Defluoridation of water via electrically controlled anion exchange by polyaniline modified electrode reactor

Enhancing fluoride removal from wastewater using Al/Y amended sludge biochar

This study explored the potential of utilizing aluminum and yttrium amended (Al/Y amended) sewage sludge biochar (Al/Y-CSBC) for efficient fluoride removal from wastewater. The adsorption kinetics of fluoride on bimetallic modified Al/Y-CSBC followed the pseudo-second-order model, while the adsorption isotherm conformed to the Freundlich equation. Remarkably, the material exhibited excellent fluoride removal performance over a wide pH range, achieving a maximum adsorption capacity of 62.44 mg·g-1. Moreover, Al/Y-CSBC demonstrated exceptional reusability, maintaining 95% removal efficiency even after six regeneration cycles. The fluoride adsorption mechanism involved ion exchange, surface complexation, and electrostatic adsorption interactions. The activation and modification processes significantly increased the specific surface area of Al/Y-CSBC, leading to a high isoelectric point (pHpzc = 9.14). The incorporation of aluminum and yttrium metals exhibited a novel approach, enhancing the adsorption capacity for fluoride ions due to their strong affinity. Furthermore, the dispersing effect of biochar played a crucial role in improving defluoridation efficiency by enhancing accessibility to active sites. These findings substantiate the significant potential of Al/Y-CSBC for enhanced fluoride removal from wastewater.

A comparative study of confinement and layer modified Zr-based MOFs for the efficient removal of Cr(vi) from wastewater

The GO- and SBA-15-modified UiO-66 adsorbents were developed for removal of trace Cr(vi) from wastewater and investigated to understand the effect of different hybrid ways on the absorption activity and reaction mechanism. The characterization results confirmed that the UiO-66 nanoparticles could be encapsulated by the SBA-15 matrix and anchored onto GO layers. Due to different exposure modes, the adsorption results showed that the GO-modified UiO-66 had better Cr(vi) trapping performance with the maximum removal efficiency of 97% within 3 min, presenting one of the most efficient Cr(vi) removal materials. Kinetic models showed that the adsorption process included fast, exothermic, spontaneous and pseudo-secondary chemical adsorption. By comparison with the Freundlich and Temkin model, the results revealed that the adsorption process of Cr(vi) by UiO-66@SBA-15 involved some multi-layer physical adsorption, while Cr(vi) was adsorbed onto the UiO-66@GO surface. The mechanism study also found that the fixation of Cr was the chemical action of UiO-66 on GO. Additionally, the encapsulated way increases the protection of UiO-55 from surface damage. In all, both hard-core-shell UiO-66@SBA-15 and piece UiO-66@Go increase the absorption activity of Cr(vi), but the different hybrid ways lead to different activities, absorption processes and regeneration abilities.

Anchored growth of highly dispersed LDHs nanosheets on expanded graphite for fluoride adsorption properties and mechanism

In this study, a new composite with layered double hydroxides (LDHs) anchored grown on expanded graphite (EG) interlayers was prepared by vacuum-assisted intercalation and hydrothermal method. Both sides of EG were completely covered by highly dispersed LDHs nanosheets and formed a sandwich-like structure. The unique structure made expanded graphite/layered double hydroxides (EG/LDHs) composites which had excellent F^- adsorption performance. The adsorption performance of F^- on EG/LDHs was evaluated, and the results indicated that the adsorption process was consistent with the pseudo-second-order kinetic model and the Langmuir model. Pseudo-second-order kinetic model indicated that the adsorption sites were the main factor in the adsorption process. Moreover, the maximum adsorption capacity (Q_m) reached 63.21 mg·g^-1 at 30 min at room temperature, which was better than most of the same type of adsorbents. The highly dispersed of LDHs anchored growth on EG overcame the disadvantage of aggregation, which exposed more adsorption sites and improved the removal efficiency of F^-. In addition, the effects of pH, anion interference, different water quality, and regeneration tests on the EG/LDHs composites were also analyzed, showing that the composites have good stability.

Wood-Based Self-Supporting Nanoporous Three-Dimensional Electrode for High-Efficiency Battery Deionization

Developing highly efficient advanced battery deionization (BDI) electrode materials at a low cost is vital for seawater desalination. Herein, a high-efficiency wood-based BDI electrode has been fabricated for seawater desalination, benefiting from the self-supporting three-dimensional (3D) nanoporous structure and rich redox-active sites. The finely tuned rich electrochemical redox active C═O groups on the surface of the wood electrode derived from the facile thermochemical conversion of lignin play a crucial role in the Faradaic cation removal dynamics of BDI. Coupling the 3D wood electrode and a polyaniline-modified wood electrode as the cathode and anode, an all-wood-electrode-based deionization battery has been successfully assembled with a state-of-the-art ion removal capacity of up to 164 mg g^-1 in seawater. Our work reported an example of utilizing wood as the BDI electrode via fine-tuning the redox-active sites, demonstrating a novel resource utilization pathway of converting cheap biomass into BDI electrodes for highly efficient seawater desalination.

Enhanced Cr(vi) removal by hierarchical CoFe2O4@SiO2–NH2via reduction and adsorption processes

CoFe2O4@SiO2–NH2 shows excellent removal performance towards Cr(vi) due to its excellent electrostatic effect as well as the reduction process originating from the –CH group.

Selective Removal of Perfluorobutyric Acid Using an Electroactive Ion Exchanger Based on Polypyrrole@Iron Oxide on Carbon Cloth

Perfluorobutyric acid (PFBA) is one type of perfluoroalkyl and polyfluoroalkyl substances (PFASs) and is widely used as an industrial compound. The removal of PFBA has attracted considerable scientific interests in recent decades because it causes environmental pollution and human diseases. Currently, the adsorption method has been used commonly to remove PFASs from wastewater. However, it is usually limited by the inevitable "secondary waste" produced in this treatment process. In this work, PFBA can be effectively removed by synergistic electrical switching ion exchange (ESIX) and a new type of nanostructured ion exchanger. Herein, the nanostructured ion exchanger has been designed and synthesized by coating a polypyrrole (PPy)@Fe₂O₃ nanoneedle on carbon cloth (PPy@Fe₂O₃ NN-CC). Results show that the PPy@Fe₂O₃ NN-CC nanocomposite enhances ion exchange speed and efficiency, which ensures its high adsorption capacity and rapid regeneration property, thereby reducing secondary waste. Moreover, ESIX based on the PPy@Fe₂O₃ NN-CC nanocomposite has high selectivity for adsorption of PFBA over other common anions in water, such as Cl^-, SO₄^2-, and NO₃^-.

Metal-organic frameworks/alginate composite beads as effective adsorbents for the removal of hexavalent chromium from aqueous solution

Industrial waste discharge comprising heavy metals into potable water bodies induces many health hazards. This study investigates the role of metal-organic frameworks (MOFs) doped alginate beads (MOFs@ABs) as potential adsorbents for Cr(VI). Effects of pH, stirring rate, temperature, initial chrome concentration, and particles dosage on Cr(VI) adsorption are studied to evaluate adsorption ability of UiO-66@ABs for Cr(VI) removal from aqueous solution. The adsorption kinetics follows pseudo second order and the equilibrium isotherm is consistent with Langmuir isotherm model. The maximum adsorption capacity of UiO-66@ABs calculated from the model conforms to the experimental results. The desorption experiment of Cr(VI) adsorbed UiO-66@ABs (82%) demonstrates satisfactory regeneration efficiency. Based on our findings and comparative controlled experiments, the superiority of UiO-66@ABs promises their potential application in Cr(VI) removal from wastewater.

Mechanism of simultaneous removal of aluminum and fluoride from aqueous solution by La/Mg/Si-activated carbon

La/Mg/Si-activated carbon derived from palm shell has been a suitable material for removal of aluminum and fluoride from aqueous solution. In the study, the mechanism of simultaneous removal of aluminum and fluoride by La/Mg/Si-activated carbon (La/Mg/Si-AC) was investigated to understand its high efficiency. It was found that the removal of aluminum and fluoride by La/Mg/Si-AC was favored at lower pH compared to the point of zero charge of La/Mg/Si-AC and high temperature. Adsorption capacity of Al(OH)₄^- was about 10 times higher than that of F^- due to the strong binding affinity of Al(OH)₄^- on protonated surface and competition between F^- and OH^- toward charged adsorption site. Kinetics results showed that the aluminum and fluoride adsorption were explained using the pseudo-second-order kinetic model and intra-particle diffusion model. Adsorption process of Al(OH)₄^- and F^- was driven by the potential rate-limiting step involved in mass transport process occurred on the boundary diffusion layer of porous adsorbent surface. Electrostatic interaction between protonated surface of La/Mg/Si-AC and negatively charged ions (i.e., Al(OH)₄^- and F^-) as well as ion-exchange between hydroxide and ionic metal species were important mechanisms in the process of aluminum and fluoride adsorption. Driving forces for adsorption of individual Al(OH)₄^- and F^- were not entirely different. Identifying the dominant mechanism will be helpful in understanding the adsorption process and developing new adsorbent.

Electrically Switched Ion Exchange Based on Carbon-Polypyrrole Composite Smart Materials for the Removal of ReO4- from Aqueous Solutions

A simple and rapid process of ReO₄^- (as a surrogate of TcO₄^-) removal from aqueous solutions based on the electrically switched ion exchange (ESIX) method has been demonstrated in this work. Activated carbon-Polypyrrole (AC-PPy) was synthesized from activated carbon and pyrrole by electrodeposition method which was served as an electrically switched ion exchanger for ReO₄^- removal. The characterization results show that the AC-PPy composite exhibited an excellent loading capacity and a high stability for ions uptake and release. Chronoamperometric studies show that the ESIX treatment could be completed within 60 s, demonstrating the rapid uptake and release of ions. Uptake and release of ReO₄^- was verified by electrochemical quartz crystal microbalance with dissipation shift (EQCMD) studies. By modulating the electrochemical potential of the AC-PPy, the uptake and release of ReO₄^- ions can be controlled. Similar trends of uptake and release of ReO₄^- were observed in cyclic voltammetry (-0.4 to 0.8 V) for five cycles with the EQCMD. X-ray photoelectron spectroscopy (XPS) confirmed the process of ReO₄^- removal in the AC-PPy composite. Conclusively, the smart material shows excellent efficiency and selectivity for the removal of ReO₄^- from aqueous solutions.

The Extremely High Adsorption Capacity of Fluoride by Chicken Bone Char (CBC) in Defluoridation of Drinking Water in Relation to Its Finer Particle Size for Better Human Health

The ingestion of fluoride-contaminated water causes serious health issues in people all over the world. In the current study, the adsorption of fluoride onto chicken bone char (CBC) was investigated as a defluoridation technique. Finer-sized CBC with a diameter of 106–212 µm was used to investigate the fluoride adsorption capacity onto CBC. Results revealed that finer-sized CBC yielded an unusually high fluoride adsorption capacity of 11.2 mg/g at the equilibrium fluoride concentration of 10 mg/L. The study shows that CBC can be utilized in the defluoridation of drinking water and that finer-sized CBC enhances ion exchange to perform a higher adsorption capacity.

Redox-electrodes for selective electrochemical separations

Redox-active materials hold great promise as platforms for selective liquid-phase separations. In contrast to capacitive electrodes that rely purely on double-layer charge for deionization, redox-modified electrodes can be used to control Faradaic reactions at the interface to selectively bind various charged and uncharged molecules, thus modulating surface interactions through electrochemical potential solely. These electrodes can be composed of a range of functional materials, from organic and organometallic polymers to inorganic crystalline compounds, each relying on its own distinct ion-exchange process. Often, redox electrochemical systems can serve as pseudocapacitors or batteries, thus offering an advantageous combination of adsorption selectivity and energy storage/recovery. This review summarizes redox-interfaces for electrosorption and release, outlines methods for preparation and synthesis, discusses the diverse mechanisms for interaction, and gives a perspective on the future of redox-mediated separations.

Electrocatalytic reduction of bromate based on Pd nanoparticles uniformly anchored on polyaniline/SBA-15

A nano-composite electrocatalyst of Pd nanoparticles (Pd-NPs) anchored on polyaniline (PANI) supported by mesoporous SBA-15 (Pd-NPs/PANI/SBA-15), was synthesized using an in situ chemical method. Transmission electron microscopy showed that the Pd-NPs were homogeneously dispersed. Fourier-transform infrared and X-ray photoelectron spectroscopies confirmed that the Pd-NPs in the metallic state (Pd(0)) were predominantly immobilized on nitrogen sites in the PANI chains. The electrochemical performance of Pd-NPs/PANI/SBA-15 for electrocatalytic reduction of bromate (BrO3(-)) in an acidic medium was investigated by cyclic voltammetry (CV) and amperometric measurement. The reduction peak in the CV curves in the region 0.12 to -0.22V (vs. SCE) corresponded to response of BrO3(-) electroreduction, and the reduction peak current was well fitted linearly to the BrO3(-) concentration. It is proposed that the bromate ions diffuse to the Pd-NPs active sites and then the electrocatalytic reduction occurred with the H(+) doped in PANI. Furthermore, by amperometric measurement, Pd-NPs/PANI/SBA-15 showed relatively high sensitivity with respect to BrO3(-) concentration in the range of 8μmolL(-1) to 40mmolL(-1). Continuous CV for 200 cycles proved that Pd-NPs/PANI/SBA-15 had excellent electrocatalytic stability. These results show that Pd-NPs/PANI/SBA-15 is effective for electrocatalytic reduction of BrO3(-) and has great potential for the fabrication of BrO3(-) electrochemical sensor.

Facile preparation of ion-imprinted composite film for selective electrochemical removal of nickel(II) ions

A facile unipolar pulse electropolymerization (UPEP) technique is successfully applied for the preparation of ion-imprinted composite film composed of ferricyanide-embedded conductive polypyrrole (FCN/PPy) for the selective electrochemical removal of heavy metal ions from wastewater. The imprinted heavy metal ions are found to be easily removed in situ from the growing film only by tactfully applying potential oscillation due to the unstable coordination of FCN to the imprinted ions. The obtained Ni(2+) ion-imprinted FCN/PPy composite film shows fast uptake/release ability for the removal of Ni(2+) ions from aqueous solution, and the adsorption equilibrium time is less than 50 s. The ion exchange capacity reaches 1.298 mmol g(-1) and retains 93.5% of its initial value even after 1000 uptake/release cycles. Separation factors of 6.3, 5.6, and 6.2 for Ni(2+)/Ca(2+), Ni(2+)/K(+), and Ni(2+)/Na(+), respectively, are obtained. These characteristics are attributed to the high identification capability of the ion-imprinted composite film for the target ions and the dual driving forces resulting from both PPy and FCN during the redox process. It is expected that the present method can be used for simple preparation of other ion-imprinted composite films for the separation and recovery of target heavy metal ions as well.

Separation of low concentration of cesium ion from wastewater by electrochemically switched ion exchange method: experimental adsorption kinetics analysis

A series of experiments were performed to evaluate the continuous separation of cesium based on an electrochemically switched ion exchange (ESIX) process using a diaphragm-isolated reactor with two identical nickel hexacyanoferrate/porous three-dimensional carbon felt (NiHCF/PTCF) electrodes as working electrodes. The effects of applied potential, initial concentrations and pH values of the simulation solutions on the adsorption of cesium ion were investigated. The adsorption rate of cesium ion in the ESIX process was fitted by a pseudo-first-order reaction model. The experiments revealed that the introduction of applied potential on the electrodes greatly enhanced the adsorption/desorption rate of Cs(+) and increased the separation efficiency. H(3)O(+) was found to play a dual role of electrolyte and competitor, and the adsorption rate constant showed a curve diversification with an increase in pH value. Also, it was found that the electrochemically switched adsorption process of Cs(+) by NiHCF/PTCF electrodes proceeded in two main steps, i.e., an ESIX step with a fast adsorption rate and an ion diffusion step with a slow diffusion rate. Meanwhile, the NiHCF/PTCF film electrode showed adsorption selectivity for Cs(+) in preference to Na(+).

Electrochemical removal of fluoride from water by PAOA-modified carbon felt electrodes in a continuous flow reactor

A novel poly(aniline-co-o-aminophenol) (PAOA) modified carbon felt electrode reactor was designed and investigated for fluoride removal from aqueous solutions. This reactor design is innovative because it operates under a wider pH range because of coating with a copolymer PAOA ion exchange film. In addition, contaminant mass transfer from bulk solution to the electrode surface is enhanced by the porous carbon felt as an electron-conducting carrier material compared to other reactors. The electrically controlled anion exchange mechanism was investigated by X-ray photoelectron spectroscopy and cyclic voltammetry. The applicability of the reactor in the field was tested through a series of continuous flow experiments. When the flow rate and initial fluoride concentration were increased, the breakthrough curve became sharper, which lead to a decrease in the breakthrough time and the defluoridation capacity of the reactor. The terminal potential values largely influenced fluoride removal by the reactor and the optimal defluoridation efficiency was observed at around 1.2V. The breakthrough capacities were all >10mg/g over a wide pH range (pH 5-9) with an initial fluoride concentration of 10mg/L. Consecutive treatment-regeneration studies over a week (once each day) revealed that the PAOA-modified carbon felt electrode could be effectively regenerated for reuse. The PAOA-modified carbon felt electrode reactor is a promising system that could be made commercially available for fluoride removal from aqueous solutions in field applications.