《环境化学》课程PPT教学课件讲稿（英文版）Unit 13 Water in Soil：Neutralization and Watershed Buffering

Water in soil: Neutralization and Watershed Buffering

Water in Soil: Neutralization and Watershed Buffering

pH of a buffer and buffer capacity Example: For a buffer solution consisting of 0. 1 M acetic acid and o 1M sodium acetate the ph of the solution is 4.75. HAc=h+Ac Ka=LH IAc 1 [HAc] [H]=Ka· HAc] LAc =10-470.1 4.75 f 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?

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. 4.75 4.75 10 0.1 0.1 10 [ ] [ ] [ ] [ ] [ ][ ] − − − + + − + − = • = = • = = + Ac HAc H Ka HAc H Ac Ka HAc H Ac 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?

pH of a buffer and buffer capacity( Continued) After addition of HCl, the new [ac]=0.09M, [Hac]=0. 11M m1=ka。Hg LAC 0.11 =10 -4.75 10 4.66 0.09 pH=46 ypH=475-4.66=0.09 The addition of 0.01M strong acid to pure water would lower ph by 5 units(from 7 to 2)

pH of a buffer and buffer capacity (Continued) After addition of HCl, the new [Ac- ]=0.09M, [HAc]=0.11M 4.75 4.66 0.09 4.66 10 0.09 0.11 10 [ ] [ ] [ ] 4.7 5 4.6 6  = − = = = • = = • − − − + pH pH Ac HAc H Ka The addition of 0.01M strong acid to pure water would lower pH by 5 units (from 7 to 2)!

pH of a buffer and buffer capacity( Continued) The ph of a buffer depends on the pka of the buffer acid not its concentration HA=H+A Ka HTA I HA H]=。lHA [A J ph=PKa-loe(A

pH of a buffer and buffer capacity (Continued) • The pH of a buffer depends on the pKa of the buffer acid, not its concentration. [ ] [ ] log [ ] [ ] [ ] [ ] [ ][ ] − − + + − + − = − = • = = + A HA pH pKa A HA H Ka HA H A Ka HA H A

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 12-0.8-0.400.40.81.2 log CNaA CHA

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

Water pH and Well-being of Fish Species 」 Smallmouth bass lost La。 whitefish lost Loagpowe sucker lot The ability of a Lake trout lost water body to support its normal 3 complement of =4 biological species 五 can be critically affected by the Big Moose Lake pH of the water. New Yor state U.S.A 8801900190019401960190 Dashed line: lake pH, solid line: upwind So2 emission from the U.s. industrial midwest

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

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 watersheds natural buffering capacity delayed the onset of ph decline Implication: Polluting activities may be far displaced in time from their environmental effects

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

Decline in soil solution ph over time in response to atmospheric acid inputs ◆·吧身一品一 Carbonate Cxt +c0. buffering Cation exchange i HC, M-K buffering range 二学…“·…d======== buffering range T 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

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

Watershed buffering: carbonate buffering Caco,+h,CO=Ca+2HCO H,C0=H*+HCO% Kegs iCa IHCO, l-=lot Ka=lH jAco 6.4 [ CO3 [H2CO3I H2CO3]=10M PH=pKa-log lH, co31 HCO3 I 2[Ca]=[HCO3 6.40+197=8.37 [BCO3]=10 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

Watershed buffering: carbonate buffering HCO M Ca HCO H CO M H CO Ca HCO Keq CaCO H CO Ca HCO 3.0 0 3 3 2 4.9 2 3 4.4 1 2 3 2 3 2 3 2 3 2 3 [ ] 10 2[ ] [ ] [ ] 10 10 [ ] [ ][ ] 2 − − + − − − + − + − = = = = = + = + 6.40 1.97 8.37 [ ] [ ] log 10 3 2 3 6.4 [ ] [ ][ ] 2 3 3 2 3 3 = + = = − = = = + − − + − + − HCO H CO pH pKa Ka H CO H HCO H CO H HCO Acidic rainwater can be neutralized by exposure to calcareous soils, with a concomitant significant increase in the concentration of calcium ion in solution. That’s how underground caves are formed

Watershed buffering: cation exchange buffering H+[M(SiO4)]=M*+[2H(SiO4)l M=Ca, Mg t, Na, Kt The buffer capacity of clay soils is usually limited because of the limited exchangeable sites occupied by the cations Nat 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

Watershed buffering: cation exchange buffering + + + + + + = + = + M Ca Mg Na K H M SiO M H SiO x n x , , , [ ( )] [2 ( )] 2 2 4 4 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