Effects of UV-visible irradiation on natural organic matter from the Amazon basin.

The aim of this work was to evaluate the effects of UV-visible irradiation on organic carbon concentration, fluorescence intensity and metal transport ability of the natural organic matter (NOM) at fixed pH values and under several types of atmosphere (air, N(2), O(2)). The water samples were obtained from various sampling sites along the Rio Negro (Amazon basin) and the NOM metal transport ability was determined with regard to Cu(2+). As a first step we measured the concentration of total organic carbon (TOC) and its fluorescence intensity before and after an irradiation for a given time. The results demonstrated that when photodegradation happened, the dominant reactions involved O(2) and the fluorescent sites were among the first to be altered. Then we determined by spectrofluorometry the complexing capacity (C(L)) before and after irradiation. Fluorescence and complexing site densities (C(L)/TOC ratio) increased after a short irradiation time (20 min, i.e. before the photodegradation happened) and decreased after longer irradiation times. Therefore, we conclude that a change of the macromolecular structure activates previously inactive fluorophores and complexing sites prior to photodegradation.

Excitation-emission fluorescence matrix to study pH influence on organic matter fluorescence in the Amazon basin rivers.

A study of the three-dimensional excitation-emission fluorescence matrix (EEFM) of organic matter from the Amazon basin rivers is reported in this paper. The EEFM, applied to the fractions obtained from sequential tangential ultrafiltration (STUF), give spectroscopic informations on the fluorescent properties of particulate (>0.22 microm), colloidal and dissolved (< 5 kD) organic matter. STUF process does not seem to alter the characteristic peaks position of humic material. Two characteristic excitation-emission peaks were observed in all fractions of the samples investigated, one stimulated by UV excitation (peak A) and one by visible excitation (peak C). Increasing the pH, fluorescence intensities of the fluorophores A and C, respectively, I(A) and I(C), responded in the same manner to pH, increasing with increasing pH. However, the fluorophore A seems more sensitive to this pH variation because its fluorescence intensity enhancement is always higher. Thus the chemical behaviour of the two fluorophores must be somewhat differentiated. The evolution of the I(A)/I(C) ratio with pH is studied for all samples. This investigation showed a linear relation between I(A)/I(C) and pH. Moreover, this behaviour is water type (black or white) and molecular size independent. Indeed, this linear relationship is observed for all samples and all fractions.