BEHAVIOR OF TRACE METALS IN AQUATIC SYSTEMS: EXAMPLE CASE STUDIES (Cont

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BEHAVIOR OF TRACE METALS IN AQUATIC SYSTEMS
EXAMPLE CASE STUDIES (Contd)

• Environmental Biogeochemistry of Trace Metals
• (CWR6252)

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4. Interaction of Aqueous Mercury Species with
Solid Phases
4.1. Surface Properties of Colloidal Particles

• Specific surface area Typical measured values
  for natural particles are
• Kaolinite
• 5 to 20 m2/g
• Montmorillonite
• 700 800 m2/g
• Fulvic and Humic Acids
• 700 to 10000 m2/g
• Determines the extent of sorption capacities of
  particles

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4.1.1. THE ELECTRICAL DOUBLE LAYER
Zeta potential the electrical potential that
exists at the surface of a particle, which is
some small distance from the surface. The
development of a net charge at the particle
surface affects the distribution of ions in the
neighboring interfacial region, resulting in an
increased concentration of counter ions close to
the surface. Each particle dispersed in a
solution is surrounded by oppositely charged ions
called fixed layer. Outside the fixed layer,
there are varying compositions of ions of
opposite polarities, forming a cloud-like area.
Thus an electrical double layer is formed in the
region of the particle-liquid interface.
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The double layer may be considered to consist of
two parts (1) - an inner region which includes
ions bound relatively strongly to the surface
(2) an outer region, or diffuse region, in
which the ion distribution is determined by a
balance of electrostatic forces and random
thermal motion. The potential in this region
decays with the distance from the surface, until
at a certain distance it becomes zero
Adsorption based on electrostatics physical
process where charge density on both the colloid
and solution determine the extent of
sorption Particle-.Na K(aq) ? particle-.K
Na(aq)
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Specific adsorption
Fe
Fe-OH
O
Hg(H2O)22
Hg
O
Fe
Fe-OH
2H3O

• Forming of specific covalent chemical bonds
  between the solution species and the surface
  atoms of the particles
• Covalent binding of a cation to the surface
  shifts the particle pzc to a lower value, while
  binding of an anionic produces an upward shift.

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Types and Size Classification of Particles in the
Hydrosphere

• Diameter (m)

10-10
10-8
10-5
10-2
Molecules
Clay minerals.humic acids
Suspended sediments
Bacteria
Viruses
Algae
SOLUBLE
COLLOIDAL
PRECIPITATED
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Functional Groups Commonly Found on Particles

• Functional groups on natural particles can
  interact with
• H, OH-, metal ions, and other ligands when Lewis
  acid sites (e.g. ?Al and ?Fe) are available
• Many inorganic particles (oxides and silicates)
  contain hydroxo groups, carbonates, and sulfides
  which are exposed
• Surfaces of humic acids are characterized
  primarily by carboxylic and phenolic-OH groups
• Biological surfaces contain primarily
• COOH, -NH2, and OH groups
• These groups have the ability to bind protons and
  metal ions

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QUANTITATIVE DESCRIPTIONS OF ADSORPTION
Adsorption of Hg(II) onto silica (SiO2).
Experimental data points and equilibrium model
line (Tiffreau et al.)
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Example Adsorption Patterns of Metal Cations
Extent of surface complex formation measured as
mol of the metal ion adsorbed to the iron oxide
surfaces as a function of pH (Dzombak and Morel,
1990)
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Example Adsorption Patterns of Oxyanion Forming
Elements
Extent of surface complex formation with metal
ions adsorbed to the iron oxide surfaces as a
function of pH (Dzombak and Morel, 1990)
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Removal of Metal Solution and Phase Distribution
Aggregation/Coagulation/Flocculation Kd - BCF

• Coagulation in natural waters refers to the
  aggregation of particles due to electrolytes
  (e.g. coagulation of suspended solids as salinity
  increases toward the mouth of a river estuary)
• Flocculation is important in water treatment when
  iron (FeSO4), FeCl3) and aluminum (Al2(SO4)3)
  salts are used to destabilize colloids and to
  form polymers and precipitates (Fe(OH)3 and
  Al(OH)3 that promote flocculation

• Distribution Coefficient (Kd)
• Bio-concentration Factor (BCF)
• hydrophilic vs. hydrophobic compounds

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Effects of Redox Chemistry on Fate of Metals in
Aquatic Systems Hg as Example

• Atmosphere

Hg
?????????
Water
Hg
?????????
?????????
Sediments
Hg
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5. Aspects of Remediation of Contaminated Waters
Using Metals
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Mechanisms of Remediation with Metals and
Importance of particle size
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• Use of Zero Valent Iron (ZVI) as a Case Study

• Remediation mechanisms based on interaction of
  the pollutant or water with the bare ZVI surfaces
• 1.1. Mechanism-1 Direct reduction at the metal
  surface
• 1.2. Mechanism-2 Reduction by ferrous iron
  Fe(II) produced after Fe0 corrosion
• 1.3. Mechanism-3 Reduction by hydrogen with
  catalysis

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1.1. Mechanism-1 Direct reduction at the metal
surface
Electrons are transferred from Fe(0) to the
adsorbed pollutant at the metal-water
interface Fe0 ? Fe2 2e- 2RX(organic
pollutant) 2H 2e-? 2RH 2X- _______________
_______________
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1.2. Mechanism-2 Reduction by ferrous iron
Fe(II) produced after Fe0 corrosion
1. Fe(0) is corroded 2. Fe(II) is formed and
electrons are transferred 3. Oxidation up to
production of Fe(III) 2Fe0 ? 2Fe2 2e- 2H2O
2e- ? H2 2OH- 2Fe2 ? 2Fe3 2e- 2RX
(organic ) 2H 2e-? 2RH 2X-
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Hydrogen from the anaerobic corrosion of Fe(II)
could react with the pollutant if an effective
catalyst is present
1.3. Mechanism-3 Reduction by H2 with catalysis
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Bare ZVI surfaces vs. Oxide layers

• Hydrogenation plays a minor role in most systems
  as iron surfaces become very quickly oxidized and
  covered with precipitates
• Oxide layers formed at the iron surfaces become
  more important in ZVI-based remediation process

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• 2. Oxide Layer Formation at ZVI surfaces and
  Mediation of Electron Transfer from Fe0 to
  adsorbed pollutants
• 2.1. Mechanism-1 Direct electron transfer from
  Fe(0) to the pollutant in a corrosion pit
• 2.2. Mechanism-2 Oxide film mediated electron
  transfer from Fe(0) to pollutant by acting as a
  semi-conductor
• 2.3. Mechanism-3 Oxide layer as a coordinating
  surface containing sites of Fe(II) that interact
  with the pollutant

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2.1. Mechanism-1 Direct electron transfer from
Fe(0) to the pollutant in a corrosion pit
Deficiency in oxide layer coating Direct
electron transfer from metallic
iron Interaction with pollutant similar to
those described earlier with bare ZVI
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2.2. Mechanism-2 Oxide film mediated e-
transfer from Fe0 to pollutant by acting as a
semi-conductor
In this case, the oxide layer acts as a
semiconductor, allowing electron transfer From
the metallic iron to the pollutant adsorbed on
it. The breakdown of the Pollutant occurs at the
oxide layer.
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2.3. Mechanism-3 Oxide layer as a coordinating
surface Fe(II) that interact with the pollutant
Adsorption and immobilization predominates
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6. METAL INTERACTIONS WITH BIOLOGICAL SYSTEMS

• Implications for Toxicity

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6.1. Electronegativity (En) and toxicity of
chemical compounds
Pauling Electronegativity (En Zeff/r2)
The En difference between two atoms in a chemical
compound determines the degree of charge
separation or polarity and therefore the degree
of solubility in aqueous versus organic solvent.
Examples NaCl and CCl4 Na 0.82 Cl 2.96
C1.9 ? DEn (NaCl) 2.9-0.822.14 and DEn
(CCl4) 2.9-1.9 1.0
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6.2. TWO MAJOR FEATURES OF CHEMICALS ASSOCIATED
WITH TOXICITY

• Lipophilicity (solubility in lipids)
• Electrophilic reactivity (reactivity toward
  electron-rich nucleophiles such SH groups)

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6.3. METALLOIDS AND BIOLOGICAL EFFECTS

• The following are metalloids with known toxicity
  and quite well-studied toxicity mechanisms
• Arsenic (As)
• Selenium (Se)
• Tin (Sn)
• Antimony (Sb)
• Tellurium (Te)
• They have high En and provide primarily
  covalently bound compounds and form acidic
  (amphoteric) hydroxides

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6.3.1. ARSENIC

• As compounds react readily with nucleophiles
• Most As-compounds behave like organic
• compounds, w/ tetrahedral configuration and
  covalent centers
• Can be methylated to
• Produce As-C bonds

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6.3.1.1. Methylation of Arsenic

• Typical reactions of the Challenger mechanism.
• The top line indicates a mechanism for the
  reduction, As(V) to As(III), resulting in an
  unshared pair of electrons on As. Structures are
  as follows R1 R2 OH ?arsenate R1 CH3, R2
  OH ?methylarsonate R1 R2 CH3
  ?dimethylarsinate.
• The bottom line indicates the methylation of an
  As(III) by S-adenosyl methionine or SAM shown in
  abbreviated form as CH3-S-(C)2. A proton is
  released and SAM is converted to
  S-adenosylhomocysteine abbreviated form,
  S-(C)2.

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Challenger mechanism Conversion of arsenate to
trimethylarsine (A) Arsenate (B) arsenite
(C) methylarsonate (D) methylarsonite (E)
dimethylarsinate (F) dimethylarsinite (G)
trimethylarsine oxide (H) trimethylarsine. Top
line structures show As(V) intermediates.
Vertical arrows reduction of As(V) to As(III)
species shown in bottom line Diagonal arrows
indicate the methylation steps by SAM
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• Expanded version of the Challenger mechanism
  Roles of different components both in the cells
  themselves and in the surrounding medium. The
  double vertical lines indicate cell walls.
• Phosphate transport system
• thiols and/or dithiols
• active transport system
• active/passive transport
• passive diffusion.
• Abbreviations MMAV, methylarsonic acid DMA,
  dimethylarsinic acid TMAO, trimethylarsine
  oxide.

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6.3.2. SELENIUM

• Shares many of the properties of sulfur and
  arsenic
• Its compounds are covalent
• Selenite (Se4) and selenate (Se6) are most
  stable oxidation states
• Replaces S in cysteine and methionine
• Accumulation plants makes forage toxic to animal
• Evapoconcentrated in aquatic systems
• Example of The San Joaquim Valley, CA and
  suggested remediation

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6.3.3. TELLURIUM

• Not particularly toxic
• The most notable result of Te exposure/intake is
  a very strong body odor called Tellurium Breath
  from biochemical reduction and methylation to the
  garlic-ordored dimethyl-telluride

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6.3.4. TIN

• The most metallic of the metalloids
• Elemental Sn is safe (e.g. tin cans) and stannous
  fluoride is approved for use in toothpaste
• However, TBT tributyltin is extremely toxic
• TBT-oxide and chloride used as antifouling, but
  highly toxic to aquatic biota. Shellfish are
  killed with levels as low as 10 to 20 ng/L or ppt.

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6.4. METALS IN BIOLOGICAL SYSTEMS

• Similar to metalloids, the toxicity of metals is
  governed by their degree of (1) lipo-solubility
  and (2) electrophilic reactivity
• The following are elements with well-studied
  toxicity mechanisms
• Mercury (Hg)
• Lead (Pb)
• Thallium (Tl) and Bi
• Transition metals (Cr, Mn, Co, Ni, Cu, Zn, Mo,
  Ag, and Cd)
• Radioactive elements (Uranium (U) and Radium
  (Ra))

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6.4.1. Metals with naturally produced
methyl-compounds

• Mercury (Hg)
• Lead (Pb)
• Thallium (Tl)
• Gold (Au)
• Platinum (Pt)
• Palladium (Pd)
• Elements with stable alkyl-compounds in
  natural systems

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6.4.2.Mercury

• Forms primarily covalent bonds in both inorganic
  and organic compounds, which increase
  liposolubility
• High affinity for SH groups
• Treatment in case of Hg-poisoning
• Inorganic Hg species Dimercaprol (intramuscular)
  penicillamine (orally)
• Methyl-Hg Binding resins

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6.4.3. Lead (Pb)

• Binds to -SH containing substrates
• Inhibits HEME biosynthesis (?low hemoglobin)
• Replaces Ca in bones and biochemical processes,
  affects ATP synthesis (mitochondrial ATP)
• Disturbance of Ca-metabolism alters brain
  neurotransmitter functions and inhibits Na/K
  ATPase
• Treatment Ca-EDTA is used, but not efficient if
  brain poisoning

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6.4.4. Thallium (Tl) and Bismuth (Bi)

• THALLIUM used in electronics and its sulfates
  were used as poison for rats
• Symptoms hair loss (Alopecia). Was used in
  depilatories at some point
• Toxicity due to competition with K and effect on
  Na/K
• Treated with BAL (British anti-lewisite)
• BISMUTH no outstanding toxicity. Pepto-bismol
  (anti-acid)

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6.4.4. Radium (Ra)

• Was used to produce numerals on clocks, phones,
  and other instruments
• Similar to Ca
• Radioactive decay produces RADON (Rn)

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4.5. Cu, Zn, Cd, Mo, Cr, Mn, Ni

• Mn Manganism ressembles parkinsonism and is due
  to exposure to airborne MnO2 or water with
  16-18ppm Mn.
• MMT methylcyclopentadienylmanganese tricarbonyl
  C6H8Mn(CO)3 in non-leaded fuels? Mn3O4.