Polymer Enhanced Ultrafiltration (PEUF) for the Removal of Metals from Water: Influence of Competing Solutes and Chelators

There is considerable potential for processes involving Polymer Enhanced Ultrafiltration (PEUF) to remove metals from water. The process involves the use of membrane and a powerful metal chelating polymer materials such as Polyethylenimne (PEI), this have the capability of selectively binding metals so reducing their concentrations in water to sub ppm levels. This paper reports the investigation of the competition for metal between PEI and other simple naturally occurring substances that could interfere with the PEUF process. Using ultrafiltation membrane (Nadir 30,000 molecular weight cut off) the filtration and retention of metals ions in the presence of 1 g/l PEI (750,000 Da) and the additives 2mM EDTA, Nitriloacetic acid (NTA), sodium acetate, ammonium chloride and ammonium sulphates was investigated. The metal ions investigated were: Cu2+, Zn2+, Ni2+, Cr6+, Co2+ and Cd2+. The metals in the resultant permeate and retentates were then analysed by OES ICP.The results of these experiments showed that the interactions between the metals, PEI and the additional competitors (cheletors or salts), as compared with PEI alone, are complex. The effects of the additions were especially pronounced at low metal ion concentrations. In some cases this led to negative rejection, for example Cu2+in the presence of EDTA, while in others e.g. Cd2+and Zn2+, additions of EDTA led to enhanced retention over PEI alone. Using NTA as a competitor enhanced retention at high metal concentrations (<30 ppm) under the standard conditions.It is clear from these results that the interaction of the PEI, metal, and addition of competing substances leads to a complex set of interactions that generally lead to reduced metal retention in the system. The results suggest that when these processes are applied to water cleaning, such as landfill and other leachates, care must be taken to pre-treat the water to avoid potential problems with indigenous chelators, especially low molecular weight organics. However, under certain conditions, the use of specific additives has the potential to be exploited to enhance purification for these metals using PEUF. By manipulating the process environment metals can be enriched relative to one another in the permeate or retentate of the PEUF process. The study also shows that interactions must therefore not only involve the metal-PEI or metal-competitor but in some cases must also involve the binding competitor to PEI and/or the metal simultaneously.