Fe(II)EDTA-NO reduction by a newly isolated thermophilic Anoxybacillus sp. HA from a rotating drum biofilter for NOx removal
Introduction
Nitric oxide (NO) is the primary form (95%) of nitrogen oxides (NOx) emitted from fuel combustion; NOx are major contributors of acid rain formation and ozone layer depletion (Gomez-Garcia et al., 2005, Van Durme et al., 2008). The traditional methods of NO treatment are physicochemical methods, such as scrubbing and selective catalytic reduction (Shi et al., 1996, Wu et al., 2007). The major drawbacks of traditional methods are high cost and possibility of second pollution. Chemical absorption–biological reduction (BioDeNOx) is a cost-effective and eco-friendly process for NO removal (Jin et al., 2005). The rotating drum biofilter (RDB) coupled with BioDeNOx has been successfully applied to remove a large number of NO (Chen et al., 2009, Jun et al., 2008). The NO removal efficiency has been reported to improve from 70.56% to 80.15% as the temperature increases from 32.5 °C to 40.5 °C (Chen et al., 2013). RDB was developed as a type of biological reactor to effectively remove NOx and solve operational problems, such as uneven distribution of nutrients and filter blocking (Chen et al., 2007, Yang et al., 2003).
In such an integrated process, NO is chelated by Fe(II) ethylenediaminetetraacetic acid (EDTA) (the most efficient solution complexation to bind NO) via Eqs. (1), (2) to form Fe(II)EDTA-NO, which enhances NO mass transfer from gas to water, and subsequently reduced to N2 by denitrification bacteria. In this process, Fe(III)EDTA is produced according to Eq. (3).
Fe(II)EDTA-NO and Fe(III)EDTA, which function as electron acceptors in biochemical processes, are reduced by heterotrophic denitrifying bacteria and iron-reducing bacteria according to Eqs. (4), (5), respectively.
In past decades, some bacteria and inoculated sludge for Fe(II)EDTA-NO reduction have been isolated and studied at low and mesophilic temperatures. Dong et al. (2014) reported that Paracoccus denitrificans ZGL1 isolated from activated sludge of a municipal wastewater treatment plant can simultaneously reduce Fe(II)EDTA-NO and Fe(III)EDTA at a mesophilic temperature (30 °C). However, the optimal growth temperature (usually between 10 °C and 35 °C) of the isolated bacteria is not matched with the temperature of flue gas after wet desulfurization (usually between 50 °C and 60 °C). Previous studies identified the need to correlate temperature with the reduction process of Fe(II)EDTA-NO. Four types of sludge inoculated from mesophilic and psychrophilic reactors have been used for Fe(II)EDTA-NO reduction experiments at thermophilic temperatures (55 °C); although three of them exhibited activity, the NO reduction rates (1.6, 0.8, and 0.3 mmol N gVSS− 1 d− 1, respectively) showed great differences (van der Maas et al., 2003). Previous studies mainly focused on the effects of electron donors or acceptors and carbon source on the Fe(II)EDTA-NO reduction process, but the mechanism of temperature still needs to be elucidated.
Redox cycling of FeEDTA plays an important role in the biological denitrification process of the BioDeNOx system (van der Maas et al., 2003), so the biological reduction of Fe(III)EDTA is a key step in the integrated system. Some denitrifying bacteria were confirmed to simultaneously reduce Fe(II)EDTA-NO and Fe(III)EDTA (Dong et al., 2012, Dong et al., 2014, Li et al., 2007, Li et al., 2013). Previous studies have reported that Fe(III)EDTA can function as an electron acceptor and competitor to Fe(II)EDTA-NO during reduction (Kazumi et al., 1995, Kieft et al., 1999). Thus, the inhibition of Fe(III)EDTA on Fe(II)EDTA-NO should be eliminated to maintain high rates of Fe(II)EDTA-NO reduction.
In this study, a new denitrifying bacterium for simultaneous reduction of Fe(II)EDTA-NO and Fe(III)EDTA was isolated and identified from an RDB at 55 °C. We aimed to elucidate the mechanism underlying the oxidation of Fe(II)EDTA during NO removal. Given that temperature influences the reduction process, the effects of temperature on the reduction rates of Fe(II)EDTA-NO and bacterial growth were investigated. The role of temperature in the chemical absorption–biological reduction integrated process was also determined.
Section snippets
Chemicals
EDTA disodium salt (Na2EDTA, 99.95%) and d-glucose (99.95%, cell culture tested) were obtained from Guangdong Guanghua Chemical Factory Co., Ltd., China. FeCl3·6H2O was purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Nitric oxide (99.95%) and N2 (99.999%) were obtained from Zhejiang Jingong Gas Co., China. All chemicals used in this study were of analytical-grade reagents that were commercially available, and used without further purification.
Culture medium
Modified denitrification medium (g·L
Characterization of Anoxybacillus sp. HA
The strain HA was identified as Gram-positive, rod-shaped, white, and opaque. The 16S rDNA gene of HA was amplified in the laboratory and sequenced by Shanghai Sangon Biological Engineering Co., Ltd. The phylogenetic tree displayed the relative similarities and differences between HA and other Anoxybacillus spp. on the basis of BLAST DNA sequence analysis (Fig. 1). In the phylogenetic tree, HA had 95% similarity to Anoxybacillus contaminans (NR029006.1). By combining the results of the
Discussion
In recent years, several Fe(II)EDTA-NO-reducing bacteria growing under mesophilic conditions have been reported. For example, P. denitrificans ZGL1 and Pseudomonas sp. DN-2 exhibit Fe(II)EDTA-NO reduction capacity at 30 °C and 40 °C, respectively. In the present study, we isolated the thermophile A. contaminans HA, which can simultaneously reduce Fe(II)EDTA-NO and Fe(III)EDTA performance a high NO removal efficiency of 98.7%.
Based on the reduction mechanism of Fe(II)EDTA-NO in the chemical
Conclusions
In summary, the experimental results confirmed that HA simultaneously reduced Fe(II)EDTA-NO and Fe(III)EDTA. Fe(II)EDTA could serve as an electron donor, and was bio-oxidized to form Fe(III)EDTA. Fe(II)EDTA-NO was more competitive than Fe(III)EDTA as an electron acceptor, and the appearance of Fe(III)EDTA slightly affected the reduction rate of Fe(II)EDTA-NO. The optimal temperature for Fe(II)EDTA-NO reduction or microbial growth ranged from 50 °C to 60 °C. The appropriate temperature for the
Acknowledgments
The research was supported by the National Natural Science Foundation of China (No. 21277125) and the China Postdoctoral Science Foundation (No. 2013M53148 1) and Program for Changjiang Scholars and Innovative Research Team in University.
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