Conformations and aggregate structures of sorbed natural organic matter on muscovite and hematite

Abstract

In-solution atomic-force microscopy was used to characterize molecular dimensions and aggregate structures of natural organic matter (NOM) sorbed to the basal-plane surfaces of muscovite and hematite as a function of pH (3–11), ionic strength (0.001–0.3 M), NOM concentration (20–100 mgC L −1), and cation electrolyte identity (Ca, Na, Li added as Cl-salts). Electrolyte identity and concentration exerted important effects on image stability, particle size, and sorption density. On mica, the presence of either Ca 2+ or Li + led to more stable images than Na +. For example, at pH 3, we were unable to obtain stable images of NOM on mica in NaCl. However, at pH 3 in CaCl 2 and LiCl solutions, we observed adsorbed spheres with diameters appropriate for single molecules. A higher apparent adsorption density was observed in CaCl 2 than in LiCl, consistent with previous reports that Ca enhances NOM adsorption. In LiCl, the spheres often were aggregated into small groups whereas in CaCl 2, they were mostly isolated. At intermediate pH and relatively low NOM concentrations on mica, larger spherical aggregates were observed, but at higher NOM concentrations, we observed ring structures with nanoporosity that could be important for partitioning of organic pollutants. At high pH on mica, we observed an highly ordered array which most probably indicates reordering of the mica surface structure as Si released by dissolution interacts with NOM. This indicates that NOM may play an important role in phyllosilicate diagenesis. On hematite at pH 4 and in the presence of high dissolved iron concentration, large spherical NOM aggregates were observed, consistent with observations by Myneni et al. (1999) using in solution X-ray imaging. Overall our results demonstrate that NOM sorbs in complex structures and aggregates. Therefore, current models of NOM sorption are likely overly simplistic and require further direct verification.