Simultaneous removal of nitrate and heavy metals in a continuous flow nitrate-dependent ferrous iron oxidation (NDFO) bioreactor
Graphical abstract
Introduction
Removal of nitrogen is an important part of wastewater treatment. Biological wastewater treatment such as nitrification, denitrification, and anaerobic ammonium oxidation (anammox) are commonly used to remove nitrogen from wastewater (McCarty, 2018). In addition to nitrogen, various heavy metals are present in wastewater, especially in industrial wastewater (Azimi et al., 2017). Since high concentrations of heavy metals may cause negative health and environmental consequences (Azimi et al., 2017; Jacob et al., 2018), heavy metals need to be removed from wastewater before being discharged to the surrounding waterways. Presence of heavy metals can also influences the microbial N removal processes. For example, Zinc is known to inhibit anammox reaction at >10 mg/L (Zhang et al., 2018), although this level of Zn is not inhibitory for ammonia oxidizing bacteria (Zhang et al., 2017). Therefore, removal of heavy metals is also important for stable N removal during wastewater treatment processes.
Removal technologies for heavy metals include chemical precipitation, ion exchange, adsorption, and membrane filtration (Azimi et al., 2017). However, these processes may require costly chemicals and materials and also generate a large quantity of secondary wastes (Jacob et al., 2018). Biological treatment may offer more cost-efficient and environment-friendly approach to remove heavy metals (Jacob et al., 2018).
Some microorganisms are capable of producing iron oxides, to which various heavy metals can be adsorbed (Weber et al., 2006a; Bryce et al., 2018; Liu et al., 2019). Under anoxic, nitrate-rich and circumneutral pH conditions, iron oxides can be produced via anaerobic nitrate-dependent ferrous iron (Fe[II]) oxidation (NDFO) reaction. Production of iron oxides itself is not necessarily catalyzed by enzymes such as Fe(II) oxidoreductase: i.e., Fe(II) could be oxidized by reactive N-species that are formed during biological nitrate reduction (Ishii et al., 2016; Bryce et al., 2018; Liu et al., 2019). Nonetheless, this reaction is useful for the simultaneous removals of nitrate and heavy metals (Park et al., 2014; Bryce et al., 2018; Liu et al., 2018). The iron oxide products of NDFO, such as ferrihydrite and amorphous and poorly crystalline Fe(III) oxides, have reactive surfaces that can adsorb various heavy metals, and therefore, can be used to co-precipitate heavy metals (Lack et al., 2002; Weber et al., 2006a; Bryce et al., 2018; Liu et al., 2019; Wang et al., 2019). However, application potential of NDFO reaction to the simultaneous removal of nitrate and heavy metals in actual industrial wastewater is still limited (Tian and Yu, 2020).
The objective of this study was to examine the usefulness of NDFO reaction for the simultaneous removal of nitrate and heavy metals from industrial wastewater. To meet this objective, we used batch culture tests and laboratory-scale continuous-flow bioreactor experiments. As an industry wastewater for testing, we used the coke-oven wastewater discharged from a steel-manufacturing plant because it frequently contains high levels of N (as ammonia), heavy metals such as Zn, cyanides, thiocyanite, and organic contaminants such as phenol (Toh and Ashbolt, 2002; Vázquez et al., 2006). Partial nitrification (nitritation) followed by anammox reaction has a strong potential to be applied to remove N from coke-oven wastewater (Oshiki et al., 2018). While organic contaminants in coke-oven wastewater can be removed by denitrifying bacteria co-existed with anammox bacteria (Oshiki et al., 2018), inhibition of anammox reaction by heavy metals, especially Zn, remains an issue. We hypothesized that the NDFO reaction can be used to remove Zn and other heavy metals from coke-oven wastewater, thereby easing the toxicity for anammox bacteria if used as a pretreatment.
Section snippets
Bacterial culture used
An NDFO strain, Pseudogulbenkiania sp. strain NH8B (Tago et al., 2011; Ishii et al., 2016), was used in this study. Strain NH8B was grown under denitrifying conditions in 10 ml anoxic bicarbonate-buffered basal medium (pH 7.4) supplemented with 5 mM nitrate and 10 mM acetate in test tubes with butyl rubber stoppers, as previously described (Ishii et al., 2016). The basal medium contained 0.25 g/L of NH4Cl, 0.6 g/L of NaH2PO4, 0.1 g/L of KCl, 2.52 g/L of NaHCO3, and 1/100 vol each of the trace
Batch experiment for the removal of nitrate and heavy metals from a basal medium
When Pseudogulbenkiania sp. strain NH8B was present in the medium, oxidation of Fe(II) and reduction of nitrate occurred simultaneously (Fig. 2A), suggesting that the NDFO reaction occurred similar to a previous study (Ishii et al., 2016). Nitrate and Fe(II) were removed at 0.0160 ± 0.0015 mM/h and 0.0437 ± 0.0058 mM/h respectively, when cell concentration was 5 × 107 cells/ml. The NDFO reaction also occur when Zn was present (Fig. 2B) with nitrate and Fe(II) removal rates of
Credit author statement
Kazuki Jokai: Investigation, Writing - original draft. Tomomi Nakamura: Investigation, Satoshi Okabe: Conceptualization, Supervision. Satoshi Ishii: Conceptualization, Formal analysis, Writing- Reviewing and Editing, Supervision, Funding acquisition
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported, in part, by Steel Foundation for Environmental Protection Technology, Japan, and by the Minnesota’s Discovery, Research and InnoVation Economy (MnDRIVE) initiative of the University of Minnesota.
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