–F.. The addition of HDTMA to 100% CEC of vermiculite resulted in a material with XRD peaks at 2.88, 1.88 and 1.39 nm, and corresponding interlayer d-spacings of 1.91, 0.91 and 0.42 nm. However, after increasing the HDTMA concentration to 200% CEC, the only XRD peak appeared at 1.98 nm, which is constituent with two layers Ž200% CEC. of HDTMA being present in the interlayers of highly charged vermiculite ŽLagaly and Weiss, 1969; Lagaly, 1982. . 3.2. Effect of temperature and solÕents on HDTMA–clay preparation The effect of temperature on clay expansion was not as significant for SWy-1 and soil clay as the concentration effect. However, with the addition of l00%
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Fig. 2. FT-IR spectra between 2700 and 3100 cmy1 for the samples containing ŽA. 60%, ŽB. 100%, ŽC. 150%, ŽD. 200%, ŽE. 250%, ŽF. 300% of the CEC added to SWy-1, and ŽG. HDTMA.
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Fig. 3. Adsorption isotherm of 2,4-DP on HDTMA–clay complexes.
CEC of HDTMA to vermiculite, the XRD peak corresponding to the 2.88 nm d-spacing becomes more intense for the samples prepared at 808C, as compared to that of the otherwise similarly prepared sample at 308C. A comparison of the d-spacing for the samples prepared from methanol, ether, acetone and chloroform by adding l00% and 200% CEC to clays, shows that low polarity of chloroform results in greater d-spacing values in both l00% and 200% CEC HDTMA–clay samples. When the HDTMA–clay complexes were treated with various solvents, chloroform produced the largest swelling of the clay Ž Lagaly, 1994.. 3.3. Sorption of 2,4-DP by HDTMA–clay Sorption isotherm of 2,4-DP adsorbed by HDTMA–clay Ž 200% CEC. is shown in Fig. 3. The Freundlich isotherm equation fits quite well with the experimental data Ž K f s 1.828–11.474 l kgy1, n f s 1.130–1.273, R s 0.919– 0.944. ŽTable 1. ŽJanes and Boyd, 1991. . The K f and n f values show the trend
Table 1 Freundlich 2,4-DP adsorption parameters of O-MON2, O-SCB2 and O-VER2 Sample
K f Žl kgy1 .
nf
R
O-MON2 O-SCB2 O-VER2
11.474 1.828 2.680
1.130 1.454 1.273
0.944 0.919 0.924
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Table 2 The d oo1 of HDTMA–clays equilibrated with various concentrations of 2,4-DP 200% CEC clay
Initial 2,4-DP concentration Žm M . 0
0.04
0.20
0.40
0.80
1.60
2.25 b – – – – 1.90 b – 1.01
2.70 b 2.30 1.37 – – – 1.20 1.02
2.72 b 2.19 1.33 2.66 b 1.31 – 1.25 1.00
2.71b 2.02 1.33 2.69 b 1.31 – 1.25 1.00
d-spacing Žnm. O-MON2
O-SCB O-VER2
a b
2.21b –a – 2.03 b – 2.06 b 1.51 1.38
2.19 b – – 2.61b 1.32 1.99 b 1.42 1.01
–: Not detected. Symbol indicates the majored d-spacing.
of O-MON2) O-VER2) O-SCB2 and O-SCB2) O-VER2) O-MON2, respectively. The 2,4-DP adsorption by three types of HDTMA–clay show a quite clear linear relationship with the Freundlich equation. In general, HDTMA– montmorillonite sorbed more 2,4-DP than HDTMA–vermiculite. The d-spacing and adsorption isotherm ŽTable 2. show that maximum adsorption of 2,4-DP by O-MON2, O-SCB2 and O-VER2 are 160, 140 and 130 mmol kgy1 HDTMA– clay, respectively ŽFig. 3.. Eight successive sorptions of 2,4-DP by HDTMA– clay with 200% CEC of HDTMA showed more adsorption than that by 100% CEC HDTMA–clay Ž Hsu, 1996. and showed the similar trend of O-MON2) O-SCB2) O-VER2 ŽFig. 4.. The effect of pH on adsorption of 2,4-DP by
Fig. 4. Successive adsorption curves of HDTMA–clay complexes Žcumulative adsorption..
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Fig. 5. Effect of pH on adsorption of 2,4-DP by HDTMA–clay complexes.
HDTMA–clay is shown in Fig. 5. In general, the maximum 2,4-DP adsorption occurs at a pH level of approximately 3. When the pH is less than 3, 2,4-DP is present as a molecular phase Žp K a of 2,4-DP s 3.0., whereas when the pH exceeds 3, it is mostly of ionic type.
4. Discussion X-ray diffractograms of the samples containing 60% to 250% CEC of HDTMA added to SWy-1 show the most prominent d-spacing Ž d oo1 . in the range of 1.48 to 2.11 nm. However, when 300% CEC of HDTMA is added to clay, the XRD pattern not only shows 2.11 nm peak but also exhibits 2.68 and 1.32 nm peaks, corresponding to the HDTMA molecules, thus indicating that excess HDTMA is precipitated on the external sites of the clay. The ordering of HDTMA–bentonite complexes may be enhanced by exposing the clay to HDTMA in excess of CEC if care is taken to remove the excess salt by washing with water or ethanol. Ethanol and ethanol–water mixtures yield well-dispersed suspensions and well-oriented aggregates for X-ray analysis Ždata not shown. . The d-spacings in the 100% and 200% of CEC SWy-1 samples as well as soil clay, and vermiculite do not show evidence of interaction of excess HDTMA with clays. From these data, it can be seen that the layer and pseudolayer of HDTMA penetrate into the interlayer of montmorillonite, paraffin and that the pseudolayer of HDTMA penetrate into the interlayer of vermiculite used for 2,4-DP sorption studies Ž Boyd et al., 1988a,b; Xu and Boyd, 1994. . For various
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organic solvents with different polarities, the HDTMA reaction shows that chloroform, with the lowest polarity, offers the best expansion for HDTMA for either 100% or 200% CEC added to the three types of clays. The infrared spectra show that the C–H stretching vibrations at 2929 and 2856 cmy1 for the sample containing 60% CEC gradually shifted to lower frequencies for the sample containing 300% CEC Ž Fig. 2. , thus indicating bonding between tetrahedral sheets Ž Si–O. at weak concentration of HDTMA molecules. At large loading of HDTMA, the spectra are similar to those of solid HDTMA Ž Fig. 2G. because the chains are densely packed. However, the N–H stretching peak at 3017 cmy1 appears only after addition of HDTMA at concentrations of higher than 100% CEC to the clays, thus indicating that the free molecules of HDTMA exceed the clay saturation ŽFig. 2C–G. . Zhang et al. Ž1993. have proposed that sorption of quaternary amines on montmorillonite involves at least three types of reactions, viz. a cation-exchange reaction, adsorption of ion pair and tail–tail interactions Ž Xu and Boyd, 1994. . 4.1. Sorption of 2,4-DP by HDTMA–clays Cumulative sorption of 2,4-DP by different clays is shown in Fig. 4. In general, the trend of adsorption for 200% CEC added to HDTMA–clays is O-MON2) O-SCB2) O-VER2. After eight successive treatments, the amount of 2,4-DP adsorbed was found to be relatively low, and the adsorption of 2,4-DP at low concentrations by HDTMA–clay resulted in expansion of the interlayer of clay. HDTMA–clay containing 200% CEC of HDTMA adsorbed more 2,4-DP than did the HDTMA–clay with 100% CEC Ž Hsu, 1996. . This suggests that for HDTMA–clay complexes, new adsorption sites on the external surface were exposed by expanding the silicate layer due to the strong ring–tail attraction as more and more 2,4-DP molecules were adsorbed. As a qualitative example, 200% CEC added HDTMA–clay samples Ž O-MON2 and O-SCB2. exhibited limited d-spacings when the 2,4-DP concentrations were less than 1.60 mM Ž Table 2. , whereas, the O-VER2 sample exhibited limited expansion when the 2,4-DP concentration was less than 0.20 mM. Due to the entrapment of 2,4-DP molecules, cross-linkage in herbicide, and the high charge density of vermiculite, these 2,4-DP molecules adsorbed on the wedge of tetrahedral sheets, so small concentrations of 2,4-DP are able to penetrate into the interlayer of HDTMA–clay. On the other hand, due to the large concentrations of 2,4-DP molecules at the cross-linkages and entrapped coil molecules, along with the high charge of vermiculite, the 2,4-DP molecules are adsorbed on the wedge of the interlayer, and hence, the herbicide cannot penetrate into the interlayer. Low surface charge montmorillonite, however, can allow large concentrations of 2,4-DP to penetrate into the interlayer with small residual charge on the wedge. Hence, with increasing 2,4-DP concentration, the basal d-spacings of HDTMA–montmorillonite complexes increased from 2.15 to 2.71 nm. The van
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der Waal area for cosmo solvation Ž water. is 1.094 nm2 and the length of 2,4-DP is 0.603 nm. Variation in 2,4-DP adsorption as a function of pH of the equilibrated solution is plotted in Fig. 5 for O-MON2, O-SCB2, and O-VER2. Although the 2,4-DP adsorption on O-VER was much less than that on O-MON in the pH range of 1–8 ŽHermosin and Cornejo, 1993. , the amount of 2,4-DP adsorbed on HDTMA–clay decreased continuously with pH increase from 3 to 8, and it also reduced markedly as the pH decreased from 3 to 1.5. The maximum 2,4-DP sorption was thus observed to occur near a pH of 3. Sorption of 2,4-DP by HDTMA–clay decreased as pH decreased below 3 since the fraction of molecular 2,4-DP increased Žp K a s 3.0.. On the other hand, adsorption on HDTMA– clay decreased with increasing in pH from 3 to 8 since the ionic form of 2,4-DP increased. Because the sorption of the anionic form is favoured Ž Fig. 5. , overall sorption is reduced under these conditions. Since the degree of dissociation of 2,4-DP is very small at low pH, adsorption of 2,4-DP by the HDTMA–clay interlayer is greater in H-bonding than the molecular adsorption involving hydrophobic partition. At low pH, it may be involved partial dissolution of the HDTMA–clays. HDTMA molecules appear to combine to form an aggregate and reduce the surface area, and charge reversal occurs at more than 100% CEC of HDTMA, consequently reducing the number of accessible sites ŽCowan and White, 1957; Schnitzer, 1986; Chandar et al., 1987.. In addition, Ghosh and Schnitzer Ž1980., and Schnitzer Ž 1995. have noted that the three parameters controlling the molecular characteristics of organics are the concentration of the species of organic materials, pH of the system, and ionic strength of the medium. However, due to the high pH levels ŽpH ) p K a . during preparation of HDTMA–clay, the clay contains residual negative charge and more COOy functional groups, which cause repulsive force between the HDTMA–clay and the 2,4-DP molecules.
5. Conclusions Most of the HDTMA–bentonites were prepared by single exposure of the clay to HDTMA solution containing CEC equivalent of the clay, or some fraction of it. For Wyoming bentonite, the samples thus prepared displayed very broad XRD peaks at 1.4–1.8 nm. Preparation of HDTMA–clays were affected by the HDTMA concentrations, temperature, solvents, and type of clays. Sorption of 2,4-DP was studied for characterizing the surface heterogeneity of these HDTMA–clays. The Freundlich equation functions quite well to describe the experimental data, particularly for small molecules. Sorption of 2,4-DP by HDTMA–clays were affected by the concentration in adsorbates and the pH in suspensions.
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Acknowledgements We thank Professor A.R. Mermut, University of Saskachewan, for helpful discussions and the National Science Council of Taiwan, Republic of China for financial support under grants NSCa 84-2621-P-002-035 and 86-2621-B-002022-A07.
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Sorption of 2,4-dichlorophenoxy propionic acid by organo-clay complexes Y.H. Hsu, M.K. Wang ) , C.W. Pai, Y.S. Wang Graduate Institute of Agricultural Chemistry, National Taiwan UniÕersity, Taipei, 10764, Taiwan Received 7 October 1998; received in revised form 6 July 1999; accepted 16 July 1999
Abstract Wyoming montmorillonite ŽSWy-1, MON., Charng-Bin ŽSCB. soil montmorillonite, and W.R. Grace vermiculite ŽVER. have been used as adsorbents to prepare organo-clays. Sorption of 2,4-dichlorophenoxy propionic acid Ž2,4-DP. by organo-clays is greater after adding of hexadecyltrimethyl-ammonium ŽHDTMA. to 200% CEC of the clay than after adding the same amount of HDTMA to 100% CEC of clay ŽHDTMA–clay.. The Freundlich adsorption isotherm describes 2,4-DP sorption by HDTMA–clays Ž K f s 1.828–11.474 l kgy1, n f s l.130–1.454, R s 0.919– 0.944. quite well. Low surface charge montmorillonite can allow a high concentration of 2,4-DP to penetrate into the interlayer with low residual charge on the wedge. Thus, with increasing 2,4-DP concentration, the basal d-spacings of HDTMA-montmorillonite complexes increased from 2.15 to 2.71 nm. However, increasing the concentration of 2,4-DP did not result in further swelling of vermiculite owing to the high surface charge. In general, HDTMA–montmorillonite ŽO-MON. sorbed more 2,4-DP than HDTMA–soil montmorillonite ŽO-SCB. and HDTMA– vermiculite ŽO-VER.. Thus, the interlayer sorption of 2,4-DP exhibited a critical level, beyond which further expansion of the clays does not occur. The equilibrated pH in the suspension affected the adsorption by HDTMA–clay complexes. The maximum 2,4-DP adsorption occurs at a pH level of approximately 3 corresponding to p K a s 3.0 of 2,4-DP in the adsorption–pH curve. q 2000 Elsevier Science B.V. All rights reserved. Keywords: hexadecyltrimethyl-ammonium clays ŽHDTMA–clays.; 2,4-dichlorophenoxy propionic acid Ž2,4-DP.; montmorillonite; sorption; vermiculite
1. Introduction Soil is known to function as a chemical as well as a biological filter that lessens the environmental impact of organic chemicals introduced into the )
Corresponding author. Tel.: q886-2-2363-0231 ext. 2491 or 3066; Fax: q886-2-2366-0751; E-mail: [email protected] 0169-1317r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 1 3 1 7 Ž 9 9 . 0 0 0 3 9 - 3
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biosphere by various processes. Soil acts as a first line of defense against the leakage of these compounds into the groundwater. Soil achieves this by absorbing organic chemicals, as well as biological andror chemical degradation. Consequently, to predict the behavior of organic pollutants in soils, and to evaluate the risk that a particular chemical might leak into groundwater, it is necessary to understand the nature and extent of sorption and degradation making it an important subject for research Ž McBride, 1994. . Adsorption and desorption of organic molecules in the soils is primarily controlled by the chemical properties of the molecules and surface properties of the specific soil. The 2:1 layer silicates are important constituents of soil to prepare organoclays for 2,4-dichlorophenoxy propionic acid Ž 2,4-DP. sorption. Vermiculite, with its high surface charge density is mostly substituted in the tetrahedral sheets of the clays; while montmorillonite, with its small surface charge, is substituted mostly in the octahedral sheet Ž Dixon and Weed, 1989. . Inorganic cations in the interlayers can be exchanged with organic or inorganic cations. Hydrophobic clays can be changed to organophilic clays by organic chemicals. Such organophilic clays have various applications in paper, petroleum Žoil-based drilling fluids for deep wells. , catalytic, water treatment, and several other industries, and also act as sorbents for a great variety of organic pollutants, such as phenols ŽJordan et al., 1950; Theng, 1974; Mortland et al., 1986; Boyd et al., 1988a. , and some weakly acidic pesticides which have been detected in surface waters ŽWauchope, 1978; Hermosin and Cornejo, 1992, 1993. . The organo-clays have been shown to act as partition media in adsorption of these organic pollutants ŽMortland, 1970; Boyd et al., 1988a; Janes and Boyd, 1991. . The HDTMA–montmorillonite Ž O-MON. , HDTMA–soil montmorillonite ŽO-SCB., and HDTMA–vermiculite Ž O-VER. were selected to act as sorbents for adsorption of 2,4-DP. 2,4-DP is a metabolic of herbicide and inhibits the growth of broad leaf weeds in soil. Thus, the objectives of this study were to investigate the sorption capacity and mechanism of 2,4-DP by three types of HDTMA–clays. These results may be applicable in the black soils typical of eastern Taiwan since clay fractions in black soils contain large amounts of montmorillonite.
2. Materials and methods 2.1. Materials 2.1.1. Clays, soils and chemicals The reference clay mineral, namely Na-montmorillonite Ž SWy-1. was obtained from Source Clay Repository Ž Clay Minerals Society Ž CMS. , Columbia, MO., and zonalite vermiculite was obtained from W.R. Grace Repository Ž W.R.
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Grace and Co., Cambridge, MA. . The black soil sample was collected from surface horizon of the Charng-Bin ŽSCB. in eastern Taiwan, which is known as an abundant source of montmorillonite Ži.e., 60%. and small amount of vermiculite Ži.e., 3%. in clay fractions Ž Pai et al., 1999. . The CEC of untreated original SWy-1 ŽMON., soil clay ŽSCB. and vermiculite ŽVER. were 80.5, 54.0, and 71.3 cmol kgy1, respectively The soil samples were first treated with H 2 O 2 , and dithionite–citrate–bicarbonate solutions to remove organic matter, free metal oxides and hydroxides ŽMehra and Jackson, 1960. . All the samples were then ultrasonicated and fractionated to less than 2 mm of clays, prepared as Na-saturated clays, and then freeze dried. Cation-exchange capacities of clays ŽŽ CarMg. exchange. were determined as described by Jackson Ž1979. . The clay samples were characterized, saturated with Mg and K, and prepared in the form of slurries on petrographic slides for X-ray diffraction ŽXRD. analysis. For the Mg-saturated SWy-1 and soil clays, the XRD peak shifted from 1.4 to 1.85 nm after glycerol-solvation; whereas with the K-saturated vermiculite heated at 1058C, the d-spacing of the XRD peak shifted from 1.4 to 1.0 nm. The HDTMA and 2,4-DP were obtained from Sigma. 2.2. Methods 2.2.1. Sorption and determinations of HDTMA Bromophenol blue Ž BPB. stock standard solution Ž 5 = 10y4 M. was prepared by dissolving accurately weighed amounts of dye in 5 ml of 0.01 M NaOH solution and diluting it to 200 ml with distilled water. For HDTMA determination, the pH of the filtrate was adjusted to 8.0 with 0.1 M phosphate buffer solution. Appropriate amounts of BPB and HDTMA solutions were mechanically shaken for 20 min with 5 ml chloroform ŽYamamoto, 1995. . After phase separation, the absorption of the organic phase was measured against that of chloroform at wavelength 608 nm. A GBC911 UV–Vis spectrophotometer was used for recording the spectra and determining the absorbance measurements of the solutions placed in quartz cells with 10-mm path length. 2.2.2. Preparation and characterization of HDTMA–clays The clay samples Ž 0.1 g. were first treated three times with HDTMA Ž 30 ml of 60%, 100%, 150%, 200%, 250% and 300% of the CEC to clays. to equilibrate within 8 h. The clay-organic suspensions were then centrifuged, washed with an ethanol–water solution Ž 50:50. five times, and then freeze-dried. Different concentrations of HDTMA were obtained by adding 100% or 200% of the CEC to clays and are referred as O-MON1, O-MON2; O-SCB1, O-SCB2; and O-VER1, O-VER2, respectively. The effect of temperature was investigated in the range 308C, 408C, 608C, 708C and 808C. Methanol, ether, acetone, and chloroform Ž 10 ml. were also used for adsorption of HDTMA and the results
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were compared with those of water. After each treatment, the HDTMA–clays were then prepared as slurries and dropped onto petrographic slides for XRD analysis Ž Jackson, 1979. . The oriented samples were examined by X-ray diffractometer, using a Rigaku Miniflex with CuK a radiation generated at 30 kV and 10 mA. The XRD patterns were recorded in the range of 2 to 3082 u at a scanning speed of 182 u miny1. For FT-IR spectroscopic analysis, the HDTMA–clays were also freeze-dried and prepared as KBr disc Ž Biorad FTs-7 IR.. 2.2.3. 2,4-DP sorption by HDTMA–clays For the 2,4-DP adsorption isotherm studies, 0.02 g of HDTMA–clay was placed in 50 ml centrifuge tube and supplemented with 10 ml of 0, 0.04, 0.20, 0.40, 0.80 and 1.60 mM of 2,4-DP solutions and shaken 24 h. Small amounts of suspension were dropped onto petrographic slide for XRD analysis. The other suspensions were centrifuged at 24,000 = g and 258C for 30 min and had their 2,4-DP concentration determined in supernatants. Successive adsorptions of 2,4-DP by HDTMA–clays were prepared by treatment of 0.1 g of HDTMA–clay with 10 ml of 0.5 mM of 2,4-DP, shaken for 4 h. The HDTMA–clay was first separated from the supernatant by centrifugation and then, the old supernatant was removed and replaced by fresh 10 ml of 2,4-DP solution. After eight successive treatments, the complexes were prepared as slurries and dropped onto petrographic slides for XRD analysis. Sorption of 2,4-DP at different pH levels was measured using 0.02 g of HDTMA–clay and 10 ml of 1 mM 2,4-DP solution in 0.01 M CaCl 2 . The pH of the suspensions was initially adjusted to pH levels of 1, 2, 3, 4, 5, 6, 7 and 8 with 0.1 M HCl or NaOH. These suspensions were then shaken for 24 h and centrifuged under the same conditions as the adsorption isotherm experiments. The equilibrium pH and 2,4-DP concentrations were then determined in the supernatants. The amount of adsorbed herbicide was calculated from the difference between the initial and equilibrium 2,4-DP concentrations, as determined by UV spectroscopy at 230 nm ŽBogus et al., 1990; Hermosin and Cornejo, 1993. . All the experimental determinations were done in duplicate. A blank sample Ž 0.02 g of sample with 5 ml of 0.01 M CaC1 2 . was used for background correction.
3. Results 3.1. Preparation of HDTMA–clays The basal spacing Ž d oo1 . and interlayer d-spacings obtained from d-spacing of the XRD analysis of Na-montmorillonite ŽSWy-1. treated with HDTMA ŽMON. are summarized in Fig. 1. Interlayer d-spacings of HDTMA–clays were
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Fig. 1. Basal Ž d oo1 . and interlayer d-spacings of HDTMA–clay ŽSWy-1..
calculated from the corresponding d oo1 minus basal spacing of 2:1 clays Ž i.e., 0.96 nm. ŽDixon and Weed, 1989. . For addition of 150% CEC of montmorillonite ŽSWy-1. , basal d-spacing as well as interlayer d-spacing exhibit maximums at 2.25 and 1.31 nm, respectively. The basal d-spacing then reaches a constant value when more HDTMA is added to the clays. Sorption characterization of HDTMA by soil montmorillonite Ž SCB. is similar to that of MON, with the only difference that HDTMA was observed to be unable to penetrate into the interlayer completely. The XRD data of the SCB clay showed that aluminum hydroxy-interlayered compounds were present in the interlayer. The XRD peak in the Mg-saturated SCB-clay shifted to 1.7 nm when glycerol solvation after the sample was pre-treated with 0.3 M Na-citrate Ž Pai et al., 1999. . Although the properties of soil clay were characterized as montmorillonite, its clay intercalated with HDTMA molecules is not expandable as that of SWy-1. Infrared spectra of HDTMA–clay complexes show that C–H stretching at 2929 and 2856 cmy1 for the 60% CEC clay sample gradually shifted to 2917 and 2849 cmy1 for the 300% CEC clay sample ŽFig. 2A–F.. The addition of HDTMA to 100% CEC of vermiculite resulted in a material with XRD peaks at 2.88, 1.88 and 1.39 nm, and corresponding interlayer d-spacings of 1.91, 0.91 and 0.42 nm. However, after increasing the HDTMA concentration to 200% CEC, the only XRD peak appeared at 1.98 nm, which is constituent with two layers Ž200% CEC. of HDTMA being present in the interlayers of highly charged vermiculite ŽLagaly and Weiss, 1969; Lagaly, 1982. . 3.2. Effect of temperature and solÕents on HDTMA–clay preparation The effect of temperature on clay expansion was not as significant for SWy-1 and soil clay as the concentration effect. However, with the addition of l00%
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Fig. 2. FT-IR spectra between 2700 and 3100 cmy1 for the samples containing ŽA. 60%, ŽB. 100%, ŽC. 150%, ŽD. 200%, ŽE. 250%, ŽF. 300% of the CEC added to SWy-1, and ŽG. HDTMA.
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Fig. 3. Adsorption isotherm of 2,4-DP on HDTMA–clay complexes.
CEC of HDTMA to vermiculite, the XRD peak corresponding to the 2.88 nm d-spacing becomes more intense for the samples prepared at 808C, as compared to that of the otherwise similarly prepared sample at 308C. A comparison of the d-spacing for the samples prepared from methanol, ether, acetone and chloroform by adding l00% and 200% CEC to clays, shows that low polarity of chloroform results in greater d-spacing values in both l00% and 200% CEC HDTMA–clay samples. When the HDTMA–clay complexes were treated with various solvents, chloroform produced the largest swelling of the clay Ž Lagaly, 1994.. 3.3. Sorption of 2,4-DP by HDTMA–clay Sorption isotherm of 2,4-DP adsorbed by HDTMA–clay Ž 200% CEC. is shown in Fig. 3. The Freundlich isotherm equation fits quite well with the experimental data Ž K f s 1.828–11.474 l kgy1, n f s 1.130–1.273, R s 0.919– 0.944. ŽTable 1. ŽJanes and Boyd, 1991. . The K f and n f values show the trend
Table 1 Freundlich 2,4-DP adsorption parameters of O-MON2, O-SCB2 and O-VER2 Sample
K f Žl kgy1 .
nf
R
O-MON2 O-SCB2 O-VER2
11.474 1.828 2.680
1.130 1.454 1.273
0.944 0.919 0.924
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Table 2 The d oo1 of HDTMA–clays equilibrated with various concentrations of 2,4-DP 200% CEC clay
Initial 2,4-DP concentration Žm M . 0
0.04
0.20
0.40
0.80
1.60
2.25 b – – – – 1.90 b – 1.01
2.70 b 2.30 1.37 – – – 1.20 1.02
2.72 b 2.19 1.33 2.66 b 1.31 – 1.25 1.00
2.71b 2.02 1.33 2.69 b 1.31 – 1.25 1.00
d-spacing Žnm. O-MON2
O-SCB O-VER2
a b
2.21b –a – 2.03 b – 2.06 b 1.51 1.38
2.19 b – – 2.61b 1.32 1.99 b 1.42 1.01
–: Not detected. Symbol indicates the majored d-spacing.
of O-MON2) O-VER2) O-SCB2 and O-SCB2) O-VER2) O-MON2, respectively. The 2,4-DP adsorption by three types of HDTMA–clay show a quite clear linear relationship with the Freundlich equation. In general, HDTMA– montmorillonite sorbed more 2,4-DP than HDTMA–vermiculite. The d-spacing and adsorption isotherm ŽTable 2. show that maximum adsorption of 2,4-DP by O-MON2, O-SCB2 and O-VER2 are 160, 140 and 130 mmol kgy1 HDTMA– clay, respectively ŽFig. 3.. Eight successive sorptions of 2,4-DP by HDTMA– clay with 200% CEC of HDTMA showed more adsorption than that by 100% CEC HDTMA–clay Ž Hsu, 1996. and showed the similar trend of O-MON2) O-SCB2) O-VER2 ŽFig. 4.. The effect of pH on adsorption of 2,4-DP by
Fig. 4. Successive adsorption curves of HDTMA–clay complexes Žcumulative adsorption..
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Fig. 5. Effect of pH on adsorption of 2,4-DP by HDTMA–clay complexes.
HDTMA–clay is shown in Fig. 5. In general, the maximum 2,4-DP adsorption occurs at a pH level of approximately 3. When the pH is less than 3, 2,4-DP is present as a molecular phase Žp K a of 2,4-DP s 3.0., whereas when the pH exceeds 3, it is mostly of ionic type.
4. Discussion X-ray diffractograms of the samples containing 60% to 250% CEC of HDTMA added to SWy-1 show the most prominent d-spacing Ž d oo1 . in the range of 1.48 to 2.11 nm. However, when 300% CEC of HDTMA is added to clay, the XRD pattern not only shows 2.11 nm peak but also exhibits 2.68 and 1.32 nm peaks, corresponding to the HDTMA molecules, thus indicating that excess HDTMA is precipitated on the external sites of the clay. The ordering of HDTMA–bentonite complexes may be enhanced by exposing the clay to HDTMA in excess of CEC if care is taken to remove the excess salt by washing with water or ethanol. Ethanol and ethanol–water mixtures yield well-dispersed suspensions and well-oriented aggregates for X-ray analysis Ždata not shown. . The d-spacings in the 100% and 200% of CEC SWy-1 samples as well as soil clay, and vermiculite do not show evidence of interaction of excess HDTMA with clays. From these data, it can be seen that the layer and pseudolayer of HDTMA penetrate into the interlayer of montmorillonite, paraffin and that the pseudolayer of HDTMA penetrate into the interlayer of vermiculite used for 2,4-DP sorption studies Ž Boyd et al., 1988a,b; Xu and Boyd, 1994. . For various
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organic solvents with different polarities, the HDTMA reaction shows that chloroform, with the lowest polarity, offers the best expansion for HDTMA for either 100% or 200% CEC added to the three types of clays. The infrared spectra show that the C–H stretching vibrations at 2929 and 2856 cmy1 for the sample containing 60% CEC gradually shifted to lower frequencies for the sample containing 300% CEC Ž Fig. 2. , thus indicating bonding between tetrahedral sheets Ž Si–O. at weak concentration of HDTMA molecules. At large loading of HDTMA, the spectra are similar to those of solid HDTMA Ž Fig. 2G. because the chains are densely packed. However, the N–H stretching peak at 3017 cmy1 appears only after addition of HDTMA at concentrations of higher than 100% CEC to the clays, thus indicating that the free molecules of HDTMA exceed the clay saturation ŽFig. 2C–G. . Zhang et al. Ž1993. have proposed that sorption of quaternary amines on montmorillonite involves at least three types of reactions, viz. a cation-exchange reaction, adsorption of ion pair and tail–tail interactions Ž Xu and Boyd, 1994. . 4.1. Sorption of 2,4-DP by HDTMA–clays Cumulative sorption of 2,4-DP by different clays is shown in Fig. 4. In general, the trend of adsorption for 200% CEC added to HDTMA–clays is O-MON2) O-SCB2) O-VER2. After eight successive treatments, the amount of 2,4-DP adsorbed was found to be relatively low, and the adsorption of 2,4-DP at low concentrations by HDTMA–clay resulted in expansion of the interlayer of clay. HDTMA–clay containing 200% CEC of HDTMA adsorbed more 2,4-DP than did the HDTMA–clay with 100% CEC Ž Hsu, 1996. . This suggests that for HDTMA–clay complexes, new adsorption sites on the external surface were exposed by expanding the silicate layer due to the strong ring–tail attraction as more and more 2,4-DP molecules were adsorbed. As a qualitative example, 200% CEC added HDTMA–clay samples Ž O-MON2 and O-SCB2. exhibited limited d-spacings when the 2,4-DP concentrations were less than 1.60 mM Ž Table 2. , whereas, the O-VER2 sample exhibited limited expansion when the 2,4-DP concentration was less than 0.20 mM. Due to the entrapment of 2,4-DP molecules, cross-linkage in herbicide, and the high charge density of vermiculite, these 2,4-DP molecules adsorbed on the wedge of tetrahedral sheets, so small concentrations of 2,4-DP are able to penetrate into the interlayer of HDTMA–clay. On the other hand, due to the large concentrations of 2,4-DP molecules at the cross-linkages and entrapped coil molecules, along with the high charge of vermiculite, the 2,4-DP molecules are adsorbed on the wedge of the interlayer, and hence, the herbicide cannot penetrate into the interlayer. Low surface charge montmorillonite, however, can allow large concentrations of 2,4-DP to penetrate into the interlayer with small residual charge on the wedge. Hence, with increasing 2,4-DP concentration, the basal d-spacings of HDTMA–montmorillonite complexes increased from 2.15 to 2.71 nm. The van
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der Waal area for cosmo solvation Ž water. is 1.094 nm2 and the length of 2,4-DP is 0.603 nm. Variation in 2,4-DP adsorption as a function of pH of the equilibrated solution is plotted in Fig. 5 for O-MON2, O-SCB2, and O-VER2. Although the 2,4-DP adsorption on O-VER was much less than that on O-MON in the pH range of 1–8 ŽHermosin and Cornejo, 1993. , the amount of 2,4-DP adsorbed on HDTMA–clay decreased continuously with pH increase from 3 to 8, and it also reduced markedly as the pH decreased from 3 to 1.5. The maximum 2,4-DP sorption was thus observed to occur near a pH of 3. Sorption of 2,4-DP by HDTMA–clay decreased as pH decreased below 3 since the fraction of molecular 2,4-DP increased Žp K a s 3.0.. On the other hand, adsorption on HDTMA– clay decreased with increasing in pH from 3 to 8 since the ionic form of 2,4-DP increased. Because the sorption of the anionic form is favoured Ž Fig. 5. , overall sorption is reduced under these conditions. Since the degree of dissociation of 2,4-DP is very small at low pH, adsorption of 2,4-DP by the HDTMA–clay interlayer is greater in H-bonding than the molecular adsorption involving hydrophobic partition. At low pH, it may be involved partial dissolution of the HDTMA–clays. HDTMA molecules appear to combine to form an aggregate and reduce the surface area, and charge reversal occurs at more than 100% CEC of HDTMA, consequently reducing the number of accessible sites ŽCowan and White, 1957; Schnitzer, 1986; Chandar et al., 1987.. In addition, Ghosh and Schnitzer Ž1980., and Schnitzer Ž 1995. have noted that the three parameters controlling the molecular characteristics of organics are the concentration of the species of organic materials, pH of the system, and ionic strength of the medium. However, due to the high pH levels ŽpH ) p K a . during preparation of HDTMA–clay, the clay contains residual negative charge and more COOy functional groups, which cause repulsive force between the HDTMA–clay and the 2,4-DP molecules.
5. Conclusions Most of the HDTMA–bentonites were prepared by single exposure of the clay to HDTMA solution containing CEC equivalent of the clay, or some fraction of it. For Wyoming bentonite, the samples thus prepared displayed very broad XRD peaks at 1.4–1.8 nm. Preparation of HDTMA–clays were affected by the HDTMA concentrations, temperature, solvents, and type of clays. Sorption of 2,4-DP was studied for characterizing the surface heterogeneity of these HDTMA–clays. The Freundlich equation functions quite well to describe the experimental data, particularly for small molecules. Sorption of 2,4-DP by HDTMA–clays were affected by the concentration in adsorbates and the pH in suspensions.
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Acknowledgements We thank Professor A.R. Mermut, University of Saskachewan, for helpful discussions and the National Science Council of Taiwan, Republic of China for financial support under grants NSCa 84-2621-P-002-035 and 86-2621-B-002022-A07.
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