Competitive adsorption behaviors of arsenite and fluoride onto manganese-aluminum binary adsorbents

The combined contamination of arsenic (As) and fluoride (F) in groundwater poses severe health risks. However, little information is available on the simultaneous removal of As(III) & F, not to mention their competition behaviors. Hence, we investigated the characteristics of competitive adsorption between As(III) and F onto the surface of Mn-Al binary oxides (MABO). The co-adsorption behaviors of As(III/V) & F, including isotherm and kinetic characteristics, were influenced by As speciations, solution pH, and Mn/Al molar ratios. With sequential addition of As(V) & F, the non-exchangeable fraction for each ion was approximately equal to their initially adsorbed amounts, respectively. Moreover, the adsorption amount of As(V) or F in the rapid reaction stage was attributed to the surface complexation formation. By contrast, when As(III) and F were adsorbed sequentially, the As(III) molecules adsorbed in the rapid reaction stage was more vulnerable and easily replaced by F ions. Additionally, the non-exchangeable fraction for adsorbed As was mainly As(V), which was converted from As(III) via redox reactions. These results suggested that the competition between As(III) and F is quite different with that between As(V) and F onto MABO surfaces. Furthermore, F complexation formation not only inhibits the adsorption of As(III), but also suppresses the oxidative reactivity of MABO.