Migration and transformation of endogenous heavy metals from animal manure biochar in acid soil

Abstract: Animal manure treatment can be widely used a promising pyrolysis technology, due to the complete destruction of pathogens, full decomposition of antibiotics, the value-added energy, and biochar products, heavy metal immobilization, as well as the remarkable reduction in the waste stream volume. Biochar is one type of solid carbon-rich by-product of the animal manure pyrolysis (<800 ℃), indicating the abundant porous structure and high levels of mineral elements. The biochar can be used as the soil amendment and fertilizer, in order to improve the uptake of nitrogen, phosphorus, and potassium for the better water-holding capacity and the less susceptibility to erosion. Extensive research has indicated that the biochar can be an e?ective adsorbent for the contaminants, such as heavy metals and pesticide residues. However, the high levels of heavy metals were the key limiting factor for the application of animal manure. Fortunately, pyrolysis can be expected to concentrate the heavy metals in biochars. In addition, the animal manure biochar can also be alkaline to generally improve the acid soil with high ash contents. Previous studies have demonstrated that the biochar application for the remediation of soils can reduce the bioavailability of heavy metals, thus reducing the potential for the heavy metals to be uptaken by agricultural crops. However, acid soil can increase the bioavailability of heavy metals in biochar. The long-term application of animal manure biochar to the cropland can also increase the heavy metal accumulation in soil, and even enter the food chain as a threat to human health. Therefore, it is essential to evaluate the ecotoxicity of heavy metals in the biochar for scientific utilization. More importantly, the pig production was approximately 0.5 billion heads annually in China, accounting for nearly 1/2 of the global production. The concentrations of heavy metals, especially Cu and Zn in pig manure were significantly higher than those in other animal manure. Taking the pig manure biochar as the research object, this study aims to explore the interfacial behavior of heavy metals at the surface of biochar and soil particles. A pot experiment was conducted using biochar and acid soil in the laboratory. Scanning Electronic Microscope (SEM), physical absorption, X-ray Diffraction Pattern (XRD), X-ray Photoelectron Spectroscopy (XPS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) were utilized to identify the morphology, pore structure, crystal composition and elements of acid soil and biochars, respectively. The basic properties (pH, EC, Cu, Zn, and PO43-) of soil pore water were characterized after the experiment. Results showed that the main fraction of Cu and Zn in the pig manure biochar was the oxidizable extraction, accounting for 79.37% and 53.43%, respectively. Mineral elements of biochar were found on the surface of biochar particles as the oxides. The specific surface area and pore volume of biochar increased after incubated with the acid soil. The contents of Cu, P, K on the surface of biochar decreased. There was a significant increase in the pH and EC values, as well as the concentration of Cu, Zn, and PO43- of soil pore water, after the biochar was applied to the acid soil. The concentrations of PO43- and Cu were in the range of 2.26-298.00 mg/L and 1.81-2.86 μg/L, respectively. Consequently, the carbonate and phosphate alkaline salt in the biochar neutralized the H + in the acid soil, and then released into the acid soil, leading to the release of Cu that precipitated and adsorbed by carbonate, phosphate in biochar. Some Cu was released into the soil solution, while another Cu was formed from the mineral compounds (Cu4(SO4(OH)6H2O)H2O,Cu(SiO3)H2O) that combined with the soil components. The smaller particle size of biochar greatly contributed to facilitating the release of Cu. The largest release amounts of PO43- and Cu in the biochar were achieved on the 30th day incubated with acid soil. However, the Zn in the biochar was difficult to release in the acidic soil environment, where the biochar adsorbed Zn to reduce the bioavailability after being applied to the acidic soil. The finding can provide a strong reference to better understand the transformation and potential environmental risk of heavy metals in pig manure biochars.