Understanding Selectivity in Solute–Solute Separation: Definitions, Measurements, and Comparability

Abstract

The development of membranes capable of precise solute−solute separation is still in its burgeoning stage without a standardized protocol for evaluating selectivity. Three types of membrane processes with different driving forces, including pressure-driven filtration, concentration difference-driven diffusion, and electric field-driven ion migration, have been applied in this study to characterize solute−solute selectivity of a commercial nanofiltration membrane. Our results demonstrated that selectivity values measured using different methods, or even different conditions with the same method, are generally not comparable. The cross-method incomparability is true for both apparent selectivity, defined as the ratio between concentration-normalized fluxes, and the more intrinsic selectivity, defined as the ratio between the permeabilities of solutes through the active separation layer. The difference in selectivity measured using different methods possibly stems from the fundamental differences in the driving force of ion transport, the effect of water transport, and the interaction between cations and anions. We further demonstrated the difference in selectivity measured using feed solutions containing singlesalt species and that containing mixed salts. A consistent protocol with standardized testing conditions to facilitate fair performance comparison between studies is proposed.

Nanopores: synergy from DNA sequencing to industrial filtration – small holes with big impact

“…35,86,87 Establishment of standardized protocols for measuring selectivity, including flow rate, pressure and voltage range as well as ionic concentrations, could also link data collected for single nanopores and membranes. 13 Finally, new experimental approaches may enable performing the same selectivity measurements with both single nanopores and membranes. For example, modified mass-spectrometry measurements, where ions are delivered from a nanopore directly into high vacuum and focused, could potentially detect ions in the permeate of individual nanopores.…”

“…This approach is not applicable to single nanopores due to the miniscule flow through an individual pore which is in the range of femto to attoliters per second. As pointed out recently, 13 extrapolation of transport properties of individual nanopores to membranes containing many nanopores may not be straightforward. The same membrane can exhibit very different selectivities when probed using the two different measurement approaches.…”

“…(ii) New modeling methods are necessary to link existing and new data on transport and selectivity in both single nanopores and multipore membranes. 13 (iii) Single nanopore studies have to learn from multipore membranes on using non-aqueous and complex media in order to expand the sensing range. In the following, we first highlight the main…”

Applying Transition-State Theory to Explore Transport and Selectivity in Salt-Rejecting Membranes: A Critical Review

“…Reverse osmosis (RO) and nanofiltration (NF) have been the dominant processes for water desalination and advanced purification processes for many years. , The key component in these processes is a dense polymeric (commonly polyamide) membrane, possessing free-volume elements (or voids) in the size range of 0.1–2 nm that facilitate the separation of different ionic and molecular species . Because of their smaller size and neutral charge, water molecules permeate through the membrane faster than ions, allowing for excellent water-salt selectivity. , Size and charge effects also play a role in ion–ion selectivity, which is mostly exploited for water softening and sulfate removal due to the prominent differences between monovalent and divalent ions. − Other, more complex transport mechanisms can promote selectivity between akin species such as two monovalent anions, which can be beneficial for some applications (e.g., removing the excess of nitrate or fluoride from drinking water while keeping the unharmful chloride in the water). − Elucidating such molecular-level mechanisms is the focus of current research efforts, which are a key step toward the design of single-species selective membranes that can enhance the sustainability of current processes and expand the use of NF and RO membranes to applications beyond water treatment (e.g., resource recovery, energy devices, and sensing). ,− …”

Humic Acid Modified Selective Nanofiltration Membrane for Efficient Separation of PFASs and Mineral Salts

“…Nanofiltration (NF) membrane with controllable selectivity recently has shown great prospect in precise solute–solute separation at the subnanometer scale. , The sieving pore diameter of NF membranes is around several angstroms to less than two nanometers, which has the potential for separating EMPs from mineral salts . However, current NF membranes are still not perfectly optimized for the precise separation of EMPs and mineral salts, though some NF membranes have preliminarily separated mineral salts from EMPs. − For example, Boo et al fabricated a selective nanofiltration via interfacial polymerization of trimesoyl chloride and a mixture of piperazine and bipiperidine to separate PFOA and calcium .…”