N. Malengreau et G. Sposito, SHORT-TIME DISSOLUTION MECHANISMS OF KAOLINITIC TROPICAL SOILS, Geochimica et cosmochimica acta, 61(20), 1997, pp. 4297-4307
Previous research on the short-time dissolution behavior of kaolinitic
Oxisols suggested pH-dependent kinetics involving ligand-promoted dis
solution, metal readsorption, and colloidal dispersion, with soil orga
nic matter conjectured to play a decisive role. A novel combination of
spectroscopy, lightscattering, and batch dissolution experiments, con
ducted at controlled pH and ionic strength over five dissolution perio
ds ranging from 1 to 12 h, was applied to evaluate this mechanism for
samples of a representative kaolinitic Oxisol; collected at both fores
ted and cultivated field sites (leading to significant differences in
organic matter content and field soil pH). The overall characteristics
of the pH-dependent net release kinetics of Al, Fe, and Si by the soi
l samples, for any dissolution period in the range investigated, were
determined by the pH value at which colloid dispersion commenced, whic
h decreased significantly as the soil organic matter content increased
. Plots of log(Si/Al released) (or Si/Fe released) vs. -log [H+] ([H+]
is proton concentration) were superimposable for all dissolution peri
ods studied, rising to a plateau value above the point of zero net cha
rge of the soils (pH 3.2). Light-scattering and X-ray diffraction data
showed conclusively that this plateau represented the release of sili
ceous colloids containing kaolinite and X-ray amorphous material. X-ra
y diffraction, UV-visible diffuse reflectance spectroscopy, and electr
on spin resonance spectroscopy, applied to the soil samples before and
after dissolution, and after conventional chemical extractions to rem
ove Al, C, Fe, and Si, showed that kaolinite and iron oxide phases (th
e latter being highly Al-substituted and present in both coatings and
occlusions) were essentially unaltered by dissolution, even at -log [H
+] = 2, whereas substantial dissolution loss of soil quartz occurred.
Diffuse reflectance spectroscopy gave strong evidence that C in these
soils occurs principally in discrete solid phases, not as a reactive c
oating on mineral surfaces. Copyright (C) 1997 Elsevier Science Ltd.