Removal of Fluoride from Drinking Water by Sorption Using Diatomite Modified with Aluminum Hydroxide

Approaches for the Efficient Removal of Fluoride from Groundwater: A Comprehensive Review

Contamination of groundwater with fluoride represents a significant global issue, with high concentrations posing serious public health threats. While fluoride is a critical element in water, excessive levels can be detrimental to human health and potentially life-threatening. Addressing the challenge of removing fluoride from underground water sources via nanotechnological approaches is a pressing concern in environmental science. To collate relevant information, extensive literature searches were conducted across multiple databases, including Google Scholar, PubMed, Scopus, Web of Science, the American Chemical Society, Elsevier, Springer, and the Royal Society of Chemistry. VOS Viewer software version 1.6.20 was employed for a systematic review. This article delivers an exhaustive evaluation of various groundwater fluoride removal techniques, such as adsorption, membrane filtration, electrocoagulation, photocatalysis, and ion exchange. Among these, the application of nanoparticles emerges as a notable method. The article delves into nano-compounds, optimizing conditions for the fluoride removal process and benchmarking their efficacy against other techniques. Studies demonstrate that advanced nanotechnologies-owing to their rapid reaction times and potent oxidation capabilities-can remove fluoride effectively. The implementation of nanotechnologies in fluoride removal not only enhances water quality but also contributes to the safeguarding of human health.

A review on fluoride contamination in groundwater and human health implications and its remediation: A sustainable approaches

Contamination of drinking water due to fluoride (F-) is a major concern worldwide. Although fluoride is an essential trace element required for humans, it has severe human health implications if levels exceed 1.5 mg. L-1 in groundwater. Several treatment technologies have been adopted to remove fluoride and reduce the exposure risk. The present article highlights the source, geochemistry, spatial distribution, and health implications of high fluoride in groundwater. Also, it discusses the underlying mechanisms and controlling factors of fluoride contamination. The problem of fluoride-contaminated water is more severe in India's arid and semiarid regions than in other Asian countries. Treatment technologies like adsorption, ion exchange, precipitation, electrolysis, electrocoagulation, nanofiltration, coagulation-precipitation, and bioremediation have been summarized along with case studies to look for suitable technology for fluoride exposure reduction. Although present technologies are efficient enough to remove fluoride, they have specific limitations regarding cost, labour intensity, and regeneration requirements.

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.

Preparation of aluminium-hydroxide-modified diatomite and its fluoride adsorption mechanism

As the current excessive accumulation of fluoride (F^-) in the environment can be hazardous to human health, it is essential to remove fluoride from wastewater. In this study, diatomite (DA) was used as a raw material and modified using aluminum hydroxide (Al-DA) for use in the adsorption of F^- from water bodies. SEM, EDS, XRD, FTIR, and Zeta potential characterization analyses were carried out; adsorption tests and kinetic fitting were performed, and the effects of pH, dosing quantity, and presence of interfering ions on the adsorption of F^- by the materials were investigated. The results show that the Freundlich model effectively describes the adsorption process of F^- on DA, which therefore involves adsorption-complexation interactions; however, the Langmuir model effectively describes the adsorption process of F^- on Al-DA, corresponding to unimolecular layer adsorption mainly via ion-exchange interactions, that is, adsorption is dominated by chemisorption. Aluminum hydroxide was shown to be the main species involved in F^- adsorption. The efficiency of F^- removal by DA and Al-DA was over 91% and 97% for 2 h, and the adsorption kinetics were effectively fit by the quasi-secondary model, suggesting that chemical interactions between the absorbents and F^- control the adsorption process. The adsorption of F^- was highly dependent on the pH of the system, and the maximum adsorption performance was obtained at pH 6 and 4. The optimal dosage of DA and Al-DA was 4 g/L. Even in the presence of interfering ions, the removal of F^- on Al-DA reached 89%, showing good selectivity. XRD and FTIR studies showed that the mechanism of F^- adsorption on Al-DA involved ion exchange and the formation of F-Al bonds.

Efficient removal of fluoride on aluminum modified activated carbon: an adsorption behavioral study and application to remediation of ground water

In recent times, ground water contamination by toxic elements is of great concern and it is to be addressed for consumption of human, animal, and plant growth. In this context, we have synthesized an adsorbent by modifying commercially available activated carbon with aluminum and tested for de-fluoridation studies. The activity results suggested that the optimized adsorbent is highly efficient in removing the fluoride from ground water. Adsorption maxima are obtained over a wide pH range from 4 to 9, with a contact time of 15 minutes at a dosage of 4 g/L. The results also revealed that the synthesized adsorbent is suitable for application in ground water without any pH adjustment and has exhibited 85%-95% tolerance for common anions in the range of 100-500 mg/L. Equilibrium adsorption isotherm models as well as kinetics of adsorption were applied for the system. An adsorption capacity of 20.4 mg/g and fast kinetics observed are most favorable for defluoridation. Reuse of adsorbent over repeated cycles was investigated. Residual amount of aluminum in treated water is found to be negligible. The removal of toxic elements like Pb, Cd, Cr, Cu, Ni, Zn, As, and Se under the optimized experimental conditions has also been investigated. Al-AC found to be a highly promising material for removal of fluoride and toxic metals from drinking water.

Green Synthesis of Hydroxyapatite Nanoparticles Using Monoon longifolium Leaf Extract for Removal of Fluoride from Aqueous Solution

Hydroxyapatite (Ca10(PO4)6(OH)2) calcium phosphate is a robust and viable magnetic material for the treatment of polluted air, water, and soil. Because of its unique structure and appealing properties such as high adsorption capabilities, acid-base adaptability, ion-exchange capability, and thermal stability, hydroxyapatite (HAp) has a lot of potential in the field of environmental management. An aqueous extract of Monoon longifolium leaves was used for the preparation of hydroxyapatite nonparticles as the adsorbent for fluoride ion removal from aqueous solution in this work, resulting in bio-based hydroxyapatite nanoparticles. The prepared adsorbent was characterized by using instrumental techniques such as TGA/DTA, XRD, AAS, FT-IR, and UV-Vis spectroscopy as well as SEM. The batch adsorption approach was used to determine the optimum adsorption efficiency of HAp NPs under various experimental conditions. As a result, the best removal efficiency corresponds to 0.75 g HAp NPs, 15 mg/L, and pH 7 at 50 minutes (96%). The equilibrium adsorption data were better fitted into the Freundlich isotherms (R2 = 0.99), and the pseudo-second-order kinetic model was found to be suitable (R2 = 0.99) for the kinetic model. Fluoride ion adsorption on HAp NPs is spontaneous, endothermic, and possible at temperatures over 318 K, according to thermodynamic calculations. The results hint at a conclusion that the synthesized HAp NPs were an efficient adsorbent for the removal of fluoride ions and the overall process can be an economical choice for scaled-up water treatment processes.

Tailored design of a novel composite foam of sodium alginate used for fluoride ion removal

Fluoride is an essential micronutrient for humans. Nonetheless, when the amount of fluoride ion is greater than required, it will cause skeletal fluorosis and dental fluorosis to threaten human health. In this paper, a series of sodium alginate (SA)-based foam materials are prepared by freeze-drying technique and anchored with the nano-activated alumina (nAl₂O₃) in the SA to obtain a novel adsorbent of SA-nAl₂O₃ foam used for fluoride ions removal. The SA-nAl₂O₃ foam morphology was further explored and confirmed that nAl₂O₃ existed stably in the SA. The adsorption results showed that the maximal fluoride ion adsorption capacity was 5.09 mg/g with 20 mg/L fluorine solutions at a pH of 3. The adsorption isotherm fitted adequately to the Langmuir isotherm model, which demonstrated that the adsorption process is closer to monolayer adsorption. The adsorption kinetics behavior of SA-nAl₂O₃ foam was described by a pseudo-second-order model, and the adsorption process occurred by chemisorption. Adsorption thermodynamics analysis emphasized that the adsorption process was spontaneous and endothermic. The main mechanism of the foam is ion exchange. The SA-nAl₂O₃ foam exhibited excellent regeneration performance and stability after three cycles.

Adsorption Characteristics and Charge Transfer Kinetics of Fluoride in Water by Different Adsorbents

Water containing high concentrations of fluoride is widely distributed and seriously harmful, largely because long-term exposure to fluoride exceeding the recommended level will lead to fluorosis of teeth and bones. Therefore, it is imperative to develop cost-effective and environmentally friendly adsorbents to remove fluoride from polluted water sources. In this study, diatomite (DA), calcium bentonite (CB), bamboo charcoal (BC), and rice husk biochar (RHB) were tested as adsorbents to adsorb fluoride (F^‐) from water, and this process was characterized by scanning electron microscopy (FEI-SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR). The effects of pH, dosage, and the initial mass concentration of each treatment solution upon adsorption of F^‐ were determined. Kinetic and thermodynamic models were applied to reveal the mechanism of defluoridation, and an orthogonal experiment was designed to obtain the optimal combination of conditions. The results show that the surfaces of CB, BC, and RHB have an irregular pore structure and rough surface, whereas DA has a rich pore structure, clear pores, large specific surface area, and high silica content. With regard to the adsorption process for F^‐, DA has an adsorption complex electron interaction; that of CB, BC, and RHB occur mainly via ion exchange with positive and negative charges; and CB on F^‐ relies on chemical electron bonding adsorption. The maximum adsorption capacity of DA can reach 32.20 mg/g. When the mass concentration of fluoride is 100 mg/L, the pH value is 6.0 and the dosage is 4.0 g/L; the adsorption rate of F^‐ by DA can reach 91.8%. Therefore, we conclude that DA soil could be used as an efficient, inexpensive, and environmentally friendly adsorbent for fluoride removal, perhaps providing an empirical basis for improving the treatment of fluorine-containing water in the future.

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.

Functional activated carbon: from synthesis to groundwater fluoride removal

Developing green and functional adsorbents for the removal of inorganic pollutants from industrial wastewater is still a great challenge. Activated carbons (ACs) are promising eco-friendly materials for adsorption applications. This study reports on the preparation and functionalization of AC and its application for fluoride removal from water. Activated carbon was prepared from date stems, and the material was employed as a support for different modifications such as incorporation of Al(OH)₃, in situ dispersion of aluminum particles (Al⁰) and grafting of 3-(aminopropyl)triethoxysilane (APTES). The resulting functional adsorbents were fully characterized by Fourier transform infrared spectroscopy, scanning electronic microscopy, energy dispersive X-ray fluorescence, X-ray diffraction, differential scanning calorimetry and zeta potential analysis. The results evidenced successful surface modifications. All adsorbents had affinity for the removal of fluoride ions (F⁻). The highest F⁻ removal rate was up to 20 mg g⁻¹ for AC-Al(OH)₃. Removal of fluoride ions obeyed Langmuir isotherms and a second-order kinetic model, and reached 99% uptake. The AC-Al(OH)₃ adsorbent was successfully used to treat a groundwater solution contaminated by fluoride ions. These results open an interesting avenue for developing eco-friendly functionalized AC for adsorption applications.

Conversion and surface modification of date stems to obtain a relevant adsorbent to remove fluoride contamination.

Comparison of Water Defluoridation Using Different Techniques

Fluoride pollution in subsurface water is a significant problem for different nations across the world because of the intake of excessive fluoride caused by the drinking of the contaminated subsurface. Water pollution by flouride can be attributed to the natural and human-made agents. Increased levels of fluoride in drinking water may result in the irretrievable demineralization of bone and tooth tissues, a situation called fluorosis, and other disorders. There has long been a need for fluoride removal from drinking water to make it safe for human use. Among the various fluoride removal methods, adsorption is the method most popularly used due to its cheap cost, ease of utilization, and being a scalable and simple physical technique. According to the findings of this study, the highest concentration of fluoride (0.1–15.0 mg/L) was found in Sweden and the lowest (0.03–1.14 mg/L) in Italy. We collected the values of adsorption capacities and fluoride removal efficiencies of various types of adsorbents from valuable released data accessible in the literature and exhibited tables. There is still a need to find the actual possibility of using biosorbents and adsorbents on a commercial scale and to define the reusability of adsorbents to decrease price and the waste generated from the adsorption method. This article reviews the currently available methods and approaches to fluoride removal of water.

Optimization of Fluoride Adsorption on Acid Modified Bentonite Clay Using Fixed-Bed Column by Response Surface Method

Using small-scale batch tests, various researchers investigated the adsorptive removal of fluoride using low-cost clay minerals, such as Bentonite. In this study, Column adsorption studies were used to investigate the removal of fluoride from aqueous solution using acid-treated Bentonite (ATB). The effects of initial fluoride concentration, flow rates, and bed depth on fluoride removal efficiency (R) and adsorption capability (qe) in continuous settings were investigated, and the optimal operating condition was determined using central composite design (CCD). The model's suitability was determined by examining the relationship between experimental and expected response values. The analysis of variance was used to determine the importance of independent variables and their interactions. The optimal values were determined as the initial concentration of 5.51 mg/L, volumetric flow rate of 17.2 mL/min and adsorbent packed-bed depth of 8.88 cm, with % removal of 100, adsorptive capacity of 2.46 mg/g and desirability of 1.0. This output reveals that an acid activation of Bentonite has made the adsorbent successful for field application.

Recycling electro-coagulated sludge from textile wastewater treatment plants as an adsorbent for the adsorptions of fluoride in an aqueous solution

This research investigated the high content of iron-based materials from recycled electro-coagulated (EC) sludge for the adsorptive removal of fluoride, and the properties of the material were characterized. The thermal activation of EC sludge in which the unwanted impurity was removed by beneficiation and thermally activated at 500 °C, and was used for fluoride removal. Basic operating parameters (mixing time, adsorbent dosage, adsorbate concentration, solution pH, and temperature) were examined to evaluate the optimum de-fluoridation capacity (DC). The functional groups, the crystalline structure, and surface morphology of thermally treated and raw EC sludge were analyzed using FTIR, XRD, and SEM, respectively, and demonstrates that thermally activated EC sludge contains significant content of magnetite and hematite. The optimum DC was recorded as 5.12 mg of F⁻/gm with experimental conditions: mixing time = 20 min, adsorbent dosage = 0.3 gm/100 ml, initial fluoride concentration = 1 mg/L, and pH = 5 at the temperature of 353 K. The Langmuir isotherm model was fitted, and the capacity is calculated as 6.43 mg/g. The adsorption process follows the Pseudo-Second-order kinetic models. It can be concluded that the prepared adsorbents have excellent fluoride removal capacity, and EC sludge can be used as an alternative adsorbent for de-fluoridation.

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Iron-based oxides and hydroxides from the EC sludge were recovered and prepared for fluoride ion adsorption.

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EC sludge as an iron-based adsorbent was synthesized by thermal activation at 500 °C.

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Iron oxide adsorbents could efficiently remove fluoride ions from synthetically prepared water solutions.

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The adsorption of fluoride followed a Langmuir isotherm pseudo-Second-order kinetic model.

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The prepared adsorbents were regenerated in an aqueous solution and the reusability efficiency was up to the 4th cycle.

Immobilization of recombinant Escherichia coli cells expressing glucose isomerase using modified diatomite as a carrier for effective production of high fructose corn syrup in packed bed reactor

To improve the operational stability of glucose isomerase in E. coli TEGI-W139F/V186T, the immobilized cells were prepared with modified diatomite as a carrier and 74.1% activity of free cells was recovered after immobilization. Results showed that the immobilized cells still retained 86.2% of the initial transformational activity after intermittent reused 40 cycles and the yield of D-fructose reached above 42% yield at 60 °C. Moreover, the immobilized cells were employed in the continuous production of High Fructose Corn Syrup (HFCS) in a recirculating packed bed reactor for 603 h at a constant flow rate. It showed that the immobilized cells exhibited good operational stability and the yield of D-fructose retained above 42% within 603 h. The space-time yield of high fructose corn syrup reached 3.84 kg L^-1 day^-1. The investigation provided an efficient immobilization method for recombinant cells expressing glucose isomerase with higher stability, and the immobilized cells are a promising biocatalyst for HFCS production.

Electrocoagulation as a Promising Defluoridation Technology from Water: A Review of State of the Art of Removal Mechanisms and Performance Trends

Fluoride ions present in drinking water are beneficial to human health when at proper concentration levels (0.5–1.5 mg L−1), but an excess intake of fluoride (>1.5 mg L−1) may pose several health problems. In this context, reducing high fluoride concentrations in water is a major worldwide challenge. The World Health Organization has recommended setting a permissible limit of 1.5 mg L−1. The application of electrocoagulation (EC) processes has received widespread and increasing attention as a promising treatment technology and a competitive treatment for fluoride control. EC technology has been favourably applied due to its economic effectiveness, environmental versatility, amenability of automation, and low sludge production. This review provides more detailed information on fluoride removal from water by the EC process, including operating parameters, removal mechanisms, energy consumption, and operating costs. Additionally, it also focuses attention on future trends related to improve defluoridation efficiency.

Acid Modification of Diatomite-Based Sorbents

In this work, the effect of acid pre-treatment (hydrochloric acid, HCl) and calcination of diatomite, a silicon dioxide-material from natural sources, was studied with the aim to obtain diatomite-based sorbents with specific physicochemical properties. For this, acid pre-treatments with HCl at different calcination conditions, namely HCl concentration (0.5, 1 M) and calcination temperatures (from 600 to 900 °C) were studied. Morphological features different from those of natural diatomite were obtained. It has been found that treatment of diatomite with 0.5 M HCl at 800 °C showed a specific pore volume of 0.008 cm3/g, and a specific surface area of 19.26 m2/g, while the treatment of diatomite with 1.0 M HCl showed a specific pore volume of 0.011cm3/g, and a specific surface area of 25.57 m2/g. The performance of the acid pretreatment of diatomite for adsorption of Pb ions from water was also studied.