Competition of Sorption and Degradation Reactions during Glyphosate Degradation by Ferrihydrite/δ-Manganese Oxide Composites

Ever since its introduction into the herbicidal industry in the 1970s, glyphosate has been a very effective weed control herbicide and its application has steadily increased; however, at the same time, its toxicity has been increasingly questioned. In the present research, ferrihydrite/δ-MnO₂ composites were synthesized at different Mn/Fe molar ratios and competition of sorption and degradation of glyphosate on these minerals was analyzed using colorimetric, NMR, and high-performance liquid chromatography (HPLC) methods. Our results showed that the degradation of glyphosate was accomplished exclusively by δ-MnO₂, whereas both ferrihydrite and δ-MnO₂ played important roles in the sorption reaction. The difference, however, is that glyphosate that sorbed onto δ-MnO₂ underwent rapid degradation. Competition between sorption versus degradation was controlled by the molar ratio of Mn and Fe. For example, at low Mn/Fe ratios (<0.0167), the dominant mechanism of glyphosate removal from the aqueous solution was through sorption by ferrihydrite, although the extent of removal was low. When the Mn/Fe ratio was higher than 0.0167, the degradation reaction dominated. The direct CP bond cleavage to generate glycine was the major degradation pathway and the aminomethylphosphonic acid (AMPA) pathway was minor. Changes in the Mn/Fe ratios incurred no detectable influence on the preference for either of the two degradation pathways. Present results highlighted the additive effects of the two oxides that are commonly present in soils on the degradation and sorption of glyphosate thus reducing the negative impacts of glyphosate in the environment.