Coupling Process Of TiO₂ Photocatalytic And Biological To Degrade Of Environmental Endocrine Disruptors-Bisphenol A

As a quite important raw material in chemical engineering, Bisphenol A (BPA for short) is in increasing amount released into environment due to its wide application. Considered to be a kind of recalcitrant compounds, BPA could reach a relatively high concentration in vivo via bioaccumulation effect, which greatly threatens not only water quality but also human beings. Current solutions towards BPA in environment are mainly of biodegradation. Unfortunately, normal treatment processes installed in our existing wastewater treatment plants are not able to fully remove BPA from water.To address this problem, a combined photocatalytic-biological degradation method is proposed based on mere low-efficient biodegradation. Such a combination is considered to be a superb way to deal with the BPA in water because: photocatalysis could break down recalcitrant compounds into easy-biodegradable products, while biodegradation could efficiently degrade and mineralize those small products. Traditional collaborated photocatalytic and biodegradation processes take process in different reactors sequentially. Contrary to that, the combined degradation process discussed in this article takes effect in the same place and at the same time, which is a mechanism largely increase the degradation efficiency.First step of this research is to investigate and optimize the immobilization method of TiO₂. The polyurethane foam was selected as proposed carrier to meet the requirements of reactor setup. Due to its irresistant to high-temperature treatment, kinds of low-temperature immobilization methods were investigated and discussed. Superficial morphology, photocatalytic performance, crystal phase of immobilized TiO₂ as well as its microscopic morphology was four major factors taken into evaluation to assess the immobilization techniques. Besides, degradation kinetics was also considered. The results proved that: the microwave-assisted liquid phase deposition was a promising method to obtain fixed TiO₂ films on carriers in low temperature. The TiO₂ film was evenly distributed and smooth; mixed crystal phase (anatase and rutile) was found in generated TiO₂; Rhodamine B's degradation rate was up to 80% in 180 minutes. There was no apparent detached photocatalyst even after 6 times repeated tests, while the degradation rate was relatively constant. That demonstrates such an immobilized photocatalyst carriers has a long lifetime.Horizontal Circulating Bed Photocatalytic Reactor (HCBPR for short) was specailly designed for achieving combined photocatalytic and biodegradation. Derived from criculating bed reactor and three-phase fluidized bed, HCBPR has its advantages in providing optimized reacting place for degradation: carriers' circulation were achieved by bubbling air and water around the central-installed UV light, ensuring efficient degradation; proper amount of bubbling air could also provide necessary oxgen to microorganisms' metabolism; porous carriers act like barriers shelter microorganisms from threatens of both UV light and free hydro radicals. Hydrodynamic characteristics study further pointed out that there was lowest dead zone rate and good hydraulic mixing when the bubbling rate was 0.1 m³/h.In additon, we investigate the photocatalytic degradation process of BPA in HCBPR. Four factors were selected to optimize best operation conditions. They were initial BPA concentration, initial pH, dosage of photocatalytic carriers (defined by filling ration), temperature. Results revealed that optimized conditions were as follows: initial BPA concentration = 10 mg/L, initial pH > 7, filling ration = 1%, under which BPA was completely degradaed within 6 hours. We should also notice the temperature had little effect on degradation rate.Finally, combined degradation of BPA in HCBPR was investigated. During the inoculation lasting for 15 days in separate aeration reactor, total biomass on photocatalyst carriers declined at first and returned to a constant value, demonstrating the inoculation process was done. Then the reaction was operated throughout 6 different stages characterized by UV light was on/off, biofilm carriers had/had no photocatalyst. It was fully proved that with the assistance of biodegradation, overall degradation rate of BPA was 10% higher than pure photocatalytic degradation. Granted the threatens exerted by UV light and free hydro radicals, microorgranisms still survived and conducting metabolism, which was revealed by apparent declined COD and increased BPA degradation rate. From the research, it is worthwhile to notice the microorganism became more resistant to threatens after repeated different operation conditions.