SILICATES

Discrete anions

Olivine

MgSiO₄

SILICATES

Chains (SiO₃)n

Pyroxenes

MgSiO₃

Each SiO₄ shares 2 oxygens

2 non-bridging oxygens

SILICATES

Double Chains Si₄O₁₁^6-

Tremolite

Ca₂Mg₅Si₈O₂₂(OH)₂

single chains are cross linked

SiO₄ units shares 2 or 3 oxygens alternately

Alternate SiO₄ units share an oxygen with the neighbouring chain

SILICATES

Sheets Si₄O₁₀^4-

Micas

Mg₃Si₄O₁₀(OH)₂

Each SiO₄ unit shares 3 oxygen

1 non-bridging oxygen

SILICATES

3D Polymers

Framework Silicates

Each SiO₄ unit shares 4 oxygens

WEATHERING

Physical

• wind, water, ice
• temperature change
• pressure of plant roots
• increases surface area which increases rate of chemical weathering

WEATHERING

Chemical

• water and oxygen attack primary minerals resulting in the removal of ions
• chemical composition becomes more stable and secondary minerals can be formed
• solution pH is important
• bacteria can catalyse chemical weathering

Essential elements

• Required for...
• Deficiency...
• Toxic at...

• Required for biological functions
• Deficiency in these elements can have adverse effects
• Become toxic at elevated levels
• Fe Cu Zn Co

Non-Essential Elements

• Biological Function
• Examples

• No known biological function
• As, Cd, Hg, Pb, Sb

Exposure Pathways

Aquatic Organisms

4 things

1. diet
2. sediment ingestion
3. dermal
4. gills

Exposure Pathways

Humans

5 things

1. diet
2. drinking water
3. soil/dust ingestion (children)
4. dermal
5. occupational

Exposure duration

Acute

1. short term exposure
2. less than 24 hours
3. generally high conc. of contaminant needed to cause effect

Exposure duration

Chronic

1. Long term duration
2. Months to years
3. Repeated exposure to low conc. of a contaminant can cause effect

Mechanisms of toxic effects of PTEs

5 things

• mimicry
• metal mediated oxidative damage
• induce gene expression
• form DNA and protein adducts
• impair function(s) of biomolecules

Mechanisms of toxic effects of PTEs

Mimicry

• act as mimics of essential elements and bind to physiological sites
• Cd Cu Ni can replace Zn

Mechanisms of toxic effects of PTEs

Metal mediated oxidative damage

• act as catalytic centres for redox reactions with oxygen or other
• displacement of redox essential elements from cellular ligands causing oxidative stress

the distribution of an element between different chemical and physical forms.

many species and several phases may be involved

Speciation

Physical

• dissolved
• colloidal (~1nm - 2µm)
• particulate (< 0.45µm)

Speciation

Chemical

• free ion
• complexed ion
• adsorbed ion
• oxidation state e.g. As(III)/As(V)

Free ion activity model

4 steps

1. Diffusion of M^z+ to the biological surface
2. Diffusion of M^z+ through the outer protective layer
3. Sorption/surface complexation to binding sites in the protective layer or at sites on the outer surface of the membrane
4. transport of the metal across the membrane

Factors which influence trace element speciation

7 things

pH (master variable)

Trace element concentration(s) ~10-10 to 10-6 M

Types & concentrations of inorganic ligands

-CO32-, SO42-, S2-, Cl-, F-

Types and concentrations of organic ligands

- simple organic molecules e.g. acetic acids, amino acids

- macromolecular compounds e.g. humic & fulvic acids

Types and concentrations of solid phases present

- humic substances, oxide/hydroxide, clay, biological material surfaces

Ionic strength

Redox potential

• removes metals from solution
• metals are less biologically available

• Releases metals into the water column
• Increases availability for uptake by organisms
• Desorbed metals may be transported by water flow
• Desorption of metals can be caused by increased salinity, decreased redox potential and decreases in pH