Preventing membrane fouling for water and wastewater treatment has drawn considerable attention in recent years. Natural organic matter (NOM), including humic acid (HA), is a well-recognized foulant for membrane fouling. The implementation of novel photocatalytic membrane system holds promise for fouling mitigation by producing a series of reactive oxygen species (ROS) to oxidize NOM, meanwhile the system can also potentially utilize renewable solar energy to reduce the energy and chemical footprint as well as maintenance cost. However, molecular-scale understanding of photocatalysis for fouling mitigation is largely underexplored. We prepared partially oxidized purified humic acid (OHA) by photocatalysis, and investigate its physical and chemical properties and fouling behavior on polyvinylidene fluoride (PVDF) ultrafiltration membranes, in comparison with purified humic acid (PHA). Flux tests demonstrated that the OHA fouled the membranes less than PHA, though at the same concentration (represented by chemical oxygen demand, COD). Atomic force microscopy (AFM) indicated that OHA formed smaller aggregates on the membranes than PHA. Contact resonance force microscopy provided mechanical characterization for the foulants: OHA is softer (i.e., smaller Young’s modulus) and more viscous than PHA. These results supported the argument that photocatalytic oxidation of humic acid mitigated membrane fouling from both topographic and mechanical point of view. Contact angle analysis for fouled membranes suggested that OHA is slightly more hydrophilic than PHA (46.1° ± 0.9° vs. 51.0° ± 1.2°), and X-ray photoelectron spectroscopy (XPS) showed that OHA had a slightly higher oxygen content than PHA (O/C atomic ratio of 0.45 vs. 0.35). Though PHA was extensively oxidized in photocatalysis (COD degraded by 50%) for preparing OHA, a small amount of O was introduced to the backbone of the humic acid. These results may indicate that depolymerization of the humic acid into small fragments, rather than the introduction of O-containing functional groups to the humic acid, is the mechanism for the photocatalysis of the humic acid. Future molecular size analysis by size-exclusion chromatography will provide more insights into this aspect. Our study will shed light on the development of a photocatalytic membrane system for mitigating NOM fouling, and the knowledge can also be translated into understanding how oxidation can reduce NOM and biological fouling for the membranes.

Mitigation of humic acid fouling on ultrafiltration membranes in a photocatalytic system

255th National Meeting (2018)

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