Resolving early stages of homogeneous iron(III) oxyhydroxide formation from iron(III) nitrate solutions at pH 3 using time-resolved SAXS.

Small angle X-ray scattering (SAXS) measurements coupled to a stopped-flow device has permitted the observation of the kinetics of Fe(III) oxyhydroxide (FeOx) formation and transformation from around 1 s to 30 min after initiation under environmentally relevant conditions at pH 3. The Unified Model approach was used to determine the evolution of multiple key parameters (particle scattering mass, mean particle volume, particle concentration, particle dimensionality, and particle size) for two separate structural levels as a function of time, with the results obtained enabling clarification of the mechanisms underlying FeOx formation and transformation under these conditions. Colloidal primary particles (radius of gyration 2-10 nm) that were observable by SAXS formed within 1 s of stopping the flow and subsequently grew over several minutes, first by cluster-cluster addition and then by a monomer-addition mechanism. Aggregation of these primary particles via a secondary cluster-cluster addition mechanism simultaneously resulted in a distinct population of larger (25-40 nm radius of gyration) secondary particles. The primary particles evolved into compact spheroidal forms with fractally rough surfaces, while the secondary particles were relatively open mass fractal structures. Comparison of the observed rates of these processes with those predicted for Fe polymerization indicates that kinetics of primary particle formation were likely controlled initially by rates of exchange between water molecules coordinated with Fe and those in the bulk solution. These findings provide new insights into the mechanisms underlying FeOx formation and transformation, and the kinetics of these mechanisms, at pH 3.

Hydroxyl radical production by H2O2-mediated oxidation of Fe(II) complexed by Suwannee River fulvic acid under circumneutral freshwater conditions.

The Fenton reaction, the oxidation of ferrous iron by hydrogen peroxide (H(2)O(2)), is typically assumed to be a source of hydroxyl radical (HO(•)) in natural systems, however, formation of HO(•) in this process is strongly dependent upon solution pH and the ligand environment, with HO(•) only formed when Fe(II) is organically complexed. In this study we examine the formation of HO(•) when Fe(II)-NOM complexes are oxidized by H(2)O(2) using phthalhydrazide as a probe for HO(•). We demonstrate that HO(•) formation can be quantitatively described using a kinetic model that assumes HO(•) formation occurs solely from the reaction of Fe(II)-NOM complexes with H(2)O(2), even though this reaction is sufficiently slow to play only a negligible role in the overall oxidation rate of total Fe(II). As such, NOM is seen to play a dual role in circumneutral natural systems in stabilizing Fe(II) toward oxidation by H(2)O(2) while enabling the formation of HO(•) through this oxidation process.

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 ((1)O(2)) 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.

Iron redox transformations in continuously photolyzed acidic solutions containing natural organic matter: kinetic and mechanistic insights.

In this work, the various pathways contributing to the formation and decay of Fe(II) in photolyzed acidic solutions containing Suwannee River fulvic acid (SRFA) are investigated. Results of experimental and computational studies suggest that ligand to metal charge transfer (LMCT), superoxide-mediated iron reduction and interaction with reduced organic species that are present intrinsically in SRFA each contribute to Fe(III) reduction with LMCT the most likely dominant pathway under these conditions. Fe(II) oxidation occurs as a result of its interaction with a variety of light-generated species including (i) short-lived organic species, (ii) relatively stable semiquinone-like organic species, and (iii) hydroperoxy radicals. While not definitive, a hypothesis that the short-lived organic species are similar to peroxyl radicals appears most consistent with our experimental and modeling results. The semiquinone-like organic species formed during photolysis by superoxide-mediated oxidation of reduced organic moieties are long-lived in the dark but prone to rapid oxidation by singlet oxygen ((1)O2) under irradiated conditions and thus play a minor role in Fe(II) oxidation in the light. A kinetic model is developed that adequately describes all aspects of the experimental data obtained and which is capable of predicting Fe(II) oxidation rates and Fe(III) reduction rates in the presence of natural organic matter and light.

Decoupling between water column oxygenation and benthic phosphate dynamics in a shallow eutrophic estuary.

Estuaries are crucial biogeochemical filters at the land-ocean interface that are strongly impacted by anthropogenic nutrient inputs. Here, we investigate benthic nitrogen (N) and phosphorus (P) dynamics in relation to physicochemical surface sediment properties and bottom water mixing in the shallow, eutrophic Peel-Harvey Estuary. Our results show the strong dependence of sedimentary P release on Fe and S redox cycling. The estuary contains surface sediments that are strongly reducing and act as net P source, despite physical sediment mixing under an oxygenated water column. This decoupling between water column oxygenation and benthic P dynamics is of great importance to understand the evolution of nutrient dynamics in marine systems in response to increasing nutrient loadings. In addition, the findings show that the relationship between P burial efficiency and bottom water oxygenation depends on local conditions; sediment properties rather than oxygen availability may control benthic P recycling. Overall, our results illustrate the complex response of an estuary to environmental change because of interacting physical and biogeochemical processes.

Calcium Carbonate Prenucleation Cluster Pathway Observed via In Situ Small-Angle X-ray Scattering.

For more than 150 years, our understanding of solid-phase mineral formation from dissolved constituent ions in aqueous environments has been dominated by classical nucleation theory (CNT). However, an alternative paradigm known as non-classical nucleation theory (NCNT), characterized by the existence of thermodynamically stable and highly hydrated ionic "prenucleation clusters" (PNCs), is increasingly invoked to explain mineral nucleation, including the formation of calcium carbonate (CaCO3) minerals in aqueous conditions, which is important in a wide range of geological and biological systems. While the existence and role of PNCs in aqueous nucleation processes remain hotly debated, we show, using in situ small-angle X-ray scattering (SAXS), that nanometer-sized clusters are present in aqueous CaCO3 solutions ranging from thermodynamically under- to supersaturated conditions regarding all known mineral phases, thus demonstrating that CaCO3 mineral formation cannot be explained solely by CNT under the conditions examined.

Improving the Measurement of Iron(III) Bioavailability in Freshwater Samples: Methods and Performance.

The toxicity of iron(III) in fresh waters has been detected at concentrations above the iron solubility limit, indicating a contribution of colloidal and particulate forms of iron(III) to the toxicity response. Current water quality guideline values for iron in fresh water are based on analytical determinations of filterable or total iron. Filtration, however, can underestimate bioavailable iron by retaining some of the colloidal fraction, and total determinations overestimate bioavailable iron measurements by recovering fractions of low bioavailability from suspended solids (e.g., iron oxides and oxyhydroxides) naturally abundant in many surface waters. Consequently, there is a need for an analytical method that permits the determination of a bioavailable iron fraction, while avoiding false-negative and false-positive results. Ideally, a measurement technique is required that can be readily applied by commercial laboratories and field sampling personnel, and integrated into established regulatory schemes. The present study investigated the performance of pH 2 and pH 4 extractions to estimate a bioavailable iron(III) fraction in synthetic water samples containing iron phases of different reactivities. The effects of aging on fresh precipitates were also studied. The total recoverable, 0.45-µm filtered, and pH 4 extractable fractions did not discriminate iron phases and age groups satisfactorily. Contrastingly, the pH 2 extraction showed specificity toward iron phases and aging (0.5-2-h interval). Extraction times above 4 h and up to 16 h equally recovered >90% of the spiked iron regardless of its age. Furthermore, <1% of the well-mineralized iron was targeted. The present study shows that a pH 2 dilute-acid extraction is a suitable candidate method to operationally define iron fractions of higher bioavailability avoiding false-negative and false-positive results. Environ Toxicol Chem 2023;42:303-316. © 2022 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Fungi increases kelp (Ecklonia radiata) remineralisation and dissolved organic carbon, alkalinity, and dimethylsulfoniopropionate (DMSP) production.

Fungi are key players in terrestrial organic matter (OM) degradation, but little is known about their role in marine environments. Here we compared the degradation of kelp (Ecklonia radiata) in mesocosms with and without fungicides over 45 days. The aim was to improve our understanding of the vital role of fungal OM degradation and remineralisation and its relevance to marine biogeochemical cycles (e.g., carbon, nitrogen, sulfur, or volatile sulfur). In the presence of fungi, 68 % of the kelp detritus degraded over 45 days, resulting in the production of 0.6 mol of dissolved organic carbon (DOC), 0.16 mol of dissolved inorganic carbon (DIC), 0.23 mol of total alkalinity (TA), and 0.076 mol of CO2, which was subsequently emitted to the atmosphere. Conversely, when fungi were inhibited, the bacterial community diversity was reduced, and only 25 % of the kelp detritus degraded over 45 days. The application of fungicides resulted in the generation of an excess amount of 1.5 mol of DOC, but we observed only 0.02 mol of DIC, and 0.04 mol of TA per one mole of kelp detritus, accompanied by a CO2 emission of 0.081 mol. In contrast, without fungi, remineralisation of kelp detritus to DIC, TA, dimethyl sulfide (DMS), dimethylsulfoniopropionate (DMSP) and methanethiol (MeSH) was significantly reduced. Fungal kelp remineralisation led to a remarkable 100,000 % increase in DMSP production. The observed substantial changes in sediment chemistry when fungi are inhibited highlight the important biogeochemical role of fungal remineralisation, which likely plays a crucial role in defining coastal biogeochemical cycling, blue carbon sequestration, and thus climate regulation.

Effects of pH, chloride, and bicarbonate on Cu(I) oxidation kinetics at circumneutral pH.

The oxidation kinetics of nanomolar concentrations of Cu(I) in NaCl solutions have been investigated over the pH range 6.5-8.0. The overall apparent oxidation rate constant was strongly affected by chloride, moderately by bicarbonate, and to a lesser extent by pH. In the absence of bicarbonate, an equilibrium-based speciation model indicated that Cu(+) and CuClOH(-) were the most kinetically reactive species, while the contribution of other Cu(I) species to the overall oxidation rate was minor. A kinetic model based on recognized key redox reactions for these two species further indicated that oxidation of Cu(I) by oxygen and superoxide were important reactions at all pH values and chloride concentrations considered, but back reduction of Cu(II) by superoxide only became important at relatively low chloride concentrations. Bicarbonate concentrations from 2 to 5 mM substantially accelerated Cu(I) oxidation. Kinetic analysis over a range of bicarbonate concentrations revealed that this was due to formation of CuCO(3)(-), which reacts relatively rapidly with oxygen, and not due to inhibition of the back reduction of Cu(II) by formation of Cu(II)-carbonate complexes. We conclude that the simultaneous oxygenation of Cu(+), CuClOH(-), and CuCO(3)(-) is the rate-limiting step in the overall oxidation of Cu(I) under these conditions.

Impact of Natural Organic Matter on H2O2-Mediated Oxidation of Fe(II) in Coastal Seawaters.

Whereas the oxidation of inorganic Fe(II) by H(2)O(2) in seawater has been well studied, the oxidation of Fe(II) complexes with natural organic matter (NOM) by this ubiquitous oxidant has received little attention. Suwannee River fulvic acid (SRFA), a proxy for terrestrial NOM, is shown to have a much smaller impact upon Fe(II) oxidation kinetics in seawater than the strong effect previously observed in freshwater conditions. However, the oxidation kinetics of Fe(II) in seawater and freshwater can be quantitatively described employing the same mechanistic kinetic model, except that the apparent formation constant of Fe(II)-SRFA complexes is substantially decreased under conditions representative of estuarine and river-influenced coastal waters. This implies that the same basic processes occur in both systems, with differences between Fe(II) oxidation kinetics in seawater and freshwater largely attributable to effects of ionic strength and matrix composition. This was confirmed with studies employing NaCl solutions with or without Mg(2+)/Ca(2+) addition demonstrating that both ionic strength and divalent cations effect a decrease in the Fe(II)-binding affinity of SRFA. The impact of NOM upon iron redox transformation kinetics is therefore greatly influenced by changes in both ionic strength and the presence of cations able to compete with Fe(II) for binding sites.

Kinetics of Cu(II) reduction by natural organic matter.

The kinetics of Cu(II) reduction by Suwannee River fulvic acid (SRFA) at concentrations from 0.25 to 8 mg L(-1) have been investigated in 2 mM NaHCO(3) and 0.7 M NaCl at pH 8.0. In the absence of oxygen, SRFA reduced Cu(II) to Cu(I) in a biphasic manner, with initial rapid formation of Cu(I) followed by a much slower increase in Cu(I) concentration over time. When present, oxygen only had a noticeable effect on Cu(I) concentrations in the second phase of the reduction process and at high [SRFA]. In both the absence and presence of oxygen, the rate of Cu(I) generation increased with increasing [SRFA]. At 8 mg L(-1) [SRFA], nearly 75% of the 0.4 µM Cu(II) initially present was reduced to Cu(I) after 20 min, although the yield of Cu(I) relative to [SRFA] decreased at [SRFA] > 1 mg L(-1). Two plausible kinetic modeling approaches were found to satisfactorily describe the experimental data over a range of [SRFA]. Despite some uncertainty as to which approach is correct, common features of both approaches were complexation of Cu(II) by SRFA and reduction of Cu(II) by two different electron donor groups within SRFA: a relatively labile electron donor (with a concentration of 1.1 × 10(-4) equiv of e(-) (g of SRFA)(-1)) that reduced Cu(II) relatively rapidly and a less labile donor (with a concentration of 3.1 × 10(-4) equiv of e(-) (g of SRFA)(-1)) that reduced Cu(II) more slowly.

Reactive oxygen species in the world ocean and their impacts on marine ecosystems.

Reactive oxygen species (ROS) are omnipresent in the ocean, originating from both biological (e.g., unbalanced metabolism or stress) and non-biological processes (e.g. photooxidation of colored dissolved organic matter). ROS can directly affect the growth of marine organisms, and can also influence marine biogeochemistry, thus indirectly impacting the availability of nutrients and food sources. Microbial communities and evolution are shaped by marine ROS, and in turn microorganisms influence steady-state ROS concentrations by acting as the predominant sink for marine ROS. Through their interactions with trace metals and organic matter, ROS can enhance microbial growth, but ROS can also attack biological macromolecules, causing extensive modifications with deleterious results. Several biogeochemically important taxa are vulnerable to very low ROS concentrations within the ranges measured in situ, including the globally distributed marine cyanobacterium Prochlorococcus and ammonia-oxidizing archaea of the phylum Thaumarchaeota. Finally, climate change may increase the amount of ROS in the ocean, especially in the most productive surface layers. In this review, we explore the sources of ROS and their roles in the oceans, how the dynamics of ROS might change in the future, and how this change might impact the ecology and chemistry of the future ocean.

Crop fertilisation potential of phosphorus in hydrochars produced from sewage sludge.

Sewage sludges are a rich underused source of phosphorus (P) which contributes to environmental degradation, yet if recaptured, could return significant amounts of P to agricultural systems. Hydrothermal carbonisation (HTC) can efficiently recover P, with the added ability to transform P species into potentially more desirable forms for direct application to crops. P dynamics in hydrochars have primarily examined P speciation and chemical extractability as indicators of P bioavailability, but few studies directly evaluate the agronomic effectiveness of hydrochars as P fertilisers. As such, there is a clear need to assess the suitability of hydrochar as a source of bioavailable P in plant systems and the influence of HTC synthesis conditions. Response Surface Modelling of HTC synthesis conditions (pH, temperature and time), revealed initial pH significantly influence P distribution. Mild conditions of 180 °C for 30 min at pH 8.0 maximised P recovery (99%) along with carbon (62%) and nitrogen (43%) in hydrochars. Systematic characterisation of hydrochar P by chemical extraction and P L2,3-edge X-ray absorption near edge spectroscopy revealed H2O, NaHCO3 and NaOH- P fractions were significantly (p < 0.05) reduced in all hydrochars, while HCl-P fraction increased with HTC temperatures at pH 7. In contrast, P L2,3-edge XANES spectra were remarkably similar in raw sludges and corresponding hydrochars, regardless of HTC temperature or pH, revealing P was predominantly present as ferric phosphate with some hydroxyapatite. Multiple linear regression modelling suggested a significant relationship between chemical extractability and P bioavailability to wheat present in the raw sludges and hydrochars. This research provides further insight into the potential to use hydrothermal treatment for recovery and agricultural reuse of P, the importance of operational conditions on P transformation and the relationship between P speciation and bioavailability. The value of sewage sludge in a more sustainable global P cycle is also highlighted.

Phthalhydrazide chemiluminescence method for determination of hydroxyl radical production: modifications and adaptations for use in natural systems.

The hydroxyl radical is formed through a variety of processes pertinent to natural and anthropogenic systems. Here we report development of a simple and sensitive trap-and-trigger chemiluminescence method based upon the hydroxylation of phthalhydrazide to 5-hydroxy-2,3-dihydro-1,4-phthalazinedione, which emits chemiluminescence when oxidized under alkaline conditions. Cu(III) is employed as an oxidant and is shown to be relatively insensitive to a range of interferences likely to be encountered. The method has been standardized by use of γ-radiolysis of water as a primary source of hydroxyl radical, with a convenient secondary calibration procedure developed that uses Fenton chemistry. Detection limits varied from 7.4 nM (at pH 3) and 6.2 nM (at pH 8.1) of accumulated hydroxyl radical production in a simple 10 mM NaCl matrix to around 30 nM in an artificial seawater medium, due to competition for hydroxyl between the phthalhydrazide probe and bromide. The method has been used to characterize the kinetics of the Fenton system employed for calibration and is shown to be consistent with published models of this process over time scales of several hours. The application of this method to a range of matrices and for photochemical studies is also described.

Iron uptake by toxic and nontoxic strains of Microcystis aeruginosa.

Iron uptake by microcystin-producing and non-microcystin-producing strains of Microcystis aeruginosa was investigated through short-term uptake assays. Although strain-specific differences were observed, the siderophore-independent Fe uptake kinetics were essentially similar (e.g., maximum uptake rates of 2.0 to 3.3 amol·cell(-1)·h(-1)) for the wild-type toxic strain PCC7806 and a genetically engineered mutant unable to produce microcystin.

Oxic and Anoxic Organic Polymer Degradation Potential of Endophytic Fungi From the Marine Macroalga, Ecklonia radiata.

Cellulose and chitin are the most abundant polymeric, organic carbon source globally. Thus, microbes degrading these polymers significantly influence global carbon cycling and greenhouse gas production. Fungi are recognized as important for cellulose decomposition in terrestrial environments, but are far less studied in marine environments, where bacterial organic matter degradation pathways tend to receive more attention. In this study, we investigated the potential of fungi to degrade kelp detritus, which is a major source of cellulose in marine systems. Given that kelp detritus can be transported considerable distances in the marine environment, we were specifically interested in the capability of endophytic fungi, which are transported with detritus, to ultimately contribute to kelp detritus degradation. We isolated 10 species and two strains of endophytic fungi from the kelp Ecklonia radiata. We then used a dye decolorization assay to assess their ability to degrade organic polymers (lignin, cellulose, and hemicellulose) under both oxic and anoxic conditions and compared their degradation ability with common terrestrial fungi. Under oxic conditions, there was evidence that Ascomycota isolates produced cellulose-degrading extracellular enzymes (associated with manganese peroxidase and sulfur-containing lignin peroxidase), while Mucoromycota isolates appeared to produce both lignin and cellulose-degrading extracellular enzymes, and all Basidiomycota isolates produced lignin-degrading enzymes (associated with laccase and lignin peroxidase). Under anoxic conditions, only three kelp endophytes degraded cellulose. We concluded that kelp fungal endophytes can contribute to cellulose degradation in both oxic and anoxic environments. Thus, endophytic kelp fungi may play a significant role in marine carbon cycling via polymeric organic matter degradation.

Oxygen and superoxide-mediated redox kinetics of iron complexed by humic substances in coastal seawater.

Complexes with terrestrially derived humic substances represent one of the most reactive pools of dissolved Fe in natural waters. In this work, redox kinetics of Fe-humic substance complexes (FeL) in simulated coastal seawater were investigated using chemiluminescence techniques with particular attention given to interactions with dioxygen (O2) and superoxide (O2•-). Although rate constants of FeIIL oxidation by O2 (5.6-52 M-1 · s-1) were 4-5 orders of magnitude less than those for O2•- (6.9-23 × 105 M-1 · s-1),O2 is likely to outcompete O2•- for FeIIL oxidation in coastal seawaters where steady-state O2•- concentrations are generally subnanomolar. Rate constants for FeIIIL reduction by O2•- of 1.8-5.6 × 104 M-1 · s-1 were also determined. From the balance of FeIIL oxidation rates and O2•- -mediated FeIIIL reduction rates, steady-state FeIIL concentrations were estimated to be in the subpicomolar to picomolar range, which is generally lower than measured in situ Fe(II) concentrations under relevant conditions. This suggests that (i) processes other than O2•- -mediated reduction (such as photochemical ligand-to-metal charge transfer) may be responsible for Fe(II) formation, (ii) the in situ ligands differ significantly from the humic substances used in this work, and/ or (iii) the influence of other environmental factors such as pH and temperature on Fe redox kinetics may have to be considered.

Sorption of phosphate and silicate alters dissolution kinetics of poorly crystalline iron (oxyhydr)oxide.

Iron (oxyhydr)oxides (FeOx) control retention of dissolved nutrients and contaminants in aquatic systems. However, FeOx structure and reactivity is dependent on adsorption and incorporation of such dissolved species, particularly oxyanions such as phosphate and silicate. These interactions affect the fate of nutrients and metal(loids), especially in perturbed aquatic environments such as eutrophic coastal systems and environments impacted by acid mine drainage. Altered FeOx reactivity impacts sedimentary nutrient retention capacity and, eventually, ecosystem trophic state. Here, we explore the influence of phosphate (P) and silicate (Si) on FeOx structure and reactivity. Synthetic, poorly crystalline FeOx with adsorbed and coprecipitated phosphate or silicate at low but environmentally relevant P/Fe or Si/Fe ratios (0.02-0.1 mol mol-1) was prepared by base titration of Fe(III) solutions. Structural characteristics of FeOx were investigated by X-ray diffraction, synchrotron-based X-ray absorption spectroscopy and high-energy X-ray scattering. Reactivity of FeOx was assessed by kinetic dissolution experiments under acidic (dilute HCl, pH 2) and circum-neutral reducing (bicarbonate-buffered ascorbic acid, pH 7.8, Eh ∼ -300 mV) conditions. At these loadings, phosphate and silicate coprecipitation had only slight impact on local and intermediate-ranged FeOx structure, but significantly enhanced the dissolution rate of FeOx. Conversely, phosphate and silicate adsorption at similar loadings resulted in particle surface passivation and decreased FeOx dissolution rates. These findings indicate that varying nutrient loadings and different interaction mechanisms between anions and FeOx (adsorption versus coprecipitation) can influence the broader biogeochemical functioning of aquatic ecosystems by impacting the structure and reactivity of FeOx.

The characterization of iron (III) in seawater and related toxicity to early life stages of scleractinian corals.

Currently toxicity data for iron (Fe) in seawater are limited; furthermore, these data are of poor quality as a result of the importance of Fe solubility in test solutions being overlooked. The present study characterized the solubility and lability of Fe(III) in seawater and then examined the effects of Fe(III) on the fertilization success and larval survival of the tropical marine scleractinian corals Acropora spathulata and Platygyra daedalea. We present the first assessment of the effects of Fe on the early life stages of scleractinian corals. Concentrations of both soluble and labile forms of Fe were very low, with dissolved Fe concentrations ≤0.195 mg/L in bioassay test solutions and chemical determinations revealing labile Fe concentrations ≤1.21 mg/L. For fertilization experiments, the median effect concentration (EC50) value for total Fe was 25 mg/L for the most sensitive species, P. daedalea, whereas the EC50 values for A. spathulata ranged between 40 and 66 mg/L. The median lethal concentration value for P. daedalea larval survival was 47 mg/L Fe after 72-h exposure. We provide Fe toxicity data for tropical marine keystone species that could be used to help generate more reliable guideline values for Fe in marine waters. Environ Toxicol Chem 2018;37:1104-1114. © 2017 SETAC.

Production of reactive oxygen species on photolysis of dilute aqueous quinone solutions.

We have examined the generation of the reactive oxygen species (ROS) superoxide and hydrogen peroxide (H2O2) by irradiation of dilute aqueous solutions of disodium anthraquinone-2-6-disulfonate (AQDS) with simulated sunlight. Irradiating a solution of AQDS in 2 mM NaHCO3 and 0.01 M NaCl produced superoxide and H2O2 at nanomolar concentrations. Experiments in which initial concentrations of dioxygen, H2O2, the superoxide radical trap nitroblue tetrazolium and the electron donor dimethyl sulfoxide were varied suggested that the interaction of solvent water with photo-excited quinone moieties produces dioxygen-reducing radicals, and that these are the primary source of ROS in the system. A kinetic model for ROS production is proposed based on our experimental data.