Role of Ionic Charge Density in Donnan Exclusion of Monovalent Anions by Nanofiltration Razi Epsztein, † Evyatar Shaulsky, † Nadir Dizge, †,‡ David M. Warsinger, † and Menachem Elimelech* ,† † Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06520-8286, United States ‡ Department of Environmental Engineering, Mersin University, Mersin 33343, Turkey * S Supporting Information ABSTRACT: The main objective of this study is to examine how the charge densities of four monovalent anionsfluoride (F - ), chloride (Cl - ), bromide (Br - ), and nitrate (NO 3 - ) influence their Donnan (charge) exclusion by a charged nanofiltration (NF) membrane. We systematically studied the rejection behavior of ternary ion solutions containing sodium cation (Na + ) and two of the monovalent anions as a function of the pH with a polyamide NF membrane. In the solutions containing F - and Cl - or F - and Br - ,F - rejection was higher than Cl - or Br - rejection only when the solution pH was higher than 5.5, suggesting that F - (which has a higher charge density) was repelled more strongly by the negatively charged membrane. The order of change in the activation energy for the transport of the four anions through the polyamide membrane as a response to the increase of the membrane negative charge was the following: F - > Cl - > NO 3 - > Br - . This order corroborates our main hypothesis that an anion with a smaller ionic radius, and hence a higher charge density, is more affected by the Donnan (charge)-exclusion mechanism in NF. We conclude with a proposed mechanism for the role of ionic charge density in the rejection of monovalent anions in NF. ■ INTRODUCTION Compared to reverse osmosis (RO) membranes, nanofiltration (NF) membranes are unique in having varying selectivity toward different electrolytes. 1-5 Rejection mechanisms in NF include mainly steric (size) and Donnan (charge) exclusion. 6,7 Species with much larger hydrated size than the membrane pore size are sterically retained, 8 while transport within the pores of species with size similar to that of the membrane pores may be hindered. 9 A membrane with fixed charged groups repels ions with the same charge (co-ions) and attracts ions with the opposite charge (counterions). 10,11 These rejection mechanisms result in high selectivity of NF membranes for the passage of monovalent ions, 12,13 which is exploited in various applications for removing selectively multivalent ions and small organics. 1,3,7,14 Nanofiltration membranes are also capable of removing monovalent ions to different extents. 15-18 However, the difference in selectivity for the passage of different monovalent ions is usually small, and the mechanism for such a difference is relatively poorly understood. The net charge of different monovalent ions is the same (i.e., -1 or +1). Also, the minor difference in the hydrated radius of specific monovalent anions 19 may sometimes fail to explain differences in selectivity for different anions as in the commonly observed case of chloride (Cl - ) and nitrate (NO 3 - ). 20-22 Both anions have a net charge of -1 and similar hydrated radii, but Cl - is rejected more favorably than NO 3 - by NF membranes. An alternative explanation for the difference in selectivity for different ions with similar net charges and hydrated radii is based on the so-called dehydration phenomenon. 17,23-27 According to this theory, an ion that approaches the membrane pore can strip and rearrange temporarily the water shells surrounding it so it can fit more easily into the pore. The extent to which dehydration occurs depends specifically on the hydration energy of the ion: the higher the hydration energy, the more difficult for the ion to undergo dehydration. 17,24 For monatomic ions, the hydration energy depends on both the ionic size and charge. In general, higher ionic charge and smaller ionic size result in higher hydration energy. 25,28,29 The dehydration phenomenon, which is associated more with the size-exclusion mechanism, can partially explain some differences in the rejection of ions with the same charge and similar hydrated radii, including the case of Cl - and NO 3 - . 30 However, some observations cannot be adequately explained solely by the size-exclusion mechanism. For example, fluoride (F - ) has a smaller ionic radius, larger hydrated size, and higher hydration energy compared to Cl - , 19,29 suggesting higher rejection of F - than Cl - by NF membranes. However, reports on F - rejection by NF compared to Cl - are inconsistent and Received: December 12, 2017 Revised: February 25, 2018 Accepted: March 6, 2018 Published: March 6, 2018 Article pubs.acs.org/est Cite This: Environ. Sci. Technol. 2018, 52, 4108-4116 © 2018 American Chemical Society 4108 DOI: 10.1021/acs.est.7b06400 Environ. Sci. Technol. 2018, 52, 4108-4116 Downloaded via TECHNION-ISRAEL INST OF TECHNOLOGY on June 14, 2020 at 09:25:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.