Hybrid capacitive deionization of NaCl and toxic heavy metal ions using faradic electrodes of silver nanospheres decorated pomegranate peel-derived activated carbon

Na2MnSiO4/C as hybrid capacitive deionization electrode material to enhance desalination performance

The utilization of Na2MnSiO4 as a Faraday electrode in hybrid capacitive deionization (HCDI) is investigated to achieve efficient desalination. The Na2MnSiO4/C (NMSO) materials were fabricated via a simple sol-gel method, in which the synthesis process was modulated by adjusting the volume ratio of ethanol to water. When maintaining the volume ratio of water to ethanol at 3:1, the resultant NMSO-3/1 exhibited expected salt adsorption capacity of 31.06 mg g-1 and salt adsorption rate of 20.43 mg g-1 min-1. This distinguished desalination performance was mainly attributed to its inherent multiple redox pairs, as well as the integration of ethanol, which enhanced both specific capacitance and hydrophilicity of the material. This study opens a new perspective for the development of highly efficient materials in HCDI.

High-performance electrosorption of lanthanum ion by Mn3O4-loaded phosphorus-doped porous carbon electrodes via capacitive deionization

Transition-metal-oxide@heteroatom doped porous carbon composites have attracted considerable research interest because of their large theoretical adsorption capacity, excellent electrical conductivity and well-developed pore structure. Herein, Mn3O4-loaded phosphorus-doped porous carbon composites (Mn3O4@PC-900) were designed and fabricated for the electrosorption of La3+ in aqueous solutions. Due to the synergistic effect between Mn3O4 and PC-900, and the active sites provided by Mn-O-Mn, C/PO, C-P-O and Mn-OH, Mn3O4@PC-900 exhibits high electrosorption performance. The electrosorption value of Mn3O4@PC-900 was 45.34% higher than that of PC-900, reaching 93.02 mg g-1. Moreover, the adsorption selectivity reached 87.93% and 89.27% in La3+/Ca2+ and La3+/Na+ coexistence system, respectively. After 15 adsorption-desorption cycles, its adsorption capacity and retention rate were 50.34 mg g-1 and 54.12%, respectively. The electrosorption process is that La3+ first accesses the pores of Mn3O4@PC-900 to generate an electric double layer (EDL), and then undergoes further Faradaic reaction with Mn3O4 and phosphorus-containing functional groups through intercalation, surface adsorption and complexation. This work is hoped to offer a new idea for exploring transition-metal-oxide @ heteroatom doped porous carbon composites for separation and recovery of rare earth elements (REEs) by capacitive deionization.

Copper hexacyanoferrate/carbon sheet combination with high selectivity and capacity for copper removal by pseudocapacitance

The efficient capture of copper ions (Cu2+) in wastewater has dual significance in pollution control and resource recovery. Prussian blue analog (PBA)-based pseudocapacitive materials with open frameworks and abundant metal sites have attracted considerable attention as capacitive deionization (CDI) electrodes for copper removal. In this study, the efficiency of copper hexacyanoferrate (CuHCF) as CDI electrode for Cu2+ treating was evaluated for the first time upon the successful synthesis of copper hexacyanoferrate/carbon sheet combination (CuHCF/C) by introducing carbon sheet as conductive substrate. CuHCF/C exhibited significant pseudocapacitance and high specific capacitance (52.92 F g-1) through the intercalation, deintercalation, and coupling of Cu+/Cu2+ and Fe2+/Fe3+ redox pairs. At 0.8 an applied voltage and CuSO4 feed liquid concentration of 100 mg L-1, the salt adsorption capacity was 134.47 mg g-1 higher than those of most reported electrodes. Moreover, CuHCF/C demonstrated excellent Cu2+ selectivity in multi-ion coexisting solutions and in actual wastewater experiments. Density functional theory (DFT) calculations were employed to elucidate the mechanism. This study not only reveals the essence of Cu2+ deionization by PBAs pseudocapacitance with promising potential applications but also provides a new strategy for selecting efficient CDI electrodes for Cu2+ removal.

Electro-intensified simultaneous decontamination of coexisting pollutants in wastewater

The treatment of wastewater has become increasingly challenging as a result of its growing complexity. To achieve synergistic removal of coexisting pollutants in wastewater, one promising approach involves the integration of electric fields. We conducted a comprehensive literature review to explore the potential of integrating electric fields and developing efficient electro-intensified simultaneous decontamination systems for wastewater containing coexisting pollutants. The review focused on comprehending the applications and mechanisms of these systems, with a particular emphasis on the deliberate utilization of positive and negative charges. After analyzing the advantages, disadvantages, and application efficacy of these systems, we observed electro-intensified systems exhibit flexible potential through their rational combination, allowing for an expanded range of applications in addressing simultaneous decontamination challenges. Unlike the reviews focusing on single elimination, this work aims to provide guidance in addressing the environmental problems resulting from the coexistence of hazardous contaminants.

Layered hydrated-titanium-oxide-laden reduced graphene oxide composite as a high-performance negative electrode for selective extraction of Li via membrane capacitive deionization

In this work, we initially prepared layered lithium titanate (Li2TiO3) using a solid-state reaction. Then Li+ of Li2TiO3 were acid-eluded with Hydrochloric acid to obtain hydrated titanium oxide (H2TiO3). Different weight percentages (50%, 60%, 70%, 80%, and 90%) of the as-prepared H2TiO3 were deposited on a conductive reduced graphene oxide (rGO) matrix to obtain a series of rGO/ H2TiO3 composites. Of the prepared composites, rGO/H2TiO3-60% showed excellent current density, high specific capacitance, and rapid ion diffusion. An asymmetric MCDI (membrane capacitive deionization) cell fabricated with activated carbon as the anode and rGO/H2TiO3-60% as the cathode displayed outstanding Li+ electrosorption capacity (13.67 mg g-1) with a mean removal rate of 0.40 mg g-1 min-1 in a 10 mM LiCl aqueous solution at 1.8 V. More importantly, the rGO/H2TiO3-60% composite electrode exhibited exceptional Li+ selectivity, superior cyclic stability up to 100,000 s, and a Li+ sorption capacity retention of 96.32% after 50 adsorption/desorption cycles. The excellent Li+ extraction obtained by MCDI using the rGO/H2TiO3-60% negative electrode was putatively attributed to: (i) ion exchange between Li+ and H+ of H2TiO3; (ii) the presence of narrow lattice spaces in H2TiO3 suitable for selective Li+ capture; (iii) capture of Li+ by isolated and hydrogen-bonded hydroxyl groups of H2TiO3; and (iv) enhanced interfacial contact and transfer of large numbers of Li+ ions from the electrolyte to H2TiO3 achieved by compositing H2TiO3 with a highly conductive rGO matrix.

Preparation of high performance porous carbon by microwave synergistic nitrogen/phosphorus doping for efficient removal of Cu2+ via capacitive deionization

Sargassum biochar has potential advantages as an electrode material due to its natural microscopic pore channels. However, conventional pyrolysis method is prone to thermal damage to the biochar, and incapable to form a complete pore structure resulting in poor biochar electrode performance. In this study, a strategy of microwave pyrolysis coupled with KOH activation was used to prepare nitrogen/phosphorus double-doped graded porous biochar (STC) using ammonium dihydrogen phosphate as dopant. The carbon material STC-1.24-800 prepared by the optimal parameters had a high specific surface area (SSA) of 1367.6 m² g ^-1 and a total pore volume of 1.499 cm³ g^-1. The precise inside-out heating characteristics of microwave facilitated the generation of suitable meso-micropore distribution ratios in carbon, and the graded porous structure provided abundant active sites for charge accumulation and ion diffusion. The doped nitrogen/phosphorus atoms responding to the microwave field, generated spin to promote microwave absorption, introducing surface structural defects to produce electron density differences. The change in the nature of the electron donor and its electron density enhanced the electrical conductivity and chemical stability of STC. Nitrogen/phosphorus polar surface functional groups improved hydrophilicity and wettability. STC-1.24-800 had a higher specific capacitance of 531 F g^-1 and exhibits great cycle performance in capacitive deionization (CDI) applications (1.0 V, 50 mg L^-1 Cu²⁺) as well as adsorption performance (56.16 mg g ^-1). The present work can provide a novel feasible idea for preparing diatomically doped graded porous biochar for CDI electrode application by microwave irradiation.

Electro-oxidation of heavy metals contaminated water using banana waste-derived activated carbon and Fe3O4 nanocomposites

The main objective of this study was to banana waste-derived activated carbon (BWAC) make a high pore surface area was prepared and composited with Fe₃O₄ via a facile hydrothermal method. Various physiochemical characteristics of the prepared samples were evaluated using XRD, FTIR, FESEM, Raman Spectroscopy and XPS analysis. In addition, cyclic voltammetry and electrochemical impedance spectroscopy analyses were performed to determine the electrochemical properties of the prepared samples. The Fe₃O₄/BWAC sample showed a higher capacitance (285 F g^-1) than BWAC at the same scan rate of 10 mV s^-1. The capacitive deionization (CDI) cell configuration was varied, and its electro-sorption and defluoridization efficiencies were analyzed during the lead (Pb²⁺) removal 90%. An asymmetric combination of electrodes in the CDI cell exhibited better heavy metal removal performance, possibly due to the synergistic effect of the high surface area and the balance between the active adsorption site and the overlapping effect of the EDL. As a result, Fe₃O₄/BWAC could be a potential resource for supercapacitors and CDI electrodes, and the novel Fe₃O₄/BWAC nanocomposites outstanding performance suggests that they could be helpful for future energy storage and environmental applications.

Adsorption of As(III) and As(V) by Fe/C composite nanoparticles synthesized via a one-pot hydrothermal approach without the addition of carbon sources

Arsenic remediation from contaminated water has become a serious issue worldwide. Carbon-encapsulated Fe nanoparticle composites (Fe/C CNPs) were created utilizing a one-pot hydrothermal process with ferrocene and no carbon sources. The Fe/C CNPs produced were characterized using a variety of techniques. As(III) and As(IV) (V) were modeled using a pseudo-second-order kinetic model. The Langmuir model described As(III) adsorption on Fe/C CNPs with an extreme adsorption ability of 5.85 mg g^-1, indicating monolayer adsorption. On the other hand, (V) adsorption was well matched with the Freundlich model, with a high adsorption volume of 5.05 mg g^-1, demonstrating multilayer adsorption onto the surface of Fe/C CNPs. These findings imply that the Fe/C CNPs generated can be utilized to remediate As-contaminated water.

Cu-based MOF-derived architecture with Cu/Cu2O nanospheres anchored on porous carbon nanosheets for efficient capacitive deionization

The design of high-performance electrode materials with excellent desalination ability has always been a research goal for efficient capacitive deionization (CDI). Herein, a hybrid architecture with Cu/Cu₂O nanospheres anchored on porous carbon nanosheets (Cu/Cu₂O/C) was first synthesized by pyrolyzing a two-dimensional (2D) Cu-based metal-organic framework and then evaluated as a cathode for hybrid CDI. The as-prepared Cu/Cu₂O/C exhibits a hierarchically porous structure with a high specific surface area of 305 m² g^-1 and large pore volume of 0.55 cm³ g^-1, which is favorable to accelerating ion migration and diffusion. The porous carbon nanosheet matrix with enhanced conductivity will facilitate the Faradaic reactions of Cu/Cu₂O nanospheres during the desalination process. The Cu/Cu₂O/C hybrid architecture displays a high specific capacitance of 142.5 F g^-1 at a scan rate of 2 mV s^-1 in 1 M NaCl solution. The hybrid CDI constructed using the Cu/Cu₂O/C cathode and a commercial activated carbon anode exhibits a high desalination capacity of 16.4 mg g^-1 at an operation voltage of 1.2 V in 500 mg L^-1 NaCl solution. Additionally, the hybrid CDI exhibits a good cycling stability with 18.3% decay in the desalination capacity after 20 electrosorption-desorption cycles. Thus, the Cu/Cu₂O/C composite is expected to be a promising cathode material for hybrid CDI.

Co-Co3O4 encapsulated in nitrogen-doped carbon nanotubes for capacitive desalination: Effects of nano-confinement and cobalt speciation

Capacitive deionization (CDI) has gained increasing attention as an environmentally friendly and energy-efficient technology for brackish water desalination. However, traditional CDI electrodes still suffer from low salt adsorption capacity and unsatisfactory reusability, which inhibit its application for long-term operations. Herein, we present a facile and effective approach to prepare Co and Co₃O₄ nanoparticles co-incorporating nitrogen-doped (N-doped) carbon nanotubes (Co-Co₃O₄/N-CNTs) via a pyrolysis route. The Co-Co₃O₄ nanoparticles were homogeneously in-situ encapsulated in the inner channels of the conductive CNTs to form a novel and efficient CDI electrode for the first time. The encapsulation of Co-Co₃O₄ nanoparticles in CNTs not only inhibits the Co leaching but also significantly enhances the desalination capacity. The morphology, structure, and capacitive desalination properties of the Co-Co₃O₄/N-CNTs were thoroughly characterized to illuminate the nano-confinement effects and the key roles of the interaction between cobalt species in the CDI performance. The co-existing metallic cobalt and cobalt oxides act as the roles of effective active sites in the CDI performance. As a consequence, the optimum Co-Co₃O₄/N-CNTs electrode displays an outstanding desalination capacity of 66.91 mg NaCl g^-1 at 1.4 V. This work provides insights for understanding the nano-confinement effects and the key roles of the interaction between cobalt species on the CDI performance.

Recent advances in waste-derived functional materials for wastewater remediation

Water pollution is a major concern for public health and a sustainable future. It is urgent to purify wastewater with effective methods to ensure a clean water supply. Most wastewater remediation techniques rely heavily on functional materials, and cost-effective materials are thus highly favorable. Of great environmental and economic significance, developing waste-derived materials for wastewater remediation has undergone explosive growth recently. Herein, the applications of waste (e.g., biowastes, electronic wastes, and industrial wastes)-derived materials for wastewater purification are comprehensively reviewed. Sophisticated strategies for turning wastes into functional materials are firstly summarized, including pyrolysis and combustion, hydrothermal synthesis, sol-gel method, co-precipitation, and ball milling. Moreover, critical experimental parameters within different design strategies are discussed. Afterward, recent applications of waste-derived functional materials in adsorption, photocatalytic degradation, electrochemical treatment, and advanced oxidation processes (AOPs) are analyzed. We mainly focus on the development of efficient functional materials via regulating the internal and external characteristics of waste-derived materials, and the material's property-performance correlation is also emphasized. Finally, the key future perspectives in the field of waste-derived materials-driven water remediation are highlighted.

Metal-organic framework derived carbon nanoarchitectures for highly efficient flow-electrode CDI desalination

Flow-electrode capacitive deionization (FCDI) has shown a robust desalination performance, in which the electrode materials play a crucial role. However, commercial activated carbon (AC) commonly with relatively poor conductivity, which can be a limit to the desalination process. To address this issue, we successfully prepared ZIF-8 derived nanocarbon materials (Zx, X = 0, 1, 2, 3, the number representing the activator ratio) via a pyrolysis activation procedure as electrode materials for FCDI desalination. The results manifested that Z3 achieved desalination rates of 0.0403 and 0.094 mg min^-1 cm^-2 in the isolated closed cycle (ICC) and the short-circuited closed cycle (SCC) mode, respectively, at 1.2 V with only 5 wt% carbon loading. The desalination rate of Z3 in the SCC mode was improved with flow rates and influent salt concentrations increase, reaching 0.278 mg min^-1 cm^-2 under a continuous operation. In the ICC mode, it was found that the adsorption capacity of the Zx sample was positively correlated with its specific surface area. The superior performance of Z3 could be attributed to the high conductivity, large specific surface area and well-developed pores. Overall, this work provided new insights and references for electrode material's application to FCDI desalination.

Surface tuned polyethersulfone membrane using an iron oxide functionalized halloysite nanocomposite for enhanced humic acid removal

Nanomodification of ultrafiltration (UF) membranes has been shown to be a simple and efficient technique for the preparation of high-performance membranes. In this work, an iron oxide functionalized halloysite nanoclay (Fe-HNC) nanocomposite was prepared and used as a nanofiller for polyethersulfone (PES) membranes. The effect of Fe-HNC concentration on the filtration performance of the membrane was investigated by varying the nanocomposite dosage (0-0.5 wt %) in the casting dope. Various characterization studies showed that the incorporation of Fe-HNC nanocomposites improved the membrane morphology and enhanced the surface properties, thermal stability, mechanical strength, hydrophilicity, and porosity. The permeability to pure water and filtration of humic acid (HA) were significantly improved by incorporating Fe-HNC into the PES membranes. The membrane with Fe-HNC loading of 0.1 wt % exhibited the highest pure water permeability (174.3 L/(m² h bar)) and removal of HA (90.1 %), which were 1.8 times and 29 % higher, respectively than the pristine PES membrane. Moreover, fouling studies showed the enhanced antifouling ability of the Fe-HNC nanocomposites modified PES membranes, especially against irreversible fouling. Continuous membrane regeneration-based fouling removal studies from HA showed that the PES/0.1 wt % Fe-HNC membrane exhibited a high fouling recovery of 70.4 % with very low reversible and irreversible fouling resistance of 9.61 % and 14.78 %, respectively, compared to the pristine PES membrane (fouling recovery: 40.4 %; reversible fouling: 21.7 %; irreversible fouling: 20.1 %). Overall, the Fe-HNC nanocomposite proved to be an effective nanomodifier for improving the permeability of PES membranes and the antifouling ability to treat HA polluted aqueous streams.

Catalytic oxidation and deionization of nitrite and nitrate ions using mesoporous carbon-supported nano-flaky cobalt and nickel oxyhydroxides

The composite electrode of NiCo oxide supported by porous carbon was synthesized for nitrite oxidation and nitrate electro-sorption. The crystal structure and chemical state of the Co and Ni oxyhydroxides which were precipitated on loofah-derived activated carbon (AC) using hypochlorite were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and BET surface area. The voltammetry showed that the redox couple of Co(II)/Co(III) and Ni(II)/Ni(III) as the mediator catalytically transferred the electrons of NO₂^-/NO₃^-; the Ni site had a relatively high transfer coefficient and diffusive current, while the Co site was better in the capacitive removal of the nitrite and nitrate compounds. A batch electrolysis of nitrite ions was operated under constant anodic potential mode (0 to + 1.5 V vs. Ag/AgCl) to assess the performance of the composite electrodes. The adsorption capacity of NiCo/AC (Ni = 5% and Co = 5% on AC by weight) was 23.5 mg-N g^-1, which was twice that of AC substrate (7.5 mg-N g^-1), based on a multilayer adsorption model. The steady-state kinetics of the consecutive reaction were derived to determine the rate steps of the electrochemical oxidation of NO₂^- and adsorption of NO₃^-.

Using Pomegranate Peel and Date Pit Activated Carbon for the Removal of Cadmium and Lead Ions from Aqueous Solution

Some agricultural byproducts are useful for solving wastewater pollution problems. These byproducts are of low cost and are effective and ecofriendly. The study aim was to investigate the possibility of using pomegranate peel (PP) and date pit (DP) activated carbon (PPAC and DPAC, respectively) as sorbents to remove Cd(II) and Pb(II) from aqueous solutions. Agricultural wastes of DPs and PPs were subjected to carbonization and chemical activation with H3PO4 (60%) and ZnCl2 and used as adsorbents to remove Cd(II) and Pb(II) from their aqueous solutions. The physical characterizations of PPAC and DPAC, including determination of surface area, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Fourier-transform infrared spectroscopy, were performed. The following factors affected adsorption: solution pH, adsorbent dosage, initial metal ion concentration, and contact time. These factors were studied to identify the optimal adsorption conditions. The results showed that the maximum adsorptions of Cd(II) and Pb(II) were achieved at pH ranging from 6 to 6.5, 90 min contact time, and 0.5 g/L for PPAC and 1 g/L for DPAC dosage. Furthermore, the adsorption efficiencies for both Pb(II) and Cd(II) were higher for PPAC than for DPAC. However, the recorded Qmax values for PPAC were 68.6 and 53.8 mg/g for Pb(II) and Cd(II) and for DPAC were 34.18 and 32.90 mg/g for Pb(II) and Cd(II), respectively. The Langmuir isotherm model fit the adsorption data better than the Freundlich model. Kinetically, the adsorption reaction followed a pseudo-second-order reaction model, with qe ranging from 12.0 to 22.37 mg/g and an R2 value of 0.99.