Separation of heavy metal ions from multi-component solutions using ultrafiltration enhanced with complexing polymer

Polymer-enhanced ultrafiltration (PEUF) is a new, attractive membrane technique enabling the separation of low molecular weight compounds from aqueous solutions. PEUF is especially interesting in terms of heavy metal ions removal from industrial effluents. In the presented work, the technique was applied for the separation of heavy metal ions (Cu(II), Cd(II), Zn(II), Ni(II), and Cr(III)) from their equimolar five-component solution. Polyethyleneimine (PEI), a water-soluble polymer containing amine groups, was chosen to enhance metal ions separation in the ultrafiltration. The effect of pH, polymer-to-total metals molar ratio C_PEI /C_Ms, and the presence of the accompanying ions (formula presented) on the separation efficiency of the heavy metal ions was investigated. Then, ultrafiltration concentration of the heavy metal ions was conducted at selected process parameters followed by the decomposition of polymer-metal complexes (pH = 2) and recovery of metal ion concentrates. Both C_PEI /C_Ms and pH strongly influenced the PEUF effectiveness. At a moderate pH (pH = 5) and lower PEI dose (C_PEI /C_Ms = 1), the preferential Cu(II) separation from the multi-component mixture was observed. Applying a higher PEI dose (C_PEI /C_Ms = 7), very high rejection coefficients (0.98–1.00) of all heavy metal ions can be achieved. At a lower polymer dose, the tested accompanying ions diminished permeate flux and metal ion retention coefficients. The effect was much less visible at a higher C_PEI /C_Ms value. Ultrafiltration concentration of polymer-metal complexes with subsequent decomplexation–ultrafiltration step (pH = 2) enabled recovery of 67%–91% of concentrated metal ions.