Heavy metals (Hg, Cd etc.) have the predilection to "occupy" the thiol groups - SH and those thiol groups constitue actif sites of a great number of enzymes, so they are at a great risk to be inactivated by heavy metals, also those heavy metals have the tendency to occupy the place of "useful cations" like magnesium, zinc, manganese etc., this makes the metallo enzymes inactif or less performant, so supplementing with these useful oligo-elements increases the possibility to displace heavy metals from these sites.
It is good to supplement in addition to Mg, Zn, Mn with Se and alpha lipoic acid (both have 2 thiols to bind firmly Hg, Cd), in addition to MSM (only 1 thiol) and of course vit. C helps to evacuate them quickly...