oxide for example results in the inclusion of various functional groups that are relatively negatively charged e g carboxylic acid groups or polar e g carbonyl groups polyvalent cations will be attracted to and captured by these groups in one embodiment the filter material is covalently modified with thiol groups according to known diazonium chemistry procedures thiol groups are naturally excellent at capturing positively charged ions notably doubly charged mercury cations although other metallic cations ions such as the aforementioned ca 2 zn 2 etc contained in downhole brines will also be readily captured by thiol groups other functional groups such as disulfide groups carboxylic acid sulfonic acid groups may also be used for their ability to capture polyvalent cations particularly doubly charged cations other functional groups include chelating ligand groups such as iminodiacetic acid iminodiacetic acid group n 5 amino 1 carboxy t butyl pentyl iminodi t butylacetate group n 5 amino 1 carboxypentyl iminodiacetic acid group n 5 amino 1 carboxypentyl iminodiacetic acid tri t butyl ester group aminocaproic nitrilotriacetic acid group aminocaproic nitrilotriacetic acid tri tert butylester group 2 aminooxyethyliminodiacetic acid group and others that would be recognized by those of ordinary skill in the art in view of the disclosure herein the graphene based materials could also be functionalized to filter anions e g with quaternary ammonium quaternary phosphonium ternary sulfonium cyclopropenylium cations or primary secondary ternary amino or other groups these groups are.