Michèle Negre

Analysis of carboxyl groups in soil humic acids by a wet chemical method, Fourier-Transform Infrared spectrophotometry, and solution-state carbon-13 nuclear magnetic resonance. A comparative study

Carboxyls are important functional groups that affect polarity and reactivity in humic acids (HAs). Carboxyls were analyzed in eight soil HAs by three methods based on widely differing principles: (i) wet chemical analysis, (ii) Fourier-Transform Infrared spectrophotometry (FT-IR), and (iii) liquid-state 13 C Nuclear Magnetic Resonance (13C NMR). The objective was to uncover the suitability of each of these methods for the analysis of COOH groups in HAs and the extent to which the three methods agreed with each other in quantitative measurements of COOH groups in HAs. In regard to reaction mechanisms, the chemical Ca-acetate method is based on ion-exchange of H of COOH for Ca of Ca-acetate. From FT-IR spectra of HAs, COOH groups were determined by totaling absorbances at 1720-1710 cm−1 (COOH) and 1620-1600 cm−1 (COO−), whereas from 13C NMR spectra of HAs, COOH groups were computed by integration of 175-185 ppm area. Good correlations were found between the three methods although the COOH values computed by 13C NMR were higher, as a result of the inclusion of small amounts of esters, amides, and lactones, than those obtained by the other two methods. Thus, depending on the equipment and facilities available, soil scientists have a choice of methods that can be used for determining COOH groups in HAs.

INTERACTION OF IMIDAZOLINONE HERBICIDES WITH SOIL HUMIC ACIDS. EXPERIMENTAL RESULTS AND MOLECULAR MODELING

Adsorption and desorption isotherms of the herbicides imazapyr, imazethapyr and imazaquin on a soil humic acid have been performed at pH 2.8 and 4.0 (below and above the pKa of the herbicides). At both pH, adsorption increased according to the lipophilic character of the molecules (imazapyr < imazethapyr ≪ imazaquin). The extent of adsorption was higher at pH 2.8 than at pH 4.0 because of the partial ionization of the carboxylic groups of both herbicides and humic acids at increasing pH. Desorption of imazapyr and imazethapyr was nearly complete at pH 4 and higher than 60% at pH 2.8 while desorption of imazaquin was 45 and 8% at pH 4 and 2.8, respectively. No differences between adsorption isotherms at 10°C and 25°C were observed a pH 4.0 indicating that adsorption involved very weak bonds while at pH 2.8, adsorption was higher at 10°C than at 25°C indicating an exothermic process. The isosteric enthalpy of adsorption of each herbicide was low (about − 1 kJoule mole−1) suggesting that low energetic bonds were involved. Adsorption on different humic acids has indicated that for each herbicide, the extent of adsorption expressed as Kd was correlated with the amount of carboxylic and aromatic groups of humic acids suggesting that hydrogen bonding and/or charge-transfer complexes formation could take place. Molecular modeling and geometry optimization of humic acid and soil organic matter (SOM) herbicide complexes were also performed. The results obtained with this theoretical approach gave a consistent chemical interpretation of the experimental results. To the best of our knowledge this is the first report to contribute to a better understanding of site-specific bonding of herbicides in SOM complexes by nanochemical modeling and distinct energy descriptors.

Adsorption of the herbicides imazapyr, imazethapyr and imazaquin on soils and humic acids

Abstract Adsorption of the imidazolinone herbicides imazapyr, imazethapyr and imazaquin have been studied on two humic acids and on ten soils. On humic acids, adsorption was promoted by low pH values and was in the order imazapyr < imazethapyr < imazaquin corresponding to the increasing lipophilicity of the molecules, in agreement with the octanol‐water partition coefficient. On soils, the correlations between the Freundlich coefficient Kf of the isothermes and the soil characteristics provided evidence that for each herbicide studied, the soil components mainly involved in the process were organic matter, amorphous iron oxides and iron‐organic matter complexes. Adsorption was promoted by low pH values. However, the order of adsorption in soils was imazapyr ≍ imazaquin < imazethapyr though imazaquin exhibited the highest affinity for soil humic acids. An hypothesis for this behaviour could be that the big and rigid quinoline moiety of the imazaquin molecule does not allow it to reach the adsorption sites ...

Effect of the chemical composition of soil humic acids on their viscosity, surface pressure, and morphology

Humic acids were extracted from two histosols, an andosol, an inceptisol, a rice field soil, and a sludge-amended cambisol and characterized by elemental analysis, acidic groups titration, E4/E6 determination, Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (13C NMR) spectroscopy, viscosity and surface pressure measurements, and scanning electron microscopy. Relevant compositional differences between humic acids depending on their formation conditions were observed. Humic acids from the histosols and the andosol were largely aromatic and rich in acidic functional groups. The inceptisol humic acid contained about the same amount of aliphatic and aromatic moieties and was particularly abundant in soil, probably because it is stabilized by strong interactions with the inorganic components. The paddy soil humic acid was mainly aliphatic and poor in acidic groups because of the partial anaerobic conditions of the rice field. The sludge-amended soil humic acid was similar to humic acids from rapidly decomposed organic matter: low content of carboxylic groups and high content of proteinaceous material. The chemical composition affected, to some extent, the physical behavior of the humic acids. The surface tension was negatively correlated with the concentration of carboxylic groups. The presence of polysaccharides and proteinaceous moieties promoted elongated conformations of humic molecules in solution and directly related to the intrinsic viscosity and consequently to the molecular dimension of humic molecules. The largest aggregates in dry state were observed in humic acids with, besides a high aromaticity, a high content of carboxylic groups.

Processed vs. non-processed biowastes for agriculture: effects of post-harvest tomato plants and biochar on radish growth, chlorophyll content and protein production.

The aim of this work was to address the issue of processed vs. non-processed biowastes for agriculture, by comparing materials widely differing for the amount of process energy consumption. Thus, residual post harvest tomato plants (TP), the TP hydrolysates obtained at pH 13 and 60 °C, and two known biochar products obtained by 650 °C pyrolysis were prepared. All products were characterized and used in a cultivation of radish plants. The chemical composition and molecular nature of the materials was investigated by solid state 13C NMR spectrometry, elemental analysis and potentiometric titration. The plants were analysed for growth and content of chlorophyll, carotenoids and soluble proteins. The results show that the TP and the alkaline hydrolysates contain lignin, hemicellulose, protein, peptide and/or amino acids moieties, and several mineral elements. The biochar samples contain also similar mineral elements, but the organic fraction is characterized mainly by fused aromatic rings. All materials had a positive effect on radish growth, mainly on the diameter of roots. The best performances in terms of plant growth were given by miscanthus originated biochar and TP. The most significant effect was the enhancement of soluble protein content in the plants treated with the lowest energy consumption non processed TP. The significance of these findings for agriculture and the environment is discussed.

ADSORPTION OF IMIDAZOLINONE HERBICIDES ON FERRIHYDRITE-HUMIC ACID ASSOCIATIONS

Adsorption of the imidazolinone herbicides imazapyr, imazethapyr and imazaquin was studied on two binary systems (ferrihydrite-humic acid) prepared by treating ferrihydrite (Fh) immediately after its precipitation with a soil humic acid (HA) at different loadings (4% and 8% HA content), and on a blank ferrihydrite sample prepared in the same way, but without HA addition. Imidazolinone adsorption on pure Fh and on the 4% Fh-HA decreased with increasing of the herbicide hydrophobicity (imazaquin<imazethapyr<imazapyr), whereas on the 8% Fh-HA system the order of adsorption was nearly the same, indicating that higher amounts of HA on Fh increase the hydrophobicity of the surface and thus increases its affinity for the herbicides, specially for imazaquin. Adsorption was also enhanced by low pH values and slight changes in pH enhanced the contribution of each component in adsorption. Both the nature of the surface exposed and its ability to interact with herbicides are greatly altered by the mutual interaction of Fh and HA. The results show that albeit artificial polyphasic associations are more representative of the real situation than single model components and are thus better indicators of the behaviour of a given herbicide in the soil.

Removal of fenhexamid and pyrimethanil from aqueous solutions by clays and organoclays.

The ability of a sodium montmorillonite (CLONa) and two commercial available organoclays having interlayer organic cations possessing different functional groups (CLO20A and CLO30B) was investigated for adsorbing two pesticides namely fenexamid (FEX) and pyrimethanyl (PMT). The two organoclays displayed a higher affinity with the pesticides than the unmodified clay, but the improvement in adsorption capacity varied according to the characteristics of the pesticide and the interlayer organic cation. FEX was adsorbed to a greater extent than PMT by both organoclays, which may be due to the higher hydrophobicity of FEX thereby indicating considerable hydrophobic interaction between the adsorbent/adsorbate systems. Our findings may find application in the removal of water-soluble pesticides from aquifers.

Organo-clays and nanosponges for acquifer bioremediation: Adsorption and degradation of triclopyr

To avoid the problem of groundwater contamination, mitigation techniques have been proposed that consist of creating barriers made of suitable materials that can facilitate the adsorption and degradation of the pollutants. This study aims at evaluating the capacity of two organo-clays (Dellite 67 G and Dellite 43 B) and one nanosponge to adsorb the herbicide, triclopyr. Triclopyr was chosen because it is a good example of a moderately mobile, leacheable molecule. The rate of degradation of the molecule in the soil, both with and without the presence of the materials under examination, was also determined. Both the organo-clays adsorbed more than 90% of the herbicide. The nanosponge and the soil adsorbed less than 10% triclopyr. When the soil was added with the two organoclays, adsorption increased to 92%. When added to the soil, the materials accelerated the degradation of triclopyr. The half-life in soil was 30 days, whereas in soil with Dellite 67 G and Dellite 43 B it was 10 and 6 days respectively. The addition of the nanosponge to the soil decreased the half life by 50%. These results lead us to suggest that they be used in creating reactive barriers for the remediation of soils and aquifers.

Fate and degradation of triasulfuron in soil and water under laboratory conditions

The behavior and fate of triasulfuron (TRS) in water and soil systems were examined in laboratory studies. The degradation of TRS in both buffer solution and soil was highly pH-sensitive. The rate of degradation could be described with a pseudo first-order kinetic and was much faster at pH 4 than at pH 7 and 9. Aqueous hydrolysis occurred by cleavage of the sulfonylurea bridge to form 2-(2-chloroethoxy) benzenesulfonamide (CBSA) and [(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] (AMMT). AMMT was unstable in aqueous solutions in any pH condition but it degraded more quickly at pH 4 and 9. CBSA did not degrade in aqueous solutions or in enriched cultures but it underwent a quick degradation in the soil. The rates of TRS degradation in sterile and non-sterile soils were similar, suggesting that microorganisms played a minimal role in the breakdown process. This hypothesis is supported by the results of studies on the degradation of TRS by enriched cultures during which the molecule underwent a prevalently chemical degradation.

Bioherbicidal activity of a germacranolide sesquiterpene dilactone from Ambrosia artemisiifolia L.

ABSTRACT Ambrosia artemisiifolia L. (common ragweed) is an invasive plant whose allelopathic properties have been suggested by its field behaviour and demonstrated through phytotoxicity bioassays. However, the nature of the molecules responsible for the allelopathic activity of common ragweed has not been explored. The main objective of this study was to identify the phytotoxic molecules produced by A. artemisiifolia. A preliminary investigation has indicated that a methanol extract of A. artemisiifolia completely inhibited the germination of cress and radish. Semi-preparative fractionation of the methanol extract allowed separating of phytotoxic fraction which contained a single compound. The structure of this compound was elucidated by liquid chromatography–mass spectrometry (LC-MS)/MS, high-resolution mass spectral, nuclear magnetic resonance, and Fourier transform infrared spectra as sesquiterpene lactone isabelin (C15H16O4). The effect of pure isabelin was tested on four different weed species, confirming the inhibitory activity of molecule. The results indicate directions for the future studies about herbicidal specific activity of isabelin, as pure molecule or in the crude extract, as a potential candidate for biological weed control.