(36b) Acid Activation of Ferrate(VI) for Oxidation of Organic Compounds in Aqueous Solution

The unexpected enhancement of oxidation of selected organic compounds by adding simple acids (e.g. HCl and HNO₃) to the high-valent iron-based oxidant, ferrate(VI) (Fe^VIO₄^2-, Fe(VI)), is demonstrated. The activation of Fe(VI) was demonstrated by selecting organic compounds that have sluggish reactivity with Fe(VI) such as caffeine (psychostimulant, CAF), acesulfame potassium (artificial sweetener, ACE) and atenolol (β-blocker, ATL). Significantly, these pollutants exist in a variety of water systems (CAF, ATL and ACE have been detected in surface waters, groundwater and influents and effluents of wastewater treatment plants). The activated Fe(VI) resulted in increased removal of CAF, ACE, and ATL by 30 %. CAF and ACE could be removed almost completely at the highest amount of acid added. In the case of ATL, which was not removed by Fe(VI) alone, the enhancement due to H⁺ ions, was also seen at the studied Fe(VI) to ATL molar ratios (4.0 and 10.0) but removal was not that high (~10 and 30 % at Fe(VI) to ATL molar ratio of 4.0 and 10.0 at 444 and 713 µM HCl respectively). Similar percentage removal efficiencies of CAF in the presence of HCl, HNO₃ and acetic acids were observed under anoxic and oxic conditions indicating that dissolved oxygen played no role in causing the enhancement of oxidation of CAF by the activation of Fe(VI) by H⁺ ions. The aforementioned enhancement may be attributed to the increasing formation of reactive species, Fe(V)/Fe(IV) due to addition of H⁺ ions to the Fe(VI)-organic mixed solution since Fe(V) is about 3 orders of magnitude more reactive than Fe(VI), probably because of its partial free-radical character. The findings have wider implications in purifying pollutants in water and wastewater. The results demonstrated that the removal of pollutants can be accomplished at a shorter time scale and also at lower oxidant to organic pollutant molar ratios by adding H⁺ ions than the removal of the molecules without the addition of acids. Activated Fe(VI) could decontaminate the sluggishly reactive or non-reactive pollutants. Both CAF and ACE had very low reactivity with Fe(VI) and took several hours to be oxidized. However, activated Fe(VI) was able to remove these molecules over much shorter times (e.g. 30 minutes).