Influence of Organic Complexation on the Adsorption Kinetics of Nickel in River Waters

The kinetics of Ni adsorption in rivers of widely different chemical characteristics have been studied by monitoring the uptake of 63Ni by suspended sediment particles. The rate and extent of adsorption was critically dependent on the presence and concentration of dissolved organic matter (DOM), defined analytically as the concentration of dissolved organic carbon (DOC). Thus, adsorption was greatest in experiments in which the DOM was decomposed by UV-oxidation and least in experiments in which Ni was preequilibrated with filtered river water before addition of particles. The extent of adsorption arising from the latter approach displayed a clear, inverse dependency on the concentration of DOC in the sample. These observations were interpreted and modeled in terms of the competing effects of DOM and particle sorption sites for dissolved Ni. Adsorption onto suspended particles in the absence of dissolved complexing ligands was adequately described by a reversible, two-stage reaction and a single set of adsorption rate constants. Forward and reverse rate constants defining Ni complexation with DOM of about 8 x 10(4) h(-1) M(-1) and 10 h(-1), respectively, were derived from data-fitting. The experimental and model results indicate that the toxicity, availability, and transport of Ni in freshwater environments are largely dictated by the concentration of DOM and the speciation of Ni entering the watercourse.