BAGIAN 4: HIDROSFER 3. LOGAM BERAT DI DALAM AIR - 43 logam di air

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KIMIA LINGKUNGAN

BAGIAN 4: HIDROSFER

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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

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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

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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

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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

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BIOACCUMULATION OF THE HEAVY

METALS

 the only one of the four heavy metals (Hg, Pb, Cd and As) that is indisputedly capable of doing biomagnifcation  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

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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:

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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  difuses from the lungs into bloodstream 

crosses the blood-brain barrier  enter the brain 

serious damage to the central nervous system 

difculties with coordination, eyesight and tactile senses

 adequate ventillation is required _ the equilibrium

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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

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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  fowing

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

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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 

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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

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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

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MERCURY:

METHYLMERCURY FORMATION

 mercuric ion Hg2+ with anions that are more

capable forming covalent bonds (than are nitrate, oxide or sulfde ions)  forms

covalent molecules rather than ionic solid

 HgCl₂is a molecular compound  Cl- ions

form a covalent compound with Hg2+

 the methyl anion, CH_3-, with Hg2+  the

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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 

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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

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MERCURY:

BIOGEOCHEMICAL CYCLE

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MERCURY:

BIOGEOCHEMICAL CYCLE

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THE MERCURY CYCLE: MAJOR PROCESSES

Hg(0) Hg(II)

Atomic wt. 80 Electronic shell: [ Xe ] 4f14_5d10_6s2

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GLOBAL MERCURY CYCLE (NATURAL)

Inventories in Mg Rates in Mg y-1

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GLOBAL MERCURY CYCLE

(PRESENT-DAY)

DAY)

Inventories in Mg Rates in Mg y-1

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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

N. America accounts for only 7% of global anthro. emission (2000)