Evaluation of Raw and Calcined Eggshell for Removal of Cd²⁺ from Aqueous Solution

The obtained hen eggshells were pulverized using a grinder (JL-1000, Hibell, WhaSung, Korea) after peeling off completely eggshell membrane. And then, a portion of eggshell powder was calcined at 900 ℃ for 2 h in a rotary kiln. The resulting powders were sieved through sieves ranging in mesh size from 100 to 200 and dried in an oven at 60 ℃ for 3 days. An analytical grade reagent Cd(NO₃)₂·4H₂O was supplied by Daejung Chemicals & Metals Co., LTD, Siheung City, Korea. A stock solution containing 1000 mg L^-1 of Cd²⁺ was made by diluting the above-mentioned reagent with distilled water. Subsequently, the desired Cd²⁺ concentrations were prepared from a stock solution by stepwise dilutions.

The image of samples was observed by Scanning Electron Microscope (SEM, VEGA II LMU, Tescan, Brno, Czech). Same samples were analyzed using Field Emission Scanning Electron Microscope (FE-SEM, MIRA 3, Tescan, Brno, Czech) coupled with Energy Dispersive X-ray (EDX, X-Max^N, Oxford, UK) to investigate the component elements as well as the weight percentage corresponding to each element. The observation on crystalline composition of samples was carried out using X-ray Diffractometer (XRD, Ultima IV, Rigaku, Tokyo, Japan). The thermal decomposition according to the temperature variation of samples was performed by Thermogravimetry and Differential Thermal Analysis (TG/DTA, TG-1280, Rigaku, Tokyo, Japan). The measured values of specific surface area, pore volume and pore size of samples were performed on a specific surface area analyzer (SSAA, 3 Flex, Micrometrics, Atlanta, GA, USA). The specific surface area was acquired using BET (Brunauer, Emmett and Teller) equation. The volume and size of pore were determined by BJH (Barret, Joyner and Halenda) method. A pH meter (Radiometer, PHM 250 ion analyser, Woonsocket, USA) combined with a glass electrode was employed for the pH determination of samples (the ratio of sample and distilled water 1 : 2.5, agitation for 10 min).

All the adsorption experiments were conducted by using a batch mode. For adsorption kinetic experiments, a certain amount (1.0~10.0 g) of samples was added into 500 mL of solutions containing 50 mg L^-1 of Cd²⁺, respectively. And then, the Cd²⁺ solutions were shaken at 150 rpm and at room temperature. An aliquot of solutions was removed at fixed time intervals of 0.5, 1, 2, 4 and 8 h. Afterwards each was centrifuged at 3000 rpm for 30 min and filtered through a 0.47 μm membrane filter paper. In the adsorption isotherm experiments, a 2.5 g of samples was mixed with the 500 mL of Cd²⁺ solutions ranging of 25 to 200 mg L^-1. Following the adsorption, the rest of the process followed the same experimental procedures as the kinetic study. The concentrations of Cd²⁺ in solutions were analyzed with an inductively coupled plasma-atomic emission spectrometry (ICP-AES, Flame Modula S, Spectro, Kleve, Germany). The adsorption amount of Cd²⁺ was determined by the difference in initial and final concentration of Cd²⁺.

SEM images of raw and calcined eggshell are presented in Fig. 1(a) and 1(b), respectively. As seen in Fig. 1(a), the surface morphology of raw eggshell is shaped with the aggregated cubic-like calcite crystals. By contrast, calcined eggshell has size different irregular forms with the presence of some pores (Fig. 1(b)). EDX spectra representing elemental constituents of raw and calcined eggshell were given in Fig. 2(a) and 2(b). The spectrum of Fig. 2(a) showed that raw eggshell consists mainly of C, O and Ca as major elements along with other elements such as P, S, Na, Mg, K and Cl as minor constituents. The peaks of calcined eggshell shown in Fig. 2(b) are well matched with those of raw eggshell, except difference in peak height of C and Ca. Table 1 represents the weigh percentages for the elemental composition of raw and calcined eggshell as quantified by EDX. It was revealed that the content of C in calcined eggshell is very lower level compared to that of raw eggshell.

Figure1.

SEM image of (a) raw and (b) calcined egg shell.

Figure2.

EDX spectra of (a) raw and (b) calcined egg shell.

Fig. 3(a) and 3(b) show the TG/DTA profiles of raw and calcined egg shell according to the temperature increase. As seen in Fig. 3(a), almost all moisture (1.42%) and volatile organics (5.14%) were removed in temperature between 25 and 600 ℃. Subsequently, when temperature arrives at around 800℃, the thermal decomposition of ESP proceeded further by means of the evolvement of CO₂. It is considered that the weight loss (40.85%) of ESP is due to the transformation of CaCO₃ to CaO. The TG/DTA curve of Fig. 3(a) was parallel with X-axis since 800 ℃. It suggests that CaCO₃ consisting of ESP is almost completely converted to CaO as shown in Fig. 3(b).

Figure3.

TGA profile of (a) raw and (b) calcined egg shell after heating at 900 ℃ for 2 h.

The XRD patterns of raw and calcined egg shell are shown in Fig. 4(a) and 4(b). The obtained XRD patterns were also compared to those of the typical CaCO₃ (calcite) and CaO (lime). All peaks of raw and calcined egg shell are perfectly matched with the peaks corresponding to the phase of typical calcite and lime, respectively. The main peaks of ESP and CESP appeared at 2θ = 29.5 and 37.4, respectively. The sharp and high peaks represent that the raw and calcined egg shell are good crystalline solids. From the given XRD patterns, it was revealed that, with the increase in temperature, CaCO₃ transforms gradually to CaO by evolving CO₂. Consequently, the composition of calcined egg shell at and above 900 ℃ mainly consisted of lime (CaO) as the active ingredient.

Figure4.

XRD patterns of (a) raw and (b) calcined egg shell after heating at 900 ℃ for 2 h.

The textural characteristics of raw and calcined egg shell are summarized in Table 2. Raw and calcined eggshell were nearly the same in the BET surface area. Whereas, the total pore volume and average pore diameter of raw eggshell were 4 times higher than those of calcined eggshell. The measured pH values of raw and calcined egg shell were 5.81 and 12.55, respectively.

Fig. 5(a) and 5(b) represent the concentration of Cd²⁺ remained in aqueous solution after adsorbing onto each 2.5 g of raw and calcined eggshell for initial Cd²⁺ concentration in the range of 25~200 mg L^-1 with variation of contact time. With the passing of contact time, the adsorption trend of Cd²⁺ by raw and calcined eggshell by occurred differently. As for raw eggshell, the adsorption rate of Cd²⁺ was quite slow and complete removal was not achieved even after 8 h. On the contrary, the adsorption rate of Cd²⁺ onto calcined eggshell was occurred remarkably fast from the early beginning of time (0.5 h) regardless of the initial concentrations of Cd²⁺. It suggests that calcined eggshell shows a higher affinity for Cd²⁺ than ESP.

Figure5.

Adsorption trend of Cd²⁺ from aqueous solution by (a) raw and (b) calcined eggshell for initial Cd²⁺ concentration in the range of 25~200 mg L^-1 with variation of contact time (0.5~ 8 h).

Removal efficiencies (%) of Cd²⁺ by raw and calcined eggshell at 8 h of contact time are shown in Fig. 6. Removal efficiencies of Cd²⁺ by raw eggshell were decreased gradually from 96.72% to 22.89% by increasing the initial Cd²⁺ concentration. In contrast, calcined eggshell showed removal efficiencies higher than 99% irrespective of initial Cd²⁺ concentration. As listed in Table 2, the pH values of raw and calcined eggshell were recorded as 5.81 and 12.55, respectively. Their pH values were actually unchanged during adsorption process. It may be explained that Cd²⁺ can be completely removed from solution in the basic pH region around 12. In conclusion, it is thought that the removal of Cd²⁺ is more effectual by calcined eggshell with a high alkaline environment than raw eggshell.

Figure6.

Removal efficiency (%) of Cd²⁺ from aqueous solution by raw and calcined eggshell at 8 h of contact time.

The adsorption trend of Cd²⁺ with variation of raw and calcined eggshell dosage was displayed in Fig. 7(a) and Fig. 7(b). Adsorption experiments were performed with Cd²⁺ concentration of 50 mg L^-1 by varying the dosage from 1.0 to 10.0 g. From the Fig. 7(a), it was observed that adsorption rate was much slow at an addition of 1.0 g of ESP. Thereafter, the adsorption rate of Cd²⁺ proceeded a little more quickly until the dosage of raw eggshell increased up to 10.0 g. On the other hand, adsorption rate of Cd²⁺ by calcined eggshell behaved differently as compared with raw eggshell. The rapid adsorption of Cd²⁺ was observed at an addition of 1 g of calcined eggshell dosage. Overall Cd²⁺ was more favorably adsorbed onto calcined eggshell than to raw eggshell. As it is observed, an optimum dosage of raw and calcined eggshell for almost entire removal of Cd²⁺ from aqueous solution is approximately 5.0 g and 1.0 g, respectively, at 8 h run.

Figure7.

Adsorption trend of Cd²⁺ from aqueous solution by dosage (1~10 g) of (a) raw and (b) calcined eggshell with variation of contact time (0.5~8 h).

Removal efficiencies (%) of Cd²⁺ according to the changes of raw and calcined eggshell dosage (1~10 g) at 8 h of contact time are shown in Fig. 8. In case of calcined eggshel, removal efficiencies of Cd²⁺ were over 99% even at relatively low dosage. By contrast, raw eggshell was required the further additional dosage, causing an increase in removal of Cd²⁺ from 34.6% to 99.48%. It indicates that removal of Cd²⁺ is more effective in calcined eggshell.

Figure8.

Removal efficiency (%) of Cd²⁺ from aqueous solution by dosage (1~10 g) of raw and calcined eggshell at 8 h of contact time.

The pseudo-first-order (Eq. 1) and pseudo-second-order (Eq. 2) kinetic models² were fitted to the experimental adsorption data. They can be expressed as the linearized forms, respectively, as shown below:

Where q_e (mg g^-1) and q_t (mg g^-1) are the amount of adsorbed Cd²⁺ per gram of raw and calcined eggshell at equilibrium and a given time t, respectively and k₁ (min^-1) and k₂ (g mg^-1 min^-1) are the pseudo-first-order and the pseudo-second-order rate constants. The values of log(q_e-q_t) and k₁ and t/q_t and k₂ are determined from the slope and intercept.

The correlation coefficient R² acquired from experimental adsorption data is widely used to validate the better kinetics model. Typically, kinetic model with relatively higher R² value is more acceptable. Besides, the experimental value (q_{e, exp}) and calculated value (q_{e, cal}) obtained from the fitted kinetic plot are also assessed to adopt better kinetics model. Plots of the pseudo-first-order and pseudo-secondorder kinetics model obtained from experimental data were provided in Fig. 9 and 10.

Figure9.

The pseudo-first-order plots for adsorption of Cd²⁺ onto raw and calcined eggshell.

Figure10.

The pseudo-second-order plots for adsorption of Cd²⁺ onto raw and calcined eggshell.

It was found that, a better fit, with regard to R², was displayed for the pseudo-second-order kinetic model, where all the R² values were very close to unity. Table 3 shows the fit of adsorption data to pseudo-first-order and pseudosecond- order kinetic models. In addition to R², the calculated value of q_e,cal for the pseudo-second-order kinetics model corresponded roughly with the actual value of q_e,exp. From these results, it indicates that the adsorption processes of Cd²⁺ on raw and calcined eggshell follows more the pseudo-second-order kinetics than the pseudo-first-order kinetics at initial Cd²⁺ concentrations of 50 mg L^-1.

The adsorption data of Cd²⁺ onto raw and calcined eggshell were applied to Freundlich and Langmuir isotherm models. The Freundlich isotherm model assumes the multilayer adsorption of adsorbate onto the surfaces with a highly heterogeneous nature.¹⁵ The Freundlich isotherm equation is expressed as the following log function form:

Where, K_F (mg g^-1) and n are associated with the maximum adsorption capacity of the adsorbent and an intensity of affinity between the adsorbate and adsorbent, respectively. The K_F and n values are determined from the intercept and slope of the linear plot of log q_e vs log C_e, respectively. Ce (mg L^-1) is the concentration of adsorbate in solution at equilibrium.

The Langmuir isotherm model is regarded as the mono-layer adsorption occurring on a homogeneous surface without the interaction between the adsorbed adsorbates. The linear form of Langmuir isotherm equation is given as:

Where, Q (mg g^-1) and K_L (L mg^-1) are the Langmuir isotherm constants related to the maximum adsorption capacity of adsorbent and an intensity of affinity between the adsorbate and adsorbent, respectively. The Q and K_L values are determined from the intercepts and slope of the linear plot of C_e/q_e vs C_e, respectively.

The Freundlich and Langmuir isotherms of Cd²⁺ for raw and calcined eggshell are depicted in Fig. 11 and 12. The correlation coefficient R² obtained from experimental data is commonly used to determine the best fitting isotherm. Compared to Freundlich isotherm, the adsorption result of raw eggshell was better represented by the Langmuir isotherm on account of the higher R² value over the Cd²⁺ concentration range studied. On the other hand, it was shown that adsorption pattern of Cd²⁺ on calcined eggshell followed the Freundlich isotherm well. Isotherm constants (K_F, n, Q, K_L) produced from the linear plots of Freundlich and Langmuir isotherms were listed in Table 4. The values of Langmuir constants, K_F and n, of Cd²⁺ adsorption onto raw eggshell were 18.40 and 0.68, and Freundlich constant values regarding calcined eggshell, Q and K_L, were 54.43 and 2.31, respectively.

Figure11.

Freundlich and Langmuir isotherm plots for adsorption of Cd²⁺ onto raw eggshell.

Figure 12.

Freundlich and Langmuir isotherm plots for adsorption of Cd²⁺ onto calcined eggshell.