Application Of Redox Mediators To Accelerate The Decolorization Of Azo Dye By Photosynthetic Bacteria

Dyeing stream is a big challenge in industrial wastewater treatment due to its high chromaticity, high toxicity and biological refractory. Processes employing photosynthetic bacteria (PSB) are demonstrated to take advantages in stress resistance and capability of degrading refractory compounds. In this dissertation, an anoxic photo-rotating biological contactor (PRBC) was developed and combined with an aerobic moving bed biofilm reactor (MBBR) to treat azo dye wastewater, and its treatment efficiency was evaluated. Then the characteristics and decolorization efficiency of photosynthetic bacteria isolated from the anoxic PRBC and the role of redox mediator in the biodecolorization process were studied, respectively. Furthermore, the treatment performance of the anoxic PRBC was enhanced by supplement redox mediators.The anoxic PRBC has no PSB reflux and its luminous decay is low. Azo dye wastewater was treated by the combined process of the anoxic PRBC and the aerobic MBBR. The anoxic PRBC was successfully started up within 35 days. After the success of start-up, the decolorizaiton rate and COD removal rate were both stabilized at approximate 92%. It was found that the stable phototrophic biofilm consisted of two layers: inner layer of photosynthetic bacteria and outer layer of filamentous bacteria, and filamentous bacteria played an important role in the attachment of the biofilm to the disks.The suggested conditions for dyeing wastewater treatment by PRBC-MBBR were: anoxic, illumination time≥6h/d and HRT≥10h, under which the total color and COD removal efficiencies were 92% and 91% respectively. PRBC-MBBR resisted high shocking load of dyes; color and COD removal efficiencies of up to 86% and 90% were obtained even under 7.2×105 mg/m3·d of dyes, and the color removal was mainly attributed to the anoxic PRBC, while the sequential MBBR ensured the water quality of the final effluent. The essential concentration of co-substrate for the decolorization by PRBC-MBBR was low. When concentrations of the dye and the co-substrate were 100 and 1000 mg/L, total decolorization efficiency of 90% was achieved.Denaturing Gradient Gel Electrophoresis (DGGE) and 16S rRNA sequences analysis revealed that the phototrophic biofilm developed in this study consisted Rhodopseudomonas palustris, Rhodomicrobium vannielii, Chlorobium limicola, Chlorobium phaeobacteroides, uncultured cyanobacterium and other bacteria. Sulfate, light and oxygen strongly affected the phototrophic bacterial community. Sulfate was the key factor for switching between purple and green bacteria; high concentration of sulfate was an inhibitor for purple bacteria but a stimulator for green ones. Increases in light time and oxygen supply enhanced the predominance of photosynthetic bacteria in the system, whereas did not change the community structure.An efficient bacterial decolorizer Rhodopseudomonas palustris W1 was isolated from the anoxic PRBC. The results of batch tests of decolorization suggested that to enhance strain W1 growth and its decolorization efficiency, suitable pH level (≤10), carbon source and its concentration (glutamine or lactate recommended,≥500 mg/L when lactate was selected), nitrogen source and its concentration (NH4Cl recommended,≥100 mg/L), salinity (NaCl concentration≤5%), RB5 concentration (≤700 mg/L), and light presence were required. Under that optimal condition, over 95% of color removal efficiency could be achieved in 20 h. The optimal pH for azoreductase from strain W1 was 7.0, and temperature was 40℃. Moreover, strain W1 exhibited high reductase activity of anthraquinone-sulfonate (AQS) and lawsone, but relatively low of 1-2-7-triamino-8-hydroxy-3-6-naphthalinedisulphate (TAHNDS).The biodecolorization of Reactive Black 5 (RB5) by strain W1 was studied and autocatalysis was observed, that is the decolorization metabolites (DM) from RB5 accelerated its own further decolorization. At a suitable DM concentration, the decolorizaton rate was improved to 70% in 5 h. DM from RB5 can also catalyze the decolorization of other dyes. The decolorization rate of Direct Yellow 11 (DY11) was increased by 2.2-fold by supplementing an optimized amount of DM, compared with unamended controls. Cyclic voltammetric(CV) tests revealed that DM could be a rodox mediator. The results of FT-IR and HPLC-MS showed that the active component in DM as a redox mediator was TAHNDS. In the decolorization of a dye mixture of RB5 and AR1, RB5 was decomposed before AR1, and the DM released catalyzed the decolorization of AR1, increasing its rate by 2-fold.DM can mediate the chemical reduction of RB5 by sulfide produced in the anaerobic bioreduction of sulfate. But sulfide did not contribute to the biodecolorization of DY 11 since no redox mediator was produced in this group of azo dyes. Sulfide was oxidized to elemental sulfur during the chemical decolorization of dyes. Under anaerobic condition, sulfur could be readily reduced to sulfide again by W1, which reduced azo dyes repeatedly.When employing RB5 as an endogenic redox mediator, the color and COD removal efficiencies were up to 91 and 94% respectively under 100 mg/L of AR1 and 150 mg/L of RB5 and in the presence of co-substrate. Without co-substrate, RB5 inhibited the decolorization of AR1. Sulfate accelerated the decolorization of RB5 and elemental sulfur produced during the reduction of sulfide accumulated in the biomass. However sulfate had inhibition in the COD removal and decolorization of DY11 because DY11 could not produce redox mediators during its degradation.AQS pillared hydrotalcite was supplemented as an immobilized redox mediator in PRBC-MBBR system. Under HRT of 5h and initial AR1 concentration of 100mg/L, the color and COD removal efficiencies of the two-stage process stabilized at 89.9 and 90% respectively in 18d. In addition, the AQS pillared hydrotalcite improved the sulfate reduction efficiency from 56% to 90%.