1 Fluoride in water, health implications and plant-based remediation strategies

Abstract

The high prevalence of dental fluorosis and bone mineralization deficiency as a result of exposure to fluorides has increased in Kenya over the years due to consumption of water with elevated levels of fluoride. The World Health Organization (WHO) provides a guideline of 1.5 mg/L level of fluoride in drinking water. However, majority of studies carried out in Kenya over the last 40 plus years have indicated very high levels of fluoride in drinking water in various regions, with a prevalence in dental fluorosis observed in children and adults living in Rift valley and central regions due to basaltic and volcanic rocks. Unfortunately, this trend of fluoride-induced enamel changes has been observed in other regions such as Nairobi and Machakos which were originally presumed to contain low fluoride levels. This study sought to analyse the applicability of Maerua subcordata root powder (MSRP) in the removal of fluorides in borehole drinking water. Fresh Maerua subcordata roots were peeled to obtain the white flesh, chopped into small pieces, dried and ground into powder. The process parameters varied were; fluoride ion concentration [F−] (0–12 mg/L), adsorbent dosage (0–200 g/L) and equilibration time (30–240 min) [F−] were hence analysed before and after treatment using ion selective electrode (ISE) fluoride meter. Results indicated that MSRP is a viable plant in fluoride treatment with approximately 68% fluoride ion removal efficiency. An MSRP dosage of 200 g/L was found optimal in [F−] reduction while a 2 mg/L [F−] concentration recorded the highest reduction of [F−]. The optimal equilibration time was found to be 30 min. The results can be used to develop a low-cost column for treatment of high fluoride waters in rural areas using MSRP. Borehole samples were treated with MSRP using the optimized conditions; however their reduction levels were lower than the [F−] standards used. It is envisaged that with further modification and/or doping with zero-valent iron nanoparticles, its efficiency will be improved.

1.1 Introduction

Water is very critical for human survival. However, only 3% is fresh and in a drinkable state even though 70% of the earth is covered with water, and sadly, one in three people globally do not have access to clean drinking water [1]. Limited supply of fresh water can be attributed to high demand, climate change and pollution. The later can be attributed to primary and/or secondary sources of pollution. According to World Health Organisation (WHO), about 3.4 million people die annually due to water-borne diseases. These water pollutants include heavy metals, anions, organic matter, pathogens and other emerging contaminants such as personal care products. Of the various pollutants, fluoride is particularly of great concern since it is a naturally available, necessary element for human life but excess intake can lead to adverse health effects [2]. Fluorine is an element of the halogen family that forms inorganic and organic compounds called fluorides.

Fluoride compounds such as fluorspar (CaF2), fluorapatite (Ca5[PO4]3[F,Cl]), topaz (Al2SiO4[F,OH]2) and cryolite [Na3AlF6] occur through geogenic processes such as dissolution of fluorine containing minerals. The ionic form of fluorine is the 13th most abundant element in the earth crust. There are 416 fluoride bearing rock minerals with major primary mineral source being apatite [Ca5(PO4)3F] [3]. Other fluoride containing minerals include fluorite (CaF2), igneous zircon (ZrSiO4), biotite (K(MgFe)3(AlSi3O10)(FOH)2) and hornblende (CaNa)2(Mg,F,Al)5(Al,Si)8O22(OH)2. Fluoride occurrence has also been linked to high Ca levels, presence of thermal waters and volcanic activity. The latter produces magmatic fluorine mainly as hydrogen fluoride (HF) [4]. Regions with K, Mg, Ca and bicarbonate ions (HCO3 −) have also been associated with high concentration of fluorides. These regions also tend to have pH levels above 7 [5, 6]. Fluoride ions and fluoride compounds may be produced by manmade processes. These anthropogenic sources have been linked to poor industrial waste disposal, thermal decomposition of coal, oil refining, steel, brick and phosphatic fertilizer manufacturing processes [7, 8].

Fluoride ion (F−) may be deleterious or beneficial to cells or organs [3]. This may depend on the level of intake, enamel development age, and duration of exposure. The dual nature of fluorides poses a great concern to health-care professionals, toxicologists and geo-environmentalists. Exposure to high levels of fluorides in drinking water [9], brick tea [10] and coal-burning [11] have been associated with the development of skeletal and dental fluorosis among other effects [2]. Moreover, acute high-level exposure to fluoride water contamination can lead to seizures and muscle spasms. It is estimated that more than 70 million people suffer from fluorosis worldwide. On the other hand, fluoride produced synthetically is mainly used in drinking water, toothpaste, mouthwashes and various chemical products to prevent fluorosis. This is because fluoride displaces the hydroxide ions from Ca5(PO4)3OH mineral found in bones and teeth to form a much harder and stronger Ca5(PO4)3F resistant to acidic attack. Ironically, too much fluoride can lead to dental fluorosis (1.5–2.0 mg/L) or skeletal fluorosis (4–8 mg/L), which can damage bones and joints, lead to thyroid problems and neurological effects [7]. With high levels of fluorides, the Ca5(PO4)3OH mineral is converted to Ca5F10 plus phosphate ions (PO4 3−).

Harmful effects of fluorides are not only limited to human beings but ecological effects on flora and fauna have been documented [12]. Studies have shown accumulation of fluoride in the plants foliage and livestock forages resulting into decreased plant growth and yield, thus directly affecting agricultural outputs and in turn endangering both humans and animals through the food chain [13]. Furthermore, fluorides also accumulate in insect tissue causing severe damage. D’ Addabbo et al. [14] reported on the impact of volcanoes on amphibian living freshwater organisms. Elsewhere, chronic fluorosis in grazing animals from groundwater or geothermal waters have been reported [15]. This can be attributed to accumulation of fluoride ions (F−) in their bones and into the soft tissue, leading to metabolic malfunction in animals [16]. Social-economic implications resulting to a decline in government water supply programs especially in fluoridated areas have been reported [17]. It is therefore important to come up with strategies that will aid in removal of fluorides in drinking water.

Studies have indicated regions with high fluoride levels within the Great Rift Valley. These mainly include the Middle East and East African countries. The latter include Kenya, Sudan, Ethiopia, Tanzania and Uganda [3, 18]. In Kenya, fluorspar, apatite and hornblende are found in Kerio Valley, Mirima hills and Central Kenya. In addition, other documented regions with high fluoride levels in Kenya include Turkana, Southern Rift Valley areas, Central and Eastern Regions. A Kenyan survey conducted in 2017 indicated that the mean [F−] across 8 provinces was 4.14 ± 8.63 mg/L. This is higher than the recommended World Health Organization limit of 1.5 mg/L [18]. The highest fluoride reported in ground waters of the volcanic areas of the Nairobi, Rift Valley and Central Provinces was 30–50 mg/L [5]. Certain regions in Kenya have recorded very high fluoride levels. For example, Lake Elementaita (1640 mg/L) and Lake Nakuru (2800 mg/L) have the highest fluoride levels in the world [18]. Sadly, fluorosis and other health issues associated with ingestion of fluoride in drinking water are not only confined to the people living in these regions but noticeable effects have been seen in other surrounding regions too hence raising an important toxicological and geo-environmental concern....