Surface functionalization of nanofiltration membrane by catechol-amine codeposition for enhancing antifouling performance

Nanofiltration (NF) technology is promising for treatment of wastewater containing micropollutants (MPs) because it can retain both MPs and small organics while permeate inorganic salts. However, both MPs adsorption and organic fouling formation deteriorate NF membrane performance. In this work, a novel surface functionalization strategy is proposed to tackle both two issues. Based on catechol-amine chemistry, a commercially available NF membrane is modified by tannic acid-3-aminopropyltriethoxysilane (TA-APTES) coating and subsequent horseradish peroxidase (HRP) immobilization. Taking aflatoxin B1 (AFB1) as an example of MPs, its adsorption on the modified NF membrane is greatly inhibited by coating induced pore narrowing (enhancing size exclusion to MPs), surface barrier effect (alleviating the interaction between MPs and membrane) and enzymatic self-cleaning ability (degrading MPs adsorbed on the membrane), resulting in higher and more stable AFB1 rejection (similar to 90%). The flux decline and irreversible fouling are also reduced by the amphiphilic property of the immobilized HRP on the membrane surface when treating corn soaking solution. Although the on-line self-cleaning operation in the presence of hydrogen peroxide can alleviate concentration polarization by interfacial catalytic reaction, the flux decline and irreversible fouling become severer possibly due to the pore blocking by the degraded products. The off-line self-cleaning after the filtration enables to remove loose fouling layer, but the dense fouling layer impedes the accessibility of hydrogen peroxide to the immobilized HRP, limiting its self-cleaning ability. Therefore, catalytic intensity, enzymatic products and substrate accessibility to enzyme should be considered when constructing a self-cleaning NF membrane to deal with combined fouling.