| Chloronitrobenzenes (CINBs) as the synthesis chemical intermediates, are widely used in the manufactures of chemicals, medicines, dyes, pesticides, etc. These compounds are teratogenic, carcinogenic, mutagenic, refractory, which could be easily accumulated in sediments and soils, and subsequently threaten the human health and social security via food chains. Because of the serious environmental issue, these compounds have been decleared to be priority pollutants by EPA, EEC and China. Consequently, there are significant demands of efficient and cost-effective technologies for CINBs wastewater treatment and soil remediation. Based on the advantages of anaerobic biological treatment and catalytic properties of bioelectrochemistry, the reductive transformation and dechlorination characteristics of p-ClNB, performance of combined bioelectrode-UASB system, and community structure of anaerobic slude and electrode biofilm are investigated. The main results are as follows:1. The enhanced reductive transformation and dechlorination of p-ClNB were studied in batch systems with bioelectrode and anaerobic sludge. Results showed that p-ClNB was firstly transformed into p-chloroaniline (p-ClAn), and then dechlorinated. The reduction process of nitro group followed first-order dynamics. Under the condition of the voltage of 0.5 V, initial p-ClNB concentration of 30 mg-L’1 and sludge concentration of 3 g·L-1,the conversion rate constants of individual anaerobic sludge system and bioelectrode-anaerobic sludge system were (0.286±0.07) h-1 and (0.598±0.06) h-1, respectively. The strengthen factor Q’reached 2.093, showing that the reductive transformation and dechlorination of p-ClNB were enhanced by intergrated bioelectrode and anaerobic sludge system.The reductive transformation and dechlorination of p-ClNB were affected by the applied voltage, system pH and carbon source of wastewater. When the applied voltage increased from 0.25 V to 0.75 V, the rates of nitro-group reduction and dechlorination increased. However, the reductive transformation and dechlorination process of p-ClNB was improved when the applied voltage further increased to 1 V. The wastewater addition of 3 gCOD·L-1 enhanced the reductive transformation of p-ClNB into p-ClAn with the conversion rate constant of 0.5785 h-1,while remarkably slowed the dechlorionation rate of p-ClAn. The dechlorination of p-ClAn didn’t occur until p-ClNB transformed into p-ClAn completely, while the p-ClNB reduction and p-ClAn dechlorination occurred simultaneously without the addition of organic wastewater. Besides, the suitable pH (7.0-7.4) could also improve the overall reductive transformation and dechlorination ofp-ClNB.According to results of GC/LC-MS analysis and related studies, the possible pathway of p-ClNB transformation in coupled system was proposed: p-ClNBâ†' p-chlotonitrosobenzeneâ†'p-chlorophenylhydroxylamineâ†' p-ClAnâ†'subsequent dechlorination, and the dechlorination was the main rate-limiting step.2. A novel bioelectrode-UASB coupled system was developed to investigate the performances of p-ClNB wastewater treatment. The combined system was stably operated at p-ClNB loading of 60 g·m-3·d-1,COD loading of 2.1 kgCOD·m-3·d-1, HRT of 36 h and applied voltage of 5.0 V (external current of 0-2 mA), and 99.9% of p-ClNB and 96.3% of COD were removed and the effluent concentration of p-ClNB and COD were lower 0.1 mg·L-1 and 120 mg·L-1, respectively. The p-ClAn removal rate of bioelectrode-UASB combined reactor (R2) and control UASB (R1) were 40.39%±9.26% and 31.07%±8.60%, and the effluent average pH values of R2 and R1 were (7.35±0.23) and (7.13±0.19), respectively. Results indicated that the bioelectrode could enhance the reductive transformation and dechlorination of p-C1NB by anaerobic sludge, and also improve the stability of UASB. When the applied voltage of R2 increased to 6.5 V, the external current increased from (0.40～0.65) mA to (0.50～1.75) mA, and the p-ClAn removal rate reached 52.26%±13.01% correspondingly. However, the external current decreased to (0.25～0.53) mA when the applied voltage further increased to 7.5 V, and the p-ClAn removal rate decreased to 49.09%±9.10%. It is speculated that the applied voltage of 7.5 V was too high that disturbed the microbial metabolism of biofilm and electron transfer. Furthermore, researches on the properties of anaerobic sludge granule found that granular sludge with more EPS in R2 had the larger average diameter (D50) of 0.792 mm, meanwhile D50 of R1 was only 0.352 mm, indicating that the applied field could promote the sludge granulation. The treatment performance of two UASB reactors without external additional carbon source was investigated. The p-CINB was completely transformed in both reactors, and the removal rate of p-ClAn in R1 and R2 were 53.31%±5.65 and 87.69%±10.60%, respectively.To further reveal the reductive transformation and dechlorination mechanism in bioelectrode-UASB combined system, Batch experiments were conducted with inital p-ClAn concentration of 40 mg·L-1, sludge concentration of 3 g·L-1, and applied voltage of 0.75 V. The ratio of H2 in R2 was 5.59 μmol·bottle-1, which is higer than that in Rl (2.45 μmol·bottle-1), and the ratio of CH4 decreased. Analysis was conducted that the bioelectrode facilitates the H2 generation and change of electron transfer pathway to enhance the sebsequent dechlorination ofp-ClAn.3. PCR-DGGE and 454 pyrophosphate sequencing technologies were applied to investigate the microbial communities in anaerobic sludge and electrode biofilm. Compared with control UASB, the microbial community of anaerobic sludge in bioelectrode-USAB system was more abundant, and Firmicutes sp. and Planctomycetes sp. were enriched to enhance p-ClNB transformation. Thereinto, Anaerolineae sp. SJA-15 had high homology with the typical reductive dechlorinating bacteria of Dehalococcoides ethenogenes, which is speculated to be the main p-ClNB dechlorinating microbe in R2. Another dominant microbe was Sarcina. sp, and it could produce hydrogen as electron donor under anaerobic condition for improving the reduction of nitro group, dechlorination ofp-ClAn and production of CH4.The contrastive analysis of electrode biofilm showed that the fermenters (Kosmotoga sp.,16.8%), homoacetogens (Treponema sp.,2.5%) and anode-respiring bacteria (ARB) (Geobacter sp.,4.0%) were enriched on the bioanode, and the syntrophic interaction of these microorganisms could make the anode maintaining high electron transfer level and coulombic efficiency. On the biocathode, Sarcina. sp (24.4%) was the dominant species, and the proportion was close to that in anaerobic sludge (25.0%). The results indicated that the applied voltage facilitated the enrichment of Sarcina.sp on biocathode, and then improves the ability of hydrogen generation and reductive transformation of p-ClNB in bioelectrode-UASB combined system.In summary, the integration of bioelectrodes and UASB system could enhance the reductive transformation and dechlorination of chlorinated nitroaromatics via the generation of hydrogen as the electron donor, micro electric field domesticating metabolic activity, and synergy of bioelectrodes and anaerobic sludge. This study provides the feasible process for efficient treatment of toxic refractory organic wastewater. |
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