Nitrate, arsenic and fluoride removal by electrodialysis from brackish groundwater

Evaluation Study of Ammonium Removal from Groundwater by Electrodialysis: Case Study of Real Groundwater from the City of Kenitra in Morocco

In this work, we investigated the feasibility of ammonium removal by electrodialysis (ED), a well-known electro-membrane process, based on the selective migration of anions and cations through anion exchange membranes (AEMs) and cation exchange membranes (CEMs). ED experiments are performed using a laboratory pilot. The ion exchange membranes (IEMs) pair used is AXE/CMX, AXE as an AEMs and CMX as a CEMs. The first tests are performed with real groundwater solutions from Kenitra city (Morocco), spiked with an initial concentration of 3 mg/L NH4Cl. The results gave a specific demineralization (SD) to NH4 + ions of 84.60 %; for a demineralization rate (DR) of 80 % and a time of 60 min. The multivariate principal component analysis (PCA) presents a total inertia of 99.23 %, the majority of the variables of which are positively correlated on the C1 axis with a variance of 95.5 % than that of C2 of 3.78 %. The quality of the diluted water determined by the Legrand-Poirier method showed that the water was aggressive and that the addition of 4.54 mg/L Ca2+ was necessary to balance the water and make it fit for human consumption.

Regulating the Selectivity of Nitrate Photoreduction for Purification or Ammonia Production by Cooperating Oxidative Half-Reactions

The removal and conversion of nitrate (NO3-) from wastewater has become an important environmental and health topic. The NO3- can be reduced to nontoxic nitrogen (N2) for environmental remediation or ammonia (NH3) for recovery, in which the tailoring of the selectivity is greatly challenging. Here, by construction of the CuOx@TiO2 photocatalyst, the NO3- conversion efficiency is enhanced to ∼100%. Moreover, the precise regulation of selectivity to NH3 (∼100%) or N2 (92.67%) is accomplished by the synergy of cooperative redox reactions. It is identified that the selectivity of the NO3- photoreduction is determined by the combination of different oxidative reactions. The key roles of intermediates and reactive radicals are revealed by comprehensive in situ characterizations, providing direct evidence for the regulated selectivity of the NO3- photoreduction. Different active radicals are produced by the interaction of oxidative reactants and light-generated holes. Specifically, the introduction of CH3CHO as the oxidative reactant results in the generation of formate radicals, which drives selective NO3- reduction into N2 for its remediation. The alkyl radicals, contributed to by the (CH2OH)2 oxidation, facilitate the deep reduction of NO3- to NH3 for its upcycling. This work provides a technological basis for radical-directed NO3- reduction for its purification and resource recovery.

Fluorine Modification Promoted Water Dissociation into Atomic Hydrogen on a Copper Electrode for Efficient Neutral Nitrate Reduction and Ammonia Recovery

Electrocatalytic nitrate reduction to ammonia (NITRR) offers an attractive solution for alleviating environmental concerns, yet in neutral media, it is challenging as a result of the reliance on the atomic hydrogen (H*) supply by breaking the stubborn HO-H bond (∼492 kJ/mol) of H2O. Herein, we demonstrate that fluorine modification on a Cu electrode (F-NFs/CF) favors the formation of an O-H···F hydrogen bond at the Cu-H2O interface, remarkably stretching the O-H bond of H2O from 0.98 to 1.01 Å and lowering the energy barrier of water dissociation into H* from 0.64 to 0.35 eV at neutral pH. As a benefit from these advantages, F-NFs/CF could rapidly reduce NO3- to NH3 with a rate constant of 0.055 min-1 and a NH3 selectivity of ∼100%, far higher than those (0.004 min-1 and 9.2%) of the Cu counterpart. More importantly, we constructed a flow-through coupled device consisting of a NITRR electrolyzer and a NH3 recovery unit, realizing 98.1% of total nitrogen removal with 99.3% of NH3 recovery and reducing the denitrification cost to $5.1/kg of N. This study offers an effective strategy to manipulate the generation of H* from water dissociation for efficient NO3--to-NH3 conversion and sheds light on the importance of surface modification on a Cu electrode toward electrochemical reactions.

Efficient and Selective Electrochemical Nitrate Reduction to N2 Using a Flow-Through Zero-Gap Electrochemical Reactor with a Reconstructed Cu(OH)2 Cathode: Insights into the Importance of Inter-Electrode Distance

Electrochemically converting nitrate, a widely distributed nitrogen contaminant, into harmless N2 is a feasible and environmentally friendly route to close the anthropogenic nitrogen-based cycle. However, it is currently hindered by sluggish kinetics and low N2 selectivity, as well as scarce attention to reactor configuration. Here, we report a flow-through zero-gap electrochemical reactor that shows a high performance of nitrate reduction with 100% conversion and 80.36% selectivity of desired N2 in the chlorine-free system at 100 mg-N·L-1 NO3- while maintaining a rapid reduction kinetics of 0.07676 min-1. More importantly, the mass transport and current utilization efficiency are significantly improved by shortening the inter-electrode distance, especially in the zero-gap electrocatalytic system where the current efficiency reached 50.15% at 5 mA·cm-2. Detailed characterizations demonstrated that during the electroreduction process, partial Cu(OH)2 on the cathode surface was reconstructed into stable Cu/Cu2O as the active phase for efficient nitrate reduction. In situ characterizations revealed that the highly selective *NO to *N conversion and the N-N coupling step played crucial roles during the selective reduction of NO3- to N2 in the zero-gap electrochemical system. In addition, theoretical calculations demonstrated that improving the key intermediate *N coverage could effectively facilitate the N-N coupling step, thereby promoting N2 selectivity. Moreover, the environmental and economic benefits and long-term stability shown by the treatment of real nitrate-containing wastewater make our proposed electrocatalytic system more attractive for practical applications.

Nitrogen compounds removal from brackish water by electrodialysis at fixed electric potential and dynamic current density operations

Nitrogen (N) compounds can occur in water resources from natural and anthropogenic activities. It is ideal that these contaminants be removed before water consumption. As water quality has been affected by increased salinity and pH variation, more advanced and robust technologies such as electrodialysis (ED) can be considered for simultaneous desalination and pollutant removal. In this context, the removal of N-species (NO3-, NO2-, NH4+, and CH4N2O) from brackish water by ED was investigated for different feed water quality, considering increased salinity (0 - 10g/L NaCl) and pH variation (3 - 11), under limit current density (LCD) at fixed electric potential condition. The applied electric potential (5 - 25V) under, at, and over the LCD at fixed electric potential and dynamic current density (DCD), as a percentage of LCD (0.4 - 1.2), were analyzed to improve the process. In addition, energy efficiency in the form of specific energy consumption (SEC) and current efficiency (CE) were assessed for ED at fixed electric potential and DCD. The results showed that, at extreme pH of the feed water, the removal of NO2- and NH4+ can be affected, while NO3-was the most stable compound with pH variation. An increase in feed water salinity just slightly impacted the removal of N-compounds, due to the similar characteristics of the ions in the water. The increase in electric potential at fixed electric potential or DCD increased the removal and molar flux of N-compounds. However, operating over the LCD increased the SEC of the ED process while changes in removal were not significant. DCD procedures resulted in higher CE and shorter run time of the experiments. Therefore, ED proved to be a suitable treatment technique to produce fresh water due to the selective removal of the studied ions, especially at 15V (fixed electrical potential) and 0.8 LCD (DCD) related to removal, molar flux, and run time to achieve guidelines.

Recent breakthroughs on the development of electrodeionization systems for toxic pollutants removal from water environment

Since ecosystems are becoming inherently polluted, long-term contaminant removal methods are required. Electrodeionization, in particular, has recently been demonstrated as an effective approach for eliminating ionic compounds from contaminated water sources. Being a more environmentally friendly technology is most likely the main reason for its eminence. It uses electricity to replace toxic contaminants that are conventionally used to regenerate and hence reducing the toxins associated with resin regeneration. In wastewater treatment, continuous electrodeionization system overcomes several limitations of ion exchange resins, notably ion dumping. This prospective assessment delves into the mechanism, principle, and theory of electrodeionization system. It also focused on the design and applications, particularly in the removal of toxic compounds, as well as current advances in the electrodeionization system. Recent breakthroughs in electrodeionization were comprehensively discussed. Further developments in electrodeionization systems are also projected, with improved efficiency at the time of functioning at lower costs because of reduced energy use, proving them desirable for commercial usage with a broad array of applications across the globe.

Selectively transport and removal of fluoride ion by pillar[5]arene polymer-filled nanochannel membrane

Excess fluoride ions in groundwater accumulate through the roots of crops, affecting photosynthesis and inhibiting their growth. Long-term bioaccumulation also threatens human health because it is poorly degradable and toxic. Currently, one of the biggest challenges is developing a unique material that can efficiently remove fluoride ions from the environment. The excellent properties of functionalized pillar[5]arene polymer-filled nanochannel membranes were explored to address this challenge. Constructing a multistage porous nanochannel membrane, consisting of microscale etched nanochannels and nanoscale pillar[5]arene cross-linked polymer voids. A fluoride removal rate of 0.0088 mmol ⋅ L-1  ⋅ min-1 was achieved. Notably, this rate surpassed the rates observed with other control ions by a factor of 6 to 8.8. Our research provides a new direction for developing water fluoride ion removal materials.

Performance of three anion-exchange membranes in fluoride ions removal by electrodialysis

The performance of three anion-exchange membranes (AEMs) in the fluoride ions reduction by electrodialysis (ED) is performed on real and synthetic water. The electric potential method measures the potential difference (PD) between two synthetic anion solutions separated by ACS, AFN and AXE membranes. The selectivity of these three AEMs coupled with the membrane CMX, is a cation-exchange membrane (CEM) towards different ions. The removal rate is influenced by the thickness of the polarization layer (PL) which reduces the material transfer and provides an additional barrier. The greater the thickness δ of the PL, the longer the passage time and consequently the removal rate of anions is small. Using the unstirred layer model, δ for each ion will be determined. According to the potential measurement method, none of the tested AEMs are selective to fluoride ions and the order of selectivity is as follows: AFN> AXE> ACS. Best membrane couple selected for fluoride ion removal is ACS/CMX and ion selectivity follows the order: Cl-> NO-3>F-> HCO-3> SO42-. For ACS membrane, both the demineralization rate (DR) and δ of fluoride ions are influenced by the initial concentration of the co-ion according to the following order: NO-3> Cl-> HCO-3> SO2-4.

Ex-situ electrochemical characterisation of fixed-bed denitrification biocathodes: A promising strategy to improve bioelectrochemical denitrification

The worldwide issue of nitrate-contaminated groundwater requires practical solutions, and electro-bioremediation offers a promising and sustainable treatment. While it has shown potential benefits, there is room for improvement in treatment rates, which is crucial for its further and effective implementation. In this field, electrochemical characterisation is a valuable tool for providing the foundation for optimising bioelectrochemical reactors, but applying it in fixed-bed reactors is challenging due to its high intrinsic electrical resistance. To overcome these challenges, this study employed the easy and swift eClamp methodology to screen different process parameters and their influence on the performance of fixed-bed denitrifying biocathodes composed of granular graphite. Granules were extracted and studied ex-situ under controlled conditions while varying key operational parameters (such as pH, temperature, and nitrate concentration). In the studied biocathode, the extracellular electron transfer associated with denitrification was identified as the primary limiting step with a formal potential of -0.225 ± 0.007 V vs. Ag/AgCl sat. KCl at pH 7 and 25 °C. By varying the nitrate concentration, it was revealed that the biocathode exhibits a strong affinity for nitrate (KMapp of 0.7 ± 0.2 mg N-NO3- L-1). The maximum denitrification rate was observed at a pH of 6 and a temperature of 35 °C. Furthermore, the findings highlight a 2e-/1H+ transfer, which holds considerable implications for the energy metabolism of bioelectrochemical denitrifiers. These compiled results provide valuable insights into the understanding of denitrifying biocathodes and enable the improvement and prediction of their performance.

The function of doping nitrogen on removing fluoride with decomposing La-MOF-NH2: Density functional theory calculation and experiments

Fluoride is an important pollutant in wastewater, and adsorption is an effective way to remove fluoride. Because nitrogen plays an important role in adsorbent materials, computational models were developed to understand the changes in work function resulting from nitrogen doping. La-N-C-800°C, was prepared by pyrolyzing La-MOF-NH2 to verify the influence on the performance of removing fluoride by electrosorption. Material and electrochemical performance tests were performed to characterize La-N-C-800°C. Adsorption kinetics, adsorption thermodynamics, initial concentrations, pH, and ions competition were investigated using La-N-C-800°C for fluoride removal. In addition, density functional theory was applied to evaluate the function of nitrogen. When nitrogen atoms were added, the density of states, partial density of states, populations, and different orbits of charge were calculated to discover deep changes. Nitrogen strengthened the carbon structure and La2O3 structure to remove fluoride. In addition, nitrogen can also act as an adsorption site in the carbon structure. These results provide design ideas for improving the performance of adsorbent materials by doping elements.

New easily recycled carrier based polyurethane foam by loading Al-MOF and biochar for selective removal of fluoride ion from aqueous solutions

The production of Integrated circuits (ICs) generates wastewater with a high concentration of residual fluoride ions, necessitating highly efficient fluorine removal methods. In this study, a novel composite carrier was developed using a hydrothermal synthesis method to load Al-MOF and biochar (BC) onto polyurethane foam (PUF), resulting in the composite foam of Al-MOF-PUF@BC. The results showed that the composite carrier exhibited a stable fluoride removal effect, with a maximum adsorption capacity of 16.52 mg/g at room temperature. The adsorption isotherm curves were consistent with the Langmuir isotherm model, and the adsorption kinetics were well-described by the pseudo-first-order model. The mechanism of fluorine adsorption on Al-MOF-PUF@BC was ligand exchange with hydroxyl groups and the formation of FAl bonds. Density functional theory (DFT) calculations revealed that the adsorption energy reached -246.7 eV, indicating stable adsorption for fluoride ions. The composite foam demonstrated excellent regenerative properties and was effective for fluoride removal in actual IC wastewater.

Investigation on Applying Biodegradable Material for Removal of Various Substances (Fluorides, Nitrates and Lead) from Water

Sapropel was used as a biodegradable material for water treatment. Sapropel is a sedimentary layer of a mix of organic and inorganic substances accumulated in the bottoms of lakes for thousands of years. It is a jelly-like homogeneous mass and has properties of sorption. Sapropel is used as a biosorbent and an environment-friendly fertiliser, and it is used in building materials and in the beauty industry as well. In water, there are abundant various solutes that may cause a risk to human health. Such substances include fluorides, nitrates and lead in different sources of water. The goal of this investigation is to explore and compare the efficiencies of removal of different pollutants (fluorides, nitrates and lead) from aqueous solutions upon using sapropel as a sorbent. In this research, various doses of sapropel (0.1, 0.5, 1, 5, 10, 20, 50, 100 and 200 g/L) and various mixing times (15, 30, 60, 90 and 120 min) were used for removal of fluorides, nitrates and lead from aqueous solutions. It was found that the maximum efficiency (up to 98.57%) of lead removal from aqueous solutions by sapropel was achieved when the minimum doses of it (0.1 and 0.5 g/L) were used. The most efficient removal of fluorides (64.67%) was achieved by using 200 g/L of sapropel and mixing for 120 min. However, sapropel does not adsorb nitrates from aqueous solutions.

Efficient purification of a nitrate and chlorate mixture in water via photoredox activated intermediate coupling-decoupling pathway

Nitrate (NO3-) is a widespread contaminant that threatens human health and ecological safety. Meanwhile, the disinfection byproducts chlorate (ClO3-) is generated inevitably in conventional wastewater treatment. Therefore, the contaminants mixture of NO3- and ClO3- are universal in common emission units. Photocatalysis technology is a feasible approach for the synergistic abatement of contaminant mixture, where matching suitable oxidation reactions is a potential strategy to improve the photocatalytic reduction reactions. Herein, formate (HCOOH) oxidation is introduced to facilitate the photocatalytic reduction of the NO3- and ClO3- mixture. As a result, high purification efficiency of NO3- and ClO3- mixture are achieved, evidenced by 84.6% e--dependent removal of the mixture at a reaction time of 30 min, with 94.5% N2 selectivity and 100% Cl- selectivity, respectively. Specifically, by the close combination of in-situ characterizations and theoretical calculations, the detailed reaction mechanism is revealed, in which the intermediate coupling-decoupling route from NO3- reduction and HCOOH oxidation is established by the chlorate-induced photoredox activation, leading to the significantly enhanced efficiency for the wastewater mixture purification. The practical application of this pathway is established for simulated wastewater to show its wide applicability. This work provides new insights into photoredox catalysis technology for its environmental application.

Effective reduction and recovery of As(III) and As(V) from alkaline wastewater by thiourea dioxide: Efficiency and mechanism

For alkaline wastewater with high arsenic concentration, the traditional lime precipitation inevitably produces large amounts of hazardous waste. Herein, a heat-activated reduction method employing thiourea dioxide (TDO) as the reductant was proposed to efficiently remove and recover As(III)/As(V) from alkaline wastewater in the form of valuable As(0). More than 99.9% of As(III)/As(V) (2-400 mM) were reduced to As(0) with a high purity of more than 99.5 wt% by TDO within 30 min. The highly reductive eaq- and SO2- radical generated during TDO decomposition contribute to the arsenic reduction, and the contribution ratios of eaq- and SO2- radical were estimated to be approximately 57.6% and 42.4% for As(III) removal and 62.2% and 37.8% for As(V) removal, respectively. The arsenic reduction was greatly improved by increasing pH and temperature, which could accelerate the cleavage of C-S bond in TDO for the eaq- and SO2- formation. The presence of dissolved oxygen, which can not only scavenge eaq-/SO2- but also directly oxidize SO22-, had a negative effect on the arsenic removal. The presence of CO32- slightly suppressed the arsenic removal due to the eaq- scavenging effect while SiO32-, PO43-, Cl-, SO42- and NH4+ had negligible effects. The proposed method was a potential technology for the efficient removal and reduction of arsenic in alkaline wastewater.

A cascade electro-dehydration process for simultaneous extraction and enrichment of uranium from simulated seawater

Uranium extraction from seawater has become a crucial issue that has raised tremendous attention. The transport of water molecules along with salt ions through an ion-exchange membrane is a common phenomenon for typical electro-membrane processes such as selective electrodialysis (SED). In this study, a cascade electro-dehydration process was proposed for the simultaneous extraction and enrichment of uranium from simulated seawater by taking advantage of water transport through ion-exchange membranes and the high permselectivity of membranes for monovalent ions against uranate ions. The results indicated that the electro-dehydration effect in SED allowed 1.8 times the concentration of uranium with a loose structure CJMC-5 cation-exchange membrane at a current density of 4 mA/cm². Thereafter, a cascade electro-dehydration by a combination of SED with conventional electrodialysis (CED) enabled approximately 7.5 times uranium concentration with the extraction yield rate reaching over 80% and simultaneously desalting the majority of salts. Overall, a cascade electro-dehydration is a viable approach, creating a novel route for highly effective uranium extraction and enrichment from seawater.

Biologically-induced synthetic manganese carbonate precipitate (BISMCP) for potential applications in heavy metal removal

Heavy metal pollution of water is a burning issue of today's world. Among several strategies involved for heavy metal remediation purpose, biomineralization has shown great potential. Of late, research has been focused on developing effective mineral adsorbents with reduced time and cost consumption. In this present paper, the Biologically-Induced Synthetic Manganese Carbonate Precipitate (BISMCP) was produced based on the biologically-induced mineralization method, employing Sporosarcina pasteurii in aqueous solutions containing urea and MnCl₂. The prepared adsorbent was characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), SEM-energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD) and BET surface area analyzer. EDX analysis showed the elements in the crystal BISMCP were Mn, C, and O. XRD result of BISMCP determined the crystal structure, which is close to rhodochrosite (MnCO₃). Spectral peaks of FTIR at 1641.79 cm^-1 confirmed the appearance of C[bond, double bond]O binding, with strong stretching of CO₃^2- in Amide I. From the six kinds of BISMCP produced, sample MCP-6 has the higher specific surface area by BET analysis at 109.01 m²/g, with pore size at 8.76 nm and higher pore volume at 0.178 cm³/g. These specifications will be suitable as an adsorbent for heavy metal removal by adsorption process. This study presents a preliminary analysis of the possibility of BISMCP for heavy metals adsorption using ICP multi-element standard solution XIII (As, Cr, Cd, Cu, Ni, and Zn). BISMCP formed from 0.1 MnCl₂ and 30 ml of bacteria volume (MCP-6) produced a better adsorbent material than others concentrations, with the adsorption efficiency of total As at 98.9%, Cr at 97.0%, Cu at 94.7%, Cd at 88.3%, Zn at 48.6%, and Ni at 29.5%. Future work could be examined its efficiency adsorbing individual heavy metals.

Effects of elevated arsenic and nitrate concentrations on groundwater resources in deltaic region of Sundarban Ramsar site, Indo-Bangladesh region

An attempt has been adopted to predict the As and NO₃^- concentration in groundwater (GW) in fast-growing coastal Ramsar region in eastern India. This study is focused to evaluate the As and NO₃^- vulnerable areas of coastal belts of the Indo-Bangladesh Ramsar site a hydro-geostrategic region of the world by using advanced ensemble ML techniques including NB-RF, NB-SVM and NB-Bagging. A total of 199 samples were collected from the entire study area for utilizing the 12 GWQ conditioning factors. The predicted results are certified that NB-Bagging the most suitable and preferable model in this current research. The vulnerability of As and NO₃^- concentration shows that most of the areas are highly vulnerable to As and low to moderately vulnerable to NO₃. The reliable findings of this present study will help the management authorities and policymakers in taking preventive measures in reducing the vulnerability of water resources and corresponding health risks.

Combined Electrodialysis and Electrocoagulation as Treatment for Industrial Wastewater Containing Arsenic and Copper

In copper smelting processes, acidic effluents are generated that contain inorganic contaminants such as arsenic and copper. Nowadays, the treatment of wastewater is done by physicochemical methods without copper recovery. Electrodialysis is an alternative process that can recover copper. Moreover, when electrocoagulation is applied to remove arsenic from wastewater, a more stable final sludge of less volume is obtained. The present research studies the application of a combined electrodialysis and electrocoagulation process to (1) recover Cu and (2) precipitate and remove arsenic simultaneously in the same batch reactor, using synthetic wastewater that simulates wastewater from a copper smelter. Copper and arsenic could be removed and separated by the electrodialysis part, and the electrocoagulation of arsenic was verified. With electrodialysis, the arsenic and copper removals were 67% and 100%, respectively, while 82% of the arsenic arriving at the electrocoagulation part of the cell could be precipitated and removed by this process. Initial concentrations were around 815 mg L^-1 Cu and 7700 mg L^-1 As. The optimal current was found to be 1.36 A due to the shorter treatment times necessary to get removal percentages, recovery percentages and energy/removed copper mass ratios in the same ranges as the values achieved with a current of 1.02 A. In summary, the combined process is a promising tool for simultaneous copper recovery and arsenic removal.

Microbially induced calcium precipitation coupled with medical stone-coated sponges: A targeted strategy for enhanced nitrate and fluoride removal from groundwater

The coexistence of nitrate and fluoride in groundwater is of high concern due to its potential environmental impacts and health risks. Medical stone-coated sponges, as a microbial activity promoter and slow-release calcium source, were introduced into an immobilized bioreactor for enhanced removal of nitrate and fluoride. Under the hydraulic retention time of 3 h, nitrate, fluoride, and calcium contents of 16.5, 3.0, and 100 mg L^-1, the average removal efficiencies of nitrate, fluoride, and calcium reached 99.49%, 74.26%, and 70.43%, respectively. Co-precipitation and chemisorption were the mechanisms for fluoride and calcium removal. Medical stone load improved the competitiveness of dominant bacteria and electron transport activity, accelerated the denitrification process, and stimulated biofilm formation. High fluoride level (5.0 mg L^-1) inhibited the nitrate removal and aromatic protein production. The fluoride content changes altered the carbon source preference of the microbial community, which preferred to use amino acids and carbohydrates under a higher fluoride content. The introduction of medical stones significantly accelerated the fluoride and nitrate removal, providing a new insight for the application of microbially induced calcium precipitation technique in the remediation of low-calcium groundwater.

Recovery of Fluoride-Rich and Silica-Rich Wastewaters as Valuable Resources: A Resource Capture Ultrafiltration-Bipolar Membrane Electrodialysis-Based Closed-Loop Process

Traditional technologies such as precipitation and coagulation have been adopted for fluoride-rich and silica-rich wastewater treatment, respectively, but waste solid generation and low wastewater processing efficiency are still the looming concern. Efficient resource recovery technologies for different wastewater treatments are scarce for environment and industry sustainability. Herein, a resource capture ultrafiltration-bipolar membrane electrodialysis (RCUF-BMED) system was designed into a closed-loop process for simultaneous capture and recovery of fluoride and silica as sodium silicofluoride (Na₂SiF₆) from mixed fluoride-rich and silica-rich wastewaters, as well as achieving zero liquid discharge. This RCUF-BMED system comprised two key parts: (1) capture of fluoride and silica from two wastewaters using acid, and recovery of the Na₂SiF₆ using base by UF and (2) UF permeate conversion for acid/base and freshwater generation by BMED. With the optimized RCUF-BMED system, fluoride and silica can be selectively captured from wastewater with removal efficiencies higher than 99%. The Na₂SiF₆ recovery was around 72% with a high purity of 99.1%. The aging and cyclic experiments demonstrated the high stability and recyclability of the RCUF-BMED system. This RCUF-BMED system has successfully achieved the conversion of toxic fluoride and silica into valuable Na₂SiF₆ from mixed wastewaters, which shows great application potential in the industry-resource-environment nexus.

State-of-the-art of research progress on adsorptive removal of fluoride-contaminated water using biochar-based materials: Practical feasibility through reusability and column transport studies

Fluoride (F^-) is one of the essential elements found in soil and water released from geogenic sources and several anthropogenic activities. Fluoride causes fluorosis, dental and skeletal growth problems, teeth mottling, and neurological damage due to prolonged consumption, affecting millions worldwide. Adsorption is an extensively implemented technique in water and wastewater treatment for fluoride, with significant potential due to efficiency, cost-effectiveness, ease of operation, and reusability. This review highlights the current state of knowledge for fluoride adsorption using biochar-based materials and the limitations of biochar for fluoride-contaminated groundwater and industrial wastewater treatment. Biochar materials have shown significant adsorption capacities for fluoride under the influence of low pH, biochar dose, initial concentration, temperature, and co-existing ions. Modified biochar possesses various functional groups (-OH, -CC, -C-O, -CONH, -C-OH, X-OH), in which enhanced hydroxyl (-OH) groups onto the surface plays a significant role in fluoride adsorption via electrostatic attraction and ion exchange. Regeneration and reusability of biochar sorbents need to be performed to a greater extent to improve removal efficiency and reusability in field conditions. Furthermore, the present investigation identifies the limitations of biochar materials in treating fluoride-contaminated drinking groundwater and industrial effluents. The fluoride removal using biochar-based materials at an industrial scale for understanding the practical feasibility is yet to be documented. This review work recommend the feasibility of biochar-based materials in column studies for fluoride remediation in the future.

Enhanced removal of fluoride, nitrate, and calcium using self-assembled fungus-flexible fiber composite microspheres combined with microbially induced calcium precipitation

Self-assembled fungus-flexible fiber composite microspheres (SFFMs) were firstly combined with microbially induced calcium precipitation (MICP) in a continuous-flow bioreactor and achieved the efficient removal of fluoride (F^-), nitrate (NO₃^-), and calcium (Ca²⁺). Under the influent F^- of 3.0 mg L^-1, pH of 7.0, and HRT of 8 h, the average removal efficiencies reached 77.54%, 99.39%, and 67.25% (0.29, 2.03, and 8.34 mg L^-1 h^-1), respectively. Fluorescence spectrum and flow cytometry analyses indicated that F^- content significantly affected the metabolism and viability of bacteria. SEM images showed that flexible fibers and intertwined hyphae provided effective locations for bacterial colonization in SFFMs. The precipitated products were characterized by XRD and FTIR, which revealed that F^- was mainly removed in the form of calcium fluoride and calcium fluorophosphate (CaF₂ and Ca₅(PO₄)₃F). High-throughput analysis at different levels demonstrated that Pseudomonas sp. WZ39 acted as the core strain, which played a crucial role in the bioreactor. The mechanism of enhanced denitrification was attributed to minor F^- stress and bioaugmentation technology. This study highlighted the superiorities of SFFMs and MICP combined remediation and documented a promising option for F^-, NO₃^-, and Ca²⁺ removal.

Bamboo Chopstick Biochar Electrodes and Enhanced Nitrate Removal from Groundwater

The nitrate pollution of groundwater can cause serious harm to human health. Biochar electrodes, combined with adsorption and electroreduction, have great potential in nitrate removal from groundwater. In this study, bamboo chopsticks were used as feedstocks for biochar preparation. The bamboo chopstick biochar (BCBC), prepared by pyrolysis at 600 °C for 2 h, had a specific surface area of 179.2 m2/g and an electrical conductivity of 8869.2 μS/cm, which was an ideal biochar electrode material. The maximum nitrate adsorption capacity of BCBC-600-2 reached 16.39 mg/g. With an applied voltage of 4 V and hydraulic retention time of 4 h, the nitrate removal efficiency (NRE) reached 75.8%. In comparison, the NRE was only 32.9% without voltage and 25.7% with graphite cathode. Meanwhile, the average nitrate removal rate of biochar electrode was also higher than that of graphite cathode under the same conditions. Therefore, biochar electrode can provide full play to the coupling effect of adsorption and electroreduction processes and obtain more powerful nitrate removal ability. Moreover, the biochar electrode could inhibit the accumulation of nitrite and improve the selectivity of electrochemical reduction. This study not only provides a high-quality biochar electrode material, but also provides a new idea for nitrate removal in groundwater.

Recent advances in the source identification and remediation techniques of nitrate contaminated groundwater: A review

Researchers have long been committed to identify nitrate sources in groundwater and to develop an advanced technique for its remediation because better apply remediation solution and management of water quality is highly dependent on the identification of the NO₃^- sources contamination in water. In this review, we systematically introduce nitrate source tracking tools used over the past ten years including dual isotope and multi isotope techniques, water chemistry profile, Bayesian mixing model, microbial tracers and land use/cover data. These techniques can be combined and exploited to track the source of NO₃^- as mineral or organic fertilizer, sewage, or atmospheric deposition. These available data have significant implications for making an appropriate measures and decisions by water managers. A continuous remediation strategy of groundwater was among the main management strategies that need to be applied in the contaminated area. Nitrate removal from groundwater can be accomplished using either separation or reduction based process. The application of these processes to nitrate removal is discussed in this review and some novel methods were presented for the first time. Moreover, the advantages and limitations of each approach are critically summarized and based on our own understanding of the subject some solutions to overcomes their drawbacks are recommended. Advanced techniques are capable to attain significantly higher nitrate and other co-contaminants removal from groundwater. However, the challenges of by-products generation and high energy consumption need to be addressed in implementing these technologies for groundwater remediation for potable use.

Study of the Seawater Desalination Performance by Electrodialysis

The global scarcity of freshwater resources has greatly contributed to the development of desalination technologies, in which electrodialysis desalination is one of the most widely used and highly regarded methods. In this work, the first step was to design and assemble a experiment module for electrodialysis desalination. The ion removal efficiency and single membrane mass transfer flux of electrodialysis desalination were investigated. The results show that the desalination performance of the module is improved by increasing the voltage gradient, increasing the concentration of seawater and electrolyte and decreasing the membrane surface flux and that the optimum operating conditions for the module at 24 V operating voltage are feedstock concentration of 35 g/L, electrolyte concentration of 1.42 g/L which and system flow rate of 15 L/h. The results of the study will help to better investigate electrodialysis desalination technology.

A highly active copper-nanoparticle-based nitrate reduction electrocatalyst prepared by in situ electrodeposition and annealing

In recent years, copper-based electrodes have attracted intense attention for the electrochemical reduction of nitrate (NO₃^-), the so-called ECRN. However, these electrodes suffer from low activity and selectivity. Herein, we report a novel Cu-based electrode (IE-Cu-400) for the ECRN fabricated by loading Cu-based nanoparticles onto graphite felt using in situ electrodeposition followed by annealing. Compared with traditional Cu-based electrodes, the IE-Cu-400 is comprised of smaller particles and the copper is present in a high oxidation state (Cu²⁺ in CuO). During operation, the CuO is converted to Cu, which is the active ECRN species. In addition, an increased surface area and high density of grain boundaries resulting from the reduction of CuO were observed for IE-Cu-400. This resulted in a 3.38-fold increase in the NO₃^- removal rate and a 1.36-fold increase in NH₄⁺ selectivity. Further analyses revealed that the enhanced ECRN performance of IE-Cu-400 is linked to its increased number of active sites, as well as its improved adsorption and reduction ability for NO₂^-. Moreover, IE-Cu-400 displays high stability for the ECRN. Finally, the produced NH₄⁺ was effectively oxidised to N₂ with approximately 100% selectivity via chlorination. Hence, the two-stage treatment strategy (i.e. ECRN by IE-Cu-400 + chlorination treatment) presented here shows great potential for the complete electrocatalytic denitrification of water. Further, this work highlights the beneficial effect of decreasing the particle size and controlling the surface oxidation of Cu-based catalysts simultaneously for enhancing the ECRN and offers new suggestions for the design of high-performance electrode materials for the ECRN.

Enhancing nitrate removal efficiency of micro-sized zero-valent iron by chitosan gel balls encapsulating

The activity of micro-sized zero-valent iron (MZVI) material for nitrate removal in neutral pH and low C/N ratios water needs to be improved. In this study, micro-sized zero-valent iron@chitosan (MZVI@CS) material was synthesized through embedding MZVI particles into chitosan (CS) gel by sol-gel method, and was used for deep removal of NO₃^--N in the absence of organic carbon sources and neutral pH. The NO₃^--N removal rate of MZVI@CS was 0.37 mg-N·L^-1·d^-1 (dosage of 1%, initial pH = 7, 25 °C, initial nitrate concentration = 15 mg-N·L^-1), which was 11.33 times higher than that of MZVI. The apparent activation energy (E_a) of MZVI@CS with nitrate was 38.23 kJ·mol^-1. MZVI@CS can remove nitrate effectively at a low concentration (15 mg-N·L^-1). A stable denitration rate (0.37-2.28 mg-N·L^-1·d^-1) could be maintained under weak acidic, neutral and alkaline conditions (pH = 5-9). More than 80% of reduced nitrate was converted to N₂, and only a small amount was converted to NH₄⁺ or NO₂^-. The gel structure of MZVI@CS eliminated the agglomeration between MZVI particles while the forming of Fe-CS chelates reduced the formation of iron oxide and solved the problems of passivation, hence successfully strengthened the NO₃^--N removal efficiency of MZVI. Therefore MZVI@CS has great application potential in NO₃^--N deep removal of water bodies with neutral pH and low C/N ratios.

As and [Formula: see text] cooccurrence in drinking water: critical review of the international scenario, physicochemical behavior, removal technologies, health effects, and future trends

Drinking water contaminated with As and [Formula: see text] is increasingly prevalent worldwide. Their coexistence can have negative effects due to antagonistic or synergistic mechanisms, ranging from cosmetic problems, such as skin lesions and teeth staining, to more severe abnormalities, such as cancer and neurotoxicity. Available technologies for concurrent removal include electrocoagulation ~ adsorption > membranes > chemical coagulation > , and among others, all of which have limitations despite their advantages. Nevertheless, the existence of competing ions such as silicon > phosphate > calcium ~ magnesium > sulfate > and nitrate affects the elimination efficiency. Mexico is one of the countries that is affected by As and [Formula: see text] contamination. Because only 10 of the 32 states have adequate removal technologies, more than 65% of the country is impacted by co-presence problems. Numerous reviews have been published concerning the elimination of As or [Formula: see text]. However, only a few studies have focused on the simultaneous removal. This critical review analyzes the new sources of contamination, simultaneous physicochemical behaviors, available technologies for the elimination of both species, and future trends. This highlights the need to implement technologies that work with actual contaminated water instead of aqueous solutions (55% of the works reviewed correspond to aqueous solutions). Similarly, it is necessary to migrate to the creation of pilot, pre-pilot, or prototype scale projects, because 77% of the existing studies correspond to lab-scale research.

Portable Seawater Desalination System for Generating Drinkable Water in Remote Locations

A portable seawater desalination system would be highly desirable to solve water challenges in rural areas and disaster situations. While many reverse osmosis-based portable desalination systems are already available commercially, they are not adequate for providing reliable drinking water in remote locations due to the requirement of high-pressure pumping and repeated maintenance. We demonstrate a field-deployable desalination system with multistage electromembrane processes, composed of two-stage ion concentration polarization and one-stage electrodialysis, to convert brackish water and seawater to drinkable water. A data-driven predictive model is used to optimize the multistage configuration, and the model predictions show good agreement with the experimental results. The portable system desalinates brackish water and seawater (2.5-45 g/L) into drinkable water (defined by WHO guideline), with the energy consumptions of 0.4-4 (brackish water) and 15.6-26.6 W h/L (seawater), respectively. In addition, the process can also reduce suspended solids by at least a factor of 10 from the source water, resulting in crystal clear water (<1 NTU) even from the source water with turbidity higher than 30 NTU (i.e., cloudy seawater by the tide). We built a fully integrated prototype (controller, pumps, and battery) packaged into a portable unit (42 × 33.5 × 19 cm³, 9.25 kg, and 0.33 L/h production rate) controlled by a smartphone, tested for battery-powered field operation. The demonstrated portable desalination system is unprecedented in size, efficiency, and operational flexibility. Therefore, it could address unique water challenges in remote, resource-limited regions of the world.

Steric hindrance effect on the excited-state proton transfer process: TDDFT study on the fluorescent sensing mechanism of a fluoride sensor

An understanding of the excited-state process and the sensing mechanism for specific anions can be helpful for the design and synthesis of fluorescent sensors in analytical chemistry and biotechnology. Here, we theoretically investigated the fluorescent response mechanism of a reported acylhydrazone-based fluorescent sensor (Soft Matter, 2019, 15, 6690) for fluoride recognition using the time-dependent density functional theory approach. At the M06/TZVP/SCM level, the vertical excitation energies, which were calculated based on the ground state and first singlet-state geometries of the sensor molecule, agreed well with the experimental ultraviolet-visible and fluorescence spectra. Therefore, the time-dependent density functional theory method was considered reasonable and effective. According to the frontier orbital analysis and an excited-state potential energy scan, we proposed an excited-state proton transfer mechanism for the sensor-fluorine complex, where the steric hindrance leads to a high potential barrier. The excited-state proton transfer process facilitates sensor molecule deprotonation, alleviates its steric hindrance effect and expands its conjugated system. As a result, the fluorescence emission band of the sensor molecule was red-shifted significantly with the addition of fluoride anion. Based on this fluorescence difference, the sensor could be used for fluoride anion identification. This work provides a strategy to study sensor-analyte interactions in the excited state and offers an approach to tune the fluorescence emission wavelength of sensor molecules in anionic environments.

Enhanced arsenic removal using lateritic bauxite modified by heating and blending

The use of bauxite to remove arsenic from wastewater has been confirmed to be effective, but its removal efficiency in literature is not up to 90%, hence the need to devise a more effective method to remove arsenic from wastewater. In this study, a novel material was prepared by thermally modifying and blending yellow and red lateritic bauxite to form thermally modified and blended yellow and red lateritic bauxite (TYB + TRB). The adsorption isotherm, morphology and chemical composition of the novel material were determined by the Langmuir and Freundlich models, Scanning Electron Microscopy (SEM) and X-ray Fluorescence (XRF), respectively. Application of the novel material in arsenic removal resulted in >97% removal efficiency within 60 min. The arsenic adsorption by TYB + TRB conformed to the Freundlich model. The SEM image depicted a compacted earth material after use of the TYB + TRB to remove arsenic from the wastewater. The XRF results also showed a drastic reduction in the chemical composition of the novel bauxite except Ti, thus suggesting the occurrence of multiple mechanisms during the arsenic removal. This study demonstrated the potential of TYB + TRB to be developed and used as the most suitable material for wastewater treatment.

A review on recent advances in electrodeionization for various environmental applications

The growing contamination of ecosystems necessitates the development of long-term pollution-removal technologies. Electrodeionization, in notably, has newly proven as an efficient method for removing ionic chemicals from polluted waterways. The fact that continuous electrodeionization is a greener technique is most probably the biggest cause for its success. It replaces the toxic chemicals typically required to replenish resins with electric power, therefore eliminating the wastewater involved with resin renewal. In water treatment, electrodeionization solves some of the drawbacks of ion exchange resin beds, particularly ion dumping as beds expire. This comprehensive review explores the theory, principles, and mechanisms of ion movement and separation in an electrodeionization unit. Also, it investigated the construction and usage, notably in removing heavy metal and its current developments in electrodeionization unit. Recent advances in Electrodeionization like polarity reversal, Resin wafer Electrodeionization, membrane free Electrodeionization, and electrostatic shielding with novel materials and hybrid process along with Electrodeionization were addressed. Further advancements are expected in electrodeionization systems that exhibit better efficacy while running at lower costs due to decreased energy usage, rendering them appealing for industrial scale up across a wide range of applications across the world.

Synergistic removal of fluoride from groundwater by seed crystals and bacteria based on microbially induced calcium precipitation

A new hypothesis that seed crystals (SC) and bacteria based on microbially induced calcium precipitation (MICP) synergistically remove fluoride (F^-) from groundwater was proposed, with a focus on evaluating the defluoridation potential of this method and revealing its F^- removal mechanism. The crucial conditions were optimized to reduce preparation and operation costs. SC furnished more available binding sites due to the existence of bacteria, and the reuse experiments showed that the defluoridation efficiency of SC still remained a high level after 14 cycles (70.10%), with a residual F^- concentration of 0.96 mg L^-1. The SEM-EDS, FTIR and XRD analyses indicated the predominant F^- removal mechanism of SC could be ascribed to the chemisorption, ion exchange, and co-precipitation. Moreover, ion exchange and co-precipitation (PO₄^3- involvement) were validated more contributive than chemisorption (CaCO₃ and CaSO₄ involvement). As a feasible, reusable, and eco-friendly technique, SC suggests promising applications in the treatment of fluoride-contaminated groundwater.

Evaluation of a Smectite Adsorption-Based Electrostatic System to Decontaminate F− Rich Thermal Waters

Several studies have shown the presence of fluoride levels much higher than the 1.5 mg/L threshold concentration recommended by WHO in the spring waters and wells of the Ethiopian Rift Valley. Available defluoridation techniques can be costly, present complicated technical aspects, and show limited effectiveness. Therefore, it is necessary to devise innovative, sustainable, and effective solutions. This study proposes an alternative method of intervention to the known techniques for removing fluoride from water, particularly suitable for smaller rural communities. In particular, in this work, the possibility to use electromagnetic fields as a physical method for removing the excess fluoride was investigated. The study was carried out by developing a multiphysics model used for studying and envisaging the design of a device. In this framework, the combination of this approach with the use of highly reactive smectite clay was numerically studied. The results obtained, although preliminary, indicate that the proposed system could significantly impoverish the waters of the Rift Valley from fluoride, with the consequence of obtaining a resource suitable for human consumption, in particular for rural communities. However, further theoretical investigations and experimental phases will be necessary to achieve the desired results.

Effect of magnetic field on calcium - silica fouling and interactions in brackish water distribution systems

Fouling growth in brackish water distribution systems (BWDS), especially calcium-silica fouling, is inevitable issue in brackish water desalination, chemical and agricultural industry, eventually threaten the cleaner production process and environment. Magnetic Field (MF) has been a greener and effective technology to control calcium carbonate fouling. However, the effects of MF on composite calcium-silica fouling are still elusive. Therefore, this paper assessed the effect of MF on calcium and silica fouling. We found that MF not only significantly reduce the calcium carbonate fouling, but also obviously decreased the silica fouling. The MF reduced the calcite fouling reached 38.2%-64.3% by changing water quality parameters to trigger the transformation rate of CaCO₃ crystal from compact calcite to looser aragonite, as well as increase the unit-cell parameters and chemical bond lengths of calcite and aragonite. The MF also decreased the content of silica fouling (silica and silicate) reached 22.4-46.3% by reducing the concentration of soluble silica and accelerating the flocculation settlement to form large size solid particles in BW. Furthermore, MF broke the synergistic interactions among calcium and silica fouling. In addition, the anti-fouling ability of permanent MF was higher by 12.3-35.1% than electric MF. Overall, these findings demonstrate that MF is an effective and chemical-free technology to control calcium-silica fouling in BWDS, and provide a new perspective for sustainable application of brackish water.

Combining electro-bioremediation of nitrate in saline groundwater with concomitant chlorine production

Groundwater pollution and salinization have increased steadily over the years. As the balance between water demand and availability has reached a critical level in many world regions, a sustainable approach for the management (including recovery) of saline water resources has become essential. A 3-compartment cell configuration was tested for a new application based on the simultaneous denitrification and desalination of nitrate-contaminated saline groundwater and the recovery of value-added chemicals. The cells were initially operated in potentiostatic mode to promote autotrophic denitrification at the bio-cathode, and then switched to galvanostatic mode to improve the desalination of groundwater in the central compartment. The average nitrate removal rate achieved was 39±1 mgNO₃^--N L^-1 d^-1, and no intermediates (i.e., nitrite and nitrous oxide) were observed in the effluent. Groundwater salinity was considerably reduced (average chloride removal was 63±5%). Within a circular economy approach, part of the removed chloride was recovered in the anodic compartment and converted into chlorine, which reached a concentration of 26.8±3.4 mgCl₂ L^-1. The accumulated chlorine represents a value-added product, which could also be dosed for disinfection in water treatment plants. With this cell configuration, WHO and European legislation threshold limits for nitrate (11.3 mgNO₃^--N L^-1) and salinity (2.5 mS cm^-1) in drinking water were met, with low specific power consumptions (0.13±0.01 kWh g^-1NO₃^--N_removed). These results are promising and pave the ground for successfully developing a sustainable technology to tackle an urgent environmental issue.

Batch fluidized bed reactor based modified biosynthetic crystals: Optimization of adsorptive properties and application in fluoride removal from groundwater

A batch fluidized bed reactor (BFBR) with modified biosynthetic crystals (MBC), derived from Pseudomonas sp. HXF1, was investigated for the treatment of the groundwater containing fluoride (F^-). Impacts of different hydraulic retention time (HRT), pH, and initial F^- concentration on F^- removal were examined and the maximum defluorination efficiency was recorded as 95.20%. Moreover, recycling experiments were performed to evaluate the stability of repeated use. BFBR/MBC system showed a long-term effective treatment outcome with low fluctuation in the concentrations of residual Ca²⁺ and F^-. The formed precipitates were characterized by SEM, XPS, XRD, and FTIR. The defluorination mechanisms of BFBR/MBC system were defined as the chemisorption and induced crystallization of Ca₅(PO₄)₃F on the MBC surface. As a feasible, economical, and environment-friendly technique, the method has a long-term value, which suggests promising applications in F^- removal and resourceful treatment.

Denitrification potential of sodium alginate gel beads immobilized iron-carbon, Zoogloea sp. L2, and riboflavin: Performance optimization and mechanism

A kind of gel beads loaded with iron-carbon powder (Fe-C), Zoogloea sp. L2, and riboflavin (VB2) were prepared through cross-linking of sodium alginate (SA) to establish an immobilized bioreactor. The optimal ratio of SA beads was adjusted by orthogonal experiment. The change of oxidation-reduction potential (ORP) and the concentration of Fe²⁺ and Fe³⁺ showed that the addition of VB2 as a redox mediator can promote denitrification. Under the optimal conditions (carbon to nitrogen (C/N) ratio = 2.0, pH = 7.0, and hydraulic retention time (HRT) = 8 h), the nitrate removal efficiency (NRE) of bioreactor reached 98.48% (1.99 mg L^-1h^-1). Furthermore, Fourier transform infrared spectrometer (FTIR), Fluorescence excitation-emission matrix (EEM), X-ray diffraction (XRD), and gas chromatography (GC) analysis revealed that the immobilization and denitrification of the immobilized bioreactor were excellent. High throughput sequencing also showed that Zoogloea played a vital role in nitrate removal.

Investigation of kinetics and adsorption isotherm for fluoride removal from aqueous solutions using mesoporous cerium-aluminum binary oxide nanomaterials

Herein, we report the synthesis of Ce–Al (1 : 1, 1 : 3, 1 : 6, and 1 : 9) binary oxide nanoparticles by a simple co-precipitation method at room temperature to be applied for defluoridation of an aqueous solution. The characterization of the synthesized nanomaterial was performed by XRD (X-ray diffraction), FTIR (Fourier transform infrared) spectroscopy, TGA/DTA (thermogravimetric analysis/differential thermal analysis), BET (Brunauer–Emmett–Teller) surface analysis, and SEM (scanning electron microscopy). Ce–Al binary oxides in 1 : 6 molar concentration were found to have the highest surface area of 110.32 m² g⁻¹ with an average crystallite size of 4.7 nm, which showed excellent defluoridation capacity. The adsorptive capacity of the prepared material towards fluoride removal was investigated under a range of experimental conditions such as dosage of adsorbents, pH, and initial fluoride concentration along with adsorption isotherms and adsorption kinetics. The results indicated that fluoride adsorption on cerium–aluminum binary metal oxide nanoparticles occurred within one hour, with maximum adsorption occurring at pH 2.4. The experimental data obtained were studied using Langmuir, Freundlich, and Temkin adsorption isotherm models. The nanomaterial showed an exceptionally high adsorbent capacity of 384.6 mg g⁻¹. Time-dependent kinetic studies were carried out to establish the mechanism of the adsorption process by pseudo-first-order kinetics, pseudo-second-order kinetics, and Weber–Morris intraparticle diffusion kinetic models. The results indicated that adsorption processes followed pseudo-second-order kinetics. This study suggests that cerium–aluminum binary oxide nanoparticles have good potential for fluoride removal from highly contaminated aqueous solutions.

Mesoporous Ce–Al binary oxide nanomaterials prepared with a surface area of 110.32 m² g⁻¹ showed defluoridation capacity at pH 2.4, exhibited maximum adsorption capacity of 384.6 mg g⁻¹ and a removal efficiency of 91.5% at a small dose of nanoadsorbent.

Microbially induced calcium precipitation based simultaneous removal of fluoride, nitrate, and calcium by Pseudomonas sp. WZ39: Mechanisms and nucleation pathways

A simultaneous denitrifying and mineralizing bacterium, Pseudomonas sp. WZ39 was isolated for fluoride (F^-), nitrate (NO₃^--N), and calcium (Ca²⁺) removal. Strain WZ39 exhibited a remarkable defluoridation efficiency of 87.49% under a pH of 6.90, F^- and Ca²⁺ concentration of 1.99 and 201.88 mg L^-1, respectively. EEM, SEM-EDS, XRD, and FTIR analyses elucidated the chemical adsorption and co-precipitation with calcium salt contributed to the removal of F^-. The mechanisms of biomineralization were also investigated by determining the role of bound and unbound extracellular polymeric substances (EPS), cell wall, and calcium channel in nucleation. The results showed that bacteria can promote nucleation on the templates of cell walls or EPS through the electrostatic effect. The presence of the calcium channel blocker inhibited the transport of intracellular Ca²⁺ to the extracellular environment. The outcome of the present research can provide a theoretical basis for the understanding of MICP phenomenon and the efficient treatment of F^- containing groundwater.

Simultaneous removal of calcium, fluoride, nickel, and nitrate using microbial induced calcium precipitation in a biological immobilization reactor

In this research, an immobilized biofilm reactor was established for the simultaneous removal of calcium (Ca²⁺), fluoride (F^-), nickel (Ni²⁺), and nitrate (NO₃^--N) by microbial induced calcium precipitation (MICP). The operating parameters of the reactor, hydraulic retention time (HRT: 4, 8, and 12 h), influent Ca²⁺ concentration (36.0, 108.0, and 180.0 mg L^-1), and influent Ni²⁺ concentration (0.0, 3.0, and 6.0 mg L^-1) were discussed. Under the HRT of 12 h, influent Ca²⁺ concentration of 180.0 mg L^-1, and influent Ni²⁺ concentration of 3.0 mg L^-1, the removal ratios of Ca²⁺, F^-, Ni²⁺, and NO₃^--N reached 45.31%, 79.55%, 85.11%, and 55.29%, respectively, which was the reactor stable operation performance. The SEM revealed the morphology of calcium-precipitated bio-crystals. XPS showed the Ca²⁺ and Ni²⁺ precipitate components and XRD further revealed the formation of CaCO₃, Ca₅(PO₄)₃OH, and NiCO₃ precipitation. Nitrogen (N₂) was the main gas produced in the reactor. Fluorescence spectroscopy manifested that extracellular polymers played an important role in the organism nucleation. High-throughput sequencing exhibited that Acinetobacter sp. H12 was the dominant bacterial group. This study provided a new insight for simultaneous remediation of Ca²⁺, F^-, Ni²⁺, and NO₃^--N in water bodies.

Sustainable disposal of seawater brine by novel hybrid electrodialysis system: Fine utilization of mixed salts

Sustainable seawater brine treatment demands an essential paradigm shift for effective recovery of resources and high value utilization of mixed-salts. Here, a novel hybrid electrodialysis (ED) system was proposed that integrated an innovative hybrid selective ED (HSED) and a developed selective bipolar membrane ED (SBMED). The HSED process allowed simultaneous recovery of major divalent cations and anions from seawater brine when NaCl was selectively enriched. Then, the impure NaCl-rich stream was fed directly into the SBMED process for acid/base preparation without any purification pretreatment. Detailed analysis of the HSED process showed that increasing unit voltage from 2.33 V to 2.67 V would improve the removal ratio of Ca²⁺, Mg²⁺ and SO₄^2- from 54.7%, 41.4% and 13.3% to 78.9%, 76.6% and 32.1%, respectively. In addition, the increment of initial concentration of product streams promoted the transport of various ions from the feed and middle compartments. The fine utilization performance, in terms of ionic removal ratio and fractionation ratio of divalent ions in the HSED process, was more limited by the initial concentration of product streams. Furthermore, the SBMED stack was found to have nearly identical performance over five cycles, indicating that the presence of a trace amount of hardness cations did not induce scaling. The current study thus provided a novel suitable strategy with a perspective of fine utilization for practical applications in sustainable disposal of seawater brine.