| Industrial wastewater containing toxic refractory organics is discharged enormously in China, and is the major problem for the water environment treatment and drinking water security. Chloronitrobenzenes (CINBs) as typical xenobiotics and important chemical intermediates, have been widely used as for synthesis of medicines, dyes and pesticides. CINBs have mutagenicity and genotoxicity, and could be accumulated in sediments and soils and then threaten ecological security and human health due to their refractory and biological accumulation. Therefore, it is important to develop efficient and cost-effective technologies for the control of toxic organic contaminants. Based on the advantages of anaerobic biological treatment and catalytic reduction of bioelectrochemistry, the bioanode acclimatization and characteristic evaluation, synergistic reductive transformation mechanism of p-ClNB by bioelectrode and anaerobic sludge, and performance of bioelectrode-UASB coupled process are investigated in this paper. The main results are as follows:1. In order to enhance the performance of bioelectrode, the impacts of bioanode acclimatization on the performance of bioelectrode and reductive transformation of p-ClNB were investigated. In comparison with electrode acclimatization in open circuit, the internal resistance of bioanode acclimated in MFC was2.5Ω, only one quarter of control electrode; With the same applied voltage (0.5V), the efficient current of bioanode-MEC was between8-10mA, while that of control anode-MEC was always below1mA; The reductive transformation of accorded with pseudo-first order kinetic equation with the rate constant (k) was (0.256±0.017) h-1in bioanode-MEC, which was1.65times as that in control anode-MEC (0.155±0.008) h-1. The results confirmed that quick start of MEC and efficient degradation of target contaminant could be achieved by bioanode acclimatization in MFC. In addition, biocathode was quickly acclimatized and bioelectrode-anaerobic sludge system was successfully established.2. Based on the enhancing p-ClNB reductive trancformation of bioelectrode, the characteristics of reductive transformation and influencing factors of bioelectrode-anaerobic sludge coupled system for p-ClNB degradation were studied. Results showed that p-ClNB was transformed into p-chloroaniline (p-C1An) more rapidly in coupled system than anaerobic sludge system. With applied voltage of0.5V and sludge concentration of3g·L-1, the reaction rate constant (Q) was (0.328±0.012) h-1in coupled system, whereas (0.124±0.005) h-1in anaerobic sludge system and the value of strengthening factor α(kElectrode+Sludge/kSludge) was2.65. The synergistic effect was apparently affected by applied voltage, electrode surface area, dose of sludge and organic carbon source. The reductive transformation rate of p-ClNB increased when the applied voltage raised from0.2V to0.5V, while decreased from (0.328±0.012) h-1to (0.295±0.008) h-1when the voltage was added up to0.8V. It’s speculated that excessively high applied voltage would affect the metabolism and electron transfer of microorganisms adhered to the electrodes. Decrease of organic carbon source didn’t lead to significant decline in p-ClNB reduction rate, on the contrary, resulted in an increase in cathode coulomb efficiency, which means there is internal electron donor existing in coupled system. Increasing electrode surface area (10-40cm2) and the dose of anaerobic sludge (1-3g·L-1) would all strengthen the degradation of p-ClNB. Besides, with the same applied voltage, the increase of sludge dose would enhance the reinforcement of p-ClNB reduction in coupled system.3. Based on the synergistic effect of bioelectrode and anaerobic sludge, a bioelectrode-UASB coupled system was conducted to investigate the properties of anaerobic granule sludge and the performance of p-ClNB reductive transformation. Results showed that stable COD removal (99%), efficient p-ClNB transformation (0.328h-1) and excellent system stability were achieved in bioelectrode-UASB coupled system (R2) operated with influent COD and p-ClNB loading rate of2.1-4.2kgCOD·m-3·d-1and60g·m-3·d-1. Compared to UASB (R1), sludge granulation in bioelectrode-UASB coupled system (R2) was distinctly faster and mean particle size of sludge was much bigger. The EPS concentration ((268.9±13.45) mg·L-1) and surface hydrophobicity (contact angle103.3°±1.2°) of granule sludge in R2were both higher than that in R1. Gas production and composition analysis showed that gas production of two reactors were (1.98±0.12) L·kgCOD-1·d-1and (2.48±0.1) L·kgCOD-1·d-1respectively with the influent COD loading rate of4.2kgCOD·m-3·d-1, while H2production in R2was only1/4～1/2of R1and R2/R1for H2production continued to decline to1/6～1/4with the decrease of the COD loading rate. In consideration of H2and CH4dynamics by bioelectrodes and anaerobic sludge in coupled system, bacth experiments were conducted for mechanism research. Results indicated that H2production was promoted in bioelectrode-anaerobic sludge system, and H2was consumed as internal electron donor for p-ClNB reduction. Furthermore, CV results of biocathode showed a positive shift in reductive peak potential and a dramatic increase in reductive peak current, which mean catalytic reduction ofp-ClNB by biocathode.In summary, the integration of bioelectrodes to UASB reactor favors the participation of both electrochemical and microbial degradation, reflects catalytic reduction by biocathode, produces H2as an electron donor, maintains appropriate pH and ORP, and increases EPS concentration and contact angle on sludge surface, all of which may play an essential role in promotion of sludge granulation and enhancement of p-ClNB reduction. This paper may provide a novel integrated technology for the treatment of recalcitrant wastewaters. |
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