The aim of the study was to examine physicochemical properties of three cationic surfactants and to evaluate the effectiveness of their removal with the use of polymeric membranes. The experiments were performed in a laboratory scale set-up with the use of Microdyn-Nadir® nanofiltration and ultrafiltration membranes. Cetrimonium bromide (CTAB), benzalkonium chloride (BAC) and Tequat LC90i (TEAQ) were chosen for the test. In the experiments, surfactant solutions in a wide range of concentration were treated (50–3,000 mg L−1). The experimental research included evaluation of the effect of membrane type and solution parameters (surfactant type and concentration, presence of inorganic compounds) on the process efficiency (retention coefficient and permeate flux). It was shown that surfactant removal by means of the pressure-driven membrane processes is an extensive issue and its efficiency depends on many factors. Ultrafiltration and nanofiltration membranes proved to be usable in CTAB removal (separation exceeded 90%); however, the process effectiveness was affected by surfactant concentration, membrane polymer type and membrane pore size. Separation obtained for BAC was on the lower level – the use of nanofiltration membranes brought maximum retention of 70%. TEAQ separation was very high and reached 100% with the use of ultrafiltration membranes. Mineral salt addition led to significant drop in surfactant retention.
The usefulness of ultrafiltration process for removal of quaternary ammonium compound has been assessed. Intersep Nadir membranes with a cut-off of 5 kDa made of cellulose and polyethersulfone were tested. The concentration of cationic surfactant in the feed solutions amounted to 50-3500 mg/L. It was found that polymer type affects the removal efficiency of cationic biocide. The polyethersulfone membranes allowed to remove above 63% of BAC from the feed solution of concentration equal to 100 mg/L, while for cellulose membranes the highest retention (i.e. 27%) was obtained for process with 3500 mg/L. During the tests surfactant affected on membranes' hydraulic properties. For experiments with polyethersulfone membranes, the relative flux obtained was in the range 80-20% (depending on pollutant concentration) of distilled water flux. Cellulose membranes were less fouling susceptible. Moreover, in experiment with surfactant concentration equal to 1000 mg/L an increase in membrane permeability (comparing to distilled water flux) was pronounced.
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