PROTON SURFACE-CHARGE DETERMINATION IN SPODOSOL HORIZONS WITH ORGANICALLY BOUND ALUMINUM

Net proton surface charge densities were determined in O, E, Bh, and B s horizons of a sandy till, Spodosol from Denmark, by means of acid-ba se titration combined with ion adsorption in 0.005 M Ca(NO3)(2) and in dependent permanent charge determination. The release of organic anion s exceeded the adsorption of NO3-, resulting in a desorption of anions in all horizons. Data were found to obey the law of balance between s urface charges and adsorbed ions only when charges pertaining to Al an d organic anions released during the titration experiments were accoun ted for, in addition to charges pertaining the potential determining i ons(PDT) H+ and OH- and the index ions Ca2+ and NO3-. It was furthermo re shown that the point of zero net proton charge (PZNPC) in soils hig hly depends on the concentration of organically bound Al. Approaches p reviously used in soils, in which adsorbed Aln+ has been ignored (i.e. , considered equivalent to nH(+) as a PDI), resulted in a PZNPC of 4.1 in the Bs horizon. If instead organically bound Al was accounted for as a counter-ion similar to 3/2Ca(2+), a PZNPC of 2.9 was obtained for the same Bs horizon. Based on PZNPC values estimated by the latter ap proach, combined with a weak-acid analog, it was shown that organic pr oton surface charges buffered pH with a similar intensity in the O, E, Bh, and Bs horizons of this study. Because the acidity of Al adsorbed to conjugate bases of soil organic acids is substantially weaker than the acidity of the corresponding protonated form of the organic acids , the point of zero net proton charge (PZNPC) will increase if the con centration of organically adsorbed Al increases at the expense of adso rbed H. This means that PZNPC values determined for soils with unknown concentrations of organically adsorbed Al are highly operational and not very meaningful as references. Copyright (C) 1998 Elsevier Science Ltd.