Enhanced electrosorption selectivity of phosphate using an anion-exchange resin-coated activated carbon electrode

Enhanced phosphate recovery in R-MCDI systems: Synergistic effects of modified electrodes and membrane-electrode-current collector assembly

Efficient phosphorus (P) recovery is critical for sustainable wastewater management and resource reuse. This study optimized a reservoir of membrane capacitive deionization (R-MCDI) system by integrating acid-modified activated carbon cloth (ACC) electrodes and a membrane-electrode-current collector assembly (MECA) configuration. Acid modification enhanced the electrode's specific surface area, microporosity, and carboxyl group content, while reducing charge transfer resistance, significantly improving P recovery and selectivity. The ACC-42 electrode achieved optimal performance, achieving a 52% P recovery efficiency and low energy consumption of 8.8 kWh/kg P. The MECA configuration further amplified P recovery by optimizing electric field distribution and maximizing electrode utilization, achieving a fourfold higher recovery rate (0.081 μmol·cm-2·min-1) while reducing energy consumption by 59% compared to alternative setups. Multi-cycle operations validated the system's robustness, with P concentrations reaching 397 mg/L in the electrode chamber and a nearly 15-fold increase in selectivity for P over sulfate. This study highlights the synergistic effects of electrode modification and assembly configuration in enhancing R-MCDI performance, providing a scalable and energy-efficient solution for nutrient recovery in wastewater treatment.

Nitrogen-doped activated carbon composite electrode for deionization of phosphate removal and DFT model adsorption of phosphates

Phosphate discharge in sewage can result in water eutrophication, posing a threat to aquatic ecosystems. Membrane capacitive deionization (MCDI) has demonstrated outstanding performance and significant potential for salt removal and nutrient recovery. In this study, a nitrogen-doped activated carbon electrode material (NAC) was synthesized through one-step pyrolysis to selectively remove phosphate from MCDI. At a voltage of 1.2V, a flow rate of 20 mL/min, and a pH of 6.51, the phosphate adsorption capacity of the NAC electrode was determined to be 1.60 mg/g. The study revealed that NAC pHpzc increased from 4.14 to 6.44, effectively broadening the pH range for phosphate removal. In the presence of competing ions (NO3-, Cl-, and SO42-) at a concentration of 0.5 M, the electroadsorption capacity of phosphate decreased to 1.21 mg/g, 1.14 mg/g, and 1.02 mg/g, respectively. The kinetic parameters of adsorption indicated that NAC electroadsorbed phosphate through physical adsorption, with the maximum adsorption capacity achieved at 303K. Data from the Freundlich isothermal model suggested that phosphate adsorption by the NAC electrode involves a multilayer adsorption process. A carbon structure model of density functional theory (DFT), incorporating doped nitrogen, was constructed based on XPS analysis. Following nitrogen doping, the electrostatic potential (ESP) of unsaturated carbon atoms became more positive, enhancing the ability of nitrogen-doped activated carbon to adsorb phosphate. This study provides compelling evidence that nitrogen doping facilitates the adsorption of phosphate by carbon materials.

Enhanced phosphorus electrosorption using Fe, N-co-doped porous electrode via capacitive deionization

Excessive phosphorus discharge causes water eutrophication and disturbs the homeostasis of aquatic ecosystems. Capacitive deionization (CDI) has been proven to be a more energy-efficient and environmentally friendly technology for removing phosphorus. Raw carbon (Raw C) electrodes are widely used in CDI. However, the phosphorus removal capacity of most unmodified Raw C still needs to be enhanced. Therefore, the Fe, N-co-doped carbon prepared in this study was expected to further improve the phosphorus removal performance. Herein, the optimal electrode with 5% Fe (FeNC) had an approximately 2.7 times higher adsorption capacity than Raw C. At a low concentration (5 mg P/L), FeNC exhibited a high maximum removal capacity of 4.28 mg P/g. Under reversed voltage, the phosphorus was easily desorbed by deionized water. Ion competition studies showed that coexisting ions adversely affected phosphorus adsorption onto FeNC in the order SO42- > NO3- > Cl-. Furthermore, the energy consumption of FeNC was calculated to be as low as 0.0069 kWh/g P and 0.023 kWh/m3 water under 1.2 V. More importantly, phosphorus removal by FeNC during CDI was demonstrated in simulated natural water from the Jinjiang River (Chengdu, China). This study indicated that FeNC is expected to be a potential electrode for CDI dephosphorization.

Sustained Phosphorus Removal and Enrichment through Off-Flow Desorption in a Reservoir of Membrane Capacitive Deionization

In this study, we used a membrane capacitive deionization device with a reservoir (R-MCDI) to enrich phosphorus (P) from synthetic wastewater. This R-MCDI had two small-volume electrode chambers, and most of the electrolyte was contained in the reservoir, which was circulated along the electrode chambers. Compared with conventional MCDI, R-MCDI exhibited a phosphate removal rate of 0.052 μmol/(cm2·min), approximately double that of MCDI. This was attributed to R-MCDI's utilization of OH- alternative adsorption to remove phosphate from the influent. Noticing that around 73.9% of the removed phosphate was stored in the electrolyte in R-MCDI, we proposed a novel off-flow desorption operation to enrich the removed phosphate in the reservoir. Exciting results from the multicycle experiment (∼8 h) of R-MCDI showed that the PO43--P concentration in the reservoir increased all the way from the initial 152 mg/L to the final 361 mg/L, with the increase in the P charge efficiency from 5.5 to 22.9% and the decrease in the energy consumption from 28.2 to 6.8 kW h/kg P. The P recovery performance of R-MCDI was evaluated by viewing other similar studies, which revealed that R-MCDI in this study achieved superior P enrichment with low energy consumption and that the off-flow desorption proposed here considerably simplified the operation and enabled continuous P enrichment.

Advanced capacitive deionization for ion selective separation: Insights into mechanism over a functional classification

Due to the diversity and variability of harmful ions in polluted water bodies, the selective removal and separation for specific ions is of great significance in water purification and resource processes. Capacitive deionization (CDI), an emerging desalination technology, shows great potential to selectively remove harmful ionic pollutants and further recover valuable ions because of the simple operation and low energy consumption. Researchers have done a lot of work to investigate ion selectivity utilizing CDI, including both theoretical and experimental studies. Nevertheless, in the investigation of selective mechanisms, phenomena where carbon materials exhibit entirely opposite selectivity require further analysis. Furthermore, there is a need to summarize the specific chemical reaction mechanisms, including the formation of hydrogen bonds, complexation reactions, and ligand exchanges, within selective electrodes, which have not been thoroughly examined in detail previously. In order to fill these gaps, in this review, we summarized the recent progress of CDI technologies for ion selective separation, and explored the selective separation mechanism of CDI from three aspects: selective physical adsorption, specific chemical reaction, and the utilization of selective barriers. Additionally, this review analyzes in detail the formation process of chemical bonds and ion conversion pathways when ions interact with electrode materials. Finally, some significant development prospects and challenges were offered for the future selective CDI systems. We believe the review will provide new insights for researchers in the field of ion selective separation.

Emerging electro-driven technologies for phosphorus enrichment and recovery from wastewater: A review

The recovery of phosphorus from wastewater is a critical step in addressing the scarcity of phosphorus resources. Electro-driven technologies for phosphorus enrichment have gathered significant attention due to their inherent advantages, such as mild operating conditions, absence of secondary pollution, and potential integration with other technologies. This study presents a comprehensive review of recent advancements in the field of phosphorus enrichment, with a specific focus on capacitive deionization and electrodialysis technologies. It highlights the underlying principles and effectiveness of electro-driven techniques for phosphorus enrichment while systematically comparing energy consumption, enrichment rate, and concentration factor among different technologies. Furthermore, the study provides a thorough analysis of the capacity of various technologies to selectively enrich phosphorus and proposes several methods and strategies to enhance selectivity. These insights offer valuable guidance for advancing the future development of electrochemical techniques with enhanced efficiency and effectiveness in phosphorus enrichment from wastewater.

Cation selectivity of activated carbon and nickel hexacyanoferrate electrode materials in capacitive deionization: A comparison study

In this study, the electrosorption selectivity of porous activated carbon (AC) and nickel hexacyanoferrate (NiHCF), which represent two working mechanisms of capacitive electrosorption and redox intercalation, was investigated to separate cations in capacitive deionization (CDI). The cyclic voltammetry diagrams of AC showed the rectangular shape of double-layer charging, while that of NiHCF showed separated peaks associated with redox reactions. The specific capacitance of NiHCF was 143.6 F/g in 1 M NaCl, which was almost two times higher than that of AC. Cation selectivity experiments were conducted in single-pass CDI for a multi-cation solution. The electrosorption preference of the AC cathode was determined by a counterbalance between the ionic charge and hydrated size, reflecting the selectivity coefficient of different cations over Na⁺ in the range of 0.86-2.63. For the NiHCF cathode, the cation selectivity was mainly dominated by the hydrated radius and redox activity. Notably, high selectivities of K⁺/Na⁺ ≈ 3.57, Na⁺/Ca²⁺ ≈ 9.97, and Na⁺/Mg²⁺ ≈ 18.92 were obtained. A significant improvement in the electrosorption capacity and monovalent ion selectivity can be achieved by utilizing the NiHCF electrode. The study demonstrates the fundamental aspects and promising opportunities of CDI in regard to ion selectivity.