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A Review of Metal oxide Nanomaterials for Fluoride decontamination from Water Environment
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ScienceDirect Materials Today: Proceedings 18 (2019) 1146–1155
www.materialstoday.com/proceedings
ICN3I-2017
A Review of Metal oxide Nanomaterials for Fluoride decontamination from Water Environment a
b
Ms. Disha Khandare , Dr. Somnath Mukherjee
a Civil Engineering Department, Government Polytechnic, Nagpur-440030, India. b
Civil Engineering Department, Jadavpur University, Kolkata-700032, India.
Abstract
Fluoride pollution in water emerges a challenging problem to environmental researchers especially in regions where people depend on groundwater for drinking. Natural water bodies are experienced Fluoride impurities from geogenic and anthropogenic sources. Fluoride contamination in drinking water sources has been recognized as a major problem in many countries worldwide especially in several parts of India, China, Sri Lanka, South Africa, Tanzania, Argentina, East Africa, part of South Africa, Turkey and some part of South America. Intrusion of Fluoride with drinking water manifests several health effects such as dental caries and teeth mottling besides skeletal fluorosis. Due to clinical manifestations caused by drinking Fluoride contaminated water, the World Health Organization (WHO) has recommended 1.5 mg L-1. Hence, it is very much needed to supply water with safe F¯. Various physico-chemical methods are available for defluoridation of water, out of which adsorption method is common and widely be used .to remove fluoride from water. Till date, activated alumina is most conventional adsorbing material that are conveniently being used for this purpose. However, activated alumina performed well in acidic environment and regeneration issue poses a complex problem. Other traditional adsorbents though are able to uptake Fluoride from water environment the low sorption capacities and efficiencies restricted their wide application. A major breakthrough took place in recent years due to application of nanomaterial in water purification. As compared with traditional materials used, nanostructure based adsorbents exhibited much higher Corresponding author. Tel.:+ 91 9422819119; fax: +91-712-256-4483. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i-2017).
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performance and efficiency of water decontamination as well as providing a sustainable approach to safe water supply. Nonmaterial is being used throughout the globe as possible solution for removal of fluoride from water as an engineering tool. In the past ten years, many researchers have dealt with low cost and effective adsorbent nanomaterials for the removal of fluoride from aqueous solution and contaminated water. Among them Iron, Magnesia, Alumina, Cerium and calcium, Titania, Silica and Zirconium based metal oxide nanomaterials and composites have proven themselves as excellent adsorbents due to their unique features. The increased surface area of the metal oxide nanoparticles highly favours fluoride adsorption. Their high adsorption capacity, non-toxic nature, limited solubility in water and good desorption potential makes metal oxides a material of choice. The present paper discusses use of nanomaterial and composite nanomaterials for enhanced removal of fluoride contaminated water. The present study also highlights the various key factors (pH, agitation time, initial fluoride concentration, temperature, particle size, surface area, etc) which governs efficacy of different materials in the removal of fluoride from water. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i-2017).
Keywords: Nanomaterials; Nanocomposite; Adsorption; Fluoride Removal.
1. Introduction Presence of Fluoride in sub surface water has emerged a challenging problem to water scientists and environmental researchers especially for the regions where people primarily depend on groundwater for drinking. Natural water bodies are also experienced Fluoride contamination from geogenic and anthropogenic attributes. Due to clinical manifestations caused by drinking Fluoride contaminated water, the World Health Organization (WHO) has recommended 1.5 mg L-1. Hence, it is very much needed to supply water with safe F¯ within the stipulated limit. In the past ten years, numerous attempts are endeavored to develop low cost and effective adsorbents inclusive of nanomaterials or nanocomposites for the removal of Fluoride from water. As compared with traditional materials used, nanostructure based adsorbents exhibited much higher efficiency and faster rate of removal in water treatment.[1] Nanotechnology exhibited a great potential in improving the performance and overall efficiency of water decontamination as well as providing a sustainable approach to safe water supply. Nanotechnology is considered as the vision of the next industrial revolution. Due to the large specific area and fast adsorption kinetics rate, the nano-scale adsorbents have attracted great attention for effective fluoride removal. Recently, metal oxide based nano- sorbents such as iron oxide, aluminum oxide, titanium oxide, manganese oxide, zirconium oxide has been focused by some researchers [2] .The nano sized metal oxides are classified as the promising tools for engineers for heavy metals removal from aqueous systems [3]. The present review aims to focus some important issues and to provide an overview of fluoride removal by applications of advanced metal oxides nanomaterials.
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2. Metal oxide nanomaterials Nanosized metals and metal oxides have received ever-increasing attentions because of their high performance and low cost for contaminants removal. [1,4]. Various inorganic nanomaterial play a vital role in the removal of fluoride from aqueous solution out of which Iron, Cerium, Alumina, Titania, Magnesia, Zirconium, and Ccalcium based metal oxide nanomaterials and composites attribute an important role in defluorination processes. [3-6]. Nanosized metals and metal oxides mainly include nanosized zero-valent iron, ferric oxides, aluminium oxides, manganese oxides, titanium oxides, magnesium oxides and cerium oxides [2]. Existing literatures mentioned that number of nanosized metals and metal oxides exhibit favourable sorption towards the contaminants such as arsenic [5,6], cadmium [7,8], chromium[9,10], uranium[11], and other common pollutants such as phosphate[12] and organics [13, 14] in terms of high capacity of removal and selectivity of ions. The oxides of nanomaterials are rich in functional groups at their surface, highly stable for adsorbing fluorides from aqueous solution. The increased surface area of the metal oxide nanoparticles highly favour Fluoride adsorption.[1721].Their high adsorption capacity, non-toxic nature, limited solubility in water and good desorption potential makes a material of choice for defluoridation [15,16]. Fluoride adsorption efficacy is influenced by various factors viz. the effect of contact time, adsorbent dosage, adsorbate concentration, pH and temperature [16]. 2.1. Iron based nanomaterials In recent years, there is a growing interest in the use of iron oxides nanoparticles for the removal of heavy metal due to their simplicity and availability. Iron based nanomaterials have been successfully used as sorbent materials in the removal of various heavy metals from water systems [22]. Various literatures entail that magnetic magnetite (Fe3O4) and magnetic maghemite ( -Fe2O4) and nonmagnetic hematite ( -Fe2O3) are often used as nanoadsobnents The use of nanosized iron oxides for water and wastewater treatment is receiving growing interest because of their strong sorption capability, operational simplicity, resourcefulness and reasonable cost. Magnetic nanosized iron oxide sorbents also offered a viable and convenient solution by utilizing an external magnetic field [1, 3] Pathak et al. (2003) used the nano-sized powder of inorganic oxides such as Fe3O4, Al2O3 and ZrO2 and sonicated them in water. After that they combined all the inorganic oxides and impregnated in the matix of activated charcoal through adsorption. The material obtained then was used as the adsorbing packing bed for the removal of trace of fluoride/arsenite and arsenate from industrial wastewater and aldehyde from perfume grade alcohol respectively having concentration upto 0.01-0.02 mg/L..The thermolysis of polymeric based aqueous precursor solution of sucrose and polyvinyl alcohol was mixed with respective cations homogeneously. It was calcined to precursor 2
powder at 200-500°C for 2h to get required oxides. A phase of particle diameter (20 nm and 38nm) with 200 m /g surface area was obtained by this process. [23] Wu et. al. (2007) prepared a novel Fe-Al-Ce tri-metal oxide for fluoride removal from water and explored them in both of batch and column study. The tri-metal oxide was developed by co-precipitation process of Fe(II), Al(III) and Ce(IV) salt solution with a molar ratio of 1:4:1 under alkaline condition. It was observed that more than 90% of
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o
fluoride removal was achieved for tri-metal oxide calcined below 600 C temperature. The pH range of 6-6.5 found to be optimum and higher rate of adsorption was also taking place in the pH range of 5.5-7.0. A high adsorption capacity of 178 mg/g was obtained at pH of 7.0. The adsorption data was well fitted to the Langmuir two site adsorption. The Langmuir maximum defluoridation capacity was 185 mg/g and 229 mg/g for one-site and two-site adsorption model respectively. The presence of phosphate or arsenate reduced fluoride adsorption efficiency highly. The defluoridation was not significantly affected by presence of sulphate and chloride though nitrate above 50mg/l affected defluoridation efficiency. Desorption study revealed about 97% fluoride recovery was obtained by treating with spent adsorbent with NaOH solution at pH of 12.2. [24]. Biswas et.al. (2010) synthesized hydrous iron (III) –chromium (III) bimetal mixed oxide (HICMO) adsorbent for fluoride removal from water environment. The optimum pH for maximum fluoride adsorption was found to be 3.0 and then rate of adsorption declined in the pH range of 3-5. The adsorption followed the Langmuir isotherm model and the pseudo-second-order kinetic. The maximum Langmuir adsorption capacity of HICMO was found to be 16.34 mg/g. The thermodynamic study revealed that the adsorption process was spontaneous and endothermic in nature. [25] Wu et. al. (2011) prepared 3-5 mm granulated nano-adsorbent of Fe-Al-Ce tri-metal hydroxide (FAC) immobilized in porous polyvinyl alcohol (PVA) matrix via cross-linking with boric acid. The mechanically sound and good adsorbent were developed by a FAC concentration of 12% and PVA concentration of 7.5% resulted into fluoride adsorption capacity of 4.46 mg/g at pH of 6.5 with an initial fluoride concentration of 19mg/l and dose of 2.0g/l for 100ml fluoride solution. [26] Dhillon (2015) synthesized hydrous hybrid Fe-Ca-Zr oxide nanoadsorbent for defluoridation of water. The adsorbent possessed the maximum adsorption capacity of 250 mg/g with an optimal dose of 0.25 mg/l at pH-7.0.The adsorption kinetics followed Freundlich and Dubinin-Radushkevitch isotherms and data followed pseudo-secondorder kinetic model. This composite adsorbent could also able to remove E-coli strain from water. [27] Christina and Viswanathan (2015) prepared two different adsorbents viz. Fe3O4 nanoparticle immobilized in sodium alginate matrix (FNPSA) and Fe2O3 nanoparticles and saponified orange peel residue that were immobilized in sodium alginate matrix (FNPSOPR) for defluoridation of water. The adsorption process for both the adsorbent followed Langmuir isotherm and pseudo-first-order kinetic model. The maximum adsorption capacity of FNPSA and FNPSOPR were attained as 58.24 and 80.33 mg/g respectively. [28] 2.2 Aluminium based Nanoparticles It has been well reported in literature that metal oxides, of aluminium have been found to be excellent sorbent for anions removal from aqueous solutions. Since negatively charged anions are sorbed on the positively charged surface of metal oxides. Fluoride has been proven to be strongly adsorbed onto various aluminium-containing mineral surfaces.[29] Suriyaraj (2012) prepared hybrid Al2O3/Bio-TiO2nanocomposite impregnated on hermoplastic polyurethane membrane for defluoridation of water. The maximum adsorption capacity of this nanocomposite impregnated
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membrane was observed to be 1.9 mg/g and time- concentration adsorption data followed Langmuir isotherm and pseudo-second-order-kinetic model. [30] Chen et. al. (2013) prepared granulation of Fe-Al-Ce nano-adsorbent for fluoride removal from drinking water. The granulation of Fe-Al-Ce nano-adsorbent was synthesized by using aluminium, zirconium, and titanium and silica sol. The inorganic binders like zirconium and titanium reduced the adsorption capacity of Fe-Al-Ce adsorbent whereas aluminium and silica sol. showed enhanced adsorption capacity. The granulation of Fe-Al-Ce by extrusion o
with aluminium sol. calcined at 500 C had maximum fluoride removal over 90% and much higher than granulation by acrylic-styrene copolymer latex as a binder. [31] Adeno et al. (2014) prepared nanoscale aluminium oxide hydroxide as nano-AlOOH for defluoridation from water. The rate of adsorption was found to be very fast as most of the adsorption took place within the first 30 min. For initial fluoride concentration of 20mg/l with an optimum adsorbent dose of 1.6g/L the equilibrium was achieved within 60 min. It was observed that the removal of fluoride increases with the increase in adsorbent dose. The fluoride removal efficiency was increased at pH range between 3 and 8, later it decreases with increase in pH but the optimum pH was found at 7.0. The experimental data fitted well with Langmuir isotherm model and followed pseudo-second-order rate equation. The Langmuir maximum adsorption capacity was 62.5 mg/g with initial fluoride concentration of 20 mg/L. [32] Qiao et.al. (2014) synthesized Al-Fe (hydr) oxides with different Al/Fe molar ratios of co-precipitation method for simultaneous removal of arsenate and fluoride from water in the pH range of 5.0-9.0. The removal of fluoride was more at acidic pH and maximum at pH 6.0 and deceased onward. The fluoride removal was preliminary due to ligand exchange and the surface hydroxyl group density of Al-Fe (hydr) oxides beside its largest pHpzc. [33] Woyessa et. al. (2014) synthesized hydrous aluminium (III)-Iron (III)-Manganese (IV) ternary mixed oxide by coprecipitation method for defluoridation of water. The maximum adsorption was achieved by Al: Fe: Mn sample of ratio 75:15:10. The optimum pH range was found to be 6-7. The kinetic data was well described with the pseudosecond-order equation and equilibrium data was also fitted well with the Freundlich isotherm model. At pH range of o
6-7, the maximum adsorption capacity of hydrous Al-Fe-Mn was 23.99 mg/g and a temperature of 25 C with initial fluoride concentration of 20 mg/l. The presence of phosphate ions reduced the fluoride adsorption capacity greatly whereas carbonates, sulphate, nitrate and chloride ions affected on adsorption capacity was not significant. [34] 2.3 Titania based Nanoparticles Photocatalytic degradation is an emerging and promising technology, and has been successfully applied in the contaminant degradation in water and wastewater [35]. Titanium dioxide (TiO2) was focus of some studies in recent years, because of its photocatalytic effects which decompose organic chemicals and capacity of killing bacteria [36]. Nanosized TiO2 is also widely explored in the removal of micro-pollutants [37]. More and more efforts are devoted to recover TiO2 nanoparticles from the treated wastewater, especially when they are used in suspension. Li et. al. (2010) used Ti-Ce, Ti-La and TiO2 by hydrolysis-precipitation and hydrolysis for defluoridation of water. The defluoridation capacity of Ti-Ce, Ti-La and TiO2 were 9.6, 15.1 and 1.7 mg/g respectively. The equilibrium for three adsorbent was achieved within 4 hours. With the increase in pH from 3 to 9.5 sorption capacity was decreased.
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The adsorption kinetic was well explained with the Langmuir isotherm model, that followed the pseudo-secondorder kinetic model [39]. Suriyaraj et. al. (2014) carried out defluoridation investigation of water with various phases of TiO2 nanoparticle synthesized using a metal resistant Bacillus NARW11 bacterial species. It was observed that, anatase phase of TiO2 nanoparticle showed increased adsorption capacity (Qo =0.85 mg/g) when compared to other phases. The data were fitted onto Langmuir and Dubinin-Radushkevitch isotherms. Desorption of fluoride was carried out using initial NaOH concentration of 10%. Maximum desorption (64%) rate was observed with native TiO2 nanoparticle when compared to other phases [40]. 2.4 Cerium based Nanoparticles Variety of Cerium nanoparticles are being used worldwide for industrial applications such as catalysis, solar energy devices, optical display technology and corrosion prevention and biomedical applications [41]. CeO2 is widely used as a fuel additive in the automotive industry and a UV blocking agent in the cosmetic industry Studies on the solubility of CeO2 nanoparticles are scarce, but they are generally considered to be insoluble[42]. Recently attempts are made to explore its potential in the field of water and wastewater decontamination Xiuru et. al. (1998) used CeO2-TiO2/SiO2 surface composite by sol-gel method to coating CeO2-TiO2 on SiO2 substrate for fluoride removal from water. The adsorption capacity was found to be 21.4 mg/g and percentage removal was observed 85.6% [43]. Liu et. al. (2010) explored Al-Ce hybrid adsorbent by co-precipitation method using 0.2 mol L-1 AlCl3 and 0.05 3
o
mol L-1Ce (NO3) was precipitated and was dried at 80 C . The optimum pH of Al-Ce hybrid adsorbent was found to be 6.0. The adsorbent had a high adsorption capacity was up to 27.5 mg/g and the Langmuir adsorption capacity was 91.4 mg/g. The equilibrium fluoride concentration of 1mg/l. [44]. Ping et.al. (2010) used Ce-La binary hydroxide (CLH) adsorbent for defluoridation of industrial wastewater. It was found that adsorption capacity of 77.4 - 89.5 mg/g was achieved in the optimum pH range of 4-8. The equilibrium was reached after 120 minutes. The experimental data followed Langmuir isotherm model and the pseudo-secondorder kinetic model. The Langmuir maximum adsorption capacity was 84.2 mg/g at pH 7.0. The presence of co 2-
-
2-
2
-
3-
anions affected the defluoridation capacity in the order of HPO4 >HCO3 ≥ SiO3 > SO4 -> Cl , NO [45]. Deng et. al. (2011) used Mn-Ce oxide adsorbent by co-precipitation method. The process of calcining the Mn-Ce o
powder and pseudo-boehmite was used for granulation. The optimum Ce/Mn ratio was 1:1 and calcined at 300 C. The adsorption efficiency of 85.1 mg/g was observed. The maximum adsorption capacity of fluoride on powdered and granulated Mn-Ce oxide adsorbent was 137.5 and 103.1 mg/g respectively whereas the adsorption capacity of powder and granular Mn-Ce adsorbent were 79.5 and 45.5 respectively. It was observed that, adsorption on granulate Mn-Ce oxide was fast in first 1h and it reached in equilibrium after 3h [46]. Zhang et. al. (2013) prepared CeO2/Al2O3 composite adsorbent for defluoridation of water. The CeO2/Al2O3 composite was prepared by a simple chemical co-precipitation method. The optimum fluoride adsorption was obtained at a pH range of 3-10. The defluoridation capacity was affected by presence of co-anions in the order of Cl2
2
2
2-
< NO3-< SO4 -< CO3 -< C2O4 -< HPO4
. The kinetic of adsorption followed the pseudo-second-order kinetic
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model and adsorption isotherm was best fitted by Redlich-Peterson model. The defluoridation capacity of unmodified and NTP modified CeO2/Al2O3 composite was 24.1 mg/g and 37.0 mg/g respectively for initial fluoride concentration of 120mg/l with an adsorbent dose of 2g/l and adsorption time of 24 h. The chemisorptions was responsible for fluoride removal [47]. Ghosh et. al. (2015) studied defluoridation performance of agglomerated (140-290µm) Ce(IV)-Zr(IV) mixed oxide nanoparticle (̴50 nm) from ground water in packed bed column. The breakthrough was well described by Thomas kinetic model and the ground water with initial fluoride concentration of 3.0 mg/l was treated effectively by bed 3
capacity (542.43 mg/cm ) and length of mass transfer zone (0.6039cm) as calculated by bed depth service time model. [48] 2.5 Manganese Based Nanoparticles Magnesium oxide or magnesia (MgO) is a well-known adsorbent showing extremely high defluoridation capacity. [49]. Nanosized manganese oxides (NMnOs) exhibited superior adsorptive performance towards certain contaminants than other metal oxides because of its polymorphic structures and higher specific area[1]. MgO particles are effective in the removal of contaminants from aqueous solutions. [50] Maliyekkal et. al. (2010) prepared nano-magnesia by self-propagated combustion of magnesium nitrate trapped in cellulose fibre. The adsorption process followed pseudo-second-order equation and equilibrium data fitted well with Freundlich isotherm model and the maximum Langmuir adsorption capacity of nano-magnesia was 267.82 mg/g. At higher pH, the adsorption was decreased slightly. Phosphate showed great competitive factor for removal of fluoride followed by bicarbonate and nitrate. [51]. Dash et. al. (2013) prepared manganese oxide modified aluminium oxy (hydroxide) (MOAOH) adsorbent by sol-gel method for defluoridation of water. The fluoride removal of 94.8% was obtained at pH range of 5-7. The experimental data are fitted well with Langmuir isotherm model and adsorption process followed pseudo-secondorder kinetic model. The Langmuir maximum adsorption capacity was observed to be 18.62 mg/g.They used 1% NaOH solution for desorption of 86.20% fluoride was in 3h [52]. Devi et. al. (2014) used nano-sized magnesium oxide (nano-MgO) for fluoride removal from water. The surface 2
area of adsorbent was 92.46 m /g. The maximum fluoride removal of 90% was obtained with 0.6 g/l dose of adsorbent. The influence of pH on adsorbent was negligible. The experimental data were fitted well with Freundlich isotherm model which indicated that the adsorption was multilayer. The adsorption followed the pseudo-secondorder kinetic model. Desorption of 95% was achieved by 1M HCl. The presence of co-anions on reduction of adsorption was found in order of hydroxide (76%) < sulphate (83%) < bicarbonate (84%) < chloride (89%) [53]. Dayananda et. al. (2015) used MgO nano-particle loaded with mesoporous alumina (MgO@Al2O3) for defluoridation from water. The maximum fluoride removal was obtained by 40 wt. %MgO nanoparticle loaded on mesoporous Al2O3 . The maximum fluoride removal of nearly 90% was attained by 40 wt. %MgO nanoparticle loaded on mesoporous Al2O3 with initial fluoride concentration of 10 mg/l. The adsorption kinetics was well explained with both Langmuir and Freundlich isotherm models. The kinetics of adsorption was noted as pseudosecond-order rate equation and the adsorption process was based on chemisorption. The loaded mesoporous Al2O3 (40Mg@Al2O3) nanoparticles efficiently could reduced 5 and 10 mg/L of fluoride to approximately 1 mg/L [54].
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Minju (2015) used magnesium oxide coated magnetite nanoparticles for defluoridation of water. The fluoride removal of 98.6% for 13.6 mg/L of fluoride solution was achieved at optimum conditions of pH 6.0, contact time of 120 minutes and the adsorbent dose of 2 g/L. The experimental data fitted well with Langmuir isotherm and pseudo-second-ordered kinetic model. [55] 2.6 Other Nanoparticles Chang et. al. (2006) used the super magnetic nano-scale adsorbent of bayerite/SiO2/Fe3O4 adsorbent for defluoridation of water. The adsorbent was prepared through the three sequential steps such as chemical precipitation of Fe3O4, coating of SiO2 on Fe3O4 and finally coating of bayerite (Al (OH)3) on SiO2/Fe3O4 by adopting the sol-gel (MASG) or homogeneous precipitation (MAHP) methods. The sorbent ac The increased surface area of the metal oxide nanoparticles highly favor Fluoride adsorption.[17-21]. hieved physicochemical stability at pH range of 6-8. The MASG was found to be most effective adsorbent with the adsorption capacity of 38 g/kg at pH 6.0 as. [56] He et. al. (2014) used zirconium-based nanoparticle for the defluoridation from drinking water. The optimum pH was found to be 4 when the fluoride removal was carried out at pH range between 3.0 and 10.0. The adsorption process reached the equilibrium within 4 hrs and it was noticed that the equilibrium data fitted well with Langmuir isotherm model. The maximum adsorption capacity was observed as 97.48 and 78.56 mg/g at the optimal pH and -
3-
neutral pH respectively. The adsorption capacity decreased due to the presence of HCO3 and SiO2
ions. The
presence of co ions like phosphate, nitrate and organic matters had no effect on fluoride removal. The ion-exchange mechanism was equally responsible for adsorption of fluoride by zirconium-based nanoparticle [57]. Balarak (2015) used SiO2 nanoparticle for defluoridation of water with maximum adsorption capacity of 49.95 mg/g at pH of 6 with a contact time of 20 minutes and initial fluoride concentration of 25 mg/l. Defluopridation data were fitted well with Langmuir isotherm and followed pseudo-second-order-kinetic models. [58] -
Edris Bazrafshan, et.al. (2016) investigated the adsorption of F onto CuO nanoparticles was. The results reveal -
that CuO nanoparticles are an effective adsorbent for the removal of F from aqueous solutions. The equilibrium -
time was observed to be 80 min. The removal capacity of adsorbent was found to be 357 mg F /g. The adsorption -
kinetics followed the pseudosecond-order kinetic model. The equilibrium data for the adsorption of F on CuO nanoparticles were best represented by the Freundlich isotherm [59] 3. Conclusions and Perspectives In the present paper, the most recently explored metal oxide nanoparticles (metal oxides of Iron, Alumina, Titania, Cerium, Magnesia, Silica, Zirconium etc.), were highlighted to address their fluoride removal capacity as investigated by different environmental scientists and researchers . Considering the current scenario of development and application of nanomaterials, use of metal oxide nanomaterial appears to be an extremely promising tool for water and wastewater treatment. According to the literature available, metal oxides and hydroxides and its binary or tri-metal combination possesses higher adsorption capacities for fluoride ions. Among the oxides and hydroxides, various iron, titanium, magnesium and aluminium oxides and are most frequently tested and showed the highest adsorption capacities and a high selectivity for fluoride ions.
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Use of metal oxide nanomaterials has potential of being the most promising alternative for fluoride decontamination of water environment. Though many technologies for fluoride removal are available, further studies are still to be needed to offer a cost effective, user friendly solution to address the worldwide problem associated with fluoride contamination of water. Till date only a few metal of nanomaterials have emerged commercially. From literature we can conclude that several metal nanomaterials may have adverse effects on the environment and human health. The future research should be focused to reduce their potential toxicity to the environment and human health. More attempts should be made to come up with an engineering application of use of metal nano oxides as solution for environmental problems like fluoride contamination of water. Achieving the economical efficiency of metal and metal oxide nanomaterials can be a challenging area to work with. 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ScienceDirect Materials Today: Proceedings 18 (2019) 1146–1155
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ICN3I-2017
A Review of Metal oxide Nanomaterials for Fluoride decontamination from Water Environment a
b
Ms. Disha Khandare , Dr. Somnath Mukherjee
a Civil Engineering Department, Government Polytechnic, Nagpur-440030, India. b
Civil Engineering Department, Jadavpur University, Kolkata-700032, India.
Abstract
Fluoride pollution in water emerges a challenging problem to environmental researchers especially in regions where people depend on groundwater for drinking. Natural water bodies are experienced Fluoride impurities from geogenic and anthropogenic sources. Fluoride contamination in drinking water sources has been recognized as a major problem in many countries worldwide especially in several parts of India, China, Sri Lanka, South Africa, Tanzania, Argentina, East Africa, part of South Africa, Turkey and some part of South America. Intrusion of Fluoride with drinking water manifests several health effects such as dental caries and teeth mottling besides skeletal fluorosis. Due to clinical manifestations caused by drinking Fluoride contaminated water, the World Health Organization (WHO) has recommended 1.5 mg L-1. Hence, it is very much needed to supply water with safe F¯. Various physico-chemical methods are available for defluoridation of water, out of which adsorption method is common and widely be used .to remove fluoride from water. Till date, activated alumina is most conventional adsorbing material that are conveniently being used for this purpose. However, activated alumina performed well in acidic environment and regeneration issue poses a complex problem. Other traditional adsorbents though are able to uptake Fluoride from water environment the low sorption capacities and efficiencies restricted their wide application. A major breakthrough took place in recent years due to application of nanomaterial in water purification. As compared with traditional materials used, nanostructure based adsorbents exhibited much higher Corresponding author. Tel.:+ 91 9422819119; fax: +91-712-256-4483. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i-2017).
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performance and efficiency of water decontamination as well as providing a sustainable approach to safe water supply. Nonmaterial is being used throughout the globe as possible solution for removal of fluoride from water as an engineering tool. In the past ten years, many researchers have dealt with low cost and effective adsorbent nanomaterials for the removal of fluoride from aqueous solution and contaminated water. Among them Iron, Magnesia, Alumina, Cerium and calcium, Titania, Silica and Zirconium based metal oxide nanomaterials and composites have proven themselves as excellent adsorbents due to their unique features. The increased surface area of the metal oxide nanoparticles highly favours fluoride adsorption. Their high adsorption capacity, non-toxic nature, limited solubility in water and good desorption potential makes metal oxides a material of choice. The present paper discusses use of nanomaterial and composite nanomaterials for enhanced removal of fluoride contaminated water. The present study also highlights the various key factors (pH, agitation time, initial fluoride concentration, temperature, particle size, surface area, etc) which governs efficacy of different materials in the removal of fluoride from water. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i-2017).
Keywords: Nanomaterials; Nanocomposite; Adsorption; Fluoride Removal.
1. Introduction Presence of Fluoride in sub surface water has emerged a challenging problem to water scientists and environmental researchers especially for the regions where people primarily depend on groundwater for drinking. Natural water bodies are also experienced Fluoride contamination from geogenic and anthropogenic attributes. Due to clinical manifestations caused by drinking Fluoride contaminated water, the World Health Organization (WHO) has recommended 1.5 mg L-1. Hence, it is very much needed to supply water with safe F¯ within the stipulated limit. In the past ten years, numerous attempts are endeavored to develop low cost and effective adsorbents inclusive of nanomaterials or nanocomposites for the removal of Fluoride from water. As compared with traditional materials used, nanostructure based adsorbents exhibited much higher efficiency and faster rate of removal in water treatment.[1] Nanotechnology exhibited a great potential in improving the performance and overall efficiency of water decontamination as well as providing a sustainable approach to safe water supply. Nanotechnology is considered as the vision of the next industrial revolution. Due to the large specific area and fast adsorption kinetics rate, the nano-scale adsorbents have attracted great attention for effective fluoride removal. Recently, metal oxide based nano- sorbents such as iron oxide, aluminum oxide, titanium oxide, manganese oxide, zirconium oxide has been focused by some researchers [2] .The nano sized metal oxides are classified as the promising tools for engineers for heavy metals removal from aqueous systems [3]. The present review aims to focus some important issues and to provide an overview of fluoride removal by applications of advanced metal oxides nanomaterials.
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2. Metal oxide nanomaterials Nanosized metals and metal oxides have received ever-increasing attentions because of their high performance and low cost for contaminants removal. [1,4]. Various inorganic nanomaterial play a vital role in the removal of fluoride from aqueous solution out of which Iron, Cerium, Alumina, Titania, Magnesia, Zirconium, and Ccalcium based metal oxide nanomaterials and composites attribute an important role in defluorination processes. [3-6]. Nanosized metals and metal oxides mainly include nanosized zero-valent iron, ferric oxides, aluminium oxides, manganese oxides, titanium oxides, magnesium oxides and cerium oxides [2]. Existing literatures mentioned that number of nanosized metals and metal oxides exhibit favourable sorption towards the contaminants such as arsenic [5,6], cadmium [7,8], chromium[9,10], uranium[11], and other common pollutants such as phosphate[12] and organics [13, 14] in terms of high capacity of removal and selectivity of ions. The oxides of nanomaterials are rich in functional groups at their surface, highly stable for adsorbing fluorides from aqueous solution. The increased surface area of the metal oxide nanoparticles highly favour Fluoride adsorption.[1721].Their high adsorption capacity, non-toxic nature, limited solubility in water and good desorption potential makes a material of choice for defluoridation [15,16]. Fluoride adsorption efficacy is influenced by various factors viz. the effect of contact time, adsorbent dosage, adsorbate concentration, pH and temperature [16]. 2.1. Iron based nanomaterials In recent years, there is a growing interest in the use of iron oxides nanoparticles for the removal of heavy metal due to their simplicity and availability. Iron based nanomaterials have been successfully used as sorbent materials in the removal of various heavy metals from water systems [22]. Various literatures entail that magnetic magnetite (Fe3O4) and magnetic maghemite ( -Fe2O4) and nonmagnetic hematite ( -Fe2O3) are often used as nanoadsobnents The use of nanosized iron oxides for water and wastewater treatment is receiving growing interest because of their strong sorption capability, operational simplicity, resourcefulness and reasonable cost. Magnetic nanosized iron oxide sorbents also offered a viable and convenient solution by utilizing an external magnetic field [1, 3] Pathak et al. (2003) used the nano-sized powder of inorganic oxides such as Fe3O4, Al2O3 and ZrO2 and sonicated them in water. After that they combined all the inorganic oxides and impregnated in the matix of activated charcoal through adsorption. The material obtained then was used as the adsorbing packing bed for the removal of trace of fluoride/arsenite and arsenate from industrial wastewater and aldehyde from perfume grade alcohol respectively having concentration upto 0.01-0.02 mg/L..The thermolysis of polymeric based aqueous precursor solution of sucrose and polyvinyl alcohol was mixed with respective cations homogeneously. It was calcined to precursor 2
powder at 200-500°C for 2h to get required oxides. A phase of particle diameter (20 nm and 38nm) with 200 m /g surface area was obtained by this process. [23] Wu et. al. (2007) prepared a novel Fe-Al-Ce tri-metal oxide for fluoride removal from water and explored them in both of batch and column study. The tri-metal oxide was developed by co-precipitation process of Fe(II), Al(III) and Ce(IV) salt solution with a molar ratio of 1:4:1 under alkaline condition. It was observed that more than 90% of
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o
fluoride removal was achieved for tri-metal oxide calcined below 600 C temperature. The pH range of 6-6.5 found to be optimum and higher rate of adsorption was also taking place in the pH range of 5.5-7.0. A high adsorption capacity of 178 mg/g was obtained at pH of 7.0. The adsorption data was well fitted to the Langmuir two site adsorption. The Langmuir maximum defluoridation capacity was 185 mg/g and 229 mg/g for one-site and two-site adsorption model respectively. The presence of phosphate or arsenate reduced fluoride adsorption efficiency highly. The defluoridation was not significantly affected by presence of sulphate and chloride though nitrate above 50mg/l affected defluoridation efficiency. Desorption study revealed about 97% fluoride recovery was obtained by treating with spent adsorbent with NaOH solution at pH of 12.2. [24]. Biswas et.al. (2010) synthesized hydrous iron (III) –chromium (III) bimetal mixed oxide (HICMO) adsorbent for fluoride removal from water environment. The optimum pH for maximum fluoride adsorption was found to be 3.0 and then rate of adsorption declined in the pH range of 3-5. The adsorption followed the Langmuir isotherm model and the pseudo-second-order kinetic. The maximum Langmuir adsorption capacity of HICMO was found to be 16.34 mg/g. The thermodynamic study revealed that the adsorption process was spontaneous and endothermic in nature. [25] Wu et. al. (2011) prepared 3-5 mm granulated nano-adsorbent of Fe-Al-Ce tri-metal hydroxide (FAC) immobilized in porous polyvinyl alcohol (PVA) matrix via cross-linking with boric acid. The mechanically sound and good adsorbent were developed by a FAC concentration of 12% and PVA concentration of 7.5% resulted into fluoride adsorption capacity of 4.46 mg/g at pH of 6.5 with an initial fluoride concentration of 19mg/l and dose of 2.0g/l for 100ml fluoride solution. [26] Dhillon (2015) synthesized hydrous hybrid Fe-Ca-Zr oxide nanoadsorbent for defluoridation of water. The adsorbent possessed the maximum adsorption capacity of 250 mg/g with an optimal dose of 0.25 mg/l at pH-7.0.The adsorption kinetics followed Freundlich and Dubinin-Radushkevitch isotherms and data followed pseudo-secondorder kinetic model. This composite adsorbent could also able to remove E-coli strain from water. [27] Christina and Viswanathan (2015) prepared two different adsorbents viz. Fe3O4 nanoparticle immobilized in sodium alginate matrix (FNPSA) and Fe2O3 nanoparticles and saponified orange peel residue that were immobilized in sodium alginate matrix (FNPSOPR) for defluoridation of water. The adsorption process for both the adsorbent followed Langmuir isotherm and pseudo-first-order kinetic model. The maximum adsorption capacity of FNPSA and FNPSOPR were attained as 58.24 and 80.33 mg/g respectively. [28] 2.2 Aluminium based Nanoparticles It has been well reported in literature that metal oxides, of aluminium have been found to be excellent sorbent for anions removal from aqueous solutions. Since negatively charged anions are sorbed on the positively charged surface of metal oxides. Fluoride has been proven to be strongly adsorbed onto various aluminium-containing mineral surfaces.[29] Suriyaraj (2012) prepared hybrid Al2O3/Bio-TiO2nanocomposite impregnated on hermoplastic polyurethane membrane for defluoridation of water. The maximum adsorption capacity of this nanocomposite impregnated
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membrane was observed to be 1.9 mg/g and time- concentration adsorption data followed Langmuir isotherm and pseudo-second-order-kinetic model. [30] Chen et. al. (2013) prepared granulation of Fe-Al-Ce nano-adsorbent for fluoride removal from drinking water. The granulation of Fe-Al-Ce nano-adsorbent was synthesized by using aluminium, zirconium, and titanium and silica sol. The inorganic binders like zirconium and titanium reduced the adsorption capacity of Fe-Al-Ce adsorbent whereas aluminium and silica sol. showed enhanced adsorption capacity. The granulation of Fe-Al-Ce by extrusion o
with aluminium sol. calcined at 500 C had maximum fluoride removal over 90% and much higher than granulation by acrylic-styrene copolymer latex as a binder. [31] Adeno et al. (2014) prepared nanoscale aluminium oxide hydroxide as nano-AlOOH for defluoridation from water. The rate of adsorption was found to be very fast as most of the adsorption took place within the first 30 min. For initial fluoride concentration of 20mg/l with an optimum adsorbent dose of 1.6g/L the equilibrium was achieved within 60 min. It was observed that the removal of fluoride increases with the increase in adsorbent dose. The fluoride removal efficiency was increased at pH range between 3 and 8, later it decreases with increase in pH but the optimum pH was found at 7.0. The experimental data fitted well with Langmuir isotherm model and followed pseudo-second-order rate equation. The Langmuir maximum adsorption capacity was 62.5 mg/g with initial fluoride concentration of 20 mg/L. [32] Qiao et.al. (2014) synthesized Al-Fe (hydr) oxides with different Al/Fe molar ratios of co-precipitation method for simultaneous removal of arsenate and fluoride from water in the pH range of 5.0-9.0. The removal of fluoride was more at acidic pH and maximum at pH 6.0 and deceased onward. The fluoride removal was preliminary due to ligand exchange and the surface hydroxyl group density of Al-Fe (hydr) oxides beside its largest pHpzc. [33] Woyessa et. al. (2014) synthesized hydrous aluminium (III)-Iron (III)-Manganese (IV) ternary mixed oxide by coprecipitation method for defluoridation of water. The maximum adsorption was achieved by Al: Fe: Mn sample of ratio 75:15:10. The optimum pH range was found to be 6-7. The kinetic data was well described with the pseudosecond-order equation and equilibrium data was also fitted well with the Freundlich isotherm model. At pH range of o
6-7, the maximum adsorption capacity of hydrous Al-Fe-Mn was 23.99 mg/g and a temperature of 25 C with initial fluoride concentration of 20 mg/l. The presence of phosphate ions reduced the fluoride adsorption capacity greatly whereas carbonates, sulphate, nitrate and chloride ions affected on adsorption capacity was not significant. [34] 2.3 Titania based Nanoparticles Photocatalytic degradation is an emerging and promising technology, and has been successfully applied in the contaminant degradation in water and wastewater [35]. Titanium dioxide (TiO2) was focus of some studies in recent years, because of its photocatalytic effects which decompose organic chemicals and capacity of killing bacteria [36]. Nanosized TiO2 is also widely explored in the removal of micro-pollutants [37]. More and more efforts are devoted to recover TiO2 nanoparticles from the treated wastewater, especially when they are used in suspension. Li et. al. (2010) used Ti-Ce, Ti-La and TiO2 by hydrolysis-precipitation and hydrolysis for defluoridation of water. The defluoridation capacity of Ti-Ce, Ti-La and TiO2 were 9.6, 15.1 and 1.7 mg/g respectively. The equilibrium for three adsorbent was achieved within 4 hours. With the increase in pH from 3 to 9.5 sorption capacity was decreased.
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The adsorption kinetic was well explained with the Langmuir isotherm model, that followed the pseudo-secondorder kinetic model [39]. Suriyaraj et. al. (2014) carried out defluoridation investigation of water with various phases of TiO2 nanoparticle synthesized using a metal resistant Bacillus NARW11 bacterial species. It was observed that, anatase phase of TiO2 nanoparticle showed increased adsorption capacity (Qo =0.85 mg/g) when compared to other phases. The data were fitted onto Langmuir and Dubinin-Radushkevitch isotherms. Desorption of fluoride was carried out using initial NaOH concentration of 10%. Maximum desorption (64%) rate was observed with native TiO2 nanoparticle when compared to other phases [40]. 2.4 Cerium based Nanoparticles Variety of Cerium nanoparticles are being used worldwide for industrial applications such as catalysis, solar energy devices, optical display technology and corrosion prevention and biomedical applications [41]. CeO2 is widely used as a fuel additive in the automotive industry and a UV blocking agent in the cosmetic industry Studies on the solubility of CeO2 nanoparticles are scarce, but they are generally considered to be insoluble[42]. Recently attempts are made to explore its potential in the field of water and wastewater decontamination Xiuru et. al. (1998) used CeO2-TiO2/SiO2 surface composite by sol-gel method to coating CeO2-TiO2 on SiO2 substrate for fluoride removal from water. The adsorption capacity was found to be 21.4 mg/g and percentage removal was observed 85.6% [43]. Liu et. al. (2010) explored Al-Ce hybrid adsorbent by co-precipitation method using 0.2 mol L-1 AlCl3 and 0.05 3
o
mol L-1Ce (NO3) was precipitated and was dried at 80 C . The optimum pH of Al-Ce hybrid adsorbent was found to be 6.0. The adsorbent had a high adsorption capacity was up to 27.5 mg/g and the Langmuir adsorption capacity was 91.4 mg/g. The equilibrium fluoride concentration of 1mg/l. [44]. Ping et.al. (2010) used Ce-La binary hydroxide (CLH) adsorbent for defluoridation of industrial wastewater. It was found that adsorption capacity of 77.4 - 89.5 mg/g was achieved in the optimum pH range of 4-8. The equilibrium was reached after 120 minutes. The experimental data followed Langmuir isotherm model and the pseudo-secondorder kinetic model. The Langmuir maximum adsorption capacity was 84.2 mg/g at pH 7.0. The presence of co 2-
-
2-
2
-
3-
anions affected the defluoridation capacity in the order of HPO4 >HCO3 ≥ SiO3 > SO4 -> Cl , NO [45]. Deng et. al. (2011) used Mn-Ce oxide adsorbent by co-precipitation method. The process of calcining the Mn-Ce o
powder and pseudo-boehmite was used for granulation. The optimum Ce/Mn ratio was 1:1 and calcined at 300 C. The adsorption efficiency of 85.1 mg/g was observed. The maximum adsorption capacity of fluoride on powdered and granulated Mn-Ce oxide adsorbent was 137.5 and 103.1 mg/g respectively whereas the adsorption capacity of powder and granular Mn-Ce adsorbent were 79.5 and 45.5 respectively. It was observed that, adsorption on granulate Mn-Ce oxide was fast in first 1h and it reached in equilibrium after 3h [46]. Zhang et. al. (2013) prepared CeO2/Al2O3 composite adsorbent for defluoridation of water. The CeO2/Al2O3 composite was prepared by a simple chemical co-precipitation method. The optimum fluoride adsorption was obtained at a pH range of 3-10. The defluoridation capacity was affected by presence of co-anions in the order of Cl2
2
2
2-
< NO3-< SO4 -< CO3 -< C2O4 -< HPO4
. The kinetic of adsorption followed the pseudo-second-order kinetic
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model and adsorption isotherm was best fitted by Redlich-Peterson model. The defluoridation capacity of unmodified and NTP modified CeO2/Al2O3 composite was 24.1 mg/g and 37.0 mg/g respectively for initial fluoride concentration of 120mg/l with an adsorbent dose of 2g/l and adsorption time of 24 h. The chemisorptions was responsible for fluoride removal [47]. Ghosh et. al. (2015) studied defluoridation performance of agglomerated (140-290µm) Ce(IV)-Zr(IV) mixed oxide nanoparticle (̴50 nm) from ground water in packed bed column. The breakthrough was well described by Thomas kinetic model and the ground water with initial fluoride concentration of 3.0 mg/l was treated effectively by bed 3
capacity (542.43 mg/cm ) and length of mass transfer zone (0.6039cm) as calculated by bed depth service time model. [48] 2.5 Manganese Based Nanoparticles Magnesium oxide or magnesia (MgO) is a well-known adsorbent showing extremely high defluoridation capacity. [49]. Nanosized manganese oxides (NMnOs) exhibited superior adsorptive performance towards certain contaminants than other metal oxides because of its polymorphic structures and higher specific area[1]. MgO particles are effective in the removal of contaminants from aqueous solutions. [50] Maliyekkal et. al. (2010) prepared nano-magnesia by self-propagated combustion of magnesium nitrate trapped in cellulose fibre. The adsorption process followed pseudo-second-order equation and equilibrium data fitted well with Freundlich isotherm model and the maximum Langmuir adsorption capacity of nano-magnesia was 267.82 mg/g. At higher pH, the adsorption was decreased slightly. Phosphate showed great competitive factor for removal of fluoride followed by bicarbonate and nitrate. [51]. Dash et. al. (2013) prepared manganese oxide modified aluminium oxy (hydroxide) (MOAOH) adsorbent by sol-gel method for defluoridation of water. The fluoride removal of 94.8% was obtained at pH range of 5-7. The experimental data are fitted well with Langmuir isotherm model and adsorption process followed pseudo-secondorder kinetic model. The Langmuir maximum adsorption capacity was observed to be 18.62 mg/g.They used 1% NaOH solution for desorption of 86.20% fluoride was in 3h [52]. Devi et. al. (2014) used nano-sized magnesium oxide (nano-MgO) for fluoride removal from water. The surface 2
area of adsorbent was 92.46 m /g. The maximum fluoride removal of 90% was obtained with 0.6 g/l dose of adsorbent. The influence of pH on adsorbent was negligible. The experimental data were fitted well with Freundlich isotherm model which indicated that the adsorption was multilayer. The adsorption followed the pseudo-secondorder kinetic model. Desorption of 95% was achieved by 1M HCl. The presence of co-anions on reduction of adsorption was found in order of hydroxide (76%) < sulphate (83%) < bicarbonate (84%) < chloride (89%) [53]. Dayananda et. al. (2015) used MgO nano-particle loaded with mesoporous alumina (MgO@Al2O3) for defluoridation from water. The maximum fluoride removal was obtained by 40 wt. %MgO nanoparticle loaded on mesoporous Al2O3 . The maximum fluoride removal of nearly 90% was attained by 40 wt. %MgO nanoparticle loaded on mesoporous Al2O3 with initial fluoride concentration of 10 mg/l. The adsorption kinetics was well explained with both Langmuir and Freundlich isotherm models. The kinetics of adsorption was noted as pseudosecond-order rate equation and the adsorption process was based on chemisorption. The loaded mesoporous Al2O3 (40Mg@Al2O3) nanoparticles efficiently could reduced 5 and 10 mg/L of fluoride to approximately 1 mg/L [54].
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Minju (2015) used magnesium oxide coated magnetite nanoparticles for defluoridation of water. The fluoride removal of 98.6% for 13.6 mg/L of fluoride solution was achieved at optimum conditions of pH 6.0, contact time of 120 minutes and the adsorbent dose of 2 g/L. The experimental data fitted well with Langmuir isotherm and pseudo-second-ordered kinetic model. [55] 2.6 Other Nanoparticles Chang et. al. (2006) used the super magnetic nano-scale adsorbent of bayerite/SiO2/Fe3O4 adsorbent for defluoridation of water. The adsorbent was prepared through the three sequential steps such as chemical precipitation of Fe3O4, coating of SiO2 on Fe3O4 and finally coating of bayerite (Al (OH)3) on SiO2/Fe3O4 by adopting the sol-gel (MASG) or homogeneous precipitation (MAHP) methods. The sorbent ac The increased surface area of the metal oxide nanoparticles highly favor Fluoride adsorption.[17-21]. hieved physicochemical stability at pH range of 6-8. The MASG was found to be most effective adsorbent with the adsorption capacity of 38 g/kg at pH 6.0 as. [56] He et. al. (2014) used zirconium-based nanoparticle for the defluoridation from drinking water. The optimum pH was found to be 4 when the fluoride removal was carried out at pH range between 3.0 and 10.0. The adsorption process reached the equilibrium within 4 hrs and it was noticed that the equilibrium data fitted well with Langmuir isotherm model. The maximum adsorption capacity was observed as 97.48 and 78.56 mg/g at the optimal pH and -
3-
neutral pH respectively. The adsorption capacity decreased due to the presence of HCO3 and SiO2
ions. The
presence of co ions like phosphate, nitrate and organic matters had no effect on fluoride removal. The ion-exchange mechanism was equally responsible for adsorption of fluoride by zirconium-based nanoparticle [57]. Balarak (2015) used SiO2 nanoparticle for defluoridation of water with maximum adsorption capacity of 49.95 mg/g at pH of 6 with a contact time of 20 minutes and initial fluoride concentration of 25 mg/l. Defluopridation data were fitted well with Langmuir isotherm and followed pseudo-second-order-kinetic models. [58] -
Edris Bazrafshan, et.al. (2016) investigated the adsorption of F onto CuO nanoparticles was. The results reveal -
that CuO nanoparticles are an effective adsorbent for the removal of F from aqueous solutions. The equilibrium -
time was observed to be 80 min. The removal capacity of adsorbent was found to be 357 mg F /g. The adsorption -
kinetics followed the pseudosecond-order kinetic model. The equilibrium data for the adsorption of F on CuO nanoparticles were best represented by the Freundlich isotherm [59] 3. Conclusions and Perspectives In the present paper, the most recently explored metal oxide nanoparticles (metal oxides of Iron, Alumina, Titania, Cerium, Magnesia, Silica, Zirconium etc.), were highlighted to address their fluoride removal capacity as investigated by different environmental scientists and researchers . Considering the current scenario of development and application of nanomaterials, use of metal oxide nanomaterial appears to be an extremely promising tool for water and wastewater treatment. According to the literature available, metal oxides and hydroxides and its binary or tri-metal combination possesses higher adsorption capacities for fluoride ions. Among the oxides and hydroxides, various iron, titanium, magnesium and aluminium oxides and are most frequently tested and showed the highest adsorption capacities and a high selectivity for fluoride ions.
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Use of metal oxide nanomaterials has potential of being the most promising alternative for fluoride decontamination of water environment. Though many technologies for fluoride removal are available, further studies are still to be needed to offer a cost effective, user friendly solution to address the worldwide problem associated with fluoride contamination of water. Till date only a few metal of nanomaterials have emerged commercially. From literature we can conclude that several metal nanomaterials may have adverse effects on the environment and human health. The future research should be focused to reduce their potential toxicity to the environment and human health. More attempts should be made to come up with an engineering application of use of metal nano oxides as solution for environmental problems like fluoride contamination of water. Achieving the economical efficiency of metal and metal oxide nanomaterials can be a challenging area to work with. 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