Extractability and dissolution kinetics of pure and soil-added synthesized aluminium hydroxy sulphate minerals

Various extractants used in current analytical procedures for the fractionation of sulphur (S) in soils were compared with respect to their ability to dissolve Al hydroxy sulphate minerals of defined composition (basaluminite, K alunite, mixtures of basaluminite and Na alunite). The minerals were synthesized and aged in the laboratory at 20°C and 50°C. The dissolution kinetics at 20°C of these Al hydroxy sulphates in deionized water, 0.02 M HCl and 0.02 M NaOH were also investigated. The dissolution stability of the Al hydroxy sulphate minerals increased in the order basaluminite < K alunite < Na alunite. The dissolving power of the reagents used increased in the order H₂O ≤ 0.016 M KH₂PO₄ << 0.02 M HCl ≃ acidic NH₄ oxalate ≃ 0.5 M NaHCO₃ < Na₂CO₃/NaHCO₃ < 0.1 M NH₄F < 0.5 M NH₄F < 0.05 M NaOH ≤ 0.05 M LiOH. In Al hydroxy sulphate-containing soils, inorganic S is probably underestimated and ester sulphate S overestimated, if the inorganic S pool is assessed by extraction with KH₂PO₄, Ca(H₂PO₄)₂ or NaHCO₃ solutions. The dissolution of all studied Al hydroxy sulphates, particularly that of K and Na alunite, in H₂O and 0.02 M HCl is strongly delayed by kinetic restraints. Thus it seems unlikely that Al³⁺ or SO₄^2- activities in soil solutions are strictly regulated by precipitation/dissolution equilibria of these minerals except for horizons with extraordinarily slow seepage water movement.