Contaminant Hydrogeology III

1. Contaminant Hydrogeology III Гидрогеология Загрязнений и их Транспорт в Окружающей Среде Yoram Eckstein, Ph.D. Fulbright Professor 2013/2014 Tomsk Polytechnic University Tomsk, Russian Federation Fall Semester 2013

2. Basic Environmental Chemistry

3. Chemical concentration Unless two substances are fully miscible there exists a concentration at which no further solute will dissolve in a solution. At this point, the solution is said to be saturated. If additional solute is added to a saturated solution, it will not dissolve (except in certain circumstances, when supersaturation may occur). Instead,phase separation will occur, leading to eithercoexisting phases or a suspension. The point of saturation depends on many variables such as ambient temperature, pressure and the precise chemical nature of the solvent and solute.

4. Chemical concentration Mass versus volume Some units of concentration — particularly the most popular one, molarity — require knowledge of a substance's volume, which unlike mass is variable depending on ambient temperature and pressure. Therefore, volumes are not necessarily completely additive when two liquids are added and mixed. Volume-based measures for concentration are therefore not to be recommended for non-dilute solutions or problems where relatively large differences in temperature are encountered (e.g. for phase diagrams).

5. Chemical concentration Therefore, unless otherwise stated, all the measurements of volume are assumed to be at a standard state temperature and pressure (for example 25 degrees Celsius at 1 atmosphere or 101.325 kPa). The measurement of mass does not require such restrictions.

6. Chemical concentration mg/L (of the solution) µmg/L

7. Chemical concentration mg/kg (of the solvent) µmg/kg

8. Units of chemical concentration Normality (N) Normality is equal to the gram equivalent weight of a solute per liter of solution. A gram equivalent weight or equivalent is a measure of the reactive capacity of a given molecule. Normality is the only concentration unit that is reaction dependent.

9. Units of chemical concentration Equivalent weight The weight of a substance that will combine with or replace one mole of hydrogen or one-half mole of oxygen. The equivalent weight is equal to the atomic weight divided by the valence.

10. Units of chemical concentration • Molality • Molarity • Mass/volume • Mass/mass • Equivalents • Normality

11. Chemical kinetics and chemical equilibrium

12. The Second Law of Thermodynamics In a chemical reaction, only part of the energy is used to do the work. The rest of the energy is lost as entropy.

13. Gibbs Free Energy Gibbs free energy G is the amount of energy available for work for any chemical reaction. G = H – TS where: H is the enthalpy S is the entropy T is the absolute temperature

14. Gibbs Free Energy aA + bB ↔ cC + dD as this system proceeds toward equilibrium, the change in Gibbs free energy per each additional mole reacting is: ΔG = ΔGo + R T lnQ

15. Gibbs Free Energy ΔG = ΔGo + R T lnQ ΔGo is the standard free energy change characteristic for a given reaction R is the gas constant T is the absolute temperature and where [C], [D], [B] and [A] are molar concentrations

16. Gibbs Free Energy once the reaction has reached equilibrium ΔG = ΔGo + R T lnQ = 0 and ΔGo = -R T lnKeq

17. Gibbs Free Energy

18. Gibbs Free Energy Some reactions are spontaneous because they give off energy in the form of heat (ΔH < 0). Others are spontaneous because they lead to an increase in the disorder of the system (ΔS > 0). Calculations of ΔH and ΔS can be used to probe the driving force behind a particular reaction.

19. Gibbs Free Energy Example: Calculate ΔH and ΔS for the following reaction and decide in which direction each of these factors will drive the reaction. N2(g) + 3 H2(g) ↔2 NH3(g)

20. Solution Using a standard-state enthalpy of formation and absolute entropy data table, we find the following information: Compound ΔHo(kJ/mol) So(J/mol-K) N2(g) 0 191.61 H2(g) 0 130.68 NH3(g) -46.11 192.45

21. Solution (cont’d) The reaction is exothermic (ΔHo< 0), which means that the enthalpy of reaction favors the products of the reaction: ΔHo= ΔHo (products) - ΔHo (reactants) = = [2 mol NH3 x 46.11 kJ/mol] - [1 mol N2 x 0 kJ/mol+ + 3 mol H2 x 0 kJ/mol] = -92.22 kJ

22. The entropy of reaction is unfavorable, however, because there is a significant increase in the order of the system, when N2 and H2 combine to form NH3. ΔSo= So(products) - So(reactants) = = [2 mol NH3 x 192.45 J/mol-K] – - [1 mol N2 x 191.61 J/mol-K + + 3 mol H2 x 130.68 J/mol-K] = -198.75 J/K Solution (cont’d)

23. Other concepts • Principle of Electroneutrality The principle expresses the fact that all pure substances, including natural waters carry a net charge of zero.

24. Other concepts • Principle of Electroneutrality • Ci - Ai • Analytical error (%) = · 100 • Ci+ Ai • Aimeq/L Acceptable difference • 0 - 3.0 ± 0.2 % • 3.0 - 10.0 ± 2% • 10. – 800. ± 5%

25. Other concepts In chemical thermodynamics, activity is a measure of the “effective concentration” of a species in a mixture, in the sense that the species' chemical potential depends on the activity of a real solution in the same way that it would depend on concentration for an ideal solution. The difference between activity and other measures of composition arises because molecules in non-ideal gases or solutions interact with each other, either to attract or to repel each other. The activity of an ion is particularly influenced by its surroundings. • Chemical activity

26. Other concepts Activities should be used to define equilibrium constants but, in practice, concentrations are often used instead. The same is often true of equations for reaction rates. However, there are circumstances, e.g. in highly concentrated brines, where the activity and the concentration are significantly different and, as such, it is not valid to approximate with concentrations where activities are required. • Chemical activity

27. Other concepts Deviations from ideality are accommodated by modifying the concentration of an ion Ci by an activity coefficient. • Chemical activity coefficient where fzis the activity coefficient; I is the ionic strength and z is the electric charge of the ion i and ion activity

28. Ionic Strength (I) and Activity ( ) I = 0.5 Σmi zi2 zi = electric charge of the ion i mi = equivalent concentration of the ion i a = ionic radius A ~ f(t) & B ~ f(t)

29. Chemical kinetics • First-order kinetics

30. Error in measurements • Mean • Standard deviation

31. Chemical partition into phases • Solubility and vapor pressure; • The ideal gas law:

32. Chemical partition into phases

33. Chemical partition into phases • Henry’s Law Constants

34. Chemical partition into phases Ranges of Henry’s law constants for some classes of organic pollutants

35. Chemical partition into phases • Polarity, sorption and solubility; • Kow=[n]octanol/[n]water

36. Chemical partition into phases Kow – octanol/water partition coefficient octanol – CH3(CH2)7OH Has both hydrophobic and hydrophilic character ("amphiphilic") Therefore a broad range of compounds will have measurable Kow values

37. Chemical partition into phases The Kow, or Octanol - Water partition coefficient, is simply a measure of the hydrophobicity (water repulsing) of an organic compound. The more hydrophobic a compound, the less soluble it is, therefore the more likely it will adsorb to soil particles.

38. Chemical partition into phases Kow can be determined by adding a known amount of contaminant to a bottle consisting of equal volumes of octanol and water. The coefficient is determined by calculating the concentration in the octanol phase compared to the concentration in the water phase.

39. Chemical partition into phases The Kow of a compound can also be used to find the Koc of a particular contaminant. Koc is the partition coefficient of the contaminant in the organic fraction of the soil. Koc depends on the physico-chemical properties of the contaminant, not the percent of organic matter in the soil. One such relationship between Kow of aromatic compounds and Koc is: Log Koc = 1.00 (Log Kow) - 0.21

40. Chemical partition into phases A separate equation is used for every class of compound to determine the organic partitioning coefficient from the octanol-water partitioning coefficient of the compound.

41. Chemical partition into phases Kow – octanol/water partition coefficient Importance: • Method of quantifying the hydrophobic character of a compound • Can be used to estimate aqueous solubility • Huge database of Kow values available • Can be used to predict partitioning of a compound into other nonpolar organic phases: • other solvents • natural organic material (NOM) • biota (like fish, cells, lipids, etc.)

42. Chemical partition into phases Ranges of Kowconstants for some classes of organic pollutants

43. Chemical partition into phases • Sorption is the common term used for both absorption and adsorption. These terms are often confused. Absorption is the incorporation of a substance in one state into another of a different state (e.g., liquids being absorbed by a solid or gases being absorbed by water). Adsorption is the physical adherence or bonding of ions and molecules onto the surface of another molecule. It is the most common form of sorption used in cleanup. Unless it is clear which process is operative, sorption is the preferred term.

44. Adsorption and absorbtion • Adsorption As3+sorbing to the negative charges on the surface of clay minerals • Absorbtion As3+ replacing idiomorphically Fe3+ in iron-oxides

45. d Adsorbate = material being adsorbed Adsorbent = adsorbing material

46. Types of adsorption • Exchange adsorption (ion exchange)– electrostatic due to charged sites on the surface. Adsorption goes up as ionic charge goes up and as hydrated radius goes down. • Physical adsorption: Van der Waals attraction between adsorbate and adsorbent. The attraction is not fixed to a specific site and the adsorbate is relatively free to move on the surface. This is relatively weak, reversible, adsorption capable of multilayer adsorption.

47. Types of adsorption • Chemical adsorption: Some degree of chemical bonding between adsorbate and adsorbent characterized by strong attractiveness. Adsorbed molecules are not free to move on the surface. There is a high degree of specificity and typically a monolayer is formed. The process is seldom reversible. • Generally some combination of physical and chemical adsorption is responsible for activated carbon adsorption in water and wastewater.

48. ADSORPTION EQUILIBRIA • If the adsorbent and adsorbate are contacted long enough an equilibrium will be established between the amount of adsorbate adsorbed and the amount of adsorbate in solution. The equilibrium relationship is described by isotherms.

49. ADSORPTION EQUILIBRIA • qe = mass of material adsorbed (at equilibrium) per mass of adsorbent. • Ce= equilibrium concentration in solution when amount adsorbed equals qe. • qe/Ce relationships depend on the type of adsorption that occurs, multi-layer, chemical, physical adsorption, etc.

50. Sorption column experimental setup