Green Chemistry and the Ten Commandments

7.8. Why Natural Waters Contain Alkalinity and Calcium

Most surface water and groundwater contains water alkalinity, defined as the ability
of water to react with and neutralize acid (H+). This is because such water contains
bicarbonate ion, HCO 3 - , which can react as follows with H+ ion:

HCO 3

-

+ H

+

(^) ←→ CO 2 (aq) + H 2 O (7.8.1)
This is a useful reaction because, among other reasons, it acts to neutralize pollutant
acids, such as those from acid rain, that get into water. This water obtains alkalinity
when dissolved carbon dioxide in the water reacts with calcium carbonate mineral
(Figure 7.5), the same process by which the water acquires water hardness in the form
of dissolved Ca2+ ion:
CO 2 (aq) + CaCO 3 (s) + H 2 O ←→ Ca
2+
(aq) + 2HCO 3

-

(7.8.2)

Rainfall

Infiltrating

water

CaCO 3

CO 2

CO 2

CaCO 3 + CO 2 + H 2 O Ca2+ + 2HCO 3 -

Ca2+

HCO 3

Figure 7.5. Water containing dissolved carbon dioxide from the atmosphere or from bacterially produced
sources underground reacts with limestone, CaCO 3 , to produce dissolved Ca2+ (water hardness) and
dissolved HCO 3 - (water alkalinity).

7.9. METALS IN WATER

Although we commonly write the formulas of metal ions dissolved in water with
simply the symbol and charge of the metal, such as Ca2+ for calcium discussed above,
metal ions are always bound to water molecules in aqueous solution. There are usually 6
water molecules bound to each metal ion, so that dissolved calcium ion can be represented
as Ca(H 2 O) 6 2+.

170 Green Chemistry, 2nd ed