Study On Anaerobic Digestion Enhancement Of Contaminated Rice Straw And Adsorption Characteristics Of Biogas Residue Biochar

Heavy metal pollution in farmland soil will lead to low concentration of heavy metals in crop straws during agricultural production.The direct return of straw biomass containing heavy metals to the field will cause secondary pollution and it is urgent to treat it harmless and resourceful.Anaerobic digestion technology can recover biomass to produce clean energy.It is a feasible technology to treat crop straw.However,a large amount of lignocellulose contained in straw is difficult to degrade and the efficiency of methane production is low,which restricts the development of straw anaerobic digestion.At the same time,the biogas residue produced by anaerobic digestion contains heavy metals and other pollutants.The use of biogas residue as organic fertilizer will deteriorate the soil or water environment and limit the resource utilization of biogas residue.Therefore,this study took contaminated rice straw as the research object,used zero-valent iron（ZVI）and activated carbon（AC）to improve the anaerobic digestion,and prepared biogas residue biochar（BC,P-BC）through pyrolysis to investigate the adsorption characteristics of biochar for lead and cadmium and the potential to stable remediation of heavy metal contaminated soil.The aim is to provide a new idea for resource utilization of contaminated rice straw.The main research results were as follow:（1）ZVI and AC could effectively promote the anaerobic digestion of contaminated rice straw to produce methane and release heavy metals.Compared with the control,the peak daily methane yield and cumulative methane yield of the anaerobic digestion system increased by 24.5%,11.5%and 20.1%,11.3%respectively after adding 1 g ZVI and AC.Among them,the cumulative methane yield of ZVI and AC system reached 116 and 107m L·g^-1VS,respectively.ZVI and AC could also further promote release of heavy mentals,and the release rates of Cd,Pb,and Zn could reach45.0%～67.5%,37.5%-50.0%and 52.9%～66.4%,respectively.In addition,ZVI and AC were beneficial to the growth and reproduction of dominant microbial species in the anaerobic digestion system,and promoted the enrichment of hydrolytic acidification bacteria such as Dojkabacteria,Dechlorobacter and Lativibrio in the anaerobic digestion system,and increased the relative abundance of hydrogen methanogens and acetic methanogens respectively,which increased the methane yield of anaerobic digestion of contaminated rice straw.Therefore,ZVI and AC enhanced the anaerobic digestion process mainly by promoting the hydrolysis and acidification of organic matter and enriching the methanogenesis pathway.（2）Biochar had good adsorption performance which prepared by anaerobic digestion of biogas residue from contaminated rice straw.The biogas residue biochar could effectively remove Cd and Pb in the solution.The co-pyrolysis of calcium dihydrogen phosphate and ZVI could significantly improve the adsorption of Pb.The biochar prepared by pyrolysis at 300℃had the best adsorption effect on heavy metals.The removal rate of Cd by BC_ZVI reached 74.8%,while the removal rate of Pb by PBC_ZVI with mass ratio of 10 reached 91.6%.The adsorption of Cd and Pb by biogas biochar conformed to Langmuir equation and pseudo-second-order kinetic model.They were monolayer chemical adsorption on the surface of adsorbent,which is controlled by chemical rate.The maximum adsorption capacity of BC_CK and PBC_ZVI for Cd and Pb were 30.3 and 271mg·g^-1,respectively.After the co-pyrolysis of biogas residue and calcium dihydrogen phosphate,P-O-P,PO₄^3-phosphate group were successfully introduced into biochar,which was more conducive to the adsorption of heavy metals.Except for Fe and Zn,the leaching concentration of other heavy metals in biogas residue biochar was close to zero,and the leaching concentration of all heavy metals in PBC was lower than that in BC,which was lower than the standard limit of Identification Standards for Hazardous Wastes-Identification for Extraction Toxicity（GB5085.3-2007）.Moreover,the addition of ZVI and AC were also more beneficial to stabilizing heavy metals in biogas residue biochar,indicated that biogas residue biochar had low potential toxicity and could be used for adsorb/stabilize heavy metals.（3）The application of biogas residue biochar could improve the physicochemical properties of soil and reduce the available content of heavy metals in soil.The application of biogas residue biochar in soil significantly increased the contents of p H,alkali-hydrolyzed nitrogen,available phosphorus and available potassium（P<0.05）,which provided richer nutrients for soil.PBC had a more significant impact on soil properties,while ZVI and AC had a negative impact on improving soil fertility.The application of biogas residue biochar could stabilize Cd and Pb in soil,transform heavy metals from unstable state to stable state,and reduce their environmental risk.The stabilization effect of 2%BC_ZVI and PBC_ZVI was the best.After application into soil,the acid extractable content of Cd and Pb decreased by 26.4%and 46.8%,respectively,and the residue content increased by 10.4%and 17.7%,and the available contents of heavy metals Cd and Pb in the contaminated soil decreased significantly.These results indicated that biogas residue biochar application could be used as one of the effective measures to control the potential pollution risk of heavy metal contaminated soil in industrial and mining areas.In conclusion,the adding of ZVI and AC could improve the methane production efficiency and heavy metal release rate of contaminated rice straw anaerobic digestion.Anaerobic digested biogas residue could be pyrolyzed into biochar with high adsorption capacity,and calcium dihydrogen phosphate could further improve the adsorption capacity of biogas residue biochar.Biogas residue biochar could reduce the environmental risk of heavy metals in contaminated soil.This study provided a feasible technical scheme for resource utilization of contaminated rice straw and potential risk control of heavy metal contaminated soil in mining and smelting areas.