Effects Of Different Oxygen Supply Methods On The Reduction Of Fe³⁺ By Humic Substance In Sludge Composting

Humic substances（HSs）have the ability to transfer electrons.This feature makes humic substances have the ability to reduce heavy metals in the natural environment.Native reducing capacity（NRC）is an important indicator to measure its reducing ability.HSs will be formed in the process of aerobic composting of sludge,but there is a lack of research on the effect of different oxygen supply methods on HSs reduction ability in different stages of aerobic composting.Therefore,this study sets up two comparative experiments.Group 1:keeping the same amount of ventilation,setting up continuous ventilation（T1）and intermittent ventilation（T2）aerobic composting;group 2:continuous ventilation（T3）and turning over（T4）aerobic composting.Collect samples at different stages of composting,extract and purify HSs including FA,HA and Hy I（the HSs below only contain FA,HA and Hy I）.In this study,ferric citrate（Fe Cit）was used as the electron acceptor to characterize the reduction ability of HSs,electrochemical methods were used to characterize the electron transfer ability（EAC,EDC and ETC）,and the changes in the chemical composition and structure of compost HSs were analyzed by spectroscopy.This study explored the effects of different oxygen supply methods on the humus reduction ability of sludge aerobic composting.The NRC of FA and HA derived from aerobic composting at different stages of T1 and T2treatment is stronger than that of Hy I,and the NRC and ETC of the two treatments in high temperature and cooling period are stronger than those in other stages.In general,the NRC of FA,HA,and Hy I in T2 treatment was stronger than that in T1,and the NRC of FA,HA and Hy I in the two groups had a peak around the 24th day.The NRC _total in each stage of T2 treatment was larger than that of T1,but the NRC _total of the two treatments had a peak on the 24th day,which was 30.37 and 36.36 mmole^-/mol C,respectively.Both NRC and ETC of HSs are positively correlated with C=C double bond content and humus-like components,and negatively correlated with protein-like components.Protein-like and microbial metabolites are gradually transformed into humus-like substances,which improves the humus degree of HS,degree of chemical and aromatization during composting.These results show that intermittent ventilation is more conducive to the improvement of compost humification and the improvement of HS’s reduction ability.The order of the native reducing capacity of FA,HA and Hy I in the same stage of T3 and T4 aerobic composting is FA>HA>Hy I.The NRC_FA in T3 treatment was significantly greater than that in T2.In the early stage of composting,the NRC_HA of T4 treatment was slightly larger than that of T3,while in the composting stage,the NRC_HA of T3 treatment was larger than that of T4 treatment.The NRC_{Hy I} of T4 treatment is always slightly larger than T3 treatment.The NRC _total of the T4 treatment was greater than that of the T3 treatment.Both NRC _total of two treatments reached the maximum values of 33.20 and 35.86 mmole^-/mol C on the 17th day,and there was a slight decrease in the maturity stage.The NRC and ETC of FA and HA are negatively correlated with their protein-like components.The NRC_{Hy I} is positively correlated with the content of humus-like substances.The protein-like substances in HSs are negatively correlated with microbial metabolites,residual substances and humus-like substances,which indicates that protein-like and microbial metabolites are gradually transformed into humus-like substances during the composting process.This transformation enhances the humification,aromatization and humus-like substances of HSs.The degree of polymerization improves the redox activity of HSs.These results show that turning over aerobic composting is more conducive to the improvement of its HS’s reduction ability.