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KIMIA LINGKUNGAN

KIMIA LINGKUNGAN. BAGIAN 4: HIDROSFER 3. LOGAM BERAT DI DALAM AIR. COMMON FEATURES. heavy metals  near the bottom of the periodic table the densities  high compared to other common materilas

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KIMIA LINGKUNGAN

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  1. KIMIA LINGKUNGAN BAGIAN 4: HIDROSFER 3. LOGAM BERAT DI DALAM AIR

  2. COMMON FEATURES • heavy metals  near the bottom of the periodic table • the densities  high compared to other common materilas • as water pollutants and contaminants in food  the most part transported from place to place via the air, as gases or as species adsorbed or absorbed in suspended particulate matter

  3. TOXICITY OF THE HEAVY METALS • mercury vapor is highly toxic  Hg, Pb, Cd and As are not particularly toxic as the condensed free elements • Hg, Pb, Cd and As  dangerous in the form of their cations and also when bonded to short chains of carbon atoms • biochemically, the mechanism of their toxicity action arises from the strong affinity of the cations for sulfur  ‘sulfhydryl’ groups, -SH, readily attach themselves to ingested heavy metals cations or molecules that contains the metals

  4. TOXICITY OF THE HEAVY METALS • sulfhydryl’ groups  occur commonly in the enzymes that control the speed of critical metabolic reactions in the human body • the toxicity for Hg, Pb, Cd and As  depends very much on the chemical form of the element  upon its speciation  example: the toxicity of metallic lead, lead as the ion Pb2+, and lead in the form of covalent molecules differ substantially

  5. TOXICITY OF THE HEAVY METALS • for some heavy metals such as Hg  the form that is the most toxic  having alkyl groups attached to the metal  many such compounds are soluble in animal tissue and can pass through biological membranes • the toxicity of a given concentration of heavy metal present in a natural waterway  depends on the pH and the amounts of dissolved and suspended carbon  interactions such as complexation and adsorption may well remove some of the metal ions from potential biological activity

  6. BIOACCUMULATION OF THE HEAVY METALS • the only one of the four heavy metals (Hg, Pb, Cd and As) that is indisputedly capable of doing biomagnification  Hg • the extent to which a substance accumulates in a human or in any other organisms depends on: • the rate of intake  R  at which it is ingested from the source • the rate of elimination  kC  the mechanism by which it is eliminated, that is, its sink. C  organism’s concentration of the substance

  7. BIOACCUMULATION OF THE HEAVY METALS • if none of the substance is initially present in an organism  C = 0  initially rate of elimination is zero  the concentration builds up solely due to its ingestion • as C rises  the rate of elimination also rises  eventually matches the rate of intaje if R is a constant  once this equality achieved, C does not vary thereafter  steady state • under steady state conditions: rate of elimination = rate of intake  kC = R the steady state value for the concentration is: Css = R/k

  8. MERCURY:THE FREE ELEMENT • employed in hundreds of applications  its unusual property of being a liquid that conducts electricity well • the most volatile of all metals  its vapor is highly toxic  diffuses from the lungs into bloodstream  crosses the blood-brain barrier  enter the brain  serious damage to the central nervous system  difficulties with coordination, eyesight and tactile senses • adequate ventillation is required  the equilibrium vapor pressure of mercury is hundreds of times the maximum recommended exposure

  9. MERCURY: MERCURY AMALGAMS • mercury readily forms amalgam  solutions or alloys with almost any other metal or combination of metals  example: the “dental amalgam”  is prepared by combining approximately equal proportions of liquid mercury and a mixture that is mainly silver and tin • in working some ore deposits  tiny amounts of elemental gold or silver are extracted from much larger amounts of dirt by adding elemental mercury to the mixture  this extracts gold or silver by forming an amalgam  is then heated to distill of the mercury

  10. MERCURY: THE CHLORALKALI PROCESS • amalgam of sodium and mercury  some industrial chloralkali plants  converts aqueous sodium chloride into the commercial products chlorine and sodium hydroxyde (and hydrogen) by electrolysis: •  to form pure solution of NaOH  flowing mercury is used as the negative electrode (cathode) of the electrochemical cell  produce metallic sodium by reduction  removed from NaCl solution without reacting in the aqueous medium : Hg • Na+(aq) + e- Na (in Na/Hg amalgam)

  11. MERCURY:THE CHLORALKALI PROCESS • the reactivity of sodium dissolved in amalgams is greatly lessened than its free state form  highly reactive elemental sodium in Na-Hg amalgam does not react with the water in the original solution  amalgam is removed  induced by the application of a small electrical current  to react with water in a separate chamber  produce salt-free sodium hydroxyde  the mercury is then recovered and recycled back to the original cell

  12. MERCURY:THE CHLORALKALI PROCESS • the recycling of mercury is not complete  enter the air and the river  to be oxidized to soluble form by the intervention of bacteria that present in natural waters  becomes accessible to fish

  13. MERCURY: IONIC MERCURY • the common ion mercury  the 2+ species  Hg2+ mercuric or mercury (II) ion  example: HgS  very insoluble in water • most of the mercury deposited from the air  in the form of Hg2+ • in natural waters  Hg2+ is attached to suspended particulates and is eventually deposited in sediments

  14. MERCURY: METHYLMERCURY FORMATION • mercuric ion Hg2+ with anions that are more capable forming covalent bonds (than are nitrate, oxide or sulfide ions)  forms covalent molecules rather than ionic solid • HgCl2 is a molecular compound  Cl- ions form a covalent compound with Hg2+ • the methyl anion, CH3-, with Hg2+ the volatile molecular liquid dimethylmercury, Hg(CH3)2

  15. MERCURY:METHYLMERCURY FORMATION • the process of dimethylmercury formation occurs in the muddy sediments of rivers and lakes, especially under anaerobic conditions  anaerobic microorganisms convert Hg2+ into Hg(CH3)2 pathway of production and fate of dimethylmercury and other mercury species in a body of water • the less volatile ‘mixed’ compounds CH3HgCl and CH3HgOH  written as CH3HgX  methylmercury  more readily formed in the same way as dimethylmercury

  16. MERCURY:METHYLMERCURY FORMATION • methylmercury production predominates in acidic or neutral aqueous solutions • methylmercury is more potent toxin than are salts of Hg2+ ingestion of CH3HgX  converted to compounds in which X is a sulfur-containing amino acid  soluble in biological tissue  cross both the blood-brain barrier and the human placental barrier  methylmercury the most hazardous form of mercury, followed by the vapor of the element

  17. MERCURY:BIOGEOCHEMICAL CYCLE

  18. MERCURY:BIOGEOCHEMICAL CYCLE

  19. ANTHROPOGENIC PERTURBATION: fuel combustion waste incineration mining THE MERCURY CYCLE: MAJOR PROCESSES Atomic wt. 80 Electronic shell: [ Xe ] 4f14 5d10 6s2 oxidation Hg(II) Hg(0) reduction highly water-soluble volatilization evapo- transpiration volcanoes erosion deposition particulate Hg oxidation Hg(II) Hg(0) biological uptake reduction uplift burial SEDIMENTS

  20. GLOBAL MERCURY CYCLE (NATURAL) Inventories in Mg Rates in Mg y-1 Selin et al. [2007]

  21. GLOBAL MERCURY CYCLE (PRESENT-DAY) Inventories in Mg Rates in Mg y-1 Selin et al. [2007]

  22. CONTRIBUTIONS TO N. AMERICAN MERCURY DEPOSITION FROM THE GLOBAL vs. REGIONAL POLLUTION POOL N. America accounts for only 7% of global anthro. emission (2000) Global pool (lifetime ~ 1 y) Hg(0) Hg(II) Hg(0) emission (53%) N. American boundary layer External anthropogenic Oceans Land reduction Regional pollution pool Hg(II) Hg(II) emission (47%) NORTH AMERICA cycling and re-emission

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