Degradation Of P-Chloronitrobenzene By Heterogeneous Catalytic Ozonation

The contradiction between the rapid rough development of economy and the weak consciousness of environmental protection leads to serious water pollution with more and more emerging organic contanminants in our country. Thus, the removal of trace toxic organic pollutants becomes an urgent matter for further improvemnet of the drinking water quality in our country. In general, chemical oxidation is the most widely used method for the drinking water treatment plants to deal with the refractory organic pollutants. However, some of the contaminants can also not be effectively removed by common oxidants (e.g., potassium permanganate, liquid chlorine, sodium hypochlorite and ozone, etc.). Heterogeneous catalytic ozonation is an effective method to strengthen ozone oxidation of the refractory organic matter. And thus, mastering the basic rule of heterogeneous catalytic ozonation can help us to utilize heterogeneous catalytic ozonation technology more correctly and also find the economic and efficient catalysts which can be applied to the water treatment.In the present work, commercial nano titanium dioxide (n-TiO2), the prepeared mesoporous silica (SBA-15). carbon black (CB), titanium and carbon composite pellets (TiO2-C) and cerium oxide (CeO2) were selected as catalysts to investigate the efficacy and mechanism of the degradation of parachloronitrobenzene (pCNB) which is toxic and easily detected in the natural water by heterogeneous catalytic ozonation using batch experiments.The results showed that the decomposition of ozone followed the first order reaction kinetics generally and two phases of the ozone decomposition could be observed in all of the systems. In the initial phase, O3 decomposition did not follow the first order reaction kinetics which might be resulted from the direct ozonation of by-products of ozonated pCNB. And in the second phase, it followed the pseudo-first-order reaction kinetic models (R2≥98.4%). n-TiO2, CB and TiO2-C could promote the ozone decomposition and pCNB removal. However, both SBA-15 and CeO2 had an inhibitory effect on the degradation of pCNB. The most notable inhibition could be observed in the SBA-15/O3 system and pCNB could be removed only by 35% after 20 min reaction, which was less than that in single ozonation by ca.21.6%. The increase of the reaction temperature, the initial concentration of pCNB and also the dosage of ozone solution could increase the apparent reaction rate constants of pCNB degradation. The change of the dosage of catalysts had different influence on ozone decomposition and pCNB degradation in the different systems. The pH values of the solution played an important role on the chain reaction of ozone decomposition and the activity of catalysts. The anions such as Cl-1, HCO3-, SO42- and HPO42- would weaken the activity of catalysts and pCNB removal through complexation or capturing hydroxyl radical (HO). A few of humid acid (HA) would promote the chain reactions to degrade pCNB more rapidly. Excessive consumption of O3 would occur in the presence of a large amount of HA (5 mg.L-1), which resulted in the reduction of pCNB reduction.The aggregation of n-TiO2 was discovered under different pH values. Moreover, the results of further experiments indicated that the aggregation of n-TiO2 could affect its catalytic activity and thus pCNB degradation. Both the most rapid aggregation rate and the lowest catalytic activity was obtained near the point of zero charge (pHpzc) of w-TiO2 and it could be speculated that the relative pCNB removal rate was correlative inversely with the aggregation rate under different pH values. The aggregation of n-TiO2 resulted in the change of surface states and then the catalytic activity. O3, pCNB and NO3- had less effect on n-TiO2 aggregation, while both HA and PO43- notably decreased the zeta potential, inhibiting the aggregation of n-TiO2. Moreover, on the basis of the study of the adsorption of pCNB by catalysts and the effect of radical inhibitor-tertiary butanol (TBA) on the systems, it could be concluded that the removal of pCNB relied on the oxidation by HO· and also the adsorption by the catalysts. The adsorption of pCNB and also the by-products resulted in the different catalytic effect in the different catalytic systems (inhibit/promote pCNB removal).The results from the mineralization effect of pCNB, the catalytic degradation effect of pCNB in tap water, the leaching of metal ions, and also the reuse of the catalysts indicated that all of the catalysts were stability and also practicability.