Unexpected toxicity decrease during photoelectrochemical ...

Environ Chem Lett (2012) 10:177–182 DOI 10.1007/s10311-011-0340-4

ORIGINAL PAPER

Unexpected toxicity decrease during photoelectrochemical degradation of atrazine with NaCl Geoffroy R. P. Malpass • Douglas W. Miwa Ricardo L. Santos • Eny M. Vieira • Artur J. Motheo

•

Received: 20 July 2011 / Accepted: 4 December 2011 / Published online: 20 December 2011 Ó Springer-Verlag 2011

Abstract This report shows an unexpected toxicity decrease during atrazine photoelectrodegradation in the presence of NaCl. Atrazine is a pesticide classified as endocrine disruptor occurring in industrial effluents and agricultural wastewaters. We therefore studied the effects of the degradation method, electrochemical and electrochemical photo-assisted, and of the supporting electrolyte, NaCl and Na2SO4, on the residual toxicity of treated atrazine solutions. We also studied the toxicity of treated atrazine solutions using Artemia nauplii. Results show that at initial concentration of 20 mg L-1, atrazine was completely removed in up to 30 min using 10 mA cm-2 electrolysis in NaCl medium, regardless of the electrochemical method used. The total organic carbon removal by the photo-assisted method was 82% with NaCl and 95% with Na2SO4. The solution toxicity increased during sole electrochemical treatment in NaCl, as expected. However, the toxicity unexpectedly decreased using the photo-assisted method. This finding is a major discovery because electrochemical treatment with NaCl usually leads to the formation of toxic chlorine-containing organic degradation by-products.

G. R. P. Malpass (&) Department of Chemical Engineering, Institute of Technological and Exact Sciences, Federal University of Triaˆngulo Mineiro, Avenida Doutor Randolfo Borges Juńior, 1250, Univerdecidade, Uberaba, MG CEP 38064-200, Brazil e-mail: [email protected] D. W. Miwa R. L. Santos E. M. Vieira A. J. Motheo Department of Physical Chemistry, Saõ Carlos Chemistry Institute, University of Saõ Paulo, P.O. Box 780, Saõ Carlos, Saõ Paulo CEP 13560-970, Brazil

Keywords Atrazine Electrochemical oxidation Photo-assisted electrochemical oxidation Toxicity Dimensionally stable anodes Artemia nauplii

Introduction Detailed chemical analysis of an effluent is not sufficient to predict toxicity. A large number of chemical compounds are difficult to detect using commonly available chemical analyses and, even when they can be detected, the toxicity of many substances is not always completely understood. Different chemicals, when combined together in the same effluent or under a certain set of environmental conditions, can have unknown additive (synergistic) effects—even when the toxicity of each individual chemical is well known. So, the development of effluent treatment methods that both efficiently remove target compounds and subsequent degradation products is extremely important (Gogate and Pandit 2004). The removal of pollutants by electrochemical methods have received considerable interest in recent years (MartinezHuitle and Ferro 2006; Martinez-Huitle and Brillas 2009). Electrochemical degradation can be classified as either: (a) direct (by electron transfer) or (b) indirect (through the formation of oxidizing/reducing agents at the electrode) (Panizza and Cerisola 2009). In both these processes, the aim is to produce active species at a rate and in quantities that enable rapid degradation of the pollutant species. Atrazine is a triazine herbicide that is applied to a wide range of crops (e.g., corn, sugarcane, and Christmas trees). In aquatic environments, atrazine is highly toxic to aquatic invertebrates, making it necessary to control its release into the environment. Several recent studies have detailed the degradation of atrazine using electrochemical advanced

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Environ Chem Lett (2012) 10:177–182

oxidation processes. The majority of these studies employ boron-doped diamond electrodes such as Borra`s et al. (2010) or other electrode materials to generate highly oxidizing species (e.g., O3, Vera et al. 2009). Balci et al. (2009) have studied the mechanistic aspects of atrazine degradation at boron-doped diamond electrodes and achieved organic carbon removal rates of 82%. Recently published work by our group has investigated the degradation of the pesticides atrazine (Malpass et al. 2010a, b, 2007, 2006) and carbaryl (Malpass et al. 2009; Miwa et al. 2006) by electrochemical (EC) and photoassisted electrochemical (photo-assisted—application of an electrical current and simultaneous application of UV–vis radiation at the electrode surface) methods. However, in many cases, for degradation to occur at appreciable rates, it is often necessary to add NaCl to the reaction mixture. This has the dual aim of (a) increasing the conductivity of the effluent (supporting electrolyte) and (b) producing Cl2 at the anode (Eq. 1), which can subsequently form free chlorine species (Eqs. 2 & 3) and act in the removal of organic load. 2Cl ! Cl2 þ 2e

ð1Þ

main focus of this study is the demonstration that it is possible to use NaCl as a supporting electrolyte to improve the generation of oxidizing species and, at the same time, to reduce the observed toxicity against a given test organism. As the samples from the experiments contain high concentrations of ions, a salt tolerant organism, Artemia nauplii (Artemia sp.), was adopted as the test organism. Since samples from experiments contain high concentrations of ions, a salt-tolerant organism such as Artemia sp. is suitable for measuring toxicity due to other sources. Other advantages of using this crustacean are that the organism can easily be procured, hatching is not complicated, simple equipment can be used for the measurements, and only small sample volumes are needed for the test procedure (Nunes et al. 2006).

Experimental Electrodes and reactor

The concentration of the weak acid, HOCl, and its conjugate base (ClO-) is dependent on the pH of the solution:

The electrochemical degradation assays were conducted in a single compartment photoelectrochemical filter-press-cell using as the working electrode a Ti/Ru0.3Ti0.7O2 mixed oxide electrode, as described elsewhere (Malpass et al. 2007). All measurements were taken under temperature control (25 ± 2°C). The UV source was a 250 W high pressure Hg lamp (kMAX = 254 nm).

HOCl $ OCl þ Hþ ðpKa ¼ 7:5 at 25 CÞ:

Analyses

and Cl2 þ H2 O ! HOCl þ HCl

ð2Þ

ð3Þ

Subsequently, the HOCl or OCl- can act to remove the organic load in a process that mimics a traditional chlorination process. The use of NaCl in this way can be problematic as subsequent reactions can result in the formation of chlorine-containing organic degradation byproducts, which can be toxic and/or carcinogenic (Choi et al. 2004). The presence of such compounds can significantly increase the toxicity of treated effluents (Alves et al. 2010). The literature demonstrates that the extent of chlorinated DBP formation during electrochemical treatment is dependent on the overall organic load and the chloride concentration (Rajkumar et al. 2005; Aquino Neto and De Andrade 2009). However, it is interesting to note that, in recent studies using NaCl, the phytotoxicity (using lettuce (Lactuca sativa) seeds as the test organism) of real textile effluent increased significantly during electrochemical treatment, but did not change appreciably during photo-assisted treatment (Alves et al. 2010). The aim of the present communication is to report the effect of the electrochemical treatment method (pure or assisted by simultaneous UV irradiation) and supporting electrolyte (NaCl or Na2SO4) on the acute toxicity of solutions containing the pesticide atrazine is presented. The

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Samples were collected at different time intervals during the degradation process and analyzed. Atrazine was analyzed by HPLC (Shimadzu LC-10AD VP) with a reverse phase column (LC-18, Supelcol). The eluent was acetonitrile/water (1:1, v/v). The concentration of atrazine was monitored using an ultraviolet detector (SPD-10A VP) at k = 221 nm. Total organic carbon values were obtained using a TOC-VCPH, Shimadzu equipment. Procedure A volume of 300 cm3 of the solution to be treated was added to the electrolyte reservoir and was pumped through the system in batch mode. Samples were collected at different time intervals during the degradation process and analyzed for atrazine concentration, total organic carbon, and acute toxicity. All electrolyses in the current study were performed at 10 mA cm-2, with or without simultaneous UV irradiation as stated. A total of 13 samples were submitted to testing. Solutions of the salts employed (NaCl and Na2SO4) and atrazine were tested before mixing and after treatment.


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Atrazine solutions Atrazine was obtained from Fluka (PESTANALÒ, analytical standard). The atrazine-containing solutions (20 mg L-1) plus salt were prepared with the aim of imitating the concentration levels found in effluent from atrazine-producing industries. As the concentrations are quite high, the samples do not represent ‘‘environmental samples’’ themselves, but samples that maybe released into the environment. Toxicity assays Artificial seawater of 35 ± 1%, prepared with pharmaceutical-grade sea salt (Tropic MarinÒ, Germany), was used as the hatching medium as for toxicity assays. The hatching procedure followed that described in the ARC test—standardized short-term toxicity test with A. nauplii (Vanhaecke and Persoone 1981). After the hatching period, an aliquot of the nauplii was transferred to a Petri dish for subsequent manual distribution to the test flasks that contained the atrazine solutions (treated and untreated). Each sample received 35 g L-1 of marine salt, and subsequently, 10 cm-3 was transferred to a recipient and 10 A. nauplii were added. Subsequently, the sample was left in the dark at 25 ± 1°C for 24 and 48 h. The acute test was based on the determination of mortality (immobility) of instar II–III nauplii of Artemia sp. strain. The total number of immobile brine shrimps for each test solution (four replicates) was recorded after 24 and 48 h, and the percentage of the total number of dead organisms in each test solution was compared to the control. The sample was considered toxic when the mortality in each solution was higher than 20%. The following samples were studied for acute toxicity: before treatment: (I) 0.033 mol L-1 Na2SO4; (II) 20 mg L-1 atrazine; (III) 0.10 mol L-1 NaCl; (IV) 0.033 mol L-1 Na2SO4 ? 20 mg L-1 atrazine; (V) 0.10 mol L-1 NaCl ? 20 mg L-1 atrazine and after treatment: (VI) sample (I) treated electrochemically; (VII) sample (III) treated electrochemically; (VIII) sample (IV) treated electrochemically;(IX) sample (IV) treated photochemically; (X) sample (IV) treated with the photo-assisted electrochemical method; (XI) sample (V) treated electrochemically; (XII) sample (V) treated photochemically; (XIII) sample (V) treated with the photo-assisted electrochemical method.

Results and discussion Effect of electrolyte on degradation First, it is important to demonstrate to the effect of the two electrolytes used in this study on the rate of atrazine removal

[Atrazine] (mg/L) 20

15

10

5

0 0

30

60

90

120

150

180

Time (min) Fig. 1 Atrazine concentration trend during electrolysis at 10 mA cm-2 of aqueous solutions containing 20 mg L-1of atrazine and supporting electrolyte. (open circle) photo-assisted (filled circle) electrochemical in 0.10 mol L-1 NaCl and (open square) photoassisted (filled square) electrochemical in 0.033 mol L-1 Na2SO4. (open triangle) Photochemical in 0.033 mol L-1 Na2SO4

(Fig. 1). It can be observed from Fig. 1 that the rate of atrazine removal during purely electrochemical degradation is much greater when NaCl is used as the supporting electrolyte. This observation can be attributed to the in situ generation of Cl2 at the anode and subsequent formation of dissolved free chlorine species, which continue to remove organic species (Eqs. 1–3). The results for the purely photochemical treatment in sulfate media are also given in Fig. 1, and it can be seen that the extent of removal is limited, being very similar to the extent of electrochemical removal. The photochemical removal in NaCl was for purpose identical to removal in Na2SO4 (Fig. 1). When the photo-assisted technique is applied, there is substantial increase in the rate of atrazine removal when Na2SO4 is used as the supporting electrolyte, but in the presence of NaCl, the removal of atrazine is not greatly enhanced compared to the electrochemical method. However, the main difference can be seen when the extent of total organic carbon abatement is considered. This is given in Fig. 2, where *95% of organic carbon can be observed to be removed over an electrolysis time of 3 h for the photo-assisted technique compared to *85% in the case of the electrochemical technique using NaCl. These levels of organic carbon removal are much higher than those generally reported in the literature where the final degradation product of atrazine is cyanuric acid, which is a recalcitrant species. Cyanuric acid is added to swimming pools in order to stabilize the free chlorine present via the formation of chlorinated isocyanurate/isocyanuric acid species. Cyanuric acid degradation is believed to occur via the process (Wojtowicz 2001):

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180


Total Organic Carbon removal (%)

80

(b) (b)

60

(a)

40

20

(a) 0

Electrochemical

(a)

(b)

Photo-chemical

Photo-assisted

Fig. 2 Extent of total organic carbon removal over 3 h of degradation by different methods as indicated in the y-axis. The electrochemical and photo-assisted methods were performed by electrolysis at 10 mA cm-2. Initial solutions contained 20 mg L-1 atrazine and a 0.033 mol L-1 Na2SO4 or b 0.10 mol L-1 NaCl. Figure shows that photo-assisted electrochemical removal is much more efficient than for isolated photochemical or electrochemical methods

2ðHNCOÞ3 þ 9ClO ! 3N2 þ 6CO2 þ 9Cl þ 3H2 O:

ð4Þ

It is suggested that the removal of cyanuric acid under natural sunlight conditions is slow. However, the isocyanurate/isocyanuric acid species usually have values of kMAX in the range of 215–220 nm, and their photochemical degradation may be accelerated by the kind light source used in this study (250 W, kMAX = 254 nm), which may explain the higher organic carbon removal rates observed here. In the case of Na2SO4, organic carbon removals of *82 and 25% for the photo-assisted and electrochemical techniques are obtained, respectively. It might be thought that it would be desirable to avoid the use of NaCl as the supporting electrolyte for organic degradation as the formation of toxic organo-chloride byproducts has been observed by previous authors (Rajkumar et al. 2005; Aquino Neto and De Andrade 2009). Taking into account the above, in most studies, Na2SO4 is chosen as the supporting electrolyte in the photo-assisted method, considering that there is less chance of producing such toxic species and the removal rate, although not as rapid as in NaCl, is still considerable. In the present study, it was decided to test this hypothesis in terms of the acute toxicity of the pesticide-containing solutions using both Na2SO4 and NaCl as supporting electrolytes. Acute toxicity tests Initial tests The first stage of the tests involved testing the toxicity of the untreated solutions: (I) 0.033 mol L-1 Na2SO4, (II)

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20 mg L-1 atrazine and (III) 0.10 mol L-1 NaCl and mixtures of the salts and atrazine at the same concentrations (solutions IV and V for Na2SO4 and NaCl, respectively). Typically, the salt-only solutions did not present acute toxicity over a 48-h period. Atrazine solutions (at 20 mg L-1) presented toxicity over 48 h, and when the salts are mixed with atrazine, only the mixture with NaCl presents toxicity (sol. V). In order to verify the effect of electrochemical treatment of salt-only containing solutions on the final toxicity of the treated solutions, 3 h electrolyses (at 10 mA cm-2) were performed on the salt-only solutions (sol. VI and sol. VII for Na2SO4 and NaCl, respectively). After electrolysis, both treated solutions presented elevated toxicity compared to the original solution. These results demonstrate that the electrochemical treatment itself can lead to the formation of toxic species, even when only the supporting electrolyte is present. Effect of treatment types The effect of the different treatment types on the extent of toxicity is given in Table 1. It can be observed that when both NaCl and Na2SO4 are used as supporting electrolytes during electrochemical treatment (sol. VIII and XI), the final solutions present toxic effects. Although detailed mechanistic studies were not carried out, it is probable that the cause of the increased toxicity in the case of NaCl is due to the formation of organo-chloride degradation by-products (Choi et al. 2004). When the photo-assisted method is applied, the toxicity in the case of both electrolytes is reduced to zero (sol. X and XIII). When allied with the rate of atrazine and the extent of organic carbon removal, the photo-assisted results are extremely promising as almost complete organic carbon removal is possible with much lower toxicity resulting when the electrochemical method only is used. Based on the above, two facts should be noted: 1.

2.

Photo-assisted treatment results in a substantial increase in the rate/extent of atrazine and organic carbon removal. This has been demonstrated in previous studies (Malpass et al. 2007). Photo-assisted treatment, under the conditions employed here, results in total detoxification of the atrazine-containing solution, in the presence of both supporting electrolytes.

From the results presented here and in previous papers, it is apparent that the combined method is capable of achieving results beyond those of simply combining the electrochemical and photochemical methods. The results can be rationalized by considering the formation of


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Table 1 Results for the toxicity tests before and after different types of treatment Electrolyte: Na2SO4 (0.033 mol L-1)

Electrolyte: NaCl (0.10 mol L-1)

Sample composition

Type of treatment

Toxicity

Sample composition

Type of treatment

Toxicity No

Na2SO4

None

No

NaCl

None

Na2SO4 ? Atrazinea

None

No

NaCl ? Atrazinea

None

Yes

Na2SO4

Electrochemical

Yes

NaCl

Electrochemical

Yes

Na2SO4 ? Atrazinea

Electrochemical

Yes

NaCl ? Atrazinea

Electrochemical

Yes

a

Photochemical

Yes

NaCl ? Atrazinea

Photochemical

No

Na2SO4 ? Atrazinea

Photo-assisted

No

NaCl ? Atrazinea

Photo-assisted

No

Na2SO4 ? Atrazine

The table shows that the toxicity of the treated solution varies according to the type of treatment applied a

20 mg L-1

electrochemical reaction intermediates that subsequently interact with UV light to produce radicals. There are two principal reaction intermediates that can be produced from electrolysis of the electrolytes under study. In NaCl solutions, the well-known set of reactions occurs, with the first step being the formation of elemental chlorine and dissolved free chlorine species at the anode (Eqs. 1–3). However, the free chlorine species in solution, HOCl and OCl-, can additionally undergo photolysis to form radicals (Feng et al. 2007): OCl þ hm ! O þ Cl

O þ H2 O ! OH þ OH

ð5Þ

ð6Þ

or HOCl þ hm ! OH þ Cl:

ð7Þ

In the case of sulfate, persulfates may be generated electrochemically at the anode (Goldin et al. 2009): 2 2SO2 4 ! S2 O8 þ 2e :

ð8Þ

The persulfate species may then be photolyzed to produce the sulfate radical SO 4 , which in turn produces hydroxyl radicals. S2 O2 8 þ hm ! 2SO4

SO 4

þ H2 O ! OH þ

ð9Þ HSO 4

ð10Þ

The sulfate radical is, itself, a strong oxidizing agent (Stanbury 1989) and has been reported to be more effective than hydroxyl radicals (Criquet and Leitner 2009). Hence, during photo-assisted treatment, in both NaCl and Na2SO4 electrolytes, hydroxyl radicals maybe produced and increase the rate of oxidation. This possibility represents an alternative explanation for the high rates of pollutant removal encountered in this and other studies performed by the authors.

Conclusions The present study has shown that high atrazine removal rates, significant organic carbon destruction, and reduction in effluent toxicity are possible under the conditions studied, even when NaCl is employed as the supporting electrolyte. This is an interesting fact as many studies in the literature strive to produce advanced (and costly) electrode materials that produce radicals that oxidize the pollutant. On the other hand, chlorine evolution is a relatively facile reaction at most electrode materials. This combined with the use of simultaneous UV radiation results in an effective effluent treatment system. Acknowledgments The authors wish to thank the financial support from the Brazilian funding organization CNPq (Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico).

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