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Pollutants and environmental compartments

Pollutants and environmental compartments. 1(ii) Physico -chemical properties of pollutants and their influence on their behaviour in the environment. Aims. To provide overview of molecular properties of pollutants in the environment:

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Pollutants and environmental compartments

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  1. Pollutants and environmental compartments 1(ii) Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  2. Aims • To provide overview of molecular properties of pollutants in the environment: • Vapour pressure – theoretical background, molecular interactions governing vapour pressure, availability of experimental vapour pressure data and estimation methods • Activity coefficient and solubility in water – thermodynamic consideration, effect of temperature and solution composition on aqueous solubility and activity coefficients, availability of experimental data and estimation methods Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  3. Outcomes • Students will be able to: • estimate relevant physico-chemical properties of pollutants from their structure • predict reactivity of pollutants and possible environmental behavior of pollutants Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  4. Vapour pressure • Definition: • Pressure of a substance in equilibrium with its pure condensed (liquid or solid) phase – pº • Why is it important? • Air/water partitioning • Air/solid partitioning • When is it important? • Spills • Pesticide application Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  5. Ranges of pº (atm) • PCBs – 10-5 to 10-9 • n-alkanes – 100.2 to 10-16 • n-C10H22 ~ 10-2.5 • n-C20H42 ~ 10-9 • benzene ~ 10-0.9 • toluene ~10-1.42 • ethylbenzene ~ 10-1.90 • propylbenzene ~ 10-2.35 • carbon tetrachloride ~ 10-0.85 • methane 102.44 • Even though VP is “low”, gas phase may still be important. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  6. Phase diagram and aggregate state Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  7. Thermodynamic considerations(deriving the van’t Hoff equation) • In equilibrium the change in chemical potential in the two systems is equal : where S = molar entropy and V = molar volume Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  8. Liquid-vapor equlibrium • For a liquid vaporizing, the volume change can be assumed to be equal to the volume of gas produced, since the volume of the solid or liquid is negligible: where H12 = Hvap (gas) or Hsub (solid) = energy required to convert one mole of liquid (or solid) to gas without an increase in T The van’t Hoff equation Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  9. Integration assuming DHvap is constant over a given temperature range leads to: • If the temperature range is enlarged DHvap is not constant: Antoine equation Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  10. Solid-vapor equilibrium • For sublimation: DHsub = DHmelt (~25%) + DHvap (~75%) • Still use liquid phase as reference: • Hypothetical subcooled liquid = liquid cooled below melting point without crystallizing Importantfor solubility Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  11. Molecular interactionsaffecting vapor pressure • Molecule:molecule interactions in condensed phase (l or s) have greatest affect on VP: • strong interactions lead to large DHvap, low VP • weak interactions lead to small DHvap, high VP • Intermolecular interactions can be classified into three types: • van der Waals forces (nonpolar) • Polar forces • Hydrogen bonding Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  12. Vapor Pressure Estimation Technique based on regression of lots of VP data, best fit gives: H-bonding ability size polarizability pressure in Pa, where: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  13. H-bonding ability Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  14. Refractive index • Refractive index (response to light) is a function of polarizability Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  15. Trouton’s rule • At their boiling points, most organic compounds have a similar entropy of vaporization: • exception: strongly polar or H-bonding compounds • Kistiakowsky’s expression gives slightly more accurate predictions: • KF = 1 for apolar and many monopolar compounds • For weakly bipolar compounds (e.g., esters, ketones, nitriles), KF = 1.04 • Primary amines KF = 1.10, phenols KF = 1.15, aliphatic alcohols KF = 1.30 • At Tb: • So, if we know Tb, we can estimate Hvap (at the boiling point) fairly accurately. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  16. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  17. Estimating vapor pressure at other T • Important: DHvap is not constant. • Especially if Tb is high (> 100ºC), the estimate of DHvapfrom Trouton/Kistiakowsky may not be valid. • Empirically, DHvapis a function of the vapor pressure: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  18. From a data set of many compounds, Goss and Schwarzenbach (1999) get: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  19. Less empirically, assume DHvap is linearly proportional to T (i.e. assume that the heat capacity, vapCp is constant): • Substitution into the Clausius-Clapeyron equation and integration from Tb to T gives: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  20. Substitution in previous equation gives: • Generally: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  21. Inserting Kistiakowsky’s expression, the following equation is obtained: • KF is the Fishtine factor, usually 1, but sometimes as high as 1.3 • OK for liquids with Tb < 100ºC • High MW compounds, need correction for intermolecular forces (bar) Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  22. Aqueous Solubility Air KH = PoL/Csatw A gas is a gas is a gas T, P Kow = Csato/Csatw • Equilibrium partitioning of a compound between its pure phase and water • Will lead us to Kow and Kaw Koa = Csato/PoL Koa KH Octanol PoL Water NOM, biological lipids, other solvents T, chemical composition Fresh, salt, ground, pore T, salinity, cosolvents Kow Pure Phase (l) or (s) Csato Csatw Ideal behavior Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  23. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  24. Relationship between solubility and activity coefficient • Organic liquid dissolving in water: • At equilibrium: for the organic liquid phase for the organic chemical in the aqueous phase At saturation! Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  25. If we assume: xiL = 1 and giL = 1 • The relationship between solubility and activity coefficient is: • The activity coefficient is the inverse of the mole fraction solubility for liquids or Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  26. Solids • additional energy is needed to melt the solid before it can be solubilized: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  27. Gases: • solubility commonly reported at 1 bar or 1 atm (1 atm = 1.013 bar) • O2 is an exception • the solubility of the hypothetical superheated liquid (which you might get from an estimation technique) may be calculated as: theoretical “partial” pressure of the gas at that T (i.e. > 1 atm) Actual partial pressure of the gas in the system Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  28. Concentration dependence of g • g at saturation  g at infinite dilution • However, for compounds with g > 100 assume: • g at saturation = g at infinite dilution, i.e. solute molecules do not interact, even at saturation Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  29. Molecular picture of the dissolution process • The two most important driving forces in determining the extent of dissolution of a substance in any liquid solvent are: • an increase in entropy of the system • compatibility of intermolecular forces. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  30. Ideal liquids: • For ideal liquids in dilute solution in water, the intermolecular attractive forces are identical, and Hmix = 0. The molar free energy of solution is: Gs ,Gmix = Gibbs molar free energy of solution, mixing (kJ/mol) TSmix = Temperature  Entropy of mixing (kJ/mol) R = gas law constant (8.414 J/mol-K) T = temperature (K) Xf, Xi = solute mole fraction concentration final, initial • for dilute solutions mole fraction of solvent  1 Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  31. Nonideal liquids: • The intermolecular attractive forces are not normally equal in magnitude between organics and water: Ge = Excess Gibbs free energy (kJ/mol) He, Se = Excess enthalpy and excess entropy (kJ/mol) He = intermolecular attractive forces; cavity formation (solvation) Se = cavity formation (size); solvent restructuring; mixing Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  32. For small molecules, enthalpy term is small (± 10 kJ/mol) • Only for large molecules is enthalpy significant (positive) • Entropy term is generally unfavorable • Water forms a “flickering crystal” around the compound, which fixes both the orientation of the water and of the organic molecule Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  33. Solubility estimation techniques • Activity coefficients and water solubilities can be estimated a priori using molecular size, through molar volume (V, cm3/mol). Molar volumes can be approximated: Ni = number of atoms of type i in j-th molecule ai = atomic volume of i-th atom in jth molecule (cm3/mol) nj = number of bonds in j-th molecule (all types) • Solubility can approximated using a LFER of the type: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  34. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  35. This type of LFER is only applicable within a group of similar compounds: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  36. Another estimation technique – universal – valid for all compounds/classes/types: molar volume describes vdW forces refractive index describes polarity Vapour pressure additional polarizability term cavity term H-bonding Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  37. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  38. Factors Influencing Solubility in Water • Temperature • Salinity • pH • Dissolved organic matter (DOM) • Co-solvents Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  39. Temperature effects on solubility • Generally: • as T , solubility  for solids. • as T , solubility can  or  for liquids and gases. • BUT For some organic compounds, the sign of Hs changes; therefore, opposite temperature effects exist for the same compound! • The influence of temperature on water solubility can be quantitatively described by the van't Hoff equation as: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  40. Solids: • Liquids: • Gases: Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  41. The effect of salinity • As salinity increases, the solubility of neutral organic compounds decreases (activity coefficient increases) • Ks = Setschenow salt constant (depends on the compound and the salt) typical seawater [salt] = 0.5M Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  42. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  43. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  44. The effect of pH • pH effect depends on the structure of the solute. • If the solute is subject to acid/base reactions then pH is vital in determining water solubility. • The ionized form has much higher solubility than the neutral form. • The apparent solubility is higher because it comprises both the ionized and neutral forms. • The intrinsic solubility of the neutral form is not affected. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  45. The effect of DOM • DOM increases the apparent water solubility for hydrophobic compounds. • DOM serves as a site where organic compounds can partition, thereby enhancing water solubility. • Solubility in water in the presence of DOM is given by the relation: • [DOM] = concentration of DOM in water, kg/L • KDOM = DOM/water partition coefficient • Again, the intrinsic solubility of the compound is not affected. Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  46. The effect of cosolvents • the presence of a co-solvent can increase the solubility of hydrophobic organic chemicals • co-solvents can completely change the solvation properties of “water” • examples: • industrial wastewaters • “gasohol” • engineered systems for soil or groundwater remediation • HPLC Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  47. Solubility increases exponentially as cosolvent fraction increases. • Need 5-10 volume % of cosolvent to see an effect. • Extent of solubility enhancement depends on type of cosolvent and solute: • effect is greatest for large, nonpolar solutes • more “organic” cosolvents have greater effect propanol>ethanol>methanol Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

  48. Bigger, more non-polar compounds are more affected by co-solvents • Different co-solvents behave differently, behavior is not always linear • We can develop linear relationships to describe the affect of co-solvents on solubility. These relationships depend on the type and size of the solute Environmental Processes / 1(ii) / Physico-chemical properties of pollutants and their influence on their behaviour in the environment

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