ANALYSIS OF PROTON BINDING BY A PEAT HUMIC-ACID USING A SIMPLE ELECTROSTATIC MODEL

Detailed potentiometric titration data were collected for a purified p eat humic acid (PPHA) over a range of pH (pH 3.5-10.5) and KNO3 backgr ound electrolyte concentrations (0.001-0.3 M). The data were analyzed following the master curve approach which includes both an electrostat ic double layer model and a model for the intrinsic heterogeneity of t he PPHA. Spherical and cylindrical double layer models gave equally go od fits to the data. A salt dependence observed around pH 5 could not be completely removed by taking into account the electrostatic interac tions. Hysteresis was observed to a much greater extent in the first t itration cycle compared with the second cycle. This suggested that som e slow and only partly reversible aggregation was occurring possibly a s a result of the aggregation created during the purification of the h umic acid. Titration curves for fully redispersed samples fitted the m aster curve approach (surface charge vs. surface pH) reasonably well b ut still displayed an ionic strength dependence at a pH of less than 5 which could not be accounted for using the simple electrostatic model . Heterogeneity analysis of the master curve showed that the affinity distribution had two peaks centred at log K(H)int approximately 4 and log K(H)int approximately 8 to 9. The total number of weak acid sites titrated between pH 3.5 and 10.5 was approximately 3.5 eq kg-1 but the total number of sites estimated from the isotherm analysis was 5.3-5. 8 eq kg-1. Double Toth and double Langmuir-Freundlich isotherms fitted the data almost equally well but the implied distribution of sites be tween the more acidic ''carboxylic'' sites and the weakly acidic ''phe nolic'' sites varied with the isotherm chosen. An important source of uncertainty in the analysis was in estimating the charge on the humic acid at its initial pH of about pH 3.