Chemosphere 109 (2014) 49–55

Contents lists available at ScienceDirect

Chemosphere journal homepage: www.elsevier.com/locate/chemosphere

Application of electrochemical advanced oxidation processes to the mineralization of the herbicide diuron Angelo R.F. Pipi a,b, Ignasi Sirés a, Adalgisa R. De Andrade b, Enric Brillas a,⇑ a Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain b Departamento de Química, Laboratório de Eletrocatálise e Eletroquímica Ambiental, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo (FFCLRP, USP), Av. Bandeirantes 3900, 14040-901 Ribeirão Preto, SP, Brazil

h i g h l i g h t s  Greater diuron degradation using BDD/air-diffusion cell compared to Pt/air-diffusion.  Mineralization up to 93% by UVA photoelectro-Fenton in a 100 mL stirred tank reactor.  Mineralization up to 70% by solar photoelectro-Fenton in a 2.5 L pre-pilot flow plant.  Oxalic and oxamic acids detected as final carboxylic acids. +



 Release of NH4 and Cl , partially oxidized to ClO3

a r t i c l e

i n f o

Article history: Received 11 January 2014 Received in revised form 6 March 2014 Accepted 10 March 2014

Keywords: Electrochemical oxidation Diuron Electro-Fenton Solar photoelectro-Fenton Water treatment



and ClO4 ions.

a b s t r a c t Here, solutions with 0.185 mM of the herbicide diuron of pH 3.0 have been treated by electrochemical advanced oxidation processes (EAOPs) like electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), electro-Fenton (EF) and UVA photoelectro-Fenton (PEF) or solar PEF (SPEF). Trials were performed in stirred tank reactors of 100 mL and in a recirculation flow plant of 2.5 L using a filter-press reactor with a Pt or boron-doped diamond (BDD) anode and an air-diffusion cathode for H2O2 electrogeneration. Oxidant hydroxyl radicals were formed from water oxidation at the anode and/or in the bulk from Fenton’s reaction between added Fe2+ and generated H2O2. In both systems, the relative oxidation ability of the EAOPs increased in the sequence EO-H2O2 < EF < PEF or SPEF. The two latter processes were more powerful due to the photolysis of intermediates by UV radiation. In the stirred tank reactor, the PEF treatment with BDD was the most potent method, yielding 93% mineralization after 360 min at 100 mA cm2. In the flow plant, the SPEF process attained a maximum mineralization of 70% at 100 mA cm2. Lower current densities slightly reduced the mineralization degree in SPEF, enhancing the current efficiency and dropping the energy consumption. The diuron decay always obeyed a pseudo-first-order kinetics, with a much greater apparent rate constant in EF and SPEF compared to EO-H2O2. Oxalic and oxamic acids were detected as final carboxylic acids. Ammonium and chloride ions were also released, the latter ion being partially converted into chlorate and perchlorate ions at the BDD surface. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Recently, several electrochemical advanced oxidation processes (EAOPs) are being developed for water remediation (MartínezHuitle and Ferro, 2006; Panizza and Cerisola, 2009; Klavarioti et al., 2009). These methods are based on the in situ generation

⇑ Corresponding author. Tel.: +34 934021223; fax: +34 934021231. E-mail address: [email protected] (E. Brillas). http://dx.doi.org/10.1016/j.chemosphere.2014.03.006 0045-6535/Ó 2014 Elsevier Ltd. All rights reserved.

of OH, which reacts with most organics up to their mineralization to CO2, water and inorganic ions due to its high standard redox potential (E°(OH/H2O) = 2.80 V/SHE). The most powerful EAOPs generate H2O2 from the two-electron reduction of injected O2 (Brillas et al., 2009; Sirés and Brillas, 2012):

O2 þ 2Hþ þ 2e ! H2 O2

ð1Þ

Good efficiencies for H2O2 generation from reaction (1) have been reported for carbonaceous cathodes like carbon sponge (Özcan et al., 2008), carbon-felt (Dirany et al., 2012), graphite-felt

50

A.R.F. Pipi et al. / Chemosphere 109 (2014) 49–55

(Panizza and Oturan, 2011), boron-doped diamond (BDD) (Ramírez et al., 2013) and carbon-PTFE O2 or air-diffusion electrodes (ElGhenymy et al., 2013). The most popular EAOP is anodic oxidation or electrochemical oxidation (EO), where organics are oxidized at high current by physisorbed M(OH) formed during the anodic water discharge to O2 (Martínez-Huitle and Ferro, 2006; Aquino Neto and De Andrade, 2009a,b):

M þ H2 O ! Mð OHÞ þ Hþ þ e

ð2Þ

The best anode known for EO is the BDD thin film (Panizza and Cerisola, 2009). When an undivided cell with an O2 or air-diffusion cathode is used, the method is so-called EO with electrogenerated H2O2 (EO-H2O2) (Sirés et al., 2007; Cavalcanti et al., 2013). The oxidation ability of EO-H2O2 is enhanced in electro-Fenton (EF) by adding a small amount of Fe2+ to the solution at pH ca. 3, thus reacting with generated H2O2 to give OH and Fe3+ in the bulk from Fenton’s reaction (3) (Brillas et al., 2009):

Fe2þ þ H2 O2 ! Fe3þ þ  OH þ OH

ð3Þ

In photoelectro-Fenton (PEF), the solution is irradiated with artificial UVA light (Sirés et al., 2007; Anotai et al., 2011; De Luna et al., 2012; Garcia-Segura et al., 2012). When sunlight is used, the method is so-called solar PEF (SPEF) (Flox et al., 2007; Almeida et al., 2011; Salazar et al., 2012). In both EAOPs, the radiation causes: (i) the photolysis of Fe(OH)2+, the pre-eminent Fe3+ species at pH  3, from photo-Fenton reaction (4), and (ii) the photodecarboxylation of some generated Fe(III)-carboxylate complexes from reaction (5).

FeðOHÞ2þ þ hm ! Fe2þ þ  OH

ð4Þ

FeðOOCRÞ2þ þ hm ! Fe2þ þ CO2 þ R

ð5Þ

Our group has demonstrated that in EF, PEF and SPEF, aromatics are more quickly destroyed by OH in the bulk than by BDD(OH) (Flox et al., 2007; Ruiz et al., 2011; Salazar et al., 2012), allowing the use of less powerful anodes like Pt (Almeida et al., 2011; El-Ghenymy et al., 2013). However, more research efforts are necessary to clarify the effect of the anodes in such EAOPs, as well as to gain a better understanding of their potential scale-up from small stirred tank reactors to flow plants for their application at industrial level. Phenylurea herbicides are applied to the pre- and postemergence control of weeds in many crops and also on roads and railways. Among phenylureas, diuron (3-(3,4-dichlorophenyl)1,1-dimethylurea, see chemical structure in Fig. 1) is widely used in many formulations and has been found in natural waters (Oturan et al., 2008). It presents toxic effects on mammals and might have serious consequences on human health and reproduction (Giacomazzi and Cochet, 2004). The destruction of diuron has been

H3C

N

CH3

C HN

O

Cl Cl Fig. 1. Chemical structure of the herbicide diuron.

tested by physicochemical methods (Bouras et al., 2007), AOPs like Fenton (Gallard and De Laat, 2001), photo-Fenton (Djebbar et al., 2008), photolysis (Djebbar et al., 2008) and TiO2 photocatalysis (Katsumata et al., 2009), and microbiological degradation (Tixier et al., 2000). Higher degradation rates for diuron have been found by EAOPs. Polcaro et al. (2004) reported an almost total mineralization of 41.8 mg L1 diuron at pH 7 by EO with BDD in an undivided flow cell at 15–51 mA cm2. No aromatic byproducts were detected and stoichiometric amounts of Cl and NH+4 ions were released. Almost total mineralization was also found by treating 27.6 mg L1 diuron in 0.05 M Na2SO4 with 0.2 mM Fe3+ at pH 3.0 by EF in an undivided Pt/carbon-felt reactor at 250 mA (Oturan et al., 2008, 2010). In previous work, we studied the EO degradation of 50 mg L1 diuron in 1.5 M Na2SO4 or in 1.5 M Na2SO4 + NaCl using a divided cell with a dimensionally stable anode at 25– 100 mA cm2 (Pipi et al., 2013), but almost total mineralization was only feasible in the presence of Cl due to the more potent destruction by chlorinated active species formed from anodic oxidation of Cl. This paper aims to study the comparative mineralization of a 0.185 mM (42.9 mg L1) diuron solution at pH 3.0 by EO-H2O2, EF, PEF and SPEF in undivided cells with an air-diffusion cathode. The oxidation ability of these EAOPs in 100 mL stirred tank reactors with a Pt or BDD anode was clarified. The treatments were further scaled-up to a 2.5 L flow plant with a BDD/air-diffusion filter-press cell, which was coupled to a solar photoreactor for SPEF. 2. Experimental 2.1. Chemicals Reagent grade diuron (>98% purity) from Sigma–Aldrich was used as received. Anhydrous sodium sulfate, ferrous sulfate heptahydrate and oxalic and oxamic acids were of analytical grade from Merck and Fluka. Solutions degraded in stirred tank reactors were prepared with high-purity water obtained from a Millipore Milli-Q system (resistivity > 18 MX cm), whereas those treated in the flow plant were prepared with deionized water. The initial solution pH was adjusted to 3.0 with analytical grade sulfuric acid from Merck. 2.2. Electrolytic systems The electrolytic trials at laboratory scale were conducted in open, undivided and cylindrical tank reactors containing a 100 mL solution and with a double jacket for circulation of external thermostated water at 25 °C. The solution was stirred with a magnetic bar at 800 rpm to ensure mixing and the transport of reactants toward/from the electrodes. The cells contained either a 3 cm2 Pt sheet from SEMPSA or a 3 cm2 BDD thin-film electrode from Adamant Technologies as the anode and a 3 cm2 carbon-PTFE air-diffusion cathode from E-TEK. The cathode was mounted as described elsewhere (Isarain-Chávez et al., 2010), being fed with an air flow of 1 L min1 for H2O2 generation. The trials were performed at constant current density (j) provided by an Amel 2053 potentiostat–galvanostat. In PEF, the solution was irradiated with a Philips TL/6W/08 lamp, which emitted UVA light with kmax = 360 nm, supplying a photoionization energy of 5 W m2 determined with a Kipp & Zonen CUV 5 radiometer. Treatments with a 2.5 L recirculation flow plant were performed as reported elsewhere (Flox et al., 2006, 2007; Ruiz et al., 2011). The diuron solution was degraded at 25 °C and liquid flow rate of 200 L h1. The electrolytic reactor was an undivided filter-press cell with a 20 cm2 BDD thin-film anode from Adamant Technologies and a 20 cm2 carbon-PTFE air-diffusion cathode from E-TEK. The cathode was fed with air pumped at an overpressure of

51

A.R.F. Pipi et al. / Chemosphere 109 (2014) 49–55

8.6 kPa. A constant j of 50, 100 and 150 mA cm2 was provided to the cell by an Agilent 6552A DC power supply, yielding average potential differences of 10.6, 16.5 and 20.3 V, respectively. The solar photoreactor connected to the cell in SPEF consisted of a polycarbonate box of 24 cm  24 cm  2.5 cm (600 mL of irradiated volume), with a mirror at the bottom and tilted 41° (local latitude). The SPEF trials started from the noon in sunny and clear days during the summer of 2013 in our laboratory of Barcelona. The average UV irradiation intensity (300–400 nm) supplied by sunlight was 30–32 W m2, determined with a Kipp & Zonen CUV 5 radiometer. Before the use of both electrolytic systems, the electrodes were polarized in 0.05 M Na2SO4 solutions of pH 3.0 at 100 mA cm2 for 180 min in order to remove the impurities from the BDD anode surface and activate the catalytic layer of the air-diffusion cathode. 2.3. Apparatus and analytical procedures The solution pH was measured on a Crison GLP 22 pH-meter. The samples withdrawn from electrolyzed solutions were neutralized at pH 7–8 to quench the degradation process and filtered with 0.45 lm PTFE filters from Whatman. The mineralization of diuron solutions was monitored from their dissolved organic carbon (DOC) abatement, determined with a Shimadzu VCSN TOC analyzer. From these data, the mineralization current efficiency (MCE) at a given current I (A) and electrolysis time t (h) was estimated as follows (Salazar et al., 2011):

MCEð%Þ ¼

nFV s DðDOCÞexp 4:32  107 m I t

100

ð6Þ

Pt/air-diffusion and BDD/air-diffusion cells at 100 mA cm2 for 360 min. In the two latter processes, 0.5 mM Fe2+ was added to the solution to generate OH via Fenton’s reaction (3) and/or photo-Fenton reaction (4). In these trials, the solution pH remained practically unaltered, slightly decreasing up to 2.7–2.8 probably due to the formation of carboxylic acids (Brillas et al., 2009). Fig. 2a depicts that DOC was very slowly removed by EO-H2O2 using Pt, only being reduced by 30% in 360 min. In contrast, Fig. 2b evidences a higher DOC abatement of 82% using BDD, as expected if higher amounts of BDD(OH) compared with Pt(OH) are formed from reaction (2), thus destroying diuron and its oxidation products more efficiently. The greater oxidation ability of BDD can also be observed in Fig. 2b for the EF process, where OH produced in the bulk accelerated the DOC removal. However, the enhancement for EF was more relevant using Pt (see Fig. 2a) since Pt(OH) alone exhibited a low ability to attack the organic matter. Thus, EF yielded final mineralization of 72% for Pt and 87% for BDD. Fig. 2a and b also show a more pronounced DOC decay by PEF, leading to 88% and 93% mineralization using Pt and BDD, respectively. The quicker DOC removal in PEF can be related to the additional photolysis of Fe(III) complexes with intermediates like short-chain carboxylic acids via reaction (5) (Garcia-Segura and Brillas, 2011; Salazar et al., 2012). The oxidation ability of the EAOPs then rose in the sequence EO-H2O2 < EF < PEF, regardless of the anode used, although the replacement of Pt by BDD always improved the mineralization rate. For this reason, further study was only made considering the more potent BDD/air-diffusion reactor that allows a greater mineralization of diuron solutions.

where F is the Faraday constant (96,487 C mol1), Vs is the solution volume (L), D(DOC)exp is the experimental DOC decay (mg L1), 4.32  107 is a conversion factor (3600 s h1  12,000 mg mol1) and m is the number of carbon atoms of diuron (9 atoms). The number of electrons (n) consumed per each diuron molecule was taken as 36 considering that its mineralization leads to carbon dioxide and chloride and ammonium ions, as proposed by Polcaro et al. (2004), from reaction (7):

a

20

15



C9 H10 Cl2 N2 O þ 17H2 O ! 9CO2 þ 2Cl þ 2NHþ4 þ 36Hþ

10

ð7Þ

ECDOC ðkW h g1 DOCÞ ¼

Ecell I t V s DðDOCÞexp

ð8Þ

where Ecell is the average potential difference of the cell (V). The diuron removal was followed by reversed-phase HPLC with a Waters 600 LC fitted with a Waters Spherisorb ODS 2 5 lm, 150 mm  4.6 mm, column at 35 °C and coupled with a Waters 996 photodiode array detector set at k = 249 nm. The mobile phase was a 50:50 (v/v) acetonitrile/water mixture at 0.5 mL min1. Generated carboxylic acids were detected by ion-exclusion HPLC   and the contents of NH+4, Cl, ClO 3 , ClO4 and NO3 ions were determined by ion chromatography as reported elsewhere (Randazzo et al., 2011; Salazar et al., 2012). 3. Results and discussion 3.1. Degradation of diuron in stirred tank reactors To check the oxidation ability of the EAOPs regarding diuron and the influence of anode materials, 100 mL of 0.185 mM of this herbicide (near saturation) in 0.05 M Na2SO4 of pH 3.0 were comparatively treated by EO-H2O2, EF and PEF in stirred

5 -1

In the flow plant, the energy consumption per unit DOC mass (ECDOC) was calculated as follows (Almeida et al., 2011; Ruiz et al., 2011):

DOC / mg L

þ 36e

0

b

20

15

10

5

0

0

60

120

180

240

300

360

420

Time / min Fig. 2. DOC abatement with electrolysis time for the treatment of 100 mL of 0.185 mM diuron solutions in 0.05 M Na2SO4 at pH 3.0 and 25 °C by (s,d) electrochemical oxidation with electrogenerated H2O2 (EO-H2O2), (h,j) electroFenton (EF) with 0.5 mM Fe2+ and (4,N) photoelectro-Fenton (PEF) with 0.5 mM Fe2+ and a 6 W UVA light of kmax = 360 nm. The cell was a stirred tank reactor equipped with a: (a) Pt or (b) boron-doped diamond (BDD) anode and an airdiffusion cathode, all of them with 3 cm2 area. Current density: 33.3 mA cm2 (filled symbols) and 100 mA cm2 (hollow symbols).

A.R.F. Pipi et al. / Chemosphere 109 (2014) 49–55

a

DOC / mg L

-1

20

15

10

5

0

b 10

% MCE

8 6 4 2 0

c

12 10

2 BDDð OHÞ ! 2 BDD þ O2 þ 2Hþ þ 2e

ð10Þ

EC

Fe2þ þ  OH ! Fe3þ þ OH

ð9Þ

DOC

-1

The influence of j on the oxidation ability of the EAOPs tested in the BDD/air-diffusion cell was investigated to clarify the role of generated oxidizing species, mainly BDD(OH) and OH. Fig. 2b shows a quicker DOC abatement for EO-H2O2 and EF from 68% to 82% and from 80% to 87%, respectively, when j increased from 33.3 to 100 mA cm2. This trend can be ascribed to the production of greater quantities of BDD(OH) from the acceleration of reaction (2), and/or of H2O2 from the increase in rate of reaction (1) enhancing the generation of OH via Fenton’s reaction (3) (Isarain-Chávez et al., 2010; Sirés and Brillas, 2012). In contrast, Fig. 2b shows a similar DOC abatement at 33.3 and 100 mA cm2 until 180 min of PEF, indicating that the mineralization kinetics is controlled by the photolysis of intermediates formed. At longer time, DOC was more rapidly reduced at 100 mA cm2 probably because the remaining products were more quickly destroyed with the higher amounts of generated BDD(OH). Despite the positive effect of increasing j on diuron mineralization in the EAOPs, the current efficiency calculated from Eq. (6) dropped drastically. For example, MCE values of 2.4%, 3.7% and 5.0% were obtained after 180 min of EO-H2O2, EF and PEF at 33.3 mA cm2, being reduced to 1.2%, 1.5% and 1.7% at 100 mA cm2. The very low efficiencies found in all cases may be related to the formation of highly recalcitrant oxidation products. The loss of efficiency with rising j can be explained by the decay in the effective concentration of BDD(OH) and OH due to the greater acceleration of their wasting reactions, making the processes more inefficient. These parasitic reactions involve primordially the oxidation of BDD(OH) to O2 by reaction (9) and the reaction of OH with either Fe2+ by reaction (10) or H2O2 by reaction (11) (Sirés et al., 2007; Özcan et al., 2008; Brillas et al., 2009).

/ kWh g DOC

52



H2 O2 þ OH !

HO2

þ H2 O

ð11Þ

3.2. Diuron mineralization in a flow plant

8 6 4 2 0

0

60

120

180

240

300

360

420

Time / min Since the above study showed the superiority of the BDD/ air-diffusion cell to destroy the herbicide in all the treatments, the processes were scaled-up to a 2.5 L flow plant using these electrodes. In the plant, EO-H2O2 and EF were tested in the dark, whereas PEF was replaced by SPEF to assess its viability at industrial scale. The initial solutions with 0.185 mM diuron in 0.05 M Na2SO4 at pH 3.0 were treated between 50 and 150 mA cm2 for 360 min. In EF and SPEF, 0.5 mM Fe2+ was added to the initial solution. The solution pH only underwent a slight decrease during the treatments reaching final values near 2.7–2.8, similarly to that found in the comparative trials made in the stirred tank reactors. Fig. 3a shows that the relative oxidation ability of the three EAOPs in the plant at 100 mA cm2 also increased as EO-H2O2 < EF < SPEF. This indicates again that the destruction ability of BDD(OH) formed from reaction (2) in EO-H2O2 was enhanced in EF by the action of OH generated from Fenton’s reaction (3) and additionally in SPEF by the photolysis of intermediates under UV irradiation from sunlight. However, these processes were less potent than in the stirred tank reactor under similar conditions. After 360 min of electrolysis, for example, DOC was reduced by 32% and 53% for EO-H2O2 and EF, respectively, values much lower than 82% and 87% obtained in the stirred tank reactor (Fig. 2b). For SPEF, only 70% of mineralization was reached in 240 min, without attaining greater DOC decay at longer time, which was substantially smaller than 93% found for PEF in the stirred tank reactor at 100 mA cm2 (Fig. 2b). The loss in oxidation power in the plant

Fig. 3. (a) DOC removal, (b) mineralization current efficiency and (c) energy consumption per unit DOC mass vs. electrolysis time for the treatment of 2.5 L of 0.185 mM diuron solutions in 0.05 M Na2SO4 at pH 3.0 and 25 °C using a flow plant containing a one-compartment BDD/air-diffusion filter-press cell with 20 cm2 electrodes at 100 mA cm2 and liquid flow rate of 200 L h1. Method: (s) EO-H2O2, (h) EF with 0.5 mM Fe2+ and (}) solar PEF (SPEF) with 0.5 mM Fe2+. In SPEF, the electrolytic reactor was coupled to a solar photoreactor of 600 mL of irradiated volume.

can be explained because its electrochemical reactor produces BDD(OH) radicals that can practically act over the organics when they flow through the electrodes, representing a small volume (22.4 mL) compared with 2.5 L of the treated solution. Results of Fig. 2b prove the large action of BDD(OH) during all the mineralization processes in the stirred tank reactor, where the electrodes are always in contact with the 100 mL solution, thus destroying the organics more rapidly and yielding quicker mineralization. The MCE values depicted in Fig. 3b show an almost constant efficiency of about 1.2% for EO-H2O2 and 2.3% for EF after 120 min of electrolysis, meaning that organics were mineralized at a constant rate, with similar action of BDD(OH) and OH in EF. In contrast, the MCE for SPEF attained 11% as maximal at 60 min, whereupon it decayed drastically to 3.9% at 240 min. This suggests the initial formation of intermediates that are rapidly photolyzed by sunlight, further remaining in the solution very recalcitrant species that are slowly removed by hydroxyl radicals. Accordingly,

53

A.R.F. Pipi et al. / Chemosphere 109 (2014) 49–55 Table 1 Percentage of DOC removal, mineralization current efficiency and energy consumption per unit DOC mass determined after 240 min of degradation of 2.5 L of a 0.185 mM diuron solution in 0.05 M Na2SO4 at pH 3.0 by different EAOPs using a recirculation flow plant.

a

% DOC removal

% MCE

ECDOC (kW h g1 DOC)

EO-H2O2

50 100 150

15 21 32

1.7 1.2 1.2

5.6 12.2 15.3

EFa

50 100 150

32 42 59

3.5 2.3 2.2

2.6 6.1 8.3

SPEFa

50 100 150

65 70 70

7.2 3.9 2.6

1.3 3.7 7.0

2.0 ln (co /c )

Current density (mA cm2)

a

0.15

[Diuron] / mM

Method

0.20

1.5 1.0 0.5

0.10 0.0 0

60 120 180 240 300 360 420 Time / min

0.05

0.00

0

0.20

0.5 mM Fe2+ was added to the solution.

60

120

180

b

240

300

360

420

5

3.3. Decay kinetics for diuron and evolution of generated carboxylic acids and inorganic ions The kinetics of the reaction between diuron and generated hydroxyl radicals (BDD(OH) and OH) by the EAOPs in the flow plant was followed from the herbicide decay determined by reversedphase HPLC, where it displayed a peak at retention time of 3.7 min. Fig. 4a evidences that the herbicide underwent a very slow destruction by BDD(OH), being only reduced by 81% after 360 min of EO-H2O2. This can explain the low DOC removal achieved by this procedure in Fig. 3a. In contrast, Fig. 4b shows a very fast attack of diuron by OH disappearing at similar rate in 6 min by EF and SPEF, indicating a small participation of photo-Fenton reaction (4) for  OH generation in the latter process. This behavior corroborates a slow mineralization of the herbicide solution in EF and PEF due to the formation of highly recalcitrant oxidation products. The inset panels of Fig. 4a and b show that diuron decayed obeying a pseudo-first-order kinetics. From this analysis, a low apparent rate constant (k1) of 7.3  105 s1 (R2 = 0.997) for EO-H2O2 was found, whereas a much higher and similar k1 value around 1.3  102 s1 (R2 > 0.986) was obtained for EF and SPEF. This suggests the

[Diuron] / mM

high energy consumptions were obtained from Eq. (8) for all these processes, as presented in Fig. 3c. The lowest ECDOC values were obtained for SPEF, attaining a final value of 3.7 kW h g1 DOC (52.8 kW h m3) after rising from a minimal of 1.3 kW h g1 DOC (13.2 kW h m3) at 60 min, just when the maximum MCE was reached (Fig. 3b). The effect of j on all the treatments made in the flow plant was also explored. Results of Table 1 highlight that for EO-H2O2 and EF, an increase in j always caused greater mineralization because of the generation of more quantities of BDD(OH) and OH. However, the potent action of UV irradiation provided by sunlight in SPEF allowed a similar DOC decay of 70% at 100 and 150 mA cm2, only slightly decreasing to 65% at 50 mA cm2. The application of low j values enhanced the current efficiency and decreased drastically the energy consumption, as can be seen in Table 1. In all the processes, the highest MCE with the lowest ECDOC were found at the smallest j of 50 mA cm2. The best treatment was then found for SPEF at 50 mA cm2 with 7.2% current efficiency and 1.3 kW h g1 DOC (16.9 kW h m3) energy consumption at 240 min. The aforementioned findings indicate a very difficult mineralization of diuron solutions by EAOPs with BDD due to the formation of highly recalcitrant products. For the more potent SPEF process, low j values should be applied at industrial scale to become a viable process with acceptable mineralization degrees, relatively greater efficiencies and lower energy costs.

ln (co /c )

4

0.15

3 2 1

0.10

0

0

1

2 3 4 Time / min

5

6

0.05

0.00

0

1

2

3

4

5

6

7

Time / min Fig. 4. Decay of diuron with electrolysis time under the conditions described in Fig. 3. In plot (a), EO-H2O2 process. In plot (b), (h) EF and (}) SPEF treatments with 0.5 mM Fe2+. The inset panels show the kinetic analysis assuming a pseudo-firstorder reaction for diuron.

reaction of the herbicide with a constant concentration of hydroxyl radicals in each EAOP. The oxidation of diuron with hydroxyl radicals leads to demethylated derivatives like 3-(3,4-dichlorophenyl)-1-methylurea, 3(3,4-dichlorophenyl)urea and 3,4-dichloroaniline (Polcaro et al., 2004; Oturan et al., 2008). The degradation of these compounds and other aromatic intermediates is expected to give short-chain carboxylic acids (Brillas et al., 2009). This behavior was confirmed by detecting generated oxalic and oxamic acids by ion-exclusion HPLC, presented in Fig. 5a and b, respectively. In EO-H2O2, oxalic acid was accumulated up to 0.31 mM in 120–180 min to be further removed to 0.07 mM at 360 min under the action of BDD(OH) (Martínez-Huitle and Ferro, 2006), but oxamic acid was not detected, suggesting a low oxidation ability of this EAOP to destroy N-derivatives. In EF and SPEF, both acids formed Fe(III) complexes that were slowly oxidized primordially by BDD(OH) (Panizza and Cerisola, 2009; Garcia-Segura and Brillas, 2011). This can be observed in Fig. 5a and b for EF, where maximum contents of 0.31 mM for oxalic acid and 0.04 mM for oxamic acid were found, slightly decreasing to 0.24 and 0.03 mM, respectively, at 360 min. For SPEF, the Fe(III)-oxalate complexes were rapidly photodecarboxylated via reaction (5) (Garcia-Segura and Brillas, 2011) and oxalic acid disappeared in 90 min. In contrast, the Fe(III)-oxamate complexes were much more slowly photolyzed and a final content of 0.02 mM was obtained. A simple mass balance from the data of Fig. 5a and b allows concluding that the detected carboxylic acids contributed with 1.7, 6.5 and 0.5 mg L1 of DOC to the final solutions of EO-H2O2, EF and SPEF, respectively, corresponding to 12.6%, 69% and 8% of the remaining DOC. A large proportion of

54

A.R.F. Pipi et al. / Chemosphere 109 (2014) 49–55

[Oxalic acid] / mM

0.4

0.37 mM of initial Cl, indicating a rapid dechlorination of all the intermediates. The total release of Cl ion during diuron mineralization agrees with the proposed reaction (7). Fig. 6 highlights that at 360 min of SPEF, the Cl concentration was reduced to 0.16 mM (44% of initial Cl) since this ion was transformed into 0.046 mM  ClO 3 (12% of initial Cl) and 0.12 mM ClO4 (31% of initial Cl).

a

0.3

0.2

4. Conclusions 0.1

0.0

b

[Oxamic acid] / mM

0.05 0.04 0.03 0.02 0.01 0.00

0

60

120

180

240

300

360

420

Time / min Fig. 5. Evolution of the concentration of (a) oxalic and (b) oxamic acids detected during the (s) EO-H2O2, (h) EF and (}) SPEF treatments of 2.5 L of 0.185 mM diuron solutions under the same conditions of Fig. 3.

undetected recalcitrant products is then present in the final solutions of all the EAOPs. The solutions treated in the flow plant were also analyzed by ion chromatography to determine the released inorganic ions. Its + initial N was expected to be converted into NO 3 and NH4 ions (Bril+ las et al., 2009). However, only NH4 ion was pre-eminently released in the potent SPEF process, as proposed in the mineralization reaction (7). Chlorinated derivatives were expected to be mineralized with loss of Cl as primary ion, which could be oxidized to ClO 3 and ClO 4 ions at the BDD anode (Randazzo et al., 2011). Fig. 6 exemplifies this behavior for SPEF. As can be seen, 0.34 mM of Cl ion was released in 60 min, very close to the content of

[Chlorinated ions] / mM

0.4

It has been demonstrated that 100 mL of 0.185 mM diuron at pH 3.0 can be mineralized up to 93% after 360 min of PEF using a stirred BDD/air-diffusion tank reactor at 100 mA cm2 due to the combined action of BDD(OH), OH and UVA radiation. Lower oxidation ability was found for EF, whereas EO-H2O2 was less powerful because only BDD(OH) acted as the main oxidant. The reaction of organics with BDD(OH) was quicker than with Pt(OH), and then, the comparative EAOPs in a stirred Pt/air-diffusion cell yielded poorer degradation. The increase in j always favored the destruction of organics but with smaller MCE. The scale-up of all these processes to a 2.5 L flow plant with a BDD/air-diffusion filter-press cell yielded smaller mineralization degrees than those obtained in a stirred cell because of the different design of the systems. In the plant, the best EAOP was SPEF, yielding the most favorable results for 0.185 mM diuron at 50 mA cm2, with 65% mineralization, 7.2% current efficiency and 1.3 kW h g1 DOC (16.9 kW h m3) energy consumption at 240 min. Higher j caused a slight increase of the mineralization degree in SPEF, but substantially dropping the current efficiency and raising the energy consumption. Low current densities are then needed to make viable the SPEF process at industrial scale. The diuron decay always obeyed a pseudo-first-order kinetics. The apparent rate constant was small in EO-H2O2, which rose considerably in EF and SPEF because of the effective attack of OH on the herbicide. Oxalic and oxamic acids coming from degradation of intermediates were detected as final carboxylic acids. A large proportion of other highly recalcitrant products remained in all the final treated solutions. NH+4 and Cl ions were released during diuron mineralization, although the latter ion  was partially oxidized to ClO 3 and ClO4 ions on BDD. Acknowledgments The authors acknowledge funding from MICINN (Ministerio de Ciencia e Innovación, Spain) through CTQ2010-16164/BQU project, co-financed with FEDER funds, and also thank support from the FAPESP (Fundação de Amparo a Pesquisa do Estado de São Paulo, Brazil) through BEPE scholarship (2012/00736-4). References

0.3

0.2

0.1

0.0 0

60

120

180

240

300

360

420

Time / min  Fig. 6. Time-course of the concentrations of (d) Cl, () ClO 3 and (.) ClO4 ions detected during the SPEF process of 2.5 L of a 0.185 mM diuron solution with 0.5 mM Fe2+ under the conditions given in Fig. 3.

Almeida, L.C., Garcia-Segura, S., Bocchi, N., Brillas, E., 2011. Solar photoelectroFenton degradation of paracetamol using a flow plant with a Pt/air-diffusion cell coupled with a compound parabolic collector: process optimization by response surface methodology. Appl. Catal. B – Environ. 103, 21–30. Anotai, J., Singhadech, S., Su, C., Lu, M., 2011. Comparison of o-toluidine degradation by Fenton, electro-Fenton and photoelectro-Fenton processes. J. Hazard. Mater. 196, 395–401. Aquino Neto, S., De Andrade, A.R., 2009a. Electrooxidation of glyphosate herbicide Ò at different DSA compositions: pH, concentration and supporting electrolyte effect. Electrochim. Acta 54, 2039–2045. Aquino Neto, S., De Andrade, A.R., 2009b. Electrochemical degradation of glyphosate Ò formulations at DSA anodes in chloride medium: an AOX formation study. J. Appl. Electrochem. 39, 1863–1870. Bouras, O., Bollinger, J.-C., Baudu, M., Khalaf, H., 2007. Adsorption of diuron and its degradation products from aqueous solution by surfactant-modified pillared clays. Appl. Clay Sci. 37, 240–250. Brillas, E., Sirés, I., Oturan, M.A., 2009. Electro-Fenton process and related electrochemical technologies based on Fenton’s reaction chemistry. Chem. Rev. 109, 6570–6631. Cavalcanti, E.B., Garcia-Segura, S., Centellas, F., Brillas, E., 2013. Electrochemical incineration of omeprazole in neutral aqueous medium using a platinum or

A.R.F. Pipi et al. / Chemosphere 109 (2014) 49–55 boron-doped diamond anode: degradation kinetics and oxidation products. Water Res. 47, 1803–1815. De Luna, M.D.G., Veciana, M.L., Su, C.-C., Lu, M.-C., 2012. Acetaminophen degradation by electro-Fenton and photoelectro-Fenton using a double cathode electrochemical cell. J. Hazard. Mater. 217–218, 200–207. Dirany, A., Sirés, I., Oturan, N., Özcan, A., Oturan, M.A., 2012. Electrochemical treatment of the antibiotic sulfachloropyridazine: kinetics, reaction pathways, and toxicity evolution. Environ. Sci. Technol. 46, 4074–4082. Djebbar, K.E., Zertal, A., Debbache, N., Sehili, T., 2008. Comparison of diuron degradation by direct UV photolysis and advanced oxidation processes. J. Environ. Manage. 88, 1505–1512. El-Ghenymy, A., Cabot, P.L., Centellas, F., Garrido, J.A., Rodríguez, R.M., Arias, C., Brillas, E., 2013. Mineralization of sulfanilamide by electro-Fenton and solar photoelectro-Fenton in a pre-pilot plant with a Pt/air-diffusion cell. Chemosphere 91, 1324–1331. Flox, C., Cabot, P.L., Centellas, F., Garrido, J.A., Rodríguez, R.M., Arias, C., Brillas, E., 2006. Electrochemical combustion of herbicide mecoprop in aqueous medium using a flow reactor with a boron-doped diamond anode. Chemosphere 64, 892–902. Flox, C., Garrido, J.A., Rodríguez, R.M., Cabot, P.L., Centellas, F., Arias, C., Brillas, E., 2007. Mineralization of herbicide mecoprop by photoelectro-Fenton with UVA and solar light. Catal. Today 129, 29–36. Gallard, H., De Laat, J., 2001. Kinetics of oxidation of chlorobenzenes and phenylureas by Fe(II)/H2O2 and Fe(III)/H2O2. Evidence of reduction and oxidation reactions of intermediates by Fe(II) and Fe(III). Chemosphere 42, 405–413. Garcia-Segura, S., Brillas, E., 2011. Mineralization of the recalcitrant oxalic and oxamic acids by electrochemical advanced oxidation processes using a borondoped diamond anode. Water Res. 45, 2975–2984. Garcia-Segura, S., Garrido, J.A., Rodríguez, R.M., Cabot, P.L., Centellas, F., Arias, C., Brillas, E., 2012. Mineralization of flumequine in acidic medium by electroFenton and photoelectro-Fenton processes. Water Res. 46, 2067–2076. Giacomazzi, S., Cochet, N., 2004. Environmental impact of diuron transformation: a review. Chemosphere 56, 1021–1032. Isarain-Chávez, E., Arias, C., Cabot, P.L., Centellas, F., Rodríguez, R.M., Garrido, J.A., Brillas, E., 2010. Mineralization of the drug beta-blocker atenolol by electroFenton and photoelectro-Fenton using an air-diffusion cathode for H2O2 electrogeneration combined with a carbon-felt cathode for Fe2+ regeneration. Appl. Catal. B – Environ. 96, 361–369. Katsumata, H., Sada, M., Nakaoka, Y., Kaneco, S., Suzuki, T., Ohta, K., 2009. Photocatalytic degradation of diuron in aqueous solution by platinized TiO2. J. Hazard. Mater. 171, 1081–1089. Klavarioti, M., Mantzavinos, D., Kassinos, D., 2009. Removal of residual pharmaceuticals from aqueous systems by advanced oxidation processes. Environ. Int. 35, 402–417. Martínez-Huitle, C.A., Ferro, S., 2006. Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes. Chem. Soc. Rev. 35, 1324–1340. Oturan, N., Trajkovska, S., Oturan, M.A., Couderchet, M., Aaron, J.-J., 2008. Study of the toxicity of diuron and its metabolites formed in aqueous medium during

55

application of the electrochemical advanced oxidation process ‘‘electroFenton’’. Chemosphere 73, 1550–1556. Oturan, M.A., Edelahi, M.C., Oturan, N., El kacemi, K., Aaron, J.-J., 2010. Kinetics of oxidative degradation/mineralization pathways of the phenylurea herbicides diuron, monuron and fenuron in water during application of the electro-Fenton process. Appl. Catal. B – Environ. 97, 82–89. Özcan, A., Sahin, Y., Koparal, A.S., Oturan, M.A., 2008. Carbon sponge as a new material for the electro-Fenton process. Comparison with carbon felt cathode and application to degradation of synthetic dye Basic Blue 3 in aqueous medium. J. Electroanal. Chem. 616, 71–78. Panizza, M., Cerisola, G., 2009. Direct and mediated anodic oxidation of organic pollutants. Chem. Rev. 109, 6541–6569. Panizza, M., Oturan, M.A., 2011. Degradation of Alizarin Red by electro-Fenton process using a graphite-felt cathode. Electrochim. Acta 56, 7084–7087. Pipi, A.R.F., Aquino Neto, S., De Andrade, A.R., 2013. Electrochemical degradation of Ò diuron in chloride medium using DSA based anodes. J. Braz. Chem. Soc. 24, 1259–1266. Polcaro, A.M., Mascia, M., Palmas, S., Vacca, A., 2004. Electrochemical degradation of diuron and dichloroaniline at BDD electrode. Electrochim. Acta 49, 649– 656. Ramírez, C., Saldaña, A., Hernández, B., Acero, R., Guerra, R., Garcia-Segura, S., Brillas, E., Peralta-Hernández, J.M., 2013. Electrochemical oxidation of methyl orange azo dye at pilot flow plant using BDD technology. J. Ind. Eng. Chem. 19, 571– 579. Randazzo, S., Scialdone, O., Brillas, E., Sirés, I., 2011. Comparative electrochemical treatments of two chlorinated aliphatic hydrocarbons. Time course of the main reaction by-products. J. Hazard. Mater. 192, 1555–1564. Ruiz, E.J., Arias, C., Brillas, E., Hernández-Ramírez, A., Peralta-Hernández, J.M., 2011. Mineralization of Acid Yellow 36 azo dye by electro-Fenton and solar photoelectro-Fenton processes with a boron-doped diamond anode. Chemosphere 82, 495–501. Salazar, R., Garcia-Segura, S., Ureta-Zañartu, M.S., Brillas, E., 2011. Degradation of disperse azo dyes from waters by solar photoelectro-Fenton. Electrochim. Acta 56, 6371–6379. Salazar, R., Brillas, E., Sirés, I., 2012. Finding the best Fe2+/Cu2+ combination for the solar photoelectro-Fenton treatment of simulated wastewater containing the industrial textile dye disperse blue 3. Appl. Catal. B – Environ. 115–116, 107– 116. Sirés, I., Brillas, E., 2012. Remediation of water pollution caused by pharmaceutical residues based on electrochemical separation and degradation technologies: a review. Environ. Int. 40, 212–229. Sirés, I., Centellas, F., Garrido, J.A., Rodríguez, R.M., Arias, C., Cabot, P.L., Brillas, E., 2007. Mineralization of clofibric acid by electrochemical advanced oxidation processes using a boron-doped diamond anode and Fe2+ and UVA light as catalysts. Appl. Catal. B – Environ. 72, 373–381. Tixier, C., Bogaerts, P., Sancelme, M., Bonnemoy, F., Twagilimana, L., Cuer, A., Bohatier, J., Veschambre, H., 2000. Fungal biodegradation of a phenylurea herbicide, diuron: structure and toxicity of metabolites. Pest. Manage. Sci. 56, 455–462.