Morphology, mineralogy and surface chemistry of manganiferous oxisols near Graskop, Mpumalanga Province, South Africa

[摘要] ENGLISH ABSTRACT: In the humid Graskop region of Mpumalanga Province, South Africa, there is ananomalous body of highly weathered black, manganiferous oxisols derived from dolomite.With Mn contents as high as 17%, potential large-scale Mn release is an environmentalconcern under current, acid generating, forestry practices. This study aims at establishingthe factors which may affect the stability of the manganiferous oxisols of Graskop and inthe process, investigating some of the morphological, mineralogical and chemicalproperties of these unique soils.Typically, the soils show a reddish, nodule-rich horizon, containing 3-4% Mn, gradingthrough a red and black mottled zone into a black (5YR 2.5/1) apedal subsoil with >7%Mn. The Mn gradient down the profile as well as the abundant nodule content of the uppersubsoil horizons implies that Mn mobilization and redistribution are active pedogenicprocesses. The exceptional Mn content of these soils is complemented with Fe and Alconcentrations of up to 10 and 8%, respectively, and anomalously high trace element levelsin particular Ni and Zn (as high as 541 and 237 mg kg-1, respectively) which are at theupper limit of cited world natural maxima for soils. The Mn mineral lithiophorite[(Al,Li)MnO2(OH)2], dominates the mineralogy of the soils with accessory amounts ofbirnessite, gibbsite, goethite, hematite, maghemite, kaolinite, aluminous chlorite and mica -a mineral suite reflecting that of well weathered soils.With the pH of the soil being at or close to the point of zero charge (4.5-5.5) the soils showisoelectric equilibrium. The very low buffer capacity results in metal dissolutioncommencing with the first increment of titrated acidity. Manganese dissolution is relativelyminor considering the large potential for release and is highly overshadowed by Al release.The apparent resilience of the Mn phase to added acidity may relate to the overwhelmingpoise of the soils which maintains robust, oxic conditions despite the usual instability ofMn oxides at low pH.Manganese release and soil redox properties are substantially affected by drying especiallyin the organic rich topsoils. Using various redox analyses, evidence is shown forinvolvement of Mn(III)-organic complexes in the drying reactions. Using this andinformation gained in a real time, attenuated total reflectance Fourier transform infrared(ATR-FTIR) spectroscopic study, a mechanism is suggested which may account for theobserved Mn release and the loss of Cr oxidising capacity commonly observed in driedsoils. The information provided by the ATR-FTIR study showed the decrease in surfacepH of a clay film, from 5 to below 2, as well as the shift in coordination nature of sorbedoxalate from a more outer-sphere association to a more inner-sphere associationconcomitant with the removal of free water from the clay surface. This spectroscopicevidence for these chemical changes which accompany surface drying not only providesfurther insight into the reactions involving Mn oxides in soils but also highlights thesuitability of ATR-FTIR for real time, in situ investigation into the chemistry of the dryingwater interface.From these results it is concluded that Mn release from the manganiferous oxisols, underacid generation of the kind known to occur in pine plantations, is less that anticipated. Onthe other hand, desiccation of the topsoil results in substantial Mn release with a suggestedmechanism which involves a Mn(III) intermediate.

[发布机构] Stellenbosch University