Rapid and extremely high adsorption performance of porous MgO nanostructures for fluoride removal from water

Advances and future perspectives of water defluoridation by adsorption technology: A review

Fluoride contamination in water sources poses a significant challenge to human health and the environment. In recent years, adsorption technology has emerged as a promising approach for water defluoridation due to its efficiency and cost-effectiveness. This review article comprehensively explores the advances in water defluoridation through adsorption processes. Various adsorbents, including natural and synthetic materials, have been investigated for their efficacy in removing fluoride ions from water. The mechanisms underlying adsorption interactions are elucidated, shedding light on the factors influencing defluoridation efficiency. Moreover, the review outlines the current state of technology, highlighting successful case studies and field applications. Future perspectives in the field of water defluoridation by adsorption are discussed, emphasizing the need for sustainable and scalable solutions. The integration of novel materials, process optimization, and the development of hybrid technologies are proposed as pathways to address existing challenges and enhance the overall efficacy of water defluoridation. This comprehensive assessment of the advances and future directions in adsorption-based water defluoridation provides valuable insights for researchers, policymakers, and practitioners working towards ensuring safe and accessible drinking water for all.

Amendment of Sargassum oligocystum bio-char with MnFe2O4 and lanthanum MOF obtained from PET waste for fluoride removal: A comparative study

The use of biomass and waste to produce adsorbent reduces the cost of water treatment. The bio-char of Sargassum oligocystum (BCSO) was modified with MnFe2O4 magnetic particles and La-metal organic framework (MOF) to generate an efficient adsorbent (BCSO/MnFe2O4@La-MOF) for fluoride ions (F-) removal from aqueous solutions. The performance of BCSO/MnFe2O4@La-MOF was compared with BCSO/MnFe2O4 and BCSO. The characteristics of the adsorbents were investigated using various techniques, which revealed that the magnetic composites were well-synthesized and exhibited superparamagnetic properties. The maximum adsorption efficiencies (BCSO: 97.84%, BCSO/MnFe2O4: 97.85%, and BCSO/MnFe2O4@La-MOF: 99.36%) were achieved under specific conditions of pH 4, F- concentration of 10 mg/L, and adsorbent dosage of 3, 1.5, and 1 g/L for BCSO, BCSO/MnFe2O4, and BCSO/MnFe2O4@La-MOF, respectively. The results demonstrated that the experimental data adheres to a pseudo-second-order kinetic model. The enthalpy, entropy, and Gibbs free energy were determined to be negative; thus, the F- adsorption was exothermic and spontaneous in the range of 25-50 °C. The equilibrium data of the process exhibited conformity with the Langmuir model. The maximum adsorption capacities of F- ions were determined as 10.267 mg/g for BCSO, 14.903 mg/g for the BCSO/MnFe2O4, and 31.948 mg/g for BCSO/MnFe2O4@La-MOF. The KF and AT values for the F- adsorption were obtained at 21.03 mg/g (L/mg)1/n and 100 × 10+9 L/g, indicating the pronounced affinity of the BCSO/MnFe2O4@La-MOF towards F- than other samples. The significant potential of the BCSO/MnFe2O4@La-MOF magnetic composite for F- removal from industrial wastewater, makes it suitable for repeated utilization in the adsorption process.

Efficient fluoride removal using nano MgO: mechanisms and performance evaluation

In this study, highly efficient fluoride removal of nano MgO was successfully synthesized using a simple hydrothermal precipitation method. Hexadecyl trimethyl ammonium bromide (CTMAB) was utilized as a surfactant. Its long-chain structure tightly wrapped around the precursor crystal of basic magnesium chloride, inhibiting the growth of precursor crystals, reducing their size, and improving crystal dispersion. This process enhanced the adsorption capacity of nano MgO for fluoride. The adsorption performance of nano MgO on fluoride was investigated. The results indicate that pseudo-second-order kinetics and the Langmuir isotherm model can describe the adsorption behavior for fluoride, with a maximum adsorption capacity of 122.47 mg/g. Methods such as XRD, SEM, XPS, and FTIR were employed to study the adsorption mechanisms of the adsorbent. Additionally, factors potentially affecting adsorption performance in practical applications, such as pH and competing ions, were examined. This study enhances our profound understanding of the defluorination effectiveness and mechanisms of nano MgO.

Development of innovative and green adsorbents for in situ cleanup of fluoride-polluted groundwater: Mechanisms and field-scale studies

In this study, the magnesium oxide (MgO)-based adsorbents [granulated MgO aggregates (GA-MgO) and surface-modified MgO powder (SM-MgO)] were developed to remediate a fluoride-contaminated groundwater site. Both GA-MgO and SM-MgO had porous, spherical, and crystalline structures. Diameters for GA-MgO and SM-MgO were 1-1.7 mm and 1-10 μm, respectively. The pseudo second-order dynamic adsorption and the Freundlich isotherm could be applied to express the chemical adsorption phenomena. The monolayer adsorption was the dominant mechanism at the initial adsorption period. During the latter part of fluoride adsorption, the multilayer adsorption became the dominant mechanism for fluoride removal from the water phase, which also resulted in the increased adsorption capacity. Higher hydroxide, phosphate, and carbonate concentrations caused a decreased fluoride removal efficiency due to the competition of sorption sites between fluoride and other anions with similar electronic properties. Fluoride removal mechanism using GA-MgO and SM-MgO as the adsorbents was mainly carried out by the chemical adsorption. Reaction paths contained two main processes: (1) formation of magnesium hydroxide after the reaction of MgO with water, and (2) the hydroxyl group of the magnesium hydroxide was replaced by fluoride ions to form magnesium fluoride precipitation. Results from column tests show that up to 61 and 73% of fluoride removal (initial fluoride concentration = 9.3 mg/L) could be obtained after 50 pore volumes of groundwater pumping with GA-MgO and SM-MgO injection, respectively. The GA-MgO system could be applied to contain and remediate fluoride-contaminated groundwater, and SM-MgO could be applied as an immediate fluoride removal alternative to achieve a rapid pollutant removal for emergency responses. Up to 71% of fluoride removal (fluoride concentration = 10.8 mg/L) could be obtained with GA-MgO injection after 30 days of operation. The developed GA-MgO system is a potential and green remediation alternative to contain the fluoride plume significantly.

Successful removal of fluoride from aqueous environment using Al(OH)3@AC: column studies and breakthrough curve modeling

In this study, we discuss the removal of fluoride from water through column adsorption methods using Al(OH)3@AC as a functional granular activated carbon. The height of the bed, fluoride concentration, and flow rate are the experimental factors used to obtain the breakthrough curves. As the flow rate increased, the breakthrough and saturation times decreased. The analysis of simplified column models, such as the Adams-Bohart, Thomas, and Yoon-Nelson models, revealed that the Clark model best described the adsorption process when fitting the experimental data. The obtained breakthrough curves agreed with the corresponding experimental data. The highest capacity for adsorption obtained during the column procedure was found to be 41.84 mg g-1 with a bed height of 3 cm, an initial fluoride concentration of 10 mg L-1 and a flow rate of 7.5 mL min-1.

Facile preparation, characterization and application of novel sugarcane bagasse-derived nanoceria-biochar for defluoridation of drinking water: kinetics, thermodynamics, reusability and mechanism

Although expensive, rare-earth oxides are well known for being powerful defluoridation agents. Being costlier, cerium is used as a hybrid adsorbent in conjunction with a prudent and environmentally benign substance like biochar. The novel CeO2/BC (surface area 260.05 m2/g) composite was shaped using the facile chemical precipitation technique without any cross-linkers. Surface properties of synthesised CeO2/BC were investigated using powder XRD, FTIR, BET, pH point of zero charge and SEM. According to XRD analysis, immobilized Ce is primarily in form of CeO2, while pristine biochar is in an amorphous state. Batch mode adsorption tests were carried out with different solution pH, F- initial concentration, adsorbent dosage and contact time and counter anions. CeO2/BC can be used in a varied pH range (2-10) but shows maximum removal at pH 4. The Langmuir adsorption isotherm and a pseudo-second-order kinetic model are best fitted to support the adsorption process with a maximum Langmuir adsorption capacity of 16.14 mg/g (F- concentration 5 to 40 mg/L). The removal phenomenon is non-spontaneous in nature. The plausible mechanism of fluoride uptake was explained using XPS and pHPZC, and it was demonstrated that the fluoride was mainly removed by ion exchange and electrostatic attraction. The adsorbent could be successfully used up to fourth cycle after regenerating.

Adsorption and photodegradation of organic contaminants by silver nanoparticles: isotherms, kinetics, and computational analysis

In view of the widespread and distribution of several classes and types of organic contaminants, increased efforts are needed to reduce their spread and subsequent environmental contamination. Although several remediation approaches are available, adsorption and photodegradation technologies are presented in this review as one of the best options because of their environmental friendliness, cost-effectiveness, accessibility, less selectivity, and wider scope of applications among others. The bandgap, particle size, surface area, electrical properties, thermal stability, reusability, chemical stability, and other properties of silver nanoparticles (AgNPS) are highlighted to account for their suitability in adsorption and photocatalytic applications, concerning organic contaminants. Literatures have been reviewed on the application of various AgNPS as adsorbent and photocatalyst in the remediation of several classes of organic contaminants. Theories of adsorption have also been outlined while photocatalysis is seen to have adsorption as the initial mechanism. Challenges facing the application of silver nanoparticles have also been highlighted and possible solutions have been presented. However, current information is dominated by applications on dyes and the view of the authors supports the need to strengthen the usefulness of AgNPS in adsorption and photodegradation of more classes of organic contaminants, especially emerging contaminants. We also encourage the simultaneous applications of adsorption and photodegradation to completely convert toxic wastes to harmless forms.

Study of Modified Magnesium Phosphate Cement for Fluoride Removal

In this study, we used a novel composite material based on magnesium phosphate cement (MPC) to explore the retention of fluoride from used water. Dead-burned magnesium oxide (MgO), ammonium dihydrogen phosphate (NH4H2PO4), and a few retarders were used to create this particular substance. Several studies have corroborated the performance of using aluminum in the capture of fluoride. From this perspective, we attempted to reinforce our matrix with different quantities of aluminum, which increased the resistance of the composite in water. The optimal conditions that were obtained were evaluated and scrutinized using a range of techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier transforms infrared spectroscopy (FTIR), and Brunauer-Emmett-Teller (BET). The adsorbents demonstrated a powerful ability to remove fluoride from contaminated water and the defluoridation capacity was evaluated at 4.84 mg/g. Equilibrium modeling was carried out, and the experimental data were expressed in accordance with the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms.

Removal of oxytetracycline from wastewater by biochar modified with biosynthesized iron oxide nanoparticles and carbon nanotubes: Modification performance and adsorption mechanism

The pollution problem of oxytetracycline (OTC) from wastewater becomes more serious, so an efficient, economical, and green adsorption material is urgently explored. In this study, the multilayer porous biochar (OBC) was prepared by coupling carbon nanotubes with iron oxide nanoparticles synthesized by Aquabacterium sp. XL4 to modify corncobs under medium temperature (600 °C) conditions. The adsorption capacity of OBC could reach 72.59 mg g-1 after preparation and operation parameters were optimized. In addition, various adsorption models suggested that OTC removal resulted from the combined effect of chemisorption, multilayer interaction, and disordered diffusion. Meanwhile, the OBC was fully characterized and exhibited a large specific surface area (237.51 m2 g-1), abundant functional groups, stable crystal structure, high graphitization, and mild magnetic properties (0.8 emu g-1). The OTC removal mechanisms mainly included electrostatic interactions, ligand exchange, π-π bonding reactions, hydrogen bonds, and complexation. pH and coexistence substance experiments revealed that the OBC possesses a wide pH adaptation range and excellent anti-interference ability. Finally, the safety and reusability of OBC were confirmed by repeated experiments. In summary, OBC as a biosynthetic material shows considerable potential for application in the field of purifying new pollution from wastewater.

Synthesis of Composites for the Removal of F- Anions

This work presents the synthesis of amine and ferrihydrite functionalized graphene oxide for the removal of fluoride from water. The synthesis of the graphene oxide and the modified with amine groups is developed by following the modified Hummer's method. Fourier transform infrared spectrometry, X-ray, Raman spectroscopy, thermogravimetric analysis, surface charge distribution, specific surface area and porosity, adsorption isotherms, and the van't Hoff equation are used for the characterization of the synthesized materials. Results show that the addition of amines with ferrihydrite generates wrinkles on the surface layers, suggesting a successful incorporation of nitrogen onto the graphene oxide; and as a consequence, the adsorption capacity per unit area of the materials is increased.

A novel edible colorant lake prepared with CaCO3 and Monascus pigments: Lake characterization and mechanism study

Monascus pigments (MPs) were adsorbed using calcium carbonate to produce CaCO₃-MPs lakes. The fundamental properties and formation mechanism of the lakes were investigated. Results indicated that CaCO₃ displayed a high enough affinity for the MPs to form colorant lakes, while the MPs tended to transform the CaCO₃ crystals from calcite to vaterite. The adsorption of MPs by CaCO₃ followed the Freundlich isothermal model with n value higher than 1, confirming it as physical adsorption. The ΔG⁰ (-29 to ∼-33 kJ/mol) and ΔH⁰(30-55 kJ/mol) indicated that lake formation was a spontaneous and endothermic process. UV/Vis spectroscopic analysis verified the complex formation between Ca²⁺ and MPs via physical bonding, suggesting a possible attraction between the Ca²⁺ and glutamate residues of the MPs. EDS showed that the MPs were trapped inside the particles. FTIR spectroscopy and XPS further confirmed that the physical bonding was the primary driving force behind the lake formation.

In-situ modified biosynthetic crystals with lanthanum for fluoride removal based on microbially induced calcium precipitation: Characterization, kinetics, and mechanism

In this research, in-situ modified biosynthetic crystals with lanthanum (BC-La) were synthesized based on anaerobic microbially induced calcium precipitation (MICP) and investigated its capacity for groundwater defluoridation under various operational conditions. The kinetic and thermodynamic models were simulated to explore the effect of the material on the removal of fluoride ion (F^-) under various parameters (pH, initial concentration of F^-, and temperature). BC-La had the maximum F^- adsorption capacity of 10.92 mg g^-1 and 96.66% removal efficiency. The pseudo-second-order kinetic model and Langmuir isotherm model were the best kinetic and isotherm models for F^- removal from BC-La, which indicated that F^- were mainly spontaneously removed through chemisorption and adsorption processes. The specific surface area was 54.26 m² g^-1 and the average pore size was 9.0670 nm. BC-La mainly contained LaCO₃OH, LaPO₄, CaCO₃, Ca₅ (PO₄)₃OH, and F^- was mainly removed through ion exchange with the material surface. Moreover, OH^-, PO₄^3-, and CO₃^2- significantly influenced the F^- removal. This work suggested a novel method for in-situ modification of anaerobic biosynthetic crystals, which improved the defluoridation effect of traditional biosynthetic crystals, increased the stability of the BC-La and allowed to remove F^- from groundwater consistently.

Iron functionalized silica particles as an ingenious sorbent for removal of fluoride from water

The paucity of safe drinking water remains a global concern. Fluoride is a pollutant prevalent in groundwater that has adverse health effects. To resolve this concern, we devised a silica-based defluoridation sorbent from pumice rock obtained from the Paka volcano in Baringo County, Kenya. The alkaline leaching technique was used to extract silica particles from pumice rock, which were subsequently modified with iron to enhance their affinity for fluoride. To assess its efficacy, selected borehole water samples were used. Scanning electron microscopy, X-ray diffraction, Fourier transform infrared and X-ray fluorescence spectroscopy was used to characterize the sorbent. The extracted silica particles were 96.71% pure and amorphous, whereas the iron-functionalized silica particles contained 93.67% SiO₂ and 2.93% Fe₂O₃. The optimal pH, sorbent dose and contact time for defluoridation of a 20 mg/L initial fluoride solution were 6, 1 g and 45 min, respectively. Defluoridation followed pseudo-second-order kinetics and fitted Freundlich's isotherm. Fluoride levels in borehole water decreased dramatically; Intex 4.57-1.13, Kadokoi 2.46-0.54 and Naudo 5.39-1.2 mg/L, indicating that the silica-based sorbent developed from low-cost, abundant and locally available pumice rock is efficient for defluoridation.

B-site metal modulation of phosphate adsorption properties and mechanism of LaBO3 (B = Fe, Al and Mn) perovskites

La-based adsorbents are widely used for controlling phosphate concentration in water bodies. In order to explore the effect of different B-site metals regulating La-based perovskites on phosphate adsorption, three La-based perovskites (LaBO₃, B = Fe, Al, and Mn) were prepared using the citric acid sol-gel method. Adsorption experiments showed that LaFeO₃ exhibited the highest adsorption capacity for phosphate, which was 2.7 and 5 times higher than those of LaAlO₃ and LaMnO₃, respectively. The characterization results demonstrated that LaFeO₃ has dispersed particles exhibiting larger pore size and more pores than LaAlO₃ and LaMnO₃. Spectroscopy analysis and density functional theory calculation results showed that different B-positions cause a change in the type of perovskite crystals. Among them, the differences between lattice oxygen consumption ratio, zeta potential and adsorption energy are the main reasons for the differences in adsorption capacity. In addition, the adsorption of phosphate by La-based perovskites were well fitted with Langmuir isotherm and pursues the pseudo-second-order kinetic models. The maximum adsorption capacities were 33.51, 12.31 and 6.61 mg/g for LaFeO₃, LaAlO₃ and LaMnO₃, respectively. The adsorption mechanism was mainly based on inner-sphere complexation and electrostatic attraction. This study provides an explanation for the influence of different B sites on phosphate adsorption by perovskite.

Synthesis and Characterization of Porous MgO Nanosheet-Modified Activated Carbon Fiber Felt for Fluoride Adsorption

In the present work, the porous MgO nanosheet-modified activated carbon fiber felt (MgO@ACFF) was prepared for fluoride removal. The MgO@ACFF was characterized by XRD, SEM, TEM, EDS, TG, and BET. The fluoride adsorption performance of MgO@ACFF also has been investigated. The adsorption rate of the MgO@ACFF toward fluoride is fast; more than 90% of the fluoride ions can be adsorbed within 100 min, and the adsorption kinetics of MgO@ACFF can be fitted in a pseudo-second-order model. The adsorption isotherm of MgO@ACFF fitted well in the Freundlich model. Additionally, the fluoride adsorption capacity of MgO@ACFF is larger than 212.2 mg/g at neutral. In a wide pH range of 2-10, the MgO@ACFF can efficiently remove fluoride from water, which is meaningful for practical usage. The effect of co-existing anions on the fluoride removal efficiency of the MgO@ACFF also has been studied. Furthermore, the fluoride adsorption mechanism of the MgO@ACFF was studied by the FTIR and XPS, and the results reveal a hydroxyl and carbonate co-exchange mechanism. The column test of the MgO@ACFF also has been investigated; 505-bed volumes of 5 mg/L fluoride solution can be treated with effluent under 1.0 mg/L. It is believed that the MgO@ACFF is a potential candidate for a fluoride adsorbent.

A new basic burning raw material for simultaneous stabilization/solidification of PO43--P and F- in phosphogypsum

Phosphogypsum (PG) contains a lot of soluble phosphate (PO₄^3--P) and fluorine ion (F^-), which seriously has hindered the sustainable development of the phosphorous chemical industry. In this study, a new burning raw material (BRM) as an intermediate product in the cement production process was used for PO₄^3--P and F^- stabilize in PG. The stabilizing mechanism of PO₄^3--P and F^- were investigated by Fourier Transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD), Scanning Electron Microscopy (SEM), X-ray fluorescence (XRF) and X-ray spectroscopy system (XPS). The effect of PG and BRM weight ratio, solid-to-liquid ratio, reaction time, and reaction temperature on the concentrations of PO₄^3--P and F^- were studied. The results showed that the concentration of F^- in the PG leaching solution was 8.65 mg/L and the stabilizing efficiency of PO₄^3--P was 99.78%, as well as the pH of the PG leaching solution was 8.12 when the weight ratio of PG and BRM was 100:2, and the solid to liquid ratio was 4:1, reacting for 24 h at the temperature of 30 ℃. PO₄^3--P and F^- were mostly solidified as Ca₅(PO₄)₃F, CaPO₃(OH), Ca₅(PO₄)₃(OH), Ca₂P₂O₇·2H₂O, CaSO₄PO₃(OH)·4H₂O, CaF₂, and CaFPO₃·2H₂O. Leaching test results indicated that the concentrations of PO₄^3--P, F^- and heavy metals were less than the integrated wastewater discharge standard (GB8978-1996). This study provides a new harmless treatment method for PG.

Synthesis and Characterization of New Composite Materials Based on Magnesium Phosphate Cement for Fluoride Retention

In this research work, new composite materials based on magnesium phosphate cement (MPC) were developed to evaluate the retention of fluorine from wastewater. This material was prepared with dead burned magnesia oxide (MgO), ammonium dihydrogen phosphate (NH₄H₂PO₄), and some retarding agents. We chose to synthesize with hydrogen peroxide instead of water; alumina and zeolite were also added to the cement. The obtained optimal conditions were studied and analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), BET, and thermogravimetric analysis (TGA). The adsorbents showed a strong ability to remove fluoride from contaminated water, and the best defluoridation capacity was evaluated as 2.21 mg/g for the H₂O₂ cement. Equilibrium modeling was performed, and the experimental data were presented according to the isotherms of Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich.

Comparison of fluorine removal performance and mechanism of spheroidal magnesium oxide before and after lanthanum modification

Firstly, spherical magnesium oxide was synthesized by simple magnesium salt and specific reaction conditions. Then, lanthanum-modified spherical magnesium oxide (LSMO) was prepared by impregnation of lanthanum salt. The adsorption mechanism of the adsorbent was investigated by XRD, SEM, XPS, and FT-IR. Through the study of fluorine removal performance, for the solution with fluoride ion concentration of 10 mg·L^-1, the fluorine removal efficiency of lanthanum-modified spherical magnesium oxide (15LSMO) (93.1%) with 15% impregnation mass ratio is higher than that of SMO (82.7%). In addition, in the pH range of 2-11 or in the presence of interfering ions, the fluoride removal effect of 15LSMO still meets the fluoride removal efficiency of more than 90%. The research enhanced the profound insights into the effect and mechanism of fluorine removal of lanthanum modified materials.

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.

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.

Simultaneous adsorption of tetracycline, ammonium and phosphate from wastewater by iron and nitrogen modified biochar: Kinetics, isotherm, thermodynamic and mechanism

The simultaneous removal of various pollutants in wastewater is increasingly deserved attention. In this study, an efficient adsorbent Fe/N@BC was synthesized by Fe-N co-modification. The adsorbability of Fe/N@BC was evaluated using a mixture with tetracycline (TC), NH₄⁺-N and PO₄^3-P. In comparison to BC, N@BC and Fe@BC, Fe/N@BC exhibited an excellent performance for simultaneously absorbing TC, NH₄⁺-N and PO₄^3-P. The pseudo-first-order was used to describe the adsorption process of NH₄⁺-N and PO₄^3-P, while the pseudo-second-order could be well fitted to TC adsorption data. The adsorption isotherms of TC, NH₄⁺-N and PO₄^3-P were more in line with Sips model (Adj.R² > 0.97). The maximum adsorption capacities of Fe/N@BC towards TC, NH₄⁺-N and PO₄^3-P were 238.94, 111.87 and 165.02 mg g^-1, respectively, which were 1.31-1.91 times than that of BC, N@BC and Fe@BC. The simultaneous adsorption mechanism mainly involved pore filling, electrostatic interaction, ion exchange, surface complexation, surface precipitation, H bond and π-π interaction. Furthermore, after six cycles, the removal efficiencies of TC, NH₄⁺-N and PO₄^3-P were 75.3, 66.1 and 64.5% by Fe/N@BC, highlighting its promising potential to adsorb multi-pollutants from aqueous solution.

Removal of fluoride ions from aqueous solution by metaettringite

Fluoride contamination is a major problem in wastewater treatment. Metaettringite (which has previously shown enhanced anion adsorption) was investigated as a possible adsorbent to remove fluoride from low-concentration solution (25 mg-F/L). The fluoride removal properties of ettringite and metaettringite were first compared at pH 10, and metaettringite was found to be more effective. The dominant reaction mechanism for fluoride adsorption in metaettringite was found to be recrystallization of metaettringite by rehydration; this was accompanied by precipitation of calcium fluoride. The adsorption kinetics followed the pseudo-second order model. Metaettringite was also able to remove fluoride effectively in low pH environment (i.e., at pH 3.5). The influence of coexistence of sulfate ions in solution on the fluoride removal performance was investigated, and a small decrease in performance was noted. The residual fluoride concentrations obtained with higher doses of metaettringite were lower than those specified by the Japanese effluent standard (non-coastal areas: 8 mg-F/L; coastal areas: 15 mg-F/L). The fluoride removal capacity of metaettringite was compared with those of other solid materials. The observed maximum capacity was 174.7 mg-F/g-metaettringite. In the case of high fluoride concentration solution, the main removal mechanism will be changed to calcium fluoride precipitation. In general, metaettringite is regarded as promising material for fluoride removal in wastewater treatment.

Application of Magnesium Oxide Media for Remineralization and Removal of Divalent Metals in Drinking Water Treatment: A Review

The post-treatment of soft and desalinated waters is an integral step in the production of quality drinking water. Remineralization is therefore often essential in order to stabilize the effluent for distribution and to attain mineral levels that fulfill aesthetic and health goals. According to the World Health Organization, magnesium (Mg2+) is a nutrient essential to human health. This review summarizes the effectiveness of magnesium oxide (MgO) media for soft water remineralization, as well as its potential for divalent metal removal (e.g., Mn, Cu, and Zn), which is of particular interest in small or residential applications. We present MgO sources, properties, and dissolution mechanisms. Water treatment applications are then reviewed, and the available design models are critically appraised in regard to remineralization and contaminant removal processes. In addition, we review the process operation challenges and costs. Finally, we discuss the use of MgO in combination with calcite and address the technical advantages and limitations compared to other available methods.

Rapid and efficient adsorption of tetracycline from aqueous solution in a wide pH range by using iron and aminoacetic acid sequentially modified hierarchical porous biochar

The object of this work was to synthesize an iron and aminoacetic acid sequentially modified hierarchical porous biochar (AC-Fe@HPBC) for tetracycline (TC) removal from aqueous solution. Results showed that AC-Fe@HPBC had a larger surface area (362.5370 m²/g), developed microporous structure (0.1802 cm³/g), and numerous functional groups, which provided more adsorption sites. The maximum adsorption capacity towards TC by AC-Fe@HPBC was 457.85 mg/g, 1.43, 1.29 and 1.20-fold than that of HPBC, AC@PHBC and Fe@HPBC, respectively, and the super-fast adsorptive equilibrium was achieved within 10 min. Additionally, introducing amino and carboxyl functional groups on the AC-Fe@HPBC surface significantly broadened the operation pH range (3-11). Site energy analysis indicated TC and AC-Fe@HPBC had stronger adsorption affinity at a higher temperature. The adsorption mechanism involved pore filling, surface complexation, H-bond and π-π interaction. Moreover, the reusability experiments proved AC-Fe@HPBC as an effective adsorbent for TC removal from aqueous solution.

Fluoride ions sorption using functionalized magnetic metal oxides nanocomposites: a review

Fluoride is an anionic pollutant found superfluous in surface or groundwater as a result of anthropogenic actions from improper disposal of industrial effluents. In drinking water, superfluous fluoride has been revealed to trigger severe health problems in humans. Hence, developing a comprehensive wastewater decontamination process for the effective management and preservation of water contaminated with fluoride is desirable, as clean water demand is anticipated to intensify considerably over the upcoming years. In this regard, there have been increased efforts by researchers to create novel magnetic metal oxide nanocomposites which are functionalized for the remediation of wastewater owing to their biocompatibility, cost-effectiveness, relative ease to recover and reuse, non-noxiousness, and ease to separate from solutions using a magnetic field. This review makes an all-inclusive effort to assess the effects of experimental factors on the sorption of fluoride employing magnetic metal oxide nanosorbents. The removal efficiency of fluoride ions onto magnetic metal oxides nanocomposites were largely influenced by the solution pH and ions co-existing with fluoride. Overall, it was noticed from the reviewed researches that the maximum sorption capacity using various metal oxides for fluoride sorption was in the order of aluminium oxides >cerium oxides > iron oxides > magnesium oxides> titanium oxides, and most sorption of fluoride ions was inhibited by the existence of phosphate trailed by sulphate. The mechanism of fluoride sorption onto various sorbents was due to ion exchange, electrostatic attraction, and complexation mechanism.

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.

Recently Developed Adsorbing Materials for Fluoride Removal from Water and Fluoride Analytical Determination Techniques: A Review

In recent years, there has been an increase in public perception of the detrimental side-effects of fluoride to human health due to its effects on teeth and bones. Today, there is a plethora of techniques available for the removal of fluoride from drinking water. Among them, adsorption is a very prospective method because of its handy operation, cost efficiency, and high selectivity. Along with efforts to assist fluoride removal from drinking waters, extensive attention has been also paid to the accurate measurement of fluoride in water. Currently, the analytical methods that are used for fluoride determination can be classified into chromatographic methods (e.g., ionic chromatography), electrochemical methods (e.g., voltammetry, potentiometry, and polarography), spectroscopic methods (e.g., molecular absorption spectrometry), microfluidic analysis (e.g., flow injection analysis and sequential injection analysis), titration, and sensors. In this review article, we discuss the available techniques and the ongoing effort for achieving enhanced fluoride removal by applying novel adsorbents such as carbon-based materials (i.e., activated carbon, graphene oxide, and carbon nanotubes) and nanostructured materials, combining metals and their oxides or hydroxides as well as natural materials. Emphasis has been given to the use of lanthanum (La) in the modification of materials, both activated carbon and hybrid materials (i.e., La/Mg/Si-AC, La/MA, LaFeO3 NPs), and in the use of MgO nanostructures, which are found to exhibit an adsorption capacity of up to 29,131 mg g−1. The existing analytical methodologies and the current trends in analytical chemistry for fluoride determination in drinking water are also discussed.

Bivalve shells (Corbula trigona) as a new adsorbent for the defluoridation of groundwater by adsorption-precipitation

Defluoridation of groundwater was performed in a batch reactor using bivalve shell powder (BSP) as adsorbent. The physicochemical characteristics of BSP, studied by Fourier Transform Infrared, X-ray Diffraction and Inductively Coupled Plasma-Optical Emission Spectrometry after dissolution, have shown that BSP was mainly composed of crystalline CaCO₃ (∼97.8%). The effects of pH, initial fluoride concentration, adsorbent dose and contact time on the adsorption capacity of BSP were investigated. For an initial fluoride concentration of 2.2 mg/L and with 16 g/L of BSP, after 8 hours of treatment, 27.3% were eliminated at pH 7.5 versus 68% at pH 3, highlighting the efficiency of the adsorption process. The difference in adsorption capacity as a function of pH was correlated to the pHpzc of the BSP, which was equal to 8.2. Thus, at pH below pHpzc, electrostatic attraction between the fluoride anions and the positively charged adsorbent could justify the adsorption mechanism. Fittings of experimental data have evidenced that the adsorption kinetics were of pseudo-second order whereas the adsorption isotherms were of Langmuir type. The chemical precipitation of calcium fluoride was also revealed to occur upon release of Ca²⁺ from partial dissolution of CaCO₃ in acidic conditions.

Isolation of biosynthetic crystals by microbially induced calcium carbonate precipitation and their utilization for fluoride removal from groundwater

Biosynthetic crystals (BC) were prepared through microbially induced calcium carbonate precipitation (MICP) for fluoride (F^-) removal from the groundwater. Batch experiments were conducted to evaluate the fluoride adsorption capacity and the impacts of critical factors (organic matter, pH, initial fluoride concentration and BC dosage) on defluorination efficiency of BC. The maximum adsorption amount and defluorination efficiency were recorded as 5.10 mg g^-1 and 98.24%, respectively. The adsorption kinetics and isotherms studies showed that pseudo-second-order kinetic model and Freundlich isotherm model were best fitting to the reaction. Adsorption thermodynamic parameters indicated a spontaneous, endothermic and thermodynamically favorable adsorption process. Moreover, the mechanism of F^- removal by BC was further analyzed by SEM, XPS, XRD and FTIR. The method can cope with the problem of applying the external organic substances in MICP, and avoid the microbial safety risk in the effluent. As an economically and environmentally friendly adsorbent, BC can be used for F^- removal from groundwater.

Masking Ability of Various Metal Complexing Ligands at 1.0 mM Concentrations on the Potentiometric Determination of Fluoride in Aqueous Samples

Fluoride is a common anion present in natural waters. Among many analytical methods used for the quantification of fluoride in natural waters, potentiometric analysis is one of the most widely used methods because of minimum interferences from other ions commonly present in natural waters. The potentiometric analysis requires the use of ionic strength adjusting buffer abbreviated as TISAB to obtain accurate and reproducible data. In most of the reported literature, higher concentrations of strong metal chelating ligands are used as masking agents generally in the concentration range of 1.0 to 0.01 M. In the present study, effectiveness of the masking agents, phosphate, citrate, CDTA ((1,2-cyclohexylenedinitrilo)tetraacetic acid), EDTA (ethylenediaminetetraacetic acid) HE-EDTA ((hydroxyethyl)ethylenediaminetriacetic acid)), triethanolamine, and tartaric acid at 1.0 mM in TISAB solutions was investigated. The experimental data were compared with a commercially available WTW 140100 TISAB solution as the reference buffer. According to the experimental data, the reference buffer always produced the highest fluoride concentrations and the measured fluoride concentrations were in the range of 0.611 to 1.956 mg/L. Out of all the masking agents investigated, only CDTA performed marginally well and approximately a quarter of the samples produced statistically comparable data to the reference buffer. All the other masking agents produced significantly low concentrations compared to the reference buffer. The most probable reasons for the underestimation of fluoride concentrations could be shorter decomplexing time and lower masking agent concentrations.

Facile synthesis of chitosan-modified ZnO/ZnFe2O4 nanocomposites for effective remediation of groundwater fluoride

This study explores the possibility of developing an eco-friendly adsorbent for effective remediation of groundwater fluoride, a well-known health hazard affecting more than 25 nations on the various continents. A facile and milder approach has been adopted to synthesize chitosan-modified ZnO/ZnFe₂O₄ nanocomposites. The synthesized materials have been characterized by different spectroscopic, microscopic, and diffractometric techniques. X-ray photoelectron spectroscopy and X-ray diffraction studies have confirmed the formation of pure and highly crystalline ZnO/ZnFe₂O₄ nanocomposites. The presence of surface-adsorbed chitosan in the modified ZnO/ZnFe₂O₄ has been confirmed by FT-IR and thermogravimetric analysis. The results from microscopic and BET surface area analysis of ZnO/ZnFe₂O₄ nanocomposites indicated that chitosan plays a crucial role in modulating the surface morphology and surface properties of the nanocomposites. The nanocomposites exhibit excellent adsorption performance in the remediation of groundwater fluoride. Experimental conditions have been systematically designed to evaluate the optimum adsorption condition for fluoride, and the results have been analyzed with various non-linear models to describe the kinetics and isotherms of adsorption. The adsorption primarily follows Lagergren pseudo-first-order kinetics, and the Langmuir adsorption capacity is varied from 10.54 to 13.03 mg g^-1 over the temperature range 293-323 K. The thermodynamics study reveals that the adsorption process is endothermic and spontaneous. The mechanism of adsorption has been proposed based on the spectroscopic analysis of the fluoride-loaded adsorbent. The adsorption is non-specific in nature as co-existing anion can reduce its fluoride removal capacity. The effect of the co-existing anions on adsorption of fluoride follows the trend PO₄^3- > CO₃^2- > SO₄^2- > Cl^-. The adsorbent can be reused successfully for the 5th consecutive cycles of adsorption-desorption study. This study offers a very promising material for remediation of groundwater fluoride of affected areas.