Simultaneous removal of arsenic and fluoride by freshly-prepared aluminum hydroxide

The coexistence of arsenic (As) and fluoride (F) in some underground waters creates challenges in the simultaneous removal of these two toxic elements. This study investigates the effect of fluoride at different molar ratios of fluoride to arsenic (As) (RF:As) on the removal of arsenic [i.e., arsenite (As(III), arsenate (As(V)] by freshly-prepared aluminum hydroxide (AlOxHy), and that of arsenic at different molar ratios of arsenic to fluoride (RAs:F) on fluoride removal. In single pollutant solutions, the removal of neutral As(III) is independent on pH at RAs(III):Al ≤ 0.70:1 and is much lower than that of As(V). The optimum As(V) removal is at weak acidic pH of 5 and 6 whereas that of fluoride is at pH 7 and 8. Fluoride at RF:As(V) > 35:1 significantly impairs the removal of As(V) with more significant inhibition at elevated pH. The negatively-charged As(V) inhibits fluoride removal to a larger extent than the neutral As(III) does. The adverse effect of fluoride on As(V) removal is mainly attributed to the lowered ζ-potential, which is controlled by the combined effects of pH and RF:As(V). In relative terms, the removal of fluoride is highly pH dependent, although RAs(V):F does show some effects. The oxidation of As(III) to As(V) and the adjustment of pH to weak acidic range is well preferred to achieve the simultaneous removal of As and F by AlOxHy adsorption.