Structure-Dependent Oxidation Mechanisms of Glyphosate by Manganese Oxides: Role of Mineralogy and pH

Abstract

The environmental persistence of glyphosate and its transformation into aminomethylphosphonic acid (AMPA) necessitate a mechanistic understanding of its abiotic degradation in natural environments. Manganese (Mn) oxides are ubiquitous redox-active minerals capable of oxidizing glyphosate; however, how Mn oxide mineralogy and pH regulate degradation kinetics, pathway selectivity, and interfacial redox processes remains unclear. Here, we investigated glyphosate oxidation by three representative Mn oxides, layered birnessite, disordered-layered vernadite, and tunnel-structured cryptomelane, across pH 3–7. Vernadite exhibited the highest and most sustained oxidative reactivity, cryptomelane showed rapid but strongly pH-dependent oxidation, and birnessite displayed slower yet persistent reactivity. Mineral structure controlled product selectivity: under near-neutral conditions, layered Mn oxides favored glycine formation through C–N bond cleavage adjacent to the phosphonate group, whereas tunnel-structured cryptomelane promoted AMPA accumulation through cleavage near the carboxyl group. Acidic conditions (pH 3 and 5) enhanced AMPA accumulation across all Mn oxides, suggesting increased persistence risks. Spectroscopic and kinetic analyses demonstrated that glyphosate oxidation involved surface complexation with Mn(IV), interfacial electron transfer, and propagation through transient, labile Mn(III) intermediates. These findings reveal that Mn oxide mineralogy and pH jointly govern bond-selective glyphosate oxidation and AMPA accumulation, advancing mechanistic predictions of glyphosate fate in soil and aquatic environments.