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Clays and Clay Minerals, Vol.47, No. 3, 366-374. 1999.

M O D E L I N G OF O R G A N I C A N D I N O R G A N I C C A T I O N S O R P T I O N BY ILLITE TAMARA POLUBESOVA AND SHLOMO NIR The Seagram Center for Soil and Water Sciences, The Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel Abstraet--Sorption of several organic and inorganic cations on illite (Clay Minerals Society Source Clay Imt-2) was determined experimentally and results compared to model calculations. The cations studied were crystal violet (CV+), benzyltrimethylammonium (BTMA§ benzyltriethylammonium (BTEA+), Ca 2+, Mg 2+, K +, Na +, Cs +, and Li +. The adsorption-model calculations involved a solution of the electrostatic Gouy-Chapman equations. The model considered specific adsorption and sorption/exclusion in the double-layer region in a closed system. Model calculations considered the simultaneous presence of four to six cations in the system. The adsorption of CV included formation of neutral and charged complexes. The adsorption attained 0.37 mol kg i or 150% of the cation exchange capacity (CEC) of illite in aqueous suspension. The adsorption of BTMA and BTEA did not exceed the CEC and was reduced with an increase in ionic strength. The sorption of CV below the CEC was rather insensitive to the ionic strength because of the large binding coefficients and was only slightly reduced in NaC1, CsC1, or Na2SO4 solutions. When added in amounts exceeding the CEC in high ionic strength, 0.667 M NaNO 3, NaC1, or CsC1 solutions, the adsorbed quantities of CV increased to three times the CEC. At high sulphate concentrations (0.333 M Na2SO4), the adsorption was below the CEC. Model calculations yielded satisfactory simulations for the adsorption, particularly for cations added in amounts approaching or exceeding the CEC. The binding coefficients for formation of neutral complexes followed the sequence: CV > Ca > BTMA > BTEA > Cs > Mg > K > Na > Li. Model calculations also suggested that sites were present which bound exchangeable cations, particularly K +, Na +, and Mg 2+, very tightly. Key Words--Benzylalkylammoium, Cation Adsorption Model, Crystal Violet, Illite, Ionic Strength, Source Clay Imt-2, Montmorillonite.

INTRODUCTION L i k e m o n t m o r i l l o n i t e , the clay m i n e r a l illite is a m a j o r c o m p o n e n t o f the clay f r a c t i o n in arid soils ( F a n n i n g e t al., 1989) w h i c h are a b u n d a n t in desert areas, s u c h as the N e g e v in Israel ( R a v i k o v i t c h e t al., 1972). A d s o r p t i o n o f organic cations o n illite m a y affect the fate o f a g r o c h e m i c a l s in soils. This a d s o r p t i o n o n clays c h a n g e s the n a t u r e o f the clay surface, t r a n s f o r m i n g it f r o m h y d r o p h i l i c to h y d r o p h o b i c . T h u s , the m o d i f i e d clay surface c a n sorb o r g a n i c m o l e c u l e s o f low solubility in w a t e r (Lee e t aL, 1989; J a y n e s a n d B o y d , 1991; Lagaly, 1987, 1994; Z h a n g et al., 1993). B e n zylalkylammonium cations pre-adsorbed on montmorillonite are v e r y effective for the s o r p t i o n o f h y d r o p h o b i c h e r b i c i d e s , s u c h as a l a c h l o r (E1-Nahhal et al., 1998). T h e nature o f c l a y - o r g a n i c c a t i o n i n t e r a c t i o n s was r e v i e w e d p r e v i o u s l y ( M o r t l a n d , 1970; T h e n g , 1974; Serratosa, 1979; Yariv et al., 1989). O r g a n i c cations are a d s o r b e d o n the clay m i n e r a l surface b y n e u tralization o f the n e g a t i v e electrical c h a r g e r e s p o n s i b l e for the c a t i o n e x c h a n g e c a p a c i t y ( C E C ) o f the clay ( M o r t l a n d , 1970) a n d a b o v e the C E C b y h y d r o p h o b i c i n t e r a c t i o n s ( M a r g u l i e s e t al., 1988; J a y n e s a n d B o y d , 1991; R y t w o et al., 1995). T h e m e c h a n i s m s o f the o r g a n o - c l a y interactions d e p e n d o n the structure o f the m o l e c u l e s , the nature o f the f u n c t i o n a l groups, the m o lecular weight, a n d the g e o m e t r y o f the o r g a n i c toolCopyright 9 1999, The Clay Minerals Society

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ecules ( T h e n g , 1974; Serratosa, 1979; Yariv et aI., 1989). T h e r e is little i n f o r m a t i o n o n a d s o r p t i o n o f o r g a n i c cations o n illite. Q u a t e r n a r y a m m o n i u m cations c a n b e a d s o r b e d o n illite in e x c e s s o f the CEC. T h e m e c h a n i s m s i n v o l v e d in the a d s o r p t i o n are c a t i o n e x c h a n g e ( C E C ) ( G r i m , 1947; J a y n e s a n d B o y d , 1991). T h e a d s o r p t i o n o f cationic p o l y m e r s o c c u r r e d p r i m a r i l y o n e x t e r n a l surfaces o f illite ( B e n - H u t et aL, 1992). T h e m e c h a n i s m o f a d s o r p t i o n of m o n o v a l e n t o r g a n ic c a t i o n s - - c r y s t a l v i o l e t ( C V ) a n d b e n z y l a l k y l a m m o n i u m o n m o n t m o r i l l o n i t e , w a s studied e x p e r i m e n tally a n d t h e o r e t i c a l l y ( R y t w o et al., 1995; P o l u b e s o v a e t aL, 1997). T h e s e organic cations w e r e c h a r a c t e r i z e d b y e x t r e m e l y large ( C V ) a n d v e r y large b i n d i n g coefficients ( B T M A ) , respectively. T h e m a i n o b j e c t i v e o f this w o r k is to study the a d s o r p t i o n o f the c a t i o n i c dye CV and the benzylalkylammonium cations, B T M A a n d B T E A , o n illite b y c o m b i n i n g e x p e r i m e n tal m e t h o d s a n d m o d e l calculations. T h i s c o m b i n a t i o n will p r o v i d e i n f o r m a t i o n o n the a d s o r p t i o n characteristics of o r g a n i c c a t i o n s w h o s e affinity to the clay m a y v a r y b y several orders o f m a g n i t u d e . In addition, B T M A a n d B T E A w e r e e m p l o y e d in the d e s i g n o f o r g a n o - c l a y s as a basis for h y d r o p h o b i c h e r b i c i d e form u l a t i o n s to r e d u c e leaching. To a c h i e v e this objective, w e d e t e r m i n e d a d s o r p t i o n c h a r a c t e r i s t i c s o f the

Vol. 47, No. 3, 1999

Adsorption of organic and inorganic cations on illite

BTMA

BTEA

CH3

~

CH z CH3

~

CH 3

M W = 185.7

CIHzCH~ M W =227.78

CV

fNtC H :)~

{C H ~l2

N[C H 31~

MW=408.0

Figure 1. Structural models of the molecules of CV, BTMA, and BTEA.

exchangeable cations Ca > , Mg 2+, K +, and Na +. For completeness, we also determined the binding coefficients of Cs + and Li +. Our study also provides information about competition for adsorption on illite between organic and inorganic cations. M ATER I A L S A N D M E T H O D S Illite (IMt-2) from Silver Hill, Montana, obtained from the Source Clays Repository (Clay Minerals Society) was used. The mineralogy and chemical properties of Silver-Hill illite were characterized by Hower and Mowatt (1966) and by Srodofi and Eberl (1984). The clay is a mixture of illite and illite-smectite (Srodoff and Eberl, 1984). We refer to the sample as "illite". The sample was used without further purification. CV was purchased as the chloride salt from Fluka Chemic A G (Buchs, Switzerland). Benzyltrimethylammonium (50 wt. % in water solution) was obtained as the chloride salt and benzyltriethylammonium (99%) was obtained as the bromide salt from Aldrich Chemical Company (Milwaukee, Wisconsin). Analytical grade salts were used; LiC1 and MgCI2 were purchased from J.T. Baker, Inc. (Phillipsburg, New Jersey). NaC1 and KC1 were obtained from Frutarom Laboratory Chemicals (Haifa, Israel); N a 2 S O 4 w a s obtained from Analar B D H Chemicals (Poole, England), CsC1, CaC12, and NaNO3 were purchased from M e r c k (Darmstadt, Germany). All materials were used without further treatment or purification. The structural models of CV, BTMA, and B T E A are shown in Figure 1. Adsorption isotherms

The adsorption of CV, BTMA, and B T E A on drysieved illite (fraction