Study On Treatment Technology Of Organic Pollutants By The Supercritical Water Oxidation

After exceed the critical point of water（T=374.3℃，P=22.05MPa）, thewater becomes a non-polarsolvent with higher diffusivity and excellenttransport properties, which is known as the supercritical water. Supercriticalwater oxidation（SCWO） is a process which is based on supercritical water,with oxygen or hydrogen peroxide as the oxygenant, and oxidize the organicpollutants completely into CO₂and H₂O.The supercritical water oxidation（SCWO） is an innovative and promising technology for fast, complete andefficient destruction of organics in wastewater, which almost can be aplliedto deal with any waste liquid including organic pollutants.In order to guide the industrialization of SCWO equipments,12aromaticcompounds were treated subcritically/supercritically under the pressure of24MP and excess hydrogen peroxide by a set of continuous SCWOexperimental apparatus.We sampled the solution under different tempraturesat specific times and determined the TOC of samples. The removal rates of9aromatic compounds were then measured in terms of their TOC removalefficiency. The reasult shows that12compounds can beremoved effectivelyfrom water by SCWO. Then the dynamic parameters of oxidation reactionwere determined by the power exponential function. It’s shown that theactivation energies of the12organic compounds are mainly between23.309-54.341kJ/mol, which proves that the temperature effect of SCWOdiffers between different structures.In order to study the reaction mechanism and degradative pathway oforganic pollutions by SCWO, the intermediate product of Aniline andNitrobenzene were analyzed and identified systematically by GC-MS aftertreated by SCWO process. It turns out that the oxidation reaction dominates, however, coupling reaction, hydrolysis reaction, pyrolytic reaction andisomerization reaction have also happened in SCWO. Take the structure andcharacteristics of aniline into consideration, the pathway of aniline is shownas below：Little is known about the effects of the molecular structure descriptors onthe appropriate temperature of organic compounds when treated by SCWO,not at all the main effect mechanism. To study the effect mechanism ofmolecular characteristics on efficient temperature in SCWO, twelve chemicalcompounds in supercritical water were investigated in this work. Thetemperature at removal ratio of90%(T_R90) was selected as an index toindicate the temperatures needed for various organic chemicals. Furthermore,their structures were optimized by DFT-B3YP/6-311g（d,p）method, as well astheir quantum chemical descriptors. and nine of those theoretical quantumchemical descriptors were selected to describe their characterisics. Correlationcoefficients between T_R90and descriptors were analyzed. Result shows thatoxidation thermodynamics of organic compounds is more than others relatedto the value of qH+, the most positive NBO （Natural Bond Orbital） partialcharge of hydrogen atom, indicating that the electron population of Hydrogenis the key features which affects the degradation effeiciency of organics bySCWO.Other than that, the highest conversion slope of the Temprature-effectcurves, which was defined as the imax, is another index to suggest the greatesttemperature period in which affects the most for each organic compound’sdestruction. And the value of imaxis a key thermodynamic indicator of organicpollutants in SCWO process. Correlation coefficients between imaxandmolecular descriptors were studied. The result shows that oxidation thermodynamics of organic compounds is more negative related to theirdipole moments and most positive partial charge of hydrogen atoms which areconnected to carbon atoms （qC-H+）, while positive related to their totalenergy and their energy of highest occupied molecular orbital （EHOMO）,whichmeans the higher theie energy and EHOMO, the bigger the imax, andcorrespondingly the temperture affects more on their degradation effect.The treatment effect of industrial wastewater by the SCWO testequipment was investigated. The COD concentration in effluent decreasedfrom20000mg/L to lower than20mg/L, with the COD removal ratio morethan99.9%.