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Solar Energy 87 (2013) 127–135

Solar photocatalytic mineralization of 2,4-dichlorophenol andmixtures of pesticides: Kinetic model of mineralization

Thomas Janin a,c,⇑, Vincent Goetz a, Stephan Brosillon b, Gael Plantard a

a Laboratoire PROcedes, Materiaux et Energie Solaire, PROMES-CNRS UPR8521, Rambla de la Thermodynamique, Tecnosud,

66100 Perpignan, Franceb IEM (Institut Europeen des Membranes), UMR 5635 (CNRS-ENSCM-UM2), Universite Montpellier 2, Place

E. Bataillon CC47, F-34095 Montpellier, Francec Societe Resolution, Espace Entreprises Mediterranee, Plein Sud Entreprises, 66600 Rivesaltes, France

Received 19 July 2012; received in revised form 21 September 2012; accepted 20 October 2012Available online 21 November 2012

Communicated by: Associate Editor Gion Calzaferri

Abstract

Phytosanitary run off from agricultural fields represents highly disseminated local sources of pollution and corresponds to relativelysmall volumes which need to be treated (around 5 m3 for a farm of a few tens of hectares). The simplicity, the robustness and the lowenergy consumption of the solar heterogeneous photocatalytic process make it well adapted to this application. Mineralization of phy-tosanitary effluent with this solar eco-technology was tested with a solar laboratory set up for a few tens of litres. Feasibility was exploredstep by step with preliminary solar experiments performed on 2,4-dichlorophenol (DCP) followed by solar mineralization carried out ontwo representative commercial pesticides and three real phytosanitary waste products which were directly collected from a vineyard area.The total organic carbon profiles showed advanced mineralization but the presence of a threshold must be underlined at about 70% ofmineralization yield in the case of the two pesticides as well as for the phytosanitary wastewater. A complementary eco-toxicity measure-ment demonstrated that this threshold is not penalizing regarding the detoxification performance. The kinetic law of mineralization, tak-ing into account the discontinuous solar UV irradiation developed and validated for DCP, was applied with success to the commercialsolutions and the wastewater from agricultural activities. Scaling up of the process based on the validated kinetics associated with solardata leads to an annual treatment capacity of 3.5 m3 for a reactor surface of 1 m2.� 2012 Elsevier Ltd. All rights reserved.

Keywords: Solar energy; Heterogeneous photocatalysis; Pesticides; Kinetic model

1. Introduction

Heterogeneous photocatalysis belongs to the family ofadvanced oxidation processes (AOPs) considered for thetreatment of bio-recalcitrant organic molecules. As withother AOPs, this process allows mineralization of a great

0038-092X/$ - see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.solener.2012.10.017

⇑ Corresponding author at: Laboratoire PROcedes, Materiaux etEnergie Solaire, PROMES-CNRS UPR8521, Rambla de la Thermody-namique, Tecnosud, 66100 Perpignan, France. Tel.: +33 616350193; fax:+33 468682213.

E-mail address: tj@residusolution.com (T. Janin).

number of organic pollutants thanks to the production ofhydroxyl radicals. In the case of heterogeneous photocatal-ysis, these highly oxidative species are produced by a solidphotocatalyst under UV irradiation. The treatment occursat ambient pressure and temperature without any additionof chemical substances. As a consequence, combined withthe direct use of the solar energy (Malato et al., 2000a), thisprocess is almost energy self-sufficient and permits thedesign of a water treatment which is simple, robust andinexpensive to set up and run (Malato et al., 1996, 2000a,2004, 2009). This process is fully within the framework ofsustainable development and integrates the concept of

Nomenclature

I UV radiation intensity (W m�2)QUV accumulated UV energy (kJ l�1)_r rate of mineralization (mg l�1 s�1)S surface of photocatalytic media (m2)t time (s)TOC total organic carbon (mg l�1)TOClimit

asymptotic TOC concentration (mg l�1)

Vr reactor volume (l)VT total fluid loop volume (l)Vtank tank volume (l)a photocatalytic media coefficient including the

reactor designb constant of Langmuir–Hinshelwood model

(l mg�1)

128 T. Janin et al. / Solar Energy 87 (2013) 127–135

green chemistry. Considering its relatively low treatmentcapacity, it is particularly well adapted to applications thatgenerate low volumes of polluted effluents which can con-taminate a very large volume of raw water. The phytosan-itary run off from agricultural fields belongs to thiscategory and is very representative of small local sourcesof pollution. In France small-scale farms with a surfacearea of less than 50 ha, generating run off volumes ofbetween 2 and 10 m3, represent more than 50% of the totalnumber of farms. These discharges consist of mixtures ofseveral commercial pesticides with relatively high concen-trations. These originate from the washing of the sprayers(after treatment of the plots with pesticides) in special areasbefore being discharged in the water network. The Frenchinter-ministerial decree of the 12 September 2006 makes itmandatory to detoxify this wastewater. This decree will beextended in 2014 throughout the European Union.

It has already been proven that photocatalytic oxidationunder natural UV provides a feasible route for the destruc-tion and the mineralization of pure pesticides and/or mix-tures (Herrmann and Guillard, 2000; Malato et al., 2000b;Oller et al., 2006; Pichat et al., 2004). Since photocatalysisis presented in the literature as “a very promising” process,it is necessary to enhance the knowledge about the effi-ciency of this unit functioning in real conditions i.e. forthe treatment of real agricultural wastewater. The mainoxidant in the photocatalyst reaction is the hydroxyl radi-cal. Based on the non-selectivity of the hydroxyl radical, alarge number of subsequent by-products having very differ-ent chemical structures are produced. The establishment ofthe complete chemical pathway from the initial molecule tothe complete mineralization of the by-products is impossi-ble. In addition the lack of standard compounds does notpermit the quantification of by-products and thus it isimpossible to calculate the linking kinetic constants. Fittingexperimental data with a kinetic model of degradation iswidespread for a single molecule. Conversely, in the caseof mixtures of pesticides, mathematical expressions of thekinetics are rarely found. Li Puma and Brucato (2007)and Li Puma et al.(2007) developed a rigorous formulationrepresentative of the degradation of herbicide mixtures foractive molecules. It is based on the coupling between the

photon absorption and scattering in the reactor and localkinetics. The agricultural waste is composed of numerouscommercial pesticides, including the active molecules butalso the additives meant to increase the wetting ability onleaves, the solubility in water, etc. In addition, the effluentto be treated is made up of various effluents collected overtime or in different forms. In that case, the total organiccarbon (TOC) present in the solution, that takes intoaccount the by-products of the mineralization steps, isprobably one of the most adapted indicators. Followingthe rare attempts in the literature (Brosillon et al., 2011;Malato et al., 2000a), the objective of this paper is to testthe possibility of the development of a simple and robustmodel, representative of the TOC profiles during the min-eralization of pesticide mixtures. It is essential to get mod-els based on global parameters like TOC, which permits thedesign of the solar photocatalytic process. TOC is an inter-esting parameter because it represents all the organic prod-ucts and by-products present in solution throughout thecourse of the reaction.

As already mentioned previously, agricultural wastewa-ter is very complex. In order to have a better understand-ing, we have established a kinetic model for themineralization of a simple solution and then applied thisto the treatment of increasingly complex solutions to finishwith real wastewater. A first measurement with a solar lab-oratory, set up for a few tens of litres, was carried out with2,4-dichlorophenol (DCP), a well-known organic pollutantfound in pesticides. These preliminary tests performed on alarge range of DCP concentrations served as a base for thedevelopment of the mineralization kinetics expression. In asecond step, the possibility of the extension of the model tothe mineralization of pesticides was explored through thecomparison between experimental and simulated TOCprofiles in the case of two commercial pesticides, Scalaand Pirimor respectively based on pyrimethanil and pirim-icarb active molecules. Pyrimethanil is a fungicide belong-ing to the aniline-pyrimidine family. Studies dealing withphotocatalysis of this substance are rare in the literature(Aguera et al., 2000; Arana et al., 2008; Oller et al.,2007). Pirimicarb, from the carbamates family, is a selectiveinsecticide, and only one article deals with photocatalysis

T. Janin et al. / Solar Energy 87 (2013) 127–135 129

of this pollutant (Szulbinski and Malato, 2001). Finally,solar experiments were performed with three real and prac-tical phytosanitary run off samples directly collected from avineyard in the Languedoc–Roussillon area (France). Inthis last case, in addition to monitoring TOC evolution,aquatic eco-toxicity measurements on one agriculturalrun-off sample were performed at the end of the photocat-alytic treatment to check the detoxification efficiency. Themodelling of the mineralization rate makes it possible toscale up the solar photocatalytic reactors. Scaling up of aprocess adapted for the treatment of few cubic meters ofphytosanitary wastewaste is proposed at the end of thepaper.

2. Experimentation

2.1. Chemicals and materials

The TiO2 catalyst, the material most widely employedfor photocatalysis, was used in an immobilized form. Thephotocatalytic medium (Paper Grad 1048), manufacturedby the Ahlstrom company, consisted of TiO2 (MilleniumPC-500) as a coating on non-woven paper. Its specific sur-face area, calculated with reference to the mass of the totalphotocatalytic medium, was equal to 98 m2 g�1. More pre-cisely, this photocatalytic material consisted of cellulosicfibers (38 g m�2), TiO2 (16.7 g m�2) and SiO2 (13.3 g m�2). SiO2 was used as an inorganic binder for titaniumdeposited on the paper fibers. It is transparent with UVlight and photostable. Such photocatalytic material hasalready been tested for the photocatalytic treatment oforganic pollutants (Brosillon et al., 2008; Goetz et al.,

Table 1Composition and function of the two commercial pesticide solutions (a) and t

Compositio

Pirimor PirimicarbButanedioic(2-ethylhexTalc (Mg3HUnknown p

Scala PyrimethanLignin, reabisulfite anPropan-1,2Unknown p

Composition

Active molecule

Effluent 1 PyriproxifenMyclobutanil

Effluent 2 GlyphosateFlazasulfuronOryzalin

Effluent 3 MetiramKresoxim-methylAlphamethrin

2009; Guillard et al., 2003; Lhomme et al., 2005; Pichatet al., 2004).

2,4-dichlorophenol (PESTANAL) with a purity of99.4% was purchased from Riedel de Haen. Commercialpesticide solutions of pirimicarb (Pirimor) and pyrimetha-nil (Scala) were supplied by an authorized dealer. Table1a identifies the significant presence of unidentified prod-ucts in the formulations. Three mixtures of pesticides werecollected from an agricultural area. Their qualitative com-position, active molecules and commercial designation aregiven in Table 1b. The first effluent was composed mainlyof one commercial insecticide (pyriproxyfen) and one com-mercial fungicide (myclobutanil). The second mixture camefrom a herbicide treatment in the vineyard. It was com-posed of three commercial herbicides (glyphosate, flazasul-furon and oryzalin). Finally, the last effluent sample wascomposed of two commercial fungicides (Metiram andKresoxim-methyl) and one insecticide (Alphamethrin).

The objective is to develop a model representative of themineralization of complex mixtures with an initial compo-sition which is not always perfectly defined (the agriculturalwaste came directly from the vineyard and resulted fromdifferent uses of the field). Also, all the results are presentedaccording the total organic carbon profiles during the solarexperiments. The TOC compositions of all the liquid sam-ples taken during the solar experiments were determinedwith a Shimadzu TOC analyzer (ON-LINE TOC-VCSH).For solutions composed of a single product, DCP, pirimi-carb and pyrimethanil, the concentration profiles of theactive molecules were also monitored during the outdoorexperiments by HPLC-UV. In all the cases, the active mol-ecules had completely disappeared from the solution at theend of the treatment.

he three mixtures of pesticides collected from an agricultural area (b).

n Concentration (%)

50 (p/p)acid, sulfo-, 1,4-bis

yl) ester, sodium salt5–10 (p/p)

2(SiO3)4) 20–30 (p/p)roducts 10–25 (p/p)

il 37.4 (p/p)ction products with sodiumd formaldehyde

4 (p/p)

-diol 2 (p/p)roducts 56.6% (p/p)

Function

Commercial solution

Admiral InsecticideSysthane Max Fungicide

Acrux HerbicideKatana HerbicideSurflan Herbicide

Polyram FungicideStroby FungicideFastac Insecticide

Fig. 1. Picture of the solar photocatalytic reactor.

0

5

10

15

20

25

0 150 300 450 600 750 900Time (min)

TOC

(mg.

l-1)

0

10

20

30

40

50

60

UV

radi

atio

n (W

.m-2

)

Fig. 2. Photolytic (j) and photocatalytic (D) mineralization of DCP inthe solar reactor. UV radiation (–).

130 T. Janin et al. / Solar Energy 87 (2013) 127–135

2.2. Solar photocatalytic reactor

All the experiments were performed under natural sun-light with a flat panel photoreactor shown in Fig. 1. Inorder to carry out the experiments in different working con-ditions (initial concentration, mass flow rate, etc.) but withthe same solar irradiation, a laboratory solar set-up con-sisting of three identical but independent reactors wasdesigned. Each reactor consists of a flat solar “panel”30 cm by 100 cm by 2 cm. The photocatalytic materialwas fixed on the irradiated wall of the reactor (0.21 m2).The sample passed through the reactor from the bottomto the top and the whole reactor was filled with water(2 cm thickness). In these conditions, the reactor volumewas 6 l. This panel was oriented southwards and inclinedat an angle of about 42�, in order to maximize the averagesolar irradiation throughout the year. Furthermore, to pre-vent evaporation of compounds and to ensure the equip-ment was watertight, the reactors were covered by aspecial Polymethyl Methacrylate (PMMA) which transmitsUV radiation. The PMMA reduced the intensity of the UVradiation by about 15%. This material has been alreadyused for solar detoxification (Dillert et al., 1999; Gulyaset al., 2005; Plantard et al., 2012) to provide a cheaper solarphotoreactor. The system operated in batch mode. Waterpollutants were stored in a tank (stainless steel with a max-imum volume of 50 l). The effluent was then fed into thereactor via a pump and was recirculated into the tank.

To monitor the solar UV radiation, a UV radiometer(UVA-Sensor CT-UVA 3) was oriented southwards andtilted at 42�. The probe recorded the spectral range from310 to 400 nm. The measurement errors were estimated at 5%.

3. Results and discussions

3.1. Solar photocatalytic mineralization of 2,4-

dichlorophenol

After a preliminary outdoor experiment with solar irra-diation (Fig. 2) which demonstrated that the TOC removal

by photolysis was very low, less than 20%, a series of mea-surements were carried out with initial TOC concentrationsranging from 6 to 153 mg l�1, corresponding to initial DCPconcentrations from 13 to 365 mg l�1. The uncertainties(sampling, analysis and accuracy) equal to 5% have beenevaluated in the experimental values. Experimentationwas performed over several successive days. The night per-iod results, when the irradiation and mineralization rateswere equal to 0, were removed. Fig. 3 illustrates the factthat the UV profiles were not the same throughout theexperimental period. The period of disturbance, or lowradiation intensity is due to clouds. As an example, the sec-ond and the third days (Fig. 3a) were sunny while the fifthday was cloudy.

For the high level concentrations (47–153 mg l�1 TOC),a change in the color of the solution during the experimentswas induced by the presence of by-products. Several chro-matogram peaks appeared, revealing the presence of sev-eral intermediary molecules such as phenol. The browncolor was probably due to the presence of hydroquinonein solution. For the three lowest concentrations studied(6–18 mg l�1 of TOC), total mineralization was almostachieved (>70%). Bayarri et al. (2005) have suggested thatat the end of mineralization, malic and acetic acids, whichare the last compounds detected, develop a low affinity forthe TiO2.

3.2. Kinetics

Taking into account the total volume of the liquid pres-ent in the closed fluid loop VT (l) and the reactor volumeVr, the material balance applied to the TOC in the processyield the following expression:

dTOC

dt¼ � V r

V T_r ð1Þ

with VT = Vr + Vtank (the total volume of the plant) and _r(mg l�1 s�1) the mineralization rate per unit of volume ofreactor.

This material balance supposes that the concentrationgradient inside the reactor was very small, and conse-

0

4

8

12

16

20

24

0 5 10 15 20 25 30 35Time (h)

TOC

(mg.

l-1)

0

10

20

30

40

50

60

UV

radi

atio

n (W

.m-2

)

0

40

80

120

160

0 5 10 15 20 25 30 35

Time (h)

TOC

(mg.

l-1)

0

10

20

30

40

50

60

UV

radi

atio

n (W

.m-2

)

(a)

(b)

Fig. 3. TOC degradation in the solar reactor as a function of time.Comparison between experimental points and model. (a) Low range ofinitial TOC concentrations (6 mg l�1 (e); 11 mg l�1 (h); 19 mg l�1 (D)).(b) High range of initial TOC concentrations (47 mg l�1 (D); 86 mg l�1

(s); 153 mg l�1 (e)).

T. Janin et al. / Solar Energy 87 (2013) 127–135 131

quently, that the overall system reactor could be assimi-lated to a perfectly-mixed reactor.

With the TiO2 implementation selected: (i) the mineral-ization happens at the surface of the flat photocatalyticmedia (Fig. 1) whose surface is equal to the irradiated sur-face of the reactor; (ii) the ratio of the photocatalytic sur-face by the reactor volume is, as for any catalyticreaction, to be taken into account in the formulation ofthe kinetics. As currently in the literature, there is no uni-versal kinetic law representing the complex photocatalyticmechanism, all the expressions are based on the reaction’stwinned dependence of the kinetics of the reaction on thelevels of the pollutant concentration and the intensity ofradiation (Brosillon et al., 2008; Correia et al., 2011; Eme-line et al., 2000; Goetz et al., 2009). In this context, the gen-eral formulation selected to be representative of the rate ofmineralization _r was (Eq. (2)).

_r ¼�a � S

V r� IðtÞn � ðTOC� TOClimitÞm

1þ b � ðTOC� TOClimitÞð2Þ

with S the surface of the photocatalytic media (m2), Vr theirradiated reactor volume (l) and I the UV radiation (W m�2) directly measured as a function of the time by the UVsensor. The photocatalyst activity, the transmission of the

flux of photons through the PMMA panel, and the irradi-ation extinction in the water solution are carried out in thekinetic constant a (whose unit depends of the value of n

and m). The other constants, b, n and m contribute tothe formulation of a global expression that includes the dif-ferent laws which describe the kinetics of degradation andmineralization of numerous pollutants. For example, thepseudo first-order kinetic (when n and m are equal to 1and b equal to 0) describes the direct and proportionaldependence of the reaction rate with regard to the irradia-tion and the TOC concentration. With the objective ofobtaining a kinetic law valid for a broad range of concen-trations and irradiations, Emeline et al. (2000) suggested akinetic law (when b = 0) which is of m order with respect tothe concentration and n order with respect to the UV lightintensity. The authors highlighted the interdependence ofm and n in the case of phenol degradation as a functionof the working conditions. Finally, this global law couldgive an expression close to the well known Langmuir–Hinshelwood (L–H) kinetic model (when n and m are equalto 1). Basically, according this formulation, the organiccompound has to be pre-adsorbed on the photocatalystsurface prior to UV illumination.

The notion of asymptotic TOC concentration(TOClimit), due to non interaction between the catalystand the by-products, was added to the previous expres-sions. This additional parameter provides the ability tohave an expression likely to be representative even whena threshold is reached during the mineralization process.The presence of such a threshold has been experimentallyreported in the literature (Aguera et al., 2000; Malatoet al., 2000a).

In the case of 2–4 DCP, TOClimit was set equal to 0.Mineralization was complete if the time duration of theexperimentation was long enough. The identification ofthe unknown parameters, a, n, m and b, was performedwith an optimization method using a trust-region-reflectivealgorithm. This algorithm, available in a Matlab modulus,is a subspace trust-region method and is based on the inte-rior-reflective Newton method. The criterion to be mini-mized, that compares the calculated concentrationprofiles against the experimental results, is:

CRIT ¼ 1

k

Xk

i¼1

TOCcal � TOCexp

TOCexp

� �2

ð3Þ

with k being the number of experimental points.This method does not merely take into account the ini-

tial rate of mineralization alone. The identification of theunknown parameters is obtained by taking into accountall the experimental points and not only the initial concen-trations. Indeed, the great majority of the literature pre-sents kinetic models only based on the initial kinetic (atthe beginning of the reaction). Our work presents the orig-inality of giving a kinetic model which could be usedthroughout the reaction. The best result was obtained for

132 T. Janin et al. / Solar Energy 87 (2013) 127–135

the following values: a = 3.17 � 10�5 l J�1; b = 0.0197 l mg�1; n � m � 1.

Consequently, the kinetic law which gives the best resultto describe the experimental results is very similar to theLangmuir–Hinshelwood law. Finally, the resulting modelcan be written as follows (Eq. (4)):

dTOC

dt¼ �

a � SV T� IðtÞ � ðTOC� TOClimitÞ

1þ b � ðTOC� TOClimitÞð4Þ

with TOClimit = 0 in the case of 2,4-dichlorophenol.Figs. 3 and 4 give the comparison between experimental

results and TOC profiles calculated thanks to this kineticlaw. For the sake of clarity, the diagram of dispersion isseparated into two, one for the low range of initial concen-trations and one for high range. Even if the model slightlyoverestimates the low concentrations, the simulated con-centration profiles are in reasonably good agreement withthe experimental ones. The definition of a representativekinetic leads to a correct profile of mineralization as a func-tion of time. The model developed includes the operatingparameters of the reactor such as the total volume to betreated and the surface area of the photocatalytic medium.As a consequence, such a formulation offers the opportu-nity for the scaling-up and the dimensioning of solar pro-cesses. These data are extremely rarely found in theliterature. Moreover it should be mentioned that the range

0

4

8

12

16

20

0 5 10 15 20Calculated concentrations (mg.l-1)

0

40

80

120

160

0 40 80 120 160

con

cent

ratio

ns (m

g.l-1

)Ex

perim

enta

l c

once

ntra

tions

(mg.

l-1)

Expe

rimen

tal

Calculated concentrations (mg.l-1)

(a)

(b)

Fig. 4. Diagram of dispersion for the comparison of the experimentalresults and the calculated results. (a) Low range of initial TOCconcentrations (6 mg L�1 (e); 11 mg l�1 (h); 19 mg l�1 (D)). (b) Highrange of initial TOC concentrations (47 mg l�1 (D); 86 mg l�1 (s);153 mg l�1 (e)).

of TOC concentration tested is very large since the initialTOC concentration ranged from 6 to 153 mg l�1.

3.3. Mineralization of commercial pesticide solutions and

actual agricultural wastewaters

3.3.1. Commercial pesticide solution

After the study of the photocatalysis of a model pollu-tant, the study was made more complex. The investigationof the treatment of two commercial solutions containingpesticides and additives was carried out; one contained pir-imicarb as the active molecule (commercial formulationname = Pirimor) and the other contained pyrimethanil(Scala). Basically, the role of the additives in commercialpesticide formulations is to improve the efficiency of thepesticides and to enhance their solubility in water.

These two commercial pesticides are characterized bythe addition of very few organic compounds. A preliminaryoutdoor experiment consisted of monitoring the degrada-tion of active molecules (pirimicarb and pyrimethanil) dur-ing the separate treatment of the two commercial solutions(Fig. 5). The results are expressed as a function of accumu-lated UV energy received by each reactor per unit volumeof solution to be treated, an expression commonly usedwhen working with the sun (Gernjak et al., 2004; Sarriaet al., 2005). In both cases, the active molecules completelydisappeared from the solution at the end of the treatment.Nevertheless, a significant difference in degradation ratesmust be underlined. Fig. 6 shows the changes in TOC con-centration during photocatalysis of the commercial solu-tion of pyrimethanil (Scala) for three different initialconcentrations. Despite the presence of additives andunknown products in the commercial formulation (Table1a), a progressive decrease of TOC concentration wasobserved for the three concentrations. The commercialsolution was mineralized during the first 50 h. However,a threshold concentration (6 mg l�1) was reached after50 h of irradiation, for whatever the initial concentration.Consequently, the mineralization yield differed. It reached60% for an initial concentration of 15 mg l�1 whereas themineralization yield reached 82% for an initial concentra-tion of 32 mg l�1. The existence of this threshold concen-

0

4

8

12

16

20

24

0 20 40 60 80QUV (kJ.l-1)

Con

cent

ratio

ns (m

g.l-1

)

Fig. 5. Changes to commercial solutions of pyrimethanil (Scala) (s) andpirimicarb (Pirimor) (D) during the degradation.

0

5

10

15

20

25

30

35

40

0 15 30 45 60 75 90Time (h)

TOC

(mg.

l-1)

Fig. 6. TOC concentration changes during the degradation of thecommercial solution of pyrimethanil for three initial concentrations.Comparison between experimental points and model.

0

4

8

12

16

20

0 15 30 45 60 75 90Time (h)

TOC

(mg.

l-1)

Fig. 7. TOC concentration changes during the degradation of a mixture ofthe two commercial pesticide solutions (Pirimor – pirimicarb (D); Scala –pyrimethanil (�); mixture (s)). Comparison between experimental pointsand model. The measurement errors (5%) are not represented in the figurefor the sake of clarity.

0

5

10

15

20

25

0 25 50 75 100 125 150QUV (kJ.l-1)

TOC

(mg.

l-1)

0,2

0,4

0,6

0,8

1

TOC

/TO

Ci

(a)

(b)

T. Janin et al. / Solar Energy 87 (2013) 127–135 133

tration is probably not due to the presence of recalcitrantby-products, which would appear at a concentration pro-portional to the initial concentration. Several assumptionsmay explain such behavior like a low affinity between theby-products and the catalyst and/or a saturation of the sur-face of the catalyst. The objective was to test the validity ofthe model developed for 2,4-dichlorophenol mineralizationin the case of the commercial pesticide solutions. By retain-ing the kinetic expression and the kinetic constants a and band by imposing a threshold concentration in the modelpreviously established for DCP (TOClimit = 6 mg l�1), thesimulated concentration profiles are in reasonably goodagreement with the experimental results, despite a slightoverestimation for the highest concentration.

The next step was the mineralization of (i) the secondcommercial pesticide formulation (Pirimor), containingpirimicarb and (ii) a mixture of the two commercial solu-tions of pirimicarb and pyrimethanil (for an identical initialTOC). Regarding the TOC profiles, representative of therates of mineralization, the differences previously under-lined in the case of the active molecule degradations(Fig. 5) vanished. Similarly, the mineralization of the mix-ture followed an evolution identical to both commercialsolutions treated separately (Fig. 7) with a plateau around6 mg l�1 reached after 50 h. These experimental resultshighlight all the interest of the total organic carbon, thattakes into account the by-products of the mineralizationsteps, as a global indicator of the state of advancementof the treatment of a polluted effluent. The model appliedto these mineralization kinetics, by imposing a TOClimit

concentration equal to 6 mg l�1, is in accordance with theexperimental data. It tends to demonstrate the ability ofthe suggested kinetic law to be adapted for commercial pes-ticides and mixtures of pesticides.

00 15 30 45 60 75

Time (h)

Fig. 8. (a) TOC concentration changes during the degradation of threereal agricultural wastewaters (effluent 1 (s); effluent 2 (h); effluent 3 (D)).(b) Application of the model. The measurement errors (5%) are notrepresented in the figure for the sake of clarity.

3.3.2. Real agricultural wastewater – toxicity assays

Finally, the degradation of three real agricultural efflu-ents was studied. When the TOC profiles during solar pho-tocatalysis are analyzed, the mineralization yield is above75%, which could be considered as a very high level of min-eralization (Fig. 8a). Nevertheless, the presence of a thresh-

old must be noted, in the same way as for themineralization of the two commercial pesticide solutions.The same assumptions, as made previously, may explainsuch a trend. With TOClimit directly obtained from theexperimental measurements and the same constants deter-mined previously (a and b), the kinetic law developed forDCP mineralization can be successfully applied to thesethree effluents. These results obtained in the case of severalmixtures demonstrate the non-selectivity of the photocata-lytic process, the necessity of using the TOC value to scale-

0

100

200

300

400

500

January

FebruaryMarch Ap

rilMayJune Ju

ly

August

September

October

November

December

Volu

me

(lite

rs)

0

10

20

30

40

QUV

(MJ.

l-1)

Fig. 9. Amount of monthly UV energy received in Perpignan (grey), withan inclination of 42� and south orientation and monthly treatmentcapacities calculated with Ahlstrom media (black).

134 T. Janin et al. / Solar Energy 87 (2013) 127–135

up a photoreactor and the ability of a kinetic law to beadapted for real agricultural wastewater.

In order to validate the efficiency of the treatment, twodifferent measurements of aquatic eco-toxicity were carriedout on the third effluent by a laboratory specializing in theanalysis of water and the environment (CARSO Labora-tory, Lyon (France)). Acute and chronic toxicity were mea-sured, respectively by Daphnia mobility and the growth ofAlgae. The test on Daphnia highlights the initial level oftoxicity of the effluent with an equitox equal to 21 m3

(Table 2). The Equitox represents the level of toxicitywhich, in 1 m3 of water, immobilizes 50% of Daphnia after24 h. Above 10 m3, the solution is considered as toxic. Inaddition, only 13.6% of the effluent was sufficient to inhibitthe growth of 50% of the algal population whereas at theend of treatment, more than 98% of the effluent was neces-sary. The photocatalytic treatment resulted in obtaining atotally harmless effluent even though all the by-productsformed during photocatalysis degradation were not com-pletely mineralized. Thus, the total mineralization is notsystematically necessary to obtain the complete detoxifica-tion of a polluted solution.

3.4. Photoreactor design

In order to develop tools for scaling up a process forindustrial application, the kinetic law obtained in the caseof the third sample of agricultural wastewater was usedto define the treatment ability of the solar reactor. Thanksto the model, simulations were performed with irradiationdata from Meteonorm software (Perpignan (south ofFrance)). According to the literature, few studies have dealtwith the operation of a photocatalysis unit compared tostudies focusing on the chemical pathway or kinetics. TheUV energy received annually in Perpignan, for an inclina-tion of 42� and a south orientation, is about 280 MJUV m�2. Fig. 9 represents the distribution of the energy receivedper unit of surface area throughout the year. November,December, January and February deliver the lowestamount of energy, with a monthly average of 10 MJUV m�2. On the other hand, the accumulated UV energy duringthe summer months is 3.5 times higher than that in winter.The kinetics of mineralization and, consequently, the treat-ment efficiency, depend directly on the UV radiation. Here,the treatment capacity is defined as the maximum volumewhich can be treated with a mineralization rate of 70%,for a surface area of the photocatalytic medium fixed at

Table 2Ecotoxicity tests on the third effluent from agricultural wastewater, beforeand after treatment.

Ecotoxicological analyses Before treatment After treatment

Algae CEr 10 72 h 7.8% >98%Algae CEr 50 72 h 13.6% >98%Daphnia CE50 24 h 4.79% >90%Equitox 21 m3 <1 m3

1 m2 and an initial TOC concentration at 20 mg l�1. Thesimulations were carried out for every month of the year(Fig. 9). About 100 l can be treated in December as com-pared to 500 l in July. This contrast is directly explainedby the difference in the amount of energy received (abouta factor of 4.6 between the two months). The addition ofall these monthly volumes reaches an annual volume of3500 l m�2. These results highlight the capacity of the reac-tor for treating small volumes of highly polluted wastewa-ter at a reasonable cost of processing. This is suitable forthe treatment of occasional pollution such as agriculturalwastewater.

4. Conclusions

In this study, the different experiments have shown thata solar photocatalytic reactor with an industrial mediumusing immobilized TiO2 can be efficient in degrading com-plex mixtures of pesticides. A photocatalyst is absolutelynecessary for an advanced mineralization. A representativekinetic law has been identified, thanks to a formulationinspired by a modified Langmuir–Hinshelwood. This lawinvolves reactor parameters such as the surface area ofthe catalyst, the total volume to be treated and, also, work-ing conditions, corresponding to the UV radiation whichfluctuates and the level of concentrations. The fits betweenmodels and experimental results are in reasonable agree-ment over a broad range of TOC. Only very few previousstudies have investigated so wide a range of TOC concen-trations. The model applied to all mixtures of pesticides(commercial pesticide solutions and actual agriculturalwastewaters), including a threshold concentration foradvanced mineralization, gives satisfactory results forthese. This model can be a tool for scaling-up purposes.Solar photocatalysis with supported TiO2 is a convenientand cheap solution for decontaminating agricultural waste-water. This AOP is useful for obtaining advanced mineral-ization and considerably reduces the toxicity ofwastewater. Finally, the simulations made it possible to

T. Janin et al. / Solar Energy 87 (2013) 127–135 135

define the treatment capability of the solar reactor at a spe-cific geographical site.

Acknowledgement

The authors thank Jean-Pierre Cambon, CNRS engi-neer (Promes, UPR 8521), for his participation in thisstudy.

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