A significant advantage of membrane capacitive deionization (MCDI) lies in its ability to achieve medium to high water recovery rates. A prototype of MCDI unit demonstrated a recovery around 68% while consistently achieving salt removal efficiencies of ≥ 90% from feed water with a total dissolved solids (TDS) concentration of 1490 mg/L. However, the presence of coagulant-derived multivalent ions, particularly Fe2+, Fe3+, and Al3+, poses a challenge to long-term salt rejection efficiency. When Fe3+ or Al3+ was present at concentrations near 10 mg/L in feed water with a TDS of ~400 mg/L, the residual iron or aluminum concentration in the treated water exceeded the permissible limits defined by drinking water standards. Despite high removal efficiencies (> 90%) for key cations including Na+, Ca2+, Mg2+, Al3+, Fe2+, and Fe3+, regeneration studies revealed a distinct desorption trend: Mg2+ > Na+ > Ca2+ > Al3+ > Fe2+ ≈ Fe3+. This trend indicates that Fe3+ and Fe2+ are the most strongly retained and thus the most scale-forming ion in MCDI systems, followed by Al3+. Salt adsorption capacity of NaCl is 0.66-4.14 mg/g and modeled using the modified Donnan model effectively described the nonlinear adsorption behavior and also for all other systems with and without coagulant ions. Due to the presence of divalent ions, Donnan potential decreased compared to NaCl system without coagulant ions. The presence of coagulant ions further decreased the Donnan potential. Energy consumed 68.2-78.6 kT/ion and mostly increased to 60.6-101.3 kT/ion during partially choked condition. Post-operational surface analyses using x-ray photoelectron spectroscopy (XPS) and energy-dispersive spectroscopy (EDS) confirmed the accumulation of these metal ions on the carbon electrode surfaces. The observed deposition of oxide and hydroxide of coagulant ions significantly impacts long-term MCDI performance, underscoring the need for pretreatment strategies and electrode material optimization to enhance the sustainability and effectiveness of MCDI in domestic water purification applications.
Mazumder et al. (Wed,) studied this question.