Acid hydrolysis

Continental water contains dissolved species that render it acidic. The acidity comes from a variety of sources: from the dissociation of atmospheric CO2 in rainwater—and particularly from dissociation of soil-zone CO2 (Section 4.4.2) — to form H2CO3, and natural and anthropogenic sulphur dioxide (SO2) to form H2SO3 and H2SO4 (see Boxes 3.7 & 3.8). Reaction between a mineral and acidic weathering agents is usually called acid hydrolysis. The weathering of CaCO3 demonstrates the chemical principle involved:

CaCO3(S) + H2CO3(aq) ^ Ca2+ ) + 2HCO-(aq) eqn. 4.11

The ionic Ca-CO3 bond in the calcite crystal is severed and the released CO2-anion attracts enough H+ away from the H2CO3 to form the stable bicarbonate ion HCO-. Note that the second HCO- formed in equation 4.11 is left over when H+ is removed from H2CO3. Bicarbonate is a very weak acid, since it dissociates very slightly into H+ and CO32-, but it is not quite dissociated enough to react with carbonate. Overall, the reaction neutralizes the acid contained in water. The reaction is dependent on the amount of CO2 available: adding CO2 causes the formation of more H2CO3 (eqn. 4.7), which dissolves more CaCO3 (forward reaction in eqn. 4.11); conversely, lowering the amount of CO2 encourages the reverse reaction and precipitation of CaCO3. Stalactites and stalagmites forming in caves are an example of CaCO3 precipitation induced by the degassing of CO2 from groundwater. This response to varying CO2 is a clear example of Le Chatelier's Principle (see Box 3.2).

Acid hydrolysis of a simple silicate, for example the magnesium-rich olivine, forsterite, is summarized by:

Mg2SiO4(s) + 4H2CO3(aq) ^ 2Mg2+q) + 4HCO-(aq) + H4&O4(aq) eqn. 4.12

Note that the dissociation of H2CO3 forms the ionized HCO-, which is a slightly stronger acid than the neutral molecule (H4SiO4) released by the destruction of the silicate.

The combined effects of dissolving CO2 into soilwater (eqn. 4.7), the subsequent dissociation of H2CO3 (eqn. 4.8) and the production of HCO- by acid hydrolysis weathering reactions (eqns. 4.11 & 4.12) mean that surface waters have near-neutral pH, with HCO- as the major anion.

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