Mercury distribution and speciation in biochar particles reacted with contaminated sediment up to 1030 days: A synchrotron-based study.

A previous long-term microcosm experiment showed mercury (Hg) in the aqueous phase of contaminated sediment was effectively stabilized through the addition of biochar. The present study focuses on the application of synchrotron-related methods to evaluate the distribution and speciation of Hg in the biochar particles reacted for 235, 387, and 1030 days. The study provided more information on Hg stabilization mechanisms in addition to the information obtained by the previous studies. Confocal micro-X-ray fluorescence imaging (CMXRFI) and micro-X-ray fluorescence (micro-XRF) maps show that mercury co-exists with S, Cu, Fe, Mn, and Zn on the surface and inside the particles of biochar. Extended X-ray absorption fine structure (EXAFS) modeling shows that Hg is in an oxide form on the surface of an iron (hydro)oxide particle from fresh sediment and in Hg-sulfide forms in biochar samples. S X-ray absorption near-edge structure (XANES) analyses show that sulfide is present within the biochar particles. After amendment with biochars, a fraction of the Hg originally present in unstable forms (dissolvable, HgO, colloidal, nano, etc.) in the sediment was likely stabilized as less soluble Hg-sulfide phases on the surface or within the biochar particle. These results suggest Hg accumulation by the biochar particles renders it less potential for transport and bioavailability.