Introduction

With the energy and environment crisis, biomass energy plays an important role in economic development and environmental protection (Zhang and Long 2010). WF, a natural cellulose composite of botanical origin, as an abundant, low-density and low-cost biomass energy, is used extensively to produce wood-filled polymer composites and can be added to commodity matrices in considerable amounts, thus offering economically advantageous solutions (Butylina et al. 2011; Ichazo et al. 2001; Xie et al. 2009; Dányádi et al. 2007). As there are lots of hydroxyl groups on the surface of WF, attempts have often been made to modify it chemically in order to improve its dimensional stability and mechanical properties (Devi and Maji 2012; Devi et al. 2003). In most of the applications, WF is used as structural material for some functional materials to improve its economic additional value (Dányádi et al. 2007), and these applications always correspond to the development trend of biomass resources (Si et al. 2013a).

The applications of TiO2 and its modification to photocatalytic degradation of dyes (Zhang et al. 2011a; Kordouli et al. 2015), various organic pollutants (Man et al. 2015; Yasin et al. 2015) and reduction in heavy metals (Djellabi and Ghorab 2014; Ma et al. 2015) have been reported by many researchers. To improve the photocatalytic degradation efficiency of TiO2, β-CD was added to the photocatalytic reaction. Based on the hydrophobic inner cavity of β-CD, the charge transfer rate from the photoexcited semiconductor to electron acceptors is accelerated and the photocatalytic substrates onto the TiO2 surface are concentrated (Willner et al. 1994). Anandan and Yoon (2004) studied the photocatalytic degradation of Nile red using β-CD/TiO2 colloids, and the photocatalytic efficiency of β-CD/TiO2 was 1.8 times higher than that of the pure TiO2. Wang et al. (2006a, b, c) reported that the photocatalytic degradation of bisphenol A, bisphenol F and bisphenol E by TiO2 was enhanced in the presence of β-CD in suspended solution.

With the properties of low cost, abundance, renewability, biodegradability, low density and easy processability (Mai et al. 2004; Trovatti et al. 2010), WF is an ideal carrier. However, for its low economic additional value and lack of functional applications, the modification of WF becomes a hot issue in wood and biomass researches. In this study, a novel composite of WF/β-CD/TiO2 was synthesized by immobilizing TiO2 on the matrix of WF/β-CD, which was prepared in previous works (Si et al. 2013a, b; Wang et al. 2014). This composite is not only a modified WF composite which preserves the good properties of WF, but also a kind of TiO2 photocatalytic composite. In this composite, polycarboxylic acid played the role as “bridge” connecting WF, β-CD and TiO2 together, and β-CD was used as a “channel” for capturing photocatalytic substrate and transferred it to TiO2 surface for improving the degradation efficiency. To the author’s knowledge, there were no reports on the effects of β-CD in TiO2 photocatalytic systems using WF as the carrier. Meanwhile, β-CD/TiO2, WF/TiO2 and TiO2 were prepared and tested under the same experimental conditions. In contrast, the kinetics of photocatalytic degradation was also studied.

Materials and methods

Materials

Poplar wood flour (100 mesh) was purchased from Lianyungang Wood Co., Ltd. (China) and activated in 20 % (wt%) sodium hydroxide (NaOH) for 1 h, then rinsed thoroughly with 65 °C distilled water and dried at 80 °C for 24 h. β-CD was purchased from TCI (Japan). TiO2 (anatase, <25 nm particle, 99.8 % metal basis), methyl orange (MO) and ammonium hydroxide (NH3·H2O) were obtained from Shanghai Aladdin Co., Ltd. (China). Citric acid (CA), sulfuric acid (H2SO4) and hydrochloric acid (HCl) were bought from Beijing Chemical Reagent Co., Ltd. (China). Sodium carbonate (Na2CO3·10H2O) and sodium bicarbonate were used to prepare buffer solution (pH 10.5) and provided by Tianjin Guangfu Technology Development Co., Ltd. (China). Sodium hypophosphite was used as catalysts and obtained from Tianjin Fuchen Chemical Reagent Co., Ltd. (China). Ammonium sulfate ((NH4)2SO4), hexamethylene tetramine, sodium dihydrogen phosphate, disodium hydrogen phosphate, cresol red, zinc powder, phenolphthalein and sodium hydroxide were all purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (China). Ethylene diamine tetraacetic acid disodium salt (EDTA) was supplied by Beijing Yili Chemical Reagent Co., Ltd. (China).

Instruments

SEM (JSM-7500F, JEOL, Japan) was employed to analyze the surface morphology and composition of the synthesized samples. TEM (JEM2100, JEOL, Japan) was used for observing the ultrastructure changes of the samples. XPS (PHI5000, Thermo Electron, Boston, MA, USA) was used to analyze the elements and chemical composition in the specimens. Photocatalytic degradation of MO by WF/β-CD/TiO2 was carried out using a 250-W high-pressure mercury lamp supplied by Jiguang special lighting electrical appliance factory (Shanghai, China). The progress of photocatalytic degradation was monitored by observing the disappearance of the absorption peak of MO at 463 nm on an ultraviolet and visible spectrophotometer (TU-1901, Beijing, China).

Preparation of β-CD/TiO2, WF/TiO2, WF/β-CD and WF/β-CD/TiO2

β-CD/TiO2: Photoinduced assembly method (Jun et al. 2005; Zhou et al. 2009) was used in this procedure. In details, β-CD/TiO2 composite was synthesized by the addition of TiO2 (2.0 g) and β-CD (1.8 g) into 100 mL of distilled water, followed by irradiating by a 250-W high-pressure mercury lamp for 30 min (Montalti et al. 2006). The precipitation was centrifuged and washed with 100 mL of distilled water for 2–3 times until the β-CD could not be detected by phenolphthalein probe technology (Mäkelä et al. 1987; Wang et al. 2011). Then the products were dried in a vacuum oven at 50 °C for 12 h.

WF/TiO2: The suspension containing WF and TiO2 in a molar ratio of 1:1 was irradiated by a 250-W high-pressure mercury lamp for 60 min, centrifuged, washed and dried in a vacuum oven at 50 °C for 12 h.

WF/β-CD: The composite of WF/β-CD was prepared according to previous work (Wang et al. 2014). In brief, activated WF (50 g) was impregnated with 500 mL of the aqueous solution containing β-CD (80 g/L), CA (100 g/L) and phosphatic catalysts (30 g/L). The impregnation of WF remained under ultrasonic shaking for 20 min and stirring for 6 h in order for the adsorption saturation to be achieved. After storing overnight, the WF was filtrated, squeezed, pre-dried at 80 °C for 12 h, and crosslinked at 180 °C for 7 min. In the end, the raw products were sufficiently washed by warm water (60 °C) and alcohol alternately, dried at 105 °C, and then, the product weight was recorded.

WF/β-CD/TiO2: WF/β-CD (1 g) obtained from above procedure was dispersed into 100 mL of distilled water. Then, another 100 mL of aqueous solution containing TiO2 (10 g/L) was added, photoinduced for 30 min using a 250-W high-pressure mercury lamp and centrifuged, washed and dried at 50 °C for 12 h to obtain the final products.

All the products were ground and well homogenized in the ball mill (100 mesh) prior to use.

Determination of the active β-CD by means of phenolphthalein probe technology

For evaluation of the encapsulation ability of β-CD on TiO2 and WF/TiO2, Mäkelä et al. (1987) put forward a method to measure β-cyclodextrin concentrations due to the decolorization of phenolphthalein upon complexation with cyclodextrins. In addition, previous work on phenolphthalein probe technology was applied here (Wang et al. 2011; Si et al. 2013a). The absorbance of the calibration solutions of CDs was measured at the wavelength of 553 nm at room temperature.

The contents of β-CD (c CD) could be calculated by Eq. 1.

$$ \Delta A = 1.565c_{\text{CD}} + 0.011 $$
(1)

where ΔA denotes the difference between the UV absorbance of diluted separation filtrates and that of β-CD blank (phenolphthalein solutions without addition of β-CD).

The contents of active β-CD on TiO2 and WF/TiO2 were evaluated by Eq. 2

$$ {\text{Active }}\beta {\text{ - CD content }}(\% ) \, = \, \frac{{(c - c_{0} ) \times 0.1 \times M}}{m} \times 100\% $$
(2)

where c represents the active β-CD concentration of modified WF material (mol L−1), c 0 represents the β-CD concentration of β-CD blank grafting wood flour (impregnated solution without addition of β-CD) (mol L−1), M represents the β-CD molecular weight of 1135 g mol−1 and m denotes the weight of dry β-CD/TiO2 and WF/β-CD/TiO2 (g).

Determination of TiO2 by means of back titration method

In order to completely determine the contents of TiO2 in WF/TiO2, β-CD/TiO2 and WF/β-CD/TiO2, a self-designed back titration method was described. Three steps of dissolution–conversion–back titration were applied here.

First, 0.1 g of composite was placed in an oven until completely carbonized and then transferred to a muffle furnace and heated at 800 °C for 2 h to afford the white powder. After the heat treatment, the samples were cooled by quenching in air to room temperature, and then, 5 g of (NH4)2SO4 and 10 mL of 98 % (wt%) H2SO4 were added. The mixture was dissolved in 100 mL of distilled water after digestion.

Then, 10 mL of aqueous solution obtained from above was accurately moved into a 250-mL conical flask; 20 mL of 0.01 mol L−1 EDTA standard solution and two drops of cresol red indicator were added. The pH of the solution was adjusted by adding 0.1 mol L−1 NaOH solution drop by drop until the color changed from red to yellow. The solution was boiling for 2 min and then cooled to room temperature. After adding 2–4 drops of xylenol orange, 1:1 (v/v) NH3·H2O was used to adjust the solution from yellow to reddish; 10 mL of hexamethylene tetramine buffer was added; the solution changed bright yellow.

Excess EDTA was titrated by 0.01 mol L−1 standard solution of zinc, using the sharp color change of indicator, from bright yellow to shallow brown red, shaken well and allowed to stand for 1–2 min to determine the endpoint.

The amounts of TiO2 were calculated by Eq. 3.

$$ m = M \times (0.0002 - cV) $$
(3)

where M represents the TiO2 molecular weight of 79.87 g mol−1, c is the concentration of zinc standard solution (0.01 mol L−1) and V denotes the back titration amounts of zinc standard solution (mL).

Photocatalytic experiments

Figure 1 shows the schematic representation of the self-designed photocatalytic reactor, and Fig. 2 displays the chemical structure of MO. In order to understand the effect of WF/β-CD/TiO2 composite on photocatalytic degradation, keeping the contents of TiO2 as a constant, the control groups of TiO2, β-CD/TiO2 and WF/TiO2 were tested, respectively.

Fig. 1
figure 1

Schematic diagram of photocatalytic reactor

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Fig. 2
figure 2

Chemical structure of methyl orange

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TiO2 (0.0971 g), β-CD/TiO2 (0.1 g), WF/TiO2 (0.1084 g) and WF/β-CD/TiO2 (0.1671 g) were mixed with 100 mL of 0.1 mmol L−1 MO solution in a 150-mL quartz test tube, respectively. Prior to irradiation, the reaction mixture was remained in the dark and stirred for 30 min to obtain adsorption equilibrium. After that, the quartz test tubes were immediately irradiated with a 250-W high-pressure mercury lamp in the self-designed photocatalytic reactor (Fig. 1). During the experiment, quartz test tubes were removed from the reactor one by one at different time intervals. Prior to analysis, each sample in the tube was centrifuged. The reaction progress was monitored by TU-1901 spectrophotometer at 463 nm. Every solution was tested three times.

In the UV–Vis absorption spectra, the absorbance (A) and concentration (c, mol L−1) were in accordance with Lambert–Beer law as shown in Eq. 4.

$$ A = \varepsilon bc $$
(4)

where ε represents the molar absorption coefficient (L mol−1 cm−1) and b denotes the optical path length (cm).

The photocatalytic degradation efficiency R (Zhang et al. 2010; Song et al. 2014; Wang et al. 2006a) was evaluated by Eq. 5.

$$ R = {{(c_{0} - c)} \mathord{\left/ {\vphantom {{(c_{0} - c)} {c_{0} }}} \right. \kern-0pt} {c_{0} }} \times 100\% = {{(A_{0} - A)} \mathord{\left/ {\vphantom {{(A_{0} - A)} {A_{0} \times 100\% }}} \right. \kern-0pt} {A_{0} \times 100\% }} $$
(5)

where c 0 and c represent the concentration (mol L−1) of MO before and after photocatalytic degradation, and A 0 and A denote the absorbance of MO before and after photocatalytic degradation.

Results and discussion

Characterization

The SEM images of TiO2, WF, WF/TiO2 and WF/β-CD/TiO2 are listed in Fig. 3. In Fig. 3, the unmodified TiO2 is seen to have a nanoscale spherical shape (Fig. 3a) and agglomerated together (Fig. 3b). On the other hand, the unmodified WF (Fig. 3c) showed a homogeneously layered structure and exhibited a smooth surface. By photoinduced assembly method, on the surface of WF/TiO2 (Fig. 3d) and WF/β-CD/TiO2 (Fig. 3e) lots of irregularly shaped crystals could be observed, which had similar morphology to the agglomerated TiO2 (Fig. 3b), and the homogeneous layered structure of WF still could be seen under the irregularly shaped crystals. However, no obvious difference was observed between the surfaces of WF/TiO2 (Fig. 3d) and WF/β-CD/TiO2 (Fig. 3e).

Fig. 3
figure 3

SEM images of TiO2 (×70,000) (a), TiO2 (×5000) (b), WF (×5000) (c), WF/TiO2 (×5000) (d) and WF/β-CD/TiO2 (×5000) (e)

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For further investigation of the elements composition of the irregularly shaped crystals on WF/TiO2 and WF/β-CD/TiO2, XPS was used to scan the surfaces of the WF/TiO2 and WF/β-CD/TiO2. The results are shown in Fig. 4a (WF/TiO2) and Fig. 4b (WF/β-CD/TiO2), respectively. In Fig. 4, besides the peaks of O1 s (530.6 eV) and C1 s (284.7 eV), the peaks of Ti2p (459.3 eV) were observed, which confirmed that the irregularly shaped crystals on the WF or WF/β-CD were TiO2. This phenomenon also demonstrated that the loading of TiO2 onto the surface of WF or WF/β-CD by photoinduced assembly method was feasible. However, the presence of β-CD in WF/β-CD/TiO2 could not be confirmed by XPS, because β-CD had no significant difference in elementary composition to WF.

Fig. 4
figure 4

XPS spectrum of WF/TiO2 (a) and WF/β-CD/TiO2 (b)

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The composite of WF/β-CD/TiO2 was further examined using TEM, as shown in Fig. 5. TiO2 particles were agglomerated together as shown in Fig. 5a, with a diameter ranging from tens to several hundred nanometers. In Fig. 5b, the dark-field imaging is the carrier material WF, and it could be clearly seen that TiO2 particles were attached, embedded and interwoven on the surface of WF. The results were in accordance with those of SEM and XPS analysis. However, the presence and the determination of β-CD should be further proved by other means.

Fig. 5
figure 5

TEM images of TiO2 (a) and WF/β-CD/TiO2 (b)

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Determination of the active β-CD on WF/β-CD/TiO2 composite

Due to the phenomenon of purple phenolphthalein solution becoming colorless in the presence of β-CD (Mäkelä et al. 1987), the surface active β-CD contents on the WF/β-CD/TiO2 were determined by the previous research works (Wang et al. 2011). Keeping the solutions in the dark and the pH 10.5 unchanged, the purple phenolphthalein solution faded by mixing the WF/β-CD/TiO2 powder. After storing overnight, the surface active β-CD contents on the WF/β-CD/TiO2 were determined as 3.5 % (wt%), which not only demonstrated a successful grafting of β-CD macromolecules but also verified the encapsulating ability of the material of WF/β-CD/TiO2 composite.

Determination of TiO2 on WF/β-CD/TiO2 composite

The contents of TiO2 in WF/β-CD/TiO2 composite were detected by means of back titration method. Gravimetric method was not applicable, because after the process of ashing (800 °C 2 h), the coexistence of rutile and anatase phase led to the uncertainty of residue’s composition. By means of back titration method, the contents of TiO2 in WF/β-CD/TiO2 composite were detected as 0.0581 ± 0.0008 g/0.1 g (five times of determination); the recoveries of TiO2 standard (0.5000 g) were all more than 95 %.

Photocatalytic performance

The photocatalytic effects of TiO2, β-CD/TiO2, WF/TiO2 and WF/β-CD/TiO2 on the degradation of MO under the 250-W high-pressure mercury lamp were performed in triplicate. It could be seen from Table 1 that the results were expressed as a mean value (c/c 0) of the three experiments and the standard deviations (SD) were below 6 %. Figure 6 shows the photocatalytic degradation curves of methyl orange by TiO2, WF/TiO2, β-CD/TiO2 and WF/β-CD/TiO2. As shown in Fig. 6, all the materials were kept in MO solutions with isoconcentration and stirred in the dark for 30 min to reach adsorption equilibrium. After dark adsorption, the c/c 0 values of MO by TiO2, WF/TiO2, WF/β-CD/TiO2 and β-CD/TiO2 were 0.9952, 0.9663, 0.8794 and 0.8394, respectively. The weak interaction between TiO2 and MO led to a slight adsorption of MO (0.0048, 1−c/c 0). Besides, WF also gave no obvious contribution to the dark adsorption of MO, for the c/c 0 values of WF/TiO2 were in the range of 0.9513–0.9813, which were much higher than those of β-CD/TiO2 (0.8244–0.8544) and WF/β-CD/TiO2 (0.8694–0.8894). It meant that β-CD/TiO2 and WF/β-CD/TiO2 exhibited an improved adsorption ability compared to TiO2 or WF/TiO2, which demonstrated a relative strong binding affinity between β-CD and MO. This strong binding affinity had been proven by the work of Kompany-Zareh et al. (2012). Due to the host–guest interaction, MO could be encapsulated by β-CD and adsorbed onto the surface of TiO2. In other words, MO was accumulated on the surface of TiO2 which would make MO degrade more quickly in the presence of superoxide anion free radicals and hydroxyl radicals.

Table 1 Photocatalytic degradation behavior of methyl orange by TiO2, β-CD/TiO2, WF/TiO2 and WF/β-CD/TiO2
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Fig. 6
figure 6

Photocatalytic degradation curves of methyl orange by TiO2, WF/TiO2, β-CD/TiO2 and WF/β-CD/TiO2

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After dark adsorption, the degradation efficiencies (R, Eq. 5) and degradation rates of MO by TiO2, β-CD/TiO2, WF/TiO2 and WF/β-CD/TiO2 were studied. The degradation rate was calculated by Eq. 6.

$$ {\text{Degradation rate}} = \frac{{{\text{d}}(c/c_{0} )}}{{{\text{d}}t}} $$
(6)

where c 0 and c represent the concentration of MO before and after photocatalytic degradation (mol L−1) and t represents the photocatalytic degradation time of MO.

In Fig. 6, compared to TiO2, the degradation efficiency (R) of MO was obviously decreased by the introduction of WF. This was probably due to the decrease in reaction areas of TiO2, since part of the surface of TiO2 was occupied by WF. However, the introduction of WF to photocatalytic composites provides plasticity, diversity to the materials and enlarges the application fields. In order to solve the aforementioned problem, β-CD was used in this research. The degradation rates of MO were decreased in the order of β-CD/TiO2, WF/β-CD/TiO2, TiO2 and WF/TiO2 as shown in Fig. 6. Moreover, the degradation efficiencies (R) of MO were almost 100 % in 21 min using β-CD/TiO2 and WF/β-CD/TiO2. However, the degradation time of TiO2 and WF/TiO2 was increased 66 and 186 % compared to that of WF/β-CD/TiO2. These results indicated that β-CD could obviously accelerate the photocatalytic efficiency even in the presence of WF.

The possible pathways of photocatalytic degradation of MO by TiO2 were likely to be as follows (Anas et al. 2015; Zhang et al. 2011a).

$$ \begin{aligned} {\text{TiO}}_{2} + h\nu \to \, h^{ + } + {\text{ e}}^{ - } \hfill \\ {\text{MO}} + h\nu \to {\text{ MO}}^{*} \to {\text{ MO}}^{ \cdot + } + {\text{ e}}^{ - } \hfill \\ {\text{e}}^{ - } + {\text{ O}}_{2} \to {\text{ O}}_{2}^{ \cdot - } \hfill \\ h^{ + } + {\text{H}}_{ 2} {\text{O}} \to { \cdot }{\text{OH}} + {\text{ H}}^{ + } \hfill \\ {\text{O}}_{2}^{ \cdot - } + {\text{ MO}} \to {\text{ degraded products}} \hfill \\{ \cdot }{\text{OH}} + {\text{ MO}} \to {\text{ degraded products}} \hfill \\ {\text{MO}}^{ \cdot + } \to {\text{ degraded products}}\hfill \\ \end{aligned} $$

The photocatalytic degradation of MO was mainly caused by the superoxide anion free radicals (\( {\text{O}}_{2}^{ \cdot - } \)) and the hydroxyl radicals (\({ \cdot }{\text{OH}} \)), which was generated through the charge transfer complex between β-CD and TiO2 under a 250-W high-pressure mercury lamp (Zhang et al. 2011b). In addition, MO molecules absorbed photons to form an excited state (MO*), whose longevity could be extended in this state because of the inclusion interaction with β-CD (Wang et al. 2006a, c). That way, MO molecules could turn into MO cation radicals (\( {\text{MO}}^{ \cdot + } \)) for the photocatalytic degradation.

A concise model of β-CD/TiO2 degradation of MO is proposed in Fig. 7. Since β-CD can include MO in its cavity and be adsorbed onto the surface of TiO2, it plays a role as “channel” to facilitate the charge transfer from the excited state of MO molecules to the conduction band of TiO2 and accumulate to higher concentration which makes MO degrade more easily (Wang et al. 2006c).

Fig. 7
figure 7

Role of β-CD in TiO2 photocatalytic reaction system

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Kinetic model

Three kinetic models can be applied to investigate the mechanism of the adsorption process as shown in Fig. 8. The pseudo-first-order kinetic model and pseudo-second-order kinetic model are based on the assumption that the rate-limiting step may be chemical sorption or chemisorption involving valence forces through sharing or exchange of electrons between sorbent (WF/β-CD/TiO2) and sorbate (MO). The intraparticle diffusion kinetic model which highlights the importance of mass transfer in the composite material has also been successfully applied to describe the sorption kinetics of MO (Zhang et al. 2015; Maruszewska and Podsiadły 2014). The rate equations of the three different models are listed below.

Fig. 8
figure 8

Plots of pseudo-first-order kinetic model (a), pseudo-second-order kinetic model (b), and intraparticle diffusion model and (c) for the photodegradation of MO by WF/β-CD/TiO2

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The pseudo-first-order kinetic model:

$$ \ln \;c = - k_{1} t + \ln c_{0} . $$
(7)

The pseudo-second-order kinetic model:

$$ {t \mathord{\left/ {\vphantom {t c}} \right. \kern-0pt} c} = {1 \mathord{\left/ {\vphantom {1 {(k_{2} c_{0}^{2} )}}} \right. \kern-0pt} {(k_{2} c_{0}^{2} )}} + {t \mathord{\left/ {\vphantom {t {c_{0} }}} \right. \kern-0pt} {c_{0} }}. $$
(8)

The intraparticle diffusion kinetic model:

$$ {{(c_{0} - c)} \mathord{\left/ {\vphantom {{(c_{0} - c)} {c_{0} }}} \right. \kern-0pt} {c_{0} }} = k_{m} t^{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0pt} 2}}} + A $$
(9)

where c 0 is the original concentration (mmol L−1) of MO, c represents its concentration (mmol L−1) at time t (min) and k 1 (min−1) is the rate constant of pseudo-first-order kinetic model; k 2 (L mmol−1 min−1) is the rate constant of pseudo-second-order kinetic model, k m (mmol L−1 min−1/2) is the rate constant of intraparticle diffusion kinetic model, while A is the intercept of the fitted curve.

The kinetic parameters for the different rate equations were determined by linear and nonlinear curve fittings, and the results are listed in Table 2. It was found that the value of R 2 for intraparticle diffusion kinetic model was extremely high (>0.99), greater than those of pseudo-first-order and the second-order models. Hence, the intraparticle diffusion kinetic model gave better correlations for the degradation of MO. It also implied that the rate-limiting step may be the mass transfer in the composite of WF/β-CD/TiO2. However, the regression line was not passing through the original, indicating that the mass transfer was not the only process during the photocatalytic degradation reaction; the encapsulation of MO by β-CD and some free radical reactions may play parts in the photocatalytic degradation process on account of the intercept (0.2996) in the fitted curve.

Table 2 Parameters of three kinetic models
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Recycle experiments

The regeneration performance of WF/β-CD/TiO2 and WF/TiO2 was tested by carrying out five cycles of photocatalytic degradation experiments. After recycling, washing, drying and grinding, WF/β-CD/TiO2 and WF/TiO2 were repeatedly photocatalytic degraded for 21 and 60 min under the same conditions, respectively. The results are shown in Fig. 9. It could be seen that WF/β-CD/TiO2 still had high degradation efficiency for MO after five cycles, while WF/TiO2 did not. This phenomenon was attributed to the participation of polycarboxylic acid in the synthesis process of WF/β-CD (Fig. 10). TiO2 could be loaded onto the WF/β-CD composite more stable via residual carboxyl afforded by polycarboxylic acid in the presence of in situ photoinduced reaction. Thus, WF/β-CD/TiO2 composite retained its stable structure during the recycle experiments.

Fig. 9
figure 9

Regeneration performance of WF/β-CD/TiO2 (a) and WF/TiO2 (b)

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Fig. 10
figure 10

Schematic drawing of WF/β-CD/TiO2

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Conclusion

The carrier composite of WF was coated with β-CD and TiO2 by two steps of synthesization. The contents of active β-CD and TiO2 in WF/β-CD/TiO2 were 3.5 % and 0.0581 g/0.1 g, respectively; 100 mL of 0.1 mmol L−1 MO can be completely photocatalytic degraded in 21 min by WF/β-CD/TiO2, similar to the effectiveness of β-CD/TiO2 (20 min). The introduction of WF has no effect on the photocatalytic activity of β-CD/TiO2, but provides an ideal biomass carrier for further processing TiO2 composite into photocatalytic products. The kinetics of photocatalytic degradation of MO by WF/β-CD/TiO2 was fitted to the model of intraparticle diffusion, but it was not the only process during the photocatalytic degradation reaction. In addition, the composite of WF/β-CD/TiO2 showed a good photocatalytic activity and regeneration property after five recycle experiments.