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Journal of Environmental Management 162 (2015) 46e52

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Research article

Use of solar distillation for olive mill wastewater drying and recoveryof polyphenolic compounds

Sotirios Sklavos, Georgia Gatidou, Athanasios S. Stasinakis*, Dias HaralambopoulosDepartment of Environment, University of the Aegean, University Hill, Mytilene 81 100, Greece

a r t i c l e i n f o

Article history:Received 17 April 2015Received in revised form23 June 2015Accepted 14 July 2015Available online xxx

Keywords:Olive mill wastewaterTreatmentRecoveryValorisationAntioxidants

* Corresponding author.E-mail address: astas@env.aegean.gr (A.S. Stasinak

http://dx.doi.org/10.1016/j.jenvman.2015.07.0340301-4797/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Olive mill wastewater (OMW) is characterized by its high organic load and the presence of phenoliccompounds. For first time, a solar distillator was used to investigate the simultaneous solar drying ofOMW and the recovery of phenolic compounds with antioxidant properties in the distillate. Two ex-periments were conducted and the role of thermal insulation on the performance of the distiller wasstudied. The use of insulation resulted to higher temperatures in the distillator (up to 84.3 �C and78.5 �C at the air and sludge, respectively), shorter period for OMW dewatering (14 days), while itincreased the performance of distillator by 26.1%. Chemical characterization of the distillate showed thatpH and COD concentration gradually decreased during the experiments, whereas an opposite trend wasnoticed for conductivity and total phenols concentration. Almost 4% of the total phenols found initially inOMW were transferred to the distillate when an insulated solar distillator was used. Gas chromato-graphic analysis of collected distillates confirmed the presence of tyrosol in all samples; whereashydroxytyrosol was found only in fresh collected distillate samples. Further experiments should beconducted to optimize the process and quantify the concentrations of recovered phenolic compounds.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Olive mill wastewater (OMW) is a major by-product of olive oilproduction industry. More than 10 million m3 OMW are producedper year in the Mediterrarean climate region; while the seasonalityof olive oil production and the characteristics of wastewater(Chemical Oxygen Demand, COD > 80 g L�1, Total Suspended Solids,TSS > 20 g L�1, pH 4e5, phenolic compounds 1.5e10 g L�1) make itsmanagement difficult and costly (Niaounakis and Halvadakis,2006).

So far, several physicochemical and biological methods havebeen tested for OMW treatment (Niaounakis and Halvadakis, 2006;Paraskeva and Diamadopoulos, 2006; Stasinakis et al., 2008;Ochando-Pulido et al., 2013; Rahmanian et al., 2014). The applica-tion of physicochemical processes in full-scale systems is prob-lematic as none of these methods alone and with reasonable costcan decrease sufficiently the toxicity and organic load of OMW(Paraskeva and Diamadopoulos, 2006). Moreover, the application ofbiological processes usually requires chemical pretreatment or/and

is).

significant dilution of OMW (Niaounakis and Halvadakis, 2006;Nesseris and Stasinakis, 2012). So far, very few studies have suc-cessfully used undiluted raw OMW during anaerobic digestion(Sampaio et al., 2011; Gonçalves et al., 2012). On the other hand, thedrying of OMW in open evaporation ponds with retention time ofseveral months is a common, low-cost practice in rural areas of theMediterranean (Jarboui et al., 2010). However, this method requireslarge areas and causes significant nuisance due to the emitted odorsand the possible contamination of aquifers. To enhance OMWdrying avoiding the aforementioned effects, in a preliminary study,Potoglou et al. (2004) used a solar distillator and obtained a solidresidual with 15% humidity after 9 days retention time.

Beyond the aforementioned difficulties on OMW management,this type of wastewater seems to have significant economical in-terest. Specifically, so far, more than 50 different phenolic com-pounds have been identified in OMW (Obied et al., 2007a;Rahmanian et al., 2014). These compounds are water soluble, thusit is expected to be found at much higher concentrations in OMWcomparing to olive oil (Rodis et al., 2002). Moreover a great numberof scientific articles have proved their bioactive properties,including antioxidant, antiflammatory, neuroprotective, antiagingand antiatherogenic effects (Obied et al., 2007b; C�ardeno et al.,2013). Due to their ability to modulate cell death, they have also

Fig. 1. Three dimensional representation of the laboratory solar distillator used in thisstudy.

S. Sklavos et al. / Journal of Environmental Management 162 (2015) 46e52 47

been proposed as chemopreventive and therapeutic agents(Giovannini and Masella, 2012). Among them, hydroxytyrosol andtyrosol seem to be the most abundant phenolic compounds inOMW either from two-phase or three-phase olive mills, beingdetected at concentrations as high as 898 mg L�1 and 388 mg L�1,respectively (Bertin et al., 2011). It is worth mentioning thathydroxytyrosol and tyrosol are amongst the compounds approvedby the European Food Safety Authority for their ability to protectlow density lipoproteins (LDL) particles from oxidative damage(EFSA, 2011).

Due to the above, food industries, cosmetics and drugs haveshown considerable interest in the recovery of phenolic com-pounds contained in OMW. During the last years, several articleshave been published, suggesting methods for the recovery of thesecompounds from OMW. The applied methods include adsorptiononto resins and other sorbent materials, solvent extraction, super-critical fluid extraction, selective concentration by ultrafiltrationand other membrane systems (Galanakis et al., 2010; El-Abbassiet al., 2011; Scoma et al., 2011; Ena et al., 2012; Kalogerakis et al.,2013; Rahmanian et al., 2014; Zagklis et al., 2015). The mainweaknesses of these methods that hamper their implementation infull-scale are the requirement for OMW pretreatment in order toreduce significantly solids concentration, as well as their high costeither due to the use of chemicals for pre-concentration and/orextraction of target compounds or due to the significant energyrequirements.

The main objective of this study was to investigate the use ofsolar distillation for simultaneous OMW drying and recovery ofselected phenolic compounds. For this reason, a lab-scale solardistillation unit was used in the absence (Experiment A) andpresence of thermal insulation (Experiment B). The temperatures ofambient air, vapor inside the distillator and retentate (sludge) aswell as the solar radiation were constantly monitored duringdifferent experiments. The quantitative and qualitative character-istics of the distillate were studied, while the presence of tyrosoland hydroxytyrosol in the distillate was investigated using GCeMStechniques. Building on the work of Potoglou et al. (2004), this isthe first study where solar distillation systematically has been usedfor simultaneous OMW drying and phenolic compounds recovery.

2. Materials and methods

2.1. OMW and reagents

OMW was collected by a three-phase, olive oil mill located inLesvos Island, Greece and stored at 4 �C until the start of the ex-periments. The characteristics of OMW used during the experi-ments are presented in Table S1.

Analytical standards of hydroxytyrosol [2-(3,4-dihydroxyphenyl)ethanol, C8H10O3, �98%] and tyrosol [2-(4-hydroxyphenyl) ethanol, C8H10O2, >99.5%] were purchased fromExtrasynthese (France) and SigmaeAldrich (U.S.A.), respectively.Bis(trimethylsilyl) trifluoroacetamide (BSTFA) þ 1% trimethyl-chlorosilane (TMCS) solution, used for silylation, were purchasedby Supelco (USA). Oasis HLB cartridges (500mg/6 cc) were suppliedby Waters (U.S.A.), while Folin-Ciocalteu's phenol reagent waspurchased from Merck (Germany).

2.2. Solar distillator

The solar distillator used in both experimental periods consistedof the liquid basin where the OMW was placed, the glass coverwhere vapors condensed and the supporting structure (Fig. 1,Fig. S1). Excepting the glass cover, the entire apparatus was made ofgalvanized steel sheets, with a thickness of 2mm and an iron frame.

The glass cover was 3mm thick and had an inclination of 30�, facingdirectly south at latitude of 38� north.

During Experiment B, all four sides and the floor of the solardistillator were covered with panels of extruded polystyrene foam(thickness: 70 mm) to improve thermal efficiency (Fig. S1b). Thismaterial is widely used for improving the thermal insulation ofbuildings. According to themanufacturer (Fibran, 2014), its thermalconductivity after 25 years is 0.034Wm�1 K�1, while Papadopoulos(2005) reported that this value varies between 0.025 and0.035 W m�1 K�1.

2.3. Experimental design

At the beginning of each experiment, 13 L of OMW were placedin the solar distillator. Experiment A was conducted between 24thof September and 10th of October 2013 (duration: 16 days), whilstthe Experiment B was performed from the 14th to the 28th ofOctober 2013 (duration: 14 days). Apart from periods of precipita-tion, the solar distillator was uncovered for 12 h every day(8:00e20:00). Measurements of temperature (ambient air, air in-side the distillator, and sludge inside the distillator) and solar ra-diation were recorded on a 24 h basis during both experimentalphases. Temperatures were measured using T-type (copper-constantan) thermocouples at 6 s intervals and were averagedevery minute with a Campbell Scientific CR10X data logger. Inci-dent solar radiation was measured with a CM11 Kipp & Zonenpyranometer which had identical tilt and the same orientationwiththe glass cover of the solar distillator. This variable was measured,averaged and recorded in the same way as the aforementionedtemperature measurements. At the end of each 24 h period, thedistillate was collected in a dark glass bottle, measured in a volu-metric cylinder and stored at 4 �C. Soon after collection, distillatesamples were analyzed for pH, electrical conductivity, COD andtotal phenols. Distillate samples that were collected in Days 13, 15(Experiment A) and Days 10, 12 (Experiment B) were also analyzedfor the existence of hydroxytyrosol and tyrosol. Regarding OMWsludge samples, during the first experiment, these samples werecollected from the liquid basin at the end of Days 4, 11 and 16.

Fig. 2. Variation of solar radiation flux (Solar Radiation), ambient temperature (TC1Ambient Temp), air temperature inside the distillator (TC2 Air Temp in Still) and sludgetemperature inside the distillator (TC3 Sludge Temp) in a typical day (Day 2) ofExperiment A.

S. Sklavos et al. / Journal of Environmental Management 162 (2015) 46e5248

During the second experiment, sludge sampling was done at theend of Days 6 and 11, as sludge had been completely dried andsolidified by the end of Day 14. Samples of retentate sludge wereanalyzed for TSS.

2.4. Analytical methods

pH and conductivity were measured using a digital calibratedpH-meter (CRISON micro pH 2000 3662) and a digital calibratedconductivity meter (CONSORT C932), respectively. Concentrationsof COD, Total Solids (TS) and TSS were determined according toStandard Methods for the Examination of Water and Wastewater(APHA, 1998). Total phenols were determined using the spectro-photometric method associated with the Folin-Ciocalteau's phenolreagent (Box, 1983; Stasinakis et al., 2008). Specifically, a 10 mLsample was mixed with 1.5 mL of a sodium carbonate solution(200 g L�1) and 0.5 mL of the FolineCiocalteau reagent. The finalsolution was kept in the dark for 1 h (20 �C) and was stirred every15 min. Afterwards the absorbance of the solutionwas measured ata wavelength of 750 nm (Hach spectrophotometer, model DR/2400).

Isolation of hydroxytyrosol and tyrosol from distillates wasperformed by solid phase extraction (SPE) procedure. Analysis wasbased on a method developed by Jerman Klen and Vodopivec(2011). Briefly, Oasis HLB cartridges were conditioned with meth-anol (2 mL) and acidic (pH ¼ 2) ultra pure water (2 mL) followed by1.5 mL of distillates. Extraction was performed without using anyvacuum manifold. Cartridges were washed with 10 mL of hexaneand finally the target compounds were eluted with 2 � 5 mL ofmethanol. The eluents were evaporated to dryness under gentlestream of nitrogen. Dry residues were derivatized with 200 mL ofBSTF þ 1% TMCS at 25 �C for 30 min (Saitta et al., 2002).

An Agilent 7890A gas chromatography system connected to anAgilent mass spectrometer 5975C inert XL MSD (Agilent technol-ogies, USA) was used for the identification of target phenoliccompounds in distillates. The system was supported by a 7683series autosampler (Agilent technologies, USA). The separation ofthe compounds was achieved by using a DB5MS capillary column(60 m) with a film thickness of 0.25 mm and internal diameter of0.32 mm (Agilent technologies, USA). The carrier gas was heliumand maintained at a constant flow rate of 1 mL min�1. A samplevolume of 1 mL was injected in splitless mode at an inlet temper-ature of 280 �C. The column temperature, after preliminary ex-periments, was programmed as followed: from 80 �C to 275 �C at20 �Cmin�1 and hold in this temperature for 4min. TheMS transferline temperature was maintained at 280 �C, whereas the ion sourcetemperature was 230 �C. For the qualitative analysis, the full scanmode was used, monitoring the mass range from 50 to 550. For theselected ion monitoring (SIM) mode, the most abundant ions wereselected for each compound from its spectrum. The chosen ions forSIM were 73, 282, 267, 193 and 179 for tyrosol (tr ¼ 9.47 min) and370, 355, 267, 193 and 179 for hydroxytyrosol (tr ¼ 10.75 min). Theselected ions of the target compounds after trimethylsilylationwere in agreement with those reported elsewhere (Saitta et al.,2002). A typical chromatogram of the target compounds andtheir mass spectra in SIM mode are shown in Fig. S2.

3. Results and discussion

3.1. Operation of the solar distillator

Fig. 2 shows the fluctuations of measured temperatures andsolar radiation during a typical day of the Experiment A. At the startof the day (8:00 am), all measured temperatures were almost 17 �C,while at the end of the day (20:00 pm) ranged between 25 and

27 �C. As it was expected, ambient temperature displayed the leastvariation during the day, climbing to 30 �C at about 15:00 andbegan diminishing after 17:30. On the other hand, the temperatureof the air inside the solar distillator showed the greatest increase,reaching 58 �C (13:15), while sludge temperature reached 49 �C(14:00). The intensity of solar radiation on the tilted surface of theglass was 133W/m2 at the beginning of the day; it reached 1032W/m2 at 13:30 and began to diminish rapidly after 14:00. Its fluctua-tion between 15:00 and 16:00 was caused by intermittent cloudcover, while it was practically zero by 18:30.

The average daily ambient temperature and the total solar ra-diation were slightly higher in Experiment A (temperature:19.8 ± 4.7 �C; solar radiation: 21.5 ± 6.2 MJ/m2) comparing toExperiment B (temperature: 18.9 ± 1.4 �C; solar radiation:19.3 ± 4.8 MJ/m2), but no statistical difference was found usingStudent's t-test. Information on the variance of maximum andaverage temperatures of ambient air and solar radiation duringdifferent days of both experimental cycles can be found in Fig. S3.During the first days of Experiment A, the maximum temperaturewas ranged around 32 �C and the solar radiation was higher than25 MJ/m2. Afterwards, the maximum temperatures were graduallydeclined to 19 �C (Day 11) and increased again to 28 �C up to theend of the experiment (Day 16). The lower solar radiation intensityobserved in Day 7 and Day 8 of Experiment A was due to theextensive cloud and the significant precipitation occurred (Fig. S3).Regarding Experiment B, the maximum ambient temperaturesranged between 23 and 30 �C. The lower solar radiation intensityobserved in Day 3, Day 4 and Day 5 of Experiment B was due to thecloudy weather.

Beside the slightly higher ambient temperatures and total ra-diation observed in Experiment A, the use of thermal insulationresulted to higher temperatures in the distillator during Experi-ment B. Specifically, the average air temperature in the distillatorwas 27.5 ± 7.5 �C and 29.8 ± 3.8 �C in Experiment A and B,

S. Sklavos et al. / Journal of Environmental Management 162 (2015) 46e52 49

respectively. Moreover, excepting Days 3e5 where much highersolar radiation was recorded in Experiment A (Fig. S3), themaximum air temperatures inside the distillator were higher inExperiment B comparing to Experiment A (Fig. 3a). It is worthmentioned that temperatures higher than 80 �C were achievedduring 5 days of Experiment B; whereas the highest temperaturerecorded in Experiment A was almost 62 �C (Fig. 3a). Similarly tothe above, the highest sludge temperatures were achieved inExperiment B, exceeding 73 �C during the last five days of theexperiment. In Experiment A, the highest recorded sludge tem-perature was 56 �C (Fig. 3b).

The cumulative production of distillate and the available energyduring both experimental periods are presented in Fig. 4. In bothexperiments, the production of distillate was almost 11.5 L. Despitethe fact that the total amount of solar radiation was less by 21.3% inExperiment B, the total distillate production was completed twodays earlier comparing to Experiment A. Due to the extensive cloudcover and the protection of the apparatus from precipitation,distillate production was practically nil at Days 7 and 8 of Experi-ment A, while it was significantly reduced at Days 3e5 of Experi-ment B. Using the data presented in Fig. 4, it was calculated that45.7 mL and 57.6 mL of distillate per MJ of incident solar radiationon the tilted glass surface were produced during Experiment A andB, respectively. This means that the thermal insulation increasedthe performance of the solar distillator by 26.1%. In a previousstudy, Potoglou et al. (2004) reported a maximum of 55.6 mL ofdistillate per MJ of incident solar radiation, on a surface identicallyinclined, during a similar experiment performed with water insummer at a geographical location which was approximately 1.1�

Fig. 3. Maximum and average temperatures of air (a) and sludge (b) inside the dis-tillator during different days of Experiments A and B.

closer to the equator.

3.2. Characteristics of the distillate and remaining sludge

The pH values in the distillate that was collected duringdifferent days of the experiments presented a similar decreasingtrend, starting from values of 5.07 (Day 1, Exp A) and 3.91 (Day 1,Exp B) and reaching 3.40 and 3.14 at the end of Experiment A and B,respectively (Fig. S4). On the other hand, an opposite trend wasnoticed for the electrical conductivity with lower values at the startof the experiment and higher values at the end (Fig. S4), indicatingthe gradual dissolution of ionic species in the liquid. Amongdifferent experiments, higher conductivity values were observed inExperiment B. In this experiment, the slight increase of conduc-tivity observed during the first 8 days was followed by a dramaticincrease to 512 mS cm�1 up to Day 14.

COD concentrations in the distillate were gradually decreasedduring the experiments starting from almost 31,000 mg L�1 (Day 1)to 4800 and 7800 mg L�1 at the end of Experiment A and B,respectively (Fig. 5a). These results indicate that volatile organiccompounds are transferred to the distillate; whereas their occur-rence seems to be higher during the first phase of solar distillation.Surprisingly to the above results, total phenols concentrationspresented a different trend, being almost stable during the firstdays of the experiments and increasing rapidly later (Fig. 5b).Specifically, during the first 10 days of Experiment A, total phenolsconcentration was almost stable ranging between 0.9 and1.5 mg L�1, while afterwards it was rapidly increased up to4.3 mg L�1. Much higher total phenols concentrationwas noticed inExperiment B, starting from 11 mg L�1 and reaching 42.5 mg L�1 atDay 14 (Fig. 5b).

To calculate the fraction of total phenols that transferred to thedistillate, Eq. (1) was used.

DistillatedTotalPhenolsð%Þ ¼ 100�Pn

i¼1 Ci � Vi

COMW � VOMW(1)

where Ci is the concentration of total phenols in the distillate in Dayi, Vi is the volume of the distillate produced in Day i, COMW is theconcentration of total phenols in OMW at the start of the experi-ment and VOMW is the volume of OMW added in the distillator atthe start of the experiment.

As it can be seen in Fig. S5, 0.21% of the initial total phenols werecollected in the distillate up to the end of experiment A; whereasthis fraction was almost 20-fold higher in Experiment B, reaching3.9%. These results indicate the positive effect of the higher tem-peratures achieved in the 2nd experimental period on total phenolsrecovery.

Regarding the sludge that remained at the bottom of the solardistillator, the concentration of TSS was gradually increased withtime, reaching 44 g L�1 and 122 g L�1 at Day 11 of Experiment A andB, respectively. It should be mentioned that the initial concentra-tion of TSS in OMWwas much lower in Experiment A (Table S1). Atthe last day of both experiments, the sludge has been solidified andstuck on the bottom of the distillator (Fig. S6).

3.3. Identification of phenolic compounds in the distillate

GCeMS analysis of distillates indicated the presence of tyrosolin all samples. The confirmation of its presence was based on thecomparison of both mass spectrum and retention time of tyrosol inthe sample and in standard solution. As an additional confirmationtool, the obtained mass spectra were compared with commerciallyavailable NIST MS mass spectra (MS) library using Agilent MSDChemstation software for peak identification. After completion of

Fig. 4. Cumulative graph of distillate volumes and available solar radiation per unit of tilted surface during Experiments A and B.

Fig. 5. Changes of COD (a) and total phenols concentrations (b) in distillate samplescollected during different days of Experiments A and B.

S. Sklavos et al. / Journal of Environmental Management 162 (2015) 46e5250

each run, the mass spectrum of derivatized tyrosol was matchedwith the NIST MS library and the results showed similarity higherthan 95% in all cases. Side by side comparison of the obtained massspectra of tyrosol fromNISTMS library and analysis of distillates aregiven in Figs. S7 and S8.

Since the analysis of the distillates during the current study wasfocused only to the identification of the two phenols in the samples,concentration levels of the detected tyrosol can not be preciselycalculated. Nevertheless, comparison of peak areas of the

compound in the samples and in the standard solutions was uti-lized as a primary indication of the amount of the recovered phe-nols from OMW after solar distillation. Based on the aboveapproximation, during the Experiment A, peak areas of tyrosolfound to be 6 and 23 times higher in the samples taken on Days 13and 15, respectively, compared to the standard solution of 1 mg L�1

(Fig. 6a). Similarly, during the Experiment B, peak areas of thecompoundwere about 60 and 90 times higher in the samples takenon Days 10 and 12, respectively, in comparison with the peakobserved in standard solution (Fig. 6b). Having in mind that themaximum air temperatures inside the distillator during theExperiment B were higher compared to those achieved during theExperiment A (Fig. 3a), the estimated results suggest that muchhigher amounts of tyrosol can be recovered at temperatures as highas 80 �C.

In contrast to tyrosol, hydroxytyrosol was not detected indistillate samples of both experiments. According to the literature,tyrosol is a compound resistant to air/oxygen, bacterial and enzy-matic degradation (Niaounakis and Halvadakis, 2006), whilehydroxytyrosol is sensitive to several environmental factors. In aprevious study, Zafra-G�omez et al. (2011) investigated the stabilityof hydroxytyrosol in aqueous solutions at different temperatures(�20 �C, 4 �C and 25 �C). Their results showed no changes on theinitial concentration (2.5 mg L�1) of the compound in the solutionsafter one week of storage at �20 �C. On the contrary, after two daysof storage at room temperature (25 �C) the concentration ofhydroxytyrosol was decreased notably possibly due to oxidation.According to Cilliers and Singleton (1989) phenolate anions directlyreact with triplet oxygen to form a semiquinone. Moreover, Vognaet al. (2003) and de Lucia et al. (2006) suggested that hydro-hytyrosol autoxidises to o-quinone or tautomeric p-quinonemethides. These products after water addition undergo couplingwith a second o-quinone leading to the formation of dimmers(methanooxocino benzodioxinone derivatives). In order to inves-tigate if the storage of distillate before analysis could play a key rolein the presence of hydroxytyrosol, a short experiment was con-ducted using the insulated solar distillator in November 2013. Inthis case, the distillator was operated as in Experiment B but thedistillate samples were analyzed the same day of their production.According to the results, apart from tyrosol another small peak wasalso observed in the same retention time with hydroxytyrosol(Fig. S9). Additionally, similarity of about 85% between the massspectrum of the above detected peak in the distillate and thatincluded NIST MS library mach was observed (Fig. S10).

Fig. 6. Recovered tyrosol after solar distillation of OMW during Experiment A (a) and B (b).

S. Sklavos et al. / Journal of Environmental Management 162 (2015) 46e52 51

4. Conclusions

The use of solar distillation resulted to OMW dewatering in aperiod of 14 d and to the production of distillate containingphenolic compounds. Tyrosol was identified in all analyzed distil-late samples; whereas hydroxytyrosol was found occasionally. Theuse of thermal insulation improved the performance of solardistiller and enhanced the recovery of phenolic compounds in thedistillate. Solar distillation seems to be a promising, low-cost, eco-friendy process for OMW treatment and recovery of antioxidantcompounds. Further research should be conducted to optimize theprocess and quantify the concentrations of recovered compounds.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jenvman.2015.07.034.

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