Intensificatión of photocatalytic degradation processes in aqueous effluents

In: http://hdl.handle.net/10578/12485

Abstract

This Thesis has been mainly developed in the laboratories of IMAES research group in the Chemical Engineering Department at the University of Castilla-La Mancha (UCLM). It is a part of a research line dealing with the elimination of emerging contaminants in wastewater using advanced oxidation processes. It has been supported by the "Ministerio de Economia y Competitividad" (MINECO, Spanish Government) through the project CTM2013-44317-R entitled "Tratamiento en planta piloto de efluentes acuosos industriales mediante sonofotocatálisis UV/solar" and UCLM funding GI20142907. In addition, some parts of this research were carried out at the Department of Chemical Engineering of the University of Bath (United Kingdom), thanks to University of Bath International Research Fund Future Research Leaders Incubator Scheme, and at the School of Engineering of the University of Edinburgh, through a research visit supported by MINECO. Over the past 15 years, increasing numbers of studies have identified numerous different water sources containing trace, but accumulating, toxic chemicals. Many of such chemicals are thought to be recalcitrant, inhibitory or toxic to biological treatment in both wastewater treatment works and in the environment and so may be able to enter our water cycle. Photocatalysis is a promising degradation technology for such compounds, however due to inefficient and costly reactor systems, it has not yet been widely taken up by Industry. This project seeks to rectify this, by comparing a wide range of photocatalytic reactor technologies that have been integrated with further intensification technologies to increase the efficiency and effectiveness of the pollutant degradation. This includes: different reactor configurations (falling film photoreactor, solar compound parabolic collectors (CPC), a spinning disc reactor.) and intensifications processes (use of ultrasound (US), ferrioxalate and sulfate radicals-based advanced oxidation processes). This Thesis is focussed on comparing and optimizing the key parameters controlling the reaction kinetics and mechanisms of the reactors to determine the optimal degree of treatment of these types of accumulating toxic biologically recalcitrant chemicals in wastewaters, using antipyrine and carmamazepine as model compounds. The treatment efficiency and effectiveness, neural network optimization as well as technoeconomic analysis were used to determine the optimal intensification technology of photocatalytic process. Finally, real industrial effluents were also tested to prove the validity of studied processes. Overall, the main novelty of this work will be the first extensive study comparing a wide range of photocatalytic process intensification technologies. ANTIPIRINE The removal of antipyrine (AP) was evaluated under a sono-photo-Fenton system (UV/H2O2/Fe/US) in a synthetic wastewater reaching 79% of mineralization in 50 minutes in optimal conditions ([H2O2]0 = 500 mg/L; [Fe2+]0 = 27 mg/L; Amplitude = 20% and Pulse length = 1). The radical reaction in the bulk solution was found be the primary mineralization pathway (94.8%), followed by photolysis (3.65%), direct reaction with H2O2 (0.86%) and reaction by ultrasonically generated oxidative species (0.64%). Complete mineralization of reaction intermediates refractory towards hydroxyl radical was attained using persulfate anions simultaneously activated by heat energy (thermally, ultrasound) and UV-C light. The SO·4-based mineralization process enables another reaction pathway generating more easy degradable derivatives reaching more than 99% of total organic carbon (TOC) removal in 120 min under selected optimal operating conditions ([S2O82-]0 = 1200 mg/L; Temperature = 50 ªC; Amplitude = 10%; pH = 2.8). This demonstrated that activated persulfate-based oxidation system is a potential alternative to degrade intermediate compounds, which are refractory to hydroxyl radicals. The degradation of AP in aqueous solution using UV-A-LED (365 nm) photo-Fenton reaction intensified with ferrioxalate complexes and with the addition of persultate was also studied. A complete degradation of AP and 93% of mineralization of AP solution was reached in 2.5 and 60 min, respectively. In the last step of reaction, different intermediates difficult to be degraded such as 2-butenedioic acid, butanedioic acid, 4-oxo-pentanoic acid, acetate and formate may be generated. Finally, the photodegradation of AP solutions was also studied and optimized in a novel photocatalytic spinning disc reactor (SDR). A heterogeneous process (UV/H2O2/TiO2) was selected. TiO2 was immobilized on the surface of a glass disc using the sol-gel method. AP was completely degraded under the optimal conditions: pH = 4; [H2O2]0 = 1500 mg/L; Disc speed = 500 rpm; Flowrate = 25 mL/s. In addition, regeneration of the disc (up to 10 cycles) was performed with no loss in efficiency. The value of the apparent volumetric rate constant was found to be 6.9·10^4 s-1 with no apparent mass transfer limitation. Based on the main identified intermediates, a reaction mechanism was proposed for antipyrine photodegradation: firstly, cleavage of the N-N bond of penta-heterocycle leads to the formation of two aromatic acids and N-phenylpropanamide. An attack to the C-N bond in the latter compound produced bencenamina. Finally, the phenyl ring of the aromatic intermediates was opened and molecular organic acids were formed. CARBAMAZEPINE Different sytems including UV/H2O2/Fe/US, UV/H2O2/US and UV/H2O2 were used to study the photodegradation process of the antiepileptic drug carbamazepine (CBZ). An important synergistic effect between sonolysis and UV irradiation of 27.7% was quantified using the pseudo-first-order rate constants for carbamazepine degradation. An empirical model that includes the scavenger effect was applied and satisfactorily reproduced both degradation and mineralization of CBZ. It was found that the carbamazepine photodegradation occured mainly through a radical mechanism in two steps: during the first 10–15 min, CBZ was completely degraded, whereas TOC barely changed, confirming that intermediates were not easy to mineralize. From that moment, intermediates were formed and HO· radicals were responsible for increased mineralization rate with a gradual decrease in the scavenger effect (kscv = 0.0004 min^-1 mM^-1). 93% of mineralization in 35 minutes was reached when the initial conditions were [Fe2+]0 = 15 mg/L and [H2O2]0 = 680 mg/L. A study of the flow pattern inside the reactor showed that improvement in mineralization rate with US radiation cannot be attributed to a positive effect in mixing. On the other hand, \textit{in situ} chemical oxidation of CBZ was also performed using persulfate anions simultaneously activated by heat energy (thermal, ultrasound), UV-C light, Fe2+ ions and hydrogen peroxide. Nearly complete mineralization (99%) was reached in 90 min. The mineralization process of carbamazepine solutions can be described using pseudo-second-order kinetics in the studied system. A lab thin film reactor was tested to remove CBZ in a photo-Fenton system assisted with ultrasound radiation (UV/H2O2/Fe/US) achieving 89% of mineralization in 35 min. The synergism between the UV process and the sonolytic one was quantified as 55.2%. The sono-photodegradation of CBZ was also tested in a thin film pilot plant reactor and compared with a 28-L UV-C conventional pilot plant and with a solar Collector Parabolic Compound (CPC). At pilot plant scale, a UV/H2O2/Fe/US process reaching 60% of mineralization would cost 2.1 and 3.8 €/m^3 for the conventional and thin film plant, respectively. In the solar process, electric consumption accounts for a maximum of 33% of total costs. Thus, for 80% TOC removal, the cost of this treatment was about 1.36 €/m^3. However, the efficiency of the solar installation decreases in cloudy days and cannot be used during night, so that a limited flow rate can be treated. REAL WASTEWATER FROM BEVERAGE INDUSTRY Real industrial wastewater from beverage industry was studied during solar photo-Fenton system intensified with ferrioxalates complexes in a CPC plant. 70.6% and 96.6% of TOC was removed after 55 and 125 min, respectively, under optimal conditions (H2O2 flowrate = 460 mL/h}; (COOH)2 flowrate = 2100 mL/h}; [Fe2+]0 = 150 mg/L;} pH = 2.79; Medium solar power = 35.8 Wh). It was found that solar power was the main factor affecting mineralization during the first 60 Wh of accumulated solar energy due to the generation of hydroxyl radicals. However, solar power is unimportant at the end of the process (150 Wh of accumulated energy), when the molecular reaction mechanism between H2O2 and the intermediates was predominant. The overall mineralization process (k = 0.0096 min^-1) occured due to the contributions of the photo-Fenton process (k = 0.0044 min^-1) and the ferrioxalate photochemistry (k = 0.003 min^-1)}. The synergism between both processes was 22.9% based on the pseudo-first-order rate constants for TOC removal. A wastewater with similar characteristics was treated in a conventional UV-C pilot plant using a photo-Fenton process intensified with persulfate. Under optimal conditions (pH = 2.9; [H2O2]0 = 4000 mg/L}; [Fe2+]0 = 375 mg/L), 53% of mineralization was achieved after two hours. The remaining TOC was mainly composed of acetate and formate, whose decarboxylation was limited via hydroxyl radical reactions. Thus, persulfate was added to the system after 2 h to obtain a more efficient decarboxylation by sulfate radicals. The combined treatment with UV-C irradiation and thermally activated persulfate enhanced the mineralization efficiency. Under the best conditions, 76% mineralization was achieved in 4 h: first 2 h under photo-Fenton reaction (UV-C/H2O2/Fe) and UV-C/H2O2/Fe/S2O82-/Thermal process in the second two hours (65 ªC; [S2O82-]0 = 1500 mg/L). WASTEWATER EFFLUENT FROM PHARMACEUTICAL INDUSTRY Finally, a ferrioxalate-assited photo-Fenton process was used to treat an industrial wastewater effluent from a pharmaceutical laboratory in a CPC semi-industrial plant. More than 79% of TOC was removed in 2 h when the aquous effluent containing up to 400 mg/L of organic carbon concentration. It could be seen that most of TOC removal occurs early in the first hour of reaction. Then the degradation shows a slower rate due to the generation of acetates which are more difficult to eliminate.