Competitive separation of heavy metals from binary solutions by polymer-enhanced ultrafiltration

The polymer-enhanced ultrafiltration has been proposed for the separation of Zn(II) and Cr(VI) ions from binary mixtures. Polyethyleneimine (PEI), a water-soluble polymer containing primary, secondary and tertiary amino groups, was used to bind both metal ions and enhance the ultrafiltration process. Model solutions containing Zn(II) and Cr(VI) ions with various molar ratios of both metals were used in the investigations. The impact of the main process parameters, that is, pH, the molar ratio of PEI to the total amount of both metals, the composition of the bi-ionic Zn(II)/Cr(VI) mixture and the presence of chlorides and sulphates on process effectiveness was discussed. It had been found that the Zn(II) rejection coefficient increased with increasing pH and its value also depended on the amount of PEI and the composition of the binary solution. The Cr(VI) rejection coefficient reached maximum values in the pH range of 6–8 and significantly decreased with a decrease in the PEI dose, increase in Cr(VI) amount in the bi-ionic mixture and the presence of sulphate anions. An effective simultaneous metal separation was observed at pH 6–8 in the solutions without sulphates, when the amount of PEI was sufficient to bind both metals (molar ratio C_PEI/C_M = 5). Selective separation of Zn(II) ions from the binary Zn(II)/Cr(VI) mixture was visible at pH = 10, and was favoured by both the reduction of the polymer dose and the presence of sulphate ions in the solution.