Enhanced fluoride removal from drinking water by magnesia-amended activated alumina granules

https://doi.org/10.1016/j.cej.2007.09.049Get rights and content

Abstract

This paper describes the fluoride removal potential of a novel sorbent, magnesia-amended activated alumina (MAAA) from drinking water. MAAA, prepared by calcining magnesium hydroxide impregnated alumina at 450 °C has shown high fluoride sorption potential than activated alumina from drinking water. Batch sorption studies were performed as a function of contact time, pH, initial fluoride concentration, and adsorbent dose. Studies were also performed to understand the effect of various other co-existing ions present in real ground water samples. X-ray powder diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray (EDAX) and a gas adsorption porosimetry analyses were used to characterize the physicochemical properties of MAAA. More than 95% removal of fluoride (10 mg l−1) was achieved within 3 h of contact time at neutral pH. Sorption of fluoride onto MAAA was found to be pH dependant and a decrease in sorption was observed at higher pHs. Among the kinetic models tested, pseudo-second-order model fitted the kinetic data well, suggesting the chemisorption mechanism. Among the various isotherm model tested, Sips model predicted the data well. The maximum sorption capacity of fluoride deduced from Sips equation was 10.12 mg g−1. Most of the co-existing ions studied have negligible effect on fluoride sorption by MAAA. However, higher concentrations of bicarbonate and sulfate have reduced the fluoride sorption capacity.

Introduction

Low concentrations of fluoride in drinking water have been considered beneficial to prevent dental carries. But scientists are now debating on the health benefits of fluoride even at low concentrations. It has long been known that chronic intake of excessive fluoride (>1 mg l−1) can lead to severe dental and skeletal fluorosis. It not only affects teeth and skeleton, but its accumulation over a long period can also lead to change in the DNA structure [1], [2].

Consumption of ground water containing high levels of fluoride concentration is one of the major exposure mechanism to the public in India and in many other parts of the world. Drinking water sources in India have fluoride concentration as high as 30 mg l−1. According to World Health Organization (WHO) guidelines, the fluoride concentration in drinking water should not exceed 1.5 mg l−1 [3]. However, this guideline value of fluoride is not universal.

Fluoride removal from drinking water can be achieved by chemical precipitation, adsorption onto activated alumina, membrane process and ion exchange. Among these processes, membrane and ion exchange processes are not very common due to its high installation and maintenance cost. Other two methods are very common in India, especially chemical precipitation by addition of alum and lime mixture into fluoride-contaminated water (Nalagonda technique). However, the associated problems like generation of acid/alkali water, residual aluminium and soluble aluminium fluoride complexes, generation of sludges, and relatively higher residual fluoride concentrations are of major concern [4], [5], [6].

Adsorption onto solid surface is a simple, versatile and appropriate process for treating drinking water system, especially for small communities. Adsorption technique is considered as economical and can remove ions over a wide pH range and to a lower residual concentration than precipitation. Activated alumina is one of the best available and generally used sorbent for defluoridation of drinking water [7], [8]. However, the slow rate of adsorption of commercially available activated alumina limits its use for treating large quantity of water. Many researchers have reported the potential of magnesium oxide (magnesia) to scavenge fluoride [9], [10], [11]. However, magnesia is available only as fine powder. Employing metal oxides in powder forms as sorbent has practical limitations, including difficulty in solid/liquid separation, low hydraulic conductivity and leaching of the metal/metal-oxide along with treated water. To overcome the above-mentioned limitations, more environmental friendly and efficient techniques, which can be applicable to diverse end users, need to be developed.

In this paper, the defluoridation potential of a novel sorbent magnesia-amended activated alumina (MAAA) has been investigated in batch system. The physicochemical characteristics of MAAA were examined with XRD, SEM, EDAX and gas adsorption porosimetry analysis. Attempts have also been made to understand the adsorption kinetics, equilibrium and mechanism of adsorption.

Section snippets

Sorbent preparations

MAAA was prepared by wet impregnation of commercially available AA (ACE Manufacturing and Marketing, AD-101F) with magnesium hydroxide slurry. For this, 1N NaOH solution was slowly added and mixed with 20 ml of magnesium chloride (4 M) solution until the pH rose up to ∼ 9.5. To this mixture, 20 g of AA particles (0.5–0.6 mm) were added, and allowed to settle for 30 min at room temperature. The samples were dried by evaporating excess water at 110 °C for 8 h with proper mixing and subsequently calcined

Sorbent characterization

The MAAA prepared as discussed earlier was characterized by powder XRD. Fig. 1 shows the X-ray diffraction pattern of MAAA. Analyzing the X-ray pattern using the software database published by the JCPDS; it was found that all the peaks observed in the X-ray diffraction pattern was best suiting the alumina (Al21.333O32, JCPDS – 80-0955). No peaks that correspond to magnesia and other forms of magnesium were found. This may be due to the amorphous nature of the magnesia or small volume of

Conclusions

In this study, a novel sorbent, magnesia-amended alumina has been prepared and examined for its potential in removing fluoride from drinking water system. This material has shown much higher and faster fluoride removal compared to AA. The main conclusions drawn from the present study are:

  • Among different factors tested, pH is found to be the most influential factor for fluoride uptake by MAAA. An optimum fluoride removal was observed in the pH range of 5–7.5. This pH range is very reasonable in

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