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Chemical Engineering Journal 191 (2012) 364– 368

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal

jo u r n al hom epage: www.elsev ier .com/ locate /ce j

fficient degradation of Rhodamine B in microwave-H2O2 system at alkaline pH

un Honga,b,∗, Nannan Yuana, Yanxin Wanga,b, Shihua Qia,b

School of Environmental Studies, China University of Geosciences, 388 Lumo Road, Hongshan District, Wuhan 430074, PR ChinaState Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, 388 Lumo Road, Hongshan District, Wuhan 430074, PR China

r t i c l e i n f o

rticle history:eceived 21 September 2011eceived in revised form 12 March 2012ccepted 12 March 2012

a b s t r a c t

Microwave-H2O2 (MW-H2O2) technology, one of the environmental treatment methods, was reportedto usually prefer acidic and neutral circumstances in substrates’ degradation. In this work, degradationof Rhodamine B (RhB) and methylene blue (MB) in MW-H2O2 system was found to be very competi-tive at extreme alkaline pH. This is a new finding and few studies have noticed it yet. Through radical

eywords:icrowave2O2

lkalinehodamine B

species’ tests and control experiments, degradation of RhB at strong alkaline pH in MW-H2O2 system wasconsidered to mainly undergo oxygen oxidation, and the enhancement of MW thermal effect. This workprovided a possible effective way to study the degradation at highly alkaline circumstances, and mightpromote the development of MW-H2O2 technology in alkaline wastewater treatment.

© 2012 Elsevier B.V. All rights reserved.

egradation

. Introduction

Strong alkaline wastewater with pH higher than 11 is usuallyerived from human industrial activities [1]. Traditional treatmentethods as neutralization, precipitation, wet air oxidation, Fenton,

iological process, etc. [2–4] present certain limitations to somextents, such as high biochemical oxygen demands and microor-anisms hampering due to extreme alkaline circumstances andoxicities of aromatics. Recently, the application of microwaveeating in combination with hydrogen peroxide (MW-H2O2) forollutants treatment has shown to be an efficient oxidationechnology [5,6]. MW-H2O2 process has demonstrated its highegradation efficiency [7], because it generates active species ofydroxyl radicals [8] with a redox potential of +2.8 eV that cannselectively degrade most organic compounds. Meanwhile, MW-2O2 has no need of catalyst separation and no potential risk ofnvironmental pollution, compared to some hybrid MW systems9–11].

Previous studies on MW-H2O2 system showed that high sub-trates oxidation efficiencies were usually appeared at acidic andeutral pH [12,13]. A few studies also pointed out that the high-st substrate removal rate was observed at pH 6, while the lowest

ne was at pH 11, which was inferred to be self-decompositionf hydrogen peroxide and scavenging effect of hydroxyl radicalst alkaline circumstances [14]. However, since those very limited

∗ Corresponding author at: State Key Laboratory of Biogeology and Environmen-al Geology, China University of Geosciences, 388 Lumo Road, Hongshan District,

uhan 430074, PR China. Tel.: +86 27 67883170; fax: +86 27 87436235.E-mail address: hongjun18888@163.com (J. Hong).

385-8947/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2012.03.032

work that had been conducted, it was uncertain if the degradationof some substrates in alkaline conditions by MW-H2O2 technol-ogy was weak or not, and still much more work in this area wererequired to make it clear.

In present study, we are trying to test the degradation of twokinds of dye substrates at alkaline pH by MW-H2O2 technology, dueto (1) environmental-friendly as well as highly efficient methodsfor alkaline wastewater treatment are popular among traditionalones, (2) no specific work have been done in alkaline MW-H2O2 sys-tem yet. Therefore, a detailed work on degradation of RhodamineB (RhB) in a broad pH range (from 1.5 to 12.0) in MW-H2O2 systemwas conducted to check the influence on the degradation by thosedifferent pH values. To evaluate possible role of •OH oxidation at pH12.0, scavenging tests were introduced into the experiments. Fur-thermore, the degradation rate constant in our alkaline MW-H2O2system was compared to those from previous studies, which couldexplain the degradation efficiency of the system. The investigationon MW thermal effect was performed through control experimentsto explore degradation mechanism. To verify the effectiveness ofthis alkaline MW-H2O2 system, another substrate as methyleneblue (MB) was also tested. Through above studies, we hope to pavethe way for experimental researches at alkaline pH in MW-H2O2system, and provide a possibility for future application in alkalinewastewater treatment.

2. Experimental

2.1. Chemical reagents

RhB (CAS No.: 81-88-9, molecular mass: 479.02 g/mol), MB(CAS No.: 61-73-4, molecular mass: 319.85 g/mol), H2O2 (30%),

J. Hong et al. / Chemical Engineering Journal 191 (2012) 364– 368 365

mmae

2

TMaiwirmtndm

dMwtpmt2waadp

lwtt

2

wat

Fig. 1. UV–vis absorbance spectrum for 0.021 mM of RhB at different pH.

ethanol, 1, 10-phenanthroline monohydrate, potassium dichro-ate, ferrous ammonium sulfate, H2SO4 and NaOH were of

nalytical grade. Double distilled water was used throughoutxperiments.

.2. Experimental procedures

200 mL of RhB solution were poured into a 500 mL pyrex flask.he flask was quickly placed into a MW oven (MM721AAU-PW,ade in Media Co., China) operated at a frequency of 2.45 GHz

fter addition of H2O2. Reaction was timing when MW oven wasnitiated. Temperature (T) of the reaction solution was measured

ith a thermometer to nearly approach the boiling point of watern 5 min. In order to keep the solution volume constant, a watereflux condenser was connected to the flask via a 600 mm long com-unication pipe that went through an aperture in top oven. Note

hat an aluminum tube fixed in the aperture was used to elimi-ate MW leaking. The limit on the safe stray leakage of MW powerensity was kept below 0.5 mW/cm2 (GB 10436-1989), which waseasured at 200 mm distance horizontally from the aperture.Five treatments in control experiments were designed and con-

ucted. (1) MW, in which RhB aqueous solution was irradiated byW only. (2) H2O2, in which only RhB and H2O2 were coexistentithout MW irradiation. (3) Conventional heating (CH), referred to

hat the reaction was timing when RhB solution had already beenre-heated to boiling state using an electric stove. (4) CH-H2O2-L,eant the reaction was timing when H2O2 was added into the solu-

ion that was conventionally heated at room temperature (around5 ◦C). (5) CH-H2O2-H, meant the reaction was timing when H2O2as added into the solution that had already been convention-

lly pre-heated to boiling state. All of the above were conductedt pH 12.0. While for the MW-H2O2 system we studied, the degra-ation was performed in MW field in the presence H2O2 withoutre-heating.

Experiments for degradation data analysis were done in trip-icate. Samples were withdrawn in every several minutes. Blanks

ere subtracted from corresponding experimental data. For reac-ions at different pH, either NaOH or H2SO4 was added to adjusthem.

.3. Analysis

RhB absorbance at 550 nm was determined, against distilledater as the blank, with a Unico 7200 spectrophotometer using

1 cm glass cuvette. It was then converted to RhB concentra-ion referring to a standard curve that showed a linear behavior

Fig. 2. Degradation of RhB in MW-H2O2 system at different pH ([RhB] = 0.084 mM,[H2O2] = 489.7 mM).

between the concentration and the absorbance. To assess pH influ-ences on RhB’s absorbance at 550 nm, pre-experiments as UV–visabsorbance spectrum for RhB solution with pH of 1–12, respec-tively, were conducted using a Unico UV-2800 (Fig. 1). Resultsshowed that the absorbance at 550 nm appeared negligible blueor red shift for all pH values, except pH 1. Therefore, a standardcurve at pH 1 was made separately for the measurement of RhBconcentration in this situation. All other pH values were consideredto have little influence on the measurement of RhB concentrationwhen standard curve was used.

Chemical oxygen demand (COD) was determined using potas-sium dichromate solution as oxidizer in a strong acid medium, thenby titration step using ferrous ammonium sulfate as the reducingagent and Ferroin as the indicator [15].

3. Results and discussion

3.1. Degradation at alkaline pH

Degradation of RhB by MW-H2O2 system in a broad pH range,from acidic to alkaline conditions, was firstly studied (Fig. 2). Highefficiencies were obtained at extreme acidic pH (<2.0), compared tothose in the pH range of 2.5–9.9. Meanwhile, it was very interestingto find that RhB was nearly completely decomposed in 5–10 minat extreme alkaline pH (>11.0), especially at pH 12.0. Therefore,not only strong acidic conditions, but also extreme alkaline onescould sharply accelerate RhB’s degradation in MW-H2O2 system.This finding was different from those of previous studies whichfound that acidic conditions rather than alkaline were favorable todegradation in MW-H2O2 system [8,14]. The difference betweenour work and previous studies could partly attribute to the veryfew pH values (only pH 3, 6 and 11) they had studied [14]. It wasalso possible that substrates with distinct physico-chemical char-acterization could show different degradation behaviors in alkalineMW-H2O2 system.

To verify above new finding we observed, further UV–vis spec-tral scan for RhB’s degradation with a 150 times smaller ratio of[H2O2] to [RhB] in MW-H2O2 system at pH 12.0 was performed(Fig. 3). Results showed that the absorbance of RhB at both UVand visible regions were quickly decreased in 10 min’s reaction,suggesting RhB’s chromophores and most aromatic rings had been

destroyed. COD also decreased with time, around 43% of COD wasremoved in 14 min. COD is related to total concentration of organics,and the decrease of COD reflects the degree of mineralization.

366 J. Hong et al. / Chemical Engineering Journal 191 (2012) 364– 368

Table 1Comparisons of the degradation by MW-H2O2 with other technologies.

Technologies Substratesconcentration (mM)

Initial pH Reactiontemperature (◦C)

Substrates degradation COD or TOC loss References

MW-H2O2 0.21–1.05 12.0 ∼100 0.23–0.61 min−1 43% of COD in14 min (for[RhB] = 0.63 mM)

This study

Electro-Fenton 0.01 9 – 0.0064 min−1 – [16]Ethylenediamine–CuCl2

complex plus H2O2

0.05 9.18 25 ± 1 Around 0.0144 min−1

(calculated from thedata in the paper)

– [17]

Solar light/Mo6+-TiO2 – 8.0 – 0.0004–0.0108 min−1 – [18]UV/SrTiO3/CeO2 0.1 12 25 ± 1 >90% (100 min) 50% (300 min) [22]MW/NaH PO /Pd/C 1.0 NaOH media 90–180 ca. 75% – [23]

–

MgavcditlbtMsit

idsaprR(ecmmt

F[

12.0, because hydrogen peroxide was liable to be decomposed into

2 3

: not reported.

The degradation efficiency of RhB at extreme alkaline pH in ourW-H2O2 system was also compared to those by other technolo-

ies conducted in alkaline circumstances in previous work. Resultsre summarized in Table 1. RhB’s removal rate in our system wasery fast (0.23–0.61 min−1) at comparatively high substrate con-entrations (0.21–1.05 mM), while other studies showed very lowegradation rates [16–18]. COD loss in MW-H2O2 system we stud-

ed was also efficient in such a short time (43% in 14 min). Fromhe point of view of economy when dealing with practical alka-ine wastewater, the MW-H2O2 system we studied was very simpleecause it could avoid removing solid catalysts from aqueous solu-ion after reaction, compared to other heterogeneous technologies.

oreover, our system was environmental-friendly since it con-umed only H2O2 reagent that could decompose exothermicallynto water and oxygen gas spontaneously, facing no secondary con-amination when they were discharged into the environment.

Considering different substrates might show different behaviorsn MW-H2O2 system at strong alkaline circumstance, another basicye named MB was chosen as the substrate to test the efficacy of theystem (Fig. 4). Results showed that the chromophores of MB at 613nd 662 nm were also decomposed very fast in 10 min. The decom-osition at 662 nm followed the pseudo first-order kinetics, with aate constant of 0.3663 min−1 and t1/2 = 0.88 min. As a comparison,hB’s degradation rate constants at pH 12.0 were 0.23–0.61 min−1

Table 1). This clearly indicated that MB could also be degradedfficiently in the system. Due to our work’s limitation, we did nothoose more substrates to test this system in the researches. Still,

ore work on this respect should be carried out to mature thisethod, and some related work was on-going in our lab. However,

he above results we obtained had shown that it was possible to

ig. 3. Time series of RhB decay in conditions of initial pH 12.0, [RhB] = 0.63 mM,H2O2] = 24.5 mM in MW-H2O2 system; inlet depicts the change of COD.

(dechlorination)

degrade some recalcitrant compounds in alkaline MW-H2O2 sys-tem. We deemed that this method could become a new idea forscientific researches, and might be developed to be one of the effi-cient technologies for the treatment of recalcitrant compounds inalkaline wastewater.

3.2. Radical species’ test

Formation of hydroxyl radicals as active oxidants was alwaysconsidered to be the main process in substrates’ degradation inacidic and neutral MW-H2O2 systems, according to previous stud-ies [8]. Herein hydroxyl radicals that were possibly involved inMW-H2O2 system under condition of pH 12.0 were checked.

Extingushing experiments were performed by methanol whichwas a good •OH scavenger and had been used already in manyliterature [19]. Comparatively high concentrations of methanol(184.0–220.8 mM) were used to scavenge all possible •OH that wereproduced from decomposition of H2O2 (24.5 mM) in the reaction.Supposing •OH were the main species in substrate degradation,it should be totally suppressed by this large amount of methanol.But result from extingushing experiments show that the degrada-tion percentages were only 11–57% decreased in 5 min than thatin original MW-H2O2 process (Fig. 5), which indicated that at leastthe main attack toward substrate was not •OH oxidation. On theother hand, it was probably due to the role of activated oxygendecomposed from hydrogen peroxide under such an alkaline pH as

oxygen and hydrogen in alkaline circumstances [17], which wasdifferent from that in acidic conditions. The reason why RhB degra-dation was reduced to some extent when methanol was added

Fig. 4. Time series of MB decay in MW-H2O2 system, inset is the degradation of MBas a function of time ([MB] = 0.313 mM, H2O2 = 49.0 mM, pH 12.0).

J. Hong et al. / Chemical Engineering Journal 191 (2012) 364– 368 367

Fig. 5. Extingushing experiments by methanol in MW-H2O2 system(

cm

dwtteeap2sefuaddlddH

Fi

Fig. 7. Control experiments of MW, H2O2, CH, CH-H2O2-L, and CH-H2O2-H in MW-H2O2 system ([RhB] = 0.63 mM, pH 12.0, [H2O2] = 24.5 mM if added, CH: the reactionwas timing when RhB solution had already been pre-heated to boiling state usingan electric stove, CH-H2O2-L: the reaction was timing when H2O2 was added intothe solution that was conventionally heated at around 25 ◦C, CH-H2O2-H: the reac-

[RhB] = 0.63 mM, [H2O2] = 24.5 mM, pH 12.0).

ould be explained by the consumption of activated oxygen byethanol oxidation.When H2O2 was considered to generate active oxygen to oxi-

ize substrates in alkaline circumstances, it was necessary to knowhether and to what extent H2O2 would have impacts on degrada-

ion in our MW-H2O2 system, which could also be another evidenceo explain the species that were acting on degradation. Thereforeffect of the ratio of [H2O2]/[RhB] on substrate degradation wasxamined (Fig. 6). The degradation rate constants were increaseds [H2O2]/[RhB] were raised from 46.7 to 233.3, afterwards alateau was observed when [H2O2]/[RhB] reached the range of33.3–699.5, indicating there was an optimal usage of H2O2 in theystem. It also suggested that a huge amount of H2O2 neither accel-rated nor decreased the degradation, which was totally differentrom the oxidation by •OH that would be scavenged by excess H2O2nder this situation. Thus it could confirm the deduction above thatt least •OH were not the main species responsible for substrateegradation. When O2 was thought to be responsible for substrate’segradation, RhB should be decomposed in the same way regard-

ess how much H2O2 were added. Because there would not be anyegradation enhancement when RhB had already been completelyegraded through O2 oxidation decomposed from a certain level of

2O2, though more H2O2 still existed in the solution at that time.

ig. 6. Effect of [H2O2]/[RhB] on the pseudo first-order degradation rate constantsn MW-H2O2 system ([H2O2] = 24.5–146.9 mM, [RhB] = 0.21–1.05 mM, pH 12.0).

tion was timing when H2O2 was added into the solution that had already beenconventionally pre-heated to boiling state).

3.3. MW effects

In MW magnetic field, usually polar molecules like water areforced to rotate with the magnetic field and lose energy in colli-sions. Therefore MW can heat polar solvents quickly throughoutthe aqueous solution body (MW thermal effect). By contrast, CH-based heat generates thermal gradient and is transferred with moreheat loss. In our MW-H2O2 system, temperature of the solution wasobserved to increase quickly to nearly boiling point of water in ashort time. So it was still a question if MW thermal effect couldinfluence substrate’s degradation. Therefore, several comparativetreatments were done to investigate it (Fig. 7). It was interesting tofind that the degradation by CH-H2O2-H treatment was the fastestwithin 2.5 min’s reaction, compared to those of CH-H2O2-L andMW-H2O2. Meanwhile, the treatments of H2O2, CH or MW showeda little influence on the degradation. This inferred that the tempera-ture of the reaction solution was crucial to substrate’s degradation.So MW thermal effect should play an important role in substrate’sdegradation. In alkaline MW-H2O2 system, high temperature mightaccelerate H2O2’s decomposition to release O2 and finally oxidizeRhB.

It was noted that the degradation percentage by MW-H2O2system was 12.9% higher than any of those by CH-H2O2-L and CH-H2O2-H treatments. It was possible that MW non-thermal effectwould take some effect on the degradation, since MW non-thermaleffect was supposed not to require the transfer of microwaveenergy into thermal energy, and was responsible for some unusualobservations [20]. However, this issue needed more investigationsdue to too much distribution inhomogeneities in the electromag-netic field.

Several phenomena appeared in these comparative experi-ments were necessary to be mentioned. For the treatment of CH,a negative degradation percentage was observed, although at asmall level. In fact, a color fading of the solution was found to turnfrom pink-red to deep red during this treatment, which might beascribed to the formation of some other kinds of compounds underhigh temperature. For MW treatment, no obvious RhB loss wasobserved, and this supported the fact that the energy of microwave(E < 1.0 J·mol−1 at � = 2.45 GHz) was too low to disrupt moleculebonds. For the treatment of H2O2, 11% of RhB was removed in 5 min,

which was in accordance with the results by AlHamedi et al. [21]that hydrogen peroxide could be decomposed spontaneously witha slow decomposition rate at alkaline pH.

368 J. Hong et al. / Chemical Engineering

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Fig. 8. Degradation mechanism in alkaline MW-H2O2 system.

In general, it was no doubt that MW thermal effect could playn important role in substrate’s decomposition in alkaline MW-2O2 system. Besides, it seemed that the way to heat the aqueous

olution might influence reaction rates, since MW-enhanced H2O2-ased process was superior to CH-H2O2 based systems in termsf degradation efficiency in the final stage. Still, more investiga-ions on this issue were required to explore the reason. Overall,e provided an illustration describing the mechanism of sub-

trate’s degradation at strong alkaline pH in MW-H2O2 system fromur study (Fig. 8). The main species as O2 were generated fromhe decomposition of H2O2 in alkaline MW-H2O2 system, whichhen oxidized the substrate. Meanwhile, MW thermal effect couldnhance the decomposition of H2O2, generating more O2 to acceler-te substrate’s degradation. This process was totally different fromOH oxidation observed under neutral and acidic conditions.

. Conclusion

This study demonstrated that MW-H2O2 process could effi-iently degrade refractory substrates at strong alkaline pH, via O2xidation and the enhancement of MW thermal effect. This effec-ive method was environmental-friendly, since it only consumedhemical reagent as H2O2 that could be decomposed into non-oxic substances of H2O and O2. It should be mentioned that usingdvanced MW reactors (temperature sensor, pressure control, MWbsorbing materials, multi-magnetron) would be helpful to thistudy, and we were considering updating our MW reactor. How-ver, we considered that the degradation mechanism in alkalineW-H2O2 system using those developed MW reactors would be

o difference if the same conditions (reaction temperature, pres-ure, pH in our study) were performed, because the developednes could only alter reaction rates from above mechanism deduc-ion. Anyway, we hope this work would bring great benefits tohe researches on non-biodegradable compounds’ degradation inxtreme pH conditions, and provide a possible way for future appli-ations in alkaline wastewater treatment.

cknowledgments

This work was financially supported by National Natural Sci-nce Foundation of China (50808167). We appreciate Mrs. Yue Lei

[

Journal 191 (2012) 364– 368

and Mrs. Alafei-Nama-Janice Sandrine for parts of raw data theyprovided. Also we want to thank the editor and two anonymousreviewers for their kind suggestions on this work.

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