The anodic oxidation of tetracycline was performed in an up-flow reactor, operating in batch mode with recirculation, using as anode a boron-doped diamond electrode. The influence on the degradation rate of solution initial pH (2 to 12), applied current intensity (25 to 300 A m(-2)) and type of electrolyte (sodium sulphate or sodium chloride) were investigated. For the assays run at equal current density, with sodium sulphate as electrolyte, the solution's initial pH of 2 presented the highest absorbance and chemical oxygen demand removals. Regarding the influence of current density, for equal charge passed, the organic load removal rate decreased with the increase in applied current. When sodium sulphate was used as an electrolyte, high-performance liquid chromatography (HPLC) results have shown an almost complete removal of tetracycline after a 2-h assay. HPLC results have also shown the presence of oxamic acid as one of the intermediates of tetracycline anodic oxidation. The complete removal of tetracycline was much faster in the presence of chloride ions that promoted the complete degradation of this antibiotic in 30 min. However, in the presence of chloride ions, the tetracycline mineralization is slower, as observed by the lower organic carbon removal rate when compared to that of the tetracycline degradation in the presence of sulphate.
The behaviors of the electrodes Ti/PbO2 and Ti/Pt/PbO2 as anodes in the electro-oxidation of two antibiotics-tetracycline and oxytetracycline-were evaluated at different applied current densities, to evaluate the influence of the Pt interlayer. In the preparation of the electrodes, the electrodeposited β-PbO2 phase was homogeneous; no Ti or Pt peaks were detected in the diffractograms. The β-PbO2 surface presented significant roughness when deposited over the Pt interlayer, which also conferred significant conductivity to the material. In the electro-oxidation assays, the COD, TOC and absorbance removals increased with the current density due to an increase in the concentration of hydroxyl radicals, for both electrode materials and antibiotics tested. Slightly better results were obtained with Ti/PbO2. The primary differences observed in the antibiotics concentration decay consisted of zero-order kinetics at the Ti/Pt/PbO2 anode and first-order kinetics at the Ti/PbO2 anode with a higher oxytetracycline concentration decay than the tetracycline concentration decay. A greater amount of total nitrogen was eliminated with the Ti/PbO2 electrode. At the Ti/Pt/PbO2 anode, the organic nitrogen primarily transformed into NH4(+) and the total nitrogen remained unchanged. The specific energy consumption with the Ti/Pt/PbO2 anode was significantly lower than the specific energy consumption with the Ti/PbO2 anode due to the higher electrical conductivity of the Ti/Pt/PbO2 anode. Both anode materials were also utilized in the electro-oxidation of a leachate sample collected at sanitary landfill and spiked with tetracycline, and the complete elimination of the antibiotic molecule was observed.
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