Nanofiltration of indium and germanium ions in aqueous solutions: Influence of pH and charge on retention and membrane flux

The retention of In2(SO4)3 and GeO2 with two different commercially available polymeric nanofiltration (NF) flat sheet membranes (NP010, NF99HF) was investigated between pH 2 and 12. The main objective of this experimental study is to investigate the selective separation of both indium and germanium in aqueous sulfate solution. The experiments focus on a future membrane application for winning indium and germanium from bioleaching solutions. The investigation was conducted with synthetic solutions while single salt experiments and experiments with the binary salt system were performed. Depending on pH value, ions show different speciation which strongly influences membrane charge and separation performance. Streaming potential measurements with In2(SO4)3 and GeO2 were performed to determine zeta potential as a measure of membrane charge. KCl was used as a reference system of zeta potential since it is considered inert regarding interaction with the membrane surface. Both, zeta potential profiles of single salt solutions and binary mixtures showed a remarkable interaction between indium and germanium ions and the membrane surface because the IEP was shifted to a higher pH value. The results were ascribed to specific adsorption of In3+ on the membrane surface. The nanofiltration experiments revealed that indium and germanium are separated successfully within distinct pH values which is caused by electrostatic interaction of species like In3+ and In(OH)4- and the membrane charge. Furthermore, size exclusion plays a distinctive role in the separation of In(OH)30 and Ge(OH)40. Above all, we could show that germanium can successfully be enriched in the permeate.