Preparation and characterization of algae-based biopolymer nanocomposite hydrogels for the adsorptive-photocatalytic degradation of malachite green dye

Abstract

This study combines experimental synthesis with molecular dynamic (MD) simulations to investigate the molecular level interactions governing the formation of the microalgae Ba/starch hydrogel. A molecular model representing the experimentally synthesized hydrogel was constructed, and Monte Carlo (MC) and MD simulations were performed to predict and validate the interfacial interactions and thermodynamic affinity between Ba and the starch matrix. Based on these computational insights, the purpose of this study is to prepare a highly absorbent hydrogel from microalgae and biopolymers for removing malachite green dye (MG) from wastewater via photodegradation. In this work, microalgae (Ba) + starch (Ba + St) hydrogel, microalgae Ba + chitosan (Ba + Cs) hydrogel, and Ba + St + Cs hydrogel are used as model substrates for the photocatalytic degradation of MG dye under solar radiation with a variety of reaction conditions. The surface and compositional changes of the produced photocatalyst were studied with XRD, FTIR, SEM, and UV-vis spectrophotometers in relation to the MG dye photodegradation, irradiation time, catalyst dose, dye concentration, and pH. The findings demonstrated that the decomposition progressed consistently from 5 to 360 minutes of irradiation time, reaching 11% to 99% for Ba + St hydrogel degradation and from 6% to 36% for Ba + Cs hydrogel and from 33% to 72% for Ba + St + Cs. This results from the MG dye molecule contact with the photocatalyst surface. The longer the irradiation period, the more the MG dye molecules interacted with the photocatalyst surface. As a result, the photocatalyst photodegradation efficiency increased. To reach a quicker rate of degradation, intervals of 360 minutes are employed.

This study combines experimental synthesis with molecular dynamic (MD) simulations to investigate the molecular level interactions governing the formation of the microalgae Ba/starch hydrogel.

1. Introduction

A widely used computational approach to modelling material behaviour at the atomic scale and predicting the properties of complex material systems is the Monte Carlo (MC) simulation. It provides useful information about the physical and chemical processes occurring in materials and helps to optimize the material design and processing conditions by relating the microstructure features to the macroscopic properties.¹ MC simulations can be used in various applications to metals,² alloys,³ ceramics,⁴ polymers,⁵ and advanced composite materials,⁶ also utilized in studying complex systems such as biochemical network,⁷ climate dynamics,⁸ and economic processes.¹ This approach is built on the evaluation of a large number of stochastic scenarios, and it increases the prediction accuracy, helps the system optimization and improves the decision making in uncertain situations in various fields of science and engineering.¹

Hydrogels are three dimensional (3D) biopolymeric networks with excellent water absorption and adsorption capacities due to the presence of hydrophilic functional groups.⁹ Their properties can be tuned by changing the synthesis parameters such as pH, temperature and agitation time.⁹ The crosslinking and reinforcing agents use increases their mechanical strength. The peculiar physicochemical properties of hydrogels make them highly functional, thus enabling their efficient use in various fields.¹⁰ The porosity and morphology are closely related to their wastewater treatment performance, which controls water and oxygen permeability and moisture absorption.¹¹ This sustained release reduces excessive reactive oxygen species (ROS), relieves oxidative stress, improves the microenvironment and stimulates the treatment process.¹¹ Moreover, hydrogels can be divided into synthetic and natural hydrogels according to their raw materials, among which natural hydrogels are more in line with the principles of sustainable development.¹² The synthetic polymers used for the preparation of hydrogels are generally non-biodegradable and may have some health hazards due to the presence of residual monomers, crosslinkers, and by-products formed during the polymerization.¹³ The most common natural raw materials for the synthesis of hydrogels are cellulose, chitosan, sodium alginate, starch, proteins, and their derivatives.¹⁴ In addition, hybrid hydrogels can be manufactured by mixing synthetic polymers with natural materials. In contrast, composite hydrogels are prepared by adding inorganic fillers like silica, zeolite, or calcium carbonate to the organic hydrogel matrices.¹⁴

Water is one of the worthiest supplies, playing a critical role in supporting life and keeping the environment safe, clean, and hygienic situations.¹⁵ However, when dangerous organic and inorganic pollutants are released into natural water bodies without being properly handled, human health and aquatic ecosystems are facing severe challenges. Specifically, the most harmful pollutants to aquatic and human life are synthetic organic colors,¹⁶ which the paper, textile, cosmetics, plastic, and leather industries can produce and release.¹⁷ Cause a widespread environmental catastrophe due to their unpleasant characteristics and their role as a carcinogen.¹⁸ The high solubility of textile dyes in water results in a rapid discharge of dark pigments, which blocks sunlight and causes serious harm to people and aquatic life in and near these industries' locations.¹⁹ There are over 10 000 of these dyes. When considering widespread use, the azo dye has enormous and crucial harm.²⁰

MG dye (N-methylated diaminotriphenylmethane) is an organic compound with mainly a metallic sheen of green colored crystals and one of the most potent substances in the world.²¹t is extremely poisonous and must be cleaned before being discharged into the environment. It poisons the liver and damages the intestines, kidneys, gills and mammalian cells. If breathed in it causes cancer, if it touches the skin it irritates and hurts.^22,23 Direct discharge of MG dye in hydrosphere causes environmental imbalance. This untreated water is used for irrigation which can affect the quality of crop output.²⁴ These colors are still in use due to their cheapness, availability and strength, even though they are banned in many countries.²⁵

The most important issue to be addressed is the treatment of this dye contamination before its release into water bodies. Researchers have tried out many methods to extract dyes from wastewater including reverse osmosis, precipitation, electrokinetic coagulation, microbial degradation, fungal decolorization, electrodialysis, nanofiltration, electrosorption, adsorption and filtration.²⁶ Adsorption is the most successful technique for decolorization of wastewater because of its simplicity and low cost. For example, activated carbon and clay minerals as well as hydrogels were used as adsorbing materials for the removal of dyes.²⁷

One of the more recent developments that aids in the elimination of organic contaminants from wastewater is the use of composite hydrogel algae based as a photocatalyst.²⁸ These composite capacity for photocatalysis aid in ecosystem monitoring and enhancement. In fact, under various circumstances, they eliminate the harmful organic contaminants to the greatest degree possible. These materials are the most popular since they are inexpensive and produce nearly no hazardous byproducts.²⁹

This study confirmed the interaction and fabrication between microalgae and starch hydrogel by MC and MD simulation and presents the first modification of microalgae Ba hydrogel based on starch (St), which have been synthesized and confirmed by MC and MD simulations. It also reports on the ability of the nanohydrogel composite to degrade a mixture of malachite green (MG) through photocatalysis.

2. Material and methods

2.1. Materials

The materials utilized in this study include extra pure starch derived from potatoes (St; Piochem, Egypt), chitosan (CS; Bio Basic, ON, Canada) characterized by a deacetylation degree of 96%, and Synechocystis sp. PAK 13 (Sy), which was isolated from both soil and water samples in Egyptian environments. The selected strain was procured from the Microbiology Department at the Faculty of Science, Beni-Suef University, Egypt, and subsequently cultivated without modification in 1 ml of Wuxal media (WM) mixed with 1 liter of distilled water and 5 g of sodium chloride in the microbiology laboratory at Beni-Suef University, Egypt, malachite green (MG) from Loba Chemie, and distilled water (DW).

2.2. Preparation of hydrogel composites

Two types of hydrogel composites were synthesized to investigate the role of chitosan in the structure matrix.

2.2.1. Chitosan + starch + microalgae hydrogel

Chitosan (CS) was initially solubilized in 25 ml of 1% (v/v) aqueous acetic acid. Concurrently, the starch (St) component was prepared by dissolving it in 25 ml of distilled water under mechanical stirring at 70 °C for 20 minutes. Separately, the microalgae (Sy) suspension was prepared in 25 ml of a 50 : 50 (v/v) distilled water/ethanol mixture and stirred for 1 hour at 70 °C. The three components were then combined and subjected to continuous mechanical stirring for 1.5 hours at room temperature. The resulting mixture was dried at 40 °C for 24 hours.³⁰

2.2.2. Starch + microalgae hydrogel (control)

This formulation served as a control to isolate the effect of chitosan. The preparation followed the protocol described in Section 2.2.1, with the exclusion of the chitosan component. The starch and microalgae solutions were combined, stirred for 1.5 hours at room temperature, and subsequently dried at 40 °C for 24 hours under identical conditions.³⁰

2.3. Preparation of dye stock solution

The first step is to accurately weigh 0.1 g MG dye on a digital balance. The weighed MG dye was transferred into a conical flask and dissolved in a small amount of DW. Finally, DW was added to prepare the final volume to 1000 ml, and the concentration of stock solution was 100 ppm. The solution stirred continuously for 5 min for complete dissolution and the prepared aqueous MG dye stock solution was stored for further use.

2.3.1. Photocatalytic activity

Efficient photodegradation of MG dye requires optimized operating conditions. The effect of various physicochemical parameters such as irradiation time, pH of MG dye solution, initial concentration of MG dye and catalyst dosage were systematically investigated. The photocatalytic degradation was carried out under sunlight using 25 ml aliquots of a 5 ppm MG dye solution and irradiation time ranging from 5 to 360 min. Based on these results, the effects of solution pH (3, 5, 8, and 10) and catalyst dosage (0.025–0.14 g) were further investigated using a 5 ppm MG dye solution. In addition, the effect of the initial MG dye concentration was evaluated over the concentration range of 5–40 ppm. The photocatalytic degradation of MG dye was monitored using UV-vis spectrophotometer by measuring the absorbance at the maximum absorption wavelength (λ_max = 617 nm). Each photodegradation experiment was repeated three times under identical conditions, and the result are presented mean values with error bars representing the variation among repeated measurements.

1

Here, C_o and C_f represents the initial and final concentration of MG. after the photocatalysis reaction. The experimental data obtained was processed using Origin 2024.

2.4. Computational study

The Monte Carlo dynamics (MC), and molecular dynamics (MD) simulations in this study were performed using adsorption locator and the Forcite module to investigate the formation mechanisms of the Ba + St hydrogel system. The St surface was selected as the adsorption substrate for the microalgae Ba. Periodic boundary conditions were applied to construct the simulation box with dimensions 19.5 × 19.5 × 19.5 ų. During the simulation process, the COMPASS III forcefield was employed to optimize the structures of the studied complexes. The main objective of these simulations was to identify the lowest energy most stable adsorption configurations and to elucidate the interaction mechanism of microalgae onto the St surface for the formation hydrogel.

2.5. Characterization

The crystalline structure of the prepared materials was characterized by X-ray diffraction (XRD) using a PANalytical Empyrean X-ray diffractometer with Cu-Ka radiation (λ = 1.54060 Å) at 40 kV accelerating voltage and 35 mA current. The diffraction patterns were collected in the range of 20–70° 2θ with a scanning step of 0.02°. The surface morphology was studied by scanning electron microscopy (SEM, Sigma 500, Zeiss, Germany) at 50 kV. Fourier transform infrared (FTIR) spectroscopy was performed using a Vertex 70 spectrometer (Burker, Germany) to determine the functional groups at a wave number of 400–4000 cm⁻¹. The optical properties of the samples were studied using a Shimadzu UV-vis spectrophotometer (M 160 PC) and absorption spectra were recorded in the wavelength range of 250–1400 nm at room temperature.

3. Results and discussion

3.1. Computational study

This computational research has two central objectives: (1) to construct an atomistic model representing the proposed chemical structure of a novel starch (St) and microalgae (Ba) hydrogel based on its experimentally synthesized composition and to investigate its structural stability as well as (2) to utilize molecular dynamics (MD) simulation and energy calculations to evaluate the proposed formation of a reticulated 3D structure. Rather than simulating the experimentally prepared hydrogel, the computational study was performed on a representative atomistic model constructed from the experimentally proposed composition and cross linked structure. Adsorption energy calculations, bond length analysis, and water mobility evaluation were performed to assess the stability of the modelled hydrogel network, providing basic insights into the intermolecular interactions responsible for the formation of the rigid hydrogel network, thus complementing the experimental findings.

The inherent stability of the novelty synthesized microalgae Ba with St hydrogel was originally confirmed via an extensive thermodynamic evaluation. The calculated total adsorption energy presented a noticeably negative value of −484.67 kcal mol⁻¹, supplying obvious evidence for the spontaneous and strongly exothermic interaction between microalgal Ba and the St polymer for formation hydrogel are shown in Table 1 and Fig. 1a. The large magnitude of this energy proves that the adsorption process goes beyond simply intermolecular attraction, which implies the occurrence of chemisorption.³¹ This explanation is further strengthened by the Deformation energy −460.03 kcal mol⁻¹, indicating that both the St chains and the microalgae Ba molecules exhibit major structural rearrangement to optimize interfacial contact, thus forming a highly stable and energy efficient hydrogel based approaches.

Table 1. Adsorption energy (kcal mol⁻¹), rigid adsorption energy, deformation energy, and substrate-adsorbate configurations (dEads/dNi) for the adsorption configuration of microalgae Ba on starch surface as hydrogel.

Total energy	Adsorption energy	Rigid adsorption energy	Energy deformation	Algae: dEad/dNi
−2.10854778	−484.67745168	−24.64650309	−460.03094860	−484.67745168

Fig. 1. Representative structure of microalgae Ba with St as hydrogel, (a) adsorption microalgae on St surface, (b) bond length atoms between microalgae and hydrogel, (c) simulation box of hydrogel system under water condition, and (d) simulation box of bulk water.

The transformation from a binary mixture into structured hydrogel phase is obviously confirmed by the solvent distribution profiles presented in Fig. 1c and d. In the control system containing water only was shown Fig. 1d, water molecules display the typical disordered arrangement associated with a bulk liquid state. In contrast, following the incorporation of microalgae Ba with St matrix at illustrated Fig. 1b and c, a pronounced clustering or partitioning phenomenon becomes evident. Water molecules are preferentially localized around the microalgae Ba with St interaction sites, indicating that the framework functions as a molecular templet. This spatial confinement represents the initial physical evidence of the hydrogel capacity to retain the solvent within its internal cavities, thereby modifying the local hydration environment.³²

The macroscopic characteristics of the hydrogel were quantitatively confirmed via the evaluation of Mean Square Displacement (MSD) and diffusion coefficient (D) demonstrated at eqn (2).³³ The MSD of water molecules within the hydrogel achieved a pronounced decreased to 7 Ų at 5 ps compared with 12.21 Ų measured in bulk water are illustrated at Table 2. This kinetic restriction was further substantiated by the D which decreased from 0.406 Ų per ps in the bulk water to 0.233 Ų within the hydrogel phase, corresponding to a 43% reduction in translational mobility. This phenomenon, referred to as the caging effect, arises directly from the dense cross linking and extensive hydrogen bonding network formed during the adsorption process.³⁴ The resulting matrix generates a highly tortuous diffusion pathway that markedly restricts water mobility. This immobilized water state provides definitive functional evidence of successful hydrogel formation, in which the chemical potential of the hydrogel surpasses the kinetic energy of the water molecules, thereby establishing the material as a promising candidate for high efficiency water retention.

MSD = 6Dt 2

where MSD represents of mean square displacement (Ų), D shown for diffusion coefficient (Ų), and t is time by ps.³³

Table 2. Parameters of mean square displacement (MSD) and diffusion coefficient (D).

Parameters	MSD (Å2)	D (Å2)
Water	12.68022	0.422
Adsorption system with water	7.215549	0.240

To elucidate the molecular level interactions governing hydrogel formation, visual analysis and radial distribution function (RDF) investigation were combined. As illustrated in Fig. 2a–c, the chemical anchoring is distinguished by the formation of several strong intermolecular interactions. Specifically, the detected bond lengths between O–H bond, H–O bond, and O–H bond were 1.641 Å, 1.536, and 1.873 respectively are attributed to strong hydrogen bonding between the nucleophilic functional groups of the algae and the hydroxyl enriched St backbone. Also bond length between N–O bond, and N–H bond were 2.530 Å and 1.865 Å respectively are shown at Fig. 1b. RDF of all systems below 2 Å. The occurrence of these short-range interactions confirms that the microalgae Ba doesn't simply remain as an isolated phase but functions as an effective crosslinking agent that strengthens the polymeric network. This localized coordination plays a critical role in preserving the mechanical and structural stability of the hydrogel under aqueous environments.

Fig. 2. RDF of (a) hydrogel without water condition, (b) water bulk, and (c) hydrogel under water condition.

3.2. Characterization

3.2.1. Fourier transform infrared spectroscopy (FTIR)

Fig. 3 displayed the FT-IR spectra of St, CS, Ba, Ba + St + CS hydrogel, and Ba + St. The following bands were used to describe the absorption peaks of St: O–H stretching vibration was the cause of the band at 3417 cm⁻¹. The C–H asymmetric stretching was attributed to the peak that emerged at 2920 cm⁻¹. However, because of C–H bending and C C bending, respectively, the peaks show up at 1650 cm⁻¹, and 991 cm⁻¹.³⁵ The stretching vibrations of the O–H and N–H groups, as well as C–H stretching, were represented by a broad band in the CS spectrum at 3462 cm⁻¹ and 2875 cm⁻¹, respectively. Additionally, there were peaks at 1074 cm⁻¹, 1651 cm⁻¹, and 1382 cm⁻¹ that represented the stretching vibration of the C–N bond, C–N stretching, and (C O amide) stretching, respectively.^36,37 On the other hand, the Ba spectrum displayed five peaks representing O–H stretching, N–H stretching, C–H stretching, C O stretch vibrations, and C–O stretching, respectively, at 3448 cm⁻¹, 2920 cm⁻¹, 1670 cm⁻¹, 1454 cm⁻¹, and 1028 cm⁻¹. At 3429 cm⁻¹, 1627 cm⁻¹, and 1060 cm⁻¹, the Ba + St hydrogel spectra showed three peaks that were attributed to the stretching vibration of the –OH. C O stretching, C–O–C, and so on. For O–H stretching and C N stretching, the Ba + St + CS hydrogel spectra displayed three prominent peaks at 3624 cm⁻¹, 2146 cm⁻¹, and 1650 cm⁻¹, respectively. For N–H stretching (primary NH₂).³⁸

Fig. 3. FTIR spectrum of starch St, chitosan CS, microalgae Ba, Ba + St + CS hydrogel, Ba + St hydrogel, and Ba + St hydrogel after reusability.

The FTIR spectra of both pristine starch/algae hydrogel composites, as well as those loaded with malachite green which evaluated after multiple reuse cycles (1–5 days). A clear evidence of effective sorption and structural stability. For the pristine hydrogel spectrum, at 3429 cm⁻¹, 1627 cm⁻¹, and 1060 cm⁻¹ show three peaks that were attributed to the stretching vibration of the –OH. C O stretching, and C–O–C. After loading with malachite green and subsequent reutilization, it is observed that the broad hydroxyl band splits and shifts into three clear, sharp peaks at 3465 cm⁻¹, 3415 cm⁻¹, and 3134 cm⁻¹.³⁹ This phenomenon suggests significant electrostatic interactions and hydrogen bonding between the cationic dye molecules and the oxygen/nitrogen-rich active site in the composite.^40,41 Furthermore, shifts were observed in the polysaccharide fingerprint region (from 1060 cm⁻¹ to 1022 cm⁻¹), alongside the emergence of particular aromatic/aliphatic signatures (2092 cm⁻¹, 864 cm⁻¹, and 613 cm⁻¹) and the maintenance of the main skeletal peak (1631 cm⁻¹),⁴² further confirm the active role of the carbohydrate–protein structure in dye entrapment, along with the remarkable reusability and structural stability of the starch/algae hydrogel during extended operational cycles.

3.2.2. X-ray diffraction (XRD)

Fig. 4 displays the XRD spectra of Ba. Its crystalline nature was confirmed by the XRD pattern of Ba, which showed two broad peaks at around 2θ = 9° and 19° and sharp crystalline peaks at about 2θ = 29°, 31°, 36°, 39°, 43°, 47°, and 64°. The crystallite size, determined using Braggs' equation, was 365.56 Å. Although chitosan was represented by a peak at 20° in the Ba + St + CS hydrogel's XRD pattern, the crystalline peaks were less numerous than in the Ba chart, which was depicted because of the significant drop in crystallinity that occurred after inclusion into the hydrogel.⁴³ The marked suppression of the original Ba crystalline fingerprints and the reduction in crystallite size to 110.12 Å suggest that the incorporation of the algae within the starch–chitosan network disrupts the periodic lattice order through extensive intermolecular hydrogen bonding. Therefore, the Ba + St hydrogel, which demonstrated superior dye removal efficiency, maintains a distinct crystalline–amorphous equilibrium. The sharp crystalline reflection at 2θ = 20.28° with a maximum relative intensity of 100% and an interplanar d-spacing of 4.37 Å. Additional structural markers were recorded at 16.84° and 24.11°, with relative intensities of 69.17%, and 63.54%, respectively. The crystallite size for the primary phase in the Ba + St hydrogel was determined to be 72.31 Å, with a micro-strain value of 3.02%. Interestingly, a high-angle reflection at 75.45° exhibited an exceptionally large, localized crystallite size of 20 033.69 Å. This unique structural configuration of the Ba + St hydrogel provides a more robust and accessible framework compared to the Ba + St + CS hydrogel, offering a higher density of active binding sites and optimized d-spacing for the efficient diffusion and entrapment of dye molecules, thereby justifying the enhanced adsorption performance observed for this composite.

Fig. 4. XRD of microalgae Ba, Ba + St + CS hydrogel, and Ba + St hydrogel.

3.2.3. Hydrogel morphological characteristics (SEM)

As seen in Fig. 5a(1–3), the naked hydrogels morphological surface displayed an uneven surface with voids and multilayers stacked as sheets. The MG dye can be accommodated and absorbed by the voids that exist between the layers. Additionally, it was found that the Ba + St hydrogel pore diffusion was more regular than the other hydrogels, suggesting a more organized polymer network. As a result, a highly MG dye-adsorbing porous hydrogel was created as a result of the Ba blending with the St, as shown in Fig. 5b(1 and 2). A significant amount of MG dye may be held in the hydrogel structure using this configuration. The hydrogel pores vanished more readily, and the MG dye coated the surface the more the MG dye was adsorbed.

Fig. 5. SEM of Ba + St + CS hydrogel (A1–3), and Ba + St hydrogel (B1 and 2).

3.3. Photocatalytic studies for hydrogel

As shown in Fig. 6a, the adsorption efficiency of MG dye onto Ba + St hydrogel catalyst reached 62.4% under dark condition at an initial concentration of 5 ppm after 360 min, which indicates the significant adsorption capacity of hydrogel toward MG molecules. This result highlights the significant contribution of the adsorption process to the overall MG removal mechanism. In the photolysis experiment, under sunlight irradiation, the removal efficiency was only 3%, which showed the negligible contribution of direct photodegradation for the removal of MG dye.

Fig. 6. Effect of (a) degradation time on MG dye using Ba + St hydrogel ( under dark conditions and sunlight irradiation) in comparison with Ba + Cs hydrogel, Ba + StCs and photolysis without catalyst under sunlight irradiation, (b) pH, (c) initial MG concentration, and (d) dosage of Ba + St hydrogel.

The photocatalytic degradation experiments were performed using aqueous solutions containing 5 ppm MG dye treated with Ba + St hydrogel, Ba + Cs hydrogel, and Ba + StCs composite hydrogel under sunlight irradiation for different exposure time. The gradual fading of the MG dye solution colour during irradiation, visually demonstrates the occurrence of the photocatalytic degradation process. The effect of irradiation time on photocatalytic removal efficiency of MG dye is shown in Fig. 6a. The degradation efficiency was increased gradually with the increase of the irradiation time from 5 to 360 min. The Ba + St hydrogel exhibited the highest photocatalytic performance, with MG degradation efficiency increasing from 11% after 5 min to 99% after 360 min of sunlight irradiation. In parallel, Ba + Cs hydrogel and Ba + StCs composite hydrogel increased from 6% to 36% and 33% to 72% respectively, over the same irradiation period. The improved photocatalytic efficiency can be attributed to the efficient interaction between the MG dye molecules within the hydrogel network and their subsequent interaction with the photocatalytically active sites, facilitating the degradation process.⁴⁴ Therefore, an irradiation time of 360 min was selected as the optimum reaction period for achieving efficient MG dye removal.

In Fig. 6b illustrated the effect of pH on the time dependent photocatalytic degradation of MG dye using Ba + St hydrogel photocatalyst. The photocatalytic efficiency is increased from 23% to 25% when pH is increased from 1 to 5, then it is increased to 95% at pH 8 and remains constant at 99% at pH 10. The better photocatalytic performance under alkaline conditions than the neutral and acidic media as shown in Fig. 6b, indicating the favorable effect of alkaline environments in the photocatalytic process. This elucidated for the enhanced photocatalytic activity at pH 9 and 10. At alkaline pH, the Ba + St hydrogel photocatalyst surface is negatively charged because of surface functional group deprotonation and facilitates electrostatic attraction with cationic MG dye molecules, which improved photocatalytic decolorization efficiency.^45,46 Also, the negative charge on the surface favour the formation of hydroxide ions.⁴⁶ Thus, reactive hydroxyl radicals (OH°) are generated which decompose the MG dye, hence augmenting photocatalytic activity.⁴⁶ Therefore, it can be concluded that the Ba + St hydrogel shows a higher photocatalytic efficiency for MG dye under alkaline conditions. The Ba + St hydrogel shows the best photo-decolorization of MG dye at pH 10.

A series of sample solutions with MG dye concentrations between 5 ppm and 40 ppm were generated to elucidate the correlation between initial dye concentration and the photodegradation process Fig. 6c. Each sample had a specified quantity of Ba + St hydrogel (0.025 g) in 25 ml of solution, and the photocatalytic procedure was executed at under sunlight irradiation for 6 hours. The experimental findings demonstrated that as the concentration of MG dye escalated, degradation diminished from 99% to 13% at concentrations of 5 ppm to 40 ppm, respectively. The observed decrease is attributed to the expected consumption of effective oxidizing radicals during the irradiation time of 360 min. In addition, the increase in MG concentration leads to turbidity of the reaction mixture, which blocks the light photons from reaching the active sites of the catalyst, thus reducing the efficiency of degradation.⁴⁷ This is because of the initial condition at lower concentration of MG dye, where the number of active sites of hydrogel exceeds the number of dye molecules of MG dye. As the concentration of MG increases, the amount of dye molecules is higher than the active sites on the surface of Ba + St hydrogel,⁴⁸ resulting in a decrease in degradation, as illustrated in Fig. 6c. Hydrogel is highly effective for dye degradation at low concentrations, commonly present in diverse wastewater sources.

To ascertain the correct dose of the catalyst, an MG dye solution with a specified neutral pH and concentration (5 ppm) will be utilized under sunlight. Various quantities of Ba + St hydrogel, from 0.025 g to 0.14 g, in a 25 ml solution during a duration of 6 h. The effect of Ba + St hydrogel dosage on the degrading process was evaluated with variable weigh of catalyst with range of 0.025 g to 0.14 g at 5 ppm MG dye at under sunlight irradiation are shown in Fig. 6d. The results showed that the effectiveness of degradation reduced with increasing the dose of catalyst which might be 0.025 g to 0.14 g from 99% to 70%. The maximum degradation efficiency was observed with a catalytic dosage of 0.025 gm for MG dye. The results demonstrated that a higher amount of Ba + St hydrogel affects the efficacy of the degrading process. It could be attributed to an increase in agglomeration and a decrease in active sites on the catalyst surface.⁴⁹ Reduction of active sites impacts the availability for photon adsorption to form electrons–holes, limiting the transit of light through the solution due to the turbidity, increasing the opacity of suspension and the scattering of light.⁴⁹ Additionally, when exposed to sunlight, the correct amount of catalyst can produce more reactive organic species, which will efficiently reach the MG dye molecules.⁵⁰ Since the clouding effect, high catalyst loading was unable to boost efficiency since photons might not reach the dye molecules. At 0.025 gm, the greatest percentage of degradation was obtained.

3.4. Isothermal and kinetics studies

3.4.1. Isotherm models study

Photocatalytic refers to the process of photodegradation of dyes when exposed to light irradiation. The mechanism of photodegradation is contingent upon the characteristics of the catalyst.⁵¹ Consequently, at equilibrium under constant temperature, the relationship between the Ba + St hydrogel and MG dye is characterized by the isotherm of photodegradation. A photocatalytic isotherm is the method via which the interaction mechanism between the MG Dye molecule and Ba + St hydrogel surface may be analyzed and established when the MG dye concentration attains equilibrium.⁵² The isotherm models include Langmuir, Freundlich, and Temkin models. In the process of fitting experimental data to various models, numerous parameters are derived from the optimal fit, which can elucidate the mechanism, surface properties, and affinities of Ba + St hydrogel. This study aims to analyze and identify a viable model for the photocatalytic degradation of MG dye molecules utilizing Ba + St hydrogel, based on experimental data.

The Langmuir model isotherm is employed for dye degradation processes, wherein MG Dye molecules create a monomolecular layer on the uniform surface of Ba + St hydrogel. This model posits that photocatalysis transpires on a homogenous surface, devoid of interactions among the MG dye molecules.⁵³ As per this model, Ba + St hydrogel surface comprises of identical active specific sites with equal energy and enthalpy. Thus, all the photodegradation sites have a comparable affinity for the MG dye. It signifies that a filled-up location cannot degrade further.⁵⁴ Thus, a saturation point is attained where the maximum surface deterioration occurs. The linear mathematical expression of Langmuir model is given by eqn (3).⁵⁵

3

where C_e is equilibrium concentration of adsorbent (mg L⁻¹), q_e (mg g⁻¹) and q_m (mg g⁻¹) indicates the equilibrium photocatalytic capacity and maximal degradation capacity correspondingly. b (L mg⁻¹) is Langmuir rate constant reflecting the interaction between two species.⁵⁵

The heterogeneous photodegradation characteristic of Ba + St hydrogel is assumed by the Freundlich model of isothermal degradation.⁵³ Catalytic molecules multilayer photodegradation is assumed to be the source of this.⁵⁶ The Freundlich model's mathematical statement is illustrated by eqn (4).⁵³

4

where q_e is quantity adsorbed (mol g⁻¹), C_e = equilibrium concentration of adsorbate (mol L⁻¹), K_f denotes Freundlich constant related to photodegradation capability of the catalyst and n is isotherm type.⁵³

Temkin model was established for chemisorption not for physical sorption but was utilized here to examine if some chemical phenomenon was also occurring besides the physical.⁵⁷ It is based on the concept that the heat of degradation of surface molecules diminishes linearly rather than logarithmically.⁵⁷ The photodegradation process is characterized by a consistent distribution of binding energy at Ba + St hydrogel surface and covers the interaction between the MG Dye and Ba + St hydrogel.⁵⁸ The isotherm was utilized to examine equilibrium data. It is not based on a homogenous surface; it is a function, not the constant potential of photocatalytic, eqn (5) calculated by ref. 57.

q_e = B ln(A_T) + B ln(C_e) 5

where B is Temkin constant, and A_T is Temkin parameter.

Based on the Fig. 7a–c, the Langmuir, Freundlich, and Temkin models of isotherm are utilized to determine which forms of photocatalytic isotherm are more appropriate for defining the photocatalytic process. The three photocatalytic isotherm models that are provided in the Table 3 for MG degradation and data fitting serve as a guide for deciding which sorts of isotherm models best represent the photocatalytic process. The Langmuir, Freundlich, and Temkin models all match the MG degradation data with R² values of 0.891, 0.139, and 0.095, respectively. The MG photodegradation process of Ba + St hydrogel can be best represented by the Langmuir photocatalytic isotherm since it has the highest R² value. The highest levels of MG photodegradation in Ba + St hydrogel show that this material has enormous potential for treating wastewater containing dyes.

Fig. 7. Isothermal of the photodegradation reaction for MG at the different initial concentrations of Ba + St hydrogel, (a) Langmuir, (b) Freundlich, and (c) Temkin models.

Table 3. Parameter of the photodegradation reaction for MG at the different initial concentrations of Ba + St hydrogel, Langmuir, Freundlich, and Temkin.

Isothermal models	Parameters	Values
Langmuir model	q m (mg g−1)	0.450893219
	B	−5.469886
	R 2	0.892
Freundlich model	1/n	−0.04394
	N	−22.758
	K f	0.573271
	R 2	0.1392
Temkin model	B	−0.02151
	ln(AT)	−27.6494654
	R 2	0.09553

3.4.2. kinetic study

The photocatalytic reaction kinetics were estimated by fitting the experimental data with different kinetic models including the pseudo first order model (eqn (6)),⁵⁹ and the second order model (eqn (8))^60,61 with their associated rated constants are illustrated at (Fig. 8).

6

Fig. 8. Kinetics of the photodegradation reaction for MG at the different initial concentrations of Ba + St hydrogel, (a) Pseudo first order, and (b) second order.

The half lifetime of the dye degradation at shown in eqn (7)

7

8

where C₀ = initial concentration of dye in the solution (mg L⁻¹), C_t = concentration of Dye in the solution at time t (mg L⁻¹), k₁ = pseudo-first-order rate constant (1 min⁻¹ or 1 h⁻¹), K = the rate constant (L mol⁻¹ min⁻¹) for second-order reactions.^59–61

The photodegradation of MG dye pollutants by means of photocatalysis normally follows pseudo first order kinetics. Table 4 summarize the rate constant for the reaction (K₁) and the correlation coefficient (R²). The calculated value of K₁ is 0.348 h⁻¹ as shown in table, the half lifetime is 1.9913 h and the correlation coefficient (R²) of 0.986. The result reveals a moderate photocatalytic activity for MG degradation under sunlight irradiation, which can be attributed to the fast recombination of the photogenerated charge carriers.⁶² It is also interesting to note that the photocatalytic degradation of organic compounds is a surface mediated process, the first step being the adsorption of the MG dye onto the Ba + St hydrogel surface of the photocatalyst.^62,63 The cationic MG dye can be readily adsorbed on the negatively charged surface of the catalyst. Ba + St hydrogel possesses excellent photocatalytic activity and degradation rate.

Table 4. Parameters of pseudo-first order, and second order kinetic model.

Kinetics models	Parameters	Values
The pseudo first-order	k 1	0.348
	R 2	0.986
The second-order	k 2	1.369
	R 2	0.909

3.5. Catalyst reusability

The stability and recyclability of photocatalyst are indispensable for its potential large scale application. To evaluate the reusability of the Ba + St hydrogel, the catalyst was recovered after each photocatalytic reaction, separated and thoroughly washed with distilled water. The recovered catalyst was dried in a hot air oven at 60 °C for 1 h and then used for the next cycle. A new MG dye solution was prepared for each run. The test for reusability was performed up to five times for five consecutive cycles under the same experimental conditions. Complete photodegradation was obtained within 360 min of the first, second, third, fourth, and fifth cycles. The photodegradation efficiencies were 99%, 97.77%, 97.35%, 96.70%, and 96.44% for the first, second, third, fourth, and fifth run respectively. The recycling results are shown in Fig. 9. The FTIR spectra (Fig. 3) indicate very similar peaks for the fresh Ba + St hydrogel and the sample reused for five consecutive cycles, which suggests that the Ba + St hydrogel catalyst has excellent chemical stability in the degradation reactions under sunlight irradiation.

Fig. 9. Reusability of photodegradation of MG dye using Ba + St hydrogel.

3.6. Degradation pathway of MG

The possible pathways for the photocatalytic degradation of MG by the Ba + St hydrogel are depicted in Fig. 10. The prepared photocatalyst has high catalytic activity enabling the opening of the MG molecular ring and the formation of short chain molecules. There are two mechanisms involved in MG degradation and it is unable to determine at this moment which one predominates. Finally, the degradation of MG proceeds mainly via N-demethylation and the cleavage of conjugated structures.⁶⁴

Fig. 10. Possible pathway of degradation of MG using Ba + St hydrogel, and the schematic illustration of possible charges transformation, and separation of electron hole pairs in visible region by catalytic activity of Ba + St hydrogel in MG degradation.

Moreover, the active species trapping experiments and previously reported photodegradation mechanisms of organic compounds, such as dyes,^64–67 are used to present a schematic illustration of the photocatalytic mechanism under sunlight irradiation in Fig. 10. The introduced dopants in the Ba + St hydrogel catalyst result in the structural defects, especially the oxygen vacancies and lattice distortion, which enhance the light absorption and generate the localized defect states to favour the more efficient separation of the photogenerated charge carriers. Under sunlight irradiation, the electron–hole pairs (e⁻/h⁺) are generated, and these defected sites act as trapping centers which inhibit the fast recombination of the electron and holes by spatially separating them and prolonging their lifetime and enhancing the overall photocatalytic efficiency. Photogenerated holes (h⁺) oxidize adsorbed water molecules (H₂O) or hydroxide ions to produce hydroxyl radicals (OH) and photogenerated electron (e⁻) reduce dissolved oxygen (O₂) to produce superoxide radicals (O₂⁻˙). Moreover, electrons can take part directly in the reduction and degradation of MG dye molecules. This efficient degradation of malachite green under sunlight irradiation is caused by such reactive oxygen species and direct redox reactions.^64–67

3.7. Ba + St hydrogel photocatalyst compared to other photocatalysts

MG dye has been removed previously using various photocatalysts of different origins. Table 5 shows a comparison of MG dye using biopolymer based hydrogel. The Ba + St hydrogel developed in the present study showed the highest photodegradation efficiency for MG, which is 99% removal.

Table 5. Comparison of different biopolymer-based hydrogels for evaluating photocatalytic degradation efficiency.

Photocatalyst/additive	Removal efficiency (%)	Time (h)	Ref.
Biochar and Scenedesmus obliquus algae based on sodium alginate microspheres (BAMS)	98	120	68
Enzymatic approach extracellular laccase derived a fungal strain Fusarium oxysporum HUIB02	80	20	69
Poly(acrylamide-co-acrylic acid) hydrogels	96.4	1	70
Ba + St hydrogel	99	6	This study

4. Conclusion

MD and MC simulations successfully decoded that fundamental of microalgae BA with starch matrix mechanisms governing the hydrogel formation. The strong interfacial interactions and stable crosslinking patterns occur between the microalgae and starch matrix are proved. These theoretical results corroborate that the produced Ba + St hydrogel is a highly efficient superabsorbent material for removal of MG from aqueous solutions. The synthesized Ba + St hydrogel showed excellent photocatalytic degradation performance under solar radiation which provides a promising, eco-friendly, and sustainable solution for wastewater treatment. Degradation efficiency decreased with increasing catalyst dose, from 99% to 70% for 0.025 to 0.14 g respectively. The optimum degradation efficiency for MG dye was achieved at 0.025 gm catalytic dosage. It was observed that the efficiency of degrading process is affected by a higher concentration Ba + St hydrogel. It might be due to reduction of active sites of catalyst surface and increase of agglomeration. The reduction of active sites affects the availability of photon absorption for the generation of electron holes. The maximum dye photodegradation was achieved at pH with respect to other parameters investigated, indicating that Ba + St hydrogel has a better photocatalytic performance on MG dye in alkaline conditions. The Ba + St hydrogel is optimal on photo-decolorization. Considering all the above, the obtained Ba + St hydrogel is a hydrophilic functional polymeric network system that is a promising new superabsorbent substance for the treatment of waste and industrial water.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.