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 t...