Studies On Chlorobenzenes Degradation By Photosynthetic Bacteria (Rhodobacter Sphaeroides)

Chlorobenzenes are widely used as solvents, degreaser, and intermediates in the synthesis of dyes and pesticides. They are highly toxic compounds that cause a wide variety of effects ranging from immunological disorders to adverse effects on the liver, kidney, thyroid, and lung. Once released into environment, they will accumulate in the surroundings and endanger human as well as its ecological environment over a long period of time.The investigation on biodegradation of chlorinated benzenes (chlorobenzene, o-dichlorobenzene) by the photosynthetic bacteria Rhodobacter sphaeroides was carried out. The degradation efficiency, dechlorination efficiency and some reaction parameters such as carbon source, initial pH, and chlorobenzene concentration were studied. The methods and achievements in this paper were following:Rhodobacter sphaeroides growth and their dehydrogenase activity under different concentration of chlorobenzene (CB) were studied. When the CB concentration was 80 mg/L, the growth of Rhodobacter sphaeroides was inhabited. When the CB concentration was more than 666 mg/L, its growth was completely inhabited. With the increase of the concentration of CB,the dehydrogenase activity of Rhodobacter sphaeroides was decreased gradually. A standard method of algal bioassay for evaluating the toxicity of toxic chemicals was applied further in the experiments. The lowest observed effect of their concentration (LOEC) on the growth was 40 mg/L, the 96h-EC50 of CB on the inhibition of their growth was 346.7 mg/L.Chlorobenzene would not be taken as sole carbon and energy sources by Rhodobacter sphaeroides. Degradation way of Chlorobenzene was cometabolism with the induced enzyme in the presence of the suitable carbon source.These substrates, glucose, malate, citrc acid, acetic acid, propionic acid, supported both aerobic and anaerobic growth of Rhodobacter sphaeroides well, but they had different influence on chlorobenzene degradation by Rhodobacter sphaeroides. When malate was served as carbon source, the degradation efficiency for chlorobenzene was higher under both aerobic and anaerobic in light condition.Chlorobenzene degradation was investigated with the tamed strain of Rhodobacter sphaeroides in the presence of light and in the absence of oxygen. From the results, carbon source concentration, (NH4)2SO4 concentration, yeast extract, pH, had great influence on the degradation and dechlorination of chlorobenzene and cells grown. The Lag phase of bacterial growth was obviously shorted in the presence of yeast extract. The optimal conditions of anaerobic degradation of chlorobenzene when the concentration of chlorobenzene was 100mg/L were malate.1.Og/L, (NH4)2SO4 0.1 g/L, yeast extract 1.0 g/L and pH 7.0.1,2-Dichlorobenzene was taken as an example to study the metabolic Mechanism leading to anaerobic degradation of chlorinated aromatic compounds.The results showed that 1,2-dichlorobenzene was fully degraded by the tamed strain of Rhodobacter sphaeroides under phototrophic (anaerobic in light) condition. Analysis of degradation products of 1,2-dichlorobenzene by the strain tested showed that chlorobenzene,4-hydroxybenzote, chloroform were formed. The probable degradative routs taken in the anaerobic metabolism of chlorinated aromatic compounds by Rhodobacter sphaeroides were that they are metabolized anaerobically to form chlorobenzene, and then chlorobenzene was dechlorinated and passed through 4-hydroxybenzote routes for aromatic ring cleavage.Rhodobacter sphaeroides w as able t o d egrade c hlorobenzene u nder aerobic c ondition. Aerobic degradation rates of chlorobenzene by the strain were higher than anaerobic. Chlorobenzene was fully degraded within 8h. Dechlorination rates of chlorobenzene were slower under aerobic condition. The optimal conditions of aerobic degradation of chlorobenzene when the chlorobenzene concentration was 100mg/L were malate 1.5g/L, dark, and pH 7.0. The results of enzymes assays showed that catechol-1,2dioxygenase activities and catechol-2,3dioxygenase activities were induced in cells of the strain Rhodobacter sphaeroides grown on chlorobenzene under aerobic condition, and related to concentration of chlorobenzene.The aerobic metabolism of chlorobenzene by Rhodobacter sphaeroides was ortho ring cleavage catalyzed by catechol-1,2dioxygenase. Based on the analysis of aerobic degradation products by GC-MS and dioxygenase assays of cell extracts, the degradative routs taken in the aerobic metabolism of chlorobenzene by Rhodobacter sphaeroides were deduced.They were that first chlorobenzene was oxidized and dehydrogenated to form chlorocatechol, second chlorocatechol was cleaved to form chloro-cis, cis-muconate catalyzed by catecho1-1,2dioxygenase, third chloro-cis, cis-muconate was oxidized to form chloroform, last chloroform was dechlorinated to form carbon dioxcide, chloride and water.Influences o f several different kinds o f flocculating agent o n photosynthetic b acteria Rhodobacter sphaeroides were assessed by analyzing settling performances and strength of bioflocculates.From the results, polychlorinated alumina (PAC) was finally selected as a flocculating agent to accelerate formation of granular sludge of Rhodobacter sphaeroides. Tests gave the dose of PAC in the range of 140-160mg/L. When the dose of PAC was 150mg/L, granulation was the most effective. The degradation efficiency and reaction parameters were investigated on treating chlorobenzene in water by granular sludge of Rhodobacter sphaeroides. The results showed that the optimal conditions of treatment for chlorobenzene in water by granular sludge of Rhodobacter sphaeroides were aerobic condition, temperature 30℃and pH 7.0.The chlorobenzene wastewater was treated with Sequencing Batch Reactor (SBR) filled with granular sludge of Rhodobacter sphaeroides. The results showed that the SBR system was reliable and stable. The parameters of affecting treatment efficiency were examined including chlorobenzene concentration of influent, sedimentation time, reaction time, concentration of granular sludge,concentration of dissolved oxygen,idle time. From the kinetic standpoint, the biodegradtion process of C B with SBR fits the first order equation. When chlorobenzene concentration of influentwater was 125 to 187.5mg/L, the removal rate of chlorobenzene was 90.5-95.6%.The optimal conditions were the reaction time for 6h, the sedimentation time for 1.5h, concentration of granular sludge for 4000-6000mg/L,DO 4.75-5.0 mg/L,the idle time for Oh.