Photo-Fenton at Circumneutral pH for Agricultural Water Reuse

Abstract

[eng] Water is an essential resource for life. However, the low water availability of freshwater together with the increase in its demand in the last decades, has provoked worldwide water stress. Thus, the reused wastewater can play an important role to reduce freshwater demand, for instance, in agriculture with around 70% of the total freshwater demand. Nevertheless, the reused wastewater must guarantee the safety for public health and environment. In the last decades, there is an especial concern on removing micropollutants, since they are potentially harmful substances for public health and aquatic ecosystems.

Advanced oxidation processes have been proven their efficiency in the degradation of several compounds of emerging concern, such micropollutants. Among these techniques, photo-Fenton process is one of the most effective. However, the requirement to work at acidic conditions to keep the iron in solution is one of the drawbacks of the process. Additionally, the need of irradiation rises the treatment operational costs and environmental impact since some conventional lamps comprises mercury. These inconveniences make the processes unattractive for its full scale-application.

The investigation derived from this thesis is focused on different solutions trying to solve these drawbacks. Firstly, LEDs and solar simulated irradiation were used, carrying out the photo-Fenton process more economically and ecofriendly. The employment of organic fertilizers, as a chelating agents of iron, is the second foundation on which the investigation is based. The final objective is the reuse of wastewater in agriculture, facing the current water scarcity scenario. The study of the conventional photo-Fenton with UV-A LEDs revealed the suitability of the process to remove a recalcitrant compound with high conversion efficiencies. Additionally, the results suggested that synergies on micropollutant, COD and TOC conversion were observed combining two wavelength ranges, because the treatment time was reduced by half. However, from the results obtained in the comparison between UV-A LEDs in wastewater effluents it was evidenced the necessity to optimize the system geometry when LEDs are used as irradiation source. This fact is essential in the treatment of wastewater since the turbidity and organic matter present in these matrices influence on the radiation transfer throughout the photoreactor affecting more when LEDs are used, because they are punctual sources of irradiation. The study in the use of organic fertilizers, as iron chelating agents, revealed that all tested agents (DTPA, HEDTA, EDDS and EDTA, except EDDHA) were effective in photo-Fenton process at circumneutral pH in MBR, CAS and CAS-NE effluents in removing micropollutants. The results showed that the efficiency in the degradation is linked to the stability constant of chelates with iron. Low stability corresponds to high reaction rates at initial times but also high iron release. The opposite happens with chelating agents with high stability constant with iron. The enhancement in removal kinetics and the decrease in iron release was achieved combining organic fertilizers with different stability constant. The organic fertilizers mixtures were also tested in two different wastewater effluents to study the effect of the matrix on iron release. It was observed that, in wastewater with higher turbidity, alkalinity and organic matter, the iron precipitation was higher, decreasing the efficiency of the process overall in EDDS, which have the lowest stability constant with iron. Finally, the mechanisms involved in photo-Fenton process, with iron chelates in aquatic environment, cannot be generalized, since each iron complex have its stability constant with iron and its specific absorbance in solar spectrum. The irradiation experiments without H2O2 revealed that only EDDS and DTPA were capable to generate hydroxyl radicals. Tests with dosing of non-chelated iron revealed that reaction with iron chelates are the main via to hydroxyl radical generation.

[spa] El agua es un recurso esencial para la vida. Sin embargo, la baja disponibilidad de agua dulce, junto con el incremento en su demanda, durante las últimas décadas, ha ocasionado estrés hídrico en todo el mundo. Por lo que la reutilización del agua residual podría ser un parámetro clave para hacer frente a esta situación, sobre todo en la agricultura ya que, el 70% del total de la demanda de agua dulce se destina a este sector. Ahora bien, el agua reutilizada debe garantizar la seguridad para la salud pública y el medioambiente, por lo que tiene que estar libre de los llamados microcontaminantes ya que, son sustancias potencialmente nocivas para el medio acuático y la salud pública.

El proceso foto-Fenton es uno de los procesos de oxidación avanzada más efectivos. Sin embargo, el requisito de trabajar a pH ácido para evitar la precipitación del hierro y la necesidad de irradiación son sus mayores inconvenientes para su aplicación a gran escala.

La investigación derivada de esta tesis se centra en potenciales soluciones para la aplicación del proceso foto-Fenton. En primer lugar, se utilizaron LEDs como fuente de luz para llevar a cabo un proceso más económico y respetuoso con el medio ambiente. En este sentido se estudió la importancia de la optimización de los reactores con LEDs, posibles sinergias entre diferentes longitudes de onda y comparación con lámparas convencionales. El empleo de fertilizantes orgánicos comerciales como agentes quelantes del hierro para trabajar a pH casi neutro utilizando luz solar simulada, es el segundo fundamento en el que se basa esta tesis. De esta investigación se destacó la importancia de la estabilidad de los complejos con el hierro en la cinética de degradación de los microcontaminantes y cómo esto influye en el tratamiento de efluentes con más materia orgánica. Mezclas de fertilizantes con diferentes estabilidades con el hierro fueron probadas con el fin de mejorar las cinéticas de eliminación de contaminantes y la disponibilidad de hierro disuelto al final del tratamiento. Todo ello, con el objetivo final fue la reutilización de los efluentes tratados con fines agrícolas.