Metal adsorption onto oxides from aqueous solutions: The influence of the electrolyte, ionic strength and surface coverage on surface complexation

A comprehensive model that describes metal adsorption onto mineral surfaces in different electrolytes over a range of ionic strengths and metal concentrations would be invaluable to the study of geochemical processes such as chemical weathering and trace metal cycling in the environment. In this study, data for divalent metal adsorption over a range of ionic strengths and surface coverages from different electrolytes, are reviewed and analyzed using the extended triple-layer model (ETLM). Data for metal adsorption onto oxides from different electrolytes over a wide range of ionic strengths (0.0001 M--1.0 M) reveal that transition and heavy metal adsorption is a function of the type of electrolyte. Analysis of adsorption edges with the ETLM demonstrates that transition and heavy metals form many different types of complexes on solid surfaces. In particular, ternary surface complexes of the metal and electrolyte anion (e.g., >SOHM2+--NO3-, >SOHMClO4-, and >SOHMCl+) describe transition and heavy metal adsorption over a range of ionic strengths onto solids such as goethite and corundum.;Metal adsorption data over a range of surface coverages is typically characterized by curvilinear adsorption isotherms. The isotherm curvature may be attributed to changes in surface charge and potential which depend on the predominant type of metal surface complex. The single-site ETLM is used to reinterpret adsorption isotherms and pH edges for a variety of metals and solids in different electrolyte solutions. Again, surface complexes involving both the metal cation and the electrolyte anion are often found to play a role in describing metal adsorption behavior. This single-site model can account for metal surface coverages ranging from 10-4mumol/m 2 to 10.2 mumol/m2 if appropriate adsorption reactions are chosen.;Comparisons of ETLM analyses for different datasets help verify the selection of particular surface complexes and equilibrium constants, expose discrepancies between experimental results from different laboratories, and determine the predominant surface complexes in different compositional domains. In conclusion, metal adsorption on oxide surfaces occurs by many different reactions. The type of surface complexes formed depends on the metal type, solution composition (i.e., pH, ionic strength, and electrolyte), and solid.