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BEHAVIOR OF TRACE METALS IN AQUATIC SYSTEMS: EXAMPLE CASE STUDIES (Cont’d)

BEHAVIOR OF TRACE METALS IN AQUATIC SYSTEMS: EXAMPLE CASE STUDIES (Cont’d). Environmental Biogeochemistry of Trace Metals (CWR6252). 4. Interaction of Aqueous Mercury Species with Solid Phases. 4.1. Surface Properties of Colloidal Particles.

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BEHAVIOR OF TRACE METALS IN AQUATIC SYSTEMS: EXAMPLE CASE STUDIES (Cont’d)

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

  2. 4. Interaction of Aqueous Mercury Species with Solid Phases 4.1. Surface Properties of Colloidal Particles • 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

  3. 4.1.1. THE ELECTRICAL DOUBLE LAYER 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.

  4. 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)

  5. Specific adsorption Fe Fe-OH O + Hg(H2O)22+ Hg O Fe Fe-OH + 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.

  6. Types and Size Classification of Particles in the Hydrosphere Diameter (m) 10-10 10-8 10-5 10-2 Molecules Clay minerals……….humic acids Suspended sediments Bacteria Viruses Algae SOLUBLE COLLOIDAL PRECIPITATED

  7. Functional Groups Commonly Found on Particles • Functional groups on natural particles can interact with: • H+, OH-, metal ions, and other ligands when Lewis acid sites (e.g. Al and Fe) are available • Many inorganic particles (oxides and silicates) contain hydroxo groups, carbonates, and sulfides which are exposed • Surfaces of humic acids are characterized primarily by carboxylic and phenolic-OH groups • Biological surfaces contain primarily: • –COOH, -NH2, and –OH groups • These groups have the ability to bind protons and metal ions

  8. Langmuir Freundlich QUANTITATIVE DESCRIPTIONS OF ADSORPTION Adsorption of Hg(II) onto silica (SiO2). Experimental data points and equilibrium model line (Tiffreau et al.)

  9. Example Adsorption Patterns of Metal Cations Extent of surface complex formation measured as %mol of the metal ion adsorbed to the iron oxide surfaces as a function of pH (Dzombak and Morel, 1990)

  10. Example Adsorption Patterns of Oxyanion Forming Elements Extent of surface complex formation with metal ions adsorbed to the iron oxide surfaces as a function of pH (Dzombak and Morel, 1990)

  11. Removal of Metal Solution and Phase Distribution “Aggregation/Coagulation/Flocculation – Kd - BCF” • Coagulation in natural waters refers to the aggregation of particles due to electrolytes (e.g. coagulation of suspended solids as salinity increases toward the mouth of a river estuary) • Flocculation is important in water treatment when iron (FeSO4), FeCl3) and aluminum (Al2(SO4)3) salts are used to destabilize colloids and to form polymers and precipitates (Fe(OH)3 and Al(OH)3 that promote flocculation • Distribution Coefficient (Kd) • Bio-concentration Factor (BCF) • hydrophilic vs. hydrophobic compounds

  12. Effects of Redox Chemistry on Fate of Metals in Aquatic Systems: Hg as Example Atmosphere Hg ????????? Water Hg ????????? ????????? Sediments Hg

  13. 5. Aspects of Remediation of Contaminated Waters Using Metals

  14. Mechanisms of Remediation with Metals and Importance of particle size

  15. Remediation mechanisms based on interaction of the pollutant or water with the bare ZVI surfaces • 1.1. Mechanism-1: Direct reduction at the metal surface • 1.2. Mechanism-2: Reduction by ferrous iron [Fe(II)] produced after Fe0 corrosion • 1.3. Mechanism-3: Reduction by hydrogen with catalysis Use of Zero Valent Iron (ZVI) as a Case Study

  16. 1.1. Mechanism-1: Direct reduction at the metal surface Electrons are transferred from Fe(0) to the adsorbed pollutant at the metal-water interface: Fe0 Fe2+ + 2e- 2RX(organic pollutant) + 2H+ + 2e- 2RH + 2X- ______________________________

  17. 1.2. Mechanism-2: Reduction by ferrous iron [Fe(II)] produced after Fe0 corrosion 1. Fe(0) is corroded 2. Fe(II) is formed and electrons are transferred 3. Oxidation up to production of Fe(III) 2Fe0 2Fe2+ + 2e- 2H2O + 2e-  H2 + 2OH- 2Fe2+  2Fe3+ + 2e- 2RX (organic ) + 2H+ + 2e- 2RH + 2X-

  18. 1.3. Mechanism-3: Reduction by H2 with catalysis Hydrogen from the anaerobic corrosion of Fe(II) could react with the pollutant if an effective catalyst is present

  19. Bare ZVI surfaces vs. Oxide layers • Hydrogenation plays a minor role in most systems as iron surfaces become very quickly oxidized and covered with precipitates • Oxide layers formed at the iron surfaces become more important in ZVI-based remediation process

  20. 2. Oxide Layer Formation at ZVI surfaces and Mediation of Electron Transfer from Fe0 to adsorbed pollutants • 2.1. Mechanism-1: Direct electron transfer from Fe(0) to the pollutant in a corrosion pit • 2.2. Mechanism-2: Oxide film mediated electron transfer from Fe(0) to pollutant by acting as a semi-conductor • 2.3. Mechanism-3: Oxide layer as a coordinating surface containing sites of Fe(II) that interact with the pollutant

  21. 2.1. Mechanism-1: Direct electron transfer from Fe(0) to the pollutant in a corrosion pit Deficiency in oxide layer coating Direct electron transfer from metallic iron Interaction with pollutant similar to those described earlier with bare ZVI

  22. 2.2. Mechanism-2: Oxide film mediated e- transfer from Fe0 to pollutant by acting as a semi-conductor In this case, the oxide layer acts as a semiconductor, allowing electron transfer From the metallic iron to the pollutant adsorbed on it. The breakdown of the Pollutant occurs at the oxide layer.

  23. 2.3. Mechanism-3: Oxide layer as a coordinating surface Fe(II) that interact with the pollutant Adsorption and immobilization predominates

  24. 6. METAL INTERACTIONS WITH BIOLOGICAL SYSTEMS Implications for Toxicity

  25. 6.1. Electronegativity (En) and toxicity of chemical compounds Pauling Electronegativity (En = Zeff/r2) The En difference between two atoms in a chemical compound determines the degree of charge separation or polarity and therefore the degree of solubility in aqueous versus organic solvent. Examples: NaCl and CCl4 Na: 0.82 Cl: 2.96 C:1.9  DEn (NaCl) = 2.9-0.82=2.14 and DEn (CCl4) = 2.9-1.9 = 1.0

  26. 6.2. TWO MAJOR FEATURES OF CHEMICALS ASSOCIATED WITH TOXICITY • Lipophilicity (solubility in lipids) • Electrophilic reactivity (reactivity toward electron-rich nucleophiles such –SH groups)

  27. 6.3. METALLOIDS AND BIOLOGICAL EFFECTS • The following are metalloids with known toxicity and quite well-studied toxicity mechanisms: • Arsenic (As) • Selenium (Se) • Tin (Sn) • Antimony (Sb) • Tellurium (Te) They have high En and provide primarily covalently bound compounds and form acidic (amphoteric) hydroxides

  28. 6.3.1. ARSENIC • As compounds react readily with nucleophiles • Most As-compounds behave like organic • compounds, w/ tetrahedral configuration and covalent centers • Can be methylated to Produce As-C bonds

  29. 6.3.1.1. Methylation of Arsenic Typical reactions of the Challenger mechanism. The top line indicates a mechanism for the reduction, As(V) to As(III), resulting in an unshared pair of electrons on As. Structures are as follows: R1 = R2 = OH arsenate; R1 = CH3, R2 = OH methylarsonate; R1 = R2 = CH3dimethylarsinate. The bottom line indicates the methylation of an As(III) by S-adenosyl methionine or SAM [shown in abbreviated form as CH3-S+-(C)2]. A proton is released and SAM is converted to S-adenosylhomocysteine [abbreviated form, S-(C)2].

  30. Challenger mechanism: Conversion of arsenate to trimethylarsine • (A) Arsenate; (B) arsenite; (C) methylarsonate; (D) methylarsonite; (E) dimethylarsinate; (F) dimethylarsinite; (G) trimethylarsine oxide; (H) trimethylarsine. • Top line structures show As(V) intermediates. • Vertical arrows = reduction of As(V) to As(III) species shown in bottom line • Diagonal arrows indicate the methylation steps by SAM

  31. Expanded version of the Challenger mechanism: Roles of different components both in the cells themselves and in the surrounding medium. The double vertical lines indicate cell walls. • Phosphate transport system • thiols and/or dithiols • active transport system • active/passive transport • passive diffusion. • Abbreviations: MMAV, methylarsonic acid; DMA, dimethylarsinic acid; TMAO, trimethylarsine oxide.

  32. 6.3.2. SELENIUM • Shares many of the properties of sulfur and arsenic • Its compounds are covalent • Selenite (Se+4) and selenate (Se+6) are most stable oxidation states • Replaces S in cysteine and methionine • Accumulation plants makes forage toxic to animal • Evapoconcentrated in aquatic systems • Example of “The San Joaquim Valley, CA” and suggested remediation

  33. 6.3.3. TELLURIUM • Not particularly toxic • The most notable result of Te exposure/intake is a very strong body odor called “Tellurium Breath” from biochemical reduction and methylation to the garlic-ordored dimethyl-telluride

  34. 6.3.4. TIN • The most metallic of the metalloids • Elemental Sn is safe (e.g. tin cans) and stannous fluoride is approved for use in toothpaste • However, TBT = tributyltin is extremely toxic • TBT-oxide and chloride used as antifouling, but highly toxic to aquatic biota. Shellfish are killed with levels as low as 10 to 20 ng/L or ppt.

  35. 6.4. METALS IN BIOLOGICAL SYSTEMS • Similar to metalloids, the toxicity of metals is governed by their degree of: (1) lipo-solubility and (2) electrophilic reactivity The following are elements with well-studied toxicity mechanisms • Mercury (Hg) • Lead (Pb) • Thallium (Tl) and Bi • Transition metals: (Cr, Mn, Co, Ni, Cu, Zn, Mo, Ag, and Cd) • Radioactive elements (Uranium (U) and Radium (Ra))

  36. 6.4.1. Metals with naturally produced methyl-compounds • Mercury (Hg)*** • Lead (Pb)*** • Thallium (Tl) • Gold (Au) • Platinum (Pt) • Palladium (Pd) • ***Elements with stable alkyl-compounds in natural systems

  37. 6.4.2.Mercury • Forms primarily covalent bonds in both inorganic and organic compounds, which increase liposolubility • High affinity for –SH groups • Treatment in case of Hg-poisoning: • Inorganic Hg species: Dimercaprol (intramuscular) + penicillamine (orally) • Methyl-Hg: Binding resins

  38. 6.4.3. Lead (Pb) • Binds to -SH containing substrates • Inhibits HEME biosynthesis (low hemoglobin) • Replaces Ca in bones and biochemical processes, affects ATP synthesis (mitochondrial ATP) • Disturbance of Ca-metabolism alters brain neurotransmitter functions and inhibits Na+/K+ ATPase • Treatment: Ca-EDTA is used, but not efficient if brain poisoning

  39. 6.4.4. Thallium (Tl) and Bismuth (Bi) • THALLIUM: used in electronics and its sulfates were used as poison for rats • Symptoms: hair loss (Alopecia). Was used in depilatories at some point • Toxicity due to competition with K+ and effect on Na+/K+ • Treated with BAL (British anti-lewisite) • BISMUTH: no outstanding toxicity. Pepto-bismol (anti-acid)

  40. 6.4.4. Radium (Ra) • Was used to produce numerals on clocks, phones, and other instruments • Similar to Ca • Radioactive decay produces RADON (Rn)

  41. 4.5. Cu, Zn, Cd, Mo, Cr, Mn, Ni • Mn: Manganism ressembles parkinsonism and is due to exposure to airborne MnO2 or water with 16-18ppm Mn. • MMT = methylcyclopentadienylmanganese tricarbonyl [C6H8Mn(CO)3] in non-leaded fuels Mn3O4.

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