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

KIMIA LINGKUNGAN KIMIA LINGKUNGAN

BAGIAN 4: HIDROSFER

3. LOGAM BERAT DI DALAM AIR COMMON FEATURES 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 TOXICITY OF THE HEAVY METALS 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 afnity of the cations for sulfur  ‘sulfhydryl’ groups, -SH, readily attach TOXICITY OF THE HEAVY METALS 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

Pb , and lead in the form of covalent molecules difer substantially TOXICITY OF THE HEAVY METALS 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 BIOACCUMULATION OF THE HEAVY BIOACCUMULATION OF THE HEAVY METALS 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

◦ the rate of elimination  kC  the mechanism by which it is eliminated, that is, its sink. C 

BIOACCUMULATION OF THE HEAVY

METALS 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 MERCURY: MERCURY:

THE FREE ELEMENT 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

 MERCURY: MERCURY:

MERCURY AMALGAMS 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 MERCURY: MERCURY:

THE CHLORALKALI PROCESS 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 : MERCURY: MERCURY:

THE CHLORALKALI PROCESS 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 MERCURY: MERCURY:

THE CHLORALKALI PROCESS 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 fsh MERCURY: MERCURY:

IONIC MERCURY



the common ion mercury  the 2+ species

 Hg  mercuric or mercury (II) ion  example: HgS  very insoluble in water



most of the mercury deposited from the air

 in the form of Hg

2+ 

in natural waters  Hg is attached to suspended particulates and is eventually deposited in sediments MERCURY: MERCURY:

METHYLMERCURY FORMATION METHYLMERCURY FORMATION 2+



mercuric ion Hg with anions that are more capable forming covalent bonds (than are nitrate, oxide or sulfde ions)  forms covalent molecules rather than ionic solid



HgCl is a molecular compound  Cl ions

form a covalent compound with Hg

2+ 

the methyl anion, CH , with Hg  the

volatile molecular liquid dimethylmercury, MERCURY: MERCURY:

METHYLMERCURY FORMATION METHYLMERCURY FORMATION 

the process of dimethylmercury formation occurs in the muddy sediments of rivers and lakes, especially under anaerobic conditions

 anaerobic microorganisms convert Hg into Hg(CH

3 ) 2  pathway of production and

fate of dimethylmercury and other mercury species in a body of water



the less volatile ‘mixed’ compounds CH HgCl

and CH HgOH  written as CH HgX 

3 MERCURY: MERCURY:

METHYLMERCURY FORMATION METHYLMERCURY FORMATION 

methylmercury production predominates in acidic or neutral aqueous solutions



methylmercury is more potent toxin than are

salts of Hg  ingestion of CH

3 HgX 

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 MERCURY: MERCURY:

BIOGEOCHEMICAL CYCLE

MERCURY: MERCURY:

BIOGEOCHEMICAL CYCLE

BIOGEOCHEMICAL CYCLE ANTHROPOGENIC

THE MERCURY CYCLE: MAJOR PROCESSES

PERTURBATION: Atomic wt. 80 fuel combustion

Electronic shell: waste incineration ₁₄ ₁₀ ₂

[ Xe ] 4f 5d 6s mining oxidation

Hg(II) Hg(0)

reduction highly water-soluble volatilization volcanoes evapo- deposition erosion transpiration particulate oxidation

Hg Hg(II)

Hg(0)

biological reduction uptake uplift burial

GLOBAL MERCURY CYCLE (NATURAL)

Inventories in Mg _-1 Rates in Mg y

GLOBAL MERCURY CYCLE (PRESENT-

DAY) DAY)

Inventories in Mg Rates in Mg y

CONTRIBUTIONS TO N. AMERICAN MERCURY DEPOSITION CONTRIBUTIONS TO N. AMERICAN MERCURY DEPOSITION

FROM THE GLOBAL vs. REGIONAL POLLUTION POOL FROM THE GLOBAL vs. REGIONAL POLLUTION POOL N. America accounts for only 7% of global

Global pool (lifetime ~ 1 y) anthro. emission (2000)

Hg(0) Hg(II) Hg(0) emission

N. American (53%)

External anthropogenic boundary layer

Oceans Land reduction

Regional pollution Hg(II)

) emission pool

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