Oxidative Degradation of Organic Contaminants by FeS in the Presence of O₂

Abstract

Reductive transformation of organic contaminants by FeS in anoxic environments has been documented previously, whereas the transformation in oxic environments remains poorly understood. Here we show that phenol can be efficiently oxidized in oxic FeS suspension at circumneutral pH value. We found that hydroxyl radicals (•OH) were the predominant reactive oxidant and that a higher O 2 content accelerated phenol degradation. Phenol oxidation depended on •OH production and utilization efficiency, i.e., phenol degraded per •OH produced. Low FeS contents (≤1 g/L) produced less •OH but higher utilization efficiency, while high contents produced more •OH but lower utilization efficiency. Consequently, the most favorable conditions for phenol oxidation occurred during the long-term interaction between dissolved O 2 and low levels of FeS (i.e., ≤1 g/L). Mossbauer spectroscopy suggests that FeS oxidation to lepidocrocite initially produced an intermediate Fe(II) phase that could be explained by the apparent preferential oxidation of structural S(−II) relative to Fe(II), rendering a higher initial •OH yield upon unit of Fe(II) oxidation. Trichloroethylene can be also oxidized under similar conditions. Our results demonstrate that oxidative degradation of organic contaminants during the oxygenation of FeS can be a significant but currently underestimated pathway in both natural and engineered systems.

Active Iron Phases Regulate the Abiotic Transformation of Organic Carbon during Redox Fluctuation Cycles of Paddy Soil

“…Although Fe­(II) species are the main active components that induce • OH formation during redox fluctuations, , whether Fe­(II) phases are oxidized to produce • OH in soil depends on the morphology and redox sensitivity of different Fe­(II) species, , such as aqueous Fe 2+ , ligand-complexed Fe­(II), and Fe-bearing mineral species (e.g., green rusts, FeS, Fe­(II)-bearing smectite clays). − Moreover, by altering the content and crystallinity of Fe phases, , redox cycles may also affect • OH production and therefore the geochemical behaviors of associated OC, but these processes are under-investigated. Although different Fe­(II) species strongly promote • OH production, these processes associated with soil per se are rarely investigated concurrently. ,,, This is especially the case for paddy soil, where redox fluctuations commonly occur.…”

Heterogeneous Fenton Degradation of Patulin in Apple Juice Using Carbon-Encapsulated Nano Zero-Valent Iron (CE-nZVI)

“…Additionally, studies done by Lee et al (2008) further explained the limitation of the nZVI/O 2 system and explored the use of Ni-Fe/O 2 bimetallic systems to enhance contaminant degradation [33]. Chen et al (2020) used FeS for the oxidation of phenol noting that at higher O 2 content there is an acceleration in the degradation of phenol [34]. This further supports that the degradation phenomenon is related to the O 2 distribution within the system.…”

Free Radicals Produced from the Oxidation of Ferrous Sulfides Promote the Remobilization of Cadmium in Paddy Soils During Drainage

“…The dynamics of both dissolved Cd and extractable Cd was monitored. Meanwhile, the cumulative hydroxyl free radicals (OH • ) produced in the soil suspensions were quantified using sodium benzoate …”

“…More recently, several studies have documented the oxidizing roles of FeS in both oxic natural and engineered systems. The interactions between FeS and dissolved O 2 lead to the oxidative degradation of organic contaminants , and oxidative mobilization of inorganic contaminants such as U and Tc. , These effects were attributed to the production of a hydroxyl free radical (OH • ), which is the most reactive oxidant in nature . With regard to Cd in paddy soils, theoretically, FeS could play two opposite roles, a voltaic cell effect protecting CdS from oxidation and a free radical effect promoting CdS oxidation.…”