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Abstract
The spectral emission of laser-induced plasma in water has a broadband continuum containing ultraviolet light, which can be used as a novel light source for the degradation of organic compounds. We studied the degradation process of the organic dye Rhodamine B (RhB) using plasma light source excited by the “Laser + Fe” mode. Spectral analysis and reaction kinetics modelling were used to study the degradation mechanism. The degradation process using this light source could be divided into two stages. The initial stage was mainly photocatalytic degradation, where ultraviolet light broke the chemical bond of RhB, and then RhB was degraded by the strong oxidising ability of ·OH. As the iron and hydrogen ion concentrations increased, the synergistic effect of photocatalysis and the Fenton reaction further enhanced the degradation rate in the later stage. The plasma excited by the “Laser + Fe” mode achieved photodegradation by effectively enhancing the ultraviolet wavelength ratio of the emission spectrum and triggered the Fenton reaction to achieve rapid organic matter degradation. Our findings indicate that the participation of the Fenton reaction can increase the degradation rate by approximately 10 times. Besides, the impact of pH on degradation efficiency demonstrates that both acidic and alkaline environments have better degradation effects than neutral conditions; this is because acidic environments can enhance the Fenton reaction, while alkaline environments can provide more ·OH.
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1. Introduction
Owing to the negative impact of highly developed industries, three major global crises have occurred: resource shortages, environmental pollution, and ecological damage. Water pollution has been an important issue. Wastewater discharged from industrial production poses significant harm, and certain dyes used in the printing, dyeing, and textile industries seriously endanger human health if they penetrate natural water bodies [1]. The high concentration and complex composition of organic matter in printing and dyeing wastewater make effective degradation difficult [2]. Therefore, studying the degradation of organic matter in printing and dyeing wastewater is of great significance for environmental protection.
Traditional technologies for treating water pollution can be divided into physical, chemical, and biological treatment technologies, each of which has unique advantages and limitations [3]. The physical methods include solvent extraction [4,5], activated carbon adsorption [6] and membrane separation [7,8]. The advantages of physical methods are simple principles, no chemical reaction, and easy and safe operation; however, the disadvantages are limited degradation ability and ease of secondary pollution. Chemical oxidation reactions for the degradation of organic pollutants exhibit high degradation efficiencies [9,10]. However, secondary pollution is generated during the reaction, and the degradation efficiency continues to decrease with an increase in pollutant types. Microorganisms in the natural environment oxidise and decompose organic matter and certain inorganic toxins in wastewater using biological methods, transforming them into stable and harmless inorganic substances [11]. This method does not cause secondary pollution or damage to the ecological environment; however, its degradation rate is slow, significantly affected by temperature and acidity, and can only degrade organic wastewater at lower concentrations [12]. Traditional methods for treating water pollution have drawbacks; therefore, it is important to find a more efficient, energy-saving, pollution-free, and controllable method for treating water pollution.
Photodegradation technology has attracted considerable attention owing to its advantages, such as high efficiency, energy conservation, and controllability. This technology requires ultraviolet light to irradiate organic wastewater, which combines with oxidants, such as hydrogen peroxide and oxygen, to produce hydroxyl radicals that promote the degradation of organic matter [13]. Compared with traditional mercury vapour lamps, lasers are efficient, energy-saving, controllable, and safe. Wochnowski and Fuchiwaki used ultraviolet (UV) lasers to degrade organic compounds [14,15]. In their research, the laser wavelength was concentrated in the ultraviolet region, whereas the laser plasma source contained ultraviolet components and did not require the wavelength of the laser. Laser-induced plasma is generated by the interaction between the laser beam with high intensity and matter, which has an extremely complex effects, mainly including high-temperature phase transitions and nonlinear ionization. In the former case, the melting and strong evaporation occur, which contains electrons, ions, and neutral ablative particles [16]. While for the latter one, the multi-photon ionization can provide free electrons, which can be used as ‘seed electrons’ for avalanche ionization (cascade ionization), and in this way, the drastic increase in electron density and energy ultimately leads to ionization breakdown of water [17]. The combined effects above can ultimately lead to the formation of laser plasma with high temperature and pressure, and in the process, the shock wave can be formed by plasma expansion, and at the same time, light with wide wavelength spectrum emits from electron-ion recombination. Laser-induced plasma is a completely or partially ionized gas material that contains metastable and excited states of atoms, molecules, and particles and can generate broad-spectrum radiation light from ultraviolet to infrared [18]. Thus, the ultraviolet spectrum can be used as a new light source in photodegradation technology.
In this study, a “Laser + Fe”/H2O2 mode was proposed. This mode can generate plasma as a degradation light source and ionise iron ions in this process, triggering the Fenton reaction. Fenton oxidation method is a chain reaction in which H2O2 generates strong oxidising ·OH in the presence of Fe2+ under acidic conditions and triggers more other reactive oxygen species to achieve the degradation of organic matter [19]. A combination of the Fenton method and UV/H2O2 systems can significantly enhance the oxidising ability of Fenton reagents [20]. Rhodamine B (RhB) is taken as the research object to study the degradation mechanism of organic matter in the process of “Laser + Fe” mode-excited plasma degradation by establishing a reaction kinetics model. We also studied the influence of the pH on the conditions for rapid degradation. The method can effectively degrade organic pollutants, and the study provides a reference for promoting and applying this technology.
2. Materials and methods
2.1 Materials
The chemical reagent used in this study was RhB purchased from Tianjin Kemio Chemical Reagent Co., Ltd., with a molecular formula of C28H31ClN2O3. It is a bright green powder at room temperature, and its aqueous solution is blue-light red (with strong fluorescence). Figure 1 shows the molecular structure and UV-visible absorption spectrum. The maximum absorption wavelength of RhB is 554 nm. The iron sheet used in the experiment was purchased from the High Purity Metal Materials Research Institute, with purity of 99.99% and sample size of 1 mm × 5 mm × 5 mm.
The experimental setup includes a 1064 nm Nd: YAG laser (Solar, Belarus), laser energy meter (Coherent, USA), fibre optic spectrometer (BIM-6601, Sichuan, China), and laser plasma degradation reactor, as shown in Fig. 2.
The laser pulses frequency used in experiment was 12 Hz, the pulse width was 10 ns, and the output energy of laser pulse was 219 mJ. The laser beam was divided into two using a beam splitter (Leo-1064-G0032A, Beijing, China). One beam was used to measure the laser intensity in real-time using a laser energy meter. The other beam was focused on the iron plate in the tube with RhB through a convex lens (GCL-0108, Beijing, China) to form a laser plasma degradation reactor. The focus length and diameter were 20 cm and 1.1 mm respectively. Simultaneously, two focusing lenses were set on the side of the test tube to collect the radiation light signal of the plasma and then couple it into a fibre optic spectrometer. The emission spectrum of the plasma was obtained using Brolight software.
2.2 Degradation process of plasma excited by “Laser + Fe” mode
RhB (2 mg) was added to 100 mL of deionised water and stirred for 10 min, and then 3 mL of RhB solution (20 mg/L) was mixed with 1 mL of H2O2 with a concentration of 15 g/L. The mixture was stirred in a dark environment for 10 min at room temperature and was labelled as Group A. In the control group, 3 mL of RhB solution (20 mg/L) was mixed with 1 mL of deionised water and designated as Group B. Iron sheets were added to the two sets of solutions, and the mixtures were stirred in a dark environment for 1 min at room temperature. The concentration of each component in the solution before the laser plasma action started was used as the initial concentration for the experiment. The laser was focused on the iron metal sheet surface in the mixed solution, with the liquid height of 3 cm. The metal plate was separated from the mixed solution after the laser plasma action was complete, and the solution was centrifuged multiple times at 10,000 rpm for 6 min to eliminate the influence of other impurities. After centrifugation, the upper solution was transferred to a quartz colorimetric cell. The corresponding absorption spectrum was measured using a Shimadzu UV2401 UV-visible spectrophotometer (Shimadzu, Kyoto, Japan) in the wavelength range of 300-700 nm, with a sampling interval of 0.5 nm, a resolution of 0.1 nm, and a scanning speed of 3200 nm/min. The degradation rate was calculated using Lambert Beer’s law based on the spectral intensity of RhB measured at 554 nm [21]. All data were the average values obtained through three experiments.
3. Results and discussion
3.1 Spectral analysis
The absorption spectrum of the sample obtained from the experiment was measured using a UV-visible spectrophotometer in the wavelength range 300–700 nm. The degradation rate of RhB was calculated using Lambert Beer's law. Figure 3 shows the degradation characteristics over time.
Fig. 3. (a) Absorbance and (b) degradation rate of RhB under the action of laser plasma at different times.
The spectrum shown in Fig. 3(a) corresponds to Group A samples, and it can be seen from the graph that the peak value of the RhB solution at the maximum absorption wavelength gradually decreased with the action time, indicating that RhB was decomposed gradually. Figure 3(b) shows the effect of adding H2O2 on the degradation efficiency. After comparing the two datasets, it was observed that the addition of H2O2 significantly improved the degradation rate, which were 63.267% and 7.622%, respectively, at 30 min. Moreover, there was a turning point at 10 min for the action of the laser plasma in a solution containing H2O2, and the subsequent degradation rate increased significantly, indicating a change in the degradation mechanism.
3.2 Mechanism of plasma degradation excited by the “Laser + Fe” mode
3.2.1 Degradation reaction kinetics analysis
To study the process of plasma induced degradation of organic compounds in wastewater based on the “Laser + Fe” mode, a reaction kinetics model was established for the degradation process. The differential form of the first-order reaction kinetics model is given by
where $A = A(t )$ is the concentration of RhB at time t, $\frac{{dA}}{{dt}}$ is the reaction rate, k is the reaction rate constant, $k > 0$ and does not change with the concentration of RhB. ${A_0}$ is the concentration of RhB at the initial time $t = 0$. Equation (1) is integrated to obtainBased on experimental data, the curve of $- Ln\left( {\frac{A}{{{A_0}}}} \right)$ and t can be fitted, as shown in Fig. 4.
Figure 4 shows that the RhB degradation process in Group B conformed to the first-order kinetic reaction, and the absolute residual values of the fitting values of $- \textrm{Ln}\left( {\frac{\textrm{A}}{{{\textrm{A}_0}}}} \right)$ did not exceed 0.025. The degradation process in Group A did not conform to the first-order kinetic reaction model, and the maximum absolute residual value of the fitted value was 0.176. This difference suggests that the degradation mechanism changed owing to the action of iron ions in the solution. The addition of H2O2 to Group A can form Fenton's reagent with the iron ions in the solution to participate in the degradation reaction; therefore, the degradation rate of Group A is also related to the iron ions.
To explore the changes in the iron ion concentration during the degradation process, two groups of experiments were conducted. Three mL of deionised water was mixed with 1 mL of H2O2(15 g/L) solution and designated as Group C. Correspondingly, 4 mL of deionised water was used as the control group and designated as Group D. In these two sets of experiments, deionised water was used instead of RhB to eliminate its influence. The iron sheets were added to two sets of solutions, and the solutions were stirred in a dark environment for 1 min at room temperature. Subsequently, the same laser parameters as in the RhB degradation experiment were used for Groups C and D, and samples were extracted every 5 min. The laser plasma solution was centrifuged multiple times for 6 min and 2 mL of the clear solution was extracted as a test sample. The total iron ion content in the solution after laser plasma treatment was measured using the o-phenanthroline spectrophotometer method. The principle of this method is that, in a solution with pH = 2–9, o-phenanthroline reacts with ferrous ions (Fe2+) to produce a stable orange-red complex with a maximum absorption peak at 510 nm. To measure the total iron content in the solution, an effective method was adopted to reduce iron from high-valent ions to ferrous ions using a reducing agent (such as hydroxylammonium chloride). Figure 5 shows the variation in absorbance over time for groups C and D.
Figure 5 shows that the peak value at the maximum absorption wavelength in groups C and D gradually increased under the action of laser plasma, indicating that the total iron ion concentration in the solution gradually increased and that in group C increased even faster.
FeCl2 was used to calibrate the total iron ion concentration in the solution, as shown in Fig. 6. According to Beer's law, the iron ion concentration is linearly related to the absorbance. Based on the experimental data fitting, the linear relationship is
where y is the absorbance of the calibration solution at $510\; nm$ and c is the concentration of Fe2+ in the solution.Based on the calibration results, the relationship between the total iron ion content and action time of the laser plasma was fitted as follows:
Fig. 7. Relationship between the concentration of total iron ions in solution and the action time of laser plasma.
The experimental results revealed a turning point in the degradation rate of RhB when the laser plasma was applied for 10 min, and the subsequent degradation rate increased significantly. Based on the change in the total iron ion concentration, the degradation model after 10 min of laser plasma application was modified to obtain the following equation:
where B is the total iron ion concentration at time t, B is defined as $B = {k_1}{t^2} + {k_2}t$. Integrating Eq. (5) yields where $m = \frac{{k{k_1}}}{3}$, $n = \frac{{k{k_2}}}{3}$.According to the experimental data, the fitted curve based on Eq. (6) is shown in Fig. 8, and the modified curve exhibited a better fit. The absolute residual values of the fitted values did not exceed 0.025, which is much lower than the uncorrected fitted residual, indicating that the degradation of RhB in Group A after 10 min followed a second-order kinetic reaction.
According to the reaction kinetics model of the degradation process, the degradation rate of the entire process for Group B was only linearly related to the concentration of RhB, indicating that the mechanism of degradation did not change. However, for Group A, the initial degradation rate was linearly related to the concentration of RhB, whereas that of the later one was nonlinear. The reason for this difference is that the degradation rate in the later stage for Group A was also related to the concentration of iron ions, suggesting that the degradation mechanism of Group A changed in the later stage.
3.2.2 Photocatalytic degradation
In the process of laser focusing on a liquid, the energy density near the focal point reaches a threshold and breaks through the liquid, forming a local plasma with high temperature and high pressure. Plasma excited by a laser continues to absorb the laser energy and rapidly expands, forming shock waves and bubbles [22]. In this study, iron was used as a solid wall surface to generate bubbles. The intense emission of light excited by the collapse of the bubbles had a wide wavelength range, with the highest energy concentrated in the ultraviolet region, as shown in Fig. 9.
The characteristics and reasons for the above spectral distribution strongly depend on the atomic structure of iron, which is the atomic ground state in the 5D4 orbital and outermost electron structure in 3d64s2. The d orbital is partially filled, and the electrons in the d orbital absorb a certain number of energy photons and transition from the low-energy State d orbital to the high-energy State d orbital. This transition is called the d-d electron transition, also known as the coordination field transition, which can result in a strong absorption of the incident laser [23]. The plasma formed by the laser breakdown of the liquid and iron substrates has high energy, which can easily cause electron transitions and generate a wide radiation spectrum from ultraviolet to visible light. The continuous spectrum of high-intensity radiation contains complex physical effects, including Stark and Doppler broadening, bremsstrahlung, composite, and blackbody radiations [24,25]. An iron plate, when used as an excitation source, can produce a high proportion of continuous-spectrum radiation in the ultraviolet region [26].
The ultraviolet light excited by laser plasma was involved in the degradation of RhB. It is generally believed that the UV/H2O2 process degrades organic pollutants in the following three ways. (1) Organic compounds are directly oxidised by the strong oxidising properties of H2O2. (2) Ultraviolet light directly triggers the dissociation of organic molecular bonds through effective photons for photodegradation. (3)H2O2 generates hydroxyl radical (·OH) under ultraviolet radiation and organic pollutants are oxidised by hydroxyl radicals and thus degraded [27]. RhB belongs to the category of aromatic methane dyes and is completely degraded through four processes: N-de-ethylation, chromophore cleavage, benzene ring opening, and mineralisation [28]. The maximum energy required to break the chemical bonds in RhB is approximately 615 KJ/mol (approximately 6 eV). As the energy of UV light radiation from laser plasma concentrate in the range of 200-300 nm, and the corresponding photon energy is distributed between 4.14 and 6.21 eV, which can reach the energy required to destroy the chemical bond of RhB, the UV light radiation from laser plasma can directly destroy the chemical bond of RhB and decompose it into other small molecular substances. Moreover, ultraviolet light can directly activate H2O2 and H2O to produce hydroxyl radicals, as shown in Eqs. (7) and (8).
In the formula, ·OH has a significantly strong ability to obtain electrons, and its oxidation potential is 2.8 V. It can attack organic compounds by searching for hydrogen, addition, and electron transfer to achieve the final oxidative decomposition of organic compounds [29,30].
3.2.3 Fenton reaction
Iron ions are excited though ionisation, thermal effects, and other factors during the process of degradation of RhB using plasma generated in the ‘Laser + Fe’ mode as a light source. The continuous generation of Fe ions affect the degradation characteristics. By comparing the degradation rates of the two solutions, it can be concluded that the degradation rate of Group B solution increased linearly without a significant turning point, indicating photodegradation. However, there was a significant turning point in the degradation rate of Group A at 10 min, suggesting a change in the degradation mechanism. The difference in the degradation rates of the two solutions in the first 10 min was caused by the catalytic effect of H2O2 on UV photodegradation. Considering the degradation reaction kinetics, photocatalytic action can only produce a linear relationship between the degradation rate and reactants, which contradicts the experimental results. In the actual degradation process, the laser plasma can simultaneously generate iron and hydrogen ions, and both concentrations increase with time. At a certain level, the Fenton reaction can occur, corresponding to the demarcation point at 10 min in the experiment. The synergistic effect of the Fenton reaction and photocatalysis accelerated RhB degradation. The main reaction equation for the Fenton reaction is as follows [31]:
The principle of ‘Laser + Fe’ mode excited plasma degradation of RhB is shown in Fig. 10.
It is shown in Fig. 10 that the plasma spectrum excited by the “Laser + Fe” mode explains a high proportion in the ultraviolet band (200–300 nm) and has extremely high energy of a single photon. Photons break the chemical bonds of RhB; H2O2 and H2O are activated to produce hydroxyl radicals, and the combined action of the two ultimately decomposes RhB into small-molecule substances. Furthermore, iron ions ionised by the laser plasma can trigger a Fenton reaction with H2O2 in an acidic environment. The degradation rate in the early stages of the reaction was linearly related to the concentration of RhB, which was caused by photocatalysis. The degradation rate in the later stage of action is related to both the concentration of RhB and the total concentration of iron ions in the solution, which is the synergistic effect of photodegradation and Fenton reaction.
3.3 Effect of initial pH on degradation efficiency
The concentration of H+ affects the Fenton reaction rate, thereby altering the degradation rate. Therefore, it is necessary to study the effect of pH on the decomposition efficiency. The samples used in the relevant experiments consisted of 3 mL of RhB solution (20 mg/L) mixed with 1 mL of H2O2 at a concentration of 15 g/L. The initial pH value of the solution was adjusted with 1 mol/L of HCl or NaOH in the pH range of 2.0 to 12.0, without further control during the reaction process. The experimental results are presented in Fig. 11.
It is shown from Fig. 11 that the degradation rate was the fastest under acidic conditions, followed by alkaline conditions, and was the worst under neutral conditions. Moreover, the higher the acidity or alkalinity, the faster the degradation rate. This phenomenon is caused by the significant impact of pH on the degree of the Fenton reaction; an acidic environment can promote it, while an alkaline environment has the opposite effect. At pH value between 2-6, both hydrogen ions and hydrogen peroxide in the solution react with the iron sheets to generate $\textrm{F}{\textrm{e}^{2 + }}$ and $\textrm{F}{\textrm{e}^{3 + }}$. Therefore, photocatalysis and the Fenton reaction work simultaneously under the excitation of laser plasma, and the synergistic effect leads to a rapid degradation rate of RhB, reaching approximately 91% within 4 min. For pH values ranging from 8 to 12, the only degradation mechanism is photocatalysis, because alkaline conditions do not meet the conditions for the Fenton reaction. However, alkaline conditions can provide more ·OH, which can also accelerate decomposition.
To demonstrate the Fenton reaction, the concentration of the total iron ions in the solution was measured using the ortho-phenanthroline method. The experimental results are shown in Fig. 12: the lower the pH, the higher the concentration of total iron ions. At the same pH value, the concentration of total iron ions gradually increased with action time. The total iron ion concentration could exceed $0.238 \times {10^{ - 4}}\textrm{mol}/\textrm{L}$ at 4 min in alkaline conditions, which meet the concentration required for occurrence conditions for Fenton reaction, while the pH value hindered it. Thus, there was only a photocatalytic effect and no synergistic effect of the Fenton reaction under alkaline conditions.
In summary, the pH value has a significant impact on the degradation efficiency, and the synergistic effect of photocatalysis and Fenton reaction leads to a rapid degradation of RhB under acidic conditions.
4. Conclusions
The plasma light source excited by the “Laser + Fe” mode is concentrated in the ultraviolet band. The emitted photons can directly break organic molecular bonds and decompose H2O2 and H2O to produce strong oxidising ·OH, thereby degrading organic pollutants. During the degradation process, the concentration of iron ions in the solution increased, and the Fenton reaction occurred, which accelerated the degradation process. By establishing a degradation reaction kinetics model, it was verified that the initial stage followed photocatalytic degradation, and that the later stage included a synergistic effect of the Fenton reaction and photocatalysis. After investigating the influence of pH on degradation efficiency, it was shown that acidic conditions can promote the Fenton reaction, whereas alkaline conditions accelerate the photocatalytic effect. Considering a neutral environment, acidic conditions (pH of 2–5) have the fastest degradation rate. By controlling the pH, the optimal conditions for the Fenton reaction can be obtained, thereby accelerating the decomposition of organic pollutants and providing favourable support for promoting and applying this technology.
Funding
National Key Research and Development Program of China (2022YFB3606304); National Natural Science Foundation of China (U2030108); Sichuan Province Science and Technology Support Program (24QYCX0350).
Acknowledgments
The authors would like to express their sincere gratitude to the the Sichuan Science and Technology Program for help identifying collaborators for this work
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the corresponding author upon reasonable request.
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