Removal of As(III) from Water by Cellulase Templated TiO₂: A Photocatalytic Oxidation Conjugated Adsorption Process

Liang Jiang , Lulu Xie , Jiao He ^*, Juanxue Kang , Wei Wang , Jiaqiang Wang ^*

School of Engineering, National Center for International Research on Photoelectric and Energy Materials, School of Materials and Energy, School of Chemical Sciences & Technology, Yunnan University, Kunming 650091, P.R. China

* Correspondences: Jiao He and Jiaqiang Wang

Academic Editor: Ermelinda Falletta

Special Issue: Recent Advances in TiO2 Photocatalysis and Applications

Received: July 16, 2022 | Accepted: September 02, 2022 | Published: September 09, 2022

Catalysis Research 2022, Volume 2, Issue 3, doi:10.21926/cr.2203025

Recommended citation: Jiang L, Xie L, He J, Kang J, Wang W, Wang J. Removal of As(III) from Water by Cellulase Templated TiO₂: A Photocatalytic Oxidation Conjugated Adsorption Process. Catalysis Research 2022; 2(3): 025; doi:10.21926/cr.2203025.

© 2022 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

Arsenic (As) contamination of water, and the resulting arsenic poisoning that seriously affects large groups of people, is an acute problem in many countries. The chronic toxicity of arsenic in drinking water can lead to various types of cancer, including liver, lung, kidney, bladder, and skin cancer, among others [1,2,3]. The removal of arsenic from solutions with high ionic strength is a major challenge for various water treatment technologies, including ion exchange, adsorption, reverse osmosis, and nanofiltration [4]. There are many techniques for the detection and adsorption of arsenic. For example, one study described the detection and adsorption of arsenic compounds through functional optical fibers, and the results showed that the proposed APTES-alginate-Ca⁺ system had As(V) selectivity. The adsorption of As(V) was due to the electronic stability in the molecular system and its binding energy [5]. Most changes in As adsorption are affected by the pH of the solution and the concentration of As [6]. Additionally, ion exchange and complexation strongly influence arsenic adsorption [7]. In native water, inorganic arsenic mainly exists in two oxidation states, As(V) and As(III). As(III) is more toxic than As(V); it has higher fluidity and lower affinity for the adsorbent [8]. The oxidation of As(III) species is ideal for enhancing arsenic immobilization. Therefore, common arsenic removal technologies include photocatalytic treatment of arsenide and the pre-processing of As(III) oxidation, followed by coprecipitation/adsorption of As(V) onto metal hydroxide [9]. For photocatalytic treatment, TiO₂ has been widely studied because of its non-toxicity and high photostability [10]. However, there are several methods to improve visible light absorption and the photocatalytic efficiency of TiO₂ nanoparticles, such as adding decorative metal nanoparticles, replacing non-metal atoms with metal atoms [11], and coupling with other semiconductors [12]. For As(III) oxidation treatment, chlorine compounds, ozone, H₂O₂, permanganate or manganese oxide, and Fenton reagent might be used as the oxidant for As(III) oxidation [13,14,15,16,17,18,19,20]. The photocatalytic oxidation of As(III) is an effective method. Among them, TiO₂ has been widely studied because of its non-toxicity and high photostability. For example, As(III) was oxidized to As(V) in the suspension of TiO₂ (Degussa P25) irradiated by ultraviolet light [21,22]. To remove As(III) in one step, a prior oxidization step to convert As(III) to As(V) and the following adsorption of As(V) are essential for photocatalysts. However, Degussa P25 TiO₂ has a low capacity to adsorb arsenic. Generally, increasing the specific surface area of an adsorbent can increase its adsorption ability. Therefore, developing photocatalysts with a high specific surface area is an effective method to remove arsenic in water.

Biological templates have been investigated for the synthesis of porous inorganic materials, as they can derive morphologically controllable materials with structural specificity and unique functions [23,24,25,26,27]. In another study, we showed that the specific surface area of TiO₂/SiO₂ prepared by using diatoms as the biotemplate was significantly higher than the specific surface area of conventional TiO₂/SiO₂, which might be due to the formation of many accessible pores and channels [28]. Additionally, many biomolecules, such as peptides, proteins, enzymes, nucleic acids, or antibodies, may be captured in sol-gel matrices to build functional inorganic materials [29]. Based on the synthesis of biomolecular templates, nanomaterials, such as PbS quantum dots, were synthesized by luciferase, and CdS quantum dots were synthesized by the organophosphorus acid an hydrolase biological template [30,31]. Additionally, an enzyme-mediated in-situ synthesis of Ag–TiO₂ using Escherichia coli as a biological template for dye degradation was proposed [32]. Enzymes are extremely effective in catalyzing various reactions under environmental and physiological conditions. Thus, there is great interest in using enzymes as templates to synthesize inorganic nanomaterials.

Cellulase mainly consists of endoglucanase, exonuclease, and β-glucosidase [33] and is an active protein with catalytic power [34]. Among them, endoglucanase acts mainly on amorphous and crystalline regions and truncates long-chain cellulose molecules by random hydrolysis of β-1,4-glycoside bonds to produce small molecules of cellulose with reducing ends. Exodexranase hydrolyzes the β-1,4-glycosidic bond and acts on the end of the cellulose molecule, cutting one fibrodiosacrose molecule at a time. Glucosidase reduces the feedback inhibition reaction of some hydrolysates and breaks down fibrodiose glycose into glucose molecules [35,36,37]. Cellulase can catalyze the decomposition of cellulose and some related polysaccharides. The cellulose template substantially improves thermal stability and increases the surface area. A study reported the removal of CO₂⁺ and Ni²⁺ in water by bagasse cellulose [38]. In this study, TiO₂-cellulase was prepared by using cellulase as a template under hydrothermal conditions. The TiO₂-cellulase combined the processes of photocatalytic oxidation and adsorption. Thus, it can oxidize As(III) to As(V), and also adsorb and remove As(V). It is a new strategy for removing As(III) and is different from the use of synergistic oxidant and adsorbent reported in other studies. Additionally, TiO₂-cellulase can oxidize and adsorb efficiently, and it performs the photocatalysis of As(III) more effectively than commercial Degussa P25 TiO₂.

2. Materials and Methods

2.1 Preparation of Cellulase Templated Mesoporous TiO₂ (TiO₂-cellulase)

Mesoporous TiO₂ samples were synthesized by a biomacromolecule templated hydrothermal process. Briefly, 1.0 g of cellulose from Trichoderma viride was dispersed in 30 mL of anhydrous ethyl alcohol by stirring for 30 min, and 3.0 g of TTIP was added. Then, 40 mL of distilled water was added dropwise with continued stirring. After stirring for 24 h, the mixture was transferred to a Teflon bottle and treated at 363 K under self-pressure for seven days without agitation. The mixture was filtered, and the final solid product was dried and calcinated at 673 K for 4 h. For comparison with P25 TiO₂, two photocatalysis reaction tubes were taken, and50 mL of the prepared As(III) solution was added. Next, 25 mg of TiO₂-cellulase was added to tube 1, and add 25 mg of P25 TiO₂ was added to tube 2. The samples were taken after dark reaction for 9 h, and the concentration of total As was measured. Then the samples were exposed to a UV lamp for 1 h, and measured the concentration of total As.

2.2 Characterization

Powder X-ray diffraction (XRD) experiments were conducted using a D/max-3B spectrometer with Cu Kα radiation, and scans were performed in the 2θ range of 10–90° with a scan rate of 10°/min (wide-angle diffraction). Using the Micromeritics Tristar II surface area and porosity analyzer, the distribution of the pore size, BET surface area, and pore volume were measured by nitrogen adsorption/desorption. Before conducting the analysis, the samples were degassed at 573 K for 3 h. With an acceleration voltage of 15 kV and a sample chamber pressure of 1 Torr, scanning electron microscopy (SEM) images were taken using a FEIQuanta200FEG microscope. High-resolution transmission electron microscopy (HRTEM) micrographs were obtained using a JEM-2100 microscope. FT-IR measurements were performed using a Thermo Nicolet 8700 instrument. Potassium bromide particles containing 0.5% catalyst were used in the FT-IR experiments and 32 scans were obtained for each transmission spectrum at a spectral resolution of 4 cm^–1. The dry KBr spectra were used for background subtraction. The UV-vis diffuse reflectance spectra (DRS) were measured (200–800 nm) in the air at room temperature using a Shimadzu UV-2401PC photometer.

2.3 Photocatalytic Activity

The reserved As(III) solution with a concentration of 1,000 mg/L was purchased from the National Analysis Center for Iron and Steel (NACIS, Beijing, China). In the experiment, the standard stock solution was diluted to 0.10 mg/L. Then, 2.5 mg of TiO₂-cellulase was added to 50 mL of the above-mentioned solution under agitation. A UV lamp (105 W; Tungsram, 360 nm) was used as the light source, which was located 15 cm above the reactor vessel. Before irradiation, the suspensions were magnetically stirred in the dark for about 9 h to ensure that an adsorption/desorption equilibrium was established. During specific irradiation intervals, 2 mL of the suspension was collected and filtered using a 0.45 µm microporous filter to remove the solid. The total arsenic concentration in the remaining solution was detected by a nondispersive atomic fluorescence spectrometer (AFS) (Pgeneral PF-6, Beijing). Speciation of As(III) and As(V) in the sample solution was detected by performing liquid chromatography-tandem mass spectrometry (LCMS) (Shimadzu LCMS-8030). The photocatalytic removal rate of arsenic was calculated using equation (1), where C₀ and C indicate the initial absorption and instantaneous absorption of As, respectively. The first-order rate constant k (min^–1) was calculated using equation (2), where C_e indicates the equilibrium adsorption of As and C indicates the instantaneous adsorption of As at time t.

\[ { Removal \ rate }=\frac{C_{0}-C}{C_{0}} \cdot 100 \% \tag{1} \]

\[ ln \frac{C_{e}}{C}=k t \tag{2} \]

3. Results and Discussion

3.1 Characterization of TiO₂-cellulase

In this study, TiO₂-cellulase was prepared by the hydrothermal method. The nanoparticles prepared using the hydrothermal method were highly crystalline and had larger surface areas than those prepared by the sol-gel method. The surface areas of the nanoparticles prepared by the hydrothermal and sol-gel methods were 18.2 m²·g^–1 and 9.69 m²·g^–1, respectively. This indicated that CaTiO₃ prepared by the hydrothermal method had higher photocatalytic activity [39]. The XRD pattern of the sample prepared using cellulase as the template is shown in Figure 1. The peaks of the 2θ values at 25.3°, 37.8°, 48.0°, 54.0°, 55.1°, 62.8°, 69.0°, 70.4°, 75.1°, and 82.9° corresponded to the (101), (004), (200), (105), (211), (204), (116), (220), (215), and (224) crystallographic planes of anatase TiO₂ (JCPDS, No. 84–1285), respectively. No peaks representing other phases were observed, indicating the formation of pure phases of anatase TiO₂.

Figure 1 (a) The XRD pattern of TiO₂-cellulase. (b) The FT-IR spectrum of TiO₂-cellulase. (c) The UV-Vis diffuse reflectance spectra of TiO₂-cellulase and P25 TiO₂.

The bonding properties of the functional groups in TiO₂-cellulase were determined by performing FT-IR spectroscopy, as shown in Figure 1b. The band from 500 cm^–1 to 590 cm^–1 represents the Ti-O-Ti stretching vibration in TiO₂. The absorption peak at approximately 1,620 cm^–1 was associated with the stretching and band-like vibrations of surface water molecules, including hydroxyl groups and molecular water, on the samples [40,41]. No peaks corresponding to-CH,-CH₂, or-CH₃ bonds were observed, indicating that the C elements in the TiO₂ sample did not contain organic species from the cellulase template. After calcination at 673 K, organic species from cellulase were completely removed.

To determine the band gap, the UV-vis diffuse reflectance spectra of the as-prepared TiO₂-cellulase were measured. For comparison, the diffuse reflectance UV-Vis spectra of P25 TiO₂ and TiO₂-cellulase are shown in Figure 1c. TiO₂-cellulase showed absorption in the UV region and shifted the absorption edge of TiO₂ to the visible light range. In contrast, P25 TiO₂ did not show any considerable shift in the absorption spectra. The wavelength at the absorption edge (λ) is determined as the intercept on the wavelength axis for a tangent line drawn on the absorption spectra. By applying this method, the absorption edge for TiO₂-cellulase was found to be 400 nm, corresponding to a band gap of 3.1 eV, which was lower than the band gap of P25 TiO₂. This showed that TiO₂-cellulase has a stronger light absorption ability than P25 TiO₂, which can enhance the utilization of photons and increase the production of photoexcited electron/hole pairs, thus improving the photocatalytic performance of TiO₂-cellulase.

The N₂ adsorption/desorption isotherms for TiO₂-cellulase and the corresponding pore size distribution are shown in Figure 2. The isotherms were classified as type IV with hysteresis, which is the characteristic of mesoporous materials. The isotherm of the TiO₂-cellulase sample was type IV with an H3-type hysteresis loop, associated with mesopores present in aggregates composed of primary particles, giving rise to pileup pores. The pore size distribution consisted of a single narrow peak. The Brunauer-Emmett-Teller (BET) surface area, pore volume, and average pore diameter for TiO₂-cellulase were 166.6 m²·g^–1, 0.26 cm³·g^–1, and 5.0 nm, respectively. Thus, the biotemplated process greatly influenced the BET surface area of the samples.

Figure 2 The N₂ adsorption/desorption isotherms and the pore size distribution curves (inset) of TiO₂-cellulase.

The SEM and TEM images showed that the TiO₂-cellulase nanomaterial consisted of an assemblage of dense nanoparticles (Figure 3). The HRTEM image of an enlarged rectangle area in Figure 3c shows lattice spacing of 0.35 nm, matching that of the (101) plane of anatase TiO₂.

Figure 3 The (a) SEM, (b) TEM, and (c) HRTEM images of TiO₂-cellulase.

3.2 Removal of As(III) over TiO₂-cellulase

3.2.1 Catalyst Amount Effect

The amount of TiO₂-cellulase ranged from 0 mg to 50 mg at a constant initial arsenic concentration of 1.0 mg/L after exposure to UV light for 6 h. The total arsenic concentration decreased with an increase in the amount of TiO₂-cellulase (Figure 4). When the amount of TiO₂-cellulase reached 2.5 mg, the total arsenic concentration decreased to 0.01 mg/L. Further increase in the amount of TiO₂-cellulase showed a negligible change in the total arsenic concentration. Thus, the optimal amount of TiO₂-cellulase was selected as 2.5 mg.

Figure 4 The effect of the amount of catalyst on the total As concentration.

3.2.2 Reaction Time Effect

The change in the total arsenic concentration during the irradiation of UV light in the presence of 2.5 mg TiO₂-cellulase was recorded. The results showed that the total arsenic concentration reached equilibrium after 1 h (Figure 5). The activity of the concentration of total As within 1 h with a removal rate of 81% in irradiated TiO₂-cellulase dispersions was determined, and only 40% of arsenic was adsorbed in the dark. This result suggested that UV light might improve the removal of arsenic in the presence of TiO₂-cellulase. In contrast, the photocatalytic removal rate of arsenic was only 56.8% for P25 TiO₂ under the same condition. Thus, TiO₂-cellulase showed higher removal efficiency than P25 TiO₂, which might be due to its larger surface area.

Figure 5 The effect of reaction time on the total As concentration in aqueous dispersions (catalysts = 2.5 mg).

3.2.3 Effect of pH

The effect of pH on the removal of arsenic in the dark and under irradiation is shown in Figure 6. The removal rate of arsenic increased slightly from pH 2 to 9 in the dark and did not change after that, with an increase in the pH in irradiated TiO₂-cellulase dispersions. This occurred probably because the characteristics of the TiO₂ photocatalyst can remain stable over a wide range of pH [42], indicating that pH has a negligible effect on the removal of arsenic. The results suggested wide applicability of TiO₂-cellulase.

Figure 6 Effect of pH on total As concentrations in aqueous dispersions (catalysts = 2.5 mg).

3.2.4 Variation in the Valence and Concentration

The variation in the concentration of As(III) and As(V) under visible light is shown in Figure 7, which indicated the importance of light irradiation in the removal process. The concentrations of As(III) and As(V) reached 92 µg/L and 0 µg/L, respectively, in the dark in the presence of TiO₂-cellulase. Thus, As(III) was not oxidized in the dark. When the TiO₂-cellulase dispersions were exposed to UV light, the concentration of As(III) decreased, and that of As(V) increased within 25 min. After 30 min, the As(V) concentration also decreased. These results indicated that As(III) was oxidized to As(V) under light, and TiO₂-cellulase adsorbed As(V). This occurred because the semiconductor responsible for forming oxidant radicals is activated by UV light. The mechanism is as follows [43]:

\[ \unicode{x2460} \ \mathrm{TiO}_{2} \ + \ \mathrm{h} v \rightarrow \mathrm{TiO}_{2}\left(\mathrm{e}^{-} \ + \ \mathrm{h}^{+}\right) \]

\[ \unicode{x2461} \ \mathrm{OH}^{-} \ + \ \mathrm{h}^{+} \rightarrow \mathrm{OH}^{\cdot} \]

\[ \unicode{x2462} \ \mathrm{O}_{2} \ + \ \mathrm{e}^{-} \longrightarrow \mathrm{O}_{2}{ }^{\cdot-} \]

\[ \unicode{x2463} \ \mathrm{As}(\mathrm{III}) \ + \ \mathrm{OH} \cdot \rightarrow \mathrm{As}(\mathrm{IV}) \ + \ \mathrm{OH}^{-} \]

\[ \unicode{x2464} \ \mathrm{As}(\mathrm{III}) \ + \ \mathrm{O}_{2}{}^{\cdot-}+2 \mathrm{H}^{+} \rightarrow \mathrm{As}(\mathrm{V}) \ + \ \mathrm{H}_{2} \mathrm{O}_{2} \]

\[ \unicode{x2465} \ \mathrm{As}(\mathrm{IV}) \ + \ \mathrm{O}_{2} \rightarrow \mathrm{As}(\mathrm{V}) \ + \ \mathrm{O}_{2}{ }^{\cdot-} \]

\[ \unicode{x2466} \ \mathrm{O}_{2}{ }^{\cdot-} \ + \ \mathrm{e}^{-}+2 \mathrm{H}^{+} \rightarrow \mathrm{H}_{2} \mathrm{O}_{2} \]

\[ \unicode{x2467} \ \mathrm{O}_{2}{}^{\cdot-} \ + \ \mathrm{O}_{2}{}^{\cdot-}+2 \mathrm{H}^{+} \rightarrow \mathrm{H}_{2} \mathrm{O}_{2} \ + \ \mathrm{O}_{2} \]

\[ \unicode{x2468} \ \mathrm{As}(\mathrm{III}) \ + \ \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{As}(\mathrm{V})+2 \mathrm{OH}^{-} \]

\[ \unicode{x2469} \ \mathrm{As}(\mathrm{III}) \ + \ \mathrm{h}^{+} \rightarrow \mathrm{As}(\mathrm{IV}) \ + \ \mathrm{O}_{2} \rightarrow \mathrm{As}(\mathrm{V}) \ + \ \mathrm{O}_{2}{ }^{\cdot-} \]

Figure 7 Changes in the concentration of As(III) and As(V) under visible light.

3.3 Comparison of the Removal Efficiency of As (III) between TiO₂-cellulase and P25 TiO₂

The concentration of total As was measured, and the results are shown in Table 1.

Table 1 Comparison of TiO₂-cellulase and P25 TiO₂ for the removal efficiency.

The adsorption effect of P25 TiO₂ on As(III) in the dark reaction was better than that of TiO₂-cellulase, but the removal rate of As(III) by TiO₂-cellulase was significantly higher than that of P25 TiO₂ after light reaction adsorption (Table 1). The As(III) solution with an initial concentration of 0.088 mg/L was reduced to 0.004 mg/L after adding TiO₂-cellulase, which was within the latest standard of 0.01 mg/L used in many countries.

4. Conclusions

To summarize, cellulase was successfully used as a template to synthesize TiO₂ structures with anatase phases under hydrothermal conditions, which substantially improved the thermal stability and increased the surface area. The TiO₂-cellulase effectively performed the photocatalysis of As(III), and the performance was considerably better than that of commercial Degussa P25 TiO₂. When the amount of TiO₂-cellulase reached 2.5 mg, the highest photocatalytic removal rate of 81% was achieved under UV light irradiation. The high removal rate of total arsenic facilitated by TiO₂-cellulase at different values of pH suggested its wide applicability, and we suggest that the synthetic strategy presented here might be extended to other mesoporous materials. The TiO₂-cellulase photocatalyst might also be able to effectively purify water contaminated with toxic ions, such as those of arsenic and other heavy metal elements.

Acknowledgments

The authors thank the National Natural Science Foundation of China (22062026). The authors also thank The Yunling Scholar (K264202012420), the Industrialization Cultivation Project (2016CYH04), the Basic Research Projects of Yunnan Province (202101AT070017), the Key Projects for Research and Development of Yunnan Province (2018BA065), the Key Laboratory of Advanced Materials for Wastewater Treatment of Kunming for financial support, the authors also thank the Advanced Analysis and Measurement Center of Yunnan University for the sample testing service.

Author Contributions

Jiaqiang Wang conceived and designed the experiments; Lulu Xie performed the experiments; Jiao He and Juanxue Kang analyzed the data; Wei Wang contributed reagents/materials/analysis tools; Liang Jiang wrote the paper.

Competing Interests

The authors declare no conflict of interest.