The problem of high fluoride in water sources in Africa and the rest of the developing world has exacerbated in the latest past due to increasing shortage of water. More people are being exposed to high water fluoride resulting in elevated levels of fluorosis in the societies. Fluoride (F) adsorption from solutions using a siliceous mineral from Kenya (M1) was studied on batch basis and results verified on high fluoride water using fixed-bed column experiments. About 100% batch F adsorption was achieved at 200 mg/L F concentration, 0.5 g/mL adsorbent dosage, 303-333 K, and pH 3.4 AE 0.2. Based on Giles classifications, F adsorption isotherm was found to be an H3 type isotherm. The equilibrium data was correlated to Freundlich and Langmuir models and the maximum Langmuir adsorption capacity was found to be 12.4 mg/g. Column experiments were conducted for different fluoride concentrations, bed depths, and flow rates. The F breakthrough curves were analyzed using the Thomas model and efficient F adsorption was found to occur at low flow rates and low influent concentrations. The Thomas F adsorption capacity (11.7 mg/g) was consistent with the Langmuir isotherm capacity showing that M1 could be applied as an inexpensive medium for water defluoridation.
Acid treated diatomaceous earth (ATDE) from a mining site in Kenya was evaluated for its removal of F from aqueous solutions using adsorption batch experiments. The effect of initial F concentration, adsorbent dosage, contact time, temperature, pH and competing anions was studied. The adsorption process was very fast reaching an initial equilib- rium in just 10 min. Fluoride adsorption onto ATDE increased strongly from just about 40% to over 92% when the solution temperature was raised from 293 to 303 K. The process was however, almost unresponsive to pH changes drop- ping by a margin of < 1% from 98.8% to 98% when the solution pH was raised from 1.59 to 6.89. It was obvious therefore that increase in concentration of OH<sup>-</sup> ions does not affect F adsorption onto ATDE. More so apart from the Cl<sup>-</sup> ions which marginally reduced F adsorption onto ATDE, there was no obvious effect of the SO<sub>4</sub><sup>2-</sup> , NO<sub>3</sub><sup>-</sup> and PO<sub>4</sub><sup>3-</sup> ions on F uptake by ATDE. Complete F removal (100% adsorption) could be achieved at 400 mg/L initial F concentra- tions using 0.5 g/mL ATDE batch loading ratio at 303 - 313 K and pH = 3.4 ± 0.2. The F adsorption iso- therm was well correlated to the Freundlich and Langmuir models and could be classified as H-Type according to Giles classification of isotherms. The maximum Langmuir F adsorption capacity of ATDE was 51.1 mg/g. It has been demonstrated that a diatomaceous mineral from Kenya could be use as an inexpensive adsorbent for the removal of F ions from aqueous streams
Adsorption of fluoride (F) ions from water using acid treated lateritic mineral (LM-1) from Kenya was studied by batch experiments. The effect of acid-treatment of adsorbent and change in temperature, mass of LM-1, pH and selected competing ions was evaluated. The adsorption process was strongly influenced by temperature, pH and adsorbent dosage. The percentage F removal increased the presence of the nitrate and the chlorate ions but decreased the presence of sulphates, chloride and phosphate ions. Adsorption isotherms were classified according to Giles' classification and the adsorption data validated using Langmuir and Freundlich isotherms. The data correlated to both the Langmuir and Freundlich isotherms although the data fit to the Freundlich model was somehow better. This showed that F adsorption onto LM-1 followed a mixed adsorption mechanism in which physisorption reactions involving intra-particle diffusion of F into mesoporous sites in LM-1 became increasingly important at higher concentrations and temperatures whereas ion-exchange mechanism involving surface OH -appear to dominate at low surface coverage in more alkaline conditions. With maximum adsorption capacity of 10.5 mg/g, LM-1 could be used to remove F water.
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