Fate of Contaminants in Natural & Engineered Environments

Introduction to Environmental Engineering

  • Focus on the fate of contaminants in natural and engineered environments.

Learning Objectives

  • Phase Changes: Understanding evaporation, condensation, vapor pressure, and boiling.
    • Evaporation and Condensation: Review the processes of phase change.
    • Vapor Pressure: Study the vapor pressure of different substances:
    • Organic substances
    • Impure liquids
  • Dissolution:
    • Dissolution of gases in water: Understand Henry’s Law.
    • Dissolution of nonaqueous-phase liquids (NAPLs) in water.
    • Processes of dissolution and precipitation of solids.

Kinetics of Phase Changes

  • Batch Reactor Example: Consider a sealed jar with nitrogen gas (N₂) and pure liquid water.
    • Evaporation: Molecules transition from liquid to gas.
    • Condensation: Molecules transition from gas to liquid.

Forward Reaction (Evaporation)

  • Factors affecting evaporation rate:
    1. Kinetic energy of water molecules (temperature-dependent).
    2. Surface area of the water.
    • Rate expressed as:
      Rate{evap} = k{evap}(T) imes S
    • Where S = surface area.

Reverse Reaction (Condensation)

  • Factors affecting condensation rate:
    1. Kinetic energy of gas molecules (temperature-dependent).
    2. Concentration of water vapor in the gas phase.
    3. Surface area of the liquid water.
    • Rate expressed as:
      Rate{cond} = k{cond}(T) imes C_g imes S
    • Where C_g = concentration of water vapor.

Differential Equations for Concentration Change

  • Derive an expression to analyze the change in gas-phase water concentration with time:
    • rac{dCg}{dt} = Rate{evap} - Rate_{cond}
  • Initial condition: No water vapor in gas phase at t=0.

Equilibrium Conditions

  • Demonstrates how the system transitions from disequilibrium to equilibrium (as t o ext{∞}).
  • The equilibrium concentration of the gas phase is characterized by the vapor pressure.
  • Ideal Gas Law: Relate vapor pressure to the concentration of water vapor.

Fun Facts about Vapor Pressure

  • Vapor pressure (or saturation vapor pressure) is:
    • A function of temperature, increases with temperature.
    • Determines phase change conditions (evaporation, condensation, boiling) based on atmospheric pressure and partial pressure comparisons.

Vapor Pressure Conditions

  • Super-saturated: P{partial} > P{vapor}: condensation occurs.
  • Sub-saturated: P{partial} < P{vapor}: evaporation occurs.
  • Saturated: P{partial} = P{vapor}: no net transfer occurs.

Relative Humidity

  • Defined as the ratio of actual partial pressure of water to saturation vapor pressure:
    RH = rac{P{actual}}{P{saturation}}
  • Examples demonstrate significant differences in water vapor partial pressure based on temperature across different regions.

Henry’s Law

  • Describes gas dissolution in liquids: C{aq} = kH imes P_{gas} where:
    • C_{aq} = concentration of solute in the aqueous phase.
    • P_{gas} = partial pressure of the gas above the solution.
  • Stability and temperature dependence of solubility constants.

Dissolution of NAPLs

  • Non-aqueous phase liquids are partially miscible with water, influencing environmental contamination.
  • Factors affecting dissolution based on species and temperature, but not NAPL volume.
  • Example: Analysis of toluene in water, emphasizing when NAPLs form.

Solubility Product and Precipitation

  • Solubility product constant (K_{sp}) for ionic compounds:
    • Related to the molar concentrations of ions in equilibrium.
  • Example of a precipitation process using calcium fluoride (CaF₂).

Conclusion

  • Understanding both kinetic and thermodynamic processes is crucial in environmental engineering, specifically for managing water quality and contaminant behavior in various environments.