(12c) Degradation of Acesulfame Potassium By Ferrate(VI) and HCl-Activated Ferrate(VI) in Aqueous Solution

The increasing discharge of emerging organic pollutants (e.g. pharmaceuticals and personal care products, and endocrine disrupting compounds) to the water bodies is receiving great attention due to their potential toxicity to humans and aquatic organisms. Many of these contaminants cannot be efficiently removed by conventional biological wastewater treatment processes indicated by their presence in influents and effluents of wastewater treatment plants, surface waters, and ground water across the globe, which ascertains the need to develop a more efficient water and wastewater treatment technologies. Artificial sugars, which are widely used as a low-calorie sugar supplements, are considered as emerging organic pollutants. Acesulfame potassium (ACK) is a compound extensively used as a sugar substitute in pharmaceuticals, and personal care and food products. Significantly, ACK has been detected in influents and effluents of several wastewater treatment plants worldwide at concentrations up to 80 µg/L. Hence, its removal from water is important.

The high-valent iron (oxidation state of +6), also known as ferrate(VI) (Fe(VI)), is receiving pronounced attention as a green chemical in a wide range of applications in different research fields, e.g. organic synthesis, water and wastewater treatment, and iron batteries. The oxidation, disinfection, and coagulation properties of Fe(VI) make it a multipurpose water-treatment chemical. Recently, we demonstrated the activation of Fe(VI) by acid (e.g. HCl and HNO₃) to significantly enhance the oxidative transformation of emerging pollutants in aqueous solution at slightly basic pH conditions, where Fe(VI) has decreased reactivity (E⁰ (acidic) = +2.2 V vs. E⁰ (basic) = +0.72 V). This study deals with the enhanced degradation of ACK by HCl-activated Fe(VI) at mild-alkaline environmental-relevant pH conditions.

Almost complete removal of ACK (>95%) by HCl-activated Fe(VI) has been achieved at Fe(VI) to ACK molar ratio of 8.0, and HCl to Fe(VI) molar ratio of 2.0. Comparatively, only ~65% degradation of ACK have been attained by Fe(VI) (nonactivated) at the same molar ratio of Fe(VI) to ACK of 8.0. Importantly, the Fe(VI)-decay time was much shorter in the case of HCl-activated Fe(VI) (5 minutes) compared to Fe(VI) (4 hours). In the case of activated Fe(VI), the pH of the ACK-Fe(VI) reaction mixture lowered by 0.6 pH units only, due to the addition of HCl (from an initial pH of 9.4 to 8.8). Further experiments at pH of 8.8 and 9.7, at a molar ratio of Fe(VI) to ACK of 2.0, without activation, showed that the effect of the pH is not significant at the studied pH range (i.e. 27% and 24% ACK removal efficiencies at pH of 8.8 and 9.7 respectively). The final pH (i.e. the Fe(III) formed by the reduction of Fe(VI) during the oxidation reaction was particulate enabling its removal by filtration) of the ACK-Fe(VI) and ACK-Fe(VI)-HCl reaction mixtures was similar (~7.7). Control experiments have also been done to rule out the effect of HCl and Fe(III) alone on the degradation of ACK, confirming that the enhanced oxidation is due to the activation of the chemical oxidant by HCl.

Results showed clearly the enhanced degradation of ACK by HCl-activated Fe(VI) at shorter time and environmental-relevant slightly-basic pH. Possible reason for the observed enhancement is the increased formation of iron-based reactive species, Fe(V) and Fe(IV), upon activation of Fe(VI) by HCl. It is known that Fe(V)/Fe(IV) species are 2-3 orders of magnitude more reactive than Fe(VI), thus causing enhanced oxidation of ACK. The effect of individual anions (i.e. Cl^-, HCO₃^-, and SO₄^2-), and cations (N⁺, Ca²⁺, and Mg²⁺) usually present in wastewater on the oxidation of ACK by Fe(VI) and HCl-activated Fe(VI) has been also investigated. Moreover, the oxidation of ACK by Fe(VI) with and without activation has been also studied in the presence of organic matter to evaluate the applicability of this technology under water-treatment conditions. The mineralization of ACK was explored by dissolved organic carbon measurements.