KIMIA LINGKUNGAN BAGIAN 4 HIDROSFER 3 LOGAM BERAT

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 Pb 2+, 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 p. H 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 k. C 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 k. C = 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 Na. OH flowing mercury is used as the negative electrode (cathode) of the electrochemical cell produce metallic sodium by reduction removed from Na. Cl 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 Hg 2+ mercuric or mercury (II) ion example: Hg. S very insoluble in water most of the mercury deposited from the air in the form of Hg 2+ in natural waters Hg 2+ is attached to suspended particulates and is eventually deposited in sediments

MERCURY: METHYLMERCURY FORMATION mercuric ion Hg 2+ with anions that are more capable forming covalent bonds (than are nitrate, oxide or sulfide ions) forms covalent molecules rather than ionic solid Hg. Cl 2 is a molecular compound Cl- ions form a covalent compound with Hg 2+ the methyl anion, CH 3 -, with Hg 2+ the volatile molecular liquid dimethylmercury, Hg(CH 3)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 Hg 2+ 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 3 Hg. Cl and CH 3 Hg. OH written as CH 3 Hg. X 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 Hg 2+ ingestion of CH 3 Hg. X 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: BIOGEOCHEMICAL CYCLE

MERCURY: BIOGEOCHEMICAL CYCLE

ANTHROPOGENIC PERTURBATION: fuel combustion waste incineration mining THE MERCURY CYCLE: MAJOR PROCESSES Hg(0) volcanoes erosion volatilization evapotranspiration oxidation reduction Atomic wt. 80 Electronic shell: [ Xe ] 4 f 14 5 d 10 6 s 2 Hg(II) highly water-soluble deposition oxidation Hg(0) reduction uplift particulate Hg(II) biological uptake Hg burial SEDIMENTS

GLOBAL MERCURY CYCLE (NATURAL) Inventories in Mg Rates in Mg y-1 Selin et al. [2007]

GLOBAL MERCURY CYCLE (PRESENTDAY) 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