Aluminium hydro(oxide)–based (AO) adsorbent for defluoridation of drinking water: Optimisation, performance comparison, and field testing

E Mulugeta,Bs Chandravanshi,Ca Johnson,F Zewge

doi:10.4314/wsa.v41i1.15

Abstract

In this study, the performance of different aluminium hydroxide.based adsorbents was compared in terms of fluoride adsorption capacity, potential for repetitive regeneration, surface acidity and surface site concentrations. The adsorbents were aluminium hydro(oxide) (AO), activated alumina (AA), and pseudoboehmite (PB). The AO adsorbent was synthesised at different OH:Al ratios to optimise the fluoride adsorption capacity. The AO was pilot tested in a rural community in the Ethiopian Rift Valley where groundwaters are heavily enriched with fluoride. The maximum fluoride uptake was achieved for the AO adsorbent synthesised at OH:Al ratios between 2.5 and 2.7. The fluoride adsorption capacity of the adsorbents determined from mini-column studies was found to be 10.6, 1.9, and 2.4 mg/g for AO, AA, and PB, respectively. This significant difference in fluoride adsorption capacity is strongly related to the surface acidity and surface site concentration. The surface acidity (1.57 meq/g) and surface site concentration (0.74 meq/g) for AO is higher than that for AA and PB. The elemental composition analysis showed that AO has a lower % Al2O3 content than AA and PB, but higher sulphate (19.4%) and iron (2.2%) content. High resolution 27Al Magic Angle Spinning Nuclear Magnetic Resonance (27Al MAS NMR) spectra of AO, AA, and PB were recorded, to analyse the coordination geometry of solid Al species and the results showed that aluminium is coordinated octahedrally and tetrahedrally in all cases. Regeneration experiments showed that AA and PB can be regenerated for more than 3 cycles, whereas the potential for regeneration of AO for more than 3 cycles is limited. The results from a community defluoridation plant showed that fluoride in the feed water (8.10 mg/.) is removed below 0.1 mg/.. The average adsorption capacity was determined to be 2.11 mg/g based on continuous field monitoring results obtained until the fluoride content in the treated water exceeded the breakthrough value of 1.5 mg/.. No major operational problems and complaints from the beneficiaries were experienced during operation. Keywords : fluoride, adsorption, surface acidity, regeneration, defluoridation, Ethiopia

Highlights

The level of fluoride in drinking water is a very important physicochemical factor, which must be considered when assessing water quality for human consumption
The objectives of the present work were to: (i) explore the adsorption capacity of the aluminium hydro(oxide) (AO) adsorbent as a function of OH:Al synthesis ratio, (ii) examine the solubility of AO, (iii) compare the fluoride removal and regeneration performance of AO with other aluminium hydroxide–based adsorbents, and (iv) demonstrate the performance of a high-capacity AO adsorbent in a rural community of the Ethiopian Rift Valley, where groundwaters are highly enriched with fluoride
The yield, elemental composition and fluoride adsorption capacity are listed in Table 1 as a function of the OH:Al reactant ratio

Summary

Introduction

The level of fluoride in drinking water is a very important physicochemical factor, which must be considered when assessing water quality for human consumption. According to the World Health Organization the maximum acceptable concentration of fluoride ions in drinking water is 1.5 mg/l (WHO, 2011). While a low daily dose of fluoride is considered to be responsible for preventing dental caries; a higher daily dose has been linked to permanent dental and skeletal fluorosis (Guo and Guo, 2013). Many water sources in Ethiopia contain fluoride at elevated concentrations of up to 26 mg/l (Kloos and Tekle-Haimanot, 1999). According to estimates of the Ethiopian Ministry of Water Resources, more than 11 million people in the Ethiopian Rift Valley rely on drinking water contaminated by fluoride (Adeno et al, 2014). Community solutions have strong advantages over household solutions in terms of monitoring and maintenance, but these can be offset by higher investment and lack of ownership (Bregnhoj, 1997)

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