Low Pressure Membrane Separation Process to Remove Heavy Metal Complexes

The overall objective of this investigation is to establish the rejection behavior of heavy metals in the presence of complexing agents, utilizing negatively charged ultrafiltration membranes. An extensive experimental investigation is conducted with Zn2+, Cd2+, Cu2+, and Cu1+ in the presence of cyanide, ethylenediamine tetraacetic acid, and oxalates, under insignificant concentration polarization condition. The rejection dependence of the heavy metals is found to be a function of feed metal concentration, metal types, complexing agent to metal feed molar ratio, pH and ionic strength. The dependence of rejection behavior of heavy metals and complexing agents on pH and concentration is explained in terms of metal complex species distribution and Donnan Exclusion model. For EDTA and oxalate systems, the rejections of metal are independent of initial metal concentration; whereas for the cyanide system the rejections of both metal and cyanide decrease with concentration. At transmembrane pressure of 5.6 x 105 N/m2, metal rejections range between 77% to 96%. For all cases, the rejection of metal is highly dependent on the size and charge of the complex metal species. For example, the rejections of Zn(CN)2-4 \u3e Zn(CN)31-, and Cu(EDTA)2- \u3e Cu(CN)32- \u3e Cu(C2O4)2- are observed