Inverted polarity micellar enhanced ultrafiltration for the treatment of heavy metal polluted wastewater

Abstract

Membrane separation technologies are being applied with increasing frequency in the treatment of industrial and municipal wastewater, as a means of coping with increasingly stringent environmental regulations regarding waste-water discharge. In particular, the research work presented here is driven by the need to reduce the amount of heavy metals released into aquatic environments. Micellar enhanced ultrafiltration (MEUF) is a colloidal-based waste-water treatment process. The adsorptive capacity of the Stern layer of a large, anionic surfactant micelle is used to capture heavy metals ions; the micelles are, in turn, trapped behind a polymeric ultrafiltration membrane. Experimental work has investigated the possibility of increasing the capture of pollutant ions, by collapsing the Stern and diffuse layer radii within the hydrodynamic diameter of micelles of sodium dodecyl sulphate. This involves bringing the micelles to a state of near-flocculation through the addition of multivalent ions; these ions include aluminium, which is deliberately added as a flocculant ion, and the target heavy metal ions in solution, which are represented here by zinc. The near flocculation is achieved through the combination of surfactant, aluminium and zinc ion concen trations. The electrical triple-layer compression and near-flocculation of the micelle means that a greater fraction of the metal ions present in solution are dragged along with the micelle. The high porosity and permeability of the near-flocculated (or fully flocculated) micelles also reduces the tendency for gel formation, and thus fouling, on the ultrafiltration membrane surface. A combination of titration analysis and ultrafiltration experiments has investigated the relationship between the half-lives of permeate flux decline and the micelle-zinc binding ratios. This has allowed us to determine conditions which allow an optimum separation efficiency. Overall, the experiments have demonstrated a potential for improving the efficiency of the treatment of wastewater containing heavy metal ions.