1 Introduction

The massive progress of different industrial activities, especially those that use dyes and colored materials, such as ceramics, textiles, cosmetics, painting, and leathers, exacerbates the unbridled release of their effluents directly into the surrounding water surfaces. It poses an escalating and grave threat to the environment [1, 2]. The continuous discharge of untreated effluents results in water resources contamination and ecosystem deterioration, posing a significant threat to aquatic life, disrupting natural habitats, and endangering the health of humans and wildlife [3,4,5]. Addressing this serious issue demands new technologies for eliminating different pollutants and treating and regulating the industrial effluents discharged into the aquatic environment [6, 7].

Scientists exert great efforts to overcome this issue and develop several techniques for treating, eliminating, and degrading different pollutants from wastewater through biological, adsorption, and photodegradation techniques [8,9,10,11,12,13]. Conventional approaches, including physical and chemical preparations for dye degradation, encounter notable limitations and disadvantages. These include producing undesired-size NPs, which may require extensive time, elevated temperatures or pressures, or the potential formation of hazardous by-products. These circumstances may hinder their efficacy in addressing environmental concerns associated with textile dye pollution, making them economically and environmentally unsustainable [14, 15]. Green synthesis of nanoparticles involves utilizing natural resources, such as algae, microbes, and plant extracts, to synthesize novel nanoparticles [16]. The biosynthesis of nanoparticles significantly reduces the environmental impact, minimizing toxic substances and lowering energy usage [17, 18]. Furthermore, these technologies offer large-scale production, cost-effectiveness, eco-friendliness, and reduced energy consumption, making them more accessible for diverse applications, including catalysis, medicine, and environmental remediation [19,20,21]. The biogenic synthesis of nanoparticles promotes sustainable manufacturing and sharing to provide safe, innovative, and biocompatible nanomaterials that offer a more sustainable future [22,23,24].

Cerium is a rare earth metal from the lanthanides group. It is more abundant in the Earth's crust than lead and nearly as copper abundant [25]. This crucial element finds significant applications in modern technologies and various industries. It has a wide range of uses, particularly in environmental remediation, pharmaceuticals, and the production of glasses and ceramics [26,27,28]. Cerium possesses unique properties, such as a high oxygen storage capacity and excellent reactivity. Thus, it is widely used in automobile catalytic converters to mitigate air pollution by reducing harmful emissions. Cerium is a precious and indispensable element globally due to its wide applications and abundance [29]. Researchers fabricated CeO2 with other oxides, such as CeO2/In2O3 [30], CeO2/ZnO.4CdO [31], and NiV2O6/CeO2 NPs [32]. Ceria NPs (CeO2) are widely used in several applications, such as optical processes [33], photocatalysts [5, 7, 34], and medical purposes [35, 36].

The blue-green algae S. platensis is a vital microorganism containing numerous bioactive compounds, rich nutrient content, and potential health benefits. Its protein, vitamins, and minerals make it an all-in-one nutrient source for many individuals [37]. Moreover, Spirulina has rapidly grown in various environmental conditions, contributing to its availability on a global scale. It is widely used as a potential algae-based system for wastewater treatment. Many researchers used their extract as a stabilizing and reducing agent in the eco-friendly biosynthesis of NPs [38,39,40,41,42].

Fabricating ceria NPS with the extract of S. platensis is a novel and promising technique to produce nanoparticles without harmful chemicals or intensive energy consumption. Additionally, it is considered a new approach to addressing the knowledge gap in the existing methodologies [43, 44]. Many literatures showed methylene blue degradation dye using different types of catalysts, such as biogenic ZnO [23], CuSnSe [45], TiO2@rGO [46], Er2O3-coated silicon [47], TiO2 [48], manganese dioxide modified silicon nanowires [49], TiO2/SnO2/CeO2 [50], and CeO2 [51].

The main objective of this study was to characterize the optical and morphological properties of the bio-synthesized CeO2@Sp NPs, besides demonstrating the ability of S. platensis to enhance the photodegradation efficiencies of ceria NPs to eliminate MB dye from aqueous solution under ultraviolet irradiation.

2 Materials and methods

2.1 Chemicals

Ce(NO3)3·6H2O (cerium nitrate hexahydrate), sodium hydroxide (NaOH), ammonium hydroxide (NH4OH), ethylene glycol (C2H6O2), ethanol (C2H5OH), citric acid (C6H8O7) and methylene blue (C16H18ClN3S) were of analytical grade (Sigma). All reagents were prepared using deionized water (DW).

2.2 Culturing of algal biomass

S. platensis strain was cultured according to the methods described by Zarrouk [52] and Bischoff and Bold [53] under specific conditions (24 °C, at pH 9–10, in a 16/8 h light/dark cycle and 3000 lx). The culture was shaken twice daily. After 21 days, the algal cells were harvested by centrifugation at 6000 rpm (Sigma 3–16 KL) for 10 min at 4 °C. The algal cells were rinsed several times with DW and dried at room temperature for 7 days. The algal mass was ground to a fine powder using an electric mill and sieved throughout a 63 µm sieve. 5 and 10 g of fine-dried algal biomass in two flasks containing 100 mL of distilled water were sonicated for 30 min and heated for 60 min at 90 °C with continuous stirring. After cooling, the mixtures were filtered through Whatman filter paper (No. 1). The pure filtrate was kept at 2–4 °C for further use.

2.3 Synthesis of CeO2/S. platensis NPs

The modified Pechini method [54] was used for the preparation of ceria NPs (CeO2 NPs) (Fig. 1). Deionized water (40 mL) was used to dissolve 12.5 g of Ce(NO3)3·6H2O, then ethylene glycol-citric acid (4:1) was added. CeO2@Sp NPs were prepared by adding the aqueous extract of 5 g and 10 g of Spirulina biomass to separate flasks containing cerium nitrate solution. The mixture was stirred vigorously at 90ºC, forming a white precipitate of cerium hydroxide. After 6 h of continuous stirring, the homogenized mixtures were exposed to air for 26 h at 25ºC. Subsequently, the mixtures were centrifuged at 8000 r/min for 10 min, and the precipitate was dried at 120ºC for 6 h. The yellow cerium oxide obtained underwent further calcination at 500-550ºC for about 4 h using a muffle furnace (model, Nabrotherm p 180, Germany).

Fig. 1
figure 1

Graphical schematic diagram illustrates the green and chemical preparation of CeO2 NPs

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2.4 Physical characterizations

A spectrophotometer (Mettler Toledo V 670, United States) was used for UV-absorption spectroscopy examination at 200–800 nm. Band gap energy of as-prepared photocatalysts was calculated using UV-absorption spectra based on Tauc's equation [Eq. (1)]:

$$\left( {\alpha {\text{h}}\upupsilon } \right)^{{2}} = {\text{A }}\left( {{\text{h}}\upupsilon - {\text{E}}_{{\text{g}}} } \right)$$
(1)

An X-ray diffractometer (Philips, PW-3710) was used to determine the crystallographic structure of the as-prepared catalyst. The patterns of XRD were recorded by Cu-Kα radiation (λ = 1.5408 Å) at a tube voltage of 40 kV and current of 30 mA within a scan range of 20–80° at a scan rate of 2θ = 5°/min.

Scanning electron microscope (model, JEOL, JEM-2100, Japan) at an acceleration voltage of 200 kW was used to demonstrate the morphological characteristics (SEM and TEM) and determine the elemental composition (EDX) of as-prepared ceria NPs. Different functional groups in the surface area of as-prepared ceria NPs were detected using FTIR spectroscopy (model, 6700 FTIR, Nicolet, America) at 400–4000 cm−1 range.

2.5 Photocatalytic efficiency

The photocatalytic efficiency of the as-prepared catalyst was examined by photodegradation of MB using the batch method. The stock solution of MB (1 g/L) was used to prepare a standard 100 mg/L solution. The photodegradation reactions were carried out under UV irradiation (6 mercury lamps, 11 watts of each with light intensity of 50 lx at 260 nm). Before beginning photocatalytic experiments, all mixtures were stirred in the dark for 30 min to achieve the absorption–desorption equilibrium. The Main factors affecting MB dye's photodegradation (solutions' pH, catalyst dose, time of irradiation, and MB initial concentrations) were studied to obtain the optimum adsorption conditions for maximum degradation reactions. The concentrations of the remaining MB dye were evaluated at wavelength λ = 665 nm using a Jenway 6800 UV/VIS spectrophotometer. The photocatalytic process's capacity (qe mg/g) and removal efficiency (R%) were calculated according to Eqs. (2) and (3).

$$q_{e} = \frac{{(C_{0} - C_{e} ) \times V}}{M}$$
(2)
$$_{{R{\text{\% }} = { }\frac{{\left( {C_{0} - C_{e} } \right){ }}}{{C_{0} }} \times 100}}$$
(3)

where qe photocatalytic capacity (mg/g), R removal efficiency (%), V is the volume of MB dye (L); C0 is the initial MB concentration; Ce is the equilibrium concentration of MB, and M is the mass of used catalyst (g).

2.6 Isotherms studies

Studying isotherm models provides valuable degradation process data, helping the researchers design and optimize efficient and sustainable solutions for various applications, especially environmental remediation and degradation of inorganic and organic pollutants. Langmuir, Freundlich, Dubinin, and Radushkevich's isotherm models designate the equilibrium results of the photocatalytic reactions. Langmuir model is expressed by the Eqs. (4) and (5).

$$\frac{1}{{q_{e} }} = \frac{1}{{q_{max} }} + \frac{1}{{bq_{max} }} \cdot \frac{1}{{c_{e} }}$$
(4)

Langmuir dimensionless constant (RL) was evaluated from the following equation.

$$R_{L} = { }\frac{1}{{1 + bC_{e} }}$$
(5)

where qmax (mg/g) the maximum uptake of MB dye; qe is the MB dye equilibrium concentration (mg/g); Ce mgL−1 the initial concentration of MB; and b (L/mg) constant of Langmuir.

Freundlich isotherm model is expressed as Eq. (6).

$$q_{e} = K_{f} C_{e}^{1/n}$$
(6)

where qe (mg/g) is the amount of adsorbed MB dye; Ce (g/L) MB concentration at equilibrium; Kf is the adsorbent capacity, and n is the adsorption intensity.

Dubinin and Radushkevich represented by the Eqs. (7) and (8)

$$\ln q_{e} = lnq_{m} - \upbeta \upvarepsilon ^{2}$$
(7)
$$E = \frac{1}{\sqrt 2 \beta }$$
(8)

where qe is MB concentration at equilibrium (mg/g), qm is maximum concentration of MB (mg/g), β is D-R isotherm constant (mol2 kJ−2), ε is the Polanyi potential (kJ/mol) and E mean free energy.

2.7 Kinetics studies

Studying kinetics models such as pseudo-first and pseudo-second-order reactions is essential in degradation and adsorption research. The kinetic models give much valuable data to illustrate the mechanisms of reactions between adsorbates and adsorbents, guiding the researchers to optimize reaction conditions to obtain effective adsorption processes for catalytic reactions.

Pseudo-first order and pseudo-second order models were studied according to the Eqs. (9) and (10).

$$\log \left( {C_{e} - C_{t} } \right) = logC_{e} - \frac{{k_{1} }}{0.203} t$$
(9)
$$\frac{t}{{C_{t} }} = \frac{1}{{k_{2} C_{e}^{2} }} + \frac{1}{{C_{e} }} t$$
(10)

where Ce and Ct are the concentration of MB dye (mg/g) at equilibrium and time "t" respectively; k1 (min−1) and k2 (mg/g.min) are constant rate of pseudo-first order and pseudo-second-order reactions, respectively.

3 Results and discussion

3.1 UV-spectroscopy of ceria NPs

The study of UV–VIS spectroscopy of as-synthesized ceria and green ceria NPs gives valuable information of the electronic properties and NPs behavior according to their surface resonance, quantum confinement effects, and size-dependent bandgap shifts [14]. Figure 2a–c illustrates the UV–VIS spectrum for pure and as-synthesized green ceria NPs. The maximum absorption peak for pure CeO2 was detected at 316 nm (Fig. 2a). A slight shift towards high wavelength (320 and 322 nm) was observed in the different doped ratios of the algal extract with ceria NPs (Fig. 2b and c). These shifts are according to modifications in the surface resonance and electronic band structure of green ceria NPS [55].

Fig. 2
figure 2

UV–visible spectra and the band gaps energy of a pure CeO2, b CeO2@Sp 2:1 and c CeO2@Sp 1:1 NPs

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The band gap energy (Eg) was calculated using Tauc's equation [Eq. (1)]. A plot of (ahν)2 versus hν was constructed to calculate Eg for the as-synthesized NPs (Fig. 2a–c). The calculated Eg values from this curve were 3.3, 3.15, and 2.94 eV for pure CeO2, CeO2@Sp 2:1, and CeO2@Sp 1:1 NPs, respectively. A slight decrease of Eg values for green ceria NPs than pure ceria was detected. This decrease in Eg revealed that the algal extract improved the conductance and optical characteristics of CeO2, enhancing their photocatalytic activities [50].

3.2 XRD spectra of CeO2 NPs

XRD patterns of as-prepared catalysts of ceria and green ceria NPs are given in Fig. 3. The XRD diffractograms show four major peaks with high intensity are found at 2θ equal 28.44°, 32.92°, 47.32°, and 56.14°. These peaks were assigned to (111), (200), (220), and (311) planes according to standard JCPDS NO 89-8436. These principal planes typically confirmed the formation of the fluorite crystal structure of CeO2. Hence, these peaks display well-defined and sharp peaks, revealing high purity and crystallinity formation of ceria NPs [14, 55]. The four minor peaks obtained at 2θ equal 58.18°, 69.12°, 76.85°, and 78.78° were assigned to (222), (400), (331), and (420) planes. The slight broadening of detected peaks of the as-prepared green ceria NPs confirmed the successful formation of the bio-organic crystallization phase [56].

Fig. 3
figure 3

XRD diffractogram of as synthesized ceria and green ceria NPs

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3.3 FTIR spectroscopy

FTIR spectroscopy describes the chemical composition and determines the surficial functional groups of NPs. The pure and green ceria NPs' FTIR spectra show several peaks corresponding to various molecular vibrations (Fig. 4). Pure CeO2 showed several peaks at 3934 cm−1 attributed to O–H vibration [57], 3424 cm−1 assigned to the asymmetric stretching NH2 band [58], 2923 and 2372 cm−1 belonging to C-H stretching and C=O stretching bonds, 1637 cm−1 assigned to the stretching C=C vibration bond and at 1091 cm−1 corresponding to =C–H group [34]. The broad peaks observed at 601 and 422 cm−1 revealed the formation of Ce–O bonds [59].

Fig. 4
figure 4

FTIR spectra of pure ceria and green synthesized CeO2 with spirulina algal extract with different doping ratios

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The impregnation of algal extracts at various doping ratios during the synthesis of ceria nanoparticles has notably influenced the functional groups. This often results in increased functional groups on the surface of the synthesized nanoparticles, as these extracts are abundant in bioactive compounds like polyphenols, polysaccharides, and proteins [60, 61]. The green ceria NPs showed several broad peaks at the 3930–3750 cm−1 range, assigned to the O–H vibration of alcoholic and water molecules [62]. Several peaks were observed between 2900 and 2000 cm−1 with particular wave numbers of 2921, 2850, 2343, and 2092 cm−1 attributed to carboxylic algal extract groups. Furthermore, strong peaks were found at 1635, 1619, 1531, and 1513 cm−1 assigned for C=C, carbonyl stretching C=O bond, and N–O bond [55].

3.4 SEM, EDX and TEM analyses of ceria and green ceria NPs

SEM images provide significant evidence about the morphological structure of ceria and green ceria NPs. SEM images (Fig. 5) exhibit spherical particles with irregular porous aggregation of CeO2 clusters. The presence of porosity with irregular particle aggregation is owed to releasing gases during the calcination of catalysts [63].

Fig. 5
figure 5

SEM and EDX images of a pure CeO2, b CeO2@Sp 2:1 and c CeO2@Sp 1:1

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EDX analysis is an excellent tool to determine the elemental structure of ceria NPs. Pure CeO2 nanoparticles exhibit two prominent peaks (Fig. 5), with cerium constituting 74.2% and oxygen 25.8%. Minor peaks, observed in Fig. 5a, appear to have negligible impact on the purity of the synthesized nanoparticles, confirming the formation of highly pure ceria. Green ceria nanoparticles displayed three additional minor peaks corresponding to C (3.06%), N (1.24%), and S (0.04%). These findings confirm the successful incorporation of pure CeO2 with organic compounds from the green extract of algal cells [64].

TEM images provide a high-resolution view of the morphology and structure of as-synthesized green and pure ceria NPs. The TEM images of ceria NPs appear as well-defined and often spherical particles with high contrast and uniform shape (Fig. 6).

Fig. 6
figure 6

TEM images of as-synthesized ceria NPs with different magnification power

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The observed ceria particles agglomerate closely, forming a chain-like structure.

3.5 Photodegradation experiments

3.5.1 Effect of pH

pH significantly affects both the rate and the efficiency of the photodegradation reactions through its impacts on the ionization state of the adsorbate surface. Therefore, the degradation process exhibits optimal activities within specific pH ranges, indicating a pH-dependent nature of the degradation process [4].

The effect of pH on the photocatalytic degradation of MB dye was investigated in the range of 5–12 using an initial concentration of MB of 100 mg/L mixed with 0.3 g/L of as-synthesized ceria catalysts. A remarkable increase in MB dye decomposition with pH level elevation was observed (Fig. 7) under UV light for 90 min. The maximum removal percent was achieved at pH = 11 for all ceria NPs, with the highest removal efficiency of 79.28, 87.74, and 89% for pare CeO2, CeO2@Sp. (2:1) and CeO2@Sp (1:1) NPs, respectively. The green synthesis of ceria particles with the algal extract of S. platensis improved and enhanced the degradation ability of the catalyst. However, the removal efficiency increased from 79.28% in the case of pare CeO2 to 89% using CeO2@Sp (2:1) NPs. At high pH levels, an excess of hydroxyl groups (OH) is released, generating free radicals (·OH) and accumulating negative charges on the surface of ceria nanoparticles. This accumulation creates strong electrostatic forces between the catalyst's surface and the cationic dye MB, facilitating the adsorption of dye particles. Consequently, an initiation of the degradation of dye particles occurred [65]. Pouretedal and Kadkhodaie [51], Ali et al. [46], and Kalaycıoğlu et al. [65] reported that optimum pH levels for maximum degradation of methylene blue are 11, 9, and 12, respectively (Table 1).

Fig. 7
figure 7

Effect of pH level on the degradation process of MB dye

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Table 1 Comparison of photodegradation efficiencies of some dyes using different photocatalysts
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3.5.2 Effect of time

Contact time is a critical issue in understanding the kinetics of the photodegradation reactions, which directly affect the extent to which a substance degrades. The contact time effect on MB dye photodegradation (100 mg/L) was investigated at times ranging between 5 and 120 min with fixed solutions' pH = 9 (Fig. 8). A rapid degradation rate of MB was notable at the early reaction time of 5–45 min. The elevated degradation rate of MB is owed to the rapid formation of OH radicals, leading to a high adsorption affinity of as-synthesized catalysts. Over time, the reaction rate slows down due to the coverage of the photocatalyst's active sites with MB particles. Thus, the interaction between the photocatalyst surface and released photons ceased, leading to the cessation of the production of hydroxyl radicals [66].

Fig. 8
figure 8

Effect of contact time on the photodegradation of MB dye

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The highest removal percentages of MB dye (75.5, 86.3, and 87.2%) were reached after 90 min (Fig. 8). Relative removal efficiency was achieved using green ceria NPs comparable to pare CeO2 NPs. At the same time, there is no significant variation in degradation efficiency between different doped green ceria NPs with algal extract. These findings agree with Kusuma et al. [55], Ali et al. [46], and Kalaycıoğlu et al. [65], who mentioned the highest decomposition of MB at a contact time of 90 min using CeO2, TiO2@rGO, and CeO2-NPs/GO NPs, respectively (Table 1).

3.5.3 Effect of dose

The photocatalyst dosage introduced is essential in eliminating different pollutants and remediating the hazards of organic particles during wastewater treatment. The effect of the different photocatalyst masses (0.05–0.5 g/L) on the MB dye degradation process was examined at optimum specific conditions (Fig. 9). Increasing the photocatalyst dose enhanced the catalysts' active sites, leading to an easier, faster, and more effective MB decomposition rate. MB degradation rate showed a gradual increase with the increase of ceria and green ceria NPs, reaching maximum degradation removal efficiencies of 76.4, 86.2, and 88.8% at 0.3 g/L for pare CeO2, CeO2@Sp 2:1, and CeO2@Sp 1:1 NPs, respectively. A further increase in photocatalyst dose leads to a decreased photodegradation process because of the aggregation of catalyst particles, so the solution becomes more turbid and prevents light penetration [50].

Fig. 9
figure 9

Effect of photocatalyst dose on the photodegradation of MB dye

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Zafar et al. [67] and Al-Onazi and Ali [14] postulated 0.3 g/L as the optimum catalyst dose for maximum degradation of MB using ZnO and CeO2 NPs, respectively. While Zhang et al. [68] and Pouretedal and Kadkhodaie [51] reported 4.0 g/L and 1 g/L for degradation of MB using CeO2 coated on FACs (fly ash cenospheres) and CeO2 catalyst (Table 1).

3.5.4 Effect of initial dye concentration

Different serial concentrations (5–100 mg/L) of MB dye at a fixed pH of 11 and catalyst dose of 0.3 g/L were examined to detect the optimum initial dye concentration (Fig. 10). A remarkable high removal efficiency of MB dye (98%) was observed at low dye concentrations due to fewer dye molecules and high available active sites on the catalyst surface [69]. The degradation rate gradually decreased from 98% at 5 mg/L to about 88% at 100 mg/L of MB dye. The higher the dye concentrations, the lower the light penetration into the solution and the more occupation of the photocatalyst's active sites, leading to more competition between MB molecules. Murugan et al. [70] and El-Katori et al. [71] cited 10 and 50 mg/L as the optimum concentration of MB dye using CeO2 and CdS/SnO2 NPs, respectively (Table 1).

Fig. 10
figure 10

Effect of initial concentration of MB dye on the photodegradation efficiency

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3.6 Isotherms studies

The studies of the isothermal models of organic pollutant degradation clearly illustrate the dye breakdown process at a constant temperature. Many isothermal models describe the relationship between the adsorption molecules onto the adsorbent surfaces. Langmuir isotherm model designates homogenous monolayer adsorption, while the Freundlich model pronounces multilayer adsorption. On the other hand, the Dubinin–Radushkevich isotherm considers the variation in the adsorption potential with surface coverage. Thus, it describes the adsorption in microporous and nanoporous materials.

Table 2 and Figs. 11, 12 and 13 present the calculated constants of studied isothermal models. The isothermal constants showed that the MB dye degradation using as-prepared ceria and green ceria NPs is well fitted with the FIM with R2>0.99, better than LIM with R2 < 0.8 (Table 1 and Figs. 11, 12 and 13). These results revealed that heterogeneous multilayer degradation of MB dye occurred. Ahsan et al. [1] and ElGarni et al. [2] found that the breakdown of MB and 2,4 dinitrophenol using NiO/CuO-NC and green Fe3O4 NPs are a better fit with the Freundlich model than Langmuir isotherm model. The mean free energy (E kJ/mol) of the Dubinin–Radushkevich isotherm varied between 408.2 and 500.0 kJ/mol for the MB degradation. These values revealed that a chemisorption reaction occurred [74].

Table 2 Langmuir, Freundlich and Dubinin–Radushkevich isotherm constants for MB breakdown using pure and green ceria NPs
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Fig. 11
figure 11

Langmuir isotherm model for degradation of MB dye on the surface of CeO2 and green CeO2@Sp NPs

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Fig. 12
figure 12

Freundlich isotherm model for degradation of MB dye on the surface of CeO2 and green CeO2@Sp NPs

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Fig. 13
figure 13

Dubinin Radushkevich isotherm model for the degradation of MB dye on the surface of CeO2 and green CeO2@Sp NPs

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3.7 Kinetics models

Studying kinetics models of the adsorption provides essential information about the rate of adsorption reactions, helping to understand the dynamic nature of the degradation process, optimize reaction conditions, and predict the behavior of adsorbents over time. The most used kinetics models are pseudo-first order and pseudo-second order models, especially in the remediation of the pollutants in their aqueous solutions. The pseudo-first-order model indicated a linear correlation between the rate of adsorption and the difference between adsorbate concentrations at initial and equilibrium status. The pseudo-second-order model gives a linear relation between the adsorption rate and the square of the difference between adsorbate concentrations at initial and equilibrium status.

The results of kinetic models (Table 3 and Figs. 14 and 15) showed that the degradation of MB dye followed the pseudo-second-order model with a Ce range of 83.33 and 89.29 mg/g with R2 > 0.99.

Table 3 Calculated constants of pseudo-first and pseudo-second order reactions for degradation of MB dye degradation on the surface of CeO2 and green Ceo2@Sp NPs
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Fig. 14
figure 14

Pseudo-first order model for degradation of MB dye on the surface of CeO2 and green CeO2@Sp NPs

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Fig. 15
figure 15

Pseudo-second order model for degradation of MB dye on the surface of CeO2 and green CeO2@Sp NPs

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4 Conclusion

The green synthesis of ceria (NPs) with S. platensis extracts provides an environmentally promising approach to sustainable development. Utilizing natural extracts, especially from algae, introduces bioactive compounds that enhance the resulting ceria NPs' properties and ensure the novel catalysts' eco-friendly fabrication. Physical characterization confirmed the high purity and polycrystalline formation of ceria NPs. Optical UV–VIS spectra indicated the formation of ceria NPs, evident from intense peaks at 316, 320, and 322 nm. These peaks indicated a decrease in bandgap energy with an increase in the doping ratio of the algal extract. FTIR measurements showed multi-functional groups due to impregnating pure ceria with the algal extract. EDX results confirmed the introduction of C, N, and S elements from the algal extract to green ceria NPs with different abundance ratios. The green ceria NPs showed higher removal efficiency of MB dye (R% = 86.2 and 88.8%) than pure ceria (R% = 76.4%) under specific conditions (pH = 11, 90 min contact time, 0.3 g/L catalyst dose and 100 mg/L MB initial concentration). The isothermal constants showed that the MB dye degradation is well fitted with the Freundlich model with R2>0.99, better than the Langmuir model with R2 < 0.8. Furthermore, the degradation of MB dye followed a pseudo-second-order model with Ce values ranging between 83.33 and 89.29 mg/g with R2 > 0.99.