Iodide, caesium and strontium adsorption by organophilic vermiculite

Soils represent an important filter and buffer system for many cations and anions. Generally, their sorption properties are determined by the contents and compositions of clay minerals and humic substances. A great part of the soil organic matter is associated with clay minerals forming organomineral complexes (Greenland, 1965) which are of pronounced importance with respect to element transport and/or retention and to aggregate stability. Clay minerals also interact with organic cations. The replacement of inorganic cations by alkylammonium ions causes considerable modification in the hydration and swelling properties of the clays. Research in this area has significantly increased the knowledge on the physico-chemical behaviour of both clay minerals and organo clay minerals (Weiss, 1963; Mortland, 1970; Theng, 1974; Lagaly, 1981; Rausell-Colom & Serratosa, 1987). Cation exchange capacities (CEC) of naturally occurring organo-mineral complexes were determined by Leinweber et al. (1993). It was observed in samples extracted from nine different soils that the potential CEC decreased with increasing equivalent diameters of the separated particles (i.e. fine and medium clay: 489-813 mmolc kg-l; coarse clay: 367-749 mmolc kg-1; fine silt: 2 0 2 5 8 7 m m o l c k g l ; m e d i u m s i l t : 63-345 mmolc kg-l). The CEC varied with factors associated with the soil genesis, the mineralogical composition of the <6.3 p.m sizefraction and the C and N contents of the particle size-fractions. It has been shown in extensive investigations that organophilic clay minerals can adsorb non-ionic organic compounds (Mortland et al., 1986; Jaynes & Boyd, 1991; Xu & Boyd, 1994) as well as iodide, which exists predominantly in the anionic form in terrestrial ecosystems (Lieser & Steinkopff, 1989). After modification of bentonite, vermiculite and Cretaceous clay by hexadecylpyridinium (HDPy +) and benzethonium (BE+), these organoclay minerals exhibited adsorption rates and amounts for iodide ions which are several orders of magnitude higher than those of untreated samples. Moderate increases in the adsorption parameters were found after cation exchange with hexadecyltrimethylammonium (HDTMA+), while the treatment with trimethylphenylammonium 9 1997 The Mineralogical Society