The effect of surface reactions on the relative abundances of trace metals in deep-ocean water

Abstract

An improved general approach for estimating the relative abundances of trace metals in aquatic systems is introduced. It is proposed that the relative concentrations of trace metals in deep-sea water are regulated by two opposing processes: adsorption to and transport on inorganic and organic particle surfaces, and desorption driven by complexing with dissolved ligands in sea water. The estimated behavior is based on the difference between the surface affinity of a metal and its affinity for aqueous ligands, that is, the free energy difference between surface complexation and aqueous complexation of the metal. The simplest formulations of metal behavior are based on differences in standard state free energies, e.g., standard free energy of adsorption on oxide vs. standard free energy of aqueous chloride complexing. In a specific instance, the relative concentrations of trace metals in deep-ocean water are predicted from the ratios of their first hydrolysis constants (which are surrogates for adsorption constants)and their first chloro or carbonate complexing constants. More detailed assessments of relative metal abundances in sea water must incorporate both standard-state energy term (Δ log β) and aqueous phase activities of important ligands, e.g., chloride and carbonate ions. The lack of data for the complexation of trace nelements with PO 4 3−, HPO 4 2−, and H 2PO 4 − precludes the use of potentially important phosphate-complexes in our calculations. The general approach is applied with success to examine chemical and geochemical groupings of oceanic trace metals and to assess adsorption vs. dissolved carbonate complexation as predominant processes governing the concentrations of rare earth elements (REEs) in deep-ocean water. While trace metals in rivers are largely bound to suspended matter and tend to retain upper continental crustal ratios in unpolluted environments, most trace elements in deep-sea water are enriched relative to iron, and the degree of enrichment is directly related to the difference in affinity of the metal for dissolved ligands and O-donor surface groups on particles.