Sorption of Cd2+ and Pb2+ on Aragonite Synthesized from Eggshell

In the present work, waste eggshells were used as a precursor for the synthesis of aragonite crystals through the wet carbonation method. Cadmium (Cd2+) and lead (Pb2+) were removed by the synthesized aragonite from synthetic wastewater. The influence of initial solution pH, contact time, Cd2+ and Pb2+ concentration, and sorbent dosage were evaluated. The major sorption was observed in the first 100 mins and 360 mins for Pb2+and Cd2+ respectively reaching sorption equilibrium at 720 mins (12 hr). The sorption capacity toward Pb2+ was much higher than toward Cd2+. Both heavy metals displayed high sorption capacities at initial pH 6. The pseudo-second-order kinetic model fits well with the experimental data with a higher correlation coefficient R2. Two isotherm models were also evaluated for the best fit with the experimental data obtained. Langmuir isotherm best fits the sorption of the metals on aragonite synthesized from eggshells. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) results of sorbent after sorption showed that the mechanism of sorption was dominated by surface precipitation. Therefore, aragonite crystals synthesized from waste eggshells can be a potential substitute source for the removal of Cd2+ and Pb2+ from contaminated water.


Introduction
Solid waste management, being a current vital issue, is a huge burden regarding sustainability. Indecorous management of solid wastes such as municipal, industrial, and hazardous waste are emerging threats to the environment. The rapid rise in population resulted in an increase in food waste such as eggshell, which is one of the solid wastes available in abundance with serious environmental problems. Globally, approximately 5.92 million tonnes of waste eggshell are generated per year [1]. These waste eggshells have been dumped into landfills without further treatment, causing several environmental consequences [2]. The waste is mainly made up of calcium carbonate (CaCO 3 ) with minor impurities, which has been used in further applications as a source of calcium supplements [3][4][5]. However, the effect of impurities has been minimized with further treatments such as chemical treatment and physical treatment [6,7].
Cadmium (Cd 2+ ) and lead (Pb 2+ ) are prevalent heavy metals used in industries such as the electroplating industry, metal refining industry, and battery industry [8]. Waterbodies are contaminated by heavy metals through industrial wastewater. Cd 2+ and Pb 2+ are considered threats to the environment

Synthesis of Sorbent (Aragonite)
Calcium hydroxide (Ca(OH) 2 ) synthesized from waste eggshell was used as a precursor. The synthesis of Ca(OH) 2 was described in detail in previous work [37]. The wet carbonation method was used for the synthesis of aragonite. One liter aqueous solution of calcium hydroxide and magnesium chloride hexahydrate was prepared with a molar ratio of 1:8 (Mg: Ca). The suspension was stirred continuously at 400 rpm at 80 • C, and pH was monitored continuously. After the stabilization of pH, CO 2 gas was introduced to the suspension at 50 mL/min flow rate. The solution pH gradually decreases with the injection of carbon dioxide. The reaction process comes to an end when the pH consistency of the suspension was observed. The synthesized aragonite was then washed, filtered and dried.

Sorption Experiments
Pb(NO 3 ) 2 and Cd(NO 3 ) 2 ·4 H 2 O were dissolved in de-ionized water at specified concentrations to prepare synthetic solutions in a sealed 1000 mL glass vessel. Then, 200 mL of the stock solutions were pre-equilibrated for 24 h prior to the addition of the adsorbent at room temperature. A specified amount of aragonite was added to the flasks. Then, samples were analyzed for remaining metal concentrations by ICP-OES which were collected at designated time intervals. The initial pH adjustments were made by HNO 3 (0.1 M) or NaOH (0.1 M) solutions. The influence of three parameters on the adsorption capacity have been studied-adsorbent dosage (0.1 g/L-0.5 g/L), initial pH (2)(3)(4)(5)(6), and initial metal concentration (10 mg/L-100 mg/L). First, the dosage effect was investigated with Cd 2+ and Pb 2+ concentration of 100 mg/L and solution pH (~5.8). A dosage with optimum removal efficiency was selected to study the influence of the other parameters. The influence of initial pH was then studied with Cd 2+ and Pb 2+ concentration of 100 mg/L and the previously selected dosage of aragonite. Finally, the influence of Cd 2+ and Pb 2+ concentration was investigated with the previously selected pH and dosage. The selected optimum conditions were used for the sorption kinetics and isotherm studies. Table 1 presents detailed experimental conditions. The percentage of ion (%) removal was determined at various experimental conditions by Equation (1).
where C o and C t are initial and final concentration in mg/L, respectively. The adsorption capacities were also calculated using Equation (2).
where C o is the initial and C e is the equilibrium metal concentration in mg/L, V is solution volume in liters, and S is the amount of adsorbent used in grams.

Characterization
Mineralogical and crystallographic phases of synthesized aragonite were investigated by powder X-ray diffraction (XRD) with 2θ ranging from 10 • to 90 • (BD2745N, Rigaku, Tokyo, Japan). Scanning electron microscopy (SEM), (JSM-6330F, JEOL. Co. Ltd., Tokyo, Japan) was used for crystal structure, shape, and morphological analysis of the synthesized aragonite. The specific surface area of the sorbent was conducted by Brunauer-Emmett-Teller (BET) (Quadrasorb SI, Quantachrome Instruments, Florida, USA). A Particle size analyzer (Malvern Zetasizer Nano ZS90, Cambridge, United Kingdom) was used for particle size distribution analysis. A surface charge study was conducted through zeta potential analysis (Otsuka ELS-Z, Osaka, Japan). The pH of the solution was measured by Orion Versa Star Pro (ThermoFisher Scientific, Waltham, MA, USA) pH meter with a glass electrode. Inductive coupled plasma-optical emission spectroscopy (ICP-OES) (PerkinElmer, Inc. Waltham, MA, USA) was used for the measurement of metal concentrations.

Characterizations of Sorbent
Crystal structures and mineralogical phases of waste eggshell and synthesized aragonite were determined by powder XRD. As illustrated in Figure 1a, the raw eggshell has a major phase of rhombohedral calcite with a space group R-3c (Space Group No. 167; JCPDS PDF Card No.86-0174) appeared at 2θ= 29.48. Figure 1b represents the XRD patterns of aragonite synthesized from eggshells. All the peaks of synthesized sorbent resembled aragonite phase, which implies that the transformation of calcite to a high grade (with minor calcite) aragonite crystals can be obtained from waste eggshell through wet carbonation.   The particle size distribution of aragonite synthesized from eggshell was found in the range of 10-45 µm, with the highest peak at~20 µm, as can be seen in Figure 3. The presence of nitrogen compounds from amino acids/proteins aid the synthesis of smaller particle size distribution [38,39]. The specific surface area of aragonite synthesized from eggshell was determined as high as~18.7 m 2 g -1 due to the smaller particle size obtained. Specific surface area increases as particle size decreases, resulting in higher adsorption capacity. Therefore, the higher surface area of the synthesized aragonite aids the performance of the sorbent. Zeta potential is the key parameter of a sorbent in the adsorption process. It informs the characteristics of the surface charge of particles suspended in an aqueous medium. Figure 4 illustrates the zeta potential values of synthesized aragonite with varied pH. The particles started to dissolve as pH < 6. Furthermore, no positive charge was found on the surface of the synthesized aragonite. The negative zeta potential values of the adsorbent show the suitability of adsorption of positively charged metals.

Influence of Contact Time and Sorption Kinetics
As presented in Figure 5, the sorption phase was very fast initially due to the available active sorption sites on the sorbent followed by a slow sorption phase, indicating the active sites being occupied by the sorbed metals [27]. The major sorption took place in the first 100 mins for Pb 2+ and 360 mins for Cd 2+ , and both metals reached sorption equilibrium at 12 h. This can be explained by the hydration energies of the two metals. The hydration energy of Cd 2+ and Pb 2+ are −1807 KJ/mol and −1481 KJ/mol respectively as given later in Table 4. The slow sorption process of Cd 2+ is due to the higher hydration energy than Pb 2+ , which tends to stay in the solution. The sorption mechanism was investigated through modelling the kinetics of metals sorption on synthesized aragonite with the first and pseudo-second-order kinetic models as given in Equations (3) and (4), respectively.
where k 1 (min -1 ) and k 2 (g mg -1 min -1 ) are first and second-order rate constants respectively, t (min) is the time duration of sorption, and qe (mg/g) and qt (mg/g) are adsorption capacity at equilibrium and time t, respectively. Figure 6 presents the first order and pseudo-second-order kinetic model fitted to the experimental data. The goodness of fit reports whether a model matches well with the experimental data or not. The correlation coefficient R 2 value, which is considered as a criterion for determination of good fit and other parameters, is given in Table 2. The R 2 values of the first-order kinetic model were observed lower. Moreover, the adsorption capacity of metals (especially Pb 2+ ) calculated from the model was much lower than those determined from the experiment. On the contrary, the R 2 values with the pseudo-second-order kinetic model were estimated as 0.986 for Cd 2+ and 0.999 for Pb 2+ which were much higher than the R 2 values with the first-order kinetic model. Besides, the estimated adsorption capacities (q e ) of the model were very close to those obtained from experiments. Therefore, the pseudo-second kinetic model is a good match for the sorption process, indicating that the sorption mechanism is mainly dominated by chemisorption.

Influence of Sorbent Dosage
The adsorption capacity of a sorbent toward the sorbate is directly affected by an indispensable parameter, sorbent dosage. It determines the amount of sorbent needed for the removal of a certain amount of contaminants. The effect was investigated by varying the amount of synthesized aragonite from 20 to 100 mg in 200 mL heavy metal-containing solution with Cd 2+ and Pb 2+ concentration of 100 mg/L at solution pH 5.8. As outlined in Figure 7, both the removal efficiency and sorption capacity of Cd 2+ was much lower than that of Pb 2+ . This result was also obtained from the previous study on biosorption of Pb 2+ , Cd 2+ , and Zn 2+ with aragonite and calcite mollusk shell [27]. As illustrated in the figure, the removal efficiency increased as the sorbent dosage was increased. Contrarily, it is well understood that adsorption capacity of a sorbent decreases as the sorbent dosage increases since only part of active adsorption sites is used [40][41][42]. Although the adsorption capacities of both metals were found high at lower dosages, the removal efficiency of Cd 2+ was very low. The removal efficiency of Pb 2+ remained almost indistinguishably constant (97.5-99.5%) while the sorption capacity decreased from 1007.5 mg/g to 202 mg/g with increasing the sorbent dosage from 0.1 g/L to 0.5 g/L. The removal efficiency of Cd 2+ increased from 41.7% to 86%, while the sorption capacity decreased from 500 mg/g to 180 mg/g with the same increase in dosage. The high sorption of Pb 2+ than Cd 2+ can be explained by the solubility constant Ksp in addition to other factors discussed later in the sorption mechanism section. The solubility constant Ksp of cerussite and otavite are 7.40×10 −14 and 1.0×10 −12 , respectively (as given later in Table 4). This indicates that cerussite is slightly insoluble than otavite which can be considered for the higher sorption of Pb 2+ on synthesized aragonite than Cd 2+ . One paper studied the sorption of Pb 2+ by calcite and aragonite with sizes of 100-200 um or less [43]. It was reported that the sorption of Pb 2+ on calcite is higher than aragonite for sizes of 100-200 m, whereas for the smaller size of sorbents, the opposite was found to be true. This result was also found by another study for the sorption of Cd 2+ and Pb 2+ with aragonite and calcite [27]. However, in the current study, the sorption of Pb 2+ on aragonite was also found to be as high as that of calcite. This might be due to the smaller size of the sorbent with a high surface area.

Influence of Initial pH
Another important parameter that influences adsorption capacity of a sorbent is the initial pH. The initial pH of a solution can affect the sorbent surface charge and the speciation of sorbates on the surface of the sorbent. The influence of initial pH on the sorption capacity of the metals by aragonite is presented Figure 8. The initial pH of 200 mL heavy metal-containing solution was adjusted to 2-6 with Cd 2+ and Pb 2+ concentration of 100 mg/L and 60 mg of aragonite to investigate the influence of pH on sorption capacity. At pH 2, H + is released and competes with the metal ions (Cd 2+ and Pb 2+ ) for the same active sorption sites resulting in negligible sorption capacity for both metals [44]. As pH > 2, the amount of adsorbed ions by aragonite started increasing rapidly. Although the sorption capacities at pH > 4 were all high, it reached a maximum capacity of 333.3 mg/L for Cd 2+ and 335.8 mg/L for Pb 2+ at pH 6.

Influence of Cd 2+ and Pb 2+ Concentration
Three Cd 2+ and Pb 2+ concentrations were conducted on 200 mL solution with pH 6 and sorbent dosage 60 mg. The result, as given in Figure 9, depicted that the sorption capacity decreased as Cd 2+ and Pb 2+ concentration decreased for the same reason explained earlier for the influence of sorbent dosage. As the initial metal concentration decreases, only some part of the active adsorption site will be used due to less sorbate. Besides, the removal efficiency increases with a decrease in the initial metal concentration [45]. However, in this study, the removal efficiencies of the three metal concentrations were almost the same. This was because the sorbent dosage (0.3 g/L) used was higher for the lower Cd 2+ and Pb 2+ concentrations (10 mg/L and 50 mg/L). Therefore, the effect on the removal efficiency was checked with a lower sorbent dosage (0.1 g/L) as presented in Figure 10. As a result, Pb 2+ showed only a slight decrease in removal efficiency with an increase in Cd 2+ and Pb 2+ concentration since Pb 2+ was easily removed even at lower dosages as discussed earlier. Furthermore, Cd 2+ has clearly proved that an increase in Cd 2+ and Pb 2+ concentration resulted in decreased removal efficiency.

Sorption Isotherm
The sorption isotherm was modeled with the Langmuir and the Freundlich models, as given in Equations (5) and (6), respectively.
qe = K f Ce n , n < 1 (6) where b is the Langmuir constant; q max (mg/g) and q e (mg/g) are the maximum and equilibrium adsorption capacity, respectively; C e (mg/L) is the metal concentration in the solution at equilibrium; and k f and n are the Freundlich empirical constants which measure adsorption capacity and intensity, respectively. The two isotherm models fitted with the experimental data are given in Figure 11. The parameters of each model are also summarized in Table 3. The R 2 values with the Freundlich model were found to be slightly lower than Langmuir model. Therefore, the experimental data can be very well fitted with the Langmuir isotherm model.

Sorption Mechanism
XRD analysis of sorbent after exposed to 100 mg/L of Cd 2+ and Pb 2+ for 12 h is shown in Figure 12a. The possible phases to be formed were otavite, cerussite, hydrocerussite, and lead hydroxide [27]. However, metal hydroxides were not identified in the XRD patterns since they are not stable in the presence of carbonates. Cerussite and otavite were identified as major peaks with a minor phase of aragonite. An SEM image of the sorbent after exposure to 100 mg/L of Pb 2+ and Cd 2+ is also presented in Figure 12b. The image showed an appearance of secondary layer on the surface of the sorbent, indicating that surface precipitation has taken place during sorption. Three possible processes can be involved at the CaCO 3 -water interface: Dissolution, sorption and nucleation (crystal growth). The slightly acidic solution (pH = 6), resulted in the dissolution of CaCO 3 and released CO 3 −2 (Equation (7)). Sorption of Cd 2+ and Pb 2+ also occurred in parallel with the dissolution aragonite (Equation (8)). Then nucleation (crystal growth) of cerussite and otavite resulted where the two metals ions reacted with the released CO 3 −2 (Equation (9)).
CaCO 3 (s) + Pb 2+ + Cd 2+ + CO 3 −2 → (Cd, Pb)CO 3 + Ca 2+ (8) It has been argued that the mechanism of sorption when heavy metals interact with CaCO 3 could be ion exchange, surface precipitation as carbonate and hydroxycarbonate phases, complexation, and adsorption [27,36,43,46]. The similarity of ionic radii of ions favors the mechanism ion exchange where one ion potentially substitutes another similar ion. As presented in Table 4, the ionic radius of the metals Cd 2+ and Pb 2+ are different from those of Ca 2+ in aragonite, which is evidence that a removal mechanism other than ion exchange is involved. Surface precipitation takes place with the dissolution of sorbents and precipitation of compounds on the surface of the sorbent. In this study, dissolution of aragonite took place continuously and metals (Cd 2+ and Pb 2+ ) were sorbed where cerussite and otavite precipitated on the surface of aragonite crystals. Therefore, the removal mechanism was dominated by surface precipitation.
Lattice matching is an important factor for the formation of a layer on the substrate without defects. Aragonite and cerussite have similar crystal systems (orthorhombic) and small lattice mismatches, which can result in the epitaxial growth of cerussite, whereas otavite has a trigonal crystal system that is different from aragonite, resulting in precipitation of otavite in three-dimensional prismatic crystals on the surface of aragonite.

Conclusions
Municipal solid waste such as eggshells is a threat to a sustainable environment. However, it can be converted into valuable material like aragonite and used in versatile applications, such as a filler paper and plastics. In the present study, eggshell (mainly composed of CaCO 3 , calcite) was used as a precursor for the synthesis of aragonite through wet carbonation and applied to the removal of Cd 2+ and Pb 2+ . The conversion of calcite (eggshell) to aragonite is a vital process due to the inefficiency of calcite to remove Cd 2+ . The influence of various factors was presented in this study. The sorption kinetics and isotherm were also investigated through systematic studies where the pseudo-second-order and Langmuir model well fitted the experimental data. The heavy metals were removed by surface precipitation mechanism where cerussite and otavite were precipitated out on the surface of the sorbent. All results demonstrate the effective and sustainable removal of toxic heavy metals by aragonite synthesized from eggshells. Therefore, eggshells can be a good substitute material of natural CaCO 3 for mitigating the contamination of water bodies with toxic heavy metals.