Contaminant Hydrogeology III Yoram Eckstein Ph D Fulbright
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
Basic Environmental Chemistry
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 either coexisting 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.
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).
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 k. Pa). The measurement of mass does not require such restrictions.
Chemical concentration mg/L (of the solution) µmg/L
Chemical concentration mg/kg (of the solvent) µmg/kg
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.
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.
Units of chemical concentration ØMolality ØMolarity ØMass/volume ØMass/mass ØEquivalents ØNormality
Chemical kinetics and chemical equilibrium
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.
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
Gibbs Free Energy a. A + b. B ↔ c. C + d. D as this system proceeds toward equilibrium, the change in Gibbs free energy per each additional mole reacting is: ΔG = o ΔG + R T ln. Q
Gibbs Free Energy ΔG = ΔGo + R T ln. Q 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
Gibbs Free Energy once the reaction has reached equilibrium ΔG = ΔGo + R T ln. Q = 0 and o ΔG = -R T ln. Keq
Gibbs Free Energy
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.
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. N 2(g) + 3 H 2(g) ↔ 2 NH 3(g)
Solution Using a standard-state enthalpy of formation and absolute entropy data table, we find the following information: Compound ΔHo(k. J/mol) So(J/mol-K) N 2(g) 0 H 2(g) 0 NH 3(g) -46. 11 191. 61 130. 68 192. 45
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 NH 3 x 46. 11 k. J/mol] - [1 mol N 2 x 0 k. J/mol + + 3 mol H 2 x 0 k. J/mol] = -92. 22 k. J
Solution (cont’d) The entropy of reaction is unfavorable, however, because there is a significant increase in the order of the system, when N 2 and H 2 combine to form NH 3. ΔSo = So(products) - So(reactants) = = [2 mol NH 3 x 192. 45 J/mol-K] – - [1 mol N 2 x 191. 61 J/mol-K + + 3 mol H 2 x 130. 68 J/mol-K] = -198. 75 J/K
Other concepts ØPrinciple of Electroneutrality The principle expresses the fact that all pure substances, including natural waters carry a net charge of zero.
Other concepts ØPrinciple of Electroneutrality Analytical error (%) = Ai meq/L 0 - 3. 0 - 10. 0 10. – 800. Ci - Ai Ci + Ai · 100 Acceptable difference ± 0. 2 % ± 2% ± 5%
Other concepts ØChemical activity 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.
Other concepts ØChemical activity 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.
Other concepts ØChemical activity coefficient Deviations from ideality are accommodated by modifying the concentration of an ion Ci by an activity coefficient. where fz is the activity coefficient; I is the ionic strength and z is the electric charge of the ion i and ion activity
Ionic Strength (I) and Activity ( ) I = 0. 5 Σ mi zi 2 A ~ f(t) & B ~ f(t) zi = electric charge of the ion i mi = equivalent concentration of the ion i a = ionic radius
Chemical kinetics ØFirst-order kinetics
Error in measurements ØMean ØStandard deviation
Chemical partition into phases ØSolubility and vapor pressure; ØThe ideal gas law:
Chemical partition into phases
Chemical partition into phases ØHenry’s Law Constants
Chemical partition into phases Ranges of Henry’s law constants for some classes of organic pollutants
Chemical partition into phases ØPolarity, sorption and solubility; ØKow=[n]octanol/[n]water
Chemical partition into phases Kow – octanol/water partition coefficient octanol – CH 3(CH 2)7 OH Has both hydrophobic and hydrophilic character ("amphiphilic") Therefore a broad range of compounds will have measurable Kow values
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.
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.
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
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.
Chemical partition into phases Kow – octanol/water partition coefficient Importance: Ø Method of quantifying the hydrophobic character of a compound Ø Can be used to estimate aqueous solubility Ø Huge database of Kow values available Ø Can be used to predict partitioning of a compound into other nonpolar organic phases: Ø other solvents Ø natural organic material (NOM) Ø biota (like fish, cells, lipids, etc. )
Chemical partition into phases Ranges of Kow constants for some classes of organic pollutants
Chemical partition into phases ØSorption 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.
Adsorption and absorbtion ØAdsorption As 3+ sorbing to the negative charges on the surface of clay minerals ØAbsorbtion As 3+ replacing idiomorphically Fe 3+ in iron-oxides
d Adsorbate = material being adsorbed Adsorbent = adsorbing material
Types of adsorption ØExchange 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. ØPhysical 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.
Types of adsorption ØChemical 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. ØGenerally some combination of physical and chemical adsorption is responsible for activated carbon adsorption in water and wastewater.
ADSORPTION EQUILIBRIA ØIf 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.
ADSORPTION EQUILIBRIA Øqe = mass of material adsorbed (at equilibrium) per mass of adsorbent. ØCe = equilibrium concentration in solution when amount adsorbed equals qe. Øqe/Ce relationships depend on the type of adsorption that occurs, multi-layer, chemical, physical adsorption, etc.
Sorption column experimental setup
ADSORPTION EQUILIBRIA Four possible models for isotherms
ADSORPTION EQUILIBRIA Four common models for isotherms
Langmuir Isotherm This model assumes monomolecular layer coverage and constant binding energy between surface and adsorbate. The model is: Qao is the maximum adsorption capacity (monolayer coverage) (g solute/g adsorbent). Ce has units of mg/L K has units of L/mg
BET isotherm (Brunauer, Emmett and Teller) This is a more general, multi-layer model. It assumes that a Langmuir isotherm applies to each layer and that no transmigration occurs between layers. It also assumes that there is equal energy of adsorption for each layer except for the first layer.
BET isotherm (Brunauer, Emmett and Teller) CS =saturation (solubility limit) concentration of the solute. (mg/liter) KB = a parameter related to the binding intensity for all layers. Note: when Ce << CS and KB >> 1 and K = KB/Cs BET isotherm approaches Langmuir isotherm.
Freundlich isotherm For the special case of heterogeneous surface energies (particularly good for mixed wastes) in which the energy term, “KF”, varies as a function of surface coverage we use the Freundlich model. n and KF are system specific constants.
Determination of appropriate model To determine which model to use to describe the adsorption for a particular adsorbent/adsorbate isotherms experiments are usually run. Data from these isotherm experiments are then analyzed using the following methods that are based on linearization of the models For the Langmuir model linearization gives:
Determination of appropriate model A plot of Ce/qe versus Ce should give a straight line with intercept : and slope: or:
Determination of appropriate model Here a plot of 1/qe versus 1/Ce should give a straight line with intercept 1/Qao and slope:
Determination of appropriate model For the Freundlich isotherm use the log-log version : A log-log plot should yield an intercept of log KF and a slope of 1/n.
Determination of appropriate model For the BET isotherm we can arrange the isotherm equation to get: Intercept = Slope =
Factors which affect adsorption extent (and therefore affect isotherm) Adsorbate: Solubility In general, as solubility of solute increases the extent of adsorption decreases. This is known as the “Lundelius’ Rule”. Solute-solid surface binding competes with solute-solvent attraction as discussed earlier. Factors which affect solubility include molecular size (high MW - low solubility), ionization (solubility is minimum when compounds are uncharged), polarity (as polarity increases get higher solubility because water is a polar solvent).
Factors which affect adsorption extent (and therefore affect isotherm) Adsorbate: p. H often affects the surface charge on the adsorbent as well as the charge on the solute. Generally, for organic material as p. H goes down adsorption goes up. Temperature Adsorption reactions are typically exothermic i. e. , DHrxn is generally negative. Here heat is given off by the reaction therefore as T increases extent of adsorption decreases
Factors which affect adsorption extent (and therefore affect isotherm) Adsorbent: Virtually every solid surface has the capacity to adsorb solutes. From the wastewater/water treatment point of view activated carbon (AC) is the adsorbent of choice. AC can be prepared from many sources: Wood, Lignite, Coal, Nutshells, Bone
Factors which affect adsorption extent (and therefore affect isotherm) Adsorbent: Preparation of Activated Carbon These raw materials are pyrolyzed at high temperature under low oxygen conditions (so we don’t get complete combustion). This forms a “char”. The char is then activated by heating to 300 – 1000 o. C in the presence of steam, oxygen or CO 2. Result: “Activated carbon” which is highly porous, micro-crystalline material which resembles graphite plates with some specific functional groups (e. g. COOH, OH)
Porosity of activated carbon From Macroscopic to Scanning Electron Microscope micropores: <2 nm diameter Surface area of the AC is huge. Most of the surface area is interior in micro- and macropores. Typical surface area is in the range of 300 -1500 m 2/gram.
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