Abstract
<p>The objective of this study was to investigate the production of activated carbons (AC) from cashew shells, and millet stalks and their efficiency in fluoride retention. These agricultural residues are collected from Senegal. It is known that some regions of Sénégal, commonly called the groundnut basin, are affected by a public health problem caused by an excess of fluoride in drinking water used by these populations. The activated carbons were produced by a combined pyrolysis and activation with water steam; no other chemical compounds were added. Then, activated carbonaceous materials obtained from cashew shells and millet stalks were called CS-H<sub>2</sub>O and MS-H<sub>2</sub>O respectively. CS-H<sub>2</sub>O and MS-H<sub>2</sub>O show very good adsorbent features, and present carbon content ranges between 71 % and 86 %. The BET surface areas are 942 m² g<sup>-1</sup> and 1234 m².g<sup>-1</sup> for CS-H<sub>2</sub>O and MS-H<sub>2</sub>O respectively. A third activated carbon produced from food wastes and coagulation-flocculation sludge (FW/CFS-H<sub>2</sub>O) was produced in the same conditions. Carbon and calcium content of FW/CFS-H<sub>2</sub>O are 32.6 and 39.3 % respectively. The kinetics sorption were performed with all these activated carbons, then the pseudo-first equation was used to describe the kinetics sorption. Fluoride adsorption isotherms were performed with synthetic and natural water with the best activated carbon from kinetics sorption, Langmuir and Freundlich models were used to describe the experimental data. The results showed that carbonaceous materials obtained from CS-H<sub>2</sub>O and MS-H<sub>2</sub>O were weakly efficient for fluoride removal. With FW/CFS-H<sub>2</sub>O, the adsorption capacity is 28.48 mg.g<sup>-1 </sup>with r² = 0.99 with synthetic water.</p>
Highlights
In recent years, many studies have been done in order to remove the high concentration of many contaminants from drinking water such as fluoride, lead, arsenic, copper and nitrate (Sud, Mahajan, & Kaur, 2008)
The results showed that carbonaceous materials obtained from CS-H2O and MS-H2O were weakly efficient for fluoride removal
Three activated carbons were prepared from millet stalks (MS-H2O), cashew shells (CS-H2O), food wastes and coagulation-flocculation sludge (FW/CFS-H2O)
Summary
Many studies have been done in order to remove the high concentration of many contaminants from drinking water such as fluoride, lead, arsenic, copper and nitrate (Sud, Mahajan, & Kaur, 2008). It is known that fluoride is an essential element in drinking water. This trace element is actively involved in the health of teeth, especially in the prevention of dental cavities and plays an important role in bone strength. When the fluoride concentration in drinking water is higher than 1.5 mg.L-1, it may cause harmful effects on human health namely dental fluorosis and skeletal fluorosis at concentrations above 4 mg.L-1 (Srivastav, Singh, Srivastava, & Sharma, 2013; Diallo, Diop, Diémé, & Diawara, 2015). It is estimated that more than 200 million people are exposed to drinking water with a fluoride concentration that exceeds the WHO guideline (1.5 mg.L-1) (Bhatnagar, Kumar, & Sillanpää, 2011). Many countries around the world such as India, Bangladesh, Nepal, Sénégal, USA, and Mexico are concerned by fluoride exposure because of the high numbers of their populations that present the effects caused by the high concentration of fluoride in their drinking water (Fawell et al, 2006; Bibi, Farooqi, Hussain, & Haider, 2015)
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