Interactions controlling biopolymer fouling of reverse osmosis membranes

Laboratory experiments and model calculations were performed to elucidate the fundamental interactions that control organic fouling in reverse osmosis (RO) processes. Bovine serum albumin and alginic acid were selected as model aquatic organic macromolecules (organic foulants). An extended Derjaguin-Landau-Verwey-Overbeek (DLVO) characterization analysis was used to elucidate mechanisms of organic matter fouling on a commercial, polyamide composite RO membrane. Surface tension parameters derived from contact angle analyses are used to demonstrate that the apparent thermodynamic stability of macromolecules determines and adhesive free energy between membranes and macromolecules explained the observed differences in flux decline. Further, foulant– membrane and foulant–foulant interfacial forces helped explain why hydrophilic macromolecules formed polarization layers causing minimal flux decline, while hydrophobic macromolecules formed gel (or cake) layers that led to severe flux decline.

Valuable insights into the mechanisms of organic matter fouling of RO membranes were gained by treating the organic macromolecules like colloidal particles. Interfacial forces between foulants and the membrane surface or between foulants and the fouled (modified) membrane surface offer reasonable explanation for the difference in flux decline behavior observed for two model organic foulants — BSA and AA from brown algae (AA). Monodisperse suspensions of thermodynamically stable macromolecules, like BSA, may result in a rapid initial flux loss followed by a constant limiting flux, whereas monodisperse suspensions of thermodynamically unstable macromolecules, like alginate, may result in rapid initial flux loss followed by a sustained flux decline. Therefore, long-term flux decline behavior is attributed to the relative repulsion or attraction of the macromolecule for itself. In a companion paper published in this same issue, we focus on natural organic matter (NOM) fouling of ultrafiltration membranes where the organic macromolecules tested are classically considered dissolved in nature [see Lee et al., “Natural Organic Matter (NOM) Fouling due to Foulant–Membrane Physicochemical Interactions”). The NOM used in this study has molecular weights ranging from 60 to 100 kDa and exhibits a hydrodynamic diameters of ~5–7 nm; hence, it is classically considered “colloidal,” not “dissolved.” Regardless, it appears that fouling by both dissolved and colloidal organic matter reasonably well described by the classical colloidal deposition approach. The experimental and theoretical results presented here provide new, fundamental insight into mechanisms of organic fouling. In future studies, fouling of RO and NF membranes will be studied using more complex suspensions comprising bacteria, biopolymers, geopolymers, and inorganic colloids.