J. Prietzel et C. Hirsch, Extractability and dissolution kinetics of pure and soil-added synthesizedaluminium hydroxy sulphate minerals, EUR J SO SC, 49(4), 1998, pp. 669-681
Various extractants used in current analytical procedures for the fractiona
tion 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 sy
nthesized and aged in the laboratory at 20 degrees C and 50 degrees C. The
dissolution kinetics at 20 degrees C of these Al hydroxy sulphates in deion
ized 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 th
e reagents used increased in the order H2O less than or equal to 0.016 M KH
2PO4 << 0.02 M HCI approximate to acidic NH4 oxalate approximate to 0.5 M N
aHCO3 < Na2CO3/NaHCO3 < 0.1 M NH4F < 0.5 M NH4F < 0.05 M NaOH less than or
equal to 0.05 M LiOH. In Al hydroxy sulphate-containing soils, inorganic S
is probably underestimated and ester sulphate S overestimated, if the inorg
anic S pool is assessed by extraction with KH2PO4, Ca(H2PO4)(2) or NaHCO3 s
olutions. The dissolution of all studied Al hydroxy sulphates, particularly
that of K and Na alunite, in H2O and 0.02M HCl is strongly delayed by kine
tic restraints. Thus it seems unlikely that Al3+ or SO42- activities in soi
l solutions are strictly regulated by precipitation/dissolution equilibria
of these minerals except for horizons with extraordinarily slow seepage wat
er movement.