Removal Effect And Mechanism Of Heavy Metals From Biogas Residue By High Temperature Composting Coupled With Earthworm Decomposed Composting

Nowadays,the livestock and poultry breeding industry in China is developing rapidly on a large scale and intensively.In order to promote the rapid growth of livestock and prevent their common diseases,relevant heavy metals are often added to the feed.In this context,heavy metal pollution problems also followed.The absorption efficiency of heavy metals in feed is too low for livestock and poultry,which leads to the excretion of unabsorbed heavy metals with feces.If not properly treated,it will lead to the accumulation of heavy metal pollution.In this paper,the speciation evolution characteristics of Cr and Pb in biogas residue under high temperature composting were investigated by combining high temperature composting with earthworm decomposing composting.The removal effect of Cr and Pb from biogas residue was optimized under the conditions of earthworm density,composting time and earthworm size during earthworm composting.At the same time,the traditional chemical analysis method was combined with the new synchrotron radiation infrared microscopic imaging(SR-FTIR)and synchrotron radiation microbeam X-ray fluorescence imaging(μ-XRF)to perform in situ analysis on the biogas residue particles and earthworm sections before and after composting.From the macro level and the micro level of organic functional groups inside the particles,the changes in the content and spatial distribution of the corresponding organic functional groups in the biogas residue particles before and after high temperature composting and earthworm composting were analyzed,in order to master the change rule of heavy metals and provide theoretical basis for the non-toxic treatment of biogas residue fertilizer in the future.The main research results of this paper are as follows:(1)After high temperature composting,the concentrations of Cr and Pb in biogas residue increased from 26.52 mg/kg and 32.54 mg/kg to 52.43 mg/kg and 49.51 mg/kg,respectively,showing the "Concentration Effect" of heavy metals.The bioavailability of the two heavy metals decreased.Specifically,the exchangeable and reducible states of heavy metals decreased gradually with the time of composting,while the oxidizable and residual states increased gradually,which realized the passivation of heavy metals.(2)The μ-XRF and SR-FTIR spectra of biogas residue samples before and after high-temperature composting showed that Cr and Pb mainly gathered in the interior and edge of biogas residue before and after high-temperature composting,and the analysis of specific organic functional groups showed that biogas residue was mainly composed of protein substances(2925 cm-1),Small amounts of lipids(1650 cm-1),lignocellulose(1219cm-1),and iron oxides(690 cm-1)provide complexing sites for heavy metals.After high temperature composting,the effective components of heavy metals decreased with the decrease of Cr and Pb distribution at the edge of biogas residue particles,that is,the bioavailability of heavy metals decreased.(3)The optimal culture conditions of earthworm composting were as follows:earthworm density,48 g earthworm/kg biogas residue;Incubation time: 12 days;Earthworm size,0.7~1 g/piece with dark obvious ring earthworm.Under these conditions,the enrichment coefficients of Pb and Cr in biogas residue by earthworms were more than0.68 and 0.74 respectively.The observation of ultra-thin slices of biogas residue(about 1 μm thick)combined with synchrotron radiation infrared microscopic imaging showed that the 820 cm-1 characteristic peak in the fingerprint area of biogas residue before earthworm composting was distributed at the edge and inside of the sample,and the characteristic peak might be heavy metals or heavy metals adsorbed by inorganic substances.However,earthworm composting showed a weak position at the edge of the sample and mainly appeared in the interior of the sample.The 820 cm-1 characteristic peak in the earthworm fingerprint region did not appear before composting,but the signal of the 820 cm-1characteristic peak in the earthworm fingerprint region was significantly enhanced after composting.In conclusion,the toxicity of biogas residue from pig manure can be effectively reduced by the combination of high temperature composting and earthworm composting,and the removal effect of heavy metals from biogas residue can be optimized by controlling the culture conditions of earthworm composting.In the process of earthworm composting,earthworms may mainly remove heavy metals from the edge of biogas residue in close contact with them,and the heavy metals enriched by earthworms are mainly distributed in their bodies rather than on their surfaces.In addition,synchrotron radiation microscopic imaging technology can also provide technical support for compost maturity and organic evolution characteristics.