Is conductivity measurement or inductively coupled plasma-atomic emission spectrometry reliable to define rejection of different ions?

Rejection of single salts or ions is a basic and crucial characteristic of nanofiltration (NF) membranes. The simple and most pursued method to quantify the salt concentration has been via conductivity measurement. Pitfalls exist when ions hydrolysis or feed water contains monovalent ions. This could be explained in two possible scenarios: (1) easily hydrolyzed single salts form low charged ions and reduce feed pH, resulting in increased permeate conductivity and low nominal rejection; (2) for membranes with high multivalent ion rejections (>99%) or the concentration of target ions in feed is low, conductivity measurement results in low rejection due to the passage of monovalent ions if deionized water is used for the feed solution. A correction formula by subtracting the concentration of monovalent ions in water to obtain an accurate rejection value is proposed. This work provides an accurate, simple and robust evaluation of rejection for NF membranes, which promotes fair comparison of performance in literature, reliable analysis of separation mechanisms as well as a precise determination of product purity. • Cations hydrolysis form low-charged ions and reduce feed pH, resulting in low rejection tested by conductivity measurement. • The passage of monovalent ions results in low rejection for membranes with high multivalent ion rejections. • A correction formula was proposed by subtracting the concentration of monovalent ions for accurate rejection. [ABSTRACT FROM AUTHOR]