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

Time-dependent redistribution of soil arsenic induced by transformation of iron species during zero-valent iron biochar composites amendment: Effects on the bioaccessibility of As in soils

Zero-valent iron biochar composites (ZVI/BC) are considered as effective amendments for arsenic (As)-contaminated soils. However, the mechanisms of transformation of various soil As species during ZVI/BC amendments remain unclear. This study investigated As transformation in four soils (namely, GX, ZJ, HB, and HN) treated with ZVI/BC for 65 days under two soil moisture conditions, unsaturated and oversaturated. Results showed that the 65-day treatment was divided into two stages based on the variation of labile As content. Within 2 days (stage 1), ZVI/BC addition quickly reduced labile As content by 5.91-90.3 % in soils under unsaturated conditions. During days 2-65 (stage 2), labile As ultimately decreased by 0.06-0.31 mg/kg in GX, ZJ, and HB while increasing by 22.1 mg/kg in HN soil, due to its lower pH value and Fe content. The variations of labile As were attributed to changes in multiple Fe minerals and associated As species. In stage 1, the corrosion of ZVI/BC generated amorphous Fe oxides to immobilize labile As, resulting in the accumulation of meta-stable As. In stage 2, amorphous Fe oxides were transformed into crystalline Fe oxides, resulting in the release and re-precipitation of As along with transformation, thus redistributing immobilized As into labile and stable As, which was evidenced by multiple methods, including chemical extraction, XRD, and TEM-EDX. The elevated soil moisture condition would enhance the corrosion of ZVI/BC in stage 1, further forming a reductive environment to facilitate the transformation of Fe minerals in stage 2. Besides, As bioaccessibility in soils was reduced by 10.8-38.7 % after ZVI/BC treatment in in-vitro gastrointestinal simulations. Overall, our study revealed the time-dependent transformation mechanism of soil As species and associated Fe minerals under different soil moisture with ZVI/BC treatments, and highlighted the effectiveness of ZVI/BC as a long-term amendment for As-contaminated soils.

Photoinduced oxidation of chromium picolinate to hexavalent chromium in the presence of ferric ions

The occurrence of chromium picolinate (Cr(pic)3) in environment has attracted raising concerns on its fate and the associated risks. Herein, the photoinduced oxidation of Cr(pic)3 in the presence of ferric ions (Fe(III)) under simulated sunlight and natural solar light irradiation were investigated. Cr(pic)3 was stable under dark or without Fe(III). 87.9 % of Cr(pic)3 (C0 = 1.0 μM) was degraded in the presence of 50 μM Fe(III) after 90 min simulated sunlight irradiation at initial pH of 4.0. •OH was the main cause for Cr(pic)3 oxidation, it attacked the chromium center to generate hexavalent chromium (Cr(VI)) and picolinic acid (k = 5.9 ×108 M-1·s-1). Picolinic acid could be further oxidized to NH4+ and small organics. Relative higher Fe(III) content (25 - 75 μM) and Cr(pic)3 concentration (0.5 - 2.0 μM) promoted both of Cr(pic)3 degradation and Cr(VI) accumulation. While, the degradation of Cr(pic)3 decreased with pH at the range of 3.0 - 8.0, more Cr(VI) was accumulated at pH 5.0 and 6.0. The co-existence of inorganic ions and dissolved organic matter (DOM) in river water inhibited Cr(pic)3 oxidation by scavenging the •OH formed and shielding the light. 8.0 - 16.7 μg/L of Cr(VI) was accumulated after 9.0 h simulated sunlight irradiation of Cr(pic)3 in river water matrix ([Fe(III)]0 = 50 - 100 μM). The generation of Cr(VI) under solar light was slower than that under simulated sunlight due to the weaker light intensity (43.2 - 85.0 mW/cm2 vs. 750 - 1300 mW/cm2). These results consistently suggest photoinduced oxidation of Cr(pic)3 in environment generates the toxic Cr(VI), which deserves significant attention.

Novel Insights into Sb(III) Oxidation and Immobilization during Ferrous Iron Oxygenation: The Overlooked Roles of Singlet Oxygen and Fe (oxyhydr)oxides Formation

Reactive oxygen species (ROS) produced from the oxygenation of reactive Fe(II) species significantly affect the transformation of metalloids such as Sb at anoxic-oxic redox interfaces. However, the main ROS involved in Sb(III) oxidation and Fe (oxyhydr)oxides formation during co-oxidation of Sb(III) and Fe(II) are still poorly understood. Herein, this study comprehensively investigated the Sb(III) oxidation and immobilization process and mechanism during Fe(II) oxygenation. The results indicated that Sb(III) was oxidized to Sb(V) by the ROS produced in the aqueous and solid phases and then immobilized by formed Fe (oxyhydr)oxides via adsorption and coprecipitation. In addition, chemical analysis and extended X-ray absorption fine structure (EXAFS) characterization demonstrated that Sb(V) could be incorporated into the lattice structure of Fe (oxyhydr)oxides via isomorphous substitution, which greatly inhibited the formation of lepidocrocite (γ-FeOOH) and decreased its crystallinity. Notably, goethite (α-FeOOH) formation was favored at pH 6 due to the greater amount of incorporated Sb(V). Moreover, singlet oxygen (1O2) was identified as the dominant ROS responsible for Sb(III) oxidation, followed by surface-adsorbed ·OHads, ·OH, and Fe(IV). Our findings highlight the overlooked roles of 1O2 and Fe (oxyhydr)oxide formation in Sb(III) oxidation and immobilization during Fe(II) oxygenation and shed light on understanding the geochemical cycling of Sb coupled with Fe in redox-fluctuating environments.

Inhibiting effects of humic acid on iron flocculation hindered As removal by electro-flocculation on air cathode iron anode

Activated carbon air cathode combined with iron anode oxidation-flocculation synergistic Arsenic (As) removal was a new groundwater purification technology with low energy consumption and high efficiency for groundwater with high As concentration. The presence of organic matter such as humic acid (HA) had ambiguous effects on formation of organic colloids in the system. The effects of the particle size distribution characteristics of these colloids on the formation characteristics of flocs and the efficiency of As purification was not clear. In this work, we used five different pore size alumina filter membranes to separate mixed phase solutions and studied the corresponding changes in iron and arsenic concentrations in the presence and absence of humic acid conditions. In the presence of HA, the arsenic concentration of < 0.05 µm particle size components was 1.01, 1.28, 3.07, 7.69, 2.85 and 1.24 times of that in the absence of HA. At the same time, the arsenic content in 0.05-0.1 µm and 0.1-0.45 µm particle size components was also higher than that in the system without HA, which revealed that the presence of HA hindered the flocculation behavior of As distribution to higher particle sizes in the early stage of the reaction. The presence of HA affected the flocculation rate of iron flocs from small to large particle size fractions and it had limited effect on the behavior of large-size flocs in adsorption of As. These results provide a theoretical basis for targeted, rapid, and low consumption synergistic removal of arsenic and organic compounds in high arsenic groundwater.

Embedding Fe(0) electrocoagulation in a biologically active As(III) oxidising filter bed

Long-term consumption of groundwater containing elevated levels of arsenic (As) can have severe health consequences, including cancer. To effectively remove As, conventional treatment technologies require expensive chemical oxidants to oxidise neutral arsenite (As(III)) in groundwater to negatively charged arsenate (As(V)), which is more easily removed. Rapid sand filter beds used in conventional aeration-filtration to treat anaerobic groundwater can naturally oxidise As(III) through biological processes but require an additional step to remove the generated As(V), adding complexity and cost. This study introduces a novel approach where As(V), produced through biological As(III) oxidation in a sand filter, is effectively removed within the same filter by embedding and operating an iron electrocoagulation (FeEC) system inside the filter. Operating FeEC within the biological filter achieved higher As(III) removal (81 %) compared to operating FeEC in the filter supernatant (67 %). This performance was similar to an analogous embedded-FeEC system treating As(V)-contaminated water (85 %), confirming the benefits of incorporating FeEC in a biological bed for comparable As(III) and As(V) removal. However, operating FeEC in the sand matrix consumed more energy (14 Wh/m3) compared to FeEC operated in a water matrix (7 Wh/m3). The efficiency of As removal increased and energy requirements decreased in such embedded-FeEC systems by deep-bed infiltration of Fe(III)-precipitates, which can be controlled by adjusting flow rate and pH. This study is one of the first to demonstrate the feasibility of embedding FeEC systems in sand filters for groundwater arsenic removal. Such systems capitalise on biological As(III) oxidation in aeration-filtration, effectively eliminating As(V) within the same setup without the need for chemicals or major modifications.

Occurrence and cycle of dissolved iron mediated by humic acids resulting in continuous natural photodegradation of 17α-ethinylestradiol

17α-ethinylestradiol (EE2), a synthetic endocrine-disrupting chemical, can degrade in natural waters where humic acids (HA) and dissolved iron (DFe) are present. The iron is mostly bound in Fe(III)-HA complexes, the formation process of Fe(III)-HA complexes and their effect on EE2 degradation were explored in laboratory experiments. The mechanism of ferrihydrite facilitated by HA was explored with results indicating that HA facilitated the dissolution of ferrihydrite and the generation of Fe(III)-HA complexes with the stable chemical bonds such as C-O, CO in neutral, alkaline media with a suitable Fe/C ratio. 1O2, •OH, and 3HA* were all found to be important in the photodegradation of EE2 mediated by Fe(III)-HA complexes. Fe(III)-HA complexes could produce Fe(II) and hydrogen peroxide (H2O2) to create conditions suitable for photo-Fenton reactions at neutral pH. HA helped to maintain higher dissolved iron concentrations and alter the Fe(III)/Fe(II) cycling. The natural EE2 photodegradation pathway elucidated here provides a theoretical foundation for investigating the natural transformation of other trace organic contaminants in aquatic environments.

Hydroxyl radicals in natural waters: Light/dark mechanisms, changes and scavenging effects

Hydroxyl radicals (•OH) are the most active, aggressive and oxidative reactive oxygen species. In the natural aquatic environment, •OH plays an important role in the biogeochemistry cycle, biotransformation, and pollution removal. This paper reviewed the distribution and formation mechanism of •OH in aquatic environments, including natural waters, colloidal substances, sediments, and organisms. Furthermore, factors affecting the formation and consumption of •OH were thoroughly discussed, and the mechanisms of •OH generation and scavenging were summarized. In particular, the effects of climate change and artificial work on •OH in the largest natural aquatic environment, i.e., marine environment was analyzed with the help of bibliometrics. Moreover, Fenton reactions make the •OH variation more complicated and should not be neglected, especially in those areas with suspended particles and sediments. Regarding the •OH variation in the natural aquatic environment, more attention should be given to global change and human activities.

Influence of Fe(III) source, light quality, photon flux and presence of oxygen on photoreduction of Fe(III)-organic complexes - Implications for light-influenced coastal freshwater and marine sediments

Iron(III) photoreduction is an important source of Fe(II) in illuminated aquatic and sedimentary environments. Under oxic conditions, the Fe(II) can be re-oxidized by oxygen (O₂) forming reactive O-species such as hydrogen peroxide (H₂O₂) which further react with Fe(II) thus enhancing Fe(II) oxidation rates. However, it is unknown by aquatic sediments how the parameters wavelength of radiation, photon flux, origin of Fe(III) source and presence or absence of O₂ influence the extent of Fe(II) and H₂O₂ turnover. We studied this using batch experiments with different Fe(III)-organic complexes mimicking sedimentary conditions. We found that wavelengths <500 nm are necessary to initiate Fe(III) photoreduction and that the photon flux, wavelength and identity of Fe(III)-complexing organic acids control the kinetics of Fe(III) photoreduction. The formation of photo-susceptible Fe(III)-organic complexes did not depend on whether the Fe(III) source was biogenically produced, poorly-crystalline Fe(III) oxyhydroxides or chemically synthesized ferrihydrite. Oxic conditions caused chemical re-oxidation of Fe(II) and accumulation of H₂O₂. The photon flux, wavelength and availability of Fe(III)-complexing organic molecules are critical for the balance between concurrent Fe(III) photoreduction and abiotic Fe(II) oxidation and may even lead to a steady-state concentration of Fe(II) in the micromolar range. These results help understand and predict Fe(III) photoreduction dynamics and in-situ formation of Fe(II) in oxic or anoxic, illuminated and organic-rich environments.

Effects of riboflavin and desferrioxamine B on Fe(II) oxidation by O2

Flavins and siderophores secreted by various plants, fungi and bacteria under iron (Fe) deficient conditions play important roles in the biogeochemical cycling of Fe in the environment. Although the mechanisms of flavin and siderophore mediated Fe(III) reduction and dissolution under anoxic conditions have been widely studied, the influence of these compounds on Fe(II) oxidation under oxic conditions is still unclear. In this study, we investigated the kinetics of aqueous Fe(II) (17.8 μM) oxidation by O2 at pH 5‒7 in the presence of riboflavin (oxidized (RBF) and reduced (RBFH2)) and desferrioxamine B (DFOB) as representative flavins and siderophores, respectively. Results showed that the addition of RBF/RBFH2 or DFOB markedly accelerates the oxidation of aqueous Fe(II) by O2. For instance, at pH 6, the rate of Fe(II) oxidation was enhanced 20‒70 times when 10 μM RBFH2 was added. The mechanisms responsible for the accelerated Fe(II) oxidation are related to the redox reactivity and complexation ability of RBFH2, RBF and DFOB. While RBFH2 does not readily complex Fe(II)/Fe(III), it can activate O2 and generate reactive oxygen species, which then rapidly oxidize Fe(II). In contrast, both RBF and DFOB do not reduce O2 but react with Fe(II) to form RBF/DFOB-complexed Fe(II), which in turn accelerates Fe(II) oxidation. Furthermore, the lower standard reduction potential of the Fe(II)-DFOB complex, compared to the Fe(II)-RBF complex, correlates with a higher oxidation rate constant for the Fe(II)-DFOB complex. Our study reveals an overlooked catalytic role of flavins and siderophores that may contribute to Fe(II)/Fe(III) cycling at oxic-anoxic interfaces.

Exploring the Effects of Organic Matter Characteristics on Fe(II) Oxidation Kinetics in Coastal Seawater

The iron(II) oxidation kinetic process was studied at 25 stations in coastal seawater of the Macaronesia region (9 around Cape Verde, 11 around the Canary Islands, and 5 around Madeira). In a physicochemical context, experiments were carried out to study the pseudo-first-order oxidation rate constant (k', min^-1) over a range of pH (7.8, 7.9, 8.0, and 8.1) and temperature (10, 15, 20, and 25 °C). Deviations from the calculated k_cal^' at the same T, pH, and S were observed for most of the stations. The measured t_1/2 (ln 2/k', min) values at the 25 stations ranged from 1.82 to 3.47 min (mean 1.93 ± 0.76 min) and for all but two stations were lower than the calculated t_1/2 of 3.21 ± 0.2 min. In a biogeochemical context, nutrients and variables associated with the organic matter spectral properties (CDOM and FDOM) were analyzed to explain the observed deviations. The application of a multilinear regression model indicated that k' can be described (R = 0.921 and SEE = 0.064 for pH = 8 and T = 25 °C) from a linear combination of three organic variables, k'^OM = k_cal^' -0.11* TDN + 29.9*b_DOM + 33.4*C1_humic, where TDN is the total dissolved nitrogen, b_DOM is the spectral peak obtained from colored dissolved organic matter (DOM) analysis when protein-like or tyrosine-like components are present, and C1_humic is the component associated with humic-like compounds obtained from the parallel factor analysis of the fluorescent DOM. Results show that compounds with N in their structures mainly explain the observed k' increase for most of the samples, although other components could also play a relevant role. Experimentally, k' provides the net result between the compounds that accelerate the process and those that slow it down.

Extensive production of hydroxyl radicals during oxygenation of anoxic paddy soils: Implications to imidacloprid degradation

Hydroxyl radical (^•OH) plays a critical role in driving organic pollutants degradation during redox fluctuations. Such processes have been frequently investigated in sedimentary environments, but rarely referred to the agricultural fields, such as paddy soils with frequent occurrence of redox fluctuations. Our findings demonstrated that extensive ^•OH (40.3-1061.4 μmol kg^-1) was produced during oxygenation of anoxic paddy slurries under circumstance conditions. Wet chemical sequential extractions, Mössbauer spectra, and X-ray photoelectron spectroscopy characterizations collectively corroborated that 0.5 M HCl-extracted Fe(II) (i.e., surface-bound Fe and Fe in low-crystalline minerals) contributed to more ^•OH production than aqueous Fe²⁺. The produced ^•OH can efficiently induce the oxidative transformation of organic carbon and the degradation of imidacloprid (IMP), which in turn produced the by-products, such as IMP-urea, IMP-olefin, and 6-chloronicontinic acid, via ^•OH-attacking mechanisms. Quenching experiments showed that hydrogen peroxide (H₂O₂) was the important intermediate for ^•OH formation via Haber-Weiss mechanisms during oxygenation processes. These findings indicate that abundant ^•OH can be produced during the redox fluctuations of paddy soil, which might be of great significance to predict the removal of organic contaminants and the mineralization of organic carbon in paddy fields.

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

Iron (Fe) phases are tightly linked to the preservation rather than the loss of organic carbon (OC) in soil; however, during redox fluctuations, OC may be lost due to Fe phase-mediated abiotic processes. This study examined the role of Fe phases in driving hydroxyl radical (^•OH) formation and OC transformation during redox cycles in paddy soils. Chemical probes, sequential extraction, and Mössbauer analyses showed that the active Fe species, such as exchangeable and surface-bound Fe and Fe in low-crystalline minerals (e.g., green rust-like Fe phases), predominantly regulated ^•OH formation during redox cycles. The ^•OH oxidation strongly induced the oxidative transformation of OC, which accounted for 15.1-30.8% of CO₂ production during oxygenation. Microbial processes contributed 7.3-12.1% of CO₂ production, as estimated by chemical quenching and γ-irradiation experiments. After five redox cycles, 30.1-71.9% of the OC associated with active Fe species was released, whereas 5.2-7.1% was stabilized by high-crystalline Fe phases due to the irreversible transformation of these active Fe species during redox cycles. Collectively, our findings might unveil the under-appreciated role of active Fe phases in driving more loss than conservation of OC in soil redox fluctuation events.

Northern High-Latitude Organic Soils As a Vital Source of River-Borne Dissolved Iron to the Ocean

Organic soils in the Arctic-boreal region produce small aquatic humic ligands (SAHLs), a category of naturally occurring complexing agents for iron. Every year, large amounts of SAHLs-loaded with iron mobilized in river basins-reach the oceans via river runoff. Recent studies have shown that a fraction of SAHLs belong to the group of strong iron-binding ligands in the ocean. That means, their Fe(III) complexes withstand dissociation even under the conditions of extremely high dilution in the open ocean. Fe(III)-loaded SAHLs are prone to UV-photoinduced ligand-to-metal charge-transfer which leads to disintegration of the complex and, as a consequence, to enhanced concentrations of bioavailable dissolved Fe(II) in sunlit upper water layers. On the other hand, in water depths below the penetration depth of UV, the Fe(III)-loaded SAHLs are fairly resistant to degradation which makes them ideally suited as long-lived molecular transport vehicles for river-derived iron in ocean currents. At locations where SAHLs are present in excess, they can bind to iron originating from various sources. For example, SAHLs were proposed to contribute substantially to the stabilization of hydrothermal iron in deep North Atlantic waters. Recent discoveries have shown that SAHLs, supplied by the Arctic Great Rivers, greatly improve dissolved iron concentrations in the Arctic Ocean and the North Atlantic Ocean. In these regions, SAHLs play a critical role in relieving iron limitation of phytoplankton, thereby supporting the oceanic sink for anthropogenic CO₂. The present Critical Review describes the most recent findings and highlights future research directions.

Ligand Effects on Biotic and Abiotic Fe(II) Oxidation by the Microaerophile Sideroxydans lithotrophicus

Organic ligands are widely distributed and can affect microbially driven Fe biogeochemical cycles, but effects on microbial iron oxidation have not been well quantified. Our work used a model microaerophilic Fe(II)-oxidizing bacterium Sideroxydans lithotrophicus ES-1 to quantify biotic Fe(II) oxidation rates in the presence of organic ligands at 0.02 atm O₂ and pH 6.0. We used two common Fe chelators with different binding strengths: citrate (log K_{Fe(II)-citrate} = 3.20) and nitrilotriacetic acid (NTA) (log K_Fe(II)-NTA = 8.09) and two standard humic substances, Pahokee peat humic acid (PPHA) and Suwannee River fulvic acid (SRFA). Our results provide rate constants for biotic and abiotic Fe(II) oxidation over different ligand concentrations and furthermore demonstrate that various models and natural iron-binding ligands each have distinct effects on abiotic versus biotic Fe(II) oxidation rates. We show that NTA accelerates abiotic oxidation and citrate has negligible effects, making it a better laboratory chelator. The humic substances only affect biotic Fe(II) oxidation, via a combination of chelation and electron transfer. PPHA accelerates biotic Fe(II) oxidation, while SRFA decelerates or accelerates the rate depending on concentration. The specific nature of organic-Fe microbe interactions may play key roles in environmental Fe(II) oxidation, which have cascading influences on cycling of nutrients and contaminants that associate with Fe oxide minerals.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Elucidating the Role of Dissolved Organic Matter and Sunlight in Mediating the Formation of Ag-Au Bimetallic Alloy Nanoparticles in the Aquatic Environment

Elucidating the interactions between metal ions and dissolved organic matter and deciphering mechanisms for their mineralization in the aquatic environment are central to understanding the speciation, transport, and toxicity of nanoparticles (NPs). Herein, we examine the interactions between Ag⁺ and Au³⁺ ions in mixed solutions (χ_Ag = 0.2, 0.5, and 0.8) in the presence of humic acids (HAs) under simulated sunlight; these conditions result in the formation of bimetallic Ag-Au NPs. A key distinction is that the obtained alloy NPs are compositionally and morphologically rather different from NPs obtained from thermally activated dark processes. Photoillumination triggers a distinctive plasmon-mediated process for HA-assisted reductive mineralization of ions to bimetallic alloy NPs which is not observed in its dark thermal reduction counterpart. The initial nucleation of bimetallic NPs is dominated by differences in the cohesive energies of Ag and Au crystal lattices, whereas the growth mechanisms are governed by the strongly preferred incorporation of Ag ions, which stems from their greater photoreactivity. The bimetallic NPs crystallize in shapes governed by the countervailing influence of minimizing free energy through the adoption of Wulff constructions and the energetic penalties associated with twin faults. As such, assessments of the stability and the potential toxic effects of bimetallic NPs arising from their possible existence in aquatic environments will depend sensitively on the origins of their formation.

Oxidation of Fe(II) by Flavins under Anoxic Conditions

Flavin-mediated electron transfer is an important pathway for Fe(III) reduction by dissimilatory iron-reducing bacteria. Although the mechanisms and kinetics of Fe(III) reduction by reduced flavins have been widely studied, the reaction between Fe(II) and oxidized flavins is rarely investigated. Results of this study show that under anoxic conditions, Fe(II) can be oxidized by the oxidized forms of riboflavin (RBF) and flavin mononucleotide (FMN) at pH 7-9. For instance, at pH 9, 73% of 17.8 μM Fe(II) was oxidized by 10 μM RBF within 20 min. Both the rate and extent of oxidation increased with increasing concentrations of oxidized flavins and increasing solution pH. Thermodynamic calculations and kinetic analyses implied that the oxidation of Fe(II) proceeded predominantly via the autodecomposition of Fe²⁺-RBF^- and Fe²⁺-FMN^- complexes, along with minor contributions from direct oxidation of Fe(II) by flavins and flavin radicals. Our findings suggest that the reoxidation of Fe(II) by oxidized flavins may be a rate-controlling factor in microbial Fe(III) reduction via flavin-mediated electron transfer.

Coagulation of organo-mineral colloids and formation of low molecular weight organic and metal complexes in boreal humic river water under UV-irradiation

Photodegradation of dissolved organic matter (DOM) is highly important in humic waters of peatland regions, yet the coupling between organic and organo-mineral colloids, trace metals and bioavailability of photodegraded products is poorly known. Here we studied photo-destruction of organo-mineral colloids induced by UV-irradiation of sterile-filtered mire water. We revealed two simultaneously occurring processes of transformation of DOM and trace elements speciation: (i) disintegration of high molecular weight organo-mineral colloids into lower molecular weight (<1 kDa) DOM and metal complexes and (ii) formation of particulate (>0.22 μm) aggregates of metals and organic matter. Over 26 days of UV-irradiation, up to 20% of dissolved organic carbon from peat waters was transformed into CO₂. In addition to transformation of organic compounds, sizeable change in speciation and size fractionation of many trace metals such as Fe, Pb, Cd, Co, Zn, Cu, V, La, Ni and Cr occurred. Although short-term (1 day) UV-irradiation of mire water stimulated growth of cultivable Pseudomonas sp. bacterium, the long-term exposure (26 days) of organic substrate had a negative effect on bacterial development. Therefore, while sizeable transformation of the organic and metal colloidal load of peat water may occur over first 10 days of UV-irradiation, the enhanced bioavailability of UV-treated substrate is achieved after first day of exposure. The present study demonstrates the importance of even short-term UV-irradiation on colloidal transformation and potential bioavailability of humic waters from temperate mires and highlights the need for more detailed study of coupled metal-organic matter transformation induced by sunlight exposure of mire waters.

Superoxide-driven autocatalytic dark production of hydroxyl radicals in the presence of complexes of natural dissolved organic matter and iron

We introduced superoxide as potassium superoxide (KO₂) to artificial lake water containing dissolved organic matter (DOM) without or with introduced ferric iron complexes (DOM-Fe(III)), and monitored the production rate of hydroxyl radicals as well as changes in the absorption and fluorescence properties of DOM. The introduction of KO₂ decreased the absorption by DOM but increased the spectral slope coefficient of DOM more with complexed ferric Fe than without it. The introduction of KO₂ increased the fluorescence of humic-like components in DOM without introduced ferric Fe but resulted in the loss of fluorescence in DOM with introduced ferric Fe. A single introduction of 13 μmol L^-1 KO₂ produced 10 μmol L^-1 and 104 μmol L^-1 hydroxyl radicals during a week-long experiment without and with the introduced DOM-Fe(III) complexes, respectively. The production rate of hydroxyl radicals decreased exponentially with time but levelled off and continued several days in DOM with introduced ferric Fe. These findings suggest that in the presence of DOM-Fe complexes, superoxide can trigger an autocatalytic Fenton reaction that produces hydroxyl radicals and breaks down DOM.

Influence of pH on the Kinetics and Mechanism of Photoreductive Dissolution of Amorphous Iron Oxyhydroxide in the Presence of Natural Organic Matter: Implications to Iron Bioavailability in Surface Waters

In this study, we investigate the influence of pH on the kinetics and mechanism of photoreductive dissolution of amorphous iron oxyhydroxide (AFO) in view of the recognition that the light-mediated dissolution of iron oxides controls Fe availability in many natural waters. Our results show that both ligand-to-metal charge transfer (LMCT) and photogenerated superoxide (O₂^•-) play an important role in AFO photoreductive dissolution in the presence of the chosen surrogate of natural organic matter, Suwannee river fulvic acid (SRFA). The pH dependence of LMCT-mediated AFO photoreductive dissolution is mainly controlled by the influence of pH on AFO solubility. A decrease in pH increases the concentration of the dissolved and more photolabile Fe(III)-SRFA complex present in equilibrium with AFO, a complex in which Fe(III) is readily reduced by LMCT. The pH dependence of superoxide-mediated Fe(III) reduction (SMIR) is also controlled by the influence of pH on AFO solubility with an increase in the dissolved inorganic Fe(III) concentration with the decrease in pH resulting in an increased rate of SMIR. No influence of pH was observed on the steady-state O₂^•- concentration generated on SRFA irradiation as well as the O₂^•- decay rate in the presence of SRFA, suggesting that the concentration and lifetime of O₂^•- are not important factors in controlling the pH dependence of O₂^•--mediated AFO dissolution. Overall, the results of this study show that the impact of acidification of natural waters on Fe availability will be much more pronounced when Fe is present as iron oxyhydroxide compared to that observed when organically bound Fe dominates with this effect because of the strong dependency of iron oxyhydroxide solubility on pH. The increased rate and extent of dissolution of iron oxyhydroxides on the acidification of natural waters will also have implications to the fate of other contaminants (such as heavy metals and organic compounds) that may be present on the iron oxyhydroxide surface.

Effect of Chloride and Suwannee River Fulvic Acid on Cu Speciation: Implications to Cu Redox Transformations in Simulated Natural Waters

Copper is a critical trace nutrient and, at higher concentrations, a toxicant in natural waters, with the relative rates of transformation between the Cu(I) and Cu(II) oxidation states being key to its speciation, bioavailability, and toxicity. While the influence of chloride (Cl^-) and natural organic matter on Cu speciation and associated redox transformations has been studied separately, their combined influence on Cu speciation and Cu redox transformations has not been examined. As such, in this study, we investigate the impact of Cl^- and Suwannee River fulvic acid (SRFA) on Cu(II) reduction and Cu(I) oxidation kinetics at pH 8.2. SRFA plays a dual role in providing Cu(II) reducing moieties as well as Cu ligating sites. Our results indicate that the SRFA-bound Cu(II) is less reactive than the inorganic Cu(II), and the SRFA-bound Cu(I) being much more rapidly oxidized than the inorganic Cu(I). The presence of Cl^- weakens Cu(II) binding by SRFA, thereby increasing the reactivity of Cu(II). Similarly, weakening of Cu(I) binding by SRFA and concomitant binding of Cu(I) by Cl^- stabilizes Cu(I). Our results further show that continuous formation of hydrogen peroxide occurs in the presence of Cu(II), SRFA, and Cl^- in air-saturated solution with the presence of H₂O₂ enhancing the dynamic nature of the system.

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.