Abstract

Novel magnetic alginate beads (MagAlgbeads) have been developed by incorporation of magnesium ferrite (MgFe2O4) in alginate beads with the aim of using them in the removal of strontium from aqueous solution. MagAlgbeads were characterized by transmission electron microscopy, scanning electron microscopy, X-ray fluorescence and Fourier transform infrared spectroscopy. The adsorption of strontium onto MagAlgbeads were found to depend on pH and strontium removal increases with increasing pH until pH is 6. Strontium adsorption kinetics run through pseudo-second-order model. Thermodynamically, strontium adsorption was endothermic and spontaneous. Langmuir isotherm gave good fitting for strontium removal with adsorption capacity of 505.5 mg/g. These results proved that the prepared MagAlgbeads are very efficient material for strontium adsorption.

INTRODUCTION

The large growth in various industries in different sectors that are important for scientists concerned about water pollution is considered a big environmental issue. Heavy metals in the aquatic environment are of particular interest in their endurance and biomagnifying properties and their toxicity (Gad et al. 2016). Such a dangerous heavy metal is strontium from different activities, such as the nuclear industry, where insufficient practices for the management and radioactive waste disposal, nuclear activities, and unintentional discharge of radioactive materials (Ghaemi et al. 2011; Hajdu & Slaveykova 2012). Due to its chemical similarity to calcium, strontium can follow the path of calcium in the food chain and can enter the human body. It is easily absorbed into bones and tissues causing bone sarcoma and leukemia (Chen 1997). Therefore, low concentration of strontium, if present, needs to be removed from waste solutions.

Different technologies were developed for removing metal pollutants from water and wastewater, such as precipitation, ion exchange, reverse osmosis, membrane electrolysis, oxidation, coagulation and biosorption (Shibi & Anirudhan 2005). Biosorption is an environmentally friendly technology that received more consideration because it is available and cost-effective in eliminating metal ions. So, the use of organic polymers became more important (Kousalya et al. 2010; Lakouraj et al. 2014).

Alginate consists of natural polymers with many advantages such as high molecular weight, low toxicity, low cost, selectivity, and efficiency (Abdel-Halim & Al-Deyab 2011). Sodium alginate (NaAlg) is soluble and less stable and therefore is not used directly for the adsorption process. To overcome this difficulty, many tests were done to increase the stability and sorption capacity by the synthesis of polymeric composite materials (Jiang et al. 2012) and cross-linked with other higher valence metal ions (Singh et al. 2014). According to numerous studies, the removal capacity of alginates for several heavy metals increased by coupling it with materials like carbon nanotubes (Li et al. 2010) and activated carbon (Hassan et al. 2014). Recently, magnetic particles got more attention because of their high surface area and selectivity, which results in increased adsorption. Many investigators used magnetic particles to improve intake capacity (Nethaji et al. 2013). More recently, a combination of magnetic particles with alginate beads gave an easy separation and recovery of beads from water (Hammouda et al. 2015).

As one of these magnetic adsorbents, ferrites, especially magnesium ferrites (MgFe2O4), are efficient for the removal of hazardous metals from wastewater (Navratil 2001). Furthermore, metal oxide nanoparticles are perfect adsorbents for the removal of heavy metal ions owing to their large surface area (Savage & Diallo 2005). This study aimed to prepare magnetic alginate polymeric beads and their application in strontium (Sr (II)) removal. Magnetic MgFe2O4 particles were dispersed on the alginate polymeric matrix to generate new magnetic hybrid beads. Several important parameters concerning Sr (II) adsorption were studied and optimized. Adsorption kinetics were analyzed through the application of different kinetic models. Three parameter isotherms have applied for equilibrium data to study adsorption mechanism process.

MATERIALS AND METHOD

Preparation of magnetic MgFe2O4

MgFe2O4 particles were obtained by precipitation method. 10 mol/L NaOH solution was added to 300 mL metal nitrate aqueous solution containing 1.79 g Mg (NO3)2.6H2O, 8.08 g Fe(NO3)3.9H2O until pH = 10.0. The suspension was stirred at 25 ± 1 °C for 30 min and then was heated in a water bath at 90 °C for 2 h. After cooling, the obtained brown precipitates (MgFe2O4) were thoroughly rinsed with deionized water until pH 7.

Synthesis of MagAlgbeads

Beads were prepared in cross-linking solution using calcium chloride as cross-linking agent. About 10 g of sodium alginate was added to 200 mL double distilled water and stirred vigorously for 1 h. Sodium azide (1.54 × 10−4 mol L−1) is added to prevent the solution from the development of bacteria. About 1 g of synthesized MgFe2O4 particles was added with constant stirring at 60 °C using magnetic stirrer for 10 h. The solution was then dropped using a syringe into the prepared calcium chloride (CaCl2) bath (4%, w/v). Fe-reticulated alginate beads were formed instantly. The obtained magnetic alginate beads left undisturbed in calcium chloride solution for 24 h for complete cross-linking. Finally, the magnetic alginate beads was filtered, washed with distilled water to remove the excess of un-reacted materials. Finally, the magnetic beads and dried in hot air oven at 50 °C for 24 h. The dried magnetic beads were sieved to uniform size and then used for sorption studies.

Characterization

The microstructure and surface topography of the dried beads were observed by scanning electron microscopy (SEM). After drying in an oven at 70 C for 48 h, the beads were fixed on aluminum sample holders with double-sided adhesive tape and examined with a scanning electron microscope (SEM-FEG Hitachi SU-70).

The chemical composition of the beads was evaluated by using X-ray fluorescence (XRF) techniques (using Horiba MESA-500 spectrometer). Energy dispersive XRF technique is a multi-elemental and nondestructive analysis with high-speed qualitative and quantitative analysis in a wide concentration range. XRF analyzer is based on the investigation of the elements present in the sample when these are excited by the electromagnetic radiation interacting with the sample.

Fourier transform infrared (FTIR) was used to determine the beads' surface function. The samples were characterized with infrared transmission spectra using a FTS-135 (Bio-Rad) spectrometer. The spectra were obtained from 400 to 4,000 cm−1 using pure potassium bromide (KBr) as background. The samples were first mixed with KBr and then ground in an agate mortar (Merck, for spectroscopy) at an approximate mass ratio of 1:10 (sample:KBr) in the preparation of pellets. The resulting mixture was pressed at 10 tons for 5 min, 32 scans were collected and added together, and a step size of 4 cm−1 was applied in recording the spectra. The background obtained from the scan of pure KBr was automatically subtracted from the sample spectra. All the spectra were recorded and plotted in the same scale on the transmittance axis.

Sorption experiments

Orbital shaker was used to conduct the batch adsorption experiment. The specific amount of adsorbent was added to 10 mL of Sr (II) solution of given initial concentration in 25 mL Erlenmeyer flasks. For the kinetic experiments, the adsorbent was removed using a permanent magnet.

At different time intervals (10–120 min), Sr (II) concentration was measured using an atomic absorption (Thermo Electro Corporation).

Influence of solution pH on the sorption was performed by mixing adsorbent with Sr (II) solution at room temperature for 60 min and a different initial solution pH (2–10). The initial pH was adjusted by using NaOH or HCl solutions.

The influence of temperature was investigated by doing the adsorption experiment at different temperatures (30 °C, 45 °C and 65 °C) using a thermostatic water bath (Memmert WB29 Model).

The Sr (II) adsorption capacity qe (mg/g) and efficiency (R %) was calculated from the following equation:  
formula
(1)
 
formula
(2)
where Ce and Co are equilibrium and initial concentrations, respectively, V represents the volume of the solution (mL) and m is the weight of the sorbent (mg).

RESULTS AND DISCUSSION

Characterization of MagAlgbeads

The morphology, shape, and size of MagAlgbeads are obtained by a scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM) analysis, as shown in Figure 1. SEM image reveals that MagAlgbeads have a sphere-like morphology and relatively rough surface. It consists of agglomerated spherical particles with 10–30 nm average particle size (Figure 1(a)). Agglomeration phenomenon was also observed as small particles aggregated in order to achieve lower free energy state (Rezlescu et al. 2013). This description shows that MagAlgbeads particles were very fine so gave large surface area and many active sites favorable for adsorption. MagAlgbeads were like a sphere in shape and dark brown in color due to the presence of magnesium ferrite nanoparticles.

Figure 1

(a) SEM image, (b) XRF, (c) HRTEM image and (d) SAED pattern of the MagAlgbeads.

Figure 1

(a) SEM image, (b) XRF, (c) HRTEM image and (d) SAED pattern of the MagAlgbeads.

The composition of MagAlgbeads is obtained from XRF spectrum that is given in Figure 1(b). The XRF spectrum of MagAlgbeads approves the presence of Mg, Fe, and Ca elements and validates high purity of the material.

HRTEM image displays that MagAlgbeads is accumulated from hundreds of nanoparticles with a diameter of about 10 nm, results in the formation of sphere-like and porous MagAlgbeads (Figure 1(c)).

Additionally, the polycrystalline nature of MagAlgbeads can be obviously detected from the selected area electron diffraction (SAED) pattern, as given in Figure 1(d). The SAED pattern shows clear diffraction points and can be well indexed as a pure spinel magnetic alginate beads phase, indicating a high crystallinity of magnetic alginate beads.

FTIR spectra of MagAlgbeads is shown in Figure 2. All of the absorption peaks confirm the coating of sodium alginate onto the surface of MagAlgbeads. Broadband at 3,400 cm−1 correspond to stretch vibration of hydroxyl groups, proposing the presence of a great number of hydroxy groups that enable MagAlgbeads with outstanding aqueous dispersion and stability. The weak band at 3,000 cm−1 is assigned to the stretch vibration absorption of aliphatic C-H from glucose units in alginate chains (Prabhu et al. 2009). The bands at 1,608 and 1,427 cm−1 correspond to C-O asymmetric and symmetric stretching vibrations of the carboxyl groups of alginate molecule (Dong et al. 2011). Peaks at 1,034 cm−1 are related to the C-O stretching vibrations of sodium alginate. The signals observed in the range of 1,170–1,030 cm−1 correspond to symmetric and asymmetric vibration bands of C-O-C bonds typical of polysaccharide rings. At 1,320 cm−1, a weak band appears representing the vibration of the C-O bond. Absorption peak in the range of 500–850 cm−1 is due to Fe-O, O-Fe-O, and Fe-O-Fe lattice vibrations. Peaks at lower than 500 cm−1 are due to Mg-O bond vibration.

Figure 2

FTIR spectra of MagAlgbeads.

Figure 2

FTIR spectra of MagAlgbeads.

Sorbed strontium gave broad of the band at 3,420 cm−1 in the FTIR spectra indicates the electrostatic attraction between strontium ions and the beads.

Adsorption profile

Figure 3 displays the sorption of Sr (II) as a function of contact time. Sr (II) removal increased with time and then achieved equilibrium after 60 min. This is considered a short time that is one of the main concerns in the efficient application of adsorbent in wastewater treatment. The curve is single, smooth, and continues until saturation. The curve can be represented by two main steps including 0–30 min step that represents initial fast of Sr (II) and 30–60 min that represents gradual removal, after which no more of Sr (II) uptake was observed. First stage is due to the presence of a large number of surface available sorption sites for Sr (II). The later, slower stage is owing to the competition of Sr (II) for a restricted number of remaining empty sorption sites (Pruden & Suidan 2004; Asenjo et al. 2011; Moura et al. 2011).

Figure 3

Effect of contact time on adsorption of strontium. Experimental conditions: [(V) = 20 mL; pH = 6; (m) = 0.01 g; (Co) = 75 mg/L; (T) = 30 °C].

Figure 3

Effect of contact time on adsorption of strontium. Experimental conditions: [(V) = 20 mL; pH = 6; (m) = 0.01 g; (Co) = 75 mg/L; (T) = 30 °C].

Effect of pH

pH can influence metal ions speciation and adsorbent functional groups, it was considered as an essential factor for metal ions adsorption. Adsorption increases gradually in pH range from 2 to 6 and then decreases afterward (Figure 4).

Figure 4

Effect of pH on the adsorption of strontium. Experimental conditions: [(V) = 20 mL; (m) = 0.01 g; (Co) = 75 mg/L; (T) = 30 °C].

Figure 4

Effect of pH on the adsorption of strontium. Experimental conditions: [(V) = 20 mL; (m) = 0.01 g; (Co) = 75 mg/L; (T) = 30 °C].

Sr (II) adsorption is supposed to take place through the interaction between MagAlgbeads surface groups and metal ion species in solution. It can happen through ion-exchange of strontium ion and protons of the surface hydroxyl and carboxyl groups as follows (Khalid et al. 2000):  
formula
(3)
where −COOH = carboxyl group and mH+ = number of protons released. At low solution pH, Equation (3) lies to the left due to high H+ concentration that competes with Sr (II) for binding onto the MagAlgbeads' surface. Furthermore, MagAlgbeads are positive at this pH that lowers the removal of positively charged Sr (II) on MagAlgbeads. Increasing solution pH equation precedes right, as a result of the decoration of ion exchange sites and surface –ve charge of MagAlgbeads increases. The adsorption increases due to the presence of electrostatic attraction between the negatively charged MagAlgbeads surface and the positively charged ions. Further, the solution pH after adsorption was low compared to initial solution pH. This results from the liberation of H+ ions from the MagAlgbeads surface to solution because of exchange with strontium cations. pH value of 6 was used in the next experiments to avoid any possible precipitation.

Effect of sorbent dosage

As shown in Figure 5, strontium adsorption capacity (qe) decreases and percent removal (R%) increases upon increasing the dosage of MagAlgbeads until equilibrium at 6 g/L doses. At high sorbent dosage, resistance to mass transfer of Sr (II) from the bulk solution to MagAlgbeads surface becomes important. Concurrently, strontium sorption capacity was decreased with the increase of MagAlgbeads dosage. At high sorbent concentration, there are particles aggregates that reduce the total surface area of the sorbent and consequently decrease the interaction of Sr (II) with the sorbent.

Figure 5

Effect of adsorbent dose on the uptake (mg/g) and % removal of strontium adsorption. Experimental conditions: [(V) = 20 mL; pH = 6; (m) = 0.002–0.05 g; (Co) = 75 mg/L; (T) = 30 °C].

Figure 5

Effect of adsorbent dose on the uptake (mg/g) and % removal of strontium adsorption. Experimental conditions: [(V) = 20 mL; pH = 6; (m) = 0.002–0.05 g; (Co) = 75 mg/L; (T) = 30 °C].

Kinetic study

Analysis of experimental data at the various time makes possible to calculate the kinetic parameters and take some information for designing and modeling the sorption processes.

To take some information about strontium adsorption mechanism on MagAlgbeads, the experimental data were examined using pseudo-first-order, pseudo-second-order, and intraparticle diffusion through the following equations (Ho & McKay 1998; Gad et al. 2016):  
formula
(4)
 
formula
(5)
 
formula
(6)
where qt and qe are amounts of strontium adsorbed (mg/g) at time t (min) and at equilibrium, respectively. Co and Ce are adsorbent weight per liter of solution (g/L) and adsorbate initial concentration (mg/L), V the volume of solution (mL). k1 is pseudo-first-order rate constant (min−1) which was computed from the slope and intercept of the plot of log (qe − qt) versus t (Figure 6). The plot of t/qt against t (Figure 6) using for calculating k2 which was the pseudo-second-order rate constant (g/(mg min) and ki is intraparticle diffusion (mg/g min−1/2). The initial sorption rate (mg/g min) is .
Figure 6

Linearized pseudo-first-order and pseudo-second-order plots for the sorption of strontium.

Figure 6

Linearized pseudo-first-order and pseudo-second-order plots for the sorption of strontium.

From Table 1, it can be seen that the kinetics of strontium ion sorption onto MagAlgbeads follow pseudo-second-order kinetic model with correlation coefficients higher than 0.99. Estimated qe values exactly predict strontium sorption kinetics over the whole adsorption times. This proposed that chemisorption control overall sorption rate (Singha & Das 2013) and the reaction rate is directly proportional to the number of active sites on the adsorbent surface.

Table 1

Comparison between sorption rate constants associated with different kinetic models

Model Parameter value 
 qe,exp (mg/g) 103.66 
Pseudo-first-order model k1 (min−10.04 
qe,cal (mg/g) 37.8 
R2 0.927 
Second-order model k2 (g/mg min) 1.97*10−3 
h(mg/g min) 23.5 
qe,cal (mg/g) 109.28 
R2 0.999 
Intraparticle parameter ki (mg/g min0.55.1 
R2 0.990 
Model Parameter value 
 qe,exp (mg/g) 103.66 
Pseudo-first-order model k1 (min−10.04 
qe,cal (mg/g) 37.8 
R2 0.927 
Second-order model k2 (g/mg min) 1.97*10−3 
h(mg/g min) 23.5 
qe,cal (mg/g) 109.28 
R2 0.999 
Intraparticle parameter ki (mg/g min0.55.1 
R2 0.990 

The sorption of the solution includes three steps: film diffusion, intraparticle diffusion, and adsorption at an internal sit. The overall sorption rate based on the slowest step in the adsorption process. The third step is very rapid and cannot be taken into account for the rate-determining step (Abdi et al. 2017). The pseudo-second-order kinetic model cannot recognize strontium sorption diffusion mechanism and rate-controlling step. Therefore, the experimental data were subjected to the intraparticle diffusion model (Equation (6)).

q vs. t0.5 plot of strontium sorption shows multilinearity (Figure 7), and sorption process might be covered by the combination of film and intraparticle diffusion (Chio et al. 2009).

Figure 7

Intraparticle diffusion plots for adsorption of strontium.

Figure 7

Intraparticle diffusion plots for adsorption of strontium.

Adsorption isotherms

We can understand the interaction between adsorbate and adsorbent and also get a possible insight of the adsorption capacity of the adsorbent from the experimental data in the form of adsorption isotherm. The Langmuir (Equation (7)) and Dubinin-Radushkevich (D-R) (Equation (8)) isotherm models equations were used to examine the adsorption data.  
formula
(7)
 
formula
(8)
where qmax (mg/g) is the adsorbent monolayer capacity; kL(L/mg) is Langmuir constant in relation to adsorption free energy; qm (mg/g) is the theoretical saturation capacity; and ɛ is Polanyi potential, ɛ = RT ln(1 + (1/Ce)), where T (K) is the absolute temperature and R (J/mol K) is the gas constant; β (mol2/kJ2) is a constant in relation to the mean free energy of adsorption per mole of the adsorbate. Langmuir qmax and kL can be obtained from a linear plot of Ce/qe versus Ce (Figure 8). qm and β of D-R equation can be obtained by plotting ln qe versus ɛ2 (Figure 9). Langmuir and D-R parameters for Sr (II) adsorption onto MagAlgbeads are shown in Table 2. According to the correlation coefficients, the applicability of the isotherms was compared. The higher correlation coefficient (i.e. with values nearly ≈ 1) illustrated that D-R model is more fit to explain the adsorption equilibrium of Sr (II) onto MagAlgbeads in the studied concentration range. The constant β can give an indication on mean free energy E (kJ/mol) of adsorbate molecule adsorption when it is migrated from solution to the adsorbent surface. β can be obtained using the relationship (Dubey & Gupta 2005):  
formula
(9)
The calculated E is 8.2 kJ/mol (Table 2) that corresponds to ion exchange or chemisorption process (Faghihian et al. 2013). This confirms the results obtained from the pH effect section.
Table 2

Langmuir and Dubinin-Radushkevich parameters for adsorption of Sr (II)

Model Parameter Value 
Langmuir qmax (mg/g) 505.5 
kL (L/mg) 0.007 
R2 0.999 
Dubinin-Radushkevich qm (mg/g) 24.11 
β (mol2/kJ20.0078 
E (kJ/mol) 8.2 
R2 0.983 
Model Parameter Value 
Langmuir qmax (mg/g) 505.5 
kL (L/mg) 0.007 
R2 0.999 
Dubinin-Radushkevich qm (mg/g) 24.11 
β (mol2/kJ20.0078 
E (kJ/mol) 8.2 
R2 0.983 
Figure 8

Langmuir plot of strontium adsorption.

Figure 8

Langmuir plot of strontium adsorption.

Figure 9

D-R plot of strontium adsorption.

Figure 9

D-R plot of strontium adsorption.

Effect of temperature

The adsorption of Sr (II) by MagAlgbeads depended on the time of uptake at different temperatures (30 °C, 45 °C and 65 °C) at pH 6 is shown in Figure 10. As illustrated, the equilibrium adsorption capacity of Sr (II) onto MagAlgbeads was favored at higher temperatures and increased with increasing temperature. It is endothermic adsorption.

Figure 10

The effect of temperature on strontium uptake. Experimental conditions: [(V) = 20 mL; pH = 6; (m) = 0.01 g; (Co) = 75–450 mg/L; (T) = 30–65 °C].

Figure 10

The effect of temperature on strontium uptake. Experimental conditions: [(V) = 20 mL; pH = 6; (m) = 0.01 g; (Co) = 75–450 mg/L; (T) = 30–65 °C].

Increase of adsorption with temperature means an activated chemisorption process, at low temperature, physisorption is favored by being basically exothermic processes. In chemisorption, an increase in temperature increases adsorption because the energetic barrier was overcome. In liquid phase ion-adsorption, the adsorption mechanism is more complicated depending on possible interactions between solid surface, solute, and solvent (Stavropoulos et al. 2013). Factors involved are surface functional groups, properties of ion, and properties of the solution (Srivastava et al. 2007). Temperature has an effect on some of them like solubility, ion hydration, and hydrophobicity. Their variation can alter the effects that temperature can have on adsorption reaction which can be chemical or electrostatic (Wang et al. 2010). Some parameters of the system solid-solute-solvent were affected by temperature (Table 3), which led to increasing in adsorbed amount. For example, increasing the kinetic energy of adsorbing molecules increases solubility, decreases hydrophobicity and ion hydration energy which are the factors most affected by temperature. At high temperature, ions are dehydrated, and sorption becomes more favorable.

Table 3

Characteristic parameters of the experimental data according to the Langmuir model for adsorption of strontium onto MagAlgbeads at different temperatures

Temp °C qmax mg/g kL L/mg R2 
30 505.5 0.007 0.999 
45 1,095 0.0039 0.981 
65 1,333 0.0037 0.970 
Temp °C qmax mg/g kL L/mg R2 
30 505.5 0.007 0.999 
45 1,095 0.0039 0.981 
65 1,333 0.0037 0.970 

Thermodynamic parameters

Energy and entropy considerations must be taken into account to determine what process will occur spontaneously. The following thermodynamic equations are used to calculate the values of thermodynamic parameters.  
formula
(10)
 
formula
(11)
where R = universal gas constant (1.987 cal/mol K or 8.314 j/mol k) and T = absolute temperature in Kelvin (K). ko is the thermodynamic equilibrium constant was determined by the method of Khan and Singh (1987) by plotting ln (qe/Ce) vs Ce and extrapolating to zero qe. ΔHo and ΔSo were obtained from the slope and intercept of Van 't Hoff plots (Equation (10)) of ln ko vs. 1/T (Figure 11). The calculated values of ko, ΔHo, ΔSo, and ΔGo at different temperatures are presented in Table 4.
Table 4

Thermodynamic parameters of strontium adsorption onto MagAlgbeads at different temperatures

Temperature K ko L/g ΔGo kJmol−1 ΔHo kJmol−1 ΔSo Jmol−1 K−1 
303 3.32 −3.02   
318 4.48 −3.96 10.39 44.41 
338 5.47 −4.77   
Temperature K ko L/g ΔGo kJmol−1 ΔHo kJmol−1 ΔSo Jmol−1 K−1 
303 3.32 −3.02   
318 4.48 −3.96 10.39 44.41 
338 5.47 −4.77   
Figure 11

Van 't Hoff plots of strontium adsorption.

Figure 11

Van 't Hoff plots of strontium adsorption.

Values of ko are always higher than unity, i.e. strontium has a high preference for their sorbent surfaces (Yakout et al. 2017). The endothermic nature of strontium adsorption is indicated by an increase in ko with the rise in temperature. This is confirmed by the positive enthalpy changes ΔHo for strontium. The ΔHo and ΔSo values are positive. The positive values ΔHo indicate the presence of an energy barrier in the sorption and endothermic process (Hameed et al. 2007). The positive value of entropy change ΔSo means a good affinity of Sr (II) for the sorbent and the increasing randomness at the solid–solution interface during the sorption process (Yeddou Mezenner & Bensmaili 2009). The ΔGo value decreases are negative which suggests that the adsorption process is spontaneous.

CONCLUSIONS

Magnetic alginate beads (MagAlgbeads) are effective in removal of Sr (II). FTIR, SEM and XRF analysis were used in the characterization of magnetic alginate beads. The adsorption of Sr (II) adsorption was affected by pH of the medium, contact time and adsorbate concentration. The sorption run through Langmuir isotherm and the nature of Sr (II) removal was spontaneous and endothermic. The kinetic studies showed that the pseudo-second-order and intraparticle diffusion models fit well. The mechanism of Sr (II) sorption was controlled by electrostatic adsorption.

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