Regeneration Mechanism Of Saturated Activated Carbon Fiber By The Electro-peroxone Process

Activated carbons （ACs） are the most commonly used adsorbent for removing organic contaminants from drinking water and wastewater. Due to their high surface area and well-developed porosity, ACs can efficiently remove a wide variety of pollutants （e.g., taste and odor compounds, synthetic organic compounds, and natural organic matter） from aqueous media. However, during their applications, ACs would become progressively saturated with adsorbates, and therefore lose their adsorption capacity for target pollutants. Hence, regeneration of exhausted ACs efficiently for reuse is a more cost-and energy-efficient, as well as environmentally-sustainable option.An ideal regeneration process should be capable of mineralizing the adsorbed pollutants to H₂O and CO₂, so as to restore the virgin adsorption capacity as well as eliminate the potential risks associated with the pollutants and their transformation products. By far, more than ten kinds of regeneration attempts for saturated adsorbents have been intensively demonstrated （e.g., thermal, chemical, electrochemical and ozonation regeneration）. However, each regeneration attempts has limitations more or less （e.g., high energy consumption, generate secondary pollutant or low regeneration efficiency）.Activated carbon fiber （ACF） is a new kind of efficient adsorption material developed in recent years. It is the third generation activated carbon products after powder activated carbon and granular activated carbon. Due to its small and uniform microporous aperture, it has high capacity and kinetics for orgnic pollutant adsorption. As a functional material for adsorption and separation, ACF is considered to be one of the best materials in the environmental protection and resource recycling field in the 21 st century. In this paper, ACF was used as the adsorbent; phenol and p-Nitrophenol （PNP） were selected as the model compound. By simply combining conventional ozonation and electrochemical regeneration, ozone generator effluent （O₂ and O₃ gas mixture） was sparged into an electrolysis reactor that is equipped with a carbonbased cathode to electrochemically convert the sparged O₂ to H₂O₂, the in-situ generated H₂O₂ can then react with the sparged O₃ to yield·OH. We proposed this new ACs regeneration process, called the electro-peroxone （E-peroxone） regeneration technology. Then, The performance of the different regeneration methods of conventional ozonation, electrochemical regeneration, and the novel E-peroxone process were compared in terms of the regeneration efficiency of ACF and mineralization of desorbed pollutants, and then the influence factors on regeneration efficiency in the E-peroxone regeneration process were discussed. Under the best conditions in the E-peroxone regeneration process, modifications of the structural and chemical properties of ACFs before and after regenerated were compared. According to the evolution profiles of PNP and its major transformation products during the different regeneration processes, the mechanism of ACF regeneration by E-peroxone method was overall demonstrated. In addition, the actual application prospect of E-peroxone regeneration method was evaluated preliminary by means of energy consumption calculation. The results would provide scientific basis for making prevention strategies of water organic pollutants and development of ACs regeneration technology, which is of markedly theoretical, social, environmental and applicable value.（1） H₂O₂ can be produced stably by electrochemically convert the sparged O₂ at the carbon-PTFE gas diffusion cathode in the 17 tested carbon-based materials. The current efficiency of H₂O₂ was more than 80% under the different current conditions. As a result, carbon-PTFE cathode can be used as a good cathodic material for H₂O₂ production in the E-peroxone regeneration process.（2） The optimum regeneration conditions of E-peroxone method was platinum anode, carbon-PTFE cathode,400 mL of 0.05 mol/L Na₂SO₄ electrolyte solution, current of 400 mA, ozone concentration of 65 mg/L, sparging gas flow rate of 0.4 L/min, and regeneration time of 3 h. Under these conditions, the ozonation, electrochemical, and E-peroxone regeneration restored 26.2%,98.2%, and 94.7% of the PNP adsorption capacity of ACF, respectively. In addition, regeneration efficiencies were generally within 92-97% during the twelve adsorption and regeneration cycles, and no apparent decline of the regeneration efficiency was observed. However, regeneration efficiency declined to 71.9% during the twelve adsorption and regeneration cycles when phenol was the model compound. This result was probably due to the irreversible adsorption of phenol. Moreover, the ozone concentration was the vital factor during the E-peroxone regeneration process, and regeneration efficiency declined obviously during multiple adsorption and E-peroxone regeneration cycles under high ozone concentration.（3） It can be concluded that the low regeneration efficiency obtained by ozonation regeneration can be mainly attributed to the low desorption efficiency of PNP and its derivatives, and the substantial destruction of ACF textural properties and elemental composition by ozone oxidation. On the contrary, the influence of ACF characterizations was almost neglectable, and the local basic pH near the cathode was in favor of the PNP and its derivatives desorption. As a result, both electrochemical and E-peroxone regeneration can provide an effective way to regenerate phenol-saturated ACFs. However, electrochemical regeneration could not effectively mineralize the desorbed pollutants, and gradually transformed PNP to many undesired by-products such as hydroquinone,4-nitrocatechol, hydroxyquinol and further carboxylic acid and polymers. In contrast to electrochemical regeneration, the in-situ generated H₂O₂ can then react with sparged O₃ to yield significant amounts of ·OH in the E-peroxone regeneration process. Thanks to the powerful oxidation capacity of ·OH, PNP and its transformation products can be effectively mineralized to H₂O and CO₂. As a result, the E-peroxone effluent will not pose considerable threat to receiving water bodies. Meanwhile, the concentrations of H₂O₂ in the diffusion layer of ACF are expected to be high, which may prevent O₃ from penetrating the diffusion layer to reach and then oxidize ACF surface. And it is inferred that under negative potentials, the cathode may provide some protection for the attached ACF to resist oxidation by O₃ and other oxidizing species during the E-peroxone regeneration.（4） The ACF used in this study had a maximum phenol and PNP adsorption capacity of 282.3 and 440.9 mg/g. The adsorption kinetics was very fast which reached pseudo-equilibrium in about 1 h. Compared to the conventional granular activated carbon or power activated carbon, ACF has the bigger adsorption capacity and quicker adsorption kinetics for pollutants. It is no doubt that combining the ACF and E-peroxone regeneration process is very promising in the water treatment technique.（5） The energy consumptions of the ozonation, electrochemical, and E-peroxone regeneration were 1.28,1.77 and 0.45 kWh/g TOC, respectively. Therefore, the novel of E-peroxone regeneration is an environmentally-sustainable and cost-efficiency attempt for saturated ACs.