| Bubbling-induced AOPs technology for water treatment is characterized by many advantages of application such as intense reactivity and easy operation. Two aspects should be taken into consideration for application of bubbling-induced AOPs systems, regarding the enhancement of total reaction efficiency as well as the control of hydroxyl radical (HO·) dominated oxidation pathway. Functional medium often exists in multiphase flow as solid phase, which may improve the efficiency of gas-liquid mass transfer by interfacial behaviors and simultaneously, accelerate the generation of HO-through active-surface-involved catalytic or synergistic reactions in the liquid bulk. Study and exploit the controlling-effect of active medium on chemical processes using bubbling-induced AOPs technology is therefore, of importance for the improvement in system design as well as the intensification in reaction efficiency.As main techniques for bubbling-induced AOPs processes, ozonation and internal electrolysis own dissimilar technical characteristics. Carbon nanotube (CNTs) was selected as the functional medium in this paper and methylene blue (MB) was employed as model compound. Research work was carried out in three aspects, including gas-liquid mass transfer in multiphase flow, catalytic or synergistic ozonation, and material preparation for internal electrolysis, where intensification of AOPs processes was respected as the main thread. The aim of work is to explore the application possibility of CNTs medium during chemistry and chemical engineering processes of bubbling-induced AOPs technology.Main conclusions gained in this work are summarized below,(1) The presence of CNTs medium and the analogues could enhance the rate of gas-liquid mass transfer and this "medium effect" is likely affected by three aspects, including material physicochemical properties (density and size were analyzed to be key influence factor), dosage of medium as well as energy dissipation. Possible mechanism was speculated depicting the behavior of medium, which is attributed to the frequent collision of solid medium toward liquid film resulting in the thinning effect that in turn reduced gas-liquid mass transfer resistance. The mathematical relation of solid loading and mass transfer coefficient was developed and result fits well with experimental data, which may help to verify the validity of proposed mechanism based on thinning effect of film layer. (2) CNTs medium exhibited appreciable ability for catalytic ozonation, which was reflected by comparative study with, plain ozone on generation rate of HO· and strength of mineralization, as well as GAC/O3system on decolorization of MB. But the catalytic role of CNTs can be largely confined by solution pH, i.e. basic environment favors its catalysis. The evolution of H2O2and HO· concentrations as a function of solution pH was analyzed under different operation systems, and the possible AOPs pathway for catalytic ozonation induced by CNTs was suggested, which may ascribe the enhancement of HO· yielding in the presence of CNTs to the accelerated consumption of H2O2by dissolved O3molecules. A facilitator material for acidic solution environments was prepared by depositing zero-valent iron particles on the surface of CNTs, which was proved to be capable to intensify the AOPs pathway during ozonation at low solution pH (pH3in this study).(3) A combined iron-carbon material (Fe0-CNTs,o) for internal electrolysis was prepared by a series of procedures, mainly containing oxidative modification to obtain CNTs,o, electrophoresis deposition (EPD) of CNTs,o onto the surface of zero-valent iron, and high-temperature calcinations to enhance material stability, where CNTs medium was regarded as active cathode. The vigorous performance of Fe0-CNTs,o material was validated by degradation of MB. Oxidative pretreatment step dealing with CNTs as well as proper time of EPD (about3min) are both favorable for Fe0-CNTs,o to work through internal electrolysis. Relative information of solution chemistry collected was analyzed, and results indicated that the AOPs pathway during internal electrolysis might be realized by Fenton-like chain reactions producing HO· species. The stoichiometric relation of solution pH and dissolved oxygen (DO) play important role on conducting the chemical process for the generation of HO· during internal electrolysis. |
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