Lecture 1
Chemistry:
- natural= volcanoes, oods, animal waste, algae, etc
- Anthropogenic= chemical, sewage, plastic, radioactive, waste, oil, etc
What is the most common element in the earth's crust ?
O2 -> H2O
-> CO2
-> NO2
-> Fe2O3/Fe3O4
-> O3
Chemical speciation
Behaviour and risk of chemicals -> depend of their speciation
De nitions
Speciation of an element: distribution of an element amongst de ned chemical species in a system
Chemical species: a speci c form of a chemical element, de ned by its complex structure, or oxidation
state. For example, a bottle full of water contains molecules of exactly the same chemical species.
Chemical speciation helps understand the mobility, bioavailability and toxicity of substances
Example of chemical speciation:
Chromium (Cr)
Chromium -> occurs in several oxidation states Most common ones:
1. Cr(III) occurs as the cation Cr3+ => green
2. Cr(VI) occurs as the anion chromate (CrO42-) => yellow
Due to di erent nature of the charge these chromium species behave di erent in the environment and pose
di erent risks.
Mercury (Hg)
The di erence in toxicity, mobility, and treatability of mercury is directly predicated on its molecular form.
Many regulations associated with food, water, and drug safety focus on methyl mercury
Mercury is a highly toxic heavy metal. Mercury exists in three oxidation states:
- Hg(0) (metallic), -> atmosphere
- Hg(I) (mercurous)
- Hg(II) (mercuric)
Organo-mercury compounds bioaccumulate in aquatic environments. Consumption of sh can cause
mercury toxicity => example Minamata disease in Japan = a poisoning disease that nervous system, mainly
central nervous system, is damaged by methylmercury. It is caused by the daily consumption of large
quantities of sh and shell sh that were heavily contaminated with the toxic chemical generated in chemical
factories and then discharged into the sea.
However, metallic and inorganic mercury species are less dangerous ..
For example: mercury (Hg0) for tooth llings; jewelry containing mineral cinnabar (HgS)
.. because they are poorly soluble and absorbed by a much smaller extent.
Chemical speciation
Chemical speciation of substances in the
environment:
• complex
• result of many simultaneous physical,
chemical and biological processes
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Uptake = how (metal) ions enter the cellular material
Adsorption = adhesion of (metal) ions on to a surface
Dissociation = compound breaks into its component ions
Complexation = A (metal) atom or ion bonded to one or more ligands (ions or molecules that contain one
or more pairs of electrons that can be shared with, e.g. the metal)
Precipitation = when cations and anions in aqueous solution combine to form an insoluble ionic
Changes in speciation can be described through two distinct approaches
1. Thermodynamics (very important for the exam)
For reactions that are at equilibrium
ΔG = 0 (Gibbs free energy)
Speciation can be described using mass law and mass balance equations.
At equilibrium: rate forward = rate reverse
Keq => equilibrium constant = ratio of product over reactant
NaCl (s) <-> Na+ + Cl-
Keq = [Na=][Cl-]/ [NaCl]
2. Kinetic
For reactions that are not at equilibrium ΔG 0 => Speciation changes can be described with rate law
equations.
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Kinetic description
necessary if an equilibrium is not attained (slow
reactions) or if the system it is continuously
perturbed (i.e. biological activity)
Extra info rate law equations
Rate: depends on the concentration
Rate constant (k): is independent of
concentration. One value at a given temperature
Keep in mind for equilibrium chemical speciation
In environmental systems, a single element participates in many reactions that form a complex reaction
network
The “free concentration” plays a central role in understanding speciation in environmental systems ->
most equilibrium reactions involve the free concentration
Free concentration - scenario 1
1. The free concentration follows from the solubility of a mineral
2. For a given pH and solubility product (Ksp), the free concentration can be calculated from the
corresponding mass law equation.
3. Concentration of other chemical species (e.g. Al(OH)2+ (aq)) can then be calculated using again a mass
law equation
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