Investigating the roles of MnO2-acid complexes in catalytic ozonation for enhanced micropollutants abatement in water

Abstract

[eng] A significant amount of organic pollutants has been released into the water environment with the rapid growth of the population. To remove refractory compounds from the aquatic environment, advanced oxidation processes (AOPs), such as electrochemical methods, Fenton system, irradiation and ozonation have attracted more attention. Heterogeneous catalytic ozonation (HCO), as one of the AOPs, could shorten treatment time, enhance pollutant removal efficiency, and improve ozone (O3) availability, which has gained increasing interest. Considering that catalysts used in the HCO process should be low-cost, environmentally friendly, and readily available, manganese dioxide (MnO2) composites were used as catalysts in this study to explore their catalytic activity towards pollutant removal in water. In this thesis, alpha-MnO2, beta-MnO2, and gamma-MnO2 were fabricated through a modified hydrothermal-calcination method and evaluated for their potential catalytic ozonation performance in degrading oxalic acid (OA) through semi- continuous experiments. In order to control individual variables of experimental conditions, batch experiments were further conducted to assess the catalytic activities of the catalysts. It was found that the introduction sequence of the catalyst and O3 stock solution greatly affects the efficiency of the HCO system. For example, when the alpha-MnO2 catalyst (50 mg L-1) was added to the OA (10 mg L-1) solution first, followed by O3 introduction (30 mg L-1), the OA removal rate reached 78.22%. Conversely, the efficiency was only 20.53% when the sequence weas reversed. Pre-contact between oxalic acid (OA) and the catalyst seems to have a significant impact on the efficiency of catalytic ozonation. To identify the influence of OA in the process, Cat/OA/O3 system was established using atrazine (ATZ) as a model pollutant. The effect of operational parameters on catalytic performance including the O3 concentrations (1-30 mg L-1), catalysts dosages (10-50 mg L-1), pH values (pH=3-7), and the molar ratio of catalyst to OA were investigated. Compare to O3 alone (43.6%), when the molar ratio of catalyst-to-OA increased to 1:0.8, the ATZ (5 mg L-1) degradation efficiencies within 3 min were 95.09%, 93.43%, and 96.71% for the alpha- MnO2/OA/O3, beta-MnO2/OA/O3, and gamma-MnO2/OA/O3 systems at pH 3.0, respectively. This study confirmed that the complexation between metal oxides and organic acid plays an important role in water treatment. However, the mechanism of complex in catalytic ozonation process (HCO) was not studied yet. To elucidate the mechanism, as well as the phase effect and ROS contribution for each system, a series of characterizations, pyrophosphate experiments, and probe experiments were designed. The results verified that the formation of the MnO2-OA complexes relied on the presence of Mn3+ on the catalyst surface, which enhanced electron transfer and facilitated the creation of active sites such as oxygen vacancies. This, in turn, accelerated O3 decomposition, generating more reactive oxygen species (ROS) and further promoting pollutant degradation. In addition, the crystalline phase of MnO2 did not influence the formation of MnO2-OA complexes but affected the reaction sites between the complexes and O3. For the alpha-MnO2-OA complex, more oxygen vacancies were generated on the surface, adsorbing O3 and pollutants for further reaction. For the a-MnO2-OA complex, more manganese vacancies and enhanced lattice oxygen (OL) mobility were key factors contributing to ROS production. As for the delta- MnO2-OA complex, the surface -OH groups acted as the main active site, reacting with O3 to generate more ROS, thus improving ATZ degradation. Interestingly, probe experiments suggested that in the MnO2-OA complex- mediated catalytic ozonation system, the O2•- played a major role in ATZ removal, followed by •OH, while O3 and 1O2 had a lesser effect. This thesis provides new insights into the potential applications and perspectives of MnO2 in catalytic ozonation.

[cat] Amb el ràpid creixement de la població, es generen grans quantitats de contaminants orgànics que s'alliberen al medi aquàtic. Per eliminar aquests compostos refractaris, els processos d'oxidació avan9ada (A0Ps), entre els quals destaca l'ozonització catalítica heterogènia (HC0), han demostrat la seva eficàcia, ja que redueixen el temps de tractament, milloren l'eliminació de contaminants i optimitzen l'ús de l'ozó (03). El di6xid de manganès (Mn02) és un catalitzador atractiu gracies al seu baix cost i fàcil disponibilitat. En aquest estudi, s'han sintetitzat alfa-Mn02, beta-Mn02 i delta-Mn02 mitjançant un mètode hidrotermal-calcinat modificat, i s'ha avaluat la seva eficàcia en la degradació d'àcid oxàlic (0A). S'ha comprovat que la seqüència d'addició del catalitzador i de l'03 influeix de manera determinant en l'eficiència del procés HC0: per exemple, amb a-Mn02 es va assolir un 78,22% d'eliminació d'0A quan s'afegia l'ozó a la solució amb el catalitzador, en comparació amb el 20,53% en invertir l'ordre. A més, d'altres àcids (FA, AA, CA i PA) també han mostrat capacitat per promoure l'ozonització catalítica a través de la formació de complexos amb Mn02. Per tal d'entendre el mecanisme, s'ha estudiat en detall el sistema í3- Mn02/0A/03, aconseguint un 90,4% de degradació de pCBA i un 65,2% d'eliminació d'0A en 10 minuts a pH 4,0. Les caracteritzacions (XPS, H2-TPR, etc.) indiquen que el complex í3-Mn02-0A millora la transferència electr6nica i afavoreix la formació de vacants d'oxigen i espècies reactives d'oxigen (R0S). Mitjan9ant l'ús de pirofosfat de sodi es va confirmar que el Mn3+ és fonamental en la formació d'aquest complex, mentre que els experiments amb captadors de radicals van identificar el radical •0H com l'espècie principal en la degradació de pCBA. Les diferents fases cristal·lines de Mn02 no afecten la formació del complex Mn02-0A-dependent del Mn3+-per6 sí que influeixen en la via de producció de R0S. Al sistema alfa-Mn02-0A predominen vacants d'oxigen superficial, al beta-Mn02-0A destaquen vacants de manganès i més mobilitat de l'oxigen de xarxa, i al delta-Mn02-0A són els grups -0H els principals centres actius. Els radicals 02•- i •0H s'han identificat com els responsables principals de la degradació dels contaminants.