Fe-Mn-Al metal oxides/oxyhydroxides as As(III) oxidant under visible light and adsorption of total arsenic in the groundwater environment

Abstract

Arsenite (As(III)) and arsenate (As(V)) species are responsible for arsenic toxicity in groundwater. Anoxic groundwater condition makes it concentrate with highly toxic As(III) compared to As(V). Commercial arsenic adsorbents are mainly effective to adsorb As(V), thus a costly oxidation step needs before an adsorption step. An adsorbent with strong As(III) oxidation capability at visible light and the capability to effectively adsorb As(V) from water is warranted. Ternary metal oxides/oxyhydroxides were prepared by mixing Fe, Al, and Mn-salt precursors and hydrolyzed simultaneously up to an optimized pH. The formation of oxyhydroxides Fe, Al, & Mn, specifically Mn(IV) oxidation state, can quickly oxidize the As(III) species in water; simultaneously, arsenic species get adsorbed on other (Fe, Al oxides/hydroxides) adsorption sites. The manganese compound’s energy bandgap (1.29 eV) is accountable for showing the oxidation capability of the adsorbent even under visible light irradiation. Hence, the maximum Langmuir adsorption capacity obtained was 34.34 mg/g and 21.22 mg/g for the As(III) and As(V), respectively. The Elovich kinetics concluded the chemisorption type of adsorption of arsenic species in the groundwater environment. XPS analysis confirms the oxidation of As(III) species into the As(V) species and reveals the inner sphere arsenic complexation mechanism through surface hydroxyls. As(III) removal through oxidation followed by adsorption was almost identical under visible light conditions and UV exposure. Thus, the study proposes using this adsorbent as indoor household filter media for arsenic removal without the requirement of UV irradiation.