Study On TiO₂ Photocatalytic Degradation Of Refractory Organics And Environmental Risk In Water

Refractory organics in water are type of organics with complex chemical structure and higher stability.They can be accumulated in natural environment and harm animal or human throgh food chain.The main technique of degradation of refractory organics in water are biologic degradation and chemic degradation.Though biologic degradation technique being a stage of industrialization,the degradation rate is very low because the restriction by a lot of factors and the hazard can not be eliminated absolutely.The photocatalytic technique is provided with many advantages such as lower energy consumimg, handy operating, facile reaction temperature,and complete contamination mineralizing under longgest reaction time theoretically, especially,the degradation can be reacted use sunlight directly in expectation, therefore the technique become one of the water treatment with lower energy consumimg and higher efficiency.But this method have also some limitation such as contamination mineralizing can not be progressed completely because of the request of treatment capacity to degradation rate,the result will cause serious environmental harm with a great lot of organic intermediates produced by original refractory organics in despite of the original has been tally with water quality standard.Considering that the original and intermediates can possibly cause environmental risk together in process of photocatalytic degradation of refractory organics in water,the research relating with the process such as photocatalytic theory and technics, characteristic and generation of intermediates, environmental effect of treated water, method of environmental risk assessment and risk managemant, have investigated with the thinking of risk managemant and combining of experiment and theory analysis.In order to reflect on the environmental risk of actual sewerage with photocatalytic degradation,the risk assessment and risk managemant methods of treated water in use of drinking and discharging into surface water or groundwater indirectly have discussed.The research indicate some conclusions as below:(1) The photocatalytic technique of refractory organics degradation has inimitable advantage comparing with biologic degradation technique,but the environmental harm resulted by intermediates when organics can not be mineralized completely must be consider in actual sewerage treatment.(2) WFCPR(water film circling photocatalytic reactor) developed by author is a kind of suspended knockdown photocatalytic reactor with ring and cylinder,it has many advantages such as lower energy consumimg,higher efficiency,higher stabilization and agile operation, is fit for refractory organics and ammonia-nitrogen degradation experiment and also has definite scale-up future.(3) The photocatalytic degradation experiment of phenol and ammonia-nitrogen solution by WFCPR shows that equipment parameters(light intensity, adding amount of TiO2, circulating flux) and operation parameters(reaction time, temperature, initial concentration,pH, adding amount of H2O2) influence the rate of phenol and ammonia-nitrogen, optimal equipment or operation parameters are 300W mercury lamp,160L/h circulating flux,25℃. Optimal conditions having higher degradation rate of phenol and ammonia-nitrogen synchronously are initial concentration of phenol is 60-100mg/L, initial concentration of ammonia-nitrogen is 50-60mg/L, adding amount of TiO2 is 1.5g/L, adding amount of H2O2 is 2-4ml/L,pH is 12. Coexistence substance in solution are affecting each other,phenol can advance the degradation of ammonia-nitrogen,Ca2+ and ammonia-nitrogen can restrain the degradation of phenol,but the restraining ability of Ca2+ is weaker than ammonia-nitrogen.(4) The kind and concentration of intermediates produced by refractory organics photocatalytic degradation are relation to reaction time,pH,kind and amount of catalyzer, concentration of O2 and H2O2,and coexistence organics.Longest reaction time is important factor to ensure refractory organics can be degradated completely.The more initial concentration is, the higher intermediates concentration is,and the longer reached to maximum time is.The lower pH is,the higher intermediates concentration is.When pH is over 9 intermediates concentration become lower,but have longer to arrive at maximum time.When activity of catalyzer is better, intermediates concentration is lower.O2 and H2O2 can advanced to reduce intermediates concentration.Adding light organics can intensify the degradation of refractory organics and reduce intermediates concentration,but can not affect reaching to maximum time.(5) Treated water through mineralizing imperfectly will engender different environmental effect with drinking,reusing or draining into surface water,and form risk to receiving environment or human. In normal the steps of the risk assessment are hazard identification,dose-response analysis, exposure assessment and risk characterization,and in accident the steps are hazard identification, accident frequency, aftereffect estimation, risk account and assessment.The risk of treated water can be limited to the range of acceptability in environment by adjusting technics because intermediates relation to technics parameter,but in accident the relationship is changered dut to the damaged equipment and establishment,in this case, it is necessary to establish the relationship over again then adjust technics by the established relationship.(6) The instance research of health risk of drinking water and immediate or indirect environmental rsik of runoff indicate that it is feasible to limited the risk of intermediates to the acceptability by adjusting technics.