Analysis of proton binding by a peat humic acid using a simple electrostatic model

Abstract

Detailed potentiometric titration data were collected for a purified peat humic acid (PPHA) over a range of pH (pH 3.5–10.5) and KNO3 background electrolyte concentrations (0.001–0.3 M). The data were analyzed following the master curve approach which includes both an electrostatic double layer model and a model for the intrinsic heterogeneity of the PPHA. Spherical and cylindrical double layer models gave equally good fits to the data.A salt dependence observed around pH 5 could not be completely removed by taking into account the electrostatic interactions. Hysteresis was observed to a much greater extent in the first titration cycle compared with the second cycle. This suggested that some slow and only partly reversible aggregation was occurring possibly as a result of the aggregation created during the purification of the humic acid. Titration curves for fully redispersed samples fitted the master curve approach (surface charge vs. surface pH) reasonably well but 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 KHint ∼ 4 and log KHint ∼ 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 kgt1¯. Double Tóth and double Langmuir-Freundlich isotherms fitted the data almost equally well but the implied distribution of sites between the more acidic “car☐ylic” sites and the weakly acidic “phenolic” 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.