Separation of indium using nanofiltration membranes:: A non-steady-state study

In this work, the performances of a nanofiltration membrane NTR7450 separating a multivalent cation (In3+) under non-steady and steady states were compared. The effect of both pH and concentration of the feed solution was studied. The ability of indium to form complexes even under acidic conditions could be theoretically determined using the solubility constants for various end products. Turbidity studies were carried out to determine the non-steady and steady phases of indium chloride solutions at room temperature. Two hypotheses were formulated from the turbidity studies and subsequently verified by the results of nanofiltration experiments. While InCl3 solution aged under the non-steady state, the rejection rate of indium by nanofiltration gradually increased from 64 to 88%. Under the steady state, indium rejection rate of NTR7450 was found constant and independent of the feed concentration. The results demonstrated the significant impact of indium complexation on the performance of nanofiltration. Based on the basic principles of electrochemistry, a semi-empirical model was proposed to link the change of solution structures with nanofiltration performance. The steady state of indium chloride solutions could be also reached by pH adjustment using NaOH. At increased pH, a non-steady state was not observed during nanofiltration.