Revelation of the interaction between aluminum coagulants and natural organic matters in the dual-system during nanofiltration membrane scaling

Aluminum (Al) coagulants are widely and frequently applied to coagulation pretreatment before nanofiltration. However, the excess coagulants and residual natural organic matters (NOMs) might affect the gypsum scaling during nanofiltration process. This study investigated the nanofiltration membrane fouling process under Al coagulants and specific NOMs, including humic acids (HA), bovine serum protein (BSA), and sodium alginate (SA) in a dual-system. The flux and the particle size decreased significantly when both above were present. The gypsum crystallization on the membrane surface was enhanced, while the bulk crystallization was weakened. Zeta potential and the NICA-Donnan model showed that Al coagulants competed with Ca2+ to bind HA weakening the bulk crystallization of gypsum. Meanwhile, Al coagulants decreased the salt rejection rate. Interestingly, the BSA and SA alleviated the flux decrease. Furthermore, the characterization of the fouled membrane showed that the concentration of Ca2+ and SO42− on the membrane surface increased further in the presence of Al coagulants, the grain size reached 582.0 nm when both HA and Al were present, larger than the control group and when they were present in the feed solution alone. Finally, the modified Hermia model was introduced to analyze the mechanism of the interaction between Al coagulants and NOMs. It is suggested that the control of Al coagulants and HA is necessary for the coagulation pretreatment before nanofiltration.