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Fluoride in Groundwater: Distribution, Sources, Processes, Analysis, and Treatment Techniques: A Review

Bedour Al Sabti, Dhanu Radha Samayamanthula, Fatemah M. Dashti, and Chidambaram Sabarathinam

Water Research Center, Kuwait Institute for Scientific Research, Safat, Kuwait

1.1 Introduction

Fluoride (F-) belongs to the halogen family and is a constituent in minerals such as fluorite, fluorspar, apatite, biotite, cryolite, and muscovite (Bretzler and Johnson 2015; Dehbandi et al. 2017), apart from its availabilities in plants, soil, and groundwater. Groundwater is one of the most important sources of drinking water and one of the fundamental human rights around the globe is an access to safe drinking. Contamination and unsustainable drinking water sources could affect human health, resulting in the transmission of diseases (WHO 2018). Fluoride is one of the ions which may lead to groundwater contamination if present in high concentrations. Although high F- in groundwater is a major concern that is still being debatable around the globe, fluoride is essential for the growth of the dental and skeletal frame of the body. Fluoride concentration in groundwater differs from one region to another based on aquifer material, geology, weathering rate, aquifer depth, contact time, pH, rainfall, and temperature (Brunt et al. 2004; Onipe et al. 2020). The geochemical process governs fluoride mobility through leaching from soil and rocks to the groundwater. Studies suggest that exposure to high fluoride imparts a vulnerable effect on the mental ability of children. The IQ levels of children exposed to higher F- are lower than unaffected children (Choi et al. 2012; Das and Mondal 2016). The thyroid gland is susceptible to F-, which causes an increase in thyroid-stimulating hormone (TSH) leading to a drop in Triiodothyronine (T3) and Thyroxine (T4) levels, thereby resulting in hypothyroidism (McLaren 1976; Shashi 1988; Kumar et al. 2019). Fluorosis results from a high concentration of fluoride in drinking water and depends on other sources such as dietary habits that enhance the incidence of fluorosis (Brindha and Elango 2011; Srivastava and Flora 2020). Several countries, such as West Indies, India, Poland, China, Spain, Africa, and Italy, have been reported with high fluoride concentrations (Huang et al. 2017). The geochemical data for Cameroon, Algeria, Ghana, United Kingdom, Siri Lanka, Argentina, Canada, Tanzania, Kuwait, South Africa (Silom), India (Telangana), and Brazil were collected from the literature to understand the geochemistry of F- (Table 1.1). Some of the published data for selected countries does not contain the complete analysis results. Based on the available ions in the analytical data, they were used for statistical analysis using Statistical Package for Social Sciences (SPSS) software. The same analytical data were used for different plots developed from the output results of WATEQ4F and AQUACHEM. The objective of this review is to emphasize the global distribution, sources, analysis, and treatment strategies for excessive fluoride levels in groundwater. Also, the review presents geochemical plots, statistical techniques, thermodynamic and modeling approaches to determine processes governing the fluoride release and distribution in groundwater.

Table 1.1 Lithology and analytes considered from the literature studies of various countries but clay minerals like Vermiculite have also reported to be a source of F- in groundwater due to the process of Fluoride ion.

1.2 Permissible Limits of Fluoride in Drinking Water

According to the WHO (2006), the maximum permitted level of F-in drinking water is 1.5?mg/L. While the USPHS (1987) established a range of allowable F- concentration in drinking water for regions based on their climatic conditions, because the amount of water consumed and, the amount of F- ingested is primarily influenced by the air temperature. The rise in air temperature decreases the concentration of F-. The maximum permissible level in tropical climates with temperatures above 26?°C is 1.4?mg/L. In light of the Indian subcontinent's environmental and socioeconomic situation, the F- desirable limit is established at 0.6-1.2?mg/L, and the highest allowed level in the absence of any other source is set at 1.5?mg/L for drinking water (ISI 1995). The limit was set based on the daily consumption rate of water, about 2?L/day for an adult body mass, and contains about 0.2-0.5?mg fluorine as a standard diet (WHO 1994). A range of environmental, social, cultural, economic, and other circumstances affecting possible exposure, as well as the default assumptions used to create the guideline values, will need to be taken into account when creating national drinking-water standards based on these guideline values. In addition, the environmental-based variation depends on the region, as regional diets and ambient temperature control the permissible limit (Apambire et al. 1997). Furthermore, in a country with a constant warm environment and piped water as the main drinking-water source, authorities may choose a lower health-based fluoride target than this guideline value as water consumption is predicted to be higher (Guidelines for drinking-water quality 2021). Drinking water from groundwater may be beneficial or harmful depending on the concentration level of fluoride. In recent years, countries have been developing drinking standards to decrease waterborne diseases and improve safe water resources management (Ali et al. 2019). As the concentration of F- in drinking water is different for each country, and the amount of water consumed by a person also varies concerning the climate and availability, so each region has its own standard (Figure 1.1). Drinking high fluoride groundwater is the primary reason for endemic fluorosis in the countries such as China (Guo et al. 2007). Higher F- concentration in groundwater, i.e. exceeding the permissible limit of WHO, is observed in countries like...