Effect of different organic ligands on cadmium sorption by and extractability from soils

Soil solutions contain a variety of low molecular weight dissolved organics from breakdown of plant residues and exudation from plant roots, and complexation of metals by these organic ligands can play an important role in controlling metal solubility. The effect of the organic ligands acetate, citrate, fumate, malonate, maleate, salycylate, succinate, and tartrate on the extractability and sorption of Cd was investigated in a range of Cd-treated soils varying widely in soil chemical characteristics. Experimental parameters were adjusted to investigate the effect of pH and organic ligand concentration on Cd extractability. The soils were also extracted dth NaNO3 of equivalent pH and concentration. The organic ligands always extracted more Cd than did NaNO3, indicating that organic complexation increased the extractability of Cd from soil. Extractable Cd ranged from <0.30 to 1 mu mol kg(-1); the greatest Cd concentration in each soil was released in the presence of maleate. In general, the ability to desorb Cd followed the order: maleate > fumarate = succinate > tartrate > malonate > oxalate > salicyclate when the pH of the extract was between 3.1 and 3.4; in this pH range competition for complexation between the metal ion and ligand anion is greater than the affinity of the metal ion for the soil surface. For all soils and organics, extractable Cd decreased with increasing pH of the ligand solutions. The amount of NaNO3-extractable Cd also decreased.with increasing pH, and the amount desorbed was not detectable at pH values exceeding 5.5, suggesting that at high pH values, Cd is released into soil solution only as metal-organic complexes.