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) Hg 2+ HgCl 2 HgCl + HgCl 3 - HgCl 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 E h – pH Diagram for the chemical system: H 2 O – Hg – Cl - S Water reduced Water oxidized Most surface waters Most Ground waters Hg 0 (aq) Hg(OH) 2

2.3. Hg in Water Containing Several Ligands and Competitive Cations Hg 2+ - 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 25 0 C (pH 7) IonsConc. (M) Na K+K Ca Mg Hg Cl NO SO HCO

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(CH 3 ) 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 + CH 3 Br  Diethyl-ether  CH 3 MgBr ) 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 2 H 5 I H 5 C 2 —Pb— C 2 H 5 I C 2 H 5 Tetra-ethyllead CH 3 HgX 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 ( )

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: CH 3 -  Radical: CH 3 * –Example 2: Abiotic methylation – catalyzed by:  Humic acids  Trans-metallation (Me 1 + Me 2 R  Me 1 R + Me 2 ) TEA ox TEA re d 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

4. Interaction of Aqueous Mercury Species with Solid Phases Specific surface area: Typical measured values for natural particles are: Kaolinite 5 to 20 m 2 /g Montmorillonite 700 – 800 m 2 /g Fulvic and Humic Acids 700 to m 2 /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 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 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 + Hg(H 2 O) 2 2+ Fe O O Hg + 2H 3 O +  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