Degradation of persistent organic pollutants in wastewater by periodate-based advanced oxidation processes

Water pollution is currently an important environmental issue. The increasing demand for sustainable wastewater treatment methods, especially for the removal of recalcitrant pollutants, has prompted the development of various advanced oxidation processes (AOPs) in recent years. Among them, periodate (IO4-)-based AOPs have emerged as a promising approach for the treatment of wastewater by generating various types of strong oxidative radicals (·OH, IO·, IO2·, IO3·, etc.). Periodate is a promising oxidizing agent for AOPs due to its inherent stability and ease of handling, transport, and storage. However, the number of reports on the use of periodate-based AOPs for the degradation of organic pollutants in wastewater is limited, indicating the need for further exploration and investigation. The above background is also introduced in Chapter 1. This thesis studied the feasibility of the degradation of various refractory organic compounds by two main periodate-based technologies: UV-based periodate-AOPs (photo-activated AOPs) and catalytic periodate-AOPs. In this regard, different periodate-AOPs were compared considering various operating conditions to explore the most suitable AOPs for treating recalcitrant pollutants in different types of wastewater.

In Chapter 2, the efficiency and kinetics rate of photoactivated periodate processes for the degradation of 2,4-dichlorophenol (2,4-DCP) in water was investigated. The results revealed that the degradation rate was significantly higher for UV/periodate than for UV irradiation alone. Pseudo-first-order reaction rate kinetics were obtained for all process conditions. Furthermore, the results demonstrated the marginal impact of pH on the decomposition of 2,4-DCP using the UV/periodate system, indicating that the method has the potential to be used for treating (waste)water at various pH levels. The efficiency of degradation was found to be proportional to the UV intensity. A mechanistic study revealed that iodine radicals were the dominant species responsible for the degradation of 2,4-DCP by photoactivated periodate, while ·OH and O(3P) played only a minor role. At pH 5.0, all chlorine atoms in 2,4-DCP were released as chloride ions in the UV/periodate process, resulting in total dechlorination. Based on the findings of this study, the UV/periodate system was considered a possible valuable alternative for the treatment of effluents containing chlorinated organic compounds.

The combination of periodate with UV and catalysts (i.e., TiO2) was also investigated in Chapter 3 with an enhanced impact on the degradation efficiency of the pollutants. In this chapter, the removal of Rhodamine B (RhB) was investigated using the UV/TiO2/periodate system, as well as the effects of various process parameters on the removal efficiency. The combined UV/TiO2/periodate system showed an improved removal efficiency in comparison with the UV/TiO2 and UV/IO4- processes. Scavenging experiments were conducted to identify the involved degradation mechanisms, which revealed the contribution of O2− > IO3∙ and IO4∙ > h+ > O(3P) > ∙OH to the degradation of RhB using the UV/TiO2/periodate process. The performance of the system was found to be independent of pH.

In Chapter 4, a facile precipitation process was used to synthesize CuO and different techniques were used to characterize it to effectively activate periodate. The CuO (0.5 g/L)/periodate (0.5 mM) system demonstrated a better degradation efficiency for ciprofloxacin (CIP) when compared to systems consisting of periodate alone (0.5 mM) and CuO alone (0.5 g/L), with 98% of this pollutant (20 mg/L) eliminated in a reaction time of 30 min. Compared to alkaline conditions (pH=10), the system demonstrated enhanced effectiveness for the breakdown of CIP under both acidic (pH=3) and near-neutral (pH=6) conditions. This was attributed to the more facile adsorption of periodate ions onto the surface of the CuO nanoparticles, induced by the electrostatic attraction between the periodate ions and the CuO nanomaterial. Analysis of the degraded samples with UHPLC Q-TOF MS revealed that cleavage of the piperazine ring and defluorination were the primary degradation pathways of CIP under this oxidation process. This research presented a novel and promising periodate activation technique for the removal of refractory organic contaminants in wastewater treatment.

In Chapter 5, the degradation kinetics and mechanisms of MB by a novel biochar-CuO (B-CuO)/periodate system were investigated in-depth. According to the results achieved, iodate (IO3∙) and periodate radicals (IO4∙), ∙OH and O(3P) were mostly generated at pH ≈ 7, resulting in the efficient removal of methylene blue (MB) (90% after 30 min, kobs=0.09 min-1), while the system favored the formation of O(3P) under highly alkaline pH values (pH = 11), leading to lower degradation kinetics rate of the pollutant (51% after 30 min, kobs = 0.028 min-1). Additionally, no oxidative species were found at highly acidic pH values (pH = 3), and no significant MB removal was observed (21% after 30 min, kobs = 0.0081 min-1). The ECOSAR program was employed to evaluate the predicted toxicity of the degradation products. The results showed that the system could efficiently detoxify MB.

However, there were other parameters of high importance, including the total treatment costs and subsequent environmental impacts, to be considered for transferring these technologies from lab-scale to real applications. Therefore, Chapter 6 aimed to provide discussions on the economic aspects of the studied periodate-AOPs, the toxicity of the effluents before and after treatment with such techniques, their applicability for the treatment of various types of persistent organic pollutants and to treat of real effluents and a relevant case study. All the studied systems maintained their efficiency in various water matrices, including real wastewater, confirming the applicability of the periodate-AOPs system for real water treatment processes. The applicability of different periodate-based AOP systems was evaluated for the degradation of various refractory organic pollutants (i.e., sulfamethoxazole (SMX), carbamazepine (CBZ), CIP, 17 α-ethinylestradiol (EE2) and diclofenac (DCF)) and organic dyes (i.e., RhB, methyl orange (MO) and MB). Despite differences in degradation efficiency for these pollutants, the results confirmed the applicability of the periodate-based AOP system for the efficient removal of all these pollutants within 30 minutes of reaction. These results showed that periodate-based AOP systems can effectively degrade organic pollutants with different functional groups in wastewater, such as drugs and dyes. Respirometry measurements were also used to evaluate the toxicity of the solution after reaction and ECOSAR to evaluate the toxicity of the reaction intermediates. The results revealed that the toxicity of the effluents was reduced after the treatment process. UV-based periodate-AOPs and catalytic periodate-AOPs are hence very effective in removing various refractory organic compounds from wastewater. However, there are still some challenges related to large-scale direct applications. Chapter 6 also compares UV-based periodate-AOPs and catalytic periodate-AOPs, evaluates their advantages and disadvantages, and provides some suggestions and recommendations for future research.