Chromate Adsorption by Kaolinite

Chromate (CrO4 2-) adsorption was investigated on kaolinite (0.2-2 ~m) saturated with NaC104 over a range of pH. Adsorption increased with decreasing pH because of protonation of chromate and/ or variable charge sites on kaolinite. Chemical pretreatment to remove noncrystalline and crystalline oxide contaminants affected the magnitude ofCrO42adsorption, but not the pH range over which CrO42adsorbed. Chromate adsorption at different sorbate and sorbent concentrations increased below the pHz~ for the kaolinite edge, suggesting the formation of weak surface complexes. If f r O 4 2and SO42were present at equal concentration (5.0 x 10 -7 M), the two solutes sorbed independently, suggesting binding to separate sites. The presence of excess SO42(5.0 x 10 -4 M), however, unexplainably enhanced CrO42adsorption. The adsorption of both chromate and sulfate can be described in terms of a site-binding model of the kaolinite edge, in which the edge is viewed as composite layers of AI and Si oxide. Surface complexation constants for CrO4 2on kaolinite were similar to those for alumina, pointing to the importance of A1-OH edge sites in chromate adsorption. Key Words--Adsorption, Chromate, Kaolinite, Sulfate, Surface complex. I N T R O D U C T I O N Chromate (CrO4 2-) is an anionic contaminant often associated with industrial and power generation wastes that are disposed to the larud surface. In domestic waters, chromate is a regulated constituent (U.S. Environmental Protection Agency, 1986), because it may be toxic to man and other organisms. Adequate protection of the terrestrial environment and subsurface water supplies and the mitigation of impacts where they occur require information on the chemical reactions between chromate and soil and subsurface materials that slow solute movement relative to the transporting water front. Fe and AI oxides bind CrO42effectively at low to medium pHs (MacNaughton, 1977; Leckie et aL, 1980; Davis and Leckie, 1980; Mayer and Schick, 1981; Honeyman, 1984). Like other acid anions (e.g., SeO4 2-, 5042-, SeO32-, msO42-), f rO42exhibits pH-dependent adsorption on these solids due to surface charging and aqueous solute speciation. Like oxides of AI and Fe, kaolinite also contains hydroxylated edge and defect sites, which may be important points for chromate adsorption in soils and subsurface materials. Kaolini te adsorbs acid anions, including SO42(Rao and Sridharan, 1984), MoO42(Phelan and Mattigod, 1984), PO43(Kafkafi et aL, 1967; Nagarajah et aL, 1968; Chen et al., 1973), H3BO 3 and B(OH)4(Mattigod et al., 1985), and frO42(Griffen et aL, 1977), The favored adsorption mechanism is surface complexation by aluminol groups (A1OH) exposed pr imari ly on crystal edges (Chen et al., 1973; Swartzen-Allen and Matijevic, 1974). Because of the complex surface chemistry of kaolinite, few investigators have quanti tat ively l inked its measured surface properties with its anion adsorpt ion Copyright 9 1988, The Clay Minerals Society behavior. By applying chemical models of surface complexation, however, both Riese (1982) and Liechti (1983) succeeded in using surface-ionization and electrolyte-surface-complexation constants derived from titration data to simulate metallic cation adsorption on kaolinite. Their success in stimulating metal adsorption over ranges of pH and ionic strength suggests that a comparable approach may be tenable for anion adsorption on the kaolinite. Consideration of the surface charge-potential relationship as affected by the proton and strongly complexing solutes is particularly important for anion adsorption in which the electrostatic conditions at the solid-liquid interface exert a strong influence on adsorption (Bolan and Barrow, 1984; Zachara et al., 1987). In this paper, we extend earlier published work (Griffen et al., 1987) on adsorption of frO42by kaolinite. Chromate adsorption is measured as a function o f p H and solution concentration on carefully sized and prepared kaolinite to elucidate the surface-complexation reaction. The effect of chemical pretreatment is demonstrated, as is the influence o f SO42 as a co-sorbing ion. The triple-layer model (TLM) and the chemical equil ibrium program FITEQL, in combinat ion with data from the literature on kaolinite surface properties, are used to simulate adsorption as a function o f pH, and intrinsic adsorption constants calculated for the kaolinite surface are compared to those for a luminum oxide. EXPERIMENTAL PROCEDURES Clay preparation and analysis A well-crystallized Georgia kaolinite (KGa-1, from the Source Clays Repository of the Clay Minerals Society) was used for the adsorption experiments. Ka-