Factors influencing the removal of fluoride from aqueous solution by calcined Mg-Al-CO3 layered double hydroxides

Targeted recognition and enhanced biotransformation of phytochemicals by a dual-functional cellulose-based hydrogel bioreactor

The combined technology of molecular imprinting and immobilization is a promising strategy for improving biocatalytic performance. In this work, a dual-functional cellulose-based hydrogel bioreactor with targeted recognition and transformation functions was innovatively designed by cellulose-based molecularly imprinted polymers (CM-MIPs) hydrogel coupled with layered double hydroxide immobilized enzyme (LDH-E). Firstly, CM-MIPs hydrogel was prepared by using phlorizin, 4-pinylpyridine, and cellulose microspheres as template molecule, functional monomer, and support material, respectively. Meanwhile, layered double hydroxide (LDH) taken as a carrier to immobilize β-glucosidase. The prepared LDH was layered sheet-like structure with ultra-thin thickness of approximately only 1.5 nm, and β-glucosidase was immobilized on both sides of the LDH. Further, the dual-functional bioreactor was constructed by the anion exchange, which possessed maximum targeted adsorption capacity to phlorizin of 15.25 mg/g, exhibited 1.2 folds higher transformation efficiency than LDH-E, and the phloretin content increased 26 folds to that in Lithocarpus litseifolius (Hance) Chu leaves extracts. Moreover, the transformation efficiency remained above 70 % even after five consecutive transformations. Overall, the dual-functional bioreactor has broad prospects for the application in targeted recognition and transformation of phytochemicals, and provides a new insight on multifunctional bio-based reactors in natural production field.

Remediation of arsenic and fluoride from groundwater: a critical review on bioadsorption, mechanism, future application, and challenges for water purification

In the past few decades, the excessive and inadequate use of technological advances has led to groundwater contamination, mainly caused by organic and inorganic pollutants, which are highly harmful to human health, agriculture, water bodies, and aquaculture. Among all toxic pollutants, As and F- play a significant role in groundwater contamination due to their excellent reactivity with other elements. To mitigate the prevalence of arsenic and fluoride within the water system, the use of biochar gives an attractive strategy for removing them mainly because of the substantial surface area, pore size, pH, aromatic structure, and functional groups inherent in biochar, which are primarily dependent upon its raw material and pyrolysis temperature. Researcher develops different methods like physiochemical and electrochemical for treating arsenic and fluoride contamination. Among all removal methods, bioadsorption using agricultural waste residues shows effective/feasible removal of As and F- due to its low cost, ecofriendly nature, readily available, and efficient reuse compared with several other harmful synthetic materials that demand costly design specifications. This study discusses current developments in bioadsorption methods for As and F- that use agricultural-based biomaterials and describes the prevailing state of arsenic and fluoride removal strategies that use biomaterials precisely.

Hierarchical Mg/Al hydrotalcite oxide hollow microspheres with excellent adsorption capability towards Congo red

A novel citrate anion-intercalated Mg/Al layered double hydroxide (CA-LDH) is synthesized via a one-step hydrothermal process. The synthesized CA-LDH is a hollow flower-like microsphere composed of thin nanoflakes (10 nm in thickness). After calcination, the formed Mg/Al layered double oxide (CA-LDO) hollow microspheres possess a high specific surface area of 247.8 m2 g-1 and a high pore volume of 0.97 cm3 g-1, which endow them with excellent adsorption ability towards Congo red (CR). The maximum adsorption capacity of CR onto CA-LDO can reach up to 1883 mg g-1. The significantly improved adsorption capacity of CA-LDO can be attributed to its unique structures of hierarchical hollow microspheres, in which the hierarchical porous shell layer provides enough adsorption sites to anchor the dye molecules, and the hollow core can preserve the absorbed dye. This study provides a promising novel adsorbent which can be used for efficient water remediation.

FeCa-based layered double hydroxide, a high-performance phosphorus adsorbent in constructed wetlands and ecological dams - A pilot scale study

Research studies have modified traditional substances to seek fast-acting removal of phosphorus in constructed wetlands (CWs) and ecological dams, rather than develop a brand-new nano-adsorbent. This work synthesized FeCa-based layered double hydroxide (FeCa-LDH) with a chemical co-precipitation method, and the performance, mechanism and factors of phosphorus removal were investigated. FeCa-LDH showed a marked ability to adsorb phosphorus from waste water, with a removal efficiency of 94.4% and 98.2% in CWs and ecological dams, respectively. Both FTIR and XPS spectrum evidenced that FeCa-LDH removed phosphorus via electrostatic and hydrogen-bonding adsorption, as well as a coordination reaction and interlayer anion exchange. FeCa-LDH showed a higher capacity to remove phosphorus in alkaline and neutral waste water than in acid conditions. Co-occurrence anions, which influenced the efficiency of the phosphorus removal capacity are considered in the sequence below: CO32- ≈ HCO3- > SO42- > NO3-. Innovatively, FeCa-LDH was not affected by the low-temperature limitation for CWs, and phosphorus removal efficiency at 5 °C was almost equal to that at 25 °C. These results cast a new idea on the construction, application and phosphorus removal performance of CWs and ecological dams.

Structured electroplating sludge derived membrane for one-step removal of oil, metal ions, and anions from oil/water emulsions

The effective simultaneous treatment of hazardous waste sludge and complex oil/water emulsions in one way is urgently desired but still a challenging issue. Herein, this work for the first time presents a green and efficient strategy to fabricate an electroplating sludge (ES) derived multifunctional self-supporting membrane for the one-step removal of emulsified oils, soluble metal ions, and anions in complex oily wastewater. Due to low cost of ES and sustainability of the solvent selected in fabrication process, the large-scale application of the membrane is easily to promote. The assembled hierarchical nanostructure endowed robust underwater superoleophobicity of the membrane even under various corrosive aqueous environments, as well as excellent ultra-low oil adhesion and anti-oil-fouling performance, without chemical modification. Significantly, the multifunctional membrane possessed desirable simultaneous separation efficiency for five typical oil-in-water emulsions (>99.4%, high oil/water selective wettability), including crude oil-in-water emulsion with high viscosity (>99.6%), Cu²⁺ (>96.1%, surface complexation and ionic exchange), and Cl^- (>92.7%, electrostatic attraction). Therefore, this green, low-cost, and multifunctional membrane not only allows the large-scale resource utilization of hazardous waste sludge, but also effectively solves the problems of complex oily wastewater purification.

Synthesis of stable flowerlike MgAl-LDH@MIL-88A and its adsorption performance for fluoride

MgAl-LDH@MIL-88A as an effective adsorbent was successfully prepared by a simple stirring method in water bath through loading MIL-88A onto the surface of flowerlike MgAl-LDH, which was synthesized via solvothermal method. Interestingly, the results of characterizations showed that the MIL-88A could still grow, but extrude the brucite-like layers of MgAl-LDH. The influences of initial solution pH, contact time, temperature, and co-existing ions on the adsorption performance of MgAl-LDH@MIL-88A were studied systematically by batch static adsorption experiments. It was found that MgAl-LDH@MIL-88A represented the highest adsorption loading of fluoride (14.00 mg g−1) at initial pH 7.0 in 420 min. The uptake process was described appropriately by the pseudo-second-order, the Temkin and the Freundlich isotherm models. The thermodynamic parameters confirmed the endothermic and spontaneous nature of adsorption. MgAl-LDH@MIL-88A was the green adsorbent as the residual mental contents ([Mg2+] = 1.095 mg L−1, [Fe3+] = 0.007 mg L−1, [Al3+] = 0.076 mg L−1) after adsorption met the Chinese sanitary standard for drinking water (GB 5749-2006). The mechanism of fluoride removal by MgAl-LDH@MIL-88A involved the electrostatic interactions between Fe3+ of MIL-88A and fluoride, and ligand exchange among hydroxyl groups of MgAl-LDH, carboxylate groups of the C4H4O4 and fluoride.

Rapid and enhanced adsorptive mitigation of groundwater fluoride by Mg(OH)2 nanoflakes

Fluoride is one of the most abundant anions in groundwater, posing a significant threat to the safe drinking water supply worldwide. Fluoride contamination in drinking water at levels greater than 1.5 mg L^-1 causes a variety of serious health problems. To address this problem, the current study deals with the synthesis of Mg(OH)₂ nanoflakes by a facile and simple hydrothermal method in the absence of any added template. The sizes of these nanoflakes are in the range of 90 to 200 nm. These nanoflakes are pure and crystalline, possessing hexagonal phase structures. The surface areas of Mg(OH)₂ nanoflakes are varying from 75.8 to 108.1 m² g^-1. These Mg(OH)₂ nanoflakes exhibit excellent adsorption performance for fluoride over a pH range of 2.0 to 9.0 with a maximum Langmuir adsorption capacity of 3129 mg g^-1 at pH 7.0 at 313 K which is the highest for such kind of adsorbent reported so far. The adsorption process is spontaneous and endothermic which primarily follows pseudo-second-order kinetics. The adsorbent is effective in the presence of co-existing anions and is reusable up to the fifth cycle with a minimal loss of adsorption performance. The nanoflakes can effectively remove highly concentrated groundwater fluoride to a permissible limit within a short time which increases the versatility of using these nanoflakes for practical applications.

Molecular scale assessment of defluoridation of coal-mining wastewater by calcined Mg/Al layered double hydroxide using 19F solid-state NMR, XPS, and HRTEM

Calcination is an effective way to improve the F^- adsorption capacity of layered double hydroxide (LDH) materials, however, a molecular scale understanding of the enhanced defluoridation capability of calcined LDHs (CLDH) is lacking. This study investigated the mechanisms of F^- adsorption by CLDH using ¹⁹F solid-state NMR, X-ray photoelectron spectroscopy (XPS), and high-resolution TEM. Under calcination process, LDH underwent three periods: surface dehydration below 200 °C, structural dehydroxylation at 200-400 °C, and release of interlayer carbonate groups above 400 °C. Additionally, XPS and XRD characterization showed that CLDH could not recover to the original structural symmetry even after rehydration and reconstitution. The F^- affinity was greatly enhanced for the calcined LDH, especially at high pH. At pH 10, the adsorption capacity could reach 22.0 mg F^-/g for CLDH (500 °C calcined), about 6 times larger than that of LDH. The XRD analyses revealed that the F-adsorbed CLDH had a poorer crystalline degree as the calcination temperature increased, consistent with the TEM observation of abundant defects and Mg/Al oxides on the CLDH sheets. ¹⁹F solid-state NMR spectra of the CLDH after F^- adsorption showed that the formation of surface Al-F is the predominant F^- adsorption mode at pH 7, whereas the Mg-F local coordination mode is the pronounced F^- adsorption mechanism under alkaline conditions (pH 10). The present study provided a comprehensive understanding of CLDH in F^- adsorption and suggested that calcination is a promising treatment for promoting the efficacy of polluted anion scavenging.

Removal of Fluoride from Phosphogypsum Leaching Solution with Phosphate Tailing Based Layered Double Hydroxides: Kinetics and Equilibrium Isotherms

In this work, ternary and quaternary layered double oxides (PTB-LDO3 and PTB-LDO4) based on phosphate tailings were synthesized by the coprecipitation method. The as-prepared samples were characterized and applied to remove fluorine ions from a phosphogypsum leaching solution. The results indicated that both the precursor of PTB-LDO3 and PTB-LDO4 showed a layered structure with characteristic diffraction peaks of hydrotalcite. Compared with PTB-LDO4, PTB-LDO3 exhibited better adsorption performance at pH 5–6 and a dosage of 0.04 mg L−1. The adsorption kinetics results revealed that the adsorption of fluorine by PTB-LDO3 and PTB-LDO4 reached the adsorption equilibrium in about 3 h, and followed the pseudo-second-order model. The adsorption data could be fitted better with the Langmuir isotherm with the maximum adsorption amounts of 26.03 mg g−1 and 15.66 mg g−1 for PTB-LDO3 and PTB-LDO4, respectively. The adsorption of fluorine by PTB-LDO3 and PTB-LDO4 were both spontaneous and exothermic, and exhibited excellent reusability and stability. This study provides a possibility for the combined treatment of phosphorus chemical solid waste (phosphorus tailings) and phosphorus chemical wastewater (phosphogypsum leaching liquid).

Two-dimensional ultrathin metal-based nanosheets for photocatalytic CO2 conversion to solar fuels

Artificially simulated photosynthesis has created substantial curiosity as the majority of efforts in this arena have been aimed to upsurge solar fuel efficiencies for commercialization. The layered inorganic 2D nanosheets offer considerably higher tunability of their chemical surface, physicochemical properties and catalytic activity. Despites the intrinsic advantages of such metal-based materials viz., metal oxides, transition metal dichalcogenides, metal oxyhalides, metal organic frameworks, layered double hydroxide, MXene's, boron nitride, black phosphorous and perovskites, studies on such systems are limited for applications in photocatalytic CO₂ reduction. The role of metal-based layers for CO₂ conversion and new strategies such as surface modifications, defect generation and heterojunctions to optimize their functionalities are discussed in this review. Research prospects and technical challenges for future developments of layered 2D metal-based nanomaterials are critically discussed.

Optimization, equilibrium, adsorption behaviour of Cu/Zn/Fe LDH and LDHBC composites towards atrazine reclamation in an aqueous environment

Cu-Zn-Fe Layered double hydroxides (LDH) and LDH dispersed on bamboo biochar (LDHBC) was used to study the adsorption of Atrazine by characterizing the adsorption kinetics, isotherms and response surface methodology (RSM) to reveal interactive effects of pH, adsorbent dosage and adsorbate initial concentration towards LDH optimum performance. The estimate of parameters determined for Langmuir isotherm quantities were in the range (21.84-37.91 mg/g) for LDH and (63.64-87.04 mg/g) for LDHBC. Regeneration and reusability after five cycles detected that the adsorption efficiencies of the adsorbents were reduced to 36% for LDH and 66% for LDHBC. Box Behnken design analysis could further reveal optimized conditions for higher Atrazine removal by LDH up to 74.8%. The adsorption mechanisms could be determined by π-π interactions occurring at the interfaces by hydrogen bonding and pore filling effects.

Optimization of fluoride adsorption from aqueous solution over mesoporous titania-alumina composites using Taguchi method

The optimization of fluoride removal from aqueous media was studied over the mesoporous titania-alumina composites using Taguchi method-based L25 orthogonal array experimental design. The chemical structure, surface chemistry, and morphology of as-prepared composite adsorbents were studied utilizing various analytical methods. The findings of the characterization demonstrated that the produced composites have high textural qualities, which are conducive to enhanced fluoride adsorption. The optimum conditions for maximum percentage removal of fluoride from aqueous solution were found as adsorbent type as TA75, adsorbent dose 4 g L-1 , initial concentration of fluoride 40 ppm, solution pH 3 with a treatment time of 60 min. Under the optimum conditions, 98% of fluoride adsorption was achieved. Analysis of variance revealed that the solution pH followed by the adsorbent dose was the most significant for fluoride adsorption. The Langmuir model and pseudo-second-order kinetic model fit the adsorption data well, and the TA75 adsorbent had a maximum Langmuir fluoride adsorption capacity of 34.48 mg g-1 at pH = 3. The thermodynamic information suggests that the adsorption was spontaneous and endothermic under the given operating conditions. The synergic combination of Ti-Al nanoparticles demonstrated a high percentage removal of fluoride under the optimized operating conditions. PRACTITIONER POINTS: The Taguchi method-based design of the experimental approach was implemented in the fluoride adsorption process. Mesoporous titania-alumina composites with 0 to 100 wt.% of alumina in titania were prepared and applied to remove fluoride from an aqueous solution. Solution pH was the most influential parameter for the fluoride adsorption process, while the synergistic combination of 75 wt.% alumina in titania showed the maximum adsorption capacity.

Enhanced Fluoride Uptake by Layered Double Hydroxides under Alkaline Conditions: Solid-State NMR Evidence of the Role of Surface >MgOH Sites

Layered double hydroxides (LDHs) are potential low-cost filter materials for use in fluoride removal from drinking water, but molecular-scale defluoridation mechanisms are lacking. In this research, we employed ¹⁹F solid-state NMR spectroscopy to identify fluoride sorption products on 2:1 MgAl LDH and to reveal the relationship between fluoride sorption and the LDH structure. A set of six ¹⁹F NMR peaks centered at -140, -148, -156, -163, -176, and -183 ppm was resolved. Combining quantum chemical calculations based on density function theory (DFT) and ¹⁹F{²⁷Al} transfer of populations in double resonance (TRAPDOR) analysis, we could assign the peaks at -140, -148, -156, and -163 ppm to Al-F (F coordinated to surface Al) and those at -176 and -183 ppm to Mg-F (F coordinated to surface Mg only). Interestingly, the spectroscopic data reveal that the formation of Al-F is the predominant mode of F^- sorption at low pH, whereas the formation of Mg-F is predominant at high pH (or a higher Mg/Al ratio). This finding supports the fact that the F^- uptake of 2:1 MgAl LDH was nearly six times that of activated alumina at pH 9. Overall, we explicitly revealed the different roles of the surface >MgOH and >AlOH sites of LDHs in defluoridation, which explained why the use of classic activated alumina for defluoridation is limited at high pH. The findings from this research may also provide new insights into material screening for potential filters for F^- removal under alkaline conditions.

Capacitive Removal of Fluoride Ions via Creating Multiple Capture Sites in a Modulatory Heterostructure

Fluoride pollution has become a major concern because of its adverse effects on human health. However, the removal capacity of defluorination agents in traditional methods is far from satisfactory. Herein, capacitive removal of F^- ions via creating multiple capture sites in a modulatory heterostructure has been originally demonstrated. The heterostructure of uniformly dispersed Al₂O₃ coating on hollow porous nitrogen-doped carbon frameworks was precisely synthesized by atomic layer deposition. An exceptional F^- ion removal efficiency at 1.2 V (95.8 and 92.9% in 5 and 10 mg/L F^- solutions, respectively) could be finally achieved, with a good regeneration ability after 20 consecutive defluorination cycles. Furthermore, we investigated the removal mechanisms of F^- ions by in situ Raman, in situ X-ray diffraction, and ex situ X-ray photoelectron spectroscopy measurements. The promotional removal capacity was realized by the multiple capture sites of the reversible conversion of Al-F species and the insertion of F^- ions into the carbon skeleton. This work offers an important new pathway and deep understanding for efficient removal of F^- ions from wastewater.

Fluoride removal by thermally treated egg shells with high adsorption capacity, low cost, and easy acquisition

In this study, the use of eggshells was suggested as an adsorbent for fluoride removal, and their mechanism of fluoride removal was investigated. The eggshells underwent thermal treatment to improve their adsorption capacity; 800 °C was found to be the optimal temperature for treatment. Eggshells thermally treated at 800 °C (ES-800) were mainly composed of Ca (82.4%) and C (15.9%), and the peaks of ES-800 obtained from X-ray diffraction (XRD) corresponded to calcite, portlandite, and lime. Fluorine adsorption by ES-800 reached 70% of the equilibrium adsorption amount within 15 min and gradually increased until 24 h. The maximum adsorption capacity of ES-800 at pH 7 and 25 °C was 258.28 mg/g, which is 18 times larger than that of activated alumina; this is classified as the best available technology by the United States Environmental Protection Agency. Both enthalpy and entropy increased in the process of fluoride adsorption onto ES-800. Fluoride adsorption of ES-800 decreased from 59.16 to 11.85 mg/g with an increase in pH from 3 to 11. Fluoride adsorption decreased in the presence of anions, whose impact follows the order: HPO43- > HCO3- >> SO42- > Cl-. XRD, and X-ray photoelectron spectroscopy analysis revealed that fluoride removal was achieved by the formation of calcium fluorite (CaF2). Thus, it can be concluded that eggshells can function as highly efficient adsorbents for fluoride removal, replacing bone char and activated alumina; further, their adsorption capacity can be improved by thermal treatment.

Layered double hydroxides and LDH-derived materials in chosen environmental applications: a review

With increasing global warming awareness, layered double hydroxides (LDHs), hydrotalcites, and their related materials are key components to reduce the environmental impact of human activities. Such materials can be synthesized quickly with high efficiency by using different synthesis processes. Moreover, their properties' tunability is appreciated in various industrial processes. Regarding physical and structural properties, such materials can be applied in environmental applications such as the adsorption of atmospheric and aqueous pollutants, hydrogen production, or the formation of 5-hydroxymethylfurfural (5-HMF). After the first part that was dedicated to the synthesis processes of hydrotalcites, the present review reports on specific environmental applications chosen as examples in various fields (green chemistry and depollution) that have gained increasing interest in the last decades, enlightening the links between structural properties, synthesis route, and application using lamellar materials.

Phase transition of Mg/Al-flocs to Mg/Al-layered double hydroxides during flocculation and polystyrene nanoplastics removal

Nanoplastics, a kind of emerging pollutant in natural environments, have now drawn tremendous attention worldwide. Flocculation with Mg/Al-layered double hydroxides (LDH) precursor solutions has showed great potential for removing negatively charged nanoparticles from water. In this study, the flocculation behavior and mechanism for the removal of polystyrene nanoplastics (PSNP) with Mg/Al flocs or Mg/Al LDH were systematically analyzed and investigated. During the process of flocculation, it was observed that in situ Mg/Al LDH can be gradually formed with increasing pH, in addition, PSNP were captured or attached to the surface of LDH with a turning point around pH of 5.0. In acidic solutions with pH < 5.0, the negative surface charges of PSNP were diminished mainly due to the high concentrations of hydrogen ions and the positive charges from Mg and Al ions. In a moderately alkaline solution, Mg and Al ions gradually formed crystals capturing PSNP. Electrostatic adsorption and intermolecular force are the main mechanisms via which PSNP are captured on Mg/Al flocs. Herein, PSNP removal efficiencies from water were more than 90.0%. As the problem of plastic pollution becomes more severe, in situ LDH growth flocculation can provide an efficient way for the removal of PSNP.

Application of pyridine-modified chitosan derivative for simultaneous adsorption of Cu(II) and oxyanions of Cr(VI) from aqueous solution

The bioadsorbent C1, which is a chitosan derivative prepared in a one-step synthesis, was successfully used to adsorb Cr(VI) and Cu(II) simultaneously. Here, for the first time the simultaneous adsorption of a cation and an anion was modeled using the Corsel model for kinetics and the Real Adsorbed Solution Theory model for equilibrium data. Batch studies of the adsorption of Cu(II) and Cr(VI) in single and binary aqueous solutions were performed as a function of initial solute concentration, contact time, and solution pH. The maximum adsorption capacities of C1 in single and binary aqueous solutions were 1.84 and 1.13 mmol g^-1 for Cu(II) and 3.86 and 0.98 mmol g^-1 for Cr(VI), respectively. The reuse of C1 was investigated, with Cu(II) ions being almost completely desorbed and fully re-adsorbed. For Cr(VI), the desorption was incomplete resulting in a lower re-adsorption. Energy-dispersive X-ray spectroscopy was used for mapping the distributions of Cr(VI) and Cu(II) adsorbed on the C1 surface in single and binary adsorption systems. Isothermal titration calorimetry experiments were performed for Cr(VI) and Cu(II) adsorption in single solutions. The thermodynamic parameters of adsorption showed that the adsorption of both metal ions was enthalpically driven, but entropically unfavorable.

Controlled preparation of cerium oxide loaded slag-based geopolymer microspheres (CeO2@SGMs) for the adsorptive removal and solidification of F- from acidic waste-water

A new cerium oxide loaded slag-based geopolymer microspheres (CeO₂@SGMs) was prepared by a two-step i.e. dispersion-suspension-solidification and in-situ co-precipitation method. The optimal parameters for the preparation of 0.02CeO₂@SGMs were slag (30 g), 1.7 M water glass (12.86 g), water (8 g) and 0.02 mol/L of Ce⁴⁺. 0.02CeO₂@SGMs was characterized by SEM, XRD, BET, EDX, FTIR, XPS and PSD techniques. The leaching concentration of Ca²⁺ (95.65 mg/L) was only 1/5 of the SGMs at pH 2 after the modification of CeO₂. Adsorption data fitted well with Freundlich isotherm model suggesting multilayer adsorption mechanism with a maximum adsorption capacity for F^- by 0.02CeO₂@SGMs of 121.77 mg/g at 298 K. The negative values of thermodynamic parameters (ΔH⁰ and ΔS⁰) indicated the exothermic nature of the adsorption process with reduced chaos of the whole system. 0.02CeO₂@SGMs exhibited excellent dynamic adsorption performance at 4 mL/min F^- solution flow rate. The influence of various co-existing anions on adsorption of F^- over 0.02CeO₂@SGMs followed an order of: Cl^- ≈ NO₃^- < SO₄^2- << PO₄^3-. Attributed to the facile preparation process, cost-effectiveness and environmental friendliness, the newly designed 0.02CeO₂@SGMs can be deemed of promising industrial applications for the abatement of F^- from wastewater.

Towards the Continuous Hydrothermal Synthesis of ZnO@Mg2Al-CO3 Core-Shell Composite Nanomaterials

Core-shell Zinc Oxide/Layered Double Hydroxide (ZnO@LDH) composite nanomaterials have been produced by a one-step continuous hydrothermal synthesis process, in an attempt to further enhance the application potential of layered double hydroxide (LDH) nanomaterials. The synthesis involves two hydrothermal reactors in series with the first producing a ZnO core and the second producing the Mg₂Al-CO₃ shell. Crystal domain length of single phase ZnO and composite ZnO was 25 nm and 42 nm, respectively. The ZnO@LDH composite had a specific surface area of 76 m² g⁻¹, which was larger than ZnO or Mg₂Al-CO₃ when produced separately (53 m² g⁻¹ and 58 m² g⁻¹, respectively). The increased specific surface area is attributed to the structural arrangement of the Mg₂Al-CO₃ in the composite. Platelets are envisaged to nucleate on the core and grow outwards, thus reducing the face–face stacking that occurs in conventional Mg₂Al-CO₃ synthesis. The Mg/Al ratio in the single phase LDH was close to the theoretical ratio of 2, but the Mg/Al ratio in the composite was 1.27 due to the formation of Zn₂Al-CO₃ LDH from residual Zn²⁺ ions. NaOH concentration was also found to influence Mg/Al ratio, with lower NaOH resulting in a lower Mg/Al ratio. NaOH concentration also affected morphology and specific surface area, with reduced NaOH content in the second reaction stage causing a dramatic increase in specific surface area (> 250 m² g⁻¹). The formation of a core-shell composite material was achieved through continuous synthesis; however, the final product was not entirely ZnO@Mg₂Al-CO₃. The product contained a mixture of ZnO, Mg₂Al-CO₃, Zn₂Al-CO₃, and the composite material. Whilst further optimisation is required in order to remove other crystalline impurities from the synthesis, this research acts as a stepping stone towards the formation of composite materials via a one-step continuous synthesis.

LDH-Co-Fe-Acetate: A New Efficient Sorbent for Azoic Dye Removal and Elaboration by Hydrolysis in Polyol, Characterization, Adsorption, and Anionic Exchange of Direct Red 2 as a Model Anionic Dye

A new, double hydroxide based on Co and Fe was elaborated on by forced hydrolysis in a polyol medium. Complementary characterization techniques show that this new phase belongs to the layered double hydroxide family (LDH) with Co2+ and Fe3+ ions located in the octahedral sites of the bucite-like structure. The acetate anions occupy interlayer space with an interlamellar distance of 12.70 Å. This large distance likely facilitates the exchange reaction. Acetates were exchanged by carbonates. The as-obtained compound Co-Fe-Ac/Ex shows an interlamellar distance of 7.67 Å. The adsorption of direct red 2 by Co-Fe-Ac-LDH has been examined in order to measure the capability of this new LDH to eliminate highly toxic azoic anionic dyes from waste water and was compared with that of Co-Fe-Ac/Ex and Co-Fe-CO3/A (synthesized in an aqueous medium). The adsorption capacity was found to depend on contact time, pH, initial dye concentration, and heating temperature. Concerning CoFeAc-LDH, the dye uptake reaches a high level (588 mg/g) due to the occurrence of both adsorption processes: physisorption on the external surface and chemical sorption due to the intercalation of dye by exchange with an acetate anion. The study enables us to quantify the uptake amount of each effect in which the intercalation has the most important amount (418 mg/g).

Removal and recovery of phosphate and fluoride from water with reusable mesoporous Fe3O4@mSiO2@mLDH composites as sorbents

Three core/shell/shell MgAl-LDH composites using Fe₃O₄ microspheres as the core, a SiO₂ matrix as the inner layer and a MgAl-LDH layer as the outer shell have been synthesized for the removal and recovery of phosphate and fluoride from water by a magnetic separation technique. The synthetic mesoporous MgAl-LDH composites show good magnetic separability, well-defined pore distributions, and have specific surface areas of 73 m² g^-1, 168 m² g^-1, and 137 m² g^-1 for Fe₃O₄@SiO₂@LDH350, Fe₃O₄@SiO₂@mLDH350, and Fe₃O₄@mSiO₂@mLDH350, respectively. The adsorption isotherms of both phosphate and fluoride on these MgAl-LDH composites can be well fitted with the Langmuir model. The maximum adsorption capacities of 57.07 mg g^-1 and 28.51 mg g^-1 were obtained on Fe₃O₄@mSiO₂@mLDH350 for phosphate and fluoride, respectively, much higher than those of other LDH-type materials. The adsorbed phosphate and fluoride could be successfully recovered by NaNO₃-NaOH solution, and the regenerated MgAl-LDH composites could be reused for phosphate and fluoride removal. Owing to their high adsorption capacities of both phosphate and fluoride, easy magnetic separation from solution, and good reusability, the mesoporous MgAl-LDH composites are expected to have potential applications in removal or recovery of fluoride or phosphate from water.

A simple chemical method for the synthesis of Cu2+ engrafted MgAl2O4 nanoparticles: Efficient fluoride adsorbents, photocatalyst and latent fingerprint detection

An adaptable, energy efficient chemical process is employed to synthesize Cu²⁺ engrafted MgAl₂O₄ nanoparticles (Mg_1-xCu_xAl₂O₄, x=0, 0.1, 0.3, 0.5 abbreviated as MCA0, MCA1, MCA3, and MCA5 respectively), using chelating ligand and the calcination temperature was determined by the thermogravimetric analysis of the precursor mass. They acted as good fluoride adsorbent in the presence of co-ions, different pH (2-11) via chemisorption revealed from Fourier-transform infrared spectroscopy (FTIR) and photodegraded Methylene Blue (MB). The satisfactory results were for MCA1 (specific surface area 25.05m²/g) with 97% fluoride removal at pH7.0 for the 10mg/L initial fluoride concentration for 1.5g/L adsorbent dose with 45min contact time obeying the Langmuir isotherm model with negative thermodynamic parameters and 4mmol of MCA3 with 98.51% photodegradation for 10^-5mol/LMB solution obeying pseudo-second-order and pseudo-first-order kinetics respectively. The proposed photodegradation mechanism of MB was established by the FTIR and high-performance liquid chromatography (HPLC) analysis. The nanoparticles are cubic, estimated through X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. The band gap energies, grain size, and the effective working pH were estimated by diffuse reflectance spectra (DRS), scanning electron microscope (SEM), and zero-point potential analysis respectively. A soil candle with MCA1 also fabricated for the household purpose and tested with some fluorinated field samples. The MCA3 was able to enhance the latent fingerprint on smooth surfaces.

Synthesis of (ZrO2-Al2O3)/GO nanocomposite by sonochemical method and the mechanism analysis of its high defluoridation

A nanocomposite of (ZrO₂-Al₂O₃)/GO was successfully synthesized by a simple sonochemical method in this study. A special 3D network was formed in (ZrO₂-Al₂O₃)/GO, which produced a large surface area and good distribution of metal oxide nanoparticles. The as-synthesized (ZrO₂-Al₂O₃)/GO exhibits a maximum fluoride adsorption capacity of 62.2 mg/g, and an adsorption ability of 13.80 mg/g when the F^- equilibrium concentration is 1 mg/L, both of which are higher than most previously reported defluoridation adsorbents, indicating that it is among the top adsorbents. Large amounts of drinking water contaminated by F^- can be treated by (ZrO₂-Al₂O₃)/GO to meet the WHO limit, indicating the high potential for practical application of the adsorbent. Based on the experimental analysis, the origin of the high defluoridation performance and the adsorption mechanism were discussed in detail. Due to the simple preparation, easy operation and high performance, the adsorbent and the related sonochemical method are considered to be significant for developing highly effective adsorbents.

Simplified Batch and Fixed-Bed Design System for Efficient and Sustainable Fluoride Removal from Water Using Slow Pyrolyzed Okra Stem and Black Gram Straw Biochars

Okra stem biochar (OSBC) and black gram straw biochar (BGSBC) were prepared by slow pyrolysis at 500 and 600 °C, respectively. OSBC and BGSBC were characterized using S BET, Fourier transform infrared, X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy, SEM-energy dispersive X-ray, and energy dispersive X-ray fluorescence. High carbon contents (dry basis) of 66.2 and 67.3% were recorded in OSBC and BGSBC, respectively. The OSBC surface area (23.52 m2/g) was higher than BGSBC (9.27 m2/g). The developed biochars successfully remediate fluoride contaminated water. Fluoride sorption experiments were accomplished at 25, 35, and 45 °C. Biochar-fluoride adsorption equilibrium data were fitted to Langmuir, Freundlich, Sips, Temkin, Koble-Corrigan, Radke and Prausnitz, Redlich-Peterson, and Toth isotherm models. The sorption dynamic data was better fitted to the pseudo-second order rate equation versus the pseudo-first order rate equation. The Langmuir sorption capacities of Q OSBC 0 = 20 mg/g and Q BGSBC 0 = 16 mg/g were obtained. Biochar fixed-bed dynamic studies were accomplished to ascertain the design parameters for developing an efficient and sustainable fluoride water treatment system. A column capacity of 6.0 mg/g for OSBC was achieved. OSBC and BGSBC satisfactorily remediated fluoride from contaminated ground water and may be considered as a sustainable solution for drinking water purification.

Layered Double Hydroxide Fluoride Release in Dental Applications: A Systematic Review

This systematic review appraises studies conducted with layered double hydroxides (LDHs) for fluoride release in dentistry. LDH has been used as antacids, water purification in removing excess fluoride in drinking water and drug delivery. It has great potential for controlled fluoride release in dentistry, e.g., varnishes, fissure sealants and muco-adhesive strips, etc. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) Statement was followed with two reviewers performing a literature search using four databases: PubMed, Web of Science, Science Direct and Ovid Medline with no date restrictions. Studies including any LDH for ion/drug release in dentistry were included, while assessing the application of LDH and the value of the methodology, e.g., ion release protocol and the LDH production process. Results: A total of 258 articles were identified and four met the inclusion criteria. Based on two in vitro studies and one clinical study, LDH was previously studied in dental materials, such as dental composites and buccal muco-adhesive strips for fluoride release, with the latter studied in a clinical environment. The fourth study analysed LDH powder alone (without being incorporated into dental materials). It demonstrated fluoride release and the uptake of volatile sulphur compounds (VSC), which may reduce halitosis (malodour). Conclusion: LDHs incorporated in dental materials have been previously evaluated for fluoride release and proven to be clinically safe. LDHs have the potential to sustain a controlled release of fluoride (or other cariostatic ions) in the oral environment to prevent caries. However, further analyses of LDH compositions, and clinical research investigating any other cariostatic effects, are required.

Facile Fabrication of NiCoAl-Layered Metal Oxide/Graphene Nanosheets for Efficient Capacitive Deionization Defluorination

Capacitive deionization (CDI) has aroused extensive attention as a prospective technology for different ionic species removal from aqueous solutions. Traditional studies on the adsorption and desorption of fluoride from wastewater are energy-intensive and may have harmful effects on the environment. Herein, the feasibility of fluoride removal from wastewater by CDI has been investigated. NiCoAl-layered metal oxide (NiCoAl-LMO) nanosheets and reduced graphene oxide (rGO) composites (NiCoAl-LMO/rGO) were synthesized and used as CDI electrode materials for fluoride ion removal. The as-obtained NiCoAl-LMO/rGO with unique structure and high conductivity is beneficial to the adsorption of fluoride ions. In addition, the introduction of Co element in the laminate enhances the pseudocapacitive behavior of the electrode material. As expected, the CDI system with NiCoAl-LMO/rGO composites as anode and activated carbon treated by nitric acid (H-AC) as cathode exhibits outstanding defluorination performance. The maximum adsorption capacity of NiCoAl-LMO/rGO, 24.5 mg g^-1, can be reached when the initial NaF concentration is 500 mg L^-1 at 1.4 V applied voltage. The composites also show good cycle stability over 40 consecutive cycles of the CDI defluorination process. The excellent defluorination performance of NiCoAl-LMO/rGO makes it possible for its practical application in wastewater treatment.

Mg-Al Mixed Oxide Adsorbent Synthesized Using FCT Template for Fluoride Removal from Drinking Water

To make full use of natural waste, a novel Mg-Al mixed oxide adsorbent was synthesized by the dip-calcination method using the fluff of the chinar tree (FCT) and an Mg(II) and Al(III) chloride solution as raw materials. The adsorbents were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The effects of the Mg/Al molar ratio and calcination temperature on the performance of the novel Mg-Al mixed oxide adsorbent were investigated. The optimized Mg-Al mixed oxide adsorbent had a Langmuir adsorption capacity of 53 mg/g. This adsorption capacity was higher than that of the separate Mg oxide and Al oxide. The synergy between Mg and Al is beneficial to the adsorption performance of the material. The fluoride adsorption capacity of the optimized Mg-Al mixed oxide adsorbent is only slightly affected by ions such as Cl⁻, NO₃⁻, SO₄²⁻, Na⁺, and K⁺ and is excellent for use in recycling and real water. The hydroxyl groups on the surface of the Mg-Al mixed oxide adsorbent play a key role in the adsorption of fluorine. The as-obtained novel Mg-Al mixed oxide adsorbent is an efficient and environmentally friendly agent for fluoride removal from drinking water.

Efficient Fluoride Removal and Dye Degradation of Contaminated Water Using Fe/Al/Ti Oxide Nanocomposite

The trimetallic Fe/Al/Ti (1:1:1) nanocomposite (FAT), synthesized by an adaptable tuned chemical route, offers a new approach for water treatment, for example, the de-fluoridation and photodegradation soluble dye methylene blue (MB) at pH 7. FAT acted as a good fluoride scavenger in the presence of other co-ions and within a widespread pH range (pH 2-11). The photodegradation efficiencies were >90% for different concentrations of MB solutions. The characterization of FAT includes thermogravimetric analysis, X-ray diffraction, Fourier transform-infrared spectroscopy, scanning electron microscopy, transmission electron microscopy, and ζ-potential analysis. Furthermore, the regeneration efficiencies of both the water treatments were checked, where the removal efficiency was not hampered significantly even after five batches. Spectroscopic techniques were adopted to perform the kinetic studies and to propose the probable mechanistic paths.

Defluoridation of synthetic and industrial wastewater by using acidic activated alumina adsorbent: characterization and optimization by response surface methodology

Excessive contamination of fluoride in wastewater is the cause of several chronic health problems. For this purpose, an adsorbent was prepared from alumina by acidic activation using sulfuric acid. The current research aims to find the maximum fluoride adsorption (%) from synthetic and industrial wastewater at optimum process parameters by using response surface methodology (RSM). All batch scale experiments were carried out according to the statistical-design order. Central composite design (CCD) was applied to ascertain the effect of adsorbent dose, pH, initial fluoride concentration and temperature on fluoride adsorption (%). Maximum fluoride removal was predicted based on the quadratic model developed. Validation of the model was done with negligible error. The regression coefficient of the model was found to be 0.96. From the analysis of variance (ANOVA), the factors with the greatest effect on the adsorption of fluoride were identified. Under optimized condition, the adsorbent dose 13.89 g L^-1, pH 5.52, temperature 25 °C and initial fluoride concentration 18.67 mg L^-1 resulted in 96% of maximum fluoride adsorption. Under the same optimized parameters, the fluoride adsorption from industrial wastewater found to be 92.10%.

Ultrasound-assisted synthesis of NiFe- layered double hydroxides as efficient electrode materials in supercapacitors

Under ultrasound irradiation, NiFe-layered double hydroxide (NiFe-LDH) nanostructures with three molar ratios and three dissimilar reaction times were prepared. The powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and Fourier transform infrared spectroscopy (FT-IR) were employed to characterize the synthesized nanomaterials. Using a sonochemichal route, various morphologies of the NiFe-LDH nanostructures without any impurity and variations in the structure were produced. During the optimization process, it was found that sonication time and reagent concentration in a fixed irradiation frequency can affect the size and the morphology of the produced nanostructures. Under ultrasound irradiation, non-aggregated particles with uniform, spherical morphology were obtained with molar ratios of 4:1 (Ni:Fe) with 45 W at 180 min. The NiFe-LDH samples were observed to be supercapacitor under a 6 M KOH solution. When morphologically-controlled NiFe-LDH samples were used, the pseudo-capacitive behavior of the nanostructures was tuned. After 3 h of ultrasonic irradiation, the optimized sample (NiFe-LDH spherical nanostructures with 4:1 M ratio) had a high value of specific capacitance (168F g^-1).