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

log [Cl-] (M)

Hg2+ HgCl2HgCl+ HgCl3- HgCl4

2-

-8 -6 0-4 -2

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

Eh – pH Diagram for the chemical

system:H2O – Hg – Cl - S

Water reduced

Water oxidized

Most surface waters

Most Ground waters

Hg0 (aq)

Hg(OH)22

2Hg

)aq(02HgHS

0)aq(Hg

22HgS

02HgCl

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

•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 sub-parts per trillion levels

METHYLMERCURY COMPOUNDS

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.

C2H5

              I           H5C2—Pb—C2H5

                I C2H5

   Tetra-ethyllead

                           

CH3HgX-----------------------

1.MINAMATA, JAPAN

2. IRAQ: Methyl mercury dressed germination seeds

and Hg poisoning in Iraq (Occurred in 1960’s)

Chemical rxn to produce acetaldehyde used Hg sulfate as a catalyst and discharged in wastewaters (1932-1968)

NATURAL SOURCES OF ORGANOMETALLIC COMPOUNDS

• Hg • Ge • Sn • As • Se • Te • Pd • Pt

• Au • Tl • Pb

THE FOLLOWING ARE METALS WITH

KNOWN NATURALLY PRODUCED AND

STABLE ORGANOMETALLIC COMPOUNDS

METHYLATION AND DEMETHYLATION IN AQUATIC SYSTEMS

– Example 1:

Biomethylation catalyzed by microorganisms

Carbo-anion: CH3-

Radical: CH3*

– Example 2: Abiotic methylation – catalyzed by:Humic acids Trans-metallation (Me1 + Me2R Me1R + Me2)

Hg)CH(CHHgCH

HgCHCHHg

2333

332

TEAox TEAred

bacteria

DETERMINATION OF METHYLMERCURY IN ENVIRONMENTAL SAMPLES

Aqueous Samples

Distillation - Derivatization methods GC-separation Thermodecomposition AFS detection (EPA’s Method 1630)

Solid Samples (soil/sediments and biota)

Extraction from solid phase (organic extraction, distillation) Derivatization methods GC-separation Thermodecomposition AFS detection

Stability of Methylmercury and of its Selected Complexes in Aquatic Systems

26.................................................HHgSCHCHHgS

02.21...................................................HgSCHSHgCH

91.0...............................................HgSOCHSOHgCH

1.6...............................................HgCOCHCOHgCH

63.4.........................................HHgOHCHOHHgCH

37.....................................................Hg)CH(CHHgCH

50............................................................HgCHCHHg

LogK...............................................................................REACTIONS

34

32

3

43243

33233

323

2333

332

4. Interaction of Aqueous Mercury Species with Solid Phases

•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. Surface Properties of Colloidal Particles

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 closeto 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.

4.1.1. THE ELECTRICAL DOUBLE LAYER

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-OH

Fe-OH

+ Hg(H2O)22+

Fe

Fe

O

O

Hg

+ 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