The Study Of The Wash Efficiency Of Plant Material On Removing Zn And Cd From Polluted Soil

Heavy metal contaminant in soil has been worldwide urgent environment problem. Soil washing technique is a kind of efficient soil repair technique. However, nowadays, the chemical eluents commonly consist of the problem like secondary contaminant, etc. Therefore, it is important to find an efficient and environmental friendly eluent. Based on that, this study aim to overcome the secondary contaminant and severe soil damage by traditional chemistry washing, discuss the efficiency of repairing Zinc and Cadmium polluted soil by plants material based eluents. This paper based on soil from farmland and mine as experiment soil, applied vibration washing technique, studied efficiency of removing Zinc and Cadmium from polluted soil under different eluent concentrations, pH values, washing times and washing frequencies by four different plants material(Pisum sativum, PS; Iris tectorum Maxim, IT; Broussonetia papyrifera, BP; Isatis indigolica Fort, ?) based eluents. This paper also studied the change of chemical species and nutrients of eluents before and after experiments. And this study aim to provide some theory for application of repairing Zinc and Cadmium polluted soil. This paper include the following concludes:(1) With increasing of eluent concentrations, the washing efficiencies for Zinc and Cadmium in two kinds of polluted soil increased fast at beginning and then slowly. Specifically, the biggest washing efficiencies all showed when eluent concentrations were 90 g/L. However, while considering the actual cost,50 g/L was the best choice. In this eluent concentration, washing efficiencies for Zinc and Cadmium in two kinds of polluted soil by Iris tectorum Maxim based eluents were the best. Specifically, the washing efficiencies of Zinc and Cadmium in farmland soil were 31.89% and 26.15%, separately, while the washing efficiencies of Zinc and Cadmium in mine soil were 42.94% and 36.83%.(2) With increasing of pH value, the washing efficiencies for Zinc and Cadmium in two kinds of polluted soil decreased. Although when pH=3 washing efficiencies arrived the largest, over acidic washing condition would destruct soil structure, so pH=4 was considered as the suitable pH value. Under this acidic condition, washing efficiencies for Zinc and Cadmium in two kinds of polluted soil by Iris tectorum Maxim based eluents were better than other eluents. Specially, its washing efficiencies of Zinc and Cadmium in farmland soil were 30.39%and 25.95%, separately, while the washing efficiencies of Zinc and Cadmium in mine soil were 41.40% and 36.75%.(3) With increasing of washing time, the washing process were divided into two steps, which were fast increasing step within 2 hours and slow increase from 2 to 10 hour. But the washing process by Iris tectorum Maxim based eluents towards Cadmium in two kinds of polluted soil didn't followed above rule. Above all, consider the best ratio between costs and washing efficiency,2 h was considered as the washing time for following experiments. Using equation to model washing time and washing quantity, the results showed that all model relation coefficient were larger than 0.90 and had very significant level (p<0.01) except the experiment groups which used Pisum sativum based eluents to wash Cadmium in two kinds of polluted soil.(4) When continuously washing 4 times, washing efficiencies decreased in every times, but accumulated washing efficiencies increased obviously (p<0.05). Specifically, the washing efficiency of Pisum sativum; Iris tectorum Maxim; Broussonetia papyrifera and Isatis indigotica Fort based eluents towards Zinc in farmland soil were 35.21%, 54.92%,42.92% and 46.62%, separately, while their washing efficiency towards Cadmium in farmland soil were 33.84%,47.93%,39.97% and 42.36%, separately. And the washing efficiency of Pisum sativum; Iris tectorum Maxim; Broussonetia papyrifera and Isatis indigotica Fort based eluents towards Zinc in mine soil were 60.72%,73.72%,61.79% and 67.40%, separately, while their washing efficiency towards Cadmium in mine soil were 54.02%,66.43%,58.10% and 61.39%, separately. The results showed that the accumulative washing efficiency of Iris tectorum Maxim based eluent towards Zinc and Cadmium were more than these efficiency of other plants based eluents.(5) Before washing, the chemical species of Zinc in farmland soil were mainly Fe-Mn oxides-bound Zn and residual, while the chemical species of Cadmium in farmland soil were mainly carbonates-bound Cd and Fe-Mn oxides-bound Cd. And the chemical species of Zinc in mine soil were mainly carbonates-bound Zn and Fe-Mn oxides-bound Zn, while the chemical species of Cadmium in mine soil were mainly exchangeable Cd and carbonates-bound Cd. After washing, the amount of exchangeable Zn, carbonates-bound Zn and Fe-Mn oxides-bound Zn were obvious decreased (p<0.05). The amount of each specie of Cadmium showed decrease tendency (p<0.05) except residual Cd in farmland soil.(6) Applying Pisum sativum; Iris tectorum Maxim; Broussonetia papyrifera and Isatis indigotica Fort as eluents material, the amount of soil organic matter after washing obvious increased (p<0.05), but total nitrogen and alkali hydrolysable nitrogen; total phosphorus and available phosphorus; total potassium and available potassium all decreased in some extents (p<0.05), but the decreasing were limited. And total potassium and available potassium decreased least.