On the degradation pathway of glyphosate and glycine

Abstract

The degradation in water of the most widespread herbicide, glyphosate, is still under debate. Experimental disagreements on this process exist and there are only a few theoretical studies to support any conclusions. Moreover, the relationship between glyphosate and glycine is underestimated. Besides the structural similarity, glycine is a product of glyphosate degradation; hence, their studies are complementary. In this study, two mechanisms for the decomposition of the glyphosate molecule and glycine molecule in water are proposed. These mechanisms were explored by using quantum mechanical calculations. A combined microsolvation/PCM approach was employed to find and characterize their transition states, by which the reaction pathways were determined via the IRC method. The results have shown that the degradation processes might occur via a C-C bond cleavage, through a concerted mechanism, whereby the proton transfers and the CO2 detachments occur simultaneously. The second mechanism had two consecutive steps, a decarboxylation followed by the proton transfers. The water molecules served as a conduit for the proton transfers, away from the amine group (or the phosphonate, glyphosate case). Their function was to assist the reactions in a water-mediated decarboxylation. In these particular cases, the free energy of activation was 42.68 and 42.28 kcal mol-1 for the glycine structure and the glyphosate structure, respectively. These results agreed with the photodegradation and thermodegradation of glyphosate, as well as with the spontaneous decarboxylation of glycine. A concerted mechanism might be expected to yield C-P and C-N bond cleavages in the glyphosate molecule.

Two Birds, One Stone: Coupling Hydrogen Production with Herbicide Degradation over Metal–Organic Framework-Derived Titanium Dioxide

“…Water pollution is a major environmental challenge that has attracted widespread global concerns. One of the primary sources of water pollution is herbicides emanating from agricultural activities by leaching into water channels. − Similarly, climate change has drawn tremendous attention following the combustion and gradual depletion of fossil fuels, , resulting in an upsurge for alternative renewable energy sources such as hydrogen (H 2 ) from water splitting, owing to water being a rich proton source. As a means of addressing these challenges, photocatalysis has been considered a suitable alternative approach that can utilize the earth’s abundant solar energy in the presence of a photoactive catalyst to drive high energy conversion and environmental remediation.…”

“…Many alternative approaches employing catalysts, e.g., metal oxide semiconductors such as MnO 2 , ,,− and TiO 2 , are oxidatively effective in breaking down PMG. TiO 2 is one of the most widely studied metal oxide catalysts, owing to its excellent light absorption, chemical and thermal stability, cost-effectivity, and low toxicity. ,, TiO 2 exists in three distinct polymorphs of brookite, anatase, and rutile, with anatase being the most studied, and regarded as the most technologically relevant polymorph having the bandgap of 3.2 eV. , Thus, TiO 2 is an active photocatalyst that can drive the oxidation of organic molecules, and hydrogen production, utilizing reactive oxygen species (ROS) such as singlet oxygen ( 1 O 2 ), hydrogen peroxide (H 2 O 2 ), ozone (O 3 ), and hydroxyl radicals (OH • ) as the driving forces for the advanced oxidation process (AOP). , However, these approaches often result in incomplete oxidative breakdown of PMG or lead to the formation of AMPA as one of the degradation products, which is not environmentally favorable.…”

Dissipation and effect of glyphosate during composting of organic wastes

“…Glyphosate dissipation can be achieved through biotic degradation via catabolic enzymes activity and abiotic pathways (e.g., adsorption, photolysis, and thermolysis) (Singh et al., 2020). The main glyphosate degradation pathway in soils is microbial (Lopes Catão & López‐Castillo, 2018). Given both the intense microbial activity and the diversity of microorganisms in compost (Antunes et al., 2016), biotic degradation was most likely the major route of glyphosate degradation.…”

Effect of UV-A, UV-B and UV-C irradiation of glyphosate on photolysis and mitigation of aquatic toxicity

“…Even if the UV mediated photodegradation of glyphosate is incomplete, it may subsequently increase biodegradation in aquatic environments if the transformation products are less toxic and/or more biodegradable. The two main biodegradation pathways employed by glyphosate-degrading environmental microorganisms include cleavage of the C-N and the C-P bond converting glyphosate to AMPA and sarcosine 29 . Interestingly, AMPA and sarcosine were among the transformation products detected in the present study after UV-C irradiation of glyphosate.…”