Characterising humic acid fouling of nanofiltration membranes using bisphenol A as a molecular indicator
Fouling of nanofiltration (NF) membranes by humic acids was investigated using bisphenolA (BPA) as an indicator chemical to differentiate between various mechanisms that maylead to a change in solute rejection. Three commercially available NF membranes wereinvestigated and an accelerated fouling condition was achieved with a foulant mixturecontaining humic acids in an electrolyte matrix. The effects of membrane fouling on therejection of BPA were interpreted with respect to the membrane pore sizes and the foulingcharacteristics. Results reported here indicate that calcium concentration in the feed solutioncould be a major factor governing the humic acid fouling process. Moreover, a criticalconcentration of calcium in the feed solution was observed, at which membrane foulingwas most severe. Membrane fouling characteristics were observed by their influence onBPA rejection. Such influence could result in either an increase or decrease in rejectionof BPA by the three different membranes depending on the rejection mechanisms involved.It is hypothesized that these mechanisms could occur simultaneously and that the effectsof each might not be easily distinguished. However, it was observed that their relative contributionwas largely dependent upon membrane pore size. Pore blocking, which resultedin a considerable improvement in rejection, was prominent for the more open pore sizeTFC-SR2 membrane. In contrast, the cake-enhanced concentration polarisation effectwas more severe for the tighter NF-270 and NF-90 membranes. For hydrophobic solutessuch as BPA, the formation of the fouling layer could also interfere with the solute membrane interaction, and therefore, exert considerable influence on the separation process.