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Critical Reviews in Environmental Science and Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/best20

Sulfate Radical and Its Application in Decontamination Technologies ab

a

a

b

Bo-Tao Zhang , Yang Zhang , Yanguo Teng & Maohong Fan a

College of Water Sciences, Beijing Normal University, Beijing, China b

Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY, USA Accepted author version posted online: 19 Nov 2014.

To cite this article: Bo-Tao Zhang, Yang Zhang, Yanguo Teng & Maohong Fan (2015) Sulfate Radical and Its Application in Decontamination Technologies, Critical Reviews in Environmental Science and Technology, 45:16, 1756-1800, DOI: 10.1080/10643389.2014.970681 To link to this article: http://dx.doi.org/10.1080/10643389.2014.970681

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Critical Reviews in Environmental Science and Technology, 45:1756–1800, 2015 Copyright © Taylor & Francis Group, LLC ISSN: 1064-3389 print / 1547-6537 online DOI: 10.1080/10643389.2014.970681

Sulfate Radical and Its Application in Decontamination Technologies BO-TAO ZHANG,1,2 YANG ZHANG,1 YANGUO TENG,1 and MAOHONG FAN2 Downloaded by [University of Wyoming Libraries] at 13:31 26 May 2015

1

2

College of Water Sciences, Beijing Normal University, Beijing, China Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY, USA

This article gives a broad overview about the status and progress of the sulfate radical-based decontamination technologies. Peroxymonosulfate or persulfate can be activated by energy or a catalyzer to form sulfate radical for rapid and effective degradation of contaminants in homogenous systems. Sulfate radical-based heterogeneous catalysis oxidation has been a major research focus since 2005 because solid catalysts are stable, cost-effective, and easy to recover. The application of sulfate radical in other decontamination technologies and the mechanisms involved are also described and discussed. The main points are summarized and the directions of future researches are suggested in the last section. KEY WORDS: decontamination technologies, heterogeneous catalysis oxidation, peroxymonosulfate, persulfate, sulfate radical

1. INTRODUCTION An extensive increase in urbanization and industrialization in recent decades has significantly improved the living standards of people. However, the excessive consumption of natural resources and the discharge of various contaminants from different industrial processes and domestic households Address correspondence to Bo-Tao Zhang, College of Water Sciences, Beijing Normal University, Beijing 100875, China. E-mail: [email protected] or Maohong Fan, Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, WY 82071, USA. E-mail: [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/best. 1756

Downloaded by [University of Wyoming Libraries] at 13:31 26 May 2015

Sulfate Radical and Its Application in Decontamination Technologies

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FIGURE 1. The published number change of peer-reviewed papers about sulfate radicalbased decontamination technologies with year.

pose risks to human health and ecological systems.1,2 It is imperative to develop effective technologies to treat these pollutants, especially for the high-concentration, hazardous, nonbiodegradable, or refractory compounds, which cannot be treated well using conventional processes. Recently, radicalbased decontamination technologies have been proposed since they could completely destruct organic contaminants by transforming them into harmless end products, such as CO2 and H2 O. A radical (also called a “free radical”) refers to an atom, molecule, or ion with at least one unpaired electron in the outermost open shell of electrons. On account of the possessions of the unpaired electrons, free radicals are highly active to react with various contaminants.3 The initially used radical and its precursor in the decontamination technologies are hydroxyl radical and Fenton reagent (hydrogen peroxide with iron ions). This treatment technology is restricted by acidic pH requirement (pH 2–4), instability of H2 O2 , generation of high amount of sludge in coagulation step, and loss of iron ions in water.4 Attempts have been made to develop more convenient radicalbased decontamination technologies, with cost-effective and environmentally benign processes. The development of sulfate radical-based technologies is one of these efforts. These technologies have received increasing amounts of interest from the academic and industrial communities, which is reflected in the increasing numbers of publications in peer-reviewed journals (Figure 1), patents, and international conferences dedicated to the environmental applications. The decontamination technologies of chemical oxidation are based on the oxidative power of specific chemicals. The redox potentials of some

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B.-T. Zhang et al.

TABLE 1. Redox potential of radicals and common used oxidants Oxidant/condition


F2 Acidic SO4 .− Acidic or neutral ·OH Acidic Alkaline S2 O8 2− Acidic O3 Acidic Alkaline HSO5 − Acidic H2 O2 Acidic Alkaline MnO4 − Acidic Alkaline

Half reaction F2 + 2H+ + 2e−→2HF SO4 .− + e−→ SO4 2−

Redox potential/V 3.06 2.5–3.1

·OH + H+ + e−→ H2 O ·OH + e−→ OH−

2.70 1.80

S2 O8 2− + 2e−→ 2SO4 2−

2.01

O3 + 2H+ + 2e−→O2 + H2 O O3 + H2 O + 2e−→O2 + 2OH−

2.07 1.24

HSO5 − + H+ + 2e−→SO4 2− + H2 O

1.82

H2 O2 + 2H+ + 2e−→2H2 O HO2 − + H2 O + 2e−→3OH−

1.78 0.85

MnO4 − + 4H+ + 3e−→MnO2 + 2H2 O MnO4 − + 2H2 O + 3e−→MnO2 + 4OH−

1.68 0.58

radicals and commonly used oxidants are listed in Table 1. Sulfate and hydroxyl radicals exhibit similar redox potentials under acidic conditions, which rank second only to F2 . The sulfate radical exhibits a higher standard redox potential than the hydroxyl radical at neutral pH, which makes the sulfate radical superior in mineralizing numerous organic compounds.5 The sulfate radical is also more efficient than the hydroxyl radical for degradation of many organic compounds, since it is more selective for oxidation by electron-transfer reaction. Furthermore, the sulfate radical exhibits high oxidation efficiencies in both carbonate and phosphate buffer solutions.6 The precursor of sulfate radical is peroxymonosulfate (PMS) or persulfate (PS), which is a monosubstituted or a symmetrically substituted derivative of hydrogen peroxide by sulfo moiety. PMS is commercially available as a free-flowing powder known by the trade names Oxone with the composition 2KHSO5 ·KHSO4 ·K2 SO4 . The structure of the PMS anion is HOOSO3 − with the bond lengths of the three terminal S–O and one internal S–O (peroxo) bonds similar to those in S2 O8 2− and the O–O distance similar to that in H2 O2 by X-ray analyses.7,8 Sodium PS with high stability and water solubility is preferred for use in the decontamination technologies.9 The photolysis or thermolysis of PS is commonly used in the standard methods and commercial instruments for the determination of total organic carbon (TOC), nitrogen, or phosphorus in water and wastewater.10,11 PMS and PS are thermodynamically strong oxidants (as shown in Table 1), but direct reactions of PS or PMS with most pollutants are slow. When appropriately activated, they can

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decompose to form sulfate radical, which is a strong one-electron oxidant that rapidly and effectively oxidizes many organic compounds. This review article is intended to provide a broad view of the fundamental information, status, and progress of the sulfate radical-based decontamination technologies based on publications in peer-reviewed journals during the period 2000–2013. Direct chemical oxidation of contaminants by sulfate radical in homogenous and heterogeneous systems is presented. Applications of sulfate radical in other decontamination technologies, such as hydrothermal and in situ chemical oxidation, are described. The identification of sulfate radical using chemical probes and relative mechanism researches by quantum chemical approaches are also assessed.

2. DIRECT CHEMICAL OXIDATION OF CONTAMINANTS BY SULFATE RADICAL IN HOMOGENOUS SYSTEMS PMS or PS can be activated to form sulfate radical, which is a strong oneelectron oxidant for rapid and effective degradation of contaminants. The sulfate radical generation processes can be classified into two categories: energy and catalyzer activations.

2.1. Degradation of Contaminants by Sulfate Radical in Energy Activation Systems The sulfate radical could be generated through scission of the peroxide bond of PMS or PS by energy (heat, ultraviolet (UV), ultrasound, radiolysis, etc.) via the following reactions:

2.1.1. DEGRADATION OF CONTAMINANTS ACTIVATION SYSTEMS

BY

SULFATE RADICAL

IN

THERMAL

The activation energy of the uncatalyzed reaction involving the thermal crack of the O–O bond of S2 O8 2− (Reaction (2)) was reported to be 33.5 kcal/mol. The temperature must be raised high enough to provide the required activation energy, resulting in the scission of chemical bonds and the production of sulfate radical for decontaminant processes. Liang et al. found that


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20 ◦ C was insufficient for trichloroethylene (TCE) degradation and 1,1,1trichloroethane (TCA) degradation. However, TCE and TCA were readily oxidized at elevated temperatures (40–60 ◦ C) as a result of thermally activated PS oxidation. Comparing the activation energies and degradation rate change of these compounds, the authors proposed that reactions with high activation energies are temperature sensitive.9,12 Huang et al. investigated the ability of thermally activated PS oxidation to degrade 59 volatile organic compounds in the mixture. Thirty-seven of them were completely or almost completely removed (90–100%) by 5 g/L sodium PS at 40 ◦ C in 72 hr, and most of these compounds are with carbon–carbon double bonds or with a benzene ring bonded to reactive functional groups. The degradation of recalcitrant contaminants could be limited due to competing reactions in systems with multiple components. For example, little TCA degradation was observed in this system, but TCAs could be completely degraded individually.13 Waldemer et al. found that, at 40 ◦ C, the rate of tetrachloroethene oxidation by heat-activated PS was greater than the oxidation by permanganate and the rate of thermally activated PS oxidation was about 400 times faster than permanganate at 100 ◦ C.14 The results of Hori et al. showed that thermally activated PS led to the efficient decomposition of C5 −C9 perfluorocarboxylic acids to F− ions and CO2 . However, a high temperature (150 ◦ C) was not suitable for decomposition since the formation of F− and CO2 was dramatically decreased.15 Similar results are obtained by Lee et al. who used a microwave to thermally decompose PS to destruct perfluorocarboxylic acids. The high temperature led to a fast sulfate conversation of PS, subsequently causing lower mineralization efficiency.16 Therefore, an optimal activation temperature may be obtained, depending on which compounds are present and how temperature affects their kinetics. Thermally activated PS oxidation was also applied to degrade herbicides (clomazone, paraquat, diuron, and glyphosate),17,18 polychlorinated biphenyls,19 perfluorooctane sulfonate,20 antiepileptic drug (carbamazepine),21 landfill leachate,22 and 16 priority polycyclic aromatic hydrocarbons.23 In the above researches, the kinetics of different decontaminant processes in thermally activated PS systems could be modeled using pseudo-first-order rate equations.

2.1.2. DEGRADATION OF CONTAMINANTS BY SULFATE RADICAL PHOTOCHEMICAL ACTIVATION SYSTEMS

IN

Table 2 summarizes the photo-induced sulfate radical to degrade or mineralize different kinds of contaminants from traditional pollutants (such as dyes, phenols, and herbicides) to emerging pollutants (such as endocrinedisrupting chemicals and pharmaceuticals and personal care products).24–69 It can be seen from Table 2 that PS was used as the precursor of sulfate radical in most photochemical oxidation systems. This might be because PS is more photosensitive than PMS. UV light was used instead of visible light

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254 nm 254 nm

Cylindrospermopsin (CYN)

Butylated hydroxyanisole (BHA)

Acid Black 24

C3 F7 CF = CHCOOH

Perfluorooctanoic acid (PFOA)

Bisphenol A (BPA)

Clomazone

Acetic acid

Perfluorodecanoic acid (PFDeA) CI Basic Blue 3 (BB3)

Cibacron Brilliant Yellow 3 (CBY-3); 3,4-DCP Phenol Benzoic acid (BA)

Rhodamine B (RhB)

PMS or PS

PS

PS

PS

PS

PS

PS

PS

PS PS

PS

PS PMS

PS

200–400 nm;

254 nm 254 nm

254 nm

UV UV

254 nm

UV

254 nm

254 nm or 185 nm

254 nm

254 nm

254 nm 254 nm

Perfluorocarboxylic acids (PFCAs) 2,4-dichlorophenol (2,4-DCP)

PS PMS or PS

Wavelength

Contaminant

Oxidant

Efficiency

Remarks Reference

PS, 50 mM; PFCAs, 1.35 mM RE, ∼100% (4 hr) [24] [25] 2, 4-DCP, 0.123 mM; PS or RETOC , 82% (PS, 4 hr); PMS, 1.23 mM RETOC , 74% (PMS, 4 hr) Cylindrospermopsin, 1 μM; PS RE, ∼100% (20 min, PS); [26] or PMS, 1 mM RE, ∼100% (40 min, PMS) [39] PS, 2 mM; BHA, 0.3 mM; pH, RE, ∼100% (20 min); 7.0 RETOC , ∼100% (45 min) Acid Black 24, 2.0mM REcolor, 54% (8 min); [40] RETOC, 49.8% (7 hr) C3 F7 CF = CHCOOH, 680 μM; RE, ∼100% (5 min) [41] PS, 12.5 mM PS, 407 mg/L; PFOA, 25 mg/L; RE, ∼70% (120 min, [42] 254 nm); RE, ∼90% O2 (20 min, 185 nm) RE, 88% (1 hr, 25 ◦ C); RE, [43] BPA, 0.05 mM; PS, 0.05 mM; 96% (1 hr, 35 ◦ C); RE, 25 ◦ C, 35 ◦ C, or 50 ◦ C 99% (1 hr, 50 ◦ C) PS, 0.1 M; clomazone, RE, ∼90% (20 min) [44] 0.76 mM [45] Acetic acid, 500 μM; PS, RE, ∼100% (90 min); 1775 μM; pH, 5.0 RETOC , ∼100% (90 min) PS, 0.1 mM; PFDeA, 0.1 mM RE, ∼100% (360 min) [46] BB3, 20 mg/L; PS, 1.8 mM; RE, ∼100% (20 min) [47] pH, 7.0 [48] CBY-3, 100 ppm RE3,4-DCP , ∼90%; RECBY-3 , ∼100% (60 min) PS, 84 mM; phenol, 0.5 mM RE, ∼100% (20 min) [49] BA, 9.9 μM; PMS, 100 μM; pH, RE, ∼100% (10 min) [50] 11.0 RhB, 0.02 mM; PS, 0.8 mM RE, ∼97% (30 min) [51] (Continued on next page)

Condition

TABLE 2. Degradation of contaminants by sulfate radical in photochemical activation systems


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Sulfamethazine (SMT) Trichloromethane (TCM) Diclofenac

Iopromide

2,2,3,3-tetrafluoro-1-propanol (TFP) Polyvinyl alcohol (PVA)

Remazol Red (RR) Atenolol

Phenol

Sulfamonomethoxine (SMM) Sodium dodecylbenzene sulfonate (SDBS) Dimethyl phthalate (DMP)

Antipyrine (AP)

As(III)

2,4-dichlorophenol (2,4-DCP)

Orange II

Orange II

PS

PS

PS

PS PMS

PMS or PS

PMS PS

PMS

PS

PS

PMS

PMS

PMS

Contaminant

PS PMS PMS

Oxidant

Xe lamp

Xe lamp

Solar light

UV

254 nm

254 nm

365 nm UV

UV

UV 254 nm

UV

254 nm

254 nm

254 nm 254 nm 254 nm

Wavelength

[58] [59]

RE, ∼90% (60 min) RE, ∼88% (30 min)

[27] [28] [29]

Co2+ Co2+ Co2+

[65]

[64]

[63]

[61] [62]

[60]

[57]

[56]

[55]

[52] [53] [54]

Remarks Reference

RE, 97% (10 min)

RE, 96.5% (45 min) RE, ∼85% (15 min) RE, ∼100% (120 min); RETOC , ∼80% (90 min) RE, ∼100% (30 min); RETOC , ∼100% (80 min) RETOC , 99.7% (1 hr)

Efficiency

RE, 100% (50 min; PS); RE, 100% (75 min; PMS) PMS, 10 mM; SMM, 5 mg/L RE, 96.78% (90 min) PS, 100 mM; SDBS, 5 μm; pH, RE, ∼100% (10 min) 7.0; 25 ◦ C DMP, 100 mg/L; PMS, 40 mM; RE, ∼95% (20 min) pH, 3.0 PS, 1.06 mM; AP, kobs = 1.04e−[AP]0/0.043 + 0.0265–0.1065 mM 1.34 As(III), 0.135 mM; PS, 0.1 mM; RE, 99% (1 hr) pH, 3.0 PMS, 1 mM; Co, 4 μm; RE, ∼100% (45 min) 2,4-DCP, 100 mg/L RETOC, 100% (70 min) Orange II, 0.2 mM; PMS, 20 mM; Co, 0.06 mM; O2 Orange II, 0.2 mM; PMS, RE, ∼100% (2 min) 2.7 mM; Co, 0.1 mM

PS, 0.2 mM; SMT, 0.02 mM PS, 0.5 mM; TCM, 8.8 μM PMS, 3.15 mM; diclofenac, 0.63 mM; 20 ◦ C Iopromide, 0.126 mM; PS, 2 mM; pH, 3.4 PS, 20 mM; TFP, 1.39 mM; pH, 3.0 PS, 1 mM; PVA, 20 mg/L; pH, 3.0; 25 ◦ C PS, 1000 μM; RR, 50 mg/L Atenolol, 20 μm; PMS, 80 μm; pH, 7.0; 23 ◦ C Phenol, 515 μm; PMS or PS, 5 mM; pH, 3.1; 20 ◦ C

Condition

TABLE 2. Degradation of contaminants by sulfate radical in photochemical activation systems (Continued)


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Methyl orange

Acid orange (AO7)

Imazalil

Methylene blue (MB)

Pentachlorophenol (PCP)

CI Reactive Black 5 (RB5)

Amitrole

Phenol

Phenol

Carbamazepine

MB

Phenol

PMS

PMS

PS

PS

PS or PMS

PS

PMS or PS

PS

PMS or PS

PS

PS

PMS or PS

RE, removal efficiency.

Orange II

PMS

Orange II, 0.2 mM; PMS, 20 mM; Cu, 0.6 mM; Fe, 0.6 mM Suntest lamp Methyl orange, 0.01 mM; PMS, 0.06 mM; Co, 0.02 mM Vis, >420 nm; AO7, 0.14 mM; PMS, 1.40 mM; UV, 365 nm Co2+, 14 μM 356 nm Imazalil, 25 mg/L; pH, 3.0; TiO2 , 2.5 g/L; PS, 6 g/L 220–440 nm PS, 10 mM; TiO2 , 0.6 g/L; MB, 4 mg/L UV PS or PMS, 10 mM; TiO2 , 0.2 g/L; PCP, 0.4 mM 254 nm RB5, 20 mg/L; PS, 1 mM, TiO2 , 1 g/L Xe lamp Amitrole, 1 mg/L; PS or PMS, 0.05 mM Xe lamp Phenol, 25 mg/L; ZnO, 0.4 g/L; PS, 4 g/L 253 nm Phenol, 25 mg/L; ZnO, 0.6 g/L; PS, 7.4 mM; PMS, 6.5 mM 420 nm Carbamazepine, 0.025 mM; PS, 2 mM 220–440 nm PS, 10 mM; TiO2 , 0.6 g/L; MB, 4 mg/L >380 nm Phenol, 25 mg/L; (3%) ZnO-MCM-22, 0.5 g/L; PS, 2 g/L; PMS, 2 g/L

Xe lamp

[31]

TiO2 TiO2 TiO2 TiO2

RE, ∼100% (45 min) RETOC , ∼100% (20 min) k, 0.156/min

ZnOMCM-22

TiO2

RE, ∼100% (45 min) RE, ∼95% (3 hr)

WO3

ZnO

RE, ∼100% (5 hr, PMS); RE, ∼100% (6 hr, PS) RE, ∼92% (1 hr)

ZnO

NF-TiO2

[30]

Co2+

RETOC , 73% (UV); RETOC , 37% (Vis) RE, ∼90% (5 min)

RE, ∼80% (PMS, 2 hr); RE, ∼75% (PS, 2 hr) RE, ∼95% (2.5 hr)

[67]

Co2+

RE, ∼100% (2 min)

[69]

[68]

[38]

[37]

[36]

[35]

[34]

[33]

[32]

[66]

RE, ∼100% (30 min, Cu); Cu2+ or Fe3+ RE, ∼100% (10 min, Fe)



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to activate these peroxides, which might be attributed to the following facts: UV light could provide sufficient energy to crack the peroxo bond and the absorbance of PS or PMS is in the UV region.24 The research group of Dionysiou compared the degradation efficiencies of 2,4-dichlorophenol (2,4-DCP) by hydrogen peroxide, potassium PS, and potassium PMS activated by UV light. Their results show that the transformation of 2,4-DCP and the extent of organic carbon removal efficiencies follow the order: UV/K2 S2 O8 > UV/KHSO5 > UV/H2 O2 .25 A similar order of efficiencies was also obtained in their recent research about the degradation of cylindrospermopsin using UV-254 nm activation of these three oxidants.26 The energy of the peroxide bond has been estimated to be 33.5 kcal/mol in potassium PS and 51 kcal/mol in hydrogen peroxide. The peroxide bond distances have been found to be 1.453 A˚ in solid H2 O2 , 1.460 A˚ in KHSO5 , and 1.497 A˚ in (NH4 )2 S2 O8 . It indicates that PS followed by PMS is cleaved more easily than hydrogen peroxide, and sulfate radicals might be formed more readily than hydroxyl radicals.25 Bandala et al. found that solar light increased the rate constant for the degradation of 2,4-DCP in the Co/PMS system by 33 times compared with that obtained under dark conditions, which was much higher than that obtained due to increase in temperature. No absorption was determined either for Co or PMS in the region of solar radiation (300–400 nm), but mixing these species allows them to interact to produce a transition state adduct that can absorb in the solar spectrum region.27 The synergetic effects of photochemical (simulated solar radiation) and transition metal catalysis on the azo-dye degradation efficiencies were also found by the group of Kiwi28,29 and Chen.30 Chen et al. proposed a different mechanism for the acceleration effect by visible light. PMS cannot be activated by visible light, but excited azo-dye molecules can transfer electrons to PMS or transition metal ions and, therefore, accelerate the decomposition of PMS and the catalytic cycle of metal ions.30 PMS or PS can also serve as electron scavengers in semiconductormediated photocatalysis system to accelerate the oxidation. PMS or PS could accelerate the contaminant degradation by several folds because of dual roles: increasing the quantum efficiencies through scavenging conduction band electrons and forming of sulfate radical.31 Therefore, PS (for most cases) or PMS was proved more efficient than other inorganic oxidant electron scavengers, such as H2 O2 , ClO3 −, BrO3 −, and IO4 −.32,33 The combinations of photochemical and photocatalytical oxidation show synergetic effects in TiO2 -, ZnO-, and WO3 -mediated photocatalysis systems to maximize the rate of degradation.34–38

2.1.3. DEGRADATION OF CONTAMINANTS BY SULFATE RADICAL SONOCHEMICAL ACTIVATION SYSTEMS

IN

Sonochemical activation has significant advantages over other energy activators, in that it is safer, is cleaner, and can operate under ambient conditions.

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A sonication reactor can be used for longer periods of time without replacement, and large-scale treatments are possible in flow-type reactors.70 In the course of transient collapse of cavitation bubbles resulted from ultrasonic irradiation (referred as)))) in water, the extreme condition of localized high temperatures up to 5000 ◦ C and pressures of about 500 atm is created owing to the quasi-adiabatic procedure. Sulfate and hydroxyl radicals may be formed through the decomposition processes of PMS or PS in an ultrasonic system as shown by the following reactions.71, 72 − S2 O2− 8 2SO4

(3)

HSO− 5

(4)

SO− 4

+ OH

H2 O·OH + ·H

(5)

.− 2− + S2 O2− 8 + · H → SO4 +SO4 +H

(6)

Sulfate and hydroxyl radicals are generally considered to be the main oxidizing species responsible for the destruction of organic contaminants. Li et al. employed two scavengers (methanol and tert-butyl alcohol(TBA)) to identify the radicals generated during the activated decomposition of PS by ultrasound irradiation and evaluate their contribution. The results indicated that SO4 .− was predominantly produced over ·OH for removing TCA.71 The combination of ultrasound and PS or PMS has recently received a great deal of attention as a potential alternative method for the treatment of recalcitrant or hazardous compounds. This combination was proved to be effective for achieving the enhanced oxidation of nitric oxide,72 methyl tert-butyl ether (MTBE),73 1,4-dioxane,74 arsenic(III),75 dinitrotoluenes,76 amoxicillin,77 and dyes.78

2.1.4. DEGRADATION OF CONTAMINANTS ENERGY ACTIVATION SYSTEMS

BY

SULFATE RADICAL

IN

OTHER

In the time-resolved spectrophotometry and pulse radiolysis technique, the sulfate radical has a broad optical absorption band with a maximum at 450 nm (ε = 1100 L mol−1 cm−1). The rate constants for SO4 .− reactions could be determined by the decreasing of this absorption. Radiolysis of water could lead to the simultaneous generation of oxidative (·OH, H2 O2 ) and reductive (eaq −, ·H) species: H2 O → e− aq , ·OH, ·H, H2 O2 , H2 , . . .

(7)

The interest of the PS addition is to react with the reductive species (eaq −, Reaction (8); ·H, Reaction (6)), to prevent recombination with the hydroxyl radicals and to induce the formation of new oxidative sulfate radical. .− 2− − S2 O2− 8 +eaq → SO4 +SO4

(8)

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The degradation of phenol,79 diethyl phthalate,80 cyanuric acid,81 Acid Yellow 9,82 or micropollutants (ibuprofen, progesterone, and benzotriazole)83 in aqueous solution during radiolysis was investigated. The introduction of PS significantly increased the removal efficiencies by the generation of sulfate radical. The sulfate radical would also be generated by laser photolysis of the PS solution.84 The main purpose of photolysis researches is to get the reaction kinetics parameters of the sulfate radical with organic compound. However, few publications were found about the application of laser photolysis for SO4 .− generation in decontamination technologies according to our literature searches.

2.2. Degradation of Contaminants by Sulfate Radical in Catalyzer Activation Systems Even though PMS or PS can be effectively activated by different energy activators, transition metal-based activation is the most viable for field application owing to avoiding the high requirement of energy and decreasing the complexity and the costs of the process. The catalyzer, such as variant valence transition metal ions, could activate PMS to form sulfate and other radicals through the following reactions: n+ − → M(n+1)+ +SO.− HSO− 5 +M 4 +OH n+ HSO− → M(n+1)+ +SO2− 5 +M 4 + · OH (n+1)+ n+ + HSO− → SO.− 5 +M 5 +M +H

(9) (10) (11)

The sulfate radical could be generated by initiating PS with a transition metal ion catalyzer via the following reactions: 2− n+ S2 O2− → M(n+1)+ +SO.− 8 +M 4 +SO4

(12)

n+ SO.− → M(n+1)+ +SO2− 4 +M 4

(13)

n+ S2 O2− → 2M(n+1)+ +2SO2− 8 +2M 4

(14)

Several transition metals, such as Ag(I), Ce(III), Co(II), Fe(II), Fe(III), Mn(II), Ni(II), Ru(III), and V(III), were tested for the activation of three common oxidants, H2 O2 , KHSO5 , and K2 S2 O8 , to find the favorable metal(s) for the oxidant and achieve high 2,4-DCP degradation efficiencies. Results showed that Co(II) is the most efficient metal for the activation of KHSO5 , Ag(I) for K2 S2 O8 , and Fe(III) and Fe(II) for H2 O2 .5. Since these systems consist of a transition metal coupled with an oxidant (derivatives of hydrogen peroxide) and very strong oxidizing species are generated in these processes, they are nominated as a modification of the Fenton reagent or Fenton-like



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reagent. From the comparison results of 2,4-DCP and atrazine in cobalt/PMS and Fenton reagent, Co/PMS system was consistently proven to be more efficient at the pH range 2.0−9.0 with and without the presence of buffers such as phosphate and carbonate. The extent of mineralization by the Co/PMS reagent demonstrated higher efficiencies than the Fenton reagent.6 Iron is more environment friendly and cost-effective than other variant valence transition metal ions and shows reactivity to activate both hydrogen peroxide and its derivatives, PS and PMS.85 Iron has been receiving more attention for activating these peroxo compounds, especially for PS, to generate sulfate radical. The ferrous ion-activated PS reaction requires an activation energy of 14.8 kcal/mol, which is much lower than the value for the thermal rupture of the O–O bond.86 The ferrous ion might present a great potential to rapidly generate sulfate radicals and degrade organic contaminants. However, it has been reported by Liang et al. that the oxidation of TCE is limited in Fe(II)/PS system because excess Fe(II) might act as a radical and PS scavenger according to Reaction (13) and Reaction (14). Sequential addition of Fe(II) in small increments resulted in increased TCE removal efficiencies (from 31% by adding at once to 95% by five successive additions at Fe(II)/PS/TCE molar ratios of 25/20/1).87 The multiple additions of iron solution may be impractical for field application. Different chelating agents, such as ethylenediaminetetraacetic acid (EDTA), citric acid, sodium triphosphate, hydroxypropyl-β-cyclodextrin, and 1-hydroxyethane-1,1-diphosphonic acid, have been used in the iron-activated PS system to attempt to increase the contaminant degradation effectiveness. Citric acid was found to be the most effective, resulting in nearly complete TCE degradation since citric acid might serve to buffer available ferrous ion by altering the redox conditions and then cycling the ferrous ion.88,89 Fe(III) and EDTA alone did not appreciably decompose PS, but the presence of TCE in the EDTA/Fe(III)-activated PS system can induce faster PS and TCA degradation due to iron recycling to activate PS under a higher pH condition.90

3. DIRECT CHEMICAL OXIDATION OF CONTAMINANTS BY SULFATE RADICAL IN HETEROGONOUS SYSTEMS Although Co(II)/PMS and Ag(I)/PS were proved to be the most efficient catalysis systems for the generation of sulfate radical, the adverse effect of dissolved heavy metal ions on human health is always a concern. Therefore, it would be beneficial to activate PMS or PS in a heterogeneous manner, which may prevent the problem arising from dissolved or leached heavy metal ions present in the treated water. Solid catalysts have also attracted a great deal of attention for their properties of high stability and durability, cost-effective, and easy to recover. Lots of heterogeneous catalysis systems were attempted to achieve this aim as shown in Tables 391–132 and 4.133–152

1768

Nano-Co3 O4

Co3 O4 /TiO2

Co3 O4 /TiO2 (A); Co3 O4 /Al2 O3 (B); Co3 O4 /SiO2 (C)

Co3 O4 /PTFE

Co3 O4 /RR (Raschig rings)

Co3 O4 /SiO2

Co3 O4 /MnO2

Co3 O4 /red mud

Co3 O4 /fly ash

Co3 O4 /natural zeolites

Co3 O4 /carbon aerogel

Co3 O4 /SBA-15

Co3 O4 /carbon xerogel

2,4-DCP

2,4-DCP

Orange II

Orange II

Phenol

Phenol

Phenol

Phenol

Phenol

Phenol

Phenol

Phenol

Co3 O4

Catalyst

2,4-Dichlorophenol (2,4-DCP) Acid Orange 7 (AO7)

Contaminant

Co3 O4 /SiO2 -N, 0.2 g/L; PMS, 2 g/L; phenol, 30 ppm; 25 ◦ C Catalyst, 0.5 g/L; PMS, 2 g/L; phenol, 25 ppm; 25 ◦ C Catalyst, 0.4 g/L; PMS, 2 g/L; phenol, 25 ppm; 25 ◦ C Catalyst, 0.4 g/L; PMS, 2 g/L; phenol, 30 ppm; 25 ◦ C Catalyst, 0.4 g/L; phenol, 25 mg/L; PMS, 2 g/L; 25 ◦ C Catalyst, 0.2 g/L; phenol, 50 ppm; PMS, 2 g/L Catalyst, 0.2 g/L; PMS, 2 g/L; phenol, 30 ppm; 25 ◦ C Catalyst, 0.2 g/L; PMS, 2 g/L; phenol, 50 ppm; 25 ◦ C

PMS, 2 mM; Orange II, 0.4 mM; recirculation rate, 400 mL/min

PMS, 1 mM; Orange II, 0.05 mM; pH, 7; Dark

Catalyst, 157 mg/L; 2,4-DCP, 20 mg/L; PMS, 2.67 mM; pH, 7.0 Catalyst, 0.5 g/L; PMS, 2 mM; pH, 7; AO7, 0.2 mM Catalyst, 0.1 g/L; 2,4-DCP, 50 mg/L; PMS, 150 mg/L Catalyst, 0.1 g/L; 2,4-DCP, 50 mg/L; PMS, 150 mg/L

Condition

[102]

RE, ∼100% (2 hr)

RE, 100% (40 min)

[103]

[4]

[101]

RE, ∼100% (5 hr)

RE, 100% (3 hr)

[100]

RE, 70% (90 min)

LMCo , 0.8–1.2 ppm LMCo , 18.26%

[99]

RE, 100% (60 min)

[97]

[96]

[95]

[94]

[93]

[92]

[91]

Reference

[98] LMCo , 1.6 ppm

LMCo , 0.03 mg/L LMCo , 0.05 mg/L LMCo , 0.036 mg/L A, LMCo , 0.045 mg/L; B, LMCo , 0.08 mg/L; C, LMCo , 1.4 mg/L LMCo , 0.05 ppm (8 min) LMCo , 0.5–2 ppm (15 min) LMCo , 4%

Remarks

RE, 100% (20 min)

RE, ∼78% (5 hr)

RE, ∼95% (5 min)

RE, ∼99% (2 hr)

A, Re, ∼96%; B, RE, ∼79%; C, Re, ∼98% (10 min)

RE, 75% (2 hr)

RE, ∼98% (30 min)

RE, 99.9% (30 min)

Efficiency

TABLE 3. Degradation of contaminants by sulfate radical in heterogeneous activation peroxymonosulfate systems


1769

Co3 O4 /TiO2

Co3 O4 /Bi2 O3

Co3 O4 /MCM41

Co3 O4 /MgO (A); Co3 O4 /ZnO (B); Co3 O4 /ZrO2 (C) Co3 O4 /SBA-15

Co3 O4 /graphene oxide (GO) Co3 O4 /GO

Co3 O4 /TiO2

Co3 O4

Co3 O4 /MgO/SBA-15

CO2 O3 /activated carbon (AC) Co/resin

Co/ZSM-5

CoFe2 O4

Atrazine

2,4-DCP, MB, Phenol

Caffeine

Methylene blue (MB)

Orange II

Rhodamine B (RhB)

Amoxicillin

Rhodamine B (RhB)

Phenol

Monuron

Phenol

2,4-DCP

Orange II

Phenol

Co3 O4 /TiO2 (A); Co3 O4 /Al2 O3 (B); Co3 O4 /SiO2 (C)

Phenol

Catalyst, 0.1 g/L; PMS, 0.5 g/L; phenol, 50 mg/L; 25 ◦ C; pH, 7.0 Catalyst, 0.1 g/L; PMS, 2 mM; Orange II, 0.2 mM; pH, 7.0 Catalyst, 0.1g/L; PMS, 2 mM; Orange II, 0.2 mM; pH, 7.0 Catalyst, 1.0 g/L; RhB, 0.1 mM; PMS, 0.4 mM; pH, 6.9 Catalyst, 0.06 g; amoxicillin, 0.12 mM; PMS, 0.01 M; pH, 6.0; 60 ◦ C Catalyst, 0.1 g/L; RhB, 5.0 mg/L; PMS, 50 mg/L; 25 ◦ C Catalyst, 0.2 g/L; PMS, 2 g/L; phenol, 25 ppm; 25 ◦ C Catalyst, 0.5 g/L; monuron, 0.2 mM; PMS, 1 mM Catalyst, 0.4 g/L; PMS, 2 g/L; phenol, 25 ppm; 25 ◦ C Catalyst, 0.1 g/L; 2,4-DCP, 50 mg/L; PMS, 150 mg/L

Catalyst, 0.2 g/L; PMS, 0.2 mM; caffiene, 0.05 mM Catalyst, 0.5 g/L; PMS, 0.5 mM; MB, 40 ppm

Catalyst, 20 mg; PMS, 0.1 mM; atrazine, 0.1 mM; 419 nm center light Catalyst, 0.05 g/L; PMS, 0.5 mM; MB, 20 μM; 2,4-DCP, 25 mg/L; phenol, 25 mg/L

Catalyst, 0.5 g/L; PMS, 2 g/L; phenol, 25 ppm; 25 ◦ C

RE, ∼61% (2 hr)

LMCo , [112] ∼0.03 mg/L (Continued on next page)

[111]

RE, ∼98% (6 hr)

[132]

[131]

[110]

LMCo , 80 ug/L RE, 100%; RETOC , 60% (60 min) RE, ∼89% (45 min)

RE, 100% (5 min)

[130]

[129]

RE, ∼60% (dark, 1 hr); RE, 100% (light, 1 hr) RECOD , 91.0% (45 min)

[109]

[108]

[107]

[106]

[105]

[128]

A, LMCo , ∼0.1 mg/L (30 min) LMCo , MnO2 . Although deactivation occurred on Mn2 O3 for multiple uses, the activity could be fully recovered by high-temperature calcination.119 Different crystallographic phases of MnO2 nanomaterials were also synthesized by Saputra et al. α-MnO2 presented as nanowires and showed the highest activity in activation of Oxone for phenol degradation due to high surface area, oxygen loss, and double-tunneled structure.120 In the later research work, the same group also prepared different spinel structured metal oxides, Mn3 O4 , Co3 O4 , and Fe3 O4 , and compared the catalytic performances in phenol oxidation. Mn3 O4 /PMS system exhibited similar efficiency to Co3 O4 /PMS, presenting fast and complete phenol removal in 30 min. In contrast, Fe3 O4 /PMS system presented much low degradation of phenol at less than 15% removal in 90 min.121


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Muhammad et al. loaded RuO2 on activated carbon (AC) and ZSM-5, which are effective catalysts for the activation of PMS for the production of sulfate radicals for phenol degradation. RuO2 /activated carbon had better catalytic performance of removing phenols, which was a combination process of oxidation and adsorption.122 Ji et al. synthesized CuO particles by a hydrothermal method and tested it as a heterogeneous catalyst for phenol oxidative degradation using PMS as oxidant. Their results showed that the prepared CuO catalyst exhibited an excellent catalytic activity and could react with PMS to generate more radicals to degrade organic molecules. However, commercial CuO was found to be ineffective in degrading phenol.123 The group of Wang found that reduced graphene oxide124 and structure defective graphene125 were able to effectively activate PMS to produce active sulfate radicals. The activity of graphene was not only higher than other carbon materials, such as graphite powder, carbon nanotube, and graphene oxide, but also superior to popular transition metal oxides, Co3 O4 . They also found that AC, as an alternative to graphene and metal-based catalysts, was effective in heterogeneous activation of PMS to produce sulfate radicals for the degradation of phenol, much better than H2 O2 and PS.126

3.2. Degradation of Contaminants by Sulfate Radical in Heterogeneous Activation Persulfate Systems Most researches about heterogeneous catalysts for PS activation focused on iron compounds. As mentioned earlier, Fe(II) may not be an ideal activating agent since excess quantities of Fe(II) can act as a radical and PS scavenger. Therefore, it is necessary to implement measures to prevent the rapid conversion of Fe(II) to Fe(III) and the scavenging of free radicals. As the alternation of sequential Fe(II) additions or use of chelating agents, zero valent iron (ZVI, Fe0) could serve as a slow-releasing source of dissolved ferrous ions, which would activate PS to produce sulfate radical, as described in the following reaction equations: 2Fe0 +O2 +2H2 O → 2Fe2+ +4OH−

(15)

Fe0 +2H2 O → Fe2+ +2OH− +H2

(16)

2− 2+ Fe0 +S2 O2− 8 → Fe +2SO4

(17)

Fe +2Fe

(18)

0

3+

→ 3Fe

2+

According to Reaction (18), the generation of ferrous iron and recycling of ferric iron at the ZVI surface can avoid the accumulation of excess ferrous iron and reduce the precipitation of iron hydroxides. Additionally, ZVI is inexpensive and environmental friendly, thus providing a cost-effective waste treatment option.

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ZVI ZVI ZVI ZVI ZVI

p-chloroaniline

Bisphenol A (BPA)

Sulfamethoxazole

Methyl orange

Reactive Blue 19 (RB19) Perfluorooctanoic acid

2,4,4 Trichlorobiphenyl (PCB28)

Sulfamonomethoxine (SMM) Tetracycline

ZVI

4-chlorophenol

Fe3 O4

Fe3 O4

Fe3 O4

ZVI

ZVI

ZVI

ZVI

Zero valent iron (ZVI)

Catalyst

Polyvinyl alcohol (PVA) 2,4-dinitrotoluene

Trichloroethylene (TCE) Naphthalene

Contaminant ZVI, 2 g/L; PS, 8.4 mM; TCE, 0.75 mM ZVI, 1 g/L; PS, 10 g/L; naphthalene, 10 mg/L ZVI, 250 mg/L; PS, 250 mg/L; PVA, 47.1–50.2 mg/L ZVI, 2 g/L; PS, 250 mg/L; 2,4-dinitrotoluene, 50 mg/L ZVI, 0.2 g/L; PS, 0.78 mM; 4-chlorophenol, 0.0156 mM ZVI, 0.7 g/L; PS, 2.5 mM; p-chloroaniline, 0.05 mM; pH, 4.0; 25 ◦ C ZVI, 0.167 g/L; BPA, 80 μM; PS, 2 mM ZVI, 2.23 mM; PS, 1.0 mM; sulfamethoxazole, 39.5 μM ZVI, 0.05 g/L; PS, 40 ppm; methyl orange, 10 ppm; pH, 3 ZVI, 0.8 g/L; PS, 10 mM; RB19, 0.1 mM; pH, 7.0; 30 ◦ C ZVI, 3.6 mM; PS, 5 mM; perfluorooctanoic acid, 240.7 μM; 90 ◦ C Fe3 O4 , 2.4 mM; SMM, 0.06 mM; PS, 1.2 mM Fe3 O4 , 1.0 g/L; tetracycline, 100 mg/L; PS, 200 mM Fe3 O4 , 1.0 g/L; PCB28, 2.5 μM; PS, 2.0 mM; pH, 7.0

Condition

[141]

RE, ∼100% (50 min) RE, 100%; RETOC , 54% (45 min) RE, 67.6% (2 hr)

RE, ∼90% (4 hr)

RE, 50.5% (1.5 hr)

RE, 100% (15 min)

[140]

RE, ∼95% (1 hr)

(Continued on next page)

[146]

[145]

[144]

[143]

[142]

[139]

RE, 100% (30 min)

[138]

RE, 100% (12 min)

[136]

[135]

[134]

[133]

Reference

[137]

Remarks

RE, 88% (1 hr)

RE, 91% (5 hr)

RE, 100% (2 hr)

RE, ∼100% (17 min) RE, >99% (3 min)

Efficiency

TABLE 4. Degradation of contaminants by sulfate radical in heterogeneous activation persulfate systems


1776 AC

Perfluorooctanic acid (PFOA)

RE, removal efficiency; LM, leaching of metals.

AC

Activated carbon (AC)

Pyrite (FeS2 )

Acid Orange 7 (AO7)

Methyl tert-butyl ether (MTBE) Trichloroethylene

Polyhydroquinone/Fe3 O4

Core-shell Fe-Fe2 O3

Methyl orange

RhB

Catalyst

Contaminant Fe-Fe2 O3 , 0.2 mM; methyl orange, 50 mg/L; PS, 0.2 mM; pH, 6.0 Catalyst, 0.15 g/L; RhB, 0.02 mM; PMS, 12 mM; 20 ◦ C; pH, 3.98 FeS2 , 3 g/L; MTBE, 60 mg/L; PS, 5 g/L; 20 ◦ C AC, 3 g/L; trichloroethylene, 100 mg/L; PS, 2 g/L AC, 1.0 g/L; AO7, 20 mg/L; PS, 2 g/L AC,10 g/L; PFOA, 120.6 μM; PS, 60.3 mM

Condition

[152]

[151]

RE, ∼86% (5 hr) RE, 54.9% (12 hr)

[150]

RE, 30% (12 hr)

[149]

RE, 100% (4 hr)

[147]

Reference

[148]

LMFe , ∼140 mg/L

Remarks

RE, ∼99% (1 hr)

RE, 90% (10 min)

Efficiency

TABLE 4. Degradation of contaminants by sulfate radical in heterogeneous activation persulfate systems (Continued)




1777

Liang and Lai first used ZVI as a source for ferrous ion-activated PS oxidation of TCE. In the TCE/ZVI/PS system, the rapid TCE degradation was accompanied by the fast PS decomposition and chloride ion formation as evidence of TCE mineralization. Scanning electron microscope images of ZVI before and after PS oxidation exhibited significant corrosions.133 In the later researches, ZVI-activated PS oxidation was also proved to be effective for degradation of naphthalene,134 polyvinyl alcohol,135 2,4-dinitrotoluene,136 4-chlorophenol,137 p-chloroaniline,138 bisphenol A,139 sulfamethoxazole,140 and dyes.141,142 Researching the microwave–hydrothermal decomposition of persistent perfluorooctanoic acid (PFOA), Lee et al. found that introducing ZVI into the PFOA solution with PS addition would lead to synergetic effect that accelerated the PFOA decomposition rate and reduced the reaction time.143 Although the catalytic ability of Fe3 O4 nanoparticles for PMS activation is generally low, they exhibit excellent catalytic activities to activate PS. Yan et al. synthesized Fe3 O4 nanoparticles by a modified reverse co-precipitation process. They found that reactive free radicals generated through Fe3 O4 nanoparticles mediated the activation of PS, leading to immediate degradation of sulfamonomethoxine. The addition of Fe3 O4 nanoparticles in several batches for a given total amount of the activator was favorable to enhancing the degradation efficiencies.144 Hou et al. used commercial Fe3 O4 to activate PS for removing tetracycline. The degradation efficiency of 50.5% was achieved after 1.5 hr. Ultrasound can increase the decomposition to 89% because there is a clear synergetic effect between the sonochemical and catalytic degradation of tetracycline. According to their radical scavenger experiments with the quencher 1,4-benzoquinone, superoxide radical (O2 .−) could be formed during the degradation processes.145 Interestingly, when Fang et al. conducted the experiments about the activation of PS by Fe3 O4 nanoparticles for the efficient degradation of 2,4,4 -trichlorobiphenyl, they also proved the existence of superoxide radical using electron spin resonance (ESR) technique, and the conclusion was further confirmed by quenching studies with the addition of superoxide dismutase. Increasing the oxygen concentration in the reaction solution favored the generation of O2 .− as well as the degradation efficiencies. Therefore, they proposed that superoxide radical generated by Fe3 O4 nanoparticles could activate the PS to produce more sulfate radicals as shown in the following reaction, which improved the 2,4,4 -trichlorobiphenyl degradation efficiencies.146 .− .− 2− S2 O2− 8 +O2 → O2 +SO4 +SO4

(19)

Other iron compounds were also used as heterogeneous catalysts for PS activation. Zhu et al. synthesized core-shell Fe-Fe2 O3 nanostructures by reducing the Fe3+ ions using sodium borohydride. This core-shell FeFe2 O3 nanomaterial showed much higher degradation efficiencies of methyl

1778

B.-T. Zhang et al.

orange comparing with the PS oxidation processes using Fe2+.147 The polyhydroquinone/Fe3 O4 (PHQ/Fe3 O4 ) was synthesized by Leng et al. as a heterogeneous catalyst to activate PS to effectively degrade Rhodamine B. PHQ/Fe3 O4 showed better catalytic performance than PHQ and Fe3 O4 owing to the role of quinone assisting the redox cycling of Fe.148 Liang et al. used pyrite (FeS2 ) as the ferrous ion source in activating PS to generate sulfate-free radicals as illustrated in the following reactions. 2− 2+ + 16H2 O + S2 O2− 8 +2FeS2 → 2Fe +32H +34SO4


.− 2− 2+ S2 O2− 8 +FeS2 → Fe +2S + SO4 +SO4

(20) (21)

MTBE was completely degraded when given sufficient doses of FeS2 and S2 O8 2− and sufficient reaction time.149 It has been suggested that AC containing oxygen functional groups may act as a catalyst of the electron-transfer mediator. Therefore, AC also could generate catalytic decomposition of PS with releases of organic radicals and sulfate radicals for the contaminant destruction according to the following reactions. .− − ACsurface − OOH + S2 O2− 8 → ACsurface − OO · +SO4 +HSO4

ACsurface − OH +

S2 O2− 8

→ ACsurface − O ·

− +SO.− 4 +HSO4

(22) (23)

After AC was oxidized by PS or radical, the point of zero charge, surface area, and adsorption capacity of AC reduced. However, the increased number of acidic groups on the AC surface could activate PS to destroy TCE,150 AO7,151 and perfluorooctanic acid152 during the course of simultaneous adsorption and oxidation reactions.

4. APPLICATION OF SULFATE RADICAL IN OTHER DECONTAMINATION TECHNOLOGIES 4.1. Hydrothermal Oxidation Hydrothermal oxidation treatment is the oxidation process of wastewaters at high pressure and temperature, which is an efficient alternative to conventional methods, such as biological treatments and common chemical processes, in the treatment of highly contaminated wastewaters. Wet air oxidation (WAO) and supercritical water oxidation (SCWO) are two forms of hydrothermal oxidation, which are operated at conditions below or above the critical point of water, respectively.153 Rivas et al. used two inorganic peroxides, hydrogen peroxide and PMS, as radical promoter to treat landfill in promoted WAO. Sulfate radical promoter, PMS, exhibited higher efficiency, and COD removal can be obtained in the proximity of 60% in comparison to the 20% achieved in unpromoted



1779

WAO and the 35% achieved in H2 O2 promoted WAO.154 The same research group applied the same promoted WAO system to oxidize polycyclic aromatic hydrocarbons and found that addition of free radical promoters (either hydrogen peroxide or PMS) could enhance the process by reducing the reaction time and the working temperatures and pressures.155 In most cases, the presence of catalysts (catalytic wet air oxidation, CWAO) resulted in higher oxidation efficiencies than those obtained in the simple WAO process at similar operating conditions. High treatment efficiency and low level of secondary pollution make the CWAO process attractive in treating high concentration and refractory organic pollutants. Xu et al. utilized K2 S2 O8 as the promoter to aid the AC-catalyzed WAO degradation of fulvic acid, which was carried out in a 50 mL of Teflon-lined stainless steel autoclave equipped with a magnetic stirrer. Almost complete fulvic acid conversion and 77.8% COD removal were achieved after 4 hr of treatment at 150 ◦ C and 0.5 MPa oxygen pressure. The biodegradability of BOD5 /COD ratio increased from 0.13 of raw fulvic acid solution to 0.95 after the CWAO process. Hydroxyl and sulfate radicals played major roles in the fulvic acid degradation according to radical scavenger experiments.156 When oxidation takes place under supercritical conditions (T c = 374.2 ◦ C, P c = 22.1 MPa for water), the technique is referred to as SCWO. Below the critical temperature or pressure, water is in the subcritical state. Kronholm et al. used potassium PS as oxidant in subcritical or SCWO in a continuous flow system to remove different phenols with good oxidation efficiencies. Potassium PS was clearly more efficient than hydrogen peroxide in oxidizing the model compounds, and TOC was removed more effectively by potassium PS under mild conditions.157–159 The same research group developed an online coupled PHWE (pressurized hot water extraction)–SCWO equipment (Figure 3) to extract polycyclic aromatic hydrocarbons (PAHs) from soil samples and then to destruct them with potassium PS as oxidant. This apparatus was capable of safe and effective extraction of organic compounds from the soil, whose recoveries were better than those obtained using Soxhlet extraction. Almost 100% PAH conversions were obtained, and the TOC content was clearly decreased under optimized conditions.160

4.2. in Situ Chemical Oxidation Since the first report was published in the 1990s, in situ chemical oxidation (ISCO) has been a rapidly growing research field for the remediation of contaminated sites, such as soils and groundwater, because this technology can potentially treat source zones more rapidly than other remediation processes. ISCO is a method based on the delivery of strong chemical oxidants to the subsurface for treating contaminants by converting them to substances such as carbon dioxide, water, and inorganic acids. ISCO is gaining increased


1780

B.-T. Zhang et al.

FIGURE 3. PHWE (pressurized hot water extraction)–SCWO (supercritical water oxidation) equipment. The reaction tube was flushed from the T junction. PT, preheating tube.160 C Royal Society of Chemistry. Reproduced by permission of Royal Society of Chemistry. Permission to reuse must be obtained from the rightsholder.

popularity for site remediation, with up to 50% of new cleanup efforts being ISCO based for some companies. The major oxidants used in soil and groundwater remediation are permanganate, ozone, hydrogen peroxide, and PS. Destruction of organic compounds by direct or activated PS oxidations of soil or groundwater has lots of advantages over other oxidants (more stable than H2 O2 in the subsurface, more widespread reactivity than KMnO4 , and more easily migrating to pollutant zones than O3 ).161 The most common methods for applying sulfate radicals in ISCO technology are heat and iron activation of PS. Thermal activation is technically feasible using heating technologies, such as radio frequency heating, electrical resistance heating, or steam injections, to warm up selected contaminated zones, which has been previously dosed with PS.9 This method is effective at initiating the production of sulfate radicals and has the advantage that many of the volatile organic compounds are thermally degraded. Care must be taken to ensure that contaminants do not volatilize into the soil gas and migrate off-site before degradation.161 Chelating reagents are necessary to increase Fe(II) solubility under neutral conditions and reduce the radical scavenging, as discussed in Section 2. Sulfate radical-based activated PS has been shown to be efficient at removing a range of common soil and groundwater contaminants, such as MTBE,162,163 petroleum hydrocarbons (diesel and fuel oil),164 16 PAHs,23 benzene, toluene, ethylbenzene, and xylenes (BTEX)89, 165, and chlorinated solvents.166 Successful application of ISCO at a hazardous waste site requires understanding the scavenging reactions that could take place at the site and identification of site conditions where ISCO using PS may not be applicable. Liang



1781

FIGURE 4. Comparison of 1,1,1-trichloroethane (TCA) degradation in soil systems at 60 ◦ C. foc, fraction of organic carbon.9 C Taylor & Francis. Reproduced by permission of Taylor & Francis. Permission to reuse must be obtained from the rightsholder.

et al. found that higher temperatures, higher PS dosages, and longer treatment times were required to achieve TCE and TCA degradation in soil systems than in aqueous systems. Soil constituents (e.g., fraction of organic carbon, foc) appeared to compete for sulfate radical because increased system efficiency is most likely to occur within soils with low foc levels, as shown in Figure 4.9 Liang et al. also found that the TCE degradation rate reduced when chloride levels greater than 0.2 M or an increase concentration of carbonate species at elevated pHs.167 The group of Watts investigated the potential for 13 naturally occurring minerals to mediate the decomposition of PS and generate reactive species. Only four of the minerals (cobaltite, ilmenite, pyrite, and siderite) promoted the decomposition of PS at a concentration of 28.6% (w/w). However, these minerals in the natural soil studied did not show the same reactivity, most likely due to the lower masses of the metal oxides in the soil.168,169 Do et al. found that PS/Fe(II) system degraded diesel in soil more effectively than in sand. Metal oxides in the soil matrix could activate PS and the high reactivity of PS could be due to the high density of surface-bound Fe(II) on manganese oxide.170 Crimi and Taylor compared the efficiencies of different activation PS systems in five field soil spiking BTEX and found that response of the contaminants and oxidant (extent and rate of depletion) are both contaminant specific and porous media type dependent.171 Sulfate radical-based ISCO has already been applied in remediation projects at several contaminated sites, which was reviewed by Tsitonaki et al.172 However, most of their references about this ISCO field application are conference reports, rather than peer-reviewed publications which are almost based on laboratory studies. This would hinder the use of experiences

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B.-T. Zhang et al.

to build protocols for optimizing remediation design practices and avoid common pitfalls.


4.3. Air Pollution Control Technologies A wet scrubber is the air pollutant control device used to clean exhaust contaminant vapors by dissolving and/or mixing the contaminants into a liquid medium. Since most volatile organic compounds have low water solubility characteristics, the wet scrubber method is rarely used as the primary treatment method in on-site remediation of exhaust gases. However, chemical oxidizing agents have been used as absorbent solutions to promote the rapid oxidation of contaminants in the aqueous phase, which increases absorption and destruction efficiencies in the scrubber. Liang et al. evaluated the use of citric acid chelated Fe(II)-activated PS chemical oxidation in conjunction with a wet scrubbing system, a PS oxidative scrubber (POS) system, to destroy BTEX gases. The POS system resulted in a BTEX removal efficiency of around 50%, which is superior to using either water or only PS as a scrubber solution.173 In order to develop cost-effective wet scrubber-based technologies for simultaneously controlling multipollutant from coal-fired plants, the research group of Adewuyi used aqueous sodium PS activated by heat and/or Fe2+ in a bubble reactor for absorption and oxidation of nitric oxide and sulfur dioxide. The absorption of NO by PS solution was dependent on the reaction of dissolved NO with the reactive radicals generated by the activation of PS. In general, NO conversion increased with increasing temperature and PS concentration.174 The presence of SO2 dramatically improved NO gas absorption and oxidation, while SO2 itself was completely removed.175 About 0.01 M Fe2+ activation further improved the NO conversion rate by about 10% at all temperatures.176 The authors proposed that this technology could make wet flue gas desulfurization scrubbers more cost-effective and obviate the need to install additional costly control equipment, such as selective catalytic reduction systems. Different oxidizing agents, with the standard potential higher than 1.2 V, were tested to remove mercury using oxidative membrane gas absorption at laboratory scale. Freshly prepared Na2 S2 O8 /AgNO3 solutions gave mass transfer coefficients of up to 8 × 10−3 m/s, indicating that the reaction with mercury at the interface was instantaneous.177 Xu et al. used a glass bubble column reactor to investigate the aqueous phase oxidation of gaseous elemental mercury (Hg0) by potassium PS catalyzed by Ag+ and Cu2+. Ag+ provided a better catalytic effect than Cu2+ and the mercury removal efficiency could reach 97.0% at a condition of 0.3 mM Ag+ and 5.0 mM KPS, and just 81.9% for Cu2+. Hg0 oxidation can be achieved simultaneously via two different processes, direct oxidation by K2 S2 O8 and indirect reaction by free radicals.178

1783


TABLE 5. Second-order rate constants for reactions of selected chemical probes with hydroxyl and sulfate radicals179 Reaction rate constant(M−1 s−1)


Radical probes Anisole Benzoic acid Benzene Ethanol Methanol Nitrobenzene Propanol Phenol tert-butyl alcohol, TBA

SO4 .−

·OH

4.9 × 109 1.2 × 109 (2.4–3) × 109 (1.6–7.7) × 107 3.2 × 106