Reactivity of Fe/M (M = Ce, Mn, Al) oxide-hydroxides for hydrosulfide removal in anoxic and oxic solutions

The reactivity toward hydrosulfide (HS−) of seven iron-containing oxide-hydroxides (FeMOx) with different Fe/M (M = Ce, Mn, Al) molar ratios (0.9, 0.5, 0.1) was studied for alkaline pH in a batch slurry reactor to identify the fittest material aimed at the removal of H2S from the pulp and paper atmospheric effluents. The materials were optimized by varying the calcination temperature to maximize the surface area. FeAlOx0.9 achieved the highest surface area (286.8 m2/g), followed by FeCeOx0.9 (241.5 m2/g) and FeMnOx0.9 (217.9 m2/g). In anoxic solutions, combination of Fe with small amounts of Ce or Mn improved HS− oxidation. Increasing Mn (respectively, Fe) content in FeMnOx (respectively, FeCeOx) increased HS− conversion both in oxic and anoxic solutions. The majority of FeMOx showed improved reactivity towards HS− in oxic conditions while dissolved oxygen (DO2) accelerated HS− conversion by re-oxidizing Fe(II) to active Fe(III) as well as by homogeneously converting the polysulfides into thiosulfate. The gain in efficiency of FeCeOx in oxic versus anoxic environments correlated negative-linearly with the Fe/Ce ratio, and Ce was found to enhance the electron transfer between Fe(II) sites and DO2. The gain in efficiency of FeMnOx in oxic versus anoxic solutions correlated linearly with Fe/Mn ratio suggesting reoxidation for Fe(II) and Mn(II) to occur at different rates. Therefore, even if Mn exhibited superior reactivity toward HS− than Fe, the greater reoxidation ability of Fe(II) elects FeCeOx materials for a redox-scrubbing process aiming at the removal of H2S from Kraft atmospheric effluents.