Enhancing biodegradability of priority substances (pesticides) by solar photo-Fenton

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Enhancing biodegradability of priority substances (pesticides) by solar photo-Fenton Milena Lapertota, Ce´sar Pulgarı´na, Pilar Ferna´ndez-Iba´n˜ezb, Manuel I. Maldonadob, Leonidas Pe´rez-Estradab, Isabel Ollerb, Wolfgang Gernjakb, Sixto Malatob, a

Laboratory of Environmental Biotechnology, ENAC, Swiss Federal Institute of Technology (EPFL), CH-1015 Lausanne, Switzerland Plataforma Solar de Almerı´a-CIEMAT. Carretera Sene´s km4, Tabernas (Almerı´a), 04200-Spain

b

ar t ic l e i n f o

A B S T R A C T

Article history:

In this paper, we present the photo-Fenton treatment in a solar pilot-plant scale of several

Received 14 July 2005

EU priority hazardous substances (Alachlor, Atrazine, Chlorfenvinphos, Diuron and

Received in revised form

Isoproturon) dissolved in water. The results have been evaluated not only from the point

22 December 2005

of view of contaminant disappearance and mineralisation, but also of toxicity reduction

Accepted 3 January 2006

and enhancement of biodegradability. Degradation was monitored by total organic carbon, pesticide concentration by HPLC–UV, inorganics released by ion chromatography, and

Keywords:

biodegradability by the Zahn–Wellens (Z–W) test. The total volume of the solar

Biodegradability enhancement

photoreactor, composed of compound parabolic collectors with a total area of 4.16 m2,

Pesticides treatment

was between 70 and 82 L. The treatment was shown to be effective, mineralising all of the

Photo-Fenton

pesticides tested, both alone and in mixtures. In order to find out the conditions for

Solar photocatalysis

biocompatibility using the photo-Fenton reaction as a pre-treatment step, wastewater inoculated with unacclimated municipal sludge containing pesticides after certain degradation time was evaluated by the Z–W test. Biodegradability was enhanced (70% considered biodegradable) by the photo-Fenton treatment after 12–25 min. It may be concluded that the photo-Fenton treatment consistently enhances biodegradability of wastewater containing pesticides. & 2006 Elsevier Ltd. All rights reserved.

1.

Introduction

The incapability of conventional biological wastewater treatment to effectively remove many industrial toxic pollutants shows that new treatment systems are needed. In the European Union, water policy is undergoing considerable changes at present. The adoption of the Framework Directive on water (European Commission, 2000) provides a policy tool that enables this essential resource to be sustainably protected. Among other measures, surface water deterioration must be prevented by reducing pollution from discharges and emissions of hazardous substances by 2015 (European Commission, 2002). Thermal treatments present considerable Corresponding author. Tel.: +34 9503 879 40; fax: +34 9503 650 15.

E-mail address: [email protected] (S. Malato). 0043-1354/$ - see front matter & 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2006.01.002

emissions of other hazardous compounds, and separation treatments, require post-treatment. Alternative methods to these well-established techniques involve the oxidation of pollutants with reagents such wet oxidation, potassium permanganate, hydrogen peroxide and ozone (Gogate and Pandit, 2004a,b). Among these techniques, the so-called advanced oxidation processes (AOPs) appear to be a promising field of study. These techniques (H2O2+UV, O3+UV, H2O2+O3, photo-Fenton, TiO2–UV, etc.) can provide the conversion of contaminants to less harmful compounds (Konstantinou and Albanis, 2003,2004). Even though AOPs for wastewater treatment have been shown to be highly efficient, their operation is actually quite expensive (tens of h/m3)

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(Pulgarı´n et al., 1999; Bressan et al., 2004; Da Hora et al., 2005). The combination of an AOP as a preliminary treatment, followed by an inexpensive biotreatment, would seem to be an economically attractive option (Rodrı´guez et al., 2002; Sarria et al., 2002; Amat et al., 2003; Contreras et al., 2003; Ho¨rsch et al., 2003; Bressan et al., 2004). Similarly, the only AOPs that can be driven by a cheap source of energy (the Sun) are photo-Fenton and TiO2 photocatalysis. One of the main obligations for urban wastewater treatment imposed by European Union Council Directive 91/271/ EEC is that wastewater collecting and treatment systems (generally involving biological treatment) must be provided by 31 December 2005 in all agglomerations of between 2000 and 10000 p.e. (population equivalent). Smaller agglomerations which already have a collecting system must also have an appropriate treatment system in place by the same date .The deadline for agglomerations of more than 15000 p.e. should be accomplished by the end of 2000 (European Commission, 2004). Therefore, in the near future, most of the AOP plants developed in the EU could discharge pre-treated wastewater into a nearby conventional biological treatment. In this paper, we present the Photo-Fenton treatment in a solar pilot-plant scale of several toxic compounds considered priority substances by the EU (European Commission, 2001) dissolved in water, evaluating the data not only from the point of view of contaminant disappearance and mineralisation, but also of toxicity reduction and enhancement of biodegradability. Our objective has been to demonstrate that by a proper method of evaluation of the water, at different stages of photo-Fenton treatment, it is possible to predict its biocompatibility and reach a water quality that can be sufficient for its disposal into a municipal biological process.

2.

Materials and methods

2.1.

Chemicals

Technical-grade alachlor (2-chloro-20 ,60 -diethyl-N-methoxymethylacetanilide, 95%, Aragonesas Agro S.A.), atrazine (6chloro-N-ethyl-N0 -isopropyl-1,3,5-triazine-2,4-diamine, 95%, Ciba-Geigy), chlorfenvinphos (2-chloro-1-(2,4-dichlorophenyl)ethenyl diethyl phosphate, Aragonesas Agro S.A.), diuron (1-

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(3,4-dichlorophenyl) amino-N,N-dimethyl-formamide, 98.5%, Aragonesas Agro S.A.) and isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea, Aragonesas Agro S.A.) were used as received. Initial concentrations of the pesticides were 50 mg/L of alachlor, chlorfenvinphos and isoproturon, 25 mg/L of atrazine and 30 mg/L of diuron (see Fig. 1). Analytical standards of all pesticides for chromatographic analyses were purchased from Sigma-Aldrich. Water used in the pilot plant was obtained from the Plataforma Solar de Almerı´a (PSA) distillation plant (conductivityo10 mS/cm, Cl
¼ 0:720:8 mg=L, organic carbon o0.5 mg/L). Iron sulphate (FeSO4.7H2O), hydrogen peroxide (30% w/v), and sulphuric acid for pH adjustment (around 2.7–2.8) were reagent grade.

2.2.

Analytical determinations

Mineralisation was monitored by measuring the total organic carbon (Shimazu-5050A TOC analyser). Pesticide concentration was analysed by liquid chromatography (flow 0.5 mL/ min) in a HPLC–UV (Agilent Technologies, series 1100) with a C-18 column (LUNA 5 mm, 3  150 mm, from Phenomenex): alachlor (H2O/Acetonitrile 40/60, 225 nm), atrazine (H2O/ACN 55/45, 240 nm), chlorfenvinphos (H2O/ACN 40/60, 240 nm), isoproturon (H2O/ACN 55/45, 240 nm) and diuron (H2O/ACN 40/60, 254 nm). Ammonia and anions concentration were determined by ionic chromatography. The concentration of H2O2 was frequently determined in fresh sample solutions using Merckoquant paper (Merck cat no. 1.10011.0001). When necessary, the concentration of peroxide in the reactor was found by iodometric titration (more accurate method). The initial hydrogen peroxide concentration was always 400 mg/L and controlled to avoid complete disappearance by adding small amounts as consumed. Zahn– Wellens (Z– W) tests. An adaptation of the Directive 88/ 302/EEC (European Commission, 1988) protocol is followed. Activated sludge, mineral nutrients and the test material as the sole carbon source in an aqueous solution are placed together in a 0.25 L glass vessel. The mixture is agitated and aerated at 20–25 1C in a dark room for up to 28 days. The mineral nutrient medium is produced using the following solutions. Solution A: a mixture of KH2PO4 (8.5 g/L), K2HPO4 (21.75 g/L), Na2HPO4.2H2O (33.4 g/L) and NH4Cl (38.5 g/L) at a

Fig. 1 – Structures of the pesticides studied.

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pH 7.4. Solution B: CaCl2 (27.5 g/L). Solution C: MgSO4.7H2O (22.5 g/L). Solution D: FeCl3.6H2O (0.25 g/L). One drop of concentrated HCl is added to avoid iron precipitation. The activated sludge (from a wastewater treatment plant) is maintained with sufficient aeration for 2–3 days, then is settled for about 30 min, and the majority of the water is thrown away. Settled sludge is centrifuged two or three times (10 min at 4500 rpm), removing the supernatant and adding more settled sludge. The last is done by adding mineral medium solution (distilled water with 10 mL/L of solution A and 1 mL/L of solutions B, C and D, respectively) to the biomass, centrifuging and removing the mineral medium. The required concentration of inoculum is calculated using the following expression: Inoculum concentration (mg of wet sludge/L) ¼ (3/0.09)  DOC (mg/L). Considering 3 as an appropriate ratio between the concentration of the inoculum and the DOC of the test substance, and 9% as the percentage corresponding to dry biomass. Test solutions are prepared by adding 10 ml/L of solution A, 1 mL/L of solutions B, C and D, respectively, and the calculated amount of inoculum to 240 mL of neutralized sample. Hydrogen peroxide present in the samples was removed using catalase (2500 U/mg bovine liver; 100 mg/L) acquired from Fluka Chemie AG (Buchs, Switzerland) after adjusting the sample pH to 7. The blank is prepared using distilled water instead of test water, and the same amount of inoculum added into the test solutions. The degradation process is determined at appropriate regular time, according to Eq. (1). The DOC is always determined first 3 h after the start of the test in order to detect adsorption of contaminants by the activated sludge. Loss of volume from evaporation is adjusted just before each sampling with distilled water. A vessel containing diethylene glycol, a well-known biodegradable substance recommended by Directive 88/302/EEC is run in parallel in order to check the activity of the activated sludge   ðCt
CB Þ  100; (1) Dt ¼ 1
ðCA
CBA Þ where Dt is biodegradation (%) at time t, CA is DOC in the test mixture (measured 3 h after the beginning of the test, mg/L), Ct is DOC in the test mixture at time t, CB is DOC of the blank at time t and CBA is DOC of the blank (measured three hours after the beginning of the test). The result is plotted versus time giving the biodegradation curve (substances Dt 470% are considered biodegradable).

2.3.

Experimental set-up

The reactor consists of a continuously stirred tank, a centrifugal recirculation pump, a solar collector and connecting tubing and valves (Fig. 2). Several on-line sensors and devices for heating and cooling of the process fluid are installed in the connecting tubing. The tank is a 20-L roundbottom Pyrex flask. Piping and valves are made of polypropylene. A pump (Bominox SIM-1051, 370 W, 380 V AC) with a three-phase frequency regulator was used to manipulate the flow rate. The process fluid temperature control is done by separating heating and cooling devices. With these systems, the temperature of the experiment can be controlled within

TO (2)

FROM (1) SOLAR COLLECTOR FROM (2) TANK electric Sampling valve resistance (heating) heat-exchanger (cooling) TO (1)

Pump TC

Draining valve

FC

Fig. 2 – Flow diagram of photoreactor.

10–60 1C. The solar reactor is composed of four compound parabolic collector units (concentration factor ¼ 1) with an area of 1.04 m2 (total area 4.16 m2), mounted on a fixed platform tilted 371 (local latitude) facing south. Each unit has five borosilicate–glass tubes of thickness 1.8 mm and outer diameter 50 mm (transmissivity450% at l4300 nm; 475% at l4320 nm; 490% at l4350 nm). The total illuminated volume inside the absorber tubes is 44.6 L. Glass tubes are connected with plastic unions. For performing tests adequately (mixing in the dark, changing collector illuminated area, etc.), the collector was covered with special ‘‘hand-made’’ aluminium sheets. The total reactor volume (VT) is made up of total irradiated volume (glass tubes, Vi) and the dead reactor volume (tank+tubes). All the experiments were done in the same way. Once the pesticide dissolved in water had been added to the pilot plant, it was homogenised in the system. Then a sample was taken (initial concentration, point 1 in figures) and sulphuric acid was added. After certain time a second sample was taken (point 2) and a pH of around 2.8–2.9 was confirmed. After that, iron salt was added and mixed. Finally, the necessary amount of hydrogen peroxide was added (point 3), and after a certain time a sample was taken to evaluate the dark Fenton reaction. Then the cover is removed and samples are collected at predetermined times (t). Solar ultraviolet radiation (UV) was measured by a global UV radiometer (KIPP&ZONEN, model CUV3), mounted on a platform tilted 371 (the same angle as the CPCs). With Eq. (2), combination of the data from several days’ experiments and their comparison with other photocatalytic experimental systems is possible: t30W;n ¼ t30W;n
1 þ Dtn

UV Vi ; 30 VT

Dtn ¼ tnþ1
tn ,

(2)

where tn is the experimental time for each sample, UV is the average solar ultraviolet radiation (W/m2) measured during Dtn , t30W is a ‘‘normalized illumination time’’ which refers to a constant solar UV power of 30 W/m2 (typical solar UV power on a perfectly sunny day around noon), VT is the total reactor volume (from 70 to 82 L, as selected for each test) and Vi is

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total irradiated volume (44.6 L). All the tests were performed at constant temperature (usually at 30 1C).

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Results and discussion

Several prior tests were performed with single compounds dissolved in water (Lapertot and Pulgarin, 2005). No DOC had been removed by activated sludge obtained from a secondary effluent of the local municipal treatment plant (neither specific nor acclimated activated sludge) of alachlor, chlorfenvinphos, diuron and isoproturon after 28 days. Atrazine was degraded up to 100% in 21 days. Atrazine biodegradation has been described before and can occur in three known ways. It can be dechlorinated, the other ring substituents can be removed by amidohydrolases (the end product, cyanuric acid, is then used as a source of carbon and nitrogen) and the amino groups can be dealkylated (in this mechanism dechlorination can be performed in the second step to eventually yield cyanuric acid). The most characteristic organism performing this is Pseudomonas sp. but it can also be performed by a number of bacteria (Wacket et al., 2002; Zeng et al., 2004). Despite the biodegradability of pure atrazine, it was included in the study because it has been demonstrated at both lab (Farre´ et al., 2005) and pilot-plant scales (Hincapie´ et al., 2005a,b) that when treated by photoFenton, the toxicity (measured by Vibrio fischery) of the water

containing atrazine (around 20 mg/L) increases during the process, then decreases again after a certain time. Toxicity could not be related to biodegradability, but it could be considered that if toxicity increases during the first stages of the treatment, biodegradability should decrease. However, this must be tested. As the goal of the EU is to reduce pollution from discharges of hazardous substances (European Commission, 2002), one of which is atrazine, any type of AOP applied should demonstrate the complete removal of this substance and the harmlessness of the treatment effluent. Moreover, as several of the tests reported in this article were performed with mixtures of compounds, it is recommendable to include atrazine in these mixtures. Fig. 3 shows results obtained during photo-Fenton treatment of single compounds. It has been demonstrated (Malato-Rodrignez et al., 2004) that the amount of iron necessary for carrying out photo-Fenton in solar photoreactors with a light-path length of a few centimetres (diameter of the cylindrical photoreactor) is in the range of a few tens of mg/L. According to these results, the photoreactor described in the Experimental Section was constructed using a 50-mm diameter tube installed in the focus of a CPC, which corresponds to an optimum dose of iron between 10 and 20 mg/L. The intention was to use the highest iron concentration compatible with a subsequent biological treatment (which is often very useful, mainly for avoiding iron removal at the end of the process) without losing photo-Fenton

35 (a)

(b)

Alachlor "al" Atrazine "at" Chlorfenvinphos "cl" Diuron "di" Isoproturon "is"

50

"Illumination", t = 0 (1) Only pesticide and pH adjust. (2) pH 2.8 + Fe addition (3) H2O2 addition

30

40

25

DOC, mg L-1

C, mg L-1

is-1

30

20 is-2 15 di

20 at 10 10

cl 5

is-3

al 1

2

1

3

2

3

0

0 -75

-50

-25

0

-75

25

-50

-25

0

25

50

100 150

t30W, min Fig. 3 – Disappearance of pesticides (a) and evolution of DOC (b) as a function of t30w for 20 mg/L of Fe2+. In Fig. 3(b), samples selected for performing Z–W tests shown in Fig. 4 are marked. Points 1–3 refer to the different steps in the procedures applied prior to illuminating the system (see text for details).

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efficiency. Single compound experiments shown in Fig. 3 were performed using 20 mg/L of iron, which is enough for degrading the parent compounds in the dark but not for DOC mineralisation. According to photo-Fenton reactions (Eqs. (3)–(6) show a simplification of the main reactions), 20 mg/L of Fe2+ (in the dark) produced enough oxidising species to degrade the pesticides, but not enough for substantial mineralisation without light regeneration of Fe2+. Details of the complete mechanism have been published recently by Sagawe et al. (2001). In this system, iron is a true catalyst and the rate-limiting step is the regeneration of ferrous iron. Ferric iron complexes can be photolysed upon irradiation in a ligand-to-metal charge transfer reaction by passing to a photoexcited transition state before charge transfer and complex dissociation (Eq. (5)), with the ligand as the intermediates formed during the treatment. Especially worthy of mention is the photolysis of ferric iron/aquo complexes, which apart from efficiently regenerating ferrous iron, generates further hydroxyl radicals (Eq. (6)). Depending on pH, the number of hydroxyl ions in the complex and with it, light absorption and quantum yield of the reaction varies. The mono-hydroxyl complex is the most photoactive species. This complex is predominant at a pH of around 2.8, which is one reason why this pH is frequently given as optimal for the photo-Fenton process (Pignatello, 1992) (3) Fe2þ þ H O
! Fe3þ þ d OH þ OH
2

2

Fe3þ þ H2 O2
! Fe2þ þ HO2d þ Hþ

(4)

½Fe3þ L þ hn
!½Fe3þ L
! Fe2þ þd L loca: 500 nm

(5)

½Fe3þ ðOH
Þx ðH2 OÞy  þ hn
! Fe2þ þ ðx
1ÞOH
þ y H2 O þ  OH loca: 450 nm

ð6Þ

All of the five pesticides tested were mineralised, but atrazine was the slowest. This pesticide is of interest because

complete mineralisation is not attained, as oxidation only affects the lateral chains with five of the eight carbon atoms removed as CO2. It has been concluded (Konstantinou and Albanis, 2003) that the oxidation of the lateral chains and subsequent disappearance of the initial compound is very fast, but the formation of the final product (cyanuric acid) may require long irradiation time. It is of interest that cyanuric acid is reported to have lower toxicity than atrazine and its degradation products (Hiskia et al., 2001). Cyanuric acid is biodegradable and has negligible eco-toxicity, and therefore the best treatment for atrazine should be complete transformation of atrazine into cyanuric acid as its final goal. But our previous results have shown that cyanuric acid is only produced very slowly after more than 10 h (Hincapie´ et al., 2005a,b) and in no case (Arnold et al., 1995; Balmer and Sulzberger, 1999; De Laat et al., 1999; Krisova et al., 2003) is cyanuric described as the final product of Fenton and/or photo-Fenton, or if so, only after a very long time. Therefore, determination of biodegradability before complete oxidation of the lateral chains is of special interest to reduce the AOP treatment time as much as possible. In order to find out the conditions for biocompatibility using the photo-Fenton reaction as a pre-treatment step, the biodegradability of the wastewater containing each of the five pesticides was evaluated by the Z–W test after a certain degradation time, using unacclimated municipal sludge as the initial inoculum. Taking into account toxicity measurements (Hincapie´ et al., 2005a,b) and previous experience (Sarria et al., 2003), different stages of the treatment were taken as a reference for the study of their biocompatibility. The results are shown in Fig. 4. It should be mentioned that complete disappearance of the initial compound and chloride release were considered the key-parameters for selecting the stage of treatment when the water could be discharged into a biotreatment. Another factor was the toxicity measurements,

100

80

Dt, %

60

40

Alachlor Atrazine Chlorfenvinphos Diuron Isoproturon 1 Isoproturon 2 Isoproturon 3 Diethylene glycol

20

0 0

5

10

15 time, days

20

25

30

Fig. 4 – Evolution of Dt (see Eq. (1)) of samples taken (‘‘marked’’) during the photo-Fenton experiment shown in Fig. 3(b). The control test with diethylene glycol is also shown.

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when the parent compound disappears and chlorine release is complete, toxicity sharply decreases. More than one point was selected only in the case of isoproturon (which does not contain chlorine), just after its disappearance. The combined use of toxicity and chemical measurements appears to be necessary in the quality control of an AOP. Fig. 4 shows that a certain amount of degradation of pesticide intermediates by the microorganism was attained in all cases, and more than 70% is achieved in most of them (alachlor, chlorfenvinphos, diuron and isoproturon). In the case of atrazine, only 40% could be degraded. According to the results shown in Fig. 3(b) and chloride analyses, the DOC in this sample corresponds to 70% of the initial, even though the intermediates were completely dechlorinated. As 40% of it is degraded during the Z–W test, 60% of atrazine, corresponding to the lateral chains, had to have been mineralised by photoFenton+biodegradation, remaining the triazine ring. As cyanuric acid is known to be biodegradable, other non-chlorinated atrazine intermediates must not be. A complete recent study on degradation by photo-Fenton of atrazine and its main intermediates (Chan and Chu, 2005) and a review of previous information by Konstantinou and Albanis (2003) describe non-stable chlorinated intermediates. Intermediates produced during the degradation of alachlor, chlorfenvinphos and diuron were successfully degraded, chlorfenvinphos being intermediates the most difficult, as 70% degradation was attained only after 28 days. In the case of isoproturon, which does not contain chlorine (key-parameter for selecting the stage of the photo-treatment to be evaluated by Z–W), three different times during photo-treatment were selected. The biorecalcitrance of ‘‘isoproturon 1’’ might be explained by the presence of non-biodegradable intermediates. After 7 days, the inoculum no longer had enough organic matter for its metabolism and the cells died and were destroyed, triggering an increase in DOC (a Dt decrease). This might also be caused by some type of chronic (not acute) toxic effect. Biodegradation increased during the first 7 days and decreased afterwards, meaning that DOC was removed during the first 7 days and then DOC increased again from destruction of bacterial cells. This could be explained by the presence of a small amount of isoproturon (2 mg/L, see Fig. 3(a)) or by the presence of other toxic intermediates. From toxicity tests (Hincapie´ et al., 2005a,b), it has been concluded that isoproturon is not really very toxic (50 mg/L induce 30% inhibition using a Microtoxs). Therefore, ‘‘isoproturon 1’’ must contain one or more isoproturon intermediates that produce additional toxicity after a few days in contact with the unacclimated municipal sludge. Toxicity tests also demonstrated that a very toxic intermediate was produced during the first stages of photo-Fenton treatment of isoproturon. A similar effect has been demonstrated previously during TiO2 treatment of isoproturon (Parra et al., 2002a). Therefore, ‘‘isoproturon 1’’ was not suitable selected for Z–W test. These results underline the importance of using toxicity tests (usually a quick method) for selecting the stage of an AOP treatment at which the water might be non-toxic and, presumably, biodegradable. This stage should be tested afterwards by a biodegradability test (usually a time-consuming method). In other words, toxicity tests such as Vibrio fischeri can detect toxic response in a short time (5–30 min). This

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property makes these tests useful analytical tools that make possible quick decisions regarding the advisability and need for further treatment before AOP effluent discharge into a bioreactor. In accordance with the above results it was decided they should be validated with mixtures of pesticides. It is well known that synergistic, additive and antagonistic effects appear in the response of microorganisms when chemicals are mixed (Hernando et al., 2005), therefore different results could be obtained when pure compounds are mixed. In fact, it was observed that when using a mixture of pesticides, the initial DOC concentration was higher than with the pure compounds, and the DOC content when the water was tested by Z–W was also higher, making the results of this technique more consistent. At the same time, Fe concentration was reduced to 10 mg/L to determine whether it is enough to reach a degradation level permitting discharge of the water into a biotreatment plant. At the same time, another reason for decreasing the photo-Fenton reaction rate was to avoid its mineralisation too quickly. It should be remarked that the main objective was not to mineralise the pesticides, but to increase biodegradability. And this objective should be reached before a significant mineralisation. Fig. 5 shows the experiment done in demineralised water at two different concentrations of pesticides (mixture of five pesticides). Results presented in Fig. 5 show that photo-Fenton is also an effective method for treating mixtures. Degradation of each pesticide in the mixture was as quick as in the single compound experiments and the DOC mineralisation rate in the experiment at lower concentration was quite similar to the experiments shown in Fig. 3. It should be noted that the concentration of each pesticide in the mixture was enough to obtain an initial DOC similar to that in the single compound experiments. Increasing the concentration of pesticides up to 4 times higher DOC0, made the treatment time required to reach 80% mineralisation around 3 times longer. Complete disappearance and total dechlorination (6 mg/L and 23 mg/L of chloride were expected in both the lower and the higher concentrations tested, respectively) of all pesticides was attained very easily, at both initial concentrations tested. In any case, based on these results, it was concluded that pesticide mixtures at the lower concentration was not the best choice for testing biodegradability. Higher DOC permit more precise determination of Dt . NTOT symbolises the combination of nitrogen from pesticides mineralised as + ammonia and nitrate, the molar ratio NO
3 /NH4 at t30W ¼ 110 min (Fig. 5, right) being 2:3. This ratio varied during the treatment, the quantity of ammonia was much higher at the shorter treatment time, as ammonia was slowly converted into nitrate during the process. The nitrogen mass balance was not completed, as only 60% of organic nitrogen was detected as inorganic nitrogen at the end of the treatment. This behaviour could be reasoned in the following way. From single compound experiments it was concluded that atrazine, diuron and isoproturon were responsible for the incomplete N mass balance, as nitrogen of alachlor was completely mineralised and chlorfenvinphos does not contain N. Only two N atoms from atrazine would have to be mineralised (the other three atoms are contained in the triazine ring). The very slow degradation of DOC from phenylurea pesticides (diuron

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Fe0 = 10 mg/L

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Chlorfen.

C, mg L-1

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Cl-

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Isoprot.

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NTOT

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0

25 50 t30W, min

(a)

75

1 2

3 0

0

100

-25 (b)

0

25 50 t30W, min

75

100

125

Fig. 5 – Disappearance of pesticide mixtures and evolution of DOC using two different concentrations of each pesticide: 10 mg/ L (a) and 30 mg/L (b). Points 1–3 refer to the comments inserted in Fig. 3.

80

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a

a

b

b 70

e

c

f H

d

80

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f H I

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c Dt, %

DOC, mg/L

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30

I

40 d

20 20 Fe0 = 10 mg/L

10

0

Experiment 1 Experiment 2

1 2 3

Z-W test 0

0

50 100 t30W, min

150

200

0

4

8

12 16 time, days

20

24

28

Fig. 6 – Right: evolution of Dt (see Eq. (1)) of samples (marked with a letter) taken during two photo-Fenton experiments (see left) of pesticide mixtures (30 mg/L each). Points 1–3 refer to the comments inserted in Fig. 3.

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4 0 (200 6) 108 6 – 109 4

and isoproturon) after 80–90% mineralisation (Maurino et al., 1999; Muneer et al., 1999; Parra et al., 2002b) and the very slow mineralisation of urea (Maletzky and Bauer, 1998), which can be predicted as an intermediate of diuron and isoproturon are well known. Urea contains only one C atom and two N atoms. The stable triazine ring and the formation of urea could justify not only the incomplete release of N as NH+4 or NO
3 but also the slow mineralisation rate at the end of the treatment. Fig. 6 shows the results obtained during photo-Fenton treatment of pesticide mixtures at the higher concentration and the Z–W tests applied to different selected samples. No substantial difference between the two tests was observed. It was demonstrated that wastewater samples ‘‘a’’ and ‘‘e’’ were not biodegradable prior to the treatment. With these results, the conclusion can easily be arrived at that biodegradability is enhanced during the photo-Fenton treatment. It seems that biodegradability was enhanced during Fenton in the dark (samples ‘‘b’’ and ‘‘f’’), but not enough to reach the 70% considered biodegradable. It is also important to remark that when degradation was more pronounced (‘‘c’’ and ‘‘d’’), Dt increased very quickly during the first 5 days but not up to 70%. Under these conditions, the generation of non-biodegradable (but non-toxic) intermediates during advanced stages of the photo-Fenton treatment could be inferred. The best results were obtained with samples taken after a treatment time of 12 min (‘‘H’’) and 25 min (‘‘I’’). In the first case, 70% biodegradability is attained after 18 days. For DOC ¼ 33:4 mg=L, which corresponds to a treatment time of 25 min (‘‘I’’), 70% biodegradability is attained after 10 days. This difference could be attributed to less DOC in the second case and/or to further transformation of pesticide intermediates. It is well known that more oxidised intermediates (usually more biodegradable compounds) are produced during the last stages of advanced oxidation processes. If the results obtained in Z–W tests on sample ‘‘I’’ are compared with those obtained with samples ‘‘c’’ and ‘‘d’’, they seem to be inconsistent, however, the Z–W test has an intrinsic uncertainty concomitant with any biological test. Moreover, Z–W tests have been applied to wastewaters treated with an AOP and, therefore, the composition of the wastewaters could vary substantially after a few minutes of treatment.

4.

Conclusions

 It has been demonstrated that photo-Fenton at low iron





concentrations (10–20 mg/L) is an effective method for treating priority substances. Complete disappearance and total dechlorination of all pesticides was attained very easily at different initial concentrations, alone and in mixtures. It may be concluded that photo-Fenton treatment is a very consistent method for enhancing biodegradability of wastewater containing pesticides. Biodegradability increases significantly after short photo-Fenton treatment times. Zahn–Wellens tests is considered to be a useful tool for determining the optimal DOC range in which wastewater becomes biodegradable. After only a few minutes of photo-Fenton treatment, wastewater could be discharged into a conventional

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biotreatment. In the tests shown in this paper, the consumption of hydrogen peroxide could be reduced around 10 times and the necessary photo-Fenton reaction time (i.e., photoreactor size) about 5 times if biodegradability is considered a target of the AOP treatment, instead of 80% DOC mineralisation. Both are the most important parameters related to photo-Fenton treatment costs, and will therefore drastically reduce the cost of treatment.

Acknowledgements The authors wish to thank the Spanish Ministry of Education and Science for its financial assistance for the ‘‘Fotodetox’’ Project (Contract no. PPQ 2003-07596-C03-01) and the European Commission for its financial support under the CADOX Project (Contract no. EVK1-CT-2002-00122). They are very grateful to Aragonesas Agro S.A. (Madrid, Spain) for providing the technical pesticides. The collaboration of the Wastewater Treatment Plant of Almerı´a (Mr. Eduardo Ruiz, AQUALIA) was also valuable. They also wish to thank Mrs. Deborah Fuldauer for the English language correction. R E F E R E N C E S

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