Dear Dr. Zhonghe Pang Sir,

Please have a look at this link and PDF attachment.

https://www.researchgate.net/publication/220000345_Fluoride_in_Groundwater_Causes_Implications_and_Mitigation_Measures

Good luck

The occurrence of fluoride in groundwater is mainly due to weathering and leaching of fluoride-bearing minerals from rocks and sediments. Certain characteristics such as the alkalinity of water could lead to increased concentrations of fluoride as a result from its mobilization from fluorite.

Other major sources of fluoride in groundwater are volcanic ash and fly ash (from the combustion of fossil fuels) which are readily soluble. Phosphate fertilizers used in agriculture also contribute to this type of contamination.

Source of fluoride in ground water depend upon various factors. One of the main cause is identified, in particular in concern with India is water - rock interaction.

Good day

Here hereinafter a review may support, Fluorite (CaF2)is a common fluoride mineral. This mineral has a rather low solubility and occurs in both igneous and sedimentary rock. Apatite, Ca5(CI,F,OH)(PO4)3,commonly contains fluoride. Amphiboles, such as hornblende and some of the mica's, may contain fluoride which has replaced part of the hydroxide. According to Rankama and Sahama(1950, p. 758), rocks rich in alkali metals, and also obsidian, are as a class higherin fluoride content than mostother igneousrocks. Fluoride is commonly associated with volcanic or fumarolic gases,and in some areas these may be important sources of fluoride for natural water. Reference already has been made to the occurrence of fluoride species in volcanic condensates from Hawaii (Roberson and Barnes,1978).Because of similarity of charge and radius,substitution of fluoride for hydroxide ions at mineral surfaces is an obvious possibility. These fluoride ions could be replacedby hydroxide ions in a pH-dependent fashion. Fresh volcanic ash may be rather rich in fluoride, and ash that is interbedded with other sediments could contribute later to fluoride concentrations in ground water in such areas. Aluminum fluoride and hydroxy-fluoride species occurring in rocks include cryolite (Na3AlF6) and ralstonite. The latter mineral ranges in composition from NaMgAl(F,OH)G-H2O to A~~(F,OH)G*H2O,the end members of a solid-solution series.These minerals are rare but could become fluoride sources during weathering. The analyst reports fluoride concentrations in terms of the free F-ion. This form probably is the predominant one in most water, but other varieties of dissolved fluorine are certainly possible. At low pH the form HF could occur. From dissociation constants given by Sillen and Martell (1964, p. 256-257) it would appear that below a pH of 3.5 the HF” form probably would be predominant. Data in the same reference suggest that strong fluoride complexes would be formed with aluminum, beryllium, and ferric iron and that a series of mixed fluoride-hydroxide complexes is possible with boron. or SiF4 ; however, the conditions required for stability of these two species probably are seldom reached in natural water. The solubility equilibria for cryolite given by Roberson and Hem (1969) indicate that in a solution containing 2,300 mg/L Na’ and 2.7 mg/L AL3’ the concentration of fluoride would be limited to about 30 mg/L. These represent total concentrations, including all complex ions, but it seems evident that water with this much aluminum would also require a pH at least as low as 4.0. Sincerely

Thanks, Bayan, for your detailed answer. Do you have a paper to be shared probably?

Agree with Bayan. Appreciated remark.

Sources of fluoride in groundwater - The occurrence of fluoride in groundwater is primarily ascribed to geogenic processes. The sources are usually related to the dissolution of various minerals present in rocks and soils, such as fluorite (or fluorspar), cryolite, fluoroapatite, topaz, villaumite, amphiboles such as hornblende, micas namely muscovite and biotite and also rock phosphate. Clay minerals for instance vermiculite, kaolinite, montmorillonite, illite, chlorite and spectite are also the sources of fluoride in groundwater. Among the above stated minerals, fluorite and fluorapatite have generally been considered as the foremost geogenic sources of fluoride in groundwater.

The anthropogenic activities like the application of phosphatic fertilizers and pesticides, processing of phosphatic raw materials, use of clays in ceramic industries, electroplating, aluminum smelting operations, and mining & burning of coal  may also have role in the  elevated concentration of fluoride in  groundwater.

Fluoride enrichment - The concentration of fluoride in groundwater depends on the amount of fluoride in the host rock, the residence time of water, pH, presence or absence of ion complexes and colloids, temperature, solubility of fluoride bearing minerals, anion exchange capacity of the aquifer material (OH for F), and the presence or absence of Ca and HCO3 ions in the water.

You can read my two recently published articles. You may also get a lot of useful article-references on the topic.

Article Fluoride-contaminated groundwater of Birbhum district, West ...

Article Hydrogeochemical processes and contaminants enrichment with ...

Regards

My appreciations to both Dr. Bayan and Dr. Asit for the excellent explanation.

Your question was excellently answered by respected Dr. Asit Kumar Batabyal and Dr. Bayan. i am send you some important links which can also be very interesting for your consideration.

https://www.academia.edu/1360978/Fluoride_in_Groundwater_Causes_Implications_and_Mitigation_Measures

http://www.bgs.ac.uk/research/groundwater/health/fluoride.html

http://shodhganga.inflibnet.ac.in/bitstream/10603/28511/16/16_chapter%209.pdf

Article Fluoride contamination in groundwater resources of Alleppey,...

Article Delineation of potential fluoride contamination zones in Bir...

Article Application of geospatial modeling technique in delineation ...

https://www.researchgate.net/post/how_to_know_the_source_of_fluoride_in_groundwater

Raju - All the links you provided are good. I read these literature. Thanks