Chemical and physicochemical characteristics of humic acids extracted from compost, soil and amended soil

In order to gain understanding about how "HA-like substances" from organic amendments may change some properties in the soil solution, the knowledge of chemical and physicochemical characteristics (charge development, acid-base behavior and heterogeneity) should be known. The aim of this research were (i) to study the elemental and functional composition, (ii) to determine charge behavior, acid-base properties (apparent dissociation constant and buffer capacity) and (iii) to evaluate heterogeneity of humic acids (HA) isolated from municipal solid waste compost (MWC) and from the corresponding MWC-amended soil, in comparison to those of the unamended soil HA using potentiometric titration and differential scanning potentiometry (DSP). Potentiometric titration and the first derivative of -Q versus pH (negative charge development versus pH) curves could be used to determine proton-affinity distribution and the chemical heterogeneity of the HA as well as the average pK(app) and buffer capacity in a wide range of pH. Differential scanning potentiometry allows determination of the pK(app) values in a narrower range of pH than potentiometric titrations and is another simple methodology to study acid-base behavior of HA. DSP allows us to determine seven different pK(app) values for HA-S and HA-E and four different pK(app) for HA-C. Each one of these values corresponds to known acidic groups that can be present in the macromolecule of HA.