Thermodynamic and kinetic studies of Victoria blue B removal using the photocatalyst of flower-like cadmium sulfide microspheres synthesized via hydrothermal process

Nguyen Duc Vu Quyen; Tran Ngoc Tuyen; Dinh Quang Khieu; Dang Xuan Tin; Bui Thi Hoang Diem; Nguyen Thi Ai Nhung; Nguyen Thi Thanh Hai; Dang Thi Ngoc Hoa; Ho Thi Thuy Dung; Đao Ngoc Nhiem; Le Van Thanh Son

doi:10.4491/eer.2024.564

Environ Eng Res > Volume 30(4); 2025 > Article

Quyen, Tuyen, Khieu, Tin, Diem, Nhung, Hai, Hoa, Dung, Nhiem, and Van Thanh Son: Thermodynamic and kinetic studies of Victoria blue B removal using the photocatalyst of flower-like cadmium sulfide microspheres synthesized via hydrothermal process

Research

Environmental Engineering Research 2025; 30(4): 240564.

Published online: December 24, 2024

DOI: https://doi.org/10.4491/eer.2024.564

Thermodynamic and kinetic studies of Victoria blue B removal using the photocatalyst of flower-like cadmium sulfide microspheres synthesized via hydrothermal process

Nguyen Duc Vu Quyen^1,^†

, Tran Ngoc Tuyen¹, Dinh Quang Khieu¹, Dang Xuan Tin¹, Bui Thi Hoang Diem¹, Nguyen Thi Ai Nhung¹, Nguyen Thi Thanh Hai¹, Dang Thi Ngoc Hoa², Ho Thi Thuy Dung³, Đao Ngoc Nhiem⁴, Le Van Thanh Son⁵

¹Department of Chemistry, University of Sciences, Hue University, 77 Nguyen Hue Str., Hue City, Vietnam

²University of Medicine and Pharmacy, Hue University, 6 Ngo Quyen Str., Hue City, Vietnam

³Hue Medical College, 01 Nguyen Truong To Str., Hue City, Vietnam

⁴Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay District, Hanoi, Vietnam

⁵University of Education and Science, University of Da Nang, 459 Ton Duc Thang Str., Da Nang City, Vietnam

^†Corresponding author: E-mail: ndvquyen@hueuni.edu.vn, Tel: +84-979590971, ORCID: 0000-0002-1949-5379

Received September 23, 2024 Revised November 26, 2024 Accepted December 23, 2024

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The synthesis of flower-like cadmium sulfide (CdS) with the presence of amino acid as a shaping agent is exposed in the present study. The influence of two different amino acids (alanine and phenylalanine) on the morphology and photocatalytic ability of the obtained cadmium sulfide (denoted as A-CdS and P-CdS) is compared. The characteristics of the materials are clarified by methods including X-ray diffraction (XRD), Raman spectrum, energy-dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), nitrogen adsorption/desorption isotherms (BET), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS). The flower-like microspheres with a size of about 0.5 to 1 mm were formed from nanosheets. With the 10 mg L⁻¹ solution of Victoria blue B, a photocatalytic dosage of 1 g L⁻¹, A-CdS exhibited higher removal efficiency (95,1%) than P-CdS (85,8%). The kinetic and thermodynamic investigations of Victoria blue B dye photocatalytic decomposition were carried out for A-CdS at initial Victoria blue B concentration of 20 mg L⁻¹ and CdS dosage of 1.0 g L⁻¹. The photocatalytic kinetic of Victoria blue B removal is described using Langmuir-Hinshelwood kinetic model. The endothermic and spontaneous nature of VBB decomposition has been demonstrated through the thermodynamic parameters. The catalyst is recycled to be reused five times.

Keywords: Alanine, Flower-like cadmium sulfide, Langmuir-Hinshelwood, Phenylalanine, Shaping agent, Victoria blue B

Graphical Abstract

Keywords: Alanine, Flower-like cadmium sulfide, Langmuir-Hinshelwood, Phenylalanine, Shaping agent, Victoria blue B

Introduction

One of the notable environmental problems is the existence of toxic organic dyes which are difficult to decompose. These pollutants may have accumulated in the organism, then cause acute and chronic poisoning for humans. Besides, the presence of organic dyes in aqueous solution is easily noticeable even at low concentrations. Victoria blue B (VBB) is a popular triphenylmethane dye widely used in the textile, paper, leather and photographic industries with a complex aromatic structure that makes it more stable than others such as methylene blue (MB), methyl orange (MO) or methyl red (MR) [1]. Long-term exposure to this dye would cause damage to the skin, eyes and could lead to cancer. Therefore, the treatment of dye pollution in general and VBB dye in particular is an urgent issue, although it is difficult to degrade VBB from aqueous solution by adsorption, precipitation, oxidation or biological methods [1–3].

In recent decades, scientists have developed pollution-free methods or manufactured materials used for removing organic dyes from water, such as ion exchange [4–6], membrane filtration [7, 8], physical adsorption [9], photocatalysis [10–13] and biological method [14–17]. Among them, the heterogeneous photocatalytic treatment using semiconductor photocatalyst has been one of the simple and highly effective ways with low cost and performing at normal temperature and atmospheric pressure [18].

Cadmium sulfide (CdS) with the bulk hexagonal structure (Wurtzite) has been discovered as one of the semiconductors that attracts the most attention due to its relatively narrow band gap energy (2.4 eV). This property results in the direct absorption of energy in the visible light region, so CdS is capable of generating electrons (e−) and photogenerated holes (h+) under visible light conditions [19–21]. Therefore, CdS semiconductors have many applications in sensors [22, 23], waveguides [24, 25], photocatalysts [26–30] and optoelectronic devices [31, 32].

Numerous works have been conducted to synthesize CdS material by the hydrothermal method which exhibits high crystallinity and purity, forms highly stable CdS particles and is easy to control particle size [19, 33–35]. Especially, one of the most obvious advantages of hydrothermal synthesis of CdS is low temperature for a short time such as 90°C for 16 hours and 130°C for 15 hours [36]; 100°C for 4 hours [37]; 120°C for 8 hours [38] and 120°C for 12 hours [33]. Therefore, the method requires low energy and a simple autoclave that results in inexpensiveness. Several studies have shown that amino acids, such as D-alanine, L-histidine, L-serine, L-alanine or L-arginine···, play an important role in the synthesis of CdS as a stabilizing or capping agent [39–41]. However, none have been found for evaluating the role of amino acids containing aromatic rings such as phenylalanine. The comparison of the morphology and particle size of the obtained CdS when using non-aromatic amino acids and aromatic ones has not yet been clarified in the previous studies. The employment of the resulting CdS for photocatalytic removal of many common dyes with effective degradation was demonstrated. Repo et al. successfully applied nanostructured CdS microspheres in the photocatalytic degradation of dyes with high degradation efficiency after 3 h under near UV and blue LED radiation, where, almost MB, about 88–92% of MR and 85–98% of phenol red (PR) were removed with dye concentration of 3 mg L⁻¹ and dosage of catalyst of 1 g L⁻¹ [42]. At a dye concentration of 10 mg L⁻¹, MB was also removed using the CdS nanoparticles with a degradation efficiency of 88% (catalyst’s dosage of 0.25 g L⁻¹) [43] and 80% (material’s dosage of 0.8 g L⁻¹) [44]. Qin et al. studied another dye of MO which was removed 98.8% within 90 min under UV light irradiation at MO concentration of 10 mg L⁻¹ and a catalyst’s dosage of 0.3 g L⁻¹ [45]. When using 0.1 g of CdS nanoparticles in 200 mL of crystal violet dye, about 96% of the dye was removed [46] This photocatalyst also displayed high efficiency of 87–95% toward photodegradation of reactive red azo dye (RR141) under visible light irradiation up to 240 min [47], 88.4% of Rhodamine B (RhB) in 120 min [48]. However, the studies of the photocatalytic effect of CdS on the decomposition of VBB have been still empty. The key of the present study is the durable flower-like structure of the CdS microsphere which is completely stable over 7 reuses is synthesized by hydrothermal method combined with the employment of amino acids including alanine and phenylalanine as shaping and stabilizing agents. For the first time, the influence of phenylalanine on the formation of microspheres was examined and compared with alanine. The photocatalytic decomposition of VBB using the catalyst of flower-like CdS microspheres under visible light irradiation has been clarified in terms of kinetics and thermodynamics.

Experimental

2.1. Materials

The resulting CdS was prepared from cadmium acetate (Sigma-Aldrich) and thiourea (Sigma-Aldrich) with the stabilizing agent of alanine (Sigma-Aldrich) or phenylalanine (Sigma-Aldrich) as shown in Schema 1. Solution A contained cadmium acetate in distilled-water and the pH of the solution was adjusted to 10 using NH₃ (Merck) solution. A specified amount of alanine or phenylalanine was added to the solution under stirring. Then, thiourea was added to the solution to form CdS crystal seed grown in hydrothermal equipment at a specified temperature for a defined time. The solid was filtered, washed with water, and dried to obtain CdS material.

Factors affecting the structure formation of CdS were investigated, including S:Cd molar ratio, concentration of alanine and phenylalanine, hydrothermal pH and temperature. The samples prepared with alanine and phenylalanine were denoted as A-CdS and P-CdS, respectively.

2.2 Methods

2.2.1. Characterization of material

The characteristics of the material, including crystal phase, element composition and chemical state, morphology, porosity and band gap of the obtained CdS, were evaluated by the X-ray diffraction (XRD) using RINT2000/PC device (Rigaku, Japan) with the tube anode made of Cu with Ka = 1,54 Å, the energy-dispersive X-ray spectrum (EDS) using a Hitachi S4800 device (Japan), the X-ray photoelectron spectroscopy on Axis Supra (Japan), the scanning electron microscopy (SEM) (Hitachi S4800, Japan), the N₂ adsorption and desorption using a Tristar-3030 system (USA) and the diffuse reflectance spectroscopy (DRS) using Cary 5000 (Australia) with the Tauc method, respectively.

2.2.2. Determination of Victoria blue B

The determination of VBB concentration was carried out on the Cary 60 UV Vis (Agilent) device by UV-Vis molecular absorption spectrometry at the wavelength of 610 nm. The standard curve method was employed to quantify VBB concentration.

2.2.3. Catalytic studies

VBB removal was carried out under visible irradiation from a 100W Tungsten lamp and strong stirring. With the aim of getting physical adsorption equilibrium before the photocatalysis, the mixture containing 20 mL solution of VBB 20 mg L⁻¹ and 1.0 g L⁻¹ of CdS photocatalyst was magnetically stirred in the dark for 2 hours. VBB content (C₁) was determined. After that, the irradiation was performed for the next 2 hours so that the photocatalytic process occurred. VBB was quantified (C₂) and the VBB adsorption (H_ad) and photocatalytic removal (H_ph) efficiency were calculated based on the equations as follows:

(1)

H_{a d} (%) = \frac{C_{0} - C_{1}}{C_{0}} \cdot 100

(2)

H_{p h} (%) = \frac{C_{1} - C_{2}}{C_{0}} \cdot 100

The kinetic investigations were carried out in a 500 mL beaker containing 250 mL solution of VBB with different VBB concentrations from 5 to 25 mg L⁻¹ and 1.0 g L⁻¹ of CdS photocatalyst under stirring. After 2 hours in the dark, every 10 minutes, 10 mL of the mixture including both solution and material were separated using a pipette. The VBB concentration in 10 mL of sample was determined. The Langmuir-Hinselwood model was employed for the analysis of kinetic data.

Thermodynamic experiments were done from 283 to 323 K. At each temperature, a sample containing 250 mL solution of VBB 20 mg L⁻¹ was stirred with a catalyst dosage of 1.0 g L⁻¹ and VBB was quantified every 10 minutes. Arrhenius and Eyring equations were used to calculate activation parameters. The thermodynamic parameters of the photocatalytic reaction were inferred from the Van’t Hoff plot.

Results and Discussion

3.1. Synthesis and Characterization of CdS

3.1.1. The effect of molar ratio of S:Cd

The molar ratio of S:Cd is one of the important factors that greatly affects the characteristics and applicability of the obtained CdS material. Five S:Cd molar ratios were investigated, including 1:3, 1:2, 1:1, 2:1 and 3:1. The concentration of alanine/phenylalanine, hydrothermal pH, temperature and time were fixed at 0.05 M, 10, 100°C and 12 hours. The color of A-CdS samples changed from orange to yellow when the molar ratio of S:Cd gradually varied from 3:1 to 1:3, corresponding to the reduction of sulfur amount.

The phase composition of CdS materials is presented in the XRD diagram as shown in Fig. 1(a) and Fig. 2(a). The analysis results show that diffraction peaks of hexagonal (Wurtzite) structure of CdS (space group P63mc, JCPDS No. 06-0314) corresponding to (100), (002), (101), (110), (103) and (112) lattice planes clearly appear in XRD patterns of A-CdS1:3 and P-CdS1:3 samples [49, 50]. For P-CdS samples synthesized with S:Cd molar ratios of 1:1, 2:1 and 3:1, the peaks at 2q from 22 to 30° overlap into a band. For A-CdS samples synthesized with S:Cd molar ratios of 2:1, 1:1 and 1:2, there is no peak corresponding to (101) lattice plane.

Fig. 1(b) and Fig. 2(b) present Raman spectra of materials. As can be seen, the first-order LO Raman peak and the second-order overtones of LO optical phonon clearly appear around at wave-numbers of 290 (1LO) and 583 cm⁻¹ (2LO) in all the Raman spectra of hexagonal CdS materials, respectively [51, 52]. The intensity ratio of 1LO:2LO can be attributed to the change in the grain size of CdS [52]. The higher this ratio is, the larger the grain size of the material is. The data demonstrate that with the presence of alanine, CdS microspheres are formed in a smaller size than those synthesized using phenylalanine. This can be also observed in SEM images of samples as shown in Fig. 3.

The flower-like structures are created from an arrangement of numerous nanosheets with a thickness of 5–7 nm. The microspheres is obtained with the grain size of A-CdS samples ranging from 0.5 to 1 mm, and they show some agglomerations. Several larger particles which are about 2 mm are observed in SEM images of P-CdS samples.

For both A-CdS and P-CdS samples prepared with S:Cd molar ratios of 1:3 and 3:1, the nano-sized petals are more completely formed and more orderly arranged than other ratios. Many amorphous particles are mixed between the petals in samples prepared with S:Cd molar ratios of 1:2, 1:1 and 2:1. In another observation, the more complete and smaller the flower-like microspheres are, the higher the porosity of the material is. Both A-CdS and P-CdS microspheres synthesized with S:Cd molar ratios of 1:3 possess the highest BET surface area in comparison with other ratios. For each S:Cd molar ratio, comparing to P-CdS material, a higher BET surface area of A-CdS material is obtained. According to the IUPAC classification, the obtained IV type of N₂ adsorption and desorption isotherm curves of CdS samples as shown in Fig. 1(c) and Fig. 2(c) is corresponding to the adsorption of micro-mesoporous material [53].

The change of band gap energy of CdS microspheres with different S:Cd molar ratios is shown in Fig. 1(d) and Fig. 2(d). For A-CdS samples, the band gap change is unremarkable in the range from 2.30 to 2.42 eV, in which, the lowest band gap energy of A-CdS1:3 sample is 2.30 eV. With the presence of phenylalanine in the synthesis process, S:Cd molar ratios of 1:3 and 3:1 give low band gap energies of 2.20 and 2.15 eV, respectively. With these obtained band gap energies, CdS materials are capable of absorbing visible radiation to carry out photocatalytic reactions. The lower the band gap energy is, the more effective the photocatalysis at a longer wavelength is [54, 55].

The catalytic efficiency of A-CdS and P-CdS materials for VBB removal is evaluated as shown in Fig. 1(e) and Fig. 2(e). It can be seen that the VBB removal efficiency when using CdS1:3 material as a photocatalyst is quite high, specifically, 95.1% for A-CdS1:3 and 85.8% for P-CdS1:3 with an initial VBB concentration of 10 mg L⁻¹ and the catalyst dosage of 1.0 g L⁻¹. In which, the equilibrium adsorption efficiency of the material in the dark reaches about 22 to 28%. The VBB adsorption capacity of CdS material gets maximum value of 8.80 mg g⁻¹ for A-CdS1:3 and 6,27 mg g⁻¹ for P-CdS1:3. Some other dyes such as methylene blue (MB), methyl orange (MO) or Rhodamine B (RhB) have also been adsorbed before performing the photocatalytic process with the CdS catalyst in various previous studies. They all said that the adsorption capacity value is low and equivalent to this value in the present study. Liu et al. confirmed that there was 7.2 mg g⁻¹ of MB, 5.2 mg g⁻¹ RhB and 0.8 mg g⁻¹ MO adsorbed onto CdS nanoparticles [35]. MB, RhB and MO were also adsorbed onto nano-CdS photocatalyst with adsorption capacity of 4.97 mg g⁻¹ [36], 12 g g⁻¹ [40] and 2.5 mg g⁻¹ [56], respectively. Repo et al. said that there were no phenol red (PR), 1.5 mg of MB, 0.5 mg methyl red (MR) adsorbed onto 1 g of cauliflower CdS microspheres [34]. These low adsorption efficiencies or adsorption capacities confirm that the adsorption proceeds faster than the photocatalytic reaction which is treated as the rate-determining step [57].

From the morphological results in Fig. 3, it is found that more than 50% VBB is removed when the microspheres exhibit flower-like structure in CdS1:3 and CdS3:1 samples. The reason can be predicted because the uneven surface of the microspheres results in a large number of active catalytic centers. For the remaining S:Cd molar ratios, many catalytic centers disappeared, caused by the quite condensed surface of microspheres or covered by impurities. Therefore, the VBB removal efficiency is quite low. This result also fits well with the XRD analysis in Fig. 1(a) and Fig. 2(a). For CdS1:3 and CdS3:1 samples, no crystalline phase appears except the crystalline phase of CdS nanosheets, meaning that the reaction between Cd²⁺ and S²⁻ exhibits highly efficient and the nanosheets are arranged into flower-like structures. Therefore, in the XRD patterns of CdS1:3 and CdS3:1 samples, there are no diffraction peaks of impurities. This may be the reason why many catalytic centers are formed resulting in higher photocatalytic efficiency than that of the remaining CdS samples. In conclusion, for both A-CdS (degradation efficiency of 93.9%) and P-CdS (degradation efficiency of 85.8%) microspheres, S:Cd molar ratio of 1:3 is chosen for other investigations. VBB is also removed from aqueous solution using other photocatalysts under visible light with an equivalent efficiency such as ZnO nanoparticles (93.45%) [58], V₂O₅ nanorods/graphene oxide nanocomposites (97.95%) [59].

The EDX analysis indicates the existence of cadmium, sulfur, carbon, and oxygen elements in A-CdS1:3 and P-CdS1:3 samples with their weight percentages commented in Fig. 4. This result confirms the samples containing CdS with high purity. The EDX mapping images show the homogeneity of element distributions in both samples (Fig. 4).

The elemental and chemical state information of P-CdS is further confirmed by XPS analysis as shown in Fig. 5. The characteristic Cd3d, S2p and O1s peaks are observed in Fig. 5(a). The high-resolution spectrum in Fig. 5(b) presents two peaks at 405 and 412 eV corresponding to the 3d^3/2 and 3d^5/2 with the binding energy difference of about 7 eV which are assigned to the +2 oxidation state of Cd²⁺ in CdS. The −2 oxidation state of S is inferred from the peak at 162 eV in Fig. 5(c). These results are in excellent agreement with other reported findings [60, 61].

3.1.2. The effect of concentration of alanine/phenylalanine

The role of amino acid in the synthesis of CdS nanoparticles is mentioned as a stabilizing agent [39], capping agent [40], or chelating agent [62]. The suitable concentration of alanine or phenylalanine is chosen in the range value from 0.025 to 0.075 M based on the morphology of the obtained CdS. The results show that the non-use of alanine or phenylalanine during the synthesis process results in the failure of the formation of the microspheres in CdS1:3 sample, primary nanoparticles grow into bars with a size of about 100 nm. CdS nano-bars grow into nanosheets rearranged in the form of flower-like microspheres with the increase of the concentration of alanine or phenylalanine from 0.025 to 0.075 M. Among these concentrations of alanine/phenylalanine, 0.025 M might not be the suitable one because of the incomplete arrangement of nanosheets and the roughness of the petals. The presence of alanine with its contents of 0.050 M and 0.075 M creates flowers in the size of 200–300 nm. The flowers become larger in the size of 1–2 mm when phenylalanine is used instead of alanine. The above results partly confirmed amino acid plays an important role as a shaping agent.

3.1.3. The effect of hydrothermal pH

According to previous studies, the particle size of the obtained CdS was influenced by hydrothermal pH [63–65]. Uchil J. et al. further commented that CdS formation was not detected in acidic and neutral environments, even when the synthesis time lasted up to 24 hours [63]. Therefore, herein, the pH of the hydrothermal solution is investigated from 8 to 12. The change in morphology of CdS1:3 with hydrothermal pH is checked. At all surveyed pH values, the microspheres are formed. However, at pHs of 8 and 12, both A-CdS and P-CdS microspheres get more condensed than those at pHs of 9, 10 and 11. The flower-like structure is promoted to form at pH of 9, 10 and 11. Among them, 9 and 10 might be the most suitable pHs because of the fewest impurities.

3.1.4. The effect of hydrothermal temperature

The investigation presents that the higher temperature improves the density of micropheres. At higher temperatures (140–200°C), for A-CdS1:3 samples, the petals disappear and are replaced by polymorphic crystals, microspheres seem to gather into large clusters that make their borders disappear. For P-CdS1:3 samples, although the microspheres still are not agglomerated together, their size slightly increases, the petals become thicker, the porosity of the microspheres significantly decreases and many microcrystals appear between the petals. The hydrothermal temperature of 100°C is suitable for the formation of flower-like structure.

3.2. Photocatalytic Studies of VBB Removal

3.2.1. Photocatalytic kinetic study

Based on the preliminary investigation of VBB removal efficiency in Fig. 1(e) and Fig. 2(e), A-CdS1:3 material is employed to study the kinetics and determine the thermodynamic parameters of the VBB photocatalytic removal process from aqueous solution due to its higher removal efficiency in comparison of P-CdS1:3 material. The VBB photocatalytic removal efficiency over reaction time as shown in Fig. 6(a) indicates that the equilibrium reaction time is 60 minutes for initial VBB concentrations of 5 and 10 mg L⁻¹, 70 minutes for remaining VBB contents with the photocatalyst’s dosage of 1.0 g L⁻¹. Besides, the VBB photocatalytic decomposition efficiency strongly increases in the first 20 minutes, slightly rises in the next 40–50 minutes and unremarkable changes thereafter. Because of the fixed photocatalyst’s dosage, the increase in initial VBB concentration from 5 to 25 mg L⁻¹ reduces the equilibrium VBB removal efficiency from more than 99% to more than 86%.

The kinetic data of the photocatalytic reaction of VBB decomposition is described using a well-known model, the Langmuir-Hinshelwood (LH) kinetic model [66]. The mechanism of the reaction is suggested by I. Langmuir and developed by C. Hinshelwood includes two main steps as follows:

the adsorption of reactants (VBB) on the catalytic centers of CdS (C-CdS)

$VBB + C-CdS \hat{U} VBB \dots C-CdS$
the rate-limiting step: the surface reactions of VBB with oxidation-reduction agents to form products

$VBB \dots C-CdS ® products + C-CdS$

The non-linearized and linearized forms of The LH expression are given by Eq. (3) and Eq. (4) [54]:

(3)

r = - \frac{d C}{d t} = \frac{k_{r} K C}{1 + K C}

(4)

\frac{1}{r^{0}} = \frac{1}{k_{r}} + \frac{1}{k_{r} K C}

where r⁰ and r represent the initial and instant rate of oxidation (mg L⁻¹ min⁻¹), k_r is the LH rate constant (mg L⁻¹ min⁻¹), C⁰ and C are the initial and instant VBB concentrations (mg L⁻¹), and K is the equilibrium adsorption constant (L mg⁻¹).

Zhang et al. confirm that the adsorption step proceeds much faster than the photocatalytic step [57]. In other words, VBB is weakly adsorbed on the catalyst. Therefore, with a small VBB concentration, the Eq. (3) and Eq. (4) can be rewritten as the first-order kinetic equation [66]:

(5)

r = k_{r} K C

(6)

\frac{C^{0}}{C} = k_{1} t

where k₁ is the first-order rate constant (min⁻¹).

Fig. 6(b) shows the linear plots of ln(C⁰/C) versus the reaction time t. As can be seen, for small initial VBB concentrations (5 and 10 mg L⁻¹), the value of R² (0.979 and 0.976) is lower than this for initial VBB concentrations of 15, 20 and 25 mg L⁻¹ (0.993 – 0.997). It is therefore concluded that the first-order kinetics equation describes the kinetic data at high VBB concentration better than low VBB content. The first-order reaction takes place at a rate depending linearly on VBB concentration. Therefore, the higher the initial VBB concentration is, the larger the initial rate is.

The LH kinetic equation expresses the linear correlation between 1/r⁰ and 1/C⁰ as shown in Fig. 6(c). The determination coefficient for the LH kinetic equation (0.9837) is nearly 1, indicating that LH kinetic model is well-compatible with experimental data. The equilibrium constant (K) of the adsorption of VBB onto A-CdS1:3 calculated from the intercept of the regression line is small (0.214 L g⁻¹) which reveals the weak adsorption of VBB onto the material. The LH reaction rate (k_r) is inferred from the slope of the regression line, which is 0.824 (mg L⁻¹ min⁻¹).

With the aim of determining the activation energy (E_a) and the thermodynamic parameters of the activation process including enthalpy (ΔH^#), entropy (ΔS^#) and Gibbs free energy (ΔG^#), Arrhenius and Eyring equations are employed as follows [66]:

(7)

k_{T} = l n A - \frac{E_{a}}{R T}

(8)

\frac{k_{T}}{T} = - \frac{Δ H^{#}}{R T} + l n \frac{k_{B}}{h} + \frac{Δ S^{#}}{R}

where A is the pre-exponential factor, R is the universal gas constant (8.314 J mol⁻¹ K⁻¹), T is absolute temperature (K), k_T is the rate constant equal to the rate constant in the first-order kinetic equation, k_B (1.3807.10−23 J K⁻¹) is the Boltzmann constant, and h (6.626 J s) is the Planck constant.

The results are exposed in Fig. 6(e) and Fig. 6(f). The activation parameters for VBB removal of A-CdS1:3 photocatalyst are inferred from these plots and shown in Table 1.

The low value of activation energy (6.16 kJ mol⁻¹) calculated from the Arrhenius equation implies that the diffusion of VBB to A-CdS1:3 catalyst mainly takes place at the external surface of the catalyst. The formation process of an activated complex between VBB and the catalyst is spontaneous and favorable at high temperatures due to the positive value of activation entropy variation and the enhancement of negative values of Gibbs free energy from 298 K to 323 K. The endothermic nature of the activation process is proved based on the positive value of activation enthalpy change.

3.2.2. Photocatalytic thermodynamic study

A thermodynamic study is carried out in order to examine the thermodynamic nature of the whole reaction using the Van’t Hoff equation [66] as shown in Fig. 6(d). From the slope and intercept of the linear Van’t Hoff plot, the Gibbs free energy (ΔG⁰), standard enthalpy (ΔH⁰) and entropy (ΔS⁰) is obtained in Table 1. The results divulge the spontaneous occurrence of the photocatalytic reaction of VBB removal using A-CdS1:3 catalyst because of the negative ΔG⁰ value at all surveyed temperatures. In principle, the photocatalytic efficiency decreases when the reaction temperature increases due to the enhancement of photogenerated electron-hole recombination. However, many studies have proven that this combination strongly occurs at temperatures of 343K or higher and is limited at a low-temperature range of 0–50°C, also, the increasing temperature enhances the amount of strong oxidation agent - hydroxyl radicals in water [67–70]. This is the reason why the standard ΔG⁰ value becomes more negative when the temperature rises from 283 to 323K and the standard ΔS⁰ exhibits positive in the present study. An endothermic removal is concluded based on the positive value of standard ΔH⁰.

3.3. Recyclability of Photocatalyst

The recyclability of A-CdS1:3 photocatalyst is an important index to evaluate its stability against photocorrosion, life and cost. Hereby, the photocatalyst could be reused many times for experimental cost savings. The A-CdS1:3 material is separated from the solution after treatment by centrifugation, then dried at 100°C after soaking and stirring in distilled water at 60°C for 2 hours. The photocatalytic performances after 10 times of reusing the material slightly are 95.8; 94.3; 94.1; 94.5; 92.5; 92.1; 91.3; 87.3; 81.1 and 71.9%, respectively. These results confirm that after the first 7 reuses, the photocatalytic efficiency always reaches over 90% which might be due to the stability of active photocatalytic centers and the sustainability of flower-like CdS microspheres. The photocatalytic efficiency becomes significantly lower at the 8^th, 9^th and 10^th reuses. SEM images of the CdS photocatalyst (Fig. 7) show that the flower-like structure is remained after 7 reuses and partially broken after the 10^th reuse which demonstrated that the catalytic efficiency depends heavily on the stability of the flower-like structure of CdS.

One of the notable requirements for materials used for environmental treatment is environmentally friendliness. Some previous studies proved the toxicity of CdS due to the leaching of Cd²⁺ ions from material [71–73]. In the present work, the leaching of Cd²⁺ from the flower-like CdS photocatalyst over 10 reuses was checked by trace analyzing the Cd²⁺ content in the solution after photocatalysis using flame atomic absorption spectroscopy. The results showed that the solution was almost free of Cd²⁺ according to the WHO Guidelines for Drinking Water Quality (acceptable limit of 3 mg L⁻¹) [74]. Although the Cd²⁺ level lightly increases after each reuse (2.10; 2.13; 2.29; 2.35; 2.45; 2.65; 2.48; 3.21; 3.24 and 3.54 mg L⁻¹), it is still within the acceptable limit, except the 8^th, 9^th and 10^th reuses. The participation of amino acid in the synthesis process might be one of the reasons that explains the sustainability of CdS microspheres over time. Especially, the CdS photocatalyst retains good photocatalytic activity at the photocatalyst’s dosage of 1.0 g L⁻¹ and initial VBB concentration of 20 mg L⁻¹. The reuse of the photocatalyst at least 7 times while maintaining the high VBB removal efficiency of over 90% is one of the important advantages of the photocatalytic method in this study. In particular, the regeneration of the photocatalyst by soaking and stirring the catalyst in distilled water at 60°C for 2 hours is quite simple. This combined with the short photocatalytic time according to the investigation in section 3.2.1 (maximum 70 minutes of irradiation) with a visible light source (low energy due to a long wavelength) results in low cost compared to other traditional methods such as ion exchange, biology, and oxidation...

Conclusions

Flower-like cadmium sulfide material was hydrothermally synthesized in the presence of a shaping agent of amino acid and exhibited an effective photocatalytic activity for the decomposition of Victoria blue B from aqueous solution. The flower-like structure is formed from the arrangement of nanosheets. At ambient temperature, VBB removal of CdS catalyst synthesized in the presence of alanine and phenylalanine reached more than 95% and 85%, respectively. The photocatalytic removal mechanism of VBB on CdS catalyst was well-described by the Langmuir-Hinshelwood model, in which, the movement of VBB to the external surface of CdS is controlled by diffusion. The thermodynamic parameters reveal a spontaneous and endothermic process of photocatalytic reaction. The photocatalyst is sustainable after 7 reuses and partially broken after 10 reuses.

Acknowledgements

We are grateful to the Vietnam Ministry of Education and Training for support (grant No. B2023-DHH-13).

Notes

Conflict-of-Interest Statement

The authors declare that they have no conflict of interest.

Author Contributions

N.D.V.Q. (Doctor) did experiments, treated data, wrote and edited the manuscript. D.X.T. (Master), B.T.H.D. (Bachelor), N.T.T.H. (Doctor), H.T.T.D (Master) and D.T.N.H. (Doctor) did experiments. D.Q.K, T.N.T., N.T.A.N., D.N.N. and L.V.T.S. wrote and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

References

1. Chen CC, Chen CY, Cheng CY, Teng PY, Chung YC. Decolorization characteristics and mechanism of Victoria Blue R removal by Acinetobacter calcoacetius YC210. J Hazard Mater. 2011;196:166–172. https://doi.org/10.1016/j.jhazmat.2011.09.015

2. Pare B, Singh P, Jonnalagadda SB. Degradation and mineralization of Victoria blue B dye in a slurry photoreactor using advanced oxidation process. J. Sci. Ind. Res. 2009;68:724–729.

3. Shokrollahzadeh S, Tayar S, Azizmohseni F, Safavi M, Keypour S. Fungal decolorization of toxic triphenylmethane dye by newly isolated Ganoderma fungi: Growth, enzyme activity, kinetics. Bioresour. Technol. Rep. 2023;24:101654. https://doi.org/10.1016/j.biteb.2023.101654

4. Liu CH, Wu JS, Chiu HC, Suen SY, Chu KH. Removal of anionic reactive dyes from water using anion exchange membranes as adsorbers. Water res. 2007;41:1491–1500. https://doi.org/10.1016/j.watres.2007.01.023

5. Khan MA, Khan MI, Zafar S. Removal of different anionic dyes from aqueous solution by anion exchange membrane. Membr. Water Treat. 2017;8:259–277. https://doi.org/10.12989/mwt.2017.8.3.259

6. Khana MI, Lasharic MH, Khraishehd M, et al. Adsorption kinetic, equilibrium and thermodynamic studies of Eosin-B onto anion exchange membrane. Desalin. and Water Treat. 2019;155:84–93. https://doi.org/10.5004/dwt.2019.2393

7. Abid MF, Zablouk MA, Abid-Alameer AM. Experimental study of dye removal from industrial wastewater by membrane technologies of reverse osmosis and nanofiltration. Iran. J. Environ. Health Sci. Eng. 2012;9:17. https://doi.org/10.1186/1735-2746-9-17

8. Hidalgo AM, León G, Gómez M, Murcia MD, Gómez E, Macario JA. Removal of Different Dye Solutions: A Comparison Study Using a Polyamide NF Membrane. Membranes. 2020;10:408–424. https://doi.org/10.3390/membranes10120408

9. Demirbas O, Alkan M, Dogan M. The removal of Victoria blue from aqueous solution by adsorption on a low-cost material. Adsorption. 2002;8:341–349. https://doi.org/10.1023/a:1021589514766

10. George A, Magimai Antoni RD, Venci X, et al. Photocatalytic effect of CuO nanoparticles flower-like 3d nanostructures under visible light irradiation with the removal of methylene blue (MB) dye for environmental application. Environ. Res. 2022;203:111880. https://doi.org/10.1016/j.envres.2021.111880

11. Qutub N, Singh P, Sabir S, Sagadevan S, Oh WC. Enhanced photocatalytic removal of Acid Blue dye using CdS/TiO₂ nanocomposite. Sci. Rep. 2022;12:5759. https://doi.org/10.1038/s41598-022-09479-0

12. Abdrabou D, Ahmed M, Hussein A, ElSherbini T. Photocatalytic behavior for removal of methylene blue from aqueous solutions via nanocomposites based on Gd₂O₃/CdS and cellulose acetate nanofbers. Environ. Sci. Pollut. Res. 2023;30:99789–99808. https://doi.org/10.1007/s11356-023-28999-4

13. Ravikumar S, Mani D, Chicardi E, et al. Development of highly efficient cost-effective CdS/Ag nanocomposite for removal of azo dyes under UV and solar light. Ceram. Int. 2023;49:9551–9559. https://doi.org/10.1016/j.ceramint.2022.11.123

14. Bhatia D, Sharma NR, Singh J, Kanwar RS. Biological methods for textile dye removal from wastewater: A Review. Crit. Rev. Env. Sci. Tech. 2017;47:1836–1876. https://doi.org/10.1080/10643389.2017.1393263

15. Chiong T, Lau SY, Lek ZH, Koh BY, Danquah MK. Enzymatic treatment of methyl orange dye in synthetic wastewater by plant-based peroxidase enzymes. J. Environ. Chem. Eng. 2016;4:2500–2509. https://doi.org/10.1016/j.jece.2016.04.030

16. Oturkar CC, Patole MS, Gawai KR, Madamwar D. Enzyme based cleavage strategy of Bacillus lentus BI377 in response to metabolism of azoic recalcitrant. Bioresour. Technol. 2013;130:360–365. https://doi.org/10.1016/j.biortech.2012.12.019

17. Gholami-Borujeni F, Faramarzi MA, Nejatzadeh-Barandozi F, Mahvi AH. Oxidative degradation and detoxification of textile azo dye by horseradish peroxidase enzyme. Fresen. Environ. Bull. 2013;22:739–744.

18. Pomilla FR, García-López EI, Marcì G, Palmisano L, Parrino F. Heterogeneous photocatalytic materials for sustainable formation of high-value chemicals in green solvents. Mater. Today Sustain. 2021;13:100071. https://doi.org/10.1016/J.MTSUST.2021.100071

19. Al Balushi BSM, Al Marzouqi F, Al Wahaibi B, et al. Hydrothermal synthesis of CdS sub-microspheres for photocatalytic removal of pharmaceuticals. Appl. Surf. Sci. 2018;457:559–565. https://doi.org/10.1016/j.apsusc.2018.06.286

20. Das NK, Chakrabartty J, Farhad SFU, et al. Effect of substrate temperature on the properties of RF sputtered CdS thin films for solar cell applications. Results Phys. 2020;1:103132. https://doi.org/10.1016/j.rinp.2020.103132

21. Yücel E, Sahin O. Effect of pH on the structural, optical and nanomechanical properties of CdS thin films grown by chemical bath deposition. Ceram. Int. 2016;42:6399–6407. https://doi.org/10.1016/j.ceramint.2016.01.039

22. Vanalakar (Vhanalkar) SA, Patil VL, Patil SM, Deshmukh SP, Patil PS, Kim JH. Chemical and gas sensing property tuning of cadmium sulfide thin films. Mater. Sci. Eng. B. 2022;282:115787. https://doi.org/10.1016/j.mseb.2022.115787

23. Yuting L, Jing Z, Donghui L. Preparation of cadmium sulfide nanoparticles and their application for improving the properties of the electrochemical sensor for the determination of enrofloxacin in real samples. Chirality. 2019;31:174–184. https://doi.org/10.1002/chir.23045

24. Jassim NM, Hadi N. Nonlinear Optical Properties of CdS Semiconductor nanowires. IOP Conference Series: Mater. Sci. Eng. 2018;454:012111. https://doi.org/10.1088/1757-899X/454/1/012111

25. Gopal V, Harrington JA. Metal sulfide coatings for hollow glass waveguides. Proceedings of SPIE – The international Society for Optical Engineering. 2003;4957:97–103. https://doi.org/10.1117/12.485407

26. Yuan YJ, Chen D, Yu ZT, Zou ZG. Cadmium sulfide-based nanomaterials for photocatalytic hydrogen production. J. Mater. Chem. A. 2018;6:11606–11630. https://doi.org/10.1039/C8TA00671G

27. Chen F, Cao Y, Jia D, Niu X. Facile synthesis of CdS nanoparticles photocatalyst with high performance. Ceram. Inter. 2013;39:1511–1517. https://doi.org/10.1016/j.ceramint.2012.07.098

28. He J, Yan Z, Wang J, et al. Significantly enhanced photocatalytic hydrogen evolution under visible light over CdS embedded on metal-organic frameworks. Chem. Commun. 2013;49:6761–6763. https://doi.org/10.1039/c3cc43218a

29. Cheng L, Xiang Q, Liao Y, Zhang H. CdS-based photocatalysts. Energy Environ. Sci. 2018;11:1362–1391. https://doi.org/10.1039/C7EE03640J

30. Gai Q, Ren S, Zheng X, Liu W, Dong Q, Gao R. Controllable photodeposition of nickel phosphide cocatalysts on cadmium sulfide nanosheets for enhanced photocatalytic hydrogen evolution performance. New J. Chem. 2020;44:4332–4339. https://doi.org/10.1039/C9NJ06403F

31. Alam A, Kumar S, Singh DK. Cadmium sulphide thin films deposition and characterization for device applications. Mater. Todays Proc. 2022;62:6102–6106. https://doi.org/10.1016/j.matpr.2022.04.1018

32. Ingle RV, Shaikh SF, Kaur J, Ubaidullah M, Pandit B, Pathan HM. Optical and Electronic Properties of colloidal Cadmium Sulfide. Mater. Sci. Eng. B. 2023;294:11648. https://doi.org/10.1016/j.mseb.2023.116487

33. Senasu T, Ruengchai N, Khamdon S, Lorwanishpaisarn N, Nanan S. Hdrothermal synthesis of cadmium sulfide photocatalyst for detoxification of azo dyes and ofloxacin antibiotic in wastwater. Molecules. 2022;27:7944. https://doi.org/10.3390/molecules27227944

34. Loudhaief N, Labiadh H, Hannachi E, Zouaoui M, Salem MB. Synthesis of CdS Nanoparticles by Hydrothermal Method and Their Effects on the Electrical Properties of Bi-based Superconductors. J. Supercond. Nov Magn. 2017;31:2305–2312. https://doi.org/10.1007/s10948-017-4496-4

35. Nivetha A, Mangala Devi S, Prabha I. Fascinating Physic-Chemical Properties and Resourceful Applications of Selected Cadmium Nanomaterials. J. Inorg. Organomet. Polym. Mater. 2019;29:1423–1438. https://doi.org/10.1007/s10904-019-01141-z

36. Zhang P, Gao L. Cadmium sulfide nanocrystals via two-step hydrothermal process in microemulsions: synthesis and characterization. J. Colloid Interface Sci. 2003;266:457–460. https://doi.org/10.1016/S0021-9797(03)00671-4

37. Zhang B, Jian JK, Zheng Y, Sun Y, Chen Y, Cui L. Low temperature hydrothermal synthesis of CdS submicro- and microspheres self-assembled from nanoparticles. Mater. Lett. 2008;62:1827–1830. https://doi.org/10.1016/j.matlet.2007.10.013

38. Zang J, Zhao G, Han G. Preparation of CdS nanoparticles by hydrothermal method in microemulsion. Front. Chem. China. 2007;2:98–101. https://doi.org/10.1007/s11458-007-0020-x

39. Samadi-maybodi A, Abbasi F, Akhoondi R. Aqueous synthesis and characterization of CdS quantum dots capped with some amino acids and investigations of their photocatalytic activities. Coll. Surf. A: Physicochem. Eng. Asp. 2014;447:111–119. https://doi.org/10.1016/j.colsurfa.2014.01.036

40. Talwatkar SS, Tamgadge YS, Sunatkari AL, Gambhire AB, Muley GG. Amino acids (L-arginine and L-alanine) passivated CdS nanoparticles: Synthesis of spherical hierarchical structure and nonlinear optical properties. Solid State Sci. 2014;38:42–48. https://doi.org/10.1016/j.solidstatesciences.2014.09.014

41. Chanu TI, Negi DPS. Synthesis of histidine-stabilized cadmium sulfide quantum dots: Study of their fluorescence behaviour in the presence of adenine and guanine. Chem. Phys. Let. 2010;491:75–79. https://doi.org/10.1016/j.cplett.2010.03.068

42. Repo E, Rengaraj S, Pulkka S, et al. Photocatalytic removal of dyes by CdS microspheres under near UV and blue LED radiation. Sep. Purif. Technol. 2013;120:206–214. https://doi.org/10.1016/j.seppur.2013.10.008

43. Liu F, Han C, Sun P, Wang G, Li J, Chang Q. Sperical CdS nanoparticles precipitated from a cadmium thiosulfate complex using ultraviolet light for photocatalytic dye degradation. Metals. 2023;13:554. https://doi.org/10.3390/met13030554

44. Wang DQ, Wang YS, Xie ZX, Zhang WM. Adsorption behaviors of methylene blue onto tubular cadmium sulfide polycrystals. Int. Nano Lett. 2020;10:287–292. https://doi.org/10.1007/s40089-020-00316-8

45. Qin S, Liu Y, Zhou Y, Chai T, Guo J. Synthesis and photochemical performance of CdS nanoparticles photocatalysts for photodegradation of organic dye. J. Mater. Sci: Mater Electron. 2016;28:7609–7614. https://doi.org/10.1007/s10854-017-6453-1

46. Upadhyay RK, Sharma M, Singh DK, Amritphale SS, Chandra N. Photodegradation of synthetic dyes using cadmium sulfide nanoparticles synthesized in the presence of different capping agents. Sep. Purif. Technol. 2012;88:39–45. https://doi.org/10.1016/j.seppur.2011.11.040

47. Senasu T, Nanan S. Photocatalytic performance of CdS nanomaterials for photoremoval of organic azo dyes under artificial visible light and natural solar light irradiation. J. Mater. Sci.: Mater. Electron. 2017;28:17421–17441. https://doi.org/10.1007/s10854-017-7676-x

48. Ullah H, Viglašová E, Galamboš M. Visible Light-Driven Photocatalytic Rhodamine B Removal Using CdS Nanorods. Processes. 2021;9:263. https://doi.org/10.3390/pr9020263

49. Soltani N, Gharibshahi E, Saion E. Band gap of cubic and hexagonal CDS quantum dots – Experimental and theoretical studies. Chalcogenide Lett. 2012;9:321–328.

50. Seyghalkar H, Sabet M, Salavati-Niasari M. Synthesis and characterization of cadmium sulfide nanoparticles via a simple thermal decompose method. High Temp Mat Process. 2016;35:https://doi.org/10.1515/htmp-2015-0143

51. Saleem MF, Zhang H, Deng Y, Wang D. Resonant Raman scattering in nanocrystalline thin CdS film. J. Raman spectrosc. 2016;48:224–229. https://doi.org/10.1002/jrs.5002

52. Rengaraj S, Jee SH, Venkataraj S, et al. CdS Microspheres Composed of Nanocrystals and Their Photocatalytic Activity. J. Nanosci. Nanotechnol. 2011;11:2090–2099. https://doi.org/10.1166/jnn.2011.3760

53. Cychosz KA, Thommes M. Progress in the Physisorption Characterization of Nanoporous Gas Storage Materials. Engineering. 2018;4:559–566. https://doi.org/10.1016/j.eng.2018.06.001

54. Khatun M, Mitra P, Mukherjee S. Effect of band gap and particle size on photocatalytic removal of NiSnO₃ nanopowder for some conventional organic dyes. Hybrid Adv. 2023;4:100079. https://doi.org/10.1016/j.hybadv.2023.100079

55. Kumar A, Pandey G. A review on the factors affecting the photocatalytic degradation of hazardous materials. Mater. Sci. Eng. Int. J. 2017;1:106–114. https://doi.org/10.15406/mseij.2017.01.00018

56. Szeto W, Kan CW, Yuen CWM, Chan SW, Lam KH. Effective photodegradation of methyl orange using fluidized bed reactor loaded with cross-linked chitosan embedded nano-CdS photocatalyst. Int. J. Chem. Eng. 2014;1:1–16. https://doi.org/10.1155/2014/270946

57. Zhang L, Jaroniec M. Chapter 2: Fundamentals of adsorption for photocatalysis. Interface Sci. Tech. 2020;31:39–62. https://doi.org/10.1016/B978-0-08-102890-2.00002-6

58. Santhosh AM, Yogendra K, Madhusudhana N. Photocatalytic degradation of Victoria blue B a cationic dye by synthesised zinc oxide nanoparticle. Int. J. Nanoparticles. 2020;12:327–341. https://doi.org/10.1504/IJNP.2020.10034533

59. Kamalam MBR, Inbanathan SSR, Sethuraman K, et al. Direct sunlight-dreiven enhanced photocatalytic performance of V₂O₅ nanorods/graphene oxide nanocomposites for the degradation of Victoria blue B. Environ. Res. 2021;199:111369. https://doi.org/10.1016/j.envres.2021.111369

60. Alhammadi S, Reddy VRM, Gedi S, et al. Performance of Graphene–CdS Hybrid Nanocomposite Thin Film for Applications in CuIn, Ga.Se₂ Solar Cell and H₂ Production. Nanomater. 2020;10:245. https://doi.org/10.3390/nano10020245

61. Jaryal A, Singh AK, Dhingra S, et al. Understanding the charge transfer dynamics in 3D–1D nanocomposites over solar driven synergistic selective valorization of lignocellulosic biomass: a new sustainable approach. EES catal. 2024;2:1019–1026. https://doi.org/10.1039/D4EY00077C

62. Yadav RS, Mishra P, Mishra R, Kumar M, Pandey AC. Growth mechanism and optical property of CdS nanoparticles synthesized using amino-acid histidine as chelating agent under sonochemical process. Ultrason. Sonochem. 2010;17:116–122. https://doi.org/10.1016/j.ultsonch.2009.04.011

63. Uchil J, Pattabi M. Effect of pH on the Size of CdS Nanoparticles Synthesized by Chemical Diffusion Across a Biological Membrane. J. New Mat. Electrochem. Syst. 2005;8:155–161.

64. Parvathy RA, Pajonk GM, Venkateswara RA. Effect of pH Aging, and Drying on the Size of CdS Nanocrystallites, Monolithicity, and Transparency of Nanocrystalline CdS-Doped Silica Xerogels. J. Mater. Synth. Process. 2001;9:11–18. https://doi.org/10.1023/A:1011378413488

65. Mohanraj V, Jayaprakash R, Chandrasekaran J, Robert R, Sangaiya P. Influence of pH on particle size, band-gap and activation energy of CdS nanoparticles synthesized at constant frequency ultrasonic wave irradiation. Mat. Sci. Semicon. Proc. 2017;66:131–139. https://doi.org/10.1016/j.mssp.2017.04.006

66. Nguyen DVQ, Dinh QK, Tran NT, Dang XT, Bui THD, Ho TTD. Highly effective photocatalyst of TiO₂ nanoparticles dispersed on carbon nanotubes for methylene blue removal in aqueous solution. Vietnam J. Chem. 2021;59:167–178. https://doi.org/10.1002/vjch.202000091

67. Chen YW, Hsu YH. Effect of reaction temperature on the photocatalytic activity of TiO₂ with Pd and Cu cocatalyst. Catalysts. 2021;11:966. https://doi.org/10.3390/catal11080966

68. Janus M, Kusiak-Nejman E, Morawski AW. Influence of water temperature on the photocatalytic activity of titanium dioxide. React. Kinet. Mech. Cat. 2012;106:289–295. https://doi.org/10.1007/s11144-012-0432-6

69. Katakis D, Mitsopoulou C, Vrachnou E. Photocatalytic splitting of water-increase in conversion and energy-storage efficiency. J. Photochem. Photobiol. A Chem. 1994;81:103–106. https://doi.org/10.1016/1010-6030(94)03777-9

70. Harvey PR, Rudham R, Ward S. Photocatalytic oxidation of liquid alcohols and binary alcohol mixtures by rutile. J. Chem. Soc., Faraday Trans. 1: Physical Chemistry in Condensed Phases. 1982;79:975–2981. https://doi.org/10.1039/F19837902975

71. Varmazyari A, Taghizadehghalehjoughi A, Sevim C, et al. Cadmium sulfide-induced toxicity in the cortex and cerebellum: In vitro and in vivo studies. Toxicol. Rep. 2020;7:637–648. https://doi.org/10.1016/j.toxrep.2020.04.011

72. Zheng X, Liu Y, Yang Y, et al. Recent Advances in CadmiumSulfide-Based Photocatalysts forPhotocatalytic Hydrogen Evolution. Renewables. 2023;1:39–56. https://doi.org/10.31635/renewables.022.202200001

73. Shivaji K, Sridharan K, Kirubakaran DD, et al. Biofunctionalized CdS Quantum Dots: A Case Study on Nanomaterial Toxicity in the Photocatalytic Wastewater Treatment Process. ACS Omega. 2023;8:19413–19424. https://doi.org/10.1021/acsomega.3c00496

74. Kubier A, Wilkin RT, PichleHr T. Cadmium in soils and ground water: A review. Appl. Geochem. 2019;108:104388. https://doi.org/10.1016/j.apgeochem.2019.104388

Fig. 1

XRD (a), Raman (b) spectra, N₂ adsorption and desorption linear plots (c), Tauc plots (d) and VBB removal efficiency (e) of A-CdS materials synthesized with different S:Cd molar ratios.

Fig. 2

XRD (a), Raman (b) spectra, N₂ adsorption and desorption linear plots (c), Tauc plots (d) and VBB removal efficiency (e) of P-CdS materials synthesized with different S:Cd molar ratios.

Fig. 3

SEM images of A-CdS and P-CdS microspheres prepared with different S:Cd molar ratios.

Fig. 4

EDX mapping analyses of A-CdS1:3 (a) and P-CdS1:3 (b).

Fig. 5

XPS analysis of P-CdS1:3 sample (a) and high-resolution XPS spectra of Cd3d (b) and S2p (c).

Fig. 6

Effect of reaction time to the VBB removal efficiency (a), first-order kinetic equations of VBB removal (b) of A-CdS1:3 catalyst at different initial VBB concentrations, Langmuir-Hinshelwood kinetic equation (c), Van’ Hoff plots (d), Arrhenius (e) and Eyring (f) equations for VBB removal of A-CdS1:3 photocatalyst.

Fig. 7

SEM images of A-CdS1:3 photocatalyst after the 7^th (a and b) and 10^th (c and d) reuses.

Schema 1

The synthesis process of CdS.

Table 1

Activation and thermodynamic parameters of MB removal using A-CdS1:3 photocatalyst

T (K)	E^a (kJ mol⁻¹)	ΔH^# (kJ mol⁻¹)	ΔS^# (J mol⁻¹)	ΔG^# (kJ mol⁻¹)	ΔH^o (kJ mol⁻¹)	ΔS^o (J mol⁻¹)	ΔG^o (kJ mol⁻¹)
283	6.16	3.65	389.61	−106.62	20.58	86.64	−3.94
293				−110.51			−4.81
303				−114.41			−5.68
313				−118.30			−6.54
323				−122.20			−7.41