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Electrochimica Acta 143 (2014) 180–187

Contents lists available at ScienceDirect

Electrochimica Acta

j our na l ho me pa g e: www.elsev ier .com/ locate /e lec tac ta

egradation of amaranth dye in alkaline medium by ultrasonicavitation coupled with electrochemical oxidation using aoron-doped diamond anode

illyam R.P. Barros, Juliana R. Steter, Marcos R.V. Lanza1, Artur J. Motheo ∗,1

nstituto de Química de São Carlos, Universidade de São Paulo, Avenida Trabalhador São Carlense 400, São Carlos, 13566-590, SP, Brazil

r t i c l e i n f o

rticle history:eceived 11 June 2014eceived in revised form 21 July 2014ccepted 22 July 2014vailable online 10 August 2014

eywords:maranth dyendocrine disruptororon-doped diamondltrasonic cavitationineralization

a b s t r a c t

Amaranth dye is used widely in the processing of paper, textiles, foods, cosmetics, beverages andmedicines, and effluents contaminated with this compound are discharged daily into the environment.Recent studies have shown that azo dyes, especially those such as amaranth dye that have been classifiedas endocrine disruptors, may cause adverse effects to animal and human health. This paper describes theapplication of electrochemical oxidation (with a boron-doped diamond BDD thin-film anode) coupledwith ultrasound sonolysis (20 kHz and 523 W cm−2) to the removal of amaranth dye from dilute alkalinesolution. The electrochemical and sonoelectrochemical processes (ECh and SECh, respectively) were car-ried out at constant current density (10 to 50 mA cm−2) in a single compartment cylindrical cell. Sonolysiswas virtually less useful for the decolorization and degradation of amaranth dye, whilst ECh and SEChwere more effective in degrading the dye with almost complete removal (90 - 95%) attained after 90 minof experiment at an applied current density of 50 mA cm−2. Degradation of the dye followed pseudo first-order kinetics in both processes, but the rate of reaction was faster with the SECh treatment confirminga synergistic effect between the cavitation process and the electrochemical system. Additionally, at low

−2

applied current densities (10 and 25 mA cm ), SECh was considerably more effective than ECh for theamaranth dye mineralization. Although at 35 and 50 mA cm−2, the two processes showed the respectiveremoval of total organic carbon values: (i) 85% for the ECh and 90% for the SECh at 35 mA cm−2; (ii) 96%for the ECh and 98% for the SECh at 50 mA cm−2. It is concluded that SECh presented the most favorableresults for the decontamination of wastewaters containing azo dye compounds.

© 2014 Elsevier Ltd. All rights reserved.

. Introduction

The azo dyes comprise a class of synthetic organic compoundshat are characterized by the presence of one or more -N = N- chro-

ophoric groups conjugated to an aromatic system. These dyes aresed in a wide range of industrial activities, including those associ-ted with the production and processing of leather, textiles, food,osmetics, medicines, paper and cellulose [1], and account for about0% of global synthetic dye production [2]. A number of azo dyes arearcinogenic while others undergo oxidation processes that form

y-products, such as aromatic amines, that are more toxic than theriginal dye itself [3,4].

∗ Corresponding author. Tel.: +55 16 3373 9932; fax: +55 16 3373 9952.E-mail address: artur@iqsc.usp.br (A.J. Motheo).

1 ISE Active Members

ttp://dx.doi.org/10.1016/j.electacta.2014.07.141013-4686/© 2014 Elsevier Ltd. All rights reserved.

Amaranth dye (E123) is a sulfonic acid-based napththylazo dye(Fig. 1) that is commonly used to impart a red to purple colorationto foodstuffs, cosmetics and medicines. The dye has been classi-fied as an endocrine disruptor, reportedly causing adverse effects toanimal and human health [5,6], although studies concerning its car-cinogenicity and safety are somewhat contradictory. On the otherhand, the reductive cleavage of the azo bonds in amaranth dyeproduces amines that are known to be carcinogenic [7]. While theuse of amaranth dye as a food colorant is prohibited in the USA, it ispermitted in many other countries, including Canada, because thechemical structure is quite similar to other dyes considered non-carcinogenic. In England, amaranth dye is permitted temporarilyuntil more conclusive studies become available, although the dyehas been banned voluntarily by food industries in Japan [8,9].

Over the last few decades, the production and use of syn-thetic dyes, including amaranth dye, has risen considerably with aconcomitant increase in the generation of industrial effluents con-taminated with these compounds. The development of effective

W.R.P. Barros et al. / Electrochimic

NaO3S N N

SO3NaHO

SO3Na

ttplcct

tpptnopecahpc[mm

spsbwstwcfbn(Tccssto

oatttd

of the applied current that is utilized in the reaction of interest, rep-resents an important parameter to characterize an electrochemicalprocess. Mineralization of amaranth involves the complete conver-

Fig. 1. Molecular structure of amaranth dye.

reatments for dye-containing wastewaters represents an impor-ant challenge, and this has driven the search for new methods ofrotecting the environment from the adverse impacts of these pol-

utants. The availability of an efficient process for the removal ofolor from industrial effluents would help to improve the livingonditions of the population and, consequently, the protection ofhe environment [10].

Among the advanced technologies that have been applied tohe degradation of recalcitrant organic compounds, those incor-orating the electrochemical process (ECh) appear to be the mostromising [11–14]. The principal feature of ECh technology ishe in situ production of the hydroxyl radical (•OH), a strongon-selective oxidizing agent that promotes the mineralization ofrganic pollutants at a rate controlled by diffusion [15–25]. Therocess presents a number of advantages, including high energyfficiency, versatility and ease of automation [26], but the effi-iency achieved depends to a great extent on the nature of thenodic material employed. In this context, a number of studiesave demonstrated that boron-doped diamond (BDD) thin filmsresent a larger O2 over-voltage and greater oxidizing power thanonventional anodes such as PbO2 [27], SnO2 [28] and Ti/IrO229]. Moreover, it is reported that BDD can promote the complete

ineralization of a wider range of compounds than other anodicaterials [11–13,18–22].Coupling ECh with another advanced oxidation process, such as

onolysis, can lead to an increase in effectiveness as assessed by thearameters reaction rate, mineralization efficiency and energy con-umption [30,31]. In the sonochemical process (SCh), cavitationalubbles are typically produced by the propagation of ultrasoundaves in a bulk liquid, and these bubbles can oxidize organic sub-

tances either directly, by thermolytic reactions occurring insidehem, or as a result of the formation of •OH by the sonolysis ofater [32–37]. The phenomenon, known as acoustic cavitation,

omprises three distinct and successive steps: (1) nucleation–theormation of microbubble particles suspended within the liquid; (2)ubble growth–in which the bubbles grow and expand in a man-er restricted by the intensity of the applied ultrasound wave, and3) bubble collapse - violent implosion or bubble rupture [32,38].he synergistic effect between ECh and SCh is associated with theomplex oxidation mechanism of sonolysis and the capacity of theavitational bubbles to enhance mass transfer at the electrode-olution interface and to contribute to the cleaning of the electrodeurface. Moreover, Fitzgerald et al. [39] observed that the forma-ion of hydrogen peroxide might also assist in the degradation ofrganic compounds.

The aim of the present study was to compare the efficiencyf ECh and the sonoelectrochemical process (SECh) in degradingmaranth dye in alkaline medium using a BDD thin-film anode. Tohis end, the rates of decolorization and decay in concentration ofhe dye were determined, together with the levels of mineraliza-

ion achieved and the mineralization energy efficiencies and energyemands of the systems.

a Acta 143 (2014) 180–187 181

2. Experimental

2.1. Reagents

Amaranth dye (95% pure) was purchased from Sigma-Aldrich,methanol (HPLC grade) was from Merck, ammonium acetate andpotassium hydroxide were from Synth, and potassium sulfate wasfrom Vetec. All reagents were used without further purification.Solutions were prepared with ultrapure water (resistivity > 18 M�)delivered from a Millipore Milli-Q system.

2.2. ECh and SECh procedures

ECh experiments were conducted in a temperature-controlled(25 ± 1 ◦C) single compartment Pyrex cell (capacity 400 mL) con-taining an anode (apparent geometric area 9.68 cm−2) comprisinga BDD thin-film deposited on single crystal p-type Si (100)wafer (provided by Adamant Technologies, Switzerland), a cir-cular platinum net as cathode, with apparent geometric area of10.24 cm−2 (pure platinum 99.9%: net with Ø = 36.1 mm; wireØ = 0.25 mm; space between the wires of 2.0 mm) and a saturatedcalomel electrode (SCE) as reference. In the SECh experiments,the ECh system described above was employed in conjunctionwith a Misonix model XL-2020 Sonicator® programmable ultra-sonic liquid processor operating at 20 kHz with an intensity of523 W cm−2.

Galvanostatic electrochemical oxidations were investigated ona Metrohm model PGSTAT-30 coupled to a model BSTR-10A highcurrent module and performed at constant current densities of 10,25, 35 or 50 mA cm−2 applied for 90 min. The electrolyte compriseda solution of amaranth dye (100 mg L−1) in 0.05 mol L−1 K2SO4adjusted to pH 12.

2.3. Analytical analyses

The mineralization of amaranth dye was assessed from thedecrease in total organic carbon (TOC) present in the filtered elec-trolyte (0.45 �m Millipore filter) as recorded on a Shimadzu modelTOC-VCPN analyzer. The rate of decolorization of the dye was mon-itored at 521 nm using a Varian Cary-50 UV-VIS spectrometer.

Quantitative determinations of the rate of decay of the con-centration of amaranth dye were performed by high performanceliquid chromatography (HPLC) using a Shimadzu model LC-20ATinstrument coupled to a diode array detector (model SPD-M20A)and a UV detector (model SPD-20A) set at 260 nm, and equippedwith a Phenomenex LC18 column (250 × 4.6 mm i.d.). Elution wasisocratic with a mobile phase comprising ammonium acetate(0.08 mol L−1) and methanol (70:30, v/v) supplied at a flow rateof 0.5 mL min−1.

Energy consumed EC (kWh Kg−1) during the degradation pro-cesses were calculated according equation 1, for each processes,where I is the current applied (A), U is the cell potential (V), t is thetime of electrochemical oxidation (h), V is the volume of the work-ing solution (L) and �TOC is mass of TOC removed (Kg). Observe that5 kW is the value of the incident power on the anode associated toSECh process [26]:

ECECH = UIt

1, 000V(�TOC)ECSECh = [UI + 5]t

1, 000V(�TOC)(1)

Mineralization energy efficiency MEE (%), defined as the fraction

sion of the dye and its initial breakdown products to carbon dioxideaccording to:

1 chimica Acta 143 (2014) 180–187

C

is

a

M

wfitta((

tTpM

3

3

tm

ab→[orf[

c

0

20

40

60

80

100(d)

(c)

(b)

(d)

(a)

(b)

(c)

(a)

ECh SEC h

(1 -

Abs

t / Ab

s 0) x 1

00 /

%

SCh

Fig. 3. Decolorization of amaranth dye (from solutions containing 100 mg L−1 ofanalyte in 0.05 mol L−1 K2SO4 adjusted to pH 12) monitored at 521 nm following

Fo1

82 W.R.P. Barros et al. / Electro

20H11N2O10S3− + 48H2O → 20CO2 + 107H+ + 100e−

+ 2NO3− + 3SO4

2− (2)

n which 100 electrons are involved in the degradation of 1 mol ofubstrate.

The MEE associated with the oxidation of amaranth dye solutionnd byproducts formed were calculated from the equation:

EE = �(TOC)t nF V

4.32 × 107 mIt× 100 (3)

here �(TOC)t represents the value of TOC removal (mg L−1) at thenal time, n is the number of electrons involved in the mineraliza-ion of amaranth, F is the Faraday constant (96,487 C mol−1), V ishe volume of the working solution (L), m is the number of carbontoms of amaranth dye (20 atoms), I is the applied current intensityA), t is the time interval (h) and 4.32 × 107 is a conversion factor3600 s h−1 × 12,000 mg mol−1) [26,40].

The byproducts formed after the degradation process were fil-ered and extracted with methanol by solid phase extraction (SPE).he analyses were performed by HPLC-UV-MS (Waters 2695) cou-led to a Waters 2996 photodiode array detector and a Watersicromass ZQ ESCi multi-mode ionization source.

. Results and discussion

.1. Decolorization of amaranth dye

The UV-VIS absorption spectra of amaranth dye solutions prioro electrochemical oxidation and after SCh, ECh and SECh treat-

ents are shown in Fig. 2.The characteristic absorption band of the azo group (-N = N-) of

maranth dye showed a �max of 521 nm, while two low intensityands localized between 245 and 360 nm were associated with �

�* transitions in aromatic rings conjugated with the azo bond41,42]. The reduction in intensity of the 521 nm band after 90 minf ECh or SECh treatment was directly related to the applied cur-ent density, verifying the rupture of the azo bond and the likely

ormation of byproducts, i.e. aliphatic acids, as indicated in Fig. 842,43].

The percentage decolorization of amaranth dye solution wasalculated from the initial and final absorbance values (Abs0 and

240 360 480 600 720

0.0

0.4

0.8

1.2

(f)

(A)

λ

Abso

rban

ce /

a.u

(a)

ig. 2. UV-VIS spectra of amaranth dye (from solutions containing 100 mg L−1 of analyf electrolysis. Panel (B): SECh process after 90 min of electrolysis. Electrolyte prior to e0 mA cm−2 (plot c), 25 mA cm−2 (plot d), 35 mA cm−2 (plot e) or 50 mA cm−2 (plot f).

SCh process for 90 min and following ECh or SECh process treatments for 90 min atapplied current densities of 10 mA cm−2 (plot a), 25 mA cm−2 (plot b), 35 mA cm−2

(plot c) or 50 mA cm−2 (plot d).

Abst, respectively) according to equation 4 [44], and the results aredisplayed in Fig. 3.

Decolorization (%) = (1 − Abst

Abs0) × 100 (4)

In the ECh and SECh treatments, the percentage decoloriza-tion increased as the applied current density increased. SEChshowed the highest levels of color removal of 88 and 95% after90 min of electrochemical oxidation at current densities of 35 and50 mA cm−2, respectively. While ECh was also effective in remov-ing color from amaranth dye solutions, the decolorization valuesobtained were somewhat lower at around 80 and 92%, respectively,for the same applied current densities.

3.2. Decay in amaranth dye concentration

The HPLC chromatograms presented in Fig. 4 reveal that sonol-ysis for 90 min at 20 kHz and 523 W cm−2 led to a reduction ofjust 8.3% in the concentration of amaranth dye. In contrast, EChand SECh treatments were effective in degrading the dye, and the

/ nm

240 36 0 480 60 0 72 0

Abso

rban

ce / a.u

(B)

(f)

(a)

0.0

0.4

0.8

1.2

te in 0.05 mol L−1 K2SO4 adjusted to pH 12). Panel (A): ECh process after 90 minlectrochemical oxidation (plot a), electrolyte after 90 min of SCh process (plot b),

W.R.P. Barros et al. / Electrochimica Acta 143 (2014) 180–187 183

2

4

6

8

10

0

150

300

450

600

(d)

(f)(e)

(c)

(b)

Inte

ns i

ty/

mV

Reten

tion

time / m

in

(a)(A)

2

4

6

8

10

0

150

300

450

600

(B)

Inte

nsi

ty/

mV

Reten

tion

time / m

in(e)

(d)(c)

(b)

(f)

(a)

F L−1 o −1

e and e( plots (p

otarai

Fa3T

ig. 4. HPLC chromatograms of amaranth dye (from solutions containing 100 mglectrochemical oxidation (plot a); electrolyte after 90 min of SCh process (plot b);plot c), 25 mA cm−2 (plot d), 35 mA cm−2 (plot e), or 50 mA cm−2 (plot f). Panel (B):

rocess at applied current densities as defined in panel (A).

bserved decline in the concentration of analyte was related tohe current density applied in both processes, directly. However,

long with the reduced intensity of the peak corresponding to ama-anth dye (retention time 6.3 min), a number of additional peaksppeared that were attributed to intermediate species formed dur-ng the degradation processes.

0 10 20

0.00

0.01

0.02

0.03

0.04

0.05

SCh

j / mA

k app /

min

-1

(C)

0 30 60 90

0.0

0.5

1.0

1.5

2.0

2.5

3.0(A)

-ln C

t / C

0

Time / min

ig. 5. Variation of ln Ct/C0 considering HPLC concentration decay, for the degradation of adjusted to pH 12) as a function of electrochemical oxidation time (90 min) for the ECh5 mA cm−2 (�) 50 mA cm−2. (C) Apparent pseudo first-order rate constants (kapp), distribhe unique kapp value for the sonochemical treatment is labeled SCh.

f analyte in 0.05 mol L K2SO4 adjusted to pH 12). Panel (A): electrolyte prior tolectrolytes after 90 min of ECh process at applied current densities of 10 mA cm−2

a) and (b) - as in panel (A); plots (c) to (f) - electrolytes subjected to 90 min of SECh

As shown in Fig. 4, the effect of increasing applied current onthe rate of degradation of amaranth dye was greater in SECh than

in ECh, although the removal of analyte was complete in both pro-cesses at current density of 50 mA cm−2.

The synergism observed between sonolysis and ECh may beassociated with the generation of •OH under cavitation action at the

30 40 50

cm-2

0 30 60 90

-lnC

t/

C0

Time / min

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0(B)

maranth dye (from solutions containing 100 mg L−1 of analyte in 0.05 mol L−1 K2SO4

(A) and SECh (B) processes. (+) SCh process (�) 10 mA cm−2 (©) 25 mA cm−2 (�)uted according to current densities applied during ECh (�) and SECh (©) process.

1 chimica Acta 143 (2014) 180–187

aoo

cedtsam

daotpr

3

apm

rmwmt

3

mlaw75

iS(

icetTob

3

vid(maoab

Fig. 6. Percentage of total organic carbon (TOC) removed from amaranth dye solu-tions (containing 100 mg L−1 of analyte in 0.05 mol L−1 K2SO4 adjusted to pH 12)following SCh process for 90 min, and following ECh and SECh process at differentapplied current densities for 90 min of electrolysis.

20

40

60

80(A)

j / mA cm-2

ME

E /

%

20

40

60

80

100

Energ

y co

nsu

mptio

n / k

Wh k

g-1

10 20 30 40 50

10 20 30 40 50

20

40

60

80

100

260

280

300

320

340

j / mA cm-2

(B)

En

ergy co

nsu

mptio

n / k

Wh k

g-1

ME

E /

%

84 W.R.P. Barros et al. / Electro

node surface. The excess of reactive radicals react directly with therganic compound, and this may lead to the formation of secondaryxidant species such as H2O2.

Hydroxyl radicals react with molecules of amaranth dye by suc-essive additions to unsaturated groups, hydrogen abstraction andlectron transfer, and diffusion processes control the rate of degra-ation. The −OH ions present, in the reaction medium, contribute tohe process, because under alkaline conditions the potential neces-ary for water discharge is lower and this facilitates •OH productiont the BDD anode and, consequently, the degradation of organicaterial becomes more useful.Considering the hybrid SECh process, the amaranth dye can be

egraded by: (i) •OH generated at the electrode, (ii) •OH gener-ted by sonolysis of H2O, (iii) thermolytic reactions, and/or (iv)ther oxidizing species, such as hydrogen peroxide, generated inhe bulk solution. Then, combination of sonolysis with an EChrocess shows a significant improvement in the efficiency of dyeemoval.

.3. Kinetic and energetic parameters of amaranth dye oxidation

Kinetic analysis of the degradation of amaranth dye under EChnd SECh conditions established that the reactions followed theseudo first-order kinetic model since the rates were dependentainly on the concentration of reagent (Fig. 5A and 5B).Plots of kapp (min−1) versus applied current density (Fig. 5C)

evealed that, in comparison with ECh, the oxidation reactions wereore rapid with SECh and kapp values of up to 4.6 × 10−2 min−1

ere recorded. It is observed an increase in reaction rate in basicedium was associated with an–OH-mediate reduction in poten-

ial required for the discharge of water.

.4. Mineralization of amaranth dye

The mineralization of amaranth dye was established fromeasurements of the percentage of TOC removed from the ana-

yte solution after 90 min process time (Fig. 6). Only a smallmount (around 19%) of TOC was removed by the SCh process,hile with SECh TOC removal was effective and ranged from

4% at an applied current density of 10 mA cm−2 up to 98% at0 mA cm−2.

ECh showed a similar trend of increased TOC removal withncreased applied current, but the process was less effective thanECh even though almost complete mineralization of amaranth dye95%) could be achieved at 50 mA cm−2.

The plots indicate that the amaranth mineralization valuesncrease in function of the applied current density. This behavioran be explain according the increase in the •OH radical species gen-ration. Then, as the mechanism of oxidation is controlled by massransport, for higher current densities observed greatest reductionsOC, since the oxidation reactions are displaced to the formationf secondary oxidant species that help in the oxidation process ofyproducts generated.

.5. Mineralization energy efficiency and energy consumption

Fig. 7A and 7B shows plots of mineralization energy efficiencyalues (MEE) associated with the oxidation and parallel reactionsnvolved in the degradation of organic compound versus currentensity applied during 90 min of ECh and SECh. In the both processECh and SECh), MEE values were in the region corresponding to

ass transport processes (i.e. < 100%) and declined with increasing

pplied current. This decrease can be attributed to the occurrencef secondary reactions, such as oxygen evolution and H2O2 gener-tion. A similar decline in MEE was observed with the SECh processut, as expected, the values were higher than those obtained with

Fig. 7. Values of mineralization energy efficiency (MEE) and energy consumption(EC) for the degradation of amaranth dye (from solutions containing 100 mg L−1 ofanalyte in 0.05 mol L−1 K2SO4 adjusted to pH 12) distributed according to currentdensity applied during 90 min of ECh (A) and SECh (B) process.

W.R.P. Barros et al. / Electrochimica Acta 143 (2014) 180–187 185

N-O3S N

HO

SO3-

SO3-(I)

1) azo bon d r upture2) elimination of -SO3

- gro up

NH2

OH

+

NH2

OH

mixture of isomer s

- NH2

OH

OH

OH

(II)

OH

OH

3)

O

Otau tome rs

OH

OH

OH

O

O

(III)

- H

internal cyclizat ion

O

O

O

OH

(IV)

-COOH

OH

OH

O

(V)

-COO H

OH

OH

(VI)

OH

OH

HOOH

OH

O

O (VII)

oxidative ring opening

aliphatic acids

HO

O

OH

O

(VIII )

CO2 + H2O

Fig. 8. Proposed degradation route for the electrochemical oxidation of amaranth dye (from solutions containing 100 mg L−1 of analyte in 0.05 mol L−1 K2SO4 adjusted to pH12) using BDD anode and applying 35 mA cm−2 during 5 h.( z = 166=

Eco

comcfsbs

sd

e

I) amaranth dye, m/z = 535; (II) naphthalenediol, m/z = 160; (III) phthalic acid, m/94; (VII) fumaric acid, m/z = 116 and (VIII) oxalic acid, m/z = 90.

Ch because of the synergistic effect associated with the hybrid pro-ess, an effect that was particularly significant at a current densityf 10 mA cm−2.

Considering that the limiting current density for the electro-hemical cell employed was 3.28 mA cm−2, hence a current densityf 10 mA cm−2 would was closest to the limiting current deter-ined for the electrochemical system employed and, consequently,

losest to the charge transfer region (MEE = 94.56% for SECh and 80%or ECh). Then, the organic species are oxidized on the electrodeurface, this suggests that the oxidation reactions were mediatedy •OH near to the BDD interface and by •OH present in the bulkolution.

Plots of the amount of electric energy (EC in kWh Kg−1) con-

umed during the oxidation of organic compound versus currentensity applied during 90 min of ECh and SECh.

The EC values were determined for the two processes accordingquation 1, and these increased according to increase in applied

; (IV) phthalic anhydride, m/z = 158; (V) benzoic acid, m/z = 122; (VI) phenol, m/z

current density. Is necessary to consider that the incident poweron the anode was 5 kW (sonicator operating with an intensity of523 W cm−2) for the SECh process.

Observed that higher current densities caused the substantiallydecrease in the mineralization energy efficiencies values (MEE) andraising the energy consumption, because part of the applied currentis being used (i) to degrade the dye, (ii) to degrade the byprod-ucts formed and (iii) to parallel reactions. As long as the currentapplied is above the limiting current density value, secondary reac-tions such as O2 evolution are favored and the electro-energy hasbeen utilized to the amaranth dye mineralization, preferably.

The power consumption or energy consumption increases withthe increase of the current density, and it presents higher values for

SECh process, because the increasing contribution of sonolysis. Forboth cases, at lower current densities it is observed that the highestvalues of mineralization energy efficiency, indicating that almost allof the applied current is being used to degrade the amaranth dye.

1 chimic

tSrisc1p4

ft

3

3ii

ttl1

oimiaz

ait

4

facital4e

ctstpEgc

w9

att

[

[

[

86 W.R.P. Barros et al. / Electro

Fig. 7A and 7B indicate that the energy absorption for both sys-ems studied, in general, is dominated by the contribution of theECh process. This because, according to the increase in applied cur-ent density, there is an increase in the EC values and this increases higher for the SECh system, since it considers the contribution ofonolysis. However, an increase in EC values is more evident whenomparing the ECh and SECh processes at low current densities i.e.0 mA cm−2. There is an increase more than 13 times for the SEChrocess, while at highest current densities this increase is around

times.It should be clear that the energy consumption was calculated

or the particular set up used, without any concerns about a designhat can be lead to viable operational cost.

.6. Proposed degradation route

Electrochemical oxidation of amaranth dye was performed at5 mA cm−2 using BDD anode during 3 h. Aliquots were collected

n 1 h intervals and the intermediates of degradation process weredentified by HPLC-MS.

Initially, the cleavage of the azo bond occurs by cathodic reduc-ion mechanism, in which aromatic amines are identified [45]. Inhe next step, there is the elimination of sulfonic acid groups, fol-owed by insertion of •OH, forming a mixture of isomers (1,4 and/or,2- aminenaphtol).

At this point occurs the NH2 elimination followed by insertionf •OH, undergoes the naphthalenediol (II), this intermediate isn equilibrium keto-enolic (tautomers), as shown in the proposed

echanism and after addition of •OH radicals phthalic acid can bedentified (III). The intermediate (III) can be converted into phthalicnhydride (IV) (internal cyclization) and subsequently into the ben-oic acid (V), after descarboxylation and hydroxylation processes.

Consequently, phenol (VI) is formed as a reactive intermediate,nd after the addition of •OH radicals the fumaric acid (VII) can bedentified. From this stage, the aromatic ring opening occurs andhen aliphatic acids, i.e. oxalic acid (VIII), are identified [22].

. Conclusions

Sonolysis by means of an ultrasound device was virtually inef-ective in the decolorization and degradation of amaranth dye inqueous alkaline solution producing just 8% removal in 90 min pro-ess time. On the other hand, both ECh and SECh were efficientn degrading the dye with almost complete removal (90 - 95%) ofhe analyte attained after 90 min of electrochemical oxidation at anpplied current density of 50 mA cm−2. Degradation of the dye fol-owed pseudo first-order kinetics with kapp values at 50 mA cm−2 of.6 × 10−2 min−1 for SECh and of 3.5 × 10−2 min−1 for ECh, consid-ring HPLC concentration decay.

At low applied current densities (10 - 25 mA cm−2), SECh wasonsiderably more effective than ECh regarding the mineraliza-ion of amaranth dye, although at 50 mA cm−2, the two processeshowed similar values with 95% TOC removal after 90 min of elec-rochemical oxidation. The results obtained in the present studyrovide clear evidence of a synergistic effect between sonolysis andCh since the hybrid process exhibited an oxidation power that wasreater than that expected of ECh alone, especially for the appliedurrent density of 10 mA cm−2.

In the comparison of the processes, the results most favorableere obtained for SECh at 10 mA cm−2, reaching MEE value around

5% and 80% for the ECh.

The EC values determined for the two processes increased

ccording to applied current density and the energy consump-ion values are higher considering the SECh system, accordinghe contribution of sonolysis (5 kW). Observed that higher

[

a Acta 143 (2014) 180–187

current densities caused the substantially decrease in the cur-rent efficiencies and raising the energy consumption. However,this increase is more evident when comparing the ECh andSECh processes at low current densities, i.e. 10 mA cm−2 (20.6and 263.9 kWh Kg−1, respectively), while at highest currentdensities the energy consumption values are 102.7 and 337.3 kWhKg−1, respectively for ECh and SECh processes (50 mA cm−2).

For both processes, at low current densities observed that thehighest values of mineralization energy efficiency, indicating thatalmost all of the applied current is being used to degrade thedye. While for high current densities, the values of mineralizationenergy efficiencies reduce drastically, because the applied currentis applied to degrading the amaranth dye and byproducts, for exam-ple.

The power consumption increases with the increase of the cur-rent density, and presents a more pronounced for SECh process dueto increased contribution of sonolysis process. The overall resultis that ultrasound increases the rate of electrochemical oxidationof amaranth, but the process is less efficient in terms of energyconsumption.

A mechanism for the electrochemical oxidation of amaranthdye was proposed according aromatic and aliphatic intermediatesidentified by HPLC-MS.

Acknowledgments

The authors are grateful to Conselho Nacional de Desen-volvimento Científico e Tecnológico (CNPq), Coordenac ão deAperfeic oamento de Pessoal de Nível Superior (CAPES), Fundac ãode Amparo à Pesquisa do Estado de São Paulo (FAPESP) andBioCiTec/IQSC/USP for financial support for this research. Theauthors are also indebted to Professor Roberto Gomes de SouzaBerlinck of the Universidade de São Paulo (Instituto de Químicade São Carlos) for providing access to the HPLC-UV-MS chromato-graph.

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