Attenuation Processes. Definitions. Definitions. Attenuation. The degradation process results in changed molecular structure
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1 ES/R 531 Fundamentals of Environmental Toxicology Lecture 22 Instructor: Allan Felsot Fall 2005 Abiotic/iotic Degradation & Transformation (Environmental Attenuation of Contaminants) Attenuation rocesses Transformation of parent molecule to either toxic or nontoxic product forms Reduction in concentration Distinguished from simple dilution or transport out of a specific area by change in molecular structure esticide (very toxic) Degradation roduct (nontoxic) Degradation roducts (bioavailable) Attenuation Transformation roduct (toxic) Degradation Definitions Decrease in concentration of a contaminant due to nonreversible alteration of chemical structure Mineralization iologically mediated degradation of chemical resulting in release of carbon dioxide ersistence Longevity of a contaminant residue in a medium or phase carbon dioxide water ound to rganic Matter (very low toxicity & bioavailability Detoxification Degradation resulting in loss of toxicity or biological activity Transformation Definitions artial change in structure of a contaminant due to biological or nonbiological reaction Transformation product may still retain toxicity ound residue The residue remaining after exhaustive extraction of a soil, water, or plant matrix Covalent incorporation of a transformation product into the natural biochemical matrix Degradation roducts The degradation process results in changed molecular structure Inactivation (detoxification) Activation (toxification) Mineralization (C 2 & 2 ) ound to rganic Matrix 1
2 Reaction Mechanisms Considerations The processes by which a chemical is degraded Divided into two basic mechanisms hase I (biologically or nonbiologically mediated) ydrolysis xidation Reduction hase II (biologically mediated) Conjugation Chemical reactions in the environment occur much slower than dissociation processes in solution For example, deprotonation of an acid (i.e., dissociation of a proton in response to solution p is faster than a chemical reaction) Thus, we re interested in the rates (kinetics) of the reactions and the mechanisms (what kinds of transformation products) We are also interested in how environmental variables affect rate and mechanisms Abiotic vs. iotic Reactions Location of relevant reaction type Soil & water--abiotic and biotic lants and animals--biotic only End roducts Abiotic reactions lead to other organic compounds (or speciation of metals) iotic reactions lead to other organic compounds and/or carbon dioxide Catalysts Abiotic--chemical (metals, water) & photolytic (UV) iotic--enzymes Distinguishing Abiotic and iotic Rxs. Natural (nonsterile) Sterile Definitely biotic Inconclusive Sterile vs. nonsterile experiment Concentration or % Remaining Metabolites arent Compound asic Strategy for Monitoring iodegradation & Reaction Mechanisms C 2 ound Residue % of Total Residues Extracted ones et al EQ 11: Degradation of Atrazine in xygenated Estuarine Water Atrazine Dealkyl atrazine ydroxy atrazine Days 2
3 Reaction Kinetics Rate Law=a mathematical function or differential equation describing the turnover rate of a compound as a function of the concentration First rder Characterized by Exponential Decrease in Concentration ver ower Rate Law Rate = "dc dt = kc n First rder when n = 1 d [ C ] t dt =! k [ C ] 0 or [ C ] t = [ C ] 0 e! kt Differential eq. Integrated eq. d [ C ] t dt =! k [ C ] 0 or [ C ] t = [ C ] 0 e! kt Linearization of First-rder Function seudo-first rder Reaction ln ln t = "kt + ln 0 T 1/2 = ln2/k = 0.693/k alf-life is independent of concentration First rder Zero rder Reaction Kinetics Disappearance of compound is independent of concentration Second rder A second species involved in the disappearance is rate limiting For ex., hydrolysis reaction where base is limited in concentration Can be reduced to pseudo-first order by considering that one of the reactants (for ex., water) is at a much larger concentration then the other reactant and therefore not rate limiting) zero order Zero rder first order First rder nd rder 70 2nd rder
4 yperbolic Kinetics Rate = "dc dt = kc 1 k 2 +C V max k 1 k 3 E + S < > ES < > E + k 2 k 4 v = [S]V max [S] + K m Rate of Reaction, k Enzyme or surface catalyzed. Rate slows down as increases. Velocity of Rx. 0.5 V max Concentration of Chemical, or [S] K m Substrate Concentration logarithmic 70 logistic Monod w/growth Garlon ust Tordon Arsenal Roundup 2,4-D Atrazine DDT mean median Field Dissipation alf-life (days) ydrolysis Reactions Typical ydrolysis Reaction (Nucleophilic Substitution) Nucleophilic substitutions roton, water, or hyroxyl is nucleophile Attracted to electron deficient atom p dependent Abiotic roducts same as for biotic rxs. R 2 - C C R 1 R 2 R 1 - C R 2 C R R- - + R- 4
5 Copied from Schwarzenbach et al Copied from Schwarzenbach et al R 1 R 2 N C R 3 R1 R2 R3 Substituents structure influences reaction rate k (M -1 s -1 ) p 7 C 3 C 3 C 2 C 3 4.5E-06 50,000 y Reductions Transfer of electrons to acceptor molecule (REDX rxs.) +2 C Cl 2 C 3 C 2 C 3 4.0E-06 55,000 y C 3 C 3 N 2 4.0E y C 3 N 2 6.0E02 3 h C 3 C 3 5.6E d C 3 5.0E01 33 h Cl +3 C Cl 3 C -1 p,p'-ddt + Cl- Cl C Cl e - DDE Cl C Cl 2 Cl C -1 Cl Cl - DDD xidations Removal of electrons from carbon or heteroatom Cl Conjugation N fenoxaprop-ethyl Esterase C 2 C C C 2 C 3-2 C3 C3-S-C-C=N--C-N-C3 C3 aldicarb (Temik) C3 C3-S-C-C=N--C-N-C3 +0 C3 aldicarb sulfoxide Cl Cl N N SG fenoxaprop C 2 C C Glutathione Transferase C 2 C C glutathione conjugate 4-hydroxyphenoxy-propanoic acid 5
6 hotolysis ond ond Energy (k mol -1 ) Wavelength (nm) C N C C-C C-Cl Cl-Cl r-r Whether a reactions will take place depends on the probability that a given compound absorbs a specific wavelength of light or on the probability that the excited molecular species undergoes a particular reaction Wavelength (nm) Wavelength (nm) Solar Irradiance Wavelength (nm) UV Absorption Spectrum of 2,4-D Copied from Zepp et al
7 Distribution of Diclofenac, a pharmaceutical, in a lake of Day Affects hotolysis Rate Spring Summer Fall Copied from Zepp et al Depth (m) Concentration of Diclofenac Rate of 2,4-D utoxyethyl Ester hotolysis Thermal Influence on otential for hotoloysis: Lake Turnover & Stratification Dependence of 2,4-D utoxyethyl Ester hotolysis alf-life on Season & Northern Latitude Effect of umic Materials on hotolysis Rate [rometryn] Remaining From Khan & Gamble 1983 Fulvic Acid umic Acid Distilled 2 0 Copied from Zepp et al of UV irradiation (hours) hotodegradation Is Faster in Dirty Water Malathion photolysis in distilled water (p 6) in presence of light T 1/2 = 990 hours Malathion photolysis in Suwannee River water (a lot of humic material) T 1/2 = 16 hours iochemical Ecology of iodegradation End products represent Mineralizations Transformations iochemical reactions involve catalysis by enzymes hotodegradation is only effective in soil when a chemical is at the soil surface 7
8 iodegradation Conceptualization of iodegradation Catalysis by enzymes (Copied from Schwarzenbach et al. 1993) (Copied from Schwarzenbach et al. 1993) Conceptualization of iodegradation 1. acterial cell containing enzymes takes up chemical 2. Chemical binds to suitable enzyme 3. Enzyme-chemical complex reacts, producing transformation products 4. roducts released from enzyme 5. Sorption in soil may influence processes above 6. roduction of new or additional enzyme capacity (induction, activation) 7. Growth of total microbial population, & thus biodegradation capacity Rate of iodegradation (Considerations eyond Enzyme-Substrate Interactions) Rate of delivery of substrate molecules to the microbial cells Rate of diffusion of substrate across intervening media Rate of uptake by microbial cells iochemical effects Enyzme induction De-repression of enzyme Mutation Constitutive enzyme Adaptation Effect of Sorption on iodegradation rocesses iphenyl Mineralization Anaerobic iodegradation ercent Mineralized 0 mg/l in solution of unacclimated microbes 0 mg/l in solution containing microbes & polyvinylstyrene beads Alternative electron acceptors (ie., alternative to 2 ) Methanogenesis (C 2 ; methane) Sulfate Reduction (S 4 ; hydrogen sulfate) Denitrification (Nitrate; N 2 ) (ased on Alexander 1994) ours 8
9 Microbial iochemical Ecological Strategies Mineralization Cometabolism Consortia lasmid exchange (Copied from Schwarzenbach et al. 1993) Soil Microbial iochemical Ecological Strategies erfusion Cycle Concentration of esticide Microbial Numbers Cometabolism pesticide microbial population. pesticide microbial population Mineralization Concentration Remaining Lag phase Factors Influencing Degradation Effect of xygen Concentration on Naphthalene iodegradation Concentration of chemical Temperature Moisture Sunlight Soil type and characteristics (texture, p, C) Nutrients roduct formulation ingredients ther chemicals and previous exposures Aging of residues (Copied from Schwarzenbach et al. 1993) 9
10 Effect of Contaminant Aging (Copied from Steinberg et al. 1987) (days)
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