Polythiol-functionalized alumina membranes for mercury capture

Various materials (particles, resins, etc.) for Hg2+ sorption from aqueous streams have been reported in literature. Conventional sorbents are relatively inefficient because only a fraction of the immobilized ligands are accessible for metal complexation. Thus, our approach was to use open structures (0.2 μm pore size alumina microfiltration membranes), immobilized with various ligands containing single or multiple thiol functional groups. Alumina has good chemical and thermal stability and abundant surface hydroxyl groups, necessary for chemical modification. Convective flow was used for all functionalization steps and Hg2+ sorption studies. Only 3-mercaptopropyl trimethoxy silane has been immobilized by direct silylation; the other ligands (cystine, cysteine, polycysteine, polyglutamic acid) required intermediate steps. Thus, via silylation with 3-glycidoxypropyl trimethoxy silane the membrane surface was functionalized with epoxy groups, which then reacted with the terminal amine group of each of the 4 ligands mentioned previously. In the case of polyglutamic acid, the carboxylic acid groups were activated with dihexylcarbodiimide and further reacted with cysteine, making it possible to synthesize a polythiol containing 240 repeat units. Hg2+ sorption studies on single thiol-functionalized membranes were used to analyze the interaction between Hg2+ and various functional groups. In addition, it was determined that Hg2+ bound to weak sites (disulfide, carboxylic acid) can be quantitatively removed by washing the membrane with water at pH = 3, making it possible to quantify the amount bound to the active sites (thiol). Polythiol-functionalized membranes showed high sorption capacities, high site accessibility, and fast sorption rates.