Chapter 7 Toxic Heavy Metals

1
The five main heavy metals discussed in this
chaptermercury (Hg), lead (Pb), cadmium
(Cd), chromium (Cr), and arsenic (As) present
the greatest environmental hazard due to their
extensive use, their toxicity, and their
widespread distribution.
2
Heavy Metals

• Middle bottom of periodic table higher density
  than most common materials
• Nonbiodegradable toxic nontoxic forms
  transported by air as gases or in suspended
  particulates

3
Toxicity of Heavy Metals

• Free elements only Hg is toxic
• Cations or organometallic species often quite
  toxic
• Cations bind to sulfhydryl (-SH) group (cysteine
  amino acid) in enzymes disrupting the vital
  metabolic reaction catalyzed by the enzyme

4
Compounds that Bind Heavy Metals

• BAL and Calcium salt of EDTA used for heavy metal
  poisoning

5
Speciation

• Insoluble forms pass through with little harm
• Some are acute toxins (arsenic oxide)
• Some pass through blood-brain barrier
  (organometallics)
• In water pH and dissolved and suspended C are
  important factors (complexation and adsorption
  will remove some of the metals)

6
Bioaccumulation of Heavy Metals

• Only Hg compounds biomagnify but other
  bioconcentrate (oyster and mussels Cd and Hg
  concentration 105 times water in which they live)

7
Mercury Vapor

• Used in electrical switches (before 2000
  automobile convenience trunk lights) antilock
  brakes instrument panels vapor lights
  (replaced mostly with Na) fluorescent lamps
• Most volatile of all metals, vapors are highly
  toxic
• Crosses blood brain barrier, oxidized to Hg2
  interferes with central nervous system
  (coordination, eyesight, tactile senses)
• Mercury vapors from burning coal, municipal waste
  (batteries) now rivals emissions of Hg from
  volcanoes
• Hg emissions are mostly gaseous as opposed to
  bound to particles travel a great distance
  before being oxidized and dissolved in rain

8
Mercury Amalgams

• Dental Hg/Ag/Sn small amounts of Hg released
  initially, some countries (Germany) banned Hg in
  fillings for small children and pregnant women
  release of Hg from dead human bodies that are
  cremated?
• Extract Au Ag (1g of Hg into environment for
  each 1g Ag extracted) from dirt by dissolving in
  Hg filtering vaporizing Hg
• S. C. America

9
Hg the Chloralkali Processproduction of Cl2
NaOH from NaCl

• Amalgam of Na Hg to extract Na then conversion
  to pure NaOH by reaction with water

Not all of the Hg is recovered (recycled) in this
process in 1994-95 chloralkali plants
contributed 7/150 tonnes of Hg
electrolysis
2NaCl(aq) 2H2O(l) 2NaOH(aq)
Cl2(g) H2(g) 2Na(aq) 2e-
2Na(in a Na/Hg amalgam) 2Na (in a Na/Hg
amalgam) 2H2O 2NaOH H2
Hg
In N. America the use of Hg electrodes is being
phased out replaced with membranes that only
Na can pass through
10
The 2 Ion of Mercury

• Most common ion Hg2
• e.g. Hg2 S-2 HgS(s)
• HgNO3 used to make felt (mad as a hatter Hg
  attacks nervous system but mainly kidney liver)
• HgO in mercury cell batteries hearing aids
• Most of the Hg deposited from air is Hg2, in
  waters attached to suspended particulates which
  are eventually deposited in the sediments
• The other ion mercury, Hg22, is not very toxic

Waste water in some chloralkali plants
treated this way since HgS is so insoluble.
11
Methylmercury Formationcovalent compounds of Hg

• HgCl2, HgR2
• Hg2 CH3- Hg(CH3)2
• CH3- comes from a derivative of Vitamin B12
  (methylcobalamin) with CH3- bound to Cobalt

anaerobic bacteria
12
Methylmercury

• Hg(CH3)2 is volatile escapes from water and is
  converted to monomethyl HgCH3 form as HgCH3Cl,
  HgCH3OH etc. also bind to proteins
• CH3Hg
• which may be able to cross the blood brain
  barrier and human placental barrier HgCH3 is
  the most toxic form of mercury follow by
  elemental Hg vapor HgCH3 also soluble in
  fatty tissue thus it bioaccumulates
  biomagnifies
• Figure 11-1 Cycling of mercury in freshwater
  lakes (insoluble inorganic forms are converted to
  soluble organomettalic forms by bacteria)

13
Figure 11-1 Cycling of mercury in freshwater
lakes (insoluble inorganic forms are converted to
soluble organomettalic forms by bacteria)
14
Methylmercury Accumulation

• Most all the mercury in humans is methylmecury
  80 from fish -binds to S-H groups thus
  distributed throughout the fish (not like
  nonpolar compounds which are concentrated in the
  fatty tissue)
• Bioconcentration in fish 106 to 107 higher in
  older fish
• Freshwater methylmercury increases with lower
  pH Figure 11-2

15
Lower pH higher mercury concentration,
acidification of lakes raises mercury levels
16
Methylmercury Toxicity

• T1/2 in humans about 70 days, binds to S-H
  groups, fat soluble cumulative poison
• Drop or two of pure demethylmercury on the gloved
  hands of Karen Wetterhahn, Dartmouth cancer
  researcher was fatal http//www.icanbehealthy.com/
  karen_wetterhahn.htm
• 1950s Minamata, Japan, Hg2 catalyst for
  production of PVC, leaked into bay, fish Hg
  100ppm, main diet of fish, thousands of people
  affected, hundreds died
• Symptoms target is the brain, central nervous
  system affected numbness of limbs, blurring/loss
  of vision, loss of hearing and muscle
  coordination, lethargy, irritability, coma
  death. Fetuses absorbed from mother show brain
  damage symptoms similar to cerebral palsy
• Levels in hair of pregnant mothers of 10-20 ppm
  are taken as a warning (WHO)

17
Other Sources of Methyl Mercury

• Fungicide in agriculture industry generally
  broken down in soil trapped sediment by bonding
  to sulfur compounds
• Human deaths from eating seeds that were meant
  for planting also reduction in number of birds
  of prey agriculture use curtailed in N. America
  W. Europe
• Hg2 leached from rocks

18
Mercury as in Preservatives Medications

• Salts of C6H5Hg used to preserve paint in can
  prevent mildew after application also formerly
  as a slimicide in pulp phenylmercury not as
  toxic as methyl mercury
• Mercury (especially Hg22) compounds still used
  in pharmaceuticals cosmetics because of their
  preservative antiseptic qualities

19
Safe Levels of Mercury in the Body

• 99.9 of Americans daily intake methylmercury
  below WHO safe limit
• WHO concluded that mothers wotj10-20 ppm
  methylmercury in their hair would have levels in
  their blood that represent a potential threat to
  the fetus
• Fetuses in areas where fish are a mainstay of the
  diet, such as northern Canada, are at risk

20
LeadThe Free Element

• mp 327oC bp 1740oC (Hg 357oC)
• Uses
• Flashing, roofing
• Pipes
• Bullets, shot (lead poisoning in animals)
• Sinkers
• Pb/Sn alloy solder
• Cooking vessels by the Romans distillation of
  alcohol in lead vessels
• Lead storage battery
• Not as major environmental problem in its
  elemental state as mercury

21
Pb2 in Water and Food as an Environmental Hazard
to Humans

• Most lead is extracted from ore PbS (Galena)
• Pb not soluble even in strong acids (e.g. Pb
  electrodes in lead storage batteries) but as an
  alloy with Sn it may dissolve e.g. solder in tin
  cans, lead dissolves in dilute acids such as
  canned fruits, also solder in copper pipes
  especially soft water

22
Pb2 in Water and Food as an Environmental Hazard
to Humans

• Lead pipes leach lead as Pb2 protective coating
  of PbCO3 forms Phosphate may be added to form
  lead phosphate
• 20 of lead exposure to Americans comes from
  drinking water

23
Lead Salts as Glazes Pigments

• PbO yellow solid used in pottery glaze (water
  proof, high sheen) gradually leaches especially
  under acidic conditions
• Lead pigments
• PbCrO4 yellow PbO3 red Pb3(OH)2 white (taste
  sweet) replaced with TiO2 as white pigment in
  paints (Pb banned in 1978 in indoor house paints
  in U.S.)
• Pb(AsO4)2 pesticide
• Lead stabilizers in PVC

24
Green Chemistry Replacement of Lead in
Electrodeposition Coatings

• Electrodeposition surface is dipped in a bath
  and acts as an electrode as the coating is
  applied electrophoretically has many advantages
  over spray painting, including
• lower air pollution, due to decreased solvent
  emissions,
• better corrosion protection, due to better
  coverage of poorly accessible areas,
• reduced waste, due to high transfer efficiency,
• more uniform coating thickness.

25
Green Chemistry Replacement of Lead in
Electrodeposition Coatings

• PGCC Award PPG, 2001 Yttrium for lead in auto
  paints
• High corrosion resistance of lead has lead to
  exemptions from bans
• Yttrium twice as effective as lead (Pb3O4)
• Lead oxide 120 times more toxic than yittrium
  oxide
• Also reduces nickel and eliminates chromium that
  are used in metal pretreatments
• Potential to eliminate the use of 1 million
  pounds of lead, but also 25,000 pounds of
  chromium and 50,000 pounds of nickel on an annual
  basis

26
Dissolution of Otherwise Insoluble Lead Salts

• Salts of lead (PbCO3, 1.5 x 10-15 PbS 8.4 x
  10-28) generally have low Ksp however the anions
  are relatively strong bases

Reaction of conjugate bases with water increases
solubility
27
Dissolution of Otherwise Insoluble Lead Salts

• In acidic solutions PbS PbCO3 solubilities are
  significantly increased

At pH 4, the solubility of PbS and the
concentration of Pb2 ion in water is calculated
to be 2.5 X108 M, whereas at pH2, the solubility
is 2.5X106 M.
28
Pb4 in Automobile Batteries

• Elemental lead and the lead oxide PbO2 employed
  as the two electrodes in storage batteries in
  almost all vehicles now constitute the major use
  of the element.
• Majority of used lead storage batteries are
  recycled for their lead content. Storage
  batteries that are not recycled constitute the
  main source of lead in municipal waste.

29
Tetravalent Organic Lead as Gasoline Additives

• Pb(CH3)4 Pb(C2H5)4 used as additives in
  gasoline phased out in most developed countries
  (interfered with catalytic converters) except in
  aviation fuel (Figure 11-3),
• Ethylene dichloride dibromide also added to
  combine with liberated lead to prevent buildup of
  lead deposits PbCl2 PbBr2 would then
  volatilize be eliminated in the exhaust
• PbR4 are generally volatile are readily
  absorbed through the skin converted to PbR3
  which acts as a neurotoxin (crosses blood brain
  barrier) high concentrations of PbR4 are fatal

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Pb Effects on Human Reproduction and Intelligence

• Microorganisms bioconcentrate Pb but it does not
  biomagnify in the food chain
• Some Pb directly inhaled, other through plants
  (bioconc.)
• Pb absorption increases with Ca or Fe deficiency

Pb enters through blood soft tissue
bone, replaces Ca2
32
Pb Effects on Human Reproduction and Intelligence

• High Pb general metabolic poison
• Effects
• brain function, decreased attention, retardation
• reproduction including miscarriage, still births
• Most sensitive are children lt7 immature
  blood-brain barrier effects mental development
  Figure 11-4
• Passed to children through mothers milk
• Pb levels in children Figure 11-5
• Not as dangerous as Hg but we are exposed to
  higher levels of Pb

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Environmental Sources of Cadium

• Cd is a byproduct of Zn smelting tends to be
  pollutant around Zn, Cu Pb smelters also from
  coal burning
• Used in Ni-Cd (about 5 g in each) batteries
  (calculators etc.) incineration leads to Cd in
  environment some states European countries
  outlawing such batteries

discharge
Cd(s) 2OH- Cd(OH)2(s) 2e-
charge
35
Environmental Sources of Cadium

• As a pigment CdS (yellow, Van Gogh?) CdSe, hue
  depends on size of particle used to color
  plastics
• CdS also in photoelectric devices TV screens
• Cd plated steel
• Released into environment upon incineration
  (bp765oC)

36
Human Intake of Cd

• Most exposure from food supply seafood organ
  meats high Cd most exposure from potatoes
  grains (eat much more of these)
• Plants absorb Cd from irrigation water
  (contaminant in fertilizers, sewage sludge)
• Cd uptake increases with decreasing soil pH

37
Human Intake of Cd

• Ouch-ouch degenerative bone disease (severe joint
  pain) Japan rice grown with irrigation water
  high in Cd (Jintsu River contaminated from Zn
  mining smelting operation) daily intake 600
  micrograms/day10x intake of N. Americans

38
Protection Against Low Levels of Cd

• Cd acutely toxic (about 1 gram) protection from
  low levels by the S-H rich protein
  metallothionein, complex is excreted (usual
  function regulation of Zn metabolism)
• Cd does not biomagnify
• Cumulative poison if not eliminated quickly (by
  metallothionein), lifetime in body several
  decades

39
Arsenic

• Metalloid properties intermediate between metal
  and nonmetal
• As2O3 common poison used for centuries for murder
  and suicide

40
As(III) Versus As(V) Toxicity

• to some extent mimics P (same group same
  subshell electrons), although more tendency to
  form ionic compounds found with phosphorus
  compounds in nature
• Acute poisoning due to severe vomiting diarrhea
  
• As(III) more toxic than As(V) (reduced to III in
  the body) due to bonding to S-H groups (in
  enzymes) longer retention in the body

41
Anthropogenic Sources of As in the Environment

• Contamination of phosphates
• use as a pesticide Pb3(AsO4)2, insecticide
  herbicides Na3AsO3, Cu3(AsO3)2 (also wood
  preservative)
• Mining smelting Au, Pb, Cu, Ni
• Combustion of coal 1 As in parts of China,
  global average 5 ppm)
• Iron steel production
• Copper chromated arsenic (CCA) used to pressure
  treat lumber 90 of the use of As in US is for
  wood preservatives

42
Arsenic in Drinking Water

• As (much from natural sources) in Drinking water
  (particularly groundwater) is one of the most
  serious environmental health hazards
• Bangladesh tube wells drilled by UNICEF for
  potable water (surface water high bacteria
  concentrations), high concentration of As soils
  some well exceed the 10 ppb WHO guideline by
  50X also used for irrigation thus contaminating
  rice vegetables
• Carcinogen
• Lung, skin, (liver bladder?)
• Synergistic effect between As and smoking

43
Drinking Water Standards for As

• Global average As in drinking water 2.5 ppb
• EU, US WHO limits 10 ppb Canada 25 ppb
  Australia 7 ppb

44
Removal of As from Drinking Water

• Passing water over basic alumina ion exchange
  reverse osmosis are promising
• Precipitation as an insoluble salt addition of
  ferric chloride requires oxidation to AsV

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Steady State of As in Water
46
As in Organic Other Molecular Forms

• Most are water soluble thus can be excreted
  thus less toxic

Also occurs in human liver
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As in Organic Other Molecular Forms

• Majority of daily As intake from food meat,
  seafood (mostly organic form such as As(CH3)4
  which are readily excreted), LD50 Table 11-2
• As(CH3)3 AsH3 by contrast are the most toxic
  forms of As
• Molds form As(CH3)3 from arsenic containing
  pigments in wallpaper releasing this as a gas
  into the room
• Cancer risks from As on the same level as ETS
  radon

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Chromium

• Common oxidation states Cr3 Cr6, in aerobic
  conditions Cr exists primarily as the chromate
  ion highly soluble, toxic, can dimerize to
  dichromate
• Anerobic (reducing) conditions Cr exists the 3
  ion not very toxic or soluble under basic
  neutral or even acidic conditions

50
Cr Contamination of Water

• Used for electroplating, corrosion resistance
  tanning
• Second most abundant inorganic contaminant of
  groundwater under hazardous waste sites
• MCL in US 100 ppb
• Removal most heavy metals increase the pH but
  Cr6 soluble at any pH but Cr3 low solubility so
  Cr6 usually reduced to 3

51
Green Chemistry Removing the As Cr from
Pressure Treated Wood

• Untreated exposed exterior wood decays in 3-5
  years by termites, fungi etc.
• Pressure treated wood, ptw (lasts 10 -20 times
  longer) is first placed under vacuum to dehydrate
  the wood cells then under pressure is treated
  with CCA solution (35.3 CrO3, 19.6 CuO, 45.1
  As2O3)
• 2001 7 billion board feet (enough to build
  450,000 homes) of ptw was produced. 150 million
  pound of CCA was used containing 60 million
  pounds of Cr6, and 40 million pounds of As

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Green Chemistry Removing the As Cr from
Pressure Treated Wood

• A single 12-ft long 2 X6 board contains about 27
  g of arsenic, enough to kill more than 200
  adults.
• Although the preservatives are locked the wood
  some are leached out there is also concern over
  direct contact with the wood
• US CCA treated lumber as of 1/1/04 was phased out
  for consumer use

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Green Chemistry Removing the As Cr from
Pressure Treated Wood

• CCA preservative has been replaced by quaternary
  ammonium salts (ACQ) low toxicity (unrestricted
  pesticide)
• Quaternary Ammonium salts are also used in
  detergents, and disinfectants as well as
  fungicides algaecides in lakes, rivers,
  streams, fish hatcheries potable water supplies