Nanofiltration scaling influenced by coexisting pollutants considering the interaction between ferric coagulant and natural organic macromolecules

Abstract The aim of present study was to evaluate the effects of the interaction between ferric coagulant and natural organic macromolecules (NOMs) on gypsum scaling during nanofiltration. Both crystallization process and membrane flux behavior were taken into consideration. Although bulk crystallization could be enhanced by ferric coagulant and humic acid (HA), it was hindered by the mixture of these coexisting pollutants due to their interaction in aqueous solution. According to the variation in zeta potential and the prediction with the NICA-Donnan model, Fe3+ could compete with Ca2+ for binding to HA, while the interaction between coagulant and HA reduced the concentration of active ingredients in the coagulant which could enhance scaling development. FTIR-ATR spectra showed that ferric coagulant and NOMs could promote the attachment of each other on membranes. Unlike bulk crystallization, surface crystallization could be enhanced by the combination of HA and ferric coagulant on nanofiltration membranes. Therefore, both ferric coagulant and HA could increase scaling layer resistance, and there were synergistic effects on aggravating the flux decline between them. Moreover, the effects of salt saturation and system pressure on coagulant-influenced gypsum scaling in nanofiltration were also investigated. Modified Hermia’s models were applied to theoretically verify the membrane fouling mechanisms. In addition, ferric coagulant was found to decrease the retention efficiency of ions and atrazine by nanofiltration membranes by weakening the steric hindrance.