Enhancement Of Ceramic Honeycomb Catalytic Ozonation By Ultrasound For The Degradation Of Organic Compound In Water

With the development of industry and the increase of living condition, the pollution of water environment exists wildly. Therefore, the sanitary standards and the security of drinking water have received more and more much attention throughout the world. However, the conventional processes have been shown to achieve a very limited mineralization of organic micropollutants in drinking water treatment. In recent years, heterogeneous catalytic ozonation, as a promising advanced oxidation processes (AOPs), has been investigated for degrading organic compounds in water treatment due to its high oxidation potential.Ultrasound enhanced ceramic honeycomb catalytic ozonation, as a novel AOP, was invented to decompose organic compounds in water, using a vertical three ultrasonic reactor for the first time. This enhanced process achieved the organic combination of ultrasonic degradation and heterogeneous catalytic ozonation. The synergistic effect of the combined process further improved the degradation efficiency of the ceramic honeycomb catalytic ozonation system based on the advantages of efficiency and convenience, and increased the lifetime of ceramic honeycomb catalyst. Simultaneously, it also could remedy the expensive and ineffective limits of sonolysis alone for the degradation of contaminations in water, enhancing the using efficiency of ultrasonic intensity and the degradation rate of organic compound.The degradation efficiency of nitrobenzene in aqueous solution increased with the increase of single frequency (20⁴0 kHz) in the enhanced process of ceramic honeycomb catalytic ozonation by ultrasound. The enhancement efficiency was even more pronounced in the presence of vertical triple transducers with different frequency. The larger the difference between the frequencies used by the experiment, the higher enhancement efficiency was achieved. The combined process of ultrasound/ozone/ceramic honeycomb presented the obvious synergistic effect for the ozonation of nitrobenzene, resulting in a remarkable improvement of degradation efficiency. The best enhanced efficiency could be obtained when ozone was used as dissolved gas. In addition, total ozone applied and gas flow rate presented respectively the positive correlation with the degradation efficiency of nitrobenzene. With the same dosage of applied ozone, the multiple steps addition of ozone showed the higher degradation efficiency than that obtained by adding within one step before the start of the experiment.The degradation efficiency of nitrobenzene in the combined process increased with the increase of reaction temperature, initial pH, circle flow rate, amount of catalyst, pore density of catalyst, initial concentration of nitrobenzene and ultrasonic intensity, respectively, suggesting the positive correlation. Furthermore, the degradation efficiency of nitrobenzene decreased according to the following rule: flocculated river water, tap water, sedimentated water, distilled water and superpure water. The effects of general inorganic metal ions (Na⁺,K⁺,Ag⁺,Mg²⁺,Ca²⁺,Ba²⁺,Mn²⁺,Fe²⁺,Co²⁺,Ni²⁺,Cu²⁺,Zn²⁺,Cd²⁺,Cr³⁺,Fe³⁺ and Al³⁺) and anions (NO₃^-,Cl^― ,HCO₃^-,H₂PO₄^―,SO₄^2―,CO₃^2―,HPO₄^2― and PO₄^3―) on the degradation efficiency of nitrobenzene were complex in the different processes, representing no remarkable effect, accelerating effect, restraining effect and so on, respectively. Moreover, formaldehyde and dibutyl phthalate could inhibit the degradation of nitrobenzene. The experimental results indicated that humic acid, methanol and formic acid could accelerate the degradation of nitrobenzene at the lower concentration, and the reactions were retarded at higher concentrations.The bulk crystalline phase of ceramic honeycomb catalyst was 2MgO·2Al₂O₃·5SiO₂, which was the standard structure ofα-cordierite. The determination result indicated that the density of surface hydroxyl groups existing on the surface of ceramic honeycomb was 9.13×10^-6mol·m^-2, and the pHPZC was 6.60. The introduction of ultrasound or/and ceramic honeycomb catalyst accelerated the decomposition of ozone, resulting in the concentration decreases of residual ozone and offgas ozone. It also increased the utilization efficiency of ozone, the concentrations of ^·OH and H₂O₂ formation, leading to the improvement of mineralization level of TOC. The synergistic effect was even more pronounced in the combined process of ultrasound/ozone/ceramic honeycomb.The investigation of proposed oxidation pathways suggested that the degradation of nitrobenzene seemed to occur in several steps as follows: the hydroxylation of aromatic ring, the mineralization of organic nitrogen, the transformation of aromatics to aliphatics by destruction of ring structures and the oxidation of the aliphatic chains. The major intermediary degradation products detected in the experiment were nitrophenols, multihydroxy derivatives and low molecular weight organic acids, including nitrophenol isomers of o, p, m-nitrophenol, 4-nitrocatechol, 1,4-dihydroxy-2-nitrobenzene, 3,4,5-trihydroxynitrobenzene, 2,3,5-trihydroxynitrobenzene and 2,3,4,5-tetra-hydroxynitrobenzene. Phenol, hydroquinone, benzoquinone and nitrate ion were also detected as intermediates formation, confirming that ^·OH oxidation was the major reaction pathway in the present catalytic ozonation.The results indicated that degradation of nitrobenzene followed the pseudo-first-order kinetic model, and the rate constant kU-O-C of synergistic effect was 13.62×10^-2min^-1. The pseudo-first-order rate constant exerted the positive correlation with total ozone applied, gas flow rate, amount of catalyst, pore density of catalyst, reaction temperature, initial pH, circle flow rate and ultrasonic intensity, respectively. The kinetic practical equation could be expressed as follows: -dC/dt=k₀[O₃]^0.98[Cata]^1.61T^0.54(pH)^0.98I^1.90[C]The combined process of ultrasound/ozone/ceramic honeycomb presented respectively the obvious synergistic effect for the removal of TOC,COD_Mn,UV₂₅₄ and NO₂^--N from raw water of Songhuajiang River. The proportion of low molecular weight organic compounds in raw water from Songhuajiang River was increased after the treatment by the combined process. Furthermore, the combined process increased the biodegradablility and the assimilable organic carbon (AOC) of raw water, and also increased the concentrations of NH₃-N and NO₃^–-N. The total kinds of organic compounds decreased from 152 of raw water to 27 of treated water by the combined process of ultrasound/ozone/ceramic honeycomb. The experimental results illustrated that the combined process had the remarkable synergistic effect for the removals of"three cause"organic compounds, prior controlled organic compounds of EPA and endocrine disrupting chemicals in raw water from Songhuajiang River.