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Article

Removal of Sr(II) in Aqueous Solutions Using Magnetic Crayfish Shell Biochar

by
Wenchao Lu
1,2,3,
Jundong Feng
4,
Marta Otero
5,
Tao Liao
1,2,3,* and
Liang Qiu
1,3,4,5,*
1
Institute of Agro-Products Processing and Nuclear Agricultural Technology, Hubei Academy of Agricultural Sciences, Wuhan 430064, China
2
School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430000, China
3
Key Laboratory of Cold Chain Logistics Technology for Agro-Product, Ministry of Agriculture and Rural Affairs, Wuhan 430064, China
4
Department of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
5
CESAM, Department of Environment and Planning, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal
*
Authors to whom correspondence should be addressed.
Separations 2023, 10(5), 310; https://doi.org/10.3390/separations10050310
Submission received: 22 March 2023 / Revised: 3 May 2023 / Accepted: 8 May 2023 / Published: 13 May 2023
(This article belongs to the Special Issue Advanced Methods for Separation and Extraction of Radionuclides)
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Versions Notes

Abstract

:
The cooling water of nuclear power plants and discarded crayfish shells (CS), both containing Sr(II), are waste resources that cause environmental pollution and endanger human health. In this study, magnetic biochar produced by crayfish shells (mag@CSBC) was used as an adsorbent to remove radionuclide Sr(II) in an aqueous solution and under irradiation conditions. Scanning electron microscopy, X-ray diffraction analysis, Fourier-transform infrared spectroscopy and vibration sample magnetometer analysis were used to characterize mag@CSBC. In addition, an isothermal adsorption experiment conducted under irradiation conditions determined that the maximum adsorption capacity of mag@CSBC was 21.902 mg/g, which was 1.896 mg/g higher than that from experiments conducted under conditions without irradiation and more suitable for the Freundlich isotherm model. The kinetic experiment proved that irradiation could improve the adsorption cap acity of mag@CSBC and reduce the adsorption equilibrium time. At the same time, the experiment further proved that, under irradiated conditions, the adsorption rate of mag@CSBC can reach more than 90%, and the adsorption capacity is the highest when the pH is 8 and the reaction process is exothermic. Competitive adsorption with Na(I) has a high selectivity and strong recyclability. Finally, the mechanism of Sr(II) adsorption by mag@CSBC under irradiation was studied. In conclusion, mag@CSBC, as a low-cost, easy-to-synthesize, environmentally friendly and easy-to-recycle adsorbent, can be applied in batches for the removal of Sr(II) in aqueous solutions. In particular, the concept of using irradiation technology to optimize adsorption behavior serves as an inspiration for future research.

1. Introduction

Strontium is widely found in soil and seawater and is an essential trace element in the human body. Its chemical structure is similar to Ca(II) [1], and it can protect the human heart against myocardial ischemia/reperfusion injury and prevent arteriosclerosis and thrombosis [2]. At the same time, Sr-90 is also one of the most abundant radionuclides in the cooling water of nuclear power plants, with a half-life lasting as long as 29 years [3]. After being absorbed by human body, Sr is easily accumulated in the liver, lungs and kidneys [4], damaging the genetic material in cells and leading to bone cancer and nervous system diseases [5,6]. Therefore, the highly selective removal of Sr(II) from aqueous solutions is of great significance to environmental resources and human health.
Crayfish are native to South America and now widely distributed across over 50 countries and regions, such as the US, Britain, Australia and China; they have become one of the most widely distributed aquatic products, with the largest output of any aquatic product [7]. Studies have shown that crayfish shells (CS) are an integral part of crayfish. However, in actual production, they are mostly treated as biological waste, and only a few are sold as low-value raw materials [8]. Discarded CS cause mosquitoes and flies to wreak havoc and spread diseases, produce malodorous gases, pollute water and affect human health [9]. Especially in recent years, due to the COVID-19 pandemic, crayfish have mostly been sold as cold-chain shrimp tails; the output of CS has increased, and its distribution has become more concentrated [10]. In addition, biochar (BC) has a large surface area and large pore size, playing an important role in removing metal ions from wastewater [11,12,13,14,15]. Meanwhile, crayfish shell biochar (CSBC) contains a variety of chemical groups (carboxyl, hydroxyl, amino, etc.) [16,17,18], indicating that it has a unique application potential as an adsorbent. Thus, converting waste CS into BC can not only alleviate disposal problems but also create a valuable adsorbent to adsorb Sr(II) and address the challenges of energy shortage, environmental crises and increasing customer demand [19].
At present, there are various methods and techniques for the separation and removal of Sr, such as the use of alkyd resins [20], mesoporous silica [21,22] and activated carbon (AC) [23], etc. However, synthesizing these adsorbents is expensive and time-consuming. Recently, research has been conducted on the low-cost and easy synthesis of materials containing Sr(II) adsorbents, such as pecan shell, paprika biochar, banana peels biochar, basil seed, coffee grounds and bamboo charcoal [24,25,26,27,28,29]. However, experiments using CSBC, especially mag@CSBC magnetized by magnetite, to adsorb Sr(II) have not yet been carried out. Moreover, the Sr(II) adsorption behavior of magnetic adsorbents under irradiated conditions is still unknown. Gamma rays have a large initial adsorption energy and are capable of carrying out rapid and uniform reactions at room temperature [30,31]. Compared with chemical, ultraviolet or plasma methods, gamma rays can produce a large number of free radicals, reduce the bond energy of adsorption and enhance the grafting process [32]. Therefore, after achieving the easy recovery of mag@CSBC via magnetization, we carried out an innovative exploration of methods to improve the adsorption capacity of an adsorbent via irradiation technology.
In this study, mag@CSBC was synthesized from CS, which can effectively remove radionuclide Sr(II) from an aqueous solution and has the advantage of a high recovery after adsorption. In addition, the physical and chemical properties of the adsorbent were determined using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and a vibrating sample magnetometer (VSM). Considering the isotherm model, kinetic model, adsorbent dosage, pH value, temperature, coexisting Na(I) ions and desorption and re-adsorption experiments, the adsorption behavior and regeneration availability of mag@CSBC were evaluated, and the mechanism of adsorption of Sr(II) by mag@CSBC under irradiation conditions was clarified.

2. Experimental Section

2.1. Materials

The crayfish shells used in this study came from a market (Wuhan, China). All used chemicals and reagents were of analytical grades. Strontium nitrate (Sr(NO3)2, 99.5%) as a simulated solution was purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). For synthesizing mag@CSBC, ferric chloride hexahydrate (FeCl3·6H2O, 99%), ferrous sulfate heptahydrate (FeSO4·7H2O, 99%), and ammonia solution (NH4OH, 30%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Beijing, China). To study the effect of pH and sodium chloride, salt, nitric acid (HNO3, 65%), sodium hydroxide (NaOH, 25%), and sodium chloride (NaCl, extra pure) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the solutions were prepared using deionized (DI) water (Direct-Q@5UV, ERCK MILLIPORE, Darmstadt, Germany).

2.2. mag@CSBC

The CS were washed with ultrapure water and dried in an oven at 60 °C. The dried CS were incinerated in a 600 °C muffle furnace for 2 h. The temperature was increased to 200°C in the first stage, and then a heating rate of 50 °C/min was maintained for 10 min. In the second stage, the heating rate was 5 °C/min, and the temperature was raised to 400 °C for 1 h. The heating rate in the third stage was 1 °C/min, and the temperature was maintained at 600 °C for 2 h. The biochar was kept under a continuous stream of N for 2 h to ensure a constant temperature, and the flow rate of N was 0.1 m3/h. Finally, biochar was crushed and passed through 100-mesh sieves and used as an adsorbent crayfish shell biochar (CSBC). The mag@CSBC was prepared as follows. First, 26.62 mg FeCl3·6H2O and 55.54 mg FeSO4·7H2O were added to 100 mL of ultrapure water. Then, 1g of the CSBC was dispersed in the solution and stirred for 48 h at room temperature. Thereafter, the solution was adjusted to pH 10 using NaOH and reacted for 24 h at 80 °C. Finally, to obtain mag@CSBC, the CSBCs were washed 2–3 times with distilled water and dried overnight in an oven at 60 °C.

2.3. Characterization

The element composition of the adsorbent was determined using an elemental analyzer of FLASH2000 (Thermo Scientific, Waltham, MA, USA). The specific surface area and pore properties were measured using an ASAP2460 instrument (Mack, Greensboro, NC, USA) and the N2 adsorption–desorption curve, and calculated via Barrett–Joyner–Harenda (BJH) analysis. Adsorbent morphologies were analyzed using a platinum target and Schottky field emission SEM (TESCAN, Brno, Czechia). A D8 advanced X-ray diffractometer (Bruker, Billerica, MA, Germany) equipped with a Cu Kα radiation source (λ = 0.15406 nm at 40 kV and 40 mA) was used to analyze the crystalline phase of the powder samples. Additionally, the FTIR spectrum of the samples was tested using a Nicolet iS20 spectrometer (Thermo Electron, Waltham, MA, USA) over a range of 400~4000 cm−1. Finally, the magnetic moments of adsorbents were calculated using VSM analysis (LakeShore7404, Columbus, OH, USA).

2.4. Adsorption Experiments

All the adsorption experiments were performed three times in 15 mL conical tubes (PE, HaiMen China). Except for the adsorbent dosage experiments, the 10 mL Sr(II) analog solution was treated with 20 mg of the adsorbent. All the irradiated experiments were completed in the Hubei Agricultural Products Irradiation Center, using 60Co-ray with an activity of 1.18 × 1016 Bq. The adsorption time was 30 h, and the dose of irradiation was 4000 Gy [33,34].

2.4.1. Adsorption Isotherms and Kinetics

To investigate the effect of the initial concentration of Sr(II), 20 mg of the adsorbent was used for an Sr(II) solution at different initial concentrations (10, 20, 30, 40, 50, 60, 70, 80 and 100 mg/L) and adsorbed at 120 rpm for 22 h. Kinetic experiments were performed using a 70 mg/L Sr(II) solution with sampling intervals of up to 2 h.

2.4.2. Effect of Dosage, pH, Temperature and Na(I)

Furthermore, to investigate the effect of the dosage, the amount of added adsorbent was set to 10, 20, 30, 40, and 50 mg/L for adsorption. To investigate the effect of pH, 70 ppm of Sr(II) was adjusted to a different initial pH (4, 6 and 8) using 0.1 mol/L of HNO3 and NaOH solution. To investigate the effect of temperature, the reaction temperatures were set at 25, 35, 45, 55 and 65 °C in 70 mg/L concentrations for isothermal adsorption. All of the above experiments were performed using mag@CSBC in an irradiated environment. In addition, we investigated the effects of Na(I) using 10,100 and 1000 mg/L NaCl and 60, 70 and 80 mg/L Sr(II) solutions.

2.4.3. Adsorption-Desorption Experiment

To determine the reuse potential of mag@CSBC, the adsorption–desorption efficiency of Sr(II) was determined via four reutilization experiments. For the regeneration experiments, 70 mg/L of Sr(II) solution was used in an irradiated environment. Desorption was reacted with 0.1 mol/L of nitric acid for 12 h and separated via centrifugation (3000 rpm, 10 min, 20 °C). After the adsorption reaction, all solutions were centrifuged at 3000 rpm for 10 min. The supernatant was filtered with a 0.22 µm syringe filter head (Whatman). The residual concentration of Sr(II) in the solution was determined using an atomic absorption spectrophotometer (TAS-990, China) at 690 nm.
The adsorption capacity of mag@CSBC for Sr(II) (Qe, mg/g) and the remaining percentage (r, %) of Sr(II) were calculated via Equations (1) and (2), respectively:
Q e = C 0 − C e × V / m
R = C 0 − C e / C 0 × 100 %
where C0 and Ce (mg/L) represent the initial and equilibrium concentrations of Sr(II), respectively. V (L) is the volume of Sr(II) solution, and m (g) indicates the mass of the adsorbent.

3. Results and Discussion

3.1. Characterization

3.1.1. SEM Observation

The morphology of the CSBC and mag@CSBC composites are shown in Figure 1. The figure shows that the surface of CSBC is rough and loose, with regular cracks and voids. The large pore diameter might be due to the elimination of proteins and other impurities adhering on the surface and inside the pores [35]. The pore structures of CSBC appeared to be cracked and collapsed, and the surface was attached by a number of small particles that could be decomposition products of mineral materials and humic substances in the process of pyrolysis [36]. The heterogeneous spherical particles on its surface indicate an amorphous structure of the biomass [37]. Furthermore, its regular honeycomb morphology was observed due to the presence of calcium salts and chitin-containing compounds such as CaCO3. This morphology is commonly observed in crustacean bio-materials such as shrimp and crabs [38]. The above features are conducive to convenient conditions for the adsorption of Sr(II). Furthermore, in mag@CSBC, magnetite was found to attach to CSBC, and the aggregation of spherical particles was observed. The smooth surfaces shown in Figure 1a,b are attached by magnetite, as shown in Figure 1d at 20 k× magnification. The material characteristics of the CSBC and mag@CSBC are shown in Table 1. Both materials maintain more than half of their yield, while mag@CSBC was larger, possibly because Fe3O4 was loaded onto its surface. Additionally, the values of N and H decreased with their modification, which may be related to the destruction of organic compounds in CS [39]. In addition, due to some functional groups (-OH, etc.) being covered by Fe3O4, the pH value of mag@CSBC increased [40] and the atomic ratio of H/C, O/C decreased [41]. Finally, the results of BET and average pore diameter were also because of the covered Fe3O4.

3.1.2. XRD Patterns

The Fe3O4 peaks were analyzed via an XRD study to confirm that the particles expected to be Fe3O4 (Figure 1) represent the actual magnetite. In an XRD pattern (Figure 2), various peaks representing Fe3O4 were found in mag@CSBC at 18.002°, 30.166°, 35.533°, 37.129°, 43.15°, 53.45°, 57.005°, 62.735°, 65.854°, 71.005°, 74.051°, and 75.067°, while such peaks were not found in CSBC. The reduction in solution pH under magnetization conditions promotes dissolution in CaCO3 [42]. Meanwhile, the presence of Mg(II) can also further increase the concentration of solution Ca(II). Thus, mag@CSBC significantly decreased relative to the CSBC at the peak near 30° (CaCO3) [43].

3.1.3. FTIR Spectra

The functional groups of CSBC and mag@CSBC were confirmed via an FTIR analysis (Figure 3). OH bending was observed at 3423.81 cm−1, C-H stretching was 2923.93 (anti-symmetry telescopic vibration peak of CH2) and a symmetric telescopic vibration peak of CH3 was observed at 2857.45 cm−1 [44]. The C=O, C=C symmetric, and C-O stretching bands were clearly observed at 1795.34, 1622.98 and 1110.61 cm−1, respectively. These results indicate that OH and CO may be a function of proton donors and involved in complexation with Fe [45]. Furthermore, peaks in NH amines groups were observed at 879.34 cm−1 [46]. The peaks at 1600 and 1300 cm−1 were attributed to the bands of out-of-plane bending, in-plane bending and asymmetric stretching modes of CO32− stretching bands, respectively [47]. In the fingerprint region of organic matter, increased C=C stretching suggested an increasing degree of aromatization [48]. Other unique peaks were observed in the spectrum of mag@CSBC, and they were assumed to indicate the reason for the magnetism of CSBC: a peak at 564.53 cm−1 implying magnetization with Fe-O [49,50]. Sr(II) adsorption is possible through an electrostatic interaction with highly active functional groups (OH, C-H, C-O, etc.) of CSBC [51]. In particular, the carboxyl and hydroxyl groups present in the CSBC are functional groups that can react with heavy metals in the aqueous phase [52].

3.1.4. VSM Analyses

The Fe3O4 magnetic strength was analyzed via VSM study to confirm that the particles expected to be Fe3O4 (Figure 1) represent the actual magnetite. The intensity of magnetization was confirmed as 49.691 emu/g at room temperature via VSM analysis (Figure 4), indicating greater intensity compared to the previous magnetic adsorbent material [53,54].

3.2. Adsorption Experiment

3.2.1. Adsorption Isotherms

The effect of Sr(II) initial concentration on the adsorption capacity of CSBC, mag@CSBC and mag@CSBC-γ (mag@CSBC under an irradiation environment) is shown in Figure 5. The adsorption capacity increases with the increase in the initial concentration of Sr(II). One possible reason is the driving force of initial concentration, which helps to overcome the mass transfer resistance of the adsorbate from adsorbent body to surface.
Three types of adsorption isotherm models were used to analyze the equilibrium adsorption behavior: the Langmuir, Freundlich, and Temkin models [55], which are expressed as shown in Equations (3)–(5), respectively:
Q e = Q m b C e 1 + b C e
Q e = K f C e 1 / n
Q e = R T B ln K T C e
where Qm is the maximum adsorption capacity of the adsorbent (mg/g) and b is the Langmuir adsorption constant for the free energy of the adsorption (L/mg). Further, Kf [(mg/g) (L/mg)1/n] and n are the Freundlich constant and the heterogeneity factor. KT (L/g), the gas constant R (8.31 J/mol/K), and B (L/mg) are the parameters of the Temkin isotherm.
The fitting effect of the isotherm model was evaluated by using the correlation coefficient (r2). The results of isothermal adsorption are also shown in Figure 5. The parameters calculated by fitting to the Langmuir, Freundlich, and Temkin models are listed in Table 2. The nonlinear fitted isotherm model explains the interaction between the adsorbent and the adsorbate.
The CSBC itself showed higher adsorption than the mag@CSBC composite. This observation can be interpreted as a reduction in the specific surface area of CSBC by magnetite; furthermore, it indicates that Ca(II) is present in the CSBC and is favorable for ion-exchange mechanisms with Sr(II) [56]. CSBC is a calcium-rich bio-adsorbent and the mag@CSBC covered by the magnetite results in a reduced Ca(II) concentration on the surface. Therefore, it can be interpreted that CSBC is higher in adsorption ability than mag@CSBC. The CSBC and mag@CSBC were the most suitable for the Freundlich isotherm model, with r2 values of 0.964 and 0.955, respectively. This indicates that the adsorbent surface may be multi-layer adsorption and adsorbate molecules will be adsorbed when they enter the adsorption force field. Because the adsorption force field has a certain spatial range, adsorption can be multi-molecular layer. The adsorption force decreases gradually with the adsorption layer from inside to outside [55]. Additionally, the low heat of adsorption (B < 20 J/mol) for the Temkin model confirms that the adsorption process was maybe physical adsorption. The Langmuir model also predicted the maximum adsorption amounts (Qm) of 22.670 mg/g and 20.006 mg/g for CSBC and mag@CSBC, respectively. According to the adsorption results, even at low initial concentrations, CSBC with a high Qe value was suitable for Sr(II) adsorption.
However, although though the adsorption capacity decreases when CSBC is magnetized, it is a very inexpensive and readily available biomass, and it has the advantage of reducing the cost of secondary waste treatment by allowing for the easy recovery of the waste adsorbent after adsorption. Notably, the adsorption capacity of mag@CSBC to Sr(II) is significantly enhanced under an irradiation environment, and the Qm reaches 21.902 mg/g. Especially when the initial concentration is higher or the amount of adsorbent is slightly increased, Qm comparable to CSBC can be obtained for mag@CSBC-γ. This may be because the irradiation affects the chemical and physical properties of the adsorbent surface, making it highly sensitive to Sr(II), and is conducive to the molecular adsorption on the adsorbent surface [57,58]. Another reason could be that the delocalized electrons of the surface aromatics can move upon irradiation, changing the aromatic molecules with bonded or nonbonded electrons to those with anti-bonded electrons, thus enhancing the contact probability and the adsorption trend [59]. The above is the main reason for irradiation-increased adsorption.
Table 3 lists the Qm values for Sr(II) adsorption for comparison with the results obtained in studies using other biomass materials. It can be found that the low-cost, environmentally friendly, easy-to-synthesize and reusable adsorbent has become a research hotspot. The Qm values of carboxymethyl chitosan and coffee grounds are significantly higher than those of mag@CSBC. However, the Qm values of Hybrid membranes and Paprika biochar are much lower than mag@CSBC. Although the Qm varies with the types of materials, the reason why this study is worthy of attention is that the raw materials (shrimp shell waste) used by the adsorbent have the characteristics of putrefaction, stench and environmental pollution. Mag@CSBC activated by magnetite has the advantage of easy separation. In particular, the new viewpoint that γ -ray irradiation can relatively improve the Qm of adsorbent in the adsorption process will provide new inspiration for related research.

3.2.2. Adsorption Kinetics

The experimental data were fitted by two kinetic models, namely, the nonlinear pseudo-first-order model (Equation (6)) and pseudo-second-order model (Equation (7)), which have investigated the controlling mechanism of adsorption processes. The pseudo-first-order model assumes that the number of the free adsorption position on the surface of the adsorbent decides the adsorption rate, and the pseudo-second-order model assumes that the square of the number of the free adsorption position on the surface of the adsorbent decides the adsorption rate, mainly controlled by chemical adsorption [64]:
d q t d t = k a Q e − Q t
d q t d t = k a Q e − Q t
where Qt and Qe (mg/g) are the adsorbed amount of mag@CSBC at time t and at equilibrium, respectively. ka (1/min) is the pseudo-first-order kinetic constant. kb (g/mg min) is the pseudo-second-order kinetic constant.
The adsorption behavior of Sr(II) by mag@CSBC and mag@CSBC-γ is shown in Figure 6. Parameters of the kinetic experiments and the results are summarized in Table 4. The fitting effect of the kinetics model was evaluated by using the correlation coefficient (r2).
The adsorption of Sr by mag@CSBC almost reached the adsorption equilibrium at 22 h, and the r2 of the pseudo-first-order model and the pseudo-second-order model were 0.996 and 0.983, respectively. However, under the irradiation condition, the adsorption equilibrium time of mag@CSBC-γ was 18 h, which was 4 h shorter than that under the non-irradiation condition. This indicates that when adsorbing Sr(II) by mag@CSBC, the adsorption equilibrium time can be shortened by irradiation. This may be because the mechanistic reaction of mag@CSBC and Sr(II) grafting copolymerization is triggered by gamma irradiation, which effectively shortens the crosslinking time [65]. The r2 of the pseudo-first-order model and the pseudo-second-order model are 0.972 and 0.952, respectively. Both of them are more in line with the first-order reaction, and the change of adsorption reaction with time is expected to be proportional to the concentration. The Qe of mag@CSBC and mag@CSBC-γ under the condition of adsorption equilibrium is 20.835 and 23.473 mg/g, respectively. Furthermore, for mag@CSBC-γ, the isothermal experiment of the adsorption of Sr(II) solution with an initial concentration of 70 mg/L continued to stir for enough time after adsorption for 22 h, and the obtained Qe (22.114 mg/g) was similar to the kinetic experiment value. Therefore, the results indicate that mag@CSBC quickly reacted with the Sr(II) solution to adsorb Sr(II) under irradiation conditions and confirmed that the stirring time in the isotherm experiment was sufficient.
In addition, the pseudo-first-order model and pseudo-second-order model were fitted with the whole adsorption process, but they could not describe the control step for the different adsorption course. The mechanisms for external mass transfer and intraparticle pore diffusion on the surface were confirmed through the intraparticle diffusion model based on Fick’s diffusion law [66]. This can be calculated using the intraparticle diffusion equation, as shown in Equation (8). The rate of intraparticle diffusion equation is obtained as follows:
Q t = k d i t 0.5 + C
where kdi is the constant of intraparticle diffusion rate [mg/g∙min0.5]; t is the contact time [min]; C is the constant of intraparticle diffusion [mg/g]; and Qt is the adsorbed amount at time t [mg/g].
Figure 7 indicates that the mag@CSBC and mag@CSBC-γ adsorbed Sr(II) through three stages: external diffusion (surface diffusion), intraparticle diffusion, and adsorption equilibrium (diffusion within the micropore) [67]. Parameters of the intraparticle diffusion and the results are summarized in Table 5. The fitting effect of the three stages of the intraparticle diffusion model was evaluated by using the correlation coefficient (r2).
The first paragraph indicates the position of the Sr(II) passing across the boundary layer to the outer surface of the mag@CSBC. At this time, kd is large, indicating that the adsorption occurs rapidly and can be attributed to more adsorption sites on the adsorbent. The second segment indicates that the Sr(II) crosses the outer surface of the mag@CSBC into the pore, at which point the kd decreases. This may be due to the part of the adsorbent surface site being occupied after the first phase of adsorption. The third segment indicates the adsorption of Sr(II) in the micropores of the inner surface of the mag@CSBC, when kd is minimal and the adsorption goes to equilibrium. Furthermore, C not equal to 0 indicates that internal diffusion is not the only rate-limiting step.

3.2.3. Effect of Dosage

The effect of different dosages on the adsorption rate of Sr(II) concentration in the solution is shown in Figure 8. When the initial concentration of Sr(II) in the solution is 70 mg/L, the adsorption efficiency of Sr(II) increases with the increasing amount of mag@CSBC. This may be because the surface area of adsorbent increases, and the number of adsorption sites increases accordingly [43]. With the dosage increasing from 20 mg to 50 mg, the adsorption efficiency increased from 41.07% to 91.36%.

3.2.4. Effect of pH

The effect of different pH levels on the adsorption rate of Sr(II) concentration in the solution is shown in Figure 9. Our results demonstrated that with the increasing of pH, the adsorption efficiency of mag@CSBC for Sr(II) increased. This may because, at low pH, H+ competes with metal cations for the same adsorption position [68]. When the pH increased, the ability of H+ to combine adsorption sites decreased, while positively charged strontium ions began to occupy adsorption sites on the surface of the adsorbent and therefore the adsorption capacity increased [69]. However, other studies showed that when the pH increased further (>9), due to the presence of a large number of anions in water, metal ions are surrounded by anions, forming negatively charged atomic groups that reduce the adsorption effect of strontium ions [70,71].

3.2.5. Effect of Temperature

The effect of different temperatures on the adsorption rate of Sr(II) concentration in the solution is shown in Figure 10. It is found that, with the increase in temperature from 25 °C to 65 °C, the adsorption rate of Sr(II) by mag@CSBC gradually decreases. Our results show that temperature has a significant effect on the adsorption of Sr(II) by mag@CSBC. It can be concluded that the adsorption of Sr(II) is an exothermic process.

3.2.6. Effects of Na(I)

The selective adsorption of Sr(II) in the presence of coexisting ions is influenced by Na(I), Mg(II), and Ca(II) plasma in the wastewater. In particular, Na(I) becomes an important coexisting ion affecting the removal of Sr(II) [26]. Therefore, it is necessary to clarify the effects of Na(I) during Sr(II) adsorption.
To investigate the effect of Sr adsorption during the coexistence of sodium chloride, 60, 70 and 80 mg/L of Sr(II) solution and 10, 100, 1000 mg/L concentrations of sodium chloride were chosen. As shown in Figure 11, under the influence of a coexisting ion of Na(I), the adsorption rate decreases the most with the increase in Na(I) concentration. This phenomenon was most evident in the coexistence of 60 mg/L of the Sr(II) solution and 1000 mg/L of sodium chloride. At the same time, the adsorption rate was found to be less than 50% only when 80 mg/L of Sr(II) solution and 1000 mg/L of sodium chloride coexist, but this problem can be solved by increasing the adsorbent dose. In addition, Formula (1) shows that, with the increase in Sr(II) concentration, Qe is worth increasing but is still lower than the Qe value in isotherm experiments, which indicates that Na(I) may undergo competitive adsorption with Sr(II) to seize the adsorption site on mag@CSBC. Nevertheless, mag@CSBC, with its ease of magnetic separation, still has significant advantages for removing the radionuclide Sr(II) present in seawater.

3.2.7. Adsorption–Desorption

To determine the reuse efficiency of mag@CSBC under irradiation conditions, the adsorption–desorption efficiency of mag@CSBC was analyzed in four experiments. The experimental results show that the adsorbent recovery is higher than 97%, indicating that the magnetism is well-protected by the layer-covered structure, and the adsorption–desorption efficiency is shown in Figure 12. The results show that the adsorption–desorption efficiency decreased with the number of repeats. However, the adsorption–desorption efficiency of the first three repeats was higher than 50%, and the efficiency was not less than 50% until the fourth reuse of desorption. At the same time, the experiment also proved that the absorbed Sr(II) is approximately equal to the absorbed concentration of Sr(II), which shows that almost all of the adsorbed Sr(II) can be released by desorption, which once again confirms the possibility of reusing mag@CSBC.

3.3. Mechanism

The mag@CSBC surface is a multilayer of adsorption, and Sr(II) is adsorbed when entering the adsorption force field. Because the adsorption force field has a certain spatial range, the adsorption can be multi-molecular layer. Sr(II) successively enters the adsorbent through surface diffusion, coarse pore diffusion and microdiffusion in the pores. The first stage is mainly the position of Sr(II) through the boundary layer to the outer surface of mag@CSBC. Due to the greater number of adsorption sites on the adsorbent, adsorption rapidly occurs. In the second stage, Sr(II) crosses the outer surface of the mag@CSBC into the pore, which slows down due to part of the site being occupied after the first adsorption phase. In the third stage, the Sr(II) adsorption in the micropores on the inner surface of the mag@CSBC is counterbalanced. In addition, the adsorbent surface functional groups and ions become involved in the adsorption behavior. Particularly under irradiation conditions, the chemical properties and physical properties of the adsorbent surface change, making it highly sensitive to Sr(II), which is conducive to the adsorption of molecules on the adsorbent surface. Meanwhile, the delocalized electrons of the aromatic hydrocarbon on the mag@CSBC surface can move upon irradiation, transforming the aromatic molecules with bonded or non-bonded electrons into those with anti-bonded electrons, thus improving the contact probability and adsorption trend [72].

4. Conclusions

Mag@CSBC, as a low-cost, easily synthesized, environmentally friendly and easily recycled adsorbent, can be widely applied in batches to remove Sr(II) in aqueous solutions. Meanwhile, gamma radiation irradiation technology can effectively improve the adsorption capacity of mag@CSBC and shorten the adsorption balance time. In this study, the unique physical structure and chemical properties of mag@CSBC composites were verified, proving that the magnetic strength of mag@CSBC is 49.691 emu/g, the maximum adsorption capacity is 21.902 mg/g, the adsorption rate can reach more than 90% and the highest adsorption capacity is at pH 8. The reaction process with Na(I) competitive adsorption has a high selectivity with a strong recovery. Finally, the mechanism of Sr(II) was analyzed via surface diffusion, coarse pore diffusion and inner pore microdiffusion, and this process is accompanied by the influence of adsorbent functional groups and ion exchange (Ca(II), etc.). In this study, we clarified the basic principle of irradiation technology affecting adsorption behavior by increasing the sensitivity of the adsorbent surface to Sr(II) and improving the contact probability and adsorption trend.

Author Contributions

Methodology, W.L.; Investigation, J.F. and M.O.; Writing—review and editing, L.Q.; Funding acquisition, T.L. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported in part by Key R&D Program of Guangzhou City (202103000087); Young scholars of Hubei Academy of Agricultural Sciences, 2023NKYJJ23; Open Project of Irradiation Preservation Key Laboratory of Sichuan Province, Sichuan Institute of Atomic Energy (FZBZ2022001).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare that they have no competing interest.

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Separations 10 00310 g001 550
Figure 1. SEM images of (a,b) CSBC and (c,d) mag@CSBC.
Figure 1. SEM images of (a,b) CSBC and (c,d) mag@CSBC.
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Figure 2. XRD patterns of CSBC and mag@CSBC.
Figure 2. XRD patterns of CSBC and mag@CSBC.
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Figure 3. FTIR spectra of CSBC and mag@CSBC.
Figure 3. FTIR spectra of CSBC and mag@CSBC.
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Figure 4. VSM analyses of mag@CSBC.
Figure 4. VSM analyses of mag@CSBC.
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Figure 5. Sr(II) adsorption isotherms of CSBC, mag@CSBC and mag@CSBC-γ (T = 25 °C, pH = 8, Time = 18 H, mag@CSBC = 50 mg/L).
Figure 5. Sr(II) adsorption isotherms of CSBC, mag@CSBC and mag@CSBC-γ (T = 25 °C, pH = 8, Time = 18 H, mag@CSBC = 50 mg/L).
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Figure 6. Sr(II) adsorption kinetics of mag@CSBC and mag@CSBC-γ (T = 25 °C, pH = 8, C0 = 70 mg/L, mag@CSBC = 50 mg/L).
Figure 6. Sr(II) adsorption kinetics of mag@CSBC and mag@CSBC-γ (T = 25 °C, pH = 8, C0 = 70 mg/L, mag@CSBC = 50 mg/L).
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Figure 7. Intraparticle diffusion plot for 70 mg/L of Sr(II) adsorption of mag@CSBC and mag@CSBC-γ (T = 25 °C, pH = 8, mag@CSBC = 50 mg/L, C0 = 70 mg/L).
Figure 7. Intraparticle diffusion plot for 70 mg/L of Sr(II) adsorption of mag@CSBC and mag@CSBC-γ (T = 25 °C, pH = 8, mag@CSBC = 50 mg/L, C0 = 70 mg/L).
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Figure 8. Adsorption efficiency of the different dosage of mag@CSBC (C0 = 70 mg/L, Time = 18 H, T = 25 °C, pH = 8).
Figure 8. Adsorption efficiency of the different dosage of mag@CSBC (C0 = 70 mg/L, Time = 18 H, T = 25 °C, pH = 8).
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Figure 9. Adsorption efficiency of the different pH of mag@CSBC (C0 = 70 mg/L, T = 25 °C, mag@CSBC = 50 mg/L, Time = 18 H).
Figure 9. Adsorption efficiency of the different pH of mag@CSBC (C0 = 70 mg/L, T = 25 °C, mag@CSBC = 50 mg/L, Time = 18 H).
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Figure 10. Adsorption efficiency of mag@CSBC at different temperatures (C0 = 70 mg/L, pH = 8, mag@CSBC = 50 mg/L, Time = 18 H).
Figure 10. Adsorption efficiency of mag@CSBC at different temperatures (C0 = 70 mg/L, pH = 8, mag@CSBC = 50 mg/L, Time = 18 H).
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Figure 11. Effect of Na(I) on the efficiency of Sr(II) adsorption by mag@CSBC (pH = 8, mag@CSBC = 50 mg/L, Time = 18 H, T = 25 °C).
Figure 11. Effect of Na(I) on the efficiency of Sr(II) adsorption by mag@CSBC (pH = 8, mag@CSBC = 50 mg/L, Time = 18 H, T = 25 °C).
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Figure 12. Adsorption and desorption of mag@CSBC composites (C0 = 70 mg/L, pH = 8, mag@CSBC = 50 mg/L, Time = 18 H, T = 25 °C).
Figure 12. Adsorption and desorption of mag@CSBC composites (C0 = 70 mg/L, pH = 8, mag@CSBC = 50 mg/L, Time = 18 H, T = 25 °C).
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Table
Table 1. Characteristics of the CSBC and mag@CSBC.
Table 1. Characteristics of the CSBC and mag@CSBC.
PropertyProportion
(CSBC/mag@CSBC)		PropertyProportion
(CSBC/mag@CSBC)
Yield (%)54.53/67.13Atomic ratio O/C0.823/0.741
C (%)18.7/21.22Atomic ratio H/C0.33/0.14
N (%)0.91/0.77pH value10.63/9.23
O (%)20.52/25.13BET surface area (m2/g)26.77/38.12
H (%)0.52/0.41Pore volume (cm3/g)0.12/0.18
S (%)2.94/1.32Average pore diameter (nm)22.86/14.41
Fe (%)0/9.43
Table
Table 2. Important parameters of the Langmuir, Freundlich, and Temkin isotherms models for CSBC, mag@CSBC and mag@CSBC-γ.
Table 2. Important parameters of the Langmuir, Freundlich, and Temkin isotherms models for CSBC, mag@CSBC and mag@CSBC-γ.
ModelsParametersCSBCmag@CSBCmag@CSBC-γ
Langmuir isotherm model Qm (mg/g)22.67020.00621.902
b (L/mg)0.2550.1670.207
r20.9160.8690.904
Freundlich isotherm model Kf [(mg/g) (L/mg)1/n]6.7713.7615.498
n2.8892.1512.573
r20.9640.9550.969
Temkin isotherms modelKT (L/g)4.4502.7730.831
B (L/mg)10.03710.48813.243
r20.9270.9190.939
Table
Table 3. Comparison of adsorption capacity of various adsorbents for Sr(II) removal.
Table 3. Comparison of adsorption capacity of various adsorbents for Sr(II) removal.
AdsorbentQm (mg/g)References
mag@CSBC-γ21.902This study
mag@banana peels biochar23.827[26]
carboxymethyl chitosan 105.81[60]
Fe3O4@titanate fibers37.1[61]
Melamine-trimesic acid modified attapulgite26.21[62]
Paprika biochar0.02 [25]
Hybrid membranes0.94[63]
coffee grounds69.013[28]
Table
Table 4. Parameters of the kinetic experiments and the results.
Table 4. Parameters of the kinetic experiments and the results.
ModelsParametersmag@CSBCmag@CSBC-γ
Pseudo-first-order modelQe (mg/g)20.83523.473
ka (min−1)0.1450.138
r20.9960.972
Pseudo-second-order model Qe (mg/g)26.24130.183
kb (min−1)0.0050.004
r20.9830.952
Table
Table 5. Parameters of the kinetic experiments and the results under irradiated condition.
Table 5. Parameters of the kinetic experiments and the results under irradiated condition.
Irradiation Environment and Parameter Adsorption Stage
Stage 1Stage 2Stage 3
mag@CSBCkd (mg/g∙min0.5)5.7692.1790.230
C (mg/g)−1.78910.02018.933
r20.8470.9440.984
mag@CSBC-γkd (mg/g∙min0.5)6.2943.6120.450
C (mg/g)−2.3417.05719.911
r20.8850.8340.840
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Lu, W.; Feng, J.; Otero, M.; Liao, T.; Qiu, L. Removal of Sr(II) in Aqueous Solutions Using Magnetic Crayfish Shell Biochar. Separations 2023, 10, 310. https://doi.org/10.3390/separations10050310

AMA Style

Lu W, Feng J, Otero M, Liao T, Qiu L. Removal of Sr(II) in Aqueous Solutions Using Magnetic Crayfish Shell Biochar. Separations. 2023; 10(5):310. https://doi.org/10.3390/separations10050310

Chicago/Turabian Style

Lu, Wenchao, Jundong Feng, Marta Otero, Tao Liao, and Liang Qiu. 2023. "Removal of Sr(II) in Aqueous Solutions Using Magnetic Crayfish Shell Biochar" Separations 10, no. 5: 310. https://doi.org/10.3390/separations10050310

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