Adsorption and Photocatalytic Degradation of an Industrial Azo Dye Using Colloidal Semiconductor Nanocrystals


 In this study, mercaptosuccinic acid capped CdSe nanocrystals was successfully synthesized by in-situ medium colloid and used as photocatalyst for the effective photodegradation of methylene blue from aqueous solution under visible light and sunlight irradiations.The particle size and the crystal structure of these nanocrystals were analyzed by different analytical techniques. Dye adsorption prior to photocatalysis using these nanomaterials was studied via the experimental quantification of kinetics and isotherms. These experimental data were modeled including the application of statistical physics theory to analyze the corresponding adsorption mechanism. A maximum adsorption capacity of 27.1 mg/g (80% dye removal) was observed in 10 min using an initial concentration of 30 mg/L. Statistical physics calculations indicated that the adsorption energy was lower than 40 kJ/mol. Itwas also established that the dye adsorption was associated to the electrostatic interactions and hydrogen bonding. Overall, the dye removal was a spontaneous, feasible process and exothermic. Adsorption properties of CdSe-MSA nanocrystals improved the dye photodegradation efficiency under visible light thus achieving up to 80% degradation in 60 min. The synergic effect of adsorption and photocatalytic degradation performance was mainly due to the surface area, small size (3.7 nm) and structural defects (selenium vacancies Se, interstitial of cadmium ICd), which enhanced the response of these nanomaterials inside the visible range for the photocatalytic activity. In summary, these nanocrystals are promising materials to be used in wastewater treatment under sun light for the removal of organic compounds like dyes.

Abstract: 17 In this study, mercaptosuccinic acid capped CdSe nanocrystals was successfully 18 synthesized by in-situ medium colloid and used as photocatalyst for the effective 19 photodegradation of methylene blue from aqueous solution under visible light and sunlight 20 irradiations.The particle size and the crystal structure of these nanocrystals were analyzed by 21 different analytical techniques. Dye adsorption prior to photocatalysis using these 22 nanomaterials was studied via the experimental quantification of kinetics and isotherms. 23 These experimental data were modeled including the application of statistical physics theory 24 to analyze the corresponding adsorption mechanism. A maximum adsorption capacity of 27.1 25 mg/g (80% dye removal) was observed in 10 min using an initial concentration of 30 mg/L.

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Statistical physics calculations indicated that the adsorption energy was lower than 40 kJ/mol. 27 Itwas also established that the dye adsorption was associated to the electrostatic interactions 28 and hydrogen bonding. Overall, the dye removal was a spontaneous, feasible process and 29 exothermic. Adsorption properties of CdSe-MSA nanocrystals improved the dye 30 photodegradation efficiency under visible light thus achieving up to 80% degradation in 60 31 min. The synergic effect of adsorption and photocatalytic degradation performance was 32 mainly due to the surface area, small size (3.7 nm) and structural defects (selenium vacancies 33 Se, interstitial of cadmium ICd), which enhanced the response of these nanomaterials inside the 34 visible range for the photocatalytic activity. In summary, these nanocrystals are promising      Salavati-Niasari 2017)and its molecular structure is described in Table 1.    The micro-strain ε and broadening βstrainare related by the following relation (Stokes and 245 Wilson 1944): So, the total broadening due to strain and size, in a particular peak having the hklvalue, can 248 be expressed as: The last equation can be re-arranged to get: Plotting the value of β hkl cosθ as a function of 4sinθ(see Figure 1.b) can be used to estimate Where αis the absorption coefficient, ℏνis the photon energy, A is a constant and n=1/2 is a 311 transition-dependent factor.The absorption coefficient (α) was determined from Beer- In the case of thesenanocrystals, λ max was 480 nm and the correspondingD value was 2 nm, 327 which was slightly greater than the sizes obtained from XRD and TEM results. CdSe 328 nanocrystals have λ > 400 nm indicating that they were suitable for the solar spectrum.

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The PL spectrum of purified CdSe-MSA nanocrystals was obtained using a 333 photoluminescence spectrophotometer with an excitation wavelength λexc= 325 nm at room 334 temperature. Figure 6 shows PL spectrum of CdSe nanocrystals redispersed in water.Pl   Kinetics studies on the MB adsorption were conducted at 28 °C. Figure 7 shows the pseudo-

Adsorption isotherms
397 Figure 8 and Table 4 show the results of dye adsorption isotherm where the Langmuir,

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Freundlich isotherm, Langmuir-Freundlich and Redlich-Peterson equations were used to 399 correlate these equilibrium experimental data.In particular, the Langmuir model assumes that 400 the adsorption occurs with the monolayer formation of adsorbate molecules on a 401 homogeneous adsorbent surface with uniform energies of adsorption (Langmuir 1918).This  where K LF is the adsorption affinity constant and nLF is the heterogeneity index.

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Data fitting using this isotherm outperformed the results from Langmuir, Freundlich and Double layer process with two adsorption energies: Where C1/2, C1 and C2 are the corresponding concentrations at half saturation, respectively.

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Thesethreemodelswereappliedto correlate the MB adsorption data of CdSe-MSA nanocrystals 451 and the fitting results suggested that the monolayer adsorption model (R 2 = 0.98 -0.99 and 452 RMSE = 1.84 -3.49) was the best to interpret the adsorption mechanism of MB dye, see 453 Figure 9 and Table 5. Note thatthecalculation of adsorption energy via this model was  Table 6.  The variation of these thermodynamic functions with respect to the equilibrium MB 488 concentration at different temperatures is illustrated in Figure 10.  The scheme for MB removal by thesynergistic effect of adsorption and photodegradation is 572 presented in Figure 19.As indicated, the highly reactive hydroxyl radicals ( • OH) and   The authors declare that they have no known competing financial interests or personal 620 relationships that could have appeared to influence the work reported in this paper.       Kinetics for the MB adsorption on CdSe-MSA nanocrystals and its modeling with thepseudo-rst-order equation.

Figure 8
Adsorption isotherm of MB dye on CdSe-MSA nanocrystals at pH = 7 and 300 K. The absorption spectrum of MB photodegradation without nanocrystals in visible-light irradiation        Proposal of degradation mechanism of MB dye using CdSe-MSA nanocrystals under sunlight irradiation