Proton binding to humic acids from organic amendments and amended soils by the NICA-Donnan model

The acid-base properties of humic acids (HAs) are known to significantly affect the acid-base buffering capacity of soils, thus having a marked influence on the speciation of cations in the soil solid and liquid phases. Detailed information on the proton binding behavior of humic-like acids (HALs) from organic amendments and humic acids (HAs) from amended soils is, therefore, of intrinsic interest for the evaluation of the agronomic efficacy and environmental impact of soil amendment. In this work, the acid-base properties of HLAs isolated from sewage sludge (SS) and municipal solid waste compost (MSWC), and HAs isolated from soils amended with either SS or MSWC and the corresponding nonamended control soils were investigated by potentiometric titrations at various ionic strengths (0.01, 0.05, 0.1, and 0.3 M) over the pH range from 3.5 to 10.5. The nonideal competitive adsorption (NICA)-Donnan model that describes proton binding by two classes of binding sites with low and high proton affinity, i.e., carboxylic- and phenolic-type groups, was fit to titration data, and a set of fitting parameters was obtained for each HLA and HA sample. The NICA-Donnan model successfully described the shapes of the titration curves, and highlighted substantial differences in site density and proton-binding affinity between the HLAs and HAs examined. With respect to the nonamended control soil HAs, SS-HLA and MSWC-HLA were characterized by smaller carboxylic-type and phenolic-type group contents, larger affinities for proton binding by the carboxylic-type groups, and smaller affinities for proton binding by the phenolic-type groups. Amendment with SS and MSWC determined a number of modifications in soil HAs, including decrease of acidic functional group contents, slight increase of proton affinity of carboxylic-type groups, and slight decrease of the affinities for proton binding by phenolic-type groups. These effects were more evident in the HA fraction from the SS-amended soil than in the HA fraction from the MSWC-amended soil. Thus, both organic amendments examined can be a considered as a valuable source of organic matter for soil. However, MSWC appears to be an amendment of greater quality producing a smaller impact than SS on proton-binding behavior of soil HA.