Flavins enhance the hydroxyl radical production from FeCO3 oxygenation for organic pollutant degradation

Abstract

Hydroxyl radical (•OH) produced from Fe(II) oxidation has a great influence on element transformation and the natural attenuation of contaminants. Although the rules and mechanisms of •OH production from Fe(II) oxidation have been extensively studied, the influence of coexisting compounds of flavins, which are widespread in subsurface environments and could mediate microbial Fe(III) reduction to generate Fe(II), on •OH production remains unclear. Here, we reveal that reduced/oxidized riboflavin (RBFH2/RBF) significantly enhances •OH production from FeCO3 oxidation by oxygen (O2) within laboratory air at circumneutral pH conditions, but has no evident influence on other Fe(II) species. For instance, for oxygenation of 2 g/L FeCO3 at pH 7.3, the •OH accumulation within 180 min is 32.9 μM, while it increases to 72.3 and 72.6 μM with the addition of 20 μM RBFH2 and RBF, respectively. The mechanisms of •OH production enhancement are related to the complexation ability of RBF and the redox activity of RBFH2. Without RBFH2/RBF, •OH is primarily generated from the oxidation of structural Fe(II) by O2, followed by adsorbed Fe(II). With the addition of RBFH2/RBF, RBF chelates adsorbed Fe(II) and structural Fe(II) to form Fe(II)-RBF complex, thereby elevating •OH yield. Hence, •OH production is first attributed to the oxidation of structural Fe(II) and Fe(II)-RBF complex and secondarily to the oxidation of adsorbed Fe(II) in FeCO3-RBFH2/RBF systems. Besides, the oxidation of RBFH2 secondarily contributes to •OH generation in the presence of RBFH2. For pollutant removal, the removal efficiency of sulfamethoxazole increases from 20% to 39% and 38% after the addition of 20 μM RBFH2 and RBF in FeCO3 suspension, respectively. The finding reveals the neglected catalytic effects of RBFH2/RBF in •OH production from FeCO3 oxidation by O2.