Comparing the abiotic removal of glyphosate by ß-MnO2 and δ-MnO2 colloids: Insights into kinetics and mechanisms.

ABSTRACT

Glyphosate as an effective broad-spectrum herbicide is frequently detected in various water and soil resources. Given the ubiquity of ß-MnO2 and δ-MnO2 colloids in groundwater and soil, the abiotic removal of glyphosate by MnO2 colloids was investigated. ß-MnO2 colloids exhibited superior glyphosate removal efficiency, up to 37%, compared to 21% for δ-MnO2 colloids at a pH of 4.0. Glyphosate removal involved simultaneous adsorption and oxidation process, identified by HRTEM, NH3-TPD, XPS, LC-MS, FTIR analyses and the occurrence of aminomethylphosphonic acid (AMPA) and Mn2+. Moreover, adsorption dominated the removal of glyphosate by two MnO2 colloids. The solution pH had a substantial effect on glyphosate removal. Co-existing ions in the solution, such as carbonate (CO32-), phosphate (Na2HPO4, NaH2PO4) and humic acid (HA), were also found to impede glyphosate removal. Phosphate, in particular, exhibited a strong competitive effect for adsorption sites on both MnO2 colloids. Of them, the removal of glyphosate by ß-MnO2 colloids was more prone to occur due to its higher specific surface area, abundant oxygen vacancies, and moderate acid sites. However, δ-MnO2 colloids presented a stronger oxidation capacity than that of ß-MnO2 colloids due to the quicker generation rate of Mn2+. Finally, AMPA was the same products by two MnO2 colloids in the oxidation process, revealing the degradation pathway based on the cleavage of C-N bond. Therefore, by comparing kinetics and mechanisms of glyphosate removal by ß- and δ-MnO2 colloids, this study improves us better understanding for the behavior of glyphosate in the environment.