Speciation Change And Passivation Of Heavy Metals In Municipal Sludge During Hyperthermophilic Composting

Heavy metals have become the bottleneck of municipal sludge application in agriculture,and how to reduce the risk of heavy metal pollution in land use is essential to sludge agricultural safety.Hyperthermophilic composting（HTC）（>80 ℃）,a new technology of sludge aerobic composting developed and applied in recent years,has attracted more and more attention due to its numerous advantages in application,such as high fermentation temperature,long duration under high temperature,and good product harmless effect（antibiotic removal over 90%）.Existing researches show that HTC has the effect of passivating and stabilizing heavy metals.However,the mechanisms of heavy metals of changes with HTC technique are not well established.In this paper,HTC was taken as the research object,which compared with TC,the existing and complex detection method of organic matter and microbial biomass were improved;heavy metal speciation in different composting processes was studied with comparative analysis and redundancy analysis;the effects of dissolveed organic matter（DOM）components and structure changes on heavy metal passivation were elucidated by extracting and characterizing DOM in different composting stages and combined with statistical analysis;community characteristics ofbacteria and archaea and heavy metal resistance genes were analyzed in composting process,while the influence of microbial community succession characteristics on heavy metals were discusesed in fermentation process.The implementation of this study is to explore the speciation and mechanism of heavy metals in HTC.The main results are as follows:（1）In this study,three different methods were applied for the detection of soil organic matter and microbial biomass,a dual wavelength method for the indirect determination of organic matter content based on the Cr3+ concentration generated in the process of sludge digestion,a method for the determination of organic matter content based on headspace gas chromatography（HS-GC）with redox prototype（CO₂ produced in the process of strong acid oxidation of samples）,and a method using improved substrate induced respiration based on HS-GC.Results show that three methods all had good reproducibility（RSD is less than 5%,5% and 10% respectively）and the accuracy is high（the recovery is 93-112%）.What’s more,the latter two methods can be used with batch samples.（2）The analysis of heavy metal speciation showed that the Cd EXC and Cd Carb significantly decreased in HTC and decreased 9.2% and 10%respectively,while the Cd OM increased 20.9%.T-test was used to analyze the differences of heavy metal speciation in HTC and TC.It was found that there were significant differences in heavy metal speciation in HTC and TC.For Cd speciation,the differences of Cd EXC and Cd Carb in two composts（P < 0.01）were substantial,while high bioavailability content in TC was higher compared to HTC,which further proved that the stabilization effect of Cd in HTC was better.Redundancy analysis of physical and chemical properties showed that organic matter indexes such as organic matter,total organic carbon and humus were key factors affecting the content and form of heavy metals in the composting process.（3）Studies on the evolution of DOM and its affect of action on heavy metals show that the aromatization degree and humus substances level in HTC process.Aliphatic and polysaccharide substances are decreasing over time in HTC process,while aromatic and humus substances are stable and easy to decompose.3D EEM-PARAFAC was used to analyze the composition and transformation of protein like and humus like substances in composting process.Results showed that humus like substances were dominant in DOM.The correlation between DOM evolution and heavy metals in composting process was also analyzed.It was found that the passivation of Cd and Cr in HTC process was positively correlated with humus like substance and protein like substance,while the degradation of fat and the increase of carboxyl group C promoted the stability of Cd.Analysis also showed that the active speciation of heavy metal Zn was negatively correlated with DOM humus and protein like substance,and humus and protein like substance in DOM decreased the activity of Zn.（4）The study on the response characteristics of microbial community in heavy metal passivation found that,in the HTC process,Firmicutes was dominate and in a high correlation with Zn EXC,Cr Carb,and environmental factors.Another positive correlation between Rmboutsia,Proteiniclasticum and heavy metals was also found during the test.Archaea community mainly included Euryarchaeota,Crenarchaeota,Thaumarchaeota and Dipherotrites,among which Euryarchaeota showed absolute advantage,and a positive correlation among Methanoregula and Cd,Cd Fe Mn Ox,Zn Fe Mn Ox,Zn,Zn RES,Zn Crab,WHC,Cr,Cr RES,Cd Carb,Cr OM in Euryarchaeota was confirmed by T-test.（5）The detection and analysis of heavy metal resistance genes showed that czc A was the most abundant heavy metal resistance gene in the process ofHTC,which further explained the maximum abundance of resistance genes against Zn and Cd in the process of composting.The correlation analysis between MRGs and heavy metals showed that the abundance of MGRs in HTC was greatly affected by the heavy metal concentration and morphology.