Photochemical decomposition rates of pteridines and flavins in seawater exposed to surface solar radiation

Rates of photochemical decomposition of pteridines and flavins, known to occur in reef waters of the Central Great Barrier Reef, Australia, were determined by reverse-phase liquid chromatography with fluorometric detection. The fluorescent transformation products of photodecomposition were identified. First-order photochemical rates were determined for pteridines and flavins exposed to surface solar radiation under aseptic conditions at ambient seawater temperature. Kinetic rates were determined in seawater and compared to photochemical rates determined in distilled water or distilled water with added divalent metal ions (Cu²⁺, Fe²⁺ Mn²⁺ and Mg²⁺).All pteridines were more stable in seawater than distilled water when exposed to solar radiation. With the exception of folic acid, addition of Cu²⁺ enhanced the stability of these pteridines, suggesting that natural concentrations of divalent metal ions may be responsible for diminished photochemical decomposition rates in seawater.Riboflavin and riboflavin-5′-phosphate in seawater solution were extremely sensitive to sunlight. The products of riboflavin photochemical decomposition, lumiflavin and lumichrome, proved relatively refractory to solar irradiation.     

Investigations of iron coordination and redox reactions in seawater using Fe radiometry and ion-pair solvent extraction of amphiphilic iron complexes

Iron coordination and redox reactions in synthetic and coastal seawater were investigated at nanomolar concentrations using ⁵⁹Fe radiometry and ion-pair solvent extraction of iron chelated by sulfoxine (8-hydroxyquinoline-5-sulfonate) and BPDS (bathophenanthroline disulfonate). Using sulfoxine, we determined the rate at which the monomeric Fe(III) hydroxide species present in seawater of pH 8 are complexed by the microbial siderophore deferriferrioxamine B and the synthetic chelator EDTA (ethylenediaminetetraacetic acid). Forward rate constants of 2 × 10⁶M⁻¹s⁻¹ and 20 M⁻¹s⁻¹, respectively, were obtained. The kinetics of these reactions have not been measured previously at pH values near that of seawater. Conditional equilibrium constants measured for the Fe(III)-EDTA system are consistent with published stability constants for EDTA complexes and for Fe(III) hydrolytic equilibria minus the neutral Fe(OH)₃^o species, suggesting it is not quantitatively significant near pH 8. Commercial humic acid was found to have sufficient affinity for iron to compete with Fe(III) hydrolysis in seawater, and limited evidence was obtained for an interaction with dissolved organic matter in coastal seawater.In our investigations of redox reactions using BPDS to trap Fe(II) produced in the medium, we observed enhanced photoreduction of Fe(III) by humic acid as well as reduction induced by solutes released from phytoplankton in seawater of pH 8. Although the method is sensitive enough to work at near-oceanic levels of iron, the difficulty in distinguishing Fe(II) generated by Fe(III)-BPDS interactions from Fe(II) produced by other means limits its utility. This analytical ambiguity may be generalizable to other methods which measure ferrous iron in seawater using Fe(II)-specific ligands.     

Mass solubility and aqueous activity coefficients of stable organic chemicals in the marine environment: polychlorinated biphenyls

The solubilities and aqueous activity coefficients of polychlorinated biphenyls were measured in distilled and saline water (30? salinity). Solubilities in distilled water ranged from 3 · 10^?4 g/l for dichlorobiphenyls to 6 · 10^?6 g/l for heptachlorobiphenyls; values in artificial seawater were about five times lower than the corresponding values in distilled water. In both cases, the solubilities decreased regularly with increasing degree of chlorination. The corresponding activity coefficients are inversely proportional to the chlorine content and range from 4 · 10⁷ to 4 · 10⁹ in distilled water and from 3 · 10⁸ to 1.5 · 10¹⁰ in saline water. Both the solubilities and activity coefficients agree well with those predicted from additivity considerations. The physical chemical aspects discussed in this paper can be applied in determining the solubility behavior of other stable organic molecules in the marine environment.     

海水中锰的光化学反应研究

对海水中锰的光化学反应及其影响因素进行了研究.实验结果表明,锰的光化学反应主要通过有机物为媒介进行,反应液中加入的有机物种类和浓度的改变会导致锰的光化学反应速率的改变.增加光强,有利于锰的光还原反应的进行.降低体系的pH值,可提高锰的光反应速率.锰在不同介质中光反应速率从大到小的顺序为:去离子水、人工海水、天然海水.此外,搅拌有利于锰的光反应的进行,但在体系分布已达均匀的前提下,搅拌速率的大小对锰的光反应速率几乎无影响.研究表明,通过光化学反应,海水中的锰会由四价的颗粒态转化为二价的可溶态,从而有利于浮游植物的吸收和生长.     

亚甲基蓝在水体系中的光化学降解研究

研究水体系中亚甲基蓝的光化学降解.结果表明,在高压汞灯照射下,亚甲基蓝在人工海水中降解得最快,蒸馏水次之,而在天然海水中降解得最慢.通过对比研究发现.重金属离子(Cu2+,Zn2+,Cd2+,Hg2+)和腐殖酸能够在一定程度上抑制亚甲基蓝的光降解;而丙酮能促进亚甲基蓝的光降解.由此可见,重金属离子和腐殖酸可能是造成亚甲基蓝在天然海水中降解缓慢的主要因素之一.     

Coprecipitation of zinc with silica in seawater and in distilled water

Dissolved silica can coprecipitate with zinc from seawater or distilled water that has been enriched with both elements. More than 2 ppm Si are necessary for the reaction to begin. The coprecipitation shows pH dependence. The addition of pulverized illite or natural sediment as suspended particulate material does not enhance the reaction in seawater. The organic material present in the nearshore seawater samples decreases the rate and extent of reaction, as indicated by comparisons of results of experiments using natural seawater with results obtained using UV-irradiated seawater. In unbuffered distilled water the reaction must compete with hydrolysis of zinc; however, reaction does occur, which indicates that the seawater matrix is not essential for the reaction. The coprecipitation can limit the concentration of zinc in seawater to less than the solubility concentration assumed for ZnCO₃ or Zn(OH)₂. The results suggest that a zinc silicate can precipitate directly from seawater or interstitial water as an authigenic mineral.     

Photochemical formation of glyoxylic and pyruvic acids in seawater

Glyoxylic and pyruvic acids were formed when filter-sterilized seawater was exposed to solar radiation. Production rates varied from samples collected from distinctly different regions of the sea. Humic-rich seawater from the Florida Bay exhibited net photochemical production rates (glyoxylate, 27.5 nM/W-h m⁻²; pyruvate, 12.9 nM/W-h m⁻²) that were significantly greater than net production rates for humic-poor water (glyoxylate, 3.1 nM/W-h m⁻²; pyruvate, 1.8 nM/W-h m⁻²) collected in the Gulf Stream. When seawater was not filtered, the concentrations of glyoxylate and pyruvate were found to undergo diurnal variations resulting from an imbalance between biological and photochemical processes.A depth profile of the glyoxylate concentration from several oceanic stations showed a pronounced daytime maximum in the upper 10 m; this finding is consistent with laboratory results that demonstrated that glyoxylate is formed photochemically in seawater. Pyruvate, in contrast, showed no clear trend with depth; its distribution in the water column may be primarily controlled by biological processes rather than by photochemical processes.Biological processes are generally thought to control the spatial and temporal distribution of simple organic metabolites in seawater. Our results show that photochemical processes may also be important in the marine cycling of some biochemical compounds.     

Photochemical dissolution of radionuclides from marine sediment

Laboratory experiments were carried out to assess the role of photochemical reactions upon the dissolution of ^{239 + 240}pu and ²⁴¹Am from marine sediment in sea water. Supplementary information was obtained by comparing their behaviour with that of ⁵⁴Mn under similar experimental conditions. Irradiation from natural sunlight resulted in more than a ten-fold increase in the extent of ^{239 + 240}Pu desorption relative to that observed in the dark. Remobilisation of ⁵⁴Mn from sediment was also enhanced by natural sunlight, albeit to a lesser extent than ^{239 + 240}Pu, whilst the behaviour of ²⁴¹Am was largely unaffected. Data for concentrations of dissolved ^{239 + 240}Pu(IV) and ^{239 + 240}Pu(V) species showed that only the oxidised form was significantly affected by irradiation, indicating remobilisation occurs as a result of photooxidation reactions. Further experiments were carried out using artificial light sources to establish the influence of wavelength. Data from these investigations indicated ^{239 + 240}Pu photooxidation (hence desorption) was a function of both light intensity and wavelength. Remobilisation decreased concomitant with light intensity but increased as the wavelength decreased. Similar trends were observed for photoreduction of ⁵⁴Mn, although differences were less pronounced than those observed for ^{239 + 240}Pu.     

Photochemical oxidation of dimethylsulfide in seawater

Dimethylsulfide(DMS) is generally thought to be lost from the surface oceans by evasion into the atmosphere as well as consumption by microbe.However,photochemical process might be important in the removal of DMS in the oceanic photic zone.A kinetic investigation into the photochemical oxidation of DMS in seawater was performed.The photo-oxidation rates of DMS were influenced by various factors including the medium,dissolved oxygen,photosensitizers,and heavy metal ions.The photo-oxidation rates of DMS were higher in seawater than in distilled water,presumably due to the effect of salinity existing in seawater.Three usual photosensitizers(humic acid,fulvic acid and anthroquinone),especially in the presence of oxygen,were able to enhance the photo-oxidation rate of DMS,with the fastest rate observed with anthroquinone.Photo-oxidation of DMS followed first order reaction kinetics with the rate constant ranging from 2.5×10-5 to 34.3×10-5 s-1.Quantitative analysis showed that approximately 32% of the photochemically removed DMS was converted to dimethylsulfoxide.One of the important findings was that the presence of Hg2 could markedly accelerate the photo-oxidation rate of DMS in seawater.The mechanism of mercuric catalysis for DMS photolysis was suggested according to the way of CTTM(charge transfer to metal) of DMS-Hg2 complex.     

Photochemical production of molecular hydrogen in lake water and coastal seawater

Samples of lake water and coastal seawater from Nova Scotia, Canada, were irradiated with natural or artificial sunlight to investigate the potential for photochemical hydrogen production. Hydrogen photo-production was observed in all natural water samples. Rates of hydrogen formation were highest in coloured lake water (range: 98–163 pmol L^− 1h^− 1) and lower in seawater (range: 19–45 pmol L^− 1 h ^− 1). Dilutions of the most highly coloured lake sample (Kejimkujik Lake) showed a positive linear relationship between H₂ production rates and CDOM concentration. Photo-production rates normalised to UV absorption coefficients at 350 nm indicated that the photochemical efficiency of hydrogen formation varied between samples, perhaps due to differences in the CDOM composition. Photochemical hydrogen formation was also seen in solutions of syringic acid and acetaldehyde: two low-molecular-weight carbonyl compounds found in natural waters. Photochemistry may therefore offer least a partial explanation for the persistently high levels of hydrogen observed in the low-latitude surface ocean.     

The aqueous photochemical transformation of acrylic acid

Acrylic acid in sea water is thought to occur mostly as the product of microbial cleavage of dimethylsulphoniopropionate (DMSP), but could also be a pollutant introduced by waste waters of the organic chemical industry. Solutions of acrylic acid in natural and artificial sea water, and distilled and riverine water were photolyzed using a photochemical reactor and exposure to sunlight. The transformation of acrylic acid comprises the decarboxylation of the carboxylic group and subsequent polymerization to a polyethylene type molecule. Kinetic studies showed the lowest reaction rate in distilled water and somewhat higher and very similar rates in other aqueous media. The approximately similar reaction rates in all natural waters studied suggest that inorganic ions, especially Na⁺, Mg²⁺ and halides, and dissolved organic matter (probably humics) enhance the reaction rates. On studying the influence of different concentration ranges on the reaction kinetics, an exponential increase of rates with decreasing concentration was found. The reaction rate in the sea water solution in field conditions is rather slow. In thirty days exposure about 15% of the reactant was transformed. This reaction seems to be important in the marine environment in specific conditions, especially in phycospheres and macroaggregates where higher concentrations of acrylic acid inhibit the bacterial metabolism.     

Seawater: an explanation of differential isothermal compressibility measurements in terms of hydration and ion-water interactions

Hydration, ion-water interactions, and water structure effects in seawater were studied by determining differences (Δβ) between the compressibilities of test salt solutions and the compressibilities of reference solutions. The reference solutions were distilled water and seawater (35%0), and the test salt solutions were either 0.13 m or 0.26m with respect to one of the following test salts: LiCl, NaCl, KCl, CsCl, NaF, NaI, MgCl₂, CaCl₂, BaCl₂, Na₂SO₄, K₂SO₄, and MgSO₄. The compressibility measurements (to 900 bars) were carried out at 2°C and also at 15°C using a differential method in which a pressure increase or a temperature increase causes Δβ to become less negative. At 1 bar and 15°C, the Δβ (0.26 m, distilled water reference) values ranged from ?1.14 × 10^?6 bar^? for NaI to ?3.84 × 10^?6 bar^?1 for Na₂SO₄, and the Δβ (0.26 m, seawater reference) values ranged from ?1.30 × 10^?6 bar^?1 for NaCl to ?3.04 × 10^?6 bar^?1 for Na₂SO₄. The Δβ values were used to calculate hydration numbers. Entropy of transfer, excess hydrogen bond breaking (determined by NMR), and effective radii of ions are properties which can be used to describe the influence of ions on water structure. The extent to which these properties correlate with Δβ values depends upon whether the ion is an anion or a cation, and this correlation forms the thesis that anions alter water structure in a different way than do cations.     

海水中铁(Ⅲ)-氨基酸盐配合物的光化学反应研究

系统研究了高压汞灯模拟日光照射下铁(Ⅲ)-氨基酸盐配合物在天然海水中的光化学反应情况,结果发现,在氨基酸盐配体的存在下,铁(Ⅲ)会发生光化学反应生成还原态的铁(Ⅱ),同时生成的铁(Ⅱ)会被容器内的氧再氧化为铁(Ⅲ).铁(Ⅲ)的光还原反应速率会受到配体浓度、pH和光强以及温度的影响.在氨基酸与铁(Ⅲ)浓度配比大于2的情况下,在铁(Ⅲ)氨基酸盐配合物的光还原反应初期铁(Ⅱ)浓度的增长符合一级动力学反应规律,100min后浓度趋于稳定.光强升高和pH降低都能加快光还原速率,而改变温度则基本上对光还原反应速率无影响,这证明铁(Ⅲ)的光还原反应为一自由基引发的电子转移过程.     

On the feasibility of some photochemical reactions of iodide in seawater

Aerated solutions of potassium iodide in de-ionised water, of between 5–20% (w/v), were exposed to ambient spring sunlight to estimate the rate of photochemical production of molecular iodine from iodide and oxygen in seawater. This rate cannot be measured directly as other reactions, for example the reduction of molecular iodine by organic matter, interfere. Also, a parallel photo-oxidation, initiated by organic matter in real seawater, may also occur. The experiments yield a half-life for iodide in tropical surface waters of about 29 months suggesting that the reaction is insignificant. At this rate it will not compete effectively against the reduction of molecular iodine by organic matter, and hence molecular iodine should not appear. The experiments also consider the photo-oxidation by nitrate, and iodate, a combination of nitrite and oxygen, and eliminate significant interference by chloride, bromide and the phosphate buffer. The rate of photo-oxidation with each of the first three oxidations is found to be first order with respect to oxidant concentration. The variation of photo-oxidation rate with pH is also studied, with a brief investigation without conventional oxidant, where electron cage complexes still promote photo-oxidation. The photochemical action spectrum for these reactions in sunlight is shown to extend between 300 and 425 nm. The photo-oxidation of iodide by iodate is more interesting to marine chemistry as the photo-reduction of iodate. Nevertheless, the reduction-rate is judged to be several orders too low to be significant in seawater. The mechanism of the reactions are discussed and lessons drawn on the stability of potassium iodide solutions used in iodate analysis. The KI actinometer is recommended to those studying other photochemical systems activated by UV-A light as it is linear and very simple and reliable.     

Nitrite photolysis in seawater by sunlight

Nitrite is chemically stable but photochemically unstable in seawater. The net disappearance rate in abiotic low-nitrate seawater exposed to sunlight is ～ 10% per day. The primary products are the free radicals NO and OH. Quantitative aspects of the kinetics and secondary product formation are discussed in terms of a fourteen-step reaction scheme. Possible pathways explaining the results are suggested but not unequivocally identified.The rate of reaction in various marine environments is estimated from cruise data and extrapolations to vary between 0.2–60·10^?3 moles m^?2yr^?1, with a suggested global average for comparison purposes of 1–10·10³ moles m^?2yr^?1.These results confirm and quantify our previous suggestion that nitrite photolysis represents a source of OH radical in seawater. The reaction rate is large enough that significant impacts on the geochemical cycles of dissolved organic carbon and nitrogen and heavy metals may plausibly result. Effects on marine biota and atmospheric trace gas composition are also possible. However, specific reactions coupling the nitrite system to other processes have not yet been identified or demonstrated empirically.     

Photochemical production of the hydroxyl radical in Antarctic waters

Photochemical production rates and steady-state concentrations of the highly reactive OH radical were determined in Antarctic seawater in the Weddell-Scotia Confluence during the austral spring of 1993 and along the Antarctic Peninsula during the austral summer of 1994. OH radical photoproduction rates were 30±2 nM/day and 46±2 nM/day in surface open oceanic and coastal waters, respectively. Corresponding steady-state concentrations were 2.6×10⁻¹⁹ and 4.3×10⁻¹⁹ M which are similar to those found in tropical latitudes. In-situ irradiation experiments (drifter deployments) at different depths in the upper water column indicated that multiple sources for the OH radical existed at three Antarctic stations. Ultrafiltration studies and model calculations based on wavelength-dependent OH radical quantum yields indicated that the main sources were photochemical reactions of low molecular weight dissolved organic matter (DOM), nitrate, and nitrite. Production of the OH radical from nitrate photolysis was almost exclusively UV-B dependent, while OH radical production from nitrite photolysis was mainly UV-A dependent. OH production from DOM photolysis was both UV-A and UV-B dependent. In the upper few meters at open oceanic sites, nitrate and DOM were the dominant OH radical sources, while deeper in the water column DOM and nitrite were important because of the greater importance of UV-A with depth. During non-ozone hole conditions, nitrate contributed about 33%, while DOM plus nitrite contributed about 67% of the predicted OH radical production in open oceanic surface waters. During an ozone hole (151 Dobson units), the corresponding percentages changed to about 40 and 60% for nitrate and DOM due to the higher UV-B irradiance. Model calculations predict that during an ozone hole (151 Dobson units), OH radical production in surface waters will be enhanced by at least 20%, mostly from nitrate photolysis and to a lesser extent from DOM photochemical reactions. This study indicates that ozone hole events significantly increase OH radical production, as well as the photolysis of DOM, in Antarctic waters, and that rates can be as high or higher than those at lower latitudes, especially if differences in temperature and solar irradiance are taken into account.     

Colorimetric determination of nanomolar concentrations of ammonium in seawater using solvent extraction

A solvent extraction method for measuring nanomolar concentrations of ammonium in seawater is described. The procedure is based on formation of indophenol in alkaline solution by reaction of phenol, hypochlorite and ammonia using sodium aquopentacyanoferrate as a coupling agent. Indophenol is then concentrated by extraction into n-hexanol at low pH and re-extraction into aqueous alkaline buffer. The concentration of indophenol blue is determined colorimetrically. The molar absorbance is 2.08 × 10⁵ absorbance units per molar NH₄⁺ in seawater with a precision of ± 1.9 nM NH₄⁺ (95% Cl) for concentrations ≤ 50 nanomolar. This represents a 12-fold improvement in sensitivity and a 26-fold improvement in precision over standard aqueous analyses. Calibration curves are linear to at least 2 μM NH₄⁺. Sensitivity in seawater is 97% of that found in deionized distilled water due to a slight salt effect.     

Factors influencing the oxidation,reduction, methylation and demethylation of mercury species in coastal waters

The objective of this study was to examine the redox reactions and other transformations of mercury (Hg) species in surface waters, and the factors determining the rates of these reactions. For the redox studies completed at the Chesapeake Biological Laboratory (CBL), two isotopes (¹⁹⁹Hg^II and ²⁰²Hg⁰) were added into different types of filtered water (fresh to seawater) to examine the oxidation and reduction reactions. Further studies of both the redox reactions and methylation/demethylation reactions of Hg were conducted with unfiltered water on board research vessels during cruises in May and July 2005 on the Chesapeake Bay and shelf. While CH₃¹⁹⁹Hg^II was added to allow the examination of demethylation, ²⁰¹Hg^II was used to examine both reduction and methylation, and ²⁰²Hg⁰ was used to examine oxidation. Overall, the results showed that both Hg oxidation and reduction were simultaneously occurring and were photochemically mediated in the waters investigated. In contrast to the previously assumed “unreactive” nature of Hg⁰, the studies found that the magnitude of the rate constant for Hg⁰ oxidation was greater than that for reduction, indicating its importance in estuarine and coastal waters. In addition, both experiments at CBL and on board ship showed that Hg^II reduction was similar in magnitude, suggesting that biotic processes were relatively unimportant. While no measurable methylation occurred during the incubation period during the on board studies, concentration of CH₃¹⁹⁹Hg^II decreased over the time during the experiments. It appeared that the demethylation processes were not dominantly photochemically driven, but could be microbially mediated. Further studies are needed in order to help better understand Hg redox and transformations in natural water systems.     

The fluorescence of dissolved organic matter in seawater

A total of 28 vertical profiles of seawater fluorescence was measured in the Sargasso Sea, the Straits of Florida, the Southern California Borderlands, and the central Pacific Ocean. In all cases, surface seawater fluorescence was low as a result of photochemical bleaching which occurs on the timescale of hours. Fluorescence of deep water was 2–2.5 times higher than that of surface waters, and was constant, implying a long residence time for fluorescent organic matter, possibly of the order of thousands of years. Fluorescence correlates well with nutrients (NO₃⁻, PO₄³⁻) in mid-depth waters (100–1000 m) in the Sargasso Sea and the central North Pacific, consistent with results in the central Pacific and the coastal seas of Japan. This suggests that regeneration or formation of fluorescent materials accompanies the oxidation and remineralization of settling organic particles.The various sources and sinks of fluorescent organic matter in the global oceans are assessed. The major sources are particles and in situ formation; rivers, rain, diffusion from sediments, and release from organisms are minor sources. The major sink is photochemical bleaching.     

Silica-alumina interactions in seawater

Amorphous silica can polymerize in distilled water, in 0.6 N NaCl solution and in seawater to form a colloidal suspension that contains approximately 200 ppm Si. Solid amorphous alumina can prevent this polymerization in seawater and in 0.6 N NaCl, and can inhibit but not prevent it in distilled water. This prevention of polymerization may be an important factor in authigenic mineral formation.The presence of solid amorphous alumina with solid silica in the same solutions causes the final concentrations of dissolved silica to be lower than those attained by solid silica in the absence of solid alumina. The effects are similar whether the final levels are approached from above or below the saturation concentration for amorphous silica. This indicates that the observed concentration of dissolved silica will be a function of available alumina as well as of the silica solubility.The presence of solid amorphous alumina with quartz in seawater, 0.6 N NaCl solution and distilled water causes dissolved silica levels to remain below 0.7 ppm Si for at least 38 days. The same systems in the absence of alumina approach the solubility levels of quartz within that time period.