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

Reductive Dissolution Mechanisms of Manganese Oxide Mediated by Algal Extracellular Organic Matter and the Effects on 17α-Ethinylestradiol Removal

Reductive dissolution of manganese oxide (MnOx) is a major process that improves the availability of manganese in natural aquatic environments. The extracellular organic matter (EOM) secreted by algae omnipresent in eutrophic waters may affect MnOx dissolution thus the fate of organic micropollutants. This study investigates the mechanisms of MnOx reductive dissolution mediated by EOM and examines the effects of this process on 17α-ethinylestradiol degradation. The influences of EOM concentration (1.0-20.0 mgC/L) and pH (6.0-9.0) in both dark and irradiated conditions were assessed. In the dark, EOM was found to facilitate MnOx reductive dissolution via the ligand-to-metal charge transfer (LMCT). The dissolution was further enhanced under irradiation, with the participation of superoxide ions (O2•-). Higher EOM concentrations increased the contents of available reducing substances and O2•-, accelerating the reductive dissolution. Higher pH slowed the photoreductive dissolution rates, while O2•--mediated reduction became more important. Polyphenols and highly unsaturated carbon and phenolic formulas in EOM were found to drive the reductive dissolution. Soluble reactive Mn(III) formed through reductive dissolution of MnOx effectively removed 17α-ethinylestradiol in solution. Overall, the findings regarding the mechanisms behind reductive dissolution of MnOx have broad implications for Mn geochemical cycles and organic micropollutant fate.

In situ prepared algae-supported iron sulfide to remove hexavalent chromium

The effects of algae on the removal of contaminant by iron sulfide (FeS) are still unknown. Chlorella vulgaris (CV), a remarkable algal specie, was used to prepare the CV-supported FeS (CV-FeS) and to investigate the role that CV plays in the removal of a heavy metal (i.e., hexavalent chromium (Cr(VI)) by FeS. The stabilized effect from algal extracellular polymeric substance (EPS) enhanced the reactivity of FeS due to the decrease of FeS aggregation, thus increasing Cr(VI) removal rate from 0.21 min^-1 to 0.79 min^-1. Furthermore, the strong buffering induced by the algal functional groups could effectively prevent the solution pH from increasing, which improved Cr(VI) removal because acidic solution facilitated Cr(VI) reduction by FeS. However, the complexing capacity from algal EPS made Fe(II) unavailable for Cr(VI) reduction, which led to 35% decrease of Cr(VI) removal. The Fe(II) was oxidized to α-FeOOH by Cr(VI) in the absence of CV, while the unreacted Fe(II) was detected as in the form of Fe(OH)₂ in CV-FeS. Cr(VI) was reduced to Cr(III) and S(-II) was oxidized to elemental sulfur (S₈) regardless of the CV. This work showed the different roles of algae in the removal of Cr(VI) by FeS and provided value information for the application of FeS in the polluted algae-containing water system.

In situ organic Fenton-like catalysis triggered by anodic polymeric intermediates for electrochemical water purification

Organic Fenton-like catalysis has been recently developed for water purification, but redox-active compounds have to be ex situ added as oxidant activators, causing secondary pollution problem. Electrochemical oxidation is widely used for pollutant degradation, but suffers from severe electrode fouling caused by high-resistance polymeric intermediates. Herein, we develop an in situ organic Fenton-like catalysis by using the redox-active polymeric intermediates, e.g., benzoquinone, hydroquinone, and quinhydrone, generated in electrochemical pollutant oxidation as H₂O₂ activators. By taking phenol as a target pollutant, we demonstrate that the in situ organic Fenton-like catalysis not only improves pollutant degradation, but also refreshes working electrode with a better catalytic stability. Both ¹O₂ nonradical and ·OH radical are generated in the anodic phenol conversion in the in situ organic Fenton-like catalysis. Our findings might provide a new opportunity to develop a simple, efficient, and cost-effective strategy for electrochemical water purification.

Iron Transformation and Its Role in Phosphorus Immobilization in a UCT-MBR with Vivianite Formation Enhancement

The formation of vivianite (Fe₃(PO₄)₂·8H₂O) in iron (Fe)-dosed wastewater treatment facilities has the potential to develop into an economically feasible method of phosphorus (P) recovery. In this work, a long-term steady Fe^III-dosed University of Cape Town process-membrane bioreactor (UCT-MBR) system was investigated to evaluate the role of Fe transformations in immobilizing P via vivianite crystallization. The highest fraction of Fe^II, to total Fe (Fe_tot), was observed in the anaerobic chamber, revealing that a redox condition suitable for Fe^III reduction was established by improving operational and configurational conditions. The supersaturation index for vivianite in the anaerobic chamber varied but averaged ∼4, which is within the metastable zone and appropriate for its crystallization. Vivianite accounted for over 50% of the Fe_tot in the anaerobic chamber, and its oxidation as it passed through the aerobic chambers was slow, even in the presence of high dissolved oxygen concentrations at circumneutral pH. This study has shown that the high stability and growth of vivianite crystals in oxygenated activated sludge can allow for the subsequent separation of vivianite as a P recovery product.

Extracellular electron transfer by Microcystis aeruginosa is solely driven by high pH

Extracellular electron transfer (EET) by the cyanobacterium Microcystis aeruginosa was investigated. Observations indicate that EET onto an electrode poised at + 0.6 vs. standard hydrogen electrode (SHE) is triggered by high pH, more evidently at pH levels above 9. Light intensity does not appear to affect electricity generation, indicating that this may not be a "biophotovoltaic" process. The generated current density was amplified with stepwise pH increases from approximately 5 mA m^-2 at pH 7.8 to 30 mA m^-2 at pH 10.5, for dense (0.4 mg mL^-1 dry weight) Microcystis aeruginosa suspensions with dissolved CO₂ and O₂ approaching equilibrium with atmospheric concentrations. The upsurge in current density was more pronounced (from 5 mA m^-2 at pH 7.8 to 40 mA m^-2 at pH 10.2) in the absence of the cells' natural electron acceptors, dissolved CO₂ and O₂. However, the latter effect is more likely due to competition for electrons by oxygen than to reductive stress. EET in this species is therefore a light-independent process that is enhanced by increasing pH, with reasons that are still unknown, but either related to the involvement of protons in the last step of electron transfer, or to intracellular pH control.

Phenazine-1-Carboxylic Acid-Producing Bacteria Enhance the Reactivity of Iron Minerals in Dryland and Irrigated Wheat Rhizospheres

Phenazine-1-carboxylic acid (PCA) is a broad-spectrum antibiotic produced by rhizobacteria in the dryland wheat fields of the Columbia Plateau. PCA and other phenazines reductively dissolve Fe and Mn oxyhydroxides in bacterial culture systems, but the impact of PCA upon Fe and Mn cycling in the rhizosphere is unknown. Here, concentrations of dithionite-extractable and poorly crystalline Fe were approximately 10% and 30-40% higher, respectively, in dryland and irrigated rhizospheres inoculated with the PCA-producing (PCA⁺) strain Pseudomonas synxantha 2-79 than in rhizospheres inoculated with a PCA-deficient mutant. However, rhizosphere concentrations of Fe(II) and Mn did not differ significantly, indicating that PCA-mediated redox transformations of Fe and Mn were transient or were masked by competing processes. Total Fe and Mn uptake into wheat biomass also did not differ significantly, but the PCA⁺ strain significantly altered Fe translocation into shoots. X-ray absorption near edge spectroscopy revealed an abundance of Fe-bearing oxyhydroxides and phyllosilicates in all rhizospheres. These results indicate that the PCA⁺ strain enhanced the reactivity and mobility of Fe derived from soil minerals without producing parallel changes in plant Fe uptake. This is the first report that directly links significant alterations of Fe-bearing minerals in the rhizosphere to a single bacterial trait.

Is Superoxide-Mediated Fe(III) Reduction Important in Sunlit Surface Waters?

Two major pathways are reported to account for photochemical reduction of Fe(III) in sunlit surface waters, namely, ligand-to-metal charge transfer (LMCT) and superoxide-mediated iron reduction (SMIR). In this study, we investigate the impact of Fe(III) speciation (organically complexed (Fe(III)L versus iron oxyhydroxide (AFO)) on Fe(III) reducibility by photogenerated superoxide (O₂^•-) and LMCT. To simulate conditions typical of fresh, estuarine, and coastal waters, we have used Suwannee River Fulvic Acid (SRFA) as a representative of the natural organic matter likely to associate with Fe(III). Our results show that the photolabile Fe(III)SRFA complex is reduced rapidly by LMCT, while O₂^•- does not play a role in reduction of these entities. In contrast, the relatively less photolabile AFO is reduced by both O₂^•- and LMCT. The reduction of AFO by O₂^•- occurs following the dissolution of AFO, and hence, the contribution of O₂^•- to reductive dissolution of AFO is dependent on conditions such as the age of the AFO and initial AFO concentration affecting the rate of dissolution of AFO. Our results further show that while colloidal Fe(III) (in this work, particles >0.025 μm) is reduced by O₂^•-, there is no involvement of O₂^•- in dissolved Fe(III) reduction. Overall, our results show that superoxide-mediated iron reduction will be important only in natural waters containing limited concentrations of Fe binding ligands.

Effect of light on the transformation of BDE-47 by living and autoclaved cultures of Microcystis flos-aquae and Chlorella vulgaris

Polybrominated diphenyl ethers (PBDEs) are ubiquitous and toxic contaminants found in high concentrations in watercourses, and are not well removed by conventional wastewater treatment facilities. This study aimed to evaluate the removal and transformation of BDE-47, one of the environmentally predominant PBDE congener, by a green alga (Chlorella vulgaris) and a cyanobacterium (Microcystis flos-aquae) under different light conditions. Living and autoclaved cultures were exposed to BDE-47 at a concentration of 10 μg L^-1 for 7 days. Both species removed >90% of BDE-47 very shortly after spiking. Light intensity affected the transformation of BDE-47 in living cultures of both species, since 5 to 11 times more debromination products were measured at a light intensity of 100 μmol photons m^-2 s^-1 than at 20 μmol photons m^-2 s^-1. Living cultures of M. flos-aquae transformed BDE-47 at a rate of 0.22 day^-1 while no transformation was observed in the respective autoclaved cultures. On the contrary, both living and autoclaved cultures of C. vulgaris had similar BDE-47 transformation rates of 0.05-0.06 day^-1. Debromination of BDE-47 was a predominant transformation pathway in cultures of C. vulgaris, with two times higher BDE-28 concentrations measured than in M. flos-aquae, while hydroxylation was more dominant with the cyanobacterium. Most BDE-47 and its debromination product BDE-28 were found on the cell surface of both species. These results reveal that different transformation mechanisms were involved in C. vulgaris and M. flos-aquae cultures and confirm the importance of species selection for the removal of PBDEs from contaminated environments.

Iron uptake by bloom-forming freshwater cyanobacterium Microcystis aeruginosa in natural and effluent waters

Studies on Fe uptake by phytoplankton have been often conducted using artificial culture media. However, Fe chemistry in freshwater can be influenced by riverine anthropogenic impacts and other factors causing water quality changes. In this study, therefore, Fe uptake in natural (river and reservoir) and effluent waters was investigated for the notorious bloom-forming freshwater cyanobacterium Microcystis aeruginosa. To investigate the Fe uptake mechanism, a short-term incubational assay was conducted in the presence of light, Fe(II) ligand and Fe(III) reductant, with results consistently indicating that unchelated Fe(III) is the major substrate for Fe uptake by M. aeruginosa. Further assays using various freshwater samples indicated that Fe uptake is lower in natural waters compared to that of effluent waters and, interestingly, Fe uptake was found to be limited in natural waters. These results suggest that Fe limitation can be alleviated by the inflow of effluent waters. Statistical analysis with various water quality variables indicated that Fe availability is significantly influenced by concentrations of dissolved Fe and organic matter as well as specific UV absorbance (an index of aromaticity). Overall, findings of this study highlight that watershed anthropogenic activities exert important roles in Fe uptake by freshwater cyanobacteria via alteration of Fe speciation.

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

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 O₂ 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 O₂, 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.

Effect of biochar on photosynthetic microorganism growth and iron cycling in paddy soil under different phosphate levels

The surplus of exogenous and endogenous phosphate in submerged paddy fields could increase the risk of algal blooms, the photosynthesis of which might further influence the redox processes of iron. This work investigated the effects of biochar on photosynthetic microorganism growth and iron redox under different phosphate (P) levels to understand the dynamics of P and thereby control non-point source pollution by biochar addition. Paddy soils were incubated anaerobically with phosphate and biochar addition under controlled illumination conditions to determine the variation in chlorophyll a (Chl a), ferrous iron [Fe(II)], soil pH and water-soluble phosphate (W-P) with incubation time. Biochar addition significantly inhibited the photosynthetic microorganism growth, with Chl a decreased by 4.74-15.78mg·g^-1 when compared with the control. Fe(III) reduction was significantly stimulated in response to biochar addition, while Fe(II) oxidation was inhibited because of the suppression of photosynthetic microorganism growth. The enhanced Fe(III) reduction and suppressed Fe(II) oxidation decreased the P solubility in the tested soils. These findings provide a cost-effective approach for inhibiting photosynthetic microorganism growth in paddy field and valuable insight into the effect of iron cycling on P retention for further management of eutrophication from exogenous and endogenous P loading.

Impact of Microcystis aeruginosa 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 Microcystis aeruginosa 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 Microcystis aeruginosa 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.