Assessment of a continuous-flow atmospheric air dielectric barrier discharge in the degradation of tramadol, cefixime, and carbamazepine in aqueous solutions

Abstract

Active pharmaceutical contaminants which are constantly released into both surface and ground water through wastewater treatment plants (WWTP), run-off from agricultural fields, excretion, and disposal of unused or expired medicines into sewage, have become a global concern because of their effects on the aquatic ecosystem and human health. Several studies have examined the use of non-thermal plasma reactors like the dielectric barrier discharge (DBD) in the degradation of various pharmaceutical compounds with significant degradation and mineralization efficiencies. However, most studies are either conducted in batch mode with small solution volumes or in pure synthetic solutions. Also, the working gases have mostly been pure synthetic oxygen gases, which can increase the associated cost of treatment. In this study, the performance of a continuous-flow atmospheric air dielectric barrier discharge was assessed specifically for the degradation of tramadol, cefixime, and carbamazepine, which are among the commonly discovered pharmaceuticals in the water cycle. By selecting pharmaceutical contaminants from different drug classifications, this study aimed to show the efficacy of a DBD reactor in degrading a wide range of pharmaceutical residues, irrespective of their physicochemical properties. At alternating current (AC) voltage range of 6 – 8 kV and frequency of 20 kHz, 93% degradation of tramadol was observed in 60 min, >99% degradation of cefixime in 8 min, and 92% degradation of carbamazepine in 40 min. Also, the degradation efficiency of each pollutant was susceptible to the operation conditions of the DBD, including applied voltage, initial concentration of pollutant, pH, conductivity, water matrix, and water flow rate. The chemical species generated were investigated with a spectrometer while radical scavenging experiments were used to establish their respective roles in the degradation of the pollutants. Experiments conducted in real wastewater effluent confirmed that the presence of 𝐻����𝐶����𝑂����3− used as pH buffers played a scavenging role in the degradation of analgesic tramadol in the matrix as the ion reacted with hydroxyl radicals (•OH) thereby reducing its oxidizing power. Also, a toxicity test revealed that the plasma-treated tramadol solution was less toxic to Escherichia coli as opposed to the untreated solution. A new idea was investigated, which was to understand what happens when a metal ion catalyst (Fe2+) is mixed with •OH radical scavengers. In this case, the reactor was able to still achieve significant degradation of cefixime due to the increased production of H2O2 in the aqueous solution. The reactor’s performance was also compared with UV-systems for the degradation of carbamazepine at similar experimental conditions. The degradation results obtained in 40 mins were 6.5%, 17.8%, 89%, 91%, and 98% for UV-only, UV/Fe, UV/H2O2, UV/Fe/H2O2, and plasma systems, respectively. The plasma system also had the highest energy efficiency (75.24 kWh/m3) and the least required energy cost of treatment (13 USD/m3) compared to the UV-systems considered. Thus, an assessment of the laboratory-scale studies has demonstrated the feasibility of the novel continuous-flow atmospheric air dielectric barrier reactor in the degradation of tramadol, cefixime, and carbamazepine pollutants in mono-component solutions. This technology has shown potential for field-scale studies as it can be incorporated into existing wastewater treatment plants to degrade active pharmaceutical residues that escape the various treatment stages. However, considering that pharmaceutical pollutants always exist as mixtures and not as a single component in solution, future studies should consider the efficacy of the reactor in degrading a mixture of the pollutants in different water matrices, including real pharmaceutical waste samples.