The use of sea shell (Donax trunculus) powder to remove Sr(II) ions from aqueous solutions

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.

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.

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.

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).

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.

The point of zero charge (pH_pzc) 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 (pH_i) 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 (pH_f) were measured again. Then the pH_pzc is the point where the curve of ΔpH (pH_i−pH_f) versus pH_i crosses the line (Smiciklas et al. 2000).

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.

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).

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

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.

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).

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 pH_pzc of the adsorbent (pH_pzc = 5.1). At a pH above this pH_pzc, 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 (pH_pzc) 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.

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.

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.

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.

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).

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

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

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.

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.

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, ΔG^o, ΔH^o and ΔS^o, 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.

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