When fouling becomes beneficial: Organic foulants-assisted heavy metals removal enhancement in ultrafiltration

The strong affinity between organic foulants and heavy metals (HMs) critically influences the efficiency of membrane-based wastewater treatment, necessitating an investigation of their interaction mechanisms. Herein, alginate and copper/manganese ions were selected as representative organic foulants and HMs, respectively, to elucidate their interactions and subsequent membrane fouling behaviors. The findings indicated that alginate crosslinked with various hydrolysis species of copper and manganese ions to form heterogeneous transparent extracellular polymers (TEP), in which copper ions had higher bonding strength and Gibbs free energy with alginate. Notably, these TEPs were the actual participants during ultrafiltration and determined the characteristics of the fouling layer. The fouling layer was identified as a potential reservoir for HMs, exhibiting retention capacities of 0.3732 and 0.1149 mg/(g·m²) for copper and manganese ions respectively at an ion concentration of 0.1 mM. Surprisingly, ultrafiltration achieved 100 % removal of copper and manganese ions with the assistance of the fouling layer, particularly in the treatment of electroplating wastewater where it perfectly retained multiple HMs. On the other hand, the enrichment of HMs within the fouling layer also posed a risk of secondary pollution, necessitating careful consideration during membrane cleaning. Overall, this work highlighted the positive role of fouling layer in the removal of HMs.