Mn-feroxyhyte: The role of synthesis conditions on As(III) and As(V) removal capacity

This study evaluates the critical parameters of synthesizing nanocrystalline tetravalent Mn-feroxyhyte, a single-phase arsenic adsorbent equally efficient against As(III) and As(V). The production of such Fe/Mn oxy-hydroxides is based on the oxidative coprecipitation of ferrous and manganese ions through a continuous flow process. In particular, the effect of different reaction pH, redox potential and Mn incorporation percentage on the determination of structural and surface properties and sequentially on material arsenic uptake is examined. According to the experimental observations, total arsenic adsorption is maximized at acidic synthesis pH where the surface charge density is multiplied (2.7 mmol OH−/g at pH 4) and the chemisorbed sulfate's exchange with As(V) oxy-anions is activated. Besides, the capture of As(III) which lies on the intermediate oxidation of As(III) to As(V) provided by Mn(IV) surface ions, demands a high synthesis redox potential to obtain the tetravalent state and a Fe-to-Mn ratio around 3:1. The potential technological application of the studied materials was further investigated for the case which simultaneously optimize As(III) and As(V) removal capacity (synthesis pH 4, redox potential 600 mV and 12 wt% Mn). Rapid small scale column experiments indicate an almost equal adsorption capacity for As(III) (7.6 μg/mg) and As(V) (8.7 μg/mg) at the breakthrough point (10 μg/L). Spent adsorbent of this procedure is categorized as non-hazardous waste according to TCLP or inert waste according to EN12457.