Degradation of antiviral pharmaceuticals with ozonation and ldh-based catalytic ozonation in the presence of microplastics and toxicity evaluation

Abstract

During the COVID-19 (Coronavirus disease of 2019) pandemic, various pharmaceuticals were used in high concentrations for treatment of the patients. Due to the high number of people infected, the antivirals and their metabolites have been expected to be present at municipal wastewaters at relatively high concentrations. In case of partial- or no degradation in wastewater treatment systems, their release into aquatic systems could affect non-target organisms and hence the environment adversely. For instance, Oseltamivir, an anti-influenza pharmaceutical is shown to have low biodegradability in wastewater treatment plants. Therefore, two antivirals (Oseltamivir and Favipiravir which have been used during the pandemic in a high number of countries) were subjected to ozonation and catalytic ozonation experiments for the evaluation of their treatability. In addition, ecotoxicological impacts were investigated by repeating the experiments in the presence of microplastics since co-existence of antivirals and microplastics in the environment may enhance their toxicity possibility and degree. Experiments were conducted in 250- and 500-mL reactors with two different specific ozone doses of 0.2 mg O3/mg DOCo(dissolved oxygen carbon) and 1 mg O3/mg DOCo at pH values of pH: 7±0.1 and pH: 10±0.1. These pH values were adjusted initially by using phosphate buffer and sodium hydroxide. The experiments were carried out with low and high concentrations (500 ng/L and 50 µg/L) of the antivirals in distilled water. For catalytic ozonation, 0.1 g/L of ZnFe LDH (layered double hydroxide), a nanocomposite, was added as the catalyst. ZnFe (LDH-based) was prepared by coprecipitation and applied as a catalyst in heterogeneous catalytic ozonation of the selected antivirals. The structure of the synthesized catalyst was investigated by X-ray diffraction (XRD) patterns and Fourier-transform infrared spectra. Experimental results showed that ZnFe LDH-based had an appropriate crystallization structure. The morphology was evaluated by scanning electron micros-copy (SEM) analysis to examine the surface properties and elemental composition of the membrane. To determine the surface charge of solid particles at different pH values, the point of zero charge (pHpzc) was measured. The pHpzc of the catalyst was determined to be 7.15, using the pH drift method. For toxicity evaluation, Polyethylene (PE) microplastics (MPs) were selected as one of the most commonly used and frequently detected types in the environment and 0.1 mg/L PE was added into each reactor. The presence of microplastics is important as an adsorption surface or transfer mechanism for toxicity. The antivirals and their removal percentages were determined by liquid chromatography with tandem mass spectrometry (LC-MS/MS), with the aid of isotope dilution method. Solid phase extraction (SPE) was applied for rapid, selective sample preparation and purification before the chromatographic analysis. Based on the results, Favipiravir was feasibly removed to below the detection limit even with the lower ozone dose, at neutral pH and in the absence of catalyst. Oseltamivir had a lower removal efficiency (<80%) in case of applying the lower ozone dose at neutral pH; however, it reached higher removal percentages by increasing the specific ozone dose to 1 O3/mg DOCo, elevating the pH to 10 or using the catalyst. In case of adding microplastics to the media, no effect was observed for Favipiravir (still 100%). In respect to OS, a higher removal occurred at neutral pH; However, increasing the pH of media to 10 showed no significance effect in increasing or decreasing of OS removal percentages. One possible reason could be that the increase in pH might affect the surface properties of the catalyst negatively and increasing the pH above the pHpzc would result in surface dominated with positive charges which might not favor degradation of ozone at the surface of the catalyst. These results suggest that the effect of pH is more pronounced than the presence of MPs. It is possible that the MPs could act as a solid surface where ozone degradation and free radical production could happen; leading to unintended catalytic ozonation for sets where no catalyst was present. However, when both MP and the catalyst are present at the same time, there might be too many sites compared with O3 molecules and hence there is no "added" increase in degradation. To investigate the ecotoxicological influences, soil toxicity experiment was conducted by using Enchytraeus crypticus, due to its function in soil decomposition and high sensitivity to a wide range of stressors. Based on the results, no effect was observed on E. Crypticus in the presence of 0.5% PE. No observed effect concentration (NOEC) for Favipiravir, Oseltamivir and their individual combination with PE was 12.5 mg/kg. Also, no significant difference was observed at 12.5 mg/kg for Favipiravir and Oseltamivir's mixture in the presence of PE. On the other hand, a statistically significant difference was found for the mixture containing Favipiravir and Oseltamivir compared to the controls. In this case, the NOEC value for the mixture of favipiravir and oseltamivir was <12.5 mg/kg. Favipiravir and Oseltamivir, which did not have a toxic effect individually at a concentration of 12.5 mg/kg, showed a toxic impact when mixed. This could be as a result of the potential chemical interactions between two antivirals or contributing to impede some functions such as reproduction exhibiting more stress when exposed to different kinds of unknown substances. The results of the study demonstrated that ozonation can be effective in the removal of both Favipiravir and Oseltamivir, confirming the potential of ozonation as a polishing step in wastewater treatment. Using a catalyst, instead of providing basic pHs could be more practical for real wastewater systems; however, developing techniques such as using granular form catalysts or packed column to remove and reuse of catalyst should be considered to obtain a system that can be applied in full-scale WWTPs.