Processes and catalysts for the electrochemical removal of persistent organic micropollutants from urban wastewater at mild Ph

Abstract

Organic micropollutants, including pharmaceuticals, personal care products (PCPs), pesticides, dyes and many other industrial chemicals have become one of the most solid barriers for the effective and safe wastewater remediation and reclamation. Some of these contaminants are able to act as endocrine disrupting chemicals (EDCs), thus posing emerging concerns. Their concentration in water is usually at trace level and most of them are highly resistant to conventional treatment technologies like biodegradation and physicochemical methods. The inefficient removal of micropollutants causes their occurrence in final water distribution systems, even reaching drinking water, which jeopardizes the entire biosphere and human health. Over the last decades, the electrochemical technologies, especially the electrochemical advanced oxidation processes (EAOPs), have been proven to behave as clean and effective alternatives to eliminate the organic micropollutants from wastewater effluents due to the direct or indirect generation of strong oxidizing agents such as the hydroxyl radical. However, the utilization of conventional electro-Fenton (EF) and photoelectro-Fenton (PEF) is limited by several drawbacks: (i) long time needed to destroy large contents of organics; (ii) requirement of pH adjustment to 2.5-3.5; (iii) poor electroreduction and photoreduction of Fe(III); (iv) high amount of iron catalyst required; (v) deactivation of iron species; and (vi) production of iron sludge. Aiming to overcome these disadvantages, several processes and catalysts are proposed in this Thesis to modify the conventional EF and PEF. In the first part, electrocoagulation (EC) was envisaged as a valid pre-treatment before the application of the EAOPs, thus addressing the abovementioned limitation (i). This involved the in situ generation of coagulants upon dissolution of a sacrificial anode,

yielding flocs that precipitated and adsorbed part of the organics rapidly. Furthermore, when an Fe/Fe cell was employed, the residual dissolved iron species after precipitation acted as the required catalyst for subsequent EF/PEF treatment. The efficient removal of butylated hydroxyanisole (BHA) and benzophenone-3 (BP-3) from urban wastewater was successfully achieved by means of sequential EC/EAOPs. The second part investigated, for the first time, the feasibility of employing a soluble Fe(III)–EDDS complex as homogeneous EF or PEF catalyst to destroy micropollutants. The systems allowed working at near-neutral pH, exhibiting a high quantum yield for Fe2+ generation from Fe(III)–EDDS photoreduction. The Fe(III)– EDDS-assisted EF and PEF processes exhibited excellent performance for the degradation of BHA and fluoxetine (FLX) in sulfate medium, as well as in urban wastewater. Heterogeneous EF and PEF processes using solid catalysts have been developed in the third part as promising alternatives to overcome the drawbacks (ii)-(vi). The development of new types of catalysts with high activity, stability and recyclability is still a great challenge in the field. Metal-organic frameworks (MOFs) have attracted substantial attention in recent years as ordered porous materials with many potential applications. In this Thesis, raw Fe-MOFs or their derivatives were introduced as efficient and innovative heterogeneous EF or PEF catalysts to treat micropollutants in urban wastewater. FeS2/C nanocomposite, fabricated by sulfidation and pyrolysis of an Fe-MOF precursor, as EF catalyst outperformed natural pyrite and Fe2+ due to the cooperation of homogenous and heterogeneous Fenton’s reaction. The thermal treatment of NH2-MIL(Fe)-88B gave rise to N-doped nano-ZVI@C, which exhibited superior catalytic activity in EF. Finally, the direct use of a Fe-bpydc 2D MOF as PEF catalyst yielded a fast bezafibrate decay due to the synergy between photocatalysis and Fenton’s reaction. Their unique properties conferred an unprecedented degradation ability to EF and PEF at mild pH. In conclusion, this Thesis has provided several strategies to unravel the difficulties for the future application of EAOPs at industrial scale.

Los microcontaminantes orgánicos se han convertido en un gran obstáculo para asegurar un tratamiento de aguas residuales eficaz y seguro, ya que suelen ser resistentes a las tecnologías de tratamiento convencionales. En las últimas décadas, se ha demostrado que los procesos electroquímicos de oxidación avanzada (EAOPs) constituyen alternativas limpias y efectivas para eliminar microcontaminantes orgánicos de aguas residuales, gracias a la generación del radical hidroxilo. Sin embargo, la utilización de los procesos electro-Fenton (EF) y fotoelectro-Fenton (FEF) convencionales se ve limitada por: (i) el largo tiempo requerido para eliminar elevadas concentraciones; (ii) necesidad de ajuste del pH (2,5-3,5); (iii) pobre electrorreducción y fotorreducción del Fe(III); (iv) elevada cantidad de catalizador requerida; (v) desactivación de las especies de hierro; y (vi) producción de lodos. Para superar estas desventajas, en esta Tesis se proponen varios procesos y catalizadores. En la primera parte, la electrocoagulación (EC) se propuso como pretratamiento para abordar la limitación (i), lo cual implicó la generación de flóculos como adsorbentes. Además, el hierro disuelto actuó como el catalizador en el posterior tratamiento EF o FEF. Mediante EC/EAOPs se logró una eliminación eficiente de hidroxianisol butilado (BHA) y la benzofenona-3 (BP-3) en agua residual urbana. En la segunda parte se investigó, por primera vez, la viabilidad del complejo Fe(III)– EDDS soluble como catalizador EF o FEF homogéneo. Se obtuvo un excelente rendimiento para la degradación de BHA y fluoxetina (FLX) en medio de sulfato y en agua residual. En la tercera parte se desarrollaron procesos EF y FEF heterogéneos que utilizan catalizadores sólidos para superar los inconvenientes (ii)-(vi). Se introdujeron

Fe-MOFs (i.e., metal-organic frameworks) y sus derivados como catalizadores innovadores eficientes. Se fabricó un nanocomposite de FeS2/C con un mayor rendimiento que la pirita natural y el Fe2+ en el proceso EF. El tratamiento térmico de NH2-MIL(Fe)-88B dio lugar a un nano-ZVI@C dopado, el cual exhibió una capacidad catalítica superior en EF. Finalmente, la utilización de un MOF 2D de Fe-bpydc en FEF promovió una eliminación rápida de benzafibrato. En conclusión, esta Tesis ha proporcionado varias estrategias para superar las dificultades para la aplicación de EAOPs a escala industrial.