| Organic acids are commonly present and become an important factor in affecting transformation and transfer of heavy metals in environment. Hydroponic experiments were carried out to study the effects of dissolved organic matter on toxicity and bioavailability of lead and cadmium. Batch and pot experiments were conducted to investigate the effects of citric acid(CA), oxalic acid(OA), ethylenediamine tetra-acetic acid(EDTA), humic acid(HA) and diethylenetriamine penta-acetic acid(DTPA) on adsorption and desorption, transformation and transfer and phytoavailability of lead and cadmium, and speciation distribution of them in plants by using simulated experiments. The main results were summarized as follows:Firstly, in hydroponic experiment, citric acid and malic acid decreased the toxicity of lead and increased its bioavailability and its transfer to shoot of mustard. Oxalic acid decreased the toxicity and bioavailability of lead. Citric acid and oxalic acid decreased the toxicity of cadmium, but malic acid increased the toxicity of it, and they also increased the bioavailability of it. EDTA and DTPA decreased the toxicity and bioavailability of lead and cadmium. Humic acid and soil extracts increased the uptake of cadmium and transfer to shoot.Secondly, in batch experiment, different kinds of organic acid could change the characteristics of the adsorption and desorption for heavy metals in soil. Citric acid promoted the adsorption and desorption of lead in soil. Citric acid promoted the desorption of cadmium, while oxalic acid restrained the desorption of cadmium. Humic acid increased the adsorption of lead to some extent in soil, it restrained the adsorption of cadmium and desorption of lead. EDTA and DTPA increased the adsorption and desorption of lead, EDTA restrained the adsorption of cadmium and promoted its desorption.Thirdly, in all lead treatments, the predominant species were Fe-Mn oxide-bond and organic-bond lead. The exchangeable and carbonate-bond lead changed when organic acids were added. This part of lead was taken up easily by mustard. In cadmium treatment, the predominant cadmium was carbonate-bond, Fe-Mn oxide-bond and residual species. When organic acids were added, the major species changed to carbonate-bond and Fe-Mn oxide-bond cadmium, which played an important role in uptake of plants.Finally, citric acid, low concentration of humic acid, EDTA and DTPA increased the uptake of lead by stem of mustard. Citric acid increased the uptake of lead by root, and low concentration of oxalic acid, EDTA, DTPA and humic acid increased the transference of cadmium to stem. Citric acid, oxalic acid and high concentration of humic acid decreased the transference of it to stem, but EDTA and DTPA increased that. In all lead treatment, the predominant species in root and stem were hydrochloric acid extract lead. The predominant cadmium in root and stem was sodium chloride extract. The percentage of sodium chloride and distilled water extract cadmium in root of mustard increased when EDTA was added, and citric acid increased the percentage of acetic acid extract species of it in stem of mustard.
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