Interactions between metals and humic species (HS) have been a subject of extensive studies because understanding of these interactions provides insight into a variety of environmental processes. HS are well known to be characterized by a variability of their molecular weights and functional groups; thus precise characterization of these properties are challenging. Prior research indicates that carboxylic and phenolic functional groups are major sites for metals complexation. Because these groups absorb and emit light, HS-metal interactions can be tracked by measurements of responses of HS chromophores to changes of system properties. Numerical modeling of HS-metal interactions can be carried out based on several approaches, for instance, Non-ideal Competitive Adsorption (NICA) model coupled with Donnan electrostatic sub-model but such modelling as well as the actual behaviour of HS in the environment are affected by ionic strength whose effects on HS molecular have not been quantified experimentally in sufficient detail.  In this study, we use the method of differential absorbance to ascertain effects of ionic strength and complexation with calcium and magnesium on NOM chromophores. Ionic strength variations were observed to cause consistent changes of differential linear and log-transformed spectra. These effects were consistent with predictions made based on NICA-Donnan model and indicative of increased deprotonation of NOM at higher ionic strengths. Spectroscopic measurements carried out for calcium, magnesium and, in less detail barium, showed the presence of mechanisms of hardness cation-HS interactions that were different for all these ions. Strong correlations were found to exist between the intensity of the differential spectra or other relevant parameters (e.g., spectral slopes) and the amount of hardness cations bound by HS. These results show that HS-cation interactions as well as the intrinsic chemistry of HS can be probed in detail via in situ approaches.

In situ characterization of effects of ionic strength on the chromophores of humic species and their interactions with hardness cations

248th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.