Photoreduction Of Mercury Research Articles

Enhanced photoreduction of mercury (II) ions over MnCo2O4-modified wrinkled BaSnO3 nanosheets prepared by solvothermal method under visible light

Photocatalytic reduction of heavy metal ions utilizing nanostructured semiconductor photocatalysts is a promising ecological way, thanks to its simplicity and feasibility. In this article, the improved photoreduction of mercury (II) ions (Hg2+) is achieved over wrinkled barium stannate (BaSnO3) nanosheets modified by manganese cobaltate (MnCo2O4) nanoparticles. The MnCo2O4 was coupled to solvothermally-produced BaSnO3 at 4.0–16.0 wt% by impregnation. The MnCo2O4/BaSnO3 heterojunctions showed mesoporous surfaces with 133–150 m2 g1 of surface areas. The visible light harvesting is improved owing to bandgap reduction down to 2.31 eV for 9.0 wt% MnCo2O4-impregnated BaSnO3 compared to 2.99 eV for the bare BaSnO3. The complete photoreduction of 368.30 µM Hg2+ utilizing 9.0% MnCo2O4/BaSnO3 is achieved at a dosage of 2.4 gLẌ with a rate of 69.54 µM min–1 in 30 min. This optimized photocatalyst is also recyclable five times with 97% of its initial performance. The magnificent activity of MnCo2O4/BaSnO3 is accredited to the improved visible-light harvesting and efficient charge separation by the controllable impregnation of MnCo2O4.

S-scheme AgVO3-decorated BaSnO3 heterojunction for efficient photoreduction of Mercury (II) ions under visible light

BackgroundHeavy metal waste has been slowly accumulating in water supplies due to the industry's current growth, posing a serious threat to the environment and all living things. MethodsIn this paper, we describe an unpretentious hydrothermal technique for forming barium stannate nanocrystals (BSO), which are then modified with different doses (3–12 wt.%) of AgVO3 (AVO). The structural, optical, and photocatalytic features of the resulted photocatalysts were positively impacted by the AVO loading, according to characterization studies. Significant findingThe decrease of the bandgap of BSO from 2.94 to 2.39 eV and an improvement in absorbing visible light are both confirmed by the AVO decorating of just 9.0 wt%. Additionally, under visible light, the ability of the as-prepared photocatalysts to reduce Hg2+ ions were evaluated. The complete photoreduction of Hg2+ ions over the 9% AVO/BSO nanocomposite were achieved at a higher kinetic rate constant (0.057 min−1) compared to pristine BSO NPs after 45 min of illumination. After being used five runs to reduce Hg2+ during visible illumination, the AVO/BSO heterojunctions demonstrated significant stability and outstanding photocatalytic practicability. And then, the Hg2+ photoreduction reaction's mechanism was explained. This study illustrates how modified BSO photocatalysts can be used in water remediation processes.

Novel BiVO4/ZnO heterojunction for amended photoreduction of mercury (II) ions

In this research, mercury (II) ions were eliminated via photocatalytic reduction under visible light utilizing BiVO4/ZnO heterojunction. Different percentages of BiVO4 NPs were distributed uniformly over mesoporous ZnO surface. It has been demonstrated by TEM analysis that both pure mesoporous ZnO sample and the heterojunction containing 3 wt% BiVO4 NPs consist of almost homogenous spherical particles of about 25 nm diameter. The progressed BiVO4/ZnO nanocomposite containing 3 wt% BiVO4 NPs was able to accomplish comprehensive elimination of mercury (II) ions via photocatalytic reduction under visible light owing to its significant capacity to absorb light in visible region. Such heterostructure achieved almost complete (100%) photocatalytic proficiency towards mercury (II) ions reduction by the end of 45 min from the illumination by visible light. The photo-reduction rate of Hg2+ ions via practicing BiVO4/ZnO nanocomposite containing 3 wt% BiVO4 NPs was estimated at 14 times when related to that of pure mesoporous ZnO. The successful development of p-n heterojunction between ZnO (n-type) and BiVO4 (p-type) in addition to the accessible great surface area and the remarkable visible light absorption provided by such heterojunction could be regarded as the dominant causes of the distinct photo-reduction rate of Hg2+ ions. The heterojunction afforded via incorporating BiVO4 with ZnO facilitated the separation between the created charges and hindered the rate of the recombination between such charges as well.

Promoted visible-light photocatalytic reduction of Hg2+ over CuAl2O4-decorated g-C3N4 nanoheterojunctions synthesized by solution process

This work reports an effective effort to indorse the photoreduction of mercury (II) ions (Hg2+) by a developed heterojunction photocatalyst. Nanostructured CuAl2O4-decorated g-C3N4 was synthesized by MCM-41 and F-127 assisted solution process. The CuAl2O4 nanoparticles were dispersed onto g-C3N4 at 1.0, 2.0, 3.0, and 4.0 wt%. The prepared nanostructures were substantiated large surface areas and mesoporous texture surfaces. TEM observation proved the construction of heterojunctions between CuAl2O4 (28.6 nm) and g-C3N4 networks. The photoreduction of Hg2+ over the 3.0 wt% CuAl2O4-decorated g-C3N4 below visible-light radiation was attained with a superior rate of 189.4 µmol min−1 compared to 21.95 µmol min−1 for g-C3N4 and 12.95 µmol min−1 for CuAl2O4. The dose optimization of 3.0% CuAl2O4/g-C3N4. The augmented photocatalytic performance of this novel heterojunction is referred to as confined bandgap and suppression of photogenerated carrier recombination. The sustainable photoreduction of Hg2+ over CuAl2O4/g-C3N4 for five runs was also confirmed the photostability and regeneration ability.

Distribution Characteristics of Mercury in Different Urban Constructed Wetlands

To explore the spatial and temporal distribution and the methylation characteristics of mercury in different constructed wetlands in cities, and to understand the potential ecological exposure of mercury in urban wetlands, four artificial wetlands in Chongqing were studied from March 2017 to March 2018. The water samples were collected separately in four quarters, and the mass concentration of total mercury (THg) and methyl mercury (MeHg) was researched for one year. The results showed that the THg concentration in the four wetland waters is higher than the background value of the world's lakes and reservoirs for dam construction, but it is far lower than the waters with pollution history. The THg mass concentration of the water inside the wetlands is slightly higher than in the inlet and outlet. In addition, the THg mass concentration in the aquatic plant growing area, the construction area, the cruise ship parking area, and the frequent play area has an increasing trend, indicating that urban wetlands have a trapping and converging effect of the water's THg, so human activities influence total mercury a lot. The mass concentration of MeHg in the four wetland waters was slightly higher than that in other water bodies. With the exception of the Caiyun Lake, where the mass concentration of MeHg at the outlet was higher than that of the water inlet, the other three wetlands showed lower MeHg mass concentration in the outlet than the inlet. The mass concentration of MeHg in the wetland water increased with increasing water depth. The ratio of MeHg concentration to THg mass concentration (MeHg/THg) was higher than in other waters, indicating that urban wetlands have effect on net mercury methylation from waters. The photoreduction of mercury and its absorption by aquatic plants can reduce the mercury load from urban wetlands to downstream watersheds. The THg mass concentration of the four urban wetland water bodies was high in the spring and autumn, with a slight decrease in the summer, and lowest in the winter. The mass concentration of MeHg was the lowest in winter, and in the other three seasons it was basically flat, about three times higher than in winter. This study clarifies the temporal and spatial distribution and methylation of mercury in urban wetlands. It explores the degree of disturbance of human activities on wetlands and the response characteristics, as well as the impact of wetland mercury on downstream watersheds. To avoid potential mercury exposure, measures need to be established for the construction of artificial wetlands.

Effects of ultraviolet radiation on mercury isotope fractionation during photo-reduction for inorganic and organic mercury species

Photo-reduction of mercury (Hg) is an important mechanism for removal of both Hg2+ species and monomethylmercury (MMHg) from surface waters. Large mass independent fractionation (MIF) signatures of Hg isotopes preserved in natural samples are thought to reflect MIF produced during aqueous photo-reduction by the magnetic isotope effect (MIE). Recently, Hg MIF signatures in natural samples are being used to quantify photochemical reduction in aquatic systems. However, the fractionation factors used are from laboratory experiments that did not investigate many of the environmental parameters that may affect MIF during photo-reduction. In this study, the effects of different regions of the solar spectrum on the expression of MIF caused by the MIE during Hg2+ and MMHg photo-reduction were investigated to assess how the type of radiation affects the extent and signature of MIF. Photo-reduction in the presence of DOM was carried out through exposure to the full natural solar spectrum unfiltered, with the ultraviolet B (UVB; 290–320nm) portion removed and with both the UVB and the ultraviolet A (UVA; 320–400nm) removed. There is a clear relationship between the expression and magnitude of MIF and the energy of incident radiation for both Hg2+ and MMHg photo-reduction. The experiments indicate that MIF produced during photo-reduction of Hg2+ is significantly influenced by both UVB and UVA radiation. For MMHg photodemethylation, however, UVB radiation is mostly responsible for the MIF with minor contributions from UVA. Overall, there is a lack of correlation between the observed MIF and total photo-reduction in the experiments conducted in this study, which indicates that indirect and other non-MIF producing pathways of photo-reduction are the dominant pathways by which Hg species are being photo-reduced. Extrapolating these experimental results to natural systems is difficult as the experiments were not performed at realistic Hg/DOM ratios and the distribution of ligands that Hg is bound to in the experiments likely differs from natural systems. However, the strong relationship between MIF and energy of incident radiation in this study supports Hg MIF signatures as promising tools for helping to quantify photochemical cycling of mercury, but this study also highlights the need to understand the link between MIF and total photo-reduction before this tool can be fully utilized.

Effect on the Photo-production of Dissolved Gaseous Mercury in Post-desulfurized Seawater Discharged from a Coal-Fired Power Plant

A high concentration of dissolved gaseous mercury (DGM) was detected in post-desulfurized waste seawater, which was discharged from a coal-fired power plant equipped with a seawater desulfurization system and which was located in a coastal area. A large amount of DGM was converted from other forms of mercury during transformation processes, such as photo-reduction. The present study targeted the photo-reduction of mercury and the effects of various environmental parameters on DGM production in the post-desulfurized seawater discharged. The results suggested that the photo-reduction of mercury was significantly induced under UV radiation, especially with UVB. The particulate mercury on suspended solids was easily photo-reduced and considered as an important source of DGM. It was confirmed that the suspended solids in post-desulfurized seawater could enhance the reduction process of mercury under UV radiation. The pseudo-first-order rate constants of DGM production, which were determined through the concentration gradient and trial methods, were 1.39 × 10−3 min−1 and 1.45 × 10−3 min−1, respectively. The values showed no significant difference and were both much higher than the reported results, indicating that the photo-reduction of mercury in post-desulfurized seawater deserved more attention. In addition, the initial mercury level was observed when mixing the post-desulfurized seawater with fresh seawater, and this suggested that a significant amount of initial mercury would be produced when the post-desulfurized seawater was discharged into the adjacent sea area and thus becomes another considerable source of DGM.

Photoreduction of Hg(ii) and photodemethylation of methylmercury: the key role of thiol sites on dissolved organic matter.

This study examined the kinetics of photoreduction of Hg(ii) and photodemethylation of methylmercury (MeHg(+)) attached to, or in the presence of, dissolved organic matter (DOM). Both Hg(ii) and MeHg(+) are principally bound to reduced sulfur groups associated with DOM in many freshwater systems. We propose that a direct photolysis mechanism is plausible for reduction of Hg(ii) bound to reduced sulfur groups on DOM while an indirect mechanism is supported for photodemethylation of MeHg(+) bound to DOM. UV spectra of Hg(ii) and MeHg(+) bound to thiol containing molecules demonstrate that the Hg(ii)-S bond is capable of absorbing UV-light in the solar spectrum to a much greater extent than MeHg(+)-S bonds. Experiments with chemically distinct DOM isolates suggest that concentration of DOM matters little in the photochemistry if there are enough reduced S sites present to strongly bind MeHg(+) and Hg(ii); DOM concentration does not play a prominent role in photodemethylation other than to screen light, which was demonstrated in a field experiment in the highly colored St. Louis River where photodemethylation was not observed at depths ≥ 10 cm. Experiments with thiol ligands yielded slower photodegradation rates for MeHg(+) than in experiments with DOM and thiols; rates in the presence of DOM alone were the fastest supporting an intra-DOM mechanism. Hg(ii) photoreduction rates, however, were similar in experiments with only DOM, thiols plus DOM, or only thiols suggesting a direct photolysis mechanism. Quenching experiments also support the existence of an intra-DOM photodemethylation mechanism for MeHg(+). Utilizing the difference in photodemethylation rates measured for MeHg(+) attached to DOM or thiol ligands, the binding constant for MeHg(+) attached to thiol groups on DOM was estimated to be 10(16.7).

Photoreduction of mercury(II) in the presence of algae, Anabaena cylindrical

The photochemical and biological reduction of Hg(II) in the presence of algae, anabaena cylindrical, was investigated under the irradiation of metal halide lamps placed in cooling trap for maintaining constant temperature by water circulation ( λ ⩾ 365 nm, 250 W). The photoreduction rate of Hg(II) increased with increasing algae concentration. The addition of Fe(III) and humic substances into the suspensions of algae also enhanced the photoreduction of Hg(II). Alteration of pH value affected the photoreduction of Hg(II) in aqueous solution with or without the addition of algae. The concentration of dissolved gaseous mercury (DGM), the reduced product of Hg(II), increased with increasing exposure time and then gradually approached to a steady state. The influence of initial Hg(II) concentration on the photoreduction of Hg(II) with algae was studied by irradiating the suspensions of anabaena cylindrical at pH 7.0 with initial concentrations ( C 0) of Hg(II) at 50, 100, 120, 150 and 180 μg L −1, respectively, the light-induced reduction of Hg(II) followed the apparent pseudo first-order kinetics. The initial photoreduction rate could be expressed by the equation: r A = 0.0871 + 0.00129 C 0, with a correlation coefficient R = 0.9994. The overall mercury mass balance study on the photo-reductive process revealed that more than 39.86% of Hg(II) from the algal suspension was reduced to volatile metallic mercury.

Atmospheric mercury emissions from mine wastes and surrounding geologically enriched terrains

Waste rock and ore associated with Hg, precious and base metal mining, and their surrounding host rocks are typically enriched in mercury relative to natural background concentrations (<0.1 {micro}g Hg g{sup -1}). Mercury fluxes to the atmosphere from mineralized areas can range from background rates (0-15 ng m{sup -2} h{sup -1}) to tens of thousands of ng m{sup -2} h{sup -1}. Mercury enriched substrate constitutes a long-term source of mercury to the global atmospheric mercury pool. Mercury emissions from substrate are influenced by light, temperature, precipitation, and substrate mercury concentration, and occur during the day and night. Light-enhanced emissions are driven by two processes: desorption of elemental mercury accumulated at the soil:air interface, and photo reduction of mercury containing phases. To determine the need for and effectiveness of regulatory controls on short-lived anthropogenic point sources the contribution of mercury from geologic non-point sources to the atmospheric mercury pool needs to be quantified. The atmospheric mercury contribution from small areas of mining disturbance with relatively high mercury concentrations are, in general, less than that from surrounding large areas of low levels of mercury enrichment. In the arid to semi-arid western United States volatilization is the primary means by which mercury ismore » released from enriched sites.« less

Photoreduction and evolution of mercury from seawater

The photoreduction of mercury (Hg 2+ to Hg 0) in natural seawater was investigated by means of a radiotracer ( 203Hg 2+) solution exposed to natural and simulated sunlight. Different light regimes (dark, natural daylight, and a solar simulator), and dissolved organic carbon (DOC) levels from commercially available humic acids concentrations, were tested in the laboratory to evaluate the possibility of occurrence of the reaction in the environment. The natural seawater prepared accordingly to each experimental condition was continuously purged of the Hg 0 formed, which was then re-oxidised in an acid trap and determined. The use of a solar simulator permitted the test of light intensity and wavelength dependence of the process under investigation. The reaction is dependent on the concentration of DOC in the experimental solution, increasing light intensity and decreasing wavelength. Reduction rates were in the range of 0.04–2.2% h −1 for the DOC concentrations and light regimes tested. The process might have geochemical implications for the cycling of mercury around the air–sea interface.

Photoreduction of mercury in sea water and its possible implications for Hg0 air–sea fluxes

Mercury photoreduction in the presence of humic substances was investigated in laboratory experiments by means of a radiochemical technique. The seawater samples used were from the North Sea, and the light source was natural sunlight on a summer's day at temperate latitude. The reduction reaction Hg2+→Hg0 was observed to occur at different humic concentrations and light regimes. The results presented here are in the form of the percent of initial total mercury which was reduced and degassed from the solution with time. The reaction rate increased with humic concentration and the presence of light. The reaction rates varied from near zero in the case of the dark reaction in the absence of humic material to around 2% h−1 in a 5-mg l−1 of dissolved organic carbon (DOC) irradiated solution. Elemental mercury fluxes calculated from data obtained during this work are compared with fluxes reported in the literature. The consequences of mercury reduction and evasion from the solution are discussed in the light of the fluxes of gaseous mercury which can potentially be generated from the water when the photochemical reduction reaction takes place. The photochemically driven fluxes calculated are shown to be potentially significant contributors to the fluxes reported in the literature for at least some of the environments tested.

Photoreduction of mercury(II) in aqueous solution in the presence of cyclohexene. Hydroxomercuration and two-stage photolysis

Interaction of Hg 2+ and cyclohexene in argon-saturated acidic aqueous solution leads to the formation of a hydroxomercurated compound characterized with an intense absorption band at 214 nm (ε = 1.0 × 10 4 M −1 cm −1). This reaction proved to be reversible with an equilibrium constant of 2.3 × 10 3. Photolysis of this system ( λ ir = 230 nm) produces Hg 2 2+ in the first stage with a quantum yield of 0.88. Prolonged irradiation results in the reduction of Hg 2 2+ to metallic mercury.

Photochemical Formation of volatile mercury in high Arctic lakes

AbstractFormation rates of dissolved gaseous mercury (DGM were measured in three high (74 °N) Arctic lakes and one wetland. The rate of formation of DGM was controlled by (1) the intensity of solar radiation, particularly ultraviolet‐B (280–320 nm) and ultraviolet‐A (320–400 nm) wavebands and (2) the concentration of available photoreducible Hg(II) complexes. Additions of hydrogen peroxide and of fulvic acids did not enhance DGM formation rates in our experiments. This study illustrates that photoreduction of mercury occurs in a wide range of aquatic systems even in the high arctic where radiation levels are lower but continue for the entire day during the summer.

Mechanistic steps in the photoreduction of mercury in natural waters

Elemental mercury accounts for 10–50% of the dissolved mercury in lakes but plays a vital role in the cycling of this element in lacustrine environments. The view is advanced that a large fraction of the Hg(0) is generated by photochemical reactions in the surface waters. Possible mechanisms for the Hg(0) production are discussed including homogeneous photolysis of dissolved Hg species and photoreduction processes involving organic molecules and inorganic particulates in water.