A COMPARATIVE-STUDY OF PROTON AND ALKALINE-EARTH METAL-BINDING BY HUMIC SUBSTANCES

Humic substances (HS) were extracted from the soil and water of an upl and catchment and characterised with respect to molecular weight, C, H , O, N and S contents and ash content. Three samples were obtained: pe at humic acid, aquatic humic acid and aquatic fulvic acid. The weight average molecular weights were found to increase from 2400 for aquatic fulvic acid, to 6300 for aquatic humic acid and to 16500 for peat hum ic acid. Acid-base titrations were performed to assess the proton bind ing behaviour at different ionic strengths in NaCl and in the presence of important cation competitors (calcium and magnesium). The results from the titrations were fitted to a recently developed discrete-site/ electrostatic model of cation-humic binding. Model parameters showed s ystematic trends with molecular weight and good agreement with paramet ers derived from the analysis of literature data. In the order peat hu mic, aquatic humic and aquatic fulvic acid, the milli-equivalents of c arboxylic acid groups per gram of HS (n(A)) increased. In the same ord er both the negative log(10) of the mid-range intrinsic proton dissoci ation constant for the strong acid groups (pK(A)) and the empirical co nstant, P, which accounts for electrostatic effects, decreased.