Impact of dissolved O2 on phenol oxidation by dMnO2

Although redox reactions of organic contaminants with manganese oxides have been extensively studied, the role of dissolved O2 in these processes has largely been overlooked. In this study, the oxidative degradation of phenol by d-MnO2 was investigated under both oxic and anoxic conditions. Dissolved O2 inhibited phenol degradation due to its promoting role in the reoxidation and precipitation of reduced Mn(II) to Mn(III) on the d-MnO2 surface, resulting in partial transformation of d-MnO2 to “c-disordered” H+-birnessite at pH 5.5 and feitknechtite, manganite, and hausmannite at pH 7.0 and 8.5. The reformed Mn(III) phases could reduce phenol oxidation by blocking reactive sites of d-MnO2. In addition, dissolved O2 caused a higher degree of particle agglomeration and a more severe specific surface area decrease, and hence lower reactivity of d-MnO2. These findings revealed that after reductive dissolution by phenol and reoxidation by dissolved O2 throughout continuous redox cycling, d-MnO2 became less reactive rather than being regenerated. These results can provide new insights into the understanding of the oxidation of organic contaminants by manganese oxides in the natural environment.

尽管有机污染物与锰氧化物的氧化还原反应已被广泛研究，但溶解氧在这些过程中的作用在很大程度上被忽视了。在本研究中，我们在有氧和无氧条件下研究了δ -MnO₂对苯酚的氧化降解。在pH为5.5时，溶解氧由于其促进还原态的Mn(Ⅱ)在δ -MnO₂表面重新氧化并沉淀为Mn(Ⅲ)，从而抑制了苯酚的降解，导致δ -MnO₂部分转化为“c -无序”的H⁺ -水钠锰矿；在pH为7.0和8.5时，则转化为水锰矿、黑锰矿和方锰矿。重新形成的Mn(Ⅲ)相通过堵塞δ -MnO₂的活性位点而降低了苯酚的氧化。此外，溶解氧导致了更高程度的颗粒团聚和更严重的比表面积减小，从而使δ -MnO₂的反应活性降低。这些发现表明，在整个连续的氧化还原循环中，经过苯酚的还原溶解和溶解氧的重新氧化后，δ -MnO₂的反应活性降低，而不是得到再生。这些结果可为理解自然环境中锰氧化物对有机污染物的氧化作用提供新的见解。