Assessing the degradation and transformation of emerging pollutants submitted to TiO2 photocatalysis by high-resolution mass spectrometry techniques. Are we posing a new hazard to the environment?

Abstract

In the past 50 years there has been increasing interest and concern over pharmaceutical drugs and their presence in the environment, since they and/or their metabolites could induce toxicological effects and bacterial resistance in living organisms. Conventional sewage treatment plants are not able to efficiently remove these chemicals, reason why these compounds are being introduced into the aquatic environment at concentration levels of parts per-billion (ppb) and parts per-trillion (ppt). Although these concentrations are much lower than those used in medical applications, the related potential toxic effects are still poorly known and cannot be discarded. Heterogeneous TiO2 photocatalysis is an advanced oxidation process that can achieve complete oxidation and mineralisation of such organic compounds. TiO2 nanoparticles and nanofibers have been extensively investigated as the suspended semiconductors in common heterogeneous photocatalysis set-ups. However, the complete recovery of the semiconductor is still a major industrial challenge. That is the reason why their fixation onto an inner support represents a great alternative to overcome this problem. The present PhD thesis has studied the degradation of four pharmaceutical drugs (ofloxacin, clofibric acid, diclofenac and ibuprofen) using two different set-ups (suspended and supported conditions) with three different TiO2 catalysts: synthesised TiO2 nanofibers and P25 nanoparticles and commercially available nanofibers, used as reference materials. Prior performing the photocatalytical process, all TiO2 samples were submitted to physical characterisation (BET surface area, X-Ray diffraction and SEM microscopy) showing significant differences between the synthesised and reference materials. Prepared TiO2 nanofibers, however, appeared to be the material with maximum adsorption capacity (derived from Langmuir’s isotherm) and appeared to be slightly better at degrading the pharmaceuticals under study at a concentration of 10 µg L-1. Nevertheless, when the experiments were conducted under real conditions, none of all the studied catalysts could effectively degrade the studied compounds. Additionally, this work has studied how one of the selected pharmaceuticals (ofloxacin) was degraded along the photocatalytical process by mass spectrometry techniques: (tandem) high-resolution mass spectrometry and ion mobility mass spectrometry. Due to the vast amount of information generated by these cutting-edge instruments, different data treatment techniques were applied in order to achieve a fast

identification of the generated transformation products (TPs). The degradation of ofloxacin led to the formation of 59 TPs. Chemical structures of these TPs were hypothesised on the basis of their accurate mass and product ion spectra. However, due to the lack of standards of these TPs, these hypothesised structures could not be confirmed until ion mobility, in combination with theoretical calculations, was used. This technique not only rendered additional separation in the gas phase for some of the identified TPs but also provided a tool to support the complete elucidation of the chemical structures hypothesised. Having detected and identified ofloxacin TPs, the investigation of this PhD thesis was centred on the study of their toxicological impact. Toxicity was assessed by two different end-points: luminescence, using the luminescent bacteria Vibrio-fischeri and cytotoxicity using three different human cell lines (HK-2, HepG2 and fibroblast cells). Both end-points clearly demonstrated that, even at low concentrations, the formation of these TPs led to a toxicity increase when compared to the parent drug ofloxacin. Even though this toxicity increase, complete detoxification was achieved by ensuring enough irradiation time and promoting the complete depletion of ofloxacin TPs. The individual toxicity of these TPs was then studied by fractionating the reaction crude with a semi- preparative HPLC column which restricted the number of toxic TPs. Due to the poor chromatographic resolution achieved, the chemical synthesis of the generated TPs seems to be the only option, to date, to study their individual toxicity.

A lo largo de los últimos 50 años ha habido un creciente interés acerca de los fármacos y/o sus metabolitos ya que su presencia en el medio ambiente podría inducir efectos toxicológicos adversos y/o resistencia bacteriana. Puesto que los sistemas de depuradoras de aguas son incapaces de eliminarlos, éstos son introducidos y detectados en el sistema acuático. Aunque los niveles encontrados son mucho menores que las usadas en medicina, sus efectos adversos no pueden ser descartados. La fotocatálisis heterogénea mediante TiO2 es un proceso de oxidación avanzado que puede conseguir la completa oxidación y mineralización de estos compuestos en el agua. La presente tesis doctoral se ha centrado en la degradación de cuatro fármacos (ofloxacino, ácido clofíbrico, diclofenaco e ibuprofeno) con tres catalizadores de TiO2: nanofibras sintetizadas y materiales comerciales: en forma de nanopartícula y nanofibra. Las nanofibras preparadas mostraron una mayor capacidad de adsorción y degradación de los fármacos estudiados. A pesar de los buenos resultados obtenidos experimentando en agua sintética, todos los catalizadores bajaron su rendimiento al experimentar con matriz de agua real. Adicionalmente, se estudió cómo el ofloxacino se degradaba y transformaba a lo largo del proceso fotocatalítico mediante espectrometría de masas de alta resolución y de movilidad iónica. Se detectaron e identificaron 59 productos de transformación, los cuales fueron adicionalmente estudiados mediante la técnica de movilidad iónica acompañada de cálculos teóricos. Esta última combinación no solo demostró su poder separativo sino que proporcionó una herramienta para la confirmación de las estructuras químicas propuestas. Finalmente, se estudió la toxicidad asociada a los productos de transformación del ofloxacino, evaluada mediante dos targets: luminiscencia (Vibrio-fischeri) y citotoxicidad (células humanas). Ambos ensayos demostraron que la generación de estos productos de degradación incrementaba toxicidad. Aun así, la completa detoxificación se consiguió extendiendo el tiempo de fotocatálisis. A fin de estudiar su toxicidad individual, el crudo de reacción fue fraccionado mediante HPLC semipreparativo. Sin embargo, la baja resolución cromatográfica obtenida imposibilitó el estudio individual y limitó a una fracción este aumento de la toxicidad. Actualmente, la síntesis química, parece la única vía para posibilitar el estudio individual de los productos generados.