Environmental Chemodynamics

Transcription

1 Envronmental Chemodynamcs Instructor: Gregory Möller, Ph.D. Unversty of Idaho Learnng Objectves Lst the thermodynamc functons used to descrbe the energy status of molecules n an envronmental system. Understand the relatonshp of Gbb s free energy and chemcal potental n the transfer or transformaton of chemcals n a system. Develop a basc understandng of fugacty and ts role n envronmental transformatons. Defne actvty and ts relatonshp to concentraton. Learnng Objectves, Understand the concept of energy bookkeepng and the relatonshp of Gbb s free energy to enthalpy and entropy n an phase transformaton or chemcal reacton. Develop a basc understandng of frst order and psuedo-frst order chemcal knetcs ncludng ntegrated rate expressons, half-lves and T dependence. Learnng Objectves, 3 Understand the compartment model of the ecosphere. Understand the parttonng of chemcals and how partton constants are used n descrbng envronmental systems. Understand the basc approaches to modelng chemodynamcs and the usefulness and lmtatons of model use Envronmental Chemodynamcs Thermodynamcs and knetcs of processes are mportant n a descrpton of the fate and transport of envronmental chemcals. Dynamcs and energy balance drve the system. Phase transfer and chemcal reacton dynamcs. Interfacal and nter-compartment transport. Thermodynamcs The study of systems at equlbrum. Reversble processes. Used to descrbe the energy status of molecules n an envronmental system. Thermodynamc functons. Chemcal potental, μ. Fugacty, ƒ. Actvty coeffcent, a. Gbbs free energy, G. Enthalpy, H. Entropy, S. 6 1

2 7 Chem. Potental & Gbbs Free Energy Molecules have nternal energes (vbraton, rotaton, etc.) and external energy (translaton, nteracton, etc). Energy depends on temperature, pressure and chemcal composton. Energy content of a chemcal s a populaton concept - Populaton of the chemcal and all of the other substances present (total free energy). 8 μ, G Chemcal potental s the ncremental energy (as adonal molecules) added to the total free energy of the system. G( kj) n ( mol) G T, P, n j μ ( P, T, n1, n,... n ) 1 ( kj mol ) n μ 9 Reference, Standard States Spontaneous transfer of chemcal and thermal energy wll occur untl equlbrum s reached. Chemcal potental, μ can be used to quantfy the tendency of compound to transform or transfer to another system. Absolute values for μ cannot be calculated but changes from reference states can be. Reference state (e.g. nfnte dluton, pure lqud) and standard conons (P, T) yeld a standard chemcal potental, μ as a pont of comparson for startng and fnal states of molecular change for. 1 P partal presure of gas Fugacty Fugacty, ƒ the fugtve property (fleeng tendency). The fugacty of a gas n a mxture s approxmated by ts partal pressure. ƒ θ x P then, ƒ P where: ƒ (snceθ 1) fugacty of gas θ fugacty coeffcent of gas x mole fracton of gas n n j j 11 Fugacty of Lquds and Solds Lquds and solds have vapor pressures and the fleeng tendency should be related. ƒ ƒ pure lqud pure sold () l () s whereγ actvty coeffcent (accounts for non - deal behavor) γ γ ƒ γ x ƒ ƒ γ x P pure lqud pure sold P P P referencestate vapor pressure of pure lqud(sold) pure lqud(sold) and, hence: For deal lquds, γ 1, and for water, γ 1. 1 Actvty Actvty: how actve a compound s n a gven state (e.g. soluton, T, P), compared to a reference state (e.g. pure lqud, T, P). Actvty, a s an apparent concentraton. a γ x where a and x ƒ ƒ ref actvty, γ actvty coeffcent mole fracton of.

3 13 Enthalpy and Entropy Enthalpy, h and entropy, s contrbute to γ snce they descrbe the non-deal, molecule-to-molecule nteractons n a system. Enthalpy (heat energy) s the sum of ntramolecular and ntermolecular forces for a molecule. Entropy (freedom) s the contrbuton to free energy of a molecule by ts randomness of confguraton, orentaton and translaton. 14 Energy Bookkeepng Molecular change n the envronment, such as phase changes (e.g., volatlzaton) and chemcal reacton requre energy change. g (J/mol) h - T(K) s (J/molK) μ Hence, we can calculate molar free energy changes, ΔG for envronmental processes. Can determne f t wll be spontaneous (- ΔG rxn ), or what the energy costs wll be. Can estmate equlbrum concentratons. 15 Chemcal Knetcs The study of systems whose chemcal composton or energy s changng wth tme. Thermodynamcs allows predcton of whether an envronmental process wll take place, but yelds no nformaton about ts speed. Reacton of atmospherc oxygen and ntrogen wth seawater: H O + N + 5O 4HNO 3 (.1M) ΔG kj/mol Fortunately, mmeasurably slow! 16 Reacton Pathway The mechansm of a reacton ncludes all of the ndvdual steps along the pathway of reactants to products. The rate of the reacton (how fast) may be lmted by any one of these steps. Molecular propertes of reactants and products allow calculaton of equlbrum constants for a reacton, but not rate constants. Expermentally determned. Rate of Reacton Rate of reacton of chemcals s a functon of several varables. Chemcal composton, T, P, or V. aa + bb + Κ pp + qq moles 1 d Rate R lter sec a ( A) 1 d( B) 1 d( P) 1 d( Q) b p q Concentraton Dependence Most often, the concentraton dependence of reacton rates takes a smple form. R k(a) α (B) β Λ where (A), (B) are concentratons of exponents α, β are the order of reactants, reacton wth respect to A and B, and k s the rate constant. Example : Fenton' s reacton + Fe(II) + H + HO Fe(III) + HO d[ho] 1 d[fe(ii)] R 17 Example : NO + O NO d(no ) R k(no) (O ) 18 3

4 (A) ln k(b) t 19 (A) Integrated Rate Expressons Many chemcal reactons n the envronment follow frst order or psuedo-frst order (B>>A) chemcal knetcs. d(a) k(a) Upon ntegraton : ln (A) (A) Hence a kt plot of ln(a) Pseudo frst order : vs. t wll be a lne of slope k. Half-Lves of Reactons Useful because t gves a feelng for the tme scale of the reacton. Found by nsertng (A) ½(A) nto the ntegrated rate equaton. Frst order : ½ Psuedo - frst order : Second order : t t ln.693 k k ln t½ k( B) ½ 1 k(a) 1 T Dependence of Reacton Rates The rate constant of an elementary reacton s emprcally found to have a temperature dependence. Ea Ea k Aexp or ln k ln A RT RT where A s the frequency factor or pre - exponental factor. Hence, a semlog plot of be a straght lne wth slope E rate constant vs.nverse T should commonly referred to as an Arrhenus plot. a R and ntercept ln a.ths s Complex Reactons Heterogeneous reactons. Surface effects on reactons. Compettve reactons. Combnatons of elementary reactons usng one or more of the same reactants. Consecutve reactons. Sequental processes often wth one beng the rate lmtng step. Compartments The behavor and effects of envronmental pollutants are related to ther dynamcs n the four major compartments of the ecosphere. Ar (atmosphere). Water (hydrosphere). Sol (lthosphere). Bota (bosphere). Envronmental Interfaces An nterface s where two dfferent compartments meet and share a common boundary. Factors n compartment and nterfacal dynamcs. Physcochemcal propertes of the chemcal. Transport propertes n the envronment. Chemcal transformaton

5 Compartments and Processes Ar Dffuson and dsperson. Photolyss and oxdaton. Heterogeneous reactons on arborne partculates and cloud vapor. Compartments and Processes Water Soluton, sorpton, dffuson, volatlzaton and bo-uptake. Photolyss, hydrolyss, oxdaton, metabolsm, bodegradaton. 5 6 Compartments and Processes Sol Sorpton, runoff, volatlzaton, leachng, bo-uptake. Hydrolyss, oxdaton, reducton, photolyss, metabolsm, bodegradaton. Compartments and Processes Bota Uptake, metabolsm, elmnaton, sequestraton, transport, sorpton. Decomposton, botransformaton, bodegradaton. 7 8 Env. Processes and Propertes Physcal transport. Meteorologcal. Wnd. Bo-uptake. Bomass and food chan. Sorpton Organc content of sol/sedment, aquatc suspensons. Adsorpton and 9 chemsorpton. 3 Env. Processes and Propertes Volatlzaton. Turbulence, wnd velocty, evaporaton, aeraton rate, organc matter. Runoff. Precptaton rate. Leachng. Adsorpton coeffcent. Fallout. Partculate concentraton, wnd velocty. 5

6 Env. Processes and Propertes Chemcal reacton. Photolyss. Solar rradance, transmssvty of water, ar. Oxdaton. Concentratons of oxdants and retarders. Hydrolyss. ph, sedment/sol bascty or acy. Reducton. Oxygen concentraton, ferrous on concentraton, oxdaton state Env. Processes and Propertes Bologcal. Botransformaton. Mcroorgansm populaton and acclmaton. Bodegradaton. Mneralzaton. 33 Solublty n Water Abundance of a chemcal per unt volume n the aqueous phase when the soluton s n equlbrum wth the pure compound (5 C, 1 atm) Saturated soluton, C w sat. 34 Atmospherc-Water Parttonng Equlbrum parttonng of organc chemcals between the gas phase and an aqueous soluton. Henry's law constant, H or K H s the ar-water dstrbuton rato of a dlute solute n pure water. K H P / C w Fugacty mplcatons: hgh vapor pressure and hgh fugacty n water should lead to apprecable partton from water to ar. 35 O. Solvent-Water Parttonng The octanol-water partton coeffcent. Why: The dstrbuton of organc compounds between water and natural solds can be vewed as parttonng processes. Bochemcal (sol, humcs-organc carbon) and bologcal processes are mportant pathways. n-octanol s a surrogate for studyng ths parttonng (fugacty!). K ow C octanol / C water 36 Sold-Water Parttonng Adsorpton of solute to sold surfaces. Freundlch sotherm (constant T). C s K F C 1/n w where K F s the Freundlch constant and n s an emprcally determned value. For n~1, a dstrbuton coeffcent s calculated K d C sold / C water. 6

7 37 Organc Matter-Water Parttonng Organc Matter-Water Partton Coeffcent, K om. Organc matter conssts of large polymerc globular chans. Internal regons are hydrophobc. The nternal regon of the macromolecule becomes capture or soluton regons for neutral or non-polar organc pollutants. K om C organc matter / C water. Bota-Water Parttonng Boconcentraton factor used to descrbe the parttonng of chemcals between a source (typcally water) and bota. BCF C organsm / C water Because boconcentraton s often solvaton of non-polar organc chemcals n adpose tssues, t can be vewed as a fat/water parttonng and proportonal to smlar parttonng constants such as K ow. Removal of the source wll 38 redstrbute the chemcal (depuraton). Chemodynamcs - Envronmental Systems In a compartment model of the ecosphere, chemodynamcs can be used n models to better understand the fate and transport of chemcals n the envronment. Modelng Concepts A model s an mtaton of realty whch stresses those aspects that are assumed to be mportant and omts all propertes that are consdered to be non-essental () Modelng Strengths Mathematcal models central n all of scence. Smplfcaton of complex systems. Allows for predcton of chemcal behavor. Can be used to explan feld data and observatons. Can be used to generate hypotheses. Can be used to desgn experments. Can be modfed. Allows for development of alternatve explanatons. Modelng Weaknesses Over smplfcaton. Never as good as real observatons and real data. Obsolescence. Always subject to a better model

8 43 System Boundary The World Envronmental System Model CV, Ar CV, Bota CV, Water CV, Sol Control volume, CV One Box Mass Balance Model Example: ar-water exchange of perchloroethylene (perc) n a pondfed and draned by a creek. Boundary fluxes: C Cl 4 G the exchange of perc between the water and the atmosphere (pond to atmosphere s defned as postve [+] flux). S the net removal of perc to the sedment. In stu reacton: R bodegradaton, etc. 44 One Box Model G ar-water exchange S sedmentaton R n stu reacton Mass Balance d (Mass n CV) Σ(nputs) + Σ(nternal producton) - Σ(outputs) - Σ(nternal snks) dm perc Contamnant Input, I Cl Cl Cl Cl G Area, A Contamnant Output, O I perc -O perc -G perc -R perc -S perc System Boundary 45 Sedments Total mass, M Total volume, V S R Soluton for G dm/ I -O -G -R -S Assume steady state, dm/. S, R << I, O, G. Calculate G I - O. Hence, subtractng the output from the nput mass perc over a tme perod wll yeld the estmated net loss of perc to the atmosphere by the system. Dynamc Box Models Dynamc models needed to descrbe the effects of system varables that change: dm/, multple boxes, random transport, etc. Requres descrpton of system processes. Theores, relatonshps and data for dynamc transport and transformaton processes. Example: G A υ tot (C w -C a /K' H ) A surface area of pond. υ tot a/w transfer velocty. C a,w a/w concentraton. K' H Henry s Law constant. 48 8

9 Model System Varables In stu reacton of the chemcal. Hydrolyss, photolyss, redox as a functon of ph, temperature, lght ntensty. Mass transfer of the chemcal at water surface. Wnd velocty, temperature. Reacton of the chemcal n the sedments. Sorpton, sequestraton, bodegradaton, boturbdaton. Bologcal uptake, metabolsm, sequestraton and elmnaton of the chemcal. 49 Model System Varables, Inlet of contamnants at varous depths of the water system. Outflow from the water body at the surface and subsurface. Mxng of surface water wth deeper waters. In stu producton of partculates such as phytoplankton. Sorpton dynamcs of the chemcal on suspended, resuspended and settlng partculates. 5 Parttonng and Models Compartment models requre understandng of chemcal parttonng, transformatons and transport to descrbe the equlbrum concentraton relatonshps between dfferent compartments. An understandng of these relatonshps allows an understandng and predcton of the dynamcs of chemcals n the envronment and ther eventual fate