Biogeochemistry of Iron Enrichment in Groundwater: An Indicator of Environmental Pollution and Its Management

: Iron (Fe) is one of the most biochemically active and widely distributed elements and one of the most important elements for biota and human activities. Fe plays important roles in biological and chemical processes. Fe redox reactions in groundwater have been attracting increasing attention in the geochemistry and biogeochemistry ﬁelds. This study reviews recent research into Fe redox reactions and biogeochemical Fe enrichment processes, including reduction, biotic and abiotic oxidation, adsorption, and precipitation in groundwater. Fe biogeochemistry in groundwater and the water-bearing medium (aquifer) often involves transformation between Fe(II) and Fe(III) caused by the biochemical conditions of the groundwater system. Human activities and anthropogenic pollutants strongly affect these conditions. Generally speaking, acidiﬁcation, anoxia and warming of groundwater environments, as well as the inputs of reducing pollutants, are beneﬁcial to the migration of Fe into groundwater (Fe(III) → Fe(II)); conversely, it is beneﬁcial to the migration of it into the media (Fe(II) → Fe(III)). This study describes recent progress and breakthroughs and assesses the biogeochemistry of Fe enrichment in groundwater, factors controlling Fe reactivity, and Fe biogeochemistry effects on the environment. This study also describes the implications of Fe biogeochemistry for managing Fe in groundwater, including the importance of Fe in groundwater monitoring and evaluation, and early groundwater pollution warnings


Introduction
Iron (Fe) is the fourth most abundant element in the Earth's crust [1,2], the abundance being 5.6 × 10 4 mg/kg [3]. Fe is widely distributed in atmospheric aerosols, natural water, organisms, and soil. The mean Fe content of sediment is 3.9% [4]. Fe is mainly present in sediment as free Fe oxides on the particle surfaces. Fe is present in the environment at different valences and is very geochemically active and prone to undergoing redox reactions [1,2]. Fe can participate in physical and chemical processes (global biogeochemical cycles) and metabolism affecting many elements, including As, C, Mn, N, O, P, and S [5][6][7]. Fe can also directly and/or indirectly affect organic and inorganic compound degradation, natural organic matter (NOM) evolution, mineral dissolution, and rock weathering and diagenesis. In addition, Fe as an essential nutrient participates in many physiological processes such as respiratory function, photosynthesis and DNA biosynthesis [8,9]. However, excessive Fe content threatens human health. When the human body drinks groundwater with Fe content exceeding 2-3 mg/L for a long time, it leads to chronic poisoning and hemochromatosis [10], affecting normal human life and reproduction. At the same time, the oxidation of Fe makes water turbid and odorous, and excessive Fe content in groundwater also causes pipeline rust and blockage of wells, resulting in new problems such as reduced water output. A high Fe concentration in groundwater is therefore a serious pollution problem that negatively affects drinking water safety and human health. Much attention has been paid to this problem [11,12]. Fe can be present in two states, Fe(II) and Fe(III), in natural environmental media. Fe(II) is more soluble than Fe(III), and there are many bioavailable forms of Fe(II). It has recently been found that Fe concentrations in groundwater have increased in the last few decades in many parts of the world [13,14]. While this significant difference in groundwater is limited to the natural spatial distribution of Fe in aquifers [15], it depends more on highly complex geochemical processes, which are attributed to the reduction of iron in aquifer sediments. Groundwater anomalies around the world have recently attracted attention. Abnormal increases in Fe concentrations in groundwater [16] at the regional and site scales have occurred. At the regional scale, Fe concentrations in groundwater have increased markedly in some important aquifers in China [17], the USA [18], Japan [19], and some European countries [20]. Fe concentrations in groundwater have occurred more often at the site scale, mainly in groundwater affected by landfill sites [21,22] and by other polluted sites [23]. It has been found that Fe in an aquifer can be released because of inputs of certain anthropogenic pollutants to the groundwater which alter the dynamic equilibrium between Fe in the groundwater and aquifer and cause more Fe to dissolve in the groundwater [24]. Most of these pollutants are reductive and biochemically active substances such as degradable organic matter and ammonia [25]. The oxidation process of organic matter in aquifer requires the consumption of dissolved oxygen in water, and Fe oxides with high valence replace O 2 as oxidant in a reduction environment. The reaction is based on the continuous oxidation of organic matter [26].
Organic matter (e.g., CH  Some of these reductive substances can react with Fe oxides in an aquifer and increase the soluble Fe content of the aquifer [4,28] and therefore increase the Fe concentration in the groundwater. These redox reactions can also-conversely-transform these reductive pollutants, and this is conducive to the attenuation of groundwater pollutants. Since Fe plays an important role in key biogeochemical processes, the harmful contributions of human activities to groundwater pollution may become more and more serious in the future. In order to reduce these harmful effects on aquifer-groundwater systems, it is necessary to summarize our knowledge about the biogeochemical effects of Fe, to guide further research.
Our study here reviews anomalous increases in Fe concentrations in groundwater around the world. We also focus on Fe-mediated hydrogeochemical cycles and groundwater pollution, and analyze factors affecting redox reactions. This study also discusses how biogeochemical interactions between Fe in groundwater and aquifer media can be used to warn of groundwater contamination and remediate contaminated materials, evaluating available remediation techniques. Suggestions for future research directions are given, then future research directions are proposed.

Fe Distribution in Groundwater
Groundwater quality is often affected by Fe. Fe is an essential nutrient for all biota [1,2] and is essential to physiological metabolism in humans. However, excessive exposure to Fe poses risks to human health. Consuming drinking water containing Fe at concentrations Sustainability 2022, 14, 7059 3 of 14 of 2-3 mg/L or higher for a long time causes chronic toxic effects, including hemochromatosis [29]. Fe can therefore strongly affect domestic and industrial water supplies. High Fe concentrations in groundwater have been widespread for a long time. Recent studies around the world have confirmed that Fe concentrations in groundwater in different geological environments have increased [30][31][32][33]. The World Health Organization has not yet defined an Fe concentration limit for drinking water below which risks to health are acceptable. The US Environmental Protection Agency has specified a health reference concentration for Fe of 300 µg/L, and the US Geological Survey has specified a non-mandatory health screening standard for Fe in water of 300 µg/L. High Fe concentrations have been found generally in large surveys in the USA and other countries. In a national survey of groundwater quality in the USA, Fe concentrations >300 µg/L were found in 6.9% of the groundwater samples (n = 3662) [13]. The As concentration exceeded the standard in 6.7% of the samples, and the nitrate concentration exceeded the standard in 4.1% of the samples. The proportions of samples containing high Fe concentrations and high concentrations of other metals were similar. In a study performed in the Assam-Arakan basin in Northeast India, the Fe concentrations in all 32 water samples were >28 times higher than the US Environmental Protection Agency standard [34]. Fe concentrations tend to be high in deep groundwater, and tend to be low in shallow groundwater, as shown in Table 1. It was found in a two-year study of regional groundwater in western Poland that only the Fe concentrations (0.01-4.08 mg/L) and Pb concentrations were higher than World Health Organization limits for drinking water and, therefore, groundwater in the study region was only contaminated with Fe and Pb [35]. In China, the maximum allowable limit of Fe emission in industrial wastewater is 1 mg/L, and there is no limit of Fe in farmland irrigation water quality standards for the time being. The maximum allowable limits for surface water and groundwater are 1.5 mg/L, and the class III quality standards for surface water and groundwater and sanitary standards for drinking water are 0.3 mg/L. High Fe concentrations in groundwater are widespread in North and South China, particularly on the Sanjiang Plain and in the middle and lower reaches of the Yangtze River, as shown in Table 1 [13]. Fe concentrations in groundwater are generally high in areas with lacustrine strata and floodplains that are rich in dissolved organic carbon (DOC), and Fe concentrations in such areas are clearly increasing over time.

Origin of Fe in Groundwater
Fe is an important component of natural water. Fe is widely distributed in rocks (including sedimentary, igneous, and metamorphic rocks). Sediment on plains and in basins is created through weathering, transportation and sedimentation of rocks, and is rich in Fe. Such sediment provides abundant Fe minerals in aquifers [36]. Fe in groundwater is mainly supplied through reductive dissolution of Fe-containing oxides, as shown in Figure 1a. Common Fe oxides and hydroxides in sediment are goethite (α-FeOOH), hematite (α-Fe 2 O 3 ), magnetite (Fe 3 O 4 ), and pyrite (FeS 2 ), the last being present in small amounts. Groundwater in different regions contains Fe at quite different concentrations, which can be caused by the presence of Fe oxides with different reactivities. However, our understanding of the types of oxidized Fe present in aquifer sediment is rather poor. Natural Fe-containing minerals can dissolve in water, release Fe ions, or form Fe complexes. Large amounts of Fe in sediment and plant debris can be transported to groundwater from the biosphere through wind erosion or through leaching. Fe in groundwater is mostly supplied by the aquifer and surface precipitation. Fe limits biological production in the ocean. Dissolved exogenous Fe in the ocean is mainly supplied by icebergs, rivers, and wind-borne dust, through hydrothermal activities, or by the reductive and non-reductive dissolution of sediment.

Origin of Fe in Groundwater
Fe is an important component of natural water. Fe is widely distributed in rocks (including sedimentary, igneous, and metamorphic rocks). Sediment on plains and in basins is created through weathering, transportation and sedimentation of rocks, and is rich in Fe. Such sediment provides abundant Fe minerals in aquifers [36]. Fe in groundwater is mainly supplied through reductive dissolution of Fe-containing oxides, as shown in Figure 1a. Common Fe oxides and hydroxides in sediment are goethite (α-FeOOH), hematite (α-Fe2O3), magnetite (Fe3O4), and pyrite (FeS2), the last being present in small amounts. Groundwater in different regions contains Fe at quite different concentrations, which can be caused by the presence of Fe oxides with different reactivities. However, our understanding of the types of oxidized Fe present in aquifer sediment is rather poor. Natural Fe-containing minerals can dissolve in water, release Fe ions, or form Fe complexes. Large amounts of Fe in sediment and plant debris can be transported to groundwater from the biosphere through wind erosion or through leaching. Fe in groundwater is mostly supplied by the aquifer and surface precipitation. Fe limits biological production in the ocean. Dissolved exogenous Fe in the ocean is mainly supplied by icebergs, rivers, and windborne dust, through hydrothermal activities, or by the reductive and non-reductive dissolution of sediment. Reductive dissolution of Fe oxides in sediment may also be caused by degradation of DOC. The activities of microorganisms using DOC cause a reducing environment to develop. Once O2 and NO3 − are reduced, Fe oxides/hydroxides start being reduced and are released into the groundwater, as shown in Figure 1b. If DOC is rich in N it may produce NH4 + as a byproduct. This is more common in DOC in plains and basins than elsewhere. NH4 + in groundwater is often anthropogenic (e.g., supplied through sewage discharges or agricultural practices), which is of great concern around the world [37][38][39]. Positive correlations between NH4 + and Fe contents of aquifers have been found, as shown in Figure 1c [40,41]. The hydraulic gradient also affects the Fe concentration in groundwater. Excessive exploitation of deep groundwater causes a hydraulic gradient between deep and shallow aquifers, which causes upper saline water to flow through the aquifers [42]. Anthropogenic pollutants (e.g., those released during well drilling or in factory discharges) can enter aquifers and increase the concentrations of Fe and various salts in the groundwater, as shown in Figure 1d [43]. For example, when a 120 m deep well was plugged, the total Reductive dissolution of Fe oxides in sediment may also be caused by degradation of DOC. The activities of microorganisms using DOC cause a reducing environment to develop. Once O 2 and NO 3 − are reduced, Fe oxides/hydroxides start being reduced and are released into the groundwater, as shown in Figure 1b. If DOC is rich in N it may produce NH 4 + as a byproduct. This is more common in DOC in plains and basins than elsewhere. NH 4 + in groundwater is often anthropogenic (e.g., supplied through sewage discharges or agricultural practices), which is of great concern around the world [37][38][39]. Positive correlations between NH 4 + and Fe contents of aquifers have been found, as shown in Figure 1c [40,41]. The hydraulic gradient also affects the Fe concentration in groundwater. Excessive exploitation of deep groundwater causes a hydraulic gradient between deep and shallow aquifers, which causes upper saline water to flow through the aquifers [42]. Anthropogenic pollutants (e.g., those released during well drilling or in factory discharges) can enter aquifers and increase the concentrations of Fe and various salts in the groundwater, as shown in Figure 1d [43]. For example, when a 120 m deep well was plugged, the total dissolved solid concentration in nearby groundwater increased from 778 to 6016 mg/L and the concentrations of various heavy metals (including Fe) increased [44].
High Fe concentrations in groundwater have complex causes, as shown in Figure 2. It is important to improve our understanding of these causes. Redox reactions in a sedimentwater system allow Fe oxide to be reduced, meaning the Fe 2+ concentration in interstitial water in sediment will increase continually and rapidly. Surface oxidation will occur on Fe-containing minerals in an aqueous medium. Fe 2+ in an Fe-containing mineral will first diffuse from the lattice to the crystal surface and react with oxygen to give FeOOH. H 2 O or O 2 will then react with S 2 2− or S 2− through the FeOOH layer to give SO 4 2− and other compounds. Microorganisms will cause Fe-S coupling. Fe(II) will combine with H 2 S produced through the reduction of sulfate in sediment to give FeS and FeS 2 . Dissolved organic matter (e.g., aniline, natural dissolved organic matter (DOM), and phenol) in a sediment-water system can form complexes with Fe ions, compete for adsorption sites on mineral surfaces, form water complexes with Fe ions, change the redox potentials of the adsorption surfaces, change the Fe morphology, and affect Fe migration in the aquifer. Organic functional groups adsorbed to surface active sites on Fe oxides will form precursor complexes, which will decompose to give Fe(II) and free radicals, then the free radicals will be oxidized or coupled to give dimers or polymers. It is important to understand the mechanisms through which high Fe concentrations in groundwater are achieved to allow the Fe cycle in sediment-water systems to be understood.
It is important to improve our understanding of these causes. Redox reactions in a sediment-water system allow Fe oxide to be reduced, meaning the Fe 2+ concentration in interstitial water in sediment will increase continually and rapidly. Surface oxidation will occur on Fe-containing minerals in an aqueous medium. Fe 2+ in an Fe-containing mineral will first diffuse from the lattice to the crystal surface and react with oxygen to give FeOOH. H2O or O2 will then react with S2 2− or S 2− through the FeOOH layer to give SO4 2− and other compounds. Microorganisms will cause Fe-S coupling. Fe(II) will combine with H2S produced through the reduction of sulfate in sediment to give FeS and FeS2. Dissolved organic matter (e.g., aniline, natural dissolved organic matter (DOM), and phenol) in a sediment-water system can form complexes with Fe ions, compete for adsorption sites on mineral surfaces, form water complexes with Fe ions, change the redox potentials of the adsorption surfaces, change the Fe morphology, and affect Fe migration in the aquifer. Organic functional groups adsorbed to surface active sites on Fe oxides will form precursor complexes, which will decompose to give Fe(II) and free radicals, then the free radicals will be oxidized or coupled to give dimers or polymers. It is important to understand the mechanisms through which high Fe concentrations in groundwater are achieved to allow the Fe cycle in sediment-water systems to be understood.

Biogeochemical Reaction of Fe in Groundwater
Migration of Fe in a water-rock system will mainly be controlled by adsorption-desorption processes at the sediment surfaces, and by precipitation-dissolution processes. The interface between groundwater and the aquifer medium is a strongly reducing environment. When the groundwater level increases, the Fe ion concentration in the soil water will increase and Fe(III) will be converted into Fe(II). Phosphorus, heavy metals adsorbed on iron oxides (e.g., Co, Cu, Mn, Pb, etc.), humus and organic pollutants are released.
In an aquifer, reduction of Fe oxides to produce Fe(II) will almost entirely be catalyzed by microbes [45]. The Fe valence state change will be closely related to microbial activity. In a natural system, Fe will be involved in many electron exchange reactions. Some autotrophic microorganisms (e.g., Thiobacillus ferrooxidans) obtain the energy required for metabolism and assimilate carbon dioxide by being involved in oxidation processes. In an anaerobic environment, Fe will be an important electron acceptor during the decomposition of organic matter by microorganisms. Some microorganisms can degrade Fe compounds and release the inorganic ions that are produced. It has recently been found

Biogeochemical Reaction of Fe in Groundwater
Migration of Fe in a water-rock system will mainly be controlled by adsorptiondesorption processes at the sediment surfaces, and by precipitation-dissolution processes. The interface between groundwater and the aquifer medium is a strongly reducing environment. When the groundwater level increases, the Fe ion concentration in the soil water will increase and Fe(III) will be converted into Fe(II). Phosphorus, heavy metals adsorbed on iron oxides (e.g., Co, Cu, Mn, Pb, etc.), humus and organic pollutants are released.
In an aquifer, reduction of Fe oxides to produce Fe(II) will almost entirely be catalyzed by microbes [45]. The Fe valence state change will be closely related to microbial activity. In a natural system, Fe will be involved in many electron exchange reactions. Some autotrophic microorganisms (e.g., Thiobacillus ferrooxidans) obtain the energy required for metabolism and assimilate carbon dioxide by being involved in oxidation processes. In an anaerobic environment, Fe will be an important electron acceptor during the decomposition of organic matter by microorganisms. Some microorganisms can degrade Fe compounds and release the inorganic ions that are produced. It has recently been found that Fe-reducing bacteria contain membrane-bound electron transporters that perform the final electron transfer in reactions with Fe oxides, but the membrane proteins involved have not yet been isolated. The general formula of the microbial dissimilatory reduction reaction of Fe is shown below [5]. During geochemical cycling (Figure 3), valence state transition of Fe at depth will cause Fe to circulate near the interface [46,47]. This will be accompanied by early diagenetic reactions, including organic matter mineralization and trace element fixation or activation. The aquifer-groundwater environment is suitable for studying Fe-related redox effects and is readily affected by the oxygen concentration, temperature, and organic matter (see the Section 3).
final electron transfer in reactions with Fe oxides, but the membrane proteins involved have not yet been isolated. The general formula of the microbial dissimilatory reduction reaction of Fe is shown below [5].  (Figure 3), valence state transition of Fe at depth will cause Fe to circulate near the interface [46,47]. This will be accompanied by early diagenetic reactions, including organic matter mineralization and trace element fixation or activation. The aquifer-groundwater environment is suitable for studying Fe-related redox effects and is readily affected by the oxygen concentration, temperature, and organic matter (see the Section 3). Groundwater is an important medium in which Fe transformations occur. Fe transformations strongly affect Fe migration and bioaccumulation. The hypoxic natural groundwater environment usually causes the following: a sharp increase in Fe(II) concentration; a reduction and dissolution of Fe(III) minerals in the sediment; and accumulation of Fe(II) in anoxic groundwater [2]. This is the main groundwater quality problem [13,14,48]. The (water-containing medium)-groundwater interface is affected by redox condition fluctuations that control the Fe, P, and S concentrations and forms in the groundwater and pore water, as shown in Figure 3 [49]. Poorly crystalline active Fe oxyhydroxide will become enriched at this interface [50] and may become a source of Fe to the groundwater. Microbes associated with this process will oxidize organic carbon [51,52]. This results in an Fe-rich layer at the redox boundary of the sediment [50].
Various chemical and physical transformations occur when Fe enters groundwater [53]. Fe migration in groundwater is mainly controlled by adsorption-desorption of Fe at sediment surfaces and by precipitation-dissolution processes [8]. In the near-neutral conditions of natural groundwater, adsorption-desorption of Fe at suspended particle surfaces will predominantly control Fe migration. In a reducing environment, interactions between Fe and As, S, and other elements will produce FeAsS and FeS precipitates. Dissolution of these minerals will also strongly affect Fe migration in the groundwater. Various biological, chemical, and physical factors will play vital roles in all of these processes.
Fe entering groundwater will continue to be geochemically active. Fe in the water body will mainly be present as Fe(II), Fe(OH) + , Fe(OH) 2+ , and Fe(III) plasma. The solubilities of Fe(II) and Fe(III) will increase as the groundwater acidity increases and Eh decreases. At a groundwater pH < 2, the total Fe concentration can be much higher than 100 mg/L, and at a groundwater pH > 4, the total Fe concentration will be between <0.01 and 10 mg/L Groundwater is an important medium in which Fe transformations occur. Fe transformations strongly affect Fe migration and bioaccumulation. The hypoxic natural groundwater environment usually causes the following: a sharp increase in Fe(II) concentration; a reduction and dissolution of Fe(III) minerals in the sediment; and accumulation of Fe(II) in anoxic groundwater [2]. This is the main groundwater quality problem [13,14,48]. The (water-containing medium)-groundwater interface is affected by redox condition fluctuations that control the Fe, P, and S concentrations and forms in the groundwater and pore water, as shown in Figure 3 [49]. Poorly crystalline active Fe oxyhydroxide will become enriched at this interface [50] and may become a source of Fe to the groundwater. Microbes associated with this process will oxidize organic carbon [51,52]. This results in an Fe-rich layer at the redox boundary of the sediment [50].
Various chemical and physical transformations occur when Fe enters groundwater [53]. Fe migration in groundwater is mainly controlled by adsorption-desorption of Fe at sediment surfaces and by precipitation-dissolution processes [8]. In the near-neutral conditions of natural groundwater, adsorption-desorption of Fe at suspended particle surfaces will predominantly control Fe migration. In a reducing environment, interactions between Fe and As, S, and other elements will produce FeAsS and FeS precipitates. Dissolution of these minerals will also strongly affect Fe migration in the groundwater. Various biological, chemical, and physical factors will play vital roles in all of these processes.
Fe entering groundwater will continue to be geochemically active. Fe in the water body will mainly be present as Fe(II), Fe(OH) + , Fe(OH) 2+ , and Fe(III) plasma. The solubilities of Fe(II) and Fe(III) will increase as the groundwater acidity increases and Eh decreases. At a groundwater pH < 2, the total Fe concentration can be much higher than 100 mg/L, and at a groundwater pH > 4, the total Fe concentration will be between <0.01 and 10 mg/L [9] (see Section 3.1). Fe can form chelates in organic-rich groundwater. In an oxidizing environment, organic matter can cause Fe to reach a high concentration in the solution (see Section 3.3). In an oxidizing environment, Fe(II) can be oxidized to Fe(III) [9], as shown below. In fact, Fe(III) is very unstable. Fe(III) is only stable in solution at 25 • C and at pH < 2.19 [16]. As the temperature increases, Fe(III) becomes more unstable. In contrast, hematite is stable in this environment, as shown in the equation below. 2Fe(II) + 3H 2 O→Fe 2 O 3 + 6H + + 2e − The Fe cycle processes involved in Fe(II) release mean that the oxidation of reduced Fe in water and sediment can lead to the production of reactive oxygen species [54,55], which can cause oxidative damage in fish, as shown in Figure 3. Reduction of Fe oxide and the release of Fe(II) may negatively affect the groundwater environment but this is an important part of the geochemical Fe cycle in natural groundwater.

Factors Affecting Fe Biogeochemistry
Groundwater and aquifer conditions will cause changes in pH, O 2 concentration, organic matter concentration and microbial action during the redox process. These processes cause the valence state of Fe in the groundwater and aquifer to change and therefore affect the geochemical processes that occur. Changes in the environment can cause the Fe equilibrium between the solid and dissolved states to move.

pH
Fe ions will mainly be present as inorganic and organic complexes in natural water, and the Fe valence and state will be affected by pH [56]. In a sediment-water system, Fe ions will mainly be present in an ion exchangeable state, bound to carbonates, bound to Fe oxides/hydroxides, bound to organic matter, bound to sulfides, and in a residual state. Fe bound to carbonates will be very sensitive to pH [57,58]. At pH < 9, Fe in water will mainly be present as Fe(II). A decrease in pH will accelerate the dissolution of carbonates and hydroxides. Fe bound to carbonates will be released into the water. The Fe-O bonds in the crystal lattices of Fe-bearing minerals will readily be destroyed in an acidic environment to release structural Fe or interlayer Fe and promote Fe dissolution [59]. When pH < 9, ferrous ions mainly exist in the form of Fe 2+ (Figure 4a). In the natural water system, these will mainly exist in the form of hydrolysis products. Fe 3+ only exists in acidic environments in the natural system (Figure 4b). The Fe oxide/hydroxide adsorption capacity will also be related to pH. It has been found that an increase in pH and in oxidation-reduction potential increases the ability of Fe oxides to adsorb ions and the ability of the interaction zone to remove toxic pollutants [60]. Low pH (high concentration of H + ) will destroy the lattice structure of Fe-containing minerals in sediments and transform the morphology of Fe oxides. Therefore, it is possible to release Fe(II) and other high-valent heavy metals (such as As(V)) adsorbed on the mineral surface, creating body conditions for the enrichment of heavy metals in groundwater [61,62]. The pH will determine the distributions of the combined states of Fe(II) and Fe(III), and the Eh will determine the relative proportions of Fe(II) and Fe(III), as shown in the equations below. It has been found that in soil in a wetland, large amounts of Fe(III) are reduced to Fe(II) only at Eh < 200 mV [58].
where Eh is the redox potential of the system, E 0 is the standard redox potential, R is the universal gas constant, T is the temperature in degrees Kelvin, n is the number of electrons transferred, F is the Faraday constant, a is the oxidized ionic species. where Eh is the redox potential of the system, E 0 is the standard redox potential, R is the universal gas constant, T is the temperature in degrees Kelvin, n is the number of electrons transferred, F is the Faraday constant, a is the oxidized ionic species.

Redox Potential
There are redox substances other than oxygen in the environment that can affect the biogeochemistry of Fe. Reducing substances will decrease the Eh and indirectly affect the Fe geochemical cycle. Reducing substances in groundwater can supply electrons during oxidation-reduction processes in sediment. Reducing substances that are common in groundwater include chlorides (e.g., FeCl2 and SnCl2), NH3, SO2, and H2S [63]. However, reducing substances will consume dissolved oxygen, and Fe minerals act as oxidants to allow redox reactions to continue once the dissolved oxygen has been consumed. Functional groups can interact with microorganisms and adsorb minerals and therefore inhibit precipitation of minerals. Reducing micropolluting organic matter and NOM have been detected in surface water and wastewater in many countries [64]. When persulfate (PMS) is added, such reducing organic matter can promote the transformation of Fe(III) to Fe(II) through phenolquinone functional groups in NOM. This can cause micropollutants to be removed during the coagulation process. Interactions between DOM and Fe oxide mainly occur through the formation of coordination bonds between Fe(III) and hydroxyl and carboxyl groups in the DOM [65]. This facilitates reduction and dissolution of Fe oxides and promotes the re-release of As and Pb. In summary, reducing substances affect the biogeochemical reactions of Fe in an aquifer-groundwater system by changing the Eh and the oxidation-reduction processes that occur.

Organic Matter
Organic matter acts as a source of carbon and strongly affects biogeochemical processes. Organic colloids can affect the crystal forms, particle sizes, and specific surface areas of Fe oxide/hydroxide minerals. The stabilities of minerals make it possible for metal complexation and oxidation-reduction reactions to occur, and these indirectly affect the Fe geochemical cycle. NOM is an example of this type of organic matter. The redox transformation rate of Fe in water is generally assumed to be affected by NOM in the water. NOM decreases the activity of Fe(II) and therefore weakens the Fe cycle. Cheng et al. [66] found that, at a NOM/Fe(III) content ratio of 1, the NOM will effectively coat the Fe oxide particles, but that at a NOM/Fe(III) content ratio of 10, the Fe oxide that is produced will cover the outer surfaces; and heavy metal (e.g., As(V)) adsorption to the Fe oxide surfaces will be accelerated. More mesopores (pore sizes 2-10 nm) will form, increasing the As(V) adsorption rate, as the NOM content increases. The redox properties of NOM are mainly related to the presence of quinone functional groups, the presence of which has been confirmed by nuclear magnetic resonance spectroscopy [66], fluorescence spectroscopy [67,68], and electrochemical methods [68]. Godowska et al. [69] suggested a catalytic

Redox Potential
There are redox substances other than oxygen in the environment that can affect the biogeochemistry of Fe. Reducing substances will decrease the Eh and indirectly affect the Fe geochemical cycle. Reducing substances in groundwater can supply electrons during oxidation-reduction processes in sediment. Reducing substances that are common in groundwater include chlorides (e.g., FeCl 2 and SnCl 2 ), NH 3 , SO 2 , and H 2 S [63]. However, reducing substances will consume dissolved oxygen, and Fe minerals act as oxidants to allow redox reactions to continue once the dissolved oxygen has been consumed. Functional groups can interact with microorganisms and adsorb minerals and therefore inhibit precipitation of minerals. Reducing micropolluting organic matter and NOM have been detected in surface water and wastewater in many countries [64]. When persulfate (PMS) is added, such reducing organic matter can promote the transformation of Fe(III) to Fe(II) through phenolquinone functional groups in NOM. This can cause micropollutants to be removed during the coagulation process. Interactions between DOM and Fe oxide mainly occur through the formation of coordination bonds between Fe(III) and hydroxyl and carboxyl groups in the DOM [65]. This facilitates reduction and dissolution of Fe oxides and promotes the re-release of As and Pb. In summary, reducing substances affect the biogeochemical reactions of Fe in an aquifer-groundwater system by changing the Eh and the oxidation-reduction processes that occur.

Organic Matter
Organic matter acts as a source of carbon and strongly affects biogeochemical processes. Organic colloids can affect the crystal forms, particle sizes, and specific surface areas of Fe oxide/hydroxide minerals. The stabilities of minerals make it possible for metal complexation and oxidation-reduction reactions to occur, and these indirectly affect the Fe geochemical cycle. NOM is an example of this type of organic matter. The redox transformation rate of Fe in water is generally assumed to be affected by NOM in the water. NOM decreases the activity of Fe(II) and therefore weakens the Fe cycle. Cheng et al. [66] found that, at a NOM/Fe(III) content ratio of 1, the NOM will effectively coat the Fe oxide particles, but that at a NOM/Fe(III) content ratio of 10, the Fe oxide that is produced will cover the outer surfaces; and heavy metal (e.g., As(V)) adsorption to the Fe oxide surfaces will be accelerated. More mesopores (pore sizes 2-10 nm) will form, increasing the As(V) adsorption rate, as the NOM content increases. The redox properties of NOM are mainly related to the presence of quinone functional groups, the presence of which has been confirmed by nuclear magnetic resonance spectroscopy [66], fluorescence spectroscopy [67,68], and electrochemical methods [68]. Godowska et al. [69] suggested a catalytic mechanism for reactions involving phenol/quinone-containing organic matter in an Fe(III)/PMS system monitoring hydroxyquinone/benzoquinone and Fe valence state changes and inhibition of carbamazepine degradation by various quenchers. Quinone can generate semiquinone and Sustainability 2022, 14, 7059 9 of 14 phenolic compounds through spontaneous redox reactions, and these groups can promote the Fe cycle through ligand-to-metal electron migration and activate PMS to generate SO 4 − , which accelerates the Fe cycle further [69]. The oxidation-reduction potential caused by hydroxyl radicals generated during Fe(II) oxidation can reach 2.8 eV. Hydroxyl radicals can quickly react with and dissolve DOM. Therefore, Fenton or Fenton-like reactions are often used to remove DOM in practical applications [69]. Melton et al. [70] identified a special case in which Fe(III) (hydroxy)oxides were formed through oxidation and rapid hydrolysis of Fe-containing minerals and were adsorbed to/co-precipitated with DOM to form mineral-related organic matter, leading to degradation of DOM. In summary, organic matter controls Fe(III) reduction and migration rates in the aquifer medium under hypoxic conditions, and phenol/quinone groups and their oxidation products can be activated by promoting the Fe cycle and adding PMS. This will ultimately promote the degradation of pollutants.

Microorganisms
The Fe cycle in the terrestrial environment is driven by both non-biochemical reactions and microbe-mediated processes [71]. Microorganisms can produce Fe chromatin, transforming Fe into an absorbable organic chelated form. The reaction is extremely specific for Fe and leads to Fe being absorbed and assimilated. Fe(II) can be oxidized to Fe(III) through biotic or abiotic reactions in aerobic or micro-oxygenated environments, and Fe(III) will quickly precipitate as ferric hydroxide, ferric oxide (Fe 2 O 3 or Fe 2 O 3 ·H 2 O), or hydroxyl ferric oxide (FeOOH)) [72]. Huang et al. [73] performed in situ tracing experiments and found that reduction and dissolution occurred through biocatalysis. Biological respiration consumes organic matter and oxygen and creates a reducing environment, meaning Fe oxides will be reduced [4]. Many microorganisms, particularly Geobacter spp. and Shewanella spp., can reduce Fe(III) using electron donors such as acetate, H 2 , and lactate) [71]. Fe(III) is the terminal electron acceptor. Fe(III) minerals are outside microbial cells, so Fe(III)-reducing microorganisms can transfer electrons from the redox carrier in the plasma membrane to the outside of the cell [5,71], and most of the dissolved Fe will remain as Fe(III), usually strongly complexed with organic ligands. Under hypoxic conditions, Fe(III)-reducing microorganisms can reduce Fe(III) oxides, and microbe-mediated Fe(III) oxide reduction can give Fe-bearing minerals, such as siderite, in aqueous or solid phase. In summary, interactions between Fe-bearing minerals and microorganisms involving electron exchange can occur. Fe-bearing minerals act as electron acceptors and electron-conducting media for microorganisms, meaning the microorganisms can mediate the biogeochemical roles of Fe in the environment.

Environmental Management Based on Fe in Groundwater
The potential for using Fe biogeochemical processes in groundwater to manage the environment has attracted much attention in recent years. Fe biogeochemical processes could be used to monitor and evaluate groundwater and warn of groundwater pollution.

Groundwater Monitoring and Assessment
There are many ways in which Fe can be released from sediment in an aquifer to the groundwater [74]. Anthropogenic pollutants can cause Fe to be released from sediment in an aquifer, and this has been the subject of much research in recent years. Anthropogenic pollutants have strongly increased Fe concentrations in groundwater. Those pollutants produced by industrial wastewater treatment have played an especially large role in the iron enrichment of groundwater in the current era. First, industrial wastewater includes direct discharge of high-concentration heavy metal wastewater [75] which significantly increases the Fe concentration in groundwater; second, industrial wastewater includes a large number of pollutants (usually nitrogen, acid-base substances, DOC, COD, BOD, etc.) [36]. These anthropogenic pollutants further complicate hydrogeochemical processes [33], and the released soluble Fe can contaminate groundwater. Soluble Fe released from minerals can pollute groundwater and pose health risks to millions of people around the world. The mechanisms by which Fe is released from sediment have been identified in many studies, but little is known about the participation of pollutants in the groundwater Fe cycle after therelease of Fe from sediment in aquifers, or the mechanisms that are involved. Different pollutants have different properties, and so reduce Fe in sediment to different degrees. This means that the Fe in groundwater is unevenly distributed. Groundwater in some areas may be undrinkable because of the Fe concentration. For example, Broclawik et al. [76] found that methane contributes to the release of Fe compounds from sediment. Wang et al. [77] found that humic acid contributes to the release of Fe from sediment (increasing the Fe ion concentration in solution) by microorganisms.
Rapid and continuous economic development has caused anthropogenic pollutants to be released. These pollutants can affect the cycles of elements in the environment and negatively affect water quality and the groundwater environment [78]. It is therefore important to control anthropogenic pollution for more reasons than just the direct effects of the pollutants themselves. It is important to improve methods for monitoring and evaluating groundwater. New methods for selecting groundwater monitoring indicators, including Fe, have recently been proposed [79,80]. The number of studies of Fe in different regions has increased exponentially in recent years [76,81]. Groundwater monitoring and evaluation of Fe concentration levels will provide reference data for assessing groundwater pollution, developing remediation methods, and assessing the environmental behavior of Fe.

Early Warning of Groundwater Pollution
Early warning of groundwater pollution could effectively prevent such pollution and protect groundwater; and is the basis of groundwater pollution control and protection measures [82]. Research into early warnings of water pollution started in the 1970s [83]. Sensors and organisms for monitoring water quality in real time are key to early warning systems. Groundwater management departments can be supported by international realtime monitoring and by early warnings of groundwater problems based on groundwater vulnerability and on pollution risk assessment results, providing timely and reliable warning information before groundwater has deteriorated. Using levels of Fe concentration as an early warning of groundwater pollution reflects real groundwater pollution and will directly improve the accuracy of any early warning system. It would also allow a theoretical basis for early warning levels and thresholds, and facilitate groundwater protection and management. Fe has been used as an early warning factor around the world. Boateng et al. [84] used a US Environmental Protection Agency risk assessment model to calculate the risks posed to human health by groundwater in Ethiopia and determined a risk threshold for Fe in groundwater. As indicated in the information about the Fe cycle given in Section 2.3, pollutants entering an aquifer in water can chemically interact with Fe in the aquifer-groundwater system and interrupt natural water-rock interactions, making the hydrogeochemical processes more complicated [33]. Decreasing the risks posed by pollutants will require groundwater monitoring networks to be established and potential sources of pollutants to be identified [85]. In summary, decreasing the risks posed by pollutants requires potential sources of pollution to be identified, contaminated site monitoring and early warning systems to be improved, and groundwater pollution control and remediation measures to be implemented [43]. Protecting and improving groundwater quality will depend on preventing pollution, blocking the routes by which pollutants enter aquifers, remediating contaminated areas, monitoring groundwater, characterizing pollutant plumes, verifying remediation effects, and monitoring dynamic changes in groundwater quality.

Management and Prospects of Fe Pollution
The biogeochemistry of Fe in groundwater and the aquifer medium has important implications for managing Fe in groundwater. First, anthropogenic pollutants can alter the cycles of elements. Fe in groundwater should be monitored. Secondly, pollutants will interact with Fe and affect the Fe cycle, thereby changing the Fe concentration in groundwater. The Fe concentration in groundwater can therefore be used to provide an early warning of groundwater pollution.
This study argues that the biogeochemistry of Fe in groundwater and aquifers will become an important research topic, and the behavior of Fe in groundwater and aquifers in a period of environmental pollution and global change will also receive attention. Information about Fe in groundwater and aquifers will also be used to help control and remediate groundwater pollution. Research and characterization methods will be developed, and the mechanisms involved will become clearer. However, the behavior of Fe is very complicated, is affected by various environmental conditions, and is closely related to the geochemical cycles of other elements and to the fates of pollutants in the environment. The behavior of Fe will also be affected by global climate change. There will therefore be opportunities and challenges in future research.

Conclusions
It is not easy to detect groundwater pollution because most groundwater is not readily monitored. Therefore, increases in Fe concentrations in groundwater have not yet attracted widespread attention. Fe is a major component of the Earth's crust and a common trace element in groundwater. Biogeochemical Fe processes are often accompanied by changes in valence and morphology. These changes generally involve Fe (hydroxy)oxide in an aquifer being reduced to Fe(II) and entering the groundwater. Conversely, Fe(II) in groundwater can be oxidized to insoluble Fe(III), which can adhere to the surfaces of aquifer particles. Changes in the biogeochemistry of the underground environment can cause the Fe concentration in groundwater to increase and interfere with the dynamic equilibrium between Fe in groundwater and the aquifer, causing more Fe to enter the groundwater from the aquifer. Inputs of anthropogenic pollutants to groundwater drive these changes because most pollutants emitted into the environment are biochemically active and can react with active Fe after migrating into groundwater. These reactions usually involve microorganisms. The conditions that affect biochemical effects (e.g., acidity, alkalinity, redox conditions, and organic matter content) therefore affect the Fe cycle.