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Lab for Any Lec H01001

by: Schuyler Nikolaus

Lab for Any Lec H01001 CHEM 212

Marketplace > George Mason University > Chemistry > CHEM 212 > Lab for Any Lec H01001
Schuyler Nikolaus
GPA 3.72

Gregory Foster

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Gregory Foster
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This 39 page Class Notes was uploaded by Schuyler Nikolaus on Monday September 28, 2015. The Class Notes belongs to CHEM 212 at George Mason University taught by Gregory Foster in Fall. Since its upload, it has received 13 views. For similar materials see /class/215243/chem-212-george-mason-university in Chemistry at George Mason University.


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Date Created: 09/28/15
Sorption of Organic Substances t0 Aquatic Solids Sorption to Solids Given the largest environmental compartments are the atmosphere the hydrosphere and the lithosphere the soilwater and soilair interfaces are equally important to the airwater interface Processes that control the uptake of organic compounds by solids are extremely important in determining distribution in time and space and this can be particularly important in determining transport velocities since solids are subject to diiTerent physical laws than water molecules For the most part the solids of significance to understanding sorption are small clay particles which may be partially coated with natural organic matter The aquatic environment is normally considered to consist of three phases water settleable particles and nonsettleable particles sometimes referred to as colloids The significance of sorption is profound Sorption in uences many exchange processes such as airwater exchange because it affects the freely dissolved concentrations of organic compounds in natural systems Only the freely dissolved compounds are subject to exchange processes across interfaces other than sorption This could have a dramatic effect on volatilization rates for example is the compound was extremely sorptive to aquatic particles because the solidbound fraction is not free to move across the airwater interface Sorption is normally modeled as an equilibrium process between dissolved solutes in air or water and solidassociated solutes 1 TERMS A Adsorption The interphase accumulation or concentration of substance at a surface or interface Concentration in boundary region is greater than in the interior or contiguous phase B Absorption The accumulation of substance whereby the concentration is evenly distributed throughout the contiguous phase C Sorption General expression of the process of adsorption or absorption without specifying the mechanism of interphase transfer The material being sorbed is the sorbate and the sorbing phase is termed the sorbent D Sediment Fragmented geological material that originates from the disintegration of rocks and is transported by or suspended in or deposited by air or water or is accumulated in beds by other natural processes Normally classi ed as bed sediments sedimented particles or suspended sediments suspended in water in aquatic systems Colloid dispersion of particles of one substance the dispersed phase throughout another substance or solution continuous phase Particle sizes range from l pm to 1 nm 11 Sediment Lithology 1 J Geologic materials are derived from silicate rocks Silicates make up 90 of the minerals in the earth39s crust Bedrock may be in the form of igneous metamorphic or sedimentary in origin Bedrock is subject to physical and chemical weathering processes Reaction and fragmentation produces soils and sediments Weathering reactions also form dissolved phase ions is natural waters The general reaction scheme for chemical weathering is as follows bedrock atmospheric input gt altered rock clays ions 3 Clays are negrained hydrous silicates derived om bedrock Primary elemental composition is O Si and Al Clay structure is closedpacked sheets of O atoms with Si and Al ions in interstitial spaces 0 atoms arranged in a tetrahedral layer on top of an octahedral layer 111 Organic Geochemistry low MW acids 25 pollutants 5 humic amp fulvic acids 50 neutrals and bases 20 A All natural waters contain organic matter the major fraction of organic material is in the form of humic and fulvic acids 50 of OM contaminants and pollutants are lt5 and typically much lower B Important terms 1 DOWDOC dissolved organic mattercarbon material which passes through a 045 pm lter operationally de ned 2 POMPOCSOMSOC particulate organicmattercarbon suspended organic mattercarbon material retained on a 045 pm lter 3 T 0C total organic carbon sum of POC DOC 4 VOC volatile organic carbon organics trapped from a water sample by stripping techniques 5 COM colloidal organic matter ranges in size from 045 pm to 1 nm in spherical diameter dif cult to measure and seldom used term but extremely important in geochemical processing 6 allochthonous terrestrial sources outside of aquatic system streams and rivers 7 autochthonous sources within an aquatic system eg algae and microbs large lakes estuaries and oceans DOC varies according to water source range is seawater lowest to bogs highest average values are shown below Bog water 33 mgL River water 7 mgL Precipitation 1 mgL Sea water 05 mgL C Aquatic Humic Substances AHS high molecular weight refractory yellowblack amorphous macromolecular acids which show appreciable solubility in water 1 Fulvic acids AHS which range from 500 2000 amu corresponds to 8090 of the AHS action yellow in color Soluble in water at pH lt2 2 Humic acids AHS which range om 2000 100000 amu commonly associated with clay minerals and FeMn oxyhyroxides corresponds to 10 of AHS action HS39s precipitate om water at pH lt2 3 Humin humic substances found in soil that is insoluble in water very high molecular weight 4 AHS play role in the a chemical weathering processes b pollutant transport c regulating acidity of natural waters D Sources and origin of organic matter vegetation largest C source 1 Lils soils are large sources of org matter the extent depends on vegetation present prairiegtforestgtarid soil show order of OM runoff prairie has much nonwoody OM in soil organics ushed to streams with precipitation Figure below adopted from Leenheer J et al 1995 Environ Sci Technol 29 399 CH H H Ho I 2 CH3 COOH H C 39 0 C00quot 0 I o H H o 3 2 II 043 CH2 C CH2 H H o 0 gt250 HOOC CHZ H l C O H H MOOC OH H o CO I II H CHg T C CHz CHz CHZ COOH CH3 H 0 H000 OH O H H 0 COO H coon H H H H O H I 0quot coon H H H H n H H CH3 H 0 quot0 f O HOOC OH H H 20 600 llnlllll l flun chumml mmlnlu lA n fnluir spill mnlnInn B 0 ow coon H H H OH 1 leaching of plant material into water 2 leaching of plant material through soil with biochemical modi cation 3 leaching of soil humic substances into water d lysis of algal cells with photochemical modification e UV oxidation of surface active OM followed by polymerization reactions 2 Humus formation theories a lignin breakdown with by product polymerization b cellulose dehydration oxidation and polymerization c lignin fragmentation d condensation of sugars and amino acids 3 Chemical Characterization With amorphous structure no de nitive structure possible Characterized by presence of important functional groups on AHS Because the structure of DOC is so complex and variable it is often described in terms of functionality present Acidic groups 1 Carboxyl group COOH The carboxyl group ionizes COOH H20 lt gt H30 C0039 The pKa for carboxyl groups is 47 In surface water carboxylate ion predominates Carboxylate ion profoundly increases water solubility 1 carboxyl group per every 7 carbon atoms in AHS 2 Enol group CCOH pKa varies from 6 to 9 Often ionize in natural water 3 Phenol group ArOH Phenol has pKa of 9 electron withdrawing groups increase Ka and make more acidic eg Cl N02 Neutral groups 1 Hydroxyl group OH pKa is 14 and does not ionize but does increase water solubility by forming Hbonds with water 2 Ether group COC Increases water solubility slightly is present a l per every 30 C atoms 3 Aldehyde group COH and ketone group CO Increases water solubility 4 Ester group COOC Formed from carboxylic acids increases water solubility Basic groups 1 Amine group NH2 Amines are proton acceptors and form cations in neutral water NH2 HZOlt gt H3O NH3 Also form hydrogen bonds Amine groups greatly increase water solubility Relative solubility of different functional groups shows importance of carboxyl group in enhancing solubility Implications ofAHS AHS play a role in the mobilization transport segregation and deposition of trace metals in soils and sediments Play a key role in the weathering of rocks an minerals Transport metals in natural waters Can reduce oxidized forms of certain metal ions Can affect bioavailability of metal ions Example AHS binding reactions with metal ions in solution o C39OH ICI OMOH o or w o 0 4 M2 I H OH 0 M O 0 Lo 0 I M2 gt4 H C OH O C o r r 2 CO COH 39 M2H20 0 H OH OH 0 For organic pollutants AHS enhances their solubility in water IV SORPTION ISOTHERMS FOR SOILS AND SEDIMENTS Sorption of quanti ed and evaluated by tting sorption data to one of several models The three most widely used sorption models which include the Langmuir isotherm the Freundlich isotherm and the BET isotherm All three isotherms describe sorption as a dynamic equilibrium established for an organic compound often referred to as the sorbate between the sorbent and the solvent The features of each model are described below A Langmuir Isotherm Cs CssatILCw1 KLCW CS Concentration of chemical in solid material mgkg C3630 monolayer saturation concentration of solid material mgkg KL constant related to binding energy of sorbate to sorbent surface Temp dependent Lmg CW concentration of chemical in water mgL Langmuir Model Assumptions a sorbent has fixed quotnumberquot of sites saturable b all sites have equal binding enthalpies independent of the extent of coverage c maximum sorption is a monolayer on surface of substrate Langmuir Isotherm 0 8 5 Saturated Region a go 0 6 Nonlinear Region 2 0 4 U 0 2 Cw mgL Figure 1 Langmuir isotherm Csmmx 850 KL 015 Linear form of Langmuir equation is given as 1Cs 1Csmax Csmax KLCW The Langmuir isotherm works best for process such as ion or ligand exchange The sorbents tend to be mineral in nature with a xed number of active binding sites Note the saturation of the sorbent at high concentrations of solute in the water phase For the linear region of the Langmuir curve can de ne distribution constant Kd CsmaxKL Kd CsCW at equilibrium B BET isotherm PVP0 P 1VmC C1VmCPP0 3 P equilibrium pressure of sorbate atm V volume of gaseous sorbate sorbed L P0 liquefaction or saturation pressure of sorbate atm Vm volume of gaseous sorbate sorbed when the entire surface is covered with a complete monolayer C constant related to heat of sorption The BET isotherm has been used to study the sorption of organic compounds such as pesticides with relatively high vapor pressures especially when the interaction occurs on mineral surfaces BET Model Assumptions a related to Langmiur except that sorption may occur at multiple layers b each layer is modeled by Langmuir isotherm c second layer may form before first is saturated d sorption occurs at fixed sorption sites C Freundlich isotherm This is an empirically derived model and has limited theoretical foundation Cs KFCwn 4 K Freundlich distribution constant Lkg n exponential term of water concentration Freundlich Isotherm 250 200 1 50 1 00 CS mgkg 50 0 005 01 015 02 Cw mgL Figure 2 Freundlich isotherm Kd 1000 n 085 The linear form of Freundlich is log CS log KF n logCW 5 The mechanism of sorption is mixed between xed sites and a pseudo partition process many layers of xed sites exist all with different enthaplies 77777 no mechanism can be deduced from this relation The Fruendlich isotherm is an emperical relation D Linear isotherm Specialized case of Langmuir and Freundlich n 1 isotherms C Kde 6 This model is consistent with a partitioning mechanism for sorption The solute is homogeneously distributed throughout each phase Linear Isotherm 200 50150 1 Fe E 100 d 50 0 0 005 01 015 02 Cw mgL Figure 3 Linear isotherm Kd 1000 The linear isotherm is most commonly used to determine empirically derived distribution constants Linear isotherm works best for chemicals with very low water solubilities eg HOCs E Distributed sorption models In many cases a single sorption model is not adequate to completely describe the sorption behavior of organic solutes across all concentration ranges observed in nature A mixture of models may be applied referred to as distributed reactivity models The simplest cases involve a combination of two sorption mechanisms such as 1 Linear and Fruendlich Cs Kde KFCWn 2 Linear and Langmuir Cs Kde CsmaxKLCw1 KLCW Simple distributed models work best when mixed mechanisms are involved in sorption such as OM partitioning and mineral site F Organic carbon normalized sorption constants K0c 1 It has been shown that organic substances are more enriched in solids with higher organic carbon contents 2 Sorption constants are often normalized to the amount of organic carbon in the solid phase as K00 Kdfoc Koc CooCW 10 f0c weight fraction g g of organic carbon in solid phase COC concentration of chemical in solid phase mgkg normalized to oc content eg Csfoc CW concentration of chemical in water eg mgL Nonlinearity is often observed in experimental K0c determinations but usually at relatively high concentrations of CW From eq 8 above COC KOCCW lsotherms between COC and CW for organic chemicals appear linear up to the region of COC kgkg Kcch 001 above which isotherms show nonlinearity Reduction of K0c is thought to be due to saturation by the HOCs of natural organic matter in the solids Less binding sites are available at relatively high COC values G Sorption partitioning theory Sediment organic matter behaves as organic sovent octanol Treat sorption as partitioning process CLWVWYLW Ci0V0 Yi0 C1 concentration of ith compound in water w or organic matter 0 mol L v molar volume of water w or organic matter 0 Lmol y activity coef cient of compound in water w and org matter 0 K00 Ci0Ciw Vinw VOYi0p0 log K0c log SLW log v0 log y log p0 13 log K0c log SLW log vopo 14 p0 bulk density of organic matter kgL and assuming yLw gtgt no G Sorption variables 1 Temperature like partitioning theory as temperature inceases sorption decreases because AHS is negative 2 Total suspended matter concentrations TSM as TSM increases sorption decreases Colloidal materials are released om particles which enhance the solubility of dissolved phase solutes 3 pH for ionizable solutes sorption is very pH dependent Typically the neutral form is the most sorptive As the action of the ionic form increases sorption decreases V MASS FRACTIONATION Organic chemicals in the aquatic environment distribute between water and solids according to thermodynamic principles and geochemical parameters A Liguid to solid ratio The effects of the liquidsolid ratio in de ning the distribution of organics between sediments and water ocw vawvaw CSMS 15 ocw mass fraction g of chemical in water g of chemical in water and solids of compound in water phase C concentration of chemical in water w or solid s phases molL and molkg respectively VW volume of water in system L MS Mass of suspended sediment kg per unit volume of water VW Note that as Cs and T SM increase ocw decreases Cs Kde 16 Substitution of eq 16 into eq 15 yields ocw CwVwCWVW CdeMs 17 ocw VwVW KdMs 18 For surface water systems the following relation is useful dividing eq 18 through by VW for a system consisting of water and particles aw 1 KdITSM391 lt19 and thus Otp 1 OLW 0cp mass fraction in particles 20 For groundwater systems the following form of above eq is useful aw 1 Kdlpsa clgtltIgtquot 21 pS bulk density of solid kgL I porosity of system volume watertotal volume B Mass balance relations and fractional composition 3 phase model 1 Introducing colloids Colloids are small particles in size range of 1 pm to 1 nm colloids cannot be efficiently filtered from water like suspended sediments 2 Colloid composition Clay minerals Oxyhydroxides of Fe and Mn Humic and fulvic acids in water Very small microorgams eg viruses CLO crgm Colloids can play important role in sorption of organic substances om water esp those with substantial OM ie foe 3 Colloids comprise some of dissolved organic carbon DOC fraction in natural waters 4 Colloid sorption can be described much like that for sediments Kdoc CdocCW Kdoc can be estimated via KOW or SW using available LFERs 5 Effect of presence of colloids in natural waters i Apparent Kp39s TSP tend to decrease when DOC increases ii Very hydrophobic Koc39s tend to be overestimated in natural waters when using KOW as estimator iii Colloids enhance water solubility of hydrophobic organics thereby increasing amount in apparent dissolved phase Fractional Composition of Organics in Water 3 phase system model Mass balance can be expressed by the following equation stating that the organic compound is distributed between dissolved particulate and colloidal phases Ct Cw Cp Cc C phase concentration dissolved w particulate p and colloidal c mgL TSM total suspended matter concentration use kgL as unit DOC dissolved organic carbon concentration use kgL as unit Kd mass based particlewater distribution constant Lkg Kdoc DOCwater partition coefficient Lkg z KOO f0c action of organic carbon in particulate phase gg Ct CW CWTSMKd CWDOCKdoc 24 a actional mass distribution in the dissolved phase ocw ocW 1 TSM1KdDOCKdocquot 25 b actional mass distribution in the particulate phase ocp ocp 1 1ITSM1Kd DOCJKdocyuTSMiKuquot 26 c actional distribution in the colloidal phase 0cc one 1 ITSM1KdD0C1Kdoc 1D0C1Kdocquot 27 in most applications the following substitution is made in the above eqs for particle phase concentrations Iocfoc Id 20 Note that 0tw 01 one 1 6 Correction of eld measured distribution constants Field studies that measure particlewater distribution constants typically assume that Kd ts a linear isotherm measured values are typically ltl ppbxthus C Krdneas 7P C the measured meas CW includes both truly dissolved and colloidal fractions using most analytical methods need to mathematically correct for amount in colloids to derive actual act Kd the measured Kd is dependent on amount of chemical associated with colloids C K P 1 28 1 Cw 9 70Kdocwoci Kd Rearranging for the actual Kd yields K3 2 Kg 1 Km DOC Typical values of DOC run from 05 mgL estuarine to 10 mgL riverine the Kdoc term in the above eq becomes important when Kdoc values approach thresholds 21 For example for the multiplier of 1 Kd0cDOC to be gt5 the following apply 05 gt8 X l gt729 5 gt8 x l gt629 10 gt8 x 1 gt529 Assuming Kdoc 041KOW Only very hydrophobic organic contaminants will be substantially affected by DOC in most surface water environments VI SORPTION OF ORGANIC VAPORS TO SOILS Vapors of organic compounds sorb to soils The sorption is modeled as shown above using BET isotherms The magnitude of sorption depends on a temperature and b relative humidity Since soils are mineral in nature and represent polar surfaces water binds very tightly to the surface of soil particles Dry soil is an effective sorbent for organic vapors Sorption coefficients for vapor phase organic substances are generally modeled by the following process model In K A BT 26 This model is a form of the van t Hoff equation and the constant B is defined as the heat of sorption of the vapors to the solids The moisture content of the soil is extremely important in determining the degree of sorption You may have noticed the esh smell in the air after in rains This is due to the release of sorbed plant green leaf volatiles om dry soil after rainfall Dry soil is important is the vadose 22 zone of ground water and for the uptake of air phase organic substances from the atmosphere VII LINEAR FREE ENERGY RELATIONSHIPS Correlations between sorption and Swhq and KOW are routinely performed The rules governing estimations of K0c from physical properties of organic compounds are similar to that described for octanolwater partition coefficients Examples of LEFRs for the sorption of hydrophobic organic compounds is shown below Correlations with reference state water solubility log Kom 093 log Stwhq 017 aromatic hydrocarbons log Kom 056 log Stwhq 097 phenyl ureas log Kom 070 log Stwhq 035 chlorinated hydrocarbons Correlations with octanolwater partition coefficients log K0m 088 log KOW 027 chlorinated hydrocarbons log K0m 081 log KOW 025 chlorophenols log K0m 112 log KOW 015 phenyl ureas log Kom 101 log KOW 072 aromatic hydrocarbons VIII SORPTION KINETICS A Desorption of organic pollutants om sediments shows a kinetic hysteresis effect for aged sediments This is not observed for recently spiked sediments 23 Sediments behave as aggregates which have macropores and micropores Aging allows greater pollutant diffusion into aggregate micropores which is diffuses out of sediment phase very slowly Aging theories i Micro and macropores in the mineral fraction of sediments trap HOCs in the micropores ii Natural organic matter N OM shows two domains One domain is labile and shows reversible sorption kinetics This labile fraction is thought to behave as a gel phase and chemically is composed of predominantly aliphatic carbon The gel phase is highly uid and allows easy access to HOCs The second domain is characterized as more crystalline and is chemically is enriched in aromatic carbon and is more highly crosslinked This second domain shows some level of irreversible sorption kinetics and accounts for the slow desorption kinetics observed in aged sediments The aromatic enriched carbon is the product of sediment diagenesis where organic matter is altered by microorganism during sediment aging Soot carbon The aerosols produced om C combustion contain condensed soot carbon highly enriched in aromatic rings Soot carbon is thought to be crystallinelike and appears to sequester HOCs to various extents Soot carbon clearly shows enhanced sorptive capacity relative to natural organic matter ii39 H 24 Sample Problems 1 What are the major sorption isotherms used to characterize the sorption of organic substances to solids How can you differentiate each one through a linear plot Answer Linear Freundlich and Langmuir isotherms Though inspection of axis labels type of isotherm can be identi ed For example linear plots results as linear CS vs CW Fruendlish is log CS vs log CW and Langmuir is lCS vs lCW 2 A Freundlich isotherm yields the linear equation below for the sorption of compound Z to Potomac River sediments What is the Kd for compound Z If the f0c of Potomac River sediment is 014 what is the K0c for Z log CS 092 log CW 369 Answer According the Fruendlich equation 369 log Kd Kd 4900 Lkg note units K0c Kdf0c 4900014 35000 Lkg 3 A 445 L sample of Potomac River water was collected and analyzed for the organochlorine pesticide lindane yhexachloro cyclohexane in the dissolved and particulate phases The volume based ie massvolume concentrations were found to be 16 ngL in the dissolved phase and 99 ngL in the particulate phase The total suspended matter TSM in Potomac River water was 297 mgL and the suspended organic carbon SOC was 36 mgL a What is the fraction of lindane in the dissolved phase of river water b What is the mass based massmass concentration of 25 lindane in the particle phase c Estimate the Kd and K0c of lindane in Potomac River sediments Answer a the mass fraction of lindane in the dissolved phase is make sure units are similar for all measurements ocw CWCs CW 16 ngLl6 ngL 99 ngL 062 62 b CS mass based conc Cp volume based concTSM X A unit constant 99 ngL297 mgL X 1000 mgg 330 ngg c CS Kd CW and Kd CsCW assuming Kd is constant 333 nggl6 ngL X 1000 gkg 20800 Lkg Koo Kdfoc KdSOCTSM 2080035297 176500 Lkg 4 The concentrations of benzylnbutyl phthalate BBP a plasticizer in river water TSP 229 mgL foc 016 DOC 46 mgL are 19 ngL and 089 ngL in the quotdissolvedquot Cd and particulate phases Cp respectively Estimate the Kd for BBP using a 3phase model that corrects for binding to colloids a Cp aWCdeTSM where Cd CW Cc Thus Kd Cp aWCdTSM aw l KdOCDOC391 for colloids and water Must estimate Kdoc as 041KOW 04100491 33326 Lkg 26 Kd Koc foe Kd Cp 1 KdocD0C391Cd TSPD 089 ngL1 33326 Lkg x 46 x 10396 kgL391 19 ngL x 229 x 10395 kgL 236 x104 Lkg 27 GasParticle Partitioning Aerosol 28 The Atmosphere and GasParticle Partitioning The atmosphere is a major pathway for the transport and deposition of natural and anthropogenic organic chemicals including pollutants Vast quantities of organic contaminants are released into the atmosphere either directly or indirectly from a variety of sources For example many f1eldapplied agricultural pesticides volatilize directly into the atmosphere or after attatching to soil material are blown into the air by the winds Polycyclic aromatic hydrocarbons PAH are emitted into the atmosphere by automobiles forest res and many other combustion sources Polychlorinated biphenyls PCB s are released from leaking old electrical tranformers and land lls into the air Once airborne stable chemicals such as PCBs can be transported thousands of kilometers from their original sources contaminating even remote regions of the earth For example PCBs and a large number of organochlorine compounds like DDT and chlordane insecticides have been detected in air water and biota over the open oceans in Antarctica and the Arctic all regions well removed from possible sources of the pollutants see for example Bidleman et al 1989 A complete assestment of total inputs of organic pollutants into an aquatic environment must therefore consider contributions from atmospheric depositional processes 1 Aerial Distribution And Removal Of Trace Organics Semivolatile organic pollutants are found in the atmosphere in the gas vapor phase G or associated with particles P quot n VAPOR PARTICLE ADSDHBED on HEMGVAL Mlmsunw RAINIENGW Dar amour FALLDUTJI SCAVENGIMG SEAVENGING GAE EXCHANGE 29 Theory suggests that the distribution of SOCs between the gas G and particle P phases depends on several factors including 1 particle size distribution 2 particle surface area 3 organic content of particles 4 vapor pressure of the SOCs 5 the functional groups of a SOCs are also likely important for polar compounds since they can possibly undergo chemical adsorption in addition to physical adsorption and absorption Before proceeding with our discussion of gasparticle distributions of semivolatile organic compounds let39s consider several other important aspects of atmospheric chemistry 11 ATMOSPHERIC COMPOSITION AND CHEMISTRY Atmospheric Gases The atmosphere is composed primarily of two dominant gases nitrogen and oxygen with much lesser amounts of other gases see table below from Seinfeld 1986 Although the concentrations of the gases has remained remarkably stable over time the atmosphere is really a dynamic system with the gases constantly being exchanged with the oceans vegetation and other biological organisms Measurable organic pollutant atmospheric concentrations usually range from femtogram per cubic meter of air fgm3 up to ugm3 30 Atmospheric Gases Gas Residence Time Ar Ne Kr Xe 209 460 165 332 00502 058 033 III ATMOSPHERIC PARTICLES Aerosols in the atmosphere have several important implications 1 Pose a human health hazard at high concentrations and size distributions 2 Scatter and absorb Visible radiation thus altering Visibility can alter earth s climate 3 Provide particles in the atmosphere where important reactions and interactions can occur Aerosols originate om l condensation of atmospheric gases 31 2 wind action on the earth s surface A Fine aerosol are those with mass median diameters D lt 2 pm originate predominantly from the condensation of precursor gases in the atmosphere Subdivided into two size fractions 1 Ultrafine aerosols D 0002 to 002 pm which arise from gas toparticle conversions and some combustion processes Lifetimes are short often minutes due to rapid coagulation and condensation forming 2 Fine aerosols D 002pm to 2 pm Arise from condensation of ultrafine aerosols Aerosols rarely grow to larger than 2 pm because they are too large to grow by condensation and coagulation This action is also called accumulation mode because there particle size is stable in the atmosphere a Because of the nature of their source these small particles generally contain far more organics than coarse particles b Represent a small portion of the total particle number eg 5 but a significant portion eg 50 of the aerosol mass c Too small to settle out rapidly longer atmospheric lifetimes than coarse particles d Removed relatively slowly by incorporation into cloud droplets followed by rainout or by washout scavenged by falling raindrops during precipitation OR by dry deposition to surfaces by eddy diffusion or advection 3 Coarse particles D gt 2 pm produced mostly by mechanical processes such as grinding wind or erosion a Maj or deposition processes are gravitational settling and washout by precipitation 32 b Can be transported long distances by convective processes c Usually predominantly inorganic in composition d Most biological particles pollen spores etc are in the coarse range IV GasParticle Partitioning Of SOCs The distribution of airborne organic compounds between the Gas G and Particle P phases is really only important for SOC since compounds classi ed as volatile VOC or nonvolatile N OC are typically found entirely in the gas or particleassociated phases respectively A SOC bound to atmospheric particles consist of l Nonexchangeable fraction which is strongly adsorbed to active sites or embedded in the particle matrix and not in equilibrium with its gas phase Some polar organics may predominately eXperience irreversible chemical adsorption and never be available for exchange Little work has been done to look at the phase distributions of polar organics in the atmosphere 2 Exchangeable action which is more loosely bound and controlled by the SOC gasphase concentration vapor pressure in the air 33 Sorption Models JungePankow Adsorption Model bl c9fR c6 1 where bl CpCg Cp 2 1 the fraction of particlebound SOC Cp C g concentration of chemical in particle and gas phase ngm3 air FR solute reference fugacity liquid phase vapor pressure 9 the average total particle surface area per volume of air cm2cm3 9 ranges from ca 42 x 10 7 for clean continental background to 35 x 10396 for rural air to 11 x 105 for urban air c term that depends on SOC molecular weight particle surface concentration for monolayer coverage and the difference between the heat of desorption from the particle surface and the heat of vaporization of the sorbate Junge assumed c 013 torrcm 172 Pacm 17 x 104 atmcm depending on pressure units also eq 2 can be expressed as bl CpTSPCg CpTSP 3 Cp concentration in particle phase ngug particles Cg concentration of chemical in gas phase ngm3 air TSP total suspended particulate concentration ugm3 The particlegas partition coefficient may be introduced into the fractional distributional equations as Divide rhs of eq 3 by Cg 34 p KplTSPl1 KpTSP 4 Kp particlegas partition coef cient Kp CpCg 5 Cp concentration in particle phase ngug particles Cg concentration of chemical in gas phase ngm3 air Equation 4 may be used to predict I which can be used for atmospheric transport and deposition modeling The JungePankow model and eq I assume physical adsomtion only since this equation was derived using Langmuir isotherm assumptions Combining eq 1 and 5 log Kp log c9TSP 10g fR 6 A plot of log Kp vs log fR has a theoretical slope of l and an intercept related to the particle speci c surface area Problems associated with the adsorption model include dif culties in measuring 9 and Atsp and uncertainties in the parameter c Slopes often differ from 1 because of sampling artifacts nonequilibrium effects or thermodynamic factors Kp m3ug is a partitioning constant TSP is the total suspended particle concentration ugm3 and F ngm3 and A ngm3 are the operationally de ned particulateassociated and gaseous concentrations of the compound of interest respectively Blowoff loses or adsorption gains to the lter may occur biasing the ratio Despite artifact problems measurements of A F and TSP have been used to estimate the GP distribution using Kp of nonpolar SOC and the factors controlling it 35 OctanolAir Partition Coefficient 1K Model This model assumes partitioning between gas phase and liquidlike lm on particles that can be modeled using Koa Kp 10 9Koafom VIoctMomYoctYomPg 7 where Koa COCA ratio of SOC conc at equil in octanol and air dimensionless fom action of particle organic matter wtwt Moot molecular weight of octanol Mom molecular weight of particle organic matter yoct activity coef cient of SOC in octanol yom activity coef cient of SOC in particle organic matter lm pg density of octanol 820 kg m3 Assuming similar molecular weights for octanol and the particle organic matter then yoctyom H l Kp 12 X 10 12KoafomMoctMom 8 or in loglog form Log Kp 10g Koa 10 fom log MoctMom 1192 9 Plot of log Kp vs log Koa has a theoretical slope of 1 and an intercept related to the absorbing properties of the aerosol The molecular weight ratio is often dropped because it contributes very little in magnitude to the constant providing 36 Log log Koa log fom Koa can be measured directly or it can be estimated as KoaK0w KAW 11 the ratio of the octanolwater partition coef cient to the dimensionless Henry s law constant Koa is considered to be more reliably determined or estimated than is the fR of a compound that is a solid at ambient temperatures 37 Sample Problems 1 If 42 of an organic chemical is associated with atmospheric aerosols calculate the particlegas partition coefficient of the chemical given an air TSP of 291 ugm3 Answer the distribution of chemicals in air may be estimated using the JungePankow model as Ip KpTSP1 KpTSP solving for Kp yields KP p1 pTSP 042058 x 291 ugm3 0026 m3ug 2 Estimate the particle mass fraction of PCB 194 in Fairfax City air with a TSP of 883 ugm3 f0m 41 at 25 OC separately using the JungePankow model and the K0a model Answer PCB 194 properties are as follows P0 161 x 103910 atm 11 158 C 431 K POL eXpln 161 x 103910 atm 67443110298 K 1 326 x109 atm SW 00002 mgL 465 x1010 molL KOW 2 10767 KH 161 x 103910 atm465 x 103910 molL 0346 atmLmol 38 JungePankow Model Log Kp log Ip log c9TSP log fR log17 X 10394 cmZcm3 11 X 10395 atmcm 883 ugm3 log 326 x 10399 atm 1067 849 218 m3ttg Kp 0006558 m3ttg 45p KpTSPKPTSP 1 00065588830006558883 1 037 aw c161 Log Kp log Koa log f0m 1192 Koa KOWKaw 10739670346 Latrnmol00821 LatrnmolK x 298 K 331 x 109 log Kp 952 log 41 1192 179 Kp 001632 m3ttg 45p KpTSPKPTSP 1 001632883001632883 1 059 The two models provide somewhat different results 39


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