Salt concentration dependence of ionic conductivity in ion exchange membranes

A novel layer-by-layer heterogeneous cation exchange membrane for heavy metal ions removal from water

A novel layer-by-layer (LbL) cation exchange membrane was prepared for heavy metal ions removal from water via electrodialysis. LBL membranes fabricated by coating of [chitosan-co-activated carbon nanoparticles] layer on polyvinyl chloride-based heterogeneous cation exchange membrane. Betterment in adherence of layers was achieved through glutaraldehyde cross linking. FTIR, FESEM, 3D-surface images and BET analysis were used for LBL membrane characterization. Membrane surface hydrophilicity, flux, membrane potential, transport number, and their permselectivity were studied. FTIR spectra confirm LbL formation decisively. FESEM images and BET analysis demonstrated that coating of second layer on PVC membrane led to a compact structure. LbL membrane showed smoother and more hydrophilic surface compared to pristine membrane. The transport number and permselectivity increased by deposition of second layer whereas sodium flux showed up-down trend. ED experiment showed good ability in heavy metal ions removal for LBL membrane that follows (Cu²⁺> Ni²⁺> Pb²⁺) sequence. EDX analysis showed a competitive adsorption for heavy metal ions on LBL membrane as (Pb²⁺> Cu²⁺≥Ni²⁺). The effect of ultrasonic waves on regeneration of fouled membranes by heavy metals was investigated. The results showed improved performance for the regenerated membrane. Mechanical resistance also improved by utilizing of ACNs in chitosan layer.

Concentration Dependencies of Diffusion Permeability of Anion-Exchange Membranes in Sodium Hydrogen Carbonate, Monosodium Phosphate, and Potassium Hydrogen Tartrate Solutions

The concentration dependencies of diffusion permeability of homogeneous (AMX-Sb and AX) and heterogeneous (MA-41 and FTAM-EDI) anion-exchange membranes (AEMs) is obtained in solutions of ampholytes (sodium bicarbonate, NaHCO₃; monosodium phosphate, NaH₂PO₄; and potassium hydrogen tartrate, KHT) and a strong electrolyte (sodium chloride, NaCl). It is established that the diffusion permeability of AEMs increases with dilution of the ampholyte solutions, while it decreases in the case of the strong electrolyte solution. The factors causing the unusual form of concentration dependencies of AEMs in the ampholyte solutions are considered: (1) the enrichment of the internal AEM solution with multiply charged counterions and (2) the increase in the pore size of AEMs with dilution of the external solution. The enrichment of the internal solution of AEMs with multiply charged counterions is caused by the Donnan exclusion of protons, which are the products of protolysis reactions. The increase in the pore size is conditioned by the stretching of the elastic polymer matrix due to the penetration of strongly hydrated anions of carbonic, phosphoric, and tartaric acids into the AEMs.

Physico-chemical processes

The review scans research articles published in 2018 on physico-chemical processes for water and wastewater treatment. The paper includes eight sections, that is, membrane technology, granular filtration, flotation, adsorption, coagulation/flocculation, capacitive deionization, ion exchange, and oxidation. The membrane technology section further divides into six parts, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis/forward osmosis, and membrane distillation. PRACTITIONER POINTS: Totally 266 articles on water and wastewater treatment have been scanned; The review is sectioned into 8 major parts;  Membrane technology has drawn the widest attention from the research community.

Modelling of Ion Transport in Electromembrane Systems: Impacts of Membrane Bulk and Surface Heterogeneity

Artificial charged membranes, similar to the biological membranes, are self-assembled nanostructured materials constructed from macromolecules. The mutual interactions of parts of macromolecules leads to phase separation and appearance of microheterogeneities within the membrane bulk. On the other hand, these interactions also cause spontaneous microheterogeneity on the membrane surface, to which macroheterogeneous structures can be added at the stage of membrane fabrication. Membrane bulk and surface heterogeneity affect essentially the properties and membrane performance in the applications in the field of separation (water desalination, salt concentration, food processing and other), energy production (fuel cells, reverse electrodialysis), chlorine-alkaline electrolysis, medicine and other. We review the models describing ion transport in ion-exchange membranes and electromembrane systems with an emphasis on the role of micro- and macroheterogeneities in and on the membranes. Irreversible thermodynamics approach, “solution-diffusion” and “pore-flow” models, the multiphase models built within the effective-medium approach are examined as the tools for describing ion transport in the membranes. 2D and 3D models involving or not convective transport in electrodialysis cells are presented and analysed. Some examples are given when specially designed surface heterogeneity on the membrane surface results in enhancement of ion transport in intensive current electrodialysis.