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Hybrid photoactive materials from municipal sewage sludge for thephotocatalytic degradation of methylene blue†

Juan Matos,*a Maibelin Rosales,a Andreına Garcıa,a Cesar Nieto-Delgadob and Jose R. Rangel-Mendezb

Received 2nd June 2011, Accepted 30th August 2011DOI: 10.1039/c1gc15644f

To our knowledge this is the first manuscript describing a one step synthesis to produce anorganic/inorganic hybrid material prepared by carbonization of municipal sewage sludge (ACRM)that shows photoactivity for the photocatalytic degradation of methylene blue (MB). The hybridmaterial (ACRM) exhibited a mesoporous texture while XRD and SEM-EDX showed Fe2O3 andFe3C crystallites. Photodegradation of MB was studied under two different lamps and results werecompared against those obtained with a commercial TiO2. The photocatalytic tests showed thatthe hybrid material is photoactive and the binary composite TiO2–ACRM showed higher apparentfirst-order rate constants for the degradation of MB than those obtained on pure TiO2. A synergyeffect between TiO2 and ACRM was estimated at about 5 and 8 under lamps with UV-vis andalmost pure visible light, respectively. It can be concluded that the photoactivity of TiO2–ACRM

relative to neat TiO2 was up to one order of magnitude higher, suggesting that iron phases in thehybrid material photo-assist the TiO2 in the photodegradation of the methylene blue.

1. Introduction

A significant quantity of the total world production of dyesand azo-dyes used in textiles is released into effluents withconcomitant environmental hazards.1 Different technologiesfor the removal of these organic molecules, among whichadsorption, biological and chemical degradation methods, in-cluding such advanced oxidation technologies as heterogeneousphotocatalysis, have been employed. TiO2 has been used in awide range of photocatalytic reactions because of its efficientband gap energy and intrinsic properties. The combination ofphotocatalysis and adsorption with different supports, such assilica, alumina, zeolites, clays or activated carbon,2–14 has beenemployed to enhance the photocatalytic efficiency of titaniaand an interesting review about this topic by Zou and co-workers15 has recently been published. The novel synthesisof carbon-supported nanoparticles of TiO2 have received in-creased attention for the degradation of different dyes, suchas Chromotrope 2R,5 Orange-II,6 Rhodamine-B,7 Direct Blue-53,8 and Methylene Blue.9 Our group has previously pointedout that surface functionalization of activated carbon plays

aEngineering of Materials and Nanotechnology Centre, VenezuelanInstitute for Scientific Research (IVIC), 20632, Caracas 1020-A,Venezuela. E-mail: jmatos@ivic.gob.ve; Fax: +58-212-5041166;Tel: +58-212-5041166bDivision of Environmental Sciences, Instituto Potosino de InvestigacionCientıfica y Tecnologica, C.P. 78216, San Luıs Potosı, S.L.P., Mexico† Electronic supplementary information (ESI) available: Fig. S1–S6 andTables S1–S2. See DOI: 10.1039/c1gc15644f

an important role in the enhancement of TiO2 photoactivitytowards the degradation of aromatic molecules, such as phenoland 4-chlorophenol10–13 and more recently in the photo oxidationof 2-propanol.16 Also, our group9,14,16 and other groups5,6,17 havesuggested that specific functional groups on the carbon surfacecould photoassist the oxidation process occurring on the TiO2

surface. Regarding this subject, two recent reviews18,19 containmany works showing that both texture and surface properties ofcarbon materials are responsible for the enhancement of TiO2

photoactivity. On the other hand, different waste treatmentoptions for municipal solid waste have been studied.20–23 Forexample, Eriksson and co-workers20 have shown the differentpossibilities with combinations of incineration, materials re-cycling of separated plastics and cardboards containers, andbiological treatment of biodegradable waste. On the other hand,Bandosz and Pietrzak21 have reported the interaction of NO2

with sewage sludge-based composite adsorbents. They pointedout that these adsorbents are complex and consist in two phases,inorganic and organic, which depends on the type of sewagesludge, but they conclude that both parts of this hybrid materialtake part in NO2 retention and conversion on the surface. Also,Zhang and co-workers22 reported the synthesis of an adsorbentfrom municipal solid waste for the effective removal of As(V)from an aqueous phase. Finally, in a recent work, Kargbo23

reported biodiesel production from municipal sewage sludges asa promising route to the displacement of petroleum-based dieselfuel. Having all these applications in mind, in this work wepresent the synthesis of a hybrid carbon-based material frompyrolysis of municipal sewage sludge and a kinetic study of

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methylene blue (MB) photodegradation as a model dye as afunction of two different lamps with photon flux from UV toUV-vis light, this was performed in order to verify the potentialapplication of these materials in the treatment of waste watersby heterogeneous photocatalysis in the presence and absence ofstandard TiO2.

2. Experimental

2.1. Materials and synthesis of hybrid materials

Analytical grade Methylene Blue (MB) was purchased fromSigma-Aldrich. The commercially available P25–TiO2 fromDegussa was employed as a standard photocatalyst. It consistedof mainly anatase phase (ca. 80%), non-porous polyhedralparticles of ca. 30 nm mean size and BET surface area of about45 m2.g-1. The sludge was collected from a local wastewatertreatment plant (San Luis Potosı, Mexico) that deals witha mixture of municipal and industrial effluents. Prior to thecarbonization process, the sludge was sun dried for five days.Then 5 g of sludge were placed in a container made of stainlesssteel mesh of 0.178 mm opening (10 L ¥ 2 W ¥ 2 H cm), whichwas located at the central part of a tubular furnace of 40 cmlong as indicated in the schematic set-up for the synthesis inFig. 1. The carbonization process was conducted for 3 h at800 ◦C with a heating rate of 10 ◦C min-1 and 1.5 L min-1 of N2

was passed through the tubular quartz reactor to maintain aninert atmosphere and to extract the combustion gasses, whichare trapped in an acetone trap. Finally, after the carbonizationprocess ended, the treated sample was allowed to cool down toroom temperature in a nitrogen atmosphere: this sample was

Fig. 1 The experimental set-up for the synthesis of hybrid photoactivematerials.

denoted as ACRM. The ACRM was treated with CO2 for 1 h at800 ◦C, as described elsewhere10 with a heating rate of 10 ◦Cmin-1 and a CO2 flow of 0.1 L min-1, and this sample was denotedas ACRM-CO2.

2.2. Chemical analysis

The ashes of the ACRM material was obtained in a ther-mogravimetric analyzer Thermo Cahn, model Versa ThermHigh Sensitivity Series 81547. The analysis was conductedfrom ambient temperature to 900 ◦C with a heating rate of10 ◦C min-1 under air flow and high purity helium purge gas.Samples of the ACRM material and the ash were digested in amixture of nitric–hydrochloric acids within a microwave ovenfrom Milestone (model Ethos 1 series 131659). In both cases,identification and quantification of the elements Ag, Al, B,Ba, Cr, Cu, Ga, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Sc, Si, Sr,Zn was performed in an inductively-coupled plasma atomicemission spectrophotometer (ICP-OES) from Varian (model730ES-Axial, series EL07114070). Quantification of these ele-ments was performed by using a multielemental standard fromHigh Purity (E.P.A. 2007) at five different concentrations. Thedetermination of Ca, Fe and K was performed in an atomicabsorption (AA) spectrophotometer from Perkin Elmer (modeloAanalyst 400, series 201S3090705). In these cases, quantificationwas performed by using calibration curves with five differentconcentrations from standards from FisherScientific (for Fe)and Ricca Chemical Company (for Ca and K). The wavelengthand the correlation factor (R2) of the linear regression for allelements detected are showed in Table 1. Wavelengths of otherelements, such as As, Be, Cs, Cd, Co, Ge, In, La, Sb, Se, Sn,Ti, Tl, and V, were also explored but they were not detected.Analysis was done in triplicate for the samples and ashes andthe reproducibility of results was better than 99%.

2.3. Characterization

Characterization by adsorption–desorption N2 isotherms, X-raydiffraction (XRD), scanning electron microscopy (SEM) withenergy-dispersive X-ray analysis (EDX), and Fourier transforminfrared (FTIR) was performed. Adsorption–desorption N2

isotherms were recorded at 77 K. The full isotherms in therange of 4 ¥ 10-3 to 84 kPa were measured in a MicromeriticsASAP-2020. Equivalent surface area, micropore volume, andmean pore widths were obtained by Brunauer–Emmet–Teller(BET)24 Harkins–Jura (HJ)25 and Horvath–Kawazoe (HK)26

methods, respectively. HJ25 and HK26 methods were employedbecause these provide an accounting for the adsorbed layer

Table 1 Textural properties of the solids: BET surface area (SBET), micropore volume (V m), total pore volume (V t), ratio V m/V t, and mean porewidth (W pore)

Material SBET (m2 g-1) V ma (cm3 g-1) V t

b (cm3 g-1) V m/V t W poreb (A)

TiO2 45.2 ± 0.2 0.0024 0.1525 0.02 578ACRM 45.9 ± 0.2 0.0178 0.0776 0.23 484TiO2–ACRM 31.5 ± 0.3 0.0096 0.3433 0.03 1077ACRM-CO2 33.3 ± 0.1 0.0097 0.0730 0.13 531TiO2–ACRM-CO2 30.3 ± 0.2 0.0032 0.3344 0.01 949

a Estimated from Harkins–Jura method.25 b Estimated from Horvath–Kawazoe method.26

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on pore walls when calculating pore size distribution andbecause these are very useful when different types of pores areinvolved. Therefore, HJ and HW methods are proper for thepresent case because materials prepared from municipal sewagesludge are complex and consist of several inorganic and organicphases21 that simultaneously contain slits and spherical pores.Fourier transform Infra-red (FTIR) experiments were made ona spectrophotometer Magna-IR 560 from Nicolet. The powderswere mixed with KBr in a 5% (w/w) mixture. The mixed powderwas pressed to tablets of 1 cm diameter at 10 tons for 1 min.The transparent tablets were inserted in the apparatus and thespectra were recorded from 4000–400 cm-1 with a resolutionof 5 cm-1. A KBr reference spectrum and CO2 from ambientair have been subtracted from every spectrum. Powder X-raydiffraction (XRD) patterns were recorded in the range 2q =5–90◦ on a D-5005 diffractometer from Siemens with Cu-Ka(1.54056) radiation and processed with the Diffrac-Plus-Evaprogram.

2.4. Photoreactor, light source and photocatalytic tests

The experimental set-up for the photocatalytic tests has beenreported elsewhere9 but it can be summarized as follows. Anopen to air batch photoreactor was employed. It consists of a200 mL cylindrical flask made of Pyrex with a bottom opticalwindow of 6 cm in diameter. Irradiation was provided usingtwo different lamps (Fig S1, supplementary material†), a Hglamp (LHg) and a metal halide lamp (LMH). Irradiation wasfiltered by a Pyrex circulating water cell (thickness ca. 2.0 cm)to remove IR beams and prevent any heating of the suspension.The UV (from 320–390 nm) and visible light (from 400–780 nm)components in each lamp were estimated by integration of theradiation spectrum of each lamp and radiation was verifiedwith a pyranometer (Solar Light PMA2100) and total photonfluxes were estimated at each lamp (Table S1, supplementarymaterial†).

The photocatalytic tests were performed at 25 ◦C by adding62.5 mg of TiO2 and 6.25 mg of ACRM or ACRM-CO2 under stirringin 125 mL of methylene blue (MB), [25 ppm (78.2 mmol L-1)initial concentration]. These mixture of binary materials formedin situ are denoted by TiO2–ACRM and TiO2–ACRM-CO2. Thesamples were maintained in the dark for 60 min in order tocomplete adsorption at equilibrium prior to the UV-irradiationand then the suspension was irradiated. After centrifugation MBaliquots were analyzed using an UV-spectrophotometer PerkinElmer (Lambda 35) at 664 nm and the MB concentrations wereestimated using a standard calibration curve. Photoactivity testswere done in triplicate in all samples and the reproducibility ofresults being better than 97%.

3. Results and discussion

3.1. Chemical analysis

After the pyrolysis of the ACRM material from ambient to900 ◦C under air flow, the remnant ashes indicated a totalinorganic composition of about 69%. This value is in agreementwith those commonly reported (between 70–80%) and discussedelsewhere by Bandosz and Pietrzak.21 Table S2 (supplementarymaterial†) summarizes the results obtained from ICP and AA

chemical analysis for dry samples of ACRM and for the ashes.It is important to remark that some elements were lost duringcalcinations in air. For example, Si showed a dramatic reductionfrom 45.86 g kg-1 of dry sample to values lower than the limit ofdetection of the equipment after calcinations. Also, Na showedan important reduction from 33.39 g kg-1 of dry sample to0.47 g kg-1 of dry sample after calcinations. Also, K sufferedan important reduction from 19.94 to 8.89 g kg-1 of dry sampleafter calcinations. These results can be mainly attributed tometal oxides, salts and spinel-like structures contained in thesematerials21 and probably, potassium and mainly sodium silicatessalts were dissolved during water evolution in the calcinationstep and therefore extracted out from the sample. By contrast,Al and Ca did not show important reductions and it should benoted that Fe and P elements showed an increase proportionfrom 83.34 to 89.55 g kg-1 for Fe and from 28.34 to 31.14 gkg-1 for P after calcinations. In general, it should be pointed outthat the most important inorganic composition in the hybridmaterial ACRM corresponds to Fe (18%), Ca (17%), Si (10%),Na (7%), P (6%), and Al (5%). As we discuss as follows, thechange in the chemical proportion of Fe suggest an importantinteraction of this element with carbon during calcinations.

3.2. Characterization

3.2.1. Texture. Fig. S2 (supplementary material†) showsthe adsorption–desorption N2 isotherms of the bare materialsTiO2, ACRM, and ACRM-CO2. All three isotherms correspond totype 2 adsorption isotherms. For TiO2 a small and vertical hys-teresis loop can be attributed to aggregation of nanoparticles.16

This type of isotherm corresponds to a non-porous or relativelylarge porous material in agreement with a low surface area, avery low micropore volume, and with a large mean pore width,as reported in Table 1. Also, it can be seen in Fig. S2† thatACRM and ACRM-CO2 also showed type 2 adsorption isothermsbut in these cases more important vertical hysteresis loops canbe seen, which can be attributed to an important contributionof micro- and mesoporous volume to the total volume of pores.For example, micropore volume was 0.0178 and 0.0097 cm3

g-1 for ACRM and ACRM-CO2, respectively, which corresponds toabout 23% and 13% (Table 1) of the total pore volume in thesesamples. This value of about 23% of micropore volume is ingood agreement with values reported by Bandosz and Pietrzak21

of about 31% for materials also obtained from sewage sludge. Itshould be pointed out that after gasification with CO2 flow, thesample ACRM-CO2 showed a remarkable decrease in BET surfacearea and in the micropore volume in comparison with valuesobtained for ACRM, suggesting that this treatment destroyed animportant quantity of micro- and mesoporous. On the otherhand, Fig. S3 (supplementary material†) shows the adsorption–desorption N2 isotherms of the binary materials TiO2–ACRM

and TiO2–ACRM-CO2. It can be observed that both adsorption–desorption N2 isotherms are of type 2 with very similar verticalhysteresis loops to those of TiO2 alone. This result was expectedbecause the main component of the binary materials is TiO2,as is clearly indicated by the very low proportion of microporevolume in relation to the total volume of pores in these samples.Finally, it can be noted in Table 1 that even when the BET surfaceareas of TiO2–ACRM and TiO2–ACRM-CO2 samples are lower than

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that of TiO2 alone; the total volume of pores in TiO2–ACRM andTiO2–ACRM-CO2 samples is clearly higher than that observed inany of the bare materials. This result suggests that the presenthybrid materials obtained from the pyrolysis of sewage sludgecan strongly interact with TiO2, inducing an important surfaceaggregation of nanoparticles, as reported elsewhere,10 with aconcomitant decrease in surface area and increase in mean porewidths (Table 1).

3.2.2. FTIR. Fig. 2 shows the FTIR spectra of TiO2, ACRM

and TiO2–ACRM while Fig. 3 shows the FTIR spectra of TiO2,ACRM-CO2 and TiO2–ACRM-CO2. It can be seen that ACRM (Fig. 2)and ACRM-CO2 (Fig. 3) show very similar spectra with a peak at880 cm-1, which corresponds to the out-plane deformation modeof C–H in benzene rings.27 The spectra show transmittance peaksof around 1450 cm-1 attributed to carboxyl groups.22,28,29 Also,hydroxyl groups (–OH) commonly present in hybrid materialsprepared from municipal sewage sludges were confirmed byFTIR with peaks at about 1060 cm-1.22,29 In both spectra peaksbetween 500–550 cm-1 were observed, attributed to symmetricalstretching of the Fe–O bond,30,31 suggesting the presence of ironoxide phases in these samples. This peak shows less intensity inthe sample ACRM-CO2 (Fig. 3) with respect to that observed in theFTIR spectra of ACRM (Fig. 2).

Fig. 2 FTIR of TiO2, ACRM and TiO2–ACRM.

This decrease in the stretching of Fe–O bond suggeststhat during CO2 gasification, some iron oxide phases wouldsuffer some kind of chemical transformation, as we discussbelow. Finally, FTIR spectra of TiO2 show a peak at about3200–3600 cm-1, which is associated with the stretching vibrationof the –OH groups. Also, the characteristic broad peak associ-ated with the bulk titania framework11 can be observed in theregion from 400–800 cm-1. It is also important to remark that incomparison with the FTIR spectra of TiO2 alone, FTIR spectraof both TiO2–ACRM and TiO2–ACRM-CO2 showed a broadeningin the TiO2 framework region in the presence of ACRM (Fig.2) and ACRM-CO2 (Fig. 3). In concordance, the correspondingsignal associated with hydroxyl groups in the activated carbon,located at 1060 cm-1, decreased while the peak located at about

Fig. 3 FTIR of TiO2, ACRM-CO2 and TiO2–ACRM-CO2.

3200–3600 cm-1, which is associated with the stretching vibrationof the –OH groups, remarkably increases in the case of TiO2–ACRM-CO2, suggesting a strong surface interaction between bothsolids in a similar way that occurs with binary TiO2-activatedcarbon materials.11,13,14

3.2.3. XRD. Fig. 4 shows the XRD patterns of TiO2, ACRM

and TiO2–ACRM while Fig. 5 shows those of TiO2, ACRM-CO2 andTiO2–ACRM-CO2.

Fig. 4 XRD of TiO2, ACRM and TiO2–ACRM. � Fe2O3. * Fe3C.

The broad and large peak at about 15.9◦ in both XRD patterncorresponds to the sample holder. XRD patterns in Fig. 4 andFig. 5 show that commercial TiO2 P25 crystallizes with a mixtureof anatase (A) and rutile (R) phases32 with the main peak atabout 26.4◦ correspond to A(101). This and other peaks fromTiO2 clearly remain in the XRD patterns of the binary solidsTiO2–ACRM (Fig. 6) and TiO2–ACRM-CO2 (Fig. 7). It must bepointed out that the XRD patterns of ACRM and ACRM-CO2 showthe presence of iron crystallites in the present materials. The

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Fig. 5 XRD of TiO2, ACRM-CO2 and TiO2–ACRM-CO2. * Fe3C.

Fig. 6 SEM images of the ACRM hybrid material obtained frommunicipal sewage sludge.

peaks at about 30.0◦, 34.5◦, 43.6◦ correspond to an iron carbidephase Fe3C,33 in agreement with the ICDD target number 85-1317, while the peaks at about 42.3◦, 43.6◦ and 47.7◦ correspondto iron oxide Fe2O3,34 in agreement with ICDD target number85-0987. It should be noted that the peak at about 30.0◦ hasalso been reported for spinel-type structure of magnetite34 andthe peak at about 47.7◦ has also been reported as iron carbide

Fig. 7 The kinetics of the disappearance of MB under Hg lamp (A).Linear regression from the kinetic data (B).

Fe–C.33 It can be seen that the XRD pattern of ACRM-CO2

(Fig. 7) does not show the corresponding peaks of iron oxidephases observed in the XRD pattern of ACRM sample. Inother words, during CO2 gasification the surface oxides in theACRM materials showed an evolution to an iron reduced phaseas iron carbides. This fact appears contradictory because ofthe oxidative chemical nature of carbon dioxide, however, thisphenomenon has been already reported by Karasawa35 for thesynthesis of inorganic membranes together with iron carbide viaoxidation of iron by carbon dioxide dissolved in water. Karasawaindicates that since the electronegativity of carbon is larger thanthat of hydrogen (2.55 against 2.20, Pauling’s scale), reducediron phases such as carbides or elemental iron crystallites takeoxygen from CO2 and then the carbon atom bonds to theiron. The organometallic compound forms a membrane andvarious atoms in carbonated water make covalent bonds withthe part of the membrane in which it will be destroyed toproduce iron carbide. As a novel CO2 reduction technology,Karasawa35 discusses the synthesis of iron carbide nanoparticlesby means of fixation of CO2 with iron fine particles at hightemperatures (600–900 K). This fixation is perfectly possible inthe present conditions of CO2 gasification (atmospheric pressureand 1073 K). All these features permit us to conclude thatmaterials obtained from the pyrolysis of municipal sewage sludgeare very complex hybrid materials21 and therefore a perfectelucidation of the crystalline structure is very difficult. Therelated patterns of calcium salts were not indexed in the XRDpatterns of Fig. 4 and Fig. 5 because the noise of the XRDpatterns is high and the small peaks corresponding to CaOpatterns at diffraction angles of about 25.7◦, 51.4◦ and 77.1◦

are much diluted in spite of the high content of calcium (TableS2, supplementary material†). In addition, it can be pointed out

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that after CO2 treatment of the hybrid material, the diffractionpeaks at 25.7◦, and 77.1◦ were not detected, indicating thatduring the CO2 treatment of the hybrid materials, it wouldbe possible to form a more diluted calcium phase. However,from the above characterization an important presence of ironphases, such as carbides and oxides, can be established. Therole of these species will be discussed in the following sectionbecause iron phases play an important role in the heterogeneousphotocatalytic process in the aqueous36,37 and gas phases.38

3.2.4. SEM-EDX. Fig. 6A shows the SEM observations ofthe hybrid material prepared from the pyrolysis of the municipalsewage sludge. As can be seen, the hybrid material exhibitdifferent particles anchored onto the ACRM surface. Fig. 6B–6D show the predominant morphology of the ACRM surfaceobserved during the SEM analyses. The particles on Fig. 6Bare covering small portions of the material, and in accordancewith the energy-dispersive X-ray analysis (EDX) these parti-cles are mainly composed of silicon and oxygen (34 and 47wt%, respectively), with small portions of calcium, iron andaluminum. On the other hand, Fig. 6C shows spherical particlesthat are widely dispersed on the hybrid material surface. TheEDX analysis revealed that these particles are mainly composedof carbon, iron and oxygen (25, 37 and 23 wt%, respectively),with small amounts of silicon, aluminium and calcium. Thechemical composition and EDX analyses suggest that theseparticles could be iron carbides formed during carbonizationof the municipal sewage sludge, as discussed above. Previousstudies have reported that iron carbides have this sphericalmorphology.39–41 Finally, very small particles were found onthe ACRM surface (Fig. 6D) and the chemical composition ofthese particles consists of iron, oxygen and silicon. According tothe elemental composition of the ACRM material, it is expectedthat these particles are composed of diverse iron and siliconoxides/carbides that remarkably influence the catalytic activityof the TiO2, as we show in the following.

3.3. Methylene blue degradation

3.3.1. Adsorption in the dark. Preliminary studies of MBadsorption at 20 ◦C were performed on ACRM, TiO2–ACRM,ACRM-CO2, TiO2–ACRM-CO2, and on TiO2 P25 alone for compar-ative purposes. Figure S4 (supplementary material†) shows thekinetics of MB adsorption in the dark during 90 min. In all cases,most of the adsorption occurred within 30 min but in order toensure a proper equilibrium of adsorption in the dark, a longerperiod (60 min) was considered prior to the photodegradationexperiments. Table 2 contains a summary of results of MBadsorbed in the dark (%Ads). It can be seen that adsorptionon ACRM and TiO2–ACRM (28 and 30%, respectively) are lightlyhigher than that on TiO2 alone (25%) while the MB adsorbed inthe dark on ACRM-CO2 and TiO2–ACRM-CO2 are similar than thatadsorbed on TiO2. This suggest that the gasification under CO2

flow influences the number and the type of adsorption sites forMB of ACRM in agreement with the decrease in surface area ofACRM-CO2 in comparison with ACRM (Table 1). Herrmann and co-workers1 have shown that the total number of adsorption sites forMB is remarkably influenced by the presence of different surfacefunctionalities on TiO2. As we discussed above, the gasificationunder CO2 flow clearly induced changes in the surface species of

Table 2 Summary of kinetic results obtained under UV-Visible irradi-ation (LHg)

Material %Adsa kapp¥10-3 (min-1) R2 frelb IF

c

TiO2 25 12.10 0.9220 1.00 1.00ACRM 28 2.46 0.8242 0.20 —TiO2–ACRM 30 71.84 0.9847 5.94 4.93ACRM-CO2 25 2.73 0.8625 0.23 —TiO2–ACRM-CO2 26 35.03 0.9885 2.90 2.36

a After 60 min of adsorption in the dark to achieve equilibrium. b frel =kapp-i/kapp-TiO2. c IF = kapp-i/(kapp-TiO2+ kapp-AC).

the hybrid materials and therefore, differences in the adsorptionof MB would be expected.

3.3.2. Photocatalytic tests. Fig. 7–8 show the kinetics ofMB disappearance and the linear regression of kinetic data ob-tained under the UV (LHg) and UV-vis (LMH) lamps, respectively.

Fig. 8 Kinetics of disappearance of MB under a MH lamp (A). Linearregression from the kinetic data.

It can be seen from Fig. 7A, and Fig. 8A that direct photolysiswas negligible under irradiation with any of two lamps. UnderLHg lamp (Fig. 7A) the total disappearance of MB is achievedafter 45 min and 90 min reaction on TiO2–ACRM and TiO2–ACRM-CO2, respectively, while TiO2 alone required about 150 min.In the case of irradiation with the LMH lamp (Fig. 8A), the totaldisappearance of MB is achieved after 60 min and 180 minreaction on TiO2–ACRM and TiO2–ACRM-CO2, respectively, whileTiO2 alone required a longer irradiation of about 360 min. Inother words, it can be noted from Fig. 7A and Fig. 8A that TiO2

requires longer irradiation time than TiO2–ACRM and TiO2–ACRM-CO2 materials for total disappearance of MB and this time isclearly much longer for the case of irradiation with a metal halide

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Table 3 Summary of kinetic results obtained under almost pure visibleirradiation (LMH)

System %Adsa kapp¥10-3 (min-1) R2 frelb IF

c

TiO2 25 4.60 0.9710 1.00 1.00ACRM 28 2.39 0.8737 0.52 —TiO2–ACRM 30 53.73 0.9837 11.68 7.69ACRM-CO2 25 2.49 0.9736 0.54 —TiO2–ACRM-CO2 26 11.14 0.9891 2.42 1.57

a After 60 min of adsorption in the dark to achieve equilibrium. b frel =kapp-i/kapp-TiO2. c IF = kapp-i/(kapp-TiO2 + kapp-AC).

lamp, which is characterized by a smaller proportion of photonflux from the UV region than that of the mercury lamp (Fig.S1, supplementary material†). This means that in the presenceof photons from UV-vis (LMH), the present binary materials,mainly TiO2–ACRM photocatalyst seems to be remarkably morephotoactive than TiO2 P25.

Assuming a first-order reaction mechanism, as suggested bythe kinetic trends of MB disappearance, Fig. 7B and Fig. 8Bshow the linear regression Ln(Co/Ct) = f(t), obtained from thekinetic data of Fig. 7A and Fig. 8A, respectively. A summaryof kinetic results is compiled in Tables 2 and 3, which showsthe adsorption of MB in the dark (%Ads), the apparent first-order rate constants (kapp), the linear regression factor (R2), theparameter frel defined as the photocatalytic activity relative toTiO2 P25 obtained from the ratio kapp-i/kapp-TiO2 and an interactionfactor defined by our group by IF = kapp-i/(kapp-TiO2 + kapp-AC). IF

permits us to verify any synergistic effect between both solids.Apparent first-order rate constants show that under irra-

diation with more UV-photons (LHg lamp), kapp obtained oncommercial TiO2 (12.10 ¥ 10-3 min-1) is lower than that obtainedon TiO2–ACRM-CO2 (35.03 ¥ 10-3 min-1) and much lower thanthat on TiO2–ACRM (71.84 ¥ 10-3 min-1). For the case of theirradiation with less UV-photons (LMH lamp), kapp obtained oncommercial TiO2 (4.60 ¥ 10-3 min-1) was lower than that onTiO2–ACRM-CO2 (11.14 ¥ 10-3 min-1) and much lower than thatobtained on TiO2–ACRM (53.73 ¥ 10-3 min-1). The enhancementin photocatalytic activity relative to TiO2 was estimated by frel

from the ratio of kapp-i/kapp-TiO2. Table 3 shows an enhancementin photoactivity by a factor 2.4 for TiO2–ACRM-CO2 and 11.7more than one order magnitude higher for TiO2–ACRM underirradiation with a LMH lamp. However, under irradiation with aLHg lamp an increase in relative photoactivity by a factor of 2.9for TiO2–ACRM-CO2 and 5.9 for TiO2–ACRM was detected (Table2). The present results point out the better photoactivity of thepresent materials than that of TiO2 P25 by using lamps witha radiation spectrum with photons near to the visible region.This remarkable enhancement in the photoactivity cannot beascribed only to an adsorption effect because, as we discussabove, the quantity adsorbed in the dark was practically similarin most cases (Table 2). Therefore, we suggest an additional effectdue to two different effects, firstly, the surface properties of thehybrid materials discussed above and secondly to the presenceof iron oxides and carbides.

Additional experiments were performed on ACRM andACRM-CO2 materials in absence of TiO2 in order to verifythis inference. Fig. S5–S6 (supplementary material†) show thekinetics of MB disappearance and the linear regression of

kinetic data obtained under UV (LHg) and UV-vis (LMH) lamps,respectively and kinetic results are also included in Tables 2and 3. Experiments were identically performed as those in Fig.7–8 with the irradiation starting after a preliminary period of60 min adsorption to achieve equilibrium of adsorption of MBin the dark. It should be remarked that the same period of timewas remained in the dark on both solids in order to discard amultilayer of MB adsorption and Fig. S5A and S6A† showedthat after 60 min of irradiation under LHg and LMH, respectively, avery slow but clearly marked disappearance of MB was detected,which confirms that the present materials obtained from thepyroliyis of sewage sludge are photoactive for the methyleneblue photodegradation. Therefore the linear regression of thekinetic data were performed after 60 min irradiation and it canbe seen from Table 2 that under irradiation with LHg lamp, thekapp obtained on ACRM and ACRM-CO2 are clearly lower (2.46 ¥10-3min-1and 2.73 ¥ 10-3 min-1, respectively) than that obtainedon TiO2 alone (12.10 ¥ 10-3 min-1). By contrast, under irradiationwith a LMH lamp, kapp obtained on ACRM and ACRM-CO2 are notmuch lower than that on TiO2 alone (4.60 ¥ 10-3 min-1) beingof about 2.39 x10-3 min-1 and 2.49 ¥ 10-3 min-1, respectively.In order to quantify a synergy effect between both solids, aninteraction factor (IF), earlier reported by us,14,16,42 was estimatedfrom these values [IF = kapp-i/(kapp-TiO2 + kapp-AC)]. The values ofthis synergistic effect are included in Table 2 and 3. Under a LHg

lamp the synergy factors were equal to 4.9 and 2.4 for ACRM

and ACRM-CO2, respectively, while under irradiation with the LMH

lamp the synergy factors were equal to 7.7 and 1.6 for ACRM andACRM-CO2, respectively.

3.4. General discussion

In summary, a clear enhancement in the relative photoactivity(Table 3) of one order of magnitude was found with TiO2–ACRM and the kinetic evidence of a synergy effect betweenboth solids suggest that the original composition of the hybridmaterials obtained from municipal sewage sludge is responsiblefor the photo-assistance of TiO2. It can be inferred that ironoxides detected in the FTIR spectra (Fig. 2–3), in the XRDpatterns (Fig. 4–5) and in the SEM-EDX analysis (Fig. 6)would be responsible for this effect, in agreement with resultsfrom Tryba and co-workers36,37 and Guan and co-workers.38

Tryba and co-workers36 prepared Fe–C–TiO2 photocatalystsfrom a mixture of TiO2 and FeC2O4 through heating at 673–1173 K in Ar. These catalysts contained a very small residueof carbon (0.2–3.3%) and did not show a high adsorption ofphenol, but were active in photo-Fenton reactions during itsdecomposition under UV irradiation with addition of H2O2.These authors36 proved that Fe2+ governed the photoactivity ofFe–C–TiO2 photocatalysts and for comparison they preparedFe–C–TiO2 by heating TiO2 and FeC2O4 at 823 K in air for 3 hand found that phenol decomposition was much slower in thiscase with a large amount of iron in a higher oxidation state (Fe3+)in the photocatalyst, suggesting that iron reduced phases wouldenhance the TiO2 photoactivity for phenol photodegradationby means of the photo-Fenton reaction.36 In addition, it shouldbe pointed out that remaining iron oxide species play animportant role in the photocatalytic behaviour of the presenthybrid material because under visible irradiation the oxygen

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atoms are able to transfer electron density to iron atomsand therefore Fe–O is able to give a photochemical response.This fact has been recently reported by Cha and co-workers.43

These authors report a simple approach for preparing undopedand Pt-doped Fe2O3 thin films with excellent photoactivity.These authors showed photocurrent densities of undoped andPt-doped Fe2O3 thin films up to 1.2 and 1.38 mA cm-2 at0.23 V Ag/AgCl under 1 sun illumination. Also, the influence ofaromatic and sulfur aromatic compounds upon the activity andselectivity of the heterogeneous photocatalytic reactions havebeen recently revised by Samokhvalov.44 Therefore, the influenceof such aromatic groups in the present hybrid materials is underevaluation by our group.

Taking into account that municipal sludge composition variesover the time, location and many other factors,20 it is importantto perform a prior characterization of the dry materials andits ashes in order to think about in the potential use of thesehybrid materials. In the present case, ICP analysis showed animportant quantity of iron species suggesting the possibility ofphotoactivity in agreement with previous works.36–38 It can bepointed out that we performed several preparations across aweek, taking samples from the same municipal sewage sludgeand reproducibility in the composition of the material was betterthan 97%.

The role of other important metals in the sludge is underevaluation in the present. We will present, in the near future,the influence of selective leached sludge in order to verify theinfluence of other important metals, such as Ca, Si and Al, onthe photocatalytic behavior of the hybrid materials. However,we have shown in a recent work on Ti-incorporated mesoporoussilica materials,45 that SiO2 pure materials are not photoactiveunder visible irradiation, and a similar behavior is expected forAl2O3 mesoporous materials.

Finally, it must be notice that in the present study, the Pyrexglass of the water-recirculation cell is able to absorb most UV-photons9 from the metal halide lamp because their peaks areat a wavelength lower than 360 nm (Fig. S1, supplementarymaterial†). By contrast, most of the UV-light of the mercurylamp is not absorbed by this filter because this shows maximumintensity of the emission spectra at about 380 nm. In conse-quence, the mercury lamp contains almost all UV-photons thatare mainly from visible light in the case of metal halide lamp.9

So, the present kinetic results suggest that ACRM materials arephotoactive under visible light. It’s also important to say thatthis is a preliminary study and our goal is to scale these results toa solar-plant photoreactor. In solar light, there is between 5–8%of UV-light and therefore, the results obtained in the presentstudy are perfectly extrapolated to this goal.

Although simple remediation falls outside of the scope ofGreen Chemistry, the present work is related to Green Chemistryfor two reasons. First, the present work deals with the principleof the pollution-prevention, which is one of the twelve principlesof the Green Chemistry introduced by Anastas and Warner.46

Our results showed that it is possible to control the synthesisof hybrid materials from municipal sewage sludge that are evenmore active than commercial TiO2, which is a benchmark forsuch studies. Second, green chemistry belongs front and centerin water treatment as that’s where many of our most worrisomehazards to health and the environment are turning up. For

example, Herrmann and co-workers47 proposed the terminology“environmental green chemistry as defined by photocatalysis”a few years ago to refer to those examples of selective organicsynthesis that can be achieved by heterogeneous photocatalysis.Also, Palmisano and co-workers showed48 that photocatalysis isa promising route for 21st century organic chemistry, mainly bygreen chemistry. The synthetic control of hybrid materials wouldpermit a high efficiency and selective catalysis, which requires aproper understanding of the catalyst structure and the surfaceinteractions that occur during reaction. For example, Matosand Corma have showed in a recent work49 that the interplaybetween support and metal active phase in hybrid materialsbecomes determinant in the selective catalytic hydrogenation ofphenol to cyclohexanone.

4. Conclusions

A hybrid photoactive material was prepared in a one stepsynthesis by carbonization of municipal sewage sludge (ACRM).Materials exhibited a mesoporous texture and FTIR, XRDand SEM-EDX analysis showed the presence of Fe2O3 andFe3C crystallites. Photodegradation of MB on ACRM and onTiO2–ACRM was studied under two different lamps and resultscompared against those obtained with a commercial TiO2. Thephotocatalytic tests showed that ACRM is photoactive and inspite of it exhibiting a low photoactivity, the binary compositeTiO2–ACRM showed higher apparent first-order rate constantsfor the degradation of MB than those obtained with pure TiO2.A synergy effect between TiO2 and ACRM of about 6 and 7.5was estimated under lamps with UV-vis and almost pure visiblelight, respectively. It can be also concluded that the relativephotoactivity of TiO2–ACRM was one order magnitude higherthan that of the commercial TiO2 photocatalyst, suggesting thatthe presence of iron phases, mainly iron oxides and carbides, inmaterials obtained from municipal sewage sludge photo assistedthe TiO2 in the photodegradation of methylene blue.

Acknowledgements

Authors would like to thank to the National Laboratory ofAgriculture Biotechnology, Medical and Environmental (LAN-BAMA) and to M.Sc Dulce Partida Gutierrez for the technicalsupport and to the Venezuelan Ministry of Science, Technologyand Innovation for the funds.

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