Decomposition Of P-nitrophenol In Water By Ozonization With Ultrasonic Enhancement

The toxic and bio-refractory pollutant p-nitrophenol is in wide existence, such as in wastewater, chemicals, pesticides and dyes. With the aim of providing theoretical buttress for the treatment of p-nitrophenol, this paper starts with the selection of p-nitrophenol as the model pollutant, then moves on to the study of influential factors in its decomposition, synergetic effects, kinetics and mechanism through sonolysis (US/O₂), ozonolysis (O₃) and ozonation with ultrasonic enhancement (US/O3). The conclusions are shown as follows:Under the US/O2 system, p-nitrophenol decomposition was accelerated with either the decrease of the initial concentration or pH value, or the increase of ultrasonic intensity, while the solution temperature was relatively not affective. A study of dosage influence of tertiary butanol (TBA) on the decomposition of p-nitrophenol indicated that.OH did take part in the reaction, but mainly in pyrolysis which accounts for 67.95% of the whole reaction. Phenol, p-benzoquinone, hydroquinone, catechol, o-benzoquinone, p-nitrocatechol, fumaric acid, maleic acid, formic acid, NO₃^- and NO₂^- were identified as intermediates through high-performance liquid chromatograph (HPLC) and ion chromatograph (IC). Besides, the decomposition mechanism of p-nitrophenol was also covered in this part, p-nitrophenol oxidation followed pseudo-first-order kinetic and through non-linear regression analysis, its decomposition rate was obtained ask = 0.000165×C₀^-76264 (pH)^-091439 (P)^-172456.Under the O₃ system, both the increase of O₃/O₂ gas flow, temperature, pH value (pH<8) and the drop of initial concentration expedited the p-nitrophenol decomposition, while pH value (pH>8) was relatively not affective. The stoichiometric ratio between ozone and p-nitrophenol was determined and individual liquid phase mass transfer coefficient (kl) and interfacial area per unit volume (a) were identified in the absorption of CO₂ into Na₂CO₃—NaHCO₃ system. Based on it, direct oxidation kinetics and general oxidation kinetics were studied under TBA and absent TBA respectively. It was discovered that l)the stoichiometric ratio was 3;2)when pH^3, /j-nitrophenol decomposition took place maily in ozone molecules which occured in the liquid bulk and corresponded with the slow kinetic regime; 3) O3/O2 gas flow and initial concentration hardly influenced the rate constants koand &r, while temperature and pH value exerted tremendous influence. 4)When pH<3, ho and &t were correlated by a modified Arrhenius equation of temperature and pH askD = 1780428exp(-10588/Rr)[OH-]°1877, fcT = 2209010exp(-10813/Rr)[OH-]°m" ; 5)the rate constants for the non-dissociated (A&) and dissociated (kB.) were 27L/(mol-s) and 4.6 X 106 L/(mol-s) respectively. Besides, the intermediates were identified through HPLC and IC; first, nucleophilic substitution reaction (Sn2) occurred between O3 and />-nitrophenol which resulted in hydroquinone, p-benzoquinone, catechol, o-benzoquinone following the lose of — NO2. As the reaction proceeded, these intermediates experienced several ring openings in further oxidization, forming succedent fumaric acid, maleic acid, oxalic acid and formic acid, which finally led to the production of CO2 and H2O.Under the US/O3 system, climbing O3/O2 gas flow, ultrasonic intensity, pH value (pH<8), temperature and declining initial concentration are conducive to the acceleration of /Miitrophenol decomposition, while pH value (pH>8) was relatively not affective. Synergetic effects could be observed in the US/O3 process, enhancing the removal ofp-nitrophenol at a rate (/) between 150% and 191%, much higher than 1. In order to explore the possible synergetic mechanism, volumetric mass transfer coefficient in liquid phase (kvo) was measuredd in the instantaneous kinetic regime; the influence of ultrasonic on the ozone concentration in water ,on ozone decomposition and that of ozone on hydrogen peroxide (H2O2) production were studied as well; finally a comparison was made between the enhancement factors of /7-chlorophenol, /?-nitrophenol and chlorobenzene and their rate constants in direct reaction with ozone. A conclusion was arrived at that ultrasound decomposing ozone followed by -OH augmentation was the key cause of the synergetic effect. The kinetics studies indicated that when pH<3, the indirect reaction between -OH and /?-nitrophenol took up a vital position which amounted up to 0.354 to 0.505 of the whole reaction; it...