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

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

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

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

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

Copper Inhibition of Triplet-Induced Reactions Involving Natural Organic Matter

The triplet excited state of natural organic matter (³NOM*) is an important reactive intermediate in sensitizing transformation of a wide range of environmentally relevant organic compounds, but the impact of trace metals on the fate and reactivity of ³NOM* is poorly understood. In this study, we investigate the effect of low concentrations of copper on ³NOM*-mediated oxidation (electron transfer) and energy transfer reactions. The oxidative efficiency of ³NOM* from Suwannee River NOM (SRNOM) and the widely used model triplet sensitizer 4-carboxybenzophenone were determined by measuring the photooxidation of 2,4,6-trimethylphenol (TMP). The pseudo-first-order photooxidation rate constants of TMP decreased markedly in the presence of trace amounts of Cu­(II) (25–500 nM) with the decrease associated with the continuous reduction of the oxidation intermediates of TMP (i.e., TMP^•_(−H)) by the photochemically produced Cu­(I). A kinetic model is developed that adequately describes the Cu inhibition effect in TMP photooxidation in irradiated SRNOM solutions. The ³NOM* energy transfer ability was assessed by measuring the isomerization of sorbic acid with the rate of this process markedly retarded in the presence of significantly higher (micromolar) concentrations of Cu­(II) than previously used. This result is attributed to (i) decreased formation of high energy ³NOM* due to formation of Cu–NOM complexes and (ii) increased loss of ³NOM* as a result of quenching by Cu. Since ³NOM* is the precursor to singlet oxygen (¹O₂) formation, the steady-state concentrations of ¹O₂ also decreased in the presence of micromolar concentrations of Cu­(II) with the quenching rate constant of ³NOM* by Cu calculated to be 1.08 × 10¹⁰ M^–1 s^–1

Oxidative Dissolution of Silver Nanoparticles by Chlorine: Implications to Silver Nanoparticle Fate and Toxicity

The kinetics of oxidative dissolution of silver nanoparticles (AgNPs) by chlorine is investigated in this work, with results showing that AgNPs are oxidized in the presence of chlorine at a much faster rate than observed in the presence of dioxygen and/or hydrogen peroxide. The oxidation of AgNPs by chlorine occurs in air-saturated solution in stoichiometric amounts with 2 mol of AgNPs oxidized for each mole of chlorine added. Dioxygen plays an important role in OCl^–-mediated AgNP oxidation, especially at lower OCl^– concentrations, with the mechanism shifting from stoichiometric oxidation of AgNPs by OCl^– in the presence of dioxygen to catalytic removal of OCl^– by AgNPs in the absence of dioxygen. These results suggest that the presence of chlorine will mitigate AgNP toxicity by forming less-reactive AgCl(s) following AgNP oxidation, although the disinfection efficiency of OCl^– may not be significantly impacted by the presence of AgNPs because a chlorine-containing species is formed on OCl^– decay that has significant oxidizing capacity. Our results further suggest that the antibacterial efficacy of nanosilver particles embedded on fabrics may be negated when treated with detergents containing strong oxidants, such as chlorine

Mechanistic Insights into Free Chlorine and Reactive Oxygen Species Production on Irradiation of Semiconducting Silver Chloride Particles

Silver halide based plasmonic absorbers are excellent photocatalysts under visible light irradiation. In this work we have investigated the kinetics and mechanism of generation of free chlorine and reactive oxygen species (ROS), both of which may play a role in organic degradation, on irradiation of AgCl(s). Our work shows that generation of free chlorine occurs via oxidation of Cl^– by photogenerated holes in the valence band. Photogenerated electrons in the conduction band reduce Ag⁺ to form Ag⁰ atoms which coalesce and aggregate to form silver nanoparticles (AgNPs). Superoxide and its more stable disproportionation product hydrogen peroxide (H₂O₂) are also presumably formed as a result of the reduction of dioxygen by photogenerated electrons; however, H₂O₂ was not detected, possibly as a result of its reaction with free chlorine and AgNPs. Confirmation that this reaction in all likelihood occurred was provided by quantification of the formation of singlet oxygen (¹O₂), a product of this reaction. On the basis of our experimental observations, we have developed a kinetic model that adequately describes the generation of free chlorine, ROS, and AgNPs on irradiation of AgCl(s)

Chlorine-Mediated Regeneration of Semiconducting AgCl(s) Following Light-Induced Ag⁰ Formation: Implications to Contaminant Degradation

AgCl(s) is a semiconductor that has been reported to photocatalytically degrade organic contaminants under visible light irradiation. On AgCl(s) irradiation, electrons are excited from the valence to the conduction band leaving holes in the valence band. While the photogenerated holes may degrade organic contaminants, photogenerated electrons in the conduction band reduce Ag­(I) to form Ag(0) atoms which coalesce to form silver nanoparticles (AgNPs) that may aggregate further to form nonreactive Ag(0) assemblages. This continuous reduction of AgCl(s) to Ag(0) on irradiation results in a decrease in the concentration of AgCl(s), thereby reducing the rate of generation of holes and thus decreasing the efficacy of photoirradiated AgCl(s) as an oxidant. If this decrease in oxidizing ability is to be prevented, a pathway for continuous oxidation of Ag(0) to Ag­(I) during the irradiation process is required in order to restore the activity of the AgCl(s) photocatalyst. The results of investigations described here show that chlorine, which is formed as a byproduct during AgCl(s) irradiation, oxidizes Ag(0) to Ag­(I) thereby, potentially, leading to the regeneration of AgCl(s). Furthermore, these freshly regenerated AgCl(s) particles have higher reactivity than the original aged AgCl(s) particles, thereby resulting in faster degradation of organic contaminants. Our results further show that although in situ generated chlorine is able to achieve some degree of redox cycling of Ag between the +0 and +1 redox states, externally added chlorine further enhances the turnover frequency of silver, thereby resulting in more effective degradation of organic contaminants

Mechanism and Kinetics of Dark Iron Redox Transformations in Previously Photolyzed Acidic Natural Organic Matter Solutions

Stable organic species produced on irradiation of Suwannee River Fulvic Acid (SRFA) are shown to be important oxidants of Fe­(II) in aqueous solutions at acidic pH, with rate constants substantially larger than those for oxygenation of Fe­(II) under the same conditions. These Fe­(II)-oxidizing species, which are formed during photolysis by superoxide-mediated oxidation of reduced organic moieties that are present intrinsically in SRFA, are long-lived in the dark but prone to rapid oxidation by singlet oxygen (¹O₂) under irradiated conditions. The intrinsic reduced organic species are able to reduce Fe­(III) at acidic pH. Although the exact identities of the organic Fe­(II) oxidant and the organic Fe­(III) reductant are unclear, their behavior is consistent with that expected of semiquinone and hydroquinone-like moieties respectively. A kinetic model is developed that adequately describes all aspects of the experimental data obtained, and which is capable of predicting dark Fe­(II) oxidation rates and Fe­(III) reduction rates in the presence of previously photolyzed natural organic matter