Elsevier

Powder Technology

Volume 227, September 2012, Pages 3-8
Powder Technology

Fe―Ti oxide nano-adsorbent synthesized by co-precipitation for fluoride removal from drinking water and its adsorption mechanism

https://doi.org/10.1016/j.powtec.2011.11.030Get rights and content

Abstract

A novel bimetallic oxide adsorbent was synthesized by the co-precipitation of Fe(II) and Ti(IV) sulfate solution using ammonia titration at room temperature. The influences of the washing and drying methods, Fe/Ti molar ratio, and calcination temperature used in the preparation on the morphology, crystallization, surface structure and adsorption capacity were investigated. An optimized Fe―Ti adsorbent had a Langmuir adsorption capacity of 47.0 mg/g, which was much higher than that of either a pure Fe oxide or Ti oxide adsorbent. There was a synergistic interaction between Fe and Ti in which Fe―O―Ti bonds on the adsorbent surface and hydroxyl groups provide the active sites for adsorption, and Fe―O―Ti―F bonds were formed by fluoride adsorption. The novel Fe―Ti adsorbent is efficient and economical for fluoride removal from drinking water.

Graphical abstract

A novel Fe―Ti oxide nano-adsorbent was synthesized by co-precipitation, which had a Langmuir adsorption capacity of 47.0 mg/g at optimized condition. The Fe―O―Ti bond in the Fe―Ti adsorbent supported the active site (Fe―O―Ti―OH) for fluoride adsorption by forming a Fe―O―Ti―F bond on the adsorbent surface.

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Highlights

► A novel and costless Fe―Ti oxide nano-adsorbent was synthesized. ► The optimized Fe―Ti adsorbent had a Langmuir adsorption capacity of 47.0 mg/g. ► Fe and Ti in the adsorbent showed a remarkable synergistic interaction. ► The formed Fe―O―Ti in Fe―Ti adsorbent provided active sites of Fe―O―Ti―OH.

Introduction

Fluoride in minute quantities is an essential component for human health and helps in the normal mineralization of bones and formation of dental enamel, but excessive intake can result in fluorosis. There are more than 20 developed and developing nations where fluorosis is endemic and WHO has given a guideline limitation of less than 1.5 mg/L of fluoride in drinking water [1]. Reverse osmosis, nanofiltration, electrodialysis and Donnan dialysis have been used for fluoride removal [2], but adsorption is considered more efficient for fluoride removal from drinking water because it is simple to operate and cost-effective [2].

Many expensive metal oxide adsorbents with high fluoride adsorption capacities have been developed, such as those using zirconium oxide [3] and rare earth metal oxide [4], but their costs are high. Goethite is a naturally occurring water purifier which is abundant in the earth crust and cheap [5]. Synthetic iron oxide is a good adsorbent for fluoride removal from contaminated water [6], [7], [8], [9], but its Langmuir adsorption capacity is low at about 16.5 mg/g [10]. Adsorbents synthesized with iron oxide that incorporate different metal ions for high adsorption performance have been studied. Aluminum (III) [11], zirconium (IV) [12], tin (IV) [13] and chromium (III) [6] ions had been introduced into iron oxide to form bimetallic adsorbents for fluoride adsorption. The synthesis processes and adsorption capacities of these bimetallic oxide adsorbents are summarized in Table 1. It was reported that a new chemical bond formed between the two metal elements through an oxygen atom increased the amount of hydroxyl groups on the adsorbent surface and adsorption capacity of the adsorbents. However, their adsorption capacities were still not high enough and the adsorbents need frequent regeneration.

Recently, a titanium-derived adsorbent was shown to be a potential selective adsorbent for fluoride ions and especially arsenic compounds [14]. Although TiO2 hardly adsorbs fluoride, it was found that titanium hydroxide (Ti(OH)4) prepared by titrating ammonia into TiOSO4 solution [14], [15] or by sol–gel hydrolysis using titanium isopropoxide [16] can exchange fluoride ions. Nano-crystalline titanium dioxide [17], iron (III)-titanium (IV) binary mixed oxide [18] and Ce―Ti [19] oxide adsorbent have been developed to remove arsenate compounds from drinking water. Deng et al. [19] reported that titanium-derived adsorbents produced by hydrolysis at a high temperature had a higher arsenate adsorption capacity than those produced by precipitation at room temperature.

The nano-adsorbent synthesized was of fine powder or a hydroxide floc. It cannot be used directly in a packed bed due to its low hydraulic conductivity and high pressure drop. It needs to be granulated into 1–2 mm granules with a certain strength that can be used directly for adsorption in a packed bed.

In this study, a novel Fe―Ti oxide adsorbent was synthesized by co-precipitation at room temperature. The synergistic interactions between Fe and Ti both during crystallization and fluoride adsorption were investigated. The synthesized adsorbent had a high adsorption capacity and was cost effective. The adsorption mechanism on the Fe―Ti oxide adsorbent was studied.

Section snippets

Materials

FeSO4•7H2O, Ti(SO4)2•4H2O and NH3•H2O used were analytical grade (Chemical Engineering Company of Beijing, China). The other chemicals used were also analytical grade reagents.

Adsorbent preparation

FeSO4•7H2O and Ti(SO4)2•4H2O were dissolved in deionized water to form a mixed solution with total molar concentration of 0.3 M. 12.5% ammonia solution was slowly titrated into the mixed solution under agitation until the pH was 6.9 ± 0.2. The slurry was aged for 48 h. After that, the precipitates were filtrated, washed and

The optimized Fe―Ti adsorbent

Fig. 1 shows a TEM image of the optimized Fe―Ti oxide adsorbent, which was prepared using a Fe/Ti molar ratio of 2:1, ethanol washing, microwave drying, and a calcination temperature of 200 °C. The nanosized structure in the adsorbent was obvious, and the primary particle size was 5–7 nm.

The fluoride adsorption isotherm is depicted in Fig. 2. A saturated fluoride adsorption capacity of 47.0 mg/g was obtained from the fitting of the Langmuir isotherm. It has been reported that the Langmuir

Conclusion

A novel Fe―Ti bimetallic oxide adsorbent was synthesized by the co-precipitation of Fe(II) and Ti(IV) sulfate solution using ammonia titration at room temperature. An optimized adsorbent, which was synthesized with a Fe/Ti molar ratio of 2:1, ethanol washed, microwave dried and calcined at 200 °C, had a Langmuir adsorption capacity of 47.0 mg/g. Fe and Ti in the Fe―Ti oxide adsorbent interacted synergistically to give increased fluoride adsorption capacity. The hydroxyl groups and Fe―O―Ti bonds

Acknowledgements

The authors wish to express their appreciation of financial support of this study by the National Natural Science Foundation of China (NSFC No. 20906055 and No. 21176134).

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