| As a new wastewater treatment technology, photoelectrocatalysis has only been studied in laboratory at present. Although a lot of work has been done relating to enlarge TiO2 thin film electrode areas, select suitable electrode substrate, and design high efficiency PC reactor, however, the low utilization efficiency of irradiation light source has been ignored in photoelectrocatalytic (PEC) process. The photo-anode employed in PEC reactors in laboratory is always immerged completely into solution, which results in the following disadvantages: first, irradiation light utilization efficiency is low; second, transfer speed is low; third, reactor setup is high energy-consuming and complex.In order to improve the utilization efficiency of light source, enhance the transfer speed and simplify the reactor structure, we developed three types of thin-film PC reactors—rotating disk thin-film PEC reactor (RPEC), gradient sheet thin-film PEC reactor (GSPEC) and dual rotating disks cell thin-film reactor (DRPC), in which utilization efficiency of light source and transfer speed was enhanced by the rotation of disk or flow of wastewater, and the reactor setup was simplified without circuiting cooling water. In this way, the overall PC degradation rate was enhanced and the cost was reduced. The detailed work is described below:1. Rotating disk thin-film PEC reactor (RPEC) with dynamic photoanode was developed. The photoanode——TiO2/Ti electrode was prepared by different methods including direct heat-oxidation, anodic-oxidation and sol-gel method. The photo-response character of the three types TiO2/Ti electrodes was investigated. These three types TiO2/Ti electrodes were installed into the RPEC, respectively, and applied to treat Rhodamine B (RB) simulated dye wastewater. In this system, their photocatalytic activity and influence factors were investigated, respectively. The TiO2/Ti electrode prepared by sol-gel method (SG-TiO2/Ti) by 550℃heat treated for 2 hours obtained the optimal photo-response and PEC efficiency. The X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analysis of the SG-TiO2/Ti electrode demonstrates that the electrode surface consisted of small uniform particulates, which were dominantly composed of anatase and their size was about 50 nm. The optimal treatment conditions for RB solution were: bias potential 0.4V, initial pH 2.5, Na2SO4 0.5 g L-1 and rotating speed 90 rpm. Under these condition, color and total organic carbon (TOC) removal efficiency of 20mg L-1 RB solution reached 97.2 % and 72.7 %, respectively. The stability and reproducibility of SG-TiO2/Ti electrode was excellent. Average decolourization efficiency of 10 times usages of identical electrode and 10 different electrodes was 68.0±1.0 % and 67.6±2.4 %, respectively, for treatment raw textile effluent.2. The comparison result of RPEC and conventional PEC reactor (CPEC) demonstrates that both the light utilization efficiency and transfer speed of RPEC was enhanced by the rotation of the TiO2/Ti electrode. On the TiO2/Ti electrode surface, thin aqueous film of several decadal microns thick formed, and the light absorption of RB solution can be avoided. In addition, the quantity of RB removed by per electrode area for RPEC treating 20-150 mg L-1RB was enhanced to 1.03-6.75 times of that of CPEC, and the enhanced times increased with RB concentration increase, and was in direct proportion to the PC activity of the TiO2/Ti electrode. RPEC can treat raw and treated textile effluents efficiently, and improve their biological degradation. Decolourization efficiency reached 81 % and 77 %, TOC removal efficiency reached 51 % and 21 %, and BOD5/COD ratio increased from 32.6 % and 34% to 48.5 % and 42.1 %, respectively, within 150 min treatment.3. Gradient sheet thin-film PEC reactor (GSPEC) with static photoanode was developed. Optimal treatment conditions of GSPEC with SG-TiO2/Ti electrode were: bias potential 0.8V, initial pH 2.5, Na2SO4 2.0 g L-1 and circulating flux 7.7L h-1. Under these condition, color and TOC removal efficiency of 20 mg L-1RB solution reached 97.3% and 76.2%, respectively. The analysis result of GC-MS demonstrates that RB was degraded into small molecule organic acid, CO2 and H2O at last, mainly by photogenerated holes or hydroxyl radical oxidation. Within 120 minutes, color and TOC removal efficiency of 50 mg L-1 Reactive Brilliant Red X-3B (RBR) solution reached 97.7 % and 85 %, respectively. Color and TOC removal efficiency of 100mg L-1 Reactive Brilliant Blue X-BR (RBB) solution reached 69.5 % and 43.2 %, respectively. Average decolourization efficiency of 30 times usages of identical electrode treating 20 mg L-1 RB solution was 86.8±5.3 %, with standard deviation of 2.5 %. Comparison result of GSPEC and CPEC is similar to RPEC since both the light utilization efficiency and transfer speed of GSPEC was enhanced by circulating flowing of wastewater. As a result, the overall degradation rate was improved obviously. GSPEC can degrade practical dye wastewater efficiently. Decolourization efficiency of both textile 1 (150 minutes) and 2 (180 minutes) reached 85 %, and TOC removal efficiency reached 55 % and 51 %, respectively.4. SG-TiO2/Ti electrode was doped by adding NH4F into sol-gel. The response wavelength of the doped SG-TiO2/Ti electrode was extended from 400 nm to 440 nm. However, its photo-activity under UV irradiation decreased. GSPEC, with both TiO2/Ti electrode and doped TiO2/Ti electrode, can use free light source—solar light as irradiation light source, and the energy cost was further reduced. 5. In order to reduce the energy cost and improve treatment efficiency, dual rotating disks cell thin-film reactor (DRPC) was developed, in which Cu electrode was manufactured as disk and fixed on the same axis of TiO2/Ti electrode. Thus, photogerenated electrons can be driven from TiO2 to Ti substrate by Schottky Barrier between metal Ti and N-type semiconductor (TiO2) instead of by bias potential, and then to Cu surface, where the photogerenated electrons were captured by dissolved oxygen in solution on the Cu disk surface and transformed to H2O2 through one step or several steps. H2O2 can participate in dye oxidation. As a result, dual electrodes oxidation can be realized, treatment efficiency improved and energy cost decreased. Decolourization efficiency of ten kinds of dye reached 16.9 %-99.9 % by 30 minutes DRPC treatment with UV light irradiation. DRPC can degrade textile effluent 2 efficiently. Color and TOC removal efficiency within 135 minutes reached 90 % and 49 %, respectively.6. The comparison result of DRPC, RPEC and GSPEC demonstrates that DRPC is super to RPEC and GSPEC in treating high concentration dye wastewater. DRPC has higher efficiency and lower energy cost than RPEC and GSPEC.7. Exponential function was employed to establish the kinetics model of RPEC: ( )0.5383 0.6670 0.8766 0.4666 0.6065Ct = C0 exp 0.06040I Q E R C0 ?t The model can describe the pseudo first order of RB solution well degraded by RPEC. The apparent rate coefficients of the three thin-film reactors were in order below: DRPC >RPEC > GSPEC, which was consistent with the order that quantity of RB removed by per electrode area.The results demonatrate that the mass transfer and irradiation light utilization efficiency was enhanced by the rotation of the photoanode or the circulating flowing of wastewater in the three types of thin-film reactors developed in this paper. They are more effective and energy-saving than conventional PEC reactor, in which photoanode was completely immerged into wastewater. The significance of this research is to establish PC procedures that show promise of being industrialized in the future for real wastewater treatment.
|
PDF Full Text Request