Complexation of cadmium to sulfur and oxygen functional groups in an organic soil

Cadmium (Cd) is a toxic trace element and due to human activities soils and waters are contaminated by Cd both on a local and global scale. It is widely accepted that chemical interactions with functional groups of natural organic matter (NOM) is vital for the bioavailability and mobility of trace elements. In this study the binding strength of cadmium (Cd) to soil organic matter (SOM) was determined in an organic (49% organic C) soil as a function of reaction time, pH and Cd concentration. In experiments conducted at native Cd concentrations in soil (0.23 mu g g(-1) dry soil), halides (Cl, Br) were used as competing ligands to functional groups in SOM. The concentration of Cd in the aqueous phase was determined by isotope-dilution (ID) inductively-coupled-plasma-mass-spectrometry (ICP-MS), and the activity of Cd2+ was calculated from the well-established Cd-halide constants. At higher Cd loading (50054,000 mu g g(-1)), the Cd2+ activity was directly determined by an ion-selective electrode (ISE). On the basis of results from extended X-ray absorption fine structure (EXAFS) spectroscopy, a model with one thiolate group (RS-) was used to describe the complexation (Cd2+ + RS- reversible arrow CdSR+; log K-CdSR) at native Cd concentrations. The concentration of thiols (RSH; 0.047 mol kg(-1) C) was independently determined by X-ray absorption near-edge structure (XANES) spectroscopy. Log KCdSR values of 11.2-11.6 (pK(a) for RSH = 9.96), determined in the pH range 3.1-4.6, compare favorably with stability constants for the association between Cd and well-defined thiolates like glutathione. In the concentration range 500-54,000 mu g Cd g(-1), a model consisting of one thiolate and one carboxylate (RCOO-) gave the best fit to data, indicating an increasing role for RCOOH groups as RSH groups become saturated. The determined log K-CdOOCR of 3.2 (Cd2+ + RCOO- reversible arrow CdOOCR+; log K-CdOOCR; pK(a) for RCOOH = 4.5) is in accordance with stability constants determined for the association between Cd and well-defined carboxylates. Given a concentration of reduced sulfur groups of 0.2% or higher in NOM, we conclude that the complexation to organic RSH groups may control the speciation of Cd in soils, and most likely also in surface waters, with a total concentration less than 5 mg Cd g(-1) organic C. (c) 2006 Elsevier Inc. All rights reserved.