Phytoremediation On Zinc Smelting Areas Western Guizhou, China

The widely spread indigenous zinc smelting sites in western Guizhou, China, with a history of 300 years, have caused serious pollution of heavy metals of lead (Pb), zinc (Zn) and cadmium (Cd) in slags due to low recovery rate. A large amount of fumes rich in heavy metals and sulfur dioxide, have destroyed vegetation around the smelting areas and have released heavy metals into soil, plant and water body. Although indigenous zinc smelting activities have been abolished due to heavy environmental pollution in 2004, 20 millions tons of open dumped slags and 1200 hectares of polluted land were environmental concerns at present with high risk to the local ecosystem. It is of importance to understand revegtation on smelting wasteland and remediation on heavy metals in the smelting areas. This study is conducted to: 1) analyze distribution and transfer of heavy metals and degradation of soil and vegetation in the polluted environments; 2) explore limited factors on revegetation, substrate amendment methods and strongly-tolerant plant; 3) screen hyperaccumulators for heavy metals and study their accumulation properties and mechanism. The main research results were as follows.Concentrations of Pb, Zn and Cd in slags averaged at 4632 mg/kg, 8968 mg/kg, and 58 mg/kg, respectively, whereas 234 mg/kg Pb, 400 mg/kg Zn and 9.6 mg/kg Cd occurred in soil around the smelting areas. The geochemical leaching test showed that Pb, Zn and Cd in slags have low mobility and bioavailability because their concentrations presented small percentages (all less than 2 %) in the exchangeable fraction, whereas the contaminated soils had higher mobility and bioavailability for the metals. The Pb, Zn and Cd concentrations in exchangeable and carbonate fractions in slags were high and posed potential environmental risks. Concentrations of Pb, Zn and Cd were high in the local stream water but low in groundwater. In the stream water, Pb, Zn and Cd were significantly concentrated in the suspended sediments, and indicated that metal-rich erosion process of slag and contaminated soil contributed to metal mobility into stream water. To remediate the local environment polluted by Zn smelting activity, revegetation is an effective approach to control dispersion of Pb, Zn and Cd in the indigenous zinc smelting areas.Compared to background soil, slags have higher pH values and EC values, lower CEC values, lower organic matter and nitrogen. The pH values and bioavailable P contents in contaminated soil decrease compared to background soil. The indigenous zinc smelting activities destroyed vegetation around the smelting areas. The natural restoration for vegetation is quite slow on smelting waste land, especially on slags. Therefore, it is important to accelerate revegetaion by artificial measures so as to control soil and water erosion and dispersion of heavy metals.Pot experiments were applied to understand the limited factors on revegetation and substrate amendment on smelting waste land by planting Lolium perenne, Trifolium pretense and Robinia pseudoacacia. The limited factors to revegetation on newly-produced slags were salt-alkali stress, low contents of organic matter and nutrients (total N, bioavailable N and total K). The salt-alkali stress in slags was remarkably mitigated, their soil nutrients were improved after long-term eluviation. Therefore, the constraints to revegetation in slag decreased remarkably after 20 years. The slag still has high porosity and aeration, however its capacity of water provision and retention were poor with low field capacity and wilting coefficient. Therefore, droughty stress was constraint to revegetation in slag. The constraints to revegetation in contaminated soil were toxicity of heavy metals and low bioavailable P.Through pot and field experiments, some findings were obtained: 1) alien soil amendment on slag could mitigate salt-alkali stress, improve nutrients in new smelting residue, and increase water provision and retention. Revegetation could be obtained in newly-produced slag by alien soil and stong-tolerant plants. 2) Alien soil or hydrogel applied in old slag could increase water provision and retention and accelerate revegetation. 3) Applying lime in contaminated soil could improve available nutrients and reduced toxicity of heavy metals, improving growth of plant.A new Zn-hyperaccumulator, Corydalis davidii, was found in the smelting areas through screening in fields and hydroponic solutions. Corydalis davidii has strong tolerance for Pb, Zn and Cd, even though high accumulation for Pb and Cd. 70～90 percent of Pb, Zn and Cd accumulated in cell wall. Antioxidant enzymes (SOD, POD, CAT) are important protection systems of Corydalis davidii. Activities of antioxidant enzymes increase with increase of Pb and Cd concentrations in lower concentrations, but decrease in higher metal concentrations. Activities of antioxidant enzymes slightly increase with increase of Zn concentration.