Effect of pig manure with bentonite on chromium passivation in anaerobic fermentation

Abstract: Heavy metals such as chromium (Cr) have widely been used as feed additives in large-scale pig farms with the development of the feed industry in recent years. Most heavy metals have posed increasingly serious pollution and subsequent resource utilization of livestock and poultry manure, due to the low digestion and absorption of heavy metals. Anaerobic fermentation is an effective way to treat livestock and poultry manure. However, heavy metals still remain in the biogas fertilizer after treatment, where heavy metal contamination depends mainly on their forms. Taking heavy metal Cr as the research object, this study aims to investigate Cr passivation mechanism in the pig manure with bentonite after anaerobic fermentation. Pig manure was selected as the raw material, where the natural/microwave bentonite with 5% dry matter content was added (microwave heating 2, 4 and 8 min). The duration of anaerobic fermentation was 40 days at the temperature of 35℃, inoculum quantity of 30% raw material, 10% total solid, and pH value of 7. Five treatments were included: CK group was fermented with pig manure alone; TP group, the natural bentonite was added to pig manure; 2WP group was the pig manure with 2 min microwave bentonite; 4WP group was the pig manure with 4 min microwave bentonite; 8WP group was the pig manure with 8 min microwave bentonite. Each treatment was repeated 4 times for high accuracy. As such, the objective of the experiment was to explore the biogas production characteristics of natural/microwave bentonite added to pig manure, morphological changes of heavy metal Cr, and passivation effect in anaerobic fermentation. Fourier Transform Infrared spectrometer (FTIR), and scanning electron microscope/energy dispersive spectrum (SEM-EDS) were utilized to characterize microstructure and morphology of compounds in the biogas residues after anaerobic fermentation. The results show that the total gas production, the average volume fraction of methane, the proportion of Cr residual state, and the effective passivation effect of pig manure with bentonite in anaerobic fermentation were significantly higher than those of pig manure alone. Specifically, these indexes in the treatment of short-time microwave modification (2 and 4 min) were significantly higher than those of natural bentonite treatment(P<0.05).When the microwave heating time of bentonite took longer (8 min),the above indexes were lower than those of the treatments with a short time, where the best microwave heating time of bentonite was 4 min. FTIR data showed that the contents of carbohydrates, carboxylates and aliphatic compounds were reduced in the biogas residue after anaerobic fermentation, whereas the content of aromatic compounds increased significantly. In addition, the treatment of pig manure with the bentonite promoted the conversion of organic matter into humus, thereby intensifying the humification of biogas residues. SEM-EDS images showed that the pig manure was initially composed of large irregular organic matter, and then transformed into small particles of humus after anaerobic fermentation. The addition of microwave bentonite efficiently promoted the humification process of biogas residue. The reason was that the surface of bentonite changed from smooth and dense to rough and porous after microwave heating, where more adsorption sites were provided for heavy metal Cr, thereby improving passivation ability of humus and bentonite on heavy metals. The finding can provide a scientific reference for the passivation of heavy metals using the anaerobic fermentation of livestock manure, further reducing the pollution of heavy metals in biogas fertilizer for the safe utilization of wastes.