The results for the complete set of materials are in agreement with those from previously reported studies because an acidic pH favours the soJubilisation of metals and the increase in the DOC that can bind heavy metal ions in solution (Clemente et al., 2007). In addition. the presence of a mineral fraction in the GW material may also benefit the relatively high Kd values observed for this material (Gonzalez-Nunez et al., 2011 ). The relatively high Kd (Cd) determined for this material may be due to the significant S content because Cd forms stable complexes with 5 functional groups (Karlsson et al., 2(07).