Phosphorus-induced Immobilization Of Pb, Cu, And Zn In A Multi-metal Contaminated Soil

Metal contamination can not only degrade soil fertility and hence reduce crop yield, but may also poison plant growth or influence human health though transport in the food chain. Chemical immobilization is a cost-effective method for remediation of metal contaminated soil, which is based on reduction of metal availability via ion exchange, sorption, precipitation, etc. Application of phosphorus-bearing materials in remediation of contaminated soil has been greatly receiving attention, but it is mostly focused on one single metal (e.g., Pb) immobilization. However, the real metal-contaminated soils are often a system of multi-metal combined pollution. Therefore, it has more practical significances to apply phosphorus-bearing materials in remediation of the multi-metal contaminated soils.This study was conducted by using phosphate rock tailing (PR) and triple superphosphate fertilizer (TSP) for immobilization of Pb,Cu and Zn in a multi-metal contaminated soil. The treatment effectiveness was evaluated by measuring the change in concentrations of available and toxic heavy metals through chemical extraction; determining the downward migration of Pb,Cu and Zn through column leaching experiment; exploring the immobilization mechanism through Visual MINTEQ model, and assessing the bioavailability of heavy metals through pot experiment. The main results are obtained:(1) All three P treatments (PR, TSP, and PR+TSP) are effective in reducing Pb concentrations in CaCl2 and TCLP extracts by 20.1-80.2% and 16.3-70.1%, respectively. Effects of three P treatments for Pb immobilization follows a trend of TSP>PR+TSP>PR. All P treatments can also reduce the concentration of Cu in TCLP extract by 15.1-20.6%. However, only PR treatment is effective in reducing Cu concentrations in CaCl2 extract. As for Zn, all P treatments are effective reducing Zn concentrations in CaCl2 by 15.2-40.3%, following a trend of TSP=PR>PR+TSP. The TSP treatment has little effect on concentration of Zn in TCLP , but the concentration is slightly elevated by RP and PR+TSP treatments.(2) Concentrations of P in leachate and deep-layer soil are not significantly elevated. All P treatments can immobilize Pb, retaining Pb in the top-layer contaminated soil (0-10cm). Downward of Pb is limited, with Pb concentration in deeper soil (10-20cm) being far below than the control. Phosphorus treatment reduces the total Pb in the leachate by 33.2-38.4%, compared to the control; The treatments of PR and PR+TSP can also immobilize Cu, with total leachate Cu being reduced by 11.2% and 24.2%, respectively; Zn can be immobilized by TSP, with total leachate Zn being reduced by 34.3%, whereas PR and PR+TSP even increase total leachate Zn by 87.1% and 70.3%, respectively.(3) Visual MINTEQ modeling is in agreement with chemical extraction, showing that all three P treatments reduced Pb2+ activity, with a trend of TSP>PR+TSP>PR. The activity of Pb2+ is equilibrated with Pb5(PO4)3OH and Pb concentration is significantly negatively related to P (r2=0.69, p<0.05), indicating immobilization of Pb via precitation of Pb5(PO4)3OH. The PR treatment reduces activities of Cu2+ and Zn2+, while the activities of Cu2+ and Zn2+ are elevated in the TSP and PR+TSP treatment due to reduced pH. Concentrations of Cu and Zn are significantly negatively related to pH (r2(4) The P treatment has little effect on the above-ground biomass of Chinese =0.72, p<0.05), suggesting that surface adsorption and complexation may be responsible for inactivation of Cu and Zn. cabbage and Indian mustard. However, concentrations of Pb and Zn in the above-ground part of Chinese cabbage are greatly reduced by all P treatments, with reduction of 58.4-88.1% and 5.6-36.5%, respectively. P treatment has little effect on the uptake of Zn by Chines cabbage. All P treatments reduce uptake of Pb and Zn by Indian mustard by up to 20.3% and 29.6%, respectively, wheareas no significant change is observed for Cu concentration.