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

Abstract

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.

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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 photoresponse 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

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 modification with various organic and inorganic species,^7,11 immobilization on mesoporous supports to present a high specific 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 modification 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 modification approaches may lead to cost increases or environmental 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 defined 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 variety of heavy metals.¹⁶ Accompanied with the imposition of more stringent regulations governing the disposal and use of sewage sludge, many of the traditionally accepted disposal routes have been in decline or lost, mainly due to the presence of organic pollutants and heavy metals.^17,18 This situation has been further aggravated by the continuous increase in the generation of sewage sludge around the world, with the annual production in China, the E.U., and the U.S. now exceeding 30 million, 10 million, and 5 million dry tons, respectively.^19–21 In this case, the need to develop more cost effective and environmentally benign reuse of sewage sludge is of particular concern. Considering the silica and wide variety of heavy metals present in sewage sludge, and the fact it can be converted into mesoporous adsorbents,¹⁶ it is expected that the sludge can be used 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 photocatalyst, which was implemented for the degradation and mineralization of the organic pollutant, p-nitrophenol, as the target model pollutant under visible (λ > 400 nm) light irradiation 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 only exhibited as the structure-directing templates with a high surface area but also served as the source of metal dopant, which led to a significant enhancement in the visible light absorption. The stability of the as-synthesized catalyst and its possible photocatalytic mechanism were also investigated. The results of our study provide a novel route for the synthesis of a visible-light-driven TiO₂-based photocatalyst using a simple, low-cost, and green process, and demonstrate a new approach for the potential environmentally benign reuse of sewage sludge.

Experimental

Preparation of the catalysts

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, China) was used as received. All other reagents were of analytical grade unless otherwise stated. All of the water used was prepared using a purification system (Hitech Instrument Co., Shanghai, China).

The sewage sludge support and in situ doped TiO₂ photocatalyst was synthesized using a facile one-step hydrothermal process with TiOSO₄·2H₂O as a precursor. In brief, 10 g of dewatered sewage sludge and 5 g of TiOSO₄·2H₂O were added to 30 ml of deionized water. 5 ml of HCl, which was used to enhance the dissolution of inorganic fraction, especially some heavy metals, from the sewage sludge, was also added. After being stirred overnight at room temperature, the obtained solution was transferred to a 100 ml Teflon autoclave and heated at 150 °C for 12 h in an oven. After being cooled to the ambient temperature, the resultant precipitate was recovered via centrifuging, washed thoroughly with deionized water, and then dried in air at 105 °C overnight. Finally, the dried solid was calcined at 700 °C in a muffle furnace for 5 h, and the sewage sludge-derived TiO₂-based photocatalyst was obtained and designated as SS-Ti-700. To ensure its powder form, the dried solid after calcining was ground to a fine powder with an agate mortar and pestle and filtration through a 400 mesh metal sieve. TiO₂ was also synthesized using the same procedure with no sewage sludge added and SS-700 was obtained by calcining sewage sludge at 700 °C in a muffle furnace for 5 h. The textural properties of the as-synthesized catalysts was characterized (for details, see the ESI†).

Photocatalytic organic pollutant degradation

The photocatalytic activity of the as-prepared catalyst was evaluated 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 in a quartz glass cylinder (8.5 cm in height and 5.5 cm in diameter) filled with 150 ml of p-nitrophenol (35 mg l⁻¹) solution under constant stirring (Fig. S1A†). A Philips 100 W 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 fixed next to the cylinder with 2 M NaNO₂ acting as both the cooling jacket and cutoff filter (λ > 400 nm).²² The reactions were kept at a constant temperature of 25 °C via an air conditioner (for details, see the ESI†).

The concentrations of p-nitrophenol were measured from the absorbance change at a wavelength of 317 nm with a UV-vis spectroscope (PhotoLab 6100, WTW Co., Germany). The total organic carbon (TOC) concentration was measured with a TOC analyzer (TOC-L CPH CN 200, Shimadzu Co., Japan). The intermediates of p-nitrophenol degradation were determined by 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 flow rate of 1.0 ml min⁻¹. All of the tests were conducted in triplicate.

Results and discussion

Textural properties of the as-synthesized catalysts

The surface morphology and structure of the as-synthesized catalyst SS-Ti-700 were examined using SEM and the typical images are shown in Fig. S2† and 1A. In the low magnification images (Fig. S2†), numerous particles with nanometer sized diameter were stacked and a porous structure that increased the surface area of the catalyst was displayed. The relatively high magnification image showed small islands with diameters range from about 60 to 600 nm deposited on the surface of the mesoporous scaffold templates derived from the sewage sludge (Fig. 1A). The rough mesoporous structures of the scaffold templates were attributable to the combustion, carbonization, and evaporation of the organic material, bacterial cells, and adsorbed H₂O in the sewage sludge, the SiO₂ fraction, and especially the partial dissolved of the inorganic fraction from the sewage sludge by the HCl added during the synthesization process.¹⁶ This was further evident by the increased BET surface area of the as-synthesized catalyst compared with that of the SS-700 (Table S1†).

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.

The representative N₂ adsorption-desorption results for TiO₂, SS-700, and SS-Ti-700 (Fig. 1B) showed characteristic type IV curves with sharp capillary condensation steps, which are typical features of mesoporous solids. TiO₂ showed a sharp upward type H3 hysteresis loop at P/P_o > 0.8, indicative of the 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 fine pores within individual TiO₂ nanoparticles in the mesopores 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 specific BET surface area and total pore volume of the SS-Ti-700 were 35.46 m² g⁻¹ and 0.10 cm³ g⁻¹, 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 file no. 21-1272) while the SS-Ti-700 displayed two obvious additional diffraction peaks at 2θ = 20.9° and 26.7°, corresponding to the typical crystallite structures of SiO₂ (quartz) originating from the sewage sludge.²⁴ The amount of Fe content in sewage sludge is significant, 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, file no. 33-1161); stanfieldite (Ca₄(Mg,Fe)₅(PO₄)₆) (JCPDS, file no. 20-0223), muscovite ((K,Na)Al₂(Si,Al)₄O₁₀(OH)₂) (JCPDS, file no. 34-0175), anorthite (Ca(Al₂Si₂O₈)) (JCPDS, file no. 71-0788) and hematite (α-Fe₂O₃) (JCPDS, file no. 84-0306) (Fig. S2C†), further confirmed the existence of these undissolved element.

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.

Table 1 Elemental composition of sewage sludge, the supernatant, the as-synthesized sewage sludge-derived catalysts and the P25

Element conc. (wt%)	SS-105	Supernatant^a	SS-Ti-700	P25	SS-0-700	Solution^b
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. After 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 × 100%) for comparable to that of the SS-105.b The solution in the photocatalytic reactor after the photocatalytic reaction of SS-Ti-700 for 10 h under visible light irradiation conditions.
C	32.29		0.38	<0.3	0.46
H	4.33		0.43	<0.3	0.60
N	4.62		<0.3	<0.3	<0.3
Fe	7.83	7.75	0.28	0.01	0.44	0
Si	5.75		6.65	0	30.53	0
Al	2.54	2.25	0.51	0.2	2.11	0
Na	0.74	0.48	1.45	0.5	5.85	0.02
Mg	0.48	0.46	0.08	0.01	0.68	0
Ca	2.34	1.89	0.60	0.23	3.93	0.01
Ti	0.21	0.18	48.4	59.4	0.08	0
Cr	0.10	0.10	0.02	0	0	0
Mn	0.04	0.04	0	0	0	0
Ni	0.09	0.09	0	0	0	0
Cu	0.08	0.06	0.06	0	0.06	0
Zn	0.27	0.27	0	0	0	0

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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 0.58% Fe/Ti (w/w%) ratio of the as-synthesized catalyst suggest the probable impregnation of the Fe species. The C, H, Al, Ca, Mg, and Na contents of the SS-Ti-700 were attributed to the remaining organic and inorganic material and bacterial cells in the sewage sludge acting as the scaffold templates, while the Si content suggested the probable formation of the SiO₂–TiO₂ nanostructure. The observed shoulder peak at 952 cm⁻¹ 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.

The EPR spectroscopy of the control sample SS-0-700 shows two signals at g = 4.28 and 2.35 (Fig. 2B). The weak signal at g = 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 = 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 spectroscopy of the as-synthesized photocatalyst SS-Ti-700. The obvious signal at g = 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 = 2.10 and g = 1.94, which can be assigned to Cu²⁺ ions^28,29 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 identified 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 Å) and Cr⁶⁺ (0.58 Å) ions which are most likely substituted in the Ti⁴⁺ (0.74 Å) sites within TiO₂, the Cu²⁺ (0.87 Å) 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 reflectance 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 degradation. The band gap was estimated by employing a modified 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 confirmed the framework incorporation 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 fine 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 confirmed the formation of the Si–O–Ti bond within the SiO₂–TiO₂ nanostructure.¹³

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.

For Ti 2p, two predominant peaks at 464.4 and 458.8 eV corresponded to the characteristic Ti 2p_1/2 and Ti 2p_3/2 peaks of Ti⁴⁺, respectively (Fig. 3C).¹⁰ The slight inconformity between the deconvoluted and pristine peaks at higher binding energies may indicate the different chemical environment of the titanium cations in the as-synthesized catalyst. It is worth noting that a peak at around 579.5 eV, intrinsically resulting from the Cr⁶⁺ species, was observed in the samples (Fig. S3B†).³⁰ This suggests that the shifts in the peak position for Ti 2p may have resulted from the formation of Fe³⁺ and Cr⁶⁺ species on the as-synthesized TiO₂ nanoparticles, which are highly beneficial for the promotion of visible light-induced photocatalytic activity.^10,27–30

Photocatalytic degradation of p-nitrophenol under visible light irradiation

p-Nitrophenol degradation tests were conducted under visible (λ > 400 nm) light irradiation conditions to confirm 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 π → π* from the ring of phenol, indicated the destruction of the aromatic rings. With the remarkable reduction in the absorption spectra at 200 nm during the degradation process, high mineralization was expected.

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-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 TiO₂ added P25: with TiO₂ P25 added SS-Ti-700: with SS-Ti-700 added Cr(VI): with SS-Ti-700 added and 0.1 mM Cr₂O₇²⁻ dosed as an electron scavenger and methanol: with SS-Ti-700 added and 5 ml methanol dosed as a hydroxide radical scavenger).

The concentration change profiles 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 the addition of the catalyst while the p-nitrophenol removal efficiency was 2.36 ± 0.15%, 16.24 ± 1.28%, and 21.21 ± 1.21% for the SS-700, as-synthesized TiO₂, and TiO₂ P25, respectively. A removal efficiency of 92.87 ± 2.23% was achieved for the SS-Ti-700, accompanied with a mineralization efficiency of 47.28 ± 1.75%, indicating the remarkably high visible light-driven photoactivity of the as-synthesized catalyst. Benzoquinone, 1,2,4-benzenetriol, hydroquinone, and p-nitrocatechol were identified as the main intermediates in the p-nitrophenol degradation process (Fig. S4†). The effectively catalytic ability of the SS-Ti-700 both under UV and solar light irradiation was also observed (Fig. S5†). The photoelectric conversion property of the photocatalyst materials confirmed that the metal-ion doping, which is responsible for the visible light activity of 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 (OH˙) scavenger,^32,33 the p-nitrophenol removal efficiency decreased to 27.48 ± 0.84% and 11.87 ± 0.95%, respectively (Fig. 4B), implying that the photogenerated electrons, superoxide radical (O₂˙⁻), and OH˙, 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 deactivation 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-OH˙ 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 OH˙ radicals are the active species involved in photocatalytic reaction of SS-Ti-700 under visible light irradiation conditions. The intensity of DMPO-OH˙ signal further increased in the present of UV light irradiation (c), which was in consist with the observed degradation 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

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 identified 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 confirmed by the diffuse reflectance 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 when subjected to visible light illumination. Thus the probably 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 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 Fe³⁺, to the conduction band of the TiO₂ particles.^27–30 The O₂˙⁻ generated by the reaction of O₂ with the injected photogenerated electrons (eqn (1)), and its subsequent product OH˙, 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 photogenerated electrons on the conduction band of the TiO₂ particles would be scavenged. Then no O₂˙⁻ could generate by the reaction of O₂ with the injected photogenerated electrons (eqn (1)). And its subsequent product OH˙, which was responsible for the p-nitrophenol degradation, could not generate.^32,33 The ESR spectroscopy and the removal efficiency of p-nitrophenol decreased in the presence of Cr(VI) and methanol all indicated that OH˙, subsequently product by the photogenerated electrons and holes (eqn (2) and (3)), was predominantly responsible for the p-nitrophenol degradation in the SS-Ti-700 photocatalysis process under visible light irradiation conditions.

O₂ + e⁻ → O₂˙⁻ (1)

h⁺ + OH⁻ → OH˙ (2)

h⁺ + H₂O → OH˙ + H⁺ (3)

According to the structural characterization of the as-synthesized catalyst, its photocatalytic degradation of p-nitrophenol in the presence of electron and OH˙ 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 HO˙ was predominantly responsible for the p-nitrophenol degradation, while the O₂˙⁻ itself was not remarkable.

Fig. 5 Schematic diagram of the as-prepared SS-Ti-700 and the mechanism of the photocatalytic reaction under visible light irradiation conditions.

The identified intermediates in the p-nitrophenol degradation process indicated that the pathway of OH˙ 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 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 photoreactivity³⁴ (for details, see the ESI†).

Significance 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 reactively high specific 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 leaching 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 excellent photocatalytic performance. The major potential toxicity issue during our synthesization process may be in the supernatant 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.

Our proposed approach for utilizing sewage sludge has several additional advantages: (1) the sewage sludge can play multiple roles in constructing a meso-/macroporous composite, which is crucial for photocatalytic applications because it enables the efficient transport of the reactant molecules and their products,^14,23 avoiding macroscopic aggregates of the TiO₂ 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 defined 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 be adjusted finely according to its composition to obtain photocatalyst with the highest photocatalytic activity.

Considering the huge volumes and continuous increase in the generation of sewage sludge around the world, our approach might not seem to be a viable way for large-scale disposing of it at least at the present time. The development of methods for large-scale disposing of sewage sludge is appreciated, however it doesn't mean that the initiative of eco-friendly sewage sludge reusing approach should be totally rejected. It is suggested that our approach might deserves particular attention for the eco-friendly reusing of sewage sludge, at least for the resource utilization of the heavy metals, which are the critical toxic compounds in sewage sludge.

Conclusions

We synthesized an effective visible-light-driven TiO₂-based photocatalyst, SS-Ti-700, with a designed meso-/macroporous composite using sewage sludge as both the scaffold template and dopant. The intrinsic bimodal-pore property of the SS-Ti-700 was attributable to the mesoporous assembly of the content of the sewage sludge while the efficiency of the visible-light-driven photocatalysis was achieved by in situ doping with heavy metals, which are the critical toxic compounds in sewage sludge. Kinetic analysis shows that the SS-Ti-700 exhibits a more rapid p-nitrophenol degradation at a rate five times of the sum rate by the SS-700 and the as-synthesized TiO₂ under visible light irradiation.

Acknowledgements

The authors wish to thank the National Natural Science Foundation of China (51278358, 51308401) and the China Postdoctoral Science Foundation Funded Project (2013M530209, 2014T70431) for their partial support of this study.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra12434k

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