Adsorption of Cu2+ from aqueous solution onto iron oxide coated eggshell powder: Evaluation of equilibrium, isotherms, kinetics, and regeneration capacity

Rais Ahmad; Rajeev Kumar; Shaziya Haseeb

doi:10.1016/j.arabjc.2010.09.003

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Original article

5 (

); 353-359

doi:

10.1016/j.arabjc.2010.09.003

Adsorption of Cu²⁺ from aqueous solution onto iron oxide coated eggshell powder: Evaluation of equilibrium, isotherms, kinetics, and regeneration capacity

Rais Ahmad^*, Rajeev Kumar, Shaziya Haseeb

Environmental Research Laboratory, Department of Applied Chemistry, Aligarh Muslim University, Aligarh 202002, India

*Corresponding author. Address: Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India. Tel.: +91 0571 2700920 23x3000; fax: +91 0571 2400528 olifiaraju@gmail.com (Rais Ahmad)

Received: 2010-8-20, Accepted: 2010-9-9, Published: 2010-07-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Available online 17 September 2010

Abstract

This study explored the adsorption behavior of Cu²⁺ onto iron oxide coated eggshell powder (IOESP) from aqueous solution. The effect of various operational parameters such as pH, contact time, initial adsorbate concentration, surfactant, and temperature on adsorption of Cu²⁺ ions was investigated using batch adsorption experiments. The optimum pH for Cu²⁺ adsorption was found to be 6.0. Kinetics of adsorption was found to follow the pseudo-second-order rate equation. The suitability of Langmuir and Freundlich adsorption models to the equilibrium data was investigated. The adsorption was well described by the Freundlich isotherm model indicating the presence of heterogeneous sites for Cu²⁺ adsorption. The adsorption of Cu²⁺ was increased in the presence of anionic surfactant (SDS) while cationic surfactant (CTAB) shows no significant change in adsorption capacity. Thermodynamic parameters showed that the adsorption of Cu²⁺ onto IOESP was feasible, spontaneous, and exothermic. Regeneration studies were performed using HCl, HCOOH, EDTA, and NaOH as eluting agent for Cu²⁺ desorption from saturated IOESP and the maximum regeneration was observed with HCl.

Keywords

Coating

Adsorption

Cu2+

Kinetics

Regeneration

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1 Introduction

Copper is considered as one of the most toxic metal and poses a potential threat to the human health and environment, even at low concentrations. It has been well reported that the accumulation of copper in human body causes brain, skin, pancreas and heart diseases (Veli and Alyuz, 2007). The permissible limit of copper is 2.5 mg/L in water. Wastewater from various industries, such as electroplating, plastic, metal finishing, pigments, and mining contains copper. To alleviate the problem of water pollution by copper, various methods have been used to remove copper from wastewater such as chemical precipitation, coagulation, floatation, adsorption, ion exchange, reverse osmosis, and electrodialysis (Ali and Gupta, 2007; Soliman et al., 2010; Barakat, 2010 ). The production of the sludge in the precipitation method poses challenges in handling, treating, and land-filling of the solid sludge. Ion exchange usually requires a high-capital investment for the equipment as well as high operational cost. Electrolysis allows the removal of metal ions with the advantage that there is no need for additional chemicals and also there is no sludge generation. However, it is inefficient at a low metal concentration. Membrane processes such as reverse osmosis and electrodialysis tend to suffer from the instability of the membranes in salty or acidic conditions and fouling by inorganic and organic substances present in wastewater (Dang et al., 2009a,b).

Adsorption is highly effective, economical, promising, and is widely applied for scavenging the metal bearing wastewaters. Different conventional and non-conventional adsorbents have been used for the removal of heavy metals (Gupta et al., 2009; Gupta and Rastogi, 2009). Hen eggshell is the one of the major by product of the food industry. The by-product eggshell weighs approximately 10% of the total mass (≈60 g) of hen egg, representing a significant waste from the egg-derived products processor because it was traditionally useless and commonly disposed of in landfills without any pretreatment (Stadelman, 2000; Tsai et al., 2008). Hen eggshell typically consists of ceramic materials constituted by a three-layered structure, namely the cuticle on the outer surface, a spongy (calcareous) layer and an inner lamellar (or mammillary) layer (Tullett 1987; Stadelman, 2000; Tsai et al., 2006 ). The chemical composition (by weight) of by-product eggshell has been reported as follows: calcium carbonate (94%), magnesium carbonate (1%), calcium phosphate (1%), and organic matter (4%) (Stadelman, 2000). Due to its large production, the use of hen eggshell as adsorbents has slightly increased in recent years. Chojnacka (2005) studied the adsorption of Cr(III) ions from aqueous solutions using crushed hen eggshells, and found that they had a relatively high sorption capacity as compared to other adsorbents. Vijayaraghavan et al. (2005) investigated the adsorption capacity of crushed eggshell for removal of copper ions from aqueous solutions by column process. Otun et al. (2006) reported adsorption of Pb(II), Ni(II), and Cd(II) ions from aqueous solutions using powdered eggshell.

In this work, eggshell powder surface was modified by the coating of iron oxide. Iron oxides are effective in metal removal owing to the high affinity between iron oxides and metal species (Lim et al., 2008; Sharma and Srivastava, 2010; Zhang et al., 2010 ). The adsorption characteristics of iron oxide coated eggshell powder (IOESP) have been systematically studied in batch as a function of contact time, pH, metal ion concentration, and temperature. Kinetic measurements were assessed at different Cu²⁺ concentrations and evaluated by the pseudo-first order, pseudo-second order, and intraparticle diffusion model.

2 Materials and methods

2.1

2.1 Materials

FeSO₄·7H₂O was purchased from Qualigens Fine Chemicals. N-Cetyl-N,N,N-trimethyl ammonium bromide (CTAB), HCl, HCOOH, NaOH, sodium dodecyl sulfate (SDS), and Cu(NO₃)₂, Pb(NO₃)₂, Ni(NO₃)₂ were obtained from CDH Ltd., India. Metal ion solutions were prepared by dissolving appropriate amount of metal salt in double distilled water.

2.2

2.2 Equipments

The concentration of Cu²⁺ in the aqueous solution was analyzed using atomic absorption spectroscopy (AAS) (GBC 902, Australia). The FTIR spectra of IOESP before and after Cu²⁺ adsorption were recorded in the frequency range of 400–4000 cm⁻¹ using FTIR spectrophotometer (Inter-spec 2020, Spectro lab, UK) in KBr pellets. pH measurements were made using a pH meter (Elico LI-120, India).

2.3

2.3 Preparation of iron oxide coated eggshell powder

Hen eggshell waste was washed with distilled water and dried at 100 °C. Dried waste shell was crushed and converted to a fine powder of 100 BSS mesh particles. The fine eggshell powder was washed several times with distilled water and dried at 100 °C. Then 10 g eggshell powder immersed in 50 mL solution of FeSO₄·7H₂O (10 g) and 20 mL 5 N NaOH solution was added drop wise to precipitate the iron oxide on the surface of the eggshell powder. The solution was continuously stirred for 1 h. After that, iron oxide coated eggshell powder was filtered, washed several times and dried at 100 °C.

2.4

2.4 Determination point of zero charge (pHz)

The point of zero charge (pHz) was to be investigated to find the surface charge of IOESP. For the determination of pHz, 0.1 M KCl was prepared and its initial pH was adjusted between 2.0 and 12.0 by using NaOH and HCl. Then, 25 mL of 0.1 M KCl was taken in the 100 mL flasks and 0.05 g of IOESP was added to each solution. These flasks were kept for 24 h and the final pH of the solutions was measured by using a pH meter. Graphs were plotted between pH_final and pH_initial (Sharma et al., 2009).

2.5

2.5 Batch adsorption studies

2.5.1

2.5.1 Metal selectivity study

The selective nature of IOESP was studied for Cu²⁺, Pb²⁺, and Ni²⁺. The IOESP shows the affinity for the metal ions as follows Cu²⁺ > Pb²⁺ > Ni²⁺. On the basis of this primary study, Cu²⁺ was selected for a detailed adsorption study.

2.5.2

2.5.2 Kinetic studies

Batch kinetic experiments were performed by mixing 0.05 g adsorbent to each conical flask with 25 mL of metal ion solution of 50 and 100 mg L⁻¹ concentration at 30 °C. A series of such conical flasks were then shaken at a constant speed of 80 rpm in a water bath shaker and samples were collected at different time intervals. The concentration of Cu²⁺ in the supernatant solution was analyzed using AAS. The equilibrium adsorption capacity of IOESP for Cu²⁺ was calculated from the relationship:

(1)

q_{e} = (C_{0} - C_{e}) V / M

where q_e is the equilibrium adsorption capacity (mg g⁻¹), C_e is the metal ion concentration (mg L⁻¹) at equilibrium, V is the volume (L) of solution, and M is the weight (g) of adsorbent.

2.5.3

2.5.3 Equilibrium studies

The effect of the initial metal ion concentration was determined by placing 0.05 g of the adsorbent in 25 mL of metal ion solution of different initial concentrations (10–100 mg L⁻¹) for 180 min at 30 °C. The concentration of Cu²⁺ left in supernatant solution was determined by AAS.

2.5.4

2.5.4 Effect of pH on Cu²⁺ adsorption

The influence of solution pH on the Cu²⁺ adsorption was studied over the pH range from 2 to 6. The pH was adjusted using 0.1 M HCl and NaOH solutions. In this study, 25 mL of Cu²⁺ solution of 50 and 100 mg L⁻¹ were agitated with 0.05 g of IOESP for 180 min at 30 °C. In the subsequent investigations, experiments were performed at solution pH value up to 6 to avoid any possible hydroxide precipitation.

2.5.5

2.5.5 Desorption studies

Desorption studies were performed in batch using 0.01 M and 0.1 M of HCl, HCOOH, EDTA, and NaOH. 25 mL Cu²⁺ solution of 100 mg L⁻¹ metal ion concentration was treated separately with 0.05 g of adsorbent. The adsorbent in each flask was washed several times with distilled water to remove excess of metal ions. Then Cu²⁺ saturated adsorbent was agitated with 25 mL eluent for 180 min. The amount of Cu²⁺ desorbed in the eluent solution was analyzed by AAS.

3 Results and discussion

3.1

3.1 Effect of pH

The pH value of the solution plays an important role in the adsorption of Cu²⁺ on IOESP. The adsorption of Cu²⁺ onto IOESP increases with increasing solution pH (Fig. 1). Similar results were also reported for Cu²⁺ adsorption on iron pillared montmorillonite (Li and Wua, 2010) and on Al₂O₃ (Wua et al., 2003). The effect of pH may be explained in terms of zero point of charge (pHz) of the adsorbent (pHz of IOESP is 7.9, Fig. 1 inset), at which the adsorbent is neutral. The surface charge of the adsorbent is positive when solution pH is below pHz. An increase in pH above pHz will show a slight increase in adsorption as long as the metal species are still positively charged or neutral even though the surface of the adsorbent is negatively charged. When both the surface charge of the adsorbent and metal species charge become negative, the adsorption will decrease significantly. Decrease in removal of metal ions at lower pH is apparently due to the higher concentrations of H⁺ in the solution, which compete with Cu²⁺ ions for the adsorption sites of IOESP. Generally, the positive charge of the adsorbent surface decreases with the increasing pH value, leading to the decrease in the repulsion between the adsorbent surface and Cu²⁺, thus improving the adsorption capacity (Gupta, 1998).

Effect of pH on Cu2+ adsorption on IOESP. Determination of pHz of IOESP (inset).

3.2

3.2 Adsorption kinetics

The adsorption kinetics of IOESP for Cu²⁺ at 30 °C is shown in Fig. 2 and the adsorption was quick in the beginning and then slow. The adsorption equilibrium was established in about 180 min. After this period, the amount of adsorbed metal ions did not change significantly with time. The rapid uptake of metal ions onto IOESP may indicate that most of reaction sites of the adsorbent are exposed for interaction with metal ions. Furthermore, due to the presence of hydroxyl group in adsorbent, complexation may occur between metal ion and adsorbent surface which significantly shows fast adsorption (Gupta et al., 2010a).

Effect of contact time on Cu2+ adsorption onto IOESP.

To determine the kinetics of adsorption of Cu²⁺ on IOESP, pseudo-first-order, pseudo-second-order and intraparticle diffusion models were applied. The pseudo-first-order model is given by:

(2)

\log (q_{e} - q_{t}) = \log q_{e} - k_{1} t / 2.303

where k₁ is the pseudo-first-order rate constant (min⁻¹) and q_e (mg g⁻¹) is the adsorption capacity at equilibrium and q_t (mg g⁻¹) is the amount of metal adsorbed at any time t. On the other hand pseudo-second order model is expressed as:

(3)

t / q_{t} = (1 / k_{2} q_{e}^{2}) + t / q_{e}

where k₂ (g mg⁻¹ min⁻¹) is the pseudo-second-order rate constant. The values of q_e, k₁ and k₂ for different concentrations were calculated from their respected plots (figure not shown) and the results are shown in Table 1. It can be seen that the values of correlation coefficient for pseudo-second-order are relatively higher than those for pseudo-first-order kinetic model. Additionally, experimental q_e values are very close to the calculated q_e values for pseudo-second-order kinetic model. These results implied that the adsorption of Cu²⁺could be best described by the pseudo-second order model (Gupta et al., 2010b). As shown in Table 1, the value of rate constant k₂ decreases with increasing initial concentration and surface loading. Higher surface loadings would result in less diffusion efficiency and a high competition of metal ions for fixed reaction sites, consequently lower k₂ values were observed.

Table 1 Kinetic parameters for the adsorption of Cu²⁺ onto IOESP.

Conc. (mg L⁻¹)	q_e (exp) (mg g⁻¹)	Pseudo-first-order			Pseudo-second-order			Intraparticle diffusion
		q_e	k₁	R²	q_e	k₁	R²	k_id	R²
50	21	12.56	0.018	0.9330	21.97	0.003	0.9923	0.742	0.9246
100	36	15.80	0.015	0.9348	37.17	0.002	0.9924	1.018	0.9289

Intraparticle diffusion can be estimated by using the Weber–Morris intraparticle diffusion model

(4)

q_{t} = k_{id} t^{1 / 2} + C

where k_id (g mg⁻¹ min^−1/2) is the intraparticle diffusion rate coefficient and C gives an idea about the thickness of the boundary layer. These values were determined by a plot of q_t versus t^1/2 (figure not shown) and the straight lines deviated from the origin. The difference between final mass transfer rate and initial mass transfer rate may cause the deviations of the straight lines. Also, this deviation can show that the pore diffusion is not the sole rate-controlling step. The k_id values are shown in Table 1.

3.3

3.3 Adsorption isotherms

The adsorption of Cu²⁺ onto IOESP at 30 °C was determined as a function of equilibrium metal ion concentration and the corresponding adsorption plot is shown in Fig. 3. As seen from Fig. 3, equilibrium uptake increases from 7.5 to 36 mg g⁻¹ with the increase of initial metal ion concentrations from 10 to100 mg L⁻¹. This is a result of the increase in the driving force from the concentration gradient.

Effect of concentration of Cu2+ adsorption on IOESP. Langmuir and Freundlich isotherm plot (inset).

The analysis of equilibrium data is basic a requirement for the design of adsorption systems. The experimental data were fitted to the Langmuir and Freundlich equations and the constant parameters of the isotherm equations were calculated. The Langmuir and Freundlich models were expressed by the Eqs. (5) and (6), respectively

(5)

1 / q_{e} = 1 / Q^{\circ} + 1 / {bQ}^{\circ} C_{e}

(6)

\ln q_{e} = \ln K_{F} + 1 / n \ln C_{e}

where

Q^{\circ}

(mg g⁻¹) is the maximum adsorption capacity and b (L mg⁻¹) is the binding constant which is related to the heat of adsorption. K_F (L g⁻¹) and 1/n are the Freundlich constants.

The linearized Freundlich and Langmuir plots are given in Fig. 3 inset. The slopes of the linearized Freundlich and Langmuir plots were used to calculate the adsorption constants presented in Table 2. The value of correlation coefficient for Freundlich equation (R² = 0.9841) is much higher than Langmuir (R² = 0.9451) suggesting that equilibrium data are well described by Freundlich isotherm. Furthermore, the Freundlich exponent 1/n gives an indication of the favorability of adsorption. The value of 1/n < 1.0 represents a favorable adsorption condition (Gupta et al., 1999). The value of 1/n obtained in the present study for Cu²⁺ is less than unity, indicating the favorable adsorption of metal ions onto IOESP.

Table 2 Langmuir and Freundlich isotherm parameters for the adsorption of Cu²⁺ onto IOESP.

Langmuir model			Freundlich model
$Q^{\circ}$ (mg g⁻¹)	b (L mg⁻¹)	R²	K_F (L g⁻¹)	1/n	R²
44.843	0.019	0.9451	1.353	0.723	0.9841

3.4

3.4 Effect of temperature

The effect of temperature on the adsorption behavior of Cu²⁺ was studied in the range 30–50 °C by using 0.05 g IOESP and 25 mL of 100 mg L⁻¹ metal ion solution. The adsorption capacity increased from 36 to 41.5 mg g⁻¹ when the temperature increases from 30 to 50 °C, indicating that the adsorption was endothermic in nature. The rise in adsorption capacity was due to the increase in collision frequency between adsorbent and Cu²⁺ ions.

Thermodynamic parameters like standard free energy change ( $Δ G^{\circ}$ ), enthalpy change ( $Δ H^{\circ}$ ), and entropy change ( $Δ S^{\circ}$ ) were estimated to evaluate the feasibility of the adsorption process by the following equations:

(7)

Δ G^{\circ} = - RT \ln K_{c}

(8)

\ln K_{c} = (Δ S^{\circ} / R) - (Δ H^{\circ} / RT)

where K_c is the distribution coefficient for the adsorption, R is the universal gas constant (8.314 J/mol K) and T is the absolute temperature (K). The negative

Δ G^{\circ}

values 2.378, 2.826, and 3.893 kJ mol⁻¹ for 30, 40, and 50 °C, respectively, increased with temperature, indicating the feasibility and spontaneity of adsorption process of Cu²⁺ onto IOESP. The positive value of

Δ H^{\circ}

(18.877 kJ mol⁻¹) confirmed the endothermic nature of adsorption process, while the positive value of

Δ S^{\circ}

(0.0731 kJ mol⁻¹ K⁻¹) revealed the increase in randomness at the solid/solution interface during the adsorption process (Gupta and Ali, 2000).

3.5

3.5 Effect of surfactant

Presence of surfactant in aqueous solution is likely to affect the adsorption capacity of the adsorbent. This is very important for potential applications of adsorbents, because surfactants are widely employed in industry and are commonly present in real wastewaters. In the presence of surfactants, the adsorption of metal ions onto the active sites of adsorbent is affected by the charge density of the interface. In this study, the effect of anionic SDS and cationic CTAB surfactant was studied below their critical micelle concentration (CMC). The CMC of SDS and CTAB in aqueous solution without electrolysis is 8.1 and 0.9 mM, respectively, while their micelles aggregation numbers are 62 and 61, respectively (Hinze, 1987).

The effect of surfactants on the adsorption of Cu²⁺ was studied in the concentration range from 2 to 20 mg L⁻¹. The presence of the cationic surfactant CTAB does not affect the uptake of the Cu²⁺ significantly but in the presence of anionic surfactant SDS, adsorption dramatically increases from 21 to 24.5 mg g⁻¹. It was therefore supposed that some additional adsorption mechanisms had effectively played their role in the presence of surfactants. Janos et al. (1992) and Kiraly et al. (2001) reported that SDS can strongly bind to some kinds of non-polar adsorbents forming cation exchange sites and these ion exchange sites are in dynamic equilibrium with the concentration of the surfactant in solution. Furthermore, increase in the adsorption of Cu²⁺ onto IOESP in the presence of SDS can also be explained by the ion association model, ion pairs, or slightly more complex associates or aggregates which are formed between the Cu²⁺ cationic species and anionic surfactant in solution, and then they are adsorbed onto the non-polar part of the adsorbent (Janos, 2003). Results of this study are in consonance with the previously reported work by Rao et al. (2010) for Zn(II) and Ni(II) on mustard oil cake and by Nabi et al. (2008) for the various heavy metals on acrylamide aluminum tungstate.

3.6

3.6 FTIR analysis

The FTIR spectra of IOESP before and after Cu²⁺ are shown in Fig. 4. The stretching vibrations of function group –OH usually form a broad band at 3486 cm⁻¹, shifting to 3437 cm⁻¹ after Cu²⁺ ions are adsorbed onto the surface of adsorbent. The decrease in wave number of the peak is attributed to the attachment of Cu²⁺ on –OH group. Fig. 4 shows the most significant peak of intensity of eggshell particles at 1424 cm⁻¹, strongly associated with the presence of carbonate minerals within the eggshell matrix. The carbonate bond shifts to the higher frequency from 1424 to 1432 cm⁻¹, this may result from the high electron density induced by the adsorption of Cu²⁺ onto IOESP. The peaks at about 1653, 1123, 887, and 715 cm⁻¹ are strongly associated with the presence of calcium carbonate (Tsai et al., 2006) and hydroxyl group of iron oxide. Another noticeable feature is that the spectra before and after copper sorption show a major shift in the frequencies of the absorption band from 1064 to 1044 which can be attributed to the associations of the hydroxyl group with copper ions (Lim et al., 2008). In addition, there is an obvious absorption band at a low frequency zone of 400–700 cm⁻¹, which is assigned to the stretching vibration of the Fe–O bond. The peak of Fe–O has a slight shift from 470 to 462 cm⁻¹ as Cu²⁺ ions get adsorbed onto the iron oxide.

FTIR spectra of IOESP (a) before adsorption (b) after adsorption.

3.7

3.7 Effect of coexisting metal ions on Cu²⁺ adsorption

The affinity of IOESP for Cu²⁺ in the presence of Ni²⁺ and Pb²⁺ was studied by taking 0.05 g adsorbent, 50 mg L⁻¹ of each metal ion of 25 mL. The adsorption capacity of IOESP for Cu²⁺ in binary and ternary adsorption system is shown in Fig. 5. The results clearly reveal that when two or three metal ions such as Cu²⁺ and Pb²⁺ or Ni²⁺ are present at the same time in a solution, the adsorption capacity of IOESP for Cu²⁺ decreased. This may be due to competitiveness of the metal ions for the active site of adsorbent.

Effect of coexist metals on the adsorption of Cu2+ onto IOESP. Conc. = 50 mg/L of each metal ion, vol. = 25 mL, adsorbent dose = 0.05 g, temp. = 30 °C.

3.8

3.8 Regeneration studies

In order to make the process more economical and feasible, the material is regenerated. If the material can be regenerated and reused as an adsorbent after the first cycle of the adsorption process, significant improvement in the economy of the process will be achieved. For this purpose, four different desorbing agents were investigated for the removal of Cu²⁺ adsorbed on the spent adsorbent. The regeneration capacity of the IOESP is shown in Fig. 6. The adsorption capacity decreases slightly with the increase of reused times. This may be attributed to slight damage of the original surface layer of iron oxide when the extraction of Cu²⁺ from IOESP proceeds by means of an acid treatment (Gupta et al., 1997). Fig 6 show that maximum regeneration capacity was observed with 0.1 M HCl, indicating that the most suitable desorbing agent followed by 0.1 M HCOOH, 0.1 M EDTA, and 0.01 M NaOH. The results suggested that desorption with the HCl and HCOOH was by ion exchange process. EDTA is a chelating agent which forms the complex with the metal ions (Gupta and Rastogi, 2008), that is why EDTA shows a lower regeneration compared to HCl and HCOOH.

4 Comparison of adsorption capacity of various adsorbents

The data presented in Table 3 compare the maximum monolayer adsorption capacity of the different types of low-cost adsorbents used for the removal of Cu²⁺. The value of ( $Q^{\circ}$ ) in this study is larger than that in most of the previous work.

Table 3 Comparison of maximum monolayer adsorption capacities of various adsorbents for Cu²⁺ removal.

Adsorbent	mg g⁻¹	References
Green alga Spirogyra	133.3	Gupta et al. (2006)
Eggshell	5.03	Vijayaraghavan et al. (2005)
Iron oxide-coated sand	2.17	Boujelben et al. (2009)
Iron oxide	17.08	Hsueh et al. (2007)
Rice husk	4.766	Jaman et al. (2009)
Wheat straw	7.05	Wu et al. (2009)
Cereal chaff	4.46	Han et al. (2006)
Wheat shell	8.3	Basci et al. (2004)
Wheat straw	4.45	Saeiban et al. (2008)
Soybean straw	5.40	Saeiban et al. (2008)
Corn stalk	3.75	Saeiban et al. (2008)
Corn cob	2.16	Saeiban et al. (2008)
Rice shell	1.85	Aydın et al. (2008)
Wheat shell	7.39	Aydın et al. (2008)
Lentil shell	8.98	Aydın et al. (2008)
Sawdust	8.45	Larous et al. (2005)
Iron oxide coated eggshell powder	44.843	This work

5 Conclusion

In this work, iron oxide coated eggshell powder was prepared by chemical precipitation of FeSO₄·7H₂O onto hen eggshell powder. The IOESP exhibited a remarkably enhanced adsorption capacity for Cu²⁺ than the pure iron oxide and eggshell powder. The equilibrium data fitted well to Freundlich isotherm which suggests heterogeneity in the sorption sites. Thermodynamic parameters suggested that the adsorption process was spontaneous and governed by physisorption interaction. Maximum regeneration was observed with HCl, suggesting that ion exchange was the main mechanism in desorption of metal ions. The regeneration efficiency dropped after each cycle of desorption, assisted by the physical and chemical damages that occurred on the metal binding sites.

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[23] Janos P., . Sorption of basic dyes onto iron humate. Environ. Sci. Technol.. 2003;37:5792-5798.
[Google Scholar]

[24] Janos P., Sytulik K., Pacakova V., . An ion-exchange separation of metal cations on a C-18 column coated with dodecylsulphate. Talanta. 1992;39:29-34.
[Google Scholar]

[25] Kiraly Z., Findenegg G.H., Klumpp E., Schlimper H., Dekany I., . Adsorption calorimetric study of the organization of sodium n-decyl sulfate at the graphite/solution interface. Langmuir. 2001;17:2420-2425.
[Google Scholar]

[26] Larous S., Meniai A.H., Lehocine M.B., . Experimental study of the removal of copper from aqueous solutions by adsorption using sawdust. Desalination. 2005;185:483-490.
[Google Scholar]

[27] Li S.Z., Wua P.X., . Characterization of sodium dodecyl sulfate modified iron pillared montmorillonite and its application for the removal of aqueous Cu(II) and Co(II) J. Hazard. Mater.. 2010;173:62-70.
[Google Scholar]

[28] Lim S.F., Gzheng Y.M., Wenzou S., Chen J.P., . Characterization of copper adsorption onto an alginate encapsulated magnetic sorbent by a combined FT-IR, XPS, and mathematical modeling study. Environ. Sci. Technol.. 2008;42:2551-2556.
[Google Scholar]

[29] Nabi S.A., Ganai S.A., Khan A.M., . Effect of surfactants and temperature on adsorption behavior of metal ions on organic–inorganic hybrid exchanger, acrylamide aluminum tungstate. J. Surfact. Deterg.. 2008;11:207-213.
[Google Scholar]

[30] Otun J.A., Oke I.A., Olarinoye N.O., Adie D.B., Okuofu C.A., . Adsorption isotherms of Pb(II), Ni(II) and Cd(II) ions onto PES. J. Appl. Sci.. 2006;6:2368-2376.
[Google Scholar]

[31] Rao R.A.K., Khan M.A., Jeon B.H., . Utilization of carbon derived from mustard oil cake (CMOC) for the removal of bivalent metal ions: effect of anionic surfactant on the removal and recovery. J. Hazard. Mater.. 2010;173:273-282.
[Google Scholar]

[32] Saeiban M., Klasnja M., Skrbiae B., . Adsorption of copper ions from water by modified agricultural by-products. Desalination. 2008;229:170-180.
[Google Scholar]

[33] Sharma Y.C., Srivastava, . Separation of Ni(II) ions from aqueous solutions by magnetic nanoparticles. J. Chem. Eng. Data. 2010;55:1441-1442.
[Google Scholar]

[34] Sharma Y.C., Uma, Upadhyay S.N., . Removal of a cationic dye from wastewaters by adsorption on activated carbon developed from coconut coir. Energy Fuels. 2009;23:2983-2988.
[Google Scholar]

[35] Soliman, E.M., Ahmed, S.A., Fadl, A.A., 2010. Reactivity of sugar cane bagasse as a natural solid phase extractor for selective removal of Fe(III) and heavy-metal ions from natural water samples. Arabian J. Chem. doi: 10.1016/j.arabjc.2010.06.021.

[36] Stadelman W.J., . Eggs and egg products. In: Francis F.J., ed. Encyclopedia of Food Science and Technology (second ed.). New York: John Wiley and Sons; 2000.
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[37] Tsai W.T., Yang J.M., Lai C.W., Cheng Y.H., Lin C.C., Yeh C.W., . Characterization and adsorption properties of eggshells and eggshell membrane. Biores. Tech.. 2006;97:488-493.
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[38] Tsai W.T., Hsien K.J., Hsu H.C., Lin C.M., Lin K.Y., Chiu C.H., . Utilization of ground eggshell waste as an adsorbent for the removal of dyes from aqueous solution. Biores. Tech.. 2008;99:1623-1629.
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[39] Tullett S.G., . Egg shell formation and quality. In: Wells R.G., Belyavin C.G., eds. Egg Quality—Current Problems and Recent Advances. London: Butterworths; 1987.
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[40] Veli S., Alyuz B., . Adsorption of copper and zinc from aqueous solutions by using natural clay. J. Hazard. Mater.. 2007;149:226-233.
[Google Scholar]

[41] Vijayaraghavan K., Jegan J., Palanivelu K., Velan M., . Removal and recovery of copper from aqueous solution by eggshell in a packed column. Miner. Eng.. 2005;18:545-547.
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[42] Wu Y., Zhang L., Gao C., Ma J., Ma X., Han R., . Adsorption of copper ions and methylene blue in a single and binary system on wheat straw. J. Chem. Eng. Data. 2009;54:3229-3234.
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[43] Wua C.H., Linb C.F., Mab H.W., His T.Q., . Effect of fulvic acid on the sorption of Cu and Pb onto γ-Al₂O₃. Water Res.. 2003;37:743-752.
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[44] Zhang S., Li X.Y., Chen J.P., . Preparation and evaluation of a magnetite-doped activated carbon fiber for enhanced arsenic removal. Carbon. 2010;48:60-67.
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