A Multiscale Modeling Approach to Understand the Degradation of Glyphosate under Dark and Light Conditions

Glyphosate is an organophosphate herbicide that has been used for decades in agriculture. Its potential to cause carcinogenic effects and many other neurological diseases in humans has been well reported. Due to its significant environmental impact, considerable attention has been given to the removal of glyphosate from soil. A promising route is the degradation of glyphosate induced by titanium dioxide surfaces. Even though a number of experimental and theoretical studies have already addressed this topic, the exact mechanistic pathways of the degradation process, depending on the environmental conditions, are still unknown. In this work we use a multiscale approach based on classical and ab initio molecular dynamics combined with ground-state and excited-state electronic-structure simulations to unravel the degradation pathways under dark and light conditions. A detailed chemical bond analysis of the most predominant adsorption configuration of glyphosate on TiO2 is used to discuss the molecular degradation in terms of molecular strain, charge transfer, and occupation of antibonding orbitals upon adsorption. Transition-state analyses of competitive degradation pathways suggest that aminomethylphosphonic acid and sarcosine are the main initial degradation products under dark and light conditions, respectively.

草甘膦（Glyphosate）是一种已在农业领域应用数十年的有机磷除草剂。其对人类具有致癌性及诱发多种神经系统疾病的潜在风险，已有诸多文献报道。鉴于其对环境的显著负面影响，从土壤中去除草甘膦已受到广泛关注。其中一种颇具前景的技术路径是借助二氧化钛（titanium dioxide）表面诱导草甘膦降解。尽管已有诸多实验与理论研究围绕该主题展开，但降解过程的确切机理路径仍受环境条件影响，目前尚未明确。本研究采用结合经典分子动力学与从头算分子动力学（ab initio molecular dynamics）的多尺度方法，并辅以基态与激发态电子结构模拟，以阐明黑暗与光照条件下的草甘膦降解路径。研究通过对草甘膦在TiO2表面最占优势的吸附构型开展详细的化学键分析，从分子张力、电荷转移以及吸附后反键轨道占据情况等维度，探讨了其分子降解机制。对竞争性降解路径的过渡态分析表明，氨甲基膦酸与肌氨酸分别为黑暗与光照条件下的主要初始降解产物。