Synergistic Effect On Degrading Dye Wastewater By Non-Equilibrium Plasma And Porous Carbonaceous Materials

Non-equilibrium plasma water treatment is an advanced oxidation technology, which integrates effectiveness of many active species including hydroxyl radical, hydrogen peroxide, ozone, etc., and bears preferable future application and large market potential because of its advantages such as low energy consumption, high treatment efficiency, rapid and non-selective reactivity, and free of secondary pollution, etc.. The current difficulties for widely application of this technology exist mainly in structure-designing of electrical discharge reactors and selection of appropriate adsorbents and catalysts. In this work, a novel electrical discharge reactor was designed for degrading methyl orange as the model pollutant in water. The effect of electrode configuration and influence of technological parameters on degradation efficiency, mineralization and energy efficiency were investigated. The synergistic effect of non-equilibrium plasma combined with granular activated carbon (GAC), activated carbon fiber (ACF), and TiO₂/ACF composite for treatment of methyl orange in water was investigated. The performance of GAC and ACF for adsorption of pollutants and catalytic conversion of ozone to hydroxyl radical was evaluated. The effectiveness of photocatalysis by TiO₂/ACF composite was also testified with analyzing the influence of pulsed discharge on its pore structure and surface chemistry. And the synergistic mechanism of photocatalysis with non-equilibrium plasma was discussed.With pulsed voltage supplied between the high voltage needle electrode in the aqueous phase and the ground electrode in gas phase, activate species of hydroxyl radical and hydrogen peroxide were produced in liquid phase and that of ozone in gas phase. The utilization efficiency of ozone was increased by injection of pretreatment solution and setup of mesh barrier therein. The solution was circulated in pre- and main-oxidation zone driven by a peristaltic pump through circulation pipes in order to further treat the solution in main-oxidation zone which has been pre-oxidized in pre-oxidation zone. In the main-oxidation zone, the organic pollutants were mainly degraded by hydroxyl radical attack. While in pre-oxidation zone, there exist two ozone degradation passways, which were direct electrophilic attack and hydroxyl radical attack. The degradation efficiency of 0.1 L solution with concentration of 40 mg/L was 90% with experimental error lower than 4%. The ozone utilization efficiency could be enhanced by increase of solution concentration or handling capacity, by which the reactor energy efficiency was improved. The reactor energy efficiency were 2.1, 2.8, 3.4 and 3.6 g/(kWh) for 0.1 L solutions with concentrations of 40, 60, 80, and 100 mg/L, respectively, and 4.1, 5.4, 5.9, and 5.9 g/(kWh) respectively for doubled handling capacity of solutions with above mentioned concentrations.The combined treatment by pulse discharge with GAC or ACF showed good synergistic effect due to the adsorption and catalysis of GAC and ACF. A high concentration of methyl orange on GAC and ACF surfaces was achieved by adsorption. And the surfaces performed as conversion and decompose centers of methyl orange, where the basic functional groups on GAC and ACF accelerated the conversion of ozone to hydroxyl radical, which increased methyl orange degradation and COD removal efficiency. Due to its distinct difference of pore structure and surface chemistry to GAC, ACF showed superior adsorption and catalytic performance. The adsorption and catalytic activity of both GAC and ACF do not showed observable decrease in recycle utilization. In combined treatment of 0.2 L solution with concentration of 60 mg/L, the degradation efficiency remained around 92%, and COD removal at 82%, and energy efficiency reached 8.3 g/(kWh). The GAC and ACF used were regenerated in-situ by the pulsed discharge process, with regeneration efficiency of 80% and 90%, respectively. Owing to co-oxidation of non-equilibrium plasma and ozone, the surface acidic and basic functional groups on GAC and ACF increased to various extent, and specific surface area and pore volume changed in various degrees. Therefore, it is reasonable to assert that GAC and ACF act as initiator or promoter for ozone conversion to hydroxyl radical in the process of combined treatment.The combination of pulsed discharge and TiO₂/ACF showed more distinctive synergistic effect, which was mainly ascribed to the adsorption of TiO₂/ACF and photocatalysis. The easy reaction of dissolved ozone with electrons on TiO₂ surface decreased the recombination of surface electrons with cavities, and increased photon quantum efficiency of photocatalysis to impel the reactions to complete conversion. The catalytic activity of TiO₂/ACF kept unchanged during recycling. In the combined treatment of 0.2 L solution with concentration of 80 mg/L, the degradation efficiency maintained at 97%, and COD removal reached 91%, with energy efficiency of 8.7 g/(kWh) and TiO₂/ACF regeneration efficiency of 95%. The surface morphology and property of TiO₂/ACF did not influenced by pulse discharge with minor change in pore structure. Therefore, with ACF as support of photocatalyst, adsorption and photocatalysis was combined to obtain favorable performance by interfacial mass transfer.