CTAB@BiOCl: a highly adsorptive photocatalyst for eliminating dye contamination

Abstract

The title composite was synthesized under facile conditions by hydrolysis and co-precipitation. Through comparative studies, it was found that the morphology, structure and properties were affected by hexadecyl trimethylammonium bromide (CTAB) doping. Although the surface area decreases from 53.5 to 7.5 m² g⁻¹, it was found that CTAB@BiOCl exhibits higher adsorption capacity than the isolated BiOCl, and still maintains good photocatalytic activity, which is a little worse than the isolated BiOCl has. This was caused by the lower content of BiOCl in CTAB@BiOCl, which is less than 70%. The studies show that, in high concentrations of dye-contaminated water, the composite exhibits strong adsorption capacities of 901 mg g⁻¹ to Congo Red (CR) and 699 mg g⁻¹ to Reactive Red 3 (X3B). In the low-concentration case, it is able to process photocatalysis of those dyes. In the recycling experiment, the CTAB@BiOCl composite was regenerated in situ. And CTAB in the composite was almost completely degraded after five cycles, resulting in the regenerated BiOCl. Subsequently, the surface area of the composite increases from 7.5 to 22.62 m² g⁻¹, and along with this the adsorption capacities to CR and X3B decrease obviously due to the absence of the CTAB component. In addition, the photocatalysis activity of the generated composite has been promoted to be similar to the isolated BiOCl.

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1. Introduction

Nowadays more than 700 000 tones of commercially available dyes and pigments, over 10 000 types, are produced annually, over 5% of which is released into aquatic environments.¹ As the largest group of synthetic dyes, azo dyes constitute up to 70% of all the known commercial dyes.² They all have one or more aromatic azo-groups and lead to hazardous effects on the flora and fauna when discharged into the environment. Furthermore, most azo dyes are toxic, carcinogenic and mutagenic, causing allergies, cancer and mutations in humans. Therefore, it is stringent to treat dye wastewater. In recent years, green technology has attracted increasing attention in the field of water pollution control.³ Photocatalysis has been proposed as a green technology because of its utilization of solar energy and high efficiency in degradation of organic pollutants.^4,5 Unfortunately, this method is often technically difficult to perform in some practical applications. For example, dye wastewater in high concentration, chroma and salinity would consume large amount of time and energy to be degraded in photocatalysis process. Instead, in this situation, adsorption is the most common way applied in the treatment of dye wastewater. However, there are very rare studies integrating both adsorption and photocatalysis functionalities into a single material for extending the flexibilities of usage. To achieve this purposes, we would like to strategize such types of materials containing both photocatalysis and adsorption functionalities, which is able to treat low-concentration dye contaminations predominantly by photocatalysis, and treat high-concentration dye contaminations predominantly by adsorption as well.

Zhang et al. first demonstrated that BiOCl is an efficient photocatalyst, which exhibits a higher efficiency in the photodegradation of methyl orange than commercial P25 TiO₂.⁶ Since then, BiOCl has drawn more and more attention due to its outstanding photocatalytic performance.^7–14 However, its band-gap (3.4 eV) is too wide to efficiently utilize visible light.¹⁵ Therefore, in recent years, most researches have been focusing on enhancing the efficient separation of photo-generated charge carriers and realize the overall response of solar spectra by modification, such as cocatalyst,¹⁶ doping,^17–19 semiconductor combination,^20,21 graphene use²² and so on. To our knowledge, the researches of BiOCl functioning as adsorbent are very rare. He et al.²³ have prepared BiOCl nanoplates by microwave-assisted method, which exhibit a strong photocatalytic activity with the adsorption capacity of 6.05–8.81 mg g⁻¹ to RhB. Shen et al.²⁴ have prepared BiOCl with visible light photocatalytic activity and the adsorption capacities of 12.014, 11.725 and 6.211 μmol L⁻¹ to Rh101, RhB and Rh6G, respectively. However, the adsorption capacity of BiOCl is relatively low. So there is still a big challenge to further improve the adsorption capacity of BiOCl.

Inorganic–organic (IO) composite materials combining the characteristics of both inorganic and organic substances, shows a number of advantages in pollution control.²⁵ The IO composite materials often endows the composites new properties, which extend the range of applications.²⁶ IO composite materials are often efficiently used as the sorbent of dyes, organic pollutants and metals.^27,28 Therefore, IO composite materials based on BiOCl may improve its absorption abilities by the introduction of new organic components. Surfactants are such often used organic components for that purpose. For instances, several works of our group have confirmed the aggregations of Eriochrome Blue Black R and Coomassie Brilliant Blue G250 on the IO composites based on the surfactant hexadecyl trimethylammonium bromide (CTAB) obey the Langmuir isothermal adsorption.^29,30 Guo³¹ and Shirzad-Siboni³² have prepared CTAB modified clays, which have better adsorption capacity by changing the surface properties. All the studies show CTAB is an efficient organic doping components for improvement of absorption abilities towards organic dyes. It attracted our interest to investigate whether the addition of CTAB into BiOCl scaffolds will give similar high absorption properties as well. Then, how to achieve a successful composite of CTAB and BiOCl for such conception becomes a first important issue. As we known, the synthesis of BiOCl usually includes the hydrothermal, solvothermal, sonochemical, microwave methods. But all these methods have several problems, such as high energy consumption, high pressure, and long reaction time. However, the BiOCl IO composite material has not been reported so far, new synthesis method might be established to process the preparation. Herein, we choose facile hydrolysis and coprecipitation methods to produce the CTAB@BiOCl composite at room temperature, which is less energy and time consuming. The successfully-resulted composite possesses both excellent adsorptive capacity and highly efficient photocatalytic activity as above strategized. These featured multi-functionalities endow CTAB@BiOCl to have probable potentials to treat different levels of organic pollutions by adsorption or photocatalysis.

2. Experimental

2.1. Preparation of composites

The CTAB@BiOCl composite was synthesized by a chemical hydrolysis method. All the reagents were purchased from Aladdin. Scheme 1 shows the schematic illustration for the preparation process of CTAB@BiOCl. 1 g of Bi(NO₃)₃·5H₂O and 0.3 mL of chlorhydric acid was added into 30 mL of 2-methoxyethanol. 0.5 g of CTAB was dissolved into 50 mL of deionized water, which was slowly added into above solution under vigorously stirring for 30 min. After stationarily aging for 12 h at room temperature, the composite was washed by centrifugation repeatedly with deionized water to remove the excessive CTAB. The resulted CTAB@BiOCl suspension was used to perform adsorption and photocatalysis investigations. As a comparison, BiOCl was synthesized without addition of CTAB, the regenerated composite was collected after five times of reuses in the recycle experiment.

Scheme 1 Schematic illustration for the preparation process of CTAB@BiOCl.

2.2. Characterization of materials

The Fourier transform infrared (FTIR) spectra were measured with a Fourier transform infrared spectrometer (Model NICOLET 5700, Thermo Electron Co. USA) to demonstrate that CTAB was embedded into the composite materials. The X-ray powder diffraction (XRD) patterns (Equinoxss/hyperion 2000, Bruker Co., Germany) was used to determine the material crystal structure and phases composition. A scanning electronic microscopy (SEM) (Model Quanta 200 FEG, FEI Co. USA) was used to measure the size and shape of the composite sorbents. A transmission electron microscopy (TEM) (Model TECNAI G2, S-TWIN, FEI Co. USA) was used to characterize the morphology of the CTAB@BiOCl composite material. A ζ-potential instrument (Zetasizer Nano Z, Malvern, UK) was used to determine the surface potential of the sorbents. The surface area of the materials was measured with a surface area and porosimetry analyzer (Model ASAP2020, Micromeritics Co., USA). The elemental analysis device (Model Vario EL III, Germany) was used to determine C, N and H content of the composite. A photodiode array spectrometer (Model S4100, Scinco, Korea) with the Labpro plus software (Firmware Version 060105) was used to determine the concentration of Reactive Red 3 (X3B) and Congo Red (CR) solutions.

2.3. Measurement of adsorptive and photocatalytic activity

The photocatalytic properties of BiOCl, CTAB@BiOCl and the regeneration material samples were evaluated by the degradation of X3B and CR under UV irradiation at natural pH value. The adsorptive properties were measured by examining the adsorption quantity of X3B and CR. In each experiment, about 5 mg of samples was added respectively into 10 mL of X3B and CR solution in a container. During the reaction, the solutions were stirred all the time. At each time interval, the suspensions were centrifuged (4000 rpm, 4 min) to remove the photocatalyst particles in order to assess the removal rate. The solutions in dark and without material were tested as the blank control.

3. Results and discussion

3.1. Structural and morphological characterization

The functional groups in the materials were confirmed by the FTIR spectra (Fig. 1A). By comparing the FTIR spectra of CTAB with CTAB@BiOCl, it can be observed that there are the same absorption peaks at 2920, 2850 (–CH₃) and 1450, 1380 cm⁻¹ (–CH₂–). These indicate that CTAB was embedded into the composite material. In contrast, the regenerated composite has very weak peaks in these positions, which indicates that CTABs were degraded from the CTAB@BiOCl composite after five cycles of photocatalysis. Besides, all CTAB@BiOCl, BiOCl and the regenerated composite have the absorption peak at 524 cm⁻¹. It was attributed to the characteristic symmetrical stretching vibration of the Bi–O bond.¹⁷

Fig. 1 FTIR (A) and XRD (B) of BiOCl (1), the regenerated composite (2), CTAB@BiOCl (3) and CTAB (4).

From XRD of the materials (Fig. 1B), all diffraction peaks were consistent well with the BiOCl standard (JCPDS No: 06-0249). It indicates all the samples were crystallized in the tetragonal phase. The (110) diffraction peak is much sharper and stronger than other peaks. This meant that the CTAB@BiOCl composite should favour to grow along the (110) orientation and form very thin slabs.³³ It can be further proved by the SEM and TEM images. The CTAB@BiOCl composite has some peaks between 10 and 20 degree, which further confirms the existence of CTAB.

The N₂ adsorption isotherm and pore-size distribution of CTAB@BiOCl, BiOCl and the regenerated composite are shown in Fig. 2. The N₂ adsorption–desorption shape accords with the classical type IV isotherm. It refers to the most widespread adsorption behavior of the mesoporous materials. The pore size distribution of CTAB@BiOCl is from 10 to 50 nm and that of BiOCl from 5 to 20 nm. It further proves they are mesoporous. After five recycles, the regenerated composite shows an obvious increase of the N₂ adsorption. The disappearance of CTAB increases the BET surface area of the regenerated composite from 7.5 to 22.62 m² g⁻¹, and the BET surface area of BiOCl is 53.5 m² g⁻¹. All of the adsorption isotherms exhibit the obvious hysteresis loops in accordance with type H3. It indicates the presence of slit pores formed by the flake particles accumulating.

Fig. 2 N₂ absorption–desorption isotherm and pore-size distribution (inset) for CTAB@BiOCl (A), the regenerated composite (B) and BiOCl (C).

The morphologies were determined by SEM and TEM to further investigate the relationship between structures and properties for these composites. The SEM images show that all CTAB@BiOCl (Fig. 3A), the regenerated composite (Fig. 3B) and BiOCl (Fig. 3C) process the layered structures. In CTAB@BiOCl, CTAB obviously makes the structure loose. And the regenerated composite is flower-like, which might be caused by the escapement of CTAB during photocatalysis. In addition, TEM images further prove these layered structure features. The CTAB@BiOCl and BiOCl particles are irregular in the size of 100–500 nm (Fig. 3D and F). The thickness of nanoplates is estimated to be 10–20 nm (Fig. 3E).

Fig. 3 SEM and TEM images of CTAB@BiOCl (A and D), the regenerated composite (B and E) and BiOCl (C and F).

3.2. Effect of CTAB

To further evaluate the effect of CTAB, relevant experiments are performed for the adsorption of CR and the degradation of X3B. As shown in Fig. 4A, the addition of CTAB makes the CTAB@BiOCl composite has an excellent adsorptivity, which is much better than the regenerated composite and the isolated BiOCl. It is attributed to the fact that cationic CTAB has an outstanding adsorption to anionic dyes.²⁹ Fig. 4B shows the removal rate of X3B after adsorption and irradiation for different times. When the reaction was performed under dark for 30 min to reach the absorption equilibrium, the removal rate was 46.8%, 9.3%, 20.3% for CTAB@BiOCl, BiOCl and the regenerated composite, respectively. This further proves what we said in previous article. Under UV illumination, it is notable that the regenerated composite consumes less time than BiOCl and CTAB@BiOCl to reach the removal rate of 90%. Both the increased surface area and featured flower-like structure might contribute to that enhanced efficiency. There are two reasons leading to the poor photocatalytic performance of CTAB@BiOCl. The degradation of CTAB (Table S1†) consumes extra energy in the photocatalytic reaction, and as the active photocatalyst, the BiOCl content in CTAB@BiOCl is less than 70% (Table S1†). All in all, the addition of CTAB greatly improves the adsorptivity of CTAB@BiOCl and partly reduces the photocatalytic performance.

Fig. 4 The adsorption of 420 mg L⁻¹ CR (A) and photocatalytic degradation of 40 mg L⁻¹ X3B (B) in the presence of CTAB@BiOCl, BiOCl and the regenerated composite.

3.3. Evaluation of the degradation efficiency

The adsorptive capacity and photocatalytic activity of the CTAB@BiOCl composite were tested by CR and X3B solutions. To exclude the possibility of dyes self-photolysis, the comparisons among different treatments were collected in Fig. 5A and B. It was found that both of CR and X3B can be decomposed under simulated UV light irradiation without photocatalyst addition. Within 40 min, the removal rates reached 30% and 20% respectively. Differently, with the addition of CTAB@BiOCl, the photocatalytic degradation rates of CR and X3B were over 94% under simulated UV light irradiation, and the dye molecules absorbed by CTAB@BiOCl were almost degraded. What's more, in order to test the reusability and stability of the composite, the recycle was carried out to degrade 20 mg L⁻¹ X3B for 20 min. The catalyst was collected from each cycle by centrifugation and reused. The degradation rate of X3B remains over 90% after six recycles (Fig. S1†), i.e. the photocatalytic activity of CTAB@BiOCl composite has no obvious decrease. The results indicate that the CTAB@BiOCl composite exhibited a good stability and photocatalytic activity-remaining. In addition, under dark condition, the adsorption efficiencies of CR and X3B solution by CTAB@BiOCl composite were respectively 50% and 60%.

Fig. 5 Comparison for the effect of photocatalysis, adsorption and photolysis on the removal rate of 35 mg L⁻¹ CR (A) and 40 mg L⁻¹ X3B (B), the adsorption of CR (C) and X3B (D) with various concentrations.

3.4. Evaluation of the adsorption efficiency

The removal rates of CR and X3B is less than 70% when the dye concentration is lower than 0.3 mmol L⁻¹ (Fig. 5C and D). With further increasing the dye concentration, the removal rate increased up to 100% and then decreased. In the low concentration dye solution, the interaction of sulfonic dye with cationic CTAB may be weakened which leads to the poor adsorption capacity. The ζ-potential change proved that the interaction depends on the electric charge effect (Table S2†). The adsorption of dyes on the composite obeyed the Langmuir isotherm model e.g. c_eq_e⁻¹ = q_mb⁻¹ + c_eq_m⁻¹ (Fig. S2†), where q_e and c_e are the adsorption amount and the concentration of dye at the equilibrium, q_m is the saturation adsorption amount and b the binding constant.³⁴ The q_m values were calculated to be 901 mg g⁻¹ for CR and 699 mg g⁻¹ for X3B, which are much more than those with the other sorbents.³⁵ Moreover, the removal rate of dyes approached maximum only in thirty seconds. The adsorption with the composite is much faster than that with activated carbon. The salty effect caused the dyes adsorption increased with the increasing of ionic strength at the very start, and then it achieves a balance (Fig. S3†). The removal rate of CR and X3B increased by 113% in 0.3 M NaCl and 89% in 0.5 M NaCl. Therefore, the CTAB@BiOCl composite is favorable for treatment of the high-salinity dye wastewater.

4. Conclusions

In summary, a new, multifunctional CTAB@BiOCl composite was synthesized in a facile condition and it exhibits a highly photocatalytic activity and well adsorptive capacity. For low concentration of dyes-contaminated water, the degradation of CR and X3B is up to 94% within 40 min. The recycle experiment demonstrates that the CTAB@BiOCl composite has an excellent reusability and stability. The addition of CTAB significantly improve the adsorption performance without compromising its photocatalytic activity. For high concentration of dyes-contaminated water, different from other adsorbents, in the adsorption process of CR and X3B, there is a special concentration point where the adsorption efficiency is at its peak value. The maximum adsorptions of CR and X3B are 901 and 699 mg g⁻¹, respectively. In addition, high salinity can promote the adsorption efficiency. The above-mentioned qualities make CTAB@BiOCl be able to treat low-concentration pollutants predominantly by photocatalysis, and high-concentration pollutants predominantly by adsorption. In other words, CTAB@BiOCl shows potentials in the efficient treatment of different levels of dyes-contaminated wastewater.

Acknowledgements

This work was financially supported by the National Key Technologies R&D Program of China (No 2012BAJ25B02).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27714k

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