Removal of phenyl-urea herbicides in natural waters by UF membranes: Permeate flux, analysis of resistances and rejection coefficients

The removal of four phenyl-urea herbicides (linuron, diuron, chlortoluron and isoproturon) dissolved in natural waters by UF membranes was studied in a cross-flow laboratory equipment in batch concentration mode (with recycling of the retentate stream). Two polyethersulfone UF membranes (PW and PT with MWCO of 20,000 and 5000 Da) and a thin film composite membrane (GK with MWCO of 2000 Da) were used. The influence of the main operating conditions (transmembrane pressure, initial concentration of herbicides and MWCO of the membranes) on the steady-state permeate flux was established. The fouling of the higher MWCO membrane (PW) was higher than that of the tight membranes (PT and GK). The analysis of the resistances to the permeate flux obtained revealed that the contribution of the fouling resistance to the total resistance was much lower than the inherent resistance of the clean membranes. In addition, the internal fouling resistance was more important than the external, due to irreversible adsorption on the membrane. The rejection coefficients of the phenyl-urea herbicides were evaluated, being the sequence linuron>diuron>chlortoluron>isoproturon. The herbicide mass adsorbed onto the membranes was also determined, being adsorption onto the membrane surface/pores the main mechanism of herbicides retention.