Activated carbon as effective cathode material in iron-free Electro-Fenton process: Integrated H2O2 electrogeneration, activation, and pollutants adsorption
Graphical abstract
Introduction
Contamination of groundwater by toxic and persistent organic compound (OCs) has been a global environmental concern for decades [1,2], and effective remediation and cleanup is still a challenge. The Electro-Fenton (EF) process is an efficient approach that has attracted attention in the last 20 years for wastewater treatment [[3], [4], [5]] and groundwater remediation [6,7]. In the EF process, H2O2 is produced in-situ by the reduction of dissolved oxygen on the cathode (Eq. (1)) [8,9]. Hydroxyl radicals are then generated from the catalytic decomposition of H2O2 in the presence of an iron catalyst (e.g., Fe2+, Fe3+, or iron oxides) (Eq. (2)) [10,11]. The degradation efficiency of the EF process depends on the productivity of H2O2, which is highly dependent on cathode materials.O2 + 2H+ + 2e− → H2O2 (0.695 V vs. SHE)H2O2 + Fe2+ → Fe3+ + OH + OH−
Several studies have focused on the development of efficient cathode materials. Various carbonaceous materials were considered for H2O2 electrogeneration in the EF process because they are non-toxic, conductive, chemically resistant, and exhibit high overpotential for H2 evolution with low catalytic activity for H2O2 decomposition [12]. So far, graphite [13], graphite felt [14,15], carbon felt [16,17], reticulated vitreous carbon [18,19], activated carbon fiber [20], carbon sponge [21] and carbon nanotubes [22] were proposed as cathode materials. To further enhance H2O2 production, various cathode improvements were considered including electrochemical modification [23,24], Fenton treatment [25], strong acids treatment [23,26], and plasma treatment [27]. However, these pretreatments tend to be costly, difficult to scale-up, and could generate secondary pollution to the environment.
Activated carbon (AC) is among the cheapest carbon and has been produced in large-scale quantities from different precursors such as coal, coconut shells, wood chips, bamboo, and sawdust [28]. Fabrication of AC as a cathode for H2O2 production will be significant for large-scale applications. AC powder has been used in microbial fuel cells (MFCs) [29], Li-air AC cathodes [30], and super-capacitors [31] as the electrode materials. AC powder was also introduced as a graphite electrode [22], which improved activity for H2O2 generation. However, the process is complex and expensive. In these applications, poly(fluortetraethylene) (PTFE) [12,32] and poly(vinylidene fluoride) (PVDF) [29] were used as a binder for AC powder and catalysts; and the potential release of these catalysts or binders could cause secondary pollution [33]. Furthermore, this thin layer of catalyst on the surface of AC limits the H2O2 yield.
We propose a cathode fabricated by granular activated carbon (GAC) wrapped with stainless steel (SS) mesh to overcome these challenges. The SS mesh will serve to distribute the current and maintain a relatively uniform current density even for larger electrodes. Other advantages for using this electrode include simple electrode mass adjustment by adding or removing GAC from the SS mesh, effective O2 mass transfer due to the structure of SS mesh, and a rigid cathode structure. These characters will also facilitate scale up of the electrode and the process.
H2O2 activation is very important for the success of the EF process. Usually, H2O2 is activated by Fe2+, which is supplied by the addition of ferrous salts [34,35]. However, the separation of the dissolved iron and the treatment and disposal of the resulting iron sludge are major drawbacks [36]. Thus, a metal-free system is preferable. Due to its large surface area, a well-developed porous structure, and its surface functional groups, AC has been extensively applied as an environmentally-friendly catalyst in the catalytic wet peroxide oxidation process [37,38]. The AC/H2O2 system, capable of generating hydroxyl radicals (OH) (Eqs. (3), (4))) and superoxide radical anions (HO2/O2−), has been used to degrade contaminants and regenerate AC [39,40]. More importantly, AC has long been extensively used as a strong adsorbent for various OCs in water treatment [41]. The short-lived radicals (e.g., OH has a lifetime of ∼20 ns [39]) formed on the AC surface are in close proximity to the preconcentrated OCs, which might enhance rates and efficiency of the degradation reactions [39]. Moreover, the traditional AC/H2O2 system results in oxidation-related changes in the AC surface structure. Using AC as a cathode will benefit from the electrically induced protection of the AC surface [42]. Recently, activated carbon has been examined to be effective for H2O2 electrogeneration [[42], [43], [44]]. However, its utilization for integrated H2O2 electrogeneration, H2O2 activation, and OCs adsorption deserves further investigation. Thus, the proposed ACSS cathode, which utilizes AC for H2O2 electrogeneration as well as for H2O2 activation, in an iron-free EF system will have significant implications.AC + H2O2 → AC+ + OH− + OHAC+ + H2O2 → AC + H+ + HO2
In this study, a novel and low-cost EF ACSS system using AC as the catalyst for synergetic H2O2 electrogeneration, H2O2 activation for OH generation, and for OCs adsorption/degradation is proposed. The performance of ACSS cathode on H2O2 production, H2O2 catalytic decomposition, and OH generation was investigated. The influence of adsorption on H2O2 production and activation by ACSS cathode was assessed. Furthermore, the effectiveness of the ACSS cathode for removal of organic compounds (Reactive Blue 19 (RB19)) was evaluated in a batch reactor. The stability of the prepared ACSS electrode after repeated applications was then tested. A mechanism of the synergistic functions of ACSS electrode is proposed based on the experimental results.
Section snippets
Materials and chemicals
Sodium sulfate (anhydrous, ≥99%), titanium sulfate (99.9%), and hydrogen peroxide (30%wt) were purchased from Fisher Scientific. Reactive Bule 19 (C22H16N2Na2O11S3, RB19) at 99.9% purity was purchased from Sigma-Aldrich. RB19 is selected because it's negatively charged and adsorption on ACSS cathode will be limited. Deionized water (18.2 MΩ cm) obtained from a Millipore Milli-Q system was used in all the experiments. Solution pH was adjusted by sulfuric acid (98%, JT Baker) and sodium hydroxide
Characterization
The fabrication of the ACSS cathode and the proposed treatment system are illustrated in Fig. 1(a). The SS mesh functions as a rigid holder and current distributor for GAC, thus benefits from the conductive nature of GAC and makes two electron oxygen reduction reaction (2eORR) species (O2, H+, H2O) accessible to GAC vicinity. The SEM images of the GAC exhibit analogous compact stacking structure, which is possibly derived from the morphology of the precursor. The specific surface area and
Conclusions
This study reports an efficient and cost-effective iron-free EF process enabled by ACSS electrode. The ACSS electrode achieved simultaneous H2O2 electrogeneration via anodic O2 reduction, H2O2 activation for OH generation, and organic contaminants adsorption. Both electrolyte pH and current has an important impact on H2O2 production. Results on H2O2 activation by ACSS electrode showed that neutral pH is the optimal pH for H2O2 activation and OH production. Saturation of model pollutant RB19
Acknowledgment
This work was financially supported by the US National Institute of Environmental Health Sciences (NIEHS, Grant No. P42ES017198) and National Natural Science Foundation of China (Grant No. 51776055). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIEHS, the National Institutes of Health and the National Natural Science Foundation of China. The authors thank Yan Wang, Yani Ding, and Kaikai Kou on the characterization of
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