HgCl 4 2 - Soil and Water Science
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Transcript HgCl 4 2 - Soil and Water Science
BEHAVIOR OF TRACE METALS IN
AQUATIC SYSTEMS:
EXAMPLE CASE STUDIES (Cont’d)
Environmental Biogeochemistry of
Trace Metals
(CWR6252)
PART#2
2. Metals in Water with Ligands
2.1.a. Chloride as example single ligand in
water containing Hg
Hg2+
-8
HgCl+
-6
HgCl2
-4
HgCl3-
-2
HgCl42-
0
log [Cl-] (M)
Distribution of Hg chloro-complexes in water as a function of chloride
concentration.
2.1.b. Hg in Water with Chloride as Ligand (Cont’d)
Distribution of Hg chloro-complexes in water as a function of molar concentration of chloride
2.2. REDOX and Hg in Water Containing Chloride and
Sulfide as Additional Ligands
Example
Water
oxidized
HgCl 02
Hg 22
Hg(OH)2
Hg0
(aq)
HgHS 02(aq )
Most
Ground
waters
Most
surface
waters
Hg 0(aq)
Water reduced
HgS 22
Eh – pH Diagram
for the chemical
system:
H2O – Hg – Cl - S
2.3. Hg in Water Containing Several Ligands
and Competitive Cations
• Hg2+ - A “Type B” metal cations
• The electronegativity (en) of ligand donor atoms
and the stability (n) of formed complexes with
type B metal ions vary in the following order:
• Low en
High en
S I Br Cl N O F
• High n
Low n
* Complex stability with halides decrease in the order : I- > Br- > Cl- > F
* Complexes with N-containing ligands are favored over O-containing ligands
Use of Geochemical Equilibrium Models
Example simple run using MINEQL+ for a filtered surface water sample at 250C (pH 7)
Ions
Conc. (M)
Na+
5.22
K+
0.38
Ca2+
4.50
Mg2+
1.25
Hg2+
0.05
Cl-
22.57
NO3-
8.06
SO42-
8.33
HCO3-
3.28
3. Mercury Species not easily Predicted
by Thermodynamics Equilibrium
METHYLMERCURY COMPOUNDS
•Equilibrium based thermodynamics
models fail to predict the natural
occurrence
of
organometallic
compounds formed in reactions
catalyzed by microorganisms
•Analytical techniques become the
most prevalent tool for detection of
such compounds
•Primary difficulty = detection of
naturally occurring compounds at subparts per trillion levels
Environment-Relevant Organo-metallic
Compounds of Mercury
DEFINITION
• ORGANOMETALLIC
– Compounds with at least one carbon-metal bond
[e.g., -C-Me- ]
– The first organometallic compound [Zn(CH3)2] was
synthesized in ~1848 by Frankland
(English Chemist = father of organometallic chemistry)
SYNTHESIS AND USE OF MAN MADE ORGANOMETALLIC
COMPOUNDS AND INTRODUCTION TO THE ENVIRONMENT
• SYNTHESIS OF ORGANOMETALLIC
• METAL DISPLACEMENT
• DOUBLE REPLACEMENT
• HYDROMETALLATION
• REACTIONS OF METAL WITH ORGANIC HALIDES
• EXAMPLE USES OF ORGANOMETALLIC COMPOUNDS
• Catalysts in industrial activities and release via industrial effluents
(e.g., Grignard reagent Mg + CH3Br Diethyl-etherCH3MgBr)
• Pesticides/Herbicides (e.g. Sn, Hg, and As organo-compounds)
EXAMPLES OF BANNED
ALKYL-METALS
Environmental pollution from lead (Pb)
is mainly a problem arising from the
use of tetra-alkyllead compounds as
anti-knock additives. Although this use
is diminishing, the more stable forms,
tri- and di-alkyllead are fairly persistent
in the environment.
C 2H 5
I
H5C2—Pb—C2H5
I
CH3HgX
----------------------1.MINAMATA, JAPAN
Chemical rxn to
produce
acetaldehyde
used Hg sulfate
as a catalyst
and discharged
in wastewaters
(1932-1968)
C 2H5
Tetra-ethyllead
2. IRAQ: Methyl mercury
dressed germination seeds
and Hg poisoning in Iraq
(Occurred in 1960’s)
NATURAL SOURCES OF ORGANOMETALLIC
COMPOUNDS
THE FOLLOWING ARE METALS WITH
KNOWN NATURALLY PRODUCED AND
STABLE ORGANOMETALLIC COMPOUNDS
•
•
•
•
•
•
•
•
•
•
•
Hg
Ge
Sn
As
Se
Te
Pd
Pt
Au
Tl
Pb
METHYLATION AND DEMETHYLATION IN
AQUATIC SYSTEMS
– Example 1:
Biomethylation catalyzed by microorganisms
Carbo-anion: CH3TEAox
TEAred
Hg 2 CH3 CH3Hg
bacteria
Radical: CH3*
CH3Hg CH3
(CH3 ) 2 Hg
– Example 2: Abiotic methylation – catalyzed by:
Humic acids
Trans-metallation (Me1 + Me2R Me1R + Me2)
DETERMINATION OF METHYLMERCURY IN
ENVIRONMENTAL SAMPLES
Aqueous Samples
Distillation - Derivatization methods GCseparation Thermodecomposition AFS
detection (EPA’s Method 1630)
Solid Samples (soil/sediments and biota)
Extraction from solid phase (organic extraction,
distillation) Derivatization methods GCseparation Thermodecomposition AFS
detection
Stability of Methylmercury and of its Selected
Complexes in Aquatic Systems
REACTIONS...............................................................................LogK
Hg 2 CH3 CH3Hg ............................................................ 50
CH3Hg CH3 (CH3 ) 2 Hg..................................................... 37
CH3Hg H 2O CH3HgOH H ......................................... 4.63
CH3Hg CO32 CH3HgCO 3 ............................................... 6.1
CH3Hg SO 24 CH3HgSO 4 ............................................... 0.91
CH3Hg S2 CH3HgS ................................................... 21.02
HgS CH 4 CH3HgS H ................................................. 26
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.
PART-3
• Adsorption patterns and kinetics
• Redox and bioavailability
• Remediation strategies
• Toxicity and toxicity mechanims