No abstract

Three different kinds of adsorption gels, viz., cross-linked lignophenol, cross-linked lignocatechol, and crosslinked lignopyrogallol, were prepared by the chemical modification of wood lignin. The adsorption behaviors of these gels for Au(III) along with some other metals were studied and compared to that of activated carbon. All three gels were found to be more selective for Au(III) than activated carbon with comparable adsorption capacities. Of the lignin gels, cross-linked lignophenol exhibited the highest selectivity for Au(III) and was found to be almost inert toward other metals tested. All three novel lignin gels as well as activated carbon were found to be efficient in reducing Au(III) to elemental gold, as indicated by XRD analysis of the sorbents taken after adsorption. However, a significant difference between the novel sorbents and activated carbon was found, i.e., the latter exhibited no selectivity among the metal ions tested, whereas the novel gels have a high selectivity to only Au(III). In addition, gold aggregates were visually observed in the case of the lignin gels and not in the case of activated carbon. This result provides a new approach for effective gold recovery.

Waste newsprint paper was modified with dimethylamine (DMA) to obtain a tertiary amine type adsorption gel called DMA-paper gel. This new derivative was investigated for adsorption, from hydrochloric acid medium, of gold, palladium, and platinum as well as some base metals. The gel exhibited selectivity only for precious metals with a remarkably high capacity for Au(III), i.e., 4.6 mol/kg dry gel and comparable capacities, i.e., 2.1 and 0.9 mol/kg for Pd(II) and Pt(IV), respectively. Also, Au(III) was reduced to the elemental form during adsorption. Furthermore, column adsorption and subsequent elution of the adsorbed metal ions by acidic thiourea revealed encouraging recoveries (approximately 90%), thus enhancing the scope of the gel for effective preconcentration, separation, and recovery of precious metals. The effectiveness of recovery of precious metals from real industrial liquor was also tested, and it showed highly encouraging results with respect to the stability of the gel in the harsh medium, and selectivity for the targeted metal ions in the presence of excess of other metal ions.

Waste newsprint paper was chemically modified with p-aminobenzoic acid to prepare a sorption active gel. This gel proved useful for the selective uptake of gold, palladium and platinum over other coexisting metal ions such as copper, nickel, zinc and iron. A remarkably high capacity for Au(III) was observed together with a moderate uptake of Pt(IV) and Pd(IV). Furthermore, Au(III) was reduced to elemental gold which formed aggregates. A high percentage recovery of the precious metal ions (85-95%) and regeneration of the gel was attained by using a mixture of 0.15 M thiourea and 1 M HCl. Repeated cycles of uptake and elution by using a column packed with this new sorbent indicated that the gel is suitable for industrial application in continuous mode.