Impact of <i>Microcystis aeruginosa</i> Exudate on the Formation and Reactivity of Iron Oxide Particles Following Fe(II) and Fe(III) Addition

Impact of the organic exudate secreted by a toxic strain of <i>Microcystis aeruginosa</i> on the formation, aggregation, and reactivity of iron oxides that are formed on addition of Fe­(II) and Fe­(III) salts to a solution of the exudate is investigated in this study. The exudate has a stabilizing effect on the particles formed with decreased aggregation rate and increased critical coagulant concentration required for diffusion-limited aggregation to occur. These results suggest that the presence of algal exudates from <i>Microcystis aeruginosa</i> may significantly influence particle aggregation both in natural water bodies where Fe­(II) oxidation results in oxide formation and in water treatment where Fe­(III) salts are commonly added to aid particle growth and contaminant capture. The exudate also affects the reactivity of iron oxide particles formed with exudate coated particles undergoing faster dissolution than bare iron oxide particles. This has implications to iron availability, especially where algae procure iron via dissolution of iron oxide particles as a result of either reaction with reducing moieties, light-mediated ligand to metal charge transfer and/or reaction with siderophores. The increased reactivity of exudate coated particles is attributed, for the most part, to the smaller size of these particles, higher surface area and increased accessibility of surface sites

Depassivation of Aged Fe⁰ by Divalent Cations: Correlation between Contaminant Degradation and Surface Complexation Constants

The dechlorination of trichloroethylene (TCE) by aged Fe⁰ in the presence of a series of divalent cations was investigated with the result that while no significant degradation of TCE was observed in Milli-Q water or in solutions of Ba²⁺, Sr²⁺, or Ca²⁺, very effective TCE removal was observed in solutions containing Mg²⁺, Mn²⁺, Co²⁺, Fe²⁺, Ni²⁺, Zn²⁺, Cu²⁺, or Pb²⁺. The rate constants of TCE removal in the presence of particular cations were positively correlated to the log <i>K</i> representing the affinity of the cations for hydrous ferric oxide (HFO) surface sites though the treatments with Co²⁺ and Ni²⁺ were found to provide particularly strong enhancement in TCE degradation rate. The extent of Fe­(II) release to solution also increased with increase in log <i>K</i>, while the solution pH from both experimental measurement and thermodynamic calculation decreased with increasing log <i>K</i>. While the peak areas of Fe and O XPS spectra of the passivated ZVI in the presence of Ba²⁺, Sr²⁺, and Ca²⁺ were very close to those in Milli-Q water, very significant increases in surface Fe and O (and OH) were observed in solutions of Mg²⁺, Mn²⁺, Co²⁺, Fe²⁺, Ni²⁺, Zn²⁺, Cu²⁺ and Pb²⁺, revealing that the surface oxide layer dissolution is consistent with the recovery of aged Fe⁰ with respect to TCE degradation. The depassivation process is proposed to involve (i) surface complexation of cations on surface coatings of aged Fe⁰, (ii) dissolution of the hydrated surface as a consequence of magnetite exposure, and (iii) transport of electrons from underlying Fe⁰ via magnetite to TCE, resulting in TCE dechlorination and, for some cations (Co²⁺, Ni²⁺, Cu²⁺, and Pb²⁺), reduction to their zero or +1 valence state (with potential for these reduced metals to enhance TCE degradation)

H₂O₂-Mediated Oxidation of Zero-Valent Silver and Resultant Interactions among Silver Nanoparticles, Silver Ions, and Reactive Oxygen Species

The H₂O₂-mediated oxidation of silver nanoparticles (AgNPs) over a range of pH (3.0–14.0) is investigated here, and an electron charging–discharging model capable of describing the experimental results obtained is developed. AgNPs initially react with H₂O₂ to form Ag⁺ and superoxide, with these products subsequently reacting to reform AgNPs (in-situ-formed AgNPs) via an electron charging–discharging mechanism. Our experimental results show that the AgNP reactivity toward H₂O₂ varies significantly with pH, with the variation at high pH (>10) due particularly to the differences in the reactivity of H₂O₂ and its conjugate base HO₂^– with AgNPs whereas at lower pH (3–10) the pH dependence of H₂O₂ decay is accounted for, at least in part, by the pH dependence of the rate of superoxide disproportionation. Our results further demonstrate that the in-situ-formed AgNPs resulting from the superoxide-mediated reduction of Ag⁺ have a different size and reactivity compared to those of the citrate-stabilized particles initially present. The turnover frequency for AgNPs varies significantly with pH and is as high as 1776.0 min^–1 at pH 11.0, reducing to 144.2 min^–1 at pH 10.0 and 3.2 min^–1 at pH 3.0

Light-Mediated Reactive Oxygen Species Generation and Iron Redox Transformations in the Presence of Exudate from the Cyanobacterium Microcystis aeruginosa

The photochemical properties of the organic exudate secreted by a toxic strain of Microcystis aeruginosa were studied by measuring reactive oxygen species (ROS) generation and redox transformations of iron in the presence of the organic exudate under acidic (pH 4) and alkaline (pH 8) conditions. Our results show that the organic exudate generates nanomolar concentrations of superoxide and hydrogen peroxide on irradiation with simulated sunlight in a manner consistent with that reported for terrigenous natural organic matter. The photo-generated superoxide plays an important role in Fe­(III) reduction under alkaline conditions with nearly 45% of the observed Fe­(II) generation on Fe­(III) reduction occurring via Fe­(III) reduction by superoxide while the rest of the Fe­(III) reduction occurs via a ligand-to-metal charge transfer (LMCT) pathway. In contrast, under acidic conditions, 100% of the observed photochemical Fe­(II) generation on Fe­(III) reduction occurs via a LMCT pathway. These results suggest that steady-state dissolved Fe concentrations and hence Fe availability in natural waters will significantly increase in the presence of these algal exudates. Furthermore, significant diel variation in Fe­(II) concentration is to be expected, even in acidic waters, since time scales of light-mediated Fe­(III) reduction and thermal Fe­(III) reduction differ markedly. A kinetic model is developed that adequately describes both the generation of ROS and the photochemical redox transformations of iron in the presence of M. aeruginosa exudate

Redox Transformations of Iron in the Presence of Exudate from the Cyanobacterium <i>Microcystis aeruginosa</i> under Conditions Typical of Natural Waters

Interaction of the exudate secreted by a toxic strain of the cyanobacterium <i>Microcystis aeruginosa</i> with Fe­(II) and Fe­(III) was investigated here under both acidic (pH 4) and alkaline (pH 8) conditions. At the concentrations of iron and exudate used, iron was present as dissolved iron (<0.025 μm) at pH 4 but principally as small (<0.45 μm) iron oxyhydroxide particles at pH 8 with only ∼3–27% present in the dissolved form as a result of iron binding by the organic exudate. The formation of strong Fe­(III) exudate and relatively weak Fe­(II) exudate complexes alters the reduction potential of the Fe­(III)–Fe­(II) redox couple, facilitating more-rapid oxidation of Fe­(II) at pH 4 and 8 than was the case in the absence of exudate. Our results further show that the organic exudate contains Fe­(III)-reducing moieties, resulting in the production of measurable concentrations of Fe­(II). However, these reducing moieties are short-lived (with a half-life of 1.9 h) and easily oxidized in air-saturated environments. A kinetic model was developed that adequately describes the redox transformation of Fe in the presence of exudate both at pH 4 and pH 8

Kinetic Modeling Assisted Analysis of Vitamin C‑Mediated Copper Redox Transformations in Aqueous Solutions

The kinetics of oxidation of micromolar concentrations of ascorbic acid (AA) catalyzed by Cu(II) in solutions representative of biological and environmental aqueous systems has been investigated in both the presence and absence of oxygen. The results reveal that the reaction between AA and Cu(II) is a relatively complex set of redox processes whereby Cu(II) initially oxidizes AA yielding the intermediate ascorbate radical (A•–) and Cu(I). The rate constant for this reaction was determined to have a lower limit of 2.2 × 104 M–1 s–1. Oxygen was found to play a critical role in mediating the Cu(II)/Cu(I) redox cycle and the oxidation reactions of AA and its oxidized forms. Among these processes, the oxidation of the ascorbate radical by molecular oxygen was identified to play a key role in the consumption of ascorbic acid, despite being a slow reaction. The rate constant for this reaction (A•−+O2→DHA+O2•−) was determined for the first time with a calculated value of 54 ± 8 M–1 s–1. The kinetic model developed satisfactorily describes the Cu/AA/O2 system over a range of conditions including different concentrations of NaCl (0.2 and 0.7 M) and pH (7.4 and 8.1). Appropriate adjustments to the rate constant for the reaction between Cu(I) and O2 were found to account for the influence of the chloride ions and pH on the kinetics of the process. Additionally, the presence of Cu(III) as the primary oxidant resulting from the interaction between Cu(I) and H2O2 in the Cu(II)/AA system was confirmed, along with the coexistence of HO•, possibly due to an equilibrium established between Cu(III) and HO•

Depassivation of Aged Fe⁰ by Ferrous Ions: Implications to Contaminant Degradation

Investigation of the effects of ferrous iron (Fe­(II)) on the ability of aged (iron oxide coated) Fe⁰ to degrade trichloroethylene (TCE) has revealed that, while neither aged Fe⁰ nor Fe­(II) separately were able to degrade TCE, approximately 95% of the TCE present was degraded after exposure to a mixture of aged Fe⁰ and Fe­(II) for 21 days. The rates of TCE degradation increased with an increase in Fe­(II) concentration from 0 to 1.6 mM and then reached a relative plateau. Results of Fe­(II) “adsorption” studies revealed that the equilibrium pH decreased significantly with an increase in Fe­(II) concentration. Proton release during adsorption of Fe­(II) to iron oxide coatings was identified as being responsible for promotion of surface dissolution and, concomitantly, enhancement in extent of TCE reduction by aged Fe⁰. Results of open circuit potential analysis and Tafel plot measurement showed that the corrosion potential of aged Fe⁰ (<i>E</i>_corr) in the presence of Fe­(II) decreased to levels similar to that of Fe⁰/Fe²⁺, while significant increase in corrosion current (<i>I</i>_corr) and decrease in polarization resistance (<i>R</i>_p) were found with an increase in Fe­(II) concentration. The fact that the effects of different Fe­(II) concentrations on the <i>E</i>_corr, <i>I</i>_corr, and <i>R</i>_p was decoupled from their effects on TCE degradation by aged Fe⁰ suggested that the enhancement of TCE degradation in the presence of Fe­(II) was attributable to the dissolution of the Fe­(III) oxyhydroxide layer coating the aged Fe⁰. While the presence of Fe­(II) may also lead to transformation of the Fe­(III) (oxy)­hydroxide coating to more crystalline phases, the rate of reduction of compounds such as TCE by Fe­(II) associated with the Fe­(III) (oxy)­hydroxide coating is substantially slower than that mediated by Fe⁰. These findings provide new insight into the molecular-scale interaction of aged Fe⁰ and ferrous iron with particular implications for sustaining the reactivity of Fe⁰-mediated degradation of contaminants in iron-bearing environments

Depassivation of Aged Fe⁰ by Inorganic Salts: Implications to Contaminant Degradation in Seawater

In this study, aged (iron oxide coated) Fe⁰ was applied to the degradation of trichloroethylene (TCE) in seawater. It was found that while the aged Fe⁰ was inactive with regard to TCE degradation in Milli-Q water, more than 95% of the TCE present was degraded in real and synthetic seawater solutions after exposure to aged Fe⁰ for 21 days. Results with individual salts from the synthetic seawater revealed that no significant TCE degradation was observed in the presence of Na₂SO₄, CaCl₂, and NaHCO₃. Partial TCE degradation (28.4%) was observed in 500 mM NaCl after 21 days, while a similar extent of degradation to that found in the seawater solutions was observed in 50 mM solutions of magnesium salts (MgCl₂ and MgSO₄). Results of open circuit potential analysis suggested that the Fe⁰ corrosion potential was not a key determinant of extent of TCE reduction since the corrosion potential decreased to levels similar to that of Fe⁰/Fe²⁺ in the presence of all salts examined. Lower final pH values and higher dissolved Fe­(II) concentrations were observed in the presence of magnesium salts compared to other salts. Formation of the surface complex >FeOMg⁺ was identified as being critical to protonation of surface sites, reductive dissolution of the passivating Fe­(III) oxyhydroxide layer coating the underlying Fe⁰ and enhancement in extent of TCE reduction. These findings provide insight into the molecular-scale mechanism of depassivation of aged Fe⁰ by inorganic salts with particular implications for the Fe⁰-mediated degradation of contaminants in saline natural waters such as seawater and saline groundwaters

Hydroquinone-Mediated Redox Cycling of Iron and Concomitant Oxidation of Hydroquinone in Oxic Waters under Acidic Conditions: Comparison with Iron–Natural Organic Matter Interactions

Interactions of 1,4-hydroquinone with soluble iron species over a pH range of 3–5 in the air-saturated and partially deoxygenated solution are examined here. Our results show that 1,4-hydroquinone reduces Fe­(III) in acidic conditions, generating semiquinone radicals (Q^•–) that can oxidize Fe­(II) back to Fe­(III). The oxidation rate of Fe­(II) by Q^•–increases with increase in pH due to the speciation change of Q^•– with its deprotonated form (Q^•–) oxidizing Fe­(II) more rapidly than the protonated form (HQ^•). Although the oxygenation of Fe­(II) is negligible at pH < 5, O₂ still plays an important role in iron redox transformation by rapidly oxidizing Q^•– to form benzoquinone (Q). A kinetic model is developed to describe the transformation of quinone and iron under all experimental conditions. The results obtained here are compared with those obtained in our previous studies of iron–Suwannee River fulvic acid (SRFA) interactions in acidic solutions and support the hypothesis that hydroquinone moieties can reduce Fe­(III) in natural waters. However, the semiquinone radicals generated in pure hydroquinone solution are rapidly oxidized by dioxygen, while the semiquinone radicals generated in SRFA solution are resistant to oxidation by dioxygen, with the result that steady-state semiquinone concentrations in SRFA solutions are 2–3 orders of magnitude greater than in solutions of 1,4-hydroquinone. As a result, semiquinone moieties in SRFA play a much more important role in iron redox transformations than is the case in solutions of simple quinones such as 1,4-hydroquinone. This difference in the steady-state concentration of semiquinone species has a dramatic effect on the cycling of iron between the +II and +III oxidation states, with iron turnover frequencies in solutions containing SRFA being 10–20 times higher than those observed in solutions of 1,4-hydroquinone

Mechanism Underlying the Effectiveness of Deferiprone in Alleviating Parkinson’s Disease Symptoms

Elevation in iron content as well as severe depletion of dopamine (DA) as a result of iron-induced loss of dopaminergic neurons has been recognized to accompany the progression of Parkinson’s disease (PD). To better understand the mechanism of the mitigating effect of the iron chelator deferiprone (DFP) on PD, the interplay between iron and DFP was investigated both in the absence and presence of DA. The results show that DFP was extremely efficient in scavenging both aqueous iron and iron that was loosely bound to DA with the entrapment of iron in Fe-DFP complexed form critical to halting the iron catalyzed degradation of DA and associated generation of toxic metabolites. The DFP related scavenging of dopamine semiquinone (DA^•–) and superoxide (O₂^•–) may also contribute to its positive effects in the treatment of PD