Characteristics And Influencing Mechanism Of Heavy Metals Speciations Transformation In Food Waste During Rapid Thermophilic Aerobic Fermentation

Rapid thermophilic aerobic fermentation（RTAF）has attracted much attention due to its rapid realization of resource utilization of food waste.However,food waste may contain a variety of heavy metals.After aerobic fermentation,it was widely used as fertilizer and soil conditioner,which might increase the potential risk of heavy metal pollution.In this study,speciation transformation of heavy metal and its influence mechanism during RTAF in food waste was explored,which could provide scientific basis for the prevention and control of heavy metal pollution.The main conclusions were as follows:In order to determine the influence of physicochemical factors on heavy metals,the changes of physicochemical factors and heavy metal content during RTAF were studied.The results showed that during the whole fermentation process,the fermentation temperature was continuously maintained between 66°C and 74°C,accompanied by a decrease of moisture content and an increase of p H.At the end of fermentation process,the electrical conductivity of the decomposed product was 3.57ms/cm,and the content of heavy metals Mn,Zn,Cr,Ni,Cu,Pb,Hg and Cd were 101.19mg/kg,49.38 mg/kg,21.78 mg/kg,7.39 mg/kg,7.83 mg/kg,0.13 mg/kg,0.10 mg/kg and 0.10 mg/kg,respectively.All of them could meet the criterion of the industry standard NY525-2012 for organic fertilizer,which showed that it could be safely applied to the soil.The results of redundancy analysis（RDA）showed that p H was the key factor that affects the change of heavy metals content.In order to determine the influence of dissolved organic matter（DOM）component evolution on the speciation transformation of heavy metals in different fermentation stages,the composition and evolution of DOM were analyzed.The results showed that protein-like substances were consumed and humus like substances were increased during RTAF.The results of RDA showed that the main factors that drive the speciation transformation of heavy metals were tyrosine-like substances（C3）,molecular weight(SUVA₂₅₄)and humification degree（E250/E365）.The degradation of protein-like substances such as tryptophan-like substances C1 and tyrosine-like substances C3 could significantly affect the exchangeable form of Mn,Cu and Cr.The transformation of fulvic acid substances（C2）could affect the exchangeable form of Mn,Cu,Cr,Zn,Ni and Cd.Moreover,the intensification of humification is helpful to passivation of the above six heavy metals.After fermentation,the stability of Mn and Ni decreased,however,the stability of heavy metals Zn,Cu,Cr and Cd was improved.Finally,the key enzyme activities and microbial community changes during RTAF were analyzed.The results showed that the cellulase activity gradually increased and tended to be stable,the protease activity first decreased and then increased,and the dehydrogenase and catalase activities decreased with the process of fermentation.Firmicutes,Proteobacteria and Actinobacteria were the dominant phyla and Lactobacillus,Bacillus,Ralstonia and Acinetobacter were the dominant genus bacteria during RTAF.The network analysis results of Exc-HM and microorganism at the genus level showed that Firmicutes,Proteobacteria and Actinomycetes had obvious correlations with heavy metals.Among them,Lactobacillus and Pediococcu belonging to Firmicutes showed more resistance and tolerance to heavy metals.The results of RDA between COG function and Exc-HM showed that the exchangeable heavy metals affected the cytoskeleton and coenzyme transport and metabolism of microorganisms during RTAF in food waste.