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Environ Eng Res > Volume 20(3); 2015 > Article
Lee: Oxidation of organic contaminants in water by iron-induced oxygen activation: A short review

Abstract

Reduced forms of iron, such as zero-valent ion (ZVI) and ferrous ion (Fe[II]), can activate dissolved oxygen in water into reactive oxidants capable of oxidative water treatment. The corrosion of ZVI (or the oxidation of (Fe[II]) forms a hydrogen peroxide (H2O2) intermediate and the subsequent Fenton reaction generates reactive oxidants such as hydroxyl radical (OH) and ferryl ion (Fe[IV]). However, the production of reactive oxidants is limited by multiple factors that restrict the electron transfer from iron to oxygen or that lead the reaction of H2O2 to undesired pathways. Several efforts have been made to enhance the production of reactive oxidants by iron-induced oxygen activation, such as the use of iron-chelating agents, electron-shuttles, and surface modification on ZVI. This article reviews the chemistry of oxygen activation by ZVI and Fe(II) and its application in oxidative degradation of organic contaminants. Also discussed are the issues which require further investigation to better understand the chemistry and develop practical environmental technologies.

1. Introduction

For decades, advanced oxidation processes (AOPs) have been extensively studied as effective tools to degrade refractory organic contaminants in water [16]. Various AOPs have been developed to generate reactive oxidants (mainly hydroxyl radical, OH) including thermal [2, 3, 6], photochemical [1, 5], and electrical techniques [4]. Investigators have made efforts to develop new innovative or modified AOPs to overcome conventional limitations. Also, extensive research has been conducted on the kinetics and mechanism for the degradation of contaminants by AOPs. Several AOPs based on ozonation (e.g., the conventional ozonation, the O3/H2O2 system), the Fenton reaction (e.g., the dark Fenton and electro-Fenton processes), and UV photolysis (e.g., the UV/ H2O2 system) were successfully commercialized and have been widely used in drinking water and wastewater treatment plants [3, 7, 8]. AOPs using O3 and the Fenton’s reagent have also been used as in situ chemical oxidation (ISCO) technologies for remediation of contaminated groundwater and soil [9].
In general, O3, H2O2, and H2O serve as the precursors of OH in AOPs; O3 and H2O2 are the precursors in ozonation and the Fenton process, respectively, and AOPs using TiO2 photo-catalysis, VUV, and γ-radiolysis generate OH from H2O [1012]. However, relatively recent studies have demonstrated OH to be generated by activation of O2 [1315]. Theoretically, the reduction of O2 by a three-electron transfer produces OH and reduced forms of iron, such as zero-valent ion (ZVI) and ferrous ion (Fe[II]) are known to donate electrons to activate O2 into OH. ZVI undergoes corrosion by O2 via a two-electron transfer to form H2O2, which is subsequently decomposed to OH by Fe(II) (the product of ZVI corrosion). Fe(II) reduces O2 into OH by a series of single-electron transfer reactions. Meanwhile, zero-valent aluminum (ZVA) and zero-valent copper (ZVC) can activate O2 into reactive oxidants [16, 17]. The mechanism of O2 activation by ZVC is similar to that of ZVI, whereas ZVA is suggested to directly activate O2 into OH by a tree-electron transfer mechanism.
O2 activation by ZVI was originally suggested as a potential technology for the in situ remediation of groundwater and soil. ZVI (Eo(Fe2+/Fe0) = −0.44 VNHE [18]) has been widely studied as a reducing agent for specific contaminants such as chlorinated organic solvents in the aquifer under anoxic conditions [19, 20]. Granular ZVI can be used in permeable reactive barriers, and nanoparticulate ZVI (nZVI) can be directly injected into the contaminated zone. However, the reductive process by ZVI is only applicable to a limited range of contaminants vulnerable to reduction. To broaden the spectrum of contaminants degradable by ZVI, the idea of supplying O2 together with ZVI was developed so that ZVI activates O2 into a nonselective oxidant, OH. During the past decade, the O2 activation by ZVI and Fe(II) (i.e., ZVI/O2 and Fe(II)/O2 systems) has been extensively studied with a focus on the elucidation of reaction mechanisms for oxidant production, system modifications for improving the oxidant yield, and feasibility for degrading major organic contaminants. Although the ZVI research started for groundwater remediation, the ZVI/O2 and the Fe(II)/O2 systems can also be applied to wastewater treatment as a new AOP. This article reviews literatures of the iron-induced O2 activation and application in degradation of organic contaminants in water.

2. Chemistry of Iron-Induced Oxygen Activation

The corrosion of ZVI by O2 proceeds via a two-electron transfer reaction, producing H2O2 (reaction 1) [21, 22]. The produced H2O2 is either reduced to water, by the reaction with ZVI (another two-electron transfer; reaction 2), or converted into reactive oxidants (i.e., OH or Fe[IV]), by the reaction with Fe(II) (i.e., the Fenton reaction; reaction 3) in which OH is produced by one-electron transfer and Fe(IV) by two-electron transfer.
(1)
Fe0(s)+O2+2H+Fe(II)+H2O2
(2)
Fe0(s)+H2O2+2H+Fe(II)+2H2O
(3)
Fe(II)+H2O2Fe(III)+OH+OH-(or Fe(IV)+H2O)
Literatures have suggested that the dominant oxidant from the Fenton reaction shifts from OH to Fe(IV) with increasing pH [15, 2325]; OH is the main oxidant at acidic pH, whereas an alternative oxidant (most likely Fe[IV]) is dominantly formed at neutral pH. The occurrence of Fe(IV) has been usually claimed by evidences against OH such as non-hydroxylation of aromatic compounds and the failure of OH scavengers. Fe(IV) is believed to be less reactive (more selective) than OH and of different forms such as oxo- and hydroxo-complexes as well as complexes with organic and inorganic ligands [2427].
According to studies using nZVI [15, 28, 29], the H2O2 produced by reaction 1 is rapidly consumed on the highly active surface of nZVI, minimizing conversion into reactive oxidants by Fe(II); reaction 2 is dominant over reaction 3. In the case of granular ZVI, the branching ratio of reaction 3 to reaction 2 was slightly higher compared to the case of nZVI due to less surface reactivity of granular ZVI [30].
The oxidation of Fe(II) by O2 also produces reactive oxidants by a series of electron transfer reactions (reactions 4, 5 and 3) [3133]. The reaction of Fe(II) with O2 produces superoxide radical anion (O2•−) (reaction 4). Subsequently, another equivalent of Fe(II) reduces (O2•−)into H2O2 (reaction 5), which is followed by the Fenton reaction (reaction 3). The reaction of Fe(II) with O2 is the rate-determining step, and is pH-dependent; the reaction rate increases with increasing pH (reaction 4). The solution pH should be higher than 6.6 in order for the Fe(II) oxidation to significantly proceed on an hourly time scale; the half-life of Fe(II) is calculated to be approximately 10 min at pH 7, assuming a constant concentration of dissolved oxygen (0.25 mM).
(4)
Fe(II)+O2Fe(III)+O2-
(5)
Fe(II)+O2-2H+Fe(III)+H2O2
According to a previous study [34], nZVI converts O2 into (O2•−) via the surface-bound Fe(II) as a single-electron mediator.
The oxidant yields from the corrosion of nZVI and the oxidation of Fe(II) have been measured using organic probe compounds such as methanol, ethanol, 2-propanol, and benzoic acid [15]. To measure the oxidant yields, an excess amount of the probe compound was employed into the solution containing nZVI (or Fe[II]) and O2, and the major oxidized product was quantified. The reported oxidant yields in respect to the iron added (i.e., Δ [Oxidant]/Δ [Fe0]) were only less than 10% over the pH ranges studied (pH 2–11), indicating that the nZVI corrosion mainly follows the four-electron transfer pathway (i.e., reaction 1 followed by reaction 2) and providing less chances to produce reactive oxidants by the Fenton reaction (reaction 3). In addition, under neutral pH conditions, the oxidant yields of the nZVI/O2 system were similar to those of the Fe(II)/O2 system, which implies that nZVI only plays a role as the source of Fe(II). In reactions 35, the oxidant yield of the Fe(II)/O2 system (Δ [Oxidant]/Δ [Fe(II)]) is calculated to be 33%; three molar equivalents of Fe(II) are oxidized to generate one equivalent of the reactive oxidant. Based on this calculation, the observed oxidant yields (lower than 10% for both the nZVI/O2 and the Fe(II)/O2 systems) indicate that there are additional factors that lower the oxidant yields other than the four-electron transfer mechanism (reactions 1 and 2) under neutral pH conditions.
Iron precipitation at neutral pH has been suggested to decrease the oxidant yield from the Fe(II) oxidation [15]. Insoluble Fe(III)-oxyhydoroxides can co-precipitate Fe(II) to hold a fraction of Fe(II) unreacted [15]. In addition, the precipitates can provide heterogeneous surfaces that accelerate the selective consumption of Fe(IV) (i.e., the reactive oxidant at neutral pH) by adjacent Fe(II) in the precipitate matrix [35]. In the case of granular ZVI, the surface passivation is more facilitated under neutral pH conditions, limiting the electron transfer from the ZVI core to the adsorbed O2 [14].

3. Modified Systems for Enhancing Oxidant Production

Strategies to improve the oxidant production can be established based on understanding of the factors that lower the oxidant yields in the nZVI/O2 and the Fe(II)/O2 systems. As described in the previous section, the four-electron transfer mechanism (reactions 1 and 2) was the primary cause for the low oxidant yields in the nZVI/O2 system. Once H2O2 is produced by reaction 1, the ZVI surface competes with Fe(II) for H2O2 (reaction 2 vs. reaction 3). To increase the oxidant yields, the branching ratio of reaction 3 relative to reaction 2 should increase, accomplished by either weakening the surface reactivity of ZVI toward H2O2 (suppressing reaction 2) or increasing the rate of the Fenton reaction (accelerating reaction 3). In addition, the prevention of iron precipitation at neutral pH is another strategy to improve the oxidant production from both the nZVI/O2 and the Fe(II)/O2 systems. By minimizing the formation of iron precipitates (increasing iron solubility), the availability of ZVI and Fe(II) is improved, and the undesired consumption of reactive oxidants (mostly Fe[IV]) on the heterogeneous iron surfaces is avoided.
Granular ZVI exhibited higher oxidant yields than nZVI at acidic pH, which is attributed to the lower surface reactivity of granular ZVI [30]. Similarly, the incorporation of nickel into nZVI (Ni-Fe bimetallic nanoparticles, nNi-Fe) also lowers the reactivity of nZVI surfaces toward H2O2, enhancing the oxidant yields by two-fold at maximum [36]. Among the two types of nNi-Fe tested, Ni-coated nZVI required a much lower Ni content than Ni-Fe alloy to achieve a similar degree of the enhancement. However, strategies to lower the reactivity of ZVI with H2O2 also lower the reactivity with O2 (suppressing reaction 1), which decreases the rate of oxidant production [30, 36]. Therefore, the lower surface reactivity of ZVI is capable of degrading greater amounts of contaminants at the identical dose of iron oxidized, but can lead to slower degradation of contaminants.
Investigators have reported the addition of iron-chelating agents enhancing the oxidant yields for both the nZVI/O2 and the Fe(II)/O2 systems through several different mechanisms [28]. First, iron-chelating agents form soluble complexes with iron to prevent the formation of iron precipitates. As a result, the co-precipitation of Fe(II) - Fe(III) is avoided for the Fe(II)/O2 system, and the passivation of ZVI surfaces is minimized by the washing effect (an issue only for granular ZVI). Second, the coordination of Fe(II) with ligands can change the kinetics of the Fenton reaction. The Fe(II)-complexation with oxalate and nitrilotriacetic acid (NTA) accelerate the Fenton reaction, increasing the branching ratio of reaction 3 to reaction 2 [28]. In addition, these Fe(II) complexes can alter the mechanism of the Fenton reaction, shifting the dominant oxidant from Fe(IV) to OH at neutral pH; in fact, the identity of the reactive oxidant (OH vs. reactive Fe([IV)]-ligand complexes) remains controversial, but the oxidizing power of the reactive oxidant obviously increases in the presence of ligands [28, 37, 38]. The addition of oxalate, ethylenediaminetetraacetic acid (EDTA), and NTA increased the oxidant yields for the nZVI/O2 and the Fe(II)/O2 systems by two- to seven-fold (refer to Table 1). However, these organic ligands are vulnerable to chemical oxidation, and thereby undergo oxidative destruction during the reaction. For the same reason, the organic ligands scavenge the reactive oxidants, decreasing the degradation efficiency of target contaminants.
Recently, tetrapolyphosphate (TPP) has been suggested as a stable inorganic ligand resistant to oxidation [39, 40]. The addition of TPP accelerated the degradation of organic target compounds such as atrazine and pentachlorophenol [39, 40] as well as significantly increasing the oxidant yields for both the nZVI/O2 and the Fe(II)/O2 systems (unpublished Results) (Table 1). The primary role of TPP is the formation of soluble complexes by iron-chelation that prevents the precipitation of iron at neutral pH. However, TPP was also suggested to improve the electron availability of ZVI by promoting electron transfer from the ZVI core to Fe(III)-TPP (both surface-bound and soluble species) and subsequently regenerates Fe(III)-TPP into Fe(II)-TPP [40].
Polyoxometalates (POMs), such as PW12O403− and SiW12 O404− also serve as inorganic iron-chelating agents, enhancing the oxidant production from granular ZVI and nZVI [29, 41]. In particular, POMs were found to mediate electron transfer from ZVI to O2 as electron shuttles to enhance the production of H2O2 in the bulk phase (reactions 6 and 7).
(6)
Fe0(s)+2POMFe(II)+2POM-
(7)
O2+2POM-+2H+H2O2+2POM
The electron shuttling by POMs prevents the H2O2 consumption on the ZVI surfaces (reaction 2), and thereby increases chances for H2O2 to be used in the Fenton reaction (reaction 3). As inorganic ligands, POMs are resistant to oxidation. However, they go through hydrolysis at neutral pH, leaving the stability issue unresolved [4244]. Meanwhile, natural organic matter (NOM) plays a role as an electron shuttle for the ZVI/O2 system, enhancing the degradation rate of contaminants [45].

4. Application to Removal of Water Contaminants

Iron-induced O2 activation systems, relatively new approaches compared to other AOPs, have been studied for a limited number of contaminants. Several investigators have demonstrated the degradation of organic contaminants such as phenolic compounds, herbicides, pharmaceuticals and diagnostic agents, and dyes by the ZVI/O2 and the Fe(II)/O2 systems with or without additives [13, 39, 40, 4550]. Table 2 summarizes those literature results reported in terms of the type of iron, the target compound, initial concentrations, conditions, and degradation efficiency.
In general, nZVI exhibited faster kinetics for the degradation of organic contaminants than granular ZVI due to the higher surface reactivity. In the ZVI/O2 system without any additives, acidic pH conditions favored degradation of organic contaminants. The optimum pH for degradation of pharmaceuticals and diagnostic agents was found to be approximately 3 in the ZVI/O2 system [48]. The degradation rate of Acid Orange II by the ZVI/O2 system decreased by 20-fold with increasing pH from 3 to 11 [49]. The molinate degradation by the nZVI/O2 system was 95% over 3.5 h at pH 4, but was almost negligible at pH 8.1 [45]. As stated earlier, neutral pH conditions accelerate the surface passivation of ZVI and mainly produce Fe(IV) instead of OH. These behaviors serve as major factors that inhibit the degradation of organic contaminants.
The use of additives such as EDTA, TPP, POM, and NOM (iron-chelating agents or electron shuttles) greatly enhanced the degradation rate of organic contaminants [13, 3941, 46]. In particular, the addition of iron-chelating agents enables the ZVI/O2 system to work at neutral pH. The addition of EDTA improved the degradation of phenolic compounds by the ZVI/O2 system at pH 5.5 to 6.5; without EDTA, those compounds were negligibly degraded [13]. TPP was more effective by almost 10-fold than EDTA for enhancing the degradation rate of atrazine [40]. In fact, the ZVI/O2 systems with iron-chelating agents are believed to considerably rely on the homogeneous reactions of Fe(II) released from ZVI.
The reactions of Fe(II) by O2 produces reactive oxidants at neutral pH as stated earlier. However, because of the low oxidant yields, no studies have demonstrated the significant degradation of organic contaminants by the Fe(II)/O2 system without any additives. However, the addition of TPP as an iron-chelating agent greatly enhances the degradation rate of organic contaminants by the Fe(II)/O2 system [39].
Primarily, the ZVI/O2 and the Fe(II)/O2 systems oxidize organic contaminants in water and wastewater. However, a few studies have also shown that these systems can offer additional benefits besides organic oxidation, and can be applied as various manners for different purposes. Chemical oxidation by the nZVI/O2 system can enhance the biodegradability of wastewater containing toxic biocides, improving the efficiency of the secondary biological treatment [51]. Englehardt et al. [52] showed the ZVI/O2 system can simultaneously treat ligands and heavy metals in wastewater; organic ligands were oxidized by reactive oxidants, and heavy metals were removed by co-precipitation with Fe(III)-oxyhydoroxides. Fu et al. [53] also demonstrated a dye compound and Cr(VI) can be simultaneously removed in the ZVI/O2 system via oxidation and reduction/co-precipitation, respectively. Recently, a research group has demonstrated the pentachlorophenol-contaminated soil can be effectively remediated by soil washing with TPP solution and the subsequent oxidative treatment of the wastewater by the ZVI/O2 system [54]. The same group also suggested a novel electro-Fenton system using Fe(II)-TPP complexes applicable for a wide pH range [55]. A sequential reduction-oxidation process using ZVI may also be useful to treat multiple organic contaminants; ZVI is first used to treat reductively-degradable organic contaminants such as perchlorinated compounds under anoxic conditions, and subsequently, by supplying O2 the residual ZVI and Fe(II) oxidatively degrade remaining organic contaminants. Meanwhile, reactive oxidants produced by the nZVI/O2 and the Fe(II)/O2 systems have been reported to be capable of inactivating bacteria and viruses by disrupting cell membranes or capsid proteins [56, 57]; in particular, viruses were found to be selectively vulnerable to Fe(IV). Multiple processes such as chemical oxidation and reduction, coagulation and precipitation, adsorption, air stripping, and disinfection concurrently occur during the ZVI/O2 treatment [58].

5. Research Needs

The ZVI/O2 and the Fe(II)/O2 systems as new AOPs have a short history and further studies are needed from several perspectives. First, the efficiency of these systems needs evaluation with a wider spectrum of contaminants. Different from homogeneous AOPs in which the relative degradation kinetics among contaminants is determined by the rates of reactions with OH, contaminant removal by the ZVI/O2 and the Fe(II)/O2 systems can be influenced by factors such as the affinity to heterogeneous surfaces (i.e., ZVI or iron precipitates), the availability for co-precipitation with iron, the reductive degradability by ZVI, and the reactivity with oxidants.
Second, the nature of reactive oxidants produced by the ZVI/O2 and the Fe(II)/O2 systems is not fully understood under neutral pH conditions, warranting further study. Although Fe(IV) is suggested to be the dominant oxidant from the Fenton reaction at neutral pH, different forms of Fe(IV) species can be generated depending on conditions. Particularly, in those systems with iron-chelating agents, Fe(IV)-ligand complexes can be responsible for the contaminant degradation, and the reactivity of these Fe(IV) complexes is believed to be dependent on the ligand.
Finally, modified systems should be continuously explored by examining different additives, properly surface modifications of ZVI, and potential use of external energy. Various methods of implementing the ZVI/O2 and the Fe(II)/O2 systems need to be investigated for practical application (e.g., designing different reactors, improved procedures, or hybridization with other processes).

Acknowledgments

This work was supported by “The GAIA Project” funded by the Korea Ministry of Environment (RE201402059).

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Table 1
Yields of Reactive Oxidants from Iron-induced Oxygen Activation
Type of iron Additives or Effects of additives or modifications Oxidant yieldsa (Δ [Oxidant]/ Δ [Fe(II) or Fe0]) Ref.
Granular ZVI (70 mesh) - - 6% [30]
EDTA Iron-chelating 48%
nZVI (1~100 nm) - - 5~8% [15, 30]
Ni-doping Mitigation of four-electron transfer mechanism 17~28% [36]
EDTA Iron-chelating 13~22% [28, 30]
Oxalate Iron-chelating 23% [28]
NTA Iron-chelating 37%
POM Iron-chelating and electron shuttling 40% [29]
TPP Iron-chelating 52% b
Fe(II) - - 6~11% [15, 30]
EDTA Iron-chelating 5~15% [28, 30]
Oxalate Iron-chelating 25% [28]
NTA Iron-chelating 13%
POM Iron-chelating and electron shuttling 38% [29]
TPP Iron-chelating 74% b

a Measured by the formaldehyde production from the oxidation of excess methanol, values at pH 7.

b Unpublished results.

Table 2
Degradation Efficiencies of Organic Contaminants by Iron-induced Oxygen Activation Reported in Selected Literatures
Type of iron Target compound Initial concentration Conditions Degradation efficiency Ref.
Granular ZVI (40~70 mesh) 4-Chlorophenol 1.1 mM [ZVI]0 =50 g/L, [EDTA]0 =0.32 mM
pH =5.5~6.5
Room temperature
CDa in 4 h (τb ≈ 40 min) [13]


Pentachlorophenol 0.61 mM CD in 70 h

Granular ZVI (20~40 mesh) EDTA 1 mM [ZVI]0 =50 g/L
pH =5.5~6.5, 20°C
CD in 4 h (τ ≈ 30 min) [47]

Granular ZVI (100 mesh) Acid Orange II 100 mg/L (0.29 mM) [ZVI]0 =3 g/L
pH0 =3, 5, 7, 9, 11
25°C
90% in 3 h at pH0 = 3 (τ ≈ 40 min)
75% in 3 h at pH0 = 7 (τ ≈ 75 min)
[49]

Granular ZVI (100 mesh) 4-Chlorophenol 0.1 mM [ZVI]0 = 0.2 g/L, [NOM]0 (humic or fulvic acid) =0.5 mg/L
pH0 =2.5
Room temperature
60% in 4 h w/o NOM (τ ≈ 2 h)
85% in 4 h w/ NOM (τ ≈ 40 min)
[45]


Clofibric acid 65% in 2 h w/o NOM (τ ≈ 50 min)
85% in 2 h w/ NOM (τ ≈ 30 min)

Granular ZVI (0.125~3 mm) Iopromide 100 mg/L (0.13~0.3 mM) [ZVI]0 =40 g/L
pH =3
20°C
CD in 8 h (τ ≈ 90 min) [48]
Diatrizoate 65% in 8 h (τ ≈ 4 h)
Ciprofloxacin CD in 8 h (τ ≈ 60 min)
Cefuroxime CD in 4 h (τ ≈ 30 min)
Piperacillin CD in 6 h (τ ≈ 45 min)

Ifosfamide 10 mg/L (22~38 mM) 90% in 8 h (τ ≈ 90 min)
Methotrexate CD in 2 h (τ < 10 min)

nZVI (1~100 nm) Molinate 100 mg/L (0.53 mM) [nZVI]0 = 10.7 mM (0.6 g/L)
pH0 = 4, 20°C
95% in 3.5 h (τ ≈ 12 min) [46]

nZVI (nanowires, 50~100 nm in diameter) Atrazine 70 mM [nZVI]0 = 20 mM (1.1 g/L)
[EDTA]0 or [TPP]0 =1 mM
pH0 =8, 20°C
60% in 1 h w/ EDTA (τ ≈ 45 min)
CD in 20 min w/ TPP (τ < 5 min)
[40]
Fe(II) Rhodamine B 5 mg/L (8.6 mM) [Fe(II)]0 =10 mM
[TPP]0 = 50 mM
pH0 = 7.2, Room temperature
90% in 4 min (τ ≈ 1 min) [39]

Eosin B 5 mg/L (10.4 mM) 90% in 4 min (τ ≈ 1.5 min)

Pentachlorophenol 10 mg/L (37.5 mM) 90% in 4 min (half life ≈ 2 min)

CD: Complete Degradation.

b τ indicates the half-life of the target compound during its degradation.

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