Proposal of new distribution coefficients (Kd) of potentially toxic elements in soils for improving environmental risk assessment in the State of São Paulo, southeastern Brazil

Soil solid-solution distribution coefficients (Kd) are used in predictive environmental models to assess public health risks. This study was undertaken to determine Kd for potentially toxic elements (PTE) Cd, Co, Cr, Cu, Ni, Pb, and Zn in topsoil samples (0–20 cm) from 30 soils in the State of São Paulo, southeastern Brazil. Batch sorption experiments were carried out, and PTE concentrations in the equilibrium solution were determined by High Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICPMS). Sorption data was fitted to the Freundlich model. The Kd values were either obtained directly from the slope coefficients of C-type isotherms or derived from the slope of the straight line tangent to the non-linear L-type and H-type isotherms. Stepwise multiple regression models were used to estimate the Kd values through the combined effect of a number of soil attributes [pHH2O, effective cation exchange capacity (ECEC) and contents of clay, organic carbon, and Fe (oxy)hydroxides]. The smallest variation in Kd values was recorded for Cu (105-4598 L kg-1), Pb (121-7020 L kg-1), Ni (6-998 L kg-1), as variation across four orders of magnitude was observed for Cd (7–14,339 L kg−1), Co (2–34,473 L kg−1), and Cr (1–21,267 L kg−1). The Kd values for Zn were between 5 and 123,849 L kg−1. According to median values of Kd, PTE were sorbed in the following preferential order: Pb > Cu > Cd > Ni > Zn > Cr > Co. The Kd values were best predicted using metal-specific and highly significant (p < 0.001) linear regressions that included pHH2O, ECEC, and clay contents. The Kd values reported in this study are a novel result that can help minimize erroneous estimates and improve both environmental and public health risk assessments under humid tropical edaphoclimatic conditions.