Leaching And Partitioning Of Heavy Metals During Land Application Of Sewage Sludge Compost

Land application is a way to treat sewage sludge in compliance with the concept of circular economy and eco-friendliness.However,sewage sludge contains some heavy metals which might result in near-surface soil accumulation,plant uptake or long-term groundwater impact during land application of sewage sludge.In this study,we investigated the leaching and partitioning of heavy metals during land application of sewage sludge compost?SSC?in Suzhou and Kunming cities with different types of soils.First,reference soils and SSC were comparatively characterized to investigate their similarities and differences regarding physicochemical properties,heavy metal leaching behavior and speciation.Then,aerobic and anaerobic incubation experiments were conducted to simulate arid and paddy soils.These experiments were combined with the pH-dependent leaching test to reveal the release of heavy metals under various geochemical conditions?such as pH,organic matter decomposition,reduction and oxidation?.Third,the column leaching test and field test were conducted to study the influence of repeated and periodic application of SSC on heavy metal release and partitioning.These results will provide basic and scientific information to make the decision about land application of SSC in China.Results showed that total contents of heavy metals were significantly higher in SSC when comparing with reference soils,but the heavy metal speciation were similar between SSC and reference soils.Cd,Cr,and Cu were mainly bound to particulate organic matter?POM?.As was mainly distributed onto iron and aluminum?hydr?oxides?FeOxides/AlOxides?and precipitated as Ca₂?AsO₃??PO₄?.Both of POM and FeOxides/AlOxides played dominate roles in Ni and Pb sorption.The change of pH,organic matter composition,and redox can influence release of heavy metals.Compared with reference soils,SSC experienced a more rapid rise in leaching concentrations with the increase of pH under alkaline conditions.This was mainly due to the high content of organic matter,including humic substances in SSC.At alkaline pH,when humic substances could largely dissolve,the complexes formed between heavy metals and dissolved humic substances significantly enhanced the leaching of heavy metals from SSC.However,the leachability of heavy metals across the applicable pH domain?5<pH<9?were rather low compared with the total contents.The leached ratio was below 6%for As,Cd,Cu,Ni and below 1%for Cr and Pb.The decomposition of dissolved organic matter can decrease the release of Cd and Cu which had strong affinities with organic matter.After 180 days aerobic incubation,the dissolved organic carbon?DOC?declined by 45%⁴9%.It led to the lower leaching concentration of Cd and Cu because of the less contribution of DOM-complexation.The reducing condition produced by microbial activities,particularly during the anaerobic incubation experiment,resulted in the loss of sorption sites for As on Fe hydroxide,which increased the potential risk of As release.Repetitive additions of compost to soil significantly increased the content of organic matter,which favored the formation of reducing condition due to improved microbial activities and oxygen consumption.Establishment of reducing conditions can enhance the leaching concentrations of As by approximately 1 order of magnitude.Especially for the soil rich in organic matter,the cumulative release of As under the reducing condition can reach up to 55.3%of the available content.For the soil with low organic matter content,the increased content of organic matter can improve the retention of Cu,Cd,and Pb by POM sorption.The 3-year land application of SSC in the field test greatly increased the content of organic matter,humic substances,and iron?hydr?oxides.The increase of organic matter from SSC application and organic matter dissolution at alkaline pH also led to the increase of heavy metal leaching concentration at neutral to alkaline pH.However,the plant uptake of heavy metal had no relationship with SSC application and increased leaching concentration of heavy metals from soils.The soil at the application layer?0¹0 cm?had strong retention of heavy metals and the land application of SSC will not cause the contamination of underground water in this case.