Research articleAn imperative approach for fluorosis mitigation: Amending aqueous calcium to suppress hydroxyapatite dissolution in defluoridation
Graphical abstract
Introduction
Groundwater is one of the primary sources of water for many regions of the world for our daily needs as it is fresh and generally of good microbial quality than surface water (Schmoll et al., 2006). However, due to the geological contamination, the higher level of fluoride (F−) in groundwater has been a major cause of concern for its utilization (Brindha et al., 2011; Patel et al., 2014; Rafique et al., 2015). Around 200 million people from 25 nations in the world, including most populated countries like China and India, and a significant population from Eastern Africa are worst affected by the presence of excess F− in their drinking water (Susheela, 2002; Ayoob et al., 2008). The F− presence in drinking water is providing divergent health effects on consumers depending on the amount of F− ingested (Whelton et al., 2004; Fawell et al., 2006). In a brief, consumption of water containing 0.5 mg F−/L found to be beneficial in reducing dental decay. But, exposure to excess F− 1–1.5 mg F−/L causes a group of diseases called as “fluorosis,” primarily consisting of dental and skeletal fluorosis. However, Susheela (2002) and MacDonald et al. (2011) reported the appearance of fluorosis even below the desirable limit of 1 mg F−/L prescribed by the Bureau of Indian Standards (2012). Hence, Susheela (1999, 2002, 2007) proposes the revision of the guideline value for F− is required and suggested to avoid consumption of F− contaminated water and food.
Considerable research has been done on fluoride removal or defluoridation to provide potable drinking water to fluorosis affected areas (Meenakshi and Maheshwari, 2006; Ayoob et al., 2008; Mohapatra et al., 2009; Bhatnagar et al., 2011; Izuagie et al., 2016). Several defluoridation methods have been suggested in the past; however, Nalgonda technique, use of activated alumina, and reverse osmosis are most commonly employed methods. Even though these methods can successfully remove F− well below 1 mg F−/L, the problem of fluorosis persists (Meenakshi and Maheshwari, 2006; Ayoob et al., 2008), which is due to their inability to successfully implement these methods in the fluorosis affected areas (Yadugiri, 2011). The possible demerits of these defluoridation methods briefly are as follows. The Nalgonda technique requires careful monitoring of alkalinity, and residual Al3+ and concentrations as these parameters may exceed the desirable limits (Meenakshi and Maheshwari, 2006). Defluoridated water using activated alumina exceeds the desirable limit for residual Al3+, and requires periodic regeneration of saturated alumina and environmentally acceptable disposal process for exhausted alumina (Shreyas et al., 2013). Whereas reverse osmosis requires electricity and pre-treatment of feed to avoid fouling of membrane, and reject water disposal becomes another issue (Ayoob et al., 2008; Anjaneyulu et al., 2012; Samrat et al., 2018). Nevertheless, use of these defluoridation methods will not resolve the problem of safe drinking water unless combining with an additional treatment method (Anjaneyulu et al., 2012; Samrat et al., 2018). Even though some of the defluoridation methods were implemented in the fields; the number of fluorosis affected cases is increasing although the fluorosis problem is quite old (Ayoob et al., 2008). Hence, health problems may continue to grow if the issue of F− contamination in drinking water persists.
To overcome demerits of most commonly used defluoridation methods and resolve the problem of safe drinking water, researchers used synthetic (Fan et al., 2003; Sundaram et al., 2008; Nie et al., 2012; Sani et al., 2016) and natural, in the form of bone char (Medellin-Castillo et al., 2016; Delgadillo-Velasco et al., 2017), hydroxyapatite (HAP or Ca10(PO4)6(OH)2) as a defluoridation media. The HAP is considered as a potential material for defluoridation due to its selective propensity to uptake F− and higher defluoridation capacity (He and Cao, 1996; Fan et al., 2003). However, use of bone char for water treatment may not be universally accepted due to religious beliefs, and microbiological and aesthetic problems (Fawell et al., 2006). Therefore, use of HAP synthesized using locally available and inexpensive raw materials (MacDonald et al., 2011; Yakub and Soboyejo, 2013; Kanno et al., 2014) is preferable over bone char. In order to examine its feasibility for defluoridation, few of the previous laboratory studies (He and Cao, 1996; Lessard, 2007; Sternitzke et al., 2012; Kanno et al., 2014) reported HAP dissolution during the defluoridation. However, dissolution of HAP leaches calcium (Ca2+) and phosphate () ions and increases the pH of water, which negatively affects the water quality, particularly when residual is present in excess.
Presently, no Bureau of Indian Standards for in drinking water exist. However, some of the recent studies reported toxicity of and its adverse health effects on human (Razzaque, 2011; Ritz et al., 2012; Jain and Elsayed, 2013; Brown and Razzaque, 2015; Anukam and Agu, 2017). It has been reported that impaired balance of affects musculoskeletal and cardiovascular systems, excess intake of leads to an increase in mortality and morbidity, accelerates ageing process, and causes hyperphosphatemia, which further promotes the hypocalcemia. A survey by the National Health and Nutrition Education (2005–2006) reported that estimated intake exceeded the estimated average requirement in adults (Moshfegh et al., 2009). The phosphorus present in the food mostly exists as organic phosphate with bound form, whereas in drinking water, it is in orthophosphate (inorganic) form. Thus, intestinal absorption of artificial inorganic phosphate present in the drinking water is much more than the natural phosphate present in the food (Anukam and Agu, 2017). Moreover, the absorption of in the intestine is twice of that of Ca2+ ions (Razzaque, 2011). Further, the present in drinking water reduces the bioavailability of Ca2+ and Mg2+ ions present in the diet and water by forming insoluble salts (Guéguen and Pointillart, 2000; Prasad and Bhadauria, 2013), which poses an additional problem of cation minerals deficiency. It is also vital to note that the presence of Ca2+ or Mg2+ ions helps in decreasing the bioavailability of F− (Heard et al., 2001). Hence, any reduction in the bioavailability of these divalent cations due to the presence of in water further increases the chances of F− absorption into the gastrointestinal tract. Thus, individuals drinking water containing both F− and ions might have a negative impact on their health.
Besides toxicity, the HAP dissolution during the defluoridation gradually reduces the quantity of HAP. Hence, HAP dissolution is a primary concern in the defluoridation. Thus, identifying the cause of HAP dissolution during the defluoridation facilitates the efficient use of HAP to provide safe drinking water. Hence, in the present work, an effort has been made to prevent HAP dissolution to achieve safe drinking water. A recent study reported that the addition of F− to HAP suspension containing Ca2+ and ions reduced Ca2+ concentration (Sternitzke et al., 2012). Another study observed that defluoridation did not take place in the absence of bone char (natural HAP) even though solution containing Ca2+ and F− ions was supersaturated with respect to fluorite (Jacobsen and Dahi, 1997). Moreover, the addition of Ca2+ salt to F− water enhanced the defluoridation capacity of bone char (Jacobsen and Dahi, 1997) and residual Ca2+ in the HAP suspension decreased only in the presence of F− (Sternitzke et al., 2012). Thus, by above adduced facts, we hypothesize that aqueous Ca2+ has a role when we use HAP for defluoridation. Therefore, herein, we have conducted defluoridation experiments by amending aqueous Ca2+ to F− water before contact with uncalcined HAP. The outcome from the present study provides the synergistic effect of HAP and aqueous Ca2+ for efficient defluoridation without changing the quality of drinking water. Further, we have discussed possible mechanisms of defluoridation using HAP for a case when amended Ca2+ has a role in the process of defluoridation.
Section snippets
Synthesis of uncalcined hydroxyapatite
In this study, HAP was synthesized by the wet chemical precipitation method at room temperature (27 °C). Briefly, diammonium hydrogen phosphate ((NH4)2HPO4) solution was slowly added to the Ca(OH)2 suspension with continuous stirring at 500 rpm. After complete mixing of both solutions, the suspension was additionally stirred for 10 min at 600–700 rpm. The final concentrations of Ca2+ and ions in the mixture were 777.5 and 465.6 mM, respectively for an ideal Ca/P molar ratio of 1.67.
Characterization of synthesized uncalcined hydroxyapatite
The presence of and OH− groups were confirmed from the FTIR (Fig. S1a), whereas the HAP phase formation was confirmed from the XRD (Fig. S1b). Hence, FTIR and XRD characterizations suggest that the uncalcined synthesized powder was HAP. The TEM image of the untreated HAP (Fig. S2a) shows that HAP particles are rod-like shape. The details of these characterization techniques are provided in the Supplementary material S2.1.
An actual Ca/P molar ratio and BET specific surface area of hydroxyapatite
The measured actual Ca/P molar ratio of HAP was found to be 1.61
The practical implications of the aqueous calcium amended-hydroxyapatite defluoridation method
Inexpensive and readily available raw materials such as limestone and diammonium phosphate fertilizer can be used for the synthesis of HAP media at fluorosis affected villages. However, the defluoridation using HAP without amending Ca2+ provides Ca2+-deficient drinking water with high values of pH and residual Moreover, the concentration of Ca2+ has a negative correlation with F− concentration in groundwater (Brindha et al., 2011; Rafique et al., 2015). Further, the defluoridation of
Conclusions
The present study suggests the defluoridation method for fluorosis affected rural areas where fluoride removal media can be synthesized in the field. The material used, i.e., hydroxyapatite and amending aqueous calcium to fluoride contaminated water can efficiently remove fluoride from drinking water. Briefly, the conclusions drawn from this study are summarized below:
Use of hydroxyapatite without amending aqueous Ca2+ for defluoridation provides drinking water containing and high pH, and
Acknowledgements
The authors thank the Sophisticated Analytical Instruments Facility, Department of Metallurgical Engineering and Materials Science, and Central Facility-IRCC, IIT Bombay for FTIR, XRD, and HRTEM data, respectively. The authors also thank Professor Mohammed S. Razzaque, Department of Pathology, Lake Erie College of Osteopathic Medicine, Erie, PA, USA for his helpful discussion on phosphate toxicity. This research did not receive any specific grant from funding agencies in the public, commercial,
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2021, Journal of Hazardous Materials LettersCitation Excerpt :Particularly, the fluorosis technique would be The implemented defluoridation technique should at least selectively remove excess F− from drinking water without compromising other water quality parameters. In this direction, a few of the defluoridation techniques, those based on non-toxic elements such as calcium and magnesium, have found to be potential techniques and have shown promising defluoridation capacities (Islam and Patel, 2007; Pemmaraju and Rao, 2011; MacDonald et al., 2011; Mourabet et al., 2012; Khare et al., 2019; Sankannavar and Chaudhari, 2019). However, the safe disposal of resulting F−-bearing materials is another problem that demands research.