Enhanced Transformation And Degradation Of Chlorinated Nitroaromatics By Integrated Zero-valent Iron And Microbiological Treat-ment System

Chloronitrobenzenes (CINBs), as important intermediates for the synthesis of medicines, dyes and pesticides, are largely produced and used in China. Many studies have comfirmed that C1NBs are mutagenic, genotoxic, refractory and bio-cumulative, and therefore they could be accumulated in sediments and soils and then threaten ecological security and human health. Consequently, it is very important to develop efficient and cost-effective technologies for treatment of complex wastewater containing these compounds and remediation of areas contaminated by these compounds. Based on the advantages and characteristics of anaerobic biological treatment technology and zero-valent iron (ZVI) reduction process, this paper investigated degradation of CINBs by a coupling ZVI fixed bed-sequencing batch reactor(SBR) system, synergistic reductive transformation of CINBs and its mechanism by ZVI and anaerobic sludge as well as formation and characteristics of ZVI-based anaerobic granular sludge (ZVI-AGS) in UASB. The main results are as follows:1. The reductive transformation and biodegradation characteristics of a coupling ZVI fixed bed-SBR system for treatment of 2-chloronitrobenzene (2-C1NB) wastewater were investigated in this study. Results showed that 2-C1NB was rapidly transformed into 2-chloroaniline (2-C1An) in the ZVI fixed bed, and the reductive transformation reaction accorded with pseudo-first order kinetic equation with the surface area-normalized rate constant (ksa) of (0.59±0.05) L·m-2·h-1. The results of long-term performance of the ZVI fixed bed indicated that 86.4%(v/v) of ZVI was still capable of reducing 2-C1NB to 2-ClAn after 42 days, but its reduction activity decreased gradually with corrosion of ZVI and accumulation of iron precipitates such as Fe2O3, Fe(OH)2 and so on.The coupling ZVI-SBR system was operated at 2-C1NB loadings of 287.4-1266.9 g·m-3·d-1 and COD loadings of 130.9～854.4 g·n-3·d-1, over 99.9% of 2-C1NB and 2-C1An and 92.3% of COD were removed and the effluent concentrations of 2-C1NB, 2-C1An and COD were lower than 0.1 mg·L-1,0.1 mg·L-1 and 65.0 mg·L-1, respectively. On the other hand, the removal efficiency of 2-C1NB in a control SBR without iron pretreatment was only (25.3±10.2)%, which was mainly attribute to the volatilization of the pollutant. The specific degradation rate of 2-C1An in the SBR of the coupling ZVI-SBR system was up to 0.58 g·g-1·d-1. During the decomposition of 2-ClAn, corresponding mol concentrations of NH1+-N and Cl- were synchronously released and the removal efficiency of TOC was up to 95.4%. These results demonstrated that the conducted coupling ZVI fixed bed-SBR system was a feasible approach to enhance the biodegradation and mineralization of CINBs.2. With the combination of ZVI and anaerobic sludge,4-chloronitrobenzene (4-C1NB) was quickly reduced into 4-chloroaniline (4-ClAn) and subsequently dechlorinated into aniline (AN). The strengthening factor for the pseudo-first-order transformation rate constant of 4-C1NB (Q,kZVI+sludge/(Ksludge+kZVI)) was up to 3.698. Subsequently, the dechlorination efficiency of 4-ClAn by sludge was also significantly enhanced. Results demonstrated that there was a significant synergistic effect between ZVI and anaerobic sludge.In the combined ZVI-anaerobic sludge system, the synergistic effect was largely affected by ZVI’s type as well as doses of ZVI and biomass of anaerobic sludge. Results showed that the strengthening factor (Q) for the synergistic effect of different types of ZVI with sludge was followed:Reduced ZVI (RZVI)> Industrial ZVI (IZVI)> Nanoscale ZVI (NZVI). Increasing dosage of ZVI could promote the transformation of 4-C1NB, but the dechlorination rate of 4-ClAn and the values of Q were not enhanced. Besides, NZVI and high concentration of RZVI inhibited the dechlorination of 4-ClAn. With the incresing of biomass of sludge, the k4-C1NB, Q and the dechlorination rate of 4-ClAn were significantly enhanced, and the Q value had positive relation with the mass ratio of sludge to RZVI, which suggested that anaerobic biological reaction maybe the rate-determining step in the ZVI-anaerobic sludge combined system.Based on the previous results, the long-term performance of the RZVI-anaerobic sludge combined system was evaluated. Results showed that the synergistic effect was further enhanced and the k4-C1NB of the combined system during the second addition of 4-C1NB was 2.69～9.53 times of that during the first addition. Furthermore, with a high mass ratio of sludge to RZVI,4-C1NB could be efficiently and stably transformed and dechlorinated by the combined system in a long period.3. Study on the characteristic of hydrogen (H2) release during ZVI corrosion showed that the activity of H2 released by different types of ZVI was followed as NZVI>RZVI>IZVI. The H2 produced via ZVI corrosion could be effectively consumed as electron donor by the anaerobic microbes, and the reduction of 4-C1NB and production of CH4 by microbes were consequently enhanced. Compared with other electron donors, RZVI displayed the greatest intensification on the 4-C1NB transformation, which may be attributed to the best cross-linking of RZVI with anaerobic sludge and the most effective utilization of the catalytic H2/[H] by anaerobic microorganism. However, it was found that the addition of NZVI and high concentration of RZVI (5 g·L-1) lead to a high concentration of suspended iron, which would like to precipited on cell surface and show an inhibitory effect on microbes in a long term. Fortunately, in the combined system with higher mass ratio of sludge to ZVI, the formation FeOOH and Fe3O4 sorbed by ZVI as a redox mediator could accelerate electron transfer and enhaced the reductive transformation and dechlorination of 4-C1NB.The synergistic effect in the combined system was enhanced with the increase of temperature from 10℃to 40℃. The apparent activation energy (Ea) of 4-C1NB transformation in the ZVI, anaerobic sludge and ZVI-anaerobic sludge systems were (21.8±3.8) kJ·mol-1, (65.6±3.6) kJ·mol-1 and (58.0±6.5) kJ·mol-1, respectively. According to that the Ea of ZVI-anaerobic sludge system was closed to that of anaerobic sludge system, it’s speculated that the 4-C1NB transformation by microorganism is the rate-limiting step, and the main mechanism of the synergistic transformation of 4-C1NB in the combined system was that the evolution of H2 by ZVI corrosion could be used as electron donor by anaerobic sludge and therfore accelarate the biological reductive transformation.According to the results of GC-MS and LC-MS analysis and some related reports, a possible pathway of 4-C1NB transformation by the ZVI and anaerobic sludge combined system was proposed:4-chloronitrobenzene→4-chloronitrosobenzene→4-chlorophenylhydroxylamine→4-chloroaniline→analine.4. Based on the synergistic effect of ZVI and anaerobic sludge in batch experiments, the effect of ZVI on the anaerobic biotransformation and dechlorination of chloronitrobenzenes (3,4-DC1NB and 4-C1NB) were also investigated in a ZVI-added upflow anaerobic sludge blanket (ZVI-UASB, R2). Results showed stable COD removal, C1NBs transformation and dechlorination performance were achieved in R2 when operated with influent COD and 3,4-DC1NB loadings of 4200～7700 g·m-3·d-1 and 6.0～70.0 g·m-3·d-1, and R2 showed better shock resistance and buffering capacity for acidification than control UASB (R1). Furthermore, the dechlorination of 4-ClAn to AN was achieved in R2 after 45 days’running, while it did not happen in Rl after long-term operation.A novel ZVI-based anaerobic granular sludge (ZVI-AGS) via the cross-linking of ZVI and anaerobic microorganism was successfully developed in R2. Compared with normal AGS formed in R1, the mature ZVI-AGS showed a smaller size, more compact structure and better settling ability. The higher microbial activities including C1NBs transformation and dechlorination rates, H2 and CH4 production rates were all achieved in ZVI-AGS.16S rRNA PCR-DGGE analysis indicated that the populations of bacteria and archaea in the AGS and ZVI-AGS were significantly changed during the reactor operation. Compared with AGS, the dominant functional bacteria were closely related to the functional microorganism capable of reductively dechlorinating PCB, HCB and TCE in anaerobic enrichment cultures. Results showed that the addition of ZVI in the UASB provided a suitable niche for the enrichment of anaerobic reductive dechlorinating microbial community.Overall, the addition of ZVI to UASB reactor favors the participation of both chemical and microbial degradation, and stimulates H2 production as electron donor via ZVI corrosion and maintains appropriate pH and ORP, all of which may play important roles in creating a favorable niche for the growth and enrichment of 4-C1NB reductive dechlorinating microorganism. At the same time, the reductive transformation and dechlorination of C1NBs could be enhanced in the ZVI-AGS via eliminating the steric hindrance in the transportation of electron and metabolism of [H] and intermediate products. This paper could provide a novel integrated technology for the treatment of wastewaters containing recalcitrant chlorinated nitroaromatics.