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Journal of Hazardous Materials 260 (2013) 851– 859

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

jou rn al hom epage: www.elsev ier .com/ locate / jhazmat

xidative degradation of azo dyes using tourmaline

uiping Wang ∗, Yanwei Zhang, Li Yu, Zhiyuan Zhang, Hongwen SunOE Key Laboratory of Pollution Process and Environmental Criteria, College of Environment Science and Engineering, Nankai University, Tianjin 300071,

R China

i g h l i g h t s

Tourmaline can catalytically degrade MB in a broad range of pH values.In contrast to MB and RhB, CR degradation was lower than its adsorption percentage.OR, • OOR, • R and O2

• − radicals were observed during the degradation of MB.Four new intermediate products of MB were identified using LC–MS.

a r t i c l e i n f o

rticle history:eceived 14 April 2013eceived in revised form 14 June 2013ccepted 24 June 2013vailable online 29 June 2013

eywords:ourmalineye

a b s t r a c t

This study aimed to investigate the catalyzed degradation ability of tourmaline on the dyes methy-lene blue (MB), rhodamine B (RhB), and congo red (CR) at different pH values. Interestingly, tourmalinestrongly adsorbed anionic dyes, but it did not adsorb cationic dyes. When H2O2 was introduced into thetourmaline-dye systems, the degradation percentage for CR catalysis by tourmaline was lower than thepercentage of adsorption, whereas the opposite was true for MB and RhB systems. Notably, the catalyzeddegradation decreased from 100% to 45% for MB, 100% to 15% for RhB and 100% to 25% for CR as thepH increased from 3.0 to 10.0, respectively, which was much greater than the degradation obtained forpreviously reported materials at pH values ranging from 4.0 to 10.0. Tourmaline catalytically degraded

PRntermediate products

the dyes over a broad range of pH values, which was attributed to tourmaline automatically adjusting thepH of the dye solutions to approximately 5.5 from an initial range of 4.2–10.0. An electron paramagneticresonance spin trapping technique observed peroxyl (ROO•) and alkoxy (RO•) or alkyl (R•) radicals orig-inated from the attack of •OH radicals and O2

•− radicals, indicating that these radicals were involved inthe catalyzed degradation of MB. Importantly, four intermediate products of MB at m/z 383, 316, 203 and181 were observed by LC/MS.

. Introduction

Dyes and pigments have been utilized for coloring in the tex-ile industry for many years [1]. Several types of textile dyes arevailable for use with various types of textile materials [2]. Approx-mately 15% of the world’s total production of dyes is lost during theyeing process and released with the textile effluents [3]. These col-red dye effluents pose a major threat to the environment, humanealth and aquatic life [4]. Water pollution from dyes is a seriousnvironmental problem because the dyes are difficult to degradeue to their high chemical stability [5].

Continuing interest has focused on the development of meth-ds to treat dye effluents. Among them, physical methods such asdsorption [6], biological methods such as biodegradation [7], and

∗ Corresponding author. Tel.: +86 22 23504362; fax: +86 22 23509241.E-mail addresses: wangcuipping0@163.com, wangcuiping0@163.com (C. Wang).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.06.054

© 2013 Elsevier B.V. All rights reserved.

chemical methods such as Fenton and photo-Fenton reactions [8],reductive degradations utilizing zero-valent iron [9], and photo-catalysis [10] are the most frequently used methods for treatment.However, absorption is actually just a transfer of pollutants, andthe traditional biological approach achieves poor efficiency [11].The Fenton reaction has proven effective at treating organic pollut-ants in wastewater [1]. However, the narrow pH range (pH < 3.0)required limits the wide application of the Fenton reaction, andthe Fenton reaction results in the generation of ferric ions, whichare secondary pollutants. The use of photocatalysts is a rapidlyemerging and promising technology. TiO2 and modifications ofTiO2 are the most widely used photocatalysts, and they are capableof degrading the harmful dyes in wastewater [9,12,13]. However,the use of photocatalysts also requires the presence of an additionallight source, thus limiting its practical application.

To overcome this disadvantage, several studies have beenconducted to explore heterogeneous catalysts, such as ferrihydrite[14], goethite [15,16], magnetite [17], hematite [18], Fe(III)-loadedresin [19], Fe2O3 and Fe2Si4O10(OH)2 [20], Fe(II) supported on

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52 C. Wang et al. / Journal of Hazar

eolite, Al2O3 and SiO2 [21,22], and FeO/Al2O3 [23]. These materialsan promote the oxidation of different organic compounds, suchs aromatic and aliphatic acids, phenols, aromatic hydrocarbonsnd textile dyes, with hydrogen peroxide. However, many of theseystems exhibit catalytic activity only within a narrow pH rangef 2.0–6.0 [24,14,25], or they exhibit strong iron leaching due toow pH, which results in the classical homogeneous Fenton mech-nism. It is therefore of great interest to exploit a new materialor the degradation of dyes over a wider pH range, which woulde useful for practical applications in wastewaters at different pHalues.

Tourmaline is a hydrous siliceous material. It is a natural min-ral that possesses unique physical–chemical properties, includinghe continuous release of negative ions, producing an electrostaticeld and releasing rare microelements. The most important featuremong the electrical properties of tourmaline is the generation ofpontaneous and permanent poles that, produce an electric dipole,specially in small granules with diameters of several microns oress [26]. These properties of tourmaline are related to its structure.he general formula of tourmaline is expressed as XY3Z6Si6O18BO3)W4, where the X site is commonly occupied by Na+, K+, Ca2+

r Mg2+; the Y site is occupied by Li+, Fe2+, Fe3+, Al3+, Mg2+ or Ti4+;nd the Z site is occupied by Fe3+, Cr3+, Al3+, Mg2+ or Fe2+. The W sites usually occupied by OH, but it can also be replaced by F or O. The

site is usually occupied by OH, but it can also be replaced by F or. The special structure of tourmaline suggests that it might havenique properties that are useful for catalysis. Black tourmaline is

nexpensive and has been widely distributed in China. However,espite its many unique properties, there has only been one pub-

ished study demonstrating that tourmaline can be successfullyeveloped as a novel heterogeneous catalyst for the discolorationf an active commercial dye, Argazol blue (BF-BR), in an aqueousolution [27]. In this study, the investigators explicitly measuredhe content of Fe ion leaching into the solution, characterized theourmaline after BF-BR discoloration using X-ray diffraction (XRD),ourier transform infrared spectroscopy (FT-IR) and scanning elec-ron microscopy (SEM), and investigated the parameters that couldffect it. The ability of tourmaline to degrade other dyes has noteen extensively studied, and its catalytic activity and mechanismsre not well understood.

The objectives of the current study are the following: (1) tovaluate the degradation ability of tourmaline on the organic dyesethylene blue (MB), rhodamine B (RhB), and congo red (CR) under

ifferent pH values; (2) to investigate the impact of different reac-ion factors on the degradation efficiency of the catalyst; and (3)o analyze the degradation mechanism and pathways of MB usingourmaline.

. Materials and methods

.1. Materials and reagents

Iron-rich black tourmaline in 800 nm-sized particles was pur-hased from Hongyan Mineral Products Co., Ltd, Tianjin city, China.he tourmaline was produced in Xinjiang province, China. Thehemical composition of the 800 nm-sized particle tourmaline wass follows: SiO2, 36.75%; Al2O3, 33.62%; Fe2O3, 12.19%; TiO2, 0.57%;2O3, 9.78%; FeO, 1.7%; CaO, 0.4%; MgO, 4.76%; K2O, 0.14%; Na2O,.74%; P2O5, 0.19%; H2O+, 1.0%; and MnO, 0.21%. Therefore, tour-aline used in this study belongs to schorl. The MB, RhB, CR

nd H2O2 were of laboratory grade, were purchased from Tian-

in Guangfu Fine Chemical Research Institute, China. The reagent,5-dimethyl-l-pyrroline N-oxide (DMPO), used as the spin-rapping agent in the ERP studies, was purchased from the Sigmahemical Co.

aterials 260 (2013) 851– 859

2.2. Batch experiment studies

Deionized distilled water (DD-H2O) was used to prepare all solu-tions and suspensions. Stock solutions of MB (1 g/L), RhB (1 g/L), andCR (1 mg/L) were prepared by dissolving measured amounts of thedyes in DD-H2O. Batch experiments included dye type, tempera-ture, pH effects. Further, solution pH variation, the total organiccarbon (TOC), reactive oxygen species and degradation productwere studied. The purpose was to investigate the catalytic activityand mechanisms of dyes by tourmaline.

2.3. General procedure

For the MB catalyzing experiment, 40 mg of tourmaline powderwas added to a polypropylene (50 mL) tube, followed by 100 �Lof H2O2 (30%, v/v) and, finally, 20 mL of the 50 mg/L MB solu-tion. The solution pH was adjusted by diluted aqueous solutionsof NaOH or HCl. At given time intervals, the polypropylene tubewas centrifuged to remove the tourmaline particles, and 1 mL ofthe suspension was sampled, and then diluted to the total volumeof 10 mL with deionized water immediately. Other dyes, includ-ing RhB and CR, were also used to evaluate the catalytic activitiesof tourmaline following the protocol described above for MB cat-alytic degradation. The residual concentrations of the three dyeswere monitored, each at its characteristic absorption band (gen-erally 656 nm for MB, 495 nm for RhB, and 553 nm for CR) using aCary 50 Probe UV–vis spectrophotometer. Each test was performedin triplicate.

The Fe2+ was analyzed at 510 nm via its complex with 1,10-phenanthroline using a Cary 50 Probe UV–vis spectrophotometer,and the interference from the undegraded dye was subtracted bythe absorbance at 510 nm measured before the addition of thephenanthroline ligand [28]. All the samples were analyzed imme-diately to avoid any further reactions.

Experimental procedures for the control system were the sameprotocol as the catalyzing degradation system, except for the H2O2and tourmaline. To exclude decomposition of dye by H2O2 or tour-maline, the experiments were carried out following the protocoldescribed above the catalyzing system, but without the H2O2 ortourmaline.

2.4. Solution analysis

The total organic carbon (TOC) was measured with a ShimadzuTOC-VCPH carbon analyzer coupled to a solid state combustion unit-SSM-5000A.

Liquid samples were prepared for analysis of reactive oxygenspecies followed the methods of Ma et al. [9] and Cheng et al.[19] with minor modifications. Stock solutions of DMPO (10 mM)were prepared by dissolving a given amount of DMPO in deion-ized water or methanol. Then, 2 mL of DMPO aqueous solutionor methanol solution was added to a polypropylene (10 mL) tubecontaining 40 mg of tourmaline, 100 �L of H2O2 (30%, v/v) and2 mL of the corresponding MB aqueous or methanol solution withan initial concentration of 50 mg/L. The polypropylene tube wascentrifuged after 0, 1, 60 and 240 min of reaction time, the suspen-sions were taken from the polypropylene tubes for analysis of theelectron paramagnetic resonance (EPR) signals. The EPR signals ofradicals spin-trapped by the DMPO were examined with a BruckerEMX 10/12 EPR spectrometer at the X-band using the followingoperating conditions: modulation frequency, 100 kHz; microwavefrequency, 9.82 GHz; incident microwave power, 20 mW; modula-

tion amplitude, 0.50 G; magnetic field center, 3470 G; scan width,100 G; and sweep time, 84 s for five scans. To minimize measure-ment errors, the same quartz capillary tube was used throughoutfor the EPR measurements.

dous Materials 260 (2013) 851– 859 853

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Fig. 1. MB (A), RhB (B) and CR (C) degradation under various conditions: (a) inthe presence of H2O2, (b) in the presence of tourmaline, and (c) in the presence

C. Wang et al. / Journal of Hazar

The degradation products of MB were analyzed using LC–MS.he catalyzed solutions with a pH of 3.0 described above were cen-rifuged after 0, 15 and 24 h of reaction time. Fifteen millilitersf each suspension was collected and then injected into an HLBPE column for clean-up. The column was first eluted with 2 mLf methanol three times. Next, the combined eluents were con-entrated using rotary evaporation (<30 ◦C, 0.05 Mpa) to a volumef approximately 2 mL. The residue eluents were further concen-rated to near dryness under a gentle stream of N2 and thenedissolved in 1 mL of methanol for LC–MS analysis. The liquidhromatography–mass spectrometry (LC–MS) analysis was con-ucted on an Agilent 1200 series liquid chromatograph and angilent 6410B triple quadrupole mass spectrometer using posi-

ive ion electrospray in full-scan mode. The mobile phase wascetonitrile and water with a ratio of 8:2 (v/v) containing 10 mMmmonium acetate at a constant flow rate of 0.5 mL/min. The masspectrometer was equipped with an electrospray ionization (ESI)ource. The ESI-MS source conditions were established to obtain anverage maximum intensity of the precursor ions. The optimizedarameters were as follows: scan range, 90–400; dry gas tempera-ure, 350 ◦C; gas flow, 10 L/min; nebulizer, 35 psi; capillary voltage,3000 V; and dwell time, 100 ms for MB.

. Results and discussion

.1. MB degradation with tourmaline

To examine the use of tourmaline for MB degradation, it wasecessary to conduct an experiment from which direct adsorptionas excluded. MB was not adsorbed considerably by tourmaline

Fig. 1A, curve b), nor did it react with H2O2 (Fig. 1A, curve a), whichndicated that H2O2 alone could not oxidize MB within a given reac-ion period, even though H2O2 is a strong oxidant. Notably, the MBas completely degraded by tourmaline in the presence of H2O2ithout additional light irradiation (Fig. 1A, curve c). The UV–vis

pectra of the aqueous samples collected at various degradationime intervals showed that the characteristic band for MB centeredt 665 nm decreased rapidly as the degradation time increasedinset plot in Fig. 1A), together with a slight shift at the maximumbsorption. Additionally, the solution color turned from blue toearly colorless in the tourmaline/H2O2 system. Therefore, tour-aline was able to effectively degrade MB.

.2. Degradation of other dyes and desorption with tourmaline

To validate the catalytic degradation ability of tourmaline,xperiments were conducted with the cationic dye RhB and thenionic azo dye CR as model compounds. The initial pH values ofhe aqueous dye solutions were 3.0.

RhB was not adsorbed by tourmaline (Fig. 1B, curve b), nor didt react with H2O2 (Fig. 1B, curve a), which indicated that H2O2lone could not oxidize RhB, even though H2O2 is a strong oxidant.otably, the RhB was completely degraded by tourmaline in theresence of H2O2 (Fig. 1B, curve c), which was similar to the resultrom MB.

In contrast, the adsorption of CR onto tourmaline was striking,eaching complete adsorption of the dye from an initial concen-ration of 50 mg/L after only 1 h (Fig. 1C, curve b). To investigatehis phenomenon, the zeta potential measurement for tourmalineas measured using a zeta meter (JS94H, Powereach). The zetaotential of tourmaline was 8.62 at pH 3.0, indicating that the tour-

aline surface possessed a positively charged surface. Thus, at pH

.0, tourmaline tended to adsorb a negatively charged molecule.B and RhB are cationic dyes, whereas the CR is an anionic dye.

hus, the electrostatic attraction resulted in a strong adsorption

of tourmaline and H2O2. All the reactions were performed at pH 3.0, with 40 mgof tourmaline, 100 �L of H2O2 (30%, v/v), and [MB] = 50 mg/L. The inset shows theUV/vis spectral changes recorded for (c).

of the CR to the tourmaline. In addition, CR did not react withH2O2 (Fig. 1C, curve a). Figs. 1, 2 and 3 indicate that CR exhib-ited greater adsorption onto the tourmaline surface than did MBand RhB. To confirm that CR is adsorbed onto the tourmaline andnot degraded by the tourmaline itself, the tourmaline powders inthe above adsorption systems were separated by centrifugation.Then, the tourmaline-adsorbed CR was extracted using methanol,and the tourmaline-adsorbed CR concentration was determinedby UV–vis spectrophotometry. The recovered concentration nearlyequaled the initial spiked concentration of CR (data not shown).

Thus, tourmaline played an adsorption role for the anionic azo dyeCR. Fig. 1C (curve c) showed that CR were also degraded efficientlyin the tourmaline/H2O2 system, but the degradation percentagewas lower than that of MB and RhB.

854 C. Wang et al. / Journal of Hazardous M

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Fig. 3. Formation of radicals determined by DMPO spin-trapping EPR for theMB/tourmaline/H2O2 system in aqueous solutions. The open square placed abovecertain peak signals refer to EPR signals of the DMPO oxidized and the solid cycledenoted signals due to the DMPO-•OR, DMPO-•OOR, or DMPO-•R adducts.

aterials 260 (2013) 851– 859

Therefore, tourmaline could be effective for the degradationof azo dyes. However, it should be noted that the CR degrada-tion percentage was lower than the CR adsorption. This result isconsistent with the study of Devi et al. [29], which reported thatthe anionic dyes showed low degradation rates at pH 3.0, thoughstrong adsorption of CR occurred place at this condition. This strongadsorption could result in the prevention of the Fe3+/Fe2+ releasefrom the tourmaline particles into the aqueous solution. Combinedwith the above MB, RhB and CR adsorption and degradation results,it was concluded that the dye degradation mechanisms of homo-geneous catalysis at the initial solution pH of 3.0 in the aqueousphase involved the following steps. First, Fe3+ was dissolved fromthe tourmaline surface into aqueous solution. Then, H2O2 that wasadsorbed onto the tourmaline particle surface reacted with theFe3+ to produce Fe2+ and •OOH/O2

•− radicals. Meanwhile, Fe2+ bothfrom the tourmaline structure itself and generated from the abovereaction, reacted with H2O2. The generated HO2

•/O2•− radicals are

quite reactive, and they likely contribute to the oxidation of MB.Finally, the •OH and O2

•− radicals both attacked the dye moleculesin aqueous solutions to produce the oxidized products. The homo-geneous mechanisms are described in the following reactions:

Fe(III) → Fe3+(aq) (1)

Fe(II) → Fe2+(aq) (2)

Fe3+(aq) + H2O2 → Fe2+(aq) + •OOH/O2•− + 2H+ (3)

•OH + H2O2 → HO2•/O2

•− + H2O (4)

O2•− + Fe3+(aq) → O2 + Fe2+(aq) (5)

Fe2+(aq) + H2O2 → Fe3+(aq) + •OH + OH− (6)

HO2•/O2

•− + Fe2+(aq) → HO2− + Fe3+(aq) (7)

•OH and HO2•/O2

•− + dyes → peroxide or hydroxyl intermediates

→ oxidation products (8)

Therefore, the CR adsorbed tourmaline surface decreased theFe3+/Fe2+ release from the tourmaline particles into aqueous solu-tion and further decreasing in the percentage of CR degradation byhomogeneous catalysis compared to MB and RhB.

3.3. Degradation efficiency of tourmaline at differenttemperatures

The degradation percentage of MB increased when the tempera-ture of the degradation system was increased from 15 to 35 ◦C (Fig.S1). Specifically, at 35 ◦C, the degradation percentage of MB after5 h reached 100%, while the MB degradation percentage was 73%and 65% after the same amount of time at 25 and 15 ◦C, respectively.This observation was due to the higher temperature increasing thereaction rate between the hydrogen peroxide and the catalyst, thusincreasing the rate of generation of the oxidizing species, such asthe •OH radical [30]. In addition, the higher temperature providedmore energy for the reactant molecules to overcome the reactionactivation energy [27].

3.4. Degradation efficiency of tourmaline at different pH values

The effect of the initial pH on the catalytic degradation efficiencyof MB, RhB and CR in aqueous solutions is shown in Fig. 2.

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C. Wang et al. / Journal of Hazar

The results showed that the catalyst efficiency for the degra-ation of MB was lowest at a pH of 5.8 (Figs. 2 and S2), whichas the natural pH of MB when dissolved at an initial concentra-

ion of 50 mg/L. The percentage of dye degradation increased ashe solution pH decreased from pH 5.8, which is consistent with aonventional Fenton-like system. Additionally, the MB degradationas nearly 100% at both pH 2.0 and pH 3.0. However, the percent-

ge of MB degradation declined from 60% to 45% as the solution pHncreased from pH 4.2 to 10. To date, no mineral material except forourmaline has been reported that can degrade MB at this extendedH range.

The degradation of RhB was 100% and 77% at pH 2.0 and pH 3.0,espectively, and it decreased from approximately 30% to 15% fromH 4.0 to 10.0 (Fig. 2). Hence, the tourmaline-catalyzed degradationf RhB at pH higher 3.0 was much lower than that of MB, which wasue to the different chemical structures of the cationic dyes. TheB molecule possesses a thiazine structure, which contains polar

toms such as nitrogen, a positively charged sulfur atom and a neg-tively charged chloride as the counter ion [31]. Additionally, theunctional groups C N and C S in MB, can readily undergoomolytic cleavage [29]. Therefore, MB showed a lower stabilityompared to RhB, resulting in a higher degradation percentage ofB than RhB.The catalyzed degradation of CR was nearly 100% at pH 2.0 and

6% at pH 3.0, and it ranged from almost 45 to 25% in the pH range of.0–10.0 (Fig. 2). The CR molecule possesses two SO3

− groups andwo primary amine groups ( NH2). Each of these groups attacheso one of the two naphthalene rings located at the two ends ofhe molecule. It has been reported that the azo bond ( N N ) ishe most active site to be oxidized either by •OH or •OOH/O2

•−

adicals [31]. However, at a pH of 3.0, the percentage of MB, RhBnd CR degradation followed the order of MB > RhB > CR, becausehe catalyzed degradation mechanisms of tourmaline on the dye

olecules was mainly through homogeneous catalysis in the aque-us phase, however, the CR adsorbed tourmaline surface couldrevent the Fe3+/Fe2+ release from the tourmaline particles intoqueous solution and further decreasing in the percentage of CRegradation by homogeneous catalysis compared to MB. As theH values ranged from 5.0 to 10.0, the percentage of MB, RhB andR degradation followed the order of MB > CR > RhB, and the MB,hB and CR degradation results (Fig. 2) were consistent with therends of their adsorption onto tourmaline (Fig. S3). This obser-ation was due to the steric repulsion of the carboxylate anion inB, which lowered the extent of adsorption [29]. Additionally, inhis pH range from 5.0 to 10.0, it was therefore concluded that, atH greater than 5.0, the dye oxidation was likely mainly attributedo heterogeneous catalysis at the iron tourmaline surface togetherith a homogeneous catalysis occurring in the aqueous phase, as

ewer ferrous ions were produced by catalysis (in this study, thee2+ concentrations ranged from 0.29 at pH 5.0 to 0.13 mg/L at pH0.0) compared to the lower pH values of 2.0–4.0 (the Fe2+ concen-rations ranged from 7.08 mg/L at pH 2.0 to 2.9 mg/L at pH 4.0). Theeterogeneous catalytic mechanism is described in Section 3.6.

The tourmaline-catalyzed reactivity was dependent on the pHf the solution, and acidic conditions were more favorable for theegradation of dyes. This result is similar to the results from stud-

es using common forms of iron oxide to catalyze the degradationf dyes [14,24,25]. At an acidic pH (2.0–4.0), more of the Fe ions dissolved into the solution from the tourmaline catalyst, whichhen catalyzes H2O2 to generate more OH radicals [27], resultingn a more rapid degradation of MB, RhB and CR. It was concludedhat the mechanism of homogeneous catalysis was responsible for

he degradation of the dyes under the range of initial pH values inhe aqueous phase. At neutral and alkaline pH values (6.0, 8.0 and0.0), because there is much less dissolved Fe [27], the MB, RhBnd CR degradation percentages were decreased relative to those

aterials 260 (2013) 851– 859 855

at acidic pH values (2.0–4.0). However, the MB and CR degrada-tion percentages at the natural pH 5.8 and 6.2, respectively, werelowest compared to those at pH 7.0–10.0 (9.6). On the one hand,this result could be attributed to the fast decomposition of H2O2into reactive radicals during the initial stage of the reaction, whichcan degrade the MB and CR slightly in extremely alkaline solutionscompared to acidic and neutral solution [32]; On the other hand, atpH values higher than 5.0 during the initial stage of the reaction, thedye oxidation could be mainly attributed to heterogeneous catal-ysis on the iron tourmaline surface together with a homogeneouscatalysis occurring in the aqueous phase, because the degradationresults were consistent with the MB, RhB and CR adsorption trends.The MB, RhB and CR adsorption results showed that the MB andCR adsorption capacity (q) at their natural pH values of 5.8 and6.2, respectively, were lower than those at pH 7.0–10.0 (9.6) (Fig.S3). Hence, at natural pH 5.8 and 6.2, respectively, the MB andCR degradation percentages were lower than those at any otherpH values. The differences between the results from our work andthose in previous studies could partly be attributed to the pH valuesused in the studies, because pH values used in the previous stud-ies were adjusted using acid and alkaline solution, while did notinclude the substrate natural pH. In fact, substrates with distinctphysico-chemical characterization could show different degrada-tion behaviors in acid, natural and alkaline solution systems [33].Additionally, our experimental results were similar to the studyof Filho et al. [34] that demonstrated that MB adsorption at natu-ral pH was lower than at other pH values adjusted using an aceticacid–sodium acetate aqueous buffer because the because the ionstrength (Na+) of adjusting pH aqueous solution play an importantrole in the adsorption process.

3.5. Evidence for catalyzed degradation of MB by tourmaline

3.5.1. Water polarized by tourmalineTo elucidate the effective degradation ability of tourmaline on

MB in a wider pH range, the degradation kinetics were measuredas MB was degraded by tourmaline at different initial solution pHconditions. The results are presented in Fig. S4. The overall trendindicated an increase in pH, with the final pH values being higherthan the initial levels when the initial pH < 4.2, whereas a decreasein pH to a final pH of approximately 5.5 was observed when theinitial pH was between 4.2 and 10 (Fig. S4). Hence, tourmalineadjusted the solution pH automatically, which was ascribed to theprevious observation that tourmaline particles can automaticallypolarize water because of its spontaneous and permanent poles[35]. This characteristic is clearly different from that of other miner-als, which could be one of the reasons that tourmaline can degradedyes at a wider pH range (3.0–10.0). Researchers have shownthat common forms of iron oxide (goethite, hematite, magnetite,and ferrihydrite) can catalyze oxidation reactivity for hydrocar-bons and RhB in the presence of H2O2 from pH 3.0 to 6.0, andthe catalyzed oxidation reactivity obviously decrease as pH > 4.0[14,25]. Cheng et al. [19] reported that oxidation-catalyzed MBdegradation by visible-light assisted Fe(III)-loaded resin decreasedfrom 42.3% to 9.8% as pH increased from 4.0 to 10.0, while theRhB degradation rate declined from 5.6% to 0.2%. Therefore, thetourmaline-catalyzed MB and RhB degradation was much greaterthan that obtained for previously reported materials at pH valuesranging from 4.0 to 10.0.

We analyzed the mechanisms of catalyzed degradation fromthe perspective of the tourmaline itself. To determine whether the

MB/tourmaline/H2O2 system was actually able to catalyze the reac-tion, we further investigated the production of radicals and the MBdegradation products from the reaction system in addition to thetarget compound perspectives.

8 dous Materials 260 (2013) 851– 859

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56 C. Wang et al. / Journal of Hazar

.5.2. EPR measurements for the generation of radicalsTo confirm the formation of reactive oxygen species produced

n the H2O2/tourmaline/MB system, we carried out the EPR/DMPOpin trapping experiments as previously described [19]. The resultshowed that no DMPO-•OH or DMPO-O2

•− spin adducts wereetected in the control experiments of the H2O2/MB or tourma-

ine/MB system (figure not shown), which was consistent with theesults obtained in Fig. 1.

Fig. 3 showed that seven characteristic peaks were clearlybserved in the H2O2/tourmaline/MB system, which is differentrom the characteristic peaks reported in previous studies in whichhe EPR spectra exhibited a 4-fold peak characteristic of DMPO-OH adducts with an intensity ratio of 1:2:2:1 [9,19]. By contrast,hese peaks likely belonged to DMPO-peroxide adducts and alkoxyRO•) or alkyl (R•) radicals [36,37]. These signals (Fig. 3C) originatedrom the attack of •OH radicals, which was consistent with the higheactivity of the OH radical [36]. Alkoxy (RO•) or alkyl (R•) radi-als formed from the reaction of the ROOH with the Fe3+ speciesEqs. (9)–(15)) to produce DMPO-•OR, DMPO-•OOR, or DMPO-•Rdducts with DMPO, all of which typically display a six-line EPRpectrum for the oxygen-centered and carbon-centered radical inhe aqueous system [36]. Therefore, the formation pathways ofhese radicals were proposed as follows:

OH + RH → H2O + R• (9)

OH + H2O2 → HO2•/O2

•− + H2O (10)

2•− + RH → ROOH (11)

OOH + Fe3+ → Fe2+ + ROO• + H+ (12)

OOH + Fe2+ → Fe3+ + RO• + OH− (13)

H + RO• → ROH + R• (14)

OO• (or RO• or R•) + DMPO → DMPO-radical (six-line spectra)(15

RH represents the dye compound. Additionally, the intensitiesf these peaks rapidly increased with a prolonged reaction time0–1 min), which would be expected from the Fenton reaction.owever, when the reaction time was much longer (from 1 to 10,0 and 240 min), the EPR signals gradually became weaker, becausef the low steady-state concentrations of radicals with short meanifetime [38]. This observation indicated that the generation of radi-als in the tourmaline/H2O2 system is influenced by the time ofnteraction between the tourmaline and H2O2.

To further verify whether DMPO-•OOH/O2•− radicals were

roduced in the H2O2/tourmaline/MB system, they were also mea-ured in methanol instead of water [19], because •OOH/O2

•− isnstable in aqueous solutions and is difficult to detect in water.ig. 4 shows that EPR signals of •OOH and O2

•− were observed inhe H2O2/tourmaline/methanol system, indicating that •OOH/O2

•−

ere generated in the H2O2/MB/tourmaline system. Therefore,OOH/O2

•− radicals were involved in the catalyzed degradation ofB.It can be deduced from the above results that the dyes were,

o a large degree, eliminated by oxygenic radical oxidation. Thistudy thus provides the first measurements and report on theMPO-peroxide adducts and alkoxy (RO•) or alkyl (R•) radicals and

uperoxide radicals produced during the catalyzed degradation ofyes in the tourmaline and MB and H2O2 system.

.5.3. Degradation product identificationTo confirm the tourmaline-catalyzed degradation of MB, the

egradation products of MB were also analyzed by HPLC–MS. Fig. 5

Fig. 4. Formation of •OOH/O2•− radicals determined by DMPO spin-trapping EPR

for the MB/tourmaline/H2O2 system in methanol solutions.

clearly shows that the mass spectra of the MB degradation productschanged at the different times. Initially, only the main peak, whichcorresponded to the MB cation (m/z = 284), was present (Fig. 5a).After 15 h of reaction, the peak at m/z 284 disappeared completely,and the new spectra was characterized by a common m/z 181 basepeak and a second base peak at m/z 203. Their ionic molecular masspeak was at m/z 383 (Fig. 5b), indicating that the MB structuralring was broken and that degradation products of MB had beenproduced. Furthermore, when the reaction continued to 24 h, theintensities of the peaks at m/z 181 and 203 relative to the intensityof the peak at m/z 383 decreased in comparison with the spectrumafter 15 h, and new peaks appeared at m/z 316 and 367 (Fig. 5c).At this time, other m/z signals appeared that were likely related tointermediates of the methylene blue oxidation.

To better analyze the tourmaline-catalyzed degradation path-way of MB, the intermediate compounds corresponding to theabove peaks were identified by comparison with commercial stan-dards, searching the literature [3,39], and by interpretation oftheir fragment ions in the mass spectra. Four MB reaction inter-mediates were identified as compounds containing hydroxyl andformyl groups, and their intermediate structures are presented inFig. 6(a) and (b). These intermediate structures were different fromthe intermediate structures in previous reports. Their productionwould be expected in a complete oxidation reaction pathway thatleads to MB mineralization, and the TOC data described below con-firmed this hypothesis.

Based on the changes in the fragmentation spectra of these ions(Fig. 5), it is possible to associate the presence of the ion at m/z 383as a reaction of the ion at m/z 181 with the molecule at m/z 203. Theproposed reaction between molecules at m/z 181 and m/z 203 andthe structure of the ion at m/z 383 are illustrated in Fig. 6(b). Theseresults indicated that degradation intermediates play an impor-tant role in promoting the Fenton reaction. This phenomenon isconsistent with previous research [39].

3.5.4. TOC MeasurementTo assess the ability of tourmaline to completely destroy organic

molecules in aqueous solutions, the TOC was monitored during theentire reaction process. Fig. 7 shows the variation in TOC over time

in solutions of varying initial pH. The measured TOC ceased as theMB degradation time increased in the tourmaline/H2O2 system atvarious pH values. At pH values of 2.0 and 3.0, the decrease in TOCstopped when the MB was completely discolored at 15 h, at which

C. Wang et al. / Journal of Hazardous Materials 260 (2013) 851– 859 857

Fig. 5. ESI mass spectra in the positive ion mode for monitoring the oxidat

S

N

N

CH3

CH3 N

CH3

H3C

m/z 284

S

N

N

CH3

CH3 N

CH3

H3C

HOOH

m/z 316

SO3HNCH3

H

HO

N

CHO

CHO

OHHO

m/z 181 m/z 203

(a)

NHOC OH

HOC

HO

HO3S NHCO

CH3

N

HOC

OH

SO3HN

CH3

H

HOHO

HO

mz 181 m/z 203

m/z 383

(b)

Fig. 6. (a) Scheme with intermediates proposed for the oxidation of methylene bluedye (m/z = 284) by the tourmaline and H2O2 system; (b) Proposed structure for theion m/z = 383.

ion of MB in the tourmaline/H2O2 system at different reaction times.

point the TOC had been reduced by 73% from the initial concentra-tions, while the TOC decreased from 50% to 7% at pH values of 4.0and 10.0, respectively (Fig. 7). These results further demonstratedthat MB was catalyzed by tourmaline/H2O2 over a wide pH range.

3.6. A possible catalytic mechanism

Based on previous research on the Fenton-like oxidation of dyesby mixed Fe(II)–Fe(III) bearing minerals, reaction involving bothFe(II) and Fe(III) may occur [25,27,40,41]. In conjunction with theabove results obtained under different experimental conditions, wepropose possible homogeneous and heterogeneous mechanismsfor the catalytic oxidation of MB in this heterogeneous system.

For the homogeneous catalytic mechanism, please see reactions(1)–(8) in Section 3.2.

For the heterogeneous catalytic mechanism, first, H2O2 isadsorbed onto the tourmaline surface. Then, the adsorbed H2O2reacts with the active sites on the Fe2O3 in the tourmaline structureto form surface Fe2+ and •OOH/O2

•− radicals.

[Fesurface3+ + H2O2(adsorbed)] → Fesurface

2+ + •OOH/O2•− + 2H+

(16)

Meanwhile, Fe2+, bothfrom the tourmaline structure itself andgenerated from the above reaction, reacts with the adsorbed H2O2on the tourmaline surface.

Fesurface2+ + H2O2(adsorbed) → Fesurface

3+ + •OH + OH− (17)

A competitive reaction between the substrate and the H2O2occurs at higher concentrations of H2O2. The •OH radical may react

3

6

9

12

15

18

21

1 5 9 13 17 21 25

Time /h

TO

C /m

g/L

pH=2

pH=3

pH=4.2

pH=5.8

pH=7

pH=8.3

pH=10

Fig. 7. TOC kinetics during MB degradation in the presence of tourmaline and H2O2

under different pH. All the reactions were performed at 25 ◦C, with 0.04 mg of tour-maline, 100 �L of H2O2 (30%, v/v), and [MB] = 50 mg/L.

8 dous M

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4

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58 C. Wang et al. / Journal of Hazar

ith hydrogen peroxide, producing superoxide and hydroperoxyadicals according to Eq. (18):

OH + H2O2 → HO2•/O2

•− + H2O (18)

O2•− plays an important role in the redox cycle of Fe2+ and Fe3+

n the aqueous phase and generates oxygen as a byproduct, but thenteractions of the superoxide/hydroperoxy radicals with the iron

ineral surface are not yet well understood:

2•− + Fe3+ → O2 + Fe2+ (19)

O2•/O2

•− + Fe2+ → HO2− + Fe3+ (20)

Because the pKa of HO2•/O2

•− is 4.8, the generation of theydroperoxide radical HO2

• cannot be neglected at acidic pH val-es (in this study, pH 3.0), as shown by the EPR measurements. Theenerated HO2

•/O2•− radicals are quite reactive and contribute to

he oxidation of MB. Finally, the •OH and O2•− radicals both attack

he dye molecules to produce oxidized products.

OH and HO2•/O2

•− + dyes → peroxide or hydroxyl intermediates

→ oxidation products (21)

Thus, the dye degradation percentage depends on the radi-al formation rate, which in turn depends on the sum of thective sites. Tourmaline can catalyze dyes over an extended pHange, which makes it a more effective catalyst compared withther iron oxides [24,14,42,43]. This result is because tourmalines a mixed Fe(II)–Fe(III) bearing mineral, and it has been reportedhat the presence of Fe(II) in Fe-bearing minerals can enhancehe production rate of HO• [14,25,40]. Additionally, tourmalineutomatically adjusts the solution pH, resulting in the ability toffectively degrade dyes over a wide pH range (3.0–10.0). How-ver, detailed mechanistic studies are still necessary to provide alear understanding of the oxidation mechanism between MB andhe complex that it forms Fe3+/Fe2+. Additionally, other metal ionshat play a role in the catalytic activity of the system, such as Al and

n should be tested in future works.

. Conclusions

Tourmaline can catalytically degrade the dyes MB, RhB andR over a broad range of pH values from 3.0 to 10.0. Tourmalinean automatically adjust the pH of dye solutions to approximately.5 from an initial range of 4.2 to 10.0. The tourmaline-catalyzedye degradation decreased with pH increasing, which was muchreater than that observed for previously reported materials at pHalues ranged from 4.0 to 10.0. An electron paramagnetic reso-ance (EPR) spin trapping technique showed that peroxyl (ROO•) orlkoxy (RO•) or alkyl (R•) radicals originated from the attack of •OHadicals and O2

•− radicals involved in the tourmaline-catalyzedegradation of MB. Importantly, four intermediate products of MBt m/z 383, 316, 203 and 181 were observed by LC/MS. Based onhese results, a new mechanism for the tourmaline/H2O2-catalyzedxidation MB was proposed. Therefore, tourmaline might be a suit-ble catalyst for the treatment of textile industry wastewaters thatave a wide range of pH values. To date, no material other than tour-aline, has been reported that can degrade MB at this extended pH

ange.

cknowledgements

This research was funded by the National Natural Science Foun-ation of China with Grant Numbers 20907024 and 41225014 andianjin Key Programme of Basic Research (10JCZDJC24200 and3JCYBJC20200).

[

[

aterials 260 (2013) 851– 859

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2013.06.054.

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