Iron-assisted biological wastewater treatment: Synergistic effect between iron and microbes

Highlights

• Iron-assisted biological wastewater treatment processes are summarized.

• Synergistic effects between iron and microbes in bio‑iron systems are highlighted.

• Mechanisms behind the iron-enhanced bio-treatment are described.

• Concerns on the full-scale application of iron-assisted bio-treatment are discussed.

• Potential developments beyond iron-assisted bio-treatment are envisaged.

Abstract

Iron-assisted biological wastewater treatment processes have shown a promising potential in removing various types of contaminants. Synergistic effects between iron and microbes on the contaminant degradation make the role of iron beyond that of a nutritional necessity. Exploration of the synergistic mechanisms and the interactions between iron species and microbes and their metabolic products in bio‑iron systems is therefore of significant importance. Iron, including zero-valent iron, ferrous/ferric ions and iron minerals are all reported to be capable of enhancing specific contaminant removals. Although the main role of different iron species in stimulating biological process may differ between each other, their similar transformation pathways may bring us useful information about bio‑iron systems. In this paper, an overview of iron-assisted biological wastewater treatments, including anaerobic digestion, S and Cl reduction, N and P removal, heavy metal immobilization, aromatic and halogenated hydrocarbon compounds degradation, and sludge granulation is provided. Also, the potential synergistic effects between iron and microbes involved in these processes are explored. Furthermore, the main advantages, limitations, and challenges for the development of iron-assisted treatment processes are envisaged.

Introduction

Biological wastewater treatment processes, despite of their long history and widespread application, still face tremendous challenges to meet the growing demand on improved effluent quality and process sustainability (Verstraete et al., 2009; McCarty et al., 2011; Puyol et al., 2016). Chemical intervention is hence often applied to enhance the treatment performacne of biological systems. In particular, the technologies that synerginizing microbial metabolism with iron chemistry has been intensively studied over the past two decades, which offers a promising approach to improve the efficiency and degree of pollutant removal without significantly increasing the treatment cost (Xie et al., 2017; Xu et al., 2017; You et al., 2017; Wei et al. 2018a). Iron is earth-abundant, multi-valent element with good biocompatibility (Butler, 1998; Lin et al., 2005) and environmental benignity (Sun et al., 2016c; Xu et al., 2017). To date, iron-assisted biodegradation of pollutants has been extensively used in anaerobic digestion (Cruz Viggi et al., 2014; Feng et al., 2014; Zhang et al., 2014b; Yamada et al., 2015), dehalogenation (Lin et al., 2011; Baric et al., 2012), desulfidation (Xin et al., 2008; Zhang et al., 2011; Liu et al., 2015c), heavy metal immobilization processes (Wan et al., 2010; Bai et al., 2013; Suanon et al., 2016).

Iron can enhance the removal of pollutants in biological systems by multiple ways. For instance, zero-valent iron can improve the acidogenic and methanogenic activities of anaerobic digestion systems through lowering the oxidation-reduction potential and influencing the microbial community (Liu et al., 2012a; Feng et al., 2014; Hao et al., 2017). In addition, many iron species are directly involved in microbial catabolic and anabolic metabolism (Zhang et al., 2011; Zhu et al., 2015; Wang et al., 2017b). Iron is an essential trace element of microorganisms and a key component of many enzymes (Weber et al., 2006; Bird et al., 2011; Li et al., 2014). In addition, some lower-valent iron species such as Fe(II) and Fe(0) can serve as electron donors (e.g.,) for microbes such as methanogens (Daniels et al., 1987; Belay and Daniels, 1990; Lorowitz et al., 2002), sulfate reducing bacteria (Dinh et al., 2004; Karri et al., 2005), and denitrifiers (Straub et al., 1996; Till et al., 1998), while Fe(III) can be utilized as terminal electron acceptor by dissimilatory metal-reducing bacteria such as Shewanella and Geobacter spp. (Gorby et al., 2006; Lovley et al., 2011). In all, complicated, dynamic interactions between the dosed iron (including zero-valent iron, iron hydroxides, and ferrous/ferric ions) and the functional micobes have been identified and their synergetic roles in enhanced pollutant removal have been revealed in recent studies. However, a comprehensive review in this respect is still lacking.

There are several ways of the interplay between abiotic/biotic iron transformation and pollutant bioconversion processes. Firstly, the direct degradation of contaminants by iron can not only alleviate the biotoxicity of contaminants (Kim and Carraway, 2000; Ahn et al., 2011; Li et al., 2013; Li et al., 2017d; Wang et al., 2018c), but also convert some refractory compounds into readily biodegradable substrates (Perey et al., 2002; Oh et al., 2005; Xu et al., 2014). In particular, some reactive iron minerals (e.g., green rust) may be formed from bioconversion and directly contribute to pollutant degradation (Bond and Fendorf, 2003; Pantke et al., 2012; Yin et al., 2015). Secondly, iron favors the formation of microbial aggregates (e.g., in the form of granular sludge) that are more resistant to hydraulic washout (Liu et al., 2011b; Kong et al., 2014; Liu et al., 2015c), and toxic substrates over floc sludge (Sheng et al., 2010; Tang et al., 2018b). Thirdly, in anaerobic bioreactors some iron oxides (e.g. magnetite) can facilitate the direct interspecies electron transfer among different microbial species and enhance methane production (Cruz Viggi et al., 2014; Li et al., 2015a; Jing et al., 2017; Li et al., 2017c; Chen et al., 2019). Lastly, some iron species may also serve as a sorbent to co-precipitate contaminants (Liu et al., 2015c; Liu et al., 2015d; Xiu et al., 2016). Many of these processes may take place simultaneously in iron assisted biological systems (Liu et al., 2015c; Liu et al., 2015d; Li et al., 2017a).

Enhanced biological wastewater treatment processes may be reaslized by better understanding and making a full play of the synergy between iron and microbes. In this review, the microbe‑iron interactions in iron-assisted biological wastewater treatment are comprehensively overviewed, and the challenges and opportunities for better manipulating such synergny to favor more efficient, sustainable wastewater treatment processes are highlighted.