The detoxification of dye-contaminated water by photocatalysis has become a research priority. Here, a novel hybrid material, cuprous oxide/sunflower stem pith (Cu2O/SSP), was successfully synthesized in situ, using copper hydroxide gel, prepared by ion exchange, as the precursor to Cu2O. The presence of Cu2O nanoparticles on the SSP was confirmed by scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray diffraction analyses. Using methylene blue (MB) as the target pollutant, Cu2O/SSP delivered excellent adsorption–photocatalytic degradation and was readily photoregenerated. Cu2O/SSP removed 72.7% of MB after 60 min under visible light irradiation, an increase of 15.6 % compared with unmodified SSP. SSP plays three roles in the removal of MB: it acts as an adsorbent for the MB, a carrier for the Cu2O nanoparticles and it also inhibits photocorrosion of Cu2O. The mechanism of adsorption–photocatalysis by Cu2O/SSP was investigated and a description of the mechanism is provided. This study paves the way for the detoxification of dye-containing wastewater using hybrid biomass materials.

  • Cu2O/SSP was synthesized in situ using copper hydroxide gel, prepared by ion exchange, as the precursor to Cu2O.

  • Cu2O nanoparticles were well scattered and anchored to the surface of SSP.

  • The Cu2O combined with SSP was effective at inhibiting photocorrosion of Cu2O.

  • Cu2O/SSP showed excellent adsorption–photocatalytic and photoregeneration performance.

  • The adsorption–photocatalysis mechanism for the MB detoxification was discussed.

The discharge of untreated, dye-contaminated wastewater into water bodies poses enormous risks to ecological systems (Das & Adak 2022; Goswami et al. 2022; Singh et al. 2022). Nonbiodegradable organic substances in dye-contaminated water may also accumulate in the human body, leading to ill health and, possibly, even cancer. Much effort has focused on treatment of dye-contaminated water and a number of methods, including adsorption, membrane separation, photocatalytic oxidation, ozone oxidation, and biological methods (Hassan et al. 2022) have been developed. Of these, adsorption–photocatalytic oxidation has been most widely used to detoxify dye-contaminated wastewater. In general, a synergistic combination of adsorption and photocatalysis is the most common technology used to control water pollution (Jiang et al. 2016; Li et al. 2016; Zhang et al. 2016). Recently, difunctional materials with both adsorption and photocatalytic properties have been developed, with research focusing on two main aspects: (i) photocatalysts with high specific area and (ii) difunctional materials with a porous material acting as a carrier for the photocatalyst. Photocatalysts with high specific area include bismuth oxyiodide (Hao et al. 2012) and bismuth oxyiodide/silver vanadate (Wang et al. 2015), which were synthesized for the detoxification of dye-containing wastewater. Difunctional materials include titanuim dioxide/chitosan (Gozdecka & Wiacek 2018), titanium dioxide–manganese(II) titanate/hollow activated carbon fibers (Li et al. 2017), and zinc oxide/activated carbon (Cruz et al. 2018). The selection of an appropriate adsorbent/photocatalyst pair is key for synergistic adsorption and photocatalysis.

Methylene blue (MB), one of the best-known cationic dyes, is widely used in the textile, printing, and papermaking industries. Lignocellulose materials, such as black cumin (Nigella sativa L.) seeds (Thabede & Shooto 2022), Brachiaria mutica (para grass) and Cyperus rotundus (nut grass) (Arora et al. 2022), rice husk (Vadivelan & Kumar 2005), palm kernel fiber (El-Sayed 2011), and tobacco rob residues (Wang et al. 2018) are ideal adsorbents for MB. In 2019, our group found that sunflower stem pith (SSP) is an effective and inexpensive adsorbent for the removal of MB from sewage (Liu et al. 2019a). SSP has a pseudohexagonal porous structure that serves as an excellent carrier of photocatalysts. Cuprous oxide (Cu2O), a p-type semiconductor with a narrow band gap of 2.17 eV (Kandula & Jeevanandam 2016), is a high-performance visible light-responsive photocatalyst that has been used for detoxification of MB (Chai et al. 2016). There are many advantages to using Cu2O as a photocatalyst, including low cost, low toxicity, good mobility, and high abundance (Da Costa et al. 2017) and, over the last decade, various Cu2O/lignocellulose materials have emerged. The lignocellulose materials described in the literature are primarily cotton fibers; Cu2O/cotton fiber hybrid materials have been synthesized in situ, using CuSO4 as a precursor to Cu2O and glucose as the reducing and capping agent, following reaction at 70 °C for 1 h under alkaline conditions (Montazer et al. 2015). This fiber composite was shown to have self-cleaning and antibacterial properties, but the deposition of Cu2O was relatively low. Cotton fibers have also been pretreated by TEMPO-mediated oxidation to create carboxylate groups that can anchor Cu2+, followed by either hydrazine monohydrate or hydroxylamine reduction, to prepare a Cu2O/cotton fiber composite with good antibacterial properties (Errokh et al. 2016). Using a similar approach, sodium borohydride, which reduces Cu2+ to Cu and Cu+, has been used as the reductant to prepare a Cu/Cu2O/cotton fiber composite (Marković et al. 2017).

The present study focuses on in situ preparation of Cu2O/SSP under alkaline condition at 75 °C, using copper hydroxide (Cu(OH)2) gel synthesized by ion exchange as the Cu2O precursor and glucose as the reductant. The Cu2O/SSP was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy. Under visible light irradiation, Cu2O/SSP achieved satisfactory degradation of MB. The adsorption-photocatalysis mechanism for detoxification of MB using Cu2O/SSP under visible light is discussed.

Materials

Sunflower stems were collected from farmland in Zhaoyuan County, China. SSP was separated from the bark, washed with deionized water to remove impurities, and dried in sunlight until all the moisture had evaporated. Anion exchange resin (201 × 7), glucose, copper chloride, sodium hydroxide, and sodium chloride were all analytical grade and were used without further purification. Deionized water was used in all experiments.

Preparation of Cu2O/SSP

The experimental process for the preparation of Cu2O/SSP is illustrated in Figure 1 and described in more detail below. Before the in situ synthesis of Cu2O/SSP, copper hydroxide gel was synthesized as the precursor to Cu2O and, separately, the reductant glucose was adsorbed onto the SSP. Copper chloride solution was added to anion exchange resin under continuous stirring at room temperature, immediately producing a blue copper hydroxide gel. Once the reaction was complete, the supernatant gel was separated from the resin. The reductant glucose was adsorbed onto the SSP by immersing the SSP (1 g on a dry-weight basis) in saturated glucose solution (300 mL) at room temperature for 12 h. The SSP was then removed from the solution and excess solution was removed by suction filtration. The resulting material was conditioned at room temperature at least overnight. The glucose-loaded SSP was then added to the copper hydroxide gel with vigorous stirring under alkaline conditions (pH 11) at 75 °C. After 1 h, the resulting Cu2O/SSP was removed by suction filtration, washed with deionized water, and dried at 40 °C for 7 h.
Figure 1

Preparation of Cu2O/SSP.

Figure 1

Preparation of Cu2O/SSP.

Close modal

Photocatalysis and photoregeneration

Cu2O/SSP was used to degrade MB under irradiation with a 500 W Xenon lamp (CME-SL500, Beijing Micro Energy Technology Co., Ltd), located at a distance of approximately 30 cm. Cu2O/SSP (0.4 g) was added to an aqueous solution of MB (50 mg/L, 400 mL), the mixture was stirred magnetically for 1 h in the dark to reach adsorption–desorption equilibrium, and the xenon lamp was then turned on. Aliquots (5 mL) of the reaction mixture were removed every 10 min and filtered through speed filter paper before analysis of the MB concentration by measuring absorption at 665 nm using a 722 spectrophotometer, with deionized water as the control. The concentration of MB was calculated using the Beer–Lambert law. The standard working curve (y = 0.1846x + 0.00739) was obtained, with a correlation coefficient of 0.9997. The photocatalytic efficiency η was calculated using Equation (1):
formula
(1)
where C0 is the initial concentration of MB and Ct is the equilibrium concentration of MB after the photocatalytic reaction.

Photoregeneration of Cu2O/SSP was investigated by using the adsorption–photocatalytic system for detoxification of MB until the Cu2O/SSP became saturated. The Cu2O/SSP was then separated and dried at 60 °C for 12 h in the dark. The MB-saturated Cu2O/SSP was then irradiated with visible light for 6 h using a 500 W Xenon lamp. After photoregeneration, the adsorption–photocatalytic performance of the Cu2O/SSP were evaluated as described earlier.

Characterization of materials

The surface morphology and elemental composition of the cuprous oxide were investigated using a Sigma scanning electron microscope, operating at 10 kV. FTIR spectra were recorded using potassium bromide disks, prepared using spectral grade potassium bromide pellets, with a Tensor 27 spectrometer over the range 400–4,000 cm−1. XRD patterns of the cuprous oxide were recorded using an Empyrean X-ray diffractometer from 2θ = 10° to 2θ = 80°, using Cu Kα radiations at a scan rate of 0.02 s−1.

Morphology and structure

The morphology and structure of the SSP and Cu2O/SSP were studied using SEM and XRD. The surface of the SSP has a pseudohexagonal, three-dimensional, multi-porous structure, with a relatively smooth surface (Figure 2(a) and 2(b)). SEM images of Cu2O/SSP showed that the Cu2O nanoparticles are well distributed and anchored to the surface of the SSP (Figure 2(c) and 2(d)). Agglomeration of Cu2O nanoparticles was limited by using Cu(OH)2 gel as the precursor. The microspheres of Cu2O nanoparticles are uniform, with an average diameter of ∼950 nm (Figure 2(e)). This finding is in good agreement with the XRD and FTIR results. The agglomeration of Cu2O nanoparticles does not occur in the absence of stabilizer and surfactant, which can be attributed to the well-dispersed Cu(OH)2 precursor prepared by ion exchange.
Figure 2

Morphology and structure of materials. (a) SEM image of SSP, magnification ×200; (b) SEM image of SSP, magnification ×500; (c) SEM image of Cu2O/SSP, magnification ×200; (d) SEM image of Cu2O/SSP, magnification ×500; (e) particle size distribution of Cu2O deposited on surface of SSP; (f) XRD patterns of SSP and Cu2O/SSP.

Figure 2

Morphology and structure of materials. (a) SEM image of SSP, magnification ×200; (b) SEM image of SSP, magnification ×500; (c) SEM image of Cu2O/SSP, magnification ×200; (d) SEM image of Cu2O/SSP, magnification ×500; (e) particle size distribution of Cu2O deposited on surface of SSP; (f) XRD patterns of SSP and Cu2O/SSP.

Close modal

The XRD patterns of Cu2O/SSP and SSP (Figure 2(f)) both show strong diffraction peaks at 2θ = 15.5° and 2θ = 22°, which are attributable to the crystalline structure of cellulose (Youssef et al. 2012). This suggests that the chemical structure of cellulose does not change in the SSP. The characteristic peaks at 2θ = 29°, 36°, 42°, 62°, 73° and 77° in the Cu2O/SSP samples were assigned to the {110}, {111}, {200}, {220}, {311} and {222} crystal planes, respectively, of Cu2O (JCPDS No.78-2076) (Zhang et al. 2010). All of these results indicate that Cu2O was successfully loaded into the SSP.

Adsorption–photocatalytic properties and photoregeneration of Cu2O/SSP

The mechanism for the synergistic adsorption–photocatalytic degradation of MB using Cu2O/SSP is shown in Figure 3. The process involves two steps: adsorption and photocatalysis. SSP adsorbs MB very well because of its large specific surface area (2.24 m2/g) and can also combine with MB through electrostatic attractions and Van der Waals forces. The maximum adsorption capacity of SSP for MB is 277 mg/g at 338 K (Liu et al. 2019a). During photocatalysis, pure Cu2O, which has a low band gap (2.17 eV), absorbs visible light to generate holes (h+) and electrons (e) under simulated sunlight. These holes and electrons can react with H2O and O2 in solution to produce highly reactive hydroxide radicals ( •OH) and superoxide radicals ( •O2), which degrade MB. Since Cu2O can be easily deactivated by photocorrosion during the hotoreaction (Huang et al. 2009), the stability of Cu2O is a key factor for detoxification of MB. In this work the SSP plays a sacrificial role in inhibiting photocorrosion of Cu2O since hydroxide radicals and superoxide radicals react with the SSP instead of causing self-oxidation of Cu2O. Despite this overview of the mechanism of removal of MB by Cu2O/SSP, the precise molecular structure of the MB degradation products and the SSP oxidation products formed during the adsorption–photocatalytic process needs further elucidation. The whole adsorption–photocatalytic reaction sequence can be summarized by Equations (2)–(6):
formula
(2)
formula
(3)
formula
(4)
formula
formula
(5)
formula
(6)
Figure 3

Schematic showing mechanism of removal of MB by Cu2O/SSP.

Figure 3

Schematic showing mechanism of removal of MB by Cu2O/SSP.

Close modal
The FTIR spectra of SSP, Cu2O/SSP, Cu2O/SSP loaded with MB, and Cu2O/SSP after photoregeneration are shown in Figure 4. In SSP, the peak at 3,411 cm−1 was assigned to hydroxide radicals and the characteristic peak at 2,926 cm−1 was assigned to C–H bond stretching. The adsorption peaks at 1,630 cm−1, 1,538 cm−1, and 1,425 cm−1 are characteristic of the aromatic hydrocarbon skeleton. The peaks at 1,739 cm−1 and 1,060 cm−1 were assigned to C = O bond and C–O bond stretching, respectively. The peaks centered at 727 cm−1 were attributed to out-of-plane bending vibrations of unsaturated C–H bonds. The FTIR spectra thus indicate the presence of hydroxide radicals, carboxyl groups, intermolecular hydrogen bonds, and aromatic groups in the SSP (Liu et al. 2019b). In the spectra of Cu2O/SSP and Cu2O/SSP loaded with MB, the characteristic peak at 636 cm1 was assigned to Cu–O stretching vibrations, indicating that Cu2O had been successfully loaded into the SSP. This result is in good agreement with the SEM and XRD analyses. The peak at 895 cm1 is characteristic of β-D glucose and is due to residual glucose that was not completely washed out. After photoregeneration, the intensity of the peak at 3,332 cm1 in the spectrum of Cu2O/SSP decreased significantly because the SSP was oxidized by hydroxide radicals and superoxide radicals under visible light irradiation.
Figure 4

FTIR spectra of samples.

Figure 4

FTIR spectra of samples.

Close modal
In order to analyze the photocatalytic performance and extent of photoregeneration of Cu2O/SSP, MB adsorption and the photocatalytic capacity of Cu2O/SSP, SSP and Cu2O/SSP after photoregeneration were evaluated (Figure 5). The pseudo-second order kinetics equation and Langmuir isotherm describe well the process of MB adsorption by Cu2O/SSP (data not shown). The maximum adsorption capacity (338 K) for MB, as calculated from the Langmuir isotherm, was 256 mg/g. The MB adsorption capacity of Cu2O/SSP was almost identical to that of SSP in the dark, indicating that the deposited Cu2O does not reduce the adsorption sites of SSP. SSP itself does not have photocatalytic activity but the concentration of MB gradually decreased using Cu2O/SSP as photocatalyst under visible light irradiation. SSP plays a sacrificial role in preventing photocorrosion of the cuprous oxide. SSP and Cu2O/SSP, used as adsorbents in the dark, removed 57.1% and 56.8%, respectively, of the MB, showing that SSP has an excellent adsorption capacity for MB that is unaffected by the presence of Cu2O. Under irradiation with visible light, the amount of MB removed by Cu2O/SSP gradually increased to 72.7% within 60 min as a result of photocatalytic degradation, an increase of 15.6% compared with removal by SSP alone, showing that Cu2O/SSP has excellent photocatalytic performance.
Figure 5

Adsorption–photocatalytic degradation profiles of MB by Cu2O/SSP.

Figure 5

Adsorption–photocatalytic degradation profiles of MB by Cu2O/SSP.

Close modal

The photoregeneration of Cu2O/SSP activity was also investigated. The amount of MB removed within 60 min using photoregenerated Cu2O/SSP as adsorbent in the dark was 26.8%. The capacity of photoregenerated Cu2O/SSP to adsorb MB was thus significantly reduced compared with that of fresh Cu2O/SSP. This may be because the numerous adsorption sites of the Cu2O/SSP are not regenerated by the photoregeneration method, although 49.7% of the MB was removed after 60 min under visible light irradiation. The extent of MB removal was thus increased by 22.9% compared with adsorption alone, indicating that, although the adsorption–photocatalytic performance of the photoregenerated Cu2O/SSP decreased, its photocatalytic capacity increased.

In order to evaluate the photoregeneration of Cu2O/SSP, we tested the pseudo-first order kinetics equation in the framework of the Langmuir–Hinshelwood model (Oancea & Oncescu 2008; Shokri 2021).
formula
(7)
formula
(8)
formula
(9)
formula
(10)
where γ0 is the initial rate of MB degradation, c0 is the initial concentration of MB, Kad is the equilibrium constant for MB adsorption onto Cu2O/SSP, kΓ is the reaction rate of photocatalytic oxidation, k1 is the first-order degradation constant, and t1/2 is the half-life.
Equation (8) is the linear form of Equation (7) and from the slope of this line we calculated the first-order degradation constant and half-life. As shown in Figure 6 and Table 1, the experimental data for MB photocatalytic degradation by Cu2O/SSP and Cu2O/SSP after photoregeneration fit the pseudo-first order kinetics equation in the framework of the Langmuir–Hinshelwood model, with high correlation coefficients (0.9074 and 0.9694, respectively). These results show that the pseudo-first order kinetics equation in the framework of the Langmuir–Hinshelwood model are well suited to describe the photocatalytic degradation of MB. It is clear from Table 1 that the first-order degradation constant and half-life of MB degradation by Cu2O/SSP and Cu2O/SSP after photoregeneration are very similar, demonstrating that photoregeneration can effectively recover the photocatalytic performance of the Cu2O/SSP.
Table 1

Kinetic equation parameters of MB photodegradation under visible light irradiation by Cu2O/SSP and Cu2O/SSP after photoregeneration

MaterialKinetic equation linear formk1 (min−1)R2t1/2 (min)
Cu2O/SSP y = 0.00644x + 0.49499 0.00644 0.9074 108 
Cu2O/SSP after photoregeneration y = 0.00574x − 0.03335 0.00574 0.9694 120 
MaterialKinetic equation linear formk1 (min−1)R2t1/2 (min)
Cu2O/SSP y = 0.00644x + 0.49499 0.00644 0.9074 108 
Cu2O/SSP after photoregeneration y = 0.00574x − 0.03335 0.00574 0.9694 120 
Figure 6

Pseudo-first order kinetics in the framework of the Langmuir–Hinshelwood model of MB photocatalytic degradation.

Figure 6

Pseudo-first order kinetics in the framework of the Langmuir–Hinshelwood model of MB photocatalytic degradation.

Close modal

A Cu2O/SSP composite was successfully prepared in situ and showed excellent degradation of MB by adsorption-photocatalysis under visible light irradiation, both in the pristine state and after photoregeneration. The synergistic effect of the combination of SSP and Cu2O significantly enhanced capacity for removal of MB, inhibited photocorrosion of Cu2O and allowed photoregeneration. The photocatalytic degradation of MB was well described by the pseudo-first order kinetics equation in the framework of the Langmuir–Hinshelwood model. The Cu2O/SSP composite is an excellent candidate for the widespread detoxification of azo-dye-polluted wastewater.

This work was supported by the Foundation of Key Laboratory of Pulp and Paper Science and Technology of Ministry of Education, Shandong Province of China (KF201826).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.

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