Changes in structural characteristics and metal speciation for biochar exposure in typic udic ferrisols

Recent studies on biochars confirmed their potential benefits in improving soil fertility and sequestrating contaminants. However, little information on the changes in structural characteristics and metal speciation of biochars after exposure to soils is currently available. The aim of this study was to use double experimental bags to study the transformation of ozonized biochars derived from poultry manure and drying sludge (denoted PB and SB, respectively) in typic udic ferrisols. The carbon and sulfur functional groups and chemical characteristics of the biochars were determined using spectroscopic techniques, such as Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy spectroscopy, combined with chemical extractions. Metal species were characterized using Cu K-edge X-ray absorption near-edge structure and chemical sequential fractionation schemes. The results showed that the potential changes in structural characteristics and metal species of biochars in soil were highly dependent on the composition of the biochars. PB comprised highly aromatic and chemically stable C, whereas SB contained a substantial amount of easily degradable C. Oxygen-containing groups slightly increased after incubation in the soil with either 60% water holding capacity (WHC) or flooding for 3 months. Sulfur in the biochars was predominantly inorganic S. Minerals such as K, Na, Mg, and S were mobilized from the biochars, accounting for 5–55% depending on the properties and sources of the element. Both PB and SB contained high concentrations of Cu and Zn. CuO in PB dissolved within 3 months, whereas CuS in both PB and SB was partly oxidized in the soil with 60% WHC for 9 months and adsorbed to the organic phase. Zn had relatively high mobility in both biochars, resulting in its vertical migration into soils.