The invention relates to systems and processes for treating particulate oxides of manganese useful as oxidizing sorbents for capturing or removing target pollutants from industrial gas streams, including, but not limited to, nitrogen oxides (NO_x), sulfur oxide (SO_x), mercury (Hg), hydrogen sulfide (H₂S), other totally reduced sulfides (TRS), and oxides of carbon (CO and CO₂) gases. Oxides of manganese are washed an aqueous oxidizing solution, which may be adjusted as necessary to maintain the oxidizing solution a desired range of pH (acidity) and Eh (oxidizing potential) within the MnO₂ stability area for aqueous solutions. The resulting treated oxides of manganese are equally or more efficient as sorbents as the oxides of manganese processed with the invention.