Simultaneous regeneration of cathodic activated carbon fiber and mineralization of desorbed contaminations by electro-peroxydisulfate process: Advantages and limitations

Highlights

• E-PDS achieved phenol mineralization and ACF regeneration simultaneously.

• The accumulation of byproducts could be reduced by multiple dosing PDS.

• The re-adsorption of byproducts and ROS oxidation of ACF reduced the regeneration efficiency.

• Mechanism and limitations of $S{O}_4^{•-}$ based E-PDS for ACF regeneration were explored.

Abstract

This study investigated the regeneration of phenol saturated activated carbon fiber (ACF) with a novel electro-peroxydisulfate (E-PDS) process. Compared with traditional electrochemical regeneration, E-PDS process could simultaneously regenerate the exhausted ACF and mineralize desorbed contaminants by activating PDS in water with a much lower energy consumption (1/6). According to the estimation of relative contributions involved in E-PDS process, reactive oxygen species (ROS), especially sulfate radical ($S{O}_4^{•-}$), played a dominant role in the degradation of phenol and its byproducts. It was worth noting that the accumulation of byproducts in solution increased significantly after $S{O}_4^{•-}$ concentration decreased in aqueous solution. Further study proved that the regeneration efficiency of ACF could be improved by the application of multiple doses of PDS for the effective reduction of byproduct accumulation. However, application of multiple doses of PDS could not prevent ACF from being oxidized by ROS generated in the system, subsequently leading to loss of ACF adsorption capacity. This limitation is a significant concern in treatment technologies based on carbon materials activated by peroxides and such technologies should be studied further to obtain additional insights on their potential and applicability in industrial practice. Nevertheless, the adsorption capacity of ACF remained above 40% after three regeneration cycles in the E-PDS process. Therefore, E-PDS process showed promise for further evaluation as a potentially viable approach for the regeneration of carbons saturated with organic pollutants.

Introduction

Activated carbon (AC) materials are commonly used as effective adsorbents for the removal of organic contaminants from wastewater, especially from industrial wastewater, due to their large surface area, high porosity, and large numbers of surface functional groups (Gupta and Suhas, 2009; Moreno-Castilla, 2004). However, this removal process only transfers the contaminants from the liquid phase to the solid phase, and the contaminants are not actually degraded. In addition, during the adsorption process, AC gradually loses its activity and needs treatment after it is exhausted. The exhausted AC is usually incinerated or landfilled to prevent secondary pollution (Wang and Balasubramanian, 2009). These disposal methods have resulted in the waste of resources and energy, so AC regeneration technologies that are more environmental friendly should be developed.

Currently, thermal regeneration is the most commonly used regeneration method. Thermal regeneration at high temperature can restore over 80% adsorption capacity of AC (Alvarez et al., 2004; Marques et al., 2017). However, the drawbacks of thermal regeneration are also obvious. As reported, AC could lose mass during the thermal regeneration due to the carbon burn-off. This problem could be more severe after repeated thermal regenerations (Ania et al., 2004; San Miguel et al., 2001). Moreover, this method consumes large amount of energy, which leads to high operation cost, thus affecting the application of activated carbon adsorption technology in wastewater treatment (Hutchins, 1973).

In the past decades, electrochemical regeneration appeared as a promising technology for regeneration of exhausted AC (McQuillan et al., 2018; Narbaitz and CEN, 1994). Compared with thermal regeneration, electrochemical regeneration could be operated in situ with less energy consumption. Electrochemical methods require a relatively short time for AC regeneration in a process which pollutants are oxidized by the anode (Zhang, 2002; Panizza and Cerisola, 2009; Martínez-Huitle and Panizza, 2018). In addition, AC can maintain its pore structure without apparent mass loss during electrochemical regeneration (Garcia-Oton et al., 2005; Narbaitz and CEN, 1994). However, traditional electrochemical regeneration always causes secondary pollution. A large number of studies have shown that although the regeneration efficiency could reach 70–95%, toxic byproducts (such as benzoquinone, phenolic-derived oligomers, and polymers) were produced in traditional electrochemical regeneration methods of phenol-saturated AC. These byproducts pose significant threat to human health, so the effluent needs further treatment before discharge (Hussain et al., 2013; Karimi-Jashni and Narbaitz, 2005). In addition, the transformation products which are produced by anodic oxidation could be adsorbed on the surface of the anode ( Zhan et al., 2016a, Zhan et al., 2016b). Those are the limiting factors for the practical application of electrochemical regeneration. Therefore, it is necessary to develop an environmentally friendly and energy efficient technology of AC regeneration, which could regenerate AC in situ while mineralizing the adsorbed pollutants, thus reducing regeneration cost and eliminating secondary pollution.

Peroxydisulfate (PDS) can be activated by electrochemical methods to generate sulfate radical ($S{O}_4^{•-}$), which has been shown to be efficient for the degradation of refractory organic pollutants (Bu et al., 2019; Li et al., 2019; Yan et al., 2017). $S{O}_4^{•-}$ -based processes had been used in the regeneration of phenol-spent granular activated carbon (GAC) and the regeneration efficiency could reach nearly 60%. However, the regeneration mechanism was not explored in depth (Huang et al., 2017). Although extensive research studies have confirmed that $S{O}_4^{•-}$ -based processes could mineralize pollutants effectively in water (Bu et al., 2019; Yan et al., 2017), $S{O}_4^{•-}$ -based processes seem not to perform well in the regeneration of spent GAC. No matter the approach used to optimize the operating parameters (i.e., PDS concentration, current intensity, regeneration time), the process could not achieve higher GAC regeneration efficiency (Huang et al., 2017). These previous results trigger the following question: are there some limitations presented in the $S{O}_4^{•-}$ -based processes that affect the regeneration of activated carbon? These need to be further explored. In our previous studies, the electrochemical methods employed could activate PDS more effectively with activated carbon fiber (ACF) cathode, compared with inert metal cathode (Liu et al., 2018). ACF is mainly used as an initiator rather than a catalyst in this system. The presence of electric field could prevent ACF from being destroyed by the oxidation of PDS and generated ROS after long time service. Therefore, using the spent ACF as cathode and regenerating it by electrochemical activation of PDS (E-PDS) in situ could theoretically regenerate exhausted ACF while degrading organic pollutants. This technology might reduce secondary treatment cost and eliminate the risk of environmental pollution. In addition, to the best of our knowledge, there is no research about electrochemical activation of PDS (E-PDS) to regenerate spent ACF while using spent ACF as cathode.

Thus, the purpose of this study was to assess the regeneration efficiency of ACF in electrochemical activation of PDS system (with saturated ACF as cathode). Phenol, as an important chemical in industry, was selected as the model compound. The main objectives of this study were to investigate: (i) the adsorption capacity of ACF for phenol and benzoquinone as well as the desorption of phenol from the surface of saturated ACF during E-PDS process; (ii) the degradation and re-adsorption of target organic pollutant on ACF during E-PDS process; (iii) the contribution of different ROS; (iv) the limitations of E-PDS regeneration process, and (v) the variation of the adsorption capacity and structures of ACF during four cycles regeneration.