Effects Of Different Additives On Gas-producting Characteristics And Passivation Of Heavy Metals In Pig Manure During Anaerobic Fermentation

The expansion of intensive livestock and poultry production scale has brought about a large number of fecal by-products,these untreated manure contain a large amount of excessive heavy metals(i.e.,Cu,Zn and As),which will seriously pollute soil and groundwater if applied directly.How to effectively manage and unify these livestock and poultry manure has become a special concern of our government and the scientific community.Among them,anaerobic fermentation is one of the most promising and widely used methods in organic matter treatment,because its treatment can not only produce clean energy but also stabilize heavy metals.In this study,different kinds of additives were applied to the mid-temperature anaerobic fermentation of pig manure to explore their influence on the biogas production and passivation of heavy metals during anaerobic fermentation.And the pH-dependent leaching test were conducted to investigate the effects of BC and Fe3O4/BC on the changes of the leaching concentration of heavy metals in biogas residue.The surface characteristics of pig manure before and after passivation of Fe3O4/FA were analyzed by X-ray diffraction(XRD)and scanning electron microscopy energy dispersive X-ray(SEM-EDX)to explore the passivation mechanism,so as to help optimize anaerobic fermentation,better control the environmental hazards of heavy metals in pig manure,and provide theoretical basis and technical support for reducing the risk of heavy metal release in biogas residue after anaerobic fermentation.The main conclusions are as follows:(1)Different kinds of additives were added to the anaerobic fermentation system of pig manure.The results showed that 2.5%FA,2.5%BC and 2.5%Fe3O4/FA experimental groups play an promoting role in total gas production during anarobic fermentation,and the total gas production increased by 5.10%-12.10%,while the addition of 1.0%Fe2+ and 2.5%Fe3O4/BC could inhibit the total gas production.The experimental groups of each additive showed significant promotion effect on methane production.Via adding different concentration(0.0%,0.5%,1.0%and 2.5%)of composite additives(Fe3O4/FA and Fe3O4/BC)in the anaerobic fermentation experiment,the total amount of gas production and methane production after anaerobic fermentation showed the effect of low concentration promotion-high concentration inhibition.Compared with CK group without composite additives,the addition of 0.5%(Fe3O4/FA and Fe3O4/BC)had a positive effect on biogas production.Compared with CK group,the total amount of biogas increased by 13.82%and 11.72%,and the total methane production increased by 35.13%and 32.18%,respectively.(2)Compared with the initial mixed raw materials,the concentration of Cu and Zn in each experimental group after anaerobic fermentation increased by 1.05-1.35 times and 1.02-1.22 times respectively,showing obvious"relative concentration effect".Before and after anaerobic fermentation,the content of organic bound state was the highest in the percentage of various forms of copper(Cu)(56.24%-78.42%),however zinc(Zn)was mainly distributed in the oxidized state of iron and manganese(64.75%-79.23%).And it was found that 2.5%FA and 2.5%BC had activation effect on Cu in different additives.Among them,the residual states of Cu increased by 41.25%(for 1.0%Fe2+),45.78%(for 2.5%Fe3O4/FA)and 52.40%(for 2.5%Fe3O4/BC)respectively compared with CK group,showing a good passivation effect.The percentage of residual state of Zn in the experimental group of additives increased by 20.13%-42.14%compared with CK group,indicating that additives have passivation effcct on Zn.(3)The passivation effect of composite materials with different concentrations on heavy metals in pig manure under anaerobic fermentation was further studied.The results showed that the higher the proportion of composite materials,the more obvious the passivation effect.The heavy metals of Cu and Zn were transformed from unstable state to residue state in the experimental study of adding different concentrations of Fe3O4/FA,and a better performance was found in the treatment with 2.5%Fe3O4/FA,and the percentage content of heavy metals residue state of Cu and Zn increased by 45.78%and 42.14%,respectively.The a values of Cu and Zn also decreased by 13.12%and 2.52%,respectively,Therefore,the addition of Fe3O4/FA could reduce the bioavailability of heavy metals Cu and Zn after oxygen fermentation of pig manure.The addition of different concentrations of Fe3O4/BC composites also showed similar results.Among them,0.5%and 1.0%addition had activation effect on Cu,while 2.5%addition significantly reduced the bioavailability of Cu and Zn(p<0.05).(4)The leaching experiments of CK,2.5%BC and 2.5%Fe3O4/BC after anaerobic fermentation were compared.The results showed that the leaching concentration of heavy metals in biogas residue after anaerobic fermentation was generally lower than its total concentration,and showed a significant pH dependence.The leaching curves of dissolved organic matter(DOM),Cu and Zn all showed a "V"leaching curve.The leaching concentration of Cu and Zn in 2.5%Fe3O4/BC group was the lowest in weak acid and weak alkali(pH 5-9),which showed that the addition of 2.5%Fe3O4/BC could effectively reduce the leaching risk of heavy metals in biogas residue after anaerobic digestion.(5)XRD and SEM-EDX were used to analyze the structure,surface morphology and element distribution of heavy metal residues in CK and 2.5%Fe3O4/FA experimental groups after initial pig manure and anaerobic fermentation.It was found that the mechanism of Fe3O4/FA for passivating heavy metals was mainly through the physical adsorption in the anaerobic digestion.It can convert heavy metals into stable mineral precipitation,reducing the solubility and mobility of heavy metals and thus effectively passivating heavy metals.