Evaluation of PAH and Metal Contents of Different Biochars for Use in Climate Change Mitigation Systems

Biochar, a solid byproduct generated during biomass pyrolysis or gasification in the absence (or near-absence) of oxygen, has recently garnered interest for both agricultural and environmental uses owing to its unique physical and chemical properties, such as its high surface area and porosity, and ability to adsorb a variety of compounds, including nutrients, organic contaminants, and some gases. This material is also considered "sustainable," as it can be derived from agricultural wastes and is currently being investigated for carbon sequestration applications and as a soil cover amendment for reduced greenhouse gas emissions from landfills. Ongoing research in our laboratory has shown that biochar can enhance both methane adsorption and subsequent microbial oxidation in soils, making it a promising material to mitigate residual landfill emissions for which gas recovery is not economical or feasible. Physical and chemical properties of biochar are dictated by the feedstock and production conditions (i.e., temperature, conversion technology, and posttreatment processes, if any), which vary widely across commercially produced biochars. In this study, several commercially available biochars are characterized for key physical and chemical properties relevant to the common uses of biochars in environmental applications, with special attention given to PAH and heavy metal content and leachability. A high variability in chemical composition, surface properties, and PAH and heavy metal contents among commercially available biochars was observed, underscoring the importance of prescreening biochars for the presence of PAHs and heavy metals prior to selection as a landfill cover or soil amendment.