Environmental Engineering Reference
In-Depth Information
such as larger surface area, greater density of reactive sites on the particle surfaces, short
diffusion route, and higher intrinsic reactivity of the reactive surface sites [23]. They are
also attractive because of the ease with which they can be anchored onto solid matrices
and the ability to functionalize with different functional groups to enhance their afinity
toward target molecules [11]. Because of these unique properties, it is likely that nanosized
adsorbents with strong afinity to luoride can be a useful tool in enhancing the adsorption
capacity. However, the success of the process for down-to-earth applications such as water
puriication is largely dependent on the simplicity of large-scale material synthesis, ease of
solid-liquid separation, and posttreatment handling [12].
In this chapter, the author reviews major deluoridation technologies that have been
practiced in the past for water deluoridation by giving special emphasis on nanomateri-
als as an emerging deluoridation medium. The chapter also provides a brief description
about the chemical speciation, source and occurrence of luoride in subsurface water, and
the challenges being faced in the treatment of luoride-contaminated water.
19.3 Fluoride: A Global Groundwater Threat
Fluorine is the 13th most abundant element in the earth's crust. In nature, it occurs in vari-
ous minerals such as luorospar (CaF
2
), cryolite (Na
3
AlF
6
), and luorapatite (Ca
5
(PO
4
)
3
F),
and many others. Both natural and anthropogenic sources can contribute luoride into
the environment. However, most groundwater contaminations with luoride are primarily
due to natural reasons. In groundwater, the natural concentration of luoride varies with
the physical and chemical characteristics of the aquifer, which includes porosity, acidity of
the soil and rocks, temperature, action of other chemicals, and depth of wells. In aqueous
environment, it generally occurs as luoride ion (F
−
). The undissociated hydroluoric acid
(HF) and its complexes with aluminum, iron, and boron are the other most likely soluble
forms of luoride in natural water [24]. The safe drinking water limit of luoride is <1 mg/L
[4]. However, this guideline value of luoride is not universal. The limit varies among coun-
tries and the age of persons exposed to it [25]. In India, the maximum contaminant level
is restricted to 1.5 mg/L. The presence of luoride in drinking water can be detrimental or
beneicial to mankind depending on the concentration of luoride and the duration of expo-
sure. A small amount of luoride in ingested water reduces the rate of occurrence of dental
caries, especially in children [26]. On the contrary, excess intake of luoride (>1 mg/L) may
lead to various health problems such as osteoporosis, arthritis, brittle bones, bone cancer
(osteosarcoma cancer), infertility, brain damage, Alzheimer syndrome, thyroid disorder,
and DNA damage [27-30]. Fluorosis is a common symptom of high luoride ingestion,
which is manifested by mottling of teeth in mild cases and deterioration of bones and
neurological damage in severe cases [31]. Excess luoride in groundwater is reported in
many countries from different continents, notably from North America, Africa, and Asia
[26,32-35]. According to the latest information, luorosis is endemic in at least 20 countries
worldwide [4]. In India, it was irst detected in Nellore district of Andhra Pradesh in 1937
[36] and now it is prevalent in 150 districts of 17 states [3]. Estimates show that 25 million
persons in India are affected by luorosis and approximately 66 million persons are at risk
of developing luorosis [3]. In China, cases of endemic luorosis were reported as early
as the 1930s [37]. According to a report, endemic luorosis is prevalent in 29 provinces,
municipalities, or autonomous regions in China [38]. In South Korea, excessive luoride
