Transcript Hg - Soil and Water Science

BEHAVIOR OF TRACE METALS IN
AQUATIC SYSTEMS:
EXAMPLE CASE STUDIES (Cont’d)
Environmental Biogeochemistry of
Trace Metals
(CWR6252)
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
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.
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)
Specific adsorption
Fe
Fe-OH
O
+ Hg(H2O)22+
Hg
O
Fe-OH
Fe
+ 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.
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
SOLUBLE
Algae
COLLOIDAL
PRECIPITATED
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
QUANTITATIVE DESCRIPTIONS OF ADSORPTION
Langmuir
Freundlich
X *b*C
X m
1  bC
x
 KCen
m
Adsorption of Hg(II) onto silica (SiO2).
Experimental data points and equilibrium model line
(Tiffreau et al.)
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)
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)
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
(Kd)
Kd 
Coefficient
Concentrat ion (solid )
Concentrat ion (aq)
• Bio-concentration Factor
(BCF)
•
hydrophilic vs. hydrophobic
compounds
Concentrat ion ( biota )
BCF 
Concentrat ion ( water )
Effects of Redox Chemistry on Fate of Metals
in Aquatic Systems: Hg as Example
Atmosphere
Hg
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Water
Hg
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Sediments
Hg
5. Aspects of Remediation of
Contaminated Waters Using
Metals
Mechanisms of Remediation
with Metals and Importance
of particle size
Use of Zero Valent Iron (ZVI) as a Case
Study
1.
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
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+ + 2e2RX(organic pollutant) + 2H+ + 2e- 2RH + 2X______________________________
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+ + 2e2H2O + 2e-  H2 + 2OH-
2Fe2+  2Fe3+ + 2e2RX (organic ) + 2H+ + 2e- 2RH + 2X-
1.3. Mechanism-3:
Reduction by H2 with catalysis
Hydrogen from the anaerobic
corrosion of Fe(II) could react
with the pollutant if an effective
catalyst is present
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
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
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
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.
2.3. Mechanism-3:
Oxide layer as a coordinating surface
Fe(II) that interact with the pollutant
Adsorption and
immobilization
predominates
6. METAL INTERACTIONS
WITH BIOLOGICAL
SYSTEMS
Implications for Toxicity
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 C:1.9  DEn (NaCl) = 2.9-0.82=2.14 and DEn (CCl4) = 2.9-1.9 = 1.0
6.2. TWO MAJOR FEATURES OF CHEMICALS
ASSOCIATED WITH TOXICITY
• Lipophilicity (solubility in lipids)
• Electrophilic reactivity (reactivity toward
electron-rich nucleophiles such –SH groups)
6.3. METALLOIDS AND BIOLOGICAL
EFFECTS
•
The following are
metalloids with known
toxicity and quite wellstudied 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
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
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].
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
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.
(A) Phosphate transport system
(B) thiols and/or dithiols
(C) active transport system
(D) active/passive transport
(E) passive diffusion.
Abbreviations: MMAV, methylarsonic acid; DMA, dimethylarsinic
acid; TMAO, trimethylarsine oxide.
6.3.2. SELENIUM
• Shares many of the properties of sulfur and arsenic
• Its compounds are covalent
• Selenite (Se+4) and selenate (Se+6) 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
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 dimethyltelluride
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.
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))
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
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
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
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. Peptobismol (anti-acid)
6.4.4. Radium (Ra)
• Was used to produce numerals on clocks,
phones, and other instruments
• Similar to Ca
• Radioactive decay produces RADON (Rn)
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.