2{CH 2 O} + SO 42 - + 2H
+
→ H 2 S(g) + 2H 2 O + 2CO 2 (g) (7.5.4)is one in which the sulfate ion loses oxygen (is reduced). As the H 2 S gas bubbles up
through the water, it may contact molecular oxygen and other kinds of bacteria that cause
it to undergo the following reaction in which the sulfur is oxidized with the addition of
oxygen atoms to produce SO 4
2 -
ion:H 2 S + 2O 2 → SO 42 - + 2H
+
(7.5.5)7.6. Fizzy Water from Underground
Natural waters contain dissolved gases. The oxygen that fish utilize, for example, is
contained as dissolved oxygen from the air. Pressurized carbon dioxide, CO 2 , dissolved
in groundwater in some areas causes the water to “fizz” when it is pumped to the surface
and some of the dissolved gases escape. Entrepreneurs bottle this water and sell it as
mineral water “with gas.” It may not taste so great, but since it is “natural,” some people
assume it must be good for them and pay a relatively high price for the privilege of
drinking it. Dissolved CO 2 can be dangerous as was the case with dissolved volcanic
carbon dioxide in Lake Nyos in the African country of Cameroon in 1986 when CO 2
suddenly released from the lake asphyxiated about 1,700 people. (In an effort to prevent
this tragedy from being repeated, wells have now been drilled in the bottom of the lake
to gradually release CO 2 , to the atmosphere.)
Dissolved gases are often in equilibrium with the gas phase. A common example
is dissolved oxygen, which is in equilibrium with oxygen in air. The concentration of
dissolved gas is related to the pressure of the gas in the gas phase by Henry’s Law,
which states that the solubility of a gas in a liquid is proportional to the partial pressure
of that gas in contact with the liquid. The solubility of a gas in water decreases with
increasing temperature.
Oxygen in Water
The most important dissolved gas in water is molecular oxygen, O 2. At 25 ̊C the
concentration of oxygen dissolved in water in equilibrium with atmospheric air is only
about 8 milligrams per liter of water (mg/L). It is this very low concentration of oxygen
upon which fish depend for their oxygen supply. This much oxygen is readily lost or
consumed and, without some means of replenishing it, water can rapidly become too
oxygen-deficient to support fish life. The main way in which this happens is by the
consumption of biomass ({CH 2 O}) by oxygen-utilizing bacteria:
{CH 2 O} + O 2 → CO 2 + H 2 O
(7.6.1)A simple stoichiometric calculation shows that only about 8 mg of biodegradable organic
matter (a few flakes of goldfish food, for example) is a sufficient mass to consume all the
oxygen in a liter of water initially saturated with air at 25 ̊C.
168 Green Chemistry, 2nd ed