Trace elements - bioavailability Flashcards
(23 cards)
Two criteria for something to be bioavailable:
-
- Physically available
- Chemically available
Two fractions of bioavailability
- Direct bioavailable fraction
- Potential bioavailable fraction
“dynamic process”: the levels of reactive or available metals in an environment are constantly being restored or replenished
Direct bioavailable fraction
Immediately available
- Most labile
Where?
- Free ions
- Soluble complex ions
Potential bioavailable fraction
“Bioaccessible”
- Fairly labile
Where?
- Weakly absorbed to exchange surfaces in soil
- Soluble minerals
Labile
compounds that are reactive or unstable, prone to undergo reactions or transformations.
What does this mean:
“dynamic process”: replenishment of labile metal pools
The levels of reactive or available metals in an environment are constantly being restored or replenished through natural processes, keeping them in a state where they are readily available for use by organisms or for other chemical reactions.
Total content vs. bioavailable fraction
Not bioavailable
- Irreversible bound
Potentially bioavailable
- Reversibly bound
- Dissolved
Directly bioavailable
- Dissolved
Why is total trace element concentration a poor predictor of bioavailability?
Bioavailability in soil is related to speciation, particular the free form.
Which part of the soil pollution classification system take all organisms into account?
The “Very good” fraction
The rest only worries about affects on humans
Association
molecules + molecules → more complex structure or compound
Molecules or ions combine to form a more complex structure.
Dissociation
Molecule → Smaller components
Molecules or ions separate into simpler components.
Non available content
How:
- Silicate crystal lattice
Sequestrated/irreversible bound
Where: Soil matrix
Environmental availability
- Potentially available amount in soil matrix
- Actually available amount in pore water
Potentially available amount in soil matrix
How:
- Adsorbed to soil matrix
- Very strong → weakly bound fraction
Where:
- Mineral surface
Actually available amount in pore water
How:
- Dissolved as
- Free ions
- Complexes with inorganic ligans
- DOC-complexes
Where: Pore water
Environmental bioavailability
Pass through root surface
Where: Root surface
Toxicological bioavailability
Biological amount in plant tissue
- Accumulation
- Toxic effect
Where: Plant
How to assess bioavailable fraction in soil
- Laboratory leaching test
- Coloum test
- Batch test (not good)
- Extraction based method
- Geochemical process-based method
Extraction based method
Measurement of actual available concentration in porewater
- Weak extractants/echangable by unbuffered salts/weak acids
- Seperation by diffusion
- DGT
- Direct measurement of metal activities
- Desorption by chelating angents
- Exchange by strong acids
DGT
Diffuse gradient in thin films
A method used to measure the availability of nutrients or metals in soil, by capturing and analyzing the substances that diffuse into a thin film.
It’s particularly useful for studying the bioavailability of trace elements in soils and sediments.
Which extractant is often used to test plant availability?
NH₄Ac = Ammonium Acetate
