Transcript 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

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
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
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
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
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
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
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
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
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)
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
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
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
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
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
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 bloodbrain barrier and the human placental barrier 
methylmercury the most hazardous form of mercury,
followed by the vapor of the element
MERCURY:
BIOGEOCHEMICAL CYCLE
MERCURY:
BIOGEOCHEMICAL CYCLE
ANTHROPOGENIC
PERTURBATION:
fuel combustion
waste incineration
mining
THE MERCURY CYCLE: MAJOR PROCESSES
Atomic wt. 80
Electronic shell:
[ Xe ] 4f14 5d10 6s2
oxidation
Hg(0)
volcanoes
erosion
volatilization
evapotranspiration
Hg(II)
reduction
highly water-soluble
deposition
particulate
oxidation
Hg
Hg(II)
Hg(0)
reduction
uplift
biological
uptake
burial
SEDIMENTS
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
Global pool (lifetime ~ 1 y)
Hg(0)
External anthropogenic
Oceans
Land
N. America accounts
for only 7% of global
anthro. emission (2000)
Hg(II)
Hg(0) emission
(53%)
reduction
Hg(II) emission
(47%)
NORTH AMERICA
cycling and re-emission
N. American
boundary layer
Hg(II)
Regional
pollution
pool