Ecotoxicology

ENVIRONMENTAL CHEMODYNAMICS

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- An environmental fate or chemodynamic process is the quantitative or qualitative change of a substance with time due to environmental factors. This can be a change of- mass,- concentration, - chemical structure, or- any substance property

- Chemodynamic points out the dynamic nature of processes involved.

Environmental Chemodynamics (EC)

Used for conceptual understanding and quantitative tracking of chemicals in their movements from the places of origin to interfaces of the earth’s geosystems (i.e., air, water, and soil) where elements of the ecosystem are impacted

Environmental Chemodynamics

Environmental Chemodynamics Multi-science and engineering

subject environmental chemists and

engineers, geochemists, geologists, geophysicists, agricultural chemists, chemical engineers, environmental toxicologists and biologists, soil scientists, public health professionals and other environmental scientists, practitioners and specialists

Environmental Chemodynamics One important application area: tracking of substances with

hazardous and toxic properties targeting their contact with sensitive biological species including humans.

Environmental Chemodynamics- Models Models are available in various

levels of complexity, from vignette (short) models to supercomputer versionsto quantitatively(with mathematical algorithms ) track the substances in a realtime sense

make it possible to understand the chemical behavior patterns

Transport and Fate of Toxicants in the Environment

Transport and fate modelEnvironmental factors that may

modify exposure

Exposure-Response Model

Toxicant Source(s) Toxicant Exposure Toxicant Effects

Compartments The behavior and effects of

environmental pollutants are related to their dynamics in the four major compartments of the ecosphere.

– Air (atmosphere) – Water (hydrosphere) – Soil (lithosphere) – Biota (biosphere)

İnteractions btw Compartments

compound is released into a compartment, it has the tendency to move, enter adjacent compartments in a process that happens very quickly

behavior of a chemical released into a single environmental compartment has the potential to be transported or degraded

Environmental Interfaces

An interface is where two different compartments meet and share a common boundary

Factors in compartment and interfacial dynamics.

-Physicochemical properties of the chemical

-Transport properties in the environment

-Chemical transformation

Thermodynamics Thermodynamics and kinetics of processes are

important in a description of the fate and transport of environmental chemicals.

– Dynamics and energy balance drive the system.

– Phase transfer and chemical reaction dynamics.

– Interfacial and inter-compartment transport.

Thermodynamics The study of systems at equilibrium.

– Reversible processes. Used to describe the energy status of molecules in an

environmental system. Parameters for Thermodynamic functions

– Chemical potential, μ.– Fugacity, ƒ.– Activity coefficient, a.– Gibbs free energy, G.– Enthalpy, H.– Entropy, S.

Chemical Potential & Gibbs Free Energy Molecules have internal energies (vibration,

rotation, etc.) and external energy (translation, interaction, etc).

Energy depends on temperature, pressure and chemical composition.

Energy content of a chemical is a population concept- Population of the chemical and all of the other substances present (total free energy=Gibbs Free Energy).

μ, G Chemical potential is the

incremental energy (as additional molecules) added to the total free energy of the system.

Activity Activity: how active a compound is

in a given state (e.g. solution, T, P), compared to a reference state (e.g. pure liquid, T, P).

Activity, ai is an “apparent concentration”.

Enthalpy and Entropy Enthalpy, hi and entropy, si contribute to γi

(activity coeffiecient) since they describe the non-ideal, molecule-to-molecule interactions in a system.

Enthalpy (heat energy)sum of intramolecular and intermolecular forces for a molecule.

Entropy (freedom)contribution to free energy of a molecule by its randomness of configuration, orientation and translation.

Fugacity Tendency of a compound to escape

from one environmental compartment into another one driven by a thermodynamic force

For fugacity to occur, at least two phases (compartments) must be in contact. 

Fugacity Fugacity is linearly proportional to

concentration. Similar to heat transfer (heat diffuses

from an object at a higher temperature to one at lower temperature)

Chemicals move from compartments in which they have high fugacities to those of low fugacity.

Fugacity When the fugacities of a compound

in two adjacent phases are equal, the system is in equilibrium.

C0

C1

Fugacity of Gases, Liquids and Solids Fugacity is expressed in units of

pressure. Liqids and solids have vapor

pressure

Partitioning- process of distribution among phases (must be immiscible and adjacent to each other)-Partitioning determines the distribution of a chemical among the different environmental compartments that are adjacent to the initial compartment in which the chemical was released or was initially found- studied by shaking volumes of both phases containing a determined amount of the chemical of interest, letting the system reach equilibrium and then measuring its concentration in each phase

Atmospheric-Water Partitioning Equilibrium partitioning of organic chemicals

between the gas phase and an aqueous solution.

Henry's law constant, H’ is the air-water distribution ratio of a dilute solute in pure water.

– Fugacity implications: high vapor pressure and high fugacity in water should lead to appreciable partition from water to air.

O. Solvent-Water Partitioning The octanol-water partition

coefficient. Kow = Coctanol / Cwater(partition coefficient) Atrazine-a compound dissolved in water

Octanol- a solvent

Aquous phase (water and dissolved atrazine) contact with organic phase (octanol) diffusionequilibrium

Solid-Water Partitioning Adsorption of solute to solid surfaces. Adsorption- partitioning between a solution

and a solid surface

Kd = Csolid / Cwater partitioning coefficient

adsorbent or sorbent- solid surface on which adsorption occursadsorbate or sorbate- amount of adsorbed material Adsorptive- chemical being adsorbed by the sorbent

Organic Matter-Water Partitioning Organic Matter-Water Partition Coefficient,

Kom. • Organic matter consists of large polymeric

globular chains. – Internal regions are hydrophobic. • The internal region of the macromolecule

becomes “capture” or “solution” regions for neutral or non-polar organic pollutants.

Kom = C organic matter / C water .

Biota-Water Partitioning Bioconcentration factor (BCF) used to describe the

partitioning of chemicals between a source (typically water) and biota.

BCF = C organism / C water

– Because bioconcentration is often solvation of non-polar organic chemicals in adipose tissues, it can be viewed as a fat/water partitioning and proportional to similar partitioning constants such as Kow.

• Removal of the source will redistribute the chemical (depuration).

Chemodynamics Models - Environmental Systems In a compartment model of the

ecosphere, chemodynamics can be used in models to better understand the fate and transport of chemicals in the environment.

EC Modeling Monitoring the presence and

movement of small quantities in the media is a difficult task which is both time consuming and expensive.

EC modeling- ability to make predictions in time and space that extend and enhance the laboratory and field measurements

Modeling Strengths Mathematical models central in all of

science. Simplification of complex systems. Allows prediction of chemical behavior. Can be used to explain field data and

observations. Can be used to generate hypotheses. Can be used to design experiments. Can be modified. Allows development of alternative

explanations.

Modeling Weaknesses Over simplification. Never as good as real observations

and real data. Obsolescence. – Always subject to “a better

model”.

One Box Mass Balance Model Example: air-water exchange of perchloroethylene

(perc) in a pondfed and drained by a creek. Boundary fluxes:– G → the exchange of perc between the water

and the atmosphere (pond to atmosphere is defined as positive [+] flux).

– S → the net removal of perc to the sediment.– R → biodegradation

C2Cl4

One Box Model

Mass Balance

Solution for G dM/dt = I - O - G - R - S Assume steady state, dM/dt = 0. S, R << I, O, G. Calculate G = I - O. Hence, subtracting the output from

the input massperc over a time period will yield the estimated net loss of perc to the atmosphere by the system.

Dynamic Box Models

Partitioning and Models Compartment models require understanding

of chemical partitioning, transformations and transport to describe the equilibrium concentration relationships between different compartments.

An understanding of these relationships allows an understanding and prediction of the dynamics of chemicals in the environment and their eventual fate.

Future aspects There is necessity and growing reliance of

mathematical models to track chemical movements and reactions in nature

multimedia, multiphase and multicomponent risk assessment models continuing to develop in sophistification and are finding widespread applications.

will not go out of fashion- humans continue to rely on chemical “devices” for enhancing their quality of life.