Aplicación de la ozonación catalítica como proceso de oxidación avanzada en el tratamiento de aguas residuales

Resumen

In the last decade, heterogeneous catalytic ozonation processes have attracted great research interest for the removal of organic pollutants from water and wastewater. In many cases, heterogeneous catalytic ozonation involves the transformation of aqueous ozone into very strong oxidizing free radical species (mainly hydroxyl radicals) over a solid catalyst. Hydroxyl radicals are able to unselectively react with most of the organic pollutants in water transforming them into final products (i.e., more oxidized lower molecular weight products and, eventually, carbon dioxide and water). The research group TRATAGUAS of the Department of Chemical Engineering of the University of Extremadura is currently involved in several projects dealing with the topic of catalytic ozonation. This Thesis has been carried out in this context and it focuses on the use of activated carbons, alumina-based and perovskite-type catalysts for the ozonation of some model water pollutants such as pyruvic acid, gallic acid, diclofenac, sulfamethoxazole and 17alfa-ethynylestradiol as well as two secondary effluents from full-scale wastewater treatment plants (WWTPs). The research was conducted primarily in Spain during April 2005-December 2008, (Department of Chemical Engineering, University of Extremadura). In addition, a four-month research was carried out in Germany (University Duisburg-Essen) from June to September 2008. Experimental work consisted in the following stages: 1. Preparation and characterization of catalysts. 2. Degradation of target compounds by catalytic ozonation in semi-batch mode. 3. Degradation of secondary effluents by catalytic ozonation in semi-batch mode. 4. Degradation of a municipal secondary effluent spiked with target compounds by catalytic ozonation in continuous mode (pilot-plant). Alumina-based and perovskite-type catalysts were prepared in the laboratory while commercial activated carbons were purchased from different vendors. All the catalysts were characterized before use in ozonation experiments. The following characterization techniques were applied: adsorption of nitrogen at 77 K, mercury porosimetry, helium and mercury pycnometries, infrared spectroscopy (FT-IR), thermogravimetric analysis, X-ray diffraction, determination of the point of zero charge (PZC) and analysis of surface oxygen groups (SOGs). Single ozonation showed a limited capacity to mineralize the selected target compounds. Despite of its great effectiveness to remove them, with the exception of pyruvic acid, single ozonation led to some reaction by-products that accumulated in water. Catalytic ozonation clearly improved ozone utilization efficiency as well as by-products mineralization. Generally speaking, in terms of TOC removal, the increasing order of catalytic activity was found to be as follows: Alumina based catalysts< perovskite-type catalysts < activated carbons. The basicity of the activated carbons was observed to have an impact on catalytic ozonation. Thus, the higher the PZC and the concentration of basic SOGs on the activated carbon, the higher the mineralization rate of organic pollutants. Catalytic ozonation of two secondary effluents was also carried out. These effluents were taken from a food-processing WWTP of Almendralejo (Extremadura) and the municipal WWTP of Badajoz (Extremadura). Catalytic ozonation led to higher mineralization degrees as well as lower ozone consumptions than the single ozonation process. Likewise catalytic ozonation of the target compounds in aqueous solution, activated carbons gave better results than alumina-based and perovskite-type catalysts. Catalytic ozonation was integrated as a treatment stage of a continuous-mode pilot-plant treating a secondary effluent from the municipal WWTP of Badajoz. Wastewater was spiked with some pharmaceutical compounds in order to follow the effectiveness of the process to remove them in a real wastewater matrix. According to previous results, a basic activated carbon (HydraffinP110, PZC=9.7) was selected as the most suitable material for these experiments. From the results, it was concluded that activated carbon-assisted ozonation improves the removal of COD and TOC and increases the activated carbon durability. Complementary, during a stay at the University of Duisburg-Essen (Germany) pilot plant experiments were also carried out. A secondary effluent was spiked with two organophosphate pollutants such as tris-(2-chloro-1-methyl-ethyl)-phosphate (TCPP) and tri-n-butylphosphate (TnBP), whose occurrence in rivers and effluents from sewage treatment plants is extensively reported. The prepared effluent was treated with ozone and ozone in the presence of hydrogen peroxide. Single ozonation was not able to totally remove the model compounds, especially TCPP, so most of the hydroxil radicals were scavenged by the water matrix. On the other hand, the advantage of the O3/H2O2 was only noticeable at high ozone doses when the reaction of ozone with the wastewater became slower.