Tetravalent Manganese Feroxyhyte: A Novel Nanoadsorbent Equally Selective for As(III) and As(V) Removal from Drinking Water

The development of a single-phase Fe/Mn oxy-hydroxide (δ-Fe_0.76Mn_0.24OOH), highly efficient at adsorbing both As­(III) and As­(V), is reported. Its synthesis involves the coprecipitation of FeSO₄ and KMnO₄ in a kilogram-scale continuous process, in acidic and strongly oxidizing environments. The produced material was identified as a manganese feroxyhyte in which tetravalent manganese is homogeneously distributed into the crystal unit, whereas a second-order hollow spherical morphology is favored. According to this structuration, the oxy-hydroxide maintains the high adsorption capacity for As­(V) of a single Fe oxy-hydroxide combined with enhanced As­(III) removal based on the oxidizing mediation of Mn­(IV). Ion-exchange between arsenic species and sulfates as well as the strongly positive surface charge further facilitate arsenic adsorption. Batch adsorption tests performed in natural-like water indicate that Mn­(IV)-feroxyhyte can remove 11.7 μg As­(V)/mg and 6.7 μg As­(III)/mg at equilibrium pH 7, before residual concentration overcomes the regulation limit of 10 μg As/L for drinking water. The improved efficiency of this material, its low cost, and the possibility for scaling-up its production to industry indicate the high practical impact and environmental importance of this novel adsorbent