Effects of boron mud on anaerobic fermentation of heavy metal chromium and its spectral characteristics in pig manure

Abstract: In recent years, with the development of Chinese animal husbandry, some heavy metals have been added to animal feed to speed up the growth of livestock and poultry and prevent diseases. But most heavy metals can not be absorbed and digested, resulting in increasingly serious pollution problems of heavy metals in livestock and poultry manure, which seriously affects the resource utilization of livestock and poultry manure. Anaerobic fermentation is an effective approach to deal with manure pollution. In this paper, using pig manure as the processing object, the heavy metals Cr as the research object, by adding different amounts of boron mud (boron content is 2.5%, 5%, 7.5% of dry matter content) The anaerobic fermentation period is 60 days, fermentation temperature is of 35℃, inoculum quantity is of 30% fermentation raw material, total solid is of 10% and pH value is of 7. The speciation analysis of heavy metal Cr are analyzed by BCR sequential extraction and the spectral characteristics of biogas residue before and after anaerobic fermentation are studied by Fourier transform infrared spectroscopy (FTIR), effects of different amount of boron on morphological changes, effective passivation effect and spectral characteristics before and after anaerobic fermentation residue. The aim of this study is to provide a scientific basis for the treatment of heavy metals in livestock and poultry manure. Through the above experimental research, the following main conclusions are drawn: 1) After the end of anaerobic fermentation, the effective content of heavy metal Cr in each treatment decreased. The effective state of heavy metal Cr decreased by 6.06 percentage points in pig manure alone, and the effective content of heavy metal Cr in pig manure added boron decreased by 11.33 percentage points, 13.40 percentage points and 14.98 percentage points, respectively. It is indicated that the anaerobic fermentation of pig manure and boron is beneficial to convert the heavy metal Cr from the effective state to the stable state. With the increase of the added amount of boron, the proportion of the effective Cr content to the total content of heavy metals is lower. The proportion of heavy metal Cr in the treatment of pig manure added with 7.5% boron is relatively large. 2) The passivation effect of the effective form of heavy metal Cr in pig manure alone was 24.17%, and the effective passivation effect of heavy metal Cr in pig manure was 44.32%, 55.38% and 63.79%, respectively. It is indicated that the anaerobic fermentation of pig manure added boron is beneficial to improve the passivation effect of heavy metal Cr. With the increase of boron addition, the passivation effect of heavy metal Cr is better. The significant analysis showed that the anaerobic fermentation of pig manure added boron had significant difference in the passivation of heavy metal Cr (P<0.05). The passivation effect of pig manure added with 7.5% boron was better than other treatment groups, and the difference was significant (P<0.05). 3) Fourier infrared spectroscopy showed that the materials of each treatment group had similar spectral characteristics before and after anaerobic fermentation. After anaerobic fermentation, the relative intensity of the absorption peaks of the biogas residue of each treatment group decreased at 3 408-3 450 and 2 850-2 922 cm-1, indicating that the organic matter such as amide compound, carbohydrate, protein, and aliphatic compound in the biogas residue were decomposed and reduced. The absorption peak intensity increased, indicating that the olefins, aromatic compounds and other substances in the biogas residue increased. The addition of boron during the anaerobic fermentation of pig manure is beneficial to the reduction of organic matter content, the increase of humus content, and the humification degree of organic matter is better when the amount of boron sludge is 7.5%. Adding an appropriate amount of heavy metal passivating agent to the anaerobic fermentation of pig manure can reduce the effectiveness of heavy metals in the biogas residue and the risk of heavy metal pollution.