KIMIA LINGKUNGANKIMIA LINGKUNGANBAGIAN 4: HIDROSFER3. LOGAM BERAT DI DALAM AIR

COMMON FEATURESCOMMON 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 METALSTOXICITY 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 METALSTOXICITY 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 METALSTOXICITY 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 BIOACCUMULATION OF THE HEAVY METALSMETALS 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 BIOACCUMULATION OF THE HEAVY METALSMETALS 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

= Rthe steady state value for the

concentration is: Css = R/k

MERCURY:MERCURY: THE FREE ELEMENTTHE FREE ELEMENTemployed 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: MERCURY AMALGAMS MERCURY AMALGAMSmercury 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:MERCURY: THE CHLORALKALI PROCESS THE CHLORALKALI PROCESSamalgam 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:MERCURY: THE CHLORALKALI PROCESSTHE 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:MERCURY:

THE CHLORALKALI PROCESSTHE CHLORALKALI PROCESSthe 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:MERCURY: IONIC 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:MERCURY: METHYLMERCURY FORMATION METHYLMERCURY FORMATIONmercuric 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:MERCURY: METHYLMERCURY FORMATIONMETHYLMERCURY 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:MERCURY: METHYLMERCURY FORMATIONMETHYLMERCURY FORMATIONmethylmercury 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 hazardous form of mercury, followed by the vapor of the element

MERCURY:MERCURY:BIOGEOCHEMICAL CYCLEBIOGEOCHEMICAL CYCLE

MERCURY:MERCURY:BIOGEOCHEMICAL CYCLEBIOGEOCHEMICAL CYCLE

THE MERCURY CYCLE: MAJOR PROCESSESTHE MERCURY CYCLE: MAJOR PROCESSES

Hg(0) Hg(II)

particulate

Hg

burial

SEDIMENTS

uplift

volcanoeserosion

oxidation

reduction

volatilizationevapo-

transpiration

Hg(0) Hg(II)oxidation

reduction

deposition

biologicaluptake

ANTHROPOGENIC PERTURBATION:

fuel combustionwaste incineration

mining

Atomic wt. 80Electronic shell: [ Xe ] 4f14 5d10 6s2

highly water-soluble

GLOBAL MERCURY CYCLE (NATURAL)GLOBAL MERCURY CYCLE (NATURAL)

Inventories in MgRates in Mg y-1

Selin et al. [2007]

GLOBAL MERCURY CYCLE (PRESENT-GLOBAL MERCURY CYCLE (PRESENT-DAY)DAY)

Inventories in MgRates in Mg y-1

Selin et al. [2007]

CONTRIBUTIONS TO N. AMERICAN MERCURY DEPOSITION CONTRIBUTIONS TO N. AMERICAN MERCURY DEPOSITION FROM THE GLOBAL vs. REGIONAL POLLUTION POOLFROM 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 pollutionpoolHg(II) emission

(47%)

reduction

External anthropogenicOceansLand

N. America accountsfor only 7% of globalanthro. emission (2000)

NORTH AMERICA

cycling and re-emission