A cross-linked polymeric resin which contains reacted monomer units of an aniline, a diaminoalkane, and an aldehyde, and which is functionalized with at least one dithiocarbamate moiety. A process for producing the cross-linked polymeric resin whereby an aniline and a diaminoalkane are linked together by Mannich-type polycondensation reactions with an aldehyde. The resulting Mannich-type polycondensation product is converted into the cross-linked polymeric resin through functionalization of one or more amine functional groups with dithiocarbamate moieties. A method for removing heavy metals, such as Hg(II), from an aqueous solution, whereby the cross-linked polymeric resin is contacted with the aqueous solution, and the heavy metal is thus adsorbed onto the cross-linked polymeric resin.