Release And Migration Of Heavy Metals In Sewage Sludge Compost Application For Abandoned Mining Land Revegetation

Sewage sludge is rich in organic matter and N,P,K and could be used as soil amendment to improve the status of soil organic matter,soil structural characteristics and soil water retention capacity after aerobic composting.These benefits are essential to ecological restoration of abandoned mines.However,heavy metals in sewage sludge has become the main bottleneck that limits its land application.Considering the phosphate shows the unique immobilization function for heavy metals,the phosphate tailings could be added as auxiliary materials in sewage sludge composting to limit the environmental risk of the heavy metals leakage.To verify the feasibility of the above assumption,a series of studies were carried out as followed in different composting scenarios: by testing the existing form of heavy metals,the benefits of phosphate-rich composting was analyzed;by examining the heavy metals leaching characteristic of the phosphate-rich compost,the long-term effect of it could be estimated;by analysis the heavy metals distribution among the plants,compost and surface-soil,the stability and ecological risk of the phosphate-rich compost was evaluated.The results show that: the phosphate-rich compost meets the land application limits of the heavy metals.Adding phosphate tailings greatly improved the stability of heavy metals during the composting process.The risk of heavy metals leakage of the compost relieved under the nature pH conditions(pH 6-8)and the maximum leachable amount reduced under the extreme pH conditions.The leaching experiment results shows that even in the continuous acid or nature rain conditions,the groundwater would unlikely be contaminated by the compost.Especially,the effect caused by the phosphate-rich group is more imperceptible.Under continuous rainfall conditions,the phosphate-rich surface-soil could intercept most of the leached heavy metals.The testing results indicate that little heavy metals could penetrate 25~35 cm surface-soil in natural rain condition and 25~45 cm in acid rain condition.Compost application could increase the microbial and enzyme activity in soil and promote the plant growth.Meanwhile,the concentration of the heavy metals in the plants proves that the sludge compost,especially the phosphate tailings added compost,is safe to the ecosystem.