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Chemical concentration, activity, fugacity, and toxicity. Don Mackay and Jon Arnot Trent University. McKim Conference Duluth, MN September, 2007. Outline. A refresher of some physical chemistry Four chemicals, log K OW = 2, 4, 6, 8 Two organisms, fish and mammal

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chemical concentration activity fugacity and toxicity

Chemical concentration, activity, fugacity, and toxicity

Don Mackay and Jon Arnot

Trent University

McKim Conference

Duluth, MN

September, 2007

outline
Outline
  • A refresher of some physical chemistry
  • Four chemicals, log KOW = 2, 4, 6, 8
  • Two organisms, fish and mammal
  • Bioaccumulation “experiments”
  • Toxicity “experiments”
  • Concluding thoughts
  • Part II by Jon Arnot (Thursday am)
some physical chemistry of organic liquid solutes in dilute solutions
Some physical chemistry of organic liquid solutes in dilute solutions

Raoult P = x PSL (ideal)

Raoult P = x  PSL (non-ideal)

Dalton P = y PT

so P = x  PSL =y PT

rigorous fugacity version
Rigorous fugacity version

f = x  fR = y  PT

reduces to

P = x  PSL =y PT

when f = P, fR = PSL ,  = 1

f and P are “escaping pressures” Pa

two immiscible liquids and a gas phase in equilibrium liquid solute

Air

1

2

Two immiscible liquids and a gas phase in equilibrium, liquid solute.

P f = x11PSL =x22PSL = yPT

x11=x22= yPT / PSL= P / PSL = activity

Saturation or solubility occurs when activity=1.0

Solubility is a measure of the activity coeff.

equilibrium criteria
Equilibrium criteria

P or f or activity

Partition coefficients KA1, KA2, K12

can be expressed using x1, x2,y or C1, C2, CA g/m3

e.g., KAW, KOW, KOA

Depend on 1, 2 and PSL

Can give ratios of 1/ 2

Relate C to f using C = Zf, Z is a “fugacity capacity”

Useful if solvent is complex, e.g., fish Z = 1/(PSL)

CAIR=ZAIR f C1= Z1 f C2 = Z2 f K12=Z1/Z2

what if the solute is solid or gaseous

Critical

Point

Atmospheric

Pressure

Gas

Pressure

Liquid

Vapor

Pressure

Solid

Vapor

Supercooled Liquid

Vapor Pressure

PSL

Liquid

Fugacity

Ratio

F = PSS/PSL

Vapor

Triple Point

» Melting Point

Solid

Vapor

Pressure

PSS

Temperature

Boiling

Point

What if the solute is solid or gaseous?
how do we treat solid or gaseous solutes
How do we treat solid or gaseous solutes?

The solute is in the liquid state and “thinks” it is a liquid so it behaves according to PSL

We must “correct” using F for solids

For gases we lump PSL as H, Henry’s Law constant

if molar mass is large and especially if the molecule is symmetrical
If molar mass is large (and especially if the molecule is symmetrical)

Melting point is high i.e., >> 25oC

F is low, e.g., 0.1 to 0.001

PSS << PSL SSS << SSL

F = PSS / PSL = SSS / SSL

Solubility xS can be used to estimate 

Liquids xS = 1 Solids xS = F

Be wary of reported PS and SS of solids!

QSARs and retention time methods give PSL andSSL

four chemicals a b c and d
Four chemicals A, B, C and D

log KOW = 2, 4, 6, 8

Define MW, melting point, octanol , vapor pressure

Deduce water, solubility, KAW, KOAfor both liquid and solid states

two organisms
Two organisms

100 g fish, 10% lipid

100 g mammal, 10% lipid

Respiration and feeding rates

lipid = octanol

No metabolic biotransformation

slide14

Metabolic biotransformation; kM

Respiratory exchange; k1and k2

20% NLOM

70% water

Growth dilution; kG

10% lipid

Fecal egestion; kE

Dietary intake; kD

Two organisms – same properties and processes

NLOM equivalent to 3.5% lipid

bioconcentration experiment
Bioconcentration “experiment”

CWater = 1 g.m-3 of each chemical

air is in equilibrium, so

CAir = 0.009 to 0.003g.m-3 depending on KAW

slide19

Fish BCF  0.1 KOW L.KOW

Hamelink, Neely, Veith, Mackay

why is the empirical slope 1
Why is the empirical slope < 1?
  • Insufficient time for equilibration ?
  • Bioavailability ?
  • KOW overestimated by QSARs ?
  • Water and NLOM partitioning at low KOW ?
  • Lipid  Octanol ?
  • Biotransformation and growth ?
  • Other ?
toxicity experiment
Toxicity “experiment”

Assume toxic effect at 3 mmol.kg-1 or 3 mol.m-3

i.e. narcosis

What are the water and air concentrations required to cause toxicity?

Essentially a “backward” bioconcentration experiment

concentration vs activity vs fugacity

Concentration vs activity vs fugacity

Concentrations and fugacities vary by about six orders of magnitude

Activities vary relatively little (factor of 3)

The system is striving for equi-fugacity, externally and internally.

some conclusions and points for discussion
Some conclusions and points for discussion
  • Bioconcentration in fish from water and mammals from air is well understood using KOW and KOA
  • Narcosis is well characterized by 3 mol.m-3 factor of 2
  • Uptake times can be long and can exceed short term test duration
  • Solubility of solids can limit partitioning and toxicity
  • Don’t use exposure concentrations > solubility: meaningless results!
  • Biotransformation, dynamics and growth are important.
  • Challenges of “selective” toxics: we have to get the “simple” narcosis model right first. Then we use this as a basis for developing more rational and effective methods of modeling toxicity of more challenging substances and eliminating unnecessary (but not all) animal testing.
qsars and 5 d s
QSARs and 5 D’s

Toxicity depends on:

  • Delivery to organism, i.e., Corg/Cenv
  • Distribution in organism (toxicokinetics)
  • Disruption (biochemical)
  • Dynamics and efficiency of uptake
  • Degradation of chemical in organism (biotransformation)

as well as KOW

Is a general QSAR for toxicity attempting too much? Categories?

Perhaps a set of QSARs yielding parameters that can be combined using a model?