Study On Degradation Of Waster Water With Phenol And Pentachlorophenol By Ultrasound And Coupled Methods

With the development of industry and agriculture, more artificial synthetic chemical compounds are added in environment, most of which are difficult to degrade by conventional methods. The main kinds are polychlorinated biphenyl, polychlorinated phenol, nitrophenol, which are of much contaminative. Toxicity increases if the degree of chlorination increases. Pentachlorophenol (PCP) is widely used as wood preservative, antirusting agent, insecticide and weedicide. PCP is supertoxic agent, which exists stable and restrains oxidation and phosphorylation during biologic metabolism. PCP can hurt central nervous system and liver & kidney in creature. It is one of 12 kinds persistent organic pollutants (POP's) listed by Stockholm Convention on Persistent Organic Pollutants. So it is priority controlled compound and the treatment of this part of chlorides is the hot issue in the world because ordinary chemical treatment cannot degrade these compounds absolutely.Ultrasound method to treat pollutant in wastewater is considered an potential method by lots of researchers, however their treating object is low concentration (<50ppm PCPNa) waster water. In this work, higher concentration (>100ppm PCPNa) was chosen as research object. Ultrasonic cavitation, combination method with ultrasound method, reaction mechanism, pathway and dynamics were studied in detail. The combination ultrasound method with CCl₄ degradation adjuvant in PCPNa treatment and development of ultrasonic airlift reactor are brought forth as new ideas. This article was demonstrated as follows.1. Ultrasound was added in ordinary jacket reactor. Ultrasound intensity and ultrasound energy density were measured by oscillograph. Result was analyzed by ultraviolet spectrophotometry (λ. =248nm). Comparing ultrasound method with constant temperature method (T=60℃, 200r/min, t=48h), it was concluded that ultrasound method acted as sonochemical method rather than simply increased transfer performance. In order to adapt to high request of large treatment quantity, 100ppm PCPNa was chosen. Result showed that PCPNa can be 100% decomposed, however sound intensity is more than 12W/cm² which means high energy cost (7kWh in 4hours). As low energy cost was concerned, PCPNa can be 96.7% degraded when sound intensity is 4.14W/cm²(1.76kWh in 4hours). With reaction was carried on, pH value of solution decreased, the reason of which is H was separated from organic structure with PCPNa decomposing and then, increased solution acidity. Frequency of ultrasound is a very important factor of degrading PCPNa because result showed that degrading ratio under high frequency ultrasound (500kHz )was 4 times higher than that under low frequency ultrasound (20kHz).2. During ultrasonic degrading phenol in petrochemical wastewater, temperature can influence the degrading ratio greatly, if solution temperature was higher than 50℃, the ratio was only 14.1%, while the temperature maintained 25℃, the ratio was higher than 21.5%. Degradation adjuvant CCl₄ played an important roll, when CCl₄ was added in solution, degrading ratio increasing to 84.1%, which is higher than 18.8% degrading ratio without CCl₄. the reason may be that CCl₄ became ClO₂ underultrasound, so more phenol was degraded to increase decomposition ratio. The result did not change much when metal was added as catalyst in ultrasonic degrading petrochemical wastewater. If waste water was aerated, the degradation increased, and the order is: aeration before and during reaction > aeration before reaction for 1 hour > aeration during reaction > without aeration. Because the more quantity of soluble air, the more quantity of O2, which results in more OH generated, the reaction mechanism of ultrasonic degrading phenol is by means of -OH free radical, the phenol degrading ratio with aeration is greater than that without aeration.3. Mechanisms of ultrasonic degradation of both PCP ion and phenol which are induced by free radical are also approved by the influence of free radical scavenger (CCI4) on ultrasonic reaction. According to the result data regression analysis, the ultrasonic degradations of PCP ion and phenol are both first-order reaction. The reaction rate constant of ultrasonic degrading PCP ion is 3. 48 X 10⁴s^' , and the character of ultrasonic degrading phenol is first-order as well.4. In order to apply ultrasound method to treat other kinds of industrial wastewater, ultrasound coupled with other treatment methods was discussed. Comparing the reactions of Fenton agent and H2O2 action by oneself coupled with ultrasound method, it was achieved that the result of ultrasound method coupled with H2O2 was better than that of simple the plus of each method, and Fenton agent strengthened the effect of ultrasound degradation. Moreover, treatment time of PCP ion solution and phenol is very short under combination action of ultrasound method and Fenton agent method. However the reaction under ultrasound itself cannot get such a high reaction rate. The possible reason is that quantity and strength of -OH free radical is limited in consideration of low electric-sound conversion ratio and the loss of sound energy turning to heat. But if Fenton agent or H2O2 was added in solution, the loss of -OH free radical can be highly made up.5. To develop treatment performance including increasing treatment volume and treatment rate is the key point to develop one method to be applied to industry. Gas lift internal circulation reactor has the advantages mentioned above, its mechanism and individual structure make it low cost, low shear load, low mixing time, easy capsulation, good transfer performance and high reaction rate. Whereas the chemical property of material, operation condition, reactor structure, ultrasound generator and gas lift internal circulation reactor were united to improve hydrodynamics and transfer performance. Ultrasonic airlift internal circulation reactor has degrading performance close to that of ordinary ultrasonic batch reactor, while energy density of the former is much lower than that of the latter. In ultrasonic airlift internal circulation reactor degradation ratio of phenol was 31%, degradation ratio of PCP ion was 89.1%. So this kind of reactor can be considered as a typical sonochemical reactor to apply in industry.After discussing many PCP treatment methods, it was concluded that ultrasound method should be focused, which has advantage of much treatment quantity and short time costing comparing biological method, so it can be applied to the area where are PCP polluted. Because of bad treatment result and adding other ion increasing cost to wipe out, Fenton agent method is not applicative. Large energy cost during ultrasoundtreatment should be resolved in order to be applied to industry. Better result was gained if ultrasound method coupled with ozone, Fenton agent method and photolysis.