Unveiling the selective cleavage-bond mechanism during the photocatalytic degradation process with pH-mediated BiVO4/BiPO4 dipole controlled and changed glyphosate electron cloud distribution

Abstract

The selective cleavage-bond of glyphosate is still a major challenge, especially in biodegradation and chemical degradation like advanced oxidation process (AOP). In this work, we found that the BiVO4 and BiVO4/BiPO4 heterogeneous photocatalysts generated different intermediates during the photocatalytic degradation of glyphosate. The main intermediate product by the BiVO4 photocatalyst was aminomethylphosphonic acid (AMPA, reaching to 81%) with high biological toxicity, which means glyphosate was degraded through the AMPA pathway. While the BiVO4/BiPO4 heterogeneous photocatalysts showed a significant inhibition of the accumulated AMPA (31%) at pH = 4, leading to the generation of benign products, such as sarcosine, acetic acid and PO43-. More importantly, the BiVO4/BiPO4-2 photocatalyst barely generated AMPA (less than 5%) at pH = 10. The lower AMPA concentration in our experiments was not attributed to its rapid degradation rate because AMPA could not be degraded from acidic (pH = 4) to alkaline (pH = 10). It indicated that glyphosate suffered from two cleavage-bond pathways between the BiVO4 and BiVO4/BiPO4 heterogeneous photocatalysts, which had been proved by mass spectrometry (MS) results. The attenuated total reflectance Fourier transforms infrared (ATR-FTIR) spectrum and density function theory (DFT) demonstrated that the adsorption sites of the BiVO4/BiPO4-2 photocatalyst occurred on the phosphate group of glyphosate molecules at pH = 10, which facilitated the C–P bond cleavage and inhibited the formation of AMPA. We found that pH-mediated BiVO4/BiPO4 dipole controlled and changed glyphosate electron cloud distribution, played an important role in the selective cleavage-bond of glyphosate. These results revealed both scientific and environmental significance for the efficient degradation of glyphosate through benign intermediates in soil and water.