Study On The Form Transformation Of Heavy Metal Cadmium/Chromium And Biogas Production Characteristics During Anaerobic Fermentation Of Pig Manure

Pig manure is an important organic fertilizer.However,due to the use of heavy metal additive in pig feed,the content of Cd / Cr in pig manure during anaerobic fermentation is increasing,which leads to heavy metal pollution in soil.The harm of heavy metals is closely related to their existing forms.Anaerobic fermentation is one of the most effective ways to realize the innocuity,reduction and resource utilization of pig manure.But how to reduce the risk of heavy metal pollution in the process of anaerobic fermentation has not been studied.Therefore,pig manure was used as raw material for fermentation,and chromium?Cr?and cadmium?Cd?were used as research objects in this paper.By adding different pretreated straw?physical crushing,2% H₂SO₄,2% Na OH?and straw + mixed passivator?active carbon: fly ash = 1:2,1:1,2:1?,the 60-day anaerobic fermentation test was carried out under the conditions of inoculation amount?biogas slurry?of 30%,total solid content?TS?of 10%,p H of 7,temperature of 35? and initial carbon/nitrogen ratio of 25:1.BCR was used to analyze the change of heavy metal content,form and the effect of effective passivation in biogas residue before and after anaerobic fermentation;The interaction between heavy metal and humus in fermentation process was analyzed by the method of H₂O,KCl,Na₄P₂O₇,Na OH and HNO₃.And fourier transform infrared spectroscopy was used to analyze the different stages during anaerobic fermentation of pig manure.Clarify the mechanism of combining Cr,Cd and organic matter in biogas residue during anaerobic fermentation process,reduce the bioavailability of Cr and Cd in biogas residue using pig manure as fermentation raw material,and provide theoretical support for the resource and harm reduction.The main conclusions are as follows:?1?The contents of Cd and Cr in exchangeable and reducible heavy metals were decreased,and the contents of oxidation state and residue state were increased after anaerobic fermentation.Variance analysis of heavy metal Cd and Cr content showed that: adding different pretreatment straw or straw + mixed passivation agent of different proportion had a significant effect on the reduction of the exchangeable and reducible states of heavy metals Cd and Cr?P <0.05?It shows that adding acid,alkali and physical crushing pretreatment straw or straw + mixed passivation agent can reduce the bioavailability of heavy metals Cd and Cr,thus effectively reducing the harm of heavy metals.?2?In the 4 dynamic sampling groups of pig manure,pig manure + physical crushed straw,pig manure + 2% sulfuric acid treated straw,pig manure + straw + 3% activated carbon + 3% fly ash,the heavy metal Cd is mainly the organic complex state and the organic combined state,and the heavy metal Cr is mainly the residue state,followed by the organic combined state and the organic complex state.The heavy metals Cd and Cr are mainly combined with fulvic acid?FA?before fermentation.The distribution rates of Cd and Cr combined with humic acid?HA?are significantly increased as the fermentation progresses.Adding straw or straw + mixed passivating agent in anaerobic fermentation can promote the formation of HA-Cr,and enhance the stability of heavy metals in biogas residue.Among them,the most obvious effect is to add straw + activated carbon: fly ash = 1: 1 treatment group.?3?Analysis of the fourier infrared spectroscopy found that organic compounds such as amide compounds,carbohydrates,proteins,and aliphatic compounds in biogas residues were decomposed,humus such as aromatics increased.Among the 7 treatments in this test,the addition of straw + activated carbon: fly ash = 1: 1 treatment group had the best degree of humification of organic matter.The two-dimensional infrared correlation spectrum in the wavenumber range of 1800 ? 900cm^-1 shows that there is a synergistic relationship between aromatic rings,aliphatic carboxylates,proteins and polysaccharides.In the 4 dynamic sampling groups of pig manure,pig manure + physical crushed straw,pig manure + 2% sulfuric acid treated straw,pig manure + straw + 3% activated carbon + 3% fly ash,the addition of pretreated straw or straw + mixed passivation agent was beneficial to the humification process of biogas residue in the anaerobic fermentation process of pig manure,and the addition of straw + activated carbon: fly ash =1:1 treatment group had the best degree of humification.?4?Compared with the pig manure fermentation alone,the groups adding physically crushed straw and the straw + mixed passivation agent delayed the peak of biogass production and extended the fermentation cycle during the fermentation process,but the total biogass production could be increased.Among the 7 treatments in this test,the highest total biogass production is the addition of straw + activated carbon: fly ash = 1: 1 treatment group.The methane content of the 7 treatments in this test was between 44.17% and 47.89%.Compared with the pig manure fermentation alone,the average methane content of the treatment groups adding pretreated straw or straw + passivating agent was increased.Among them,the addition of straw + activated carbon: fly ash = 2: 1 treatment group has the most obvious effect of increasing methane content.