Efficient degradation of organic pollutants with a sewage sludge support and in situ doped TiO2 under visible light irradiation conditions

Article abstract of DOI:10.1039/c4ra12434k

TiO₂ is considered to be the most promising candidate for photocatalytic organic pollutant degradation, however, it suffers from a large band gap, which means that it is active only in the UV region. Thus, in recent decades, a considerable amount of research has investigated how to expand the optical absorption spectrum of TiO₂ into the visible region. In this study, we developed a new approach that used sewage sludge as the support and dopant to construct a bimodal-pore composite visible-light-driven TiO₂ photocatalyst (SS-Ti-700). The design was based on a mesoporous assembly of the organic and inorganic compounds in the sewage sludge, which served as the scaffold templates; a SiO₂-TiO₂ nanostructure, which has been demonstrated to have markedly high photocatalytic activity; and the in situ doping of TiO₂ with transition metals (Fe, Cu, and Cr) originating from the sewage sludge, which significantly enhances visible-light absorption. Kinetic analysis showed that this arrangement exhibited rapid p-nitrophenol degradation and mineralization under visible light irradiation conditions. The possible reaction mechanism was explored by ESR spectroscopy. Our protocol demonstrates a new approach for the potential environmentally benign reuse of sewage sludge and provides a facile, cost-effective, and eco-friendly approach for synthesizing TiO₂-based mesoporous photocatalysts. This journal is

Full text of DOI:10.1039/c4ra12434k

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Efficient degradation of organic pollutants with a
sewage sludge support and in situ doped TiO2
under visible light irradiation conditions†
Cite this: RSC Adv., 2014, 4, 61036
*
Shi-Jie Yuan, Xiao-Wei Li and Xiao-Hu Dai
TiO2 is considered to be the most promising candidate for photocatalytic organic pollutant degradation,
however, it suffers from a large band gap, which means that it is active only in the UV region. Thus, in
recent decades, a considerable amount of research has investigated how to expand the optical
absorption spectrum of TiO2 into the visible region. In this study, we developed a new approach that
used sewage sludge as the support and dopant to construct a bimodal-pore composite visible-light-
driven TiO2 photocatalyst (SS-Ti-700). The design was based on a mesoporous assembly of the organic
and inorganic compounds in the sewage sludge, which served as the scaffold templates; a SiO2–TiO2
nanostructure, which has been demonstrated to have markedly high photocatalytic activity; and the in
situ doping of TiO2 with transition metals (Fe, Cu, and Cr) originating from the sewage sludge, which
significantly enhances visible-light absorption. Kinetic analysis showed that this arrangement exhibited
rapid p-nitrophenol degradation and mineralization under visible light irradiation conditions. The possible
reaction mechanism was explored by ESR spectroscopy. Our protocol demonstrates a new approach for
the potential environmentally benign reuse of sewage sludge and provides a facile, cost-effective, and
eco-friendly approach for synthesizing TiO2-based mesoporous photocatalysts.
Received 15th October 2014
Accepted 7th November 2014
DOI: 10.1039/c4ra12434k
www.rsc.org/advances
Numerous methods have been proposed to solve these
problems and to maximize the photoactivity of TiO2, such as
metal and nonmetal element doping,9,10 surface modication
Introduction
In recent decades, there has been considerable interest in using
semiconductor catalysts for heterogeneous photocatalytic
oxidation under visible light irradiation conditions, as this
approach enables the direct use of sunlight for environmental
applications of refractory organic pollutant degradation and
water disinfection.1–3 Among the applicable photocatalysts,
titanium dioxide (TiO2) is one of the most commonly used
semiconductors owing to its physical and chemical stability,
low cost, and non-toxicity.3–6 However, TiO2 is active only in the
ultraviolet (UV) region due to its large band gap (ꢀ3.2 eV).
Consequently, an appropriate strategy to expand the photo-
response of TiO2 to the visible region, which accounts for 43%
of the total energy of the solar spectrum, needs to be developed
to ensure the efficient photocatalytic activity and possible
applications of this semiconductor.1,7 The high recombination
rate of the photogenerated electrons and holes, which decreases
the photocatalytic efficiency of the material, is another problem
that limits the possible applications of TiO2.8
with various organic and inorganic species,7,11 immobilization on
mesoporous supports to present a high specic surface area,5,12
and electrochemical methods.8 Among these approaches, doping
of TiO2 with metal ions is one of the most well-known and
effective strategies for extending light absorption into the visible
region and forming charge traps for holes and/or electrons to
prolong their recombination time.5,7,10 In addition, the modi-
cation of TiO2 with mesoporous SiO2 or clay has been shown to
produce markedly higher photocatalytic activity, which can be
attributed to the high photoinduced charge carrier separation
rate, the large surface area related to the supports along with easy
diffusion of the absorbed pollutants, and the avoidance of the
macroscopic aggregates of photoactive particles that can lead to
reduced efficiency.12–14 However, these existing metal doping and
modication approaches may lead to cost increases or environ-
mental concerns. In this case, appropriate strategies are still
needed to elevate the photocatalytic efficiency of TiO2 under
visible light irradiation conditions.
Sewage sludge, the residue generated from the treatment of
wastewater, is dened as a pollutant by the U.S. Environmental
Protection Agency.15 The main constituents of sewage sludge are
organic materials, such as polysaccharides, proteins, fats, and
cellulose, and inorganic compounds in the form of silica, iron
salts, calcium oxide, alumina, magnesium oxide, and a wide
State Key Laboratory of Pollution Control and Resource Reuse, College of
Environmental Science and Engineering, Tongji University, Shanghai, 200092,
China. E-mail: daixiaohu@tongji.edu.cn; Fax: +86-21-65983602; Tel: +86-21-
65986297
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra12434k
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variety of heavy metals.16 Accompanied with the imposition of then dried in air at 105 ꢁC overnight. Finally, the dried solid was
ꢁ
more stringent regulations governing the disposal and use of calcined at 700 C in a muffle furnace for 5 h, and the sewage
sewage sludge, many of the traditionally accepted disposal sludge-derived TiO2-based photocatalyst was obtained and
routes have been in decline or lost, mainly due to the presence designated as SS-Ti-700. To ensure its powder form, the dried
of organic pollutants and heavy metals.17,18 This situation has solid aer calcining was ground to a ne powder with an agate
been further aggravated by the continuous increase in the mortar and pestle and ltration through a 400 mesh metal
generation of sewage sludge around the world, with the annual sieve. TiO2 was also synthesized using the same procedure with
production in China, the E.U., and the U.S. now exceeding 30 no sewage sludge added and SS-700 was obtained by calcining
million, 10 million, and 5 million dry tons, respectively.19–21 In sewage sludge at 700 ꢁC in a muffle furnace for 5 h. The textural
this case, the need to develop more cost effective and environ- properties of the as-synthesized catalysts was characterized (for
mentally benign reuse of sewage sludge is of particular concern. details, see the ESI†).
Considering the silica and wide variety of heavy metals present
in sewage sludge, and the fact it can be converted into meso-
Photocatalytic organic pollutant degradation
porous adsorbents,16 it is expected that the sludge can be used
The photocatalytic activity of the as-prepared catalyst was eval-
uated in terms of the photodegradation of p-nitrophenol, a
typical persistent organic pollutant that was chosen as the
model pollutant. The photocatalytic reactions were performed
as both a support and dopant to construct an effective visible-
light-driven TiO2 photocatalyst.
In this study, we developed a novel sewage sludge support
and in situ doped TiO2 nanostructured visible-light-driven pho-
tocatalyst, which was implemented for the degradation and
mineralization of the organic pollutant, p-nitrophenol, as the
tion under constant stirring (Fig. S1A†). A Philips 100 W
target model pollutant under visible (l > 400 nm) light irradia-
tion conditions. The U.S. Environmental Protection Agency has
categorized p-nitrophenol as a refractory, hazardous, and
priority toxic pollutant. The sewage sludge in our protocol not
in a quartz glass cylinder (8.5 cm in height and 5.5 cm in
diameter) lled with 150 ml of p-nitrophenol (35 mg lꢂ1) solu-
halogen lamp, which produces a continuous spectrum of light
from near ultraviolet to deep into the infrared, was used as the
visible light source and was xed next to the cylinder with 2 M
NaNO2 acting as both the cooling jacket and cutoff lter (l > 400
only exhibited as the structure-directing templates with a high
nm).22 The reactions were kept at a constant temperature of
surface area but also served as the source of metal dopant, which
ꢁ
25 C via an air conditioner (for details, see the ESI†).
led to a signicant enhancement in the visible light absorption.
The concentrations of p-nitrophenol were measured from
The stability of the as-synthesized catalyst and its possible
the absorbance change at a wavelength of 317 nm with a UV-vis
photocatalytic mechanism were also investigated. The results of
spectroscope (PhotoLab 6100, WTW Co., Germany). The total
our study provide a novel route for the synthesis of a visible-
organic carbon (TOC) concentration was measured with a TOC
light-driven TiO2-based photocatalyst using a simple, low-cost,
analyzer (TOC-L CPH CN 200, Shimadzu Co., Japan). The
intermediates of p-nitrophenol degradation were determined by
and green process, and demonstrate a new approach for the
potential environmentally benign reuse of sewage sludge.
an HPLC (1100, Agilent Inc., USA) with a mixture of water with
0.1% acetic acid and methanol (60 : 40) as the mobile phase at a
Experimental
ow rate of 1.0 ml minꢂ1. All of the tests were conducted in
Preparation of the catalysts
triplicate.
The experiments were carried out with a dewatered sewage
sludge sample obtained from the Anting wastewater treatment
plant located in Shanghai, China.19 The obtained sludge was
stored at 4 ꢁC before use. TiOSO4$2H2O (Guangfu, Tianjin,
Results and discussion
Textural properties of the as-synthesized catalysts
China) was used as received. All other reagents were of analyt- The surface morphology and structure of the as-synthesized
ical grade unless otherwise stated. All of the water used was catalyst SS-Ti-700 were examined using SEM and the typical
prepared using a purication system (Hitech Instrument Co., images are shown in Fig. S2† and 1A. In the low magnication
Shanghai, China).
images (Fig. S2†), numerous particles with nanometer sized
The sewage sludge support and in situ doped TiO2 photo- diameter were stacked and a porous structure that increased the
catalyst was synthesized using a facile one-step hydrothermal surface area of the catalyst was displayed. The relatively high
process with TiOSO4$2H2O as a precursor. In brief, 10 g of magnication image showed small islands with diameters
dewatered sewage sludge and 5 g of TiOSO4$2H2O were added range from about 60 to 600 nm deposited on the surface of the
to 30 ml of deionized water. 5 ml of HCl, which was used to mesoporous scaffold templates derived from the sewage sludge
enhance the dissolution of inorganic fraction, especially some (Fig. 1A). The rough mesoporous structures of the scaffold
heavy metals, from the sewage sludge, was also added. Aer templates were attributable to the combustion, carbonization,
being stirred overnight at room temperature, the obtained and evaporation of the organic material, bacterial cells, and
solution was transferred to a 100 ml Teon autoclave and adsorbed H2O in the sewage sludge, the SiO2 fraction, and
ꢁ
heated at 150 C for 12 h in an oven. Aer being cooled to the especially the partial dissolved of the inorganic fraction from
ambient temperature, the resultant precipitate was recovered the sewage sludge by the HCl added during the synthesization
via centrifuging, washed thoroughly with deionized water, and process.16 This was further evident by the increased BET surface
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mesopores between the TiO2 nanoparticles in the aggregates,
which were approximately 30 nm in diameter (Fig. 1C).5,14 The
resulting material from the sewage sludge directly calcined at
700 ꢁC showed a relatively wide pore size distribution, whereas
as expected, the isotherm of the as-synthesized SS-Ti-700 was a
combination of two distinct regions. At low relative pressures
between 0.2 and 0.6, the high adsorption isotherm indicated
formation of a mesopore structure. However, at high relative
pressures between 0.6 and 1.0, the curve exhibited a hysteresis
loop indicating the presence of macropores (Fig. 1B).5,23 The
pore size distributions further suggested the intrinsic bimodal-
pore properties of the SS-Ti-700. The distribution consisted of
ne pores within individual TiO2 nanoparticles in the meso-
pores region and larger pores in the macropores region, which
represent the spaces between the TiO2 nanoparticles and the
large pores originally present in the sewage sludge-based
templates, respectively (Fig. 1C), thereby indicating the truly
meso-/macroporous composite property of the material.23 The
specic BET surface area and total pore volume of the SS-Ti-700
were 35.46 m2 gꢂ1 and 0.10 cm3 gꢂ1, respectively (Table S1†).
Structural characterization of the as-synthesized catalysts
XRD was used to investigate the phase structure of the as-
prepared catalysts (Fig. 2A). The phase structure of the
obtained TiO2 was well matched with that of the anatase TiO2
(JCPDS le no. 21-1272) while the SS-Ti-700 displayed two
obvious additional diffraction peaks at 2q ¼ 20.9ꢁ and 26.7ꢁ,
corresponding to the typical crystallite structures of SiO2
(quartz) originating from the sewage sludge.24 The amount of Fe
content in sewage sludge is signicant, while the other heavy
metal content was relatively low (Table 1). Most of the heavy
metals were dissolve into the supernatant from the sewage
sludge by HCl, which might work as the dopant to construct the
visible-light-driven TiO2 photocatalyst. It should be noted that
there are still some undissolved Fe, Al, Mg, Ca, and Cu
remained in the sewage sludge (Table 1), which would partially
make a contribution to the content of the SS-Ti-700 acting as the
scaffold templates. The elemental composition of the control
sample (Table 1), SS-0-700, which was obtained by the same
synthesized procedure of catalyst with no TiOSO4$2H2O added,
couple with the present of minerals, like SiO2 (JCPDS, le no.
33-1161); staneldite (Ca4(Mg,Fe)5(PO4)6) (JCPDS, le no. 20-
0223), muscovite ((K,Na)Al2(Si,Al)4O10(OH)2) (JCPDS, le no. 34-
0175), anorthite (Ca(Al2Si2O8)) (JCPDS, le no. 71-0788) and
hematite (a-Fe2O3) (JCPDS, le no. 84-0306) (Fig. S2C†), further
conrmed the existence of these undissolved element.
Fig. 1 Textural properties of the as-synthesized catalysts. SEM image
(A) of the as-prepared SS-Ti-700. N2 adsorption–desorption
isotherms (B) and pore size distributions (C) of the SS-700, TiO2, and
SS-Ti-700. A represents absorption and D represents desorption.
In comparison with the TiO2 P25 which only trace amount
of contamination was observed, the composition of the as-
synthesized SS-Ti-700 was more complex (Table 1). The
area of the as-synthesized catalyst compared with that of the SS- 0.58% Fe/Ti (w/w%) ratio of the as-synthesized catalyst
700 (Table S1†). suggest the probable impregnation of the Fe species. The C,
The representative N2 adsorption-desorption results for H, Al, Ca, Mg, and Na contents of the SS-Ti-700 were attrib-
TiO2, SS-700, and SS-Ti-700 (Fig. 1B) showed characteristic type uted to the remaining organic and inorganic material and
IV curves with sharp capillary condensation steps, which are bacterial cells in the sewage sludge acting as the scaffold
typical features of mesoporous solids. TiO2 showed a sharp templates, while the Si content suggested the probable
upward type H3 hysteresis loop at P/Po > 0.8, indicative of the formation of the SiO2–TiO2 nanostructure. The observed
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The EPR spectroscopy of the control sample SS-0-700 shows
two signals at g ¼ 4.28 and 2.35 (Fig. 2B). The weak signal at g ¼
4.28, representing isolated Fe3+ ions in a distorted tetrahedral
coordination, was assigned to the surface-Fe3+ ions dispersed
on the surface of the material. The broad signal at g ¼ 2.35,
assigned to iron oxide aggregates, indicated the existence of
undissolved Fe content in the scaffold templates (Table 1,
Fig. 2A and B). With no obviously signal was observed for the
TiO2, several new signals were obtained on the EPR spectros-
copy of the as-synthesized photocatalyst SS-Ti-700. The obvious
signal at g ¼ 1.99, which was attributed to Fe3+ substituted Ti4+
in the TiO2 lattice,26,27 clearly evidenced the in situ doped of TiO2
with Fe3+ originate from the sewage sludge. In consideration of
the Cu and Cr content of SS-Ti-700, the slight signals at g ¼ 2.10
and g ¼ 1.94, which can be assigned to Cu2+ ions28,29 and Cr6+
ions30 at substitutional sites of TiO2 respectively, indicated the
in situ Cu2+ and Cr6+ doped of TiO2 (Fig. 2B). In this case the
dopants that are responsible for the visible light activity of the
as-synthesized SS-Ti-700 were identied as Fe3+, Cu2+, and Cr6+
ions originate from the sewage sludge directly with Fe3+ ions as
the major dopant. Different from the Fe3+ (0.69 A) and Cr6+ (0.58
˚
4+
˚
˚
A) ions which are most likely substituted in the Ti (0.74 A)
sites within TiO2, the Cu2+ (0.87 A) ions are most likely located
˚
in interstitial positions of the lattice because of the relatively
large size.27–30
To ascertain the photocatalytic activity of the catalyst in the
visible range, the diffuse reectance UV-vis absorption was
measured to investigate its optical response to visible light
irradiation (Fig. 2C). Compared with the pristine TiO2, the
distinct new absorption shoulder, which ranged from 400 to 600
nm, was probably due to the charge transfer of the in situ Fe, Cu,
and Cr doping dissolution from the sewage sludge by HCl.10,27–30
This result indicates that it is possible to use SS-Ti-700 as a
visible light photoactive catalyst for organic pollutant degrada-
tion. The band gap was estimated by employing a modied
Kubelka–Munk function.5 The band gap of TiO2 was calculated
to be 3.2 eV, which is in good agreement with the reported
values for anatase phase. The band gap of the as-synthesized
catalyst was 2.85 eV, further conrmed the framework incor-
poration of these transition metal ions.
XPS analysis was performed to further probe the atomic
composition and mass fraction of the as-synthesized catalyst
(Fig. 3A). The binding energies were calibrated with respect to
the C 1s peak at 284.6 eV. Obvious C 1s, O 1s, Ti 2p, Si 2p, and P
2s peaks and ne Fe 2p and Cr 2p peaks were all observed as
expected. As the positions of the elemental peaks depend on the
local chemical environment, high resolution scans of C, O, and
Ti were deconvoluted to obtain the corresponding functional
groups by searching for the optimal combination of Gaussian
bands (Fig. S3,† 3B and C). The four C 1s peaks, corresponding
to the C–O, C]C, C–C, and C]O species, respectively
(Fig. S3A†), along with the types of oxygen specie corresponding
to C–O (533.6 eV) and C]O (534.2 eV), all attributed to the
remaining organic material and bacterial cells in the sewage
sludge acting as the scaffold templates.31 The peaks at 529.6 and
532.8 eV were attributable to the oxygen in Ti–O–Ti and the
Si–O–Si linkage, respectively, while the peak at 531.7 eV further
Fig. 2 Characterization of the SS-700, SS-0-700, TiO2, and as-
synthesized SS-Ti-700. (A) XRD spectrum (A represents anatase and H
represents hematite). (B) EPR spectroscopy of the SS-0-700, SS-Ti-
700 and TiO2. (C) Diffuse reflectance UV-vis spectra.
shoulder peak at 952 cmꢂ1 on the FTIR spectra of SS-Ti-700
(Fig. S2D†), corresponding to a Si–O–Ti linkage,25 further
indicate the formation of Si–O–Ti binding between the SiO2
in the sewage sludge and the loaded TiO2 nanoparticles in
the as-synthesized SS-Ti-700. It is worth noting that a trace
amount of Cr and Cu were also observed.
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Table 1 Elemental composition of sewage sludge, the supernatant, the as-synthesized sewage sludge-derived catalysts and the P25
Paper
Element conc. (wt%)
SS-105
Supernatanta
SS-Ti-700
P25
SS-0-700
Solutionb
C
H
N
Fe
Si
32.29
4.33
4.62
7.83
5.75
2.54
0.74
0.48
2.34
0.21
0.10
0.04
0.09
0.08
0.27
0.38
0.43
<0.3
0.28
6.65
0.51
1.45
0.08
0.60
48.4
0.02
0
<0.3
<0.3
<0.3
0.01
0
0.2
0.5
0.01
0.23
0.46
0.60
<0.3
0.44
30.53
2.11
5.85
0.68
3.93
0.08
0
7.75
0
0
0
0.02
0
0.01
0
0
0
Al
2.25
0.48
0.46
1.89
0.18
0.10
0.04
0.09
0.06
0.27
Na
Mg
Ca
Ti
Cr
Mn
Ni
Cu
Zn
59.4
0
0
0
0
0
0
0
0.06
0
0
0.06
0
0
0
0
a
The supernatant was obtained by the same procedure of catalyst synthesize with no TiOSO4$2H2O added. In brief, 10 g of dewatered sewage sludge
and 5 ml of HCl were added to 30 ml of deionized water. Aer being stirred overnight at room temperature, the supernatant was obtained by
centrifugation. The concentration of the elemental in the supernatant was converted to wt% (elemental content/weight of SS-105 ꢄ 100%) for
b
comparable to that of the SS-105. The solution in the photocatalytic reactor aer the photocatalytic reaction of SS-Ti-700 for 10 h under visible
light irradiation conditions.
conrmed the formation of the Si–O–Ti bond within the the addition of the catalyst while the p-nitrophenol removal
SiO2–TiO2 nanostructure.13
efficiency was 2.36 ꢃ 0.15%, 16.24 ꢃ 1.28%, and 21.21 ꢃ 1.21%
For Ti 2p, two predominant peaks at 464.4 and 458.8 eV for the SS-700, as-synthesized TiO2, and TiO2 P25, respectively. A
corresponded to the characteristic Ti 2p1/2 and Ti 2p3/2 peaks of removal efficiency of 92.87 ꢃ 2.23% was achieved for the SS-Ti-
Ti4+, respectively (Fig. 3C).10 The slight inconformity between 700, accompanied with a mineralization efficiency of 47.28 ꢃ
the deconvoluted and pristine peaks at higher binding energies 1.75%, indicating the remarkably high visible light-driven
may indicate the different chemical environment of the tita- photoactivity of the as-synthesized catalyst. Benzoquinone,
nium cations in the as-synthesized catalyst. It is worth noting 1,2,4-benzenetriol, hydroquinone, and p-nitrocatechol were
that a peak at around 579.5 eV, intrinsically resulting from the identied as the main intermediates in the p-nitrophenol
Cr6+ species, was observed in the samples (Fig. S3B†).30 This degradation process (Fig. S4†). The effectively catalytic ability of
suggests that the shis in the peak position for Ti 2p may have the SS-Ti-700 both under UV and solar light irradiation was also
resulted from the formation of Fe3+ and Cr6+ species on the as- observed (Fig. S5†). The photoelectric conversion property of
synthesized TiO2 nanoparticles, which are highly benecial the photocatalyst materials conrmed that the metal-ion
for the promotion of visible light-induced photocatalytic doping, which is responsible for the visible light activity of
activity.10,27–30
the as-synthesized SS-Ti-700, can also inhibit the recombination
of the photogenerated electrons and holes (Fig. S7†).
With the addition of Cr(VI) as the electron scavenger or
methanol as the hydroxyl radical (OHc) scavenger,32,33 the p-
nitrophenol removal efficiency decreased to 27.48 ꢃ 0.84% and
11.87 ꢃ 0.95%, respectively (Fig. 4B), implying that the photo-
generated electrons, superoxide radical (O2cꢂ), and OHc, may
have participated in the SS-Ti-700 photocatalysis process. The
excellent long-term stability of the as-synthesized SS-Ti-700
catalyst was demonstrated by the lack of any obvious deactiva-
tion of it photoactivity in the six repeated experiments
(Fig. S3C†). No metal ion leaching was detected during the SS-
Ti-700 photocatalysis process (Table 1).
Fig. 4C illustrates the DMPO spin-trapping ESR spectroscopy
of aqueous solution in the presence of catalyst SS-Ti-700 under
various conditions. While no signal of DMPO-OHc adducts
characterized by intensity ratio of 1 : 2 : 2 : 1 could be observed
in the presence of catalyst SS-Ti-700 only (a), it became obvious
upon visible light irradiation (b), indicated that OHc radicals are
Photocatalytic degradation of p-nitrophenol under visible
light irradiation
p-Nitrophenol degradation tests were conducted under visible
(l > 400 nm) light irradiation conditions to conrm the catalytic
activity of SS-Ti-700. Fig. 4A illustrates the UV-vis absorption
spectra of p-nitrophenol in aqueous solution during a typical
degradation process. The decrease of the peak intensity at 317
nm is clear evidence of its continuous effective degradation. The
decline of the peak at 226 nm, which was assigned to p / p*
from the ring of phenol, indicated the destruction of the
aromatic rings. With the remarkable reduction in the absorp-
tion spectra at 200 nm during the degradation process, high
mineralization was expected.
The concentration change proles of the p-nitrophenol
degradation versus time under different conditions within 10 h
are depicted in Fig. 4B. No obvious p-nitrophenol removal was
detected under the visible light irradiation conditions without
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Fig. 3 X-ray photoelectron spectra of the as-synthesized SS-Ti-700
(A). The high resolution O 1s and Ti 2p XPS spectra from the catalyst are
shown in (B) and (C), respectively.
Fig. 4 Photocatalytic activity of the as-synthesized catalysts. (A) UV-
vis spectra of p-nitrophenol in a typical degradation process under
visible light irradiation conditions. (B) The relative concentration
profiles of p-nitrophenol and TOC during typical degradation
processes under various conditions. (C) DMPO spin-trapping ESR
spectroscopy of aqueous solution in the presence of catalyst SS-Ti-
the active species involved in photocatalytic reaction of SS-Ti-
700 under visible light irradiation conditions. The intensity of
DMPO-OHc signal further increased in the present of UV light 700 without UV/vis irradiation (a), with visible irradiation after 60 s (b),
and with UV and visible irradiation after 60 s (c). (Vis: with visible light
irradiation only SS-700: with SS-700 added TiO2: with as-synthesized
irradiation (c), which was in consist with the observed degra-
dation rates of p-nitrophenol under these conditions, indicated
the efficiency of metal-ion doping to inhibit the recombination
of the photogenerated electrons and holes.27–29
TiO2 added P25: with TiO2 P25 added SS-Ti-700: with SS-Ti-700
2ꢂ
added Cr(VI): with SS-Ti-700 added and 0.1 mM Cr2O7 dosed as an
electron scavenger and methanol: with SS-Ti-700 added and 5 ml
methanol dosed as a hydroxide radical scavenger).
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The probably mechanism of the as-synthesized catalyst
The dopants that are responsible for the visible light activity of
the as-synthesized SS-Ti-700 were identied as Fe3+, Cu2+, and
Cr6+ ions originate from the sewage sludge directly. The Fe3+
ions, as the major dopant with a larger Fe/Ti (w/w%) ratio, has
the greatest effect on the photoactivity of the SS-Ti-700 (Table 1,
Fig. 2B). As conrmed by the diffuse reectance UV-vis spectra
of the as-synthesized catalyst, the doped transition metals
would lower the band gap energy by introducing trap levels
between the valence and conduction bands of TiO2.27–30 These
trap levels are known to contribute to the excitation of electrons
Fig. 5 Schematic diagram of the as-prepared SS-Ti-700 and the
when subjected to visible light illumination. Thus the probably mechanism of the photocatalytic reaction under visible light irradiation
conditions.
mechanism of the as-synthesized catalyst in the visible light
region is due to the reduction of the band gap. Under visible
light irradiation, no electron can be excited for pure TiO2. For
during the synthesized process. The Al, Ca, Mg, and Na content
of the as-synthesized catalyst, except for acting as the scaffold
templates, may hardly have any effect on the observed photo-
reactivity34 (for details, see the ESI†).
the as-synthesized catalyst, the 3d electrons of the in situ Fe, Cu,
and Cr doping can be excited and injected from the excited
impurity level of these doped metals ions, mainly ꢂFe3+, to the
conduction band of the TiO2 particles.27–30 The O2c generated
by the reaction of O2 with the injected photogenerated electrons
(eqn (1)), and its subsequent product OHc, react with the p-
nitrophenol and produce the corresponding minerals, hence
degrading the organic pollution.
In the presence of Cr(VI) as the electron scavenger, the pho-
togenerated electrons on the conduction band of the TiO2
particles would be scavenged. Then no O2cꢂ could generate by
the reaction of O2 with the injected photogenerated electrons
(eqn (1)). And its subsequent product OHc, which was respon-
sible for the p-nitrophenol degradation, could not generate.32,33
The ESR spectroscopy and the removal efficiency of p-nitro-
phenol decreased in the presence of Cr(VI) and methanol all
indicated that OHc, subsequently product by the photo-
generated electrons and holes (eqn (2) and (3)), was predomi-
nantly responsible for the p-nitrophenol degradation in the SS-
Ti-700 photocatalysis process under visible light irradiation
conditions.
Signicance in application
Compared to the pristine TiO2, the as-synthesized TiO2-based
photocatalyst SS-Ti-700 with the designed meso-/macroporous
composite has several structural advantages. First, its reac-
tively high specic surface area results in a high adsorption
capability for organic pollutants. Second, the nice bimodal-pore
mesoporous structure facilitates the inside-adsorption of
pollutants, thus increasing the likelihood of the pollutants
diffusing to the surface of the TiO2 nanoparticles embedded in
the structure of the meso-/macroporous composite. Third, this
special mesoporous structure also promotes the outside-
diffusion of the products from the photocatalytically active
sites during photocatalytic reaction.14,23 All of the above are
favorable to enhancing photocatalytic activity.
Research on the disposal and reuse of sewage sludge as an
alternative material has attracted broad interest. However, the
existing reuse approaches may suffer from the probable leach-
ing of heavy metals.18,35 This study shows that this barrier can be
partially overcome by the in situ heavy metals doping to enhance
TiO2 photocatalytic activity under visible-light irradiation. The
as-prepared SS-Ti-700 exhibited long-term stability and excel-
lent photocatalytic performance. The major potential toxicity
issue during our synthesization process may be in the super-
natant during the thoroughly washed step which contained the
residual fraction of the metals cations (Table 1). However taking
the various metal cations with special functions (e.g., the Fe
based Fenton catalyst,24 the Mg based CO2 trapper,36 and the Zn
and Cu based methanol synthesis catalyst37) involved in the
sewage sludge and the supernatant into consideration, our
approach may pave the way for the direct in situ fabrication of
various sewage sludge-supporting mesoporous metal-based
functional materials. In fact, the residual fraction of the heavy
metals in the supernatant during our synthesization process
has also been reused as dopant in our lab.
O2 + eꢂ / O2cꢂ
h+ + OHꢂ / OHc
(1)
(2)
(3)
h+ + H2O / OHc + H+
According to the structural characterization of the as-
synthesized catalyst, its photocatalytic degradation of p-nitro-
phenol in the presence of electron and OHc scavengers, and the
EPR and ESR spectroscopy, the reaction mechanism of the SS-
Ti-700 photocatalytic system is proposed and schematically
illustrated in Fig. 5. The HOc was predominantly responsible for
the p-nitrophenol degradation, while the O2cꢂ itself was not
remarkable.
The identied intermediates in the p-nitrophenol degrada-
tion process indicated that the pathway of OHc react with
p-nitrophenol seems to occur in two steps (Fig. S6†). The carbon
in the sewage sludge, mainly in the form of biomacromolecules
and will carbonize or combust during the synthesized process,
could not act as the suitable C precursor to obtain C-doped TiO2
Our proposed approach for utilizing sewage sludge has
several additional advantages: (1) the sewage sludge can play
61042 | RSC Adv., 2014, 4, 61036–61044
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multiple roles in constructing a meso-/macroporous composite, sludge. Kinetic analysis shows that the SS-Ti-700 exhibits a
which is crucial for photocatalytic applications because it more rapid p-nitrophenol degradation at a rate ve times of the
enables the efficient transport of the reactant molecules and sum rate by the SS-700 and the as-synthesized TiO2 under
their products,14,23 avoiding macroscopic aggregates of the TiO2 visible light irradiation.
nanoparticles owing to this meso-/macroporous support and
the carbonization and combustion of the organic materials in
the sewage sludge,14 and the in situ metals doping to enhance
the photocatalytic activity under visible-light irradiation; (2)
SiO2, the special component of sewage sludge that evidently
formed the SiO2–TiO2 nanostructure through the formation of
the Si–O–Ti bonds in the SS-Ti-700, has been demonstrated to
be able to improve the photocatalytic efficiency of TiO2 with
regard to band gap changes, the generation of surface acid sites,
and increased adsorption;13 and (3) almost all of the content of
the sewage sludge was properly used for the synthesis of this
efficient and stable photocatalyst. Compared with the existing
approaches using commercial reagents as the support and
dopant which may lead to environmental concerns or cost
increases, by using sewage sludge, our protocol provides a
simple, low-cost and environmentally friendly route for the
synthesis of a visible-light-driven TiO2-based photocatalyst.
Owe to these poorly dened chemicals the sewage sludge
possessing, sewage sludge disposal is one of the most pressing
problems related to water treatment plants currently and
therefore the initiative to reuse of sludge is certainly necessary.
Although the composition of sewage-sludge and its heavy-metal
content would vary from one wastewater treatment plants to
another, sewage sludge are usually composed of all these types
of chemicals which were properly provided for the synthesis of
this efficient and stable catalyst in our synthetic method.17,21
Thus the sewage sludge we used in this work might be taken as
a representative materials. For a special sewage sludge, the
content of sewage sludge used in this synthetic method need to
Acknowledgements
The authors wish to thank the National Natural Science Foun-
dation of China (51278358, 51308401) and the China Post-
doctoral Science Foundation Funded Project (2013M530209,
2014T70431) for their partial support of this study.
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