Critical operation factors and proposed testing protocol of nanofiltration membranes for developing advanced membrane materials

Nanofiltration (NF) is an environmental-friendly and energetic-efficient technique for small molecule or ion separations compared to traditional energy-intensive separation processes. However, during the journey to discovering advanced NF membrane materials using a typical dead-end device, there is an obvious discrepancy on testing methodologies/protocols of NF membranes reported in contemporary literatures, which actually results in the significant data-reliability issues. This critical issue made the evaluation of various nanofiltration membranes so confusing and misleading because of the unfair comparison on NF performance. Therefore, it is urgent to guide the membrane society on the real factors affecting the data accuracy and standardize the protocol for nanofiltration test to develop advanced NF membrane materials. In this study, we have carried out a series of designed experiments to unify the standardized separation rate indicators of nanofiltration membranes by comparing flux, permeance, and permeability. The effects of external factors on separation efficiency (rejection) of NF membranes were investigated in detail, which is also analysed and discussed on the basic theory. The dead volume, rotor, and adsorption are proven to be the pivotal indicators for achieving accurate separation efficiency, which offers insight for reliable testing of nanofiltration membranes. Therefore, a protocol was proposed for evaluating accurate separation performance of nanofiltration membranes to obtain the reliable data, which benefits for fair performance comparison to advance NF membrane materials and makes the researchers to better understand the current confusing data reported in the literatures. Graphical abstract The accurate testing protocols of NF membranes were clarified, which is important for the development of advanced nanofiltration materials.