Influence of colloidal iron oxide and natural organic matter fouling on nanofiltration membrane performance: role of feed composition and membrane properties

Abstract

Membrane fouling is an inherent technical challenge in nanofiltration (NF) processes for drinking water production. The effect of various organic and inorganic foulants on membrane performance has been studied extensively, but mechanistic understanding of combined organic-inorganic fouling, notably of iron oxide and natural organic matter (NOM), is lacking. This study investigates iron (Fe) oxide colloidal fouling in the presence of NOM and compares its effect on the water and solute transport properties of two commercial NF membranes (polypiperazine amide NF270 versus cellulose acetate SBNF). Fe oxide alone caused no significant change (<5%) in permeate flux and ion rejection due to the high electrostatic double-layer repulsion force (FEDL) between an initially deposited discontinuous foulant layer and colloids in the bulk feed solution. The presence of NOM caused surface charge reversal (from positive to negative) of the iron oxide colloids, leading to severe flux decline (30%) and salt rejection increase for the cellulose acetate membranes. A similar flux decline (30%) was observed in fouling experiments with NOM, suggesting that NOM is the determining factor in NOM-inorganic fouling. The polypiperazine amide membrane was less prone to fouling due to its higher hydrophilicity compared to cellulose acetate. However, reduced electrostatic double-layer repulsion force during combined fouling in the presence of Ca2+ led to significant flux loss (30%). This paper highlights the synergy between membrane surface properties and foulant chemistry in explaining NF fouling mechanisms.