Introduction

The physicochemical behavior of long-lived radionuclides are crucial to the environmental pollution [1, 2]. The presence of radionuclides in wastes is a major environmental concern. Such wastes arise from technologies producing nuclear fuels, and from laboratories working with radioactive materials [3].

60Co(II) is one of the most serious radionuclides affecting the environment due to its long half-life (T1/2 = 5.27a). The radionuclides 60Co and 58Co are present in liquid wastes released from pressurized water nuclear power reactors [4]. High levels of 60Co(II) may affect several health troubles such as paralysis, diarrhea, low blood pressure, lung irritation and bone defects [5]. The permissible limits of 60Co(II) in the irrigation water and livestock wastewater are 0.05 and 1.00 mg/L, respectively [6]. Therefore, the elimination of radiocobalt from wastewaters is important to protect public health. The removal of radiocobalt from aqueous solution have been studied extensively [7–18]. Treatment processes for radionuclides contaminated water include chemical precipitation, membrane filtration, ion exchange, sorption, and co-precipitation/precipitation [19–22]. Among these methods, sorption process has been found as one of the most promising technologies in radionuclide pollution management because of its low cost, simplicity of design and operation [23–25].

Generally, activated carbon, carbon nanomaterials, clay minerals, silica gel, activated alumina and ion exchange resin have been widely used as adsorbents in the removal of radionuclides from large volume of aqueous solutions [26–31]. However, the high capital and regeneration cost of these materials limit their large-scale applications for the removal of radionuclides, which have encouraged researchers to look for low-cost alternative adsorbents [32–34]. The major advantages of biomaterial as sorbents are low cost, high efficiency, minimization of chemical or biological sludge, no additional nutrient requirement, possibility of regeneration of the biosorbent, and metal recovery [35–37]. Hydroxylapatite (HAP) as an efficient and low-cost biosorbent, which has high sorption ability for 60Co(II). HAP was found to have high removal efficiency for the retention of Cd, Zn and Co in a wide pH range of 6–9 [38]. The interaction of Co2+ with HAP was also studied from the aspect of the catalytic activities of cobalt/calcium exchanged samples for oxidative dehydrogenation of ethane [39].

It is well-known that HAP can be prepared from a variety of raw materials. The most frequently used precursors are animal bones [40, 41] and coral [42]. Eggshell waste is widely produced from houses, restaurants, and bakeries. Eggshell has a little developed porosity and pure CaCO3 as an important constituent. In this work, carbonate hydroxylapatite (CHAP) was synthesized by using eggshell from the viewpoint of recycling this waste product and minimization of contaminants. The properties of the prepared CHAP from eggshells were determined, and the potential use of CHAP in the removal of 60Co(II) was studied.

The main purposes of this study are: (1) to investigate the sorption kinetics of 60Co(II) on CHAP and to analyze the experimental data with a pseudo-second-order equation; (2) to study the sorption of 60Co(II) on CHAP as a function of pH, ionic strength, foreign ions in the absence and presence of HA/FA; (3) to calculate the thermodynamic parameters (i.e., ΔH 0, ΔS 0, ΔG 0) from the temperature dependent sorption isotherms; and (4) to presume the sorption mechanisms of 60Co(II) on CHAP.

Experimental section

Preparation of CHAP

The eggshell samples used in this study were obtained from some private restaurant in China. CHAP samples were prepared as: the eggshells were washed with distilled water for several times to remove impurity and interference material such as organics and salts. Then, they were transferred to the oven at 70 °C to dry. The pretreated eggshells were calcined in air using a muffle furnace at 900 °C (heating rate of 10 °C/min) for 1 h. The following reaction happened in the calcinations:

$$ {\text{CaCO}}_{3} \mathop{\longrightarrow}\limits^{{900\,^{ \circ } C}}{\text{CaO}} + {\text{CO}}_{2} \uparrow $$
(1)

The calcined powder was ground in an alumina mortar-pestle. This CaO powder was completely dissolved in concentrated H3PO4, kept at 30–40 °C for 2–3 h, under pH 3.0. The following reaction happened in the acidic conditions:

$$ {\text{CaO}} + {\text{H}}_{3} {\text{PO}}_{4} = {\text{CaHPO}}_{4} + {\text{H}}_{2} {\text{O}} $$
(2)

Insoluble material was separated and removed by filtration. Subsequently, 1 mol/L Ca(OH)2 was added into the solution, kept at 100 °C for 24 h, adjusting pH to 11.0 by using 0.1 mol/L NaOH. When it was naturally cooled to room temperature, the suspension was collected by filtration, and washed thoroughly with distilled water to remove the excrescent free ions, and at last rinsed with distilled water until the pH of the solution reached to about 7.0. The reaction processed following the equation:

$$ {\text{CaHPO}}_{4} + {\text{CO}}_{2} + {\text{Ca}}({\text{OH}})_{2} \to {\text{Ca}}_{10} \left\{ {({\text{PO}}_{4} )_{6 - x} ,({\text{CO}}_{3} \cdot {\text{OH}})_{x} } \right\}({\text{OH}})_{2} $$
(3)

Finally, the sample was dried at 60 °C for 24 h and ground to pass 200 meshes to get the CHAP samples.

All chemicals used in the experiments were purchased as analytical purity and used directly without any further purification. All the reagents were prepared with high-purity Milli-Q water from a Millipore Synthesis A10 water system (18.2 MΩ).

Sorption experiments

All the experiments were carried out in polyethylene centrifuge tubes by using batch technique under ambient conditions. The stock suspension of CHAP was pre-equilibrated with NaClO4 solution for 24 h, and then 60Co(II) stock solution was added into the polyethylene test tubes to achieve the desired concentrations of different components. The pH values of the systems were adjusted by adding negligible volume of 0.1 or 0.01 mol/L HClO4 or NaOH. After the suspensions were shaken for 24 h, the solid and liquid phases were separated by centrifugation at 9000 rpm for 30 min at the temperature controlled same to that in the sorption experiments. It was necessary to note that the sorption of 60Co(II) on the tube wall was negligible according to the test of 60Co(II) sorption in the absence of CHAP.

The concentration of radionuclide 60Co(II) was analyzed by liquid scintillation counting using a Packard 3100 TR/AB Liquid Scintillation Analyzer (PerkinElmer). The scintillation cocktail was ULTIMA GOLD ABTM (Packard). The amount of 60Co(II) adsorbed on CHAP was calculated from the difference between the initial concentration (C 0) and the equilibrium one (C e). The sorption of 60Co(II) was expressed in terms of sorption percentage (%):

$$ {\text{Sorption(}}\% ) = {\frac{{C_{0} - C_{\text{e}} }}{{C_{0} }}} \times 100\% $$
(4)

All experiment date were the average of triplicate determinations and the relative errors were about 5%.

Results and discussion

Kinetic sorption of 60Co(II) on CHAP

The sorption of 60Co(II) on CHAP as a function of contact time was investigated at pH = 6.5 ± 0.1 (Fig. 1). It seemed that the sorption consisted of two phases: a primary rapid phase and a second slow phase. The first rapid phase lasted approximately 2 h and accounted for the major part in the total 60Co(II) sorption. Sorption reached a plateau value in approximately 4 h, which showed saturation of the active points. According to the above results, the shaking time was fixed to 24 h for the rest of the batch experiments to assure the sorption equilibrium.

Fig. 1
figure 1

Effect of contact time on 60Co(II) sorption on CHAP and the pseudo-second-order rate equation fit (inset), pH = 6.5 ± 0.1, I = 0.01 mol/L NaClO4, T = 303.15 K, m/V = 0.6 g/L, C Co(II)initial = 1.67 × 10−4 mol/L

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In order to analyze the sorption rate of 60Co(II) on CHAP, the pseudo-second-order rate equation is used to simulate the kinetic sorption [43, 44]:

$$ {\frac{t}{{q_{\text{t}} }}} = {\frac{1}{{2Kq_{\text{e}}^{2} }}} + {\frac{1}{{q_{\text{e}} }}}\,t $$
(5)

where q t (mg/g) is the amount of 60Co(II) adsorbed on CHAP at time t, and q e (mg/g) is the equilibrium sorption capacity. K (g/(mg h)) is the pseudo-second-order rate constant.

Constant K and q e were calculated from the intercept and slope of the line obtained by plotting t/q t vs. t (the inserted figure in Fig. 1). The value of pseudo-second-order rate constant K is 0.958 g/(mg h) and the equilibrium sorption capacity q e is 9.479 mg/g. The correlation coefficient (R 2 = 0.999) of the linear plot is very close to 1, which suggests that the experimental data can be fitted very well by the pseudo-second-order model [45–47].

Effect of pH and ionic strength

The pH-dependent sorption of 60Co(II) on CHAP as a function of ionic strength is shown in Fig. 2. It is clear that sorption of 60Co(II) on CHAP is obviously affected by solution pH values. The sorption of 60Co(II) increases abruptly from ~10 to ~93% at pH 4.0–10.5, and approaches a plateau at pH > 10.5. There are two possible reactions for the removal of 60Co(II) from aqueous solutions [48]. The first mechanism is the sorption of 60Co(II) on the surfaces by ion exchange. The ion exchange can be expressed as follow:

$$ {\text{Co}}_{{ ( {\text{s)}}}}^{2 + } + \equiv {\text{S}}{-}{\text{O}}{-}{\text{Ca}}_{{ ( {\text{C)}}}}^{2 + } \to \equiv {\text{S}}{-}{\text{O}}{-}{\text{Co}}_{{({\text{C}})}}^{2 + } + {\text{Ca}}_{{({\text{s}})}}^{2 + } $$
(6)
$$ {\text{Co}}_{{ ( {\text{s)}}}}^{2 + } + 2 \equiv {\text{S}}{-}{\text{O}}{-}{\text{Na}}_{{ ( {\text{C)}}}}^{ + } \to \left( { \equiv {\text{S}}{-}{\text{O}}} \right)_{2}{-}{\text{Co}}_{{ ( {\text{C)}}}}^{2 + } + 2{\text{Na}}_{{ ( {\text{s)}}}}^{ + } $$
(7)

where subscripts (s) and (C) denote solution and CHAP phase, respectively.

Fig. 2
figure 2

Effect of ionic strength on 60Co(II) sorption on CHAP as a function of pH, T = 303.15 K, m/V = 0.6 g/L, C Co(II)initial = 1.67 × 10−4 mol/L

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The second mechanism is surface complexation by CHAP. The surface functional groups of CHAP participate in the formation of complexes in the removal of Co(II) tom CHAP.

It is generally regarded that the species of 60Co(II) in solution are important to 60Co(II) sorption. The species of 60Co(II) are strongly dependent on pH values. Figure 3 shows the relative distribution of 60Co(II) species calculated from the stability constants (log K 1 = −9.6, log K 2 = −9.2, and log K 3 = −12.7) [49]. The results demonstrate that 60Co(II) presents in the forms of Co2+, Co(OH)+, Co(OH)2 and Co(OH) −3 at various pH values. At pH < 7.5, the main species of 60Co(II) is Co2+, and 60Co(II) does not form precipitation in this pH region. In the pH > 7.5, Co(OH)+ and Co(OH) −3 become the main species. In the pH > 10.5, Co2+ mainly forms precipitation. At pH < 7.5, the predominant species is Co2+. Due to the protonation reaction (i.e., ≡SOH + H+ ↔ ≡SOH2 +) on the surfaces of CHAP, the concentration of protonated sites (≡SOH2 +) decreases with increasing pH. Thereby, the sorption of Co2+ is unfavorable as a result of coulombic repulsion at pH range 1.0–4.0. However, at high pH values, the concentration of deprotonated sites (≡SOH−) increases with increasing pH because of the surface deprotonation reaction (i.e., ≡SOH ↔ ≡SO− + H+). The deprotonated sites (≡SOH−) are more available to retain the metal ions and surface complexation between Co2+ and CHAP is facilitated, thus resulting in sharp increase of 60Co(II) sorption at pH 4.0–10.5.

Fig. 3
figure 3

Relative proportion of 60Co(II) species as a function of pH

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Figure 2 also shows the effect of ionic strength on 60Co(II) sorption to CHAP. It is well known that ClO4 − does not form complexes with 60Co(II) in solution or on CHAP surfaces. The influence of NaClO4 concentration on 60Co(II) sorption is mainly due to the competition of Na+ with 60Co(II) on CHAP surfaces. One can see that the sorption of 60Co(II) at pH < 10.5 is influenced by ionic strength obviously, whereas no drastic difference of 60Co(II) sorption to CHAP is found at pH > 10.5. The pH- and ionic strength-dependent sorption of 60Co(II) at pH < 10.5 suggests that the removal of 60Co(II) to CHAP is dominated by ion exchange or outer-sphere surface complexation, whereas the pH-dependent and ionic strength-independent 60Co(II) sorption indicates that the sorption of 60Co(II) is mainly due to inner-sphere surface complexation or surface precipitation at pH > 10.5 [50–54].

To illustrate the variation and relationship of C e, pH and q e (mg/g, the concentration of 60Co(II) on solid phase), the experimental data of 60Co(II) sorption in 0.001, 0.01 and 0.1 mol/L NaClO4 were plotted as 3D plots of C e, pH and q e (Fig. 4). On the pH–q e plane, one can see that the lines are very similar to those of pH-sorption percentages (in Fig. 2); On the pH–C e plane, the projection on the pH–C e plane is just the inverted image of the projection on the pH–q e plane; On the C e–q e plane, the projection is a straight line containing all experimental data, which is due to the same initial concentration of 60Co(II) for each experimental point. The following equation can describe the relationship of C e–q e:

$$ VC_{0} = mq_{\text{e}} + VC_{\text{e}} $$
(8)

Equation 8 can be rearranged as:

$$ q_{\text{e}} = C_{0} \frac{V}{m} - C_{\text{e}} \frac{V}{m} $$
(9)

where V is the volume and m is the mass of CHAP. Thereby, the experimental data of C e–q e lies in a straight line with slope (−V/m) and intercept (C 0 V/m). The slope and the intercept calculated from C e to q e line are −1.67 and 16.67, which are quite in accordance with the values of V/m = 1.67 L/g and C 0 V/m = 16.67 mg/g. The 3D plots show the relationship of C e, pH and q e very clearly, i.e., all the data of C e–q e lie in a straight line with slope −V/m and intercept C 0 V/m at same initial concentrations and same solid contents [55, 56].

Fig. 4
figure 4

3D plots of pH, C e and q e of 60Co(II) sorption on CHAP, T = 303.15 K, m/V = 0.6 g/L, C Co(II)initial = 1.67 × 10−4 mol/L

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Effect of foreign ions

In order to investigate the influence of background electrolyte ions on 60Co(II) sorption, the sorption of 60Co(II) on CHAP was studied as a function of pH values in 0.01 mol/L NaClO4, NaCl, NaNO3, KClO4 and LiClO4, respectively. Figure 5a shows that the sorption of 60Co(II) on CHAP is clearly influenced by the cations in the suspension. At pH < 10.5, the sorption percentage of 60Co(II) on CHAP under the same pH values are in the following sequence: Li+ > Na+ > K+, indicating that the cations can alter the surface property of CHAP, and thus can influence the sorption of 60Co(II) on CHAP [57]. The sorption of 60Co(II) on CHAP can be considered as a competition of 60Co(II) with Li+ (or Na+, K+) at CHAP surfaces. The hydration radius of the three cations is K+ = 2.32 Å, Na+ = 2.76 Å and Li+ = 3.4 Å [58, 59]. The hydration radio of K+ is smaller than those of the other two cations and therefore the influence of K+ on 60Co(II) sorption is more obvious than those of Na+ and Li+. However, at pH > 10.5, no drastic difference of 60Co(II) sorption to CHAP in LiClO4, NaClO4 and KClO4 solutions is observed, which may be attributed to the inner-sphere surface complexation or surface precipitatation at high pH values. Esmadi and Simm [59] investigated the effect of Li+, Na+ and K+ on the sorption of 60Co(II) to amorphous ferric hydroxide and similar results were also found. Xu et al. [60] studied the sorption of Th(IV) on rectorite and also found similar results.

Fig. 5
figure 5

Effect of foreign ions on 60Co(II) sorption on CHAP as a function of pH, T = 303.15 K, m/V = 0.6 g/L, C Co(II)initial = 1.67 × 10−4 mol/L

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From Fig. 5b, one can see that the sorption of 60Co(II) on CHAP is influenced by the background electrolyte anions. The effect of anions on the sorption of 60Co(II) to CHAP under the same pH values is in the following sequence: ClO4 − > NO3 − > Cl−. This phenomenon might be attributed to the facts that: (1) idiocratic sorption of Cl− to CHAP surface is a little easier than NO3 − and ClO4 −, and Cl− sorption on the surface of CHAP changes the surface properties of CHAP and decreases the availability of binding sites for 60Co(II); (2) Cl− and NO3 − can form soluble complexes with 60Co(II) (e.g., CoCl+ and CoNO3 +), whereas ClO4 − can not. 60Co(II) has higher affinity with Cl− than NO3 − and ClO4 −; and (3) the inorganic acid radical radius order is ClO4 − > NO3 − > Cl−, the smaller radius of inorganic acid radicals takes up more ionic exchange sites and results in the decrease of 60Co(II) sorption on CHAP [28].

Effect of HA/FA

The sorption of 60Co(II) on CHAP in the absence and presence of humic acid (HA) and fulvic acid (FA) as a function of pH is shown in Fig. 6. It is very interesting to notice that the presence of FA/HA decreases the sorption of 60Co(II) on HA/FA-CHAP hybrids at pH < 11.0. The surface charge of CHAP is negative and the sorption of negatively charged HA/FA on the negatively charged CHAP surface decreases with increasing pH due to electrostatic repulsion [61–63]. This causes the formation of soluble complexes of HA/FA-60Co(II) in solution, and the strong complexation ability of HA/FA with 60Co(II) results in the decrease of 60Co(II) sorption [64]. The negative effect of HA/FA on 60Co(II) sorption to HA/FA-CHAP is different to most metal ion sorption to HA/FA bound materials [56, 65–69]. In the studies mentioned above, the surface charge of the materials is positive at low pH and the sorption of HA/FA on the positively charged surface of solid particle is high. The surface adsorbed HA/FA forms strong surface complexes with radionuclides on solid surfaces, and thereby enhances the sorption of radionuclides at low pH values [70–72]. At pH > 11.0, one can see that there is no drastic difference in the sorption of 60Co(II) on bare and HA/FA-bound CHAP particles. The influence of HA/FA on 60Co(II) sorption at pH > 11.0 can be negligible because of the high sorption or precipitation of 60Co(II) at CHAP surfaces. The chemical bonds between 60Co(II) and the surface functional groups of CHAP or HA/FA functional groups may be very similar at pH > 11.0. The influence of HA/FA on Co(II) sorption is also dependent on the macromolecular structures of HA/FA and the sorption ability of Co(II) with CHAP at different pH values. At low pH values, the sorption of Co(II) on CHAP is weaker than that of Co(II) with HA/FA, whereas the sorption of Co(II) on CHAP is much strong at high pH and the presence of FA/HA can not affect Co(II) sorption on CHAP obviously [73, 74].

Fig. 6
figure 6

Effect of HA/FA on 60Co(II) sorption on CHAP as a function of pH, T = 303.15 K, m/V = 0.6 g/L, C HA = 10 mg/L, C FA = 10 mg/L, C Co(II)initial = 1.67 × 10−4 mol/L

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It is very interesting to find that the sorption curve of 60Co(II) to CHAP in the presence of HA is quite similar to that of 60Co(II) to CHAP in the presence of FA. HA and FA are chemically heterogeneous compounds that contain different types of functional groups at different proportions and configurations. HA and FA contain carboxyl groups, amine groups and phenolic groups [63], and these functional groups play an important role in affecting 60Co(II) sorption to CHAP. The samples of HA and FA were extracted from the same soil samples, and thus they might have similar functional groups such as carboxyl and phenolic groups. As is illustrated in Table 1, the quantitative concentrations of functional groups of HA are very similar to that of FA. It is clearly observed in Fig. 6 that the influence of HA is very similar to that of FA. These similar functional groups and sorption property of HA and FA may interpret the similar sorption curve of 60Co(II) to CHAP in the presence of HA/FA.

Table 1 The concentrations of functional groups of HA and FA calculated from potentiometric titration by using FITEQL 3.1
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Effect of CHAP content

The sorption of 60Co(II) on CHAP as a function of CHAP content is shown in Fig. 7. The distribution coefficient, K d, value as a function of the CHAP content is also plotted in Fig. 7.

Fig. 7
figure 7

Effect of solid content on 60Co(II) sorption on CHAP, pH = 6.5 ± 0.1, I = 0.01 mol/L NaClO4, T = 303.15 K, C Co(II)initial = 1.67 × 10−4 mol/L

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The distribution coefficient (K d ) value was derived from the following equation:

$$ K_{\text{d}} = {\frac{{C_{0} - C_{\text{e}} }}{{C_{\text{e}} }}}\frac{V}{m} $$
(10)

where C 0 and C e are defined above, m is the mass of adsorbent (g), and V is the volume of the suspension (L).

One can see that the removal of 60Co(II) from solution to CHAP increases with increasing CHAP content. With increasing solid content, more surface sites is available to bind 60Co(II) at CHAP. Thereby, more 60Co(II) ions are adsorbed on CHAP at high solid content. The K d maintains a level with increasing CHAP content, which is in consistent with the physicochemical properties of the distribution coefficient, i.e., the K d value is independent of solid content at low solid and liquid concentrations [75, 76].

Effect of temperature and thermodynamic study

Figure 8 shows the sorption isotherms of 60Co(II) on CHAP at three different temperatures. One can see that the sorption isotherm is the highest at T = 343.15 K and is the lowest at T = 303.15 K, which indicates that the sorption of 60Co(II) on CHAP is promoted at higher temperature. It is reasonable that the sorption of 60Co(II) increases with increasing temperature. Several factors may account for this phenomenon. Increased diffusion rate of adsorbate molecules into the CHAP pores due to increased temperature may account for the observed behavior [77]. Changes in the adsorbent pore sizes as well as an increase in the number of sorption sites due to the breaking of some internal bonds near the edge of the particle are expected at higher temperatures [78]. An increase in temperature may also affect an increase in proportion and activity of 60Co(II) in solution, the affinity of the ions to the surface, and therefore affect the potential of the surface [79, 80].

Fig. 8
figure 8

Sorption isotherms of 60Co(II) on CHAP at three different temperatures, I = 0.01 mol/L NaClO4, pH = 6.5 ± 0.1, m/V = 0.6 g/L

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Three different models, viz. Langmuir, Freundlich and D–R isotherm equations, are employed to simulate the sorption isotherms and to establish the relationship between the amount of 60Co(II) adsorbed on solid phase and the concentration of 60Co(II) remaining in solution.

The Langmuir isotherm model is used to describe the monolayer sorption process. It can be represented by the following equation [63, 81]:

$$ q_{\text{e}} = {\frac{{bq_{\max } C_{\text{e}} }}{{1 + bC_{\text{e}} }}} $$
(11)

Equation 11 can be expressed in linear form:

$$ {\frac{{C_{\text{e}} }}{{q_{\text{e}} }}} = {\frac{1}{{bq_{\max } }}} + {\frac{{C_{\text{e}} }}{{q_{\max } }}} $$
(12)

where C e is the equilibrium concentration of 60Co(II) remained in solution (mol/L); q e is the amount of 60Co(II) adsorbed on per weight unit of solid after equilibrium (mol/g); q max is the amount of 60Co(II) at complete monolayer coverage (mol/g) and b (L/mol) is a constant that relates to the heat of sorption.

The Freundlich isotherm model allows for several kinds of sorption sites on the solid surface and represents properly the sorption data at low and intermediate concentrations on heterogeneous surfaces [82]. The model has the following form:

$$ q_{\text{e}} = k_{\text{F}} C_{\text{e}}^{n} $$
(13)

Equation 14 can be expressed in linear form:

$$ \log q_{\text{e}} = \log k_{\text{F}} + n\log C_{\text{e}} $$
(14)

where k F (mol1−n L n/g) represents the sorption capacity and n represents the degree of dependence of sorption with equilibrium concentration.

The D–R isotherm model is more general than the Langmuir isotherm since it does not assume a homogeneous surface or constant sorption potential [59, 83]. The D–R equation has the general expression:

$$ q_{\text{e}} = q_{\max } \exp \left( { - \beta \varepsilon^{2} } \right) $$
(15)

Equation 15 can be expressed in linear form:

$$ \ln q_{\text{e}} = \ln q_{\max } - \beta \varepsilon^{2} $$
(16)

where q e and q max are defined above, β is the activity coefficient related to the mean sorption energy (mol2/kJ2), and ε is the Polanyi potential, which is equal to:

$$ \varepsilon = RT\ln \left( {1 + {\frac{1}{{C_{\text{e}} }}}} \right) $$
(17)

where R is ideal gas constant (8.314 J/(mol K)), and T is the absolute temperature in Kelvin (K).

The experimental data of 60Co(II) sorption are analyzed with the Langmuir, Freundlich and D–R models, and the results are shown in Fig. 9. The relative values calculated from the three models are listed in Table 2. It can be concluded from the correlation coefficients that Langmuir model simulates the experimental data better than Freundlich and similar to D–R models. The fact that the Langmuir isotherm fits the experimental data very well suggests almost complete monolayer coverage of the adsorbent particles. Moreover, CHAP has a limited sorption capacity, thus the sorption could be better described by Langmuir model than by Freundlich, since an exponentially increasing sorption was assumed in the Freundlich model [8]. The values of q max obtained from the Langmuir model for 60Co(II) sorption on CHAP are the highest at T = 343.15 K and the lowest at T = 303.15 K, which indicates that the sorption is enhanced with increasing temperature. In the Freundlich model, the value of n is from unity, which indicates that a nonlinear sorption takes place on the heterogeneous surfaces. The sorption capacities q max derived from the D–R model are higher than those derived from the Langmuir model. This may be ascribed to the different assumptions considered in the formulation of the isotherms [30, 45].

Fig. 9
figure 9

Langmuir (a), Freundlich (b) and D–R (c) isotherm for 60Co(II) sorption on CHAP at three different temperatures, I = 0.01 mol/L NaClO4, pH = 6.5 ± 0.1, m/V = 0.6 g/L

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Table 2 The parameters for Langmuir, Freundlich and D–R sorption isotherms of 60Co(II) on CHAP at different temperatures
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The thermodynamic parameters (∆H 0, ∆S 0 and ∆G 0) for 60Co(II) sorption on CHAP are calculated from the temperature dependent sorption isotherms. The values of enthalpy (∆H 0) and entropy (∆S 0) can be calculated from the slope and intercept of the plot of lnK d vs. 1/T (Fig. 10):

$$ \ln K_{\text{d}} = {\frac{{\Updelta S^{0} }}{R}} - {\frac{{\Updelta H^{0} }}{RT}} $$
(18)

where R and T are defined above. The Gibbs free energy change (∆G 0) is calculated from the following equation:

$$ \Updelta G^{0} = \Updelta H^{0} - T\Updelta S^{0} $$
(19)

The values obtained from Eqs. 18 and 19 are tabulated in Table 3. The evaluation of thermodynamic parameters provides an insight into the mechanism of 60Co(II) sorption to CHAP. As is expected for a spontaneous process under the experimental conditions, it is clear that the Gibbs free energy change (∆G 0) of 60Co(II) sorption on CHAP is more negative at higher temperature, which demonstrates that the spontaneity of the sorption process increases with the rise in temperature. At high temperature, cations are readily desolvated and hence its sorption becomes more favorable. A positive value of the standard enthalpy change (∆H 0) indicates that the sorption process is endothermic. The positive ∆S 0 value suggests the affinity of CHAP toward 60Co(II) ions in aqueous solutions and may suggest some structure changes on the sorbents [28].

Fig. 10
figure 10

Linear plots of lnK d versus 1/T for 60Co(II) sorption on CHAP at three different temperatures, I = 0.01 mol/L NaClO4, pH = 6.5 ± 0.1, m/V = 0.6 g/L

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Table 3 Values of thermodynamic parameters for the sorption of 60Co(II) on CHAP
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Conclusions

In this study, batch technique was adopted to investigate the sorption of 60Co(II) from simulated wastewater onto CHAP as a function of various environmental factors such as contact time, pH, ionic strength, coexisting electrolyte ions, humic substances and temperature under ambient conditions. The sorption percentage of 60Co(II) weakly increases with increasing pH values at pH < 4.0, sharply increasing at 4.0 < pH < 10.5, and then maintains the high level at pH > 10.5. The sorption of 60Co(II) is dependent on ionic strength at low pH values, and independent of ionic strength at high pH values. The thermodynamic analysis derived from temperature dependent sorption isotherms suggests that the sorption process of 60Co(II) on CHAP is spontaneous and endothermic. By integrating all the above-mentioned analysis, one can conclude that the sorption of 60Co(II) on CHAP is dominated by ion exchange or outer-sphere surface complexation at low pH values, and by inner-sphere surface complexation or precipitation at high pH values. Considering low cost, accessibility, ubiquitous presence in catering industry and large-scale applications of eggshell, one can conclude that CHAP, which is prepared by eggshell waste, has a great application potential for cost-effective material for 60Co(II) preconcentration from large volumes of aqueous solutions.