Determination of free heavy metal ion concentrations in soils around a cadmium rich zinc deposit

The study of heavy metal speciation is of interest for the evaluation of their ecotoxicological risk. Free heavy metal ion concentrations of Cu2+, Zn2+, Ni2+ and Cd2+ were studied at different soil to soil solution ratio (SSR) to discuss metal bioavailability and toxicity in circumstance. The samples were collected from two farmed soils, impacted by a cadmium rich zinc deposit, Guizhou, China. One is near a bridge (QBT for short), which is 15 km away from smelter and is regarded as polluted by fly ash from smelter, the other is located 300 m away from a smelting factory (BXT for short), and it is polluted badly by waste water from smelter. To determine the free heavy metal ion concentration, Donnan Membrane Technique (DMT) and ECOSAT program were used. The concentrations of free copper and zinc ions in QBT and all studied metal ions in BXT were nearly in direct ratio with the soil to soil solution ratio, and they increased with the increasing of SSR. The variational trends of the concentrations of free nickel and cadmium ions in QBT were contrary to those of copper and zinc ions. The measured and predicted results were compared and they agreed with each other. Individual sorbents contribution to metal binding was also predicted. The results suggest soil organic matter is the most important sorbents in the two soils, and hydrous ferric oxide takes the second place.