Chemical Processes Flashcards
(23 cards)
Significance of estuaries
Most river borne material pass through
Transition zones for continentally derived material.
- can change material flux entering oceans
- chemical distributions may influence biological processes
Anthropogenic inputs, either directly or via rivers
Estuarine Mixing
Highly stratified:
- salt wedge
- large river input, weak tidal input of salt water.
Partially mixed:
- Small river input, large tidal input
Well mixed:
- small river, v large tidal
- if estuary is wide, coriolis force may segregate river and sea water horizontally
Inverse estuary:
- salinity increases towards river, hot arid climates
Mixing and chemistry
Simplest scenario is that chemical composition reflects extent of mixing between fresh and salt water
Concentration of major ions covaries with salinity
Greatest change of conc occurs at salinity 5
Dilution of seawater constituents
All major ions and some of the minor ions in seawater are simply diluted by mixing with freshwater in the estuary
Hence plot of salinity vs concentration of that constituent is straight.
Conservative
Mixing diagrams
Plot of conc of chemical constituent against an index of conservative mixing
If constituent plots along line links river water endmember to seawater endmember (TDL), the constituent is said to show conservative behaviour.
Only represents behaviour in estuary
Processes affecting concs of minor and trace constituents
Most minor are trace dissolved constituents however do not show conservative behaviour
There distribution is affected by:
- biological processes
- adsorption/desorption on particle surfaces
- coagulation/flocculation/precipitation
- redox processes
Adsorption/desorption
Adsorption is binding of dissolved chemical species to a mineral surface by formation of surface complexes
Ability of minerals to adsorp solutes depends on the mineral and on the pH.
Surface change is a function of pH, reflecting extent of protonation of O atoms on mineral.
- At pHpzc = 0
- above = -ve
- below = +ve
Metals onto iron oxides will occur in estuaries, major control on dissolved concs
Coagulation/flocculation
Coloids are very small particles that remain suspended in aq solutions.
Main colloidal phases in rivers:
- clay minerals
- iron oxides
- humic/fulvic acids
Colloids floculate when surface charge is 0
Floc of C is a major part of organic C in sediments.
Redox processes
Estuarine sediments are usually reducing
Concs of some dissolved species may therefore be higher or lower in sediment pore waters than overlying water.
- leads to diffusion in and out of sediments
Rate of exchange increased by sediment resuspension
Addition/removal of solutes during estuarine mixing
Biological processes, adsorption, coagulation and redox may add or remove solutes
Causes a positive or negative deviation from TDL
Non-conservative
Limitations of behaviour
If residence time is short, non-con behaviour will not be detected
Mixing diagrams indicate apparently non-conservative behaviour:
- If residence time is long, and there is temporal variability in the endmembers
- If an estuary receives input from more than 1 river
- If an estuary receives additional sources of material
Fluorine
Conservative
Higher concentration in seawater
Iron
Non conservative
Loss of Fe from solution in upper part of estuary
Unlikely to be due to oxidation of Fe(II) to Fe(III) as river waters contain O and rate of Fe(II) oxidation is fast
Likely to be due to coagulation of colloidal Fe(III) as river water mixes with seawater, ~5ppt
Test iron loss
Analysis of Fe speciation in solution
Most Fe lost is Fe(III)
Conc of Fe(II) can be high
Conc of Fe(II) decreases with increasing salinity.
- loss of organic ligands via flocculation and/or effects of increased ionic strength
Silicon
Dissolved Si can be conservative and non-con
Con in march and evidence of removal in early autumn, uptake by diatoms
In some estuaries see extreme removal
Often linked to phyto
Si and residence
If resident is short, then any dissolved Si used by diatoms is replaced
River plumes
Large river plumes can’t be transported out to sea and past estuary
Can use conservative behaviour of dissolved Si to trace the plume
Si cycle
Salt marshes are a source of Si in the spring (remineralization of biogenic Si), supports further diatoms growth
Over the course of a year, marsh is a net sink
H4SiO4 > SiO2-nH2O
Other sources of dissolved Si in estuaries include desorption from resuspended seds
Anthropogenic is insignificant
Evidence from Si isotopes
Dissolved Si apparently shows conservative behaviour but:
- diatoms use lighter Si isotopes to form frustule, so remaining Si is heavier
- Si isotopes can be used to study nutrient utilization, and remineralization of biogenic Si
- isotopes analyses provide evidence for cryptic removal of light Si at salinities at >20, especially in July
Manganese
Undergoes intensive cycling due to redox reactions
Oxidizing conditions:
- sig free oxygen
- Mn(IV) is thermo-dynamically stable
- As MnO2 it has low solubility, Mn(IV) forms particles or coats
Reducing:
- O deficient
- Mn(II) is thermodynamically stable
Mn(IV) > Mn(II) it is soluble and can diffuse and advect in solution.
Mn Cycle
Many are reducing, due to high rates of burial of organic C
Mn(II) may diffuse out of the sediments, and flux is increased by resuspension of sediments.
Max concs of dissolved Mn often seen at intermediate salinities
Kinetics of Mn(II) oxidation are slow, so Mn(II) can persist at concs higher than predicted by thermodynamics.
Restricted Circulation
High concs of Mn(II) can also be found in the water column if circulation is restricted fjord
High Mn(II) in bottom waters with low oxygen
Overturning event July/Aug replenishes bottom water oxygen, and concs of Mn(II) fall
