Removal of mercury from soil matrix is not possible.  We may only reduce their eco-toxicity by using some biochars. Because biochar only adsorb the heavy metal on their surface and reduced the rate of bio-magnification. You can find a number of literature on biochar for reducing the heavy metal toxicity in soil.

the answer of Sandeep Kumae is certainly right for mercury in ion form, normally almost full bound to clay or organic matter. Some Hg contamination can be in the form of little drops of fluid Hg metal. Maybe there is an opening for other "removal" methods.

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