Biochar as soil amendment can increase soil carbon (C) sequestration and mineral nutrients; however, some of its soluble elements may also be unintentionally released during the application. In this work, eight types of biochars were derived from herbaceous, woody, and waste (tailing, manure, sludge) biomass feedstocks through slow pyrolysis at 600 °C in N2. The elemental composition, specific surface area, morphology, crystalline phases, thermal stability, surface functional groups, and pH of the point of zero charge of the biochars were determined using various methods. These properties varied significantly among the tested biochars, suggesting that feedstock type played an important role in controlling their properties. Laboratory release and toxicity characteristic leaching procedure extraction experiments were conducted to evaluate the potential release of nutritious and toxic element from biochars. Results showed that all the biochars released nutritious elements and thus, may be beneficial to plants when amended in soils. In general, biochars produced from herbaceous and woody biomass feedstocks showed low risks of releasing toxic elements. Biochar derived from sludge, however, might present ecotoxicological challenges for its environmental applications due to the release of toxic elements, such as heavy metals.
Keywords: Biochar contamination; Biochar land application; Heavy metal leaching; Nutrient release; Soil amendment.