Mercury Cycle

                

 

                

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                  Photo credit: http://www.neiwpcc.org/mercury/environment.asp

 

                 Mercury enters the environment through natural and human sources. Volcanoes, natural mercury deposits and volatilization from the ocean are natural sources of mercury. Human sources are coal combustion, chlorine alkali processing, waste incineration and metal processing. From these two types of sources, elemental mercury vapor (Hg^o) is released into the atmosphere. It circulates in the atmosphere in the gaseous form. Through photochemical oxidation, elemental mercury vapour becomes inorganic mercury. In this form, it can combine with water vapours and return to the surface of the Earth as rain. It is then deposited and accumulated in the soil. In water, the inorganic mercury can be settle into the sediment as the precipitate, mercury sulfide (HgS). Sulfate-reducing bacteria, which are found in the muddy sediments of bodies of water, take up the inorganic mercury  and convert it through metabolic processes to methylmercury. It is hypothesized that it the sulfate-reducing bacteria take up the inorganic mercury through passive diffusion of the dissolved complex, HgS. Detoxifying enzymes strip the sulfur group from the HgS and replaces it with a methyl group. 

                                                      Bacteria

HgS                           CH₃Hg(II)X + H₂S

 

The methylmercury complex is then transported to the aquatic environment by the sulfate-reducing bacteria.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                 

                                                  

 

 

Photo Credit: http://www.waukeshacounty.gov/page.aspx?SetupMetaId=14814&id=14818

 

            

            

Methylmercury is extremely bioaccumaltive. Bioaccumulation is the process in which organisms absorbs a contaminants at a higher rate than its body can eliminate them. Methylmercury bioaccumulates through the food chain. Methylmercury is attracted to the lipids of living organisms. It is stored in the fatty tissues.  First the plankton absorbs the methylmercury. Smaller fish then eat the plankton; and the larger fish consume the smaller fish. The methylmercury stays in the muscle tissue. The amount of mercury that accumulates depends on the fish size and age. The largest predators tend to have the highest concentration of mercury. For example, sharks and swordfish are classified as larger predators and tend to have high levels of mercury in their tissue. Humans tend to be at the top of the food chain, which brings them to dangerous levels of mercury.

 

 

 

 

 

 

 

 

 

 

                         

 

 

 

 

 

 

 

 

Photo Credit: http://conservationreport.com/2008/01/23/mercury-pollution-high-mercury-levels-are-found-in-tuna-sushi/

 

 

 

            

            

            

             As elemental mercury vapour combines with water vapour, it then falls back to the Earth’s surface was rain. It then enters bodies of water such as lakes, rivers and oceans. Mercury can also enter into water when it is near soil that is concentrated with mercury.

             Inorganic mercury is mostly found in drinking water. Approved methods of mercury removal from drinking water are coagulation/filtration, granular activated carbon, lime softening and reverse osmosis. Most of these methods are moderately effective in removing inorganic mercury but ineffective in removing organic mercury. In drinking water, the levels of inorganic mercury are not harmful to humans. In Canada, the concentrations of mercury in surface and drinking water are below 0.001mg/L. The mean concentration of the Great Lakes are 0.017 mg/L for Lake Erie and Huron, 0.013 mg/L for Lake Ontario and 0.018 mg/L for Lake Superior. In the Altantic provinces, 20 locations of surface water samples showed mercury concentrations in excess of 0.02 mg/L. Mean concentrations of other provinces are as follows: Alberta with 0.0002 mg/L, Newfoundland with 0.00015 mg/L and 0.00029 mg/L.

            

 

 

 

 

King’s Christian Collegiate

Christina Hong

Part B: Water

Part A: Fish and Shellfish