Varying the hydrophobicity of humic matter by a phase-transfer-catalyzed O-alkylation reaction

Investigation into the role of carboxylic acid and phenolic hydroxyl groups in the plant biostimulant activity of a humic acid purified from an oxidized sub-bituminous coal

Introduction

Humic substances (HS) are increasingly being applied as crop plant biostimulants because they have been shown to increase plant productivity, especially under environmentally stressful conditions. There has been intense interest in elucidating the HS molecular structures responsible for eliciting the plant biostimulant response (PBR). The polar and weakly acidic carboxylic (COOH) and phenolic hydroxyl (ArOH) functional groups play major roles in the acid nature, pH dependent solubilities, conformation, and metal- and salt-binding capabilities of HS. Reports on the role played by these groups in the PBR of HS found growth parameters being both positively and negatively correlated with COOH and ArOH functionalities.

Materials and methods

To investigate the role of COOH and ArOH in HS biostimulant activity we used a humic acid (HA), purified from an oxidized sub bituminous coal to prepare HAs with COOH groups methylated (AHA), ArOH groups acetylated (OHA), and with both COOH and ArOH groups methylated (FHA). The original HA was designated (NHA). The four HAs were subjected to elemental, 13C-NMR, FTIR, and EPR analyses and their antioxidant properties were assessed using the trolox equivalents antioxidant capacity assay (TEAC). 13C-NMR and FTIR analysis revealed significant alkylation/acetylation. To determine the effects of alkylating/acetylating these functional groups on the HA elicited PBR, the HAs were evaluated in a plant bioassay on corn (Zea mays L.) seedling under nutrient and non-nutrient stressed conditions. Treatments consisted of the four HAs applied to the soil surface at a concentration of 80 mg C L-1, in 50 ml DI H2O with the control plants receiving 50ml DI H2O.

Results

The HA-treated plants, at both fertilization rates, were almost always significantly larger than their respective control plants. However, the differences produced under nutrient stress were always much greater than those produced under nutrient sufficiency, supporting previous reports that HA can reduce the effects of stress on plant growth. In addition, for the most part, the HAs with the alkylated/acetylated groups produced plants equal to or larger than plants treated with NHA.

Conclusion

These results suggests that COOH and ArOH groups play a limited or no role in the HA elicited PBR. Alternatively, the HA pro-oxidant to antioxidant ratio may play a role in the magnitude of the biostimulant response.

Enhanced washing of polycyclic aromatic hydrocarbons from contaminated soils by the empowered surfactant properties of de novo O-alkylated humic matter

Aqueous solutions of humic acid (HA) derivatized by a catalyzed O-alkylation reaction with methyl, pentyl, and benzyl groups at 40, 60, and 80% of total HA acidity were used to wash off polycyclic aromatic hydrocarbons (PAHs) from two contaminated soils. The enhanced surfactant properties enabled the alkylated HA to remove phenanthrene, anthracene, fluoranthene, and pyrene from both soils more extensively than the original unmodified HA, the 60% benzylation generally showing the greatest soil washing efficiency. For both soils, all alkylated HA revealed greater PAH removals than Triton X-100 nonionic surfactant, while the benzylated and methylated HA nearly and fully matched pollutants release by the anionic SDS in the coarse- and fine-textured soils, respectively. A consecutive second washing with 60% benzylated HA removed additional PAHs, in respect to the first washing, from the coarser-textured soil, except for fluoranthene, while removal from the finer-textured soil incremented even more for all PAHs. These findings indicate that the enhanced hydrophobicity obtained by a simple and unexpensive chemical derivatization of a natural humic surfactant can be usefully exploited in the washing of polluted soils, without being toxic to the soil biota and by potentially promoting the subsequent bio-attenuation of organic pollutants.