Agar-immobilized Metal Sulfides and Their Sorption Power with Respect to Ag(I) Ions

Abstract

Thin-film systems based on sulfides of various p and d elements (MS) efficiently sorb noble and toxic metals from wastewater and spent process solutions by the ion-exchange mechanism. Previously such systems were prepared in most cases by chemical deposition of metal sulfides in the form of microcrystalline films on various supports (cellulose [1], cellulose triacetate [2], glass [3]). All these sorbents are characterized by micrometric particle size, which limits their performance in sorption of noble and toxic metals, if for no other reason than long sorption time. In [4] we suggested a procedure for preparing gelatin-immobilized metal sulfide matrix systems with quasimolecular level of MS dispersity. Sorption of metal ions, in particular, Ag(I), from wastewater and spent process solutions with such sorbents is fairly fast and complete. However, the gelatin layers exhibit relatively poor adhesion to the polyethylene terephthalate support and can exfoliate, which complicates further operation and utilization of the sorbent. Therefore, it is preferable to use polymer systems in which the nanometric particle size of the immobilized metal sulfides is preserved but no foreign support is required. One of polymeric materials suitable for this purpose is agarose, linear polysaccharide built from regularly alternating residues of 3-O-substituted -D-galactopyranose (occasionally, 6-O-methyl-D-galactopyranose) and 4-O-substituted 3,6-anhydro-L-galactopyranose. Agarose is the main component of agar natural polymer recovered from red algae [5]. Agarimmobilized metal sulfide matrices (MS AIM) are prepared in two steps. In the first step, agar is added into an aqueous solution containing an appropriate metal salt (nitrate, chloride, sulfate); the mixture is heated to 90 95 C and added with a buret or pipet into a vessel with water cooled to 10 15 C. The globules formed in the process are treated in the second step at room temperature with an aqueous solution of sodium sulfide to obtain the desired metal sulfide MS (M = Cu, Zn, Pb, etc.) by exchange reactions. These globules are fairly stable to aqueous solutions at widely varied pH (from 2 to 13) and temperature (up to 100 C) and to physicomechanical stresses. According to elemental analysis, the particle size of the sulfides implanted in MS-AIM is 10 15 nm on the average and does not exceed 20 nm.