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)

Hg2+

Hg2+Hg0

1. Hg reduction

EvasionAtmospheric

deposition

SRB

Hg2+ MeHg

SO42-HS-

2. Uptake &

methylation

Hg2+ + MeHg

3. Mobilization fromsediments

4. Uptake by phytoplankton

Phytop

Zoopl

Fish

HumansBirds

oxic

anoxic

Biomagnification

Mercury in Water/Sediment

?

MeHg Hg2+

MeHg demethylation

Runoff

SO42-

SO42-

SO42-

SO42-

Nutrients

NO3

NH4

PO4 2-

Organic matter

FOOD

OXIC

ANOXIC(No O2)

Less organic matter(Large particles)

(More O2 penetration)

Fine particles(Large surface area)(Less O2 penetration)

IDEAL CONDITIONS FOR MERCURY TRANSFORMATION!!!!

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.

Cell Membrane

Lipid bilayer

Protein channels

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

Carnivorous Fish

Birds Man

Hydrosphere

Pedosphere

Mercury in Soil

Plants Animals/Birds

Man

Atmosphere

Mercury in Air

Plants Animals/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

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

eargchcationmetalb

numberoncoordinatimetala

metalMe

OH)OH(MeOH2Me

:simplyor

OH)OH()OH(MeOH)OH(Me

3)1b(

2b

3)1b(

1a22ba2

by omitting the waters of hydration

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+)

This process can also lead to the formation of polynuclear species

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

aq3)aq(222aq

aq32aq2

3aq

OH)OH(FeOH2FeOH

OHFeOHOH2Fe

44242 )OH(FeFe)OH(

oo

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/gand 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:

OH)OH()OH(HgOH)OH()OH(Hg

OH)OH()OH(HgOH)OH(Hg

3222232

3322242

Eh-pH Diagram with water as ligand

2. Metals in Water with Ligands

2.1. Chloride as example single ligand in water containing Hg

log [Cl-]

Hg2+ HgCl2HgCl+ HgCl3- HgCl4

2-

-8 -6 0-4 -2

Distribution of Hg chloro-complexes in water as a function of chloride concentration.

Eh-pH Diagram with Cl- as ligand

PART#2