Adsorptive removal of fluoride from water samples using Zr–Mn composite material
Introduction
Fluorine is quite a common element that does not naturally occur in elemental form because of its high reactivity. Fluoride is a naturally occurring compound derived from fluorine which is the 13th most abundant element in the earth's crust [1], [2]. It exists in the form of fluorides in various minerals such as sellaite (MgF2), fluorspar (CaF2), cryolite (Na3AlF6) and fluorapatite (Ca5(PO4)3F). Thus fluorides are also found in rocks, soil, plants, animals, humans and fresh as well as ocean water [2], [3]. Therefore fluoride occurs naturally in public water systems as a result of runoff from weathering of fluoride-containing rocks and soils and leaching from soil into ground water [2], [3], [4], [5]. In addition to water, fluoride is present naturally in almost all foods and beverages but levels vary widely. However, fluoride has both beneficial and harmful effects on human health depending on its level. Among the beneficial effects of fluoride in the human body, strengthening of bones and prevention from tooth decay are significant [5]. Compared to its beneficial effect fluoride is more detrimental. Thus fluoride is a toxic chemical and it is a risk factor for thyroid hormone production in children when the exposure to fluoride occurs during intrauterine growth period [6]. A report during 2008 in Scientific American on ‘second thoughts about fluoride’ was a warning to all concerned as it revealed the risk of fluoride causing disorders affecting the teeth, bone, brain and thyroid gland [6], [7]. It has also been reported that thyroxine and triiodothyronine in serum decreased with increasing urinary fluoride in cattle. Cattle affected with fluorosis developed hypothyroidism and anemia [6]. In addition, it has been confirmed that there is significant positive relationship between fluoride intake by water and the prevalence of dental fluorosis [2], [8], [9], [10], [11]. For the general population the intake of fluoride is mainly from drinking water and to a much lesser extent from foodstuffs i.e. drinking water is the major source of daily intake of fluoride [2], [12], [13].
Water is an essential natural resource for sustaining life and environment that is thought to be available in abundance as a free gift of nature. However, over the past few decades, the ever-increasing population, urbanization, industrialization and unskilled utilization of water resources have led to the degradation of water quality, causing its reduction in per capita availability in various developing countries [13]. Thus there is a substantial shortfall in the availability of potable water in less developed or developing countries, primarily due to water contamination and pollution [14], [15], [16]. It has also been reported that about 80% of the diseases in the world are due to poor quality of drinking water, and the fluoride contamination in drinking water is responsible for 65% of endemic fluorosis in the world which affects the teeth, bone and soft tissues [14], [17], [18]. The beneficial or detrimental effects of fluoride in drinking water depend on its concentration and the total amount ingested. It is beneficial especially to young children below eight years of age when present within permissible limits of 1.0–1.5 mg/L for calcification of dental enamel. Excess fluorides in drinking water cause dental fluorosis and/or skeletal fluorosis [9], [10], [12], [13], [14], [19], [20]. Furthermore, renal failure has also been reported [21]. Therefore many countries have set a maximum allowable concentration of fluoride in drinking waters. Indian standards for drinking water recommend an acceptable fluoride concentration of 1.0 mg/L and an allowable fluoride concentration of 1.5 mg/L in potable waters [22]. However, the U.S. Environmental Protection Agency has set a maximum contaminant level for fluoride of 4.0 mg/L for drinking water for public water systems but has also set a secondary standard of 2.0 mg/L. The U.S. Public Health Service has set the optimal fluoride content in drinking water in the range of 0.7–1.2 mg/L. In 2011, the U.S. Department of Health and Human Services proposed a recommendation of 0.7 mg/L to replace the current range of 0.7–1.2 mg/L [23]. The World Health Organization has set 1.5 mg/L of fluoride as the upper limit in drinking water [20]. Thus high level of fluoride in water is a world-wide problem. According to the United Nations Environmental Program, more than ten million people across twenty five developed and developing countries have been affected by fluorosis [5]. There has been regular report on high fluoride contents in drinking water from India, Pakistan, China, Sri Lanka, West Indies, Spain, Holland, Poland, Italy, Mexico, Thailand, Eritrea (North East Africa), West Africa, Southern Africa, and North and South American countries [4], [5], [13], [14], [19], [20], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. In China, endemic fluorosis has been reported in all twenty eight provinces, autonomous regions and municipalities except Shanghai [20]. In China alone more than 1.34 million people have suffered from skeletal fluorosis due to high fluoride drinking water and another 30 million are exposed to it [4], [20], [33], [34].
The presence of fluoride and other contaminants has also been reported from different parts of India where people suffered from harmful diseases due to industrial discharge of fluorides and thus the water sources have been unsafe for human consumption as well as irrigation and industrial uses [10], [12], [13], [14], [27]. The most seriously affected provinces are Andhra Pradesh, Punjab, Haryana, Rajasthan, Gujarat, Tamil Nadu and Uttar Pradesh [10], [12], [13], [14], [19], [20], [35], [36]. The highest concentration observed to date in India is 48 mg/L in Rewari District of Haryana [20], [37]. Viswanathan et al. have reported that 50% of the groundwater sources in India have been contaminated by fluoride where more than 90% of rural drinking water supply programs are based on ground water [14]. Therefore, fluoride is the major inorganic pollutant of natural origin found in groundwater [10], [12], [13], [14], [19], [27], [30], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54]. There is a narrow margin between the preferred and detrimental doses of fluoride in the human body [33], [55]. The defluoridation is carried out when naturally occurring fluoride level exceeds the permissible limits as it is the only option to control fluoride related diseases. Therefore, the removal of fluoride from drinking water when it exceeds the recommended limit is extremely important for the public's interest. Meenakshi and Maheshwari have presented a review on the sources, ill effects and techniques available for fluoride removal [13]. Mohapatra et al. have also published a review that provides precise information on efforts made by various researchers in the field of fluoride removal for drinking water [56]. For defluoridation of water various methods have been used such as membrane separation processes [57], [58], adsorption [12], [59], [60], [61], [62], [63], [64], [65], ion-exchange [66], [67], precipitation–coagulation [68], [69], nano-filtration [70], [71], reverse osmosis [58], [72], [73], electrolytic defluoridation [74], [75], [76], electrodialysis [77], [78] and Donnan dialysis [79], [80], [81]. However, in the recent review, the fluoride removal techniques have been broadly categorized in two sections such as membrane and adsorption techniques [56]. Considering factors like cost effectiveness, flexibility and simplicity of design, ease of operation and maintenance, the conventional adsorption technique has been found to be superior technique for fluoride removal [4], [82]. As the efficiency of adsorption technique depends upon the nature of adsorbents used so, in recent past, considerable efforts have been directed toward the study of fluoride removal using various types of adsorbents like natural, synthetic and biomass materials which include activated alumina/aluminum based materials [40], [62], [66], [68], [83], [84], [85], fly ash [86], alum sludge [62], [87], algal biosorbent [41], chitosan beads [12], [47], [49], [50], [51], [53], [88], red mud [65], synthetic compounds/amberlite resin/zeolite [63], [89], [90], calcite [91], hydrated cement [92], hydrotalcite and layered double hydroxides [93], [94], clays and soils [3], [4], [19], [42], [45], [46], [61], [95], carbon based materials [82], [96], [97], [98], synthetic hydroxyapatite [9], [50], [53], [54], quick lime [99], etc. Fluoride removal from aqueous solutions using various reversed zeolites, modified zeolites and ion exchange resins based on cross-linked polystyrene as well as layered double oxides has also been of great interest as adsorbents [56]. Lately, we have published a critical review on the efficiency of different materials for fluoride removal from aqueous media [38]. We observed that activated alumina has widely been used because of its efficiency and low cost but its main disadvantage is its residual aluminum, soluble aluminum fluoride complexes, generation of sludge and having a narrow pH range of 5.0–6.0 [4], [13], [84]. In continuation of our interest on developing analytical method for fluoride removal, the present study reports the novel Zr–Mn hybrid oxide i.e. composite adsorbent material to remove fluoride from aqueous solutions where the batch experiments including sorption isotherms were also studied. Studies were carried out in synthetic fluoride solutions with the objective of establishing optimum parameters for adsorption.
Section snippets
Materials and adsorbent preparation
NaF, NaOH, HCl, and all other chemicals and reagents used were of analytical reagent grade. Sodium fluoride with purity > 99%, ZrO(NO3)2·xH2O and MnSO4·H2O were purchased from E. Merck Ltd. India. All chemicals were used without further purification. 1000 mg/L fluoride stock solution was prepared by dissolving appropriate amount of NaF in 1 L of distilled deionized water from a Milli-Q water system. Synthetic fluoride solutions were prepared by adding appropriate amounts of sodium fluoride to
FTIR studies of adsorbent
The FTIR spectra of the Zr–Mn adsorbent before and after adsorption of fluoride are depicted in Fig. 1, Fig. 2 respectively. For the adsorbent, the broad band at 3390 cm− 1 and the peak at 1627 cm− 1 are assigned to the stretching and bending vibration of adsorbed water and the peak at 1127 cm− 1 due to the bending vibration of hydroxyl group of metal oxides (M-OH) [101], [102], [103], [104], [105], [106]. Fig. 2 clearly shows that after fluoride adsorption, the band at 3390 cm− 1 was shifted to 3404 cm−
Conclusions
Zr–Mn composite material as an adsorbent for fluoride removal from water was successfully prepared via co-precipitation method. The adsorption of fluoride was dependent on pH, initial fluoride concentration, adsorbent dose as well as contact time which were optimized. Removal of fluoride efficiency increased with increase in contact time and adsorbent dose. From FTIR study it has been verified that hydroxyl groups on the adsorbent were responsible for the fluoride sorption which could be due to
Acknowledgment
The authors are thankful to University Grant Commission (UGC), New Delhi for the financial assistance via grant no. F.No. 39-734/2010 (SR).
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