Surface reactivity of poorly-ordered minerals in podzol B horizons

The surface reactivity of mineral soil horizons from three podzolised fores t soils in Scandinavia was examined. The amount of accumulated C was low, b etween 1.8 and 2.3% in the top of the B horizons. Selective extractions in combination with infrared (IR) spectroscopy and transmission electron micro scopy (TEM) indicated a predominance of poorly-ordered imogolite-type mater ials (ITM) in the oxalate extractable fraction in an examined B horizons. T he presence of well-ordered imogolite was only indicated in the Nyanget B3 horizon. A large proportion of free Fe was removed by ammonium oxalate. Com parisons of Mossbauer spectra (both at room temperature and at 4.2 K) befor e and after treatment with ammonium oxalate showed that the oxalate treatme nt resulted in a removal of a (super)paramagnetic Fe3+ phase? probably ferr ihydrite. A comparison of the Mossbauer Fe3+ parameters at room temperature and 4.2 K indicated a close intergrowth of a ferrihydrite-like oxide with a magnetically neutral matrix, e.g., allophane. The specific surface area ( SSA) was determined by N-2 adsorption before and after treatment of the sam ples with acid ammonium oxalate. The loss of SSA after oxalate treatment wa s considerable in the B horizon where only between 3.8 to 13.38 of the orig inal SSA remained after treatment. The point of zero charge salt effect (PZ SE) increased with depth in the B horizon from between 4.4 and 5.1 in the u pper horizons to between 5.7 and 7.7 in the lower part of the B horizon. Th e increased PZSE with depth paralleled a decrease in the ratio of pyrophosp hate soluble C to oxalate soluble Fe + Al. The affinity for SO42-. a goad i ndicator of the presence of active surface hydroxyls, was measured by compa ring the H+ buffering capacity of a sample titrated in 2.5 mM Na2SO4 with a sample titrated in 5 mh I NaNO3,. The buffering capacity of the soil in th e Na2SO4, electrolyte was well correlated with the amount of oxalate minus pyrophosphate soluble Fe + Al (r(2) = 0.88). The sulphate exchange capacity was considerably higher than CEC, especially in lower parts of the B horiz on. The calculated surface area of the oxalate soluble material (OSM) range d between 74 and 289 m(2) g(-1) and the calculated surface site density of the same material ranged between 0.6 to 3.3 site nm(-2). It was concluded t hat the surface reactivity in the B horizons is dominated by the poorly-ord ered variable-charge oxides resulting in a low capacity to retain cations b ut a high capacity for adsorption of weak acid anions like SO42- and organi c acids. (C) 2000 Elsevier Science B.V. All rights reserved.