1. BEHAVIOR OF TRACE METALS IN AQUATIC SYSTEMS: EXAMPLE CASE STUDIES Environmental Biogeochemistry of Trace Metals (CWR6252)

2. INTRODUCTION: I. Mercury as Example Trace Metal - Background

3. Hg Cycle

4. ARTISANAL GOLD MINING IN THE DEVELOPING WORLD

5. Artisanal Gold Mining by Hg-Amalgamation

6. GOLD EXTRACTION SITE (SLUICING)

8. Hg 2+ Hg 0 1. Hg reduction Evasion Atmospheric deposition SRB Hg 2+ MeHg SO 4 2- HS - 2. Uptake & methylation Hg 2+ + MeHg 3. Mobilization from sediments 4. Uptake by phytoplankton Phytop Zoopl Fish Humans Birds oxic anoxic Biomagnification Mercury in Water/Sediment ? MeHgHg 2+ MeHg demethylation Runoff

9. SO 4 2- Nutrients NO 3 NH 4 PO 4 2- Organic matter FOOD OXIC ANOXIC (No O 2 ) Less organic matter (Large particles) (More O 2 penetration) Fine particles (Large surface area) (Less O 2 penetration) IDEAL CONDITIONS FOR MERCURY TRANSFORMATION!!!!

10. 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

11. 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-CHNH 2 -CH 2 -SH  Methionine (met): HOOC-CHNH 2 -(CH 2 ) 2 -S-CH 3 These amino-acids serve as point of attach for CHALCOPHILIC metals to proteins S Bio-concentration/Bio-accumulation Octanol-water coefficient (K ow = [C octanol ]/[C water ])

13. Bioaccumulation & Biomagnification Mercury in Water PlanktonOmnivorous Fish Carnivorous Fish Birds Man Hydrosphere Pedosphere Mercury in Soil PlantsAnimals/ Birds Man Atmosphere Mercury in Air PlantsAnimals /Birds Man

14. Example Health Impact due to Hg Exposure

15. PART-1

16. 1. Metals in Water with no other Ligands than H 2 O 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 Z 2 /r). This relationship holds true primarily for the main group elements, and other factors become important for transition metals, especially the heavy ones by omitting the waters of hydration

17. Values of Z 2 /r and pKa 1 for aquo-complexes of a few selected metals  pKa 1 = 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 Z 2 /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. Fe 2+ ) and +2 heavier metals (e.g. Hg 2+ )  This process can also lead to the formation of polynuclear species Metal ionsZ 2 /r (nm -1 )pKa 1 Na + 8.614.48 K+K+ 6.6>14.00 Be 2+ 686.50 Mg 2+ 4711.42 Mn 2+ 4810.70 Fe 2+ 4310.10 Co 2+ 45.29.60 Ni 2+ 489.40 Cu 2+ 467.53 Zn 2+ 469.60 Cd 2+ 3711.70 Hg 2+ 343.70 Al 3+ 1335.14 Fe 3+ 1152.19 o o

18. Mercury (Hg) as Example Trace Metal  Hg is a type B metal with a very high covalent index (X 2 m *r) and a low ionic index (Z 2 /r)  Earth’s crust abundance of ~89 ng/g and mostly as Hg 0 and HgS  Stable oxidation sates: 0, +1, and +2  Most important aqueous species = Hg 2+, 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:

19. Eh-pH Diagram with water as ligand

20. 2. Metals in Water with Ligands 2.1. Chloride as example single ligand in water containing Hg log [Cl - ] Hg 2+ HgCl 2 HgCl + HgCl 3 - HgCl 4 2- -8-60-4-2 Distribution of Hg chloro-complexes in water as a function of chloride concentration.

21. Eh-pH Diagram with Cl - as ligand

22. PART#2