A critical review of existing mechanisms and strategies to enhance N₂ selectivity in groundwater nitrate reduction

Highlights

• The mechanisms and products of groundwater nitrate reduction in different treatment systems are reviewed.

• The N₂ selectivity data from different remediation systems are summarized.

• The strategies to enhance N₂ selectivity are proposed.

• The future development directions on selective nitrate removal to N₂ in groundwater are prospected.

Abstract

The pollution of nitrate (NO₃^-) in groundwater has become an environmental problem of general concern and requires immediate remediation because of adverse human and ecological impacts. NO₃^- removal from groundwater is conducted mainly by chemical, biological, and coupled methods, with the removal efficiency of NO₃^- considered the sole performance indicator. However, in addition to the harmless form of N₂, the reduced NO₃^- could be transformed into other intermediates, such as nitrite (NO₂^-), nitrous oxide (N₂O), and ammonia (NH₄⁺), which may have direct or indirect negative impacts on the environment. Therefore, increasing N₂ selectivity is a significant challenge in reducing NO₃^- in groundwater, which seriously impedes the large-scale implementation of available remediation technologies. In this work, we comprehensively overview the most recent advances in N₂ selectivity regarding the understanding of emerging groundwater NO₃^- removal technologies. Mechanisms of by-product production and strategies to enhance the selective reduction of NO₃^- to N₂ are discussed in detail. Furthermore, we proposed topics for further research and hope that the total environmental impacts of remediation schemes should be evaluated comprehensively by quantifying all potential intermediate products, and promising strategies should be further developed to enhance N₂ selectivity, to improve the feasibility of related technologies in actual remediation.

Introduction

Groundwater is the dominant water source in most regions of the world and is the only water source in some arid countries such as Saudi Arabia and India (Adimalla and Qian, 2021; Wang et al., 2009). However, due to extensive agricultural intensification and other nonagricultural nitrogen (N) sources (Xin et al., 2019), a continuous rise in nitrate (NO₃⁻) concentrations in groundwater has been widely observed worldwide, including in the United States (Glenn and James Lester, 2010), China (Zhai et al., 2017b), Japan (Hosono et al., 2013), India (Adimalla and Qian, 2021), several countries in the European Union (Angelopoulos et al., 2009), and particularly in developing countries. NO₃⁻ pollution in groundwater poses a substantial threat to human and ecological health. On one hand, ingesting NO₃⁻ may induce diseases such as methemoglobinemia in infants (blue-baby syndrome) and stomach cancer (Xu et al., 2017a). On the other hand, high concentrations of NO₃⁻ in groundwater may flow into coterminous surface water or seawater, inducing eutrophication and toxic algal blooms (Ghafari et al., 2008). Thus, developing technologies that efficiently treat NO₃⁻ pollution in groundwater has increasingly attracted researchers’ attention.

Various in and ex situ technologies involving physical, chemical and biological treatment processes have been applied to remediate NO₃⁻-contaminated groundwater (Cecconet et al., 2020; Huno et al., 2018). However, implementation of these technologies at larger scales has not occurred, because of incomplete NO₃⁻ reduction. Ideally, the reduced NO₃⁻ should be transformed into its harmless form N₂ rather than some intermediates, such as nitrite (NO₂⁻), nitrous oxide (N₂O), and ammonia (NH₄⁺), which may have direct or indirect negative impacts on the environment (Niu et al., 2019). Therefore, in addition to augmenting NO₃⁻ removal, many studies have attempted to find efficient measures to promote the selective reduction of NO₃⁻ to N₂ to decrease the negative effects of intermediates (Liu and Wang, 2019; Martínez et al., 2017; Tokazhanov et al., 2020). In addition, to characterize the complete reduction degree of NO₃⁻, a new concept of N₂ selectivity has been proposed, defined as the ratio of the N₂ yield to the amount of consumed NO₃⁻ (Wei et al., 2018).

Different technologies may display variable NO₃⁻ removal performance and N₂ selectivity. Chemical reduction, available as both in situ and ex situ remediation methods, attempts to reduce NO₃⁻ to NH₄⁺ or N₂ by employing reducing agents (Song et al., 2020). Zero-valent iron (ZVI) has been the most investigated typical reductant. Bare ZVI possesses poor N₂ selectivity (Wei et al., 2018), although the N₂ selectivity in the ZVI system can be slightly improved by adjusting the hydrochemical parameters of the solution (pH and oxidation–reduction potential [ORP]) (Yang and Lee, 2005) and the size and dose of the ZVI (Choe et al., 2000; Xiong et al., 2009). Owing to the limitations of ZVI systems, some ZVI composites have been applied to increase N₂ selectivity and reviewed by Liu and Wang (2019). Carbon materials supported ZVI (ZVI/C) that are capable of changing the pH and ORP in solution (Song et al., 2020; Wei et al., 2018) and ZVI supported metallic materials that exhibit additional catalytic reduction (Lubphoo et al., 2015; Tang et al., 2019), have substantial potential for removing NO₃⁻ from groundwater and require to develop some strategies to further enhance N₂ selectivity. Moreover, catalytic and electrocatalytic reduction have substantial potential for removing NO₃⁻ from groundwater but will more probably be adopted as ex situ remediation methods. The strategies to increase N₂ selectivity in both the aforementioned treatment systems have been reviewed in previous studies (Martínez et al., 2017; Tokazhanov et al., 2020). N₂ selectivity higher than 90% can be achieved with both the aforementioned technologies, and the undesired formation of NH₄⁺ is more substantially reduced than in the ZVI system.

Biological denitrification is another traditional method for removing NO₃⁻ and may produce some toxic denitrification species, i.e., NO₂⁻, N₂O, and NH₄⁺ (Capodici et al., 2018; Yang et al., 2012), the proportion of which in final products varies with treatment conditions. Relevant investigations and discussions have been performed on the factors influencing the accumulation of intermediates during denitrification, including the type and quantity of electron donors (Warneke et al., 2011; Yang et al., 2012), hydraulic retention time (HRT) (Wang et al., 2009; Zhai et al., 2017a), temperature (Holtan-Hartwig et al., 2002; Hu et al., 2011) and so on. In recent decades, an increasing number of researchers have focused on different combinations of conventional technologies, such as microbial electro-remediation, which couples microbial physiology with electrochemistry (Sevda et al., 2018). In the integrated system, NO₃⁻ can be converted to N₂ by heterotrophic denitrification, or by autotrophic denitrification (Cecconet et al., 2020). Studies have shown that complete denitrification in microbial electro-remediation can be achieved by adjusting the conductivity of groundwater (Pous et al., 2013; Puig et al., 2012), applying different voltage (Nguyen et al., 2015; Virdis et al., 2009) or current (Ghafari et al., 2009; Virdis et al., 2009), and using different reactor configurations (Callegari et al., 2019; Zhang et al., 2014a).

In summary, increasing N₂ selectivity is a primary challenge in reducing NO₃⁻ in groundwater, which seriously impedes the widespread use of available remediation technologies. Recently, critical reviews have been published on in and ex situ groundwater nitrate treatment technologies for improving nitrate removal efficiency. By contrast, N₂ selectivity appears to be neglected in remediation performance evaluation and optimization. Although Martínez et al. (2017) made the first attempt to summarize the strategies to enhance N₂ selectivity in the catalytic and electrocatalytic reduction of nitrate using metallic catalysts, according to our review of the literature, no literature has systematically reviewed the removal performance of NO₃⁻ by different technologies from the perspective of product composition. Therefore, this review aims to provide insights into the reduction products of NO₃⁻ by different commonly used remediation technologies in groundwater and their governing mechanisms. In addition, the latest developments in improvement strategies for N₂ selectivity are presented, and research gaps and potential further research directions are proposed. We hope that our review provides a reference for practical engineering to treat NO₃⁻-polluted groundwater such that the performance is improved and the negative effects decrease.