Introduction

The demand for clean and readily available water is increasing, especially in the face of rapid industrialization and population growth. However, challenges for water supply systems, such as climate change, water scarcity, and urbanization, are still evolving. The World Health Organization (WHO) has emphasized that water reuse is becoming an important strategy to address the water crisis (WHO, 2022). Water purification is an essential step in effectively implementing this measure into practice. Efforts have been made to develop various water treatment processes, including biological treatment, chemical coagulation, precipitation, membrane separation, oxidation, and adsorption (Marinho et al. 2019; Nidheesh and Singh 2017; Roy and Saha 2021; Yadav et al. 2019; Zhang et al. 2008a, 2021b). Among them, adsorption is often considered superior to many other water treatment processes because the design of adsorbents is facile and it is easy to operate the adsorption equipment (Qiu et al. 2020). Our previous work demonstrated an efficient biodegradable electrospun cellulose acetate (CA)/chitosan (CS) fibrous adsorbent for humic acid (HA) removal. However, it reached the maximum adsorption capacity after 1 h (Zhang et al. 2021a). Replacing or regenerating adsorbents can be a time-consuming and costly process. Numerous other approaches have been proposed to address the accumulation of HA in aqueous environments. These include aluminum salt coagulation of HA (Liu et al. 2009; Sudoh et al. 2015), electrochemical combustion of HA (Liao et al. 2008), ultrafiltration membrane separation (Szymański et al. 2016), and gamma radiation treatment (Sasaki et al. 2018). The treatment processes were often limited in terms of removal efficiency, material and energy consumption, and management of residuals and by-products (Tung et al. 2019).

Heterogeneous photocatalysis employing semiconductor titanium dioxide (TiO2) exhibited high efficacy for in-suit HA degradation (Birben et al. 2017; Liu et al. 2014; Tung et al. 2019). The remarkable photoactivity, chemical stability, non-toxicity, and abundance of TiO2 have brought it tremendous attention for environmental remediation (Khorsandi et al. 2015). However, the use of powdery TiO2 in water treatment can be challenging due to its tendency of agglomeration and subsequent reduced photocatalytic activities (Gebru and Das 2017; Wang et al. 2013). It was also observed that the solution opacity increased at a higher catalyst concentration, leading to a reduction of light penetration (Tung et al. 2019). Additional steps such as coagulation and sedimentation are required for recovery and reuse, which increase the risk of secondary contamination in the water system (Xu et al. 2013). Electrospinning is a facile and versatile technology for generating ultrafine fibrous membranes with large surface-to-volume ratios, which are desirable and reliable substrates for TiO2 immobilization (Marinho et al. 2021). Enclosing the photocatalysts within such a membrane can overcome the aforementioned limitations and facilitate the reuse of photocatalysts in subsequent cycles. Additionally, the nature of the photocatalytic process is surface-oriented, and the efficiency of photo-oxidation is inseparable from the initial adsorption of the targeted contaminants onto the surface of photocatalysts (Rahman et al. 2021; Rao et al. 2016). Previous studies have primarily focused on either the adsorptive or photocatalytic properties of TiO2 (Bi et al. 2021; Joung et al. 2006; Liao et al. 2012; Zhang et al. 2008b). It is hypothesized that by incorporating TiO2 into CA/CS membranes via one-step electrospinning, the CA/CS matrix can provide a large active surface area and high affinity for the adsorption of HA, facilitating subsequent surface-oriented photodegradation of HA, while TiO2 nanoparticles fixed in the fibers can oxidize the adsorbed HA by creating reactive oxygen species (ROS) to enable the postponed saturation point of the membrane and higher removal capacity. Therefore, this study intends to explore the synergistic effects of adsorption and photooxidation processes facilitated by TiO2 within the electrospun CA/CS matrix, which have been seldom reported in the context of HA removal and biopolymer-based fibers. The morphological and structural properties of electrospun TiO2-CA/CS fibers were studied and characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) spectroscopy. The impacts of TiO2 content, pH level, fiber composition, and reaction conditions on the removal of HA were examined, and the synergistic effect of surface adsorption and photocatalytic oxidation was confirmed by comparing the removal efficiencies of TiO2-CA, TiO2-CA/CS, and CA/CS fibers. By combining traditional membrane adsorption with nanomaterial catalysis, this work is expected to inspire new ideas for the rational design of efficient adsorbents for water treatment.

Experimental methods

Materials

Cellulose acetate (CA) tow that is used to produce cigarette filters was kindly provided by Celanese Corporation (Irving, US) with a molecular weight of 75–95 kDa, acetyl content of 39.95 wt %, and degree of substitution of ~ 2.47. Chitosan (CS) synthesized from crab shell with a molecular weight of 190–310 kDa and degree of deacetylation of 75–85%, was provided by Dr. Benjamin Simpson (Department of Food Science and Agricultural Chemistry, McGill University, Quebec, Canada). Acetic acid (CH3COOH, glacial), humic acid (sodium salt, C9H8Na2O4 45–70%), titanium dioxide (TiO2, particle size of ~ 20 nm), sodium hydroxide (NaOH, ACS reagent grade), and sulfuric acid (H2SO4, ACS reagent grade) were all purchased from Fisher Scientific (Mississauga, ON, Canada) and used as received without further purification. Deionized water was used throughout all experiments.

One-step electrospinning of TiO2-CA/CS fibrous membranes

Desired amounts of CA and CS were dissolved in 85 wt % acetic acid solution and stirred for 48 h. Different amounts of TiO2 nanoparticles were also dispersed in 85 wt % acetic acid solution by using Vortex at 3000 rpm, and the suspensions were subsequently sonicated for 90 min and added into the CA/CS solutions, which were placed in the sonication bath for another 90 min. The prepared TiO2-CA/CS composite solutions were forced through a stainless-steel needle with a diameter of 0.66 mm, and the fibers were collected on a stainless-steel drum rotating at 10 rpm. Electrospinning conditions were optimized in our preliminary experiment to allow the steady generation of fibers. The summary of sample compositions and the optimized electrospinning conditions is listed in Table 1. The electrospun fabrics were vacuum-dried in a desiccator at room temperature overnight to expel possible solvent residues. In order to confirm the synergistic effect of photocatalysis and adsorption, electrospun TiO2-CA fibrous membranes were prepared by the same method as abovementioned.

Table 1 Solution compositions and optimized electrospinning conditions of various fibrous membranes
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Characterization of TiO2-CA/CS fibrous membranes

Morphological observation of TiO2-CA/CS fibrous membranes was done using a Hitachi SU-3500 SEM (Hitachi, Tokyo, Japan) operating at 30 kV. All samples were coated with 4 nm of platinum/gold layers using a Leica EM ACE200 coater (Leica, Wetzlar, Germany) prior to the observation. To measure the fiber diameters, SEM images of various samples under a magnification of ×10 k were selected from which four hundred random positions were measured for each sample using the ImageJ image-visualization software (developed by the National Institute of Health) (Wang et al. 2017). The chemical structure of the electrospun fibers was analysed with a Varian Excalibur 3100 FT-IR spectrometer (Varian, Melbourne, Australia) equipped with an attenuated total reflectance accessory (Specac, Orpington, UK). Each FT-IR spectrum was recorded in transmittance mode as the average of 64 scans with a resolution of 4 cm−1. Uniaxial tensile testing of the fibrous membranes was carried out on an ADMET eXpert 7601 testing machine (ADMET, Norwood, MA, USA) at the fixed initial grip-separation distance of 10 mm and crosshead velocity of 1 mm min−1. Five specimens with dimensions of 30 mm × 10 mm (length × width) from each sample were measured according to the ASTM D-638-V standard (Selling et al. 2011). The thickness of each sample was measured from SEM images using the ImageJ image-visualization software. The tensile strength (σ) of the electrospun membranes was determined and calculated from the following equation:

$$\sigma =\frac{F}{A}$$
(1)

Synergistic removal of HA

The performance of TiO2-CA/CS fibrous membranes towards HA removal was investigated by batch experiments. The removal rate of HA was determined as the functions of TiO2 content, treatment time, and pH level of the solution under the optimized conditions as follows: membrane dosage of 0.3 g L−1 (approximately 20 mm × 10 mm, length × width), HA solution volume of 20 mL, HA initial concentration of 30 ppm, and stirring speed of 150 rpm (Zhang et al. 2021a). Stock solution with a concentration of 100 ppm HA was prepared and further diluted to obtain HA solutions with lower concentrations. Sulfuric acid and sodium hydroxide were used to adjust the pH levels of HA solutions. Batch experiments were carried out in 25 mL glass vials, and a 20 W UV lamp with a standard wavelength of 365 nm was used as the light source. The distance between the lamp and the water surface was 10 cm. The performance of the TiO2-CA/CS membranes towards HA removal in dark conditions was also evaluated. To calculate the HA concentration before and after the treatment, a calibration curve was prepared with a series of standard solutions with known HA concentrations. The UV-vis absorbance of the solutions at 271 nm was measured using a Hitachi UV-2000 UV-vis spectrophotometer (Hitachi, Tokyo, Japan). The removal efficiency was calculated as follows:

$$\mathrm{removal}\;\mathrm{efficiency}\;(\%)\;=\frac{\left(C_0-C_i\right)}{C_0}\times100\%$$
(2)

where C0 (mg/L) is the initial HA concentration in the solution and Ci (mg/L) is the equilibrium HA concentration.

The kinetics of photocatalyzed oxidation were studied by applying and modeling the experimental data into the Langmuir-Hinshelwood kinetic model:

$$\text{r}=-\frac{dC}{dt}=\frac{{k}_{r}KC}{1+KC}$$
(3)

where r (mg/ (L min)) represents the rate of reaction that changes with time; C (mg/L) is the concentration of HA solution at time t (min); kr (mg/(L min)) is the rate constant of the reaction; and K (L/mg) is the equilibrium constant for adsorption of the substrate onto the catalyst. Equation (3) can be integrated between the limits: C = C0 at t = 0 and C = C at t = t, which is expressed as:

$$\text{l}\text{n}\left(\frac{{C}_{0}}{C}\right)+K({C}_{0}-C)={k}_{r}Kt$$
(4)

where C0 (mg/L) is the initial HA concentration in the solution (Kumar et al. 2008).

Statistical analysis

Statistical interpretations of the results were evaluated by analysis of variance (ANOVA) followed by multiple comparison tests of the means using Duncan’s multiple-range test at a 95% confidence level. All statistical analyses were done using SPSS statistical software (version 27, IBM, Armonk, NY, USA) with a p < 0.05 considered to be significant. The results were expressed as the mean of at least three replicates ± standard deviation.

Results and discussion

Structure of TiO2-CA/CS fibrous membranes

FT-IR was employed to understand the component interactions and characteristic chemical information of TiO2-CA/CS fibrous membranes. As shown in Fig. 1, all the samples had the characteristic peaks of CA at 3500 cm−1, 1750 cm−1, and 1372 cm−1 (representing O-H stretching, C = O vibration, and CH3 groups of the acetyl moiety) (Monisha et al. 2016; Zhang et al. 2021a), and the typical infrared diffraction peaks of CS at 3350 cm−1 (-OH and -NH groups) and 1600 cm−1, 1100 cm−1, and 885 cm−1 (amine groups) (Sharaf et al. 2021). Compared to the spectrum of TiO2-CA, all the other samples showed downfield shifts from the peak at 3500 cm−1 to the broad peak at 3350 cm−1, which revealed the hydrogen bonding interactions between the amine groups of CS and acetyl groups of CA (Gopi et al. 2019). After introducing TiO2 nanoparticles, TiO2-CA and TiO2-CA/CS fibers exhibited new and similar adsorption patterns in the wavenumber range of 350 cm−1 to 750 cm−1, which was assigned to the vibration of Ti-O-Ti network and confirmed the successful incorporation of TiO2 into the CA/CS fibrous matrix (Tsiourvas et al. 2011). However, the change in TiO2 contents didn’t affect the intensities of the characteristic peaks, suggesting that the incorporation of TiO2 nanoparticles did not significantly impact the structure of CA/CS fibers.

Fig. 1
figure 1

FTIR spectra of CA/CS, TiO2-CA, 1%TiO2-CA/CS, 2%TiO2-CA/CS, and 3%TiO2-CA/CS electrospun fibers

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To investigate the effect of TiO2 on the electrospun fibers, the morphologies of the fabrics with various compositions were observed by SEM and the images are shown in Fig. 2. All the samples exhibited ultrafine and continuous fibrous structures with nano-scaled diameters, which are highly desirable for the adsorption and photocatalysis processes (Erhayem and Sohn 2014; Liu et al. 2014; Lv et al. 2011). The average fiber diameters of 1%TiO2-CA/CS, 2%TiO2-CA/CS, and 3%TiO2-CA/CS were 16.0 ± 7.7, 12.6 ± 4.2, and 15.08 ± 8.2 nm, respectively, and were much smaller than those of TiO2-CA (149.9 ± 39.2 nm) and the electrospun CA/CS fibers reported in our previous study (Zhang et al. 2021a). It might be due to the higher voltage applied, the shorter tip-to-collector distances, and consequently the enhanced electrical field during electrospinning, resulting in a better stretch of the fibers (He et al. 2015; Kiennork et al. 2015). It was observed that more concave/convex fibers and beads were presented in the 3% TiO2-CA/CS sample. These were caused by the formation of an unstable Taylor cone in the presence of high TiO2 loading amounts and the possible agglomeration of the nanoparticles (Gebru and Das 2017; Zhang et al. 2021b).

Fig. 2
figure 2

SEM images and fiber diameter distributions of (a) 1%TiO2-CA/CS, (b) 2%TiO2-CA/CS, (c) 3%TiO2-CA/CS, and (d) TiO2-CA electrospun fibers

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Mechanical properties of TiO2-CA/CS fibrous membranes

As shown in Fig. 3, the tensile strength of 1%TiO2-CA/CS, 2%TiO2-CA/CS, and 3%TiO2-CA/CS were 19.02 ± 0.91, 21.84 ± 0.85, and 13.02 ± 1.01, respectively. The addition of TiO2 nanoparticles to the CA/CS matrix remarkably improved the tensile strength (Zhang et al. 2021a). It could be explained by the transfer and diversion of force from the CA/CS fibers to the nanoparticles, and the stable interface between TiO2 and the CA/CS matrix (Habiba et al. 2019; Kochkina and Butikova 2019). The significantly higher tensile strength of the 2%TiO2-CA/CS sample corresponded to its homogeneous fibrous structure and well-dispersed TiO2 nanoparticles, resulting in better stress distribution and energy absorption (Feng et al. 2019; Zhang et al. 2021a). All the TiO2-CA/CS membranes demonstrated higher tensile strength than that of the TiO2-CA sample (2.19 ± 0.17 MPa). It can be attributed to the hydrogen bonding interactions between the amine groups of CS and the acetyl groups of CA that contributed to the retardation of loading stress (Han et al. 2019). However, the introduction of TiO2 had a negative impact on the strain of the TiO2-CA/CS membranes. With the increase of the TiO2 contents from 1 wt % to 3 wt %, the elongation at break of the membranes reduced considerably from 1.61 ± 0.11% to 0.98 ± 0.07%, because the TiO2 nanoparticles restrained the matrix flexibility and mobility. Similar phenomena were reported in TiO2-reinforced starch-based nanocomposite films (Oleyaei et al. 2016).

Fig. 3
figure 3

Mechanical properties of the electrospun fibrous membranes: (a) stress-strain curves, (b) tensile strength, and (c) elongation at break. Different asterisks on the top of each column represent significant differences (p < 0.05)

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Removal of HA

Effect of pH

The effect of initial solution pH on HA removal using electrospun TiO2-CA/CS membranes was investigated within a pH range of 4 to 12. Experiments conducted at pH < 4 were excluded due to the coagulation and precipitation of HA (Abate and Masini 2003; Brigante et al. 2007; Zhang et al. 2021a). As depicted in Fig. 4, the synergistic removal efficiency of HA using TiO2-CA/CS was pH-dependent and increased at lower pH values. The pKa values of HA and the primary amine of CS were approximately 4.0 and 6.5, respectively, and the point of zero charge value of TiO2 falls within a pH range of 6.0 to 7.5 (Laird and Koskinen 2008; Mohammed et al. 2017; Paz 2006). Therefore, at pH = 4, electrostatic attraction occurred between the deprotonated carboxylic groups of HA and the positively charged CS and TiO2 nanoparticles, contributing to the superior removal efficiency towards HA. Meanwhile, the nonpolar methyl groups of CA interacted with the hydrophobic moieties of HA through hydrophobic interaction, and the positive charges of TiO2 were favorable for transferring photo-generated electrons to the surface of TiO2 and discouraging the recombination of photoelectrons and photoholes, leading to the prolonged generation of ROS (Xue et al. 2011). It was worth noting that the 2%TiO2-CA/CS sample showed the highest removal efficiency towards HA at all pH levels, while the TiO2-CA fibers were the least efficient. It confirmed that the strong adsorption of HA onto the surface of the electrospun fibers was highly conducive to the photocatalytic process (Krasian et al. 2019).

Fig. 4
figure 4

Effect of pH on the removal of HA using electrospun TiO2-CA/CS and TiO2-CA fibrous membranes (dosage: 0.3 g/L; initial concentration of HA solution: 30 ppm; volume: 20 mL; and irradiation time: 180 min)

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Effect of irradiation time

The removal efficiencies of various TiO2-incorporated fibrous composites under UV irradiation or in the dark as a function of treatment time are illustrated in Fig. 5(a). The synergistic effect of adsorption and photocatalysis was evaluated by comparing with the photocatalysis under UV irradiation without the adsorptive sites of CS and the adsorption of TiO2-CA/CS fibers in dark conditions. All samples exhibited rapid removal of HA within the first 30 min, implying tremendously available adsorptive sites. The adsorption process typically occurs faster than the photocatalytic oxidation (Liu et al. 2014). As the adsorption process continued, the active sites became increasingly occupied, and the removal efficiencies of the samples in the dark reached the equilibrium after approximately 60 min, which was in accordance with our previous report (Zhang et al. 2021a). The TiO2-CA membrane adsorbed HA through the hydrophobic interaction between HA and CA and surface complexation of TiO2 and HA (Sun and Lee 2012). However, its removal efficiency in the dark was the lowest because of the absence of CS. It was noteworthy that 1-3% TiO2-CA/CS fibrous membranes demonstrated continuous removal of HA from the aqueous solution under UV irradiation, and the removal efficiencies were approximately 1.5 times higher than those achieved by adsorption solely within 180 min. Moreover, the removal efficiency of TiO2-CA with UV irradiation was also lower than those of the 1-3% TiO2-CA/CS membranes, demonstrating the remarkable synergistic effect of the adsorption process and photocatalytic activity towards the removal of HA. The 2% TiO2-CA/CS sample exhibited the highest efficiency due to the uniform fibrous structure, which facilitated the adsorption and degradation of HA. Table 2 summarizes several TiO2-incorporated photocatalysts for HA removal, and the fibrous TiO2-CA/CS membranes reported here showed a comparable removal efficiency at a lower dosage of 0.3 g/L.

Fig. 5
figure 5

(a) Removal efficiency of TiO2-incorporated fibers with various compositions under UV irradiation or in dark, (b) kinetics of HA removal using TiO2-incorporated fibers (pH: 4.0; dosage: 0.3 g/L; initial concentration of HA solution: 30 ppm; volume: 20 mL), and (c) SEM images of TiO2-CA/CS fibers after 180 min UV irradiation

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Table 2 List of previously reported TiO2 based photocatalysts for HA removal
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Kinetics of HA removal

To examine and compare the removal rates of HA using various TiO2-incorporated fibrous composites, the experimental data were fitted into the Langmuir-Hinshelwood kinetic model, which is the most commonly used kinetic expression of the heterogeneous catalytic processes (Kumar et al. 2008). The kinetics and fitted parameters are shown in Fig. 5(b) and listed in Table 3, respectively. The obtained square correlation coefficients (R2) for all the samples were above 0.96, suggesting that the removal of HA using TiO2-incorporated fibrous membranes was well fitted by the Langmuir-Hinshelwood kinetic model and relied on the synergistic adsorption upon the fibers and oxidation of HA by TiO2 in the matrix. The 2%TiO2-CA/CS sample exhibited a noticeably higher constant rate, whereas TiO2-CA had the lowest constant rate, which further confirmed the superior removal performance of 2%TiO2-CA/CS and the success of the synergistic strategy. The morphologies of the fibrous composites after 180 min UV irradiation are shown in Fig. 5(c). It was evident that no significant changes in the structural integrity and fibrous morphologies were observed. It demonstrated the stability of the electrospun CA/CS fibers, and the fibrous structure was important for the continuous removal of contaminants and the recovery of TiO2 nanoparticles (Li et al. 2022).

Table 3 Summary of fitted parameters of HA removal using the Langmuir-Hinshelwood kinetic model
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Conclusions

The hypothesis has been confirmed that, by one-step electrospinning, the TiO2-CA/CS fibrous membranes could extend the removal of HA from the aqueous solution via synergistic effects of adsorption and photodegradation. The removal efficiency of HA varied at different pH values, TiO2 loading amounts, and fiber compositions. Due to the ultrafine fibrous morphology, homogeneity, and uniform distribution of TiO2 catalysts, the 2%TiO2-CA/CS fibrous membrane exhibited the highest tensile strength and removal efficiency towards HA. With a low fiber dosage of 0.3 g/L, a removal efficiency of 87.7% was achieved under UV irradiation after 180 min, while the sample in the dark could only remove 54.2% of HA and the adsorption reached the equilibrium after 60 min. Hence, this work presents a promising strategy for the development of high-performance adsorbents for wastewater treatment.