SHORT-TIME DISSOLUTION MECHANISMS OF KAOLINITIC TROPICAL SOILS

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.