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Abstract
A light source from a neodymium-doped yttrium aluminium garnet laser (1064 nm) was used to excite the A3 iron plate to produce plasma and release UV light for the rapid photocatalytic degradation of methylene blue (MB) dye. The 30 nm anatase TiO2 nanoparticles were used as the photocatalysts. Plasma effectively degrades organic matter under optimal conditions: (10 mg/L MB, 2 g/L TiO2, 50 ml/L H2O2, pH = 10, and P = 70 mW); the degradation efficiency is related to the UV light source (200 nm - 400 nm) produced by plasma sonoluminescence. The effect of the initial dye concentration, catalyst dosage, laser energy, and pH value, on the degradation of dyes was studied using UV–vis spectrophotometry. Ion chromatography confirmed the mineralisation of methylene blue (MB). The results showed that laser-induced plasma (LIP) photocatalysis effectively degrades MB.
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1. Introduction
With the rapid advancement of textile technology, various dyes have been developed, but the composition of dye wastewater has become increasingly complex. In addition to hindering the penetration of dissolved oxygen into natural water, some dyes are carcinogenic and can seriously endanger life and health. Therefore, the problem of dye pollution must be urgently addressed [1,2]. Methylene blue (MB) is a typical organic pollutant found in wastewater that is discharged by printing and dyeing factories. Degrading MB is crucial for treating this wastewater.
The conventional methods for treating dye wastewater include adsorption, flocculation, sedimentation, filtration, and gas extraction. These methods only separate or concentrate pollutants or simply transfer the pollutants from one phase to another, which inevitably results in waste and secondary pollution [3–5]. Therefore, the dye pollutants must be effectively degraded.
At present, the photocatalytic removal of organic pollutants has attracted considerable attention because of its advantages such as high efficiency and energy conservation. In photocatalytic oxidation technology, light radiation is combined with oxidants, such as hydrogen peroxide and oxygen, to produce hydroxyl radicals that promote the removal of organic matter. This method requires a UV light source and a catalyst [6]. Several traditional UV light sources use xenon or mercury vapour high-pressure lamps to produce radiation over a wide wavelength range for degrading dyes. However, the prolonged use of UV lamps presents various problems, including long-term power instability, low photon utilisation efficiency, long-term irradiation of dyes for complete mineralisation, and the existence of harmful mercury [7,8].
Currently, lasers are considered a new, pollution-free, efficient, and safe light source. Initially, photons of the laser are organised coherently and emitted directionally. A substantial amount of photon energy is concentrated in a small space; thus, the energy density is high [9]. Subsequently, owing to the effects of sonoluminescence and blackbody radiation [10], laser-induced plasma (LIP) generates light radiation in a wide spectrum, from ultraviolet to infrared. Therefore, combining LIP with TiO2 to obtain a new light procedure can significantly improve the efficiency of photocatalytic oxidation technology. However, high-power UV lasers have been typically considered in previous studies. To date, the photocatalytic degradation of dyes using 1064 nm laser irradiation medium-excited plasma has not been extensively studied. Therefore, the main objectives of this study are (1) to examine the photocatalytic oxidative degradation of MB dye using 1064 nm laser excited plasma as a light source and (2) to experimentally obtain the optimal factors affecting the degradation of MB dye. Therefore, this study provides a reference for degrading hazardous waste using IR lasers.
2. Experimental
2.1. Materials
MB and 30% hydrogen peroxide (H2O2) were purchased from Aladdin Reagent Institute, Shanghai, China. MB is an aromatic compound with the molecular formula C16H18ClN3S. At room temperature, it is a solid, odourless, dark-green powder that dissolves in water to produce a blue solution. The UV–vis spectra of MB in water exhibit three absorption peaks at 247, 292, and 665 nm. The absorbance of the solution was measured at a wavelength of 665 nm to evaluate the degradation of MB. To study the effect of pH on degradation efficiency, the pH of the solution (2–12) was adjusted using 1 mol/L HCl or NaOH. The dye concentration (0.03 mmol/L-0.13 mmol/L) was determined using UV–vis spectrophotometry (Shimadzu, Kyoto, Japan) to evaluate the effect of the initial dye concentration on the degradation efficiency. The degradation products of the mixed MB solution were measured using an ICS-90 ion chromatograph (Switzerland). Deionised water was used for preparing the experimental solutions.
2.2. Experimental procedure
The experimental setup used in this study (Fig. 1) primarily consists of a 1064 nm neodymium-doped yttrium aluminium garnet (Nd:YAG) laser (Laserver, Wuhan, China), a laser energy meter (Coherent, USA), an optical fibre spectrometer (BIM-6601, Sichuan, China), and a photocatalytic reactor. The quartz reactor is a 30 mL UV-quartz glass beaker made of jgs1 material, which can transmit light of 185 nm-2500 nm wavelength, and the transmittance is 95%. The laser beam was divided into two using a beam splitter (Leo-1064-g0032a; Beijing, China) made of K9 material; one beam was diverted to the laser energy meter for the real-time measurement of its intensity, and the other was focused through a convex lens (GCL-0108, Beijing, China) on an A3 iron or T2 copper plate (thickness 1 mm, size 20 mm) in a quartz beaker containing the MB solution. In the absence of the metal plate, the beam was focused on the centre of the solution. The luminescence signal of the plasma was simultaneously obtained using lateral collection. To compare the effects of different light sources on the photocatalytic degradation of MB, an ultraviolet mercury lamp at 365 nm (SPN-CLR365, USA) was used as the control.
Prior to the laser irradiation, 30% H2O2 solution was mixed with 10 mg/L MB solution at a ratio of 1:7 and 0.04 g TiO2 particles was added; the mixed solution was stirred in the dark for 30 min. The metal sheet was placed in 20 mL of an aqueous MB solution, which was stirred in the dark for 1 min. The concentration of MB (10 mg/L) in the solution before irradiation was considered the initial concentration for measuring the degradation of MB. The mixed solution was subsequently irradiated with a laser that emits pulsed light three times per second. The mixed solution was separated from the metal plate and centrifuged at 10000 rpm for 5 min to filter the TiO2 particles. Subsequently, the transparent top layers of the solutions were transferred to quartz cuvettes to measure their absorption spectra in the wavelength range of 200–900 nm using a UV-2401 spectrophotometer (Shimadzu, Japan). According to the Beer–Lambert law, the concentration of light-absorbing substances varies linearly with absorbance corresponding to the maximum absorption peak [11]. The concentration of MB (λ_max = 665 nm) in the treated solution was determined using a concentration–absorbance calibration curve generated from the absorbance measurements of MB samples at known concentrations. Finally, the degradation products of the MB mixed solution were measured using an ICS-90 ion chromatograph (Switzerland).
2.3. Experimental results
In order to explore the degradation efficiency of MB solution by different light sources, we conducted four groups of experiments: the mixture of TiO2 and the dye was directly irradiated by plasma excited by a focused Nd:YAG laser, LIP produced by Cu and Fe metals, and a UV mercury lamp (365 nm). In the above four groups of control experiments, the dye was a 10 mg/L methylene blue solution, and the concentration of TiO2 was 2 g/L. The volume of the solution was 20 mL and the temperature was 24.5 °C, but the light source was different. The time-dependent degradation efficiency of MB was measured using UV–vis spectrophotometry (Fig. 2(a)).
Fig. 2. (a) Degradation of methylene blue (MB) dye (10 mg/L) irradiated by different sources of light. (b) Luminescence spectra of MB and the mercury lamp and those of Cu and Fe in MB.
3. Results and discussion
3.1. Plasma excited by focused Nd:YAG laser for MB degradation
As shown in Fig. 2(a), the MB removal rate increases over time. The dye can fully absorb the light source in the first 10 min, so the degradation efficiency is fast. After 10 min, the dye will be degraded into other small molecular substances, such as water and sulfoxide, which will dilute the dye and affect the degradation efficiency. The degradation rate reached 10% when Nd:YAG laser focused excited plasma irradiated the solution for 60 min. The laser irradiation of MB produces LIP, which has unique physical properties: the free electrons and ions in plasma have high kinetic energy and high temperature, and their high energy characteristics cause the dissociation of molecules to generate free radicals, leading to some chemical reactions in highly excited states with active chemical properties [12]. However, the luminescence characteristics of LIP can be exploited to use it as an induced light source. As shown in Fig. 2(b), the plasma focused on the mixed solution produces near UV light of 350 nm-400 nm. Therefore, under the action of UV light with a wavelength range of 350 nm-380 nm, H2O2 in the solution will be promoted to generate hydroxyl radicals and degrade the organic substance MB.
3.2. LIP excitation of Cu and Fe for MB degradation
As shown in Fig. 2(a), although the degradation efficiency of Cu metal sheet is lower than that of mercury lamp, the removal rate of MB is greatly improved after adding Fe metal sheet. At an illumination time of 60 min, the removal rates are 47 and 95%, respectively. The degradation efficiencies obtained are higher than those of the degradation experiment conducted directly with the LYP (black line in Fig. 2(a)), indicating that adding a metal plate promotes the degradation of organic compounds. This can be attributed to the relatively strong UV light generated by the excitation of LIP after the metal is added to the dye solution (Fig. 2(b)), which may be caused by the sonoluminescence generated by the laser pulse in the liquid. This can also be explained by the cavitation caused by the laser pulse in the liquid and the ion electron recombination of compressed bubbles at high temperatures [13]. The reasonably wide wavelength range of the sonoluminescence spectrum in liquids and its high-intensity UV emission are well known [14]. Moreover, because water molecules contain hydrogen and have a high thermal conductivity, the generated high-temperature plasma is easily quenched by water [15]. Therefore, the spectra in water exhibit only broadband and continuity characteristics; the atomic and ionic lines are completely quenched at the metal–water interfaces.
In particular, the degradation of MB using the Fe plate is considerably better than that achieved using the Cu plate under identical experimental conditions. This is because most of the continuous spectral lines generated via the excitation of Cu in MB are concentrated in the visible region, with only slight fluctuations appearing in the UV region. The continuous spectral lines produced by the excitation of Fe in MB are concentrated in the UV region, and the highest energy also appears in the same region. Therefore, compared with Cu, Fe better facilitates the production of high-energy UV light in the MB solution and promotes its degradation. The luminescence spectra of metals in solutions are generally closely related to their thermal conductivity. Typically, strong continuum emission is a complex phenomenon that includes Stark and Doppler broadening, bremsstrahlung, compound radiation, and blackbody emission [16]. Therefore, for high-density plasma, blackbody emission is used to explain the observed spectra at the metal–water interface; thus, the surface temperature of the metal is estimated in these experiments. In this study, the heat flux through the metals is dominant because of their high thermal conductivities, and the heat conduction and convection of water are negligible on a nanosecond timescale. Owing to the small heat diffusion length and radiation penetration depth, the heat conduction can be expressed as a one-dimensional equation under the surface heat flux boundary condition [17], as follows:
where K is the thermal conductivity, I(t) is the time-dependent incident laser pulse intensity, and ${R_\lambda } = \frac{{{{(n - 1)}^2} + {K^2}}}{{{{(n + 1)}^2} + {K^2}}}$ is the normal reflectivity of the surface. In this model, local thermal equilibrium was assumed, that is, $T(\textrm{elec}tron) \cong T(ion) \cong T(lattice)$; moreover, melting, vapourisation, and ablation were assumed to occur in the small skin-depth layer (<25 nm) and not significantly affect the heat diffusion process. Based on the aforementioned constant thermal properties, an analytical solution for the transient surface temperature can be obtained using Duhamel’s superposition theorem [17], as follows:Fig. 3. Transient surface temperature profiles derived using the heat diffusion model. The dashed line indicates the incident intensity I(t) (arbitrary scale).
3.3. UV-lamp-assisted degradation of MB
Figure 2(a) shows that the removal rate of MB increases with time. At an illumination time of 60 min, the degradation rate reaches 60%, which is related to the high emission peak observed around 365 nm (Fig. 2(b)). However, the degradation efficiency is lower than that in the experiment in which Fe is excited by LIP to produce intense UV light. Although the frequency of the LED ultraviolet lamp is high (50 Hz), its photons are emitted without any mode, and the light source is divergent, thereby resulting in significant energy loss. While the photons of the laser are organised coherently and emitted directionally, numerous photons are emitted in an extremely small spatial range, resulting in a high energy density [9] and intense LIP-induced UV energy owing to sonoluminescence. Additionally, in contrast to a UV lamp source, the coherence, monochromatic nature, and high directivity of a laser beam can enable the effective absorption of incident photons and improve the photodegradation rate.
3.4. Photocatalytic mechanism
The degradation products of MB measured by ICS-90 ion chromatography are shown in Table 1. When excited by UV light, TiO2 can form electron–hole pairs, and these electrons and holes can migrate to the surface of particles and undergo redox reactions with substances attached to the surface. Consequently, organic substances adsorbed on its surface can be degraded into pollution-free inorganic substances [21]. Therefore, the main photocatalytic reaction process of the laser-induced Fe sheet to produce plasma as the light source, combined with TiO2 photocatalyst to degrade dyes, can be summarised by Eqs. (3)-(9) and Fig. 4. First, TiO2 absorbs effective photons (hv≥3.2 eV) and generates conduction-band electrons and valence-band holes. Owing to the presence of oxygen in the reaction, the generated e− reacts with oxygen to produce ${\cdot} O_2^ - $:
Table 1. Ion chromatography (IC) test data
OH− in H2O is neutralized with holes to produce hydroxyl radicals:
${\cdot} O_2^ - $ is neutralised with protons:Hydrogen peroxide formation and transient oxidative disproportionation:
H2O2 reacts with e− to produce •OH:
The general equation for the degradation of MB can be expressed as follows:
In the initial step of MB degradation, •OH radicals attack the C–S+=C functional group [22]. Sulfhydryl (–S–) is a chromogenic group of MB that absorbs electrons with a considerably high electron cloud density. During UV degradation, it is initially oxidised by the •OH produced by photolysis to generate a sulfonyl group with an absorption wavelength of less than 180 nm [23]. Therefore, as the UV degradation reaction progresses, the blue colour of MB gradually fades, thereby indicating its decolourisation and decomposition. CO2, NO3−, SO42− and NH4+ appear in the reaction product, implying that the dye is mineralised by TiO2 irradiated by UV light.
Four different light sources were used to study MB degradation. It was optimally degraded when light generated by the LIP-excited Fe metal was used as the light source. This is attributed to the sonoluminescence caused by the LIP excitation of Fe, which produces high-density and high-energy UV light, resulting in a faster degradation rate than that observed in most other experiments.
4. Factors affecting degradation efficiency
4.1. Effects of initial dye concentration
By changing the initial concentration (10–50 mg/L), using Fe sheet-induced plasma as the light source with a TiO2 concentration of 2 g/L, the effect of the laser plasma on the initial dye concentration was studied. As shown in Fig. 5(a), an increased initial dye concentration decreases the degradation rate; this phenomenon is similar to the photocatalytic oxidation of other dyes [24]. The higher the initial concentration of the solution, the lower the light transmittance and the shorter the path of the ultraviolet photons released by the plasma in the solution. Consequently, the ability of the mixed solution to absorb photons is reduced, which simultaneously reduces the reaction rate. Furthermore, MB demonstrates a certain absorption effect on UV light [25], which decreases the number of effective photons received on the surface of the catalyst and the oxidising free radicals and electrons generated on the surface of the catalyst. Therefore, the degradation rate of the dye decreases with increasing concentration.
Fig. 5. Degradation rate of MB (10 mg/L) for different parameters. Effects of (a) initial MB concentration, (b) catalyst concentration, (c) initial H2O2 concentration, and (d) pH on the degradation of MB.
4.2. Effects of catalyst concentration
Fe sheet-induced plasma was used as the light source. The concentration of MB was 10 mg/L, and the dosage of photocatalyst was changed (0.5–4.0 g/L). The results are shown in Fig. 5(b). With the increase in the photocatalyst concentration from 0.5 to 2.5 g/L, the degradation rate increases from 50 to 99% within 60 min. When the photocatalyst concentration is increased to more than 2.5 g/L, the degradation rate of MB decreases. Under the experimental conditions, the optimal concentration of the photocatalyst is 2.5 g/L. The reasons for this phenomenon are as follows: the absorption capacity of UV light simultaneously increases with the increasing amount of the photocatalyst. Accordingly, the number of electron–hole pairs on the surface of the photocatalyst and the oxidative degradation capacity increase. However, when the photocatalyst supply continues to increase, the degradation rate of MB decreases. As the amount of the photocatalyst increases, the TiO2 particles distributed in the MB solution increase, thus decreasing the transmittance of the solution. This reduces the utilisation of TiO2 for UV light. Therefore, an optimal choice is obtained for the photocatalyst dosage.
4.3. Effects of initial H2O2 concentration
The concentration of H2O2 in a photocatalytic system is an important factor affecting the degradation of dyes. The concentration of hydrogen peroxide was changed (0–50 ml/L); the Fe sheet-induced plasma was used as the light source, and the concentration of TiO2 was 2 g/L. Subsequently, the effect of H2O2 concentration under the action of laser plasma was studied. As shown in Fig. 5(c), as the concentration of H2O2 gradually increases, the MB degradation efficiency also increases. This is because H2O2 can capture electrons on the surface of TiO2 during the reaction [26], and hydroxyl radicals are substantially generated during the electron transfer process. Simultaneously, holes (h+) in the reaction process of TiO2 further tend to undergo oxidation, significantly improving the photocatalytic activity of TiO2. The combined use of TiO2 and H2O2 can considerably improve the degradation rate of dyes and demonstrate a synergistic effect. The energy level difference of the TiO2 crystal is 308.8 kJ/mol (3.2 eV), which can be excited by light lower than 387.5 nm, and the O–O bond energy of H2O2 is 142 kJ/mol, which can be excited by light lower than 800 nm [27]. Therefore, the two catalysts can simultaneously accept the action of different light quanta, which can improve the utilisation of light, produce more hydroxyl radicals, and promote the degradation of MB.
4.4. Effects of initial pH of dye solution
In a photocatalytic system, the pH of the solution affects the surface properties of the photocatalyst, dissociation of dyes, and formation of hydroxyl radicals [9]. Using Fe sheet-induced plasma as the light source, the concentration of TiO2 was 2 g/L. The effect of pH on the action of the laser plasma was studied by changing the pH value (2–12). As shown in Fig. 5(d), when the pH increases from 2 to 10, the degradation efficiency increases. However, when the pH is further increased, the degradation rate is decreased. With an increasing pH, the number of negatively charged adsorption sites of TiO2 increases, while the number of negatively charged surface sites decreases. This is conducive to the adsorption of positively charged MB cations. In addition, acidic conditions present weak electrostatic interactions between the photocatalyst surface and dye cations and a weak adsorption capacity. Under alkaline conditions, the dispersion of TiO2 is enhanced, the particle size decreases, and the specific surface area increases, which facilitates the production of active hydroxyl radicals [28]. However, under highly alkaline conditions, the formation of hydroxyl radicals is inhibited and the degradation rate is reduced.
4.5. Effects of laser energy
In this study, Fe sheet-induced plasma was used as the light source, and the TiO2 concentration was 2 g/L. The effect of laser energy on MB degradation was studied by adjusting the output voltage of the laser to change its output power. The results are shown in Fig. 6(a). With an increase in laser energy, the catalytic effect of the photocatalytic reaction system is enhanced, and the degradation efficiency of MB is improved. This is mainly attributed to the increasing laser energy; the number of photons released increases along with that of photons absorbed per unit area of the mixed solution of MB and TiO2, thereby improving the degradation efficiency. Additionally, an increase in the laser energy can strengthen the intensity of the UV light released, thereby improving the penetration ability of UV light. The UV light absorbed by TiO2 increases, which stimulates the activity of the TiO2 photocatalyst and increases its catalytic oxidation of MB. However, in this study, owing to the excessive laser energy, the solution splashed; hence, the laser energy was controlled to 70 mW.
Fig. 6. (a) Effect of laser energy on the degradation of MB (10 mg/L). (b) Degradation of MB (10 mg/L) with TiO2 during repeated photo-oxidation experiments under laser-induced plasma (LIP) irradiation.
4.6. Stability of the catalyst
Furthermore, the stability and recoverability of the photocatalysts were studied. After six cycles of MB photodegradation, the catalyst maintained high catalytic activity (Fig. 6(b)), and the TiO2 particle size did not change significantly after several experiments (Fig. 7), indicating sustainable use.
5. Conclusion
In this study, the photocatalytic degradation of MB dye was achieved by laser irradiation using Fe sheet-induced plasma as the light source. This provided a new method for treating environmental organic pollutants using lasers. Under optimum conditions, the degradation rate of the MB solution can exceed 92% within 20 min of laser plasma irradiation. The enhanced photocatalytic efficiency can be attributed to the fact that when the laser breaks down the medium in the experiment, the sonoluminescence produces strong UV light, which, combined with the action of the TiO2 photocatalyst, accelerates the degradation of organic pollutants. Compared with results of the study on the degradation of nanoparticles by a UV light source, the degradation efficiency constant is significantly higher, and the UV light source described in this paper requires the use of only a traditional laser to stimulate Fe to generate plasma, which has a higher energy and photon density. Therefore, the laser-plasma-induced photocatalysis described in this paper is suitable for efficient environmental applications. Further research is currently underway to study the degradation of MB and other dyes by different metals using lasers of different energies and wavelengths.
Funding
Natural Science Basic Research Program of Shaanxi Province (No.2019JQ-852); Open Foundation of Key Laboratory of Dunhuang Medicine and Transformation (KLLDT202113); Sichuan Province Science and Technology Support Program (2021YFSY0027); National Natural Science Foundation of China (U2030108).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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