Study On The Decomposition Of Landfill Leachates Over Co/Bi Catalyst By Catalytic Wet Air Oxidation

Landfill leachates is a kind of harmful wastewater containing high concentration organic compounds. Because of its serious pollution to the circumjacent environment and substrate of the landfill, the long-term maintenance of pollution, and the secondary pollution, it is an urgent problem to collect and treat the landfill leachates effectively in city environments. At present, the main methods to treat landfill leachates include biological methods and physical-chemical methods. While the ratio of BOD5/COD is larger than 0.3, the landfill leachates presents good biodegradability, which can be disposed in biological methods. While the ratio of BOD5/COD is smaller (0.07-0.2), the landfill leachates is resistant to biological treatment because of the difficulty to remove the highly toxic organic compounds, so physical-chemical methods are much better. The physical-chemical treatment of the landfill leachates need short operating period but relatively high operating cost. So in the field of landfill leachates treatment, one of the research focuses is to find a disposal method independent to the water quality traits both home and abroad. In this research paper, catalytic wet air oxidation (CWAO) over Co/Bi catalyst was employed to remove high concentration organic compounds and ammonia nitrogen (NH3-N) with oxygen as the oxidant and to build the technical condition of disposal landfill leachates using CWAO. The resulted information suggested that the temperature and the activity of catalyst are the main factors affecting the landfill leachates removal rate by CWAO. Under the optimal conditions, the COD_Cr and NH₃-N in landfill leachates can be reduced from 40000mg/L and 500mg/L to <150mg/L and <10mg/L, respectively. Moreover, the decomposition kinetics of landfillleachates was discussed, the decomposition kinetics model was established at the same time, the decomposition mechanism of organic compounds and NH3-N in the landfill leachates was explored, and the recovery and recycle of Co/Bi catalyst was investigated.Among the factors, the removal rate of TOC in the landfill leachates increases with the increasing of the temperature, the oxygen partial pressure and the dose of the catalyst, especially the temperature and the dose of the catalyst. At high temperatures, since the main contribution to expediting the reaction rate is the temperature, the effect of oxygen partial pressure was not obvious. With the increase of the reaction solution concentration, the decomposition reaction rate of the organic compounds increases, but the removal rate of the organic compounds decreases. It can be seen from the corresponding relationship between the organic compounds and the variety of pH that with the lower reaction temperature, the time course to reach the minimum pH is longer, the minimum pH is lower, and the equilibrium pH is also lower. These results revealed that with lower reaction temperature, the reaction rate slower, so the time course to reach the minimum pH is longer; and that with lower reaction temperature, the selectivity of the decomposition was poorer, the products of organic acids were more, and so the minimum pH and ultimate pH were lower.On the bases of the studied effects and laws of the factors (temperature, oxygen partial pressure, concentration of the reaction solution and the catalyst, etc.), the exponential experiential model was developed. It can be deduced from the model that while the temperature sustained, the catalysts are the main factors affecting the reaction rate and efficiency. Furthermore, the reaction efficiency could increase by altering the dose of the catalyst and improving the capability and species of the catalyst, which will be quite important to reduce the reaction condition of CWAO.GC-MS was used to detect the species and relative contents of the semifinished organic products, which are mainly the organic acids at the percent of above 88%. The analysis of decomposition mechanisms of the catalyst surface revealed that one of the mechanisms is that activated gas phase O2 captures the hydrogen of the organic pollutants to produce free radicals, which then initiate the catalytic reaction, and...