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TOX 715: Environmental Toxicology. Environmental Transport. Transport and Fate of Toxicants in the Environment. Transport and fate model Environmental factors that may modify exposure. Exposure-Response Model. Toxicant Source(s). Toxicant Exposure. Toxicant Effects.
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TOX 715: Environmental Toxicology Environmental Transport
Transport and Fate of Toxicants in the Environment Transport and fate model Environmental factors that may modify exposure Exposure-Response Model Toxicant Source(s) Toxicant Exposure Toxicant Effects
Fugacity • “Tendency of a compound to escape from one environmental compartment into another one”
Fugacity Basics • Fugacity is to mass diffusion what temperature is to heat transfer. • Fugacity is linearly proportional to concentration. • Chemicals move from compartments in which they have high fugacities to those of low fugacity.
Fugacity Basics • When the fugacities of a compound in two adjacent phases are equal, the system is in equilibrium. • Fugacity is expressed in units of pressure.
C1 C0 Fugacity
Other Partition Coefficients: • Kow • Koc, Kb and Kpw
Partitioning Theory and the Environment • Partitioning can be used to model the distribution of an organic compound in the environment. • Examples: • Sorption • Bioconcentration.
Bioaccumulation • Bioaccumulation • general term used to describe a series of processes by which chemicals found in the environment are accumulated and concentrated in living organisms • Bioconcentration • Biomagnification
Calculating BCFs • log BCF = 0.76 log Kow – 0.23 • log BCF = log Kow – 1.32 • log BCF = 0.50 log kow – 3.457
Vapor Pressure • The pressure that the vapor of a substance exerts on its own liquid or solid state at equilibrium • 1 atm = 760 mmHg = 760 torr = 1.013 x 105 Pa.
Measuring Vapor Pressures • PV = nRT • P = nRT/V = 244.4 (n/V) atm • n/V = vapor density in moles/L • R = universal gas constant = 0.082 L atm/°K/mole • T = temperature in the generating column in °K
Henry’s Law • At equilibrium and at a determined temperature a constant relationship exists between the concentration of a chemical in air and water. • Henry’s Law Constant
General Characteristics • Physico-chemical properties of the sorbent and the adsorptive • Area of the sorbent • The lower the aqueous solubility of the adsorptive (solute) the higher the binding potential • Heat has the potential to reduce adsorption
Advection • Refers to the passive movement of a chemical as part of its presence in a medium that is in movement itself. • It can happen in the same compartment or between different compartments.
Homogeneous Advection • Example, consider water in a stream flowing at 1000 m3/h and carrying a chemical at 0.5 μg/m3. The chemical is being advected in water at a rate of 500 μg/h.
Heterogeneous Advection • Refers to the case where there is a secondary phase present inside the main advective medium. • Examples: particulate matter present in advecting river water, particles carried by wind.
Diffusion • Random movement of chemical molecules due to the presence of a state of disequilibrium. • It will transport chemicals from one place to the other one within the same compartment as well as between compartments until equilibrium is reached.
Intraphase Diffusion • Two types of diffusional intraphase transport: molecular and turbulent diffusion. • Molecular diffusion: movement of particles because of a concentration gradient. • Turbulent diffusion: happens because of the turbulent mixing of the bulk medium.
Intraphase Diffusion • Fick’s law:
Interphase Diffusion • Diffusion between two phases can be described using the following formula:
Transport in Solution • Advection • Molecular diffusion • Turbulent diffusion • Dispersion
Transport in Solution • Advection • Molecular diffusion • Turbulent diffusion • Dispersion
Transport through Soil • Vadose zone • Saturated zone • Aquaclude (basal rock)
Transport in the Vadose Zone • Chemicals are able to migrate through the vadose zone by three main mechanisms: • dissolved in solution • as gases (vapor) • adsorbed to particles
Atmospheric Transport • Volatilization • Advection • Deposition