Study On Distribution Characteristics Of Organic Functional Groups And Removal Of Heavy Metals Cu And Zn During Composting Of Pig Manure Biogas Residues

Intensive livestock and poultry farming in China results in the production of amounts of manures and the difficulty of treatments.Based on the "Thirteenth Five-Year Plan" planning,the main solutions to treat and utilize manures in large-scale livestock and poultry farms at rural regions is to produce energy(e.g.,biogas)by anaerobic digesition.However,except for production of biogas,anaerobic digesition of manures also produce large amounts of biogas residues that contain high contents of organic matter.Therefore,secondary pollution will be caused if these manures are not suitablely treated prior to their entering into environments.Thermophilic composting is an effective and economical technical way to achieve harmless,resource-based,and reduced treatment of biogas residues.However,due to the widespread use of additives in feed for the livestock and poultry industry,the content of heavy metal elements such as Cu and Zn in pig manure sludge is relatively high.Long-term application of such organic fertilizers will cause heavy metals to accumulate in the soil,which will not only cause soil pollution,but also affect food safety and even human health.Therefore,a deep understanding of the changes in the forms of heavy metals and the structure of organic matter during the compost composting process is the basis and prerequisite for the safe use of biogas residues.In this experiment,pig manure,biogas residues and rice straw of a large-scale pig farm were used as raw materials.Synchrotron radiation based infrared microscopy(SR-FTIR)was used.The spatial distribution of organic matter in biogas residue particles before and after composting was characterized.Simultaneous The four-step extraction method proposed by the European Community bureau for standard substances(BCR)was used to classify and extract the forms of heavy metals in biogas residues.Finally,by adding earthworms for composting,the effect of earthworms on the accumulation of heavy metals in biogas residue was explored.The main research results are listed as follows:(1)Synchrotron radiation infrared microscopy images showed that fat(2930 cm-1),carboxylic acid(1721 cm-1),lignin(1436 cm-1),and carbohydrates(1234,1020 and 866 cm-1)appeared in biogas residue samples before and after compost.Before biogas residue composting,the fat components outside the biogas residue sample were preferentially degraded by microorganisms during the biogas fermentation process,while carboxylic acid and lignin were rarely degraded.At the same time,the biogas fermentation process formed C=O-rich carbohydrate substance.(2)Compared with before biogas residue composting,after biogas residue composting,the characteristic peaks of 1721 cm-1 and 1234 cm-1 outside the sample almost disappeared,and new characteristic peaks formed at 1715 cm-1 and 1156 cm-1.The two characteristic peaks formed may be related to the process of compost humification.(3)Biogas residue compost has a concentrating effect on heavy metals Cu and Zn.After compost treatment,the average concentration of Cu and Zn increased by 28%and 38%,respectively.After composting,the contents of exchangeable Cu and Zn in biogas slag were significantly reduced and the concentration of residues generally increased;and the high-temperature compost had better passivation effect on Cu.(4)Vermicomposting can reduce the content of heavy metals in biogas residues,and earthworms are better at enriching Zn in biogas residues than Cu;When the cultivation density is 72 g/kg,the absorption efficiency of the earthworm to Cu reaches 72.6 mg/kg at the 12th day of cultivation;when the cultivation density is 36 g/kg,the absorption efficiency of Zn by earthworms reaches 334 mg/kg at the 4th day of cultivation.In summary,thermophilic compost of biogas residues and straw can effectively reduce the biotoxicity of heavy metals in the environment by altering their bioavailability.Vemicomposting can reduce the content of heavy metals in biogas residues,so that biogas residue can be treated harmlessly,resourcefully and safely.Furthermore,SR-FTIR has a potential to explore changes of organic matters and fractions of heavy metals during biogas residue composting.