Study On The Effect Of CaO₂-Fenton-Like System On The Composting Process Of Agricultural Waste And Sediment

With the vigorous development of agriculture and non-ferrous metal smelting in the middle and upper streams of Xiangjiang River in China,the environmental problems caused by improper disposal of agricultural waste and non-compliant discharge of industrial waste water sludge are becoming increasingly serious.Co-composting of river sediment and agricultural waste is an emerging and efficient soil remediation technology that has been widely used for different types of heavy metal pollution remediation,such as mine remediation,sediment remediation,landfill remediation,etc.The key to heavy metal passivation by this remediation technology lies in the humic substances（HS）generated during the composting process.HS contains a variety of functional groups such as carboxyl,hydroxyl and phenolic groups.These functional groups have strong complexation,adsorption and redox abilities,which contribute to the passivation of heavy metals in the composting process.However,the agricultural wastes commonly used for composting（e.g.straw,bran,etc.）have high lignocellulose content,few directly usable components and are difficult to degrade,which are not conducive to HS generation and heavy metal passivation.Therefore,exploring a more suitable and effective method to promote HS generation during composting of agricultural wastes is the key to improve the passivation effect of composting compost on heavy metals.In this paper,aerobic composting experiments were conducted by adding 0.5%CaO₂（pile Ca）and 0.5%CaO₂+0.5%nano-Fe₃O₄（pile Fe）to the piles as a co-compost of agricultural waste and Xiangjiang River sediment,respectively.The aerobic composting was carried out for62 days,and the CaO₂-Fenton-like system was successfully established in the composting system with CaO₂ and nano-Fe₃O₄ by the determination of H₂O₂ and·OH.Meanwhile,the effects of this Fenton-like system on the physicochemical parameters of the compost pile were investigated through the determination of temperature,p H,water content,organic matter content,total nitrogen and carbon to nitrogen ratio.From the results of the study,it was observed that:CaO₂ and nano-Fe₃O₄accelerated the pile warming and increased the duration of the high temperature period of the pile,which was extended by 1 and 2 days for Caand Fe,respectively;the maximum temperature was increased by 2.9℃ and 3.0℃,respectively.Since CaO₂ is an alkaline reagent,it increased the pre-compost p H and kept the pile p H in the weak alkaline range throughout.At the same time,the organic matter content of compost Caand Fe and C/N of the pile decreased faster due to the release of H₂O₂ and·OH from the Fenton-like reaction,which accelerated the composting process.TN showed a gradual increase during the composting process and increased by 1.95 g/kg,1.58 g/kg and 2.25 g/kg in CK,Caand Fe of the compost pile,respectively,at the end of composting.By measuring the total amount of heavy metals,heavy metal morphology and humus fraction during the composting process and calculating the humification parameters,this paper explored the humification process and the passivation effect of heavy metals in the composting process,and the results showed that the"concentration effect"caused by the mineralization of organic matter during 62 days of composting process,the total amount of heavy metals in the three piles The total amount of heavy metals in all three piles showed an increasing trend due to the"concentration effect"caused by the mineralization of organic matter.RES-Cu increased by 24.48%,26.55%and 27.62%in CK,Caand Fe piles,respectively,while Zn showed similar changes as Cu.23.95%,26.38%and 28.49%.During the composting process,HS showed a sharp decrease and then a slow increase;HA content showed a gradual increase;FA first decreased rapidly and then stabilized.At the end of composting,the HA of CK,Caand Fe increased to 17.85 g/kg,19.84 g/kg and 20.78 g/kg,respectively,indicating that the establishment of CaO₂-Fenton-like system promoted the production of HA in the pile.both HI and DP showed an increasing trend,indicating that small molecules of FA were gradually converted into large molecules of HA,and HS showed an enhanced aromatization and increased molecular weight trend.In addition,this paper sequenced the bacterial community in the composting process with the help of 16S DNA sequencing,analyzed the OTU,α-diversity andβ-diversity of the bacterial community by using software such as Rstudio and QIIME,and fitted the correlation between biological factors and environmental factors by using software such as Cannoco and AMOS.The results showed that theα-diversity of the compost pile increased after composting,and the addition of CaO₂ increased theα-diversity in the pile,while the simultaneous addition of CaO₂and nano-Fe₃O₄ decreased theα-diversity of the pile.Proteobacteria,Firmicutes,Bacteroidetes,Actinobacteria,Gemmatimonadetes and Chloroflexi were the dominant phyla in the composting system.The PCA results indicated that the bacterial community composition showed significant differences as the composting progressed.The bacterial communities at the family level during composting were correlated with environmental factors such as physicochemical parameters,humus and heavy metals by redundancy analysis（RDA）.The results showed that temperature,HS,HA and p H were the key factors affecting the bacterial community during the combined composting of agricultural waste and substrate,and that the CaO₂-Fenton-like system accelerated the succession of bacterial communities and enhanced the passivation of heavy metals by HS and HA;in addition,the structural equation modeling（SEM）confirmed that the Fenton reaction affected the passivation of Cu and Zn through p H,HA formation and bacterial communities.passivation by an integrated pathway.