Implications of the Calcium‐Aluminum Exchange System for the Effect of Acid Precipitation on Soils

Abstract

Abstract 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 K gt , K v nd K gp , 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 Ca 2+ (E* Ca ), are affected by solution concentration but are independent of cation exchange capacity. At least within the range of log K gt values from 1 to 3, the isotherms generated from the Gaines‐Thomas and Vanselow equations are almost identical when K v = 0.316 K gt . 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 Ca 2+ or other basic cations, as long as E* Ca is above ≈0.15. The change from Ca 2+ to Al 3+ domination is quite abrupt and will usually occur at E* Ca 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 H 2 SO 4 and/or SO 2 impacts.