Degradation of representative compounds in water by reductant-enhanced advanced oxidation processes

Abstract

In recent years, concerns about the increasing occurrence of emerging contaminants present in the natural water environment have gained widespread attention. Such compounds, which could cause adverse effects in aquatic organisms and to human health, are susceptible to poor removal during the traditional wastewater treatment methods, resulting in their entry into natural waterways. Therefore, advanced oxidation processes (AOPs) have been extensively studied for its high efficiency on degradation and mineralization of such emerging contaminants. Transition metal-based materials are widely used in AOPs for their excellent performance in peroxide activation to generate reactive oxygen species (ROS) for the elimination of organic contaminants. However, while ROS are generated by the electron transfer from low-valent transition metal ion (Mn+) to peroxide, high-valent transition metal ion (M(n+1)+) with relatively weak electron-donating ability start to accumulate in the reaction system. The slow reduction rate of Mn+ regeneration from M(n+1)+ in the reaction systems severely impede the constant ROS production, thereby inhibiting the continuous degradation of target contaminants. In order to fix the Achille’s heels, four reductant enhanced advanced oxidation processes have been proposed in this dissertation. Firstly, a visible light enhanced Co-TiO2/sulfite system was proposed for metronidazole (MNZ) degradation. Cobalt species can accelerate the oxidation of Na2SO3 and generate sulfite radicals efficiently, and sulfite reduced Co3+ to Co2+, promoting the Co3+/Co2+ redox cycle. Up to 94% of MNZ was removed after 18 min reaction with MNZ concentration, Co-TiO2 and sulfite dosage being 0.05 mM, 0.1 g/L and 5 mM, respectively. Secondly, g-C3N4 was used as a heterogenous reductant in a Fe3+/g-C3N4/peroxymonosulfate (PMS)/LED process. Under visible light, electron transfer from photo-activated g-C3N4 to Fe3+, resulting in the continuous regeneration of Fe2+ in the system, which ensures non-stopping production of radicals for MNZ degradation. When 1 mM PMS, 0.04 mM Fe3+ and 0.05 g L-1 g-C3N4 were applied, the rate constant of the Fe3+/g-C3N4/PMS/LED process at 0.07288 min-1 is around 3.6 to 6.8 times faster than that of Fe3+/PMS/LED and g-C3N4/PMS/LED processes at 0.0198 and 0.01076 min-1, respectively. Thirdly, copper doped TiO2 and zinc doped TiO2, were used to activate peroxydisulfate under UVA-LED for bisphenol S degradation. Cu-TiO2 is used as a heterogenous catalyst for peroxydisulfate activation, and Zn-TiO2 serves as a heterogenous reductant. When 5 mM peroxydisulfate and 0.3 g/L catalyst were used, the removal rate of mixed catalyst (0.2 g/L Zn-TiO2 and 0.1 g/L Cu-TiO2) is 100 % in 18 min, which is significantly better than that of 0.3 g/L Zn-TiO2 (58 %) and 0.3 g/L Cu-TiO2 (90 %). Fourthly, in order to reduce the use of metal catalyst, a metal-free catalyst, multi-walled carbon nanotubes (MWCNTs) is proposed as an alternative for PMS activation. A hybrid catalyst system consisted of MWCNT and Co-TiO2 through simple physical mixing was applied for PMS activation for antipyrine degradation. With 2 mM PMS, the removal rate of antipyrine is significantly increased from 53.24% and 86.23% to 100% in the presence of same weight of 0.2 g/L MWCNTs, Co-TiO2, and MWCNTs/Co-TiO2 (half/half) mixture, respectively in 12 min. This study not only provides a novel and efficient treatment methods for emerging contaminants, but also reveals a new perspective to use the reductants for enhancing traditional advanced oxidation processes.