Inorganic anion removal using micellar enhanced ultrafiltration (MEUF), modeling anion distribution and suggested improvements of MEUF: A review

Surfactants have been used often in environmental remediation strategies due to their special amphiphilic nature which alters surface and water interfacial properties. When the aqueous concentration of a cationic surfactant far exceeds the critical micelle concentration (CMC), a large concentration of cationic micelles will form in water. These micelles each consist of tens to hundreds of surfactant monomers, and collectively can be utilized as nano sized ion exchangers to assist with ultrafiltration separation (i.e., removal) of anionic pollutants from natural waters or wastewaters. Target anionic pollutants include nitrate, phosphate, arsenate and chromate. However, most polluted waters contain a complex mixture of anions, with these different anions competing for the micellar pseudo-phase, thus potentially reducing the overall removal efficiency of the target anions. Further, loss of surfactant monomers through the membrane also reduces process efficiency as replenishment of surfactant over time is required. In this review, the existing researches on inorganic anion removal by micellar enhanced ultrafiltration (MEUF) and similar processes are summarized. Operating condition factors are discussed, including pressure, membrane pore size, surfactant-contaminant concentration ratio, and water chemistry conditions (i.e., pH, salinity). Because most micellar surfactant - anion interactions are through outer-sphere electrostatic association, emphases in this review are given to the measurement of selectivity coefficients used for identifying the affinity of anions to the micelles, which generally decreases in the order of: Fe(CN)(6)(3-) > CrO42- > SO42- > HAsO42- > HPO42- > NO3- > Br- > NO2- > Cl- > HCO3- > H2AsO4- > H2PO4- > F- > IO3-; and to the development of a speciation model, based on these selectivity coefficients, for predicting anion distribution in micellar solutions. Ways to address improved process efficiency, as well as future challenges and opportunities, are also discussed.