Evaluation of affinity constants of Cu, Cd, Ca and H for active soil surfaces for a solid phase-controlled soil ligand model

Abstract

A new approach, derived from the concept of the biotic ligand model, was used for the determination of the affinity constants of Ca, Cu, Cd and H to the active surfaces of different kinds of soils. This approach allowed us to obtain consistent data and to integrate these values in the chemical equilibrium modelling software MINEQL+ and eventually into a solid phase-controlled soil ligand model. This could then very easily be transformed into a terrestrial biotic ligand model by adding constants for biological components. We obtained the chemical speciation of the metals of interest by integrating the initial characteristics of the soil (pH; cation exchange capacity, CEC; total metal concentrations in soil extracts; ionic strength; and CO2 pressure). Comparison of the predicted and measured values of free Cu2+ is excellent using soil-specific affinity constants as well as average values. The average affinity constants between the active soil surfaces (S) and the target cations are log KCa?S ≤ ?0.84 (±0.01), log KCu?S ≤ 5.3 (±0.1), log KCd?S ≤ 4.4 (±0.2) and log KH?S ≤ 4.1 (±0.2). External soils have been used to validate the conceptual model and the results show a very good correlation between the predicted and the measured free Cu (pCu) except for an acidic soil (pH < 5.2), highlighting the importance of integrating Al into the model. © CSIRO 2008.