Unraveling the impact of nanoscale zero-valent iron on the nitrogen removal performance and microbial community of anammox sludge
Introduction
Nanoscale zero-valent iron (NZVI) particles have been increasingly used during the past decade for environmental remediation and wastewater treatment (Crane and Scott, 2012, Tang and Lo, 2013). Chlorinated contaminants (e.g., trichloroethylene), nitro aromatic compounds (e.g., nitrobenzene), nitrate, and toxic As(V), Pb(II) and Cr(VI) can be easily reduced by NZVI to less harmful or stable compounds (Wu et al., 2013, Yang et al., 2013a). NZVI is also used to control odors in biosolid treatment because the released ferrous ions can combine with malodorous sulfur compounds (Li et al., 2007). Due to its cost-efficiency and high-reactivity, NZVI-based technology has obtained a worldwide market share; however, it has yet to gain universal acceptance because NZVI likely reacts with living organisms (Crane and Scott, 2012). Accordingly, public concerns regarding the unintended impact of NZVI on indigenous organisms are increasingly growing (Scaron et al., 2011, Wu et al., 2013). Microbial communities play crucial roles in soil, water, and sediments; thus, the potential negative impacts of NZVI on these functional microorganisms need to be weighed against the benefits of NZVI use in remediation (Scaron et al., 2011).
Recent studies have reported that the introduction of NZVI may affect microbial growth and survival both positively and negatively. The antimicrobial activity of NZVI has been observed in pure cultures against a broad range of microorganisms, including Escherichia coli (Gram-negative, G−), Bacillus subtilis var. niger (Gram-positive, G+), and Pseudomonas fluorescens (G−); however, antimicrobial activity has not observed against the fungus Aspergillus versicolor (Diao and Yao, 2009, Scaron et al., 2011). Different types of NZVI have different intrinsic properties, such as the presence of polyelectrolyte coatings and the nanoparticles’ sizes, shapes, and degrees of oxidation; these properties strongly affect the NZVI toxicity. The toxicity mechanisms of NZVI may include membrane disruption, DNA damage and protein (enzyme) inactivation (Diao and Yao, 2009, Scaron et al., 2011). The situation of mixed cultures is more complex: the impact of NZVI on the community structure and functionality is dose-dependent and differs from species to species (aerobic/anaerobic, G+/G− and so on) (Fajardo et al., 2012). For example, microbial sulfate reduction in aquifer sediment was inhibited by 0.5–3.0 g L−1 NZVI (Kumar et al., 2014). Xiu et al., (2010) observed that the presence of 1.0 g L−1 NZVI significantly stimulated the activity of methanogens but inhibited dechlorinating organisms (including Dehalococcoides) in the batch microcosms.
Previous studies mainly focused on the impact of NZVI on microorganisms in pure cultures, soils or sediments, while their impact on the microorganisms in wastewater treatment has not been well documented. These microorganisms are responsible for the removal or recovery of various pollutants such as carbon, nitrogen and phosphorus, thereby playing an important role in the function and performance of wastewater treatment. ZVI powder can serve as an electron donor for methanogens and facilitate anaerobic digestion (Karri et al., 2005); however, Yang et al. (2013a) reported that NZVI inhibited methanogens (dominated by Methanosaeta) during anaerobic digestion because it disrupted cell integrity. Wu et al. (2013) found that the addition of 200 mg L−1 NZVI significantly inhibited the NH4+-N removal rate and chemical oxygen demand (COD) removal efficiency of activated sludge.
Anaerobic ammonium oxidation (anammox) bacteria are ubiquitous in various natural environments and responsible for 9–40%, 4–37%, and 50% of the nitrogen loss in the oceans, inland lakes, and agricultural soil, respectively (Hu et al., 2011). Because anammox bacteria can directly oxidize ammonium to dinitrogen gas without oxygen and organic carbon (Lackner et al., 2014), the anammox-based processes can reduce the energy consumption of wastewater treatment (Kuenen et al., 2011). The number of full-scale anammox installations around the world exceeded 100 by early 2015 (Lackner et al., 2014). Thus, unraveling the potential impact of NZVI on anammox bacteria is of great significance in both ecology and engineering. However, to the best of our knowledge, it is unknown whether the introduction of NZVI into wastewater has negative effects on anammox bacteria.
In this study, the short- and long-term impacts of NZVI on anammox bacteria were investigated using anammox sludge. The cytotoxicity of NZVI was tested in three aspects: anammox metabolic activity, intracellular reactive oxygen species (ROS) production, and extracellular lactate dehydrogenase (LDH) activity. The nitrogen removal performance of the anammox reactor was then evaluated under the stress of ZNVI at various levels. In addition, 16S rDNA-based high-throughput sequencing was used to track the dynamics of the microbial community in anammox sludge.
Section snippets
Synthetic wastewater and seeding sludge
Anammox seeding sludge was harvested from a laboratory-scale up-flow anaerobic sludge blanket (UASB) reactor with an 80-mesh sieve that restricted the mean diameter of the seeding sludge to less than 0.2 mm. The specific anammox activity (SAA) and the extracellular polymeric substance (EPS) content of this mature anammox sludge were 334.1 ± 25.5 mgTN g−1 volatile suspended solids (VSS) d−1 and 255.6 ± 16.7 mg g−1 VSS, respectively. This parent reactor has been treating synthetic wastewater that contains
Release of iron ions from NZVI
Before entering into wastewater, the metallic nanoparticles (NPs) are likely to be partially oxidized to metal oxides because they were inevitably exposed to air (Zhang et al., 2017b). Pristine NZVI is generally very reactive, and its surface properties can easily change, depending on environmental conditions. Furthermore, NZVI already has a thin but encapsulating layer of surface oxide that was acquired naturally before entering wastewater (Crane and Scott, 2012). Here, FTIR was used to
Conclusions
Short-term exposure of anammox bacteria to NZVI up to 200 mg L−1 did not affect their metabolic activity, ROS production, and cell membrane integrity. Long-term addition of NZVI at 20 or 50 mg L−1 disturbed the performance of a continuous-flow reactor, but in time, it returned to normal, even at 200 mg L−1 NZVI. Although the presence of 10, 20, 50, and 200 mg L−1 NZVI to some extent affected microbial composition, the anammox bacteria (Candidatus Kuenenia) never lost its dominance. This adaptability
Acknowledgements
The authors wish to thank the Science and Technology Development Program of Hangzhou (No. 20170533B09) and the Natural Science Foundation of China (No. 51578204) for their partial support of this study.
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These authors contributed equally to this work.