Effects of pH and salt on nanofiltration—a critical review

Nanofiltration (NF), as a powerful tool for separation of small molecules and salts, has attracted growing attention in many applications such as wastewater treatment, water purification, desalination, food processing and bio-separation. It is well known that pH and salt conditions have significant influence on NF performance. In order to manipulate NF process and optimize its efficiency, it is very important to get insights into the effects of pH and salt on nanofiltration. This paper aims at reviewing the reports on NF performance at different pH and salt conditions, focusing on the mechanisms behind various phenomena induced by pH and salt. The effects of pH and salt on NF are mainly reflected in the variations of membrane flux/permeability, solute rejection and fouling behavior, which also depend on both solute type and solution composition. In order to explain these effects, the changes of membrane properties are evaluated by physical, chemical and mathematical characterization methods. Eight mechanisms for pH and salt effects are summarized and several practical advices for NF operation are provided. Besides, some interesting opinions such as dominant-ions, co-ions competition, salting-out induced pore swelling, charge-induced concentration polarization (CP) are reviewed to elucidate some confusing phenomena reported previously. This review intends to not only offer a clear illustration on this special issue but also to provide a guide to optimize NF separation and maintenance.