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Applies to a steady-state ocean (Input = Output)
Definition: "Average time substance spends in ocean before
removal."
Calculation:
time(res.) = mass ÷ input rate (mass/time)
....= mass ÷ output rate (mass/time)
(can use either input or output because they are equal at steady
state)
mass of dissolved substance in sea water
= concentration (g/kg) * mass of sea water (kg)
input rate by rivers (most important, easy to estimate)
= conc. in river water (g/kg) * river runoff rate (kg water /
time)
Representative values:
Residence time, years
Cl 80 million
Na 60 million
Mg 10 million
SO4 9 million
Ca 1 million
P 0.7 million
Mn 7,000
Fe, Al 100
Importance? -- indication of element's "reactivity"
in sea water
Long residence time (Cl, Na): not very reactive, removed slowly
Short residence time (Mn, Fe, Al): reactive, removed rapidly
Example of reactivity & residence time: Ca2+ vs.
Na+
River water: [Ca2+] > [Na+] Sea water: [Na+]
> [Ca2+]
Ca2+ removed rapidly as CaCO3 -- short residence
time.
Na+ removed slowly; may accumulate until a major "evaporite"
event.
RESOURCES IN SEA WATER
Lots of dissolved substances, but only a few are commercial resources.
Salt (NaCl)
Evaporation in shallow artifical ponds -- 30% of world's supply
Residual brine (after NaCl ppt.) processed for
Mg (magnesium)
Br (bromine)
Fresh water -- the most value resource!
Desalinization of sea water by ...
Change in state -- freezing, evaporation
"Reverse osmosis" -- sea water forced through a membrane
that is permeable only to H2O; salts are "filtered
out", discarded with the residual high-salinity water (brine)
left after extraction of some fresh H2O
Desalinization requires lots of energy, thus money! Important
source of fresh water in arid regions (coasts, islands) where
....
energy is cheap (or money is no problem), and
cost of alternative sources of water is high.
(Detailed notes start here)
The residence time of an element in sea water is the average time that the element spends in the oceans before it is removed. The concept of residence time applies to a steady-state ocean (where input = output). Under these conditions, the residence time of an element is defined as the mass of the element in sea water divided by either the element input rate or the element removal rate.
residence time = mass ÷ input rate = mass ÷ output rate
[Note the ratio of mass to input rate (mass/time) gives a value, the residence time, in units of time.]
The mass of an element in the oceans is easy to calculate. It is simply the concentration (in g/kg) times the mass of sea water (in kg). Most residence times are calculated using the input rate by rivers.
river input rate of element (g/time) = conc. in river water (g/kg) X river runoff rate (kg of water/time)
Residence times for element in the oceans vary widely. Here are some representative values:
Residence time, years
Cl 80 million
Na 60 million
Mg 10 million
SO4 9 million
Ca 1 million
P 0.7 million
Mn 7,000
Fe, Al 100
What does a "residence time" actually mean?
Simply stated, residence time is a measure of an element's "reactivity"
in sea water.
* A long residence time (e.g., Cl, Na) indicates that an element
is relatively unreactive in sea water; it is removed at a slow
rate.
* A short residence time (e.g., Mn, Al, Fe) indicates that an
element is removed rapidly from sea water.
The relationship between residence time and reactivity in seawater can be illustrated by contrasting the behavior of two major cations, Na+ and Ca2+.
* In river water, the concentration of Ca2+ is higher than
that of Na+
(i.e., [Ca2+] > [Na+]).
* But in sea water, just the opposite is the case (i.e., [Ca2+]
< [Na+]).
* This apparent "reversal" occurs because Ca2+ is removed
rapidly and efficiently as CaCO3. The relatively high reactivity
of Ca2+ is reflected in its comparatively short residence time
(1 m.y.). In contrast, Na+ is unreactive in sea water; it tends
to accumulate until removal by a major evaporite event.
Sea water contains almost all chemical elements in solution. However, because of low concentrations, most dissolved substances cannot be considered a commercial resource. Among those substances that are extracted from sea water are the following:
Salt (NaCl). Salt is obtained by allowing sea water
to evaporate in shallow artifical ponds. Precipitation of NaCl
from sea water accounts for about 30% of the world's supply of
salt. The residual brine remaining after NaCl precipitation is
processed for other elements:
Mg (60% of world's supply)
Br (70% of world's supply)
Water. Fresh water is the most valuable resource in
sea water!
Several techniques are used in extracting fresh, potable water
from saline sea water:
* Change of state -- freezing or evapartion to yield relatively
pure H2O
* "Reverse osmosis" -- H2O is separated from seawater
by forcing sea water to flow under pressure across a membrane
that is only permeable to H2O. That is, water passes through the
membrane but salts remain behind.
The biggest limitation to sea-water desalinization is the large amount of energy required, and the cost of that energy. At present, desalinization is an important source of fresh water only in dry coastal regions or islands where energy is cheap and the cost of alternative sources of water is high (e.g, Arabian subcontinent, certainly prosperous islands in the Caribbean).
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