Adsorptive processes applied to the defluorination of groundwater for human consumption

Abstract

Contamination of groundwater by fluoride ions can occur through both natural and anthropogenic activities, such as the discharge of industrial waste containing this compound. Thus, effective fluoride removal from groundwater is essential to ensure safe drinking water. This study evaluated the performance of adsorption techniques for defluoridating groundwater in Rio Grande do Sul, Brazil. Preliminary tests were conducted using synthetic solutions with a fluoride concentration of 5 mg.L⁻¹, applying several adsorbents. Additionally, an ultrasonic process was used to synthesize an adsorbent from activated alumina pre-treated with carbon (AACP) and modified with ZnCl₂ (AA-ZnCl₂). The AACP and AA-ZnCl₂ were characterized through BET, EDS, scanning electron microscopy, X-ray diffraction (XRD), and FT-IR analysis. A Central Composite Design and response surface methodology were applied to optimize adsorption efficiency, focusing these factors: pH and adsorbent dosage. Kinetic and isotherm adsorption tests were conducted for both AACP and AA-ZnCl₂. The results showed that AACP achieved fluoride removal efficiencies of 65.4 % in synthetic solutions and 38.6 % in groundwater. The AA-ZnCl₂ demonstrated superior performance, removing over 98 % of fluoride in synthetic solutions and 55.4 % in groundwater, across a pH range of 4 to 10, with an optimal solid dosage of 3 g.L⁻¹. For an initial fluoride concentration of 5 mg.L⁻¹, a removal efficiency of 97.4 % was achieved within 5 min of contact time. The kinetic adsorption data were best described by the pseudo-second-order model, while the Freundlich isotherm model provided the best fit for the adsorption isotherm data. The findings in this work indicate hat ZnCl₂-modified activated alumina, synthesized with ultrasonic assistance, is highly effective for defluoridating groundwater for safe human consumption being an alternative method to be implemented in an industrial scale.