PhD Final Defense – Yurui Li

Mechanisms and Applications of Faradaic-Material-Incorporated Capacitive Deionization for Selective Anion Separation

Advisor: Professor Roland Cusick

Abstract

Selective separation and recovery of dilute oxyanions from complex aqueous streams is critical for both environmental protection and resource sustainability. Phosphate and perrhenate represent two important oxyanions associated with water quality management and critical mineral recovery, but both are difficult to recover selectively because they are often present at low concentrations in the presence of more abundant competing ions. These two oxyanions were selected in this dissertation because they represent contrasting electrochemical behaviors: perrhenate is a redox-active oxyanion, whereas phosphate does not undergo a change in oxidation state within the water-stability potential window. Capacitive deionization (CDI) and membrane capacitive deionization (MCDI) provide promising electrochemical platforms for low-concentration ion separation; however, conventional carbon-based electrodes rely primarily on non-specific electrical double-layer adsorption and therefore often lack sufficient target-ion selectivity. This dissertation investigates poly(vinyl ferrocene) (PVF)-incorporated CDI/MCDI systems as Faradaic-material-based platforms for selective anion separation, with emphasis on connecting electrode design, electrochemical mechanism, reactor modeling, plant-wide integration, and techno-economic feasibility.

At the material and electrode scale, PVF-coated electrodes were developed for selective perrhenate recovery. Electrodeposited PVF films achieved higher rhenium uptake and stronger long-term retention than dip-coated PVF/CNT electrodes, retaining 69.1 ± 11.3% of initial rhenium uptake after 15,000 cycles. Surface characterization showed that perrhenate adsorption was promoted by ferrocene oxidation, while efficient regeneration required sufficiently negative desorption potential to reduce ferrocene- and rhenium-associated species. These results demonstrate that electrodeposition improves PVF attachment to carbon substrates and provides a promising strategy for increasing both selectivity and longevity in Faradaic CDI electrodes.

At the electrochemical mechanism scale, PVF–activated carbon composite electrodes were evaluated to understand how electrode composition controls charge storage and cycling stability. An equivalent-circuit model was developed to distinguish electrical double-layer and PVF-associated Faradaic contributions during galvanostatic charge–discharge operation. The model reproduced experimental profiles with R² > 0.99 and showed that activated carbon primarily increased double-layer capacity, while PVF provided redox-based charge storage associated with

selective anion interaction. Long-term cycling revealed that capacity decay was mainly caused by the loss of electrochemically active PVF, whereas chitosan treatment improved electrode stability by reinforcing the composite structure and preserving Faradaic activity.

At the reactor and plant-wide scale, PVF-incorporated MCDI was evaluated as a tertiary treatment process for phosphate removal and recovery from wastewater treatment plant effluent. Flow-cell experiments showed that PVF incorporation improved phosphate/chloride selectivity compared with conventional carbon-based MCDI, with the 1:2 PVF:AC electrode exhibiting the strongest phosphate preference. Batch reactor modeling showed that treatment performance was governed by the combined effects of adsorption capacity, selectivity, and charging time, rather than capacity alone. Plant-wide modeling further demonstrated that coupling selective MCDI with struvite precipitation can redirect phosphate from secondary effluent into a recovery stream, increase phosphorus recovery, and reduce final effluent phosphorus concentration. However, the model also showed that insufficient phosphate selectivity can cause chloride accumulation through brine recycling, increasing reactor area requirements and creating potential operational concerns. Techno-economic analysis indicated that treatment cost was dominated by capital-related material costs, and that replacing high-cost substrates, reducing membrane-related costs, and extending electrode lifetime are critical for improving economic feasibility.

Together, this dissertation demonstrates that Faradaic-material incorporation can improve CDI/MCDI performance for selective oxyanion recovery by enhancing target-ion selectivity, adding redox-based charge storage, and improving effective electrode area utilization. The work also shows that practical implementation requires coordinated optimization of material chemistry, electrode stability, operating conditions, reactor design, and downstream recovery processes. By linking electrode-scale mechanisms with plant-wide performance and cost, this dissertation provides a framework for advancing PVF-based electrochemical separation systems for phosphate, perrhenate, and other critical oxyanions in dilute aqueous streams.