Structure et evolution d'un modele d'acide humique sous l'effet de l'ozonation

Abstract

Humic acids are widely studied as complexing agents in surface waters, but little is known about the chemical structure of their potential sites of interaction with micropollutants. By means of powerful spectroscopic techniques such as u.v.-visible, infrared and electron spin resonance spectrometries run on a commercial model Fluka (AHF), we carried out the structural investigation of the concerned functional groups and studied their evolution under ozonation. Consecutively to the consumption of 1.24 mg ozone per mg of AHF, one can see (Table 1) that the decrease in phenol groups (21%) can be matched with that of aromaticity (16%). Moreover, an original study performed on ultrafiltration fractions: (1) M > 3.10 5; (2) 3.10 5 > M > 5.10 4; (3) 5.10 4 > M > 5.10 3; (4) M < 5.10 3 showed that half of the hydroxyl group disappears from fraction 1. Relatively high values of molar extinction coefficients ( ϵ 254nm = 360,000 and ϵ 203nm = 458,000) suggest a polysubstitution state for aromatic cycles and this has been confirmed by infrared spectrometry. The relative position of benzenic substituents cannot be inferred from u.v. visible data (Table 2) but the well-resolved infrared spectra (Fig. 1 and Table 3) reveal the presence of phenolic, benzoate, phtalate and salicylate groups close to semiquinonic anion radicals, aromatic units being linked by some alcanoic chains, most of them formed by four methylene groups. The proposed structural scheme (Fig. 4) is in agreement with Murray and Linder's (1983). Investigation of the infrared characteristics of the four ultrafiltration fractions shows that these functional groups are not equally distributed over the molecular weight range. Polysubstituted aromatic cycles, phenolic and quinonic structures are mainly detected in fractions (1) and (2), whereas acid and ester groups are significantly observed in fractions (3) and (4). After ozonation, the 1720, 1700 and 1350 cm −1 bands, assigned to stretching valence vibrations of aromatic esters are depressed. For fraction (I), ozonation of aromatic structures is substantial (Fig. 2 and Table 4), as it is expected for polysubstituted molecules (Decoret et al., 1984; Gilbert, 1978). Less substituted structures are divided on fractions (2) (3) and (4). Infrared spectra of fractions (3) and (4) are markedly disturbed. This reveals that quinonic and ethylenic groups increase as aromatic cycles do (780 cm −1 band). Electron spin resonance investigation (Fig. 3 and Table 5) shows that AHF and its fractions (1) and (2) are endowed with electronic paramagnetism. As g values are close to 2.0040 and as this property disappears after ozonation, it must be due to semiquinonic anion radicals partly maintaining aggregative structures (Steelink et al., 1983; Wershaw et al., 1977), leading to quinonic structures by oxidation.