Oxidative Degradation of Organic Compounds Using Zero-Valent Iron

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Environ. Sci. Technol. 2009, 43, 878–883

Oxidative Degradation of Organic Compounds Using Zero-Valent Iron in the Presence of Natural Organic Matter Serving as an Electron Shuttle

SCHEME 1. Conceptual Illustration of NOM-Mediated Oxidation in Zero-Valent Iron (ZVI) System

SEUNG-HEE KANG AND WONYONG CHOI* School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea

Received June 20, 2008. Revised manuscript received October 18, 2008. Accepted November 12, 2008.

This study aims to understand the oxidative degradation of organic compounds utilizing zerovalent iron (ZVI) which is further promoted by the presence of natural organic matters (NOMs) (as humic acid (HA) or fulvic acid (FA)) working as electron shuttle mediators. The main target substrate used was 4-chlorophenol. Both HA and FA can mediate the electron transfer from the ZVI surface to O2, while enhancing the production of Fe2+ and H2O2 that subsequently initiates the OH radicalmediated oxidation of organic compounds through Fenton reaction. The electron transfer-mediating role of NOMs was supported by the observation that higher concentrations of H2O2 and ferrous ion were generated in the presence of NOM. The NOMinduced enhancement in oxidation was observed with NOM concentrations ranging 0.1-10 ppm. Since the reactive sites responsible for the electron transfer action are likely to be the quinone moieties of NOMs, benzoquinone that was tested as a proxy of NOM also enhanced the oxidative degradation of 4-chlorophenol in the ZVI suspension. The NOM-mediated oxidation reaction on ZVI was completely inhibited in the presence of methanol, an OH radical scavenger, and in the absence of dissolved oxygen.

Introduction Over the past decade, there has been a great deal of interest in the degradation of aquatic contaminants with a new treatment method utilizing reactive zerovalent iron (ZVI). ZVI has been applied to dehalogenation of halogenated organic compounds (1-4), and the reduction of nitroaromatic compounds (5) and inorganic contaminants (6, 7). Most ZVIinduced transformation reactions are reductive, which are initiated by the direct electron transfer from metallic iron to substrates (S) with the oxidative iron corrosion accompanied (reaction 1). Fe0+Sox f Fe2++Sred E0(Fe2+⁄Fe0) ) -0.447 VNHE

(1)

Iron can be also utilized to drive oxidative reactions in the presence of H2O2 that generates OH radical through Fenton reaction (8-10). Recently, it has been demonstrated that the reduction of O2 occurring on Fe0 surface leads to OH radicalinduced oxidation in the absence of added H2O2 (11-14) via the reactions 2 and 3. * Corresponding author phone: +82-54-279-2283; fax: +82-54279-8299; e-mail: [email protected]. 878

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Fe0+O2+2H+ f Fe2++H2O2

(2)

Fe2++H2O2 f Fe3+ + HO • +HO-

(3)

The corrosive oxidation of Fe0 to Fe2+ can be accompanied by the two-electron transfer to O2 with generating H2O2 (E0(Fe2+/Fe0) ) -0.447 vs E0(O2/H2O2) ) +0.695 VNHE (15)). That is, the Fenton reagent is generated in situ in the ZVI suspension. Through this process, ZVI system can take advantage of both reductive and oxidative reactions, which makes the ZVI technology more versatile. Since both the reductive and oxidative reactions are initiated by the electron transfer on Fe0 surface, enhancing the electron-transfer rate increases the overall process efficiency. This group recently demonstrated that the oxidation reactions of organic compounds on ZVI can be accelerated by the presence of polyoxometalate (POM), a metal-oxygen cluster anion, serving as an electron shuttle (POM f POM- f POM) between ZVI surface and O2 (13). Although POM was employed as a model electron shuttle, it is hardly practical in the real-world applications. Natural organic matters (NOMs), ubiquitously present in soils, surface water, and groundwater, have many redoxactive functional groups. Although the accumulation of NOMs at the iron-water interface may adversely affect the overall process by blocking the active surface sites (7, 16-18), it may also act as an electron-transfer mediator in the ZVI process. Scheme 1 illustrates the proposed role of NOM as an electron shuttle. In a similar way, NOM, by serving as an electron transfer mediator, enhanced the reduction rate of nitroaromatic or halogenated compound with sulfide or ferrous ions as an electron donor (19, 20). The electron-transfer property of NOMs has been related to the role of quinone moieties (19, 20). The effects of NOMs on Fenton and ZVI processes, however, still remain unclear since the published results are often conflicting. Several researchers observed that NOM inhibited (21, 22) or had no significant effect (23) on the degradation of organic pollutants by Fenton or Fenton-like reactions where target substrates bound to NOM become less reactive and NOM blocks the reaction between OH radical and substrates. Conversely, several other authors reported that NOM increased the oxidation efficiencies in Fentonlike systems (24-27). The redox potential of Fe(II)/Fe(III) can be dramatically altered when complexed with NOM (28). As for ZVI-mediated reductive degradation of pollutants, Tratnyek and co-workers (18) observed an inhibitory effect of NOM in ZVI-induced reduction of CCl4 and trichloroethylene, however they found that some quinone model compounds enhanced the reduction rate, possibly by serving as an electron shuttle. Giasuddin et al. (16) reported that the ZVI-mediated removal of As(III) and As(V) was reduced in the presence of humic acid because humic acid sorbed onto ZVI surface occupied the available sorption sites for arsenic. Doong and Lai (29, 30) observed the dual roles of humic acids acting as both an inhibitor to compete for the reactive sites with tetrachloroethylene and an electron shuttle to effectively accelerate the dechlorination efficiency in the ZVI 10.1021/es801705f CCC: $40.75

 2009 American Chemical Society

Published on Web 12/29/2008

system. Liu et al. (17) have reported that Cr(VI) reduction was inhibited in the presence of humic acid. It seems that the effects of NOMs on the reactivity of ZVI highly vary depending on the kind of reaction systems. In this study, the effects of humic acid and fulvic acid on the ZVI-induced oxidation of 4-chlorophenol as a model organic substrate were investigated. The oxidation rate was indeed accelerated in the presence of humic or fulvic acid that plays the role of an electron shuttle. The electron shuttling role of NOMs in ZVI-oxidation process was investigated and discussed in detail.

Experimental Section Chemicals and Materials. Chemicals used in this study include 4-chlorophenol (4-CP, Sigma), 2-(4-chlorophenoxy)2-methylpropionic acid (or clofibric acid, Aldrich), 1,4benzoquinone (BQ, Aldrich), sodium chloride (Samchun), hydrochloric acid (Samchun), sulfuric acid (Junsei), methanol (J.T.baker), 1,10-phenanthroline (Aldrich), iron(II) perchlorate hydrate (Aldrich), sodium acetate (Merck), N,N-dethyl-1,4phenylene-diamine (DPD, Aldrich), peroxidase (Aldrich, type VI-A from horseradish 1500 unit/mg solid), 2,2′-bipyridyl (Kanto Chemical), ethylenediaminetetraacetic acid disodium salt dehydrate (Na2EDTA, Aldrich), NaHPO4 (Aldrich), and Na2HPO4 · 7H2O (Aldrich), all of which were of reagent grade and used as received. Suwannee River fulvic acid (FA) and humic acid (HA) were purchased from the International Humic Substances Society (http://www.ihss.gatech.edu). Stock solutions of HA and FA in water were prepared fresh every week and stored in a refrigerator. Deionized water used was ultrapure (18 MΩ · cm) and prepared by a Barnstead purification system. Iron powder (100 mesh) was purchased from Fischer Scientific. Experimental Procedure. Iron powder was pretreated with 1 M HCl for 5 min to remove surface impurities. The iron slurry was then washed with degassed water (nitrogen purged for 1 h) three times. Typical iron powder slurry in water was prepared at a concentration of 0.2 g/L. Aliquots of 4-CP stock solution (2 mM) and HA/FA or BQ (100 ppm) were then added to make a desired concentration (typically 0.1 mM 4-CP and 0.5 ppm HA/FA or BQ). The iron suspension was unbuffered and air-equilibrated. The initial pH of the slurry was adjusted to 2.5 with HClO4 (not HCl) solution because chloride ions generated from the degradation of 4-CP were measured. The possibility of chloride production from HClO4 degradation was ruled out because no chlorides were generated in the absence of 4-CP. Although HClO4 may act as an oxidant, the control test of HClO4+4-CP reaction in the absence of ZVI showed no degradation of 4-CP in the present experimental condition. Use of HNO3 as a pH-adjusting reagent was avoided because nitrate can be reduced by ZVI. The reaction was carried out in a 120 mL glass bottle that was stirred on a rotary shaker, and the reactor was exposed to air to continuously provide dissolved oxygen. Sample aliquots of 1 mL were withdrawn at a regular time interval from the reactor with a syringe, passed through a 0.45 µm PTFE filter (Millipore), and injected into a 2 mL vial containing 10 µL sodium sulfite (Na2SO3, 1 M) to quench residual H2O2 that might be generated in situ in the reactor. Multiple experiments were carried out for a given condition. Analysis. Quantitative analysis of 4-CP and clofibric acid were done by using a high performance liquid chromatograph (HPLC Agilent 1100) equipped with C-18 column (Agilent Zorbax 300SB) and a diode-array detector. The eluent consisted of a binary mixture of 0.1% phosphoric acid and acetonitrile (8:2 or 6:4 by volume). Quantification of chloride production from the degradation of 4-CP and clofibric acid was performed by using an ion chromatograph (IV, Dionex DX-120). The IC system was equipped with a Dionex IonPac

AS-14 (for anion analysis) and a conductivity detector. The fluorescence spectra of HA and FA were recorded using a Shimadzu RF-5301PC spectrofluorophotometer with excitation wavelength of 279 nm. The colorimetric determination of ferrous ion and H2O2 concentration was done using a UV-visible spectrophotometer (Agilent 8453E). Ferrous ion concentrations were measured spectrophotometrically (absorbance measurement at 510 nm) by the 1,10-phenanthroline method (31). The Fe(II) concentrations were calibrated both with and without HA/FA although the presence of HA/FA little influenced the analytical results. H2O2 that was generated in situ in the ZVI suspensions was analyzed with the colorimetric DPD method (32), which was further modified to minimize the interference by Fe(II) and Fe(III) (27). Bipyridine (0.01 M in 1 mM HClO4) was added to complex Fe(II) and EDTA (0.3 M) to complex Fe(III). After the addition of DPD and other reagents, the absorbance was measured at 551 nm. The determination of H2O2 concentration was little affected by the presence of HA/FA.

Results and Discussion Effect of NOMs on ZVI-Induced Oxidative Degradation. We selected 4-CP as the main target substrate because the OH radical-induced degradation of 4-CP is well documented in the literature, and our previous study demonstrated that 4-CP is degraded via the OH radical-mediated pathway in the ZVI system (13). Figure 1a shows that the presence of HA or FA clearly enhanced the degradation of 4-CP despite the fact that HA/FA may cover active surface sites on iron particles or may scavenge OH radicals in competition with 4-CP. With NOM alone in the absence of ZVI, the removal of 4-CP was completely absent. The NOM/ZVI system showed no reactivity in the absence of dissolved oxygen in N2-sparged suspension. These observations rule out the possibility of the direct reaction or complexation-mediated aggregation between 4-CP and NOM under the ambient experimental condition. The concurrent production of chloride indicates that 4-CP removal was not due to the adsorption on ZVI surface, but to the degradation. The chloride production was also enhanced in the presence of HA/FA. The chloride production mechanism in the oxidative degradation of 4-CP through the OH radical-initiated reaction is well documented in the literature (33). The oxidative degradation rate depended on [NOM] (as shown in Figure 1b) and the concentration dependency was different between HA and FA. HA exhibited the optimal effect at 0.5 ppm and the further increase in [HA] slightly reduced the enhancement effect. On the other hand, FA-mediated enhancement increased up to 2.0 ppm beyond which the activity was saturated. HA and FA were similar in their enhancement effect at [NOM] < 1.0 ppm, but FA was more effective than HA at [NOM] > 1.0 ppm. Throughout the entire NOM concentration range (up to 10 ppm) tested in this study, the presence of HA or FA enhanced the oxidation rate of 4-CP. The NOM-enhanced oxidation was also observed with clofibric acid (a herbicide component with a chlorophenoxy group) as shown in Figure 1c. As in the case of 4-CP, both the removal of clofibric acid and the production of chloride were enhanced in the presence of HA/FA. The degradation of clofibric acid was completely inhibited in the absence of O2. Figure 2 shows the repeated runs of 4-CP degradation in the reused ZVI slurry with or without FA. The oxidative capacity of ZVI slurry was gradually reduced in the repeated runs both in the presence and absence of FA. Although FA was newly added at the beginning of each cycle, the FAenhanced oxidation effect decreased as the reaction cycle was repeated. The gradual deactivation is possibly ascribed to the passivation of iron surface by either oxide layer formation or surface accumulation of NOMs as the ZVI is reused. It seems that the FA-enhanced oxidation effect is VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Repeated runs of 4-CP degradation in the same batch of ZVI slurry with or without FA ([ZVI] ) 0.2 g/L; [4-CP]0 ) 100 µM; [FA]0 ) 0.5 ppm). 4-CP and FA were newly added and pH was readjusted to 2.5 at the beginning of each cycle. dechlorination was detected at all by the HPLC analyses. In addition, the degradation of 4-CP in the ZVI/NOM slurry was completely inhibited in the absence of dissolved oxygen (N2-sparged condition). This indicates that the main degradation reaction in the ZVI/NOM system is initiated by the reactive oxygen species, not by the activated NOM species that may result from the direct redox reaction between NOM and ZVI. Another possibility is that the generation of reactive oxygen species is resulted from the redox reaction between O2 and dissolved iron species since NOM-complexed ferrous ions have a higher reduction potential than aqua-complexed ones (E(Fe(III)-NOM/Fe(II)-NOM) ) -0.2∼0.3 VNHE (28) vs E0(Fe3+/ Fe2+) ) 0.77 VNHE]). It was also reported that Fe(II)-FA complex reacted more rapidly with H2O2 than Fe(II)-aquo complex (27). Such a homogeneous reductive path (reactions 4-6) may also lead to the generation of OH radicals (34, 35). [Fe(II)-NOM] + O2 f [Fe(III)-NOM] + O2-

(4)

+

-

[Fe(II)-NOM]+ O2 + 2H f [Fe(III)-NOM] + H2O2 (5) [Fe(II)-NOM] + H2O2 f [Fe(III)-NOM] + HO • +HO- (6) FIGURE 1. (a) Degradation of 4-CP and Cl- production in ZVI slurry in the presence or absence of 0.5 ppm HA (or 0.5 ppm FA). (b) Effect of the concentration of HA and FA on the degradation rate of 4-CP. (c) Degradation of clofibric acid (CA) and Cl- production in ZVI slurry in the presence or absence of 0.5 ppm HA (or 0.5 ppm FA). Other reaction conditions: [ZVI] ) 0.2 g/L; [4-CP]0 ) 100 µM; [CA]0 ) 100 µM; pHi 2.5. more sensitively affected by the surface passivation than the normal ZVI-induced oxidation. Since this effect of NOMenhanced oxidation is relatively short-lived, it may not have a great practical value in the real life systems. Nevertheless, understanding the role of NOMs in the redox process of ZVI should be valuable in the development of ZVI technologies. Oxidative Degradation Mechanism in NOM/ZVI System. To investigate whether OH radicals are involved in the NOMenhanced oxidation, the effects of methanol addition are shown in Figure 3. In the presence of 10 mM methanol that should scavenge OH radicals quantitatively, the oxidative degradation of 4-CP was almost completely inhibited, which implies that the NOM-enhanced degradation in the ZVI system is mainly ascribed to the enhanced generation of OH radicals. This also indicates that the removal of 4-CP should not be ascribed to the reductive dechlorination (4-CP + 2e+ H+ f phenol + Cl-) as we ruled out in the previous study (13). Methanol should not affect the reductive dechlorination process, if any. No phenol that could be produced from such 880

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To test this possibility, a control experiment of 4-CP degradation was carried out in the presence of 5 mM Fe(II) and 0.5 ppm NOM in air-equilibrated solution without ZVI. As shown in Figure 3, the degradation of 4-CP in the (Fe(II) + NOM) system was completely absent, which rules out the homogeneous path and confirms that the heterogeneous electron transfer reaction between ZVI and NOM is indeed responsible for the generation of OH radicals. The most plausible explanation for the NOM-enhanced oxidation is that NOM plays the role of an electron shuttle in the ZVI redox process (reactions 7 and 8). The electron transfer from Fe0 to NOM is thermodynamically favored (E(FAox/FAred) ) 0.5 VNHE (36), E(HAox/HAred) ) 0.7 VNHE (37), E0(Fe2+/Fe0) ) -0.44 VNHE (15)). Fe0+NOMox1 f Fe2++NOMred +

NOMred+2H +O2 f NOMox2+H2O2

(7) (8)

As a result, the electron transfer from the iron surface to O2 can be accelerated by the NOM mediator, which leads to the enhanced production of H2O2 and subsequently OH radical through Fenton reaction. To confirm this, we compared the in situ generated H2O2 concentration in the presence and absence of NOM. The colorimetric determination of [H2O2] was seriously interfered by the presence of ferrous and ferric

FIGURE 3. The effects of methanol (OH radical scavenger) and dissolved O2 on the degradation of 4-CP in ZVI suspension with (a) FA and (b) HA. In the N2-saturated condition, nitrogen gas was continuously sparged throughout the reaction. The control reaction in (Fe(II)+NOM) solution without ZVI is also shown. ([ZVI] ) 0.2 g/L; [4-CP]0 ) 100 µM; [NOM]0 ) 0.5 ppm; [methanol] ) 10 mM; [Fe(II)] ) 5 mM; pHi 2.5).

FIGURE 4. Time profiles of ferrous ions dissolved from ZVI in the presence or absence of FA, HA, or BQ. ([ZVI] ) 0.2 g/L; no 4-CP added; [NOM]0 ) 0.5 ppm; [BQ]0 ) 5 µM; pHi 2.5). ions (dissolved from ZVI) whose concentration is far higher than [H2O2], although bipyridine and EDTA were added as a complexing agent of Fe(II) and Fe(III), respectively. Therefore, we measured the concentration of H2O2 only at the initial stage of reaction (after 5 min of ZVI reaction) when the interference from Fe(II) and Fe(III) is not significant. In agreement with reactions 7 and 8, significantly higher [H2O2] was detected in the presence of NOM. The presence of FA and HA (0.5 ppm) led to H2O2 generation of 14 and 16 µM, respectively, whereas only 0.6 µM H2O2 was produced in the absence of NOM. This confirms that NOM mediates the electron transfer from Fe0 to oxygen, thereby increasing the

FIGURE 5. Time-dependent variation of the fluorescence emission (λex ) 279 nm) of dissolved NOM in the presence or absence of ZVI. (a) FA (emission at λ ) 451 nm) (b) HA (emission at λ ) 464 nm). The inset shows the fluorescence emission (λex ) 279 nm) spectra of (a) FA and (b) HA solution as a function of reaction time in the ZVI suspension. ([ZVI] ) 0.2 g/L; no 4-CP added; [NOM]0 ) 0.5 ppm; pHi 2.5). generation of H2O2. Since NOMs contain quinone moieties (38), which are known to function as an effective electron shuttle (39), BQ was added as a proxy compound for NOM to see whether BQ serves as an electron shuttle in the production of H2O2. Comparing the standard reduction potentials (E0(BQ/HQ) ) 0.70 VNHE vs E0(Fe2+/Fe0) ) -0.44 VNHE) (15) indicates that the reduction of BQ by ZVI is thermochemically favored. Hence, the presence of 5 µM BQ also resulted in the generation of 7 µM H2O2 in 5 min of reaction. The corrosive dissolution of ZVI should be also enhanced with the NOM-accelerated electron transfer according to reaction 7. Figure 4 clearly shows that the dissolution of Fe2+ ions in the ZVI suspension is enhanced in the presence of FA, HA, or BQ. This supports that NOM is directly involved in the electron transfer process occurring on ZVI. The NOMmediated electron transfer from ZVI to O2 should enhance the production of Fe2+ as well as H2O2. Although quinone moieties in NOMs are known to act as electron shuttles (39), NOMs involved in the electron transfer process is not an electron transfer catalyst in a strict sense because the structure of NOMs can be irreversibly changed (or destructed) as a result of the electron transfer reaction. The different notation of NOM species (NOMox1 and NOMox2) in reactions 7 and 8 indicates that a specific NOM moiety is transformed into a different structure after mediating an electron transfer. NOMs in the ZVI system may undergo not only reductive (ZVI-mediated) but also oxidative (OH radicalmediated) processes. The reaction of NOMs and OH radicals should compete with that of OH radicals and 4-CP. HA and VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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maximal around [BQ] ) 5 µM above which the enhancing effect was reduced. BQ-induced enhancement effect was observed up to [BQ] ) 100 µM (Figure 6b). With [BQ] ) 1 mM, the degradation of 4-CP was markedly inhibited on the contrary because excess BQ molecules should scavenge OH radicals. On the basis of the rate constants of the bimolecular reaction with OH radicals [k(4-CP + OH) ) 7.6 × 109 M-1s-1 (41); k(BQ + OH) ) 1.2 × 109 M-1s-1 (43)], it is estimated that less than 1% of OH radicals are scavenged by 5 µM BQ but more than 60% should be consumed by 1 mM BQ. The dual roles of BQ as an electron shuttle and an OH-radical scavenger seem to be counter-balanced at around [BQ] ) 100 µM. The addition of 10 mM methanol as an OH radical scavenger completely inhibited the BQ-mediated oxidation of 4-CP (see Figure 6a), which also indicates that the BQ-induced enhancement is related with the enhanced generation of OH radicals. The present observation that BQ that was used as a proxy of NOM enhanced the oxidation rate in the ZVI system supports the hypothesis that NOM serves as an electron transfer mediator to enhance the generation of OH radicals.

Acknowledgments This work was supported by KOSEF grant funded by the Korean government (MEST) (No. R0A-2008-000-20068-0), the KOSEF EPB center (Grant No. R11-2008-052-02002), the Brain Korea 21 program, and Korea Ministry of Environment, The Gaia Project.

Literature Cited FIGURE 6. (a) Degradation of 4-CP in air-equilibrated iron suspensions in the presence of 5 µM BQ, production of Cl-, and the effect of methanol (OH radical scavenger) (b) Effect of BQ concentration on the degradation of 4-CP. ([ZVI] ) 0.2 g/L; [4-CP]0 ) 100 µM; [methanol] ) 20 mM; pHi 2.5). FA react with OH radicals with the rate constant of k )1.9 × 104 (mg of C/L)-1s-1 and k)2.7 × 104 (mg of C/L)-1s-1 (40), respectively, while the rate constant for (4-CP + OH) reaction is 7.6 × 109 M-1s-1 (41). Under the present experimental condition of 0.5 ppm NOM and 100 µM 4-CP, most OH radicals react with 4-CP and fewer than 2% of OH radicals should be scavenged by NOM. Therefore, the OH radical mediated-reaction of NOMs in ZVI system might be much less significant than their direct reaction with ZVI (reactions 7 and 8). Figure 5 shows that the intensity of fluorescence emission from the dissolved NOM is gradually reduced with reaction time in the ZVI suspension, which indicates that fluorescing functional groups are destructed as a result of the electron-transfer reaction with ZVI. The emission spectra in the insets of Figure 5 show that the decrease of the emission intensity in the region of 400-500 nm is accompanied by the appearance of a new band around 370 nm, which also supports the structural change of NOMs. The disappearance of the broad emission band of 400-500 nm indicates that the aromatic structure in NOMs is destructed while the appearing band centered around 370 nm is possibly related with the formation of quinones (42). Several studies reported that the quinone-containing compounds serve as an electron mediator to enhance the redox reaction rates (7, 18, 39). For example, the addition of hydroquinones or quinones in Fenton and photo-Fenton systems increased the oxidation rate of phenol, which was ascribed to their role as an electron transfer mediator (39). Since BQ should be like NOMs in the role of electron transfer mediator in the ZVI suspension, we tested the oxidative degradation of 4-CP in the presence of BQ. Figure 6 clearly shows that the presence of BQ markedly increased the degradation rate of 4-CP and the enhancement effect was 882

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