Definitions. ES/RP 532 Applied Environmental Toxicology. Reaction Mechanisms. Definitions. Abiotic vs. Biotic Reactions.

Transcription

1 ES/R 532 Applied Environmental Toxicology Lecture 5 Kinetics & Reactivity (Environmental Attenuation of Contaminants 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 Detoxification Degradation resulting in loss of toxicity or biological activity Definitions Reaction Mechanisms Transformation 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 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 Considerations 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 1

2 Distinguishing Abiotic and iotic Rxs. Natural (nonsterile) Sterile Concentration or % Remaining parent asic Strategy for Monitoring iodegradation & Reaction Mechanisms C 2 Definitely biotic bound Inconclusive metabolite 1 metabolite 2 Sterile vs. nonsterile experiment 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. 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 2

3 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 yperbolic Kinetics Rate = dc dt = k 1C 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 ydrolysis Reactions Nucleophilic substitutions roton, water, or hyroxyl is nucleophile Attracted to electron deficient atom p dependent Abiotic roducts same as for biotic rxs. 3

4 X Z L X - Z L X Z X Z - - +L - +L Copied from Schwarzenbach et al R 1 R 2 N C R 3 R1 R2 R3 k (M -1 s -1 ) p 7 C 3 C 3 C 2 C 3 4.5E-06 50,000 y 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 Copied from Schwarzenbach et al Reductions Transfer of electrons to acceptor molecule (REDX rxs.) +2 C Cl 2 xidations Removal of electrons from carbon or heteroatom 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 - -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 DDD 4

5 Cl Cl Cl N N glutathione conjugate Conjugation N SG fenoxaprop-ethyl fenoxaprop Esterase C 2 C C C 2 C 3 C 2 C C Glutathione Transferase C 2 C C 4-hydroxyphenoxy-propanoic acid 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. 5

6 Solar Irradiance Distribution of Diclofenac, a pharmaceutical, in a lake Spring Summer Fall UV Absorption Spectrum of 2,4-D Depth (m) Concentration of Diclofenac Copied from Zepp et al Thermal Influence on otential for hotoloysis: Lake Turnover & Stratification of Day Affects hotolysis Rate Copied from Zepp et al Dependence of 2,4-D utoxyethyl Ester hotolysis alf-life on Season & Northern Latitude Rate of 2,4-D utoxyethyl Ester hotolysis Copied from Zepp et al Effect of umic Materials on hotolysis Rate [rometryn] Remaining From Khan & Gamble 1983 Fulvic Acid umic Acid iochemical Ecology of iodegradation End products represent Mineralizations Transformations iochemical reactions involve catalysis by enzymes Distilled 2 0 of UV irradiation (hours) 6

7 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, and 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) Alternative electron acceptors (ie., alternative to 2 ) Methanogenesis (C 2 ; methane) 0 mg/l in solution containing microbes & polyvinylstyrene beads Sulfate Reduction (S 4 ; hydrogen sulfate) Denitrification (Nitrate; N 2 ) ours (ased on Alexander 1994) 7

8 Microbial iochemical Ecological Strategies Mineralization Cometabolism Consortia lasmid exchange (Copied from Schwarzenbach et al. 1993) Soil Microbial iochemical Ecological Strategies Factors Influencing Degradation Concentration of esticide Microbial Numbers cometabolism pesticide microbial pop'n. pesticide mineralization microbial pop'n. Concentration of chemical Temperature Moisture Sunlight Soil type and characteristics (texture, p, C) Nutrients roduct formulation ingredients ther chemicals and previous exposures TIME Aging of residues Effect of xygen Concentration on Naphthalene iodegradation Effect of Contaminant Aging (Copied from Schwarzenbach et al. 1993) (Copied from Steinberg et al. 1987) 8