(Invited) Electrooxidation Pathway and Kinetics for Aminomethyl Phosphonic Acid (AMPA) Degradation in Diluted Water Matrices

Abstract

The U.S. agriculture and related industries consume 280 million pounds of glyphosate each year. Most applied glyphosate is released into the aquatic environment and biochemically transformed into Aminomethyl Phosphonic Acid (AMPA). The generated AMPA can trigger eutrophication, bioaccumulation in food webs, potential cell damage, and embryonic mortality in living organisms. To effectively control AMPA, biological retention systems (e.g., wetlands) have been utilized to degrade AMPA. However, bioretention systems are characterized by slow degradation kinetics, a large footprint, and low resilience toward shock loads. This study proposes an alternative electrooxidation system as an abiotic, chemical-free approach to transforming organic AMPA into easy-to-separate inorganic phosphate. Our system selected a boron-doped diamond as the anode due to its wide electrochemical window and high electrochemical stability. We comprehensively explored AMPA degradation pathway, kinetics, and system energy requirement in diluted water matrices. Initial 24-h batch tests revealed more than 83% of 0.01-mM AMPA were electrooxidized by hydroxyl radicals under a chronoamperometry mode. The highest oxidation rate could reach 12 mmol AMPA m-2 h-1 in the first 6 hours. The oxidation end products are determined to be CO2, NH4 +, and PO4 3-, indicating complete mineralization of organic C, N, and P within AMPA molecules. We further evaluated three electrooxidation operating modes, including chronoamperometry (CA), chronopotentiometry (CP), and applied voltage (AV). All three operating modes exhibited >80% AMPA oxidation in 24 hours. Eventually, we selected the AV mode for a subsequent long-term test due to its higher compatibility with on-site applications. During this 30-day operation, our flow-through electrooxidation system was operated under a hydraulic retention time of 3-12 hours and was able to degrade a maximum AMPA loading of 12 mmol AMPA m-2 h-1. Our promising results confirm the capability of our electrochemical retention system to degrade AMPA in diluted water matrices and warrant future efforts to probe parasitic reactions for reduced system energy input.