A STUDY OF TERNARY COMPLEXES OF CM(III) WITH HUMIC-ACID AND HYDROXIDEOR CARBONATE IN NEUTRAL PH RANGE BY TIME-RESOLVED LASER FLUORESCENCE SPECTROSCOPY

The speciation of Cm(III) in a natural groundwater, rich in humic subs tances, is carried out in the pH range from 6.0 to 9.5 by time-resolve d laser fluorescence spectroscopy in the absence of CO2 acid under 1% CO2 partial pressure. The interaction of Cm(III) with humic substances in the neutral and near alkaline pH range is much stronger than predi cted by a thermodynamic calculation based on known stability constants for the humate complexation. This leads to a postulation for the form ation of ternary complexes of Cm with humic acid and hydroxide or carb onate ions. Stability constants for the formation of Cm(OH)HA, Cm(OH)( 2)HA and Cm(CO3)HA are determined spectroscopically by deconvolution o f fluorescence emission spectra. The formation of such mixed complexes is also evidenced by excitation spectra and lifetime measurements, an d also by ultrafiltration. The results show that the simple humic comp lexes observed below pH 6 may not be extrapolated directly to the high er pH range.