RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Oxides of iron are ubiquitous subsurface mineral constituents and control the mobility of metal ions in soils and groundwater by adsorptive retardation. In this laboratory study, iron-oxide-coated sand (IOCS) was used to model Pb chemical behavior in soil using batch reactor experiments. Several parameters were varied in order to ascertain their effects: pH, ionic strength, complexation by organic ligands, competing cations, and reaction time. Results indicated that equilibrium was reached in less than 24 hr in completely mixed batch systems. In equilibrium experiments, pH was the major factor that controlled the adsorption process, and increasing Ca²⁺ and Na⁺ electrolytes somewhat decreased Pb adsorption below pH values of 5, but had no influence at higher pH values. Using a surface complexation, triple-layer model, inner-sphere surface binding was successful in describing the effect of pH and ionic strength on Pb adsorption on IOCS over a range of Pb concentrations. EDTA and NTA greatly decreased Pb adsorption at equimolar and greater concentrations over the pH range of 3 to 10. PbEDTA²⁻ was adsorbed on the IOCS by ligand-like adsorption, increasing with decreasing pH, while PbNTA²⁻ adsorption on IOCS was metal-like, increasing with increasing pH.