Abstract

This study aimed to investigate and explore the possibility of using ground sea shell powder (Donax trunculus) (SSP) for sorption of Sr(II) ions from aqueous solutions. The maximum removal of Sr(II) removal in SSP was 60% at initial Sr(II) concentration of 25 mg/L, at pH 6.0, contact time 120 min, temperature 25 °C and volume/mass ratio equivalent to 500. Sorption data were interpreted considering the adsorption isotherms and thermodynamic parameters calculated. The maximum Sr(II) adsorption on SSP was 25.45 mg/g at pH 6.0. Freundlich isotherm and Dubinin–Radushkevich models were seen to be more compatible with the sorption equilibrium. The nature of the process was spontaneous and exothermic. The results suggest that SSP could be used as an efficient and cost-effective adsorbent to remove strontium ion.

INTRODUCTION

Nuclear power plants all over the world produce great amounts of low-level radioactive wastewater. Strontium-90 is one of the most common and toxic radionuclides present in the radioactive liquid wastes due to its complexity and long physical half-life. Given the chemical similarity of strontium to calcium, it can accumulate in bone tissues with the result that inevitable carcinogenic effects occur. Therefore, concentration and separation of Sr(II) ions from waste solutions are very crucial in terms of environment and human health.

There are many techniques to remove metal ions from aqueous solutions (Guana et al. 2011; Fuks et al. 2015), one of which is the adsorption process as an effective and economically feasible method used to remove metal ions from wastewaters and water supplies. In order to eliminate Sr ions from aqueous solutions, organic, inorganic and biological sorbents have been widely used (Moon et al. 2000). New cost-effective, easily available and highly functional adsorbents have been recently developed. Biological materials of calcium-rich content such as bones, eggshells and sea shells are cheap, effective and promising adsorbents. Dimović et al. (2009) investigated removal of the Co2+ from wastewater using processed animal bones. Fishbone was used to remove cobalt and strontium from groundwater (Park et al. 2013). The biosorption of Cu(II), Pb(II) and Zn(II) ions were studied using eggshell (Putra et al. 2014).

Sea shell is a natural material generally found in beaches. It is generally composed of outer layers of proteins, which are followed by an intermediate layer of calcite and a smooth inner layer of calcium carbonate crystals (Chowdhury & Saha 2010). Most sea shell studies on adsorption focus on textile wastes such as brilliant red HE-3B (Suteu et al. 2011), methylene blue (Suteu and Rusu 2012) and basic green 4 (malachite green) (Chowdhury & Saha 2010). There are few studies on metal adsorption in the literature. Li et al. (2013) studied Pb2+ adsorption kinetics and isotherms using sea shell powder composite, Ismail and Aris (2013) investigated Cd2+ adsorption using the biogenic CaCO3 clamshells and Baraka et al. (2010) studied Cu(II) sorption by means of natural aragonite and alkaline-treated natural aragonite sea shell.

The present work is the first report to determine the feasibility of sea shell powder (SSP) as a low-cost alternative adsorbent for removal of Sr(II) from diluted aqueous solutions. The effects of pH, initial Sr(II) concentration, contact time and temperature on strontium adsorption were investigated. Characterization of the samples, adsorption isotherms and thermodynamic parameters were also assessed and reported. Sr(II) adsorption capacity of the natural SSP and other adsorbents in the literature was also compared.

EXPERIMENTAL METHODS

Chemicals

All chemicals and reagents used in the study were of analytical grade and used without further purification. A stock solution of strontium was prepared by dissolving an appropriate amount of SrCl2.6H2O (Merck) in distilled deionized water. Considering the same chemical reactivity of radioactive and stable Sr2+ isotopes, a natural isotope of strontium (88Sr) was used instead of 90Sr in terms of economy and safety. The pH of each solution was adjusted to the required value using diluted HNO3 and NaOH solutions.

Instrumentation

Sr concentrations in aqueous solutions were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Perkin Elmer Optima DV 2000). The adsorption experiments were studied by batch technique using a thermostated shaker bath, model GFL-1083. The pHs of all solutions were measured by a Hanna Instrument, model 8521, pH meter. A Hettich Zentrifugen Rotofix 32 digital centrifuge was used to centrifuge the samples. An oven (Electro-Mag M420 model) and high temperature furnace (Carbolite Furnaces CSF-1100) were employed to dry and calcine the samples, respectively.

Preparation of sea shell powder of Donax trunculus

The molluscs (Donax trunculus) known as bivalves were collected from Izmir Bay, Turkey, and properly rinsed with tap and distilled water. The shell fraction was washed thoroughly in deionized-distilled water and dried at 105 °C for 2 h. It was then ground and sieved to 0.125 mm particle size by a set of standard sieves. Sea shell powder was calcined at 900 °C in air for 8 h to remove any organic materials and transform it into calcium oxide (CaO) (Hench & Kokubo 1998).

Characterization

The resulting sorbents were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The IR spectra of the prepared samples were recorded in the 4,000–400 cm−1 range using a Perkin Elmer BX2 FT-IR spectrometer with KBr pellet containing 2–6 mg of the sample. The samples were analyzed on a Rigaku (D/MAX-2200/PC) X-ray powder diffractometer in a flat plane geometry using a source of Cu Kα with a wavelength of 0.154 nm. The patterns were recorded in the range of 10–80° (2θ). The 2θ scanning speed was 4°/min. The surface morphology of the samples was characterized by SEM (JEOL JSM-6060). Surface characteristics of the prepared SSP were carried out by a Quantachrome Autosorb-1 instrument (QUANTACHROME, version 1.51) at 77 K. The materials were vacuum-dried at 120 °C for 6 h prior to analysis. The Brunauer, Emmett, Teller (BET) surface area, pore volume and pore size of the sorbent were then determined.

Determination of pHpzc

The point of zero charge (pHpzc) of the adsorbents was determined by batch equilibration technique using the following procedure: 20.0 mL of 0.10 mol/L KNO3 solution was placed in a closed capped Erlenmeyer flask. Initial pH values (pHi) of KNO3 solutions were adjusted from ∼4.0 to ∼11.0 by addition of 0.1 M HNO3 or KOH. A suspension of SSP was allowed to equilibrate for 24 h in a shaker thermostated at 25 °C. Then the suspensions were filtered through a Whatman filter paper no. 44 and the pH values (pHf) were measured again. Then the pHpzc is the point where the curve of ΔpH (pHi−pHf) versus pHi crosses the line (Smiciklas et al. 2000).

Batch sorption experiments

All sorption experiments were performed by the batch technique using 0.01 g of the sorbent suspended in 10 mL of strontium solution in a polyethylene flask at selected pH. The flasks were shaken at different temperatures and contact times. The effects of sorption parameters such as contact time (15–300 min), pH (2–10), Sr(II) concentration (10–50 mg/L), liquid/mass (V/m) ratio (500–2,500) and temperature (15–55 °C) on the sorption of Sr(II) were determined by changing a parameter and keeping others constant.

The supernatants were separated from the solids by centrifugation at 4,000 rpm for 5 min once the adsorption equilibrium had been established. Concentration of Sr(II) in aqueous solution was determined using a Perkin Elmer Optima 2000 DV ICP-OES. The amount of the adsorbed Sr(II) ions was calculated from the difference in the Sr(II) concentration in aqueous solution before and after the adsorption. The results were expressed as the sorption capacity of Sr(II) ions sorbed per gram sorbent (qe, mg/g) and sorption efficiency (Sorp.%), which were calculated by the following:
formula
(1)
formula
(2)
where Ci is initial and Ce equilibrium concentrations of Sr (mg/L) in solutions, W the weight of the sorbent (g) and V the volume of the aqueous phase (mL). The blank experiments found sorption of Sr on the walls of the flask to be negligible. All the experiments were performed in duplicates with experimental errors within ±3%.

RESULTS AND DISCUSSION

Characterization of SSP

An FT-IR spectrum of the SSP is shown in Figure 1(a), illustrating the peak at 573 cm−1 corresponding to CaO and that at 3,634 cm−1 to stretching mode of OH groups. Those between 2,040 and 2,252 cm−1 represent stretching mode of C=O groups, C—O stretching that at 1,424 cm−1 and C—O bending peak at 870 cm−1. Those at 573 cm−1 and at 3,634 cm−1 confirm the presence of CaO and Ca(OH)2 in the SSP adsorbent. The data of C=O and C—O bands confirm the small amount of CaCO3 in the SSP sample (Witoon 2011).

Figure 1

(a) FT-IR spectrum of the SSP, (b) XRD pattern of the SSP and (c) SEM image of SSP surface.

Figure 1

(a) FT-IR spectrum of the SSP, (b) XRD pattern of the SSP and (c) SEM image of SSP surface.

An X-ray diffractogram of the powder sample is presented in Figure 1(b). The peaks for the SSP appeared at 2θ = 32.2°, 37.3°, 53.8°, 64.1°, 67.3° and 79.6°, which were characteristics of CaO (JCPD card no. 37-1497). Ca(OH)2 was also observed at 2θ = 18.1°, 28.6°, 34.1°, 47.1°, 50.7°, 64.2°, 71.7° and 84.7° (JCPD card no. 4-0733) (Ślsarczyk et al. 2005).

SEM images of the surface of SSP are shown in Figure 1(c). The SEM figure can illustrate that SSP has a rough and irregular surface. Further, cavities of the surface showed a large exposed surface area for the sorption of Sr(II).

BET surface area, total pore volume and average pore diameter of the sorbent were determined as 66.41 m2/g, 0.0003 cm3/g and 69.56 Å, respectively. Chowdhury & Saha (2010) carried out adsorption of basic green 4 (BG 4) dye by SSP from aqueous solutions. A method involving a drying process at 383 ± 1 K for 24 hours was used to prepare sea shell adsorbent and it was found that the surface area and average pore diameter of the this non-calcined SSP were 3.6 m2/g and 38 Å, respectively. It follows from the comparison of results that the calcination process increased the surface area and pore diameter of the SSP.

The pHpzc of the SSP determined by pH measurement technique was 5.1 (Figure 2).

Figure 2

Point of zero charge of SSP.

Figure 2

Point of zero charge of SSP.

Effect of contact time

Short equilibrium time is one of the most important parameters in efficient wastewater treatment. The effect of contact time on the sorption of Sr(II) onto SSP was studied for 15–300 minutes of shaking time. Figure 3 shows sorption efficiency (Sorp.%) as a function of contact time. The sorption of Sr(II) reached a maximum value for SSP in 120 min. We concluded that there is no significant difference between 5 and 300 min. Due to ease of operation, 120 min was selected for SSP in all the further experiments.

Figure 3

The effect of contact time on the sorption of Sr(II) ions with SSP (Sr(II) conc. 25 mg/L, m: 0.01 g, V: 10 mL, T: 25 °C).

Figure 3

The effect of contact time on the sorption of Sr(II) ions with SSP (Sr(II) conc. 25 mg/L, m: 0.01 g, V: 10 mL, T: 25 °C).

Effect of pH

The pH of a solution is an important factor for the sorption process because the ionization of surface functional groups and change of the solution composition affect the sorption process. The sorption of Sr(II) on the SSP was studied using the initial pH ranging from 2 to 10. The effect of pH on the sorption of Sr(II) ions on the sorbent is shown in Figure 4. When pH is low (pH ≤ 4), the SSP sorbent has practically no more affinity to Sr(II) ions because the high concentration of H+ increases the positively charged CaOH2+ with a net positively charged surface of the sorbent (Chen et al. 2009). Therefore, much fewer Sr(II) ions are adsorbed. When pH is high (pH > 6), precipitation starts due to the formation of complexes in the aqueous solution and then the adsorption decreases (Yusan & Erenturk 2011).

Figure 4

The effect of pH on the sorption of Sr(II) ions with SSP (Sr(II) conc. 25 mg/L, m: 0.01 g, V: 10 mL, t: 120 min, T: 25 °C).

Figure 4

The effect of pH on the sorption of Sr(II) ions with SSP (Sr(II) conc. 25 mg/L, m: 0.01 g, V: 10 mL, t: 120 min, T: 25 °C).

As pH increases, functional groups are progressively deprotonated, forming a negative oxidized SSP charge. The attractive forces between the anionic surface sites and cationic metal ions easily result in the formation of metal–ligand SSP complexes (Chen et al. 2009). For the above reasons, the optimum pH of SSP was selected as 6.

Studies in the literature have suggested that strontium precipitates when working above pH 6 (Torab-Mostaedi et al. 2011). Based on this, we carried out a blank experiment. The experiment was done without adsorbent in identical conditions to the previous adsorption experiments above pH 6. The concentration of Sr(II) was measured by ICP-OES, and found to be lower than 25 mg/L in the solution; therefore we can conclude that at high pH > 6 Sr(II) is precipitated. Thus, the pH was limited to values equal to 6 because of metal precipitation at higher pH.

This behavior can be explained by the pHpzc of the adsorbent (pHpzc = 5.1). At a pH above this pHpzc, the surface of the SSP becomes negatively charged, which enhances the adsorption of positively charged Sr(II) ions through the electrostatic force of attraction, and at pH value lower than 5.1 (pHpzc) the surface of the adsorbent was positively charged, inhibiting the adsorption of Sr(II) ions due to the electrostatic repulsion between the cationic structure of strontium and adsorbent. The maximum adsorption at pH 6 may be due to the development of negative charge on the surface of the SSP.

Effect of Sr concentration

The effect of initial Sr ion concentration was studied in the range of 10–50 mg/L. The results are presented in Figure 5. It is clear from Figure 5 that the sorption amount of Sr(II) ions generally increases due to the elevations of initial Sr(II) concentration. The increase in the sorption capacity of the sorbent with increasing initial Sr(II) concentration could be accounted for by higher likelihood of collision between ions and the calcium-based sorbents (Lia et al. 2010). When the application areas are taken into account (0–100 mg/L for low and intermediate level radioactive liquid waste), Sr concentration of 25 mg/L is regarded as an appropriate value (Liu et al. 2014). Therefore, 25 mg/L Sr(II) concentration was selected for further experiments.

Figure 5

The effect of Sr(II) ions concentration on the sorption of Sr(II) ions with SSP (pH: 6, t: 120 min, m: 0.01 g, V: 10 mL, T 25 °C).

Figure 5

The effect of Sr(II) ions concentration on the sorption of Sr(II) ions with SSP (pH: 6, t: 120 min, m: 0.01 g, V: 10 mL, T 25 °C).

Effect of volume of the aqueous solution to weight of adsorbent ratio

The V/m ratio is the ratio of initial Sr volume (mL) to the amount of adsorbent (g). In experiments, the volume of Sr solution was varied between 5 and 25 mL, keeping the adsorbent amount constant: m = 0.01 g. The effect of V/m ratio on the sorption of Sr(II) ions was studied using constant sorbent dosage and different volumes varied between 5 and 25 mL Figure 6 shows the variation of the sorption yield vs. V/m ratio.

Figure 6

The effect of volume/mass ratio on the sorption of Sr(II) ions with SSP (Sr(II) conc. 25 mg/L, pH: 6, m: 0.01 g, t: 120 min, T: 25 °C).

Figure 6

The effect of volume/mass ratio on the sorption of Sr(II) ions with SSP (Sr(II) conc. 25 mg/L, pH: 6, m: 0.01 g, t: 120 min, T: 25 °C).

The sorption percentage increased as V/m ratio decreased from 2,500 to 500. The results indicated that the sorption efficiency was based on the volume of the solution, which could be explained by a decrease in concentration gradient. Considering maximum sorption yield in Figure 6, the V/m was chosen as 500 in all the experiments for later studies.

Thermodynamics studies

The thermodynamic parameters for the sorption process were calculated as follows:
formula
(3)
formula
(4)
formula
(5)
where Kd is the distribution coefficient (mL/g), ΔS° standard entropy (J/(mol K)), ΔH° standard enthalpy (kJ/mol), T the absolute temperature (K) and R the gas constant (8.314 J/(mol K); Ci is initial and Ce equilibrium concentrations of Sr (mg/L) in solutions, W the weight of the sorbent (g) and V the volume of the aqueous phase (mL). The experiments were conducted at 288.15 K, 298.15 K, 308.15 K, 318.15 K and 328.15 K. The values of ΔH° and ΔS° were determined from the slopes and intercepts of the plots of lnKd vs. 1/T (Figure 7) for the adsorbent. ΔG° was also calculated using Equation (4). The thermodynamic parameters for the adsorption of Sr(II) on SSP are illustrated in Table 1.
Figure 7

Plots of ln Kd versus 1/T for Sr sorption on SSP.

Figure 7

Plots of ln Kd versus 1/T for Sr sorption on SSP.

Table 1

Thermodynamic parameters of Sr (II) sorption on SSP

SorbentΔHo (kJ/mol)ΔSo (J/(mol K))ΔGo (kJ/mol)
288 K298 K308 K318 K328 K
SSP −8.72 0.016 −13.33 −13.49 −13.65 −13.81 −13.97 
SorbentΔHo (kJ/mol)ΔSo (J/(mol K))ΔGo (kJ/mol)
288 K298 K308 K318 K328 K
SSP −8.72 0.016 −13.33 −13.49 −13.65 −13.81 −13.97 

The study shows that negative value of ΔH° confirms the exothermic nature of the sorption process. When the value of ΔH° is lower than 40 kJ/mol, the type of sorption can be accepted as a physical process. The value of ΔH° was −8.72 kJ/mol for SSP indicating that the sorption was physical by nature and thus involved weak forces of attraction (Ho & McKay 1999), which is consistent with the results of the isotherm models. The positive value of ΔS° reflects the affinity of the sorbent material to Sr(II) and the negative values of ΔS° show that the sorption process is spontaneous(Gok et al. 2013).

Sorption isotherms

The adsorption equilibrium data of strontium onto SSP were analyzed under Freundlich, Langmuir and Dubinin–Radushkevich (D-R) sorption isotherm models.

Langmuir isotherm

This model suggests monolayer sorption on a homogeneous surface without interaction between sorbed molecules and also assumes uniform energies of sorption on the surface (Langmuir 1918). Linear form of the Langmuir equation can be expressed as follows:
formula
(6)
where qe (mg/g) is the amount of metal ions sorbed onto adsorbent, Ce (mg/L) the equilibrium concentration of strontium in solution, and Q0 (mg/g) and bL (L/mg) the Langmuir constants related to sorption capacity and sorption energy, respectively. Q0 and bL values were calculated (figure not shown) and are shown in Table 2.
Table 2

Sorption isotherm constants for the sorption of Sr on SSP

Isotherm modelsParametersValue
Langmuir Q0 (mg/g) 25.445 
bL (L/mg) 0.012 
R2 0.870 
RL 0.758 
Freundlich KF (mg/g) 0.170 
1/nF 1.337 
R2 0.973 
Dubinin–Radushkevich Xm (mg/g) 3.6 × 10−2 
E (kJ/mol) 0.59 
R2 0.969 
Isotherm modelsParametersValue
Langmuir Q0 (mg/g) 25.445 
bL (L/mg) 0.012 
R2 0.870 
RL 0.758 
Freundlich KF (mg/g) 0.170 
1/nF 1.337 
R2 0.973 
Dubinin–Radushkevich Xm (mg/g) 3.6 × 10−2 
E (kJ/mol) 0.59 
R2 0.969 

The correlation regression coefficient (R2) value is lower than that of the other models for the SSP, indicating that the Langmuir model failed to adequately describe the relationship between the sorbed amount of Sr(II) on SSP and its equilibrium concentration in the solution.

The Langmuir parameters can be used to predict the affinity between the sorbate and the sorbent using the dimensionless separation factor, RL defined by Weber & Chakravorti (1974) as follows:
formula
(7)
where bL is the Langmuir isotherm constant from non-linear regression and C0 the initial strontium concentration (mg/L). The RL value explains the sorption process as follows: RL > 1, unfavorable; RL = 1, linear; 0 < RL < 1, favorable and RL = 0, irreversible (Kiran & Kaushik 2008).

The value of RL for the present sorbent is in the range of 0–1, confirming a favorable uptake of strontium by SSP.

Freundlich isotherm

The Freundlich isotherm assumes heterogeneous surface of the sorbent and its linearized equation is expressed below (Sharma & Tomar 2008):
formula
(8)
where KF is the sorption capacity of sorbent (mg/g) and nF a constant related to sorption intensity (dimensionless). Their values are given in Table 2. The linear plot of log qe versus log Ce shows the applicability of the isotherm for strontium sorption on SSP (Figure 8(a)).
Figure 8

(a) Freundlich and (b) D-R isotherm plots for the sorption of Sr(II) ions on SSP.

Figure 8

(a) Freundlich and (b) D-R isotherm plots for the sorption of Sr(II) ions on SSP.

The sorption process was found to be favorable based on the values of 1/n. A 1/n value higher than 1 indicates that cooperative adsorption has occurred (Foo & Hameed 2010). The value of R2 from the Freundlich isotherm model (0.973) was higher than in the Langmuir isotherm model (0.870), suggesting that the Freundlich isotherm model is more suitable to explain the experimental data.

D-R isotherm

In general, the D-R isotherm model is based on experimental data to determine whether the nature of sorption processes are physical or chemical (Foo & Hameed 2010). The D-R isotherm model is applicable in low concentration ranges and can be used to describe sorption on both homogeneous and heterogeneous surfaces. The D-R isotherm depicts a sorption on a single type of uniform pores, indicating that the characteristic sorption curve is related to the porous structure of the sorbent. The linearized form of the equation is as follows:
formula
(9)
where Cads (mol/g) is the amount of solute sorbed per unit weight of solid, Xm (mol/g or mg/g) the sorption capacity, β ((mol/J)2) a constant related to energy and Ɛ the Polanyi potential which is calculated using the following:
formula
(10)
where R is a gas constant in J/(mol K) and T temperature in Kelvin. If lnCads is plotted against Ɛ2 (Figure 8(b)), β and Xm will be obtained from the slope and intercept, respectively (Table 2). The D–R isotherm model fitted to the equilibrium data for the SSP sorbent well since the R2 value was 0.969.
The mean energy of sorption (E) is also calculated by the following:
formula
(11)
E is useful to estimate the type of sorption process.

If E < 8 kJ/mol, the sorption process is considered to be physical sorption: if E is between 8 and 16 kJ/mol, an ion-exchange mechanism has occurred (Walton 1962). The E value was calculated as 0.59 kJ/mol for SSP from which it can be concluded that Sr(II) sorption on the SSP is predominantly performed by physical adsorption.

Comparative study

Table 3 shows the comparison of Sr(II) sorption capacities for various sorbents, such as natural clinoptilolite, aerobic granules, Sb(III)/Sb2O5, ZrO2-TiO2, SBA-15 (Santa Barbara Amorphous-15), graphene oxide, titanate nanofibers, hydroxyapatite (Hap), graphene oxide–hydroxyapatite (GO-Hap), biogenic hydroxyapatite. The present study found Sr(II) sorption capacity of SSP to be 25.45 mg/g with the conclusion that SSP shows a good performance of Sr(II) sorption as compared to many types of sorbents in the literature. This phenomenon suggests that the SSP could find a significantly important place in the list of cost-effective, natural and economical materials used to remove Sr(II) from the aqueous solutions.

Table 3

The comparison of various sorbents for the sorption of Sr(II)

SorbentSorption capacity (mg/g)References
Aerobic granules 37.00 Wang et al. (2015)  
PAN/NaY (polyacrylonitrile–zeolite Y) 70.23 Faghihian et al. (2015)  
Sb(III)/Sb2O5 25.70 Zhang et al. (2015a, 2015b)  
Natural clinoptilolite 9.80 Smičiklas et al. (2007)  
ZrO2-TiO2 16.00 Tel et al. (2010)  
SiO2-TiO2 12.00 Gurboga & Tel (2005)  
SBA-15 (Santa Barbara Amorphous-15) 17.67 Zhang et al. (2015a, 2015b)  
MnO2-ZrO2 nanocomposite 9.34 Ahmadi et al. (2015)  
Graphene oxide 23.83 Romanchuk et al. (2013)  
Bone char 23.74 Smičiklas et al. (2008)  
Hap
GO-Hap 
354.60
702.18 
Wen et al. (2014)  
Dolomite 1.172 Ghaemi et al. (2011)  
Sea shell powder 25.45 Present work 
SorbentSorption capacity (mg/g)References
Aerobic granules 37.00 Wang et al. (2015)  
PAN/NaY (polyacrylonitrile–zeolite Y) 70.23 Faghihian et al. (2015)  
Sb(III)/Sb2O5 25.70 Zhang et al. (2015a, 2015b)  
Natural clinoptilolite 9.80 Smičiklas et al. (2007)  
ZrO2-TiO2 16.00 Tel et al. (2010)  
SiO2-TiO2 12.00 Gurboga & Tel (2005)  
SBA-15 (Santa Barbara Amorphous-15) 17.67 Zhang et al. (2015a, 2015b)  
MnO2-ZrO2 nanocomposite 9.34 Ahmadi et al. (2015)  
Graphene oxide 23.83 Romanchuk et al. (2013)  
Bone char 23.74 Smičiklas et al. (2008)  
Hap
GO-Hap 
354.60
702.18 
Wen et al. (2014)  
Dolomite 1.172 Ghaemi et al. (2011)  
Sea shell powder 25.45 Present work 

CONCLUSION

Sorption behavior of Sr(II) on the sea shell sorbent reveals its significant dependence on the solution pH, contact time, initial strontium concentration, V/m and temperature. The maximum Sr(II) removal on SSP was 60% at pH 6.0 at initial Sr(II) concentration of 25 mg/L, contact time 120 min, temperature 25 °C and amount of sorbent equivalent to 0.01 g. The maximum sorption capacity of the SSP for Sr(II) was 25.45 mg/g. When compared with the literature values, this can be considered as a moderate sorption capacity. Sorption equilibrium was better described by the Freundlich isotherm and D-R models. The mean energy (0.59 kJ/mol) of sorption from the D-R model indicates that the sorption of Sr(II) on SSP could be a physical process. Various thermodynamic parameters, ΔGo, ΔHo and ΔSo, were calculated using the experimental data. The thermodynamics experimental data indicated the spontaneous and exothermic nature of the process. The study exhibited that Sr(II) ions were successfully adsorbed on the SSP. The experiments showed that SSP could be used as a cost-effective and environment-friendly sorbent material to remove toxic Sr(II) ions from wastewaters. We believe that these results provide new ideas for disposal of waste solutions including strontium. The evidence of our laboratory-based studies is promising and should therefore be applicable in industrial wastewater treatment.

ACKNOWLEDGEMENTS

This project was sponsored by Ege University Scientific Research Project Unit Project No. 2012 NBE 005.

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