Implications of the Calcium-Aluminum Exchange System for the Effect of Acid Precipitation on Soils1
- J. O. Reuss2
Calcium-aluminum exchange isotherms were generated for three common ion exchange equilibrium equations, and their implications relative to the probable effects of acid rainfall are examined. The equations examined were those of Gaines and Thomas (1953), Vanselow (1932), and Gapon (1933), using selectivity coefficients Kgt, Kv nd Kgp, respectively, covering the range likely to be found in soils. These isotherms, shown as fraction Al in solution as a function of the charge fraction compensated by exchangeable Ca2+ , are affected by solution concentration but are independent of cation exchange capacity. At least within the range of log Kgt values from 1 to 3, the isotherms generated from the Gaines-Thomas and Vanselow equations are almost identical when Kv = 0.316 Kgt. The shape of the isotherm generated by the Gapon equation, however, is somewhat different. Using the Gaines-Thomas equation, the system was expanded to include the Al(OH) 2+, Al(OH)2+, and H+ ions. Examination of these curves suggests that for most soils, solutions will be dominated by Ca2+ or other basic cations, as long as is above ≈0.15. The change from Ca2+ to Al3+ domination is quite abrupt and will usually occur at values between 0.05 and 0.15. These results are useful in predicting the changes in Ca-Al balance in soil solutions and leachates over time as a result of H2SO4 and/or SO2 impacts.
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