Shuai Lou‡
,
Xinbai Jiang‡,
Dan Chen,
Jinyou Shen*,
Weiqing Han,
Xiuyun Sun,
Jiansheng Li and
Lianjun Wang*
Jiangsu Key Laboratory for Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu Province, China. E-mail: shenjinyou@mail.njust.edu.cn; wanglj@mail.njust.edu.cn; Fax: +86 25 84303965; Fax: +86 25 84315941; Tel: +86 25 84303965 Tel: +86 25 84315941
First published on 11th March 2015
In this study, a membrane-free bio-contact coupled bioelectrochemical system (BC-BES) was established for the enhanced reductive transformation of p-nitrophenol (PNP). The results showed that the electric field played a key role in both PNP reduction and p-aminophenol (PAP) formation. The vast majority of PNP was reductively transformed to PAP in the biocathode of BC-BES. At a cathode potential of −1000 mV vs. Ag/AgCl and hydraulic retention time (HRT) of 8.9 h, PNP removal rate as high as 18.95 ± 0.10 mol per m3 per day could be achieved in the BC-BES with acetate as the electron donor. High PNP removal and PAP formation could be achieved at low acetate dosage, high initial PNP concentration and short HRT, indicating the strong ability of the BC-BES to resist shock loading. Furthermore, partial mineralization of PAP was observed in the anode of the BC-BES, which was beneficial for the further polishing of the BC-BES effluent. Considering the advantages of high loading rate, low acetate consumption and high system stability, it is hoped that the application of this BC-BES will enhance the reductive removal of nitrophenols from wastewaters.
To date, various physicochemical technologies such as sonolysis,4 adsorption,5 Fenton oxidation,6 nickel catalysts7 and electrochemical oxidation8 have been used for the remediation of PNP pollution. However, all these methods have significant defects such as high cost or the formation of the secondary pollution during the treatment process.9,10 Biological process is regarded as cost-effective and efficient for PNP degradation.11 However, due to the strong electro-withdrawing effect of nitro group in the PNP molecular structure, PNP is difficult to oxidize in the aerobic bioprocess.12,13 Anaerobic process, where PNP can be reductively transformed to less toxic p-aminophenol (PAP) by co-metabolism, is more appropriate for PNP removal. However, anaerobic reduction process has the inherent shortcomings, such as low degradation rate, long hydraulic retention time (HRT) and poor system stability, especially for the treatment of wastewater containing high strength PNP waste.13,14
Bioelectrochemical system (BES) is a neoteric creation which has gained much attention across the globe in the past two decades due to its high versatility in the field of wastewater treatment.15,16 In BES, microorganisms are used as catalysts for the electrochemical reactions in an anode or cathode.17 The non-conservative substances such as glucose, sodium acetate, methanol, etc., are oxidized in the anode and then the generated electrons and protons transfer from the anode to the cathode though external circuit and membrane, respectively. The inorganic or organic substances in the cathode gain electrons and protons, which initiate the reduction reaction. Attempts in terms of developing BES reactors with abiotic cathode for the reduction of oxidative compounds have been made.10,15 Compared with the traditional anaerobic biodegradation processes, it has been demonstrated that the reductive transformation of pollutants as the electron acceptor in BES was significantly reinforced.18–20 In addition, it has been suggested that the reduction reaction in the BES cathode at the presence of microbial catalyst could be enhanced, with the economic viability increased compared with the abiotic cathode.21–23 Microorganisms can not only take up electrons from cathode surface and utilize them for the subsequent electrochemical reactions, but also directly degrade organics through co-metabolic reaction.
In most of the previous studies, the BES reactors were separated into anode and cathode chambers by ion/proton exchange membrane. Due to the presence of the ion/proton exchange membrane, the internal resistance of the two-chamber BES was relatively high, which could be a serious bottleneck for energy losses.24 The cost and operational maintenance of the ion/proton exchange membrane hindered their practical applications as well. In addition, the installation of membrane could cause pH gradient especially during the long-term operation for wastewater treatment.25,26 Recently, in order to overcome these shortages, membrane-free BES has been suggested.25,27 Several studies have demonstrated that the internal resistance could be reduced and the power density could be further enhanced in the membrane-free BES.28 In addition, its adaptability to various recalcitrant compounds had got confirmed.29,30 Therefore, for the efficient treatment of PNP containing wastewater, membrane-free BES could be a favorite alternative.30
In this study, a membrane-free bio-contact coupled bioelectrochemical system (BC-BES) was designed for the enhanced reduction of PNP. The effect of acetate dosage, influent PNP concentration, hydraulic retention time (HRT) on PNP removal and PAP formation was investigated. The ability of the BC-BES to resist shock loading was assessed. Additionally, the mineralization of PNP reduction product, i.e., PAP, in the anode of the BC-BES was evaluated.
The sludge collected from an anaerobic baffled reactor treating a real wastewater containing nitroaromatic compounds was used as the inoculum of the BC-BES system. The initial mixed liquid suspended solid (MLSS) concentration in BC-BES was 8.5 g L−1. Before inoculation, the seed sludge was initially acclimatized to the influent containing 0.72 mM PNP and 9.75 mM acetate in a 10 L sequencing batch reactor.
The experimental period was divided into four phases, as shown in Table 1. At the first phase, i.e., start-up stage, the BC-BES reactor was operated in open circuit mode firstly. The influent was pumped into the bottom of the reactor continuously at HRT of 8.9 h and initial PNP concentration of 0.72 mM, resulting in a low PNP loading rate of 1.94 mol per m3 per day. The purpose of this control experiment was to evaluate the PNP removal through direct anaerobic reduction. Ten days later, the circuit of BC-BES was closed. In order to improve the PNP reduction performance and confirm the positive role of current density, the cathode potential was adjusted to −1000 mV gradually, with the PNP removal and PAP formation performance evaluated. Then, in order to further increase the PNP removal capacity in BC-BES, PNP concentration increased step by step from 0.72 mM to 3.23 mM with the corresponding PNP loading rate increased from 1.94 mol per m3 per day to 8.72 mol per m3 per day. The influent acetate dosage was remained at 9.75 mM regardless of the PNP loading rate in the influent.
Phase | Days | PNP concentration (mM) | Acetate dosage (mM) | HRT (h) | PNP loading rate (mol per m3 per day) | Cathode potential (mV) |
---|---|---|---|---|---|---|
Phase 1 | 1–10 | 0.72 | 9.75 | 8.9 | 1.94 | Open circuit |
Phase 1 | 11–96 | 0.72–3.23 | 9.75 | 8.9 | 1.94–8.72 | −50 to −1000 |
Phase 2 | 97–158 | 2.52 | 14.63–1.83 | 8.9 | 6.78 | −1000 |
Phase 3 | 159–205 | 3.23–7.19 | 4.68–10.43 | 8.9 | 8.72–19.38 | −1000 |
Phase 4 | 206–250 | 2.52 | 9.75 | 8.9–2.5 | 6.78–23.22 | −1000 |
In phase 2, in order to investigate the effect of acetate dosage on the performance of PNP reduction and PAP formation, and to assess the electron donor requirement in BC-BES, the influent acetate dosage decreased from 14.63 to 1.83 mM at influent PNP concentration of 2.52 mM and HRT of 8.9 h. BC-BES was operated at cathode potential of −1000 mV at this stage.
In phase 3, in order to evaluate the effect of the high PNP dosage on PNP removal and PAP formation, the influent PNP concentration increased step by step from 3.23 mM to 7.19 mM, while HRT was controlled at 8.9 h, resulting to the increase of PNP loading rate from 8.72 mol per m3 per day to 19.38 mol per m3 per day. The acetate was added according to the molar ratio of 1.45 mol acetate per mol PNP. The cathode potential was controlled at −1000 mV at this stage.
In phase 4, in order to test the performance of the BC-BES at low HRTs, the HRT decreased step by step from 8.9 to 2.5 h with the influent PNP concentration at 2.52 mM and the acetate dosage at 9.75 mM, resulting to the increase of PNP loading rate from 6.78 mol per m3 per day to 23.22 mol per m3 per day. BC-BES was operated at cathode potential of −1000 mV at this stage.
Each experiment lasted at least 4 days to ensure that the reactor reached a steady state, judging from the slight variation of PNP removal efficiency as well as anode and cathode potentials.
40 days later, in order to further improve the reactor performance, PNP loading rate increased gradually from 1.94 on day 40 to 8.72 mol per m3 per day on day 96. As was indicated in Fig. 2, PNP removal and PAP formation were always kept at high levels within the PNP loading range of 1.94 to 8.72 mol per m3 per day. In the BC-BES effluent, almost 100% PNP could be removed while higher than 90% of the total PNP could be transformed into PAP, confirming that PNP could be efficiently reduced into PAP in the BC-BES. Meanwhile, current density increased from 1.96 ± 0.13 A m−3 at PNP loading rate of 1.94 mol per m3 per day to 6.72 ± 0.01 A m−3 at PNP loading rate of 8.72 mol per m3 per day, due to the high availability of the electron acceptor at high PNP loading rate. In order to confirm the key role of the electric field in PNP reduction, the electric field was removed at the 55th day. What is interesting is that, PNP removal efficiency sharply decrease from 100% to 76.27%, and correspondingly, PAP formation efficiency declined from 99.56% to 56.53%. After the cathode potential was reduced back to the low level, both PNP removal efficiency and PAP formation efficiency recovered to the previous level. This phenomenon showed that the electric field played a key role in both PNP reduction and PAP formation.
For the reduction process of nitroaromatic compounds such as PNP, three steps has been proposed, with the nitroso aromatic compounds and hydroxylamine aromatic compounds as intermediates, and with aminoaromatic compounds as the end products.10 In this study, both the UV-Vis spectrum and the HPLC chromatogram confirmed that PNP could be majorly converted into the final product p-aminophenol (PAP) in the BC-BES (Fig. S2 and S3†). The reduction intermediates of PNP, such as p-nitrosophenol and p-hydroxylaminophenol, were not detectable, probably due to the more negative cathode potential adopted (as low as −1000 mV), which was beneficial for both PNP reduction and PAP formation.30 Another reason could be the inoculation of the BC-BES cathode in this study. PNP reduction and PAP formation could be enhanced at the presence of the inoculated bacteria as biocatalysts.
After the successful startup of the BC-BES, from day 30 to day 40, the PNP removal efficiency and PAP formation efficiency in the anode effluent and cathode effluent were 100% and 83.48%, 100% and 75.54%, respectively. The PNP removal efficiency and PAP formation efficiency determined in the anode effluent and in the cathode effluent during this period were nearly the same, demonstrating that PNP removal and PAP formation dominantly occurred in the cathode zone of the BC-BES.
4HO–Ar–NO2 + 3CH3COO− + 4H2O → 4HO–Ar–NH2 + 6HCO3− + 3H+ | (1) |
In order to investigate the effect of acetate dosage on PNP reduction and PAP formation, different acetate dosage conditions in BC-BES was tested. As indicated in Fig. 3a, when the acetate dosage decreased from 14.63 to 3.66 mM, the PNP removal efficiency in BC-BES was maintained at the level as high as 100%, while PAP formation efficiency decreased slightly from 98.29 ± 3.60% to 89.39 ± 2.76%. However, further decrease of acetate dosage resulted in sharp decrease of both PNP removal and PAP formation. As acetate dosage decreased from 3.66 mM to 1.83 mM, the PNP removal efficiency in BC-BES decreased sharply from 100% to 82.71 ± 5.32%. At the meanwhile the PAP formation efficiency decreased sharply from 89.39 ± 2.76% to 68.43 ± 6.18%. In addition, at acetate dosage of 1.83 mM, the anode potential was sharply increased to above 0 mV, which suggested that the electrochemically active microorganisms on the anode might be seriously suppressed (Fig. S4†). Thereafter, acetate dosage was increased back to a high level for stable reactor operation.
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Fig. 3 Effect of influent acetate dosage on PNP reduction and PAP formation (a) and acetate removal efficiency and cosubstrate usage ratio (b) in BC-BES. |
The effect of acetate dosage on acetate removal efficiency and cosubstrate usage ratio in BC-BES was shown in Fig. 3b. The cosubstrate usage ratio was significantly influenced by acetate dosage. The cosubstrate usage ratio was as high as 11.63 ± 0.40 mol COD per mol PNP when the acetate dosage was 14.63 mM, but it was reduced to 2.90 ± 0.13 mol COD per mol PNP when the acetate dosage was 3.66 mM. The acetate removal efficiency remained higher than 98% during the whole period, indicating that excessive acetate could be almost completely consumed in the BC-BES system. However, the minimal electron donor dosage of 2.90 mol-COD per mol-PNP in BC-BES was rather low, compared with conventional anaerobic process for PNP reduction, where the electron donor dosage was often higher than 20 mol COD per mol PNP.11,31,32 The result indicated that BC-BES had the advantage in terms of low requirement for electron donor, which would significantly reduce the operational cost. What's more, the acetate consumption in this BC-BES was also lower than that in double-chamber BES, where acetate consumption varied in the range of 5–8 mol COD per mol PNP.10,20,30
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Fig. 4 PNP removal and PAP formation efficiency (a) and rate (b) at various influent PNP concentrations in BC-BES. |
Both the PNP removal rate and PNP loading rate of 18.95 ± 0.10 and 17.74 ± 1.03 mol per m3 per day in this study were rather high compared to those in most anaerobic systems and BESs treating PNP containing wastewater, where the influent PNP concentration and PNP loading rate was often below 5.03 mM and 6.33 ± 0.11 mol per m3 per day, respectively.10,11,32,33 Moreover, at high influent PNP concentration and high PNP loading rate, both PNP removal and PAP formation was always high, although slight decrease was observed. The strong ability to resist shock loading in BC-BES could be attributed to the cathode potential as low as −1000 mV,34 which was much more negative than that in the open circuit conditions. The strong ability to resist shock loading in BC-BES further added up to the application attractiveness of such a system and enabled better sustainability of the BC-BES system.
As indicated in Fig. 6, PAP concentration in the anode effluent of the BC-BES was always lower than that in the cathode effluent when acetate dosage varied from 9.75 to 3.66 mM. In addition, the NH4+–N concentration in the anode effluent of BC-BES was always higher than that in the cathode effluent. As was reported by other researchers, nitrogen in the PAP structure was often transformed into NH4+ during PAP biodegradation.20 Therefore, the slight increase of NH4+–N concentration in the anode effluent was a key evidence for the PAP biodegradation in BC-BES anode. In addition, when acetate dosage was as low as 3.66 mM, 100% removal of acetate in the cathode zone was observed, however, the anode potential was well below −300 mV, indicating that the electrochemically active microorganisms on the anode did not lose their functions of electron transfer, although acetate was unavailable in the anode. This result indicated that in the anode zone there are alternative electron donors, such as PAP. However, PAP removal efficiency in the anode of the BC-BES was a bit low. Therefore, further work will be focusing on how to accelerate the bioelectrochemical oxidation of PAP in the anode zone of the BC-BES.
The BC-BES could be a favorable alternative for PNP removal compared with conventional anaerobic reduction processes and conventional double-chamber BESs. As shown in Table 2, the maximum PNP removal rate in the BC-BES were above 18.95 ± 0.10 mol per m3 per day, which is much higher than those of conventional anaerobic systems,11,32,33 double chamber BES10 and UASB-BES coupling system,30 demonstrating the high efficiency of BC-BES for PNP removal from wastewater. Compared with conventional anaerobic systems, the more negative cathode potential in BC-BES was beneficial for PNP reduction and PAP formation. Compared with the double chamber BES using abiotic cathode, the bacteria attached on cathode of BC-BES could significantly contribute to the enhanced PNP reduction and PAP formation. Biocathode played an important role in increasing the ability of BC-BES to resist shock loading. In addition, the biofilm formed on the graphite granules in the cathode zone might play a key role in PNP reduction, judge from the much more excellent performance of BC-BES than that of UASB-BES coupling system, where suspended anaerobic sludge was dominant.
Reactor | Electron donor | Maximum RRPNP (mol per m3 per day) | COD usage (mol COD per mol PNP) | Reference |
---|---|---|---|---|
a Cathode potential −1000 mV, HRT 8.9 h.b Current density 4.71 A m−3, HRT 9.0 h.c Cathodic potential −500 mV vs. SHE, HRT 2.6 h.d Anaerobic migrating blanket reactor.e Anaerobic baffled reactor.f Upflow anaerobic sludge blanket. | ||||
BC-BESa | Acetate | 18.95 ± 0.10 | 2.90 ± 0.09 | This study |
UASB-BESb | Acetate | 6.77 ± 0.00 | 2.41 ± 0.10 | 30 |
BESc | Acetate | 6.33 ± 0.11 | 7.81 ± 0.56 | 10 |
AMBRd | Glucose | <0.07 | >120 | 11 |
ABRe | Glucose | <0.64 | >20 | 32 |
UASBf | VFA mixture | 5.49 | 64 | 33 |
The low organic cosubstrate consumption of 2.90 ± 0.09 mol COD per mol PNP was another advantage of this BC-BES, probably due to the significant suppression of the biogas production with the supply of the power.30 What's more important, the energy consumption in the BC-BES was well below 0.02 kW h mol−1 PNP in the BC-BES. The low energy consumption in BC-BES was much lower than that in pure electrochemical system, which typically higher than 2 kW h mol−1 PNP.36,37 In addition, the energy consumption in BC-BES was comparable with that in the conventional double-chamber BES, which typically ranged between 0.01 and 0.10 kW h mol−1 PNP.10,20 The low energy consumption in BC-BES would significantly reduce the operational costs, exhibiting the prospect of cost-effectiveness. In addition, any expensive proton/ion exchange membrane, which was usually considered as the main costly component for BES, was not adopted in the BC-BES. This further added up to the economical attractiveness of such a system and enabled better sustainability of the BC-BES.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra17218c |
‡ These authors contributed to the paper equally. |
This journal is © The Royal Society of Chemistry 2015 |