Behavioral analysis of membrane fouling caused by coexistence of proteins and cations in membrane distillation

Membrane fouling is a key issue limiting membrane distillation (MD) technology, especially when organic and inorganic fouling coexist and produce complex interactions. In this study, Bovine Serum Albumin (BSA) was employed as a model organic foulant to represent proteinaceous contaminants. The impact of a protein-inorganic salt system on MD performance was investigated by adding inorganic salts with different valencies, namely calcium chloride (CaCl 2 ) and sodium chloride (NaCl). The results indicate that the addition of cations significantly exacerbates membrane fouling and increases the difficulty of cleaning. Na + reduces the interaction energy barrier between the membrane and BSA to the lowest level, while facilitating the adsorption of substantial quantities of hydrophobic compounds on the membrane surface, resulting in a thicker fouling layer. In contrast, the presence of Ca 2+ promotes the adsorption of small molecules onto the membrane surface, forming a more loosely structured and hydrophilic fouling layer. Compared with Na + -induced fouling, Ca 2+ -induced fouling exhibits lower overall severity but shows greater resistance to cleaning. This increased resistance is attributable to BSA Ca 2+ bridging complexes, which are more difficult to dislodge during cleaning. In the mixed inorganic salt ion system, the interaction between the two inorganic salt ions produces an intermediate level of contamination compared to the single cation system. • Coexisting cations aggravate BSA fouling; Na + causes stronger fouling than Ca 2+ or Na + –Ca 2+ mixtures. • The degree of membrane fouling followed the order of BSA < BSA-Ca 2+ < BSA-Na + -Ca 2+ < BSA-Na + . • The addition of cations significantly reduces the energy barrier between BSA and the membrane. • The complexes formed between BSA and Ca 2+ are more difficult to remove with NaOH.