2-sec-Butyl-4, 6-dinitrophenol (DNBP), one of the most important alkyl dinitrophenols, is widely used in the petrochemical industry as a polymerization inhibitor for vinyl aromatics and in agriculture as a herbicide ^{[1, 2]}. Owing to its high use in industry and agriculture, it is regularly detected in aquatic environments. Although there are benefits to using DNBP, it is now considered to exert negative influences on the environment owing to its toxicity, carcinogenicity and mutagenicity. DNBP is on a US Environmental Protection Agency blacklist for priority pollutants in water ^[3]. Although there has been much research on the removal of nitrophenol contaminants from the environment, there are only a few reports on DNBP degradation. DNBP is highly chemically stable, toxic and persistent in ecosystems ^[4-6]. Thus, removal of DNBP from aqueous solution has attracted considerable research interest.

Research on semiconducting materials that mediate photocatalytic degradation of organic pollutants for environmental remediation is an area of growing interest ^{[7, 8]}. Among semiconductor photocatalysts, TiO₂ is widely used for the destruction of organic pollutants because of its relatively high catalytic reactivity, good physical and chemical stability, low-toxicity and cost-effectiveness. TiO₂ generates electron and hole pairs (e^-/h⁺) upon irradiation of light energy equal to or greater than its band gap energy. If charge separation occurs, the photo-generated electrons and holes may migrate to the catalyst surface and participate in redox reactions with adsorbed species ^{[9, 10]}. However, TiO₂ suffers from a low adsorption capacity, rapid recombination of electron-hole pairs, and the difficulty of recycling the catalyst. These issues have limited the use of TiO₂ nanoparticles in practical applications such as wastewater treatment. Various attempts have been made to enhance the absorption capacity and photocatalytic activity of TiO₂ nanoparticles. TiO₂ supported on porous adsorbent materials is one approach that has drawn much attention. Supported TiO₂ catalysts may allow the pollutant to be concentrated near the TiO₂ particles, mediate adsorption of intermediates formed during degradation, and allow the catalyst to be recovered and reused ^[11-14]. Among support materials for TiO₂, aerogels based on silica and other metal oxides (alumina, zirconia, tungsten oxide, or a mixtures of these oxides) are considered attractive candidates owing to their special physicochemical features, including high specific surface area, porous structure, high thermal stability, easily tunable chemical properties, environmen tally-friendly nature and photo-stable inorganic frameworks ^[15]. Recently, a silica aerogel-supported TiO₂ has been used as a catalyst in photocatalytic processes ^[16]. The physicochemical properties of silica aerogel-supported TiO₂ catalysts depended on their synthesis conditions and the degree of interaction between the TiO₂ and silica aerogel support. The strong electric field present in the framework of these supports can minimize electron-hole recombination and also allows pollutants to be concentrated near the TiO₂ particles ^{[17, 18]}.

Aerogels based on silica and other metal oxides are traditionally prepared via supercritical drying processes using silicate ester, aluminosilica gel and other organometallic compounds as raw materials ^[19-21]. However, the precursors and supercritical drying techniques used for preparing the aerogels are costly and dangerous, which limits the practical and commercial viability of such supports. Fly ash is a coal combustion by-product consisting of an alumina-silica mixture with smaller amounts of iron, calcium, sodium and potassium ^[22].

The objective of our work was to investigate a TiO₂/SiO₂-Al₂O₃ aerogel composite catalyst for degradation of persistent organic contaminants in wastewater. Here, we report the synthesis of SiO₂-Al₂O₃ binary aerogels derived from industrial fly ash, synthesized by an ambient pressure drying method. The prepared SiO₂-Al₂O₃ binary aerogels were used as a support matrix for TiO₂ nanoparticles, to create a composite material with a high adsorption capacity and enhanced photocatalytic activity. The photocatalytic degradation of DNBP over our TiO₂/SiO₂-Al₂O₃ aerogel composite was tested under visible light irradiation.

A representative sample of the fly ash was collected from the Zhungeer coal mine (Inner Mongolia, China). The constituents of the fly ash used in the experiments were as follows: SiO₂ 48.16%, Al₂O₃ 21.68%, Fe₂O₃ 5.64%, TiO₂ 1.12%, CaO 2.04%, MgO 1.08%, K₂O 0.50%, Na₂O 0.12%, MnO₂ 0.02%, SO₃ 0.82%, and P₂O₅ 0.54%. The loss on ignition (LOI) was determined to be 18.22%. DNBP was used as a model alkyl dinitrophenol pollutant and obtained from Retell Fine Chemical Co. Ltd. (Tianjin, China). Tetrabutyl titanate (TBOT, ≥98.0%), polyethylene glycol 400 (PEG-400) and trimethylchlorosilane (TMCS, ≥98.0%) were obtained from Sinopharm Chemical Reagent Co. Ltd., China. All other reagents used in this study, were AR grade obtained from Tianjin Kermel Chemical Reagents Co. Ltd., China. All reagents were used as received without further purification. Double distilled water was used to prepare solutions in the experiments.

Fly ash sample and sodium carbonate were well mixed at a mass ratio of 1:1 and calcined at 900 ℃ for 3 h in a muffle furnace. After cooling to room temperature, a HCl solution (5.0 mol/L) was added to the calcined product and stirred for 3 h at room temperature to produce an aluminosilica-sol. The mixture was then filtered and the homogeneous aluminosilica-sol was collected. The pH of the obtained aluminosilica-sol was adjusted to 10.0-11.0 using a NaOH solution (5.0 mol/L) for gelation. After 30 min the aluminosilica wet gels produced were aged for 6 h at 50 ℃ to strengthen the gel network structure, and then solvent exchanged with absolute ethanol at 50 ℃ for 24 h. Surface modification was performed by immersing the aluminosilica gels in TMCS/ethanol/cyclohexane solutions for 48 h. Surface modification and solvent exchange of the aluminosilica gels were performed simultaneously and the molar ratio of TMCS:ethanol:cyclohexane was 2:5:9. The aluminosilica gels were thoroughly washed with cyclohexane to remove unreacted agents and reaction products and the surface-modified gel samples were left to dry for 24 h. The gels were dried at 80 ℃ in an oven at atmospheric pressure. The resultant aluminosilica gels were stored in a vacuum desiccator prior to further use.

The TiO₂/SiO₂-Al₂O₃ aerogel composite was prepared through an acid-catalyzed sol-gel method. A typical procedure was as follows: 0.093 g of the as-obtained SiO₂-Al₂O₃ aerogel was added to a mixture of 5 mL absolute ethanol, 2 mL acetic acid and 2.5 mL water. The mixture was sonicated for 30 min at room temperature to obtain a suspension. A 5-mL portion of TBOT was added gradually to a mixture of 10 mL anhydrous ethanol and 3 mL acetic acid under continuous stirring to prepare the titanium precursor. The titania-sol was slowly added to the suspension of SiO₂-Al₂O₃ aerogel to initiate the hydrolysis reaction. After complete hydrolysis of the titanium precursor, the colloidal solution was continuously stirred for 2 h until the formation of a TiO₂ gel. After aging for at least 12 h at room temperature, the resulting sample was then dried at 110 ℃ for 12 h, and calcined at 300 ℃ for 1 h and 450 ℃ for 2 h in a muffle furnace under ambient atmosphere. The resultant TiO₂/SiO₂-Al₂O₃ aerogel composite materials were stored in a vacuum desiccator prior to further use.

The X-ray powder diffraction (XRD) patterns of the materials were measured by a Rigaku D/MAX-2000 diffractometer using Cu K_aradiation operating at 30 mA and 40 kV with a scan rate of 2q = 4.8°/min. Fourier transform infrared spectra (FT-IR) of the samples within the 400-4000 cm^-1 frequency range were measured on a Nicolet Avatar 360 FT-IR infrared spectrometer using conventional KBr pellets. The particle size and morphologies of the samples were also determined with a transmission electron microscope (TEM, FEI Tecnai G220S-TWIN, America). The UV-vis diffuse reflectance spectra (UV-vis/DRS) of the catalysts in the range 200-800 nm were measured on a JASCO UV-550 ultraviolet spectrometer equipped with an integrating sphere with BaSO₄ as a reference. The specific surface areas of the catalysts were determined by the BET method with a Micromeritics ASAP 2010 apparatus. All the samples were previously degassed at 350 ℃ for 12 h.

The zero point charge (pH_zpc) of the TiO₂/SiO₂-Al₂O₃ aerogel catalyst was measured with 10-mL portions of NaCl aqueous solution (0.05 mol/L) that were adjusted to pH values in the range 2-11 by either HCl or NaOH solutions (0.1 mol/L) ^[23]. N₂ gas was bubbled through the NaCl solutions to remove dissolved CO₂ until the initial pH stabilized. After determination of the initial pH value, the solutions were mixed with 0.03 g of the catalyst sample for 24 h and the final pH of the solutions was then measured. The pH_zpc value was defined as the pH at which the initial and final pH values were equal. The pH of the solution was measured by using a Mettler-Toledo (model Delta 320-S) digital pH meter.

Photocatalytic degradation of DNBP was performed in a XPA-7 photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China). Cylindrical quartz photochemical reactors of 22 cm x 2 cm (height x diameter) were vertically placed at a fixed distance (3.5 cm) from a 500 W Xenon lamp (Institute of Electric Light Source, Beijing) as light source to mimic solar irradiation. For the irradiation experiments, an aqueous solution of DNBP (40 mL) and the photocatalyst were added to the photoreactor. The resultant mixture was stirred continuously in the dark for at least 2 h to ensure the adsorption/desorption equilibrium of DNBP was established. The suspensions were then irradiated by the xenon lamp with the transmission of wavelengths below 380 nm limited using glass filters, while continuous purging the reactor with air. Aliquots were withdrawn at specific time intervals during the degradation experiments and analyzed after centrifugation for 5 min at 3000 r/min to completely remove any solid particles. The filtrates were subjected to UV-visible spectrophotometric analysis to monitor the concentration of DNBP at various time intervals at a wavelength of 271 nm ^[24]. The mineralization of DNBP was monitored by measuring the total organic carbon (TOC) with an Elementary Analyzer (Multi N/C 2100S, Analytikjena) by direct injection of the aqueous solution after the photocatalysis reaction. The GC/MS system (Agilent 6890 GC and 5793 MS) was used to identify the intermediate products formed during the photooxidative process. The carrier gas flow rate was 0.5 mL/min and the oven temperature was programmed as follows: isothermal at 40 ℃ for 5 min, from 40 to 280 ℃ at 5℃/min, and isothermal at 280 ℃ for 1 min.

Fig. 1 illustrates the XRD patterns of pure TiO₂, SiO₂-Al₂O₃ aerogel, and TiO₂/SiO₂-Al₂O₃ aerogel composite. A series of intensified anatase diffraction peaks were observed in Fig. 1(1). The XRD pattern of the SiO₂-Al₂O₃ aerogel (Fig. 1(2)) showed a broad, weak peak in the range 15°-30°, attributable to amorphous SiO₂. The non-crystalline diffraction peaks of SiO₂ were so broad that peaks from Al₂O₃ were not visible in the pattern. According to Fig. 1(3), the XRD pattern of the TiO₂/SiO₂-Al₂O₃ aerogel composite contained the same characteristic peaks as anatase-type TiO₂, which indicated that the introducing the SiO₂-Al₂O₃ aerogel only slightly weakened the diffraction patterns of pure TiO₂.

Fig. 2 shows FT-IR spectra of the SiO₂-Al₂O₃ aerogel, pure TiO₂ and TiO₂/SiO₂-Al₂O₃ aerogel composite. In these spectra, the broad absorption peaks at 3440 and 1630 cm^-1 were attributed to stretching and bending vibrations of surface absorbed water and hydroxyl groups. The SiO₂-Al₂O₃ aerogel samples (Fig. 2(1)) showed characteristic peaks of Si-O-Si asymmetric and symmetric vibrations at 1080 and 762 cm^{-1 [25]}. The peak at 847 cm^-1 was attributed to a stretching vibration of Al-O-Al. The absorption peaks centered around 2960 and 1258 cm^-1 corresponded to stretching and deformation modes of C-H and Si-C bonds, respectively ^[26]. The absorption bands of the Si-C bond and C-H bonds confirmed the modification of the SiO₂-Al₂O₃ aerogel samples. As shown in Fig. 2(2), the low frequency absorption in the region 500-700 cm^-1 can be assigned to a stretching vibration of Ti-O-Ti. FT-IR spectra of pure TiO₂ and TiO₂/SiO₂-Al₂O₃ aerogel composites showed a similar shape. In Fig. 2(3), the characteristic absorption peaks of the stretching vibrations of Si-O-Si and Ti-O-Ti are visible, demonstrating the successful synthesis of TiO₂/SiO₂-Al₂O₃ aerogel composite samples.

Fig. 3 shows TEM images of the as-prepared SiO₂-Al₂O₃ aerogel and TiO₂/SiO₂-Al₂O₃ aerogel composite. Fig. 3(a) shows that the SiO₂-Al₂O₃ aerogel was porous with a regular network structure. The TiO₂/SiO₂-Al₂O₃ aerogel composite (Fig. 3(b)) featured TiO₂ particles ranging in size from 10 to 30 nm dispersed evenly on the SiO₂-Al₂O₃ aerogel. The SiO₂-Al₂O₃ aerogels improved the dispersion and reduced the average size of the TiO₂ particle agglomerates. In addition, the structural characteristics of the SiO₂-Al₂O₃ aerogel indicate its potential for enhancing adsorption of organic pollutants.

Fig. 4 shows a nitrogen adsorption-desorption isotherm of an as-synthesized TiO₂/SiO₂-Al₂O₃ aerogel composite sample, which showed a type IV-like isotherm with a type H1 hysteresis loop. This finding suggests the presence of mesoporosity in the TiO₂/SiO₂-Al₂O₃ aerogel composite matrix. According to the results calculated by BET, the surface areas of the TiO₂, SiO₂-Al₂O₃ aerogel and TiO₂/SiO₂-Al₂O₃ aerogel composite samples were 87, 401.4 and 107.8 m²/g, respectively. The lower surface area of the composite than that of the SiO₂-Al₂O₃ aerogel may be explained by TiO₂ particles in the composite shrinking the overall surface area through particle agglomeration and/or blocking pores. However, the SiO₂-Al₂O₃ aerogel in the composite gave a higher surface area than that of TiO₂ alone, confirming the supporting role of the SiO₂-Al₂O₃ binary aerogels. The high specific surface area of TiO₂/SiO₂-Al₂O₃ aerogel improved photocatalytic degradation of organic contaminants by allowing better adsorption of reactant molecules. Enhancing the concentration of reactant molecules around the TiO₂ photoactive layer will benefit photocatalytic decomposition of organic pollutants. Reactant molecules are more likely to interact quickly with short-lived hydroxyl radicals that are localized on the SiO₂-Al₂O₃ aerogel surface.

Fig. 5 shows UV-vis diffuse reflectance spectra of TiO₂ and the TiO₂/SiO₂-Al₂O₃ aerogel composite in the range of 200-800 nm. There were no major differences in the absorption of the TiO₂/SiO₂-Al₂O₃ aerogel composite compared with that of pure TiO₂. This indicates that the SiO₂-Al₂O₃ aerogels only slightly affected the absorption profile of the TiO₂. The TiO₂/SiO₂-Al₂O₃ aerogel composite exhibited slight stronger absorption in the visible region than that of the pure TiO₂ samples. This is because the SiO₂-Al₂O₃ aerogel ensured a better distribution of the TiO₂ nanoparticles to maximize their photoconversion efficiency. According to the method reported in the literature ^[27], the band gap energies (E_g) of the TiO₂/SiO₂-Al₂O₃ aerogel composite and pure TiO₂ were calculated to be 3.02 and 3.08 eV, respectively.

The effects of reaction variables such as pH, catalyst concentration, initial concentration of DNBP, irradiation time on the degradation efficiency were investigated.

Solution pH influences adsorption and dissociation of the substrate, catalyst surface charge, oxidation potential of the valence band and other physicochemical properties of the system. The effect of pH on the photocatalytic degradation efficiency of DNBP was investigated at different pH values in the range 1.86-12.89 and the results are presented in Fig. 6. During these experiments, the pH of the solution was adjusted before light irradiation, but was not controlled during the course of the reaction. The figure shows that the degradation efficiency of DNBP increased as the pH was increased from 1.86 to 4.86. For further increases in pH the degradation efficiency started to decrease. Maximum degradation was obtained at pH = 4.86 (natural pH of DNBP solution) under visible light irradiation.

The solution pH affected the formation of ^•OH radicals as illustrated in the following equations ^[28]:

$TI{{O}_{2}}+hv\to TI{{O}_{2}}+e_{cb}^{-}+h_{vb}^{+}$

(1)

$h_{vb}^{+}+{{H}_{2}}O\to \cdot O{{H}_{ads}}+{{H}^{+}}$

(2)

$h_{vb}^{+}+O{{H}^{-}}\to \cdot O{{H}_{ads}}$

(3)

$\cdot O{{H}_{ads}}+DNBP\to \cdots \deg radation products$

(4)

very low and high pH values disfavor production of ^•OH radical because of the high redox potentials of Eqs. (2) and (3). The change in solution pH also affected the TiO₂ surface. The pH-dependence of the surface charge was consistent with the pH_zpc of the photocatalyst. The pH_zpc value was determined to be approximately 4.76 for the prepared TiO₂/SiO₂-Al₂O₃ aerogel composite (Fig. 6 inset). DNBP exists mainly in its molecular forms in acidic medium, and is more likely to dissociate into anions at higher pH values because the pKa value of DNBP is 4.62. Consequently, DNBP was more favorably adsorbed to the surface of the TiO₂/SiO₂-Al₂O₃ aerogel composite under weak acidic conditions, resulting in a higher DNBP degradation efficiency.

The effect of catalyst concentration on the photocatalytic degradation efficiency of DNBP was investigated in the concentration range 2-10 g/L. Fig. 7 shows a steady decrease in the DNBP absorption peak for up to 6 g/L of the catalyst. At higher catalyst concentrations the peak absorbance was higher than that at lower concentrations, which indicates the optimal degradation efficiency was achieved at 6 g/L. The enhanced degradation efficiency from 2 to 6 g/L may be attributed to an increased number of available adsorption and catalytic sites on the catalyst surface. However, at high catalyst concentrations, agglomeration of catalyst particles may have increased light scattering and decreased the number of surface active sites, which in turn reduced the production of ·OH radicals and lowered the specific activity of the catalyst. Therefore, an optimized catalyst concentration of 6 g/L was used for further experiments.

Fig. 8 presents the effect of irradiation time on the photocatalytic degradation of DNBP under the optimized experimental conditions. In a control experiment in which DNBP was illuminated with visible light in the absence of photocatalyst no degradation was observed in solution. This confirmed that the degradation of DNBP occurred predominately via photocatalysis rather than photolysis. The photoactivity of the TiO₂/SiO₂-Al₂O₃ aerogel composite was superior to that of pure TiO₂. The DNBP photocatalytic degradation efficiencies of pure TiO₂ and TiO₂/SiO₂-Al₂O₃ aerogel composites after irradiation for 5 h were 68.2% and 99.1%, respectively. The enhanced photocatalytic activity of the TiO₂/SiO₂-Al₂O₃ aerogel composite may be attributed to its high capacity for adsorbing organic pollutants. The presence of a mixed TiOSi phase at the TiO₂/SiO₂-Al₂O₃ aerogel interface may also contribute to the high photocatalytic activity of the TiO₂/SiO₂-Al₂O₃ aerogel composite ^[29]. Interfaces of this type have been shown to possess sites with high Brönsted acidity. The strong Brönsted acid sites in the TiOSi region may activate the pollutants towards oxidation.

The effect of the DNBP concentration on the photocatalytic degradation efficiency in the presence of the TiO₂/SiO₂-Al₂O₃ aerogel composite catalyst was studied by varying the initial concentration from 0.0629 to 0.2087 mmol/L. The percentage degradation decreased as the initial DNBP concentration was increased. This may be due to blocking of photocatalytic active sites on the catalyst and reduced interaction of light with these sites. At higher DNBP concentrations a larger portion of light was absorbed by the DNBP rather than the catalyst particles, which also reduced the efficiency of the photocatalytic reaction. The photocatalytic reaction on the surface of TiO₂/SiO₂-Al₂O₃ aerogel composite catalyst was a pseudo-first-order reaction and its kinetic equation can be expressed by Langmuir-Hinshelwood model:

$r=-\frac{dC}{dt}=\frac{{{k}_{r}}KC}{1+KC}$

(5)

The integrated form of Eq. (6) is:

$t=\left( \frac{1}{{{k}_{r}}K} \right)1n\left( \frac{{{C}_{0}}}{C} \right)+\frac{{{C}_{0}}V}{{{k}_{r}}}$

(6)

where t refers to the irradiation time, C₀ is initial concentration of DNBP and C is the concentration of DNBP at time t, r represents the rate of degradation, K is the equilibrium constant for the adsorption of DNBP onto the catalyst and k_r reflects the limiting rate of the reaction at the maximum coverage in these experimental conditions.

At low initial DNBP concentrations, the second term in the Eq.(6) becomes insignificant and can be neglected, such that:

$1n\left( \frac{{{C}_{0}}}{C} \right)={{k}_{r}}Kt={{k}_{app}}t$

(7)

where k_app is the apparent rate constant for the photocatalytic degradation.

A plot of ln(C₀/C) vs t gives a straight line, as can be seen in Fig. 9, confirming the applicability of Langmuir-Hinshelwood equation with k_app = 0.52-0.13 h^-1, in the DNBP concentration range 0.0629-0.2087 mmol/L. The apparent rate constant decreased as the initial DNBP concentration was increased in the solution.

Recycling and reuse of a photocatalyst is an important factor for its practical application. Thus, we investigated reuse of our TiO₂/SiO₂-Al₂O₃ aerogel composite photocatalyst. After one cycle of DNBP photodegradation, the photocatalyst was filtrated, washed, dried at 80 ℃ for 4 h and then reused for further cycles, while maintaining other reaction conditions. The photocatalytic efficiency of the composite remained above 90% even after 5 cycles (Fig. 10). This indicates that the TiO₂/SiO₂-Al₂O₃ aerogel composite catalyst has good reusability. The reduction in the photocatalytic degradation efficiency over the 5 cycles was likely caused by fouling of the catalyst by organic intermediates generated during the photocatalytic reaction, which lowered the ·OH radical production rate.

We attempted to identify intermediate products generated during photocatalytic degradation of DNBP, in an aqueous suspension of the TiO₂/SiO₂-Al₂O₃ aerogel composite by GC-MS analysis. Samples were taken at different irradiation time, then filtered and concentrated to 5 mL by rotatory evaporation at 40 ℃. The anhydrous residue was dissolved in 25 mL of diethyl ether and sonicated for 10 min. Diethyl ether was removed under reduced pressure, and the residual mass was mixed with 2 mL of methanol and 1 mL of trimethylsulfonium hydroxide. The mixture was then agitated for 10 min at 50 ℃ and the resulting samples were injected into the GC/MS system (Agilent 6890 GC and 5793 MS) for analysis. From the GC/MS analyses, the main identified intermediates resulting from photocatalytic degradation of DNBP were 3, 5-dinitro-salicylaldehyde, 3, 5-dinitrocatechol, 4-nitrocatechol. These intermediates may undergo further degradation, eventually leading to aromatic ring opening and formation of aliphatic mono- and dicarboxylic acids, which could eventually be mineralized to CO₂, H₂O and NO₃^-.

Mineralization of refractory organic pollutants with fast kinetics is highly desirable for pollution control. We measured the TOC content of the DNBP wastewater to evaluate mineralization of the compound before and after the photochemical treatment in the presence of TiO₂/SiO₂-Al₂O₃ aerogel composite photocatalyst. The reduction of TOC in the aqueous DNBP solution after 5 h of visible light irradiation was 70.15% for the TiO₂/SiO₂-Al₂O₃ aerogel composite photocatalyst. This result confirmed destruction of DNBP and demonstrates the excellent photocatalytic performance of the prepared TiO₂/SiO₂-Al₂O₃ aerogel composite photocatalyst.

In summary, TiO₂ nanoparticles loaded on SiO₂-Al₂O₃ aerogels from industrial fly ash as raw materials were successfully prepared by a simple sol-gel method. Taking advantages of TiO₂ and SiO₂-Al₂O₃ aerogel, the as-obtained TiO₂/SiO₂-Al₂O₃ aerogel composite featured a high adsorption capacity and enhanced photocatalytic activity for degradation of DNBP under visible light irradiation. Satisfactory photocatalytic degradation efficiency could be achieved using optimized operation parameters. The photocatalytic reactions appeared to follow pseudo-first-order kinetics with the reaction rate constant from 0.52 to 0.13 h^-1 in the DNBP concentration range of 0.0629 to 0.2087 mmol/L. The TiO₂/SiO₂-Al₂O₃ aerogel composite catalyst could be reused with no obvious decline in photocatalytic efficiency observed after five cycles of catalytic degradation. The results of this study show that a TiO₂/SiO₂-Al₂O₃ aerogel composite material can be used as an efficient photocatalyst for removal of organic pollutants from water.