Water in Soil: Neutralization and Watershed Buffering

1. Water in Soil: Neutralization and Watershed Buffering

2. pH of a buffer and buffer capacity Example: For a buffer solution consisting of 0.1 M acetic acid and 0.1M sodium acetate, The pH of the solution is 4.75. If an amount of hydrochloric acid, equivalent to 10% of the acetate present, is added to the buffer, what is the new pH of the solution?

3. pH of a buffer and buffer capacity (Continued) After addition of HCl, the new [Ac-]=0.09M, [HAc]=0.11M The addition of 0.01M strong acid to pure water would lower pH by 5 units (from 7 to 2)!

4. pH of a buffer and buffer capacity (Continued) • The pH of a buffer depends on the pKa of the buffer acid, not its concentration.

5. pH of a buffer and buffer capacity (Continued) • Buffer capacity is how much acid or base the buffer can tolerate while maintaining the pH (within a 1.00 unit). • Buffer capacity is determined by the concentration of the buffer acid and its conjugated base, as well as their concentration ratio.

6. Water pH and Well-being of Fish Species The ability of a water body to support its normal complement of biological species can be critically affected by the pH of the water. Dashed line: lake pH, Solid line: upwind SO2 emission from the U.S. industrial midwest.

7. Water acidification from acid deposition:pH decline lags behind acid deposition, why? • Observation: In Big Moose Lake, pH dramatic decline lagged behind in the rise in SO2 emissions by some 70 years. • Reason: The watershed’s natural buffering capacity delayed the onset of pH decline. • Implication: Polluting activities may be far displaced in time from their environmental effects.

8. Decline in soil solution pH over time in response to atmospheric acid inputs The time-scales over which the soil solution passes from one buffering range to the next depends on the intensity of acid deposition, the nature of soil, the size of watershed, and the flow characteristics of the lake or groundwater.

9. Watershed buffering: carbonate buffering That’s how underground caves are formed Acidic rainwater can be neutralized by exposure to calcareous soils, with a concomitant significant increase in the concentration of calcium ion in solution.

10. Watershed buffering: cation exchange buffering The buffer capacity of clay soils is usually limited because of the limited exchangeable sites occupied by the cations Na+, K+, Mg2+, and Ca2+. The exchangeable pool of cations on the surface is tiny compared to the pool trapped inside the soil particles. Weathering reactions release trapped cations, but they are relatively slow compared to the rate of acidification.

11. Watershed buffering: Aluminum buffering When pH drops below 4.2, H+ dissolves the Al-containing minerals. Al-containing minerals are abundant in soils, buffer capacity in this range is rarely depleted. Al3+ is toxic to plants and aquatic organisms.

12. Water acidification from acid mine drainage • When pyrite-rich coal is mined, pyrite is exposed to air and water. • Oxidation of pyrite produces sulfuric acid. 2FeS2 + 7/2 O2 + 2 H2O  Fe2+ + 2 HSO42- Fe2+ + ¼ O2 + 1/2 H2O  Fe3+ + OH- Fe3+ + 3 H2O  Fe(OH)3 + 3H+ • Overall reaction FeS2 + 15/2 O2 + 7/2 H2O  Fe(OH)3 + 2 HSO42- Assisted by bacteria Streams receiving this drainage could have a pH as low as 3.0! Brown precipitation

13. Acid mine drainage • Solution: Neutralize with limestone CaCO3 (s) + 2 H+ + SO42-  Ca2+ + SO42- + H2O + CO2 (g)

14. Study questions • What chemical substances can serve as pH buffers? What determines the buffer capacity of a pH buffer? • Why is there not an immediate decline in pH following atmospheric input of large quantities of acidic substances? • Describe the three defense lines of watershed in resisting pH changes.