1 Introduction

Pollution of natural water with toxic heavy metals is expanding step by step as a result of industrial development and growth [3]. This incident has become a striking worldwide concern since it poses a health risk to human life [9]. The existence of these toxic heavy metals is noticeable in fluid streams and soil because of social activities [11]. Human exploring produces waste, and these waste products are treated, disposed of, gathered and discarded, which can present hazards to nature and overall well-being [5]. Because of the alarming rate of the issue of heavy metal contamination, the investigation into new and modest strategies for metal expulsion has expanded as of late [8].

Hexavalent chromium (Cr(VI)) and cadmium (Cd(II)) belong to the most toxic of heavy metals and are extremely hazardous to human health because its prolonged exposure can cause endocrine disorders, carcinogenesis, renal dysfunction, mutagenesis and bone fracture [10, 29]. Chromium is aboundantly used in electroplating metallurgy, and leather tanning, other industries, leading to the release of aqueous chromium (in stable forms trivalent chromium (Cr(III)) and Cr(VI)) to the environment [29]. Hexavalent chromium seldom occur naturally, quite soluble in aqueous systems and often find its way to the groundwater [24]. The major source of cadmium pollution includes silver-cadmium, plastics, electroplating, paint pigments, and smelter operations and nickel–cadmium battery industry [14]. The release of wastewater containing Cd(II) pollutes both the soil and water bodies.

Techniques like precipitation buoyancy, filtration, sedimentation, particle exchange, dissolvable removal, electrolytic methods, layer processing, natural method, and substance reaction have been explored [2, 32, 33]. Each approach applies its advantages and flaws. Therefore, it is essential to search for prudent and convincing methods for separation of substantial metals from aqueous solution [34]. Remediation procedures must be intended for high or great removal of impurities while downplaying costs [4]. Since most common approaches are neither practical nor conservative, a better separation method is needed for removal of heavy metal at low-cost. Bio deportation may contribute to the achievement organic expulsion can help to achieve this objective [15].

One of the approaches used for removal of contaminants from wastewater equivalent to heavy metals is adsorption [39]. Adsorption is a physicochemical technique extensively applied for removal of heavy metal since it does not require elevated operation temperature and it can removed several materials all at once [29]. Metals were expelled by adsorption on multiple products, e.g. activated coal, agricultural waste, green vegetation, minerals, etc. [42]. The adsorbent should be accessible in massive amounts, abundant in nature, readily reusability and cost-effective. Prominent of the adsorbent applied earlier is activated carbon, which exhibits remarkable performance for heavy metal removal, but activated carbon is not easy to regenerate, and expensive to maintain, leading to its narrow application [29].

Agricultural goods may be used as heavy metal sorbents particular to individual metal particles. The rural side effects of metal particle adsorption may include metal collaboration or coordination with the introduction of helpful collections in protein, lipids and sugars on cell dividers [1]. The ever-increasing global consumption of hen eggs is leading to a proliferation of large quantity of eggshells. Disposal of eggshell is a major challenge in the poultry and bakery units of the food industry. In the search for versatile, readily available, non-toxic, and cost-effective resources as potential adsorbent to remove hazardous chemicals from water, eggshells have emerged as a suitable candidate [30]. Studies have shown that eggshell is promising for removal of heavy metals, organic dyes, Polycyclic aromatic hydrocarbons (PAHs), pharmaceutics (morphine) [22, 30]. Substantially toxic heavy metals like arsenic, manganese, lead, copper, cadmium, chromium, and mercury could be removed using eggshells. Eggshell contains 94% calcium carbonate, 1% calcium phosphate, 1% magnesium carbonate, and 4% organic matter by weight [6]. Eggshells are produced mainly as a by-product of the poultry industry, making it recommendable as adsorbents. Several researchers have used eggshells to remove heavy metals [7, 17]. Elabbas et al. [17] explored the use of crushed hen eggshells for removal of Cr(VI) ions from wastewater. They revealed that eggshells had a fairly high removal capacity in comparison with other adsorbents. Baláž et al. [10] investigated Cd(II) removal capacity of crushed eggshell in wastewater in a batch adsorption system. Park et al. [38] investigated the comparative removal of Cr, and Cd from electroplating wastewaters but did not explore the adsorption kinetic and equilibrium.

This work aims to explore the comparative adsorptive performance of eggshell for Cr(VI) and (Cd(II) ions, including the adsorption kinetic and equilibrium, for the first time. This venture will unravel the adsorption equilibrium using Langmuir and Freundlich isotherms. The adsorption kinetic will also be explored using pseudo-first-order and pseudo-second-order, and the influence of pH, contact time, initial adsorbate loading, and adsorbent dose.

2 Materials and methodology

2.1 Materials

All chemical reagents used for this work were of analytical grade, high purity, and utilized with no further refinement. All the experiments were conducted with deionized water prepared in our laboratory at room temperature. Hydrochloric acid, sodium hydroxide, cadmium nitrate (AR grade) and potassium dichromate (AR grade) were bought from Sinopharm Chemical Reagent Co., Ltd. Chicken eggshells were obtained from the Covenant University cafeteria in Ogun state, Nigeria.

2.2 Adsorbent preparation

The chicken eggs were washed a couple of times with deionized water and boiled for 10 min to remove contaminated particles from it. For 3 h, the eggshells were oven-dried at 150 °C and left to cool to 25 °C (ambient temperature). The dried samples were pulverized to 80–210 μm mesh size using a jaw crusher manufactured by Denver Product, Dagenham, U.K. The powdered eggshell was kept in an vacuum container for further use at room temperature.

2.3 Characterization of adsorbent

For the FT-IR analysis, adsorbent infrared spectra were obtained using Bruker Tensor 27 FT-IR at 4 cm−1 resolution in the range of 700–4000 cm−1. The sample was intimately mixed with KBr. SEM images were generated using SEM, FEI Quanta 400 FE-SEM microscope, with an accelerator current of 20 mA and voltage of 20 kV. To excite the distinctive X-ray of the components stored in the sample, EDS analysis was conducted utilizing a concentrated electron beam operating on a tiny region of the sampled specimen.

2.4 Batch adsorption studies

The heavy metal removal experiments were carried out using batch adsorption. 5 g of the adsorbent was dispersed in 200 mL a solution containing 50 mg/L of hexavalent Cr(VI)/Cd(II) obtained by dissolving cadmium nitrate or potassium dichromate in distilled water. The aqueous solution was stirred (at 150 rpm) in a water bath shaker at ambient temperature. Afterward, the mixture was filtered, and the residual Cd(II) or Cr(VI) ions in the filtrate were observed using the Atomic Absorption Spectrometer (AAS). The quantity of Cd(II) or Cr(VI) removed at equilibrium (qe) was computed using the expression below:

$$q = \frac{{\left( {C_{0} - C_{e} } \right)V}}{W}$$
(1)

The removal efficiency was obtained as:

$$\user2{\% }removal = \frac{{\left( {C_{0} - C_{e} } \right)}}{{C_{0} }} \times 100$$
(2)

where q represents the quantity of metal ion removed per gram of adsorbent, C0 (mg/L) is the initial amount of metal ion loaded, Ce (mg/L) is the amount of metal ion at equilibrium, W (g) is the mass of adsorbent, and V (L) is the volume of the aqueous solution containing metal ions.

The percentage removal was observed at varying contact time for each adsorbate to obtain the equilibrium time. The experiment was repeated using different loading of the metal ion in the solutions (50–200 mg/L). The effect of pH (with the aid of 0.1 M NaOH and 0.1 M HCl solutions), dosage of adsorbent (5–25 g), contact time (30–150 min) and initial adsorbate loading were investigated.

2.5 Adsorption equilibrium

2.5.1 Langmuir isotherm model

This model has a fundamental hypothesis that at a site on the surface of the sorbent with uniform energy level, dissemination of adsorption occurs. If the adsorbent fills all the vacant sites, there will be no room for further adsorption [1]. If this is the case, it means that the adsorption model is monolayer in nature. The following equation represents the linear shape of the Langmuir isotherm design cab:

$$\frac{{C_{e} }}{{q_{e} }} = \frac{1}{{K_{L} }} + \frac{{a_{L} }}{{K_{L} }} \left( {C_{e} } \right)$$
(3)

where Ce is the balance temperature (mg/L), qe is the quantity of metal ion removed by the designated quantity of adsorbent (mg/g). aL (L/mg) and KL (L/g) are the constants of Langmuir isotherm. The linear charts were used to estimate the value of KL from the curve and the value of aL from the path, the linear charts acquired were used.

2.5.2 Freundlich isotherm model

This model appears to be a direct reverse of the Langmuir model, this model with a basic theory that adsorption happens on non-uniform power level dissemination heterogeneous locations. It is not of the monolayer type, unlike the Langmuir template [1].

With this equation, the Freundlich isotherm model's linear shape can be depicted:

$$\ln q_{e} = \ln K_{F} + \frac{1}{n}\ln C_{e}$$
(4)

where KF is the heterogeneity variable representing the bond distribution. As straight rows, a chart of ln qe versus ln Ce was acquired. Constants KF and 1/n were calculated from the curve and curve.

2.6 Adsorption kinetics

Adsorption rate estimation provides essential data for batch adsorption devices to be created. For choosing best operating parameters for large-scale batch methods, figures acquired on the kinetics of the solution uptake are essential [1]. Pseudo-first-order and pseudo-second-order kinetic equations used in the study were employed to investigate the kinetics of heavy metals adsorption and obtain the amount of absorbed metal ions. The Lagergren pseudo-first-order model is mainly written as:

$$\ln (q_{e} - q_{t} ) = \ln q_{e} + k_{1} t$$
(5)

where qt and qe represent the timing, and equilibrium capacities for adsorption (mg/g) and k1 is the pseudo-first-order adsorption constant (1/min). At distinct levels, plots of ln(qe − qt) against t are displayed. It is then possible to obtain k1 and qe from the path and intercept.

For pseudo-second-order, the model is as follow:

$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} qe^{2} }} + \frac{t}{{q_{e} }}$$
(6)

From a graph of t/qt against t, linear charts are achieved from which K2 and qe could be computed using the curve and intercept.

3 Results and discussion

3.1 Characterization

The surface morphology of the selected adsorbents as obtained using SEM is shown in Fig. 1. The figure highlights the SEM images obtained for the Eggshell samples before and after the removal of Cr(VI) and Cd(II). The SEM analysis also shows that the treatment of Cr(VI) and Cd(II) solutions affected the matrix of the eggshell structure. The SEM image of the eggshell sample before heavy metal adsorption shows non-adhesive appearance and agglomerates formation. Meanwhile, after adsorption, the samples show regular, adhesive appearance, could further remove Cd(II) and Cr(VI). However, there seems to be more adhesive appearance on the sample that adsorbed Cd(II) than that of Cr(VI) (Fig. 2).

Fig. 1
figure 1

SEM image of a chicken eggshell before adsorption, b chicken eggshell after Cd(II) adsorption, c chicken eggshell after Cr(VI) adsorption

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Fig. 2
figure 2

EDS elemental maps of a chicken eggshell before adsorption, b chicken eggshell after Cd(II) adsorption, c chicken eggshell after Cr(VI) adsorption

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The FT-IR spectrum (Fig. 3) of unloaded eggshell shows several distinct absorption bands showing the complicated nature of eggshells. The peaks in between 3634.01 cm−1 and 2337.8 cm−1 are traceable to the existence of hydroxyl functional group, and at 1805 cm−1, a strong C=O group. The peaks at 1635.69 and 1411.94 cm−1 represent the presence of medium C=C and C = O stretching vibration [1]. This result comforms with the result of [7] that reported the presence of carbonyl group around 1424 cm−1 wavelength. The bands at 871.87 cm−1 due to C=C, 771.55 cm−1 and 702.11 cm−1 due to C–H are typical of the in-plane and out-plane deformation modes in the presence of calcium carbonate [20]. This shows that eggshell mainly contains calcite.

Fig. 3
figure 3

FTIR spectra of before and after removal of Cr(VI) and Cd(II)

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Figure 3 also presents the FT-IR spectra of the eggshell samples after taken up Cd(II) and Cr(VI) to ascertain the functional groups that are accountable for metal ion removal. For the sample loaded with Cd (II), the spectra at 3749.74 cm−1 broaden with reduced intensity and shifted to lower wavenumber (3626.29 cm−1) after Cd (II) adsorption. For the sample loaded with Cr(VI) with that of the unloaded, it was observed that the band at 3749.74 cm−1 broaden and its intensity declined, and the band shift to lower wavenumber (3649.44 cm−1) after adsorption of Cr(VI). The observations reveal that the bands of OH groups drifted to a lower frequency. The bands of other functional groups also changed to a lower frequency. The results indicate that Cr(VI) and Cd(II) ions could have been absorbed by H and O atoms of O–H and R–COOH bonds that altered the frequency of the peaks. The enhancement in the peak found around 1411 cm−1 could indicate Cd–O and Cr–O stretching for the Cr(VI) and Cd(II) absorbed eggshell, respectively. The shifts are traceable to the variations in the counter ions related to carboxylate, R–COOH group of amino acids and OH.

3.2 Adsorption experiment

3.2.1 Effect of contact time

The outcomes presented in Fig. 4 show that the removal rate of adsorbates was swift at the initially (first 30 min at pH 4). Upon increasing the contact time to 60 min, there is no substantial change in adsorption capacity for adsorbates. The rapid increase in the adsorption rate within the first 30 min could be attributed to the presence of large number of vacant sites, which are occupied with time. Increasing the contact time even up to 150 min could not engender a meaningful enhancement in the adsorption capacity. Nevertheless, the increase in the removal capacity of Cr(VI) is higher than that of Cd(II). For Cd(II) equilibrium adsorption was attained at 90 min, while the equilibrium was attained at 120 min for Cr(VI). At equilibrium time, there is decline in the amount of active sites present and the driving force for adsorption, leading to insignificant or no more adsorption [48]. The quantity of accessible locales turns out to be less, and the adsorbent moves toward becoming swarmed inside the pores, which restricts the movement of the adsorbate [1].

Fig. 4
figure 4

Effect of contact time on the adsorption of 100 ml a Cd(II) and b Cr(VI) ions using 5 g of eggshell

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3.2.2 Effect of adsorbent dose

Figure 5 gives the influence of the dose of adsorbent on the efficiency of adsorption of Cr(VI) and Cd(II). The heavy metal ion removal efficiency was computed by taking the difference in the residual and initial concentrations. The adsorption of Cd(II) increases from 44.02 to 71.36%, and that of Cr(VI) was increased from 44.80 to 60.96% when the dose of adsorbent was varied from 5 to 25 g for every 100 mg/L of solution. This shows that the increase in adsorbent dosage significantly favors Cd(II) than Cr(VI).

Fig. 5
figure 5

Influence of adsorbent dose on the removal of 100 ml a Cd(II) and b Cr(VI) ions using 5 g of eggshell

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This tendency is likely because of the direct relationship between the adsorbent mass and the number of adsorbent particles. This observation reveals that more adsorption sites and more surface areas are available for heavy metal contact. This also implies that at higher adsorbent to solution volume ratios, superficial adsorption unto the adsorbent (eggshell) surface is swift [13].

3.2.3 Effect of pH

Figure 6 illustrates the influence of pH on the adsorption capacity of eggshell for Cr(VI) and Cd(II). Increasing the pH from 2 to 6 led to a rapid increase in the capacity for adsorption. A further rise in the value of pH declines the adsorbent performance. The influence pH can be described by an ion-exchange adsorption technique in which the significance of this work is performed through carbonate groups with cation-exchange characteristics. At low pH values, heavy metal removal was restricted with a strong dominance of hydrogen ions. This trend is possible because of the difficulty between metal ions and hydrogen on the adsorption site that limits the attraction of the metals. The increase in pH exposed the presence of carbonate groups in the eggshells to develop the negative charges on the surface of the eggshell particle, attract the cadmium cations and allow the adsorbate adsorption on the adsorbent surface. However, when the equilibrium pH is reached at 6, chromium was transformed to chromate anion due to the displacement reaction with NaOH to generate aqueous NaCl. The competition of chromate and hydroxyl anion on the adsorbent surface which engenders NaCl formation in the medium declined the adsorption of chromium on the eggshell at a pH higher than 6 [23, 29]. The values of the adsorption capacity obtained in this study are quite competitive compared to those of other natural adsorbent reported previously (Table 1).

Fig. 6
figure 6

Effect of pH; on the adsorption of 100 ml a Cd(II) and b Cr(VI) ions using 5 g of eggshell

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Table 1 Comparison of maximum adsorption capacities of Eggshell with other adsorbent
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3.2.4 Effect of initial metal loading

Figure 7 gives the influence of the initial loading of Cd(II) and Cr(VI) on the removal efficiency of eggshell. The removal of Cd(II) declined from 85 to 65%, while that of Cr(VI) reduced from 88.4 to 64.3% when the initial loading was changed from 50 to 200 mg/L. This observation reveals that adsorption efficiency reduces with increasing initial loading. The reduction in adsorption efficiency could be ascribed to the absence of adequate active site to oblige significantly more metal reachable in the solution [1].

Fig. 7
figure 7

Effect of initial metal concentration on the adsorption of 100 ml a Cd(II) and b Cr(VI) ions using 5 g of eggshell

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3.3 Adsorption equilibrium

Figure 8 presents the Langmuir and Freundlich isotherms for the adsorption of Cd(II) and Cr(VI) ion at a pH of 6. The figures give the equilibrium relations between the amount of metal ions on the liquid and solid phase. The isotherms show the information regarding the maximum adsorption capacity of the adsorbent for each adsorbate [1]. The values of KL and qm are estimated from the graph of Ce/qe versus Ce. (Fig. 7a, b), and the results are presented in Tables 2 and 3.

Fig. 8
figure 8

Langmuir model plot for the adsorption of a Cd(II) and b Cr(VI) ion, and Freundlich model plot for the adsorption of c Cd(II) and d Cr(VI) ion using eggshell

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Table 2 Freundlich and Langmuir Constants for adsorption of Cd(II)
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Table 3 Freundlich and Langmuir Constants for adsorption of Cr(VI)
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Langmuir isotherm exhibits essential features obtained from dimensionless constant known as separation factor, or equilibrium parameter (RL), indicating the adsorption nature [1].

$$R_{L} = \frac{1}{{1 + k_{L} C_{0} }}$$
(7)

The adsorption is favorable when RL is in the range of 0 to 1. The value of RL more consistent in determining the nature of adsorption. The adsorption is satisfactory if 0 < RL < 1), unsatisfactory if RL > 1, irreversible if RL = 0 or linear if RL = 1 [1]. The RL values for Cr(VI) and Cd(II) displayed in Table 4 reveal that the removal of the metal ions onto eggshell powder is favourable. Furthermore, the R2 values, which reveal the consistency of the projected outcomes with the observations show that the Freundlich model is more appropriate to describe of the removal of Cr(VI) and Cd(II). This result is in agreement with the report of [18, 47] which suggest that Freundlich model is the best for the description of the removal process of Cd(II) and Cr(VI).

Table 4 Separation factor, RL values for the Cd(II)and Cr(VI) adsorption
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For the Freundlich isotherm in Fig. 6c, d, the values of KF and n attained by removing the metal ions by eggshell can be dobtained (Table 3) from the intercept and slope of ln Ce vs. ln qe. Since the R2 values for Cd(II) and Cr(VI) ions isotherms are higher than 0.9, the model can satisfactorily describe the connection between the amount of Cr(VI) and Cd(II) ions removed by the eggshell and the concentration at equilibrium in wastewater. The adsorption of Cd(II) and Cr(VI) with eggshell is favorable since the values of n for adsorbate is in the range of 1–10 [1].

The equilibrium observations for Cd(II) reveals that the Freundlich and Langmuir model satisfactorily define the adsorption process, but Freundlich model performs better. This observation agrees with the report of [46]. Meanwhile, the Freundlich isotherm model could satisfactorily describe the adsorption of Cr(VI). This observation does not agree with the reports of [10, 14, 31, 37] which claim that the equilibrium adsorption isotherm for adsorption of Cd(II) and Cr(VI) follows Langmuir model rather than Freundlich model.

3.4 Adsorption kinetics

Tables 5 and 6 presents the value of k1 and qt determined from the graph of ln t/qt versus t. The suitability of the kinetic model was validated by estimating the R2 values (Fig. 9). The estimated qe and experimental qe are close in pseudo-first-order and pseudo-second-order kinetic model for Cd(II). Meanwhile, for Cr(VI), the values are closer for the pseudo-second-order kinetic model than pseudo-first-order model at all the initial concentrations, but the pseudo-second-order kinetic model shows a better correlation (R2 > 0.99). This observation is because the R2 values for the pseudo-second-order model (R2 > 0.99) were superior to the R2 values of the pseudo-first-order equation for Cr(VI) ions. These findings suggest that the pseudo-second-order shows a more remarkable correlation in comparison with pseudo-first-order model for Cd(II) and Cr(VI). This report agrees with those of [37, 44, 46, 47], which suggest that the pseudo-second-order kinetic expression is the best for the description of the adsorption process of Cd(II) and Cr(VI).

Table 5 Kinetic parameters for removal of Cd(II) using eggshell
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Table 6 Kinetic parameters for removal of Cr(VI) using eggshell
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Fig. 9
figure 9

a pseudo-first order and b pseudo-second order for Cd(II), c pseudo-first order and d pseudo-second order for Cr(VI)

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To study the diffusion mechanism and the probable rate-controlling step, intraparticle diffusion model was explored. Previous studies revealed that heavy metal adsorption could follow three possible steps, including adsorption, intraparticle diffusion, and external diffusion [16]. The steps could control the adsorption kinetics individually or altogether [36]. External diffusion resistance is negligible if the batch system is well-agitated. Therefore, adsorption and intraparticle diffusion could be the rate-controlling step. The possibility can be studied using Weber-Morris model (intraparticle diffusion model) by plotting of qt versus t1/2, according to the following expression [48]:

$$q_{t} = K_{id} t^{1/2} + C$$
(8)

where Kid represents the rate constant for intraparticle diffusion (mg/g min1/2), and C represents the intercept are the slope and the intercept of the plot of q t vs. t 1/2 (Fig. 10). Using this model, intraparticle diffusion is the rate-controlling step if the plot of q t versus t 1/2 is linear and pass through the origin [48]. Figure 10 and Table 7 reveals that the curves have linear characteristics but the linear plots did not pass through the origin. Therefore, the rate-controlling step is not limited to intraparticle diffusion. The process mechanism is predominant adsorption controlled.

Fig. 10
figure 10

Intraparticle diffusion plot on a Cd(II) and b Cr(VI) adsorption by Eggshell

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Table 7 Intraparticle diffusion constants at different initial concentrations of Cd(II) and Cr(VI)
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4 Conclusion

The potential utilization of eggshell as a adsorbent for the adsorption of Cr(VI) and Cd(II) from wastewater was studied, considering various adsorbate concentration, adsorbent dose and pH. Eggshell exhibits carbonyl, carboxylic, hydroxyl functional groups, which plays a substantial role in the adsorption of Cd(II) and Cr(VI). The removal of Cd(II) and Cr(VI) improved with an increased initial metal presence from 50 to 200 mg/L. The contact time also plays a vital role in the removal of Cd(II) and Cr(VI). Increasing the adsorbent dose from 5 to 25 g leads to an increase in percentage adsorption efficiency of eggshell and increases in the equilibrium adsorption capacity. The best adsorption was attained at the pH value of 6 for Cd(II) and Cr(VI).

The equilibrium observations reveal that Langmuir and Freundlich model could reasonably model the adsorption of Cd(II). However, the Freundlich model is better, and only the Freundlich model can adequately model the adsorption of Cr(VI). The best monolayer adsorption capacity of Cd(II) is 10.967 mg/g and 11.789 mg/g for Cr(VI) at 200 mg/L, at room temperature. This observation signifies that the adsorption process is adequate, and the observations agree with previous reports on removal of toxic heavy metal ions.

In view of the value of R2 and variance between the predicted and the experimental values of qe. The best model for removal of Cr(VI) and Cd(II) from wastewater is pseudo-second-order kinetic. Hence, the eggshell powder is a promising sorbent for adsorption of Cd(II) and Cr(VI) from wastewater.