Fouling mechanism of hydrophobic polytetrafluoroethylene (PTFE) membrane by differently charged organics during direct contact membrane distillation (DCMD) process: An especial interest in the feed properties

The growing attention to membrane distillation (MD) processes from various disciplines raises the demand for systematic research on MD membrane fouling. This study investigates the role of feed properties, such as feed compositions, charge properties, particle size, hydrophobicity/hydrophilcity, thermal stability and intermolecular interactions, in organic fouling of direct contact membrane distillation (DCMD). The differently charged lysozyme (LYS), sodium alginate (SA), and bovine serum albumin (BSA) were chosen as model organic foulants. The fouling data showed that the feed containing LYS and SA displayed a more severe flux decline compared to the BSA/SA solution, which was highly associated with the charge interaction. Additionally, the flux performance for the mixed foulants was partly affected by the proportion of protein (BSA or LYS) to polysaccharide (SA). It was found that the mixed feed containing less BSA caused a faster flux decline, whereas the feed containing higher amounts of LYS exhibited a more severe membrane fouling. Based on the membrane characterization analysis using FTIR, the BSA and SA showed significant conformational changes during the feed heating process, which largely aggravated the membrane fouling rate. Furthermore, the membrane fouling degree was proven to be distinctly affected by the sites where fouling occurred according to the SEM-EDX analysis. The results exhibited that SA was primarily deposited on the membrane surface, while the LYS largely penetrated into the pores, causing a more severe membrane fouling. Finally, two different fouling patterns including “egg-box model” and “multi-layer model” were suggested for the BSA/SA and LYS/SA feeds during the DCMD operation process, respectively.