Elsevier

Chemosphere

Volume 262, January 2021, 127838
Chemosphere

Simultaneous removal of nitrate and heavy metals in a continuous flow nitrate-dependent ferrous iron oxidation (NDFO) bioreactor

https://doi.org/10.1016/j.chemosphere.2020.127838Get rights and content

Highlights

  • Nitrate-dependent Fe(II) oxidation (NDFO) was used to remove nitrate and metals.

  • Batch culture assays and a continuous-flow reactor was used to analyze NDFO.

  • Nitrate, Fe, and Zn were simultaneously removed in the NDFO reactor.

  • The NDFO reactor was applied to treat coke-oven wastewater.

  • Thiocyanate/organic contaminants likely lowered the NDFO reactor performance.

Abstract

Nitrogen and heavy metals can co-occur in various industrial wastewaters such as coke-oven wastewater. Removal of these contaminants is important, but cost-efficient removal technology is limited. In this study, we examined the usefulness of nitrate-dependent ferrous iron oxidation (NDFO) for the simultaneous removal of nitrate and heavy metals (iron and zinc), by using an NDFO strain Pseudogulbenkiania sp. NH8B. Based on the batch culture assays, nitrate, Fe, and Zn were successfully removed from a basal medium as well as coke-oven wastewater containing 5 mM nitrate, 10 mM Fe(II), and 10 mg/L Zn. Zinc in the water was most likely co-precipitated with Fe(III) oxides produced during the NDFO reaction. Simultaneous removal of nitrate, Fe, and Zn was also achieved in a continuous-flow reactor fed with a basal medium containing 10 mM nitrate, 5 mM Fe(II), 4 mM acetate, and 10 mg/L Zn. However, when the reactor is fed with coke-oven wastewater supplemented with 10 mM nitrate, 5 mM Fe(II), 4 mM acetate, and 10 mg/L ZnCl2, the reactor performance significantly decreased, most likely due to the inhibition of bacterial growth by thiocyanate or organic contaminants present in the coke-oven wastewater. Use of mixed culture of NDFO bacteria and thiocyanate/organic-degrading denitrifiers should help improve the reactor performance.

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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