Venkataraman Sivasankar

Methods of Defluoridation: Adsorption and Regeneration of Adsorbents

This chapter begins with an overview of operational defluoridation methods. Main methods in operational plants are adsorption on activated alumina, separation by reverse osmosis or nanofiltration, and electrocoagulation using soluble aluminum anode. Exhausted alumina can be regenerated by washing with alkali solution followed by an acid activation. Domestic defluoridations work with small equipment using adsorption on activated alumina or separation with reverse osmosis. Adsorption on alumina and regeneration of exhausted adsorbent is a model in the research of new fluoride adsorbents. In the second part, the chapter gives recent results of research about defluoridation methods. The research of new fluoride adsorbents explores the use of fluorophilic elements dispersed in insoluble materials. These new fluoride adsorbents are characterized by the presence of double or triple oxides or nanomaterials or show composite structures. In the field of fluoride removal, there are a lot of researches of low-cost adsorbents made from renewable biomass or made from geological materials and about electrochemical and membrane processes.

Fluoride: A World Ubiquitous Compound, Its Chemistry, and Ways of Contamination

Fluoride, an incompatible lithophile and the most electronegative element, forms a number of soluble, pH-dependent formation of complexes with polyvalent metal ions in water. The interaction between water and sedimentary carbonates ultimately causes fluoride concentration gradient as a sequel of hydrogeochemistry. The occurrence of fluoride in groundwater due to fluorapatite solubility and the other governing factors such as rock chemistry, residence time, well depth, preferential pathways for the upward movement of deep groundwater, hydrologic condition of the pathways, and geologic structure have also been discussed. In this chapter, in addition to the geochemistry of fluoride, the chemistry of fluoride in water and its association with the other physicochemical parametric factors such as total dissolved solids and dissolved ions such as sodium, calcium, magnesium, arsenic, boron, and hydrogen carbonate have been elaborated. As fluoride and arsenic ions participate together in their occurrence in soil and hence water, their co-contamination has been exemplified from the research reports. Fluoride solubility as a function of evaporation, evapotranspiration, temperature, and water softening has also been accounted. The leaching aspects of soil-based adsorption–desorption mechanism and its ultimate destiny on fluoride enrichment of groundwater have also been added in the chapter.

Adsorption Investigations: Kinetic and Isotherm Models, Determination of Thermodynamic Parameters

To comprehend the mechanism of sorption onto the surface of any material, it is vital to validate and interpret the kinetic and isotherm models. The determined kinetic, isotherm, and thermodynamic parameters corroborate the nature of binding forces of fluoride with the studied carbon adsorbents. In addition, the thermodynamic factors such as enthalpy change (ΔH), entropy change (ΔS), and free energy change (ΔG) predicted the nature of fluoride sorption process, disorderliness of the fluoride sorption system, and spontaneity of the fluoride sorption reaction, respectively. The compliance of each model has been validated for its fit with the present fluoride sorption dynamics at various concentrations and temperatures.

Laboratory Investigations on the Influential Factors Governing Fluoride Adsorption–Desorption on Surface-Modified Carbons

The present chapter deals with the defluoridation experiments based on the optimization of kinetic variables such as contact time, pH, carbon dose, initial fluoride concentration, and the temperature of fluoride solution so as to explore the choice toward maximum defluoridation efficiency. The nature of sorption onto carbon adsorbents was inferred from the experimental results on fluoride uptake at varied temperatures. Considering the practical aspect of these carbon adsorbents in the defluoridation of groundwater, a system of co-ion (hydrogen carbonate, chloride, nitrate, sulfate, and phosphate) with fluoride was subject to batch studies and the inhibitory effect against fluoride removal due to every co-ion was studied. To assure the reusability and economic viability of carbon adsorbents, desorption studies were conducted using 0.05 M, 0.1 M, and 0.2 M regenerating solutions of sodium hydroxide.

Cost–benefit analyses and comparative study on sorption capacity with other modified carbons

The present chapter concentrates on the cost–benefit analyses of the chosen eight carbon adsorbents and comparison of fluoride sorption with the other reported carbon adsorbents as well. The carbon adsorbents are separated on the basis of their modification by chemical agents and conventional hydrothermal process. These carbon adsorbents under present investigation are compared with other like adsorbents to explore the difference in fluoride sorption capacity. The cost of each carbon adsorbent per gram has been assessed based on chemical and other expenses during the process of modification and carbonization syntheses.

Syntheses and Characterization of Surface-Modified Carbon Materials

Surface-modified carbons were synthesized from biomaterials such as tamarind fruit shells, watermelon seeds, adamant creeper, Indian spurge tree, and the natural biopolymer, starch using modifying agents. Modification of surface was achieved by, on one hand, dispersing fluorophilic elements such as calcium, iron, and cerium and, on the other hand, using ammonium carbonate to generate more porosity on the surface. These surface-modified carbon adsorbents were characterized for BET isotherm, iodine number, elemental analysis, Fourier transform infrared (FTIR), SEM, and XRD studies. There were no appreciable changes in the stretching frequencies, surface morphology, and diffraction patterns after the fluoride sorption onto these carbonized materials.

Proposed Mechanisms on Fluoride Sorption

Mechanisms on fluoride sorption onto different chemically modified carbons derived from biomaterials have been discussed in this chapter. The workability of calcium, iron, and cerium active centers that spread on the carbon matrix toward fluoride sorption is quite incredible. The participation of calcium, cerium, and iron compounds through plausible mechanisms has been detailed in this chapter.

Treatment of Fluorosis Disease and Prevention of Negative Effects of Fluoride Ingestion

Ingestion of fluoride in excess from drinking water, foods, or industrial pollution causes chronic toxicity known as fluorosis. Many papers consider fluorosis as an irreversible disease which has to be prevented by a fluoride level in drinking water under the WHO limit. In this chapter we focus on the reversibility of fluorosis showing that this disease is not a fate. Few papers deal with successful treatment of dental fluorosis and skeletal fluorosis in man. This treatment involves the withdrawal of fluoridated water and the use of foods supplemented with calcium and vitamins C and E. Many papers report the success in mitigation of experimental fluorosis in animals. From these results it is possible to suggest nutritional recommendations for the prevention of fluorosis diseases in endemic regions: withdraw fluoridated water, select foods with low fluoride content, and supplement diet with calcium, vitamins C and E, and antioxidant compounds arising from plants and fruits.

Health Effects of Direct or Indirect Fluoride Ingestion

Fluoride is a ubiquitous chemical element in water and foodstuffs. There is no consensus about the essentiality of fluoride. There are many countries where population drinks artificially or naturally fluoridated water. The other sources of fluoride are tea, some mineral waters, fluoridated milk, fishes, fluoridated salt, fluoridated dental products, vegetables arising from contaminated soils, and air pollution due to the coal burning. Biological alterations can be associated to contemporary biomarkers, recent biomarkers, or historic biomarkers. The main negative and visible effects of fluoride ingestion are dental fluorosis and skeletal fluorosis. The caries prevention is the only positive effect of fluoride ingestion. Fluoride ingestion can have also toxic effects on a lot of soft tissues. A recent observation is the neurotoxicity of fluoride and its effects on the intelligence quotient of children. Fluoride also exhibits toxicity against terrestrial and aquatic animals, plants, and microorganisms. The presence of coexisting substances can modify the fluoride toxicity. A lot of compounds like arsenic, calcium, magnesium, aluminum, lead, and selenium have been investigated for their synergetic or antagonist effects.

Conclusions and Recommendations for Further Researches and Development in Defluoridation Processes

Fluoride anion is present all around the world in groundwater, mainly in countries where safe water is scarce. Since fluoride is harmful the defluoridation of water is very important. This concluding chapter suggests recommendations for further researches in defluoridation and for the development of defluoridation processes.