KIMIA LINGKUNGAN BAGIAN 4: HIDROSFER 3. LOGAM BERAT DI DALAM AIR.
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KIMIA LINGKUNGAN BAGIAN 4: HIDROSFER 3. LOGAM BERAT DI DALAM AIR
COMMON FEATURES � heavy metals à near the bottom of the periodic table � the densities à high compared to other common
materilas � as water pollutants and contaminants in food à the
most part transported from place to place via the air, as gases or as species adsorbed or absorbed in suspended particulate matter
TOXICITY OF THE HEAVY METALS � mercury vapor is highly toxic à Hg, Pb, Cd and As are
not particularly toxic as the condensed free elements
� Hg, Pb, Cd and As à 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 à ‘sulfhydryl’ groups, -SH, readily attach themselves to ingested heavy metals cations or molecules that contains the metals
TOXICITY OF THE HEAVY METALS � sulfhydryl’ groups à 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 à depends very much on the chemical form of the element à upon its speciation à example: the toxicity of metallic lead, lead as the ion Pb2+, and lead in the form of covalent molecules differ substantially
TOXICITY OF THE HEAVY METALS
� for some heavy metals such as Hg à the form that is the most toxic à having alkyl groups attached to the metal à 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 à depends on the pH and the amounts of dissolved and suspended carbon à interactions such as complexation and adsorption may well remove some of the metal ions from potential biological activity
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 à Hg
� the extent to which a substance accumulates in a human
or in any other organisms depends on: ◦ the rate of intake à R à at which it is ingested from
the source ◦ the rate of elimination à kC à the mechanism by
which it is eliminated, that is, its sink. C à organism’s concentration of the substance
BIOACCUMULATION OF THE HEAVY METALS � if none of the substance is initially present in an
organism à C = 0 à initially rate of elimination is zero à the concentration builds up solely due to its ingestion
� as C rises à the rate of elimination also rises à eventually matches the rate of intaje if R is a constant à once this equality achieved, C does not vary thereafter à steady state
� under steady state conditions: rate of elimination = rate of intake à kC = R the steady state value for the concentration is:
Css = R/k
MERCURY: THE FREE ELEMENT � employed in hundreds of applications à its unusual
property of being a liquid that conducts electricity well � the most volatile of all metals à its vapor is highly toxic à diffuses from the lungs into bloodstream à crosses the blood-brain barrier à enter the brain à serious damage to the central nervous system à difficulties with coordination, eyesight and tactile senses
� adequate ventillation is required à the equilibrium
vapor pressure of mercury is hundreds of times the maximum recommended exposure
MERCURY: MERCURY AMALGAMS � mercury readily forms amalgam à solutions or alloys
with almost any other metal or combination of metals à example: the “dental amalgam” à is prepared by combining approximately equal proportions of liquid mercury and a mixture that is mainly silver and tin
� in working some ore deposits à tiny amounts of
elemental gold or silver are extracted from much larger amounts of dirt by adding elemental mercury to the mixture à this extracts gold or silver by forming an amalgam à is then heated to distill of the mercury
MERCURY: THE CHLORALKALI PROCESS � amalgam of sodium and mercury à some industrial
chloralkali plants à converts aqueous sodium chloride into the commercial products chlorine and sodium hydroxyde (and hydrogen) by electrolysis:
� à to form pure solution of NaOH à flowing mercury is used as the negative electrode (cathode) of the electrochemical cell à produce metallic sodium by reduction à removed from NaCl solution without reacting in the aqueous medium : Hg
� Na+(aq) + e- à Na (in Na/Hg amalgam)
MERCURY: THE CHLORALKALI PROCESS � the reactivity of sodium dissolved in amalgams is greatly
lessened than its free state form à highly reactive elemental sodium in Na-Hg amalgam does not react with the water in the original solution à amalgam is removed à induced by the application of a small electrical current à to react with water in a separate chamber à produce salt-free sodium hydroxyde à the mercury is then recovered and recycled back to the original cell
MERCURY: THE CHLORALKALI PROCESS � the recycling of mercury is not complete à enter the air and the river à to be oxidized to soluble form by the intervention of bacteria that present in natural waters à becomes accessible to fish
MERCURY: IONIC MERCURY � the common ion mercury à the 2+ species à Hg2+ à
mercuric or mercury (II) ion à example: HgS à very insoluble in water
� most of the mercury deposited from the air à in the
form of Hg2+ � in natural waters à Hg2+ is attached to suspended
particulates and is eventually deposited in sediments
MERCURY: METHYLMERCURY FORMATION � mercuric ion Hg2+ with anions that are more capable
forming covalent bonds (than are nitrate, oxide or sulfide ions) à forms covalent molecules rather than ionic solid
� HgCl2 is a molecular compound à Cl- ions form a
covalent compound with Hg2+ � the methyl anion, CH3
-, with Hg2+ à the volatile molecular liquid dimethylmercury, Hg(CH3)2
MERCURY: METHYLMERCURY FORMATION � the process of dimethylmercury formation occurs in the
muddy sediments of rivers and lakes, especially under anaerobic conditions à anaerobic microorganisms convert Hg2+ into Hg(CH3)2 à pathway of production and fate of dimethylmercury and other mercury species in a body of water
� the less volatile ‘mixed’ compounds CH3HgCl and
CH3HgOH à written as CH3HgX à methylmercury à more readily formed in the same way as dimethylmercury
MERCURY: METHYLMERCURY FORMATION � methylmercury production predominates in acidic or
neutral aqueous solutions � methylmercury is more potent toxin than are salts of
Hg2+ à ingestion of CH3HgX à converted to compounds in which X is a sulfur-containing amino acid à soluble in biological tissue à cross both the blood-brain barrier and the human placental barrier à methylmercury the most hazardous form of mercury, followed by the vapor of the element
MERCURY: BIOGEOCHEMICAL CYCLE
MERCURY: BIOGEOCHEMICAL CYCLE
THE MERCURY CYCLE: MAJOR PROCESSES
Hg(0) Hg(II)
particulate Hg
burial
SEDIMENTS
uplift
volcanoes erosion
oxidation
reduction
volatilization evapo-
transpiration
Hg(0) Hg(II) oxidation
reduction
deposition
biological uptake
ANTHROPOGENIC PERTURBATION:
fuel combustion waste incineration
mining
Atomic wt. 80 Electronic shell: [ Xe ] 4f14 5d10 6s2
highly water-soluble
GLOBAL MERCURY CYCLE (NATURAL)
Inventories in Mg Rates in Mg y-1
Selin et al. [2007]
GLOBAL MERCURY CYCLE (PRESENT-DAY)
Inventories in Mg Rates in Mg y-1
Selin et al. [2007]
CONTRIBUTIONS TO N. AMERICAN MERCURY DEPOSITION FROM THE GLOBAL vs. REGIONAL POLLUTION POOL
Hg(0) Hg(II)
N. American boundary layer
Hg(0) emission (53%)
Hg(II)
Global pool (lifetime ~ 1 y)
Regional pollution pool Hg(II) emission
(47%)
reduction
External anthropogenic Oceans Land
N. America accounts for only 7% of global anthro. emission (2000)
NORTH AMERICA
cycling and re-emission