An excellent fluoride sorption behavior of modified amberlite resin

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.

MOFs for the treatment of arsenic, fluoride and iron contaminated drinking water: A review

Over the last few decades, the global pollution of surface and groundwater poses a serious threat not only to human beings but also towards aquatic lives due to the presence of emerging contaminants. Among the others, the presence of arsenic, fluoride, and iron are considered as the most common toxic pollutants in water bodies. The emergence of metal organic frameworks (MOFs) with high porosity and surface area is represented as significant inclusion into the era of entrapping contaminants present in drinking water. In the present review article, an in-depth insight is provided on the recent developments in the removal of arsenic, fluoride, and iron from drinking water using MOFs. Various aspects related to the synthesis, latest technologies adopted for the modifications in the synthesis process and advanced applications of MOFs for the removal of such contaminants are explicitly discussed. A detailed insight was provided to understand the mechanism of various interactions of MOFs with arsenic and fluoride. With respect to arsenic, fluoride, and iron removal the ultrastructural morphology of MOFs is assessed based on different molecular arrangements. Further, commercial aspects of various MOFs are presented in order to highlight the process feasibility. Finally, various perspectives and challenges involved in process scale up are comprehensively narrated with an aspiration of futuristic developments. The paper will be beneficial to the readers for acquiring a piece of in-depth knowledge on MOFs and its various synthesis approaches along with remarkable achievements for the removal of arsenic, fluoride, and iron from contaminated drinking water.

Assessment of the Use of Epicarp and Mesocarp of Green Coconut for Removal of Fluoride Ions in Aqueous Solution

Fruit consumption and processing result in considerable volumes of residual biomass. Transformation of this biomass into biosorbents offers an alternative for its reuse and disposal. As the green coconut shell is a waste often discarded in landfills and dumps, generating gases and leachate, two biosorbents were developed from the epicarp and mesocarp of green coconut to adsorb fluoride ions in aqueous solution. The kinetic experiments showed that sorption of fluoride ions reached equilibrium at 300 min for both epicarp and mesocarp at temperatures of 25°C, 35°C, and 45°C. The removal efficiency of fluoride ions varied from 66.25% (at 25°C) to 77.50% (at 45°C) for the epicarp and from 90% (at 25°C) to 97.50% (at 45°C) for the mesocarp. The thermodynamic parameters of the adsorption process showed that adsorption is a spontaneous, endothermic process for both biosorbents. The adsorption was classified as chemical, with the Langmuir isotherm model best suited to the adsorption isotherms data.

Advances in coagulation technique for treatment of fluoride-contaminated water: a critical review

Fluoride contamination of groundwater has become a major concern worldwide, resulting in serious medical conditions such as dental and skeletal fluorosis. Consequently, the WHO recommends that drinking water should not contain more than 1.5 mg/l of fluoride. Various defluoridation techniques such as coagulation, reverse osmosis, activated alumina adsorption, and biosorbent adsorption have been developed. Adsorption through the activated alumina and biosorbent process is not cost effective and has regeneration problems, and the reverse osmosis process has the high initial cost which makes it unacceptable for developing countries. Coagulation is a commonly employed field technology for defluoridation, which involves the addition of aluminum salts, lime, and bleaching powder followed by rapid mixing, flocculation, sedimentation, and filtration but suffers from a limitation of high residual aluminum in treated water. This paper critically reviews the recent developments in the coagulation technique for defluoridation along with its comparison to other defluoridation techniques. The review describes the pertinent gaps in the process and throws open suggestions for extending research by citing the recent studies which may lead to the revival of the process. The description about the suspension of alumino-fluoro complexes that constitute a substantial part of the residual aluminum after alum treatment has been narrated in the paper that helps in a deeper understanding of the defluoridation mechanism. To make the process highly suitable for communities, appropriate technological interventions, such as converting it to a continuous mode of operation, replacing alum with poly-aluminum chloride (PAC), and attaching a micro-filtration unit in series of the existing process, can be done. Also, using PAC as a coagulant with sand filtration has to be considered for making the process more efficient.

Solid-phase extraction of explosive nitramines on macroreticular polymers modified by freezing with water or acetone

A novel approach is proposed to modify the porous structure and surface properties of the polymers used in solid-phase extraction. The approach involves soaking in water or acetone, followed by freezing in liquid nitrogen (77.4 K) and was employed for two polymeric materials: Amberlite XAD-7 and Amberlite XAD-16. Variations in the surface properties of the adsorbents were justified by the action of acetone and water as solvents affecting the textural and other characteristic of the materials. The initial and treated adsorbents were used in extraction of explosive nitramines from aqueous samples. The performed modifications of the polymer texture allow us to increase the recovery rate as compared with the initial adsorbents. The results were justified by the swelling of fragments of the polymers and by the additional process of sorption of nitramines. The results indicate that polymeric adsorbents can be easily modified by the soaking/freezing process and the materials can be achieved that prove usefulness for the effective separation of explosive nitramines from aqueous samples.

Sustainable approach for recycling waste lamb and chicken bones for fluoride removal from water followed by reusing fluoride-bearing waste in concrete

Sustainable management of waste materials is an attractive approach for modern societies. In this study, recycling of raw waste lamb and chicken bones for defluoridation of water has been estimated. The effects of several experimental parameters including contact time, pH, bone dose, fluoride initial concentration, bone grains size, agitation rate, and the effect of co-existing anions in actual samples of wastewater were studied for fluoride removal from aqueous solutions. Results indicated excellent fluoride removal efficiency up to 99.4% and 99.8% using lamb and chicken bones, respectively at fluoride initial concentration of 10 mg F/L and 120 min contact time. Maximum fluoride uptake was obtained at neutral pH range 6-7. Fluoride removal kinetic was well described by the pseudo-second order kinetic model. Both, Langmuir and Freundlich isotherm models could fit the experimental data well with correlation coefficient values >0.99 suggesting favorable conditions of the process. Furthermore, for complete sustainable management of waste bones, the resulted fluoride-bearing sludge was reused in concrete mixes to partially replace sand. Tests of the mechanical properties of fluoride sludge-modified concrete mixes indicated a potential environmentally friendly approach to dispose fluoride sludge in concrete and simultaneously enhance concrete properties.

Arsenic and fluoride contaminated groundwaters: A review of current technologies for contaminants removal

Chronic contamination of groundwaters by both arsenic (As) and fluoride (F) is frequently observed around the world, which has severely affected millions of people. Fluoride and As are introduced into groundwaters by several sources such as water-rock interactions, anthropogenic activities, and groundwater recharge. Coexistence of these pollutants can have adverse effects due to synergistic and/or antagonistic mechanisms leading to uncertain and complicated health effects, including cancer. Many developing countries are beset with the problem of F and As laden waters, with no affordable technologies to provide clean water supply. The technologies available for the simultaneous removal are akin to chemical treatment, adsorption and membrane processes. However, the presence of competing ions such as phosphate, silicate, nitrate, chloride, carbonate, and sulfate affect the removal efficiency. Highly efficient, low-cost and sustainable technology which could be used by rural populations is of utmost importance for simultaneous removal of both pollutants. This can be realized by using readily available low cost materials coupled with proper disposal units. Synthesis of inexpensive and highly selective nanoadsorbents or nanofunctionalized membranes is required along with encapsulation units to isolate the toxicant loaded materials to avoid their re-entry in aquifers. A vast number of reviews have been published periodically on removal of As or F alone. However, there is a dearth of literature on the simultaneous removal of both. This review critically analyzes this important issue and considers strategies for their removal and safe disposal.

Rationally designed porous polystyrene encapsulated zirconium phosphate nanocomposite for highly efficient fluoride uptake in waters

Fluoride pollution in waters has engulfed worldwide regions and an excess of fluoride intake always causes skeletal fluorosis. Herein, a novel hybrid nanomaterial ZrP-MPN was fabricated for fluoride retention by encapsulating nano-ZrP onto macroporous polystyrene materials modified with quaternary ammonium groups. The as-obtained materials exhibited favorable removal of fluoride ions from aqueous solution in presence of common anions (SO4(2-)/NO3(-)/Cl(-)) at high contents. Moreover outstanding sorption properties were also detected by involving series of commercial adsorbents (AA/magnetite/GFH/manganese sands) as references. Such satisfactory performances might be ascribed to the structural design of nanocomposite. (1) the CH2N(+)(CH3)3Cl groups enhances sorption diffusion and preconcentration in sorbent phase theoretically based on Donnan membrane principle; (2) the embedded ZrP nanoparticles also devotes to the efficient adsorption capacities due to its size-dependent specific properties. Additionally, the exhausted ZrP-MPN could be regenerated readily by alkaline solution. Thus, ZrP-MPN was a promising material for fluoride retention in waters.

Defluoridation of drinking water using adsorption processes

Excessive intake of fluoride (F), mainly through drinking water, is a serious health hazard affecting humans worldwide. There are several methods used for the defluoridation of drinking water, of which adsorption processes are generally considered attractive because of their effectiveness, convenience, ease of operation, simplicity of design, and for economic and environmental reasons. In this paper, we present a comprehensive and a critical literature review on various adsorbents used for defluoridation, their relative effectiveness, mechanisms and thermodynamics of adsorption, and suggestions are made on choice of adsorbents for various circumstances. Effects of pH, temperature, kinetics and co-existing anions on F adsorption are also reviewed. Because the adsorption is very weak in extremely low or high pHs, depending on the adsorbent, acids or alkalis are used to desorb F and regenerate the adsorbents. However, adsorption capacity generally decreases with repeated use of the regenerated adsorbent. Future research needs to explore highly efficient, low cost adsorbents that can be easily regenerated for reuse over several cycles of operations without significant loss of adsorptive capacity and which have good hydraulic conductivity to prevent filter clogging during the fixed-bed treatment process.

Tamarind (Tamarindus indica) fruit shell carbon: A calcium-rich promising adsorbent for fluoride removal from groundwater

Tamarindus indica fruit shells (TIFSs) are naturally calcium rich compounds. They were impregnated with ammonium carbonate and then carbonized, leading to ammonium carbonate activated ACA-TIFS carbon. The resulting materials and carbon arising from virgin fruit shells V-TIFS were characterized and assayed as adsorbent for the removal of fluoride anions from groundwater. The fluoride scavenging ability of TIFS carbons was due to naturally dispersed calcium compounds. X-ray diffraction (XRD) showed that TIFS carbon contained a mixture of calcium oxalate and calcium carbonate. Batch studies on the fluoride removal efficiency of TIFS carbons with respect to contact time, pH, initial fluoride concentration, and co-ion interference were conducted. Applicability of various kinetic models (viz., pseudo-first-order, pseudo-second-order, intra-particle diffusion and Elovich) and sorption isotherms were tested for batch techniques. The fluoride removal capacity of TIFS carbons was found to be 91% and 83% at a pH of 7.05 for V-TIFS and ACA-TIFS carbons, respectively. The practical applicability of TIFS carbons using groundwater samples was approved. The fluoride removal was greater in groundwater without hydrogen carbonate ions than those containing these ions. The characterizations of fluoride unloaded and loaded TIFS carbons were done by SEM and XRD studies.

Matrix supported tailored polymer for solid phase extraction of fluoride from variety of aqueous streams

Fluoride related health hazards (fluorosis) are a major environmental problem in many regions of the world. It affects teeth; skeleton and its accumulation over a long period can lead to changes in the DNA structure. It is thus absolutely essential to bring down the fluoride levels to acceptable limits. Here, we present a new inorganic-organic hybrid polymer sorbent having tailored fixed-sites for fluoride sorption. The matrix supported poly (bis[2-(methacryloyloxy)-ethyl]phosphate) was prepared by photo-initiator induced graft-polymerization in fibrous and microporous (sheet) host poly(propylene) substrates. These substrates were conditioned for selective fluoride sorption by forming thorium complex with phosphate groups on bis[2-methacryloyloxy)-ethyl] phosphate (MEP). These tailored sorbents were studied for their selectivity towards fluoride in aqueous media having different chemical conditions. The fibrous sorbent was found to take up fluoride with a faster rate (15 min for ≈76% sorption) than the sheet sorbent. But, the fluoride loading capacity of sheet sorbent (4,320 mg kg(-1)), was higher than fibrous and any other sorbent reported in the literature so far. The sorbent developed in the present work was found to be reusable after desorption of fluoride using NaOH solution. It was tested for solid phase extraction of fluoride from natural water samples.

Preparation and characterization of porous granular ceramic containing dispersed aluminum and iron oxides as adsorbents for fluoride removal from aqueous solution

Porous granular ceramic adsorbents containing dispersed aluminum and iron oxides were synthesized by impregnating with salt solutions followed by precipitation at 600°C. In the present work detailed studies were carried out to investigate the effect of contact time, adsorbent dose, initial solution pH and co-existing anions. Characterization studies on the adsorbent by SEM, XRD, EDS, and BET analysis were carried out to clarify the adsorption mechanism. The adsorbents were sphere in shape, 2-3mm in particle size, highly porous and showed specific surface area of 50.69 sq m/g. The fluoride adsorption capacity of prepared adsorbent was 1.79 mg/g, and the maximum fluoride removal was obtained at pH 6. Both the Langmuir and Freundlich isotherm models were found to represent the measured adsorption data well. The experimental data were well explained with pseudo-second-order kinetic model. Results from this study demonstrated potential utility of Al/Fe dispersed in porous granular ceramics that could be developed into a viable technology for fluoride removal from aqueous solution.

An excellent fluoride sorption behavior of ceramic adsorbent

A new material, ceramic adsorbent, has been developed and undertaken to evaluate the feasibility for fluoride removal from aqueous environment. Batch experiments were performed to study the influence of various experimental parameters such as contact time (0-48 h), initial fluoride concentration (20-100 mg/L), pH (2-12) and the presence of competing anions on the adsorption of fluoride on ceramic adsorbent. The experimental data revealed that both the Langmuir and Freundlich isotherm models fitted well with the fluoride sorption process. The maximum adsorption capacity of ceramic adsorbent for fluoride removal was 2.16 mg/g. The optimum fluoride removal was observed between pH ranges of 4.0-11.0. The sorption process was well explained with pseudo-second-order kinetic model. The fluoride adsorption was decreased in the presence of phosphate followed by carbonate and sulfate. Results from this study demonstrated potential utility of ceramic adsorbent that could be developed into a viable technology for fluoride removal from aqueous environment.