Removal of micropollutants by an electrochemically driven UV/chlorine process for decentralized water treatment
Introduction
Over the past decades, the growing gap between water demand and conventional water supply has led to a rethinking of the current centralized water infrastructure in many regions (Hodges et al., 2018; Radjenovic and Sedlak, 2015; van Loosdrecht and Brdjanovic, 2014). One emerging solution to supplement the insufficient conventional water supply involves decentralized water treatment systems that can exploit alternative water sources, such as grey water and municipal wastewater effluent. However, these alternative water sources are often contaminated with various trace organic pollutants (micropollutants) such as pharmaceuticals, consumer products, and pesticides. To minimize their potential risks to human health, micropollutants have to be sufficiently removed from the water sources prior to potable/non-potable reuse (Barazesh et al., 2015; von Gunten, 2018). In this regard, advanced oxidation processes (AOPs) have been considered potential treatment options for micropollutant removal in decentralized water treatment (Hodges et al., 2018; Radjenovic and Sedlak, 2015; von Gunten, 2018).
While conventional AOPs (e.g., O3/H2O2 and UV/H2O2) have been successfully applied in full-scale centralized water systems for decades, they do not fit well in decentralized systems due to the need of chemical transport and storage (Barazesh et al., 2015; von Gunten, 2018). In contrast, electricity-driven processes such as UV irradiation and electrochemical production of oxidants (e.g., •OH, Cl2, and H2O2) can avoid chemical management and therefore are particularly suitable for decentralized systems (Chaplin, 2019; Radjenovic and Sedlak, 2015; von Gunten, 2018). Meanwhile, the flexibility, robustness, and modular design of electricity-driven processes also make them attractive for decentralized water treatment (Barazesh et al., 2015; Chaplin, 2019; Radjenovic and Sedlak, 2015). Therefore, it has been expected that electricity-based processes may bear a huge potential in future decentralized water treatment systems (Hodges et al., 2018; Radjenovic and Sedlak, 2015; von Gunten, 2018).
Recently, the UV/chlorine (UV/Cl2) process has emerged as a promising AOP option for micropollutant removal in water and wastewater treatment (Guo et al., 2018; Jin et al., 2011; Remucal and Manley, 2016; Wang et al., 2019; Yang et al., 2016). The UV photolysis of free chlorine (HOCl/OCl−) can lead to the formation of hydroxyl radicals (•OH) and a suite of reactive chlorine species (RCS) such as Cl•, ClO•, and Cl2−•, which can in turn oxidize UV- and/or chlorine-resistant micropollutants. Therefore, the UV/Cl2 process can considerably enhance micropollutant abatement compared to individual UV photolysis and chlorination (Guo et al., 2018; Pan et al., 2017; Remucal and Manley, 2016; Wang et al., 2019; Xiang et al., 2016). Nevertheless, the need of transporting and managing liquid chlorine limits the application of the conventional UV/Cl2 process in decentralized systems.
To overcome this limitation, we proposed in situ chlorine production from anodic chloride (Cl−) oxidation for the application of the UV/Cl2 technology in decentralized water systems in this study. Cl− is a ubiquitous halide ion in almost all water matrices, including the alterative water sources proposed for decentralized systems. Therefore, this approach may offer a simple way to circumvent the barrier of the UV/Cl2 technology for decentralized water treatment. In previous studies, electrochemical production of Cl2 followed by UV irradiation has been investigated for phenol and 1,4-dioxane removal from their synthetic solutions and for ammonia removal from landfill leachate (Hurwitz et al., 2014; Kishimoto et al., 2018; Xiao et al., 2009; Ye et al., 2016). The high Cl− concentrations in the synthetic solutions and leachate (∼890–3000 mg/L) facilitate electrochemical chlorine production, which then enhances substantially the removal of target pollutants during UV irradiation. However, Cl− is typically present at considerably lower concentrations in the alternative water sources proposed for decentralized systems, e.g., ranging from several tens to a few hundreds of mg/L in grey water and municipal wastewater effluent (Lian et al., 2017; Sanchez et al., 2010; Yao et al., 2016; Yu et al., 2013; Zhang et al., 2019). Moreover, the previous studies focused on the removal of high concentrations of model compounds in their synthetic solutions (e.g., ∼88 mg/L of 1,4-dioxane in sodium chloride electrolytes). The feasibility of electrochemical chlorine generation at Cl− concentrations relevant to decentralized water treatment and its effect on micropollutant removal in real water matrix have yet to be investigated.
The objective of this study was to evaluate an electrochemically driven UV/Cl2 (refer to as E-UV/Cl2 hereafter) process that combines UV irradiation with in situ electrochemical production of Cl2 for micropollutant removal under conditions simulating decentralized water treatment. A surface water was spiked with several micropollutants that have varying UV and chlorine reactivities, then treated by UV irradiation, electrolysis, E-UV/Cl2, and conventional UV/Cl2 process. The performance of the different processes was compared in terms of micropollutant abatement efficiency, energy consumption, and chlorinated by-product formation. The mechanisms of micropollutant abatement during the E-UV/Cl2 process were assessed using a chemical kinetic approach. The effects of important process and water parameters (electrodes, current densities, and Cl− concentrations) of the E-UV/Cl2 were then evaluated systematically.
Section snippets
Chemicals and reagents
Carbamazepine, metoprolol, ciprofloxacin, and trimethoprim with purity >98% were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China) and used as the model micropollutants in this study. The second-order rate constants for the reaction of the model micropollutants with main oxidants in the UV/Cl2 system (HOCl, •OH, Cl•, ClO•, and Cl2−•), as well as their UV254 M absorption coefficient (ε254) and quantum yield (Φ254) are summarized in Table 1. All other chemicals used in this study were of
Electro-generation of chlorine
Fig. 2 shows that during electrolysis and the E-UV/Cl2 process, the concentrations of free chlorine (HOCl/OCl−) increased continuously in the surface water (added with 30–300 mg/L Cl−). These observations indicate that Cl− is stably oxidized at the RuO2/IrO2–Ti anode surface to form Cl2, which then quickly hydrolyzes to yield free chlorine (Eqs. (2), (3)) (Martinez-Huitle et al., 2015), = 28.6 s−1 (Wang and Margerum, 1994)) in the water. With increasing the initial concentration of Cl−
Conclusions
This proof-of-concept study introduces the electrochemically driven UV/Cl2 (E-UV/Cl2) process as a promising technology for micropollutant abatement in decentralized water treatment systems. By electrochemically producing chlorine from the anodic oxidation of chloride present in source waters, the E-UV/Cl2 process avoids the use of chlorine stocks and thus overcomes the main barrier of the conventional UV/Cl2 process for decentralized systems. Owing to the multiple abatement mechanisms of the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study is funded by the NSFC (National Natural Science Foundation of China) project (51878370), the National Special Program of Water Pollution Control and Management (2017ZX07202), the special fund of State Key Joint Laboratory of Environment Simulation and Pollution Control (18L01ESPC), Tsinghua University-INDITEX Sustainable Development Fund (TISD201906), and Foshan-Tsinghua Innovation Special Fund (FTISF 2019THFS01-005).
References (45)
- et al.
Kinetics of aqueous chlorination of some pharmaceuticals and their elimination from water matrices
Water Res.
(2010) - et al.
Modular advanced oxidation process enabled by cathodic hydrogen peroxide production
Environ. Sci. Technol.
(2015) - et al.
Electrochemical transformation of trace organic contaminants in the presence of halide and carbonate ions
Environ. Sci. Technol.
(2016) - et al.
Figures-of-merit for the technical development and application of advanced oxidation technologies for both electric- and solar-driven systems - (IUPAC Technical Report)
Pure Appl. Chem.
(2001) - et al.
Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (•OH/•O−) in Aqueous Solution
J. Phys. Chem. Ref. Data
(1988) Critical review of electrochemical advanced oxidation processes for water treatment applications
Environ. Sci. Proc. Impacts
(2014)The prospect of electrochemical technologies advancing worldwide water treatment
Acc. Chem. Res.
(2019)- et al.
Aqueous chlorination of the antibacterial agent trimethoprim: reaction kinetics and pathways
Water Res.
(2007) - et al.
Interactions of fluoroquinolone antibacterial agents with aqueous chlorine: reaction kinetics, mechanisms, and transformation pathways
Environ. Sci. Technol.
(2005) - et al.
The roles of reactive species in micropollutant degradation in the UV/free chlorine system
Environ. Sci. Technol.
(2014)