Conformations and aggregate structures of sorbed natural organic matter onmuscovite and hematite

In-solution atomic-force microscopy was used to characterize molecular dime nsions and aggregate structures of natural organic matter (NOM) sorbed to t he 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 ca tion electrolyte identity (Ca, Na, Li added as Cl-salts). Electrolyte ident ity and concentration exerted important effects on image stability, particl e size, and sorption density. On mica, the presence of either Ca2+ or Li+ l ed 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 CaCl2 and LiCl solutions, we observed adsorbed spheres with diameters appropriate for single molecules. A higher apparent adsorption density was observed in CaC l2 than in LiCl, consistent with previous reports that Ca enhances NOM adso rption. In LiCl, the spheres often were aggregated into small groups wherea s in CaCl2, they were mostly isolated. At intermediate pH and relatively low NOM concentrations on mica, larger sp herical aggregates were observed, but at higher NOM concentrations, we obse rved ring structures with nanoporosity that could be important for partitio ning of organic pollutants. At high pH on mica, we observed an highly order ed array which most probably indicates reordering of the mica surface struc ture 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 spher ical NOM aggregates were observed, consistent with observations by Myneni e t al. (1999) using in solution X-ray imaging. Overall our results demonstra te that NOM sorbs in complex structures and aggregates. Therefore, current models of NOM sorption are likely overly simplistic and require further dir ect verification. Copyright (C) 2000 Elsevier Science Ltd.