Nickel and copper sorption in acid forest soils

Five acid forest soils of eastern Austria were used to study nickel and copper sorption at metal concentrations ranging over five orders of magnitude. Distribution coefficients were calculated, and the Langmuir equation was fitted to the experimental isotherm data. Exchange for mono- and divalent cations is considered the primary mechanism of Ni sorption, whereas complexation on organic surfaces may contribute significantly to Cu sorption in the studied soils. Organic matter is considered the most effective sorbent in topsoils, while sorption in subsoils may involve primarily Mn-oxide and clay mineral surfaces. The soils studied showed higher affinity for Cu than for Ni; however, at low metal levels, competitive complexation of Cu with soluble organic compounds counteracted sorption in topsoil horizons. Using the van Bemmelen-Freundlich equation, general sorption density isotherms were developed for the studied soils. For Ni, similar fits were obtained by using specific surface area (SSA) and cation exchange capacity (CEC) as reference quantities; however, the quality of fit was improved when only the proportion of CEC occupied by mono- and divalent cations (CEC(2+)) was used as a reference. For Cu, the surface area-based approach yielded a better fit than the charge-based isotherms. Comparison with independent datasets from the literature showed good agreement with the proposed general sorption density isotherms, suggesting general applicability for acid soils of varying origin and composition.