Applications of new surface analysis techniques (NMA and XPS) to humic substances

This paper illustrates the potentialities and the innovative character of surface analysis techniques such as nuclear microprobe analysis (NMA) and X-ray photoelectron spectroscopy (XPS) in the research field of humic substances (HS), in particular to characterize HS/trace elements association. NMA showed its potentialities to characterize HS/trace element associations at the microscopic scale and quantification of trace elements scavenged by humic acids (HA) and distribution and thickness of HA films sorbed onto silica surfaces after sorption tests. XPS was used to determine the chemical environment of iodine in natural HS. These techniques enabled us to establish association between HS colloids and numerous trace elements (both cations and anions, especially I) and to evidence a strong affinity of the smallest HA colloids (<15 nm) for these elements. Moreover, associations between HS and I were clearly evidenced by NMA, through the study of the ternary system (SiO2/HA/I), as well as of natural HS. Chemical bonds implied in these associations may be of covalent character, as seen from XPS data. Combination of these techniques with Ion Coupled Plasma Mass Spectrometry (ICP-MS) was also necessary to analyze the aqueous solutions in contact with HS.