Investigating the Impact of Iron and Aluminum Coagulants on the Performance of Capacitive Deionization Technology for Domestic Water Purification

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.