THERMOCHEMICAL PROCESSING OF AGRICULTURAL WASTES TO VALUE-ADDED PRODUCTS AND BIOENERGY
Location: Commodity Utilization Research
Title: Screening biochars for heavy metal retention in soil: role of oxygen functional groups
Submitted to: Journal of Hazardous Materials
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 16, 2011
Publication Date: May 20, 2011
Citation: Uchimiya, M., Chang, S., Klasson, K.T. 2011. Screening biochars for heavy metal retention in soil: role of oxygen functional groups. Journal of Hazardous Materials. 190:432-441.
Interpretive Summary: Biochars can be produced by the thermochemical conversion of agricultural wastes for a wide range of agricultural and environmental applications such as the soil fertilization, site remediation, and carbon sequestration. However, when used to stabilize heavy metals in contaminated soils, the biochar amendment can result in a complex and unforeseen release of nutrients as well as undesirable elements. In this study, a statistical tool was employed to better interpret the complex data set and to determine the primary factors that control the retention and release of various elements in biochar amended soils. Statistical analysis and biochar characterization results suggested that the oxygen-containing functional groups are key to the ability of biochars to retain heavy metals, if amended on soils having minimal inherent sorption capacity.
Oxygen-containing carboxyl, hydroxyl, and phenolic surface functional groups of soil organic and mineral components play central roles in binding metal ions, and biochar amendment can provide means of increasing these surface ligands in soil. In this study, positive matrix factorization (PMF) was first employed to fingerprint the principal components responsible for the stabilization of heavy metals (Cu, Ni, Cd, Pb) and the release of selected elements (Na, Ca, K, Mg, S, Al, P, Zn) and the pH change in biochar amended soils. The PMF analysis indicated that effective heavy metal stabilization occurred concurrently with the release of Na, Ca, S, K, and Mg originating from soil and biochar, resulting in as much as an order or magnitude greater equilibrium concentrations relative to the soil-only control. In weathered acidic soil, the heavy metal (especially Pb and Cu) stabilization ability of biochar directly correlated with the amount of oxygen functional groups revealed by the O/C ratio, pHpzc, total acidity, and by the 1H NMR analysis. Equilibrium speciation calculation showed minor influence of hydrolysis on the total soluble metal concentration, further suggesting the importance of binding by surface ligands of biochar that is likely to be promoted by biochar-induced pH increase.