Effect of low temperature on abiotic and biotic nitrate reduction by zero-valent Iron

Highlights

• The rate and extent of nitrate removal by ZVI was much greater in the presence of microorganisms.

• Unbuffered ZVI reduction of nitrate is more sensitive to temperature shifts than buffered process

• Removed nitrate was completely recovered as ammonium ions by abiotic ZVI reduction.

• Less than 25% of the removed nitrate was converted to NH4+ in the ZVI-cell reactors.

• Microbial reduction of nitrate is more dominant reaction for nitrate removal at low temperatures.

Abstract

The effect of low temperatures on abiotic and biotic nitrate (NO₃⁻) reduction by zero-valent iron (ZVI) were examined at temperatures below 25 °C. The extent and rate of nitrate removal in batch ZVI reactors were determined in the presence and absence of microorganisms at 3.5, 10, 17, and 25 °C. Under anoxic conditions, NO₃⁻ reduction rates in both ZVI-only and ZVI-cell reactors declined as temperature decreased. In ZVI-only reactor, 62% and 17% of initial nitrate concentration were reduced in 6 days at 25 and 3.5 °C, respectively. The reduced nitrate was completely recovered as ammonium ions (NH₄⁺) at both temperatures. The temperature-dependent abiotic reduction rates enabled us to calculate the activation energy (E_a) using the Arrhenius relationship, which was 50 kJ/mol. Nitrate in ZVI-cell reactors was completely removed within 1–2 days at 25 and 10 °C, and 67% of reduction was achieved at 3.5 °C. Only 18–25% of the reduced nitrate was recovered as NH₄⁺ in the ZVI-cell reactors. Soluble iron concentrations (Fe²⁺ and Fe³⁺) in the ZVI reactors were also measured as the indicators of anaerobic corrosion. In the ZVI-cell reactors, soluble iron concentrations were 1.7 times higher than that in ZVI-only reactors at 25 °C, suggesting that the enhanced nitrate reduction in the ZVI-cell reactors may be partly due to increased redox activity (i.e., corrosion) on iron surfaces. Anaerobic corrosion of ZVI was also temperature-dependent as substantially lower concentrations of corrosion product were detected at lower incubation temperatures; however, microbially induced corrosion (MIC) of ZVI was much less impacted at lower temperatures than abiotic ZVI corrosion. This study demonstrated that ZVI-supported microbial denitrification is not only more sustainable at lower temperatures, but it becomes more dominant reaction for nitrate removal in microbial-ZVI systems at low temperatures.

Introduction

Waters contaminated with nitrate (NO₃⁻) are commonly treated by physio-chemical processes such as ion exchange or reverse osmosis. However, these conventional processes are costly, and generate concentrated waste streams that may cause further disposal problems (Ergas and Reuss, 2001; Shrimali and Singh, 2001). As an alternative to these conventional processes, nitrate reduction by zero-valent iron (ZVI) has been demonstrated as an effective chemical process for nitrate removal (Huang et al., 1998; Tratnyek et al., 2003; Ahn et al., 2008; Suzuki et al., 2012; Zhu and Getting, 2012; Liu et al., 2013; Zhang et al., 2017; Zhang et al., 2019). In the absence of oxygen, nitrate reduction by ZVI is thermodynamically favorable with ammonia and ferrous iron as the major products according to the following reaction (Cheng et al., 1997):

$$4{\mathrm{Fe}}^0+{{\mathrm{NO}}_3}^{-}+10{\mathrm{H}}^{+}\to {{\mathrm{NH}}_4}^{+}+4{\mathrm{Fe}}^{2+}+3{\mathrm{H}}_2\mathrm{O}$$

It has been reported that the rate of nitrate reduction by ZVI is strongly influenced by acidic conditions (Choe et al., 2004; Huang and Zhang, 2004) as various acidic species such as strong acids, acetic acid, and CO₂ gas have been used to accelerate the reaction rate (Cheng et al., 1997; Huang et al., 1998; Su and Puls, 2004; Ruangchainikom et al., 2006; Ahn et al., 2012). Although the application of acidic conditions is effective in laboratory studies, lowering the solution pH may not be economically practical and may have other adverse secondary effects on the treated water quality.

In addition to abiotic reduction by ZVI, nitrate can also be reduced to nitrogen gas (N₂) by ZVI in the presence of denitrifying bacteria (Till et al., 1998; Mansell and Schroeder, 2002; Rocca et al., 2007; Shin and Cha, 2008; An et al., 2009; Peng et al., 2015). Cathodic hydrogen (H₂) is generated from anaerobic oxidation of ZVI coupled with the reduction of proton according to the following equation (Smith et al., 1994; Till et al., 1998):

$${\mathrm{Fe}}^0+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{\mathrm{Fe}}^{2+}+2{\mathrm{OH}}^{-}$$

This hydrogen gas is a thermodynamically favorable electron donor for nitrate-respiring bacteria (Kurt et al., 1987):

$$2{{\mathrm{NO}}_3}^{-}+5{\mathrm{H}}_2\to {\mathrm{N}}_2+4{\mathrm{H}}_2\mathrm{O}+2{\mathrm{OH}}^{-}$$

Microbially induced corrosion (MIC) is a well-known process that relates the activity of microorganisms and anaerobic corrosion of iron in the environment (Stott, 1993; Xu et al., 2013; Enning and Garrelfs, 2014). The MIC process promotes corrosion of iron or carbon steel by the removal and oxidation of cathodic hydrogen through bacterial enzymatic activities (Gains, 1910; Stott, 1993; Jia et al., 2017). The consumption of cathodic hydrogen by hydrogenophilic bacteria enhanced anodic ZVI oxidation, which results in the accelerated corrosion of iron (Lino et al., 2015). This type of biocorrosion has been extensively studied, especially for sulfate- and nitrate-reducing bioreactors (Hamilton, 1985; Pedersen et al., 1988; Dowling et al., 1992; De Windt et al., 2003; Hubert et al., 2005; Mori et al., 2010; Xu et al., 2013).

Some denitrifying strains, including Pseudomonas sp. S9 and Shewanella oneidensis MR-1, have been attributed to biocorrosion of ZVI under nitrate-reducing conditions (Pedersen et al., 1988; De Windt et al., 2003). In the absence of oxygen, the cell density around the iron particles was shown to increase. The accelerated consumption of H₂ film on the iron surfaces resulted in increased solubilization of iron minerals from ZVI particles, resulting in 1.33 times more total iron solubilization than that from the abiotic control test (De Windt et al., 2003). Furthermore, the presence of microorganisms stimulated both iron corrosion and the nitrate degradation kinetics at the standard conditions, while ZVI-only system did not (Kaesche, 2003).

Nitrate removal rate by ZVI particles has been shown to be temperature-dependent in many previous studies (Ginner et al., 2004; Ahn et al., 2008; Peng et al., 2015). Nitrate and nitrite reduction by iron filings in the presence of denitrifying bacteria over a range of temperatures were examined in batch reactors under the buffered condition at pH 8.4 (Ginner et al., 2004). The reduction rates of nitrate and nitrite increased as temperature increased in the buffered-mineral solution. Moreover, the faster degradation of nitrate was observed in the presence of denitrifying bacteria, Paracoccus denitrificans, compared to the abiotic nitrate reduction by iron particles at the same temperatures. Ahn et al. (2008) reported that the increase in solution temperature up to 75 °C accelerated the rate of abiotic nitrate reduction both in batch and column reactors. In addition, they confirmed that a buffered system (pH 7.4) showed lower activation energy value for nitrate reduction than an unbuffered system (initial pH 6.7) at the same temperature, indicating the importance of maintaining solution pH. Using a mathematical model, Peng et al. (2015) demonstrated that nano-scale ZVI-supported denitrification in activated sludge cultures was enhanced as temperature increased.

Most of the reported studies on abiotic and biotic nitrate reduction by ZVI were conducted at the ambient temperature and above (Ginner et al., 2004; Liou et al., 2005; Ahn et al., 2008; Hwang et al., 2010; Peng et al., 2015). Also, little research has been conducted on nitrate reduction in the presence of ZVI and cells at low temperatures, especially with mixed cultures. Kinetic information on both abiotic and biotic nitrate reduction in the presence of ZVI at low temperatures would be valuable for the design and operation of field-scale ZVI systems, especially for winter operations.

In this study, nitrate reductions by ZVI was examined at low temperatures in the presence and absence of microorganisms. Emphasis was on investigating the role of microorganisms and effect of temperature on nitrate removal efficiency and ammonia production. The study also examined microbially induced corrosion process at the ZVI surfaces to elucidate the interactions and synergies between ZVI and microorganisms in microbial-ZVI systems.