Journal ID (publisher-id): chemical
Title: Journal of the Korean Chemical Society
Translated Title (ko): 대한화학회지
ISSN (print): 1017-2548
ISSN (electronic): 2234-8530
Publisher: Korean Chemical Society대한화학회
Over a few centuries up to today, removal of heavy metals from the wastewater has received a lot of attention throughout the world since they lead to very harmful impacts on the ecological system.1,2 Particularly, among the heavy metals, Cd has been widely known as a primary toxic substance due to its tendency to accumulate in the vital organs of the body, thereby causing the biggest risks to every living organism.3,4 Therefore, to handle such threats involved with Cd, a variety of methods such as electrolysis, ion-exchange, reverse osmosis, adsorption, chemical precipitation, ultra-filtration, oxidation/reduction and etc. have been steadily studied over a long period of time.5,6
Adsorption has been world-widely reviewed as the last option by the considerable literatures owing to its low cost, usefulness, diversity and easy and secure handling without any toxic chemical intermediates or byproducts.7 One of the key performance parameters concerning the adsorption process is the selection of adsorbents, which ultimately determine the economic validity, the enhanced efficiency and the great adaptability of the process. Commercial activated carbon has extensively proved itself over decades as an attractive adsorbent for the removal of various pollutants.8 However, because of its expensive material, many investigators have taken an interest in the thrown-away waste materials as a lower-cost alternative.9 As a result, a large number of adsorbents with a cheaper price have appeared from natural materials and/or industrial, agricultural and fishery wastes as potential adsorbents for the removal of various hazardous contaminants.10,11
Eggshells are discharged in large quantities every day from poultries, bakeries, restaurants, homes and etc. But they are classified as the general waste by Korean law and transported to the dumping grounds without any recycling. Due to this reason, many studies have been performed to search the practical use of waste eggshells in various fields.12 Waste eggshells have been used for various purposes till quite recently, such as a pH controller of acid soils, a cement additive to enhance its strength, a catalyst for biodiesel fuel production, a feed for birds in aviary and an inorganic fertilizer into soils.13,14 However, utilization of eggshell and calcined eggshell heat-treated with eggshell in wastewater treatment has not attracted so much attention worldwide. As far as we know, only a few reports have published so far.
In this work, the removal capability of Cd2+ from aqueous solution by raw and calcined eggshell was investigated and compared with each other. To optimize the conditions for maximum adsorption, the effects of different variables such as pH, initial Cd2+ concentrations, contact time and dosage of raw and calcined eggshell were studied in detail. Moreover, the experimental data were also fitted to the kinetics and isotherm models to elucidate the adsorption behavior. Finally, the characteristics of raw and calcined eggshell were analyzed using various analytical instruments.
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-MaxN, 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.
|Element||Raw eggshell Weight (%)||Calcined eggshell Weight (%)|
|Na, Mg, Cl, K||trace||trace|
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).
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.
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.
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.
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.
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.
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 qe (mg g-1) and qt (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(qe-qt) and k1 and t/qt 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 (qe, exp) and calculated value (qe, 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.
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 qe,cal for the pseudo-second-order kinetics model corresponded roughly with the actual value of qe,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.
|Concentration (50 mg L-1)||Adsorbent (g)||Pseudo-first-order Parameters||Pseudo-second-order Parameters|
|qe,exp (mg g-1)||qe,cal (mg g-1)||k1 (min-1)||R2||qe,exp (mg g-1)||qe,cal (mg g-1)||k2 (g mg-1 min-1)||R2|
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, KF (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 KF and n values are determined from the intercept and slope of the linear plot of log qe vs log Ce, 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 KL (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 KL values are determined from the intercepts and slope of the linear plot of Ce/qe vs Ce, 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 (KF, n, Q, KL) produced from the linear plots of Freundlich and Langmuir isotherms were listed in Table 4. The values of Langmuir constants, KF and n, of Cd2+ adsorption onto raw eggshell were 18.40 and 0.68, and Freundlich constant values regarding calcined eggshell, Q and KL, were 54.43 and 2.31, respectively.
In this study, raw and calcined eggshell as adsorbent were adopted for the potential usage on the removal of Cd2+ from aqueous solution and their removal capacity was evaluated each other. In addition, the general characterization of raw and calcined eggshell was also investigated using various analysis instruments. As compared with raw eggshell, use of calcined eggshell showed that the removal rate of Cd2+ happened quickly regardless of initial Cd2+ concentrations from the beginning of contact time (0.5 h) and its removal efficiency was more than 99% at 2.5 g of calcined eggshell dosage and 8 h of contact time. It is considered that calcined eggshell with a higher alkaline environment than raw eggshell is more effective for the removal of Cd2+. In terms of dosage, it was found that an optimum dosage of raw and calcined eggshell for almost complete removal of Cd2+ from aqueous solution is approximately 5.0 g and 1.0 g, respectively. The adsorption kinetics of ESP and CESP was more proper for the pseudo-second-order model on account of their higher correlation coefficient (R2) values compared to those of the pseudo-first-order model. In case of raw eggshell, the better fit was applied to the Langmuir isotherm model due to the higher R2 value over the concentration range of Cd2+ studied. On the contrary, calcined eggshell was appropriated for the Freundlich isotherm model. Their corresponding maximum adsorption capacity, KL and KF, for Cd2+ were 18.40 and 54.43 mg g-1, respectively.
This work was supported by a Research Grant of Andong National University.
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