Thermally treated sludge obtained from a coagulation–flocculation water treatment process as a low-cost and eco-friendly adsorbent for water defluorination

The excess of fluoride ions in potable water is an important public health problem. This study has evaluated the thermal treatment process of a sludge from a water treatment plant at five different temperatures (200, 300, 400, 500, and 600 °C) to find a low-cost and eco-friendly adsorbent for fluoride removal. The sludge characterization was evaluated by thermogravimetric analysis, point of zero charge, scanning electron microscopy coupled with energy dispersive spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and N2 physisorption analysis, aiming to identify changes caused by the thermal treatment and its impact on F− adsorption. All the thermally treated samples were submitted to the same operational conditions in an adsorption experiment and compared with the raw sample. The thermally treated sludge showed the best adsorbent performance at 200 °C. By the use of this adsorbent material, the fluoride removal percentage was 98.13%, and the adsorption capacity was 1.05 mg g−1, resulting in a final fluoride concentration close to zero and meeting the recommendation of the World Health Organization (WHO). These results indicate that the thermal treatment of a water treatment plant sludge could be viable. On one side, thermal treatment is an option to treat problematic waste (sludge) generated in large amounts. On the other hand, this thermally treated sludge can be used as a largely available, accessible, eco-friendly, and low-cost adsorbent for fluoride removal from aqueous media.