Long-term temporal variability in hydrogen peroxide concentrations in Wilmington, North Carolina USA rainwater

Promotion of Humic Acid Transformation by Abiotic and Biotic Fe Redox Cycling in Nontronite

The redox activity of Fe-bearing minerals is coupled with the transformation of organic matter (OM) in redox dynamic environments, but the underlying mechanism remains unclear. In this work, a Fe redox cycling experiment of nontronite (NAu-2), an Fe-rich smectite, was performed via combined abiotic and biotic methods, and the accompanying transformation of humic acid (HA) as a representative OM was investigated. Chemical reduction and subsequent abiotic reoxidation of NAu-2 produced abundant hydroxyl radicals (thereafter termed as ·OH) that effectively transformed the chemical and molecular composition of HA. More importantly, transformed HA served as a more premium electron donor/carbon source to couple with subsequent biological reduction of Fe(III) in reoxidized NAu-2 by Geobacter sulfurreducens, a model Fe-reducing bacterium. Destruction of aromatic structures and formation of carboxylates were mechanisms responsible for transforming HA into an energetically more bioavailable substrate. Relative to unaltered HA, transformed HA increased the extent of the bioreduction by 105%, and Fe(III) reduction was coupled with oxidation and even mineralization of transformed HA, resulting in bleached HA and formation of microbial products and cell debris. ·OH transformation slightly decreased the electron shuttling capacity of HA in bioreduction. Our results provide a mechanistic explanation for rapid OM mineralization driven by Fe redox cycling in redox-fluctuating environments.

The harmful cyanobacterium Microcystis aeruginosa PCC7806 is more resistant to hydrogen peroxide at elevated CO2

Rising atmospheric CO2 can intensify harmful cyanobacterial blooms in eutrophic lakes. Worldwide, these blooms are an increasing environmental concern. Low concentrations of hydrogen peroxide (H2O2) have been proposed as a short-term but eco-friendly approach to selectively mitigate cyanobacterial blooms. However, sensitivity of cyanobacteria to H2O2 can vary depending on the available resources. To find out how cyanobacteria respond to H2O2 under elevated CO2, Microcystis aeruginosa PCC 7806 was cultured in chemostats with nutrient-replete medium under C-limiting and C-replete conditions (150 ppm and 1500 ppm CO2, respectively). Microcystis chemostats exposed to high CO2 showed higher cell densities, biovolumes, and microcystin contents, but a lower photosynthetic efficiency and pH compared to the cultures grown under low CO2. Subsamples of the chemostats were treated with different concentrations of H2O2 (0-10 mg·L-1 H2O2) in batch cultures under two different light intensities (15 and 100 μmol photons m-2·s-1) and the response in photosynthetic vitality was monitored during 24 h. Results showed that Microcystis was more resistant to H2O2 at elevated CO2 than under carbon-limited conditions. Both low and high CO2-adapted cells were more sensitive to H2O2 at high light than at low light. Microcystins (MCs) leaked out of the cells of cultures exposed to 2-10 mg·L-1 H2O2, while the sum of intra- and extracellular MCs decreased. Although both H2O2 and CO2 concentrations in lakes vary in response to many factors, these results imply that it may become more difficult to suppress cyanobacterial blooms in eutrophic lakes when atmospheric CO2 concentrations continue to rise.

Interference of dissolved organic matter and its constituents on the accurate determination of hydrogen peroxide in water

Hydrogen peroxide (H₂O₂) is ubiquitous in natural waters, and plays an important role in both biological and chemical processes. This study investigated the influence of dissolved organic matter (DOM) and its substituents on the accurate measurement of H₂O₂ by peroxidase-mediated depletion of scopoletin fluorescence method which is one of the most widely used methods for the determination of low concentration H₂O₂ in water. Six DOM and its 24 substituents interfered the determination of H₂O₂ at environmentally relevant concentration of 200 nM with different levels except 2,6-dimethoxy-1,4-benzoquinone and benzoic acid, which may be associated with origin and types of DOM, and numbers and position of active functional groups in DOM constituents. Each substance concentration and the corresponding decreasing ratio to the measured H₂O₂ concentration was fitted well to the linear model (R² > 0.9), and the obtained interfering ratios (k, (mgC L⁻¹)⁻¹), expressing the degree of DOM or its substituents per unit concentration to the measurement of H₂O₂, were approximate for DOM, but the order of magnitude of k values of DOM constituents took on a large span from 10^–3 to 10^–7. When DOM levels exceed 0.1 mgC L⁻¹ or its substituent concentration is at nM level (low to 20 nM), the H₂O₂ content will be underestimated substantially. A quantitative structure–activity relationship model with remarkable stability and strong predictability for the k of DOM substituents to H₂O₂ measurement was established, and the k was related to the electron transfer capacity, hydrophobicity and stability of these compounds.

Precipitation of hydrogen peroxide during winter storms and summer typhoons

Hydrogen peroxide (H₂O₂) affects the activity of microbes, including archaea, and thereby influences the biogeochemical cycles of critical elements in marine and terrestrial environments. In this study, we measured the levels of H₂O₂ associated with three classes of extreme wet precipitation events: winter storms, tropical storms, and typhoons. In conjunction with precipitation data, the measured H₂O₂ concentration in a seawater reservoir receiving precipitation was used to estimate rainwater H₂O₂ concentration and flux. The rainwater H₂O₂ concentration during winter storms and coexisting storms (storms having combined maritime and continental origins) was a factor of 2-3 higher than the levels observed during the typhoons. Fluxes of H₂O₂ in rainwater of 6 μM min^-1 or greater resulted in H₂O₂ concentrations ~1 μM in the seawater reservoir. During all precipitation events, the H₂O₂ concentration in the seawater reservoir was dominated by wet precipitation and reached levels greater than would be produced in situ by photochemical processes. During winter and coexisting storms, the rainwater H₂O₂ concentrations were likely to have been enhanced by atmospheric photochemical reactions probably involving pollutants. An increase in the H₂O₂ concentration in surface aqueous environments during extreme precipitation events will directly affect the microbial cycling of nitrogen and organic carbon.

Characterization of reactive photoinduced species in rainwater

Rainfall is a highly effective and important carrier that can remove a majority of aerosol mass into land and marine ecosystems. The photochemically formed reactive species in the rainwater are likely dominant oxidants for organic and inorganic substances. Here, we collected rainwater samples from Oct. 2016 to Dec. 2016 in CUG campus (Wuhan, Hubei, China) and measured their formation rates, lifetimes, steady-state concentrations, and apparent quantum yields of reactive photoinduced species, including hydroxyl radical (HO•), H₂O₂, singlet oxygen (¹O₂), and chromophoric dissolved organic matter triplet state (³CDOM*) in the laboratory. Results showed that rainwater samples contained photochemical sources, like DOM, nitrate, heavy metals, etc. Quantification of HO• showed that r_HO• (the photogeneration rate of HO•) were in the range of 1.05 × 10^-10-4.56 × 10^-10 M s^-1, and [•OH]ss (the steady-state concentrations of OH•) were of 4.06 × 10^-18-2.97 × 10^-17 M for the three samples. Further investigations revealed that 10-24% of r_•OH was attributed to nitrate photolysis, suggesting DOM was possibly the prevailing source of HO•. Apparent quantum yields of H₂O₂ (Φ_H2O2) correlated negatively with E₂/E₃ (the ratio of absorption at 250 and 365 nm), while Φ_1O2 and Φ_3CDOM* increased with elevated E₂/E₃.

Variability of ethanol concentration in rainwater driven by origin versus season in coastal and inland North Carolina, USA

Rainwater ethanol concentrations were measured for one year (June 2013-May 2014) in central (Elon, NC) and coastal (Wilmington, NC) North Carolina, allowing for a comparison of the effects of coastal and marine rain on ethanol concentration and deposition both at the coast and 250 km inland. Rain samples were collected on an event basis and analyzed using enzyme oxidation and headspace solid-phase microextraction (HS-SPME). The volume-weighted average ethanol concentration at Elon (609 ± 116 nM) was higher than at Wilmington (208 ± 21 nM). Rainfall influenced by air masses originating over the Atlantic Ocean has previously been observed to be lower in ethanol concentration than terrestrial rain at the Wilmington location, and this was true during this study as well. Lower-ethanol marine and coastal air masses did not affect the concentration of ethanol in Elon rain, 250 km from the coast. This is likely due to the rapid supply of locally emitted ethanol to air masses moving over the land. No difference in rainwater ethanol concentrations was observed for Elon rain based on air mass back trajectories, most likely because all the rain was impacted by both anthropogenic and biogenic terrestrial sources typical of most inland areas. Seasonal variation in ethanol concentrations was significant in the inland location with elevated ethanol concentrations observed in fall; no seasonal variation was observed in coastal location rain. This study presents for the first time the different drivers for ethanol concentrations in rainwater from a coastal and a proximal inland location.

A 13-year study of dissolved organic carbon in rainwater of an agro-industrial region of São Paulo state (Brazil) heavily impacted by biomass burning

This work presents the first comprehensive study of DOC in rainwater in a tropical agro-industrial region in central São Paulo State. The DOC concentrations ranged from 15 to 4992μmolCL^-1, with an overall volume weighted mean (VWM) of 288±17μmolCL^-1 (n=881). The number of fire spots accumulated within each year of this study was positively correlated to the VWM concentration of DOC in rainwater. During the whole study period, higher VWM DOC concentrations were found during the dry months, despite the phasing out of agricultural fires in sugar cane plantations. The evidence suggested that inputs of atmospheric soluble organic carbon from biomass burning exceeded those from vehicular fuel combustion and biogenic sources. In most cases, dilution of DOC according to precipitation volume was minimal, showing that in-cloud processes were dominant for this species. In contrast, most of the volatile dissolved organic carbon (VDOC) appeared to be removed from the atmosphere in the first milliliter or so of rain, showing a dominance of below-cloud scavenging. VDOC contributed a significant fraction of the DOC for 62% of the samples analyzed, ranging from 5.1 to 488μmolCL^-1 (n=552). The average wet deposition flux of DOC was 49kgCha^-1yr^-1, with VDOC accounting for 10% of the total. This dissolved carbon flux is higher than the estimated world average (34kgCha^-1yr^-1). The DOC in the rainwater was mostly labile (75% on average) and rapidly bioavailable (within days to weeks), in contrast to refractory dissolved carbon found in rainwater from regions where fossil fuel combustion is the dominant source. The findings of this work indicate that biomass burning can lead to important atmospheric inputs of readily available organic matter to land and to the open ocean.

Production of Hydrogen Peroxide in Groundwater at Rifle, Colorado

The commonly held assumption that photodependent processes dominate H₂O₂ production in natural waters has been recently questioned. Here, we present evidence for the unrecognized and light-independent generation of H₂O₂ in groundwater of an alluvial aquifer adjacent to the Colorado River near Rifle, CO. In situ detection using a sensitive chemiluminescent method suggests H₂O₂ concentrations ranging from lower than the detection limit (<1 nM) to 54 nM along the vertical profiles obtained at various locations across the aquifer. Our results also suggest dark formation of H₂O₂ is more likely to occur in transitional redox environments where reduced elements (e.g., reduced metals and NOM) meet oxygen, such as oxic-anoxic interfaces. A simplified kinetic model involving interactions among iron, reduced NOM, and oxygen was able to reproduce roughly many, but not all, of the features in our detected H₂O₂ profiles, and therefore there are other minor biological and/or chemical controls on H₂O₂ steady-state concentrations in such aquifer. Because of its transient nature, the widespread presence of H₂O₂ in groundwater suggests the existence of a balance between H₂O₂ sources and sinks, which potentially involves a cascade of various biogeochemically important processes that could have significant impacts on metal/nutrient cycling in groundwater-dependent ecosystems, such as wetlands and springs. More importantly, our results demonstrate that reactive oxygen species are not only widespread in oceanic and atmospheric systems but also in the subsurface domain, possibly the least understood component of biogeochemical cycles.

Dissolved organic and inorganic matter in bulk deposition of a coastal urban area: an integrated approach

Bulk deposition can remove atmospheric organic and inorganic pollutants that may be associated with gaseous, liquid or particulate phases. To the best of our knowledge, few studies have been carried out, which simultaneously analyse the presence of organic and inorganic fractions in rainwater. In the present work, the complementarity of organic and inorganic data was assessed, through crossing data of some organic [DOC (dissolved organic carbon), absorbance at 250 nm (UV250nm), integrated fluorescence] and inorganic [H(+), NH4(+), NO3(-), non sea salt sulphate (NSS-SO4(2-))] parameters measured in bulk deposition in the coastal urban area of Aveiro. The organic and inorganic parameters analysed were positively correlated (p<0.001) except for H(+), which suggests that a constant fraction of chromophoric dissolved organic matter (CDOM) came from anthropogenic sources. Furthermore, the inverse correlations observed for the organic and inorganic parameters with the precipitation amount suggest that organic and inorganic fractions were incorporated into the rainwater partially by below-cloud scavenging of airborne particulate matter. This is in accordance with the high values of DOC and NO3(-) found in samples associated with marine air masses, which were linked in part to the contribution of local emissions from vehicular traffic. DOC of bulk deposition was the predominant constituent when compared with the constituents H(+), NH4(+), NO3(-) and NSS-SO4(2-), and consequently bulk deposition flux was also highest for DOC, highlighting the importance of DOC and of anthropogenic ions being simultaneously removed from the atmosphere by bulk deposition. However, it was verified that the contribution of anthropogenic sources to the DOC of bulk deposition may be different for distinct urban areas. Thus, it is recommended that organic and inorganic fractions of bulk deposition are studied together.

Controls on the redox potential of rainwater

Hydrogen peroxide acting as a reductant affects the redox potential of rainwater collected at the Bermuda Atlantic Time Series Station, the South Island of New Zealand, the contiguous USA, and the primary study site in Wilmington, NC. Analytical measurements of both halves of redox couples for dissolved iron, mercury, and the nitrate-nitrite-ammonium system can predict the rainwater redox potential measured directly by a platinum electrode. Measurements of these redox couples along with the pH in rain yields pe⁻ between 8 and 11; the half reaction for hydrogen peroxide acting as a reductant using typical rainwater conditions of 15 μM H₂O₂ at pH 4.7 gives pe⁻ = 9.12, where pe⁻ = negative log of the activity of hydrated electrons. Of the six rainwater redox systems investigated, only manganese speciation appeared to be controlled by molecular oxygen (pe⁻ = 15.90). Copper redox speciation was consistent with superoxide acting as a reductant (pe⁻ = 2.7). The concentration of H₂O₂ in precipitation has more than doubled over the preceding decade due to a decrease in SO₂ emissions, which suggests the redox chemistry of rainwater is dynamic and changing, potentially altering the speciation of many organic compounds and trace metals in atmospheric waters.

Applicability of hydrogen peroxide in brown tide control - culture and microcosm studies

Brown tide algal blooms, caused by the excessive growth of Aureococcus anophagefferens, recur in several northeastern US coastal bays. Direct bloom control could alleviate the ecological and economic damage associated with bloom outbreak. This paper explored the effectiveness and safety of natural chemical biocide hydrogen peroxide (H(2)O(2)) for brown tide bloom control. Culture studies showed that H(2)O(2) at 1.6 mg L(-1) effectively eradicated high density A. anophagefferens within 24-hr, but caused no significant growth inhibition in the diatoms, prymnesiophytes, green algae and dinoflagellates of >2-3 μm cell sizes among 12 phytoplankton species tested over 1-week observation. When applied to brown tide bloom prone natural seawater in a microcosm study, this treatment effectively removed the developing brown tide bloom, while the rest of phytoplankton assemblage (quantified via HPLC based marker pigment analyses), particularly the diatoms and green algae, experienced only transient suppression then recovered with total chlorophyll a exceeding that in the controls within 72-hr; cyanobacteria was not eradicated but was still reduced about 50% at 72-hr, as compared to the controls. The action of H(2)O(2) against phytoplankton as a function of cell size and cell wall structure, and a realistic scenario of H(2)O(2) application were discussed.