Abiotic Transformations in the Environment

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Understand the role of solar photons as an energy source for chemical reactions in the environment. • Describe, in general, the dynamics of excited states in producing products and photo-sensitized reactants. • Understand the major abiotic chemical reaction pathways in the environment.

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Nội dung Text: Abiotic Transformations in the Environment

Principles of Environmental Toxicology Learning Objectives • Understand the role of solar photons as an energy source for chemical reactions in the environment. • Describe, in general, the dynamics of excited states Abiotic Transformations in producing products and photo-sensitized in the Environment reactants. • Understand the major abiotic chemical reaction Principles of Environmental Toxicology pathways in the Instructor: Gregory Möller, Ph.D. environment. University of Idaho 2 Principles of Environmental Toxicology Principles of Environmental Toxicology Learning Objectives Photochemical Reactions • Describe electrophillic, nucleophillic, hydrolysis and • Endothermic environmental chemical reactions can redox reactions. get required energy of reaction from solar photons. • Summarize the basic reactions associated with the • UV-Vis energy is strong enough to break some formation of the hole in the ozone layer. chemical bonds. • Summarize the reactions – Available in the solar spectrum. associated with the • E = 1.196 x 105/λ kJ/Einstein formation of acid rock E = 2.859 x 104/λ drainage. kcal/mole photons. 3 4 Principles of Environmental Toxicology Principles of Environmental Toxicology Electromagnetic Spectrum Electromagnetic Spectrum Visible 0.4 0.7 UV IR Wavelength, µm 10-6 10-5 10-4 10-3 10-2 10-1 1 10 102 103 104 105 106 107 108 γ-Rays Ultraviolet Thermal IR Radio X-Rays Near, Mid IR Microwave 5 6 1
Principles of Environmental Toxicology Principles of Environmental Toxicology Absorption Photochemical Reactions • Photon absorption is a “quantum” event and the • Excited molecules can undergo unimolecular or specific energies required for excitation and reaction bimolecular reactions. are characteristic of the molecule. – Unimolecular: dissociation; bond breaking, – IR absorption corresponds to vibrational excitation intersystem crossing. of chemical bonds. Direct photolysis • UV absorption corresponds to CH4 + hυ (λ < 140 nm) → CH2 + H2 electronic excitation, usually – Bimolecular: chemical reaction; energy transfer. lone pair (n electrons) or Mercury sensitized delocalized π electrons. Hg(1S0) + hυ (253 nm) → Hg*(3P1) – Heteroatom, n → π* Hg*(3P1) + CH4 → Hg(1S0) + CH3 + H – Conjugation, π → π* 7 8 Principles of Environmental Toxicology Principles of Environmental Toxicology Bond Energy - Light Energy Energy Levels and Transitions υ` Light energy, λ Bond Bond energy, E J `` Calvert 2 & Pitts (kJ/mole) (nm) 10 1 O—H 465 257 5 0 H—H 436 274 A, rotational, FIR B, vibrational, NIR C—H 415 288 C C, electronic, VIS/UV C—O 360 332 υ`` J `` 2 C—C 348 344 B 10 1 C — Cl 243 492 A 5 9 10 0 Principles of Environmental Toxicology Principles of Environmental Toxicology Photoexcitation, C → C* Intermolecular Energy Transfer • Physical processes (molecule unchanged). Energy – Vibrational loss of energy (heat transfer). M1* Transfer – Energy loss by light emission (luminescence) Reaction M2* – Energy transfer promoting an electron in another chemical species (photosensitization). hυ • Chemical reactions (new products). – Fragmentation. M2 – Intramolecular rearrangement. M1 – Isomerization, dimerization. – Hydrogen atom removal. The laws of quantum mechanics – Electron transfer. govern allowed and forbidden transitions. 11 12 Schwarzenbach 2
Principles of Environmental Toxicology Principles of Environmental Toxicology Reaction Quantum Yield Photons in Natural Water Direct Sunlight Diffuse Sunlight • The fraction of excited molecules of a given compound that react by a physical or chemical pathway. moles of molecules transformed Φ r( λ ) = moles of photons (λ) absorbed by the system Surface Surface due to the presence of the compound reflection refraction Optically thin surface layer Reflective * Absorptive particles Optically thick eutrophic zone molecules 13 14 Schwarzenbach Principles of Environmental Toxicology Principles of Environmental Toxicology Direct Photolysis RQY Indirect Photolysis • In complex environmental waters and soils, unknown Compound Wavelength, nm Reaction Quantum chromophores (UC) are the primary solar photon Yield, Φr λ absorbers. • Oxygen is the most important acceptor of UC*. Phenanthrene 313 1.0 x 10-2 (Ground state triplet) 3O2 → (excited state singlet) 1O2 Anthracene 313 3.0 x 10-3 Energy required only 94 kJ mole-1. Nitrobenzene 313 2.9 x 10-5 • High energy sensitized, electrophilic 2,4,6-Trinitrotoluene 313, 366 2.1 x 10-3 photoreactants include: – Singlet oxygen, 1O2 – Hydroxyl radical, HO• – Peroxy radicals, ROO• 15 16 Schwarzenbach Principles of Environmental Toxicology Principles of Environmental Toxicology Sensitized Photoreactants Focus: Ozone Depletion • Singlet oxygen, 1O2 • CFC’s are released. – Physical quenching by water. – Enter the stratosphere where sunlight produces the breakdown products of hydrochloric acid and – Will initiate a Diels-Alder reaction. chlorine nitrate. – Low concentrations make it less important. – Heterogeneous reactions on stratospheric cloud • Hydroxyl radical, HO• surfaces then produce Cl2, which is photolyzed – Photolysis of nitrate is major pathway. into chlorine radicals by UV. – Highly reactive, DOM major sink. – Chlorine radicals catalyze – H removal, hydroxylation. the conversion of O3 into O2. • Peroxy radicals, ROO• • Decreased ozone levels – Many varieties. increase UV radiation at earth’s surface. – Not well scavenged by DOM. 17 18 3
Principles of Environmental Toxicology Principles of Environmental Toxicology The Antarctic Ozone Hole Abiotic Reactive Pathways • Electrophillic. • Nucleophillic. • Oxidation. • Reduction. NASA • Other abiotic pathways. 19 20 Principles of Environmental Toxicology Principles of Environmental Toxicology Nucleophillic and Electrophillic Environmental Nucleophiles • Covalent bonds between atoms of • The majority of environmental nucleophiles are different electronegativity are polar. inorganic and they are abundant. – Typically contains an electropositive carbon. • Because of this abundance, electrophiles are short- lived, and reactions of organic compounds with R — CH2 (δ+) — Cl (δ-) electrophiles are usually photochemically or – Such organic molecules can become the sites for biologically induced. reaction with nucleophillic (+ seeking) or electrophillic (- seeking) species. ClO 4- NO3- F- H2 O • The majority of environmental SO42- CH3COO- Cl- HCO3- chemical species that can chemically react with organic HPO4 Br- OH- I- 2- molecules are nucleophillic. CN- HS- Environmental Nucleophiles 21 22 Schwarzenbach Principles of Environmental Toxicology Principles of Environmental Toxicology Reactions With Nucleophiles Nucleophillic Substitution • SN1, substitution, nucleophillic, unimolecular. • Nucleophillic species have partial or full (-). • When encountering an organic molecule with a R3 R3 R3 R3 R3 * * RLS polar bond, the e- rich atom of the nucleophile may R2 C X R2 C X R2 C Y C R2 Y C R2 X Y form a bond with the e- deficient atom of the organic R1 R1 R1 R1 R1 molecule. • Water hydrolysis predominates. – Organic molecule typically has a “leaving” group. • SN2, substitution, nucleophillic, bimolecular. • Water (OH-) is the most important R3 R3 R2 R3 environmental nucleophile. RLS Y C X Y + R2 C X R2 + C Y X – Hydrolysis reaction transforms R1 R1 R1 the organic molecule into a • Water hydrolysis, except in salt more polar molecule. or contaminated water. 23 24 Schwarzenbach Schwarzenbach 4
Principles of Environmental Toxicology Principles of Environmental Toxicology Hydrolysis Mechanisms Other Abiotic Reactions t½, X = Cl • Alkalyation. H Schwarzenbach 340 d, SN2 R C X – Aliphatic molecules that develop a (+) center can be an alkalyating agent in an electrophillic reaction H H3 C with a nucleophile. 38 d, SN2…SN1 CH X • β-Elimination H3 C – An adjacent β carbon loses a group to a CH3 nucleophillic reaction at the α carbon, 23 s, SN1 H3C C X while increasing in unsaturation. CH3 X • Chlorination. 69 d, (SN2)…SN1 HC CH2 – Reaction of Cl2 with H2C aliphatic carbonyls X and amines. 15 h, SN1 CH2 25 26 Principles of Environmental Toxicology Principles of Environmental Toxicology Oxidation Reduction • Loss of or introduction of O into a molecule. e- • Gain of or hydrogenation. e- Crosby – Combustion = combining with oxygen. • Natural reducing agents include Fe2+, H2S, iron porphyrins, sulfhydryl compounds, hydroquinones, • Atmospheric oxidants: usually photochemical origin; and hydrated electrons. can dissolve in water. Crosby • Some reactions include Triplet oxygen O O – Reductive dechlorination. Singlet oxygen O O – Nitro group reduction. Oxygen atoms O O Cl H Ozone O O Cl Cl Cl Cl + + 2e H Cl + Hydroxyl OH Cl Cl Cl Cl N O Nitrogen dioxide 27 28 DDT DDD O Principles of Environmental Toxicology Principles of Environmental Toxicology Redox Reactions Natural Redox Processes • Depending on the redox conditions, electron Half-Reaction EH0(W), V acceptors (oxidants) or donors (reductants) that may O2(g) → H2O +0.81 EH0(W) react abiotically in a thermally favorable reaction Typical natural NO3- → N2(g) +0.74 water conditions with a given chemical, MnO2 → MnCO3 +0.52 may or may not be present in sufficient abundance NO3- → NO2- +0.42 (Schwarzenbach). 2NO3- → NH4 +0.36 – Most redox reactions in FeOOH → FeCO3 -0.05 the environment are Pyruvate → Lactate -0.19 biologically mediated. SO4-2 → HS- -0.22 S(s) → H2S -0.24 CO2 → CH4 -0.25 H+ → H2 -0.41 30 2 → Glucose 29 CO -0.43 5
Principles of Environmental Toxicology Principles of Environmental Toxicology Mapping Redox Stabilities Pourbaix Diagram - Pb Eh (Volts) • The thermodynamic stability 1.0 PbO2 fields of various species can be 0.8 2PbO*PbCO3 mapped as a function of redox Pb3O4 0.6 Pb(+2a) potential (Eh) and pH. 0.4 Water Oxidized PbSO4 – Pourbaix diagram. 3PbO*PbSO4 PbCO3 0.2 PbS Pb(OH)O(-a) • Environmental conditions will 0.0 2PbO*PbCO3 ultimately determine species. -0.2 PbS -0.4 – Caution: may be a -0.6 kinetically slow process! -0.8 Water Reduced Pb -1.0 0 2 4 6 8 10 12 14 pH 31 32 Pb - C - Fe - S - H2O - System at 25 °C Principles of Environmental Toxicology Principles of Environmental Toxicology Focus Area: Abandoned Mine Lands Pourbaix Diagram – Pb, 2 • By estimate of the former U.S. Eh (Volts) 1.0 Bureau of Mines, over 12,000 Pb6(OH)8(+4a) 0.8 2PbO*PbCO3 miles of rivers and streams and Pb3O4 0.6 Pb(HS)2(a) over 180,000 acres of lakes and PbCO3 PbOH(+a) 0.4 reservoirs are adversely PbSO4 Pb(OH)O(-a) 3PbO*PbSO4 0.2 effected by abandoned metal Pb(+2a) PbS PbO2 Pb(OH)O(-a) 0.0 and coal mines, the 2PbO*PbCO3 -0.2 corresponding mine wastes and PbS related acid mine drainage -0.4 Pb(HS)2(a) Pb(+2a) (1990). -0.6 Pb -0.8 • Currently, there are over Pb(HS)3(-a) -1.0 500,000 abandoned mines in 0 2 4 6 8 10 12 14 34the U.S. pH 33 Principles of Environmental Toxicology Principles of Environmental Toxicology Ksp for Metal Sulfides, Hydroxides Acid Production • Acid rock drainage (ARD). Ksp – Adversely impacts surface water, groundwater Metal sulfide Metal hydroxide and riparian areas. Cu 8.5 x 10-45 1.6 x 10-19 • Common problem in coal mining regions, surface Zn 1.2 x 10-23 4.5 x 10-17 mines, and hardrock mines. • Forms when pyrite (FeS2) Pb 3.4 x 10-28 1.2 x 10-15 or mascarite are exposed Cd 3.6 x 10-29 5.9 x 10-15 to weathering conditions. Fe 3.7 x 10-19 1.8 x 10-15 • Oxidation and hydrolysis. Ni 1.6 x 10-16 1.6 x 10-16 Cr(III) NA 6.7 x 10-31 35 36 6
Principles of Environmental Toxicology Principles of Environmental Toxicology Acid Rock Drainage, 2 Acid Rock Drainage • Results in the formation of soluble hydrous Fe FeS2 (s) + 7/2 O2 + H2O ↔ Fe2+ + 2SO42- + 2H+ sulfates and the production of acidity. Fe2+ + 1/4 O2 + H+ ↔ Fe3+ + 1/2 H2O • Effluent solution has elevated Fe, SO4-2, high TDS and low pH. Fe3+ + 3H2O ↔ Fe(OH)3 (s) + 3H+ • Other metals. or • Oxidation of Fe 2+ to Fe 3+ FeS2 (s) + 15/4 O2 + 7/2 H2 ↔ Fe(OH)3 (s, red) + 3H+ produces additional acid and colorful iron auto-catalytic at pH below 3.5 oxyhydroxides. FeS2 (s) + 14 Fe3+ + 8H2O ↔ 15Fe2+ + 2SO42- + 16H + 37 38 Principles of Environmental Toxicology Principles of Environmental Toxicology Sulfur Cycle Bacteria Sulfide Oxidizing Bacteria - aerobic S0 Thiobacillus thiooxidans H2 S SO42- Sulfate Reducing Bacteria - anerobic Desulfovibrio & Desulfotomaculum 39 40 Principles of Environmental Toxicology Principles of Environmental Toxicology 41 42 7
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