The remediation mechanism of heavy metals contaminated soils by the passivator prepared from phosphate tailing
The contamination of heavy metals in soil is an important environmental issue in China.Active passivation is a commonly used technique for in situ soil remediation,but the lack of passivator with high efficiency and low cost hinders the large-scale application of this technology.With a hydroxyapatite-based material(HAPPT)prepared from low-cost phosphate tailing,in this paper,we have investigated the removal mechanism of heavy metals in the Zn-Cd-Pb mixed solution and the passivation mechanism of heavy metals in soils by the HAPPT through experiments.The results show that adsorption capacity of the HAPPT on Pb is higher than that on Zn and Cd in the Zn-Cd-Pb solution,and the effect of pH on adsorption of various metals is related to different types and concentrations of various metal elements.The results of passivation experiments of heavy metals in soils show that contents of water-soluble heavy metals including Zn,Cd and Pb in soils could be significantly reduced by the added HAPPT and the reduction efficiencies of water-soluble heavy metal contents of the acid soil are higher than those of the neutral soil.The precipitation of phosphate formed by the reaction of soluble hydroxyapatite and heavy metals in soils,and,the increase of pH values in soils due to the addition of HAPPT,had promoted the formation and precipitation of carbonates and hydroxides of heavy metals,and the increased adsorption capacitiies of heavy metals by the passivated soils.In addition,after passivation of soils by the HAPPT,the exchangeable phases of heavy metals in soils had been transformed to the stable phases,such as carbonate,Fe-Mn oxides bounded and/or residual fractions.The transformation was related to the precipitation of phosphates due to the dissolution of HAPPT in acid soils.Besides,the increase of pH in soils can stimulate the precipitation of carbonate and hydroxide and can also increase metal adsorption by soil constituents such as Fe oxides.Through this study,a low-cost and highly efficient heavy metal passivation material has been developed from phosphate ore tailings.It is expected that this will provide a new technical way for the remediation of heavy metal contaminated soils.
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