Fluoride in Drinking Water and Skeletal Fluorosis: a Review of the Global Impact

When safe and adequate exposure of an essential trace element is exceeded it becomes potentially toxic. Fluoride is one classic example of such a double edged sword which both plays a fundamental role in the normal growth and development of the body for example the consumption of levels between 0.5-1.0 ppm via drinking water is beneficial for prevention of dental caries but its excessive consumption leads to development of fluorosis. PURPOSE OF REVIEW: The abundance of fluorine in the environment as well as in drinking water sources are the major contributors to fluorosis. It is a serious public health concern as it is a noteworthy medical problem in 24 nations including India yet the threat of fluorosis has not been rooted out. The review focuses on recent findings related to skeletal fluorosis and role of oxidative stress in its development. The fluoride mitigation strategies adopted in recent years are also discussed. RECENT FINDINGS BASED ON CASE STUDIES: Recent findings revealed that consumption of fluoride at concentrations of 1.5 ppm is majorly responsible for skeletal fluorosis. The sampling from rural areas showed that 80% villages are having fluoride concentrations more than the WHO permissible limits and people residing in such areas are affected by the skeletal fluorosis and also in the regions of Africa and Asia endemic fluorosis have been accounted in the majority of the region affecting approximately 100 million people. Various mitigation programmes and strategies have been conducted all over the world using defluoridation. Fluorosis is a slow and progressive malady affecting our body and a serious concern to be taken into consideration and to be dealt with effectively. The fluoride toxicity although reversible, is a slow process and the side effects lack treatment options. The treatment options available are either not approachable or affordable in the rural areas commonly suffering from the fluoride toxicity. No specific treatments are available to date to treat skeletal fluorosis affectively; therefore, prevention is one of most safest and best approach to fight fluorosis. The current review lays emphasis on the skeletal fluorosis and its prevalence in recent years. It also includes the recent findings as well as the current strategies related to combat skeletal fluorosis and provides findings that might be helpful to promote the research in the field of effective treatment for fluorosis as well as development of easy and affordable methods of fluoride removal from water.

Fluoride contamination, health problems and remediation methods in Asian groundwater: A comprehensive review

In low concentration, fluoride is considered a necessary compound for human health. Exposure to high concentrations of fluoride is the reason for a serious disease called fluorosis. Fluorosis is categorized as Skeletal and Dental fluorosis. Several Asian countries, such as India, face contamination of water resources with fluoride. In this study, a comprehensive overview on fluoride contamination in Asian water resources has been presented. Since water contamination with fluoride in India is higher than other Asian countries, a separate section was dedicated to review published articles on fluoride contamination in this country. The status of health effects in Asian countries was another topic that was reviewed in this study. The effects of fluoride on human organs/systems such as urinary, renal, endocrine, gastrointestinal, cardiovascular, brain, and reproductive systems were another topic that was reviewed in this study. Different methods to remove fluoride from water such as reverse osmosis, electrocoagulation, nanofiltration, adsorption, ion-exchange and precipitation/coagulation were introduced in this study. Although several studies have been carried out on contamination of water resources with fluoride, the situation of water contamination with fluoride and newly developed technology to remove fluoride from water in Asian countries has not been reviewed. Therefore, this review is focused on these issues: 1) The status of fluoride contamination in Asian countries, 2) health effects of fluoride contamination in drinking water in Asia, and 3) the existing current technologies for defluoridation in Asia.

A review of emerging adsorbents and current demand for defluoridation of water: Bright future in water sustainability

Fluoride contamination of groundwater is a serious problem in several countries of the world because of the intake of excessive fluoride caused by the drinking of the contaminated groundwater. Geological and anthropogenic factors are responsible for the contamination of groundwater with fluoride. Excess amounts of fluoride in potable water may cause irreversible demineralisation of bone and tooth tissues, a condition called fluorosis, and long-term damage to the brain, liver, thyroid, and kidney. There has long been a need for fluoride removal from potable water to make it safe for human use. From among several defluoridation technologies, adsorption is the technology most commonly used due to its cost-effectiveness, ease of operation, and simple physical process. In this paper, the adsorption capacities and fluoride removal efficiencies of different types of adsorbents are compiled from relevant published data available in the literature and represented graphically. The most promising adsorbents tested so far from each category of adsorbents are also highlighted. There is still a need to discover the actual feasibility of usage of adsorbents in the field on a commercial scale and to define the reusability of adsorbents to reduce cost and the waste produced from the adsorption process. The present paper reviews the currently available methods and emerging approaches for defluoridation of water.

Polypyrrole-grafted peanut shell biological carbon as a potential sorbent for fluoride removal: Sorption capability and mechanism

In this study, an effective defluoridation adsorbent was developed by depositing polypyrrole (PPy) on granular peanut shell biological carbon (BC) via in situ chemical oxidative polymerization. The variables of defluoridation process (i.e., adsorbent dosage, fluoride solution pH, and anionic interference) were tested. The mechanism was determined by isotherm and kinetic studies, Brunauer-Emmett-Teller (BET) method, scanning electronic microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy and automatic titration. The PPy-grafted BC (PPy/BC) composite performed commendably from pH 2.0 to 10.0, and exhibited high selectivity for fluoride in the presence of several co-existing anions. The experimental data were described well by a Langmuir isotherm curve, and the maximum adsorption capacity was 17.15 mg g(-1). Kinetic studies illustrated the adsorption process was accomplished via surface adsorption as well as by intraparticle diffusion. In addition, mesoporous diffusion was the rate-controlling step in intraparticle diffusion process. BET and SEM analysis revealed the sponge-like polymer adhered to the BC and plugged the pores. XPS, FTIR, and SEM confirmed that fluoride removal was accomplished via the replacement of doped ionizable chloride ions (Cl(-)) coupled with positively charged nitrogen (N(+)), computation of XPS data enabled the formulation of a three-layer-deep hypothesis for PPy.

Modification of carbon derived from Sargassum sp. by lanthanum for enhanced adsorption of fluoride

Excessive fluoride in water causes serious environmental issues and adverse impacts on human health. In this study, an innovative lanthanum-modified carbon (LMC) adsorbent rooted in Sargassum sp. was developed for fluoride removal. Excellent removal efficiency was observed over a wide pH range of 3-9. Almost 90% of fluoride adsorption occurred within the first 1h; the equilibrium was established within 4h. The maximum adsorption capacity of LMC could reach 94.34 mg/g at neutral pH, much higher than many commercial adsorbents. Although the presence of such competitive anions as SiO3(2-), SO4(2-), HCO3(-), PO4(3-) and AsO3(-) had certain interference on fluoride uptake, it was noticed that there was no significant impact in the presence of humic acid. Furthermore, according to the instrumental analysis, the fluoride removal was majorly controlled by outer-sphere complex adsorption, while electrostatic attraction and ion exchange mechanisms could also be observed in the fluoride adsorption process. The findings from this study suggest that our adsorbent may have a great potential in industrial applications.

Heterotrophic nitrification and biomineralization potential of Pseudomonas sp. HXF1 for the simultaneous removal of ammonia nitrogen and fluoride from groundwater

Pseudomonas sp. HXF1, a strain capable of heterotrophic nitrification, aerobic denitrification (HNAD), and biomineralization was identified and employed for the simultaneous removal of ammonia nitrogen (NH₄⁺-N) and fluoride (F^-). It removed 99.2% of NH₄⁺-N without accumulation of nitrous nitrogen (NO₂^--N) and nitrate nitrogen (NO₃^--N), while removed 87.3% of F^-. Response surface methodology (RSM) was used to study the best removal conditions for NH₄⁺-N and F^-. The results of nitrogen balance experiments with NH₄Cl, NaNO₂, and NaNO₃ as single nitrogen sources and amplification experiments of denitrification genes proved that the bacterial strains may remove NH₄⁺-N through HNAD. The experimental results of Scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffractometer (XRD) indicated that the way of F^- removal may be adsorption and co-precipitation. The results demonstrated that the strain HXF1 has great potential in the biological denitrification and F^- removal of groundwater.

Fluoride exposure from groundwater as reflected by urinary fluoride and children's dental fluorosis in the Main Ethiopian Rift Valley

This cross-sectional study explores the relationships between children's F(-) exposure from drinking groundwater and urinary F(-) concentrations, combined with dental fluorosis (DF) in the Main Ethiopian Rift (MER) Valley. We examined the DF prevalence and severity among 491 children (10 to 15 years old) who are life-long residents of 33 rural communities in which groundwater concentrations of F(-) cover a wide range. A subset of 156 children was selected for urinary F(-) measurements. Our results showed that the mean F(-) concentrations in groundwater were 8.5 ± 4.1 mg/L (range: 1.1-18 mg/L), while those in urine were 12.1±7.3 mg/L (range: 1.1-39.8 mg/L). The prevalence of mild, moderate, and severe DF in children's teeth was 17%, 29%, and 45%, respectively, and the majority (90%; n=140) of the children had urinary F(-) concentrations above 3 mg/L. Below this level most of the teeth showed mild forms of DF. The exposure-response relationship between F(-) and DF was positive and non-linear, with DF severity tending to level off above a F(-) threshold of ~6 mg/L, most likely due to the fact that at ~6 mg/L the enamel is damaged as much as it can be clinically observed in most children. We also observed differential prevalence (and severity) of DF and urinary concentration, across children exposed to similar F(-) concentrations in water, which highlights the importance of individual-specific factors in addition to the F(-) levels in drinking water. Finally, we investigated urinary F(-) in children from communities where defluoridation remediation was taking place. The lower F(-) concentration measured in urine of this population demonstrates the capacity of the urinary F(-) method as an effective monitoring and evaluation tool for assessing the outcome of successful F(-) mitigation strategy in relatively short time (months) in areas affected with severe fluorosis.

Performance and mechanism of fluoride adsorption from groundwater by lanthanum-modified pomelo peel biochar

To obtain an economical and effective adsorbent for fluoride removal, lanthanum-loaded pomelo peel biochar (PPBC-La) was synthesized using a facile approach. The batch adsorption experiments were investigated to determine adsorbent performance. The PPBC-La and its pristine biochar (PPBC) were characterized by scanning electronic microscopy (SEM), zeta potential, Brunauer-Emmett-Teller (BET), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) methods. Experimental results showed that the adsorption data were described well by the pseudo-second-order kinetic and Freundlich isotherm models. The maximum fluoride adsorption capacity for PPBC-La was found to be 19.86 mg/g at 25 °C and pH 6.5. The PPBC-La worked well at pH 2.4-9.6 and carried positive charge at pH < 5.8. The presence of SO₄^2-, Cl^-, and NO₃^- had a slight effect on fluoride uptake except HCO₃^- and PO₄^3-. The real groundwater study testified that 9.8 mg/L of fluoride was removed effectively at 1.0 g/L of dosage and pH 5.2. The regeneration results revealed that the PPBC-La had a good reusability. According to FTIR, XPS analysis and the anion exchange experiment, anions (NO₃^- and OH^-) exchange with fluoride ions was mainly responsible for fluoride adsorption.

Modified hydrous zirconium oxide/PAN nanofibers for efficient defluoridation from groundwater

Fluoride contamination in groundwater is a worldwide problem that is related to human health. Zirconium-based adsorbents possess satisfactory selective defluoridation capacities. However, narrow efficiency pH range, easy aggregation and difficult separation are the main obstructions in practical application. In this study, the branched polyethyleneimine (bPEI) modified hydrous zirconium oxide (HZO)/polyacrylonitrile (PAN) nanofibers (NFs) are synthesized by immobilizing bPEI-HZO into PAN nanofibers via electrospinning. The resultant bPEI-HZO/PAN NFs exhibit a wide working pH range and an excellent adsorption capacity toward fluoride (67.51 mg·g^-1) even at neutral condition, indicating non-negligible superiority in the practical application of groundwater defluoridation. This enhanced adsorption performance along with extended wider working pH range are ascribed to the optimization of the adsorbents from both composition and structure. Compositionally, the modification of bPEI improves the surface property of HZO, and thus increases fluoride capacity in alkaline groundwater. Structurally, electrospinning conquers the drawbacks of nano-adsorbents for both easy aggregation and difficult separation. In addition, the effect of co-existing ions was further investigated and the X-ray photoelectron spectroscopy (XPS) as well as fourier transform infrared spectrum (FTIR) measurements were used to clarify the fluoride adsorption mechanism. Furthermore, the dynamic adsorption and regeneration performance were accomplished through the fixed-bed column experiment. All the results indicated that bPEI-HZO/PAN NFs are promising materials for defluoridation from groundwater.

Age-sex specific disability-adjusted life years (DALYs) attributable to elevated levels of fluoride in drinking water: A national and subnational study in Iran, 2017

National and subnational burden of disease attributable to elevated fluoride levels in drinking water apportioned by sex, age group, province, and community type in Iran, 2017 were quantified based on disability-adjusted life years (DALYs). The attributable burden of disease was estimated using four input data: (1) effect size of elevated drinking water fluoride levels for dental and skeletal fluorosis, (2) population distribution of drinking water fluoride levels, (3) the threshold levels of fluoride in drinking water for contribution in dental and skeletal fluorosis, and (4) age-sex distribution of population. The attributable burden of disease was only related to dental fluorosis, because the fluoride levels were lower than the threshold value for skeletal fluorosis (4.0 mg/L) in all of the cases. The national attributable prevalence (per 100,000 people), DALYs, and DALY rate in 2017 were calculated to be 60 (95% uncertainty interval 48-69), 3443 (1034-6940), and 4.31 (1.29-8.68), respectively. The national attributable burden of disease was not significantly different by sex, but was affected by age and community type in a manner that the highest DALY rate was related to the age group 10-14 y (6.06 [1.82-12.21]) and over 66% of the national attributable DALYs occurred in rural communities. The attributable burden of disease occurred only in 10 out of 31 provinces and about 94% of the attributable DALYs were concentrated in four provinces Fars (1967 [592-3964]), Bushehr (414 [124-836]), West Azarbaijan (400 [120-808]), and Hormozgan (377 [113-761]). Implementation of fluoride-safe drinking water supply schemes in the four leading provinces can prevent most of the national health losses and partly compensate the increasing trend of disease burden from oral conditions at the national level.

Fluoride removal from groundwater by limestone treatment in presence of phosphoric acid

Fluoride removal from groundwater has been studied by addition of dilute phosphoric acid (PA) to the influent water before limestone treatment through laboratory plug-flow column experiments and bench-scale plug-flow pilot tests. In this PA-enhanced limestone defluoridation (PAELD) technique, fluoride is removed from 0.526 mM to 0.50-52.60 μM in 3 h with near neutral final pH. The presence of PA increases the fluoride removal capacity of limestone to 1.10 mg/g compared to 0.39 mg/g reported in its absence. The changes in fluoride removal with variation in initial PA concentration, initial fluoride concentration and the final pH have been found to be statistically significant with p < 0.05. The estimated recurring cost is US$ ≈0.58/m(3) water. Simple scrubbing and rinsing is a preferable method for regeneration of limestone as it is almost equally effective with lime or NaOH. Sorption of fluoride by calcium phosphates produced in situ in the reactor is the dominant mechanism of fluoride removal in the PAELD. Precipitation of CaF2 and sorption of fluoride by the limestone also contribute to the fluoride removal. High efficiency, capacity, safety, environment-friendliness, low cost and simplicity of operation make the PAELD a potential technique for rural application.

Bio-inspired materials for defluoridation of water: A review

The polluted water sources pose a serious issue concerning the various health hazards they bring along. Due to various uncontrolled anthropogenic and industrial activities, a great number of pollutants have gained entry into the water systems. Among all the emerging contaminants, anionic species such as fluoride cause a major role in polluting the water bodies because of its high reactivity with other elements. The need for water remediation has led the research community to come up with several physicochemical and electrochemical methods to remove fluoride contamination. Among the existing methods, biosorption using bio and modified biomaterials has been extensively studied for defluoridation, as they are cheap, easily available and effectively recyclable when compared to other methods for defluoridation. Adding on, these materials are non-toxic and are safe to use compared to many other synthetic materials that are toxic and require high-cost design requirements. This review focuses on the recent developments made in the defluoridation techniques by biosorption using bio and modified biomaterials and defines the current perspectives of fluoride removal specifically using biomaterials.

Mitigation of Fluorosis - A Review

Fluoride is required for normal development and growth of the body. It is found in plentiful quantity in environment and fluoride content in drinking water is largest contributor to the daily fluoride intake. The behaviour of fluoride ions in the human organism can be regarded as that of "double-edged sword". Fluoride is beneficial in small amounts but toxic in large amounts. Excessive consumption of fluorides in various forms leads to development of fluorosis. Fluorosis is major health problem in 24 countries, including India, which lies in the geographical fluoride belt. Various technologies are being used to remove fluoride from water but still the problem has not been rooted out. The purpose of this paper is to review the available treatment modalities for fluorosis, available technologies for fluoride removal from water and ongoing fluorosis mitigation programs based on literature survey. Medline was the primary database used in the literature search. Other databases included: PubMed, Web of Science, Google Scholar, WHO, Ebscohost, Science Direct, Google Search Engine, etc.

Enhanced fluoride uptake by bimetallic hydroxides anchored in cotton cellulose/graphene oxide composites

A novel hybrid nanomaterial was synthesized by embedding the bimetallic Zr and La (hydro)xides onto the cotton cellulose/graphene oxide composites (CC/GO composites), forming the Zr-La-CC/GO nanocomposites. Selective uptake of fluoride onto the Zr-La /GO hybrids in multiple competitive environments were evaluated. Morphological characteristics of Zr-La-CC/GO nanocomposites reflected the well distributions of embedded Zr and La hydroxides in the nanocomposites. Results also indicated that the encapsulated bimetallic hydroxides in Zr-La-CC/GO hybrids exhibited extremely high fluoride adsorption capacity and stability. XPS investigation exhibited the strong ZrF and LaF bonds in spent Zr-La-CC/GO nanocomposites, and the bonds were weakened at higher pH, which was consistent with the adsorption results. In addition, CC/GO composites using as the host could also exert the strong shielding effect to improve the stability of embedded La and Zr species so as only a low La dissolution (<4.2%) and almost no Zr leaching (0.1%) were observed in high HA concentration. What's more, the Zr-La-CC/GO nanocomposites have also shown great potential application for defluoridation in field.

Fluoride removal from water using a magnesia-pullulan composite in a continuous fixed-bed column

A magnesia-pullulan composite (MgOP) was previously shown to effectively remove fluoride from water. In the present study, a continuous fixed-bed column was used to examine the application of the composite at an industrial scale. The influencing parameters included bed mass (4.0, 6.0 and 8.0 g), influent flow rate (8, 16 and 32 mL/min), inlet fluoride concentration (5, 10 and 20 mg/L), reaction temperature (20, 30 and 40 °C), influent pH (4, 7 and 10) and other existing anions (HCO₃^-, SO₄^2-, Cl^- and NO₃^-), through which the breakthrough curves could be depicted for the experimental data analysis. The results indicated that MgOP is promising for fluoride removal with a defluoridation capacity of 16.6 mg/g at the bed mass of 6.0 g, influent flow rate of 16 mL/min and inlet fluoride concentration of 10 mg/L. The dynamics of the fluoride adsorption process were modeled using the Thomas and Yan models, in which the Yan model presented better predictions for the breakthrough curves than the Thomas model. Moreover, the concentration of magnesium in the effluent was monitored to determine Mg stability in the MgOP composite. Results indicated the effluent concentration of Mg²⁺ ions could be kept at a safe level. Calcination of fluoride-loaded MgOP effectively regenerated the material.

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

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

Evaluation of fluoride bioremediation and production of biomolecules by living cyanobacteria under fluoride stress condition

Application of microalgae for defluoridation has gained interest in recent years. In the present study, bioremediation of fluoride using living cyanobacteria, Starria zimbabweensis, collected from wastewater of coke-oven effluent treatment plant, Durgapur, India, has been investigated. Initially, the cyanobacterial strain was grown in BG11 medium at 25°C, 45μmol/m²/s irradiation in 18h: 6h light:dark cycle in an algal incubator. Samples were withdrawn after 2 days interval and analyzed for its dry biomass and lipid content. Optimum inoculum size of 10% and age of 16th day were assessed based on maximum dry biomass (9.307 ± 0.01g/L) and lipid (244.05 ± 0.02mg/L) production. SEM-EDX and FTIR studies of both native and fluoride treated biomass were done to emphasize the changes. During kinetic study of defluoridation, initial fluoride concentration was varied in the range of 10-50mg/L. Maximum fluoride removal (66.6 ± 0.11%) and dry biomass (18.19 ± 0.12g/L) were obtained at 10mg/L fluoride concentration using 10% of 16th day's inoculum. Biomass and lipid content were found to increase 2 and 4 folds, respectively under fluoride stress condition. Furthermore, chlorophyll, carbohydrate and protein content of the biomass were also compared between control and fluoride contaminated conditions. Fatty Acid Methyl Ester (FAME) analysis was done using Gas Chromatography (GC) to compare the lipid profile of native and fluoride loaded strain.

A comparative study on defluoridation capabilities of biosorbents: isotherm, kinetics, thermodynamics, cost estimation, and eco-toxicological study

The present study aims towards fluoride remediation from synthetic water using steam-activated carbon of Aegle marmelos (bael shell/wood apple) (BAC) and Parthenium hysterophorus (PHAC) according to batch sorption techniques. The impact of different parametric conditions viz. initial fluoride concentration (4-12 mg/L), time (0-5 h), temperature (293.15-333.15 K), adsorbent dosage (4-14 g/L), pH (4-9), and RPM (150-350) were considered for both the adsorbents. Maximum defluoridation of 89% was achieved by BAC at a concentration of 10 mg/L, adsorbent dose 6 g/L, pH 5, temperature 313.15 K, agitation speed 250 rpm, and contact time 9 h, whereas PHAC attained maximum removal of 78% at an initial concentration of 8 mg/L, adsorbent dose 10 g/L, pH 4, temperature 313.15 K, and contact time 12 h. Instrumental analysis by SEM, EDX, and FTIR confirmed about the fluoride binding ability of the adsorbents. The Langmuir isotherm model provided the best fit (R² = 0.9962 and 0.9945) to the removal process with maximum adsorptive uptake of 16.85 and 6.22 mg/g by BAC and PHAC respectively. The adsorption phenomenon was found to obey pseudo-second-order kinetics. The endothermic, spontaneous, and feasible nature of the sorption process was confirmed by the thermodynamic study. The total costs of 1 kg adsorbent preparation were calculated as 1.122 USD and 1.0615 USD which helped us in determining the economic feasibility of the adsorbents in large-scale applications. The growth of Chlorella sorokiniana BTA 9031 was also observed to be affected by the fluoride solution. Comparing the removal efficiencies of both the adsorbents, it can be concluded that BAC shell proved to be an efficient adsorbent over PHAC for fluoride elimination from aqueous solution. Graphical abstract Defluoridation of aqueous solution using biochar derived from Aegle marmelos shell and Parthenium hysterophorus.

Membrane-based technologies for zero liquid discharge and fluoride removal from industrial wastewater

Several defluoridation techniques for reducing high initial fluoride concentration (IFC) in wastewater have been tested, but only a few of them have achieved the permissible standards. This study examined the hybrid crystallization-reverse osmosis technique (HRO) in light of flux, fluoride removal efficiency, fouling tendency, mineral recovery, complying zero liquid discharge (ZLD), and effluent discharge standard (EDS). Simulated wastewater with an IFC of 6600 mg/L was utilized and the final HRO performance was compared with those of the low-pressure (30 bar) standalone reverse osmosis (SRO), nanofiltration (SNF), and membrane distillation (SMD) processes. Accordingly, the study on SRO and SNF revealed that pressure, feed pH, membrane type, and IFC were the major factors affecting performance, and SRO was unable to sufficiently defluoridate wastewater with IFC >614 mg/L, needing pretreatment. Subsequently, the HRO process was selected and it was seen that the optimum calcium dose and respective final effluent pH for attaining EDS and ZLD were 16.5 g/L & 7.1 and 19.8 g/L & 5.7 respectively. The best operating pH for all conditions in HRO was approximately 9. Additionally, HRO showed good mineral recovery tendency and less organic fouling. The overall comparisons of flux and residual fluoride for HRO, SRO, SNF, and SMD were 49.3 LMH & 1.21 mg/L; 34.9 LMH & 62 mg/L, 44.05 LMH & 301 mg/L, and 38 LMH & 0.9 mg/L respectively. Therefore, low-pressure HRO can be applied to treat wastewater with high IFC; good tendency of mineral recovery, as good as that of SMD.

Application of kinetic models to the design of a calcite permeable reactive barrier (PRB) for fluoride remediation

The kinetics of fluoride sorption by calcite in the presence of metal ions (Co, Mn, Cd and Ba) have been investigated and modelled using the intra-particle diffusion (IPD), pseudo-second order (PSO), and the Hill 4 and Hill 5 kinetic models. Model comparison using the Akaike Information Criterion (AIC), the Schwarz Bayseian Information Criterion (BIC) and the Bayes Factor allows direct comparison of model results irrespective of the number of model parameters. Information Criterion results indicate "very strong" evidence that the Hill 5 model was the best fitting model for all observed data due to its ability to fit sigmoidal data, with confidence contour analysis showing the model parameters were well constrained by the data. Kinetic results were used to determine the thickness of a calcite permeable reactive barrier required to achieve up to 99.9% fluoride removal at a groundwater flow of 0.1 m.day^-1. Fluoride removal half-life (t_0.5) values were found to increase in the order Ba ≈ stonedust (a 99% pure natural calcite) < Cd < Co < Mn. A barrier width of 0.97 ± 0.02 m was found to be required for the fluoride/calcite (stonedust) only system when using no factor of safety, whilst in the presence of Mn and Co, the width increased to 2.76 ± 0.28 and 19.83 ± 0.37 m respectively. In comparison, the PSO model predicted a required barrier thickness of ∼46.0, 62.6 & 50.3 m respectively for the fluoride/calcite, Mn and Co systems under the same conditions.

Microbial induced calcium precipitation based anaerobic immobilized biofilm reactor for fluoride, calcium, and nitrate removal from groundwater

In this study, the anaerobic quartz sand fixed biofilm reactor containing Cupriavidus sp. W12 was established to simultaneously remove calcium (Ca²⁺), fluoride (F^-) and nitrate (NO₃^-N) from groundwater. After 84 days of continuous operation, the optimum operating parameters and defluoridation mechanism were explored, and the microbial community structure under different pH environments were compared and analyzed. Under the optimal operation conditions (HRT of 6 h, initial Ca²⁺ concentration of 180 mg L^-1, and pH of 7.0), the removal efficiencies of Ca²⁺, F^-, and NO₃^-N were 58.97%, 91.93%, and 100%, respectively. Gas chromatography (GC) results indicate that N₂ is the main gas produced by the bioreactor. Three-dimension excitation emission matrix fluorescence spectroscopy (3D-EEM) showed that extracellular polymers (EPS) are produced during bacterial growth and metabolism. The results of Scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), and Fourier transform infrared spectrometer (FTIR) demonstrated that the defluoridation mechanism is attributed to the synergetic effects of ion exchange, co-precipitation, and chemisorption. The comparative analysis of the microbial community structure under different pH conditions show that Cupriavidus is the dominant bacteria in the bioreactor throughout the experiment, and it shows a prominent advantage at pH of 7.0. This research provides an application foundation for anaerobic microbial induced calcium precipitation (MICP) bioremediation of Ca²⁺, F^-, and NO₃^-N from groundwater.

Development of nanohybrid adsorbent for defluoridation from aqueous systems

The objective of present study is advancement of an efficient nanomaterial which was investigated to substantiate its efficiency, using kinetic studies to ensnare fluoride in order to make water potable. A new crystalline ZrAlCa nanohybrid adsorbent for fluoride removal was successfully synthesized by a co-precipitation method in this study. The prepared adsorbents were characterized by XRD, FT-IR, TGA, BET and FESEM and EDX. The adsorption properties of the developed adsorbent were studied using batch adsorption method which shown the noticeable fluoride removal efficiency up to 99% at near neutral pH as well as in acidic pH range. The reaction kinetics for adsorption of fluoride was established using reaction based kinetic models which fitted well with Avarami kinetic model as compared to pseudo-first-order, pseudo second-order and power function rate expression. The equilibrium isotherm modelling described adsorption process and Langmuir, Jovanovic, Temkin and Freundlich isotherms provides best fit to experimental data. The fluoride loaded adsorbent was efficiently regenerated by using an alkali solution and has no significant counter ion effect on fluoride adsorption efficiency. Interestingly, the developed nanomaterial has fluoride removal efficacy over varied concentration ranges. It has capability of reanimate and reuse the nanohybrid adsorbent makes it an attractive sustainable material.

Dental Fluorosis and Dental Caries Prevalence among 12 and 15-Year-Old School Children in Nalgonda District, Andhra Pradesh, India

BACKGROUND

Fluoride is a double edged sword. The assessment of dental caries and fluorosis in endemic fluoride areas will facilitate in assessing the relation between fluoride concentrations in water with dental caries, dental fluorosis simultaneously.

AIM

The objective of the following study is to assess the dental caries and dental fluorosis prevalence among 12 and 15-year-old school children in Nalgonda district, Andhra Pradesh, India.

SUBJECTS AND METHODS

This was a cross-sectional study. Two stage cluster sampling technique was employed to select 20 schools from Nalgonda district. The oral examination of available 12 and 15-year-old children fulfilling the inclusion and exclusion criteria was carried out to assess dental caries and fluorosis. The examination was conducted by a single trained and calibrated examiner using the mouth mirror and community periodontal index probe under natural daylight. These areas were divided into four categories, low, medium, high and very high fluoride areas based on the fluoride concentration at the time of statistical analysis. The data was analyzed using Statistical Package for the Social Sciences version 16 (IBM, Chicago, USA).

RESULTS

The caries prevalence was less among 12-year-old children (39.9% [369/924]) compared with 15-years-old children (46.7% [444/951]). The prevalence was more among females (50.4% [492/977]) than males (35.8% [321/898]). The prevalence was more in low fluoride area (60.5% [300/496]) followed by very high fluoride area (54.8% [201/367]), high fluoride area (32.4% [293/904]) and medium fluoride area (17.6% [19/108]) in the descending order. The fluorosis prevalence increased with increasing fluoride concentration with no difference in gender and age distribution.

CONCLUSION

Low fluoride areas require fluoridation or alternate sources of fluoride, whereas high fluoride areas require defluoridation. Defluoridation of water is an immediate requirement in areas with fluoride concentration of 4 parts per million and above as dental fluorosis is a public health problem in these areas with 100% prevalence.

Microencapsulated reactor for simultaneous removal of calcium, fluoride and phenol using microbially induced calcium precipitation: Mechanism and functional characterization

Fluoride ions (F^-) and phenol in groundwater have become a great hurdle to the pursuit of a healthy drinking water source. This study established a microencapsulated immobilization reactor with Aquabacterium sp. CZ3 for the simultaneous removal of nitrate (NO₃^--N), calcium (Ca²⁺), F^-, and phenol from groundwater with 100%, 67.84%, 88.67%, and 100% removal efficiencies, respectively. The three-dimensional mesh structure of microcapsules facilitated the transport and metabolism of substances, while their synergistic effect with bacteria promoted the removal of contaminants. F^- was removed by co-precipitation to generate Ca₅(PO₄)₃F and CaF₂ and adsorption. On one hand, the phenol toxicity promoted the production of extracellular polymers and improved the tolerance of bacteria; on the other hand, the degradation of phenol provided a carbon source for bacteria and promoted the denitrification. The development of microencapsulated immobilized reactor provided a clear mechanism for phenol and F^- removal under the microbially induced calcium precipitation (MICP) technique, while providing a valuable solution for the treatment of complex groundwater resources.

Temperature influence on layered double hydroxide tailored corncob biochar and its application for fluoride removal in aqueous media

Exposure to excess fluoride is a controversial public health concern as it can cause dental/skeletal fluorosis as well as renal toxicity. The study intended to evaluate the synergistic interaction of clay intercalation and thermochemical modification on corncob biochar to remove fluoride from aqueous solutions. Layered double hydroxide was assorted with thermally activated (torrefaction and pyrolysis) corncob biochar at 1:1 (w/w) ratio to obtain composites called LDH-CCBC250 and LDH-CCBC500. Physicochemically characterized adsorbents were assessed against the pH (3-9), reaction time (up to 12 h) and initial fluoride concentration (0.5-10 mg L^-1) for defluoridation. The porous structure of biochar was found to be richer compared to biocharcoal. The adsorption performance of LDH-CCBC500 was 6-fold higher compared to LDH-CCBC250 signifying the pronounced effect of thermal activation. Fluoride adsorption was pH dependent, and the best pH was in the range of pH 3.5-5.0 and there was no ionic strength dependency. Fluoride uptake by LDH-CCBC500 follows pseudo-second order and Elovich kinetic models, which suggests a chemisorption process followed by physisorption. The most expected way to eliminate fluoride by LDH-CCBC500, which had a maximum adsorption capacity of 7.24 mg g^-1, was cooperative chemical adsorption upon the Langmuir and Hills isotherm (r² = 0.99) parameters. Layered double hydroxide intercalated corncob biochar derived from slow pyrolysis is best performing in acidic waters.