RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The triple-layer model of the oxide/water interface can be used to calculate the partitioning of metals among solid and aqueous phases. The defensible use of the triple-layer model in groundwater/sediment systems requires an adequate and consistent set of intrinsic adsorption constants. In the present study, published values of p*K^int for cation adsorption on iron and manganese oxides have been used to calculate values for surface complexation constants (log K^SC) via

log K_M(OH)n^SC = pK_a2^int − p*K_M(OH)n^int − log β_1n

where pK_a2^int is the intrinsic acidity constant and β_1n is the nth cation hydrolysis constant. This transformation reduced the variation between log log K^SC values determined by different investigators. Uncertainties in acidity constants and variations in site loading with adsorbing metal are the major sources of variation in the values of p*K^int. In addition, ionic strength can affect the values of p*K^int for strongly adsorbed cations. Predictive equations based on ion size and hydrolysis behavior have been derived and missing values of p*K^int for important pollutant metals predicted. Although these equations do not explicitly account for variations in ionic strength and surface loading, they are useful for predicting values of p*K^int with uncertainties of 0.5–0.8 (α-FeOOH) and 0.4–1.5 log units (δ-MnO₂. Recently published K^SC values validate the predictive equation developed for the first and second hydrolysis products of thorium. A data base of p*K^int values is presented in which the variability in pK_a2^int values are removed and missing values estimated.