Transcript SO 4 2 - Soil and Water Science

BEHAVIOR OF TRACE METALS
IN AQUATIC SYSTEMS:
EXAMPLE CASE STUDIES
Environmental Biogeochemistry of
Trace Metals
(CWR6252)
INTRODUCTION:
I. Mercury as Example Trace
Metal - Background
Hg Cycle
ARTISANAL GOLD MINING IN THE DEVELOPING WORLD
Artisanal Gold Mining by Hg-Amalgamation
GOLD EXTRACTION SITE
(SLUICING)
Evasion
Hg0
Atmospheric
deposition
1. Hg
reduction
Hg2+
Mercury in Water/Sediment
Humans
Birds
Runoff
Hg2+ + MeHg
4. Uptake by
phytoplankton
Biomagnification
Phytop
Fish
Zoopl
3. Mobilization from
sediments
MeHg
MeHg
oxic
Hg2+
anoxic
Hg2+
2. Uptake &
SRB
methylation
HS-
SO42-
Hg2+
?
MeHg demethylation
IDEAL CONDITIONS FOR MERCURY TRANSFORMATION!!!!
Nutrients
Organic matter
SO42SO4
PO4 2NH4
2-
SO42-
NO3
SO4
2-
FOOD
OXIC
ANOXIC
(No O2)
Fine particles
(Large surface area)
(Less O2 penetration)
Less organic matter
(Large particles)
(More O2 penetration)
Cell Membrane
Protein channels
Lipid bilayer
The lipids in cell membranes are chiefly phospholipids such as phosphatidyl
ethanolamine and cholesterol. Phospholipids are amphiphilic with the hydrocarbon tail
of the molecule being hydrophobic; its polar head hydrophilic. As the plasma
membrane faces watery solutions on both sides, its phospholipids accommodate this by
forming a phospholipid bilayer with the hydrophobic tails facing each other.
Transport across cell membranes
BIOACCUMULATION AND CELL TOXICITY
Example pathway for Hg and other chalcophiles incorporation into
proteins:
Example: Two amino-acids play a key role metal toxicity:
Cysteine (cys): HOOC-CHNH2-CH2-SH
Methionine (met): HOOC-CHNH2-(CH2)2-S-CH3
These amino-acids serve as point of attach for CHALCOPHILIC metals to proteins S
Bio-concentration/Bio-accumulation
Octanol-water coefficient (Kow = [Coctanol]/[Cwater])
Bioaccumulation & Biomagnification
Mercury
in Water
Plankton
Omnivorous
Fish
Hydrosphere
Mercury
in Soil
Plants
Animals/
Birds
Man
Pedosphere
Mercury
in Air
Atmosphere
Plants
Animals
/Birds
Man
Carnivorous
Fish
Birds
Man
Example Health Impact due to Hg Exposure
PART-1
1. Metals in Water with no other
Ligands than H2O
Metals would form “AQUO COMPLEXES” of metals and even loose protons


The pH of solution is important in determining whether protons are lost
Leads to an acid-base type reaction with the following general equation
Me(H 2O)ab   H 2O  Me(H 2O)a 1(OH)( b 1)  H3O 
or simply :
Me b   2H 2O  Me(OH)( b 1)   H3O  by omitting the waters of hydration
Me  metal
a  metal coordinati on number
b  metal cation ch arg e

Deprotonation steps are favored mostly in the case of highly charged and
small radius ions (high Z2/r). This relationship holds true primarily for the
main group elements, and other factors become important for transition
metals, especially the heavy ones
Values of Z2/r and pKa1 for aquo-complexes of a
few selected metals


pKa1 = pH at which the aquo complex is
at 50% fully protonated and 50% with
less 1 proton
From the Table





+1 metal ions would occur exclusively as
fully protonated hydrated species
throughout the entire pH range
For +2 ions, deprotonation occurs more
readily for smaller species (high Z2/r)
Be(OH)+ dominates at pH of 6.5 and
above
Mg(OH)+ would need pH>11
Deprotonation becomes significant in
environmentally common situations for
+3 metals (e.g. Fe2+) and +2 heavier
metals (e.g. Hg2+)
2

Fe3aq  2H 2O  FeOHaq
 H3Oaq

2

FeOHaq
 2H 2O  Fe(OH)2(aq)  H3Oaq
This process can also lead to the
formation of polynuclear species
(H 2O)4 Fe
o
o
Fe(OH2 ) 4 4
Metal ions
Z2/r (nm-1)
pKa1
Na+
8.6
14.48
K+
6.6
>14.00
Be2+
68
6.50
Mg2+
47
11.42
Mn2+
48
10.70
Fe2+
43
10.10
Co2+
45.2
9.60
Ni2+
48
9.40
Cu2+
46
7.53
Zn2+
46
9.60
Cd2+
37
11.70
Hg2+
34
3.70
Al3+
133
5.14
Fe3+
115
2.19
Mercury (Hg) as Example Trace Metal

Hg is a type B metal with a very high covalent index (X2m*r) and a low ionic index
(Z2/r)


Earth’s crust abundance of ~89 ng/g
and mostly as Hg0 and HgS
Stable oxidation sates: 0, +1, and +2

Most important aqueous species = Hg2+, particularly under oxidized conditions

In water containing no ligands, deprotonation occurs even in moderately acidic
conditions (pH ~4) to give Hg(OH)+ and Hg(OH)2 as dominant species. For example,
using Hg with a coordination number of 4:
Hg (H 2O)24  H 2O  Hg (H 2O)3 (OH)  H3O
Hg (H 2O)3 (OH)  H 2O  Hg (H 2O)2 (OH)2  H3O
Eh-pH Diagram with water as ligand
2. Metals in Water with Ligands
2.1. Chloride as example single ligand in water containing Hg
Hg2+
-8
HgCl+
-6
HgCl2
-4
HgCl3-
-2
HgCl42-
0
log [Cl-]
Distribution of Hg chloro-complexes in water as a function of chloride
concentration.
Eh-pH Diagram with Cl- as ligand
PART#2