Insight into synergies between ozone and in-situ regenerated granular activated carbon particle electrodes in a three-dimensional electrochemical reactor for highly efficient nitrobenzene degradation
Graphical abstract
Introduction
During the past decades, electrochemical technologies had been regarded as promising methods to degrade bio-refractory organic contaminants in wastewater treatment due to the methods having high efficiency, easy operation, and environmentally friendly [1], [2], [3]. The three-dimensional (3D) electrode or bed electrode provides higher current efficiency due to enhanced conductivity, mass transfer, and adsorption of contaminants by the granular activated carbon or metal particles as 3D electrodes in electrochemical system [4], [5]. The added particles in electric field can be polarized to form a large amount of charged microelectrodes with much larger specific surface area, leading to the significant enhanced electrochemical oxidation of contaminants [6], [7].
Meanwhile, ozone could oxidize various organic pollutions. It is known that ozone is a highly selective oxidant, which can only rapidly react with double bonds and active aromatic compound such as methacrylic acid and phenols. However, for some other compounds (e.g. chlorobenzene, nitrobenzene, and iopromide) [8], [9], they were refractory to the direct oxidation of O3 (e.g. kO3 ≤ 9 × 10−2 M−1s−1, kO3 ≤ 0.75 M−1s−1, and kO3 ≤ 0.8 M−1s−1 for chlorobenzene, nitrobenzene (NB), and iopromide, respectively). To overcome this restriction, heterogeneous catalytic ozonation, as a promising OH based advanced oxidation processes (AOPs), has been applied to improve the efficiency of oxidation of ozone-resistant compounds [10], [11], [12], because OH is a strong oxidant and able to attack most organics without selectivity. Therefore, during ozonation, organic pollutants can be decayed by both direct oxidation of O3 (E0 = 2.07 eV) and oxidation of OH (E0 = 2.8 eV) [13], [14]. Specifically, some researchers have found that activated carbon can accelerate the decomposition rate of O3 for the enhancement OH generation [15], [16], [17], while other researchers, including our group, have demonstrated the combination of ozone and electrolysis can significantly enhance the generation of OH. [18], [19], [20], [21]. Meanwhile, Yuan et al. used the in situ H2O2 produced on cathode surface to react with O3, developed and firstly named the Electro-peroxone process [22].
Furthermore, Zhan et al. have found that the combination of ozone with a 3D electrochemical process with 50 g L−1 granular activated carbon (GAC) as the particle electrodes enhanced the treatment of pharmaceutical wastewater with a high organic concentration [23]. An obviously synergistic effect between 3D electrochemical process and ozonation (E-GAC-O3) for the degradation of toxic organics was observed based on the significant enhanced OH production in the E-GAC-O3 process. More importantly, the adsorption capacity of GAC or powdered activated carbon (PAC) as 3D electrodes in electro-peroxone process could be regenerated in-situ during eight or five consecutive cycles, respectively [23], [24]. The organics adsorbed on the activated carbon (AC) or desorbed in the solution could be oxidized by anode, ozone, and reactive oxidant species, especially OH, in the electro-peroxone process, in which several reaction mechanisms, including peroxone reactions, heterogeneous (AC) catalytic ozone, and the electro-reduction of ozone at cathode were mainly proposed for the generation of OH. Nevertheless, as reported other literatures, it was worthy to note that the AC acts as an initiator and/or a promoter rather than a catalyst in an AC/O3 process without electric field, since the activity of AC for ozone transformation into OH gradually declines with prolonged ozone treatment time due to the simultaneous transformation of basic surface sites (active sites for ozonation) on AC into acidic surface groups (inactive sites for ozonation) [25], [26], [27], [28]. These previous results trigger the following questions: (i) What are the relative contributions of peroxone reactions (the reaction between ozone and in situ generated H2O2 as homogeneous catalytic ozonation), the GAC catalyzed ozone decomposition (heterogeneous catalytic ozonation on GAC), the electrochemical oxidation of hydroxyl ion (electrochemical reactions at anode), and the electrochemical reduction of ozone reactions (electrochemical reactions at cathode) for generating OH in the E-GAC-O3 process? (ii) Will the in-situ regenerated GAC in the E-GAC-O3 process be lost in the activity for ozone transformation into OH with prolonged ozone treatment time? In our previous studies, the electrochemical methods employed could catalyze ozone more effectively with activated carbon fiber (ACF) as cathode, compared with inert metal cathode. The injection of free electrons from direct-current electrical source onto ACF could prevent it from being destroyed by the oxidation of ozone and generated reactive oxidative species (ROS) over long time use, and thus maintain the amount of basic surface sites on ACF (Zhang et al. 2016). However, in the E-GAC-O3 process, unlike the cathodic ACF, the added GAC particles were polarized to be bipolar microelectrodes without a direct connection to the cathode all the time. Knowledge of stability of GAC, especially the transformation of basic surface sites into acidic surface groups on GAC, during long consecutive recycles in E-GAC-O3 process, was urgently required to better understand the mechanism of GAC for catalyzing ozone as bipolar microelectrodes as well as regenerating in-situ as adsorbent in the 3D electrochemical reactor.
Herein, ozone gas was aerated into a compact 3D electrochemical reactor with GAC as 3D electrodes to construct a model E-GAC-O3 system for degrading nitrobenzene (NB) as target contaminant, since NB was refractory to the oxidation of O3, H2O2, O2−, and O3−, except for OH. The main objectives of this study were to: (i) investigate the performance of E-GAC-O3 in terms of NB removal and mineralization, and evaluate the synergy of E-GAC-O3 compared with GAC adsorption, electrolysis, ozonation, GAC-O3, E-O3, and E-GAC processes; (ii) probe the ROS (O2−, H2O2, and OH), and estimate the relative contribution of different approaches for NB removal, including direct anodic oxidation, GAC adsorption, direct ozonation, ROS oxidation, and nonradical oxidation, for NB removal; (iii) produce a quantitative analysis of peroxone reactions, GAC catalyzed ozone reactions, electrochemical oxidation of hydroxyl ion, and electrochemical reduction of ozone reactions for the generation of OH in the E-GAC-O3 process; (iv) appraise the stability of GAC during long consecutive recycles in the E-GAC-O3 process with or without cathode contact, and explore the change of textural and surface properties on GAC; (v) examine the potential for practical applications (different organic pollutants, and in different water matrices).
Section snippets
Materials
GAC and purchased chemicals are summarized in Text S1.
GAC characterization
The methods of GAC characterization are listed in Text S2.
Experimental section
The experimental reactor was an undivided 750 mL glass column (see in Fig. S1). The Pt-plating titanium (Pt-Ti) electrodes (50 mm × 35 mm) were positioned vertically and parallel to each other with an inter-electrode gap of 4.0 cm, GAC packed between the anode and cathode [29], [30], [31]. The reactor was placed in a constant temperature tank with a magnetic stirrer under the bottom. O
Comparison between processes on NB removal
The removal of NB by GAC adsorption, electrolysis, O3, E-GAC, GAC-O3, E-O3, and E-GAC-O3 processes is compared in Fig. 1(a). The results showed that both GAC adsorption and electrolysis presented a low removal efficiency of NB, just 8.94% and 31.02%, respectively, within 40 min. Besides, the combination of electrolysis and GAC as 3D electrodes (E-GAC) was still inefficient to remove NB with only 35.82% removal ratios of NB. In addition, the removal ratio of NB by O3 alone was 43.34% within
Application
To systemically evaluate the practical application of the E-GAC-O3 process, the specific energy consumption (SEC), removal of other persistent organic pollutants, and removal of NB in different water matrices were investigated as follows.
Conclusions
In this report, the combination of electrolysis, GAC, and ozone (E-GAC-O3) was systematically studied, and the process displayed high NB removal and TOC removal efficiency. In addition, this process demonstrated a significant synergy for NB removal, where 92.30% of NB removal was due to the oxidation of OH. The synergy is because more OH could be generated in E-GAC-O3 process. Further study proved that electrochemical oxidation of hydroxyl ion, peroxone reactions, GAC catalyzed ozone, and
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 work was sponsored by the 111 Project, No. B13041, Central University Basic Research Fund (2019CDXYCH0027), the National Natural Science Foundation of China (Grant number 51308563), the Chongqing Natural Science Foundation Project (cstc2019jcyj-msxmX0463), and includes funding provided under the Ministry of Education “1000 Experts” scheme.
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