Competitive Complexation of Cd(II) and Cu(II) by Water‐Soluble Organic Ligands and Na‐Montmorillonite

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Abstract
Water soluble extracts obtained from a coniferous forest litter layer (FLLE), dried Chicago sludge (CSE), and a peat soil (PE) were used as a source of organic ligands to study the influence of metal complexation on the adsorption of Cd and Cu by Na‐montmorillonite. Adsorption of 50 to 0.1µM CdT and CuT by Na‐montmorillonite (2 g L−1) was measured in the presence of the FLLE (1.8 mM total carbon) over a pH range of 4 to 8.5 after a 2‐h equilibrium period under humidified N2 gas. Equilibrium activity measurements for Cd2+ and Cu2+ were made with ion selective electrodes for experiments at 50 and 10 µM total metal (MT). After accounting for inorganic complexation, the amount of Cd complexed with organic ligands in the FLLE was much lower than that for Cu. Multiple regression equations where pMads (metal adsorbed) was tested as a function of pH and pMTS (total soluble metal concentration) showed that Cu adsorption decreased with increased pH values in contrast to adsorption in the absence of FLLE. Conversely, the adsorption of Cd was only slightly affected. Adsorption of 10 µM CuT in the presence of CSE and PE showed similar effects on adsorption, revealing the importance of water soluble complexing ligands in controlling Cu speciation.