Investigation Of Catalyzed Ozonation Of P-Chloronitrobenzene In Water By Transitional Metal Hydroxides

It is difficult for traditional water treatment process to remove trace concentration of organic pollutants, and then some disinfection by-products would be generated in the process of chlorination. Chemical oxidation has been considered to be an efficient way for the removal of organic pollutants in water, however, it is not so efficient in the destruction of recalcitrant organic pollutants. Research of metal hydroxides catalytic ozonation has recently received much attention. It is important to know the catalytic mechanism of metal hydroxides for the decomposition of ozone in water.Hydroxides of six transitional metals, prepared in laboratory, were used as catalysts in the paper. Some studies were carried out on the characterization of their structure and surface properties. The catalytic decomposition of ozone and p-chloronitrobenzene (pCNB) in water were investigated, and then some hydroxides with high catalytic activity were selected out and applied in the catalytic oxidation of trace concentration of pCNB. The mechanism of catalytic ozone decomposition was discussed. The role of the catalyst structure and surface hydroxyl in catalytic ozonation were finally summarized.In the solo ozone oxidation of pCNB, the removal efficiency gets better with increasing initial concentration of ozone and the water purity, but the initial concentration of pCNB has negative effects on its removal. Hydroxyl radical inhibitor and solution pH significantly affected the ozonation of pCNB. Removal of TOC was not so high as that of pCNB in the process, about 50 percentage points lower.The catalyst prepared in the laboratory were manganese hydroxide (MnOOH), iron hydroxide (FeOOH), cobalt hydroxide (CoOOH), nickel hydroxide (NiOOH), copper hydroxide (CuOOH) and zinc hydroxide (ZnOOH). Most materials have nano-sized monomers, all of them have certain ability and capacity for the adsorption of gas. Experimental results show that, the mainly functional groups on the catalyst surface was hydroxyl, all the hydroxides have abundant surface hydroxyl groups except CuOOH. Species of hydroxyl oxygen were found to be different in different hydroxides. Hydroxyl groups on the surface of ZnOOH, FeOOH and CoOOH mainly contain chemi-adsorbed oxygen, MnOOH and CuOOH have hydroxyl groups with the lattice oxygen, and the oxygen in NiOOH was identified as that in surface-adsorped hydroxyl groups.Iron hydroxide, cobalt hydroxide and zinc hydroxide present strong catalytic activity in the reaction. At reacting time of 20min, the catalytic removal of pCNB by FeOOH, CoOOH and ZnOOH in distilled water increases from 55% to 85%, 92% and 99%, respectively. Manganese hydroxide, copper hydroxide and nickel hydroxide hold no catalytic abilities in the ozonation of pCNB in this experiment. The dissolution of metal ions in the catalytic process can be ingnored, for the dissolved amount was much less than the limit in national standard.Efficiency and affecting factors of catalytic ozonation of pCNB by ZnOOH, FeOOH and CoOOH were investigated. Results show that, the removal of pCNB increased with the increasing reaction temperature, water purity, ozone concentration and catalyst dosage; the best removal efficiency of pCNB was obtained when the initial concentration of pCNB was 50μg/L and 100μg/L; effects of NO3-, Na+ and K+ on the catalytic ozonation of pCNB can be ignored, the degradation of pCNB were slightly promoted in the presence of Ca2+ and Mg2+, the ozonation of pCNB were slightly inhibited by high concentration of Cl-, the removal efficiency of pCNB decreased in the presence of SO42-, as hydroxyl inhibitors, PO43- and HCO3- can significantly inhibit the catalytic degradation of pCNB; the removal of pCNB decreased with the increasing concentration of humic acid; the catalytic ability of the catalyst get weaker with increasing the calcinated temperature; after five successive recycles, the catalyst remained stable in the catalytic ozonation of pCNB.Hydroxides of iron, cobalt and zinc can significantly speed up the decomposition of ozone and pCNB in water. Rate constant of ozone decomposition increased 1.25 times, 1.52 times and 1.87 times, respectively. The catalytic degradation rates of pCNB were 2 times, 3 times and 9 times higher as the solo ozonation. Tert-butanol can significantly inhibit the degradation of pCNB in these three catalytic reaction systems. Enhancements of hydroxyl free radicals were obtained in the catalyzed ozone decomposition by FeOOH, CoOOH and ZnOOH. So the catalytic oxidation of pCNB can be divided into two parts: main reaction with hydroxyl radicals and accessorial oxidation by ozone molecules. The optimal catalysis is achieved at solution pH=6.5 and pH=7.5.Surface hydroxyl groups were revealed to be important active sites on catalyst. Hydroxides, which show high catalytic activity in ozone decomposition, have abundant surface hydroxyl groups. Chemi-adsorbed oxygens were thought to be active oxygen species, hydroxyls formed on the adsorbed oxygen (structural hydroxyls or hydroxyls formed through adsorption of hydrogen ions in water) have the ability to catalyse ozone decomposition. Ozone molecules in water can be adsorbed on these active hydroxyls, then ozone decomposition with the production of·OH was promoted.