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