Chemical Properties and Effects on Pollutant Fate

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					                                                                 Properties of Matter


        Chemical Properties and
        Effects on Pollutant Fate




         Physical Characteristics                           Characteristics of Chemical
                                                                    Changes
•   Melting point           • Density                   • Reaction with acids    • Ability to act as
•   Boiling point           • Electrical conductivity   • Reaction with bases      reducing agent
•   Vapor pressure          • Solubility                  (alkalis)              • Reaction with other
•   Color                   • Adsorption to a           • Reaction with oxygen     elements
•   State (solid, liquid,     surface                     (combustion)           • Decomposition into
    gas, plasma)            • Hardness                  • Ability to act as        simpler substances
                                                          oxidizing agent        • Corrosion




                                                                                                         1
                                                      Example: PESTICIDE FATES
         Chemical Fate Processes

                                                            Application             Atmospheric
•   TRANSPORT                                                                       losses

•   TRANSFORMATION/ DEGRADATION       Photo-Decomposition                                Crop Removal
                                                                             RU
•   SORPTION                                                Volatilization        NO
                                                                                       FF
                                                                                          (   wa
•   VOLATILIZATION                                                                               ter
                                                                                                     an
                                                                                                        d   sed
                                                                                                                  im
                                                                                                                    en
•   BIOLOGICAL PROCESSES              Pest Uptake           ADSORPTION
                                                                                                                       t)




                                                              Leaching             Biological
                                      Chemical
                                      Decomposition             (water)            Decomposition




                                         Transformation and
    Chemical Transport Processes
                                        Degradation Processes

    •   RUNOFF                     • Biological Transformations due to
    •   EROSION                      microorganisms
                                     – aerobic
    •   WIND                         – anaerobic
    •   LEACHING                     – facultative
    •   MOVEMENT IN STREAMS OR     • Most Important reaction for many
        GROUNDWATER                  chemicals




                                                                                                                            2
       Transformation and                                     Transformation and
      Degradation Processes                                  Degradation Processes
                                                      • Usually assumed to follow a first-order
• Chemical Transformations                              linear function
                                                                            dC
  – Hydrolysis                                                                 = KC
  – Oxidation-Reduction                                                     dt
                                                      • This leads to an exponential decay
• Photochemical Processes                               function for the concentration
  – Only in the presence of light
  – Only on or near the surface
                                                                         C = C 0 e Kt




       Transformation and
                                                                       EXAMPLE
      Degradation Processes
                                                      • Metolachlor: TH=90 days
 • K is the Degradation Coefficient
                                                      • Initial Agricultural Application Concentration =
 • K is related to the Half-Life of the chemical in
                                                        1.5 kg/ha
   the environment (TH)
                                                      • Estimate the concentration at 120 days.
 • The Half-Life is the time required for the
   concentration of the chemical to become half
                                                                − 0.6931
   of the initial value. Usually expressed in              K=               = −0.0077 / day
   days.                                                           90
                      − 0.6931
                K=                                         C = 1.5e −0.0077(120) = 0.595 kg / ha
                         TH




                                                                                                           3
                                 SOIL-WATER ADSORPTION                               Properties Influencing
                                 PARTITIONING COEFFICIENT
                                                                                          Adsorption
         Concentration in Soil   Cs = Kd Cw
                                                 1
                                                                            •   Organic Matter Content
                                                                            •   Clay Content
    Cs                              Kd
                                                                            •   Soil Water Content
                                              Linear, Instantaneous,        •   Soil Bulk Density
                                              Reversible Adsorption Model
                                                                            •   Soil Temperature
                                   Concentration in Water
                                            Cw




Chemical Properties Influencing                                                  Organic Carbon Partitioning
         Adsorption                                                                      Coefficient

•   Electronic Structure (ionic or non-ionic)                                   K d = K oc (%OC / 100 )
•   Water Solubility
•   Solution Composition
                                                                                %OC = %OM / 1.724
•   Solution Concentration

                                                                                K d = K oc (%OM / 172.4)
•   pH




                                                                                                               4
   Chemical Transport Pathways                            Chemical Transport Pathways
• Kd > 250: The chemical is so strongly
  adsorbed that very little will be transported       • 1 > Kd > 0.1: The loss pathway will
  except with eroded soil particles.                    depend on the amount of infiltration
• 250 > Kd > 50 Transport depends on the                occurring prior to initiation of surface
  sediment concentration of the runoff.                 runoff.
• 50 > Kd > 1: Enough will stay in solution           • Kd < 0.1: The chemical is so weakly
  that most will be transported with surface            adsorbed that the first rainfall will wash it
  runoff water.                                         into the soil profile before initiation of
                                                        runoff.

3/28/2006               Loss pathways            17




                                                          Biological Processes affecting
             VOLATILIZATION
                                                                    Chemicals
  • Of concern for surface-located
    chemicals                                         •   Interception
  • Affected by                                       •   Washoff
       – temperature and soil water content           •   Uptake/Penetration
       – adsorptivity and concentration               •   Translocation
       – vapor pressure and solubility (Henrys        •   Metabolism/Decay
        Constant)




                                                                                                        5
  Reaction Quotient and Chemical                                                 Acid-Base Equilibria:
            Equilibrium                                                         Brönsted-Lowry Model
                                                                       • An acid donates a proton (H+)
          aA + bB ⎯⎯→ ←⎯⎯ cC + dD
                         k1        k2
                                                                       • A base accepts a proton


Q = reaction quotient =
                        {C}c {D}d                      =
                                                           products    • ‘HA’ is used to denote a generic acid
                        {A}a {B}b                          reactants               HA + H2O ↔ A- + H3O+
                                                                                         Where H3O+ = hydronium ion

                   K eq =
                              {C}c {D}d   =
                                              k1
                                                                       • Or
                              {A}a {B}b       k2
                                                                                  HA ↔ H+ + A-
                                                                                      Where A- = conjugate base
        At equilibrium, Q = Keq




         Brönsted-Lowry Model                                                          Strong Acids
• For an acid:
                          K eq = K a =
                                          {H }{A } +   −
                                                                       • Strong acids have relatively high Kas.
                                               {HA}
• For a base:                                                          Example:
         B- + H2O ↔ HB + OH-                                           • HClO4               Ka = 107
     where                    = base
                                 B-
                                                                       • HCl                 Ka = 103
                       HB = conjugate acid
                                                                       • H2SO4               Ka = 103
                 K eq = K b =
                                  {HB}{OH − }
                                        {B }
                                          −                            • HNO3                Ka = 101
   For every acid, there has to be a conjugate base. For every base,
   there has to be a conjugate acid.




                                                                                                                      6
Graphical Solutions (pC – pH Diagrams)
                                                   Graphical Solutions (pC – pH Diagrams)




                                         Introduction and Review of Solubility Equilibria
                                         •    When the ions of a sparingly soluble salt are brought together in solution, it
                                              is observed that if the concentration of these ions is sufficiently large, a
                                              solid is formed that will settle from the solution. For example, consider the
                                              reaction:
                                                   Ca2+(aq) + CO32-(aq) ↔ CaCO3(s)
                                         •    If the concentration of the calcium and carbonate is not excessively high,
                                              the formation of calcium carbonate may be observed to be time
                                              dependent. (It will take a period of time for visible CaCO3(s) to form).
                                         •    Precipitation has therefore been concluded to be a two-step process:
                                              1.   Nucleation – condensation of ions to very small particles.
                                              2.   Particle growth – agglomeration/growth of these ions as a result of diffusion of
                                                   ions from solution.
                                         •    In order for nucleation to occur, solute ions attraction must be strong
                                              enough to displace solvent molecules separating the ions.
                                         •    Also the collision frequency between the ions must be sufficiently high to
                                              promote nucleation.

                                             Supersaturated solutions do exist. Precipitation does not occur
                                              instantaneously.




                                                                                                                                      7
    Solubility Equilibrium: Slightly Soluble Salts
•   In general, for a slightly soluble compound CaAb, the dissolution equation is:
                   CaAb(s) ↔ aC(aq) + bA(aq)
•   And the solubility equilibrium expression for an ideal solution:


                           K sp = [C ] [A]
                                                 a         b

     Where         Ksp = solubility product (equilibrium constant)
                   [C] = cation concentration
                   [A] = anion concentration

•   Remember that the concentration of the solid is not included and is assumed to
    be 1. Ksp ALWAYS refers to the dissolution reaction and determines the
    components of a saturated solution.
•   If [C]a[A]b is less than Ksp, no precipitate forms. If [C]a[A]b is greater than Ksp,
    precipitate forms in proportion to the reverse of the dissolution reaction until
    [C]a[A]b equals Ksp.




                                                                                            Concept and Importance
                                                                                              of Bioaccumulation
                                                                                              It is a process by which persistent envi-
                                                                                           ronmental pollution leads to the uptake and
                                                                                           accumulation of one or more contaminants,
                                                                                           by organisms in an ecosystem.
                                                                                              The amount of a pollutant available for
                                                                                           exposure depends on its persistence and the
                                                                                           potential for its bioaccumulation.




                                                                                                                                          8
  Basic Factors Affecting                        Uptake of Bioaccumulants
     Bioaccumulation                                The uptake of many bioaccumulants by
    Water, soil, air, plants, and any of their   organisms is typically initiated by passive
 combinations can be an ecosystem for            transport, as chemical molecules tend to
 chemical bioaccumulation.                       move from high to low concentration.
    Bioaccumulants tend to be persistent,           This first step is affected by the bioaccu-
 stable, and lipophilic environmental            mulant’s lipophilicity and water solubility.
 pollutants.                                        Some chemicals also have a high affinity
    Chemicals tending to move freely within      for binding with proteins or the ability to
 an organism’s body are less likely to be        dissolve in fats, thus prolonging the storage
 accumulated by organisms.                       of these substances inside an organism.




 Breakdown of Pollutants                                  BAF, BCF, BMF
   The biological breakdown of chemicals is         Bioaccumulation Factor (BAF) is the ratio
called metabolism; this ability varies among     of a test chemical’s concentration in a test
individual species.                              organism’s tissues to that in the surrounding
   Some chemicals are highly fat-soluble but     medium, when all potential uptake mechanisms
                                                 are included.
are easily metabolized; these chemicals do
not accumulate in organisms.                        Bioconcentration Factor (BCF) is a specific
                                                 case of BAF, when the uptake is only from the
   Thus, biological breakdown is one of the      surrounding medium.
factors leading to one of the two specific
                                                    Biomagnification Factor (BMF) is the ratio
consequences of chemical bioaccumulation:        of a test chemical’s concentration in the tissues
bioconcentration or biomagnification.            of an organism, to that in the organism’s prey.




                                                                                                     9
   Numerical Criteria for                          Bioconcentration Factor (I)
 Bioaccumulation Potential                           Many bioconcentration factor (BCF)
                                                  assessments are based on aquatic measure-
   In the USA, chemicals are considered           ments because fish provides a rich lipophilic
bioaccumulative if they have a degradation        microenvironment for bioaccumulation.
half-life > 30 days; or
                                                     BCF is typically measured as the ratio of
   If they have a bioconcentration factor         the concentration of a chemical in a test
greater than 1,000; or                            organism to the chemical’s concentration in
   If their log Kow is greater than 4.2.          the surrounding medium.
   These values are lower (i.e., more health         For many lipophilic chemicals, BCFs can
conservative) than those set forth by Canada      be calculated using the regression equation:
and many other Western countries.                 log BCF = - 2.3 + 0.76 x (log Kow).




Bioconcentration Factor (III)
   For a bioconcentration factor (BCF) to be
estimated from a site-specific study, three (3)
conditions should be met.
   For a BCF to be estimated from a
laboratory study, five (5) conditions should
be met.
   These conditions include: sufficient
duration for observation; a subthreshold test
levels; and the use of test guidelines
acceptable to the regulatory authorities.



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