A novel approach to modifying eggshell-based adsorbent for the removal of acid red 1 and crystal violet dyes: kinetics, isotherm, and thermodynamics study

TL;DR

This study explores using modified eggshells to remove harmful dyes from water, showing they are effective and low-cost.

Contribution

A novel modification of eggshells with ferrous sulfate improves their adsorption capacity for specific dyes.

Findings

Modified eggshells achieved maximum adsorption capacities of 138 mg/g for CV and 124 mg/g for AR1.

Adsorption efficiency was strongly influenced by pH, with optimal conditions at pH 9 for CV and pH 2 for AR1.

The process is exothermic and less favorable at higher temperatures, as shown by thermodynamic analysis.

Abstract

The pollution of synthetic dyes from textile wastewater has raised environmental and health concerns, prompting the search for economical, sustainable waste-derived adsorbents. This study investigates the ability of modified eggshell powder to adsorb two deleterious dyes, the anionic Acid Red 1 (AR1) and the cationic Crystal Violet (CV), from aqueous solutions. Eggshells, a prevalent biowaste of calcium carbonate, were treated with ferrous sulfate using co-precipitation to generate modified eggshells that enhance their adsorptive characteristics. Different analyses were performed on the adsorbent, including XRD, zeta potential, and XRF. The adsorption experiments were conducted to examine the effects of varying solution pH from 2 to 9, contact duration ranging from 5 to 60 min, initial dye concentration ranging from 10 to 100 mg/L, and adsorption temperature from 20 to 60 °C. It indicates that the efficiency of dye removal was considerably influenced by pH, as the maximum adsorption for CV and AR1 dyes was found at pH 9 and pH 2, respectively. The modified eggshells exhibited a maximal adsorption capacity of 138 mg.g− 1 for CV and 124 mg.g− 1 for AR1, which increased in tandem with the dye concentration. The kinetic analysis demonstrated that CV adsorption conforms to a pseudo-second-order model, signifying the influence of pore diffusion and surface interactions, whereas AR 1 adherence aligns with a pseudo-first-order model. The Freundlich and Langmuir models were effectively aligned with isotherm modeling to validate the mechanisms of physical and chemical adsorption. As the negative entropy changes and the Gibbs free energy values indicate a non-spontaneous phenomenon, the adsorption process is exothermic and becomes less favorable at elevated temperatures. Modified eggshells treated dyes wastewater better as a low-cost adsorbent, making them a viable and environmentally beneficial alternative.

Figures

The influence of temperature on adsorption capacity and removal efficiency.

Adsorption Isotherm plots; (a) Langmuir, (b) Freundlich, (c) Temkin, and (d) Dub-Rad.

Kinetics adsorption models; (a) Pseudo-first-order, (b) Pseudo-second-order, (c) Elovich, and (d) Intraparticle diffusion.

Van’t Hoff linear relationship of the adsorption of CV and AR1 on the raw and modified eggshells.

Plot of the thermodynamic parameter Gibbs’ free energy (ΔG) at different temperatures.

Adsorption mechanism.

Reusability of adsorbent.

Adsorption cost of modified eggshells and commercial activated carbon.

Experimental procedure of adsorbent preparation.

XRD pattern of raw and modified eggshells.

Zeta-potential of eggshells before and after modification.

Particle size distribution of eggshells before and after modification.

SEM and EDX for: (a) raw, and (b) modified eggshells.

FTIR of raw and modified eggshells.

The influence of pH on removal efficiency and adsorption capacity.

The effect of initial dye concentration on adsorption capacity and removal efficiency.

Effect of contact time on the adsorption capacity and removal efficiency.

Full text

Introduction

Today, the contamination of the environment is one of the critical topics being focused on, especially the contamination of water during collection from different industries. The most common contaminants present in water are organic dyes^1^. Dyes are used in various industries, including paper, textiles, and photography. The contaminated wastewater poses a threat to the environment^2^. Industrial dyes spread diseases such as cancer, leading to an increase in mortality. The increase in industrial activities has led to a decrease in freshwater and an increase in water pollution^3^. About millions of gallons of dye wastewater are produced annually, threatening public health and the environment. These concerns have prompted worldwide initiatives to create and implement effective technology to mitigate pollutant dispersion and ensure clean water availability^4^. The contaminated dye wastewater causes a reduction of oxygen levels, sunlight blocking, and changes in the biological processes in animals and aquatic plants^5,6^.

The global production of dyes and pigments has reached approximately 700 million kilograms annually, with the textile industry being the primary consumer of these compounds. This industry not only consumes significant amounts of dyes but also uses large volumes of water and various chemicals compared to other sectors^7^. Around 200 cubic meters of water are used per ton of textile product. Due to the diverse operations involved in textile processing, the resulting wastewater is chemically complex and typically contains various hazardous substances, including dyes, sodium hydroxide, acids, starches, and heavy metals^7^. Textile dyes are complicated molecular substances. Restricting temperature, oxidation, and light renders them nondegradable in the aquatic environment. Earlier investigations indicate that cationic dyes are more hazardous and poisonous than anionic dyes, which can easily interact with the cytoplasm^8,9^. Crystal violet and Acid red 1 are commonly used in textile applications due to their intense colouration and cost-effectiveness.

Crystal violet is one of the most prevalent cationic dyes. It is also referred to as a basic dye since the chromophore of the basic molecules possesses a positive charge^10–14^. Crystal violet (CV) is a triphenylmethane dye with the chemical formula C_25_H_30_N_3_Cl. It is used in the textiles, paint, pharmaceuticals, and biotechnology industries^15^. Due to its antibacterial properties, it is a preserving material while transporting long distances of meat, aquatic products, agricultural products, and pharmaceuticals^16–18^. Despite its broad utility, CV is a toxic and potentially carcinogenic compound associated with adverse effects such as skin and eye irritation and kidney dysfunction^15^. Conversely, Acid Red 1 is an anionic, water-soluble azo dye primarily used for dyeing wool and polyamide under acidic conditions^19^. Although once approved as a food additive, it is now banned due to its degradation into harmful by-products that interfere with haemoglobin and resist biodegradation^20^.

Recycling and reusing dye-contaminated wastewater is critical for conserving water resources and minimizing the discharge of hazardous pollutants into the environment^21^. Untreated textile effluents can introduce surfactants, synthetic dyes, acids, and heavy metals into water bodies, threatening aquatic ecosystems and public health^13^. Over time, several treatment methods have been developed for dye removal, including coagulation-flocculation, chemical precipitation, membrane filtration, electrochemical oxidation, ozonation, and biodegradation^22,23^. Among these, adsorption is particularly effective and economical due to its operational simplicity, flexibility, and high removal efficiency for diverse contaminants^24^. Adsorption is one of the most commonly used methods of removing contaminated wastewater because it is inexpensive, non-toxic, easily separated, and highly effective^25,26^. It is also easily regenerated, low-cost, eco-friendly, and has a high adsorption capacity for wastewater^4,27–31^.

Numerous studies have explored the adsorption of Crystal Violet using various low-cost adsorbents with high adsorption capacities, such as orange peels^26^, sugarcane bagasse^32^, coffee husks^33^, palm kernel fibre^34^, rice husk^35^, water hyacinth^36^, date palm fibre^37^, acacia leaves^38^, palm kernel shell-derived biochar^39^, and grapefruit peel^40^. Sarabadan et al.^41^ discuss the use of natural zeolite as an adsorbent to achieve a maximum removal efficiency of 99.9% at a temperature of 25℃ and pH 10, with an adsorption capacity of 0.1 g.g^−1^. Similarly, Naderi et al.^42^ demonstrate the feasibility of the usage of Centaurea stem as a bio-adsorbent, achieving an optimal removal of CV of 98.1% with an adsorption capacity of 476.19 mg.g^−1^ at pH 12.57, temperature 38.94℃, time 19.6 min, and adsorbent dose 12.218 mg with initial concentration 36.62 mg.L^−1^.

Additionally, a magnetic chitosan nanocomposite was prepared by Massoudinejad et al.^43^, who studied the factors affecting the removal efficiency of CV. The maximum removal efficiency of the nanocomposite adsorbent is 72% at a contact time of 140 min, and the adsorbent dose is 1 g with an initial concentration of 77 mg.L^− 1^. Various studies have concentrated on developing composite materials from inorganic and organic waste. Hossain et al.^44^ studied the modification of eggshells by chemical and thermal treatments, creating a magnetic composite to improve its adsorption ability. The modified material showed high efficiency in removing lead ions and methylene blue dye from water, with removal rates above 90%.

Sarra et al.^45^ investigated the usage of calcined eggshells as an adsorbent for removing dyes from aqueous, reconstituted, and real effluents that have an adsorption capacity of 80.1, 36.3, and 64.8 mg.g^− 1,^ respectively. Also, Basaleh et al.^46^ treated eggshells with NaOH, KMnO_4_, and HNO_3_ at various concentrations and contact times to eliminate lead ions from the aqueous solutions. They discovered that the eggshell treated with KMnO_4_ has the maximum affinity for lead ions compared to HNO_3_ and NaOH. They found that the maximum adsorption capacity of modified eggshells for lead ions is 700 mg.g^− 1^ with a removal efficiency of 98%. On the other hand, Nor et al.^47^ enhanced the adsorption capacity of graphene using eggshells to increase the functional groups of hydroxyl (O-H) and carboxyl (C-O) by ultrasonification to remove fluoride from wastewater. The results indicated that the highest adsorption capacity of the modified adsorbent is 54.3 mg.g^− 1^.

Finally, Sun et al.^48^ investigated the co-pyrolysis of eggshells and corn stalks to produce a biochar adsorbent used to remove phosphorus from wastewater. The maximal adsorption capacity of phosphorus was determined to be 557 mg.g^− 1^. The thermodynamic investigation indicates that the adsorption process is endothermic.

Prior research has concentrated on the calcination of eggshells^45–50^, co-pyrolysis with biomass, blending eggshells with biochar^48,51^, the preparation of composite materials incorporating various ratios of TiO_2_^52^, and the fabrication of a ternary composite material comprising calcium alginate, gum arabic, and eggshells to improve adsorption capacity^53^, often resulting in high costs.

The novelty of this research lies in the synthesis of an effective chemically modified eggshell powder after treatment with ferrous sulfate. It serves as a low-cost and environmentally sustainable bioadsorbent for the elimination of two widely used textile dyes, Crystal Violet (CV) and Acid Red 1 (AR1), from aqueous solutions. The investigation focuses on evaluating the influence of key operational parameters and analyzing adsorption kinetics, isotherm models, and thermodynamic characteristics to gain a deeper understanding of the underlying mechanisms of dye removal.

Materials and methodology

Raw materials

Eggshells were collected from commercial restaurants and cafes. Acid Red 1 is an anionic acid dye, and Crystal Violet is a basic dye with 99.9% purity purchased from the El Morgan company for chemicals. They used the stock solution of dyes to prepare the dye stock solution. The chemical properties of these dyes are illustrated in Table 1.

Stock solutions were created by dissolving 1 g of dyes in 1 L of distilled water to create 1000 ppm solutions of AR1 and CV dyes. Standard solutions with concentrations varying from 10 to 100 ppm were subsequently generated through appropriate dilution. The pH of the solution was modified utilizing 0.1 M HCl and 0.1 M NaOH when necessary. The chemical properties of the dyes are given in Table 1. The purity of NaOH and HCl is 99% and 37% respectively, and they were purchased from Elshark Awasat company.

Table 1. General characteristics of acid red 1 and crystal Violet dyes.ItemAcid red1 dyeCrystal violet dyeTypeAnionicCationicChemical compositionC_18_H_13_N_3_Na_2_O_8_S_2_C_25_H_3_0N_3_ClChemical structure

Charge−2+ 1λmax (nm)509590pKa6.48.64Molecular mass493407

Preparation of modified eggshell

A 500 g sample of eggshell with the inner membrane was collected, washed, and dried at 90 °C for 1 h. The dried sample was ground to a less than 200-mesh sieve (74 microns), which was considered a raw eggshell sample.

The modified eggshell adsorbent was synthesized by combining 5 g of eggshell powder with 50 ml of 0.25 M ferrous sulfate solution and agitating for 15 min. A 0.1 M sodium hydroxide solution was incrementally added to the mixture until the pH reached 6. The mixture was agitated for 10 min. The mixture was subjected to filtration and rinsed with distilled water, then dried at 90 °C for one hour. The synthesized adsorbent was preserved for analysis and application in the adsorption process. The experimental methodology is depicted in Fig. 1.

Adsorption experiments

The adsorption experiments were conducted by adding 0.01 g of adsorbent to 20 ml of dye solution in a round flask. The flask was placed in a shaking water bath with a stirring rate of 200 rpm at a specific contact time and temperature. At the end of the adsorption process, the adsorbent was separated from the dye solution by centrifugation for 10 min at 2000 rpm. The ultraviolet spectrophotometer (HACH DR 2800) was used to determine the concentration of the remaining dye in the solution at the maximum wavelengths of the AR1 and CV dyes, which are 509 and 590 nm, respectively.

The factors affecting the adsorption process, including pH (2–9), temperature (20–60) ℃, initial concentration (10–100) ppm, and contact time (5–60) min were studied. The removal efficiency and the adsorption capacity were determined according to the initial and final concentrations using the following Eqs. (1) and (2). The adsorption isotherm, kinetics, and thermodynamics were investigated.

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:Removal\:\left(\%\right)=\frac{{C}_{0}-\:{C}_{t}}{{C}_{0}}\times\:100$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{q}_{t}=\frac{\left({C}_{0}-{C}_{t}\right)\:V}{M}$$\end{document}

C_o_ and C_t_ represent the initial dye concentration and the concentration at time “t” in mg.L^− 1^, respectively; V denotes the volume of the solution in liters, and M signifies the mass of the adsorbent in grams.

Fig. 1. Experimental procedure of adsorbent preparation.

Isotherm, kinetics, and thermodynamics adsorption

Langmuir, Freundlich, Temkin, and Dub -rad isotherms were applied to investigate the adsorption behaviour of the studied systems using equations from (3) to (7). The Langmuir isotherm model^54^ is expressed as:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\frac{1}{{q}_{t}}=\frac{1}{{q}_{max}}+\:\frac{1}{{b{C}_{f}\:q}_{max}}$$\end{document}

Where C_f_ (mg.L^− 1^) is the equilibrium dye concentration, q_t_ (mg.g^− 1^) is the amount of dye adsorbed at time t, q_max_ (mg.g^− 1^) is the maximum monolayer adsorption capacity, and b (L.mg^− 1^) is the Langmuir constant related to the affinity between the dye and the adsorbent.

The Langmuir model assumes monolayer adsorption on a homogeneous surface, where dye molecules are adsorbed onto specific sites after diffusing through the pores and channels of the adsorbent’s structure, displacing exchangeable ions.

The Freundlich isotherm model^55^ is represented as:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{ln\:q}_{t}=ln\:k+\:\frac{1}{n}ln\:{C}_{f}$$\end{document}

The adsorption intensity is denoted by n, the equilibrium dye concentration is C_f_ (mg.L^− 1^), the Freundlich constant K is the adsorption capacity, and the quantity of dye adsorbed at time t is q_t_ (mg.g^− 1^).

A value of n = 1 signifies a linear adsorption isotherm. If n < 1, it indicates unfavorable adsorption, whereas values of n between 1 and 10 signify favorable physical adsorption^55–57^.

The Temkin adsorption model is an empirical model that expresses the adsorbate’s adsorption behavior on the solid’s surface, as well as the impact of these interactions on the binding energy distribution and adsorption heat. This model implies that the energy of adsorption reduces linearly as surface coverage increases^58^. The Temkin adsorption isotherm model^59^ is denoted as:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:Temkin:\:{q}_{e}=B\:ln\:A+B\:ln{C}_{e}$$\end{document}

Where qe is the equilibrium adsorption capacity (mg.g^− 1^), A (L.g^− 1^), and B (J.mol^− 1^) are Temkin adsorption isotherm constants.

The Dubinin-Radushkevich (D-R) isotherm is an empirical formula used to characterize adsorption mechanisms on heterogeneous surfaces with a Gaussian energy distribution of adsorption sites. It is used to study the adsorption process of multilayer or pore-filling mechanisms. It is suitable for chemical and physical adsorption^60^.

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:D-R\::\:{q}_{e}={q}_{m}\mathrm{e}\mathrm{x}\mathrm{p}(-\beta\:{\epsilon\:}^{2})$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\epsilon\:=RTln(1+\frac{1}{{C}_{e}})$$\end{document}

Where q_e_ represents the equilibrium adsorbed amount, q_m_ represents the theoretical monolayer saturation capacity, β denotes a constant related to the adsorption energy, and ε is the Polanyi potential. R is the universal gas constant, C_e_ is the equilibrium concentration, and T is the temperature of the adsorption.

The Lagergren pseudo-first-order (PFO), pseudo-second-order (PSO), intraparticle diffusion, and Elovich models were employed to assess the adsorption kinetics of the dyes onto the adsorbent using equations from (8) to (11). The subsequent equations represent the linear equations that correspond to them^55,61^.

Pseudo-first-order equation:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\it \:\mathrm{ln}\:({q}_{e}-{q}_{t})=\mathrm{ln}{q}_{e}-\:{K}_{1}\mathrm{t}$$\end{document}

Pseudo-second-order equation:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:\frac{\mathrm{t}}{{q}_{t}}\:=\frac{1}{{\mathrm{k}}_{2}{q}_{e}^{2}}+\:\frac{1}{{q}_{e}}\:\mathrm{t}$$\end{document}

Intraparticle diffusion:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:{q}_{t}={k}_{p}{t}^{0.5}+c$$\end{document}

Elovich:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\it \:Q=\left(\frac{1}{\beta\:}\right)\mathrm{ln}\left(\alpha\:\beta\:\right)+\left(\frac{1}{\beta\:}\right)ln t$$\end{document}

qₜ (mg.g⁻¹) denotes the quantity of dye adsorbed at time t (min), qₑ (mg.g⁻¹) represents the quantity adsorbed at equilibrium, k₁ (min⁻¹) signifies the rate constant of the PFO model, k₂ (g.mg⁻¹.min⁻¹) indicates the rate constant of the PSO model, kp (mg.g⁻¹.min⁻⁰.⁵) refers to the rate constant of intraparticle diffusion, c (mg.g⁻¹) is the constant of intraparticle diffusion, α (mg.g⁻¹.min⁻¹) represents the initial absorption rate, and β is the constant of the Elovich model.

Thermodynamic characteristics, such as Gibbs free energy (ΔG), entropy (ΔS), and enthalpy (ΔH), were calculated^62^. The values of ΔH and ΔS were computed using the Van’t Hoff Eq. (12)^63^.

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\it \:\mathrm{ln}{K}_{c}=\frac{{\varDelta\:S}^{^\circ\:}}{R}-\frac{{\varDelta\:H}^{^\circ\:}}{RT}$$\end{document}

Where kc = F/(1 − F), and F = (Co − Ce)/Co^64^, R represents the universal gas constant (8.314 J/mol K), and T is the temperature in Kelvin.

The Eq. (13) was employed to estimate the values of the standard Gibbs free energy change^65^:

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\triangle G\,=\, - RT{\text{ }}lnKc$$\end{document}

Characterization of the adsorbent

The Bruker D8 Discover diffractometer (Cu Kα radiation, λ = 1.542 Å, Germany) was utilized to conduct X-ray diffraction (XRD) analysis at a voltage of 40 kV. The crystalline and phase composition of the eggshell adsorbent was analyzed before and after chemical modification by recording XRD patterns over a 2θ range of 2° to 80°. The crystallite size of the synthesized modified eggshell adsorbent was determined using the Debye–Scherrer Eq. (14)^44,66,67^.

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:D=\frac{0.90\hspace{0.17em}{\uplambda\:}}{{\upbeta\:}\mathrm{cos}{\uptheta\:}}\times\:100$$\end{document}

Where D represents the average crystallite size, λ is the wavelength of the X-ray radiation, β is the full width at half maximum (FWHM) of the dominant diffraction peak, and θ is the Bragg angle. The constant 0.90 corresponds to the shape factor typically used for spherical crystallites.

X-ray fluorescence (XRF) analysis of the modified eggshell was conducted to determine its chemical composition using AXIOS, Panalytical 2005. A zeta potential analyzer (Malvern Instruments Co. Ltd., United Kingdom) was employed to perform the zeta potential measurement, and 1.0 × 10^− 3^ M KCl was employed for adjusting the ionic strength. In contrast, the surface charge of eggshells was evaluated before and after modification using 0.1 M HCl and NaOH solutions as pH regulators.

Fourier-transform infrared spectroscopy (FTIR) was employed to identify the functional groups of both raw and treated eggshells. The FTIR model employed is the Class 1 Laser product IEC/EN 60825-1/A2:2001 Avatar series (USA) at the Egyptian Academy for Engineering and Advanced Technology^68^. The Brunauer-Emmett-Teller (BET) test was employed to ascertain the surface area and pore size distribution in both raw and treated eggshells.

Reusability test of modified eggshells

After determining the modified eggshells’ maximum adsorption capacity for CV and AR1, regeneration and reusability experiments were performed. After completing the adsorption process under optimal conditions, the adsorbent was separated and washed five times with 99% methanol. The washed adsorbent was dried in an oven to remove any traces of methanol. The adsorption capacity for both CV and AR1 was determined after each cycle.

Results and discussion

Characterization of modified adsorbent

Mineralogical and chemical analyses

X-ray diffraction (XRD) analysis revealed that the primary crystalline phase in both raw and modified eggshell samples is calcite (CaCO₃), as shown in Fig. 2. However, a noticeable decrease in the intensity of the calcite peaks was observed after modification, indicating partial structural alteration or surface coverage due to the deposition of iron-containing species. That suggests a successful interaction between the eggshell surface and ferrous ions during modification. X-ray fluorescence (XRF) analysis confirmed that the raw eggshell is predominantly composed of calcium carbonate (99%), with minor amounts of MgO (0.543%), P₂O₅ (0.213%), SO₃ (0.14%), Na₂O (0.10%), and trace levels of Al₂O₃. After modification, Fe₂O₃ was detected at 0.031%, indicating the incorporation of iron from the ferrous sulfate treatment, as illustrated in Table 2. Furthermore, Scherrer’s equation was used to estimate the average crystallite size, revealing that the crystallite size of the modified eggshell powder decreased compared to the raw material. This reduction in crystallite size is attributed to the iron deposition on the eggshell surface, which increases the number of active sites and consequently enhances the surface area available for adsorption^44^.

Fig. 2XRD pattern of raw and modified eggshells.

Table 2XRF analysis of Raw and modified eggshells.ItemCaONa_2_OMgOAl_2_O_3_Fe_2_O_3_P_2_O_5_SO_3_LOITotalRaw Eggshell55.440.1020.5430.002-0.2130.1443.56 100 Modified Eggshell55.430.1020.5420.0020.0310.2130.1443.54 100

Zeta potential

The zeta potential measurements demonstrate distinct electrochemical behaviors of eggshells before and after modification. The raw eggshells exhibit an isoelectric point (IEP) at pH 5.4, whereas the modified eggshells show a shifted IEP at pH 4.6, indicating a neutral surface charge at this lower pH value. Consequently, both samples possess negatively charged surfaces at pH values above their respective IEPs, as shown in Fig. 3. The shift toward a more negative zeta potential in the modified eggshells is likely attributed to the hydration of iron hydroxide species on their surface, which increases the density of negatively charged functional groups^69^.

Fig. 3. Zeta-potential of eggshells before and after modification.

Grain size distribution

Figure 4 shows that the median particle size (d₅₀) is 7.7 μm for the raw eggshell and 13.5 μm for the modified eggshell, while the overall average particle size of the samples is 40 μm and 50 μm, respectively. The observed increase in particle size is likely due to the aggregation of particles caused by the adsorption of iron hydroxide species on the surface of the modified eggshells.

Fig. 4. Particle size distribution of eggshells before and after modification.

SEM of adsorbent and BET analysis

Figure 5 illustrates the SEM and EDX for both raw and modified eggshells at 15000x magnification. Figure 5a shows a material with a significantly larger surface area. It has large, layered flakes or plates that are covered in a substantial amount of fine, highly aggregated nanoparticles. On the other hand, Fig. 5b reveals a cleaner, more uniform powder composed mostly of large, angular, and blocky micro-particles with sharp edges and distinct borders between them. It also demonstrates the presence of iron in the modified eggshell.

Fig. 5SEM and EDX for: (a) raw, and (b) modified eggshells.

The BET analysis was conducted on both raw and modified eggshells to ascertain pore size, pore volume, and specific surface area, as presented in Table 3. The findings demonstrated that the modified eggshells possess a surface area superior to that of the raw eggshells. The chemical treatment of eggshells results in a significant surface area, making them suitable as an adsorbent. Regarding IUPAC regulations^55^, the modified eggshells are classified as mesoporous, with pore diameters ranging from 2 to 50 nm.

Table 3BET analysis of Raw and modified eggshells.Average pore radius (nm)Surface area BET (m^2^/g)Pore volume(cm^3^/g)Total pore volume (cm^3^/g)ReferencesGeopolymer7.54511.820.0586.12 × 10^− 2^ ^55^ Physically modified seaweeds4.27414.8030.051- ^70^ Kaolin, Metakaolin, Fly ash Geopolymer4.849.50760.02387- ^71^ Raw eggshells4.550910.4670.02217672.3819 × 10^− 2^Cuurrrent StudyModified eggshells2.65617.02950.02020942.2623 × 10^− 2^Current Study

FTIR of adsorbent

Figure 6 illustrates the FTIR spectra of both raw and treated eggshells. A prominent peak is observed in the raw and modified eggshells at 3411 cm^− 1^ and 3421 cm^− 1^, respectively, corresponding to the hydroxyl groups^72,73^. The absorption peaks at 1456 cm^− 1^ and 1480 cm^− 1^ correspond to the C-O stretch, indicating the existence of carbonate. The absorbance peak at around 712 cm^− 1^ indicates the presence of CaO in the raw eggshells^74^. The peaks between 582 cm^− 1^ and 685 cm^− 1^ signify the existence of the FeO stretching band in the modified eggshells^75,76^.

Fig. 6FTIR of raw and modified eggshells.

Adsorption study

Effect of pH

Figure 7 illustrates the effect of pH on the removal efficiency and adsorption capacity of raw and modified eggshell adsorbents for Acid Red 1 (anionic dye) and Crystal Violet (cationic dye). The adsorption capacity and removal efficiency were significantly influenced by pH. In particular, the removal efficiency and adsorption of CV experienced a slight increase, while the removal of AR1 experienced a significant decrease as the pH increased from 2 to 9.

At low pH, the excess H⁺ ions compete with the cationic groups of CV for adsorption sites, reducing dye uptake. Conversely, at high pH, the abundance of OH⁻ groups on the eggshell surface enhances the adsorption of the cationic dye^77^. This behavior is also in accordance with the acid dissociation constant (pKa = 8.64) of CV, which is primarily in its cationic form in aqueous media below this pH. Therefore, the adsorption process is facilitated by the electrostatic attraction between the positively charged CV molecules and the negatively charged surface at basic pH conditions^78^. The maximum adsorption capacity and removal efficiency for CV are at pH 9 for modified eggshells.

Similarly, the favorable adsorption of Acid Red 1 at acidic pH can be attributed to the electrostatic attraction between the SO₃⁻ groups of the anionic dye and the positively charged eggshell surface at low pH. The dye’s acid dissociation constant (pKa = 6.4) confirms that AR1 mostly persists in its anionic form when the solution pH surpasses this value. Consequently, when the pH rises, the electrostatic repulsion between the negatively charged dye and the progressively negatively charged adsorbent surface diminishes dye adsorption^78^.

Partial dissolution of the eggshell releases HCO₃⁻, CO₃²⁻, and OH⁻ ions, which adsorb onto the eggshell surface, imparting a negative charge^79^. Additionally, alkali and alkaline earth metal ions (Na⁺, K⁺, Ca²⁺, Mg²⁺) can adsorb onto the surface, forming an electrical double layer that give the surface a positive charge^80^. The adsorbed ions form the Stern layer, which influences adsorption behavior.

The addition of Na^+^ ions improves the adsorption process by changing the surface characteristics and increasing the number of accessible sites for crystal violet sorption. The presence of Mg^2+^ can reduce the adsorption capacity due to its effect on the charged surface. Previous research has established the affinity order of earth elements as Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺^81^. At a lower concentration of metal ions, improving the adsorption capacity of AR1, raising the concentration to 0.4 M causes a decrease in the removal efficiency of AR1 due to the interference of metallic ions on the active binding sites of the modified eggshells^82^.

Favorable adsorption of Acid Red 1 at acidic pH is attributed to the electrostatic attraction between the SO₃⁻ groups of the anionic dye and the positively charged eggshell surface at low pH. When pH is below 6, calcium carbonate in the eggshell dissolves into Ca²⁺, further facilitating dye adsorption.

Fig. 7. The influence of pH on removal efficiency and adsorption capacity.

Effect of initial concentration

Figure 8 illustrates the influence of the initial dye concentration on the adsorption capacity and removal efficiency. The adsorption capacity improved by raising the initial dye concentration to 100 mg.L^− 1^. The concentration of dye molecules plays a critical role in the adsorption process^77^. As the initial concentration increases, the number of dye molecules available for adsorption also increases, which enhances the adsorption capacity. The removal efficiency, however, tends to decrease at higher concentrations as a result of the saturation of active sites on the adsorbent surface and the presence of excess dye molecules in the solution^83^. At elevated concentrations, the driving force for mass transfer across the concentration gradient increases, thereby facilitating the transport of dye molecules toward the adsorbent surface. It enhances the loading of the adsorption sites until the equilibrium is reached^84^. Adsorption isotherms are commonly employed to describe and analyze such adsorption behaviour and better understand the interaction between dye molecules and the adsorbent surface^85^.

Fig. 8. The effect of initial dye concentration on adsorption capacity and removal efficiency.

Effect of contact time

Figure 9 demonstrates the effect of contact time on the adsorption capacity and removal efficiency at an initial concentration of 100 mg.L^− 1^. The adsorption equilibrium was attained within 30 min. The adsorption rate was initially elevated owing to the plentiful cationic dye molecules in the solutions and the numerous vacant active sites on the adsorbent surface. As time progressed, particularly after 30 min, the adsorption rate declined as these active sites became progressively occupied. The extent of adsorption was governed by the transfer of dye molecules from the bulk solution to the available active sites. The rapid initial adsorption suggests that the process predominantly occurred on the external surface of the adsorbent, followed by slower diffusion into the internal pores^86^.

Fig. 9. Effect of contact time on the adsorption capacity and removal efficiency.

Effect of temperature

The pH of the solution occurred at 2 for AR1 and 9 for CV, with an initial dye concentration of 100 mg. L^− 1^ with an adsorbent dosage of 0.5 g. L^− 1^. Figure 10 demonstrates that the dye’s removal efficiency and adsorption capacity diminished with rising temperature, attaining their lowest values at 60 °C. This trend signifies that the adsorption process is exothermic. The decline in adsorption capacity at elevated temperatures be ascribed to thermal deactivation or disruption of active sorption sites on the adsorbent surface, along with the increased kinetic energy of dye molecules, which promotes their ability to desorb from the adsorbent surface into the bulk solution^62^.

Fig. 10. The influence of temperature on adsorption capacity and removal efficiency.

Linear regression of adsorption isotherm models

The isotherm results are presented in Figure 11; Table 4. The Freundlich model provided an excellent fit to the experimental data for raw and modified eggshell adsorbents, with correlation coefficients (R²) exceeding 0.99, indicating multilayer physical adsorption^86^. In contrast, the Langmuir model did not adequately fit the raw eggshell data for either dye but showed a high correlation (R² > 0.99) for modified eggshells. It suggests that dye adsorption onto modified eggshells follows physical and chemical adsorption mechanisms^77^.

Fig. 11. Adsorption Isotherm plots; (a) Langmuir, (b) Freundlich, (c) Temkin, and (d) Dub-Rad.

Table 4. Parameters of the adsorption isotherm models.ModelParametersRaw eggshellsModified eggshellsAR1CVAR1CVLangmuirq_m_ (mg/g)59.55273.246160.718142.947k_L_ (L/mg)0.0590.0870.0520.105 R ^2^

0.997

0.992

0.999

0.993 FreundlichK_f_, (mg/g) (mg/L)^(1/n)^7.46911.71611.33816.300n2.1672.3211.5401.627 R ^2^

0.997

0.992

0.999

0.995 TemkinA (L/g)0.4880.7020.5750.914B (J/mol)14.30517.57935.43036.361R^2^0.9890.9680.9500.925Dub–Radq_m_ (mg/g)40.68052.69780.87888.678β (mol^2^/J^2^)0.000000.000000.000000.00000R^2^0.8210.8200.7980.781

Adsorption kinetics

As illustrated in Fig. 12, the Pseudo-first-order (PFO), Pseudo-second-order (PSO), intraparticle diffusion, and Elovich models were employed to assess the adsorption kinetics of the dyes onto the adsorbent. The model parameters and correlation coefficients (R²) are summarized in Table 5. Although all models commonly describe adsorption kinetics, the PSO model provides a better fit to the experimental data for CV adsorption and PFO for AR1 using modified eggshells, as evidenced by its higher R² value, which is closer to unity. The PSO indicates that the adsorption process is more accurately described by chemisorption involving valency forces through the sharing or exchange of electrons. Additionally, the higher initial adsorption rate supports the notion that dye removal predominantly occurred on the external surface of the adsorbent before progressing into internal pores^87^.

Fig. 12. Kinetics adsorption models; (a) Pseudo-first-order, (b) Pseudo-second-order, (c) Elovich, and (d) Intraparticle diffusion.

Table 5. The parameters of the kinetics adsorption model.Kinetics ModelParametersRaw eggshellsModified eggshellsAR1CVAR1CVPseudo First Orderk_1_ (min^− 1^)0.07830.09470.12470.1027q_e_ (mg/g)58.875.2124.2141.2 R ^2^

0.9774

0.9804

0.9645

0.9528 Pseudo Second Orderk_2_ (g. mg^− 1^.min^− 1^)0.0002950.00041370.0002210.000281q_e_ (mg/g)98.398109.291187.026194.069 R ^2^

0.9141

0.9296

0.9253

0.9634 Intraparticle Diffusionk_p_ (mg.g^− 1^.min^− 0.5^)8.68610.93918.4320.363C (mg.g^− 1^)−2.7350.9386−0.54323.655 R ^2^

0.9440

0.9200

0.9203

0.9254 Elovichβ0.04560.03890.02270.0214α6.92610.44816.37620.986 R ^2^

0.9561

0.9471

0.9401

0.9421

Adsorption thermodynamics

Figure 13 illustrates the plotting of 1/T vs. ln K_C_. The change in enthalpy (ΔH) and entropy (ΔS) were derived from the slope and intercept of the linear regression, respectively, exhibiting an adequate coefficient of determination. The Gibbs free energy was estimated using Eq. (11).

Fig. 13. Van’t Hoff linear relationship of the adsorption of CV and AR1 on the raw and modified eggshells.

Table 6 shows that the values of ΔH are negative, indicating that the adsorption process is exothermic. For raw eggshells, the ΔH value is approximately − 21 kJ.mol^− 1^, suggesting a physically controlled adsorption mechanism. In contrast, for modified eggshells, the ΔH values are − 62.7 kJ.mol^− 1^ and − 80.2 kJ.mol^− 1^ for AR1 and CV dyes, respectively, which indicate a chemically controlled adsorption process. Generally, enthalpy changes are associated with chemisorption, ranging from 40 to 120 kJ/mol. The negative ΔS values recorded for all systems indicate a reduction in randomness at the solid-liquid interface during adsorption. Consequently, the adsorption process becomes less effective at elevated temperatures.

Table 6. Thermodynamic parameters.Thermodynamic parametersRaw EggshellsModified EggshellsAR1CVAR1CVEnthalpy (ΔH), kJ.mol^−1^Entropy (ΔS), J.k^−1^−21.9−83−21.7−79−62.7−208−80.2−264

In the temperature range of 20 °C to 60 °C (293 to 333 K), the Gibbs free energy change (ΔG) values for fresh eggshells were positive, indicating that the adsorption process is non-spontaneous and requires external energy input (Fig. 14). In contrast, for modified eggshells, ΔG values were negative at 293 K and 303 K, suggesting that the adsorption is spontaneous at these temperatures and does not require external energy. However, at higher temperatures 40 °C to 60 °C (313 to 333 K), ΔG values became positive, indicating that the adsorption process becomes non-spontaneous. The observed increase in ΔG with rising temperature confirms that the adsorption becomes less favorable as temperature increases.

Fig. 14. Plot of the thermodynamic parameter Gibbs’ free energy (ΔG) at different temperatures.

Adsorption mechanism

Understanding the adsorption mechanism of dyes onto the surface of adsorbents provides valuable insights for improving the efficiency of current and future adsorbent materials. As illustrated in Fig. 15, during the adsorption process, Acid Red 1 (AR1) and Crystal Violet (CV) molecules diffuse from the bulk solution to the active adsorption sites located on the surface and within the pores of the raw and modified eggshells. The modified eggshells possess a porous structure with a relatively high surface area, various functional groups, and surface defects, as characterized earlier by XRD, BET, and zeta potential analyses. These features create numerous active sites that facilitate dye adsorption. For CV, kinetic analysis demonstrated a rapid initial adsorption onto the external surface, followed by a diffusion-controlled process into the internal pores of the eggshell adsorbent. A variety of interaction mechanisms, such as electrostatic attraction, hydrogen bonding, and potential surface coordination, are involved in the adsorption process CO^− 3^, COOH^−^, OH^− 1^, Fe^+ 2^, and Ca^+ 2^, contributing to the efficient uptake of both dyes. AR1 adsorption follows a similar diffusion and surface interaction pathway, albeit with variations in affinity due to its anionic nature compared to the cationic CV.

Fig. 15. Adsorption mechanism.

Desorption and reusability of modified eggshells

The regeneration of the adsorbent is a crucial process for its re-utilization. The regeneration process can reduce the cost of the adsorbent. The results showed that the adsorption capacities of CV and AR1 were 138 and 124 mg.g^− 1^ for the first cycle, and 120 and 95 mg.g^− 1^ for the second cycle, as illustrated in Fig. 16. The adsorption capacity for CV and AR1 declined significantly, with the third cycles measuring 97 mg.g^− 1^ and 73 mg.g^− 1^, respectively, and the fourth cycles measuring 70 mg.g^− 1^ and 51 mg.g^− 1^, respectively.

Fig. 16. Reusability of adsorbent.

Comparison studies

To assess the performance of the modified eggshell adsorbent, the adsorption capacity for Crystal Violet and Acid Red 1 dyes was compared with that of other reported adsorbents, as summarized in Table 7. The findings demonstrate that the modified eggshells have an adsorption capacity similar to that of several other adsorbents, the majority of which were produced using expensive chemical processes and complicated preparation procedures. It demonstrates the promise of modified eggshells as an economical and environmentally friendly substitute for dye removal applications.

Table 7. The adsorption capacity of the adsorbent and previously reported adsorbents.DyeAdsorbentAds. Capacity (mg/g)pHAdsorption IsothermAdsorption KineticsReferenceCrystal Violet Zeolite Synthesized Chitin1428LangmuirPseudo-second order ^88^ Eragrostis Plana Nees768SipsPseudo-second order ^89^ Peanut Husk212Freundlich, D-RPseudo-second order ^90^ Xanthated rice Husk9010LangmuirPseudo-second order ^35^ Modified Avocado Shells1398FreundlichPseudo-second order ^91^ Coconut Husk545LangmuirPseudo-second order ^92^ Charred Rice Husk6210LangmuirPseudo-second order ^35^ Naturel Polysaccharide3210FreundlichPseudo-second order ^93^

Modified Eggshells

138

9 FreundlichPseudo-second order Present study Acid Red 1Modified montmorillonite364.12LiuGeneral ^94^ Grafted clay363.292Langmuir- ^95^ Black tea leaves30.032LangmuirPseudo-second order ^19^ Raw Indonesian kaolin12.86.7LangmuirPseudo-second order ^96^ Oil palm empty fruit bunch activated carbon197.627-- ^97^ Mechanically activated talc1302TemkinPseudo-second order ^78^

Modified Eggshells

124

2 FreundlichPseudo-first order Present study

Cost of modified eggshells adorbent

The adsorption cost was determined according to Eq. (15)^98^.

\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\:Adsorption\:Cost\:\left(\frac{\$}{g}\right)=\frac{Chemical\:Price+\left(Electric\:Price*t*E\right)}{R\left(\%\right)\:*{C}_{o}*V}$$\end{document}

Where R is the percentage of removal efficiency, t is the adsorption time, E is the energy consumption during the experiment, Co is the initial concentration of dyes, and V is the volume of the sample in (L).

The adsorption cost of the modified eggshells was compared to that of commercial activated carbon (AC) in the adsorption of CV^99^ and AR1^100^, as illustrated in Fig. 17. The elevated price of AR1 adsorption by activated carbon is attributed to its removal effectiveness of 17.7%, despite possessing a large surface area characterized by micropores, which inhibit dye adsorption on the adsorbent’s surface. It can be concluded that the modified eggshells have a lower adsorption cost compared to the commercial activated carbon (AC).

Fig. 17. Adsorption cost of modified eggshells and commercial activated carbon.

Conclusion

Modified eggshells have been successfully synthesized by treating them with ferrous sulfate to enhance the adsorption capacity of Crystal Violet and Acid Red 1. The surface area of the modified eggshells is 17.03 m^2^.g^− 1^, which enables high channels for mass transfer for the adsorption process. The maximum adsorption capacity (qmax) for CV and AR1 at a temperature of 20 ℃, contact time of 30 min, and initial concentration of 100 mg.L^− 1^ is 138 mg.g^− 1^ and 124 mg.g^− 1^ respectively. The modified eggshells enhance the porosity and the characteristics of the raw eggshells. The modified eggshells improve the porosity and properties of the raw eggshells. The modified eggshells have enhanced the adsorption capacity of CV and AR1 by approximately 92% and 129%, respectively, compared to the raw eggshells. The adsorption cost of modified eggshells is lower than that of commercial activated carbon for the removal of CV and AR1. The adsorbent’s reusability will improve the process’s cost-effectiveness. The utilization of modified eggshells as an adsorbent effectively eliminates CV and AR1 while concurrently addressing the daily generation of eggshells.