Abstract
This study focuses on the chemical modification of Ethiopian bentonite and commercial montmorillonite with aluminum oxi-hydroxide (AO) for the purpose of defluoridation from aqueous solution. The chemical modification was achieved by preparing AO-modified adsorbents at different pH values. The adsorbent materials were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF) and point of zero charge (PZC). From the modified adsorbents, AO-Mont at pH 5 (AO-Mont 5) showed a slight shift of d 001 at 2θ = 7.5o reflection toward the lower angle 2θ = 7.0o as a result of the Al 2 O 3 insertion into interlayer space. The elemental composition from XRF further showed the increment of Al content accompanied with a decrease in the exchangeable cations in the modified samples and higher content of Al was observed in AO-Mont 5. In addition, the PZC initially 8.5 and 2.5 for raw bentonite and montmorillonite clays were increased to 10 and 8.5 in AO-Bent 5 and AO-Mont 5, respectively. Adsorption of fluoride on these modified adsorbents was studied as a function of preparation pH, contact time, solution pH and co-existing anions. AO-Mont prepared at pH 5, which contained higher content of Al resulted better fluoride removal performance. AO-Bent 5 and AO-Mont 5 have shown five and nine fold increase in fluoride removal capacity, respectively as compared to their respective unmodified clays. The experimental data fitted well with Freundlich adsorption isotherm model which indicated that the surface of the adsorbent is heterogeneous and adsorption of fluoride followed a multilayer sorption. Hence AO-modified Bent and Mont adsorbents prepared at pH 5 can be considered as potential adsorbents for defluoridation from drinking water. KEY WORDS : Defluoridation, Aluminum oxide, Bentonite, Montmorillonite Bull. Chem. Soc. Ethiop. 2018 , 32(2), 199-211. DOI: https://dx.doi.org/10.4314/bcse.v32i2.2
Highlights
Use of groundwater for drinking purpose to urban and rural populations has been practiced for long period of time
This study explores the mechanical advantage of these clays as support for aluminum oxi-hydroxide (AO) and further compares and evaluates their performance to remove fluoride from aqueous solution
The modification process at different pH yields an optimum result when the AO-modified Mont is prepared at pH 5 (AO-Mont 5)
Summary
Use of groundwater for drinking purpose to urban and rural populations has been practiced for long period of time. Fluoride is one of these chemicals which is present in groundwater mainly due to geochemical interactions and climatic conditions [2]. These include granitic basements that contain high fluoride minerals; alkaline volcanic rocks such as East African Rift Valley where high fluoride hyper alkaline volcanic rocks are present and arid climates where groundwater infiltration and flow rates are slow. Ingestion of fluoride is beneficial at low concentrations for structural strengthening of tooth enamel and lower the rate of dental caries. Excessive exposure of fluoride can lead to a number of adverse effects such as dental and skeletal fluorosis [1, 2, 6,7,8].
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