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Some Equilibrium Approaches to Study the Retention and Release of Sulfate in Some Highly Acid German Subsoils
Author(s) -
Ludwig Bernard,
Khanna Partap,
Prenzel Jürgen
Publication year - 1999
Publication title -
journal of environmental quality
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.888
H-Index - 171
eISSN - 1537-2537
pISSN - 0047-2425
DOI - 10.2134/jeq1999.00472425002800040038x
Subject(s) - freundlich equation , langmuir , sorption , chemistry , langmuir adsorption model , soil water , thermodynamics , inorganic chemistry , adsorption , organic chemistry , soil science , geology , physics
A decrease in sulfate (SO 4 ) deposition in recent years has been predicted to affect the long‐term changes in soil SO 4 dynamics differently, depending upon the equilibrium approaches (sorption isotherms or AlOHSO 4 solubility) used. This study reassesses the usefulness of equilibrium approaches using data from batch experiments where the solution concentrations were changed in small steps. Experiments were carried out using five highly acid German subsoils. The approaches included solubilities of SO 4 minerals, the classical one‐ and two‐site Langmuir isotherms, an empirical Freundlich isotherm with a pH‐dependent coefficient and modified Langmuir isotherm approaches considering the activity of protons (H + ‐dependent Langmuir) or the square of the proton activity ((H + ) 2 ‐dependent Langmuir) in the equations. The ion activity product of AlOHSO 4 was not constant for most experiments but was a function of the SO 4 sorption for all five soils. Dynamics of the retention and release of SO 4 in these soils indicated the limitation in the use of one‐ and two‐site Langmuir isotherms considering the theoretical basis behind these isotherms. The (H + ) 2 ‐dependent Langmuir isotherm approach described SO 4 sorption better than one‐ and two‐site Langmuir isotherms, but not as well as the pH‐dependent Freundlich and the H + ‐dependent Langmuir isotherms. For describing SO 4 sorption in highly acid soils it is important to include its relationship with protons. It would mean using any of the following approaches: (i) the empirical pH‐dependent Freundlich isotherm, (ii) the H + ‐dependent Langmuir isotherm approach, or (iii) an ion activity product of AlOHSO 4 as a function of SO 4 sorption.