I think that the water-soluble fractions of biochar have been shown to be an effective tool for Hg remediation, as they can facilitate the removal of Hg from contaminated soils and sediments. Largely, this is due to the ability of the organic carbon in the biochar to form complexes with Hg and, thus, reduce its bioavailability. Additionally, the presence of the biochar may also increase the sorption capacity of soils, increasing the retention of Hg in the environment.
I'm also involved with Hg soil pollution control/remediation and are going to use biochar as one of the methods. I agree that adsorption will be the most probable mechanism to get rid of the Hg together with the complex reactions, but how are you going to get rid of the "Hg-polluted biochar" ?