Parameterization of Freundlich adsorption isotherms for heavy metals in soils from an area with intensive livestock production

Abstract

Adsorption coefficients are valuable tools used to estimate the environmental relevance of heavy metal contamination. However, their determination with batch experiments is laborious. Thus, attempts have been made to deduce these coefficients from soil parameters. However, the application of the resulting parameterized equations to different sets of samples has often yielded poor results. Hence, the objectives of the present study were (1) to deduce basic soil properties governing the coefficients of Freundlich adsorption isotherms for Cd, Cu, Ni, Pb, and Zn, and (2) to derive parameterized isotherms and examine their accuracy. For this purpose, 30 topsoil and nine subsoil samples were investigated which represented one Podzol-Cambisol-Gleysol soilscape in an area with intensive livestock production in Lower Saxony, Germany. Total background concentrations (aqua regia digestion) of heavy metals in topsoils ranged from 0.290 (Cd) to 19.2 mg kg -1 (Zn) and exhibited elevated mobilizable proportions (NH 4 EDTA pH 4.6 extract) of 17 (Ni) - 66 % (Zn) from the total concentration. Background concentrations were higher in topsoils than in subsoils by factors of 1.7 (Ni) - 28 (Zn). These differences were assigned to the special situation of heavy metal input mostly originating from animal excrements. The isotherms obtained by batch experiments showed larger coefficients K F for partition among soil solid phase and soil solution in topsoils than in subsoils by a factor of 3.5. The coefficients of the isotherms were significantly correlated with routinely determined soil properties such as cation exchange capacity and pH (R = 0.36-0.89). Parameterized isotherms were calculated for each metal by inserting the relevant parameters in multiple linear regression equations. Among these parameters were the soil pH, cation exchange capacity, total metal concentration, and the concentrations of organic carbon, clay, fine silt, and various pedogenic oxides. The K F values, separately calculated for topsoils and subsoils, agreed well with those determined by batch experiments (R = 0.63-0.97). Therefore, parameterized isotherms are valuable tools for the prediction of heavy metal partition in soils from one soilscape and for a risk assessment in the investigated, densely stocked area and similar areas.