Impact of pH on Iron Redox Transformations in Simulated Freshwaters Containing Natural Organic Matter

Abstract

The impact of the pH of natural waters on the various pathways contributing to the formation and decay of Fe(II) in the presence of Suwannee River Fulvic Acid (SRFA) is investigated in this study. Our results show that thermal Fe(III) reduction occurs as a result of the presence of hydroquinone-like moieties in SRFA with the rate of Fe(III) reduction by these entities relatively invariant with change in pH in the range 6.8−8.7. The Fe(II) oxidation rate in the dark is controlled by its interaction with O 2 and increases with increase in pH with the overall outcome that the steady-state Fe(II) concentration in the dark is strongly affected by solution pH. On irradiation, a portion of the hydroquinone-like moieties present are oxidized to form semiquinones that are capable of reducing Fe(III) and/or oxidizing Fe(II) under circumneutral pH conditions. The extent of photogeneration of semiquinones on irradiation of SRFA and the persistence of these radicals increases significantly with decrease in pH. Due to the higher concentration and longevity of these organic moieties under low pH conditions, the impact of pH on steady-state Fe(II) concentration is less pronounced in previously irradiated SRFA solution compared to that observed in dark SRFA solution. Under irradiated conditions, the rates of Fe transformation (including both Fe(II) oxidation and Fe(III) reduction) are nearly independent of pH. While ligand-to-metal charge transfer (LMCT) is the dominant pathway for photochemical Fe(III) reduction, Fe(II) oxidation under irradiated conditions mainly occurs as a result of interaction with O 2 , semiquinones and other short-lived oxidants. Overall, our data supports the conclusion that, as a result of the contribution from photogenerated organic moieties to Fe redox transformations, the steady-state Fe(II) concentration in irradiated surface waters containing natural organic matter may not be impacted significantly by changes in pH.

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.…”

“…The • OH-mediated oxidation of OC can alter its molecular structure and chemical composition via hydroxylation or the cleavage of aromatic rings 23 to finally produce low-molecular-weight compounds and CO 2 . 24 Although Fe(II) species are the main active components that induce • OH formation during redox fluctuations, 25,26 whether Fe(II) phases are oxidized to produce • OH in soil depends on the morphology and redox sensitivity of different Fe(II) species, 25,27 such as aqueous Fe 2+ , 28 ligand-complexed Fe(II), 29 and Fe-bearing mineral species (e.g., green rusts, FeS, Fe(II)-bearing smectite clays). 30−33 Moreover, by altering the content and crystallinity of Fe phases, 34,35 also affect • OH production and therefore the geochemical behaviors of associated OC, but these processes are underinvestigated.…”

Using nano zero valent iron supported on diatomite to remove acid blue dye: synthesis, characterization and toxicology test

“…At the beginning, the pH of the treatments was 7.4, however at the end of the removal experiments, NZVI-DE-1 had 8.9; and NZVI-DE-2 had 8.7; while NZVI had a more neutral pH of 7.7; and DE-1 and 2 had an acid pH of 4 and 4.6., respectively. The treatment with nanoparticles showed pH values around 8 because the dye removal is a dynamic process where Fe° nanoparticles begin to be transformed into oxides as Fe 2+ , Fe 3+ , Fe(OH) 3 and Fe(OH) 2 , which reduces the quantity of H+ and raises the pH of the liquid (Garg et al, 2018). Whereas in the treatment with only DE, the pH of the solutions finishes acidic due to the protonation of surface silanol groups where protons are forming conjugate acids that lower the pH (Lowe et al, 2015; Nosrati et al, 2017).…”

Using Nano Zero-Valent Iron Supported on Diatomite to Remove Acid Blue Dye: Synthesis, Characterization, and Toxicology Test

“…However, at the end of the removal experiments, nZVI-DE-1 had 8.9, and nZVI-DE-2 had 8.7, while nZVI had a more neutral pH of 7.7 and DE-1 and -2 had an acid pH of 4 and 4.6, respectively. The treatment with nanoparticles showed pH values around 8 because the dye removal is a dynamic process where Fe • nanoparticles begin to be transformed into oxides as Fe 2+ , Fe 3+ , Fe(OH) 3 , and Fe(OH) 2 , which reduces the quantity of H + and raises the pH of the liquid [42], whereas in the treatment with only DE, the pH of the solutions finishes acidic due to the protonation of surface silanol groups where protons are forming conjugate acids that lower the pH [43,44].…”