Synthesis and characterization of fulvic acids in tre C-H-O, C-H-O-N and C-H-O-N-S systems : UV-visible spectroscopic investigations of uranyl(VI)-fulvate complexation as a function of pH at 25 degrees C and 1 bar

Abstract

Fulvic acids containing chemical elements (1) C,H,O, (2) C,H,O,N and (3) C,H,O,N,S have been synthesized from reactions in four mixtures of pure chemical reactants: (1) C6H4(OH)2 + NaOH, (2) C6H4(OH)2 + CH3COOH +NaOH, (3) C6H4(OH)2 + NH2CH2COOH + NaOH and (4) C6H4(OH)2 + NH2CH(CH2SH)COOH + NaOH, at pH = 10.0, in the dark and under oxidative conditions close to those encountered in the nature (ambient temperature and atmospheric conditions, f and f ) O2 ( g ) CO2( g ). Qualitative and semi-quantitative analysis of synthesized fulvic acids (SFA) using a combination of analytical techniques in electron microscopy, molecular spectrometry (ATR-FTIR, UV-visible, ESI-MS, APCI-MS), and X-ray diffraction have allowed to obtain the following results : - all four reaction mixtures have produced a large number of individual fulvic acids, whose m/z ratios can be as high as 943 and are amorphous; - the SFA contain functional groups: alcohol, ether and carboxylate. The addition of compounds such as CH3COOH, NH2CH2COOH and NH2CH(CH2SH)COOH led to fulvic acids containing respectively ester, amine and thiol functional groups. Finally, a systematic study of the interactions between the U(VI) ion and fulvate ligands containing three elements (C,H,O) as a function of pH (4 - 10) has been developed using UV-visible spectroscopy. Two types of U(VI)-fulvate complexes have been identified in pH range 4 ? 7 and three types of complexes at pH = 10. The presence of absorption broadbands indicates that polyuranyl (VI) ions are part of constituents of the U(VI) - fulvate complexes.