Remediation Of Heavy Metal Contaminated Soil By Chemical Leaching And Phytoremediation Combined Technology

With the rapid development of economy, industry and mining activity in our country for recent years, the heavy metal pollution of soils is more and more widespread, and its degree is more and more serious. Heavy metal pollution in soils has the characteristics of concealment, long-term existing, irreversible. It is difficult to remediate, and easy to cause harm to human health through the way of direct exposure and enrichment of the food chain. The methods of the remediation of heavy metal contaminated soils mainly include: physical, chemical and biological methods. This study chooses the in-situ chemical leaching from chemical methods, combining with phytoremediation, in order to get economic and efficient combined technology.First of all, selection of an efficient, green and low cost leaching agent was conducted in a field experiment, from three kinds of leaching agents including: ferric chloride?FeCl₃?, glutamic acid-N, N-two acetic acid?GLDA? and the mixture of citric acid and calcium chloride?CA+CaCl₂?, considering leaching efficiency and cost factors. Then adding chemical leaching to the soil grown with hyperaccumulating plantswas studied to improve the efficiency of phyto-extraction in one hand, and in another hand, reduce the heavy metal contents in top-soil by leaching. Thirdly the in-situ chemical leaching followed by adding chemical fixative to fix the heavy metals was performed to reduce the metals contents in top-soil and to reduce the risk to the under-groundwater. Finally selection of low-accumulating cultivar of sweet corn suitable for Guangdong province was done with a field experiments to improve both the economic benefit and the enthusiasm of farmers to produce corn in contaminated areas.The main research results were as follows:?1?.The leaching efficiency of the tested agentsfollowed the decreasing order:GLDA?FeCl₃>CA+CaCl₂>CK. The removal rate of main polluants, Cd and Cu,washigh Concerning the downward migration of heavy metals, Cd and Zn showeddecrease in all layers from 0 to 60 cm, indicating that they were moved downward to the deeper soil layer by leaching agents. Pb and Cu showedalsodecrease in the top-soil, but they increased in the middle and low soil layers?20-60cm?, showingtheir poor downward mobility. Concerning remediation cost, FeCl₃ was cheaper than others and more suitable for the remediation of large-scale polluted lands. The removal efficience of GLDA was good for all the four heavy metals, but it was only feasible for special needs because of its high cost.?2?. Chemical enhancement promoted the phytoextraction rate of heavy metals by Sedum alfredii. GLDA increased phyto-extraction rate higher than CA+CaCl₂ for Cd. For Zn, Pb and Cu, CA+CaCl₂ was higer than GDLA. The cost of GLDA is higher than CA+CaCl₂. Considering benefit and cost, CA+CaCl₂ is the better choice for strengthening the phytoextraction of heavy metalsby Sedum alfredii.?3?.The content of Cd in top-soil decreased obviously after the leaching by FeCl₃, its decreasing rate was greater than 30%.KOH has a poor durability in increasing the p H in the soil as the fixing agent for heavy metals, the soil p H dropped within 60 d. The durability of Ca?OH?₂ is better than KOH, the soil p H gradually increased within 60 d, and its fixation effect on heavy metals also improved with time.?4?.The screening test results showed that the content of Cd, Pb, Cu and Zn in the seed and stem and leaves of the different cultivars of sweet corn varied obviously. Among 7 cultivars, two low accumulatingcultivars of Cd and Pb were selected, including JZY and CN-6. They could be the low accumulatingcultivars of sweet corn suitable for autumn-winter seasonsin South China especially Guangdongprovince.