Mixed α-Fe2O3/Bi2WO6oxides o Pho oassis ed
He e o-Fen on Deg ada ion o Me hyl O ange and Phenol
C. Ja amillo-Páez1*, J.A. Na ío1, M.C. Hidalgo1,
Asmae Bouziani2,Mohammed EL AZZOUZI2
1Ins i u o de Ciencia de Ma e iales de Se illa, Cen o Mix o Uni e sidad de
Se illa-CSIC, Amé ico Vespucio 49, 41092 Se illa, Spain
2Depa men o Chemis y, Facul y o Sciences Raba , Uni e si y Mohammed
V 4 A enue IBN BATTOUTA B.P.1014 RPRaba , Mo occo.
Abs ac
Mixed oxides, α-Fe2O3/Bi2WO6, we e p epa ed using a mechanical mixing
p ocedu e by adding o he Bi2WO6p e iously ob ained by hyd o he mal me hod
he co esponding amoun o a p epa ed α-Fe2O3, he la e ob ainedby he mal
decomposi ion o Fe(NO3).9H2O. The physicochemical su ace, s uc u al,
mo phological cha ac e is ics and op ical p ope ies o he samples, single and
mixed, we e de e mined by BET, XRD, FE-SEM, XPS and UV-Visible di use
e lec ance spec oscopy. UV– is di use e lec ance spec a showed ha
inco po a ing a 5%w . o α-Fe2O3 o he co esponding amoun o Bi2WO6
sample b oadened he isible ligh abso p ion o Bi2WO6as expec ed. The
pho oca aly ic ac i i y, o single and mixed ca alys s, o deg ade a selec ed dye
such as Me hyl O ange (MO) as well as he anspa en subs a e Phenol
(Ph)we e s udied, in aqueous medium (pH ≈ 5.5)unde UV and sun-like
illumina ion condi ions in he absence and p esence o H2O2. In he p esen
s udy he use o aα-Fe2O3-Bi2WO6/H2O2sys emdemons a e much highe
pho oca aly ic e iciency o deg ade bo h MO and Ph han p is ineBi2WO6o α-
Fe2O3, single o mixed. Using he sys em α-Fe2O3-Bi2WO6/H2O2, a ound 85%
o MO was deg aded in 60 min unde sun-like illumina ion whe eas 100% was
deg aded in 60 min unde UV-illumina ion. Howe e , jus a ound 30% o Ph was
deg aded in 120 min in he α-Fe2O3-Bi2WO6/H2O2sys em unde sun-like
illumina ion whe eas a ound a 95% was deg aded in 90 min unde UV-
illumina ion. Unde UV-illumina ion, he gene a ion o hyd oxyl adicals is
a o able; whe eas unde sun-like illumina ion, only he small ac ion o he UV
can p oduces he •OH.Unde illumina ion, he H2O2could eac wi h
pho oinduced elec ons om he pho oca alys s leading o he p oduc ion o
hyd oxyl adicals (•OH).
Keywo ds:Bismu h Tungs a e; I on oxide; Pho oca alysis; Pho o-Fen on;
Phenol; Dyes; Hyd oxyl adicals.
* Co esponding au ho . Tel.: +34955420998
E-mail add ess: na [email p o ec ed](J.A. Na ío).
1. In oduc ion
The inc ease o indus ial ac i i ies has become a se ious p oblem ha leads o
he augmen a ion o pollu ion inai , wa e and soil. To ace his p oblem, he
scien i ic communi y wo ks o ind new me hods o undo
hecon amina ion.Du ing he pas ew decades, a a ie y o p ac ical s a egies
ha e been implemen ed o de elop iable was ewa e ea men
echnologies[1–6].
Those echnologies a e e y appealing al e na i es o he deg ada ion o
o ganic pollu an s because hey pe mi a pa ial o comple e mine aliza ion o
pollu an s. I is based on he p oduc ion o he e y eac i e and nonselec i e
en i ies (pa icula ly he hyd oxyl adicals•OH) ha ing a highe oxidizing capaci y
han adi ional oxidan s (O2, Cl2, ClO2, H2O2, O3…)[7–9].
Pho oca aly ic deg ada ion o o ganic pollu an s o pu i y was ewa e om
indus ies and household has ecei ed ex ensi e a en ion in ecen yea s. In
pa icula , he e ogeneous pho oca alysis shows p omising po en ial in dep h-
oxida ion o pollu an s o non- oxic ino ganic molecules a ambien
empe a u e[10].
Dyes a e molecules commonly ound in eal e luen s om ex ile and o he
indus ial was ewa e s[11,12]. Dyes a e majo o ganic pollu an s, which can
cause se e e en i onmen al dis up ion and heal h damages[13]. Simila ly,
phenol is one o he mos abundan pollu an s in indus ial was ewa e s and i s
oxici y, ca cinogenici y and pe sis ence, makes hiscompound dange ous o li e
a a he low concen a ions. In his sense, he e ogeneous pho oca alysis,
among a g oup o a ailable echnologies known as ad anced oxida ion
p ocesses (AOPs), is an impo an al e na i e o emo e a wide ange o
o ganic compounds, including phenolsand dyes, in pollu ed s eams. O he
echnologies as Fen on and Pho o-Fen on ha e been success ully used in
depollu ion o wa e [14–18].
To mee he equi emen o u u e en i onmen al applica ions, in he ield o
AOPs, i is s ill essen ial o no only u he imp o e he pho oca aly ic ac i i y by
syn hesizing new pho oca alys s bu also o explo e new combined p ocesses.
The de elopmen o new pho oca alys s is a ac ing as in e es . Among hem
he Bismu h ungs a e (Bi2WO6) is a ypical n- ype di ec band gap
semiconduc o wi h a band gap o 2.8 eV and has p ospec i e applica ions o
he deg ada ion o o ganic pollu an s unde isible ligh illumina ion due o hei
low alence band and high chemical s abili y[19]. In he same con ex , i on
oxide (α-Fe2O3, hema i e) wi h a na ow band gap a 2.2 eV, abso bing he ligh
up o 600 nm and collec ing abou 40% o he sola spec um ene gy, is also
ano he o he p omising ma e ials o pho oca aly ic applica ions[20].
H2O2is a dis inc i e oxida i e agen and has been equen ly used in p ac ical
wa e ea men , because i ’s a e y common sou ce o e y ac i e hyd oxyl
adicals (•OH) by i s decomposi ion a e being illumina ed wi h ul a iole (UV)
ligh .The use o H2O2in pho ochemical p ocesses (UV/H2O2) and UV (H2O2/
Fe3+ (Pho o-Fen on)) has been in es iga ed[1,21–24], howe e , long pe iods o
UV-illumina ion a e equi ed, huspoo deg ee o mine aliza ion isob ained,
makingall hese p ocesses no pe spec i es as po en ial me hods o
was ewa e pu i ica ion.
F om ano he pe spec i e, as an elec on cap u e agen , H2O2can also eac
wi h pho ogene a ed elec ons, om a pho oca aly ic p ocess, o
p oducehyd oxyl adicals (•OH) as es ablished in eq. (1-2) in which (SC) is a
gene al semiconduc o pho oca alys [25].
(SC) + h

→ (SC)(e-CB + h+VB) (ligh abso p ion) (eq. 1)
(H2O2) +e-CB → •OH+ OH-(eq.2)
(H2O) + h+VB→ •OH+ H+(eq.3)
The o ma ion, he adso p ion and he deg ada ion o H2O2on di e en samples
(TiO2and ZnO) ha e been in es iga ed o be e unde s and i s pa icipa ion in
he pho oca aly ic eac ions [26]. In a ypical UV/H2O2/TiO2sys em, he ac i e
adical o ma ion can a ises no only om he di ec UV-pho olysiswhich akes
place h oughahomoly ic p ocess (H2O2+ h → 2 •OH) bu also om he
pho oca aly ic ones (equa ion 1 o 3).
Thus, i he pho oca alys only abso bs in he UV egion, hen he hyd oxyl
adicals pho ogene a ion, du ing he combined pho ochemical p ocess
UV/(SC)/H2O2, can ake place no only by he homogeneous pho oly ic
decomposi ion o H2O2, bu also acco ding o he p ocesses indica ed in
equa ions 1 and 2. Bu i he pho oca alys abso bs in he isible egion, he
gene a ion o hyd oxyl adicals (OH.)would be expec ed o occu , unde isible
illumina ion,by he pho ogene a ed elec on cap u e o H2O2, since he pho oly ic
decomposi ion o H2O2 equi es sho e wa eleng hs[1].
The gene a ion o H2O2and hyd oxyl adicals on Bi2WO6 o phenol deg ada ion
unde isible ligh has been epo ed [27]. F om his wo k, Au ho s concluded
ha he obse ed o ganic deg ada ion o e he i adia ed Bi2WO6in ae a ed
aqueous solu ion is due o he p oduc ion o •OH and H2O2.
Bo h pho oca alys s, Bi2WO6and α-Fe2O3, display po en ial ca aly ic ac i i y o
many chemicals o ganic deg ada ion unde condi ions o sun-like illumina ion.
Howe e , due o he high ecombina ion a e o pho ogene a ed cha ge ca ie s,
he abili y is he eo limi ed. Va ious s a egies, such as he e os uc u ed
cons uc ing a e being de eloped[28–30].
The aim o his wo k is no ocused on de eloping a α-
Fe2O3/Bi2WO6he e os uc u e, bu o use a physical mix u e o bo h ma e ials
and explo e hei pho ochemical ac i i y in he p esence o absence o H2O2in
he deg ada ion o wo selec ed subs a es.
The e o e, when H2O2is co-p esen wi h single o physically mixed Bi2WO6and
α-Fe2O3, di e en ac i i ies could be displayed because, pa allel o he
mechanism o homogeneous deg ada ion by hyd oxyl adicals pho ogene a ed
by H2O2, he e ec gene a ed by he in insic pho oca aly ic ac i i y o single o
mixed sys emsisalso expec ed o occu .
Me hyl o ange (MO) and phenol (Ph) a e used o imi a e non biodeg adable,
oxic o ganic compounds. The pho oca aly ic ac i i y, unde UV o sun-like
Illumina ion, o α-Fe2O3, Bi2WO6and mixed α-Fe2O3/Bi2WO6samples, o Me hyl
O ange and Phenol deg ada ion, in he absence and p esence o H2O2is
epo ed.
2. Expe imen al de ails
2.1 P epa a ion o α-Fe2O3, Bi2WO6and mixed α-Fe2O3/Bi2WO6samples

All he eagen s used in his p ocedu e we e analy ical g ade wi hou u he
pu i ica ion. The de ailed syn hesis p ocedu e o single α-Fe2O3, Bi2WO6and
mixedα-Fe2O3/Bi2WO6sample was as ollow: The i on oxide was p epa ed by
d ying i on(III) ni a e nonahyd a e Fe(NO3).9H2O a 120°C o 2h hen
submi ing he samples o a u he calcina ions ea men a 300°C o 2h.
The Bi2WO6was p epa ed acco ding o he me hod p e iously desc ibed [31]by
dissol ing 4.85 g o Bi (NO3)3.5H2O in 10 mL o glacial ace ic acid, and 1.7 g o
Na2WO4.2H2O in 90 mL o dis illed wa e , hen hose wo solu ions we e mixed
o ming a whi e suspension (pH≈2), which was kep unde s i ing o 1h. The
whi e suspension was ans e ed in o a Te lon ecipien insideas ainless s eel
au ocla e.The hyd o he mal ea men was done a 140°C o 20h, and hen he
p ecipi a e was il e ed, washed and d ied o e nigh a 120°C. Finally he
sample was submi ed o a calcina ion ea men a 300°C o 4h.
The α-Fe2O3/Bi2WO6mixed samples we e ob ained wi h a mechanical mixing in
aga e mo a , by adding he p epa ed α-Fe2O3 o he co esponding amoun o
Bi2WO6 o a 5w . % o i on oxide in he mix u e. This sample will he ea e be
named as BW-Fe(5)-2 indica ing a 5% o i on oxide and ha Bi2WO6was
p epa ed a pH=2.
2.2. Cha ac e iza ion o he pho oca alys s
BET su ace a eas (SBET) o all samples we e e alua ed by N2adso p ion
measu emen wi h a Mic ome i ics ASAP 2010 ins umen . Degasi ica ion o he
samples was pe o med a 150 ºC o 30 min in He low.
C ys alline phase composi ion o he samples was es ima ed by X- ay di ac ion
(XRD). XRD pa e ns we e ob ained on a Siemens D-501 di ac ome e wi h Ni
il e and g aphi e monoch oma o using Cu Kα adia ion.
The mo phology o all he samples was analyzed by ield Scanning elec on
mic oscopy (FE-SEM) using a Hi achi S 4800 mic oscope.
Ligh abso p ion p ope ies o he samples we e s udied by UV–Vis
spec oscopy. The Di use Re lec ance UV–Vis Spec a (UV–Vis DRS) we e
eco ded on a Va ian spec ome e model Ca y 100 equipped wi h an
in eg a ing sphe e and using BaSO4as e e ence. Band-gaps alues we e
calcula ed om he co esponding Kubelka–Munk unc ions, F(R∞), which a e
p opo ional o he abso p ion o adia ion, by plo ing (F(R∞)×hν)1/2 agains hν.
X- ay pho oelec on spec oscopy (XPS) s udies we e ca ied ou on a Leybold-
He aeus LHS-10 spec ome e , wo king wi h cons an pass ene gy o 50 eV.
The spec ome e main chambe , wo king a a p essu e <2×10−9To , is
equipped wi h an EA-200 MCD hemisphe ical elec on analyze wi h a dual X-
ay sou ce wo king wi h Al Kα (hν=1486.6 eV) a 120 W and 30 mA. C1s signal
(284.6 eV) was used as in e nal ene gy e e ence in all he expe imen s.
Samples we e ou gassed in he p e-chambe o he ins umen a 150 ºC up o a
p essu e <2×10−8To o emo e chemiso bed wa e .All pho oelec on spec a
we e analyzed using Casa-XPS so wa e.
2.3. Pho odeg ada ion es s
The pho oca aly ic ac i i y o he ca alys s p epa ed was es ed in he pho o-
discolo a ion o a selec ed dye, Me hyl O ange (MO), as well as on he
pho odeg ada ion o Phenol (Ph) as a selec ed anspa en , oxic
subs a e.Me hyl O ange and Phenol (Reagen Plus >99%) we e supplied by
Sigma-Ald ich.Pho oca aly ic es s we e ca ied ou using a discon inuous ba ch
sys em, his includes a 250 mL Py ex eac o en eloped by an aluminum oil,
illed wi h an aqueous suspension (100 mL) con aining ei he he single
subs a es (concen a ions: 20 ppm o MO o 50 ppm o phenol) o a mix u e o
bo h (10 ppm o MO/25 ppm phenol) and he pho oca alys (1g/L). On he
expe imen s in which H2O2is co-p esen , ei he wi h subs a es o wi h
subs a es and ca alys s, be o e illumina ion, a ce ain amoun (~ 3 mM) o H2O2
(w . 30%) was added in he medium. The mixed solu ion was magne ically
s i ed in he da k o 20 min. Sys ems we e illumina ed h ough a UV-
anspa en Plexiglas® op window ( h eshold abso p ion a 250 nm) by an
Os am Ul a-Vi alux lamp (300 W) wi h sun-like adia ion spec um and a main
line in he UVA ange a 365 nm. The in ensi y o he inciden UV-Visible ligh on
he solu ion was measu ed wi h a Del a OHM pho o- adiome e HD2102.1,
being ca. 110 W/m2whe eas he in ensi y o he inciden UV ligh on he
solu ion was o ca. 90 W/m2. In o de o a o he adso p ion–deso p ion
equilib ium be ween he ca alys s and subs a es, p io o i adia ion he
suspension was magne ically s i ed o 20 min in heda k. Magne ic s i ing and
a cons an oxygen low o 20 L/h, as an oxidan , we e used o p oduce a
homogeneous suspension o he pho oca alys in he solu ion. A ank bubble
was used as a sou ce o na u al oxygen. All pho oca aly ic es s s a ed a pH
ca. 5.5 and he o al eac ion ime was 120 min.
Du ing he Me hyl O ange pho o eac ion, samples we e collec ed a di e en
imes and in o de o e alua e he dye discolo a ion a e, he concen a ion o
Me hyl O ange du ing he pho odeg ada ion eac ion was analyzed by UV–
Visible spec oscopy, conside ing he main peak o his dye in he isible ange,
loca ed a 465 nm. Fo his analysis a UV– is spec ome y wi h a Ca y 100
(Va ian) spec ome e was used.
Phenol concen a ions we e ollowed by HPLC echnique (Agilen , 1200 Se ies)
using anElipse XDB-C18 column (4.6 x 150 mm i.d., 5 μm; Agilen ) a 40ºC.
Mobile phase was wa e /me hanol (65:35) a a low a e o 0.8 ml/min. Samples
o abou 2 mL we e emo ed pe iodically du ing he expe imen and il e ed
(Millipo e Millex 25 0.45 mm memb ane il e ) p e ious o HPLC measu emen s.
Pho olysis es s o subs a es unde illumina ion and in absence o ca alys we e
ca ied ou . Unde he expe imen al condi ions used in his wo k, subs a e
pho olysis was negligible. Rep oducibili y o he measu emen s was ensu ed by
double es ing o selec ed samples.
To al o ganic ca bon was ollowed also by means o a TOC analyze (Shimadzu
5000). Mine aliza ion deg ees (%) we e e alua ed by he TOC alues upon 2 h
o illumina ion, o all he pho o-assis ed p ocesses s udied.
3. Resul s and discussion
3.1. Cha ac e iza ion
Figu e 1 shows X- ay di ac ion pa e ns (XRD) o he p epa ed ma e ials.The
XRD o he as-p epa ed i on oxide sample showed he di ac ion peaks
co esponding o he s anda d α-Fe2O3(JCPDS no. 33-0664). The main peaks
a 24.1, 33.1, 35.6, 49.5 and 54.1º was obse ed, which co espond o (012),
(104), (110), (024) and (116) di ac ion planes o hema i e espec i ely. Fo he
Bi2WO6sample all he di ac ion peaks a e in good consis en wi h he s anda d
da a o he pu e usseli e o ho hombic Bi2WO6phase (JCPDS no. 39-0256).
ca alys s, which ob iously condi ion he capaci y o su ace adso p ion o
subs a es o di e en chemical na u es. In ac , acco ding o he esul s
epo ed in he li e a u e [26], he adso p ion capaci y o H2O2seems o be
linked o he numbe o OH g oups p esen on he su ace o solids.
Rega dless o his, i is e iden ha he Bi2WO6-H2O2sys em inc eases he
deg ees o mine aliza ion o bo h subs a es, wi h espec o hose ob ained
when only he ca alys o H2O2a e used.
A p oposed mechanism o explain he combined p ocess o Bi2WO6and H2O2,
would be he gene a ion o hyd oxyl adicals, ei he by cap u ing elec ons by
H2O2and he simul aneous oxida ion o H2O by holes, as s a ed in he
equa ions 1 o 3 in which he semiconduc o (SC) would be Bi2WO3, as well as
by he di ec pho oly ic p ocesses (H2O2+ h → 2 OH.).
3.2.3 α-Fe2O3andα-Fe2O3-H2O2
Figu e 6A shows he p ocesses in he da k and unde illumina ion, which occu
o MO subs a e in he p esence o α-Fe2O3. As can be seen in he da k, a
la ge adso p ion o MO occu s a he su ace o α-Fe2O3. A en a i e explana ion
o he adso p ion p ocess in he da k would be he es ablishmen o a weak
Lewis acid-base in e ac ion be ween he elec on densi y o he ch omopho e
g oup (-N=N-) and d-o bi als o Fe3+.A e equilib a ion, he backligh , bo h in he
UV and sun-like, gene a es a pho o-deso p ion p ocess o he MO.These pho o-
deso p ion p ocesses a e accompanied by pho oca aly ic deg ada ion
p ocesses o MO, since a e 120 min unde ligh ing condi ions TOC alues
ob ained indica e ha he e has been a pe cen age o 27.4% o

mine aliza ion(Table 1).Howe e , as he TOC measu emen is pe o med on he
liquid phase, i is also possible ha MO is no comple ely deso bed and his
appa en dec ease on TOC could no be ela ed o a mine aliza ion.
This ac is impo an since i on is he second mos abundan me al on Ea h,
and he mine al hema i e is mos o en o med in na u al wa e . Thus, a na u al
pho oca aly ic deg ada ion p ocess wi h pa icles o α-Fe2O3suspended in
wa e o sedimen s could ha e a bene icial impac o he wa e ecosys em
con amina ed by MO.
In he deg ada ion es o MO wi h α-Fe2O3, an ini ial adso p ion and la e
deso p ion unde illumina ion is obse ed (Figu e 6A).This beha io obse ed
o he MO in he p esence o α-Fe2O3, is no obse ed when Phenol is used.
Figu e 6B shows he plo o he con e sion pe cen ages o Phenol, using he
p epa ed ca alys s α-Fe2O3.As wi h he use o Bi2WO6, he use o α-Fe2O3leads
o con e sion pe cen ages which a e negligible o bo h subs a es, bo h unde
UV and sun-like illumina ion. These esul s indica e ha he as p epa ed i on
oxide sample p esen s, no only a low adso p ion capabili y o Phenol bu also
a low pho oac i i y, al hough op ical abso p ion esul s, by DRS (Fig.2), indica e
a wide op ical abso p ion bo h in he ul a iole and in he isible, being in
acco dance wi h he na ow band gap a 2.2 eV. The poo pho oac i i y, es ed
o he p epa ed α-Fe2O3, simila ly o ha obse ed o Bi2WO6, could be
associa ed wi h he high ecombina ion o cha ge ca ie s in he p epa ed
samples.I is in e es ing o no e ha al hough nosigni ican deg ees o
con e sion o phenol we e obse ed, howe e TOC alues indica e ha he e
has been a ce ain deg ee o mine aliza ion (25-35%) o phenol(Table 1).
I is in e es ing o no e ha he simul aneous p esence o he ca alys (α-Fe2O3)
and H2O2, leads oa signi ican inc ease in con e sion pe cen ages o bo h
subs a es, bo h in he UV and he sun-like (Figu es 6C and 6D) and ha hey
a e di e en om hose ob ained o he wosubs a es bo h wi honly he
ca alys (Figu es 6A and 6B) and wi h H2O2alone (Figu es 4A and 4B).These
esul s a e no su p ising i hey a e explained in he con ex o a pho oassis ed
he e o-Fen on p ocess implemen ed by he α-Fe2O3/H2O2
sys em.Thus, ega dless o whe he he H2O2-pho oly ic p ocess gene a ing •OH
adicals (equa ion 7), he α-Fe2O3/H2O2sys emwould also be ac ing by
gene a ing mo e hyd oxyl adicals ia a simul aneous he e ogeneous pho o-
Fen on mechanism in ol ing Fe3+/Fe2+ pai s, as he ollowing:
H2O2+ h → 2•OH(eq. 7)
˃Fe 3+ + e-→ ˃Fe2+ [O2] → ˃Fe 3+(eq. 8)
˃Fe2+ + H2O2+ H+→ ˃Fe3+ +•OH + H2O (eq. 9)
˃Fe2+ +•OH→ ˃Fe 3+ + OH-(eq. 10)
˃OH-+ h+→ •OH(eq. 11)
In he p esence o oxygen, e-oxida ion o ˃Fe 2+ occu s a a high a e be o e
de aching om he su ace (pho oco osion).Fo his eason, he pho oco osion
o i on oxide in pho oca aly ic eac ion (unde oxygen) is gene ally e y low wi h
his p ocess being hinde ed[35–37].
3.2.4 α-Fe2O3/Bi2WO6andα-Fe2O3/Bi2WO6-H2O2
In Fig. 7, we show he MO and Phenol con e sion plo s using he mixed α-
Fe2O3/Bi2WO6pho oca alys s bo h in he absence (Figu es 7A and 7B) o in he
p esence o hyd ogen pe oxide (Figu es 7C and 7D), unde UV and sun-like
i adia ion. I is possible o obse e ha using he mixedα-Fe2O3/Bi2WO6
pho oca alys s,MO emains almos una ec ed in solu ions unde illumina ion.
Wi h BW-Fe(5)-2 unde hese condi ions, he e is a sligh imp o emen o
Phenol con e sion, a e 120 min o illumina ion (Fig.7B), compa ed wi h ha
ob ained o MO (Figu e 7A). Howe e , he small con e sion alues obse ed
a e highe unde condi ions o UV-i adia ion han ha obse ed unde sun-like
illumina ion, being compa a i ely simila o he esul s shown in Figu es 4A and
4B, as is expec ed, since he ca alys used, BW-Fe(5)-2, is a physical mix u e o
95% Bi2WO6. Thus, we can conclude ha he mechanical mix u e o Bi2WO6
and α-Fe2O3, does no a ec he in insic pho oca aly ic beha io obse ed o
single ca alys s unde he same expe imen al condi ions.
Figu es 7C an 7D shows he a ia ion o he MO (Figu e 7C) and Phenol
(Figu e 7D) con e sion wi h ime when BW-Fe(5)-2 is usedas apho oca alys in
he co-p esence o H2O2, unde UV o sun-like illumina ion. A di e en end is
obse ed o bo h subs a es. Thus, o MO he simul aneous p esence o he
mixed ca alys s BW-Fe(5)-2 and H2O2has a ma ked in luence on he
pho oassis ed discolo a ion p ocess.This e ec is mo e ma ked unde
condi ions o UV-illumina ion o which almos 100%discolo a ionis
achie edin30 min, while high con e sion alues a e ob ained (ca. 85% a 60
min) unde sun-like i adia ion.These con e sion alues u n ou o be highe
han hose ob ained o he same subs a e (MO) in homogeneous phase wi h
ligh -H2O2(Figu e 4A), in he pho oca aly ic p ocesses using single
pho oca alys s (Figu es5A and 6A) and when he single pho oca alys s a e used
wi h H2O2(Figu es 5C and 6C).These esul s indica e a syne gis ic e ec in he
mix u e o he pho oca alys sBW-Fe(5)-2 when co-exis ing wi h H2O2unde
illumina ion, a leas o he subs a e used (MO), since his syne gis ic e ec is
no obse ed o Phenol (Figu e 7D).The mos s iking esul is ha he
syne gis ic e ec occu s e en in he isible o MO.
Using his sys em BW-Fe(5)-2 + H2O2, apa om he ela i ely high alues
ob ained o pho o-bleaching o MO, unde bo h UV and unde sun-like
illumina ion, ela i ely high alues o mine aliza ion deg ees o MO a e also
ob ained(see Table 1).
This ac could be explained by assuming a con ibu ion o pho osensi iza ion o
a dye moleculesuchas MO, asi has alsobeen obse ed when using Rhodamine
B [31].
As men ioned in he in oduc ion, due o high ecombina ion a e o cha ge
ca ie s in he single oxides, α-Fe2O3and Bi2WO6, ha e de eloped s a egies
ha lead o he sepa a ion o cha ge ca ie s, such as he de elopmen o
he e os uc u al cons uc ing o bo h ca alys s [28–30]. In hese sys ems, i has
been achie ed an imp o emen in he pho oca aly ic ac i i y compa ed wi h ha
ob ained wi h he singles ca alys s. The α-Fe2O3ac s as a hole-accep ing
semiconduc o and pho ogene a ed elec ons a e injec ed wi h high e iciency
om he conduc ion band o α-Fe2O3 o he conduc ion band o Bi2WO6. In ou
wo k we used a α-Fe2O3/ Bi2WO6composi e by a mechanical mixing p ocedu e
and in o de o achie e high pe o mance, he ex a H2O2was equi ed.
Howe e , he goal is he same in bo h cases, ha is, achie e e icien
sepa a ion o he cha ge ca ie s. In ou case, H2O2ac s as an elec on
accep o , hus gene a ing •OH adicals while he H2O can ac as holes accep o ,
gene a ing mo e •OH adicals. The ad an age, o ou me hod, can be ound in
he amoun o hyd oxyl adicals gene a ed in he p ocess, he eby inc easing
pho o-assis ed deg ada ion o he subs a es. Rega dless o his, in ou wo k we
ha e assessed he pho oca aly ic ac i i y wi h wo di e en subs a es, he MO
and Phenol and no wi h he Rhodamine B, since we ha e e idence ha he
e alua ion o pho oca aly ic ac i i y wi h Rhodamine B gene a es esul s ha a e
mo e spec acula , han when a anspa en subs a e, such as Phenol is used.
The e a e o he wo ks, ha achie e imp o ed pho o-Fen on mechanism by
inco po a ing he α-Fe2O3 o a g aphene oxide (GO) [38] o o Kaolin [39]. In
bo h cases, howe e , he imp o emen ob ained could be a ibu ed o he
syne ge ic e ec s o he adso p i e powe o GO o Kaolin and he hyd oxyl
adicals p oduced by he e ogeneous pho o-Fen on eac ions. In any case, in
hese wo ks, he e alua ion o he ac i i y is done also by using RhB.
Among he mixed ca alys BW-Fe(5)-2,in he p esence o H2O2, highe
con e sion alues o MOwe e displayed, bo h unde UV and sun-like
illumina ion. XPS analyses ha e been conduc ed on he p epa ed mixed oxides
sys em BW-Fe(5)-2, jus be o e and a e he pho o-assis ed Me hyl O ange
deg ada ion unde isible illumina ion in he p esence o hyd ogen pe oxide, by
eco e ing he ca alys powde om he eac ion sys em, a e a p olonged ime
o illumina ion (Expe imen epo ed in Figu e 7C).
Figu e 8 shows he esul s o XPS analysis o he o iginal sampleo α-
Fe2O3/Bi2WO6, in which only O(1s), W(4 ), Bi(4 ) and Fe(2p) peaks we e
de ec ed bu no peaks o esidual sodium we ede ec ed. The o e iew

spec ums o he mixed sys em demons a e ha Bi, W, O and Fe exis , u he
con i ming ha he sample was composed o Bi2WO6and Fe2O3. As shown in
Figu e 8A, he cha ac e is ic peak o O 1s a ound 530.1 eV could come om he
o e lapping con ibu ions o se e al componen s, being he iden i ica ion o he
subme ged peaks pe o med by Gaussian decon olu ion and cu e i ing: hese
peaks we e loca ed a 529.32 eV, 529.65 eV, 530.32 eV, 530.98 eV and 531.26
eV which co esponds o Fe-O, Bi-O, W-O la ice oxygen, chemiso bed wa e
and •OHhyd oxyl g oups espec i ely [40,41].
The peaks loca ed a 36.02 eV and 33.91 eV wi h a spin-o bi al sepa a ion o
2.11 eV, as shown in Figu e 8B, could be assigned o he +6 oxida ion s a e o
ungs en o he W 4 5/2 and W7/2 espec i ely [40].The XPS spec um o he Bi
4 egion displayed in Figu e 8C consis ing o wo cha ac e is ic peaks wi h
binding ene gies o 159.04 eV and 164.31 eV co espond o he signals om
double s o Bi 4 7/2 and Bi 4 5/2 in he i alen oxida ion s a e, espec i ely o
pu e Bi2WO6.
Figu e 8D p o ides XPS peaks o Fe elemen , exhibi ing wo indi idual
peaksin he Fe 2p egionloca ed a 710.42 eV and 723.75 eV, which can be
assigned o Fe 2p3/2 and Fe 2p1/2 peaks in α-Fe2O3phase, espec i ely [28]
con i ming he exis ence o Fe2O3phase on he mixed sys em which was no
de ec ed by XRD echnique. Besides wo sa elli e peaks o Fe 2p loca ed a
718.8 eV and 732.9 eV a e clea ly dis inguishable. The sa elli e peaks we e he
esul o he cha ge ans e sc eening a ibu ed o he p esence o Fe in 3+
oxida ion s a e [42,43].
Howe e , he spli ed peaks o Bi 4 7/2 and 4 5/2a ibu ed o Fe3+–O–Bi3+ linkage
a lowe ene gy alues o 157.5 eV and 162.8 eV, espec i ely[44],dono
appea , indica ing ha no in e ac ion be ween α-Fe2O3and Bi2WO6exis sin he
mixed sys em, as expec ed by he p epa a ion p ocedu e.
The used α-Fe2O3/Bi2WO6sample a e pho o-assis ed discolo a ion p ocess o
Me hyl O ange unde isible illumina ion by using H2O2, was eco e ed and,
once d ied, was subjec ed o a XPS analysiswi h he esul sbeing epo ed in
Figu e 9. I is in e es ing o no e ha , in he egion o he O(1s)peak, wo dis inc
peaks appea cen e ed a ound 526.8 eV and 530.0 eV espec i ely. By a
decon olu ion analysis and i ing, se e al subme ged peaks
canbedis inguished. A clea peak cen e ed a 526.58 eV can be a ibu ed o a
pe oxide species s abilized in he sys em whichcanno be associa ed o Na2O o
Na2O2[45]since sodium was no de ec ed by XPS. No changes we e obse ed
in he oxida ion s a es o W and Bi espec i ely. Howe e , o his sample, Fe 2p
pho oelec on peaks appea ed a ound 710.6 eV and 724.0 eV wi h sa elli e
peaks. The peaks o Fe 2p1/2 and Fe 2p3/2 le els a 724.0 eV and 710.6 eV,
espec i ely, sepa a ed 13.4 eV, e i ied he p esence o Fe in 3+ oxida ion
s a e on he eco e ed α-Fe2O3/Bi2WO6ca alys s. Howe e , as shown in Figu e
9D, a shoulde a ound 706.0 eV, no p esen on he o iginal spec a (Figu e 8D)
is obse ed. This could be asc ibed o he ans o ma ion o Fe(3+) o Fe(2+)
a e he e ogeneous pho o-Fen on eac ion [46]. This inding, oge he wi h he
p esence o O(1s) peak associa ed o pe oxide species, leadus o pos ula e he
p esence o i on(II) pe oxide,Fe(O2),s abilized in he mixed sys em.
The in e ac ion o H2O2wi h i on oxide hasno been ex ensi elys udied, and
he e a e s udies ha sugges he o ma ion o Fe(O2) by compu e
calcula ions[47,48]; a ecen s udy [49]concluded ha he da k Fen on p ocess
in ol ing Fe(II) + H2O2consis s o wo egimes, a as e ous one ha is
igge ed by he eac ion o Fe2+ + H2O2and a slow e ic one ha is domina ed
by he educ ion o Fe(III).Howe e , s abiliza ion o pe oxide species as η2Fe(II)
-O22- seems unlikely, due o he ins abili y o he pe oxide species.
O he p oposals could be made, based on esul s published in he li e a u e
[50].Pigna ello e al. [51], e idenced he o ma ion o an addi ional oxidan in he
pho oassis ed Fen on eac ion. The esul s sugges he pa icipa ion o a high-
alen oxoi on complex ( e yl) in addi ion o •OHin o ganic compound
oxida ions.They e idenced ha hyd ogen pe oxide o ms a complex wi h i on,
(Fe3+-OOH)2+[K= 1.15 x10-2], ha abso bs in he isible egion and could be he
p ecu so o he p oposed e yl complex[51].
I he o ma ion o (Fe3+-OOH)2+species is assumed hen an inc easedpho o-
con e sion p ocess is likely o occu , in bo h he UV and isible, as seen in he
esul s p esen ed in Figu es 7C and 7D.
3.2.5 Mix u es o Me hyl O ange and Phenol
Finally, we ha e s udied he simul aneous deg ada ion o MO and Phenol bo h
ina mixed solu ion, using he mixed oxide pho oca alys BW-Fe(5)-2, in he
p esence o H2O2unde sun-like illumina ion condi ions. Figu e 10 epo s he
a ia ion in he concen a ion o MO and Phenol in he mix u e MO/Phenol wi h
ime unde sun-like illumina ion, using he BW-Fe(5)-2 ca alys in he p esence
o H2O2. As no ed in Fig.10, i is possible o obse e ha in mixed solu ions o
bo h subs a es, he e is an inc ease in Phenol deg ada ion in luenced by he
simul aneous p esence o MO, while he p o ile o he con e sion plo o MO is
p ac ically he same as ha ob ained o he single subs a e unde he same
expe imen al condi ions (Fig.7C). These esul s indica e ha ega dless o he
syne gis ic e ec obse ed in he physical mix u e o he wo ma e ials s udied,
BW-Fe(5)-2 in he co-p esence o H2O2, a ma ked in luence o he simul aneous
p esence o MO o e he Phenol deg ada ion, is also obse ed.
I is obse ed ha his mixed sys em, BW-Fe(5)-2, is capable o comple ely
makingMOdisappea in he mix u e, a e 120 min unde sun-like illumina ion,
leading oa esidual amoun o TOC a he inal pe iod, indica ing a pe cen age
o 50% o mine aliza ion. F om one poin o iew, ha ing a pho osensi izing
molecule, suchas MO, has p o ed o ha e some e ec in he pho odeg ada ion
o a non-pho osensi izing one, like Phenol, since by using hese condi ions, o
single phenol a con e sion alue o ca. 30% was obse ed whe eas a alue o
ca. 60% is eached in he co-p esence o MO.
Conclusions
Mixed oxides, α-Fe2O3/Bi2WO6, we e p epa ed using a mechanical mixing
p ocedu e by adding he co esponding amoun o a p epa ed α-Fe2O3 o he
Bi2WO6p e iously ob ained by hyd o he mal me hod, he o me ob ainedby
he mal decomposi ion o Fe(NO3).9H2O.
Despi e exhibi ing po en ial op ical abso p ion capaci y in he UV- is egion,
howe e , he wosingle ca alys s showed poo pho oca aly ic ac i i y, bo h in he
UV and in he isible, possibly due o high ecombina ion a e o ca ie ’s pho o-
gene a ed cha ges. Thep epa ed oxide α-Fe2O3shows a ema kable da k
adso p ion capabili y o MO, howe e unde illumina ion condi ions i displayed
a pho odeso p ion p ocess which is accompanied by a simul aneous pho o-
Figu e 1. X- ay di ac ion pa e ns (XRD) o p is ine α-Fe2O3, Bi2WO6and mixed BW-
Fe(5)-2 samples

Figu e 2.Di used e lec ance spec a (DRS) o p is ine α-Fe2O3, Bi2WO6and mixed
BW-Fe(5)-2 samples.
Figu e 3.SEM images o p is ine Bi2WO6(A and B), p is ine α-Fe2O3(C), and mixed
BW-Fe(5)-2 samples (D).
Figu e 4. Con e sion plo s o pho ochemical discolo a ion o Me hyl O ange (A) and
pho ochemical disappea ance o Phenol (B), wi h only he p esence o H2O2unde UV
o sun-like illumina ion.
A
B
Figu e 5. Con e sion plo s o pho ochemical discolo a ion o Me hyl O ange and
Phenol disappea ance, unde UV o sun-like illumina ion: wi h only he p esence o
Bi2WO6pho oca alys (A and B) o wi h he co-p esence o bo h, Bi2WO6and H2O2(C
and D).
A
B
C
D
Figu e 6. Con e sion plo s o pho ochemical discolo a ion o Me hyl O ange and
Phenol disappea ance, unde UV o sun-like illumina ion: wi h only he p esence o α-
Fe2O3pho oca alys (A and B) o wi h he co-p esence o bo h, α-Fe2O3and H2O2(C
and D).
A
B
C
D

Figu e 7.Con e sion plo s o pho ochemical discolo a ion o Me hyl O ange and
Phenol disappea ance, unde UV o sun-like illumina ion: wi h only he p esence o
mixed BW-Fe(5)-2 pho oca alys s (A and B) o wi h he co-p esence o BW-Fe(5)-2
pho oca alys s and H2O2(C and D).
A
B
C
D
Figu e 8. XPS su ace spec a o he as-p epa ed mixed BW-Fe(5)-2 pho oca alys s.
Figu e 9. XPS su ace spec a o he eco e edBW-Fe(5)-2 sample a e being used
on he pho o-assis ed discolo a ion p ocess o Me hyl O ange unde isible
illumina ion by using H2O2.
Figu e 10. Me hyl O ange and Phenol con e sion plo s in mixed solu ion wi h
BW-Fe(5)-2 unde sun-like illumina ion, in he p esence o H2O2