Fast photocatalytic degradation of methylene blue dye using a low-power diode laser

Accepted Manuscript Title: Fast Photocatalytic Degradation of Methylene Blue Dye Using a Low-power Diode Laser Author: Xianhua Liu Yulou Yang Xiaoxuan Shi Kexun Li PII: DOI: Reference:

S0304-3894(14)00769-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.09.031 HAZMAT 16279

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

22-4-2014 1-9-2014 2-9-2014

Please cite this article as: X. Liu, Y. Yang, X. Shi, K. Li, Fast Photocatalytic Degradation of Methylene Blue Dye Using a Low-power Diode Laser, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.09.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights •Photocatalytic oxidation of methylene blue was studied under laser light irradiation.

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•Fast removal of methylene blue from aqueous solution was achieved. •The photocatalyst Ag/AgCl is efficient and stable under 443nm laser light irritation.

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•Diode laser is a good light source for photocatalytic degradation of dyes.

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Dye Using a Low-power Diode Laser

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Fast Photocatalytic Degradation of Methylene Blue

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Xianhua Liu*a, Yulou Yang a, Xiaoxuan Shia, and Kexun Li*b

a. School of Environmental Science and Engineering, Tianjin University, Tianjin, 300072,

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China. Fax: +86 (22) 27402367; Tel: +86 (22) 27402367; E-mail: [email protected]

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b. School of Environmental Science and Engineering, Nankai University, Tianjin, 300074, China. Fax: +86 (22) 23495200; Tel: +86 (22) 23495200; E-mail: [email protected]

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ABSTRACT: This study focused on the application of diode lasers as alternative light sources for the fast photocatalytic degradation of methylene blue. The photocatalytic decomposition of

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methylene blue in aqueous solution under 443 nm laser light irradiation was found to be technically feasible using Ag/AgCl nanoparticles as photocatalysts. The effects of various experimental parameters, such as irradiation time, light source, catalyst loading, initial dye concentration, pH, and laser energy on decolorization and degradation were investigated. The mineralization of methylene blue was confirmed by chemical oxygen demand analysis. The results demonstrate that the laser-induced photocatalytic process can effectively degrade methylene blue under the optimum conditions (pH 9.63, 4 mg/L MB concentration, and 1.4 g/L Ag/AgCl nanoparticles). KEYWORDS: visible laser, photocatalysis, methylene blue, degradation, decolorization

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1. Introduction Photocatalytic methods for removing of organic pollutants have gained much attention because

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of their outstanding advantages, including high efficiency and energy economy. Most prior

studies on the photocatalytic degradation of dyes from wastewater have been conducted using

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broad-spectrum radiation sources such as UV lamps and using TiO2 as a photocatalyst [1,2]. The

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traditional UV source is a mercury vapor high-pressure lamp, which is a gas discharge source. Several problems are associated with the use of UV lamps emitting radiation over a broad range

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of wavelengths. In particular, long-term power instability, low photonic efficiency, longer exposure time necessary for complete mineralization of pollutants, and presence of hazardous

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mercury are the major drawbacks [3]. A newer, safer, and more energy-efficient alternative for

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UV lamps is laser light. The photocatalytic degradation of organic contaminants using laser light

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has previously been reported by us and other groups [4-7]. However, all of these studies were conducted on high-power UV lasers. Until now, there have been no reports on the use of visible

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laser diodes as light sources for the photocatalytic degradation of dyes. A laser diode is an electrically pumped semiconductor device in which the active medium is formed by a p-n junction of a semiconductor diode similar to that found in a light-emitting diode. As laser light is coherent, monochromatic, and highly directional, incident photons from laser light sources can be absorbed more efficiently by a photocatalyst compared with photons from broadband spectral sources such as lamps [7]. Given the limitations on light absorption and photonic efficiency that are intrinsic to semiconductor photocatalysts, the development of efficient visible-light-induced photocatalysts has been an urgent issue [8]. Noble metal nanoparticles (NPs) have recently been recognized as a

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new class of efficient media suitable for harvesting light energy for chemical processes due to their strong optical absorption over a wide range of the light spectrum, including both visible light and UV light [9-10]. The strong optical absorption in the visible region is due to the

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localized surface plasmon resonance (LSPR) effect of the noble metal NPs. LSPR is the resonant photon-induced coherent oscillation of charge at the metal-dielectric interface, established when

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the photon frequency matches the natural frequency of metal surface electrons oscillating against

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the restoring force of their positive nuclei [11]. Noble metal NPs have been widely used as the active catalyst components for many important reactions, and the LSPR effect of noble metal

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NPs has recently been utilized to improve the performance of semiconductor photocatalysts [1216].

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The textile industry produces wastewater, which results in serious environmental problems [17,

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18]. In addition to the problems of aesthetic deterioration and obstruction of dissolved oxygen

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penetration into natural water bodies caused by the presence of color, some of the dyes, dye precursors, and dye degradation products are carcinogenic and mutagenic in nature. A variety of

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physical, chemical, and biological methods, such as adsorption, coagulation, membrane processes, and biological oxidation, have been developed for the treatment of dye wastewater, but these conventional processes are usually insufficient for purifying the wastewater [18]. The main objectives of this study are (1) to explore the possibility of using visible laser diodes as light sources for the photocatalytic degradation of dyes in a dispersion medium and (2) to evaluate the catalytic capabilities of Ag/AgCl NPs under laser light irradiation. Methylene blue (MB) was selected as a model compound in this work due to its wide range of applications. This study will present an opportunity to design efficient processes for hazardous waste treatment based on laser-induced photocatalysis.

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2. Experimental 2.1. Chemicals. Methylene Blue (MB), Sodium chloride (NaCl), silver nitrate (AgNO3), and

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polyvinylpyrrolidone (PVP) were purchased from Tianjin Institute of Chemical Reagents,

Tianjin, China. MB is an aromatic chemical compound with the molecular formula C16H18ClN3S.

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Its structure is given in Fig. 1. It was chosen as a simple model for a series of thiazine dyes

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widely used in the textile industry. At room temperature, it appears as a solid, odorless, dark green powder that yields a blue solution when dissolved in water. The UV-visible spectrum of

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MB in water showed three absorption maxima. As shown in Fig. 1, the peaks were observed at

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246, 292, and 664 nm, respectively. The absorption within the range 550 nm - 700 nm can be attributed to a chromophore containing a long conjugated system, whereas the absorption peak at

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292 nm can be attributed to the aromatic rings. For this reason, absorbance of a solution at 664

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nm was used to evaluate the decolorization of MB, while absorbance at 292 nm was used to evaluate the degradation of MB. The molar extinction coefficients of MB at 664 and 292 nm are

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74028 cm-1/M and 39064 cm-1/M, respectively. The pH of the solutions was adjusted using HNO3 or NaOH.

Standard chemical reagents for chemical oxygen demand (COD) measurements were purchased from Hach Company, USA. Double-distilled water was used to prepare experimental solutions. 2.2. Preparation and characterization of Ag/AgCl NPs. Polyvinyl pyrrolidone (PVP, K30) (0.225 g) was dissolved in 50 ml of 0.1 mol/L AgNO3 solution under magnetic stirring at room temperature. After stirring for 20 min, 50 ml 0.1 mol/L HCl aqueous solution was added to it drop by drop. The mixture was ultrasonicated for 10 min and stirred for 20 min at room temperature. The filtered powder was washed with deionized water and dried at 100 °C for 10 h.

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Finally, the dried powder was irradiated with a 500 W halogen tungsten lamp (Guangdong Foshan Lighting, Foshan, China) for 20 min to reduce part of the Ag+ ions in the AgCl particles

electron microscope (TEM) (FEI, Hillsboro, USA) operating at 200 kV.

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to Ag0 species. The resulting Ag/AgCl NPs were characterized by a Tecnai G2 F20 transmission

2.3. Experimental Procedure. The experimental setup is depicted in Fig. 2. It consists primarily

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of a 1 W 443 nm blue-light laser (Laserver, Wuhan, China) and a photocatalytic reactor.

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Measurements of laser energy were made using a Thorlabs S302C detector connected to a PM100USB power and energy meter (Thorlabs, USA). To compare the effect of different light

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sources on the photocatalytic degradation of MB, a 1 W 440-450 nm LED (OSRAM, Munich, Germany) was used as a control. In a typical experiment, a suspension containing the required

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volume of the Ag/AgCl NP catalyst and 4 mL of an aqueous MB solution was stirred for 30 min

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in the dark prior to irradiation of laser light. The reaction mixture was continuously stirred for

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dispersion of catalysts. The concentration of MB in the bulk solution prior to irradiation was used as the initial value for the measurements of MB degradation. Then, the suspensions were

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exposed to laser-light irradiation. After the treatment, the suspensions were centrifuged at 8000 rpm for 5 min to separate the solid photocatalyst particles, and the top transparent solutions were then transferred to a quartz cuvette for measuring their absorption spectra in the wavelength range of 200-800 nm by a UV-3000 spectrophotometer (Mapada, Shanghai, China). The concentration of MB (λmax = 292 and 664 nm) in the solution was determined using a calibration curve (concentration vs absorbance) generated from absorbance measurements of MB samples of known concentrations.

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3. Results and Discussion 3.1. Characterization of Ag/AgCl NPs. The TEM observations of the Ag/AgCl NPs (image shown in Fig. 3A) showed that the prepared Ag/AgCl NPs were mainly spherical, with an

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average diameter of 21.29 nm (Fig. 3B). The UV-Vis absorbance of the Ag/AgCl NP suspension within 300-650 nm is shown in Fig. 3C. A peak at ca. 430 nm was observed, which indicates that

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Ag/AgCl NPs can absorb light irradiated from the 443 nm laser diode. Optical absorption spectra

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of the metal NPs depend directly on the size and shape of the NPs, and it is possible to design nanostructures that can interact with the laser spectrum more effectively by manipulating these

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properties in catalyst preparation.

3.2. Photocatalytic Activity. Photocatalytic activity experiments were conducted in a simple

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photocatalytic reactor with and without the addition of photocatalyst under the irradiation of

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laser light. The concentration of MB solution was 4 mg/L, and the concentration of photocatalyst

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was 1 g/L. The color of MB after the addition of catalyst was blue under dark conditions. Fig. 4a shows the changes over time in the absorbance spectrum of the MB solution (4 mg/L) after

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treatment with Ag/AgCl NPs (1 g/L) under irradiation of laser light. The reductions in the three absorbance peaks at 246, 292 and 664 nm indicate the degradation of the dye molecule to smaller intermediates. However, no new absorption peaks appeared during the reaction, which supports the hypothesis that any intermediate products formed during the dye degradation also get successfully degraded. This phenomena, no new bands appear in the UV-Vis region during the degradation process, has also been reported by Sohrabnezhad et al [19]. The photocatalytic degradation pathway of methylene blue in water has been studied by Houas et al [20]. The main aromatic intermediates generated during the degradation process were supposed to be demethylated metabolites, sulfoxide, sulfonic acid and phenolic compounds. The UV-Vis

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adsorption peaks of these compounds are also located in the range of 200-300 nm and 500-700 nm, which are similar to those of MB. Due to their low content and the heavily overlapping absorption spectra, it is difficult to detect these intermediates using UV-Vis absorption

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spectroscopy. Furthermore, this phenomena also indicates that a rapid degradation of MB can be easily achieved in the presence of Ag/AgCl under laser light irradiation. As shown in Fig. 4b and

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4c, in the absence of photocatalysts, the degradation efficiency and decolorization efficiency of

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MB dye within 10 min were 1.02% and 2.89%, respectively, while in the presence of photocatalyst, they were 90.8% and 100%, respectively. Moreover, in the presence of

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photocatalyst, the color of the reaction mixture solution changed from blue to milky white. The MB dye degradation efficiency was significantly lower in the control experiment, in which a

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1 W LED was used for irradiation, than in the experiment that used laser light. From the above

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results, it is apparent that the laser’s role in photocatalytic degradation of MB is very important.

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Laser light differs significantly from regular light. Photons of LED light and other regular lights are emitted without any pattern, whereas photons of laser light is organized in a coherent manner.

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Additionally, regular lights are multi-chromatic, but laser light is monochromatic. It can be supposed that the unique properties of laser light make the photocatalytic process more efficient. Further mechanistic studies need to be performed to elucidate the effect of laser light. 3.3. Chemical Oxygen Demand. Mineralization of MB was studied by monitoring COD loss in the dye solution. The results in Table 1 show that as laser irradiation time increased, the degradation efficiency of MB increased and the COD value decreased. The decrease in the COD value indicates that organics were being decomposed. However, even when decolorization of MB was 100%, the COD value was still not zero. One possible explanation for this phenomenon is that some of MB molecules were decomposed into smaller organic molecules, and this

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remaining organic matter was still contributing to the non-zero COD value. These results are similar to those from other studies [3]. 3.4. Effect of Catalyst Concentration. The effect of photocatalyst concentration was studied by

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varying catalyst concentration from 0.2 to 1.8 g/L. Fig. 5 shows the effect of catalyst

concentration on the degradation of MB dye in the presence of laser light irradiation for 4 min.

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The results reveal that the degradation and decolorization efficiency of the dye increase greatly

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with increasing catalyst concentration: as catalyst concentration was raised from 0.2 to 1.4 g/L, degradation efficiency increased from 2% to 45%, and decolorization efficiency increased from

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20% to 94%. When the catalyst concentration was raised above 1.4 g/L, both the degradation and decolorization efficiency of the MB decreased. This result indicates that 1.4 g/L is the optimal

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catalyst concentration for maximizing catalytic activity under the experimental conditions used

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herein. This phenomena can be explained by considering the photocatalytic activity of the

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catalyst to be dependent on the light absorption efficiency to some extent. At lower catalyst concentrations, the catalyst surface area and absorption of light on the catalyst surface are the

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limiting factors; thus, an increase in catalyst concentration greatly enhances the process efficiency [21]. However, when the amount of catalyst is too high, the total particle concentration in the solution is so high that the irradiation field inside the reaction medium is reduced due to light scattering by catalyst particles, resulting in a decrease in photocatalytic activity [21-24].

3.5. Effect of Initial Dye Concentration. The effect of initial dye concentration under laser light irradiation was investigated by varying the initial concentration from 4 mg/L to 20 mg/L with optimum catalyst concentration. As shown in Fig. 6, with an increase in initial dye concentration from 4 mg/L to 20 mg/L, degradation decreased from 90% to 37% and decolorization decreased

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from 100% to 41%. These results indicate that degradation efficiency is inversely affected by the initial concentration of MB dye. Similar results have been reported for the photocatalytic oxidation of other dyes [25]. The possible explanation for this behavior is as follows: As the

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initial concentration of the dye increases, the path length of the photons entering the solution decreases, which results in lower photon absorption by catalyst particles, and consequently lower

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photocatalytic reaction rates [26, 27]. Moreover, the active surface area on the catalyst available

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for reaction is very crucial for the degradation to take place, but as the initial dye concentration is increased, more and more dye and dye intermediates are adsorbed on the surface of the

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photocatalyst. This adsorption leads to fewer active sites available for the generation of reactive radicals [28].

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3.6. Effect of initial pH of the dye solution. The solution pH appears to play an important role

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in the photocatalytic process of various dyes [29, 30]. The effect of pH on the adsorption

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capacity of the Ag/AgCl NPs and photocatalytic degradation of MB was investigated in the pH range of 3.0-12.0 at 4 mg/L MB and 1.2 g/L catalyst. Prior to irradiation, the initial pH of the

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each experimental solution was adjusted using HNO3 or NaOH; it was not controlled further during the course of the reaction. As shown in Fig. 7, the adsorption capacity of the Ag/AgCl NPs increased as the pH increased. The effect of the solution pH on the adsorption can be explained mainly by the modification of the electrical double layer of the solid-electrolyte interface, which consequently affects the electrostatic interactions. As the pH of the system increases, the number of negatively charged adsorbent sites increases, and the number of negatively charged surface sites decreases. These conditions favor the adsorption of positively charged MB cations. As shown in Fig. 8, the decolorization and degradation efficiency increased with increase in pH from 3 to 9.63 and then decreased with further increase in pH. The pH

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influences not only the surface properties of the photocatalyst but also dye dissociation and hydroxyl radical formation [25, 31]. At low pH, electrostatic interactions between the photocatalyst surface and dye cation are weak, resulting in minimal adsorption. In contrast,

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alkaline conditions are beneficial for the generation of active hydroxyl radicals, which are

produced on the surface of catalyst by the reaction of h+ and adsorbed OH−. However, hydroxyl

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radical formation might be getting suppressed at highly alkaline pH. The different reaction rates

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at different pH values may be correlated to the generation of active hydroxyl radicals and occupancy of active sites on the catalyst surface for the production of hydroxyl radicals by

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intermediate products. During the course of photocatalytic reactions, formation of some negatively charged species (e.g., intermediate products) can occur and compete with OH− for

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occupancy of active sites on the catalyst surface. This competition reduces the possibility of

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adsorption of OH− on the surface of the catalysts, which in turn affects the generation of active

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hydroxyl radicals, thereby decreasing the efficiency of the process [25]. 3.7. Effect of laser energy. The effects of laser energy on degradation and decolorization of MB

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are shown in Fig. 9. The degradation and decolorization of MB increased as the laser energy increased. Under laser energies of 0, 0.28, 0.64, 0.98, and 1.15 W, the degradation efficiencies at 10 min were 0, 21.94, 26.67, 57.97, and 75.57%, respectively, and the decolorization efficiencies were 0, 30.62, 40.65, 74.53, and 90.90%, respectively. The explanation for this result is that the number of photons absorbed per unit area increases with increasing laser energy, thereby enhancing the decolorization efficiency. Fig. 10 shows the linear plots of Ln (C0/C) for the photodegradation of MB using Ag/AgCl as photocatalyst under 1.15 W laser light irradiation. C0 and C are the concentration of MB when the reaction time is 0 and t, respectively. This figure indicates that MB degradation and decolorization are consistent with pseudo-first-order kinetics.

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The slopes of the plots which represent the photocatalyst reaction rate constant was calculated and listed in Table 2. The rate constants were remarkably greater than those previously reported [20, 24, 32].

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3.8. Stability of the catalyst. Stability and recyclability of the photocatalysts were also

investigated. After five cycles of MB photodegradation, the catalyst could still maintain high

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efficiency without considerable decrease in activity, as shown in Fig. 11.

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3.9. Photocatalytic mechanism. For the plasmonic photocatalyst Ag/AgCl NPs, the major photocatalytic reaction procedure under laser light irradiation can be summarized by the

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following equations, schematically shown in Fig. 12.

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h AgNPs   AgNPs*

AgNPs*   AgNPs   e 

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e   O2  O2 

(1) (2) (3) (4)

AgCl (h  )  OH   OH  AgCl

(5)

AgNPs   Cl   AgNPs  Cl 0

(6)

AgCl (h  )  Cl   AgCl  Cl 0

(7)

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h AgNPs   AgCl   AgNPs  AgCl (h  )

Cl 0  MB  Degradation products  Cl 

(8)

OH (or  O2  )  MB  Degradation products

(9)

Due to LSPR produced by the collective oscillations of surface electrons, silver nanoparticles contribute greatly to the high visible-light photocatalytic activity [11, 33-35]. Additionally, the excellent conductivity of silver nanoparticles can enhance electron translation to enhance interfacial charge transfer and efficiently stop the recombination of electron-hole pairs [36].

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Furthermore, as laser light is coherent, monochromatic, and highly directional, the incident photons can be absorbed efficiently by Ag/AgCl NPs. The free electrons from the AgNPs can be scavenged by oxygen, which is then transformed into active •O2− (reaction 3). Simultaneously,

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the photogenerated holes in the AgCl lead to the formation of active •OH. In addition to the

generation of these common photocatalytic active species, the reactive radical species Cl0 is

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formed when the photogenerated holes transfer to the AgCl surface and lead to the oxidation of

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Cl ions. These reactive radicals can accelerate the degradation of dyes [11, 37].

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4. Conclusion

The photocatalytic degradation process that occurs for dye in the presence of Ag/AgCl under

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laser light irradiation suggests a new method for the application of laser light in the treatment of

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environmental pollutants. The results herein demonstrate that under optimum conditions, over

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90% degradation and up to 100% decolorization of MB solution could be achieved within 10 min. The photocatalyst Ag/AgCl described herein is efficient and stable under laser light. These

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results demonstrate that diode lasers may be good alternative light sources for the degradation of dyes. In comparison with those previously reported, the rate constant for MB degradation in our study was remarkably higher. The observed photocatalytic enhancements in these Ag/AgCl NPs should be attributed to the efficient adsorption of incident laser light and accelerated production of active redox species. In conclusion, the laser-induced photocatalytic process described in this paper can be utilized for efficient environmental applications. Further studies are currently underway to investigate the degradation of MB and other dyes using different sizes and shapes of Ag/AgCl NPs under various laser light intensities and wavelengths.

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AUTHOR INFORMATION Corresponding Author: E-mail address: [email protected]

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* Tel: +86 (22) 27402367; Fax: +86 (22) 27402367. ACKNOWLEDGMENTS

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Financial support provided by the Natural Science Foundation of Tianjin City (No.

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12JCZDJC29600) and the Promote Marine Program from the Tianjin Oceanic Administration

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(No. KJXH2011-11) are greatly appreciated. REFERENCES

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[28] K. Hayat, M.A. Gondal, M.M. Khaled, S. Ahmed, A.M. Shemsi, Nano ZnO synthesis by modified sol gel method and its application in heterogeneous photocatalytic removal of

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phenol from water, Appl Catal A-Gen, 393 (2011) 122-129.

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[29] S. Sakthivel, B. Neppolian, M.V. Shankar, B. Arabindoo, M. Palanichamy, V. Murugesan, Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO

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and TiO2, Sol Energ Mat Sol C, 77 (2003) 65-82.

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[30] Y.Z. Wang, Solar photocatalytic degradation of eight commercial dyes in TiO2 suspension, Water Res, 34 (2000) 990-994.

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[31] H.B. Fu, C.S. Pan, W.Q. Yao, Y.F. Zhu, Visible-light-induced degradation of rhodamine B

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by nanosized Bi2WO6, J Phys Chem B, 109 (2005) 22432-22439.

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[32] Y. Sun, Conversion of Ag Nanowires to AgCl Nanowires Decorated with Au Nanoparticles and Their Photocatalytic Activity. J. Phys. Chem. C. 114 (2010) 2127–2133 [33] P. Wang, B.B. Huang, X.Y. Zhang, X.Y. Qin, H. Jin, Y. Dai, Z.Y. Wang, J.Y. Wei, J. Zhan, S.Y. Wang, J.P. Wang, M.H. Whangbo, Highly Efficient Visible-Light Plasmonic Photocatalyst Ag@AgBr, Chem-Eur J, 15 (2009) 1821-1824. [34] R.C. Jin, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Photoinduced conversion of silver nanospheres to nanoprisms, Science, 294 (2001) 1901-1903.

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[35] S. Link, M.A. El-Sayed, Spectral properties and relaxation dynamics of surface plasmon electronic oscillations in gold and silver nanodots and nanorods, J Phys Chem B, 103 (1999)

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8410-8426. [36] L. Kuai, B. Geng, X. Chen, Y. Zhao, Y. Luo, Facile subsequently light-induced route to

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highly efficient and stable sunlight-driven Ag-AgBr plasmonic photocatalyst, Langmuir, 26

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(2010) 18723-18727.

[37] G.T. Li, K.H. Wong, X.W. Zhang, C. Hu, J.C. Yu, R.C.Y. Chan, P.K. Wong, Degradation of

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Acid Orange 7 using magnetic AgBr under visible light: The roles of oxidizing species,

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Chemosphere, 76 (2009) 1185-1191.

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FIGURE CAPTIONS Fig. 1. Structural formula and UV-Vis spectra of methylene blue. Fig. 2. (A) Schematic diagram of the experimental setup and (B) the output spectrum of the

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visible laser diode used in this study.

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Fig. 3. (A) TEM image and (B) particle size distribution of the Ag/AgCl NPs. (C) UV-Vis

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spectra of Ag/AgCl NP suspension.

Fig. 4. (A) Typical absorbance spectra of MB dye after treatment with Ag/AgCl NPs under

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irradiation of laser light. (B) Decolorization and (C) degradation of MB dye (4 mg/L) with and

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without catalyst in the presence of laser light or LED light irradiation. Fig. 5. Photocatalytic degradation and decolorization of MB (4 mg/L) using different

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concentrations of catalyst. Error bars represent standard deviations of three measurements.

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measurements.

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Fig. 6. Effect of initial MB concentration. Error bars represent standard deviations of three

Fig. 7. Effect of pH on the adsorption of MB on the Ag/AgCl NPs. Error bars represent standard deviations of three measurements.

Fig. 8. Effect of pH on degradation and decolorization of MB (4 mg/L). Error bars represent standard deviations of three measurements. Fig. 9. Effect of laser energy on degradation (A) and decolorization (B) of MB (5 mg/L). Fig. 10. Linear plots of Ln (C0/C) for the photocatalytic degradation and decolorization of MB using Ag/AgCl as catalyst under 1.15 W laser light irradiation.

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Fig. 11. Degradation and decolorization of MB (4 mg/L) with Ag/AgCl during repeated photooxidation experiments under laser light.

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Fig. 12. Proposed photocatalytic mechanism of Ag plasmonic photocatalyst under laser light.

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Table. 1 Percentage degradation, decolorization, and COD values of MB Dye (4 mg/L) percentage removal Laser light irradiation time (min)

decoloration

COD value (mg/L)

with catalyst

without catalyst

with catalyst

with catalyst

0

0

0

0

0

20.83

2

0.34

29.38

2.25

39.87

16.43

4

0.52

47.01

2.70

60.39

11.60

6

0.63

51.56

2.74

62.63

10.56

8

0.96

84.32

2.89

96.39

4.30

10

1.02

93.81

2.99

100

2.01

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without catalyst

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Table. 2 Removal of MB by photocatalysis using different light sources. Catalyst

Light source

Wavelength Power

Rate constant

Reference

(min−1) mercury lamp

290 nm

125 W 0.060a

[20]

TiO2

mercury lamp

340 nm

125 W 0.025a

[20]

ZnS/CdS

halogen lamp

-

500 W 0.004a

Au/AgCl

halogen lamp

-

150 W 0.023a

Ag/AgCl

diode laser

443 nm

1.15 W 0.232a

This Study

Ag/AgCl

diode laser

443 nm

1.15 W 0.141b

This Study

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Decolorization reaction rate constant; b Degradation reaction rate constant

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TiO2

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