ADSORPTION OF COPPER AND CADMIUM ONTO SOILS: INFLUENCE OF ORGANIC MATTER (ISOTHERM, COMPLEXATION, TRACE METALS)

The sorption of copper and cadmium by three arid-zone soils at low solution concentrations (i.e. <10 mmol m('-3) for copper and <50 (mu)mol m('-3) for cadmium) was studied. The majority of previous studies of metal sorption by soils reported in the literature were conducted at higher resolution concentrations of metals. Garc(')ia-Miragaya observed previously that data for cadmium sorption at low solution concentrations conformed to the van Bemmelen-Freundlich equation. McLaren et al. reported earlier that data for copper sorption by soils could be fit to the Langmuir equation. Sorption isotherms in the present study were characterized by an S-curve which is defined by an initial slope that increases with metal concentration.; Preliminary kinetic studies indicated that copper sorption reactions were completed within one hour. The sigmoidal character of the isotherm was found always to be greater for copper than for cadmium and greatest for the soil highest insoluble organic carbon. The distribution function, K(,d), for cadmium was a monotonically decreasing function of the amount of metal sorbed. Values of K(,d) first increased and then decreased with increasing amounts of copper sorbed. Although increasing sorbent concentration decreased copper sorption, the shape of the isotherm remained unchanged.; A soil separate was prepared by successive extractions with a low electrolyte solution in order to remove most of the soluble organic matter. L-curve isotherms were observed for copper and cadmium sorption by the soil separate; the surface exhibited a high affinity for the metals relative to aqueous solution. The addition of a commercially prepared fulvic acid to the soil separate reproduced an S-curve for copper sorption. Likewise, addition of fulvic acid to the unamended soils increased the sigmoidal character of the copper isotherms. A kinetic study suggested that either fulvic acid was not sorbed by the soil separate within the time period of experiment (24 hours) or that it was readily desorbed by copper. Copper was readily desorbed by fulvic acid at concentrations that are found in the soil environment. It was proposed that soluble organic ligands can form nonsorbing, soluble complexes with copper and cadmium.; Cupric ion measurements indicated that the sorption isotherm for cupric ion was an L-curve isotherm regardless of the concentration of soluble organic matter. This suggested that the copper-fulvate complex was not sorbed by the soil surface. A mathematical description of the S-curve isotherm was derived from the combination of two macroscopic models involving the binding of metal by soluble organic ligands and metal sorption.