Mercury pollution in the ocean

Sources and chemistry of mercury pollution in the ocean[1]

Mercury is a toxic heavy metal which cycles through atmosphere, water, and soil in various forms to different parts of the world. Due to this natural cycle, irrespective of which part of the world releases mercury it could affect an entirely different part of the world making mercury pollution a global concern. Mercury pollution is now identified as a global problem and awareness has been raised on an international action plan to minimize anthropogenic mercury emissions and clean up mercury pollution. In Global Mercury Assessment - 2002 concluded that, International actions to address the global mercury problem should not be delayed”.[2] Among many environments that are under the impact of mercury pollution, the ocean is one which cannot be neglected as it has the ability to act as a “storage closet” for mercury.[3] According to a recent model study the total anthropogenic mercury released into the ocean is estimated to be around 80,000 to 45,000 metric tons and two thirds of this enormous amount is estimated to be found in waters shallower than 1000m level where many consumable fish live.[4] Mercury can get bio-accumulated in marine food chains in the form of highly toxic methyl mercury which can cause health risks to human seafood consumers.,[5][6] According to statistics, about 66% of the global fish consumption comes from ocean. Therefore, it is important to monitor and regulate oceanic mercury levels to prevent more and more mercury reaching human population through seafood consumption.,[7][8]

Sources

Mercury release occurs by both natural and anthropogenic processes. Natural processes are mainly geogenic such as volcanic activities and land emissions through soil. Volcanoes release mercury from the underground reservoirs upon eruption. Land emissions are usually observed in the regions closer to plate tectonic boundaries where soils are enriched with minerals such as cinnabar containing Mercury sulfide(HgS). This mercury is released by either natural weathering of the rocks or by geothermal reactions.[9] While natural phenomena account for a certain percentage of present-day emissions, anthropogenic emissions alone have increased mercury concentration in the environment by threefold.[10] Global Mercury Assessment 2013 states main anthropogenic sources of mercury emission are artisanal and small - scale gold mining, fossil fuel burning and primary production of non-ferrous metals. Other sources such as cement production, consumer products waste, contaminated sites and chlor-alkali industry also contributes in relatively small percentages.[10]

Mercury enters the ocean in different ways. Atmospheric deposition is the largest source of mercury to the oceans. Atmospheric deposition introduces three types of mercury to the ocean. Gaseous elemental mercury (Hg0) enters the ocean through air-water exchange. Inorganic mercury (Hg2+/HgII) and particle-bound mercury (Hg(P)) enters through wet and dry deposition. In addition, mercury enters the ocean via rivers, estuaries, sediments, and, hydrothermal vents etc.[11] These sources also release organic mercury compounds such as Methyl mercury. Once they are in the ocean they can undergo many reactions primarily grouped as; redox reactions (gain or loss of electrons), adsorption processes (binding to solid particles), methylation and demethylation (addition or removal of a methyl group).[1]

Chemistry

Photochemistry of mercury on oceanic aerosols
Microbial chemical conversions of mercury

Reduction and oxidation of mercury mostly occur closer to the ocean water surface. These are either driven by sunlight or by microbial activity. Under UV radiation, elemental mercury oxidizes and dissolves directly in ocean water or binds to other particles. The reverse reaction reduces some mercury Hg2+ to elemental mercury Hg(0) and returns to the atmosphere. Fine aerosols in the atmosphere such as ocean water droplets can act as small reaction chambers in this process providing the special reaction conditions required. Oxidation and reduction of mercury in the ocean are not very simple reversible reactions.[12] Shown below is the proposed pathway of ocean aerosol mercuric photochemistry suggesting that it occurs through a reactive intermediate:

Photo oxidation is suspected to be driven by OH. radical and reduction is driven by wind and surface layer disturbances. In the dark, mercury redox reactions continue due to microbial activity. The biological transformations are different and have a smaller rate compared to sunlight driven processes above.[1] Inorganic mercury Hg2+ and methyl mercury has the ability to get adsorbed in to particles. A positive correlation of binding is observed for the amount of organic matter vs. the concentration of these mercury species showing that most of them bind to organic matter.[13] This phenomena can determine the bioavailability and toxicity of mercury in the ocean. Some methyl mercury is released to the ocean through river run-off. However, most of the methyl mercury found in the ocean is produced in –situ (inside the ocean itself).[11] Methylation of inorganic mercury can occur via biotic and abiotic pathways. However, biotic pathways are more predominant. The reactions illustrated in a simplified scheme below are actually parts of complex enzyme driven metabolic pathways taking place inside microbial cells.

In abiotic reactions, humic substances act as methylating agents and therefore this process occur at shallow sea levels where decomposing organic matter is available to combine with inorganic mercury Hg2+.9 Interestingly, mercury methylation studies in Polar Regions have also shown a positive correlation between methylation and chlorophyll content in water showing there could also be biogenic pathways for methyl mercury production.[14] Produced methyl mercury gets accumulated in microbes. Due to the high permeability and absence of degradation for methyl mercury in other species that depend on those microbes, this very toxic compound gets biomagnified through marine food chains to the top predators. Human population consumes many types of marine fish who are top predators in the food chains which puts their health in great danger. Therefore, finding possible solutions to minimize further mercury emissions and cleaning up the already existing mercury pollution is extremely important.

Prevention and remedy

Synthetic corals

Cleaning up the existing mercury pollution could be a tedious process. Nevertheless, there is some promising ongoing research bringing hope to the challenging task. One such research is based nanotechnology. It uses synthesized aluminum oxide nanoparticles (Al2O3) mimicking the coral structures. These structures absorb heavy metal toxins effectively due to high surface/volume ratio and the quality of surface. In nature, it has been long observed corals can absorb heavy metal ions due to its surface structure and this new technique has used to nanotechnology to create “synthetic corals” which may help clean mercury in the ocean.,[15][16] The reactions involved in synthesizing this material are;

Another novel material (Patent application: PCT/US15/55205) is still under investigation which looks at the possibility of cleaning mercury pollution using orange peels as raw material. This technology produces sulfur limonene polysulphide (proposed material) using sulfur and limonene. Using industrial byproducts to manufacture this polymer makes it a highly sustainable approach.The scientists say 50% of the mercury content could be reduced with a single treatment using this polymer.[17]

In addition to the cleaning processes, minimizing usage of coal power and shifting to cleaner energy sources, reducing small scale artisanal gold mining, proper treatment of industrial mercury waste, and implementation of policies are sound approaches to reduce mercury emissions in the long term-large scale plan. Public awareness is critical in achieving this goal. Proper disposal of mercury containing items such as medicinal packaging and thermometers, using mercury-free bulbs and batteries, buying consumer products with zero or minimum mercury emission to the environment can make a significant difference in recovering world’s ecosystems from mercury pollution leaving minimum legacy of mercury pollution in the ocean for our future generations.

See also

References

  1. 1 2 3 Batrakova, N., Travnikov, O. and Rozovskaya, O. (2014) "Chemical and physical transformations of mercury in the ocean: a review". Ocean Science, 10 (6): 1047–1063. doi:10.5194/os-10-1047-2014
  2. ↑ 1. United Nations Environment Programme (UNEP), Global Mercury Assessment, (Geneva, 2002). http://www.unep.org/gc/gc22/Document/UNEP-GC22-INF3.pdf (10/22/2015)
  3. ↑ http://www.livescience.com/47222-deep-ocean-traps-mercury-pollution.html (09/2015)
  4. ↑ Lamborg, C.H.; Hammerschmidt, C.R.; Bowman, K.L.; Swarr, G.J.; Munson, K.M.; Ohnemus, D.C.; Lam, P.J.; Heimburger, L.E.; Rijkenberg, M.J.A; Saito, M.A. A global ocean inventory of anthropogenic mercury based on water column measurements. Nature [Online] 2014, 512, 65 – 68
  5. ↑ Weiner, J.G.; Krabbenhoft, D.P.; Heinz, G.H.; Scheuhammer, A.M.; In Ecotoxicology of mercury, 2nd edition, Eds; CRC: Boca Ranton, FL, 2003; ch 16
  6. ↑ Clarkson, T.W.; Magos, L.; The toxicology of mercury and its chemical compounds. Crit.Rev.Toxicol. 2006, 36 (8), 609
  7. ↑ http://www.fao.org/3/a-i4883e.pdf (10/25/2015)
  8. ↑ http://www.fao.org/3/a-i4899e.pdf (10/25/2015)
  9. ↑ Selin, N.E.; Global Biogeochemical Cycling of Mercury: A Review. Annu. Rev. Environ. Resour. 2009, 34, 43 – 63
  10. 1 2 United Nations Environment Programme (UNEP), Global Mercury Assessment: Sources, Emissions, Releases and Environmental Transport (Geneva,2013)
  11. 1 2 Mason, R.P.; Choi, A.L.; Fitzgerald, W.F.; Hammerschmidt, C.R.; Lamborg, C.H.; Soerensen, A.L.; Sunderland, E.M. Mercury biogeochemical cycling in the ocean and policy implications. Environ. Res. 2012, 119, 101 -117
  12. ↑ Qureshi,A.; O’Driscoll, N.J.; MacLeod, M.; Neuhold,Y.M.; Hungerbuhler,K. Photoreactions of mercury in surface ocean water: Gross reaction kinetics and possible pathways. Environ. Sci. Technol., 2010, 44 (2), 644 – 649
  13. ↑ 13. Boszke, L.; Glosinska, G.; Siepak, J.; Some aspects of speciation of mercury in a water environment. Pol. J. Environ. Stud. 2002, 11 (4), 285 – 298
  14. ↑ Kirk, J.L.; Lehnherr, I.; Anderson, M.; Braune, B.M.; Chan, L.; Dastoor, A.P.; Dunford, D.; Gleason, A.L.; Loseto, L.L.; Steffen, A.; St Louis, V.L.; Mercury in arctic marine ecosystems: sources, pathways and exposure. Environ. Res. 2012, 119, 64 -87 Dimethyl mercury degradation also produces some of the methyl mercury present ocean.
  15. ↑ X. Wang et al. / J. Colloid Interface Sci., 2015, 453, pp 244-251
  16. ↑ http://webnesday.com/this-fake-coral-sucks-up-mercury-pollution-for-a-cleaner-ocean/ (September, 2015)
  17. ↑ https://theconversation.com/we-created-a-new-material-from-orange-peel-that-can-clean-up-mercury-pollution-49355 (10/25/2015)
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