CADMIUM SORPTION ON SPECIMEN AND SOIL SMECTITES IN SODIUM AND CALCIUMELECTROLYTES

Although specimen smectities (e.g., SWy-1) are often used as analogues of the exchanger phase in smectitic soils, few comparisons of metal i on sorption on specimen and soil smectites have been made. In this stu dy, the sorption of Cd was measured on SWy-1 and on clay-sized separat es from two smectitic subsoils to evaluate the selectivity of specimen and soil-derived smectites for Cd. Sorption was measured in clay susp ensions (approximate to 1 mmol(c) L(-1) equivalent charge concentratio n at pH 6.0) in Na+, Ca2+, and Na+-Ca2+ perchlorate solutions across p H 4.5 to 8.5 and at ionic strengths (I) ranging from 0.005 to 0.1. Ion ic strength and electrolyte cation valence strongly influenced Cd sorp tion by SWy-1 and the soil smectites. Ion exchange dominated Cd sorpti on at low ionic strength in Na+ electrolyte (I = 0.005-0.014). Increas ing Na+ concentrations to I = 0.1 or changing the electrolyte cation t o Ca2+ at I = 0.003 to 0.006 suppressed ion exchange. When ion exchang e was suppressed, Cd sorption to both specimen and soil smectites show ed little dependence on ionic strength and increased with pH. Except a t the lowest Na+ concentration (I = 0.005), eon ditional equilibrium c onstants (K-v) for Cd2+ exchange increased with increases in both ioni c strength and pH. These increases were ascribed to Cd complexation re actions to edge sites on the layer silicates whose effects became evid ent only under conditions that suppressed ion exchange. At pH 6 and I = 0.05-0.01, SWy-1 did not exhibit any preference for Na+, Ca2+, or Cd 2+. The smectitic soil separates, in contrast, showed (i) sorption beh avior that increased sharply with pH, (ii) preference for Cd in Na+ an d Ca2+ electrolytes, and (iii) variation in K-v with ionic strength, p H, and surface coverage. The contrasting sorption behavior of the soil smectites was hypothesized to result from (i) a greater edge surface area, which increased the contribution of oxide-like complexation reac tions to Cd sorption, and (ii) the presence of minor associated organi c material and Fe oxides that functioned as co-complexants for Cd.