Oxidation of organic contaminants in water by iron-induced oxygen activation: A short review

Changha Lee

doi:10.4491/eer.2015.051

Environ Eng Res > Volume 20(3); 2015 > Article

Lee: Oxidation of organic contaminants in water by iron-induced oxygen activation: A short review

Review Article

Environmental Engineering Research 2015; 20(3): 205-211.

Published online: July 9, 2015

DOI: https://doi.org/10.4491/eer.2015.051

Oxidation of organic contaminants in water by iron-induced oxygen activation: A short review

Changha Lee^†

School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea

^†Corresponding author: Email: clee@unist.ac.kr, Tel: +82-52-217-2812 Fax: +82-52-217-2809

Received May 17, 2015 Accepted June 08, 2015

(open-access):

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

Keywords: Advanced oxidation process (AOP), Ferryl ion, Fenton reaction, Ferrous ion, Hydroxyl radical, Organic contaminants, Oxygen activation, Zero-valent iron

1. Introduction

For decades, advanced oxidation processes (AOPs) have been extensively studied as effective tools to degrade refractory organic contaminants in water [1–6]. 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 [10–12]. However, relatively recent studies have demonstrated ^•OH to be generated by activation of O₂ [13–15]. 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 (E^o(Fe²⁺/Fe⁰) = −0.44 V_NHE [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)

{Fe}^{0}_{(s)} + O_{2} + 2 H^{+} \to Fe (II) + H_{2} O_{2}

(2)

{Fe}^{0}_{(s)} + H_{2} O_{2} + 2 H^{+} \to Fe (II) + 2 H_{2} O

(3)

Fe (II) + H_{2} O_{2} \to Fe (III) + {}^{•}O H + {OH}^{-} (or Fe (IV) + H_{2} O)

Literatures have suggested that the dominant oxidant from the Fenton reaction shifts from ^•OH to Fe(IV) with increasing pH [15, 23–25]; ^•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 [24–27].

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) [31–33]. 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) + O_{2} \to Fe (III) + {O_{2}}^{• -}

(5)

Fe (II) + {O_{2}}^{• -} \to 2 H^{+} \to Fe (III) + H_{2} O_{2}

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 3–5, 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)

{Fe}^{0}_{(s)} + 2 POM \to Fe (II) + 2 {POM}^{-}

(7)

O_{2} + 2 {POM}^{-} + 2 H^{+} \to H 2 O_{2} + 2 POM

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 [42–44]. 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, 45–50]. 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, 39–41, 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]
Granular ZVI (70 mesh)	EDTA	Iron-chelating	48%	[30]
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%	[28]
	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%	[28]
	POM	Iron-chelating and electron shuttling	38%	[29]
	TPP	Iron-chelating	74%	b

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

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	CD^a 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	10 mg/L (22~38 mM)		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]
	Rhodamine B	5 mg/L (8.6 mM)
	Eosin B	5 mg/L (10.4 mM)		90% in 4 min (τ ≈ 1.5 min)
	Eosin B	5 mg/L (10.4 mM)
	Pentachlorophenol	10 mg/L (37.5 mM)		90% in 4 min (half life ≈ 2 min)

CD: Complete Degradation.

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