Indirect Photodegradation of Sulfamethoxazole and Trimethoprim by Hydroxyl Radicals in Aquatic Environment: Mechanisms, Transformation Products and Eco-Toxicity Evaluation.

Jiaoxue Yang,Xiaomin Sun,Guochun Lv,Chenxi Zhang,Zehua Wang

doi:10.3390/ijms21176276

Jiaoxue Yang, Xiaomin Sun + Show 3 more

Open Access

https://doi.org/10.3390/ijms21176276

Copy DOI

Abstract

The bacteriostatic antibiotics, sulfamethoxazole (SMX) and trimethoprim (TMP), have frequently been found in wastewater and surface water, which raises the concerns about their ecotoxicological effects. The indirect photochemical transformation has been proven to be an efficient way to degrade SMX and TMP. In this study, the reaction mechanisms of the degradation by SMX and TMF by OH radicals were investigated by theoretical calculations. Corresponding rate constants were determined and the eco-toxicity of SMX and TMP and its degradations products were predicted using theoretical models. The results indicate that the most favorable pathways for the transformation of SMX and TMP are both •OH-addition reaction of benzene ring site with lowest Gibbs free energy barriers (6.86 and 6.21 kcal mol−1). It was found that the overall reaction rate constants of •OH-initial reaction of SMX and TMP are 1.28 × 108 M−1 s−1 and 6.21 × 108 M−1 s−1 at 298 K, respectively. When comparing the eco-toxicity of transformation products with parent SMX and TMP, it can be concluded that the acute and chronic toxicities of the degraded products are reduced, but some products remain harmful for organisms, especially for daphnid (toxic or very toxic level). This study can give greater insight into the degradation of SMX and TMP by •OH through theoretical calculations in aquatic environment.

Highlights

Pharmaceuticals and personal care products (PPCPs) as emerging organic micropollutants in the environment have been receiving increased attention due to their possible adverse effects on the aquatic organism and human health [1,2,3,4,5]
Intrinsic reaction coordinate (IRC) calculations [49] are analyzed in order to verify the transition states that are connected the reactants with their corresponding products for the transformation pathway
The transformation mechanisms, rate constants, and ecological risks for the OH-initiated degradation process of SMX and TMP have been studied by quantum chemistry and computational toxicology methods

Summary

Introduction

Pharmaceuticals and personal care products (PPCPs) as emerging organic micropollutants in the environment have been receiving increased attention due to their possible adverse effects on the aquatic organism and human health [1,2,3,4,5]. Excessive amounts of SMX and TMP are discharged into water for a wide range transmission to wastewater treatment plants [10], which cannot be effectively removed by conventional wastewater treatment technologies [11,12,13,14]. This has led to widespread transport of pharmaceutical contaminants in aquatic environment around the world [15]. SMX has regularly been detected in wastewater effluents and surface water with a concentration range of 100–2500 ng/L and 60–150 ng/L, even in drinking water at 12 ng/L [16,17,18]. Previous studies have reported that the residual concentration of TMP are 0.1–5 μg/L in wastewater effluents

Objectives

Methods

Results

Conclusion