Mercury Complexation with Dissolved Organic Matter Released from Thirty-Six Types of Biochar

Previous studies show mercury (Hg) can be effectively removed from solution by biochar, but limited attention was paid on the complexation between Hg and components released from biochars, e.g. dissolved organic matter (DOM). Here, aqueous data from batch-style experiments were modeled using PHREEQC, incorporating thermodynamic constants between Hg and DOM, which was assumed to be composed of thiol, carboxylic, and phenolic functional groups. Modelling results suggest that > 99% Hg complexed with thiol groups in DOM. The modelled concentrations of Hg-DOM complexes from low-T (low-temperature, 300°C) biochars were greater than from high-T (600°C) biochars. The concentrations of Hg-DOM complexes were lower in wood-based than in agricultural residue- and manure-based biochars. Hg-DOM complexes may affect Hg speciation, bioavailability, transport, and methylation processes. This research describes a method to evaluate Hg-DOM interactions, and the results indicate extra caution regarding Hg-DOM complex formation is required in the selection of biochar for Hg remediation.