Effects of Dissolved Organic Matter and pH on Toxicity and Bioavailability of Lead

The germination test was carried out to study the effect of different organic ligands citric acid(CA),malic acid(MA),oxalic acid(OA),ethylenediamine tetra-acetic acid(EDTA),diethylenetriamine penta-acetic acid(DTPA),humic acid(HA) and soil extract(A) on toxicity and bioavailability of Pb2+ and its uptake by mustard.Mustard seeds were exposed to solutions containing Pb2+ and several kinds of dissolved organic matters,and the root lengths and the concentration of Pb2+ in mustard sprouts were measured.The results indicated that low molecular weight organic acids(citric and malic acids) decreased the toxicity of Pb2+ and increased its bioavailability.Low concentrations of the synthetic chelators ethylenediamine tetra-acetic acid(EDTA),oxalic acid(OA) and diethylenetriamine penta-acetic acid(DTPA) decreased the toxicity and bioavailability of Pb2+.In contrast,humic acid increased the toxicity of Pb2+ and decreased the bioavailability of Pb2+.The toxicity and the bioavailability of lead increased at pH5,while decreased at pH5.Consequently,the bioavailability and toxic effects of Pb2+ in soil depended on the nature of coexisting organic substances in the soil solution and the pH of soil solution.