Simultaneous removal of fluoride and arsenic from drinking groundwater using limestones from Bajío Guanajuatense, Mexico

Abstract

Numerous groundwater wells in Mexico exceed the maximum allowable fluoride (F−) and arsenic (As) drinking water concentration requirements, posing an environmental and health risk to the population that relies solely on these wells for drinking water sources. Since encouraging results have been obtained utilizing native limestones to remove some toxic elements from contaminated water, the ability of the limestone rock outcrop to the south of the Sierra de Guanajuato in Mexico to remove As and F− from groundwater was assessed. A sampling campaign was conducted in the study area, focusing on wells exhibiting elevated concentrations of arsenic (As) and fluoride (F−) in compliance with international standards. This water was employed in the treatment experiment involving limestone rocks. The rock sampling process involved a reconnaissance campaign covering the study area and outcrop points of limestone rocks. Representative limestone samples were collected and subsequently subjected to mineralogical and geochemical characterization. Using rock samples, synthetic water, and groundwater from contaminated wells in the region, batch experiments were conducted to evaluate the As and F− removal capacity of limestone. The batch testing consisted of water–rock interactions at various times with different samples of limestone rocks, grain sizes, and water containing distinct concentrations of both elements, artificial and groundwater extracted close to the limestones outcrop. The results indicate that the rock with the highest calcium carbonate (CaCO3) content and the smallest grain size (< 0.05 mm) removes the highest concentrations of both As and F−, with As removal being superior. The removal mechanisms were studied using scanning electron microscopy with energy-dispersive spectroscopy (SEM–EDS) images, saturation index calculations, and Eh–pH diagrams,. Fluoride precipitation was favored when the pH of the solution was slightly acidic, whereas sorption was favored when the pH was higher. The results obtained are encouraging for the removal of high levels of As, and to a lesser extent for F−; consequently, the use of regional limestone rocks could be a viable option for improving the water quality ingested by rural inhabitants in the study area. Calcium carbonate (CaCO3) concentration can be used to identify limestone rocks with the potential to effectively remove As and F− in other locations.