Biosynthesis of calcium oxide nanoparticles by employing Mulberry (Morus nigra) leaf extract as an efficient source for Rhodamine B remediation | Scientific Reports

Scientific Reports volume 14, Article number: 23744 (2024) Cite this article

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Green processes for synthesizing nanocomposites are a hot area of research today as traditional processes are expensive, inefficient, harmful for synthesizing organic and inorganic molecules, and unsuitable for large-scale operations. The present study investigates the capacity of green synthesized Calcium oxide nanoparticles (CaO NPs) for efficiently removing Rhodamine B. Chemical reduction was replaced with Mulberry (Morus nigera) leaf extract as an environmentally friendly reaction mechanism. CaO NPs are characterized by various analytical techniques including EDX, BET, SEM, FTIR, TGA, Zeta Potential, Point of Zero Charge (PZC), and XRD. Maximum adsorption of Rhodamine B by CaO NPs is revealed at an initial concentration of Rhodamine B of 80 ppm, a temperature of 343 K, and contact time of 60 min, 0.4 g of adsorbent at a pH value of 7. Maximum removal of Rhodamine B by CaO NPs was found to be 98.2% which is promising with this small amount of adsorbent (0.4 g). Diverse Kinetic and adsorption isotherms are employed in this study to determine the requirement and significance of the adsorption process. Various adsorption isotherms such as Freundlich, Temkin, Dubinin–Radushkevich (D–R), and Langmuir models have been employed. Among the kinetic adsorption isotherms Elovich, Intraparticle kinetic model, pseudo 1st order, and pseudo 2nd order models were applied. The current study investigates the thorough understanding of the Rhodamine B adsorption process including the mechanism of adsorption using condition optimization, characterization, and model applications. The proposed adsorbent can be employed for the green removal of Rhodamine B from wastewater of industry with maximum efficiency and favorable regeneration properties.

Technology is taking its directions to reduce materials on the Nanoscale. Due to their distinctive surface characteristics, large surface area, and tremendous surface potential, nanomaterials exhibit behavior similar to atoms1. In contrast to bulk materials, these Nano-sized materials show elevated surface-to-volume ratios. Larger particles have a lower surface volume ratio, are more stable physically, and have fewer applications across diverse fields. Modifications at the Nanoscale, impart augmented and unique properties to bulk materials, influenced by their dimensions, geometric configuration, and morphology2. They are useful individually in many fields, such as solar energy2, photochemistry, medicine3, and material science4. Calcium oxide (CaO) is widely used as a destructive adsorbent, catalyst, medicine, and waste remediation1.

The synthesis of CaO NPs and other nanomaterials like NiO, FeO, ZnO, Ag, and Au has been delineated through diverse methodologies including chemical precipitation, hydrothermal, microemulsion, sol–gel, gas phase, microwave synthesis, and electrochemical approaches. In comparison to alternative approaches such as chemical and physical methods, the green method is important because it is safe, inexpensive, and environmentally friendly5,6. Some concepts related to phytoremediation in plant extract may have something to do with the chemical mechanism underlying the production of nanomaterials, according to the literature7,8.

Future generations are thought to benefit greatly from the widespread use of metal/metal oxide nanoparticles (NPs) in consumer goods, clinical care, and other industrial applications5,9. Many people have also been interested in the photocatalytic activity of CaO NPs, gas sensing, biomedical applications, and CO2 absorption10.

The synthesis of nanoparticles using plant extracts has demonstrated cost-effectiveness and has expanded the possibilities for creating non-toxic nanoparticles. Applications for calcium oxide (CaO) nanoparticles include water purification, adsorption, catalysis, and antibacterial agents11,12,13. As nanoparticles, CaO proves to be an inexpensive, readily accessible, and non-toxic raw material for heterogeneous catalysis in a variety of reaction processes14,15,16. CaO nanoparticles have numerous other uses, including as an additive in refractories17, an adsorbent, particularly in catalysis12,18, bactericides19, in the biomedical field, and as a precursor in bioceramics20. The most important application of these nanoparticles is the purification of water by removing harmful contaminants including dye, metals, and other organic contents.

In most parts of the world, getting access to clean drinking water is becoming more difficult. People struggle with issues related to drinking water quality and availability in the majority of the world21. Water used in the textile industry is extensively released after use, fully impregnated with organic pollutants, acids, bases, heavy metal ions, and dyes. New quick coloring agents have replaced natural coloring pigments, which were once used because they produce bright colors and last longer even in the presence of heat, light, and water22. However, they also significantly increase the pollution of water. Therefore, a major contributing factor to the unprecedented escalation of water pollution in our environment is the extensive utilization of dyes in industrial processes23. As an illustration, organic dyes make up the majority of industrial effluent, and the pressing need to eliminate these kinds of pollutants is necessary. These pollutants are nonbiodegradable and the only way to get a ride from them is to remove them by employing environment-safe methods. Based on available data, colors have molecules consisting of a blend of unsaturated organic compounds and chromophores acting as color carriers24.

Chemically diverse dyes are used in industries, and a lot of these dyes contaminate water and end up in the environment. The azo dyes make up roughly 70% of all the dyestuffs in terms of weight. Because of their stability, azo dyes are used extensively. These dyes are utilized in consumer goods, printing, cosmetics, and tattooing. When these azo dyes are discharged into water, they contaminate the water a major risk to human health because they are highly carcinogenic and mutagenic25. One of the cationic dyes commonly used in diverse industries, such as paper, food, jute, leather, and textiles, is Rhodamine B (RHB). It is estimated that 20% of the total dye used during production remains in the effluent26. As a result, the release of untreated effluents from these industries ends up releasing a large amount of this dye into the environment. The compound has toxic and cancer-causing qualities in both humans and animals due to the presence of N-ethyl groups on either side of the xanthene rings27.

Due to the major risk to health as well as the environment, it is necessary to reduce or even eliminate any negative effects of this dye, so appropriate mitigation measures must be used28. Therefore, Numerous techniques, including adsorption, coagulation/flocculation, and advanced oxidation, have been used in the literature to reduce its risks. The majority of these procedures are constrained by secondary waste, cost, or complexity29,30.

The adsorption approach is superior to alternative techniques due to its low cost, ease of use, and capacity to eliminate hazardous substances efficiently. Biochar, industrial byproducts, and agri-food wastes are regarded as effective, environmentally friendly sorbents that have potential uses in the environmental field31.

The synthesis of precise morphologies and properties, such as agro-waste-derived Nanocomposites, has attracted a lot of attention recently because nanoparticles (NPs) have emerged as a favorable option for addressing the worldwide problem of removing hazardous materials from water bodies. Plant extract-based nanocomposites are effective adsorbents for a variety of pollutants due to their large surface area and distinctive physical, chemical, and optical properties. The present work represents the environmentally friendly synthesis of Calcium Oxide nanoparticles through the utilization of novel green material of Mulberry leaf extract. Instead of chemical-based reducing agents, the green extract is employed to synthesize CaO NPs with excellent properties for the remediation of Rhodamine B from wastewater. Characterization of CaO NPs reveals the presence of important functional groups, large surface area, and electrostatic attraction, which play a key role in the removal of dye from aqueous media. It is more beneficial for scientific understanding of the adsorption process and underlying mechanism of adsorption connected to Rhodamine B decontamination when different models are comprehensively applied to work.

This study focuses on SGD's goal to ensure the availability and sustainable management of water and sanitation for all. Water contamination issues of developing countries are addressed in this work to ensure the removal of hazardous contaminants from water to make it available for common people without the addition of high cost.

It is necessary to completely characterize the synthesized nanoparticles by different techniques to determine the surface morphology, functional groups present on the surface, particle size, surface area, and thermal stability. In the present work, an extensive characterization of CaO NPs was performed.

Instead of using light to create an image, the SEM process uses electrons. SEMs have allowed researchers to examine a wide variety of samples and have given insights into many new research areas, including material science and nanotechnology. SEM image of CaO NPs was obtained as shown in Fig. 1. There are visible signs of the irregular surface morphologies of nanoparticles. Large grains and fine substance particles are mixed in the powders, as shown by the SEM images of the CaO nanoparticles that were collected. Scanning electron microscopy techniques can determine the morphology, and location of individual nanoparticles by using the Joel JSM-6480 LV SEM apparatus. After being sonicated with purified water, a tiny drop of the CaO powder sample NP solution was placed on a glass slide for rinsing. SEM analyses demonstrate that the calcined product results in larger particle size values and faster grain growth. Results are in good agreement with the reported literature32.

SEM image of CaO NPs at different resolutions (A) 10 µm (B) 20 µM (C) 10 µm, at another angle (D)5 µm.

The relative counts of the detected X-rays versus energy were determined using energy-dispersive X-ray spectroscopy (EDX or EDS). It is for determining the elements both quantitatively and qualitatively present in the sample. The EDX analysis graph shows the increasing composition of Ca and O in nanoparticles. The only elements present in the prepared nanoparticles are Ca and O, along with a few other elements e.g. C, Mg, and Silicon in very minute quantities as given in the EDX spectrum in Fig. 2. Five elements including C, O, Mg, Si, and Ca were found in the EDX spectrum of nanoparticles of CaO with percentage weights of 35.02, 49.41, 1.05, 1.93, and 12.59 and atomic percentages of 45.34, 48.03, 0.67, 1.07, and 4.89 respectively33.

EDX spectra and elemental composition of CaO NPs.

Fourier transform infrared spectroscopy (FTIR), which was also employed in this experiment to support the interactions between Ca ions and hydroxyl (OH) functional groups within the secondary compound of the system, affirms the involvement of biomolecules in capping and reducing the CaO NPs. The FTIR analysis was measured in the range of 500–4000 cm−1 where it indicated vibration peaks at 3522.02 cm−1, 3465.37 cm−1, 3454.51 cm−1, 2968.45 cm−1, 2534.46 cm−1, 2222.0 cm−1, 1435.04 cm−1, 1035.77 cm−1, 887.26 cm−1, 655.60 cm−1, 555.50 cm−1, 516.92 cm−1, 470.63 cm−1. The conspicuous existence of peaks at 3522.02 cm−1, 3465.37 cm−1, and 3454.51 cm−1 provides evidence for the existence of hydroxyl groups. The peak observed at 2966 cm−1 is corresponding to the C–H stretching vibration depicting the presence of alkanes. The peak at 2534 cm−1 confirms the thiol group. The peak at 1035 cm−1 indicates the C–O stretching vibration and the peak at 1435 cm−1 shows the bending vibration of CH3 alkane. The sharply visible peak at 874 cm−1 belongs to the presence of Ca-O bonding and the peak at 655.5 cm−1 is owing to the existing Ca-O bonding, identifying that calcium oxide is present. Peaks at 555.50 cm−1, 516.92 cm−1 and 470.63 cm−1 confirm that phyto-ingredients probably confirm the stabilization of CaO NPs (Fig. 3).

FTIR spectra of CaO NPs, and Mulberry leaf extract.

The carbonation of CaO nanoparticles results in the creation of the C–O bond, as evidenced by the strong band at 1471 cm−1 and a peak at 874 cm−1 34. When a highly reactive CaO nanoparticle's surface area is exposed to air during calcination, the generation of CO2 and H2O takes place. Then further adsorption of these substances on the CaO surface as free-OH and carbonate species occur.

This indicates that oxygen, which is predominantly present on the expansive surface area of the nanoparticles, is made available by the surface –OH and the lattice oxygen of CaO nanoparticles35. Mulberry leaves comprise approximately 15–30% proteins, 2–8% lipids, 10–40% carbohydrates, and 10–37% neutral dietary fibers. The stretching vibrations of O–H and C–H are indicated by broad and intense bands observed within the ranges of 2702.27–3905.85 cm−1. The broad band at 906 cm−1–1707 cm shows the presence of flavonoid molecules.

In CaO NPs, the curve after adsorption has distinct peaks from the curve before adsorption. The peaks within the range of 3122.75–3600 cm−1 show the existence of O–H and N–H stretching vibrations. The dye adsorption is indicated by the slight modification in this region (Fig. 4). A noticeable change in the OH and COOH groups has been seen in the FTIR spectra of CaO NPs after adsorption (Fig. 4). It demonstrates that these functional groups are in charge of using electrostatic and/or H-bonding interactions to bind the Rhodamine B dye36.

FTIR spectra of Rhodamine B and CaO NPs (before and after adsorption).

Particle size and structure of CaO NPs were examined by the X-ray diffraction (XRD) pattern. Overall, good crystallinity of biosynthesized CaO NPs is indicated by the solid and well-defined diffraction peaks. For CaO NPs’ obtained XRD pattern shows that it closely matches the established (JCPDS card No. 00-044-0777). The XRD pattern of CaO NPs is showing various types of diffraction peaks (2θ) at 29, 32, 37, 42, 43,48,53, 64, 67,80 showing the indexes at (011), (111), (200), (012), (018), (116), (022), (113), (222), (400) respectively, the observed pattern suggests a cubic crystal system with a lattice parameter of (α = 4.8152 Å)37.

The crystallinity of the biosynthesized CaO NPs is calculated by using the Debye–Scherrer formula:

K is the Scherrer constant (0.9), D represents the average particle size, λ denotes the wavelength (1.54) and λß signifies full width at half-maximum (FWHM). The most prominent peak observed at 29°, has a crystalline size of 9.65 nm, whereas the average crystalline size of biosynthesized CaO NPs is calculated as 35.5 nm (Fig. 5).

XRD Spectrum of CaO NPs.

Determining the stability of colloidal dispersions needs an understanding of zeta potential, it is an important factor that regulates electrostatic interactions in particle dispersions (Fig. 6). It can strengthen the optimization of formulations for suspensions and emulsions, assisting the expectations of their prolonged stability38. To determine the stability of particle suspension, the zeta potential of the particle was carried out using a Laser Doppler Electrophoresis (LDE) apparatus (Nano Series, Malvern instrument Ltd, UK) at 25 °C. The potential was evaluated six times, with each measurement depicting the average of 15 runs with a count rate of 176.6 kbps. Mean values and standard deviations were consequently measured. The zeta potential of the nanoparticles was derived from their electrophoretic mobility using smoluchowski's equation39. Suspension of nanoparticles was prepared in 10 mg/L by Nanopore water, and constant ionic strength was adjusted by using 0.01 M KNO3. The pH of the suspension was modified by using 0.1 M HNO3 or 0.1 M KOH. The instrument automatically calculated the electrophoretic mobility (U) and zeta potential using smoluchowski's equation40.

Size distribution and zeta potential of biosynthesized CaO NPs.

Here ζ represents the zeta potential, U is the electrophoretic mobility, ε is the dielectric constant and ƞ is the viscosity of the medium. The pH values of CaO nanoparticles corresponding to the zero point of charge (pH (PZC) were calculated based on their zeta potential as a function of pH stretching a range from (2 to 10). This represents that CaO nanoparticles constitute a range of negative to positive charges in the vicinity of neutral pH, the conditions under which the experiment was carried out. In Nano pore water (pH 6.0), CaO nanoparticles have a zeta potential of − 24.1 mV. This sensation is explained by the fact that anionic functional groups or natural organic matter (NOM) are characterized by negative charges in water adsorbed onto the surfaces of CaO nanoparticles, thereby imparting a negative charge irrespective of the initial positive negative charges on these nanoparticles41.

The size analysis of CaO nanoparticles was carried out through dynamic light scattering (DLS) measurements employing the Zeta sizer Nano ZS (Fig. 6). This instrument used a 4 m W He–Ne laser with a wavelength of 633 nm and a detection angle of 173°. The particle diameter averaged based on intensity, and the polydispersity index (PDI) values, serving as an indication of distribution width, were computed through the cumulates analysis as described in ISO 1332142. The mean intensity weighted diameter of CaO nanoparticles was 156.7 nm with a PDI value of 1.000.

Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DTA) were carried out with an SDT650 thermal analyzer with a serial number (0650-0681) and IP (192.168.1.2). The sample (17.5 mg) was heated at a rate of 20 °C min−1 in a nitrogen flow (10 mL min−1) from room temperature to 800 °C. The Thermal Gravimetric Analysis of synthesized CaO NPs is shown in Fig. 7. The weight loss of the NPs was 25.946% between the temperature ranges of 15.49–430.19 °C in the first 42 min. This indicates the decomposition of calcium hydroxide (Ca(OH)2, which is generated from the reaction of calcium oxide nanoparticles with ambient atmospheric moisture. There was a little weight loss of 10.902% from temperature ranges of 677.63–793.05 °C. It was due to the decomposition of residual CaCO3, and due to the release of carbon dioxide (CO2) during the carbonization process of calcium oxide nanoparticles which reveals the prominent thermal stability and purity43,44.

TGA-DTA curves of CaO NPs.

The TGA curve of CaO NPs is given in Fig. 7 which demonstrates a positive heat flow initially, once the decomposition process starts then heat is released by the sample which is represented by the negative heat flow of the DTA curve. Almost 49.35% sample is left behind after the completion of the process which indicates the stability of synthesized particles over a wide heating range.

The traditional Brunauer–Emmet–Teller (BET) isotherm was used to calculate the nanoparticles’ surface area. The cornerstone for determining the amount of nitrogen adsorption on a particular surface is the BET isotherm. Quanta chrome ASiQwin™-Automated Gas Sorption Data Acquisition and Reduction © 1994–2017, Quanta chrome Instruments version 5.21 was used in this work. Representative nanoparticles were pre-dried at room temperature (25 ± 1 °C) in a vacuum desiccator and degassed at 90 °C for 1.6 h. analysis gas was Nitrogen, cell type 6 mm, with analysis time of 31 min and bath temperature of 77.35 K. The sample which was contained in a glass sample tube had been cooled to cryogenic temperatures. It was then exposed to the nitrogen gas at a series of carefully calibrated pressures. As adsorption progressed, the thickness of the adsorbed nitrogen film increased, with surface microspores filling before macrospores (Table 1).

A well-known approach for determining the samples’ specific surface area and for determining the micro-and/or mesoporous volumes is the t-plot. The t-plot of the solid exhibits a straight line; it can be regarded as a flat surface for the solid because it exhibits adsorption similar to the flat surface. The proportionality constant is the surface area of a solid. The intercept of the linear plot in the lower pressure range is taken as the microporous volume of the solid as a material having microspores. Since none of the linear regimes pass through the origin of the t-plot, the t-plot obtained from the adsorption isotherm in a microporous material can be divided into two regimes. Adsorption starts at low pressure in the microspores. When the microspores are filled the adsorption occurs on the external side of the microporous material. At low thickness which corresponds to low pressure, the amount of adsorption rises rapidly as the thickness increases. Larger thickness corresponds to the higher pressure; here the t-plot is linear as the adsorption only takes place on the outer surface of the microporous particle in this pressure range. In this study, the microspore volume calculated is 0.002 cc/g with a diameter < 2.6 nm indicating the microporous material45.

Multilayer adsorption is assumed in the BET theory. Thus, all layers are believed to be in equilibrium, and the Langmuir equation can be applied to each layer46. Molecules in the layer under the first layer serve as locations for molecules in layers above to absorb. BET equation is represented as follows:

Here p and Po are the equilibrium and saturation pressure of the Adsorbate at the adsorption temperature, Vm is the monolayer adsorbed gas quantity and C is the BET constant which is related to the energy of monolayer adsorption. BET analysis is carried out by plotting the graph between p/po and p/po/w (g/g). A straight line is obtained having slope = (C − 1)/(Vm C) and intercept = 1/Vm C. Values of C and Vm are calculated from slope and intercept. A total surface area and specific surface area (S) are calculated by the following equations:

N is Avogadro’s number, S is the cross-sectional surface area of the adsorbed gas molecule, v is the molar volume of gas, and a is the molar weight of the adsorbed species. Thus the linear region of the BET plot is chosen to meet the following two consistency requirements47. The pressure range selected has values of W (P/Po) increasing with P/Po. (2) The intercept of the linear plot is a positive value and the value of BET constant C is > 0. Here C > 1 and P < < 1 so adsorption is monolayer and Type I isotherm is applicable. Parameters calculated for BET analysis are given in Table 2.

BET result shows the Monolayer adsorption and large amount of adsorption at lower pressure indicates the presence of microspores48. Information about pore size and specific area of nanoparticles was obtained by N2 adsorption–desorption study. The result of N2 adsorption–desorption is expressed in Fig. 8. This isotherm gives a type I curve with a Hysteresis loop indicating the nanoparticles' presence (Fig. 8).

(a) Pore Diameter of CaO nanoparticles by SF and HK Method, (b) N2 adsorption–desorption curve.

The point of zero charge plays an important role in adsorption investigations, as pH is one of the factors that govern the adsorption of dye onto the composite surfaces. During the process of adsorption electrostatic force has a crucial role for the adsorption of dye in addition to other interactive methods. The surface of CaO NPs will attain a positive charge when the pH of the aqueous solution falls below the point of zero charge (PZC). When the pH of the aqueous solution is equal to the point of zero charge (PZC) [pH = pH (PZC)], the surface of CaO NPs maintains an electrically neutral state. Conversely, when the pH of aqueous solution exceeds the point of zero charge (PZC) [pH > pH (PZC)], the surface of CaO NPs acquires a negative charge. Rhodamine B requires a negative surface for attachment, as it attains a positive charge in the solution. Thus, the adsorption of dye is suitable at a pH level exceeding the point of zero charge where CaO NPs have a negative charge on their surface (Fig. 9).

PZC determination for CaO NPs (Before Adsorption).

The adsorption experiment was performed at variable initial concentrations, contact time, pH, amount of adsorbent, and temperature to get optimized parameters for the adsorption study.

The impact of dye concentration on the adsorption process was investigated by varying the concentrations from 20 to 100 ppm while keeping all other parameters constant. The result of this experiment revealed that the amount of Rhodamine B dye adsorbed by CaO nanoparticles increased with the increase in dye concentration. The maximum removal rate of 95.17% was found at a concentration of 80 ppm. Further, an increase in dye concentration does not favor further adsorption. This implies that the adsorbent surface reaches saturation at a dye concentration of 80 ppm. Thus, the rate of adsorption is not significantly affected by further increments in the dye concentration. A momentous number of unoccupied surfaces exist on the adsorbent surface; these sites become progressively occupied by molecules of dye as dye concentration increases. Therefore, in terms of dye adsorption, no optimistic effect is seen as the concentration of dye surpasses 80 ppm49. Thus, further increasing the dye concentration after 80 ppm, adsorption progressively reduced because there were fewer active binding sites compared to dye molecules and more competition among cationic dye molecules for the adsorbent's surface active sites50,51.

The influence of contact time on the adsorption of Rhodamine-B was examined with the contact time extending from 30 to 150 min. All experiments were conducted at 80 ppm Rhodamine-B, 0.2 g of adsorbent, 30 °C temperature, and pH 7. Adsorption of Rhodamine B increased, as the length of time in which the adsorbent and dye were in contact with each other was increased from 30 to 60 min. The maximum dye removal of 98.2% was perceived at a contact time of 60 min. However, increasing the contact time beyond this duration does not cause any additional increase in dye adsorption. This demonstrates that maximum adsorption efficiency is attained at 60 min of contact time. As equilibrium is reached at this time interval further extension of time gives no additional benefits in adsorption. This is possibly due to the restricted availability of active sites on the adsorbing surface52. Thus, the increased removal capacity at the start was attributed to the numerous adsorption sites on the CaO NPs. As time went on, Rhodamine-B became harder to adsorb, as the adsorption sites on CaO NPs were partially or completely blocked, and the Rhodamine-B concentration in the solution was low53 (Fig. 10).

Effect of different parameters on percentage adsorption of Rhodamine B.

Temperature plays an important role in adsorption study, wherein an elevation in temperature correlates with an increased percentage removal of Rhodamine-B. The experiment was performed at (30–70 °C) temperature, and other process parameters remained unchanged. As depicted by experimental work, the percentage removal of dye increases as the temperature increases from room temperature 30–70 °C, demonstrating that the process was endothermic. The highest removal of dye is observed at 70 °C. This is because the entropy of the adsorption system rises as temperature increases, resulting in an elevated adsorption rate. Furthermore, it could be due to the enhanced spread of adsorbate molecules towards the outer boundary layer. The temperature rise also accelerates the diffusion of adsorbate ions within the particle pores54. However, at 30 °C significant removals of 97.9% are achieved and by an increase in temperature, a slight change is observed (98.25%). Synthesized nanoparticles can remove maximum dye at room temperature but it is also a fact that temperature causes a change in the entropy of the system and randomness increase which causes a change in the adsorption rate.

The influence of pH on the adsorption of Rhodamine-B is provided in (Fig. 10). The experiment was carried out with varying pH including neutral, acidic, and basic conditions while keeping other parameters unchanged. The optimal removal of Rhodamine-B dye occurs at a pH of 7 as determined from the experimental outcomes. This indicates that a neutral solution with a pH of 7 above the PZC of CaO NPs optimizes the interaction between the dye molecules and the surface of the adsorbent. As the pH decreases the surface charges of the dye molecule and adsorbent both change, causing a decrease in adsorption. Moreover, at low pH, the absorbent surface displays a net positive charge due to the accumulation of H+ ions. This leads to electrostatic repulsion between Rhodamine-B molecules and the adsorbent surface, preventing the removal of Rhodamine-B55.Thus, Zeta potential measurements support the observation that, when pH decreases, a change in charge on the adsorbent's surface results in weaker interactions, and a decline in adsorption occurs, this lowers the removal efficiency of dye. On the other hand, the adsorbent surface attains a negative charge at pH above the Point of zero charge value; therefore, at pH 7, value above the PZC, positively charged dye molecules provide effective removal by creating attractive forces with the negatively charged surface of adsorbent above the point of PZC. Further, increasing the pH towards basic conditions caused a decrease in adsorption. This is because in basic conditions the surface of the adsorbent is negatively charged, and Rhodamine-B exists predominantly in its zwitterionic and dimer forms. The reduction in sorption is likely caused by the repulsive interaction between the negatively charged sorbent surface and the deprotonated carboxylic acid moiety of Rhodamine-B56.

To determine the influence of the amount of dose on the percentage removal of Rhodamine-B, the adsorbent amount was varied from 0.2 to 0.6 g, while other parameters were kept constant. Experimental results depicted that an increase in the amount of adsorbent from 0.2 to 0.4 g shows a significant increase in adsorption rate up to 98.18%, as an available surface for the adsorption process is increased, representing an improved availability of surface sites on the adsorbent and its total surface area57. It indicated that increasing the adsorbent dose created more adsorption sites for Rhodamine-B adsorption. So, more dye molecules are attached to this added amount of the sorbent. However, a further increase from 0.4 g to 0.6 g gives no increment in terms of dye removal. Since at 0.4 g a maximum number of dye molecules has been removed, thus adding more adsorbent beyond 0.4 g does not improve the dye’s removal efficiency. The significant alteration in adsorption happens when dye molecules have additional surfaces to attach to the active sites. Therefore, after achieving a considerable amount of dye removal, the amount of adsorbent won’t have a significant impact on the adsorption process (Fig. 10).

Thermodynamics studies provide insight into how temperature variations affect the process of adsorption. Various thermodynamic parameters, including Gibbs free energy change (∆G°), standard entropy change (∆S°), and standard enthalpy change (∆H°) were measured to determine the temperature dependence of the adsorption process.

∆G° is determined by the subsequent equation.

Equilibrium dye adsorption (qe) and equilibrium dye concentration are used to calculate the thermodynamic equilibrium constant (KD)

Experimental data were gathered at a range of temperatures (303 to 34 K) to determine the impact of temperature on the adsorption process. To explain the thermodynamic property, the following formula is used.

In this equation T is the absolute temperature, which is expressed in kelvin (K), KD is the distribution coefficient: (∆S°) represents the change in entropy, (∆H°) is the enthalpy change, and R represents the universal gas constant with a value of 8.314 Jmol−1 K−1. The intercept and slope from the plot relating to 1/T and ln Kd are used to calculate ∆G°, ∆H°, and ∆S°49. Thermodynamic parameters calculated for Rhodamine B dye are shown in Table 3.

The results show that the Gibbs free energy (∆G°) decreases as temperature rises. The negative value of Gibbs free energy signifies the spontaneous nature of the adsorption reaction. Positive enthalpy and entropy values indicate the endothermic nature of the reaction and an augmentation in disorderliness during the process of adsorption, respectively. As the dye molecules adhere to the surface of the adsorbent, due to heightened randomness at the solid/solution interface the value of entropy increases; a similar pattern is noted by other researchers for dye adsorption. During the adsorption process, the movement of molecules intensifies with the temperature, as a result, dye molecules adhere to the surface of the active site58. Whether the adsorption process is an associative or dissociative mechanism is represented by a sign of ΔS°. The result indicates that a dissociative mechanism is involved during the adsorption process since ΔS° has a positive value. Additionally, the literature reports that by expanding the adsorbent's surface area, elevated temperature has a favorable impact on the process of adsorption. As pore volume increases, more dye molecules will adhere to the surface of the adsorbent. Furthermore, the rate of diffusion of dye in the solution increases with the temperature rise59.

Adsorption kinetics describes the amount of time needed for a specific adsorption process as well as the extent of adsorption phenomena. In this context, several kinetic models are available, as listed in Table 4.

The determination of the R2 value was conducted for each kinetic model; the applicability of the adsorption model for the system is indicated by a value near 1.

The outcomes of the kinetic model are shown in Table 5 it shows that the pseudo-first model was successfully applied to the experimental data as evidenced by the R2 value of (0.80609). The rate of adsorption is correlated with the abundance of available adsorption sites on the surface of the adsorbent, according to the pseudo-first-order kinetic model. The rate of adsorption decreases as freely available sites fill up. A comparable pattern can be seen in the current study, adsorption increases with prolonged contact time at first, but after reaching maximum adsorption, increasing time has a negligible effect on the adsorption rate. Other researchers have also documented a comparable trend in the literature60.

Pseudo-first-order equation is represented as follows.

The rate constant for intra-particle diffusion, denoted as KdIff and K2 (g/mg.min) is the rate constant for second-order reaction. Kinetic Parameters such as qt, K2, K2diff, and c were computed by using the value of qt and t as shown in Table 4.

Only type I produces a favorable outcome in the pseudo-second-order kinetic model, displaying R2 values approaching unity. Conversely, the remaining types have lower R2 values. This model is predicated on the idea that the adsorption phenomenon, which causes attractive forces to form between the molecules of the dye and adsorbent, is the rate-limiting step60. Pseudo-second order model is represented by the following equation:

Elovich kinetic model is conceived on the notion that as the amount of adsorption of dye increases, the adsorption rate diminishes exponentially. It also offers details regarding the distinctive characteristics of the adsorption process. This model is given by the following equation:

This model has been effectively applied to the adsorption of Rhodamine B dye. Values of α and β are computed from the slope and intercept of the linear plot of qt versus lnt with the R2 value found to be high (R2 = 1), indicating the chemisorption behavior of the adsorbent (Fig. 11).

Pseudo–first, pseudo-second order, Elovich, and intra particle diffusion kinetic models.

Consequently, it can be inferred from the value of R2 that the pseudo-second-order model aligns most effectively with the experimental data. The intra-particle diffusion kinetic model proposed by Weber and Morris was also applied to kinetic data. It has the following equation:

The value of the regression coefficient (R2 = 0.8231), is showing the dye’s significant adsorption. Value of (C ≠ 0), the line in the diffusion model, however; is not passing through the origin, indicating that there exist other rate-governing steps for the adsorption reaction than the intra-particle diffusion kinetic model. This is associated with the fact that the pseud-order model is followed by the process of adsorption61.

One of the main resources for encompassing the nature of the adsorbent surface is the adsorption isotherm. On the other hand, the selection of suitable adsorption equations in different concentration ranges gives a clear insight into the surface. The following can be described with importance using the adsorption isotherm. The distribution of Adsorbate molecules in the liquid and solid phase at equilibrium, the kind and characteristics of adsorption, and how the Adsorbate and adsorbent interact, are the main aspects to contemplate. The data was fitted using different models to understand the process of adsorption, including the Langmuir, Freundlich, Dubinin–Radushkevich (D–R), and Temkin Isotherm (Fig. 12).

Adsorption isotherms applied on Rhodamine B removal.

Adsorption is assumed to take place completely at a homogenous adsorption surface by the Langmuir adsorption isotherm62. Moreover, it is assumed that no significant interactions occur between the adsorbed species. Additionally, it is assumed that the maximum adsorption corresponds to a saturated monolayer of Adsorbate molecule on the surface of the adsorbent. Thus, the Adsorbate does not move in the plane of the adsorbent surface, and adsorption energy remains constant63. The Langmuir isotherm is represented by the following equation.

The equilibrium concentration of Rhodamine-B is represented by Ce (mg/l), he is the quantity of Rhodamine-B which is adsorbed onto the surface of CaO NPs at equilibrium (mg/l), qmL is the amount of dye adsorbed maximum in monolayer form (mg/g) and KL is the Langmuir adsorption constant (l/mg). The graph is plotted between 1/Ce and 1/qe with the slope of 1/qmL and intercept as 1/qmL KL. Calculated values of qmL and KL, indicate that there is no transmigration of Adsorbate in the plane of adsorbent. Saturated monolayer adsorption of Adsorbate molecule is the maximum associated adsorption with constant energy. To determine the favorability of Langmuir isotherm onto the adsorption of Rhodamine-B, the separation factor RL was calculated.

The calculated value of RL shows that weather adsorption is: Unfavorable: RL > 1; Linear: RL = 1; Favorable: 0 < RL < 1; Irreversible: RL = 0. RL value calculated is between 0 to 1 and a high value of correlation coefficient R2 = 0.989 indicates the best fitness of the Langmuir model to adsorption of Rhodamine-B on CaO NPs. The value of all < 1 and RL < 1 indicates the favorable, homogeneous, monolayer nature of adsorption64.

The equilibrium liquid and solid phase capacity, which consists of the heterogeneous surface of the adsorbent or surface supporting sites of different affinities, is related to the Freundlich isotherm65 and Freundlich isotherm is related to multilayer adsorption66. It shows that at various concentrations, the ratio of the amount of solute that is adsorbed onto a specific amount of adsorbent to the concentration of solute in the solution is not constant67.

Freundlich isotherm is represented by the following equation.

Where qe and Ce are represented as equilibrium capacity, KF is the Freundlich adsorption constant.

Which shows the adsorption capacity, n is the empirical parameter that relates to the adsorption.

The intensity of solid adsorbent, changes with the material heterogeneity. The applicability of this Isotherm is checked by measuring the value of n. When the value of n lies between 1 and 10 (1/n is less than 1), it represents that adsorption occurred easily and the adsorbent’s surface was heterogeneous68. In this study, log qe is plotted against log Ce, and a straight line is obtained with a slope of 1/n and intercept of log KF. The values of KF and 1/n obtained for this study are 0.908510603 and 0.86821, respectively. In this study, the value of n is 1.151795073, and the correlation coefficient value is 0.92757 showing the favorable adsorption intensity. The adsorption process was reasonably easy to occur when the value of 1/n was less than 1, but it was more difficult to occur when 1/n was more than 169. The fact that the 1/n value in this study is 0.8621 thus, it suggests that CaO NPs could readily absorb Rhodamine-B. Thus value of n (n = 1.108525) greater than 1 indicates a favorable as well as chemical nature of adsorption70.

Multilayer development in microporous substances can be successfully explained by the Dubinin–Radushkevich (D–R) isotherm. It does not implicit a homogenous surface or constant adsorption potential, as this equation is more general than the Langmuir isotherm. Regardless, of this it has been widely utilized to explain the energy heterogeneity of solid surfaces at low coverage. It was used to differentiate between physical and chemical adsorption71.

The (D–R) equation can be represented as follows:

Anim DR (mg/g) represents the Polanyi potential is ε (j/mol), which is saturation capacity, and activity coefficient KDR (mol2/kj2) is the related amount of energy adsorbed.

R is the gas constant (R = 8.314 J/(mol K)) and T is the temperature in (K). The graph is plotted between ε2 and lnqe.

Slope = − KDR thus, it yields a value of KDR, and intercept = lnqmDR thus, it provides the value of lnqmDR. The mean free energy E (kJ/mol) is calculated to determine the type of adsorption process. The process of adsorption is represented as chemical adsorption if the magnitude of E is between 8 and 16 kJ/mol, and the process of adsorption is indicated as physical if the value of E is less than 8 (kJ/mol). In this study on the CaO NPs the adsorption of Rhodamine-B has a value of E 9.28 × 10^3 kJ/mol. Thus the value of E is greater than 8 (kJ/mol), the adsorption process is chemical adsorption72. The correlation coefficient value calculated in this isotherm is 0.93997. When compared to the results of Freundlich and (D–R) models (R2 = 0.92757 and 0.93997) correlation coefficient value of the linear plot of Langmuir isotherm (R2 = 0.989) is more satisfactory indicating that Langmuir isotherm model fits the experimental data very well. Additionally, it was observed that the maximum adsorption capacity qmax of Rhodamine-B dye is 57.53739931 mg/g (Table 5). The fitting of Langmuir isotherm is proved by the monolayer attachment of dye on the surface of CaO NPs29.

The Temkin isotherm was applied to the results obtained after experimentation. This Isotherm is built on the fact that during the adsorption phenomenon, the reaction heat is changed. Information about binding energy is derived from Temkin constant KT and heat of adsorption is predicted from the constant BT during the process of adsorption69.

The idea of the point of zero charge clarifies the negative charge that is present on the adsorbent's surface and provides insight into the fundamentals of adsorption. At this pH, the calculated 6-determined point of zero charge denotes a neutral charge state on the surface of the adsorbent. Nevertheless, the adsorbent's surface acquires a positive charge when the pH descends below this point; on the other hand, if the pH rises above the point of zero charge, the adsorbent surface exhibits a negative charge. As a result, maximum adsorption of Rhodamine B occurs at a pH of 7, signifying a neutral medium. With any decrease in pH below this level, the adsorption of Rhodamine B decreases. This occurrence can be elucidated by the fact that as pH rises, the positive charge on the surface of the adsorbent decreases. Electrostatic forces of attraction between the dye molecules and the charged adsorbent surface primarily control the adsorption mechanism, and this interaction thoroughly elucidates the complex process of adsorption (Fig. 13).

Adsorption mechanism for the attachment of Rhodamine B on CaO NPs.

The type of adsorption can be evaluated from the values of parameters. The positive value of (∆H°) indicates the endothermic nature of adsorption according to the literature, the process of adsorption is physical if the value of enthalpy change is < 20 kJ/mol. In this study, the value of (∆H°) is < 20 kJ/mol, which indicates the physical adsorption of Rhodamine-B dye on CaO NPs73. The type of adsorption is also evaluated by the value of E. The process of adsorption will be chemisorption in nature if the value of E lies between 8 and 16 kJ/mol. In this study, the value of E for adsorption of Rhodamine-B on CaO NPs is 9.28E + 03 which indicates the chemical nature of adsorption. The effect of increasing temperature on the process of adsorption can be used to determine the nature of adsorption. In the process of adsorption of Rhodamine-B and CaO NPs the adsorption increases with the temperature rise, which represents the chemical nature of adsorption74. These results indicate that the adsorption of Rhodamine B on CaO NPs involves chemisorption as well as a physio sorption mechanism. From the literature75, both physical as well as chemical adsorption can take place on the surface at the same time, physical adsorption of a layer of the molecule can occur on the top of an underlying chemisorbed layer.

Solid waste generated during the adsorption process is an additional significant risk to the environment. Proper disposal of this solid waste can be challenging, as it contains hazardous dyes. Scientists are working on various projects to regenerate adsorbent and lessen its environmental impact73,74. In this study, following the adsorption phenomenon, the solid waste was treated with ethanol for one hour to regenerate it. The adsorbent was used once more to remove the dye after it had been adsorbed, filtered, and dried. The Composite initially displayed 98% dye removal efficiency. Figure 14 illustrates how the adsorption efficiency progressively dropped to 74% after four regeneration cycles. After multiple reuses, synthetic composites were shown to be highly stable and effective. Because of these characteristics, composites are a great option for decontaminating water.

Regeneration of CaO NPs for cycled use of dye removal.

Environmentally friendly CaO NPs were found promising in performance for the removal of Rhodamine B dye when compared with literature as given in Table 6. As depicted by data CaO NPs synthesized in the present study have shown excellent adsorption behavior with a small amount of adsorbent utilized and a high % removal of Rhodamine B.

Mulberry was selected for reduction reaction to synthesize CaO nanoparticles due to its reported antioxidant activity (80%) and high value of free phenolics (6848.43 μg/g) present in its extract. These properties make it suitable for CaO synthesis. Leaf extract was prepared with slight modification as mentioned in previously reported literature79. Mulberry leaves were collected and thoroughly cleaned with distilled water. 20 g of leaves were precisely weighed and boiled in 400 ml of distilled water for 30 min on the burner until 2/3rd of the initial volume of extract remained. The resulting solution was filtered using Whatman no 1 filter paper. After cooling the extract obtained, was employed for the synthesis of metal oxide nanoparticles (Fig. 15). Specifically, a freshly prepared extract was utilized for the synthesis of CaONPs.

Preparation of leaf extract of Mulberry.

CaO nanoparticles (CaO NPs) were synthesized by employing the reported method with some changes to make them more effective80. For the synthesis of CaO NPs, 0.1 Molar solutions of (Ca (NO3)2.4H2O) were prepared by dissolving 1.64 g of Calcium nitrate tetrahydrate in 100 ml of distilled water. To prepare a 0.1 Molar solution of NaOH, 4 g of the given compound was dissolved in 100 mL solution. CaO nanoparticles were synthesized by adding 10 ml of mulberry leaf extract into 10 ml of calcium nitrate tetrahydrate solution and the mixture was stirred using a magnetic stirrer for 30 min. NaOH solution was gradually introduced using a dropper while stirring, leading to the formation of white precipitates identified as calcium hydroxide. The resulting precipitates were filtered, air-dried for 24 h, and subsequently washed with distilled water to eliminate the residual basicity of the solution. The precipitates were transferred to a crucible and calcined at 400 °C for three hours to obtain the CaO NPs which were further characterized for identification (Fig. 16).

Green synthesis of CaO NPs by employing Mulberry leaf extract.

The sorbent must be characterized to identify the various chemical and physical properties that affect adsorption. Thus, physical and chemical characterization techniques were employed, involving Fourier transform infrared spectroscopy (FTIR) Shimadzu AIM-8800 model was employed for functional group determination. It is possible to determine the kind of binding relationship that takes place between the sorbent and the composite surface by identifying these functional groups, which are in charge of the dye's attachment to the sorbent surface. Similarly, surface analysis using Brunauer, Emmett, and Teller (BET) was conducted, to determine synthetic composite surface area. The morphology of nanocomposites was determined using a JEOL JSM5910 model number-scanning electron microscope (SEM). SEM provides details on the adsorbent's surface.

Gas adsorption data analysis utilized the BET theory, a standard approach for assessing specific areas denoted as unit area per sample mass (m2/g). To perform BET, a gas—typically nitrogen, krypton, or argon is physically adsorbed onto the surface of the specimen at cryogenic temperature. The surface area of the material is then computed, typically at the temperature corresponding to liquid nitrogen or liquid argon conditions. X-ray diffraction (XRD) was obtained for synthesized nanocomposites to confirm the formation of the required material by employing TD-3700 for the purpose.

The following factors were modified to enhance the conditions using the parameter-optimization technique:

Initial Concentration of the Adsorbate ranged from 20 to 100 ppm.

Amount of Adsorbent was set at 0.4 g.

PH levels were varied within the range of 2 to 10.

Contact Time spanned from 30 to 150 min.

To apply synthetic composites and create the ideal conditions for dye removal, parameters must be optimized.

For batch adsorption systems, kinetic studies provide information on optimal conditions, sorption mechanisms, and possible rate-limiting stages76. For this, pseudo-first-order and pseudo-second-order linear kinetics were applied to the adsorption data. To test the effect of contact time (30–150 min) on adsorption, 100 mL of the Rhodamine B solution was used for every time interval.

An appropriate adsorption isotherm has to be designed to study the adsorption process and the equilibrium relationship between the sorbent and solvent77. Isotherms predict the right sorbent parameters and behavior for a variety of sorption systems78. To investigate isothermal behavior for the elimination of Rhodamine B using synthetic composites, the dye's initial concentration was varied (20–100 ppm) while maintaining all other parameters unchanged.

Hazardous materials, such as dyes, from watery environments can be removed by the phenomena of adsorption which serves as a remarkable solution for water decontamination. The leaf extract of Mulberry is utilized to synthesize the CaO nanoparticles in this research and serves as capping and reducing agents due to specific phytochemical properties. Synthesis of nanoparticles using leaf extract is a viable substitute for a more traditional chemical approach. The present work involves the utilization of Mulberry leaf extract for the synthesis of CaO NPs as an extremely good choice for the removal of hazardous Rhodamine B dye as depicted by results (98.25% removal). Important functional groups that help to establish the electrostatic forces of attraction between the Rhodamine B dye and green synthesized CaO NPs present on the surface of nanoparticles are identified by the different characterization techniques. It was found that 0.4 g of synthesized nanoparticles was an effective amount for the noteworthy concentration of dye removal (98.25%) within just 60 min duration. Thermodynamic models and adsorption isotherms are successfully applied to illustrate the viability of the adsorption process and its mode. Thus, the current study provides information that an affordable adsorbent that has a promising adsorption potential can be used for the removal of hazardous dye Rhodamine B from wastewater.

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

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The authors would like to extend their sincere appreciation to the Researchers Supporting Project number (RSP2024R123), King Saud University, Riyadh, Saudi Arabia.

This work was funded by the Researchers Supporting Project Number (RSP2024R123), King Saud University, Riyadh, Saudi Arabia.

Institute of Chemistry, University of Sargodha, Sargodha, 40100, Pakistan

Gulnaz Nasir, Fozia Batool, Sobia Noreen, Humaira Yasmeen Gondal, Muhammad Mustaqeem & Fayyaz Ur Rehman

The University of Sheffield, Western Bank, Sheffield, S10 2TN, UK

Sobia Noreen

Department of Chemistry, Government College University, Lahore, 54000, Pakistan

Zohaib Saeed

Department of Chemistry, Thal University, Bhakkar, 30000, Pakistan

Yasmeen Gul

Department of Botany and Microbiology, College of Science, King Saud University, 11451, Riyadh, Saudi Arabia

Hayssam M. Ali

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G.N.: Main Manuscript Writing. F.B.: Supervision, Conceptualization of Idea. S.N.: Revising Manuscript. H.Y.G.: Data Analysis. M.M.: Experimentation. Z.S.: Statistical Analysis. Y.G.: Editing. F.U.R.: Characterization. H.M.A.: Funding the Project. Authors have no objection for this work to be published.

Correspondence to Fozia Batool.

The authors declare no competing interests.

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Nasir, G., Batool, F., Noreen, S. et al. Biosynthesis of calcium oxide nanoparticles by employing Mulberry (Morus nigra) leaf extract as an efficient source for Rhodamine B remediation. Sci Rep 14, 23744 (2024). https://doi.org/10.1038/s41598-024-71172-1

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Received: 13 May 2024

Accepted: 26 August 2024

Published: 10 October 2024

DOI: https://doi.org/10.1038/s41598-024-71172-1

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