Environmental Chemodynamics

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List the thermodynamic functions used to describe the energy status of molecules in an environmental system. • Understand the relationship of Gibb’s free energy and chemical potential in the transfer or transformation of chemicals in a system. • Develop a basic understanding of fugacity and its role in environmental transformations. • Define activity and its relationship to concentration.

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Nội dung Text: Environmental Chemodynamics

Principles of Environmental Toxicology Learning Objectives • List the thermodynamic functions used to describe the energy status of molecules in an environmental system. • Understand the relationship of Gibb’s free energy Environmental Chemodynamics and chemical potential in the transfer or transformation of chemicals in a system. • Develop a basic understanding Principles of Environmental Toxicology of fugacity and its role in Instructor: Gregory Möller, Ph.D. environmental transformations. University of Idaho • Define activity and its relationship to concentration. 2 Principles of Environmental Toxicology Principles of Environmental Toxicology Learning Objectives, 2 Learning Objectives, 3 • Understand the concept of energy bookkeeping and • Understand the compartment model of the the relationship of Gibb’s free energy to enthalpy and ecosphere. entropy in an phase transformation or chemical • Understand the partitioning of chemicals and how reaction. partition constants are used in describing • Develop a basic understanding of first order and environmental systems. psuedo-first order chemical kinetics • Understand the basic including integrated rate approaches to modeling expressions, half-lives chemodynamics and the and T dependence. usefulness and limitations of model use. 3 4 Principles of Environmental Toxicology Principles of Environmental Toxicology Environmental Chemodynamics Thermodynamics • The study of systems at equilibrium. • Thermodynamics and kinetics of processes are important in a description of the fate and transport of – Reversible processes. environmental chemicals. • Used to describe the energy status of molecules in – Dynamics and energy balance drive the system. an environmental system. – Phase transfer and chemical reaction dynamics. • Thermodynamic functions. – Chemical potential, μ. – Interfacial and inter-compartment transport. – Fugacity, ƒ. – Activity coefficient, a. – Gibbs free energy, G. – Enthalpy, H. – Entropy, S. 5 6 1
Principles of Environmental Toxicology Principles of Environmental Toxicology μ, G Chem. Potential & Gibbs Free Energy • Molecules have internal energies (vibration, rotation, • Chemical potential is the incremental energy (as etc.) and external energy (translation, interaction, etc). additional molecules) added to the total free energy of • Energy depends on temperature, pressure and the system. chemical composition. • Energy content of a chemical is a population concept - Population of the ⎡ ∂G (kJ ) ⎤ ( ) ≡ μi kJ ⋅ mol −1 chemical and all of the other ⎢ ⎥ ⎣ ∂ni (mol ) ⎦ T , P ,n j ≠i substances present (total free energy). G (P, T , n1 , n2 ,...ni ) = ∑ ni μi i 7 8 Schwarzenbach Principles of Environmental Toxicology Principles of Environmental Toxicology Reference, Standard States Fugacity • Fugacity, ƒ the fugitive property • Spontaneous transfer of chemical and thermal energy (fleeing tendency). The fugacity of a gas in a mixture will occur until equilibrium is reached. is approximated by its partial pressure. • Chemical potential, μi can be used to quantify the ƒ i = θ i xi P tendency of compound i to transform or transfer to then, ƒ i ≅ Pi (since θ i ≈ 1) another system. – Absolute values for μi cannot be calculated but changes from reference states can be. where : ƒ i = fugacity of gas i – Reference state (e.g. infinite dilution, pure liquid) θ i = fugacity coefficient of gas i and standard conditions (P, T) yield a standard ni chemical potential, μi 0 as a point of comparison for xi = mole fraction of gas i = ∑nj starting and final states of molecular change for i. j 10 Pi = partial presure of gas i 9 Principles of Environmental Toxicology Principles of Environmental Toxicology Fugacity of Liquids and Solids Activity • Liquids and solids have vapor pressures • Activity: how active a compound is in a given state and the “fleeing tendency” should be related. (e.g. solution, T, P), compared to a reference state ƒ i pure liquid = γ i pure liquid ⋅ Pi 0 (l ) (e.g. pure liquid, T, P). • Activity, ai is an “apparent concentration”. ƒ i pure solid = γ i pure solid ⋅ Pi 0 (s ) where γ i = activity coefficient (accounts for non - ideal behavior) ƒi ai = γ i ⋅ xi = Pi 0 = reference state vapor pressure of i, hence : ƒ ref ƒ i = γ i ⋅ xi ⋅ ƒ i pure liquid(solid) and where ai = activity, γ i = activity coefficient ƒ i = γ i ⋅ xi ⋅ Pi 0 pure liquid(solid) and xi = mole fraction of i. For ideal liquids, γ i = 1, and for water, γ i ≠ 1. 11 12 Schwarzenbach 2
Principles of Environmental Toxicology Principles of Environmental Toxicology Enthalpy and Entropy Energy Bookkeeping • Enthalpy, hi and entropy, si contribute • Molecular change in the environment, such to γi since they describe the non-ideal, as phase changes (e.g., volatilization) and chemical molecule-to-molecule interactions in a system. reaction require energy change. • Enthalpy (heat energy) is the sum of intramolecular gi(J/mol) = hi - T(K) • si(J/molK) = μi and intermolecular forces for a molecule. • Hence, we can calculate molar free energy changes, • Entropy (freedom) is the ΔG for environmental processes. contribution to free energy – Can determine if it will be of a molecule by its spontaneous (- ΔGrxn), or what randomness of configuration, the energy costs will be. orientation and translation. – Can estimate equilibrium concentrations. 13 14 Schwarzenbach Principles of Environmental Toxicology Principles of Environmental Toxicology Chemical Kinetics Reaction Pathway • The study of systems whose chemical composition or • The mechanism of a reaction includes energy is changing with time. all of the individual steps along the pathway of reactants to products. • Thermodynamics allows prediction of whether an environmental process will take place, but yields no • The rate of the reaction (how fast) may be limited by information about its speed. any one of these steps. • Reaction of atmospheric oxygen • Molecular properties of and nitrogen with seawater: reactants and products allow 2H2O + 2N2 + 5O2 → 4HNO3 (0.1M) calculation of equilibrium ΔG0298 = -355 kJ/mol constants for a reaction, but not rate constants. – Fortunately, immeasurably slow! – Experimentally determined. 15 16 Weston Principles of Environmental Toxicology Principles of Environmental Toxicology Rate of Reaction Concentration Dependence • Rate of reaction of chemicals is a • Most often, the concentration dependence function of several variables. of reaction rates takes a simple form. Weston Weston – Chemical composition, T, P, or V. α β ℜ = k (A) (B) Λ aA + bB + Κ → pP + qQ where (A), (B) are concentrations of reactants, 1 d ( A) 1 d (B ) 1 d (P ) 1 d (Q ) ⎛ moles ⎞ exponents α, β are the order of reaction with Rate = ℜ ⎜ ⎟=− =− = = ⎝ liter ⋅ sec ⎠ respect to A and B, and k is the rate constant. a dt b dt p dt q dt Example : Fenton' s reaction Example : 2Fe(II) + 2H + + H 2O 2 → 2Fe(III) + 2H 2O 2NO + O 2 → 2NO 2 d[H 2O2 ] 1 d [Fe(II)] d (NO 2 ) ℜ=− =− ℜ= = k (NO) 2 (O 2 ) dt 2 2dt dt 17 18 3
Principles of Environmental Toxicology Principles of Environmental Toxicology Integrated Rate Expressions Half-Lives of Reactions • Many chemical reactions in the • Useful because it gives a feeling for environment follow first order or psuedo-first order the time scale of the reaction. Weston (B>>A) chemical kinetics. • Found by inserting (A) = ½(A)0 into the integrated Weston rate equation. − d (A) = k (A) dt ln 2 0.693 Upon integration : First order : t ½ = = (A) k k = −kt ln (A) 0 ln 2 Psuedo - first order : t ½ = Hence a plot of ln(A) vs. t will be a line of slope k . k ( B) 0 Pseudo − first order : 1 (A) 0 Second order : t½ = = −k (B) 0 t ln 2k (A) 0 19 (A) 20 Principles of Environmental Toxicology Principles of Environmental Toxicology T Dependence of Reaction Rates Complex Reactions • The rate constant of an elementary reaction is • Heterogeneous reactions. empirically found to have a temperature dependence. – Surface effects on reactions. • Competitive reactions. – Combinations of elementary reactions using one ⎛ − Ea ⎞ ⎛E ⎞ k = A exp⎜ ⎟ or ln k = ln A − ⎜ a ⎟ or more of the same reactants. ⎝ RT ⎠ ⎝ RT ⎠ • Consecutive reactions. where A is the frequency factor or pre - exponential factor. – Sequential processes often Hence, a semilog plot of rate constant vs. inverse T should with one being the be a straight line with slope E a R and intercept ln a. This is rate limiting step. commonly referred to as an Arrhenius plot. 21 22 Weston Principles of Environmental Toxicology Principles of Environmental Toxicology Compartments Environmental Interfaces • An interface is where two • The behavior and effects of different compartments meet and environmental pollutants are share a common boundary. related to their dynamics in the – Factors in compartment and four major compartments of the interfacial dynamics. ecosphere. • Physicochemical properties – Air (atmosphere). of the chemical. – Water (hydrosphere). • Transport properties in the – Soil (lithosphere). environment. – Biota (biosphere). • Chemical transformation. 23 24 4
Principles of Environmental Toxicology Principles of Environmental Toxicology Compartments and Processes Compartments and Processes • Air • Water – Diffusion and dispersion. – Solution, sorption, – Photolysis and oxidation. diffusion, volatilization and bio-uptake. – Heterogeneous reactions on airborne particulates and cloud vapor. – Photolysis, hydrolysis, oxidation, metabolism, biodegradation. 25 26 Principles of Environmental Toxicology Principles of Environmental Toxicology Compartments and Processes Compartments and Processes • Soil • Biota – Sorption, runoff, volatilization, leaching, – Uptake, metabolism, elimination, sequestration, bio-uptake. transport, sorption. – Hydrolysis, oxidation, reduction, photolysis, – Decomposition, biotransformation, metabolism, biodegradation. biodegradation. 27 28 Principles of Environmental Toxicology Principles of Environmental Toxicology Env. Processes and Properties Env. Processes and Properties • Physical transport. – Volatilization. – Meteorological. • Turbulence, wind velocity, evaporation, aeration rate, organic matter. • Wind. – Runoff. – Bio-uptake. • Precipitation rate. • Biomass and food chain. – Leaching. – Sorption • Adsorption coefficient. • Organic content of – Fallout. soil/sediment, aquatic • Particulate concentration, suspensions. wind velocity. • Adsorption and 29 30 chemisorption. 5
Principles of Environmental Toxicology Principles of Environmental Toxicology Env. Processes and Properties Env. Processes and Properties • Chemical reaction. • Biological. – Photolysis. – Biotransformation. • Solar irradiance, transmissivity of water, air. • Microorganism population – Oxidation. and acclimation. • Concentrations of oxidants and retarders. • Biodegradation. – Hydrolysis. • Mineralization. • pH, sediment/soil basicity or acidity. – Reduction. • Oxygen concentration, ferrous ion concentration, oxidation state. 31 32 Principles of Environmental Toxicology Principles of Environmental Toxicology Atmospheric-Water Partitioning Solubility in Water • Abundance of a chemical per unit volume in the • Equilibrium partitioning of organic aqueous phase when the solution is in chemicals between the gas phase and an aqueous equilibrium with the pure compound solution. (25 °C, 1 atm) • Henry's law constant, H or KH is the air-water • Saturated solution, Cw sat. distribution ratio of a dilute solute in pure water. KH = Pi / Cw – Fugacity implications: high vapor pressure and high fugacity in water should lead to appreciable partition from water to air. 33 34 Principles of Environmental Toxicology Principles of Environmental Toxicology O. Solvent-Water Partitioning Solid-Water Partitioning • The octanol-water partition coefficient. • Adsorption of solute to solid surfaces. – Why: The distribution of organic compounds • Freundlich isotherm (constant T). between water and natural solids can be viewed Cs = KFCw1/n as partitioning processes. where KF is the Freundlich constant and n is an – Biochemical (soil, humics-organic carbon) and empirically determined value. biological processes are • For n~1, a distribution important pathways. coefficient is calculated – n-Octanol is a surrogate Kd = Csolid / Cwater . for studying this partitioning (fugacity!). Kow = Coctanol / Cwater 35 36 6
Principles of Environmental Toxicology Principles of Environmental Toxicology Organic Matter-Water Partitioning Biota-Water Partitioning • Bioconcentration factor used to describe • Organic Matter-Water Partition Coefficient, Kom. the partitioning of chemicals between a source • Organic matter consists of large polymeric globular (typically water) and biota. chains. BCF = Corganism / Cwater – Internal regions are hydrophobic. – Because bioconcentration is often solvation of • The internal region of the macromolecule non-polar organic chemicals in adipose tissues, it becomes “capture” or “solution” can be viewed as a fat/water regions for neutral or non-polar partitioning and proportional organic pollutants. to similar partitioning constants Kom = Corganic matter / Cwater . such as Kow. • Removal of the source will redistribute the chemical 37 38 (depuration). Principles of Environmental Toxicology Principles of Environmental Toxicology Chemodynamics - Environmental Systems Modeling Concepts • A model is an imitation of reality which stresses • In a compartment model of the ecosphere, those aspects that are assumed to be important — chemodynamics can be used in models to better and omits all properties that are considered to be understand the fate and transport of chemicals in the non-essential (Schwarzenbach). environment. 39 40 Principles of Environmental Toxicology Principles of Environmental Toxicology Modeling Strengths Modeling Weaknesses • Mathematical models central in all of science. • Over simplification. • Simplification of complex systems. • Never as good as real observations and real data. • Allows for prediction of chemical behavior. • Obsolescence. • Can be used to explain field data and observations. – Always subject to “a better model”. • Can be used to generate hypotheses. • Can be used to design experiments. • Can be modified. • Allows for development of alternative explanations. Schwarzenbach 41 42 7
Principles of Environmental Toxicology Principles of Environmental Toxicology Environmental System Model One Box Mass Balance Model • Example: air-water exchange of perchloroethylene (perc) in a Control volume, CV pondfed and drained by a creek. • Boundary fluxes: C2Cl4 CV, Air CV, Water – G → the exchange of perc between the water and the atmosphere (pond to atmosphere is defined as positive [+] flux). – S → the net removal of CV, Biota CV, Soil perc to the sediment. • In situ reaction: System Boundary – R → biodegradation, etc. The World 43 44 Schwarzenbach Principles of Environmental Toxicology Principles of Environmental Toxicology One Box Model Mass Balance d (Mass in CV) G = air-water exchange air- = dt S = sedimentation Σ(inputs) + Σ(internal production) - Σ(outputs) - Σ(internal sinks) R = in situ reaction in dMperc = Iperc - Operc - Gperc - Rperc - Sperc dt Cl Cl Contaminant Contaminant Cl Cl Input, I Output, O G Area, A Total mass, M R System Total volume, V Boundary S 45 46 Schwarzenbach Sediments Principles of Environmental Toxicology Principles of Environmental Toxicology Solution for G Dynamic Box Models • Dynamic models needed to describe the effects of • dM/dt = I - O - G - R - S system variables that change: • Assume steady state, dM/dt = 0. – dM/dt ≠ 0, multiple boxes, random transport, etc. • S, R
Principles of Environmental Toxicology Principles of Environmental Toxicology Model System Variables Model System Variables, 2 • In situ reaction of the chemical. • Inlet of contaminants at various depths of the water system. – Hydrolysis, photolysis, redox as a function of pH, Schwarzenbach temperature, light intensity. • Outflow from the water body at the surface and subsurface. • Mass transfer of the chemical at water surface. • Mixing of surface water with deeper waters. – Wind velocity, temperature. • In situ production of particulates • Reaction of the chemical in the sediments. such as phytoplankton. – Sorption, sequestration, • Sorption dynamics of the biodegradation, bioturbidation. chemical on suspended, • Biological uptake, metabolism, resuspended and settling sequestration and elimination particulates. of the chemical. 49 50 Principles of Environmental Toxicology Principles of Environmental Toxicology Partitioning and Models • Compartment models require understanding of chemical partitioning, transformations and transport to describe the equilibrium concentration relationships between different compartments. • An understanding of these relationships allows an understanding and prediction of the dynamics of chemicals in the environment and their eventual fate. 51 52 9