Synthesis and performance of heterogeneous catalysts for Fenton-like and photo-Fenton-like reactions at circumneutral pH

Abstract

As an essential resource for life and most human activities, water resources protection have become one of the major focuses of scientific community, especially the increasing occurrence of emerging pollutants which have its origin in commonly used pharmaceuticals, personal care products, pesticides and some other anthropogenic origin compounds. Fenton and photo-Fenton are advanced oxidation processes (AOPs) that have been widely proved on the removal of these compounds, but they are conventionally applied at acidic pH, below 3.0, because these processes involve the use of iron (II) salts, which are not soluble above. Also, the acidic effluents must be neutralized and high amounts of iron sludge are formed in the process. Current research in the field tries to solve this drawback by focusing in two possible solutions: the use of chelating agents to keep iron soluble at circumneutral pH or the use of heterogeneous catalysts containing iron that can be easily removed and reused after its application. This work is focused on the study, application and improvement of alginate-based heterogeneous catalysts for conducting photo-Fenton reactions at circumneutral pH. Initially, the preparation process of the conventionally used Fe(III)/alginate catalyst was optimized by evaluating the effect of the parameters involved on its synthesis. Then, the synthetized catalyst was applied to the abatement of antibiotic sulfamethoxazole (SMX), which was used as reference compound. The reaction mechanism of alginate-based catalysts was proposed according to the results obtained. Also, a deep study about catalyst stability revealed a high dependence with water’s pH. Fe(III)/alginate catalyst proved to release iron when sample’s pH was below alginate’s pKa. Also, due to its organic nature, a significant HO· scavenging effect of alginate itself was observed, which reduced the effective amount of hydroxyl radicals available for the oxidation of the target compound. Fe(III)/alginate was also tested for disinfection by adjusting the conditions to ensure a minimal iron release. Thus, the contribution of homogeneous reaction was significantly avoided. The catalyst proved being able to be used for disinfection of waters with low organic loads. In order to improve the Fe(III)/alginate catalyst, some modifications were applied. The first one was the addition of a dehydration stage after its synthesis. The resulting catalyst was tested on the removal of SMX and had a similar effectiveness than the conventionally used Fe(III)/alginate but with a significantly lower iron release, which confirmed its improved stability. This more stable catalyst was also modified by including TiO2, zero valent iron (ZVI) particles or cerium on its structure with the aim of, not only increase its stability, but its

effectiveness. The TiO2 catalyst proved to increase the effectiveness of the overall removal of SMX while the other two modifications did not.

Red volcanic rocks, iron shavings from metallurgical industry, a commercial Fe/Mn/O catalyst and a self-synthetized Fe/Ce/O catalyst were also tested as heterogeneous catalysts for Fenton and photo-Fenton at acidic and neutral pH. These catalysts were selected as inorganic-based ones and were compared to the Fe(III)/alginate dehydrated catalyst when applied to photo- Fenton at neutral pH. Results proved that only volcanic rocks had a significant activity when applied for photo-Fenton at neutral pH, but the effectiveness on the removal of SMX was low in comparison to the alginate-based catalyst. All other catalysts presented too low activity or major drawbacks that made them not suitable for water purification through Fenton-like and photo-Fenton-like processes.

Los procesos Fenton y foto-Fenton son procesos de oxidación avanzada (POA) que se basan en la descomposición del peróxido de hidrógeno catalizada por sales de hierro (II), produciendo radicales hidroxilo que son altamente oxidantes. El segundo además incorpora la irradiación con luz ultravioleta en el rango A, cosa que permite acelerar el proceso debido a la recuperación del hierro (II), que durante la reacción principal pasa a hierro (III). Estos procesos se suelen llevar a cabo a pH ácido (pH<3) debido a la baja solubilidad y la poca actividad de las especies de hierro a pH cercanos a la neutralidad. Este trabajo se centra en el estudio y aplicación de un catalizador heterogéneo de Fe/alginato formado gracias a la propiedad del alginato de formar geles sólidos en presencia de cationes di y trivalentes. En este caso, el Fe(III) actúan como enlace entre cadenas de alginato. Inicialmente se estudió el proceso de fabricación de este catalizador y se propuso un proceso estándar optimizado para su uso aplicado a la reacción foto-Fenton.

Posteriormente se evaluó su aplicación a la eliminación del antibiótico sulfametoxazol y a la desinfección de bacterias Escherichia Coli. Basándose en los resultados obtenidos se propuso un mecanismo de reacción que además explicaba la inestabilidad observada experimentalmente que producía la liberación de hierro dependiendo del pH del medio de reacción, lo que desencadenaba la vía de reacción homogénea. Con la intención de mejorar la estabilidad del catalizador, se modificó el proceso de síntesis añadiendo una etapa de deshidratación controlada al final del proceso. Los resultados en su prueba para la reacción foto-Fenton aplicada a la eliminación de sulfametoxazol mostraron que el rendimiento no empeoraba pero mejoraba significativamente su estabilidad. Para aumentar su eficiencia se probó incorporar cerio, partículas de hierro monovalente y TiO2. Los resultados mostraron que solo el catalizador modificado con TiO2 mejoraba el rendimiento global de la reacción.

Por último, se probaron otros catalizadores heterogéneos (piedras volcánicas, Fe/Ce/O, Fe/Mn/O, limaduras de hierro) aplicados a la reacción foto-Fenton. Solo las piedras volcánicas mostraron actividad catalítica y ésta, en comparación con la del catalizador de alginato fue baja.