Enhanced oxidation of antibiotics by ferrate(VI)-sulfur(IV) system: Elucidating multi-oxidant mechanism
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2018 Elsevier B.V. Ferrate(VI) (FeVIO42, FeVI) is usually effective in oxidizing a wide range of antibiotics on a time scale of seconds, but some recalcitrant antibiotics such as trimethoprim (TMP) and flumequine (FLU) require a longer time and higher dose of FeVI for their removal from water. The current study presents the FeVI-sulfur(IV) (or FeVI-sulfite/bisulfite) system, which could give near-instantaneous oxidation of these two antibiotics (20.0 M) (i.e., in 15 s time scale) at a lower dose of FeVI (100.0 M) than is needed with FeVI oxidation alone. Experiments on the removal of TMP were performed as a function of the molar ratios of S(IV) to FeVI (0.215.0) and FeVI to TMP (2.030.0 M) at three different pH values (7.0, 9.0 and 11.0) under both oxic (air saturated) and anoxic (nitrogen saturated) conditions. In case of oxidation of FLU under the same conditions, the molar ratios were used at 1.0 ([S(IV)]:[FeVI]) and 5.0 ([FeVI]:[FLU]) under the same solution pH. Removal of these two recalcitrant antibiotics depended on the pH and the amount of oxygen in the reaction solution. Significantly, TMP was removed by FeVI-S(IV) system under anoxic condition, whereas the removal percentage was lower than that under oxic condition. The relative amounts of the oxidized products of TMP were also different in the oxic and anoxic environments. A set of reactions that were involved in the oxidation of TMP by the FeVI-S(IV) system may demonstrate the participation of multi-oxidizing species (FeV/FeIV, SO3[rad], SO4[rad] and [rad]OH) in the removal of TMP from water. Representative anions and natural organic matters had little or no significant interference on the removal of TMP by the FeVI-S(IV) technique in water.
