Accelerated Oxidation of Organic Contaminants by Ferrate(VI): The Overlooked Role of Reducing Additives

Abstract

This paper presents an accelerated ferrate(VI) (Fe VI O 4 2–, Fe VI ) oxidation of contaminants in 30 s by adding one-electron and two-electron transfer reductants (R (1) and R (2) ). An addition of R (2) (e.g., NH 2 OH, As III, Se IV, P III, and NO 2 –, and S 2 O 3 2– ) results in Fe IV initially, while Fe V is generated with the addition of R (1) (e.g., SO 3 2– ). R (2) additives, except S 2 O 3 2–, show the enhanced oxidation of 20–40% of target contaminant, trimethoprim (TMP). Comparatively, enhanced oxidation of TMP was up to 100% with the addition of R (1) to Fe VI . Interestingly, addition of S 2 O 3 2– (i.e., R (2) ) also achieves the enhanced oxidation to 100%. Removal efficiency of TMP depends on the molar ratio ([R (1) ]:[Fe VI ] or [R (2) ]:[Fe VI ]). Most of the reductants have the highest removal at molar ratio of ∼0.125. A Fe VI –S 2 O 3 2– system also oxidizes rapidly a wide range of organic contaminants (pharmaceuticals, pesticides, artificial sweetener, and X-ray contrast media) in water and real water matrices. Fe V and Fe IV as the oxidative species in the Fe VI –S 2 O 3 2– -contaminant system are elucidated by determining removal of contaminants in oxygenated and deoxygenated water, applying probing agent, and identifying oxidized products of TMP and sulfadimethoxine (SDM) by Fe VI –S 2 O 3 2– systems. Significantly, elimination of SO 2 from sulfonamide (i.e., SDM) is observed for the first time.