Enhanced adsorption capacity and selectivity towards strontium ions in aqueous systems by sulfonation of CO₂ derived porous carbon

Abstract

Oxygen-enriched carbon materials derived from carbon dioxide were functionalized using sulfonic acid to remove Sr²⁺ ions from aqueous solutions. Synthesized sulfonated porous carbon materials (PC-SO₃H) showed higher adsorption capacity and selectivity towards Sr²⁺ than non-functionalized porous carbons (PC). The formation of the C-SO₃H functional group in PC-SO₃H and its ability to proton exchange with Sr²⁺ was the main contributor to the enhanced performance. The maximum uptake capacity of Sr²⁺ by PC-SO₃H was 18.97 mg g⁻¹, which was 1.74 times greater than PC. PC-SO₃H removed 99.9% and 97.6% of Sr²⁺ from aqueous solutions with initial Sr²⁺ concentrations of 5 mg L⁻¹ and 10 mg L⁻¹, respectively. Sr²⁺ adsorption showed rapid kinetics, reaching the adsorption equilibrium within 1 h with high adsorption capacity at equilibrium which is 3.52 times greater than that of PC. Additionally, PC-SO₃H selectively adsorbed Sr²⁺ even in the presence of excess amounts of competing ions. Sulfonation of oxygen-enriched carbon had a significant effect on enhancing the affinity towards Sr²⁺ and suppressing adsorption towards other competing ions.

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Introduction

Nuclear power is considered a promising candidate to satisfy the increased demand for energy. Currently nuclear power accounts for 4.8% of the world's total energy supply, and the proportion of energy supplied by nuclear is anticipated to grow.^1,2 However, radioactive waste from nuclear power can cause long-term environmental and health threats. In particular, ⁸⁹Sr and ⁹⁰Sr are radioactive isotopes which are present in nuclear reactors and various forms of nuclear waste,³ and if released into the environment (e.g. nuclear incidents such as Chernobyl and Fukushima), ⁸⁹Sr and ⁹⁰Sr can present a significant public health risk following exposure to the radioactivity.^4,5 Furthermore, strontium exhibits a long half-life (28.8 years) and a high decay energy; therefore effective ways to treat strontium (Sr²⁺) contaminated environments must be considered.⁶

Adsorption is often considered a feasible and somewhat economical method for the recovery of radioactive isotopes from liquid wastes.^7–10 Numerous studies have considered the removal of Sr²⁺ from aqueous environments using various adsorbents. Clay minerals such as kaolinite¹¹ and attapulgite¹² were frequently studied due to its large edge surface and high ability for ion exchange which is common and effective mechanism for adsorption in aqueous systems.^13,14 Along with the ability for ion exchange, the porous structure is usually favored when designing effective adsorbents.^15–17 Therefore, studies moves onto various materials with those characteristics such as zeolite,¹⁸ silica,^19,20 titanosilicate,²¹ Prussian blue,²² and titanate nanotubes.²³

While carbon exhibits the right physical properties along with low cost and its abundance, removal performance of Sr²⁺ is poor with carbon-based adsorbents exhibiting low removal efficiencies below 70% (ref. 24–26) and low selectivity for Sr²⁺.^27,28 Research to improve the adsorption properties of carbon-based materials, especially the removal of Sr²⁺, has been limited but is the focus of this study.

In the current study, carbon dioxide (CO₂)-derived porous carbon materials have been functionalized with sulfonic acid to enhance removal of Sr²⁺. The sulfonic acid groups have been identified to improve the removal efficiency of heavy metal ions such as cadmium,²⁹ lead³⁰ and uranium,³¹ but there is no evidence relating to strontium adsorption. Recently, Aguila et al.³² reported a metal organic framework (MOF)-SO₃H for the removal of Sr²⁺. The authors suggested the possibility of cation exchange between the proton in SO₃H and Sr²⁺, but experimental confirmation of this hypothesis was not provided. To the best of our knowledge, this is the first study to confirm the effect of SO₃H functional group for improved Sr²⁺ removal from aqueous systems. The reaction to functionalize the carbon backbone with SO₃H groups is demonstrated and supplemented with detailed material characterization. Various comparative adsorption studies between the sulfonated porous carbon and non-functionalized porous carbon were investigated to highlight the performance characteristics of SO₃H groups, specifically Sr²⁺ adsorption, with the mechanism for ion exchange clearly demonstrated and discussed.

Experimental

1. Materials

Sodium borohydride (NaBH₄, >96%), hydrochloric acid (HCl, 37 wt%), strontium standard solution (1000 mg L⁻¹, for AAS), sodium chloride (NaCl, >99%) and seawater whose cations composition was 10 409 ppm Na⁺, 359 ppm K⁺, 1327 ppm Mg²⁺ and 176 ppm Ca²⁺ (ref. 33) were purchased from Sigma-Aldrich. Concentrated sulfuric acid (H₂SO₄, 95–98%) and potassium chloride (KCl, 99.9%) were obtained from Fluka. CO₂ used to prepare the carbon particles and acetylene and N₂O (>99.8%) for the atomic adsorption spectrometry were supplied by BOC gas. All chemicals were used without further purification. Ultrapure Milli-Q water (distilled water, DIW) with a resistivity of 18.2 MΩ cm was used in all experiments.

2. Preparation of porous carbon (PC) from CO₂

The procedure to form the CO₂-derived porous carbon was adopted from earlier studies.^34–37 5 g of NaBH₄ powder was loaded into a steel-alloy tube and the tube was placed inside the furnace (MTF 12/25/400, Carbolite Gero Ltd.). The powder was heated to 500 °C from room temperature at a heating rate of 5 °C min⁻¹. The sample was then held at this temperature for 2 h, before natural convection cooling to room temperature under a continuous flow of CO₂ at 100 mL min⁻¹. The solid product (CO₂-derived porous carbon (PC)) was then washed with 1 M HCl at 50 °C for 30 min while stirring the suspension at 300 rpm to remove any unreacted NaBH₄ and other impurities. The product was washed with DIW for 15 min and filtered. The wash and filtration processes were repeated further five times until the pH of the wash water was above pH 5.5. Finally, the product underwent 15 min washing in ethanol before evaporating the ethanol in air for 6 h at 100 °C.

3. Attachment of sulfonic acid functional group to porous carbon (PC-SO₃H)

1 g of the prepared PC was dispersed in 20 mL of H₂SO₄. The mixture was heated to 130 °C in an oil bath and held at a constant temperature for 15 h while gently stirring the suspension at 200 rpm using a magnetic stirrer. Following the hydrothermal treatment the sample was cooled to room temperature and diluted in DIW and then filtered using filter paper (grade 1). Following the filtration the product was washed with DIW for 15 min and subsequently filtered. The wash and filtration processes using DIW were once again repeated until the pH exceeded 5. Finally, the product (sulfonated porous carbon (PC-SO₃H)) was air dried for 24 h at 120 °C.

4. Characterization

Elemental analysis was conducted using a CHNS/O elemental analyzer (Thermo Fisher Scientific, Flash 2000). Fourier transform infrared (FTIR) spectra were collected using the Thermo Fisher Scientific Nicolet iS10 with an attenuated total reflection (ATR) accessory. X-ray photoelectron spectroscopy (XPS) data were acquired using a multi-purpose XPS device (Thermo Fisher Scientific, Sigma Probe) equipped with an MXR1 gun (400 μm). Scanning electron microscopy (SEM) images and energy dispersive spectroscopy (EDS) data were obtained using a field emission SEM (FEI Company, Magellan 400). Prior to the measurement (XPS and SEM) the powder samples were deposited onto a carbon tape. The nitrogen adsorption/desorption isotherms at 77 K were obtained using a Micromeritics 3Flex. The surface area for PC and PC-SO₃H was calculated according to the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was determined from a non-local density functional theory (NLDFT) method.

5. Strontium adsorption experiments

To determine the equilibrium adsorption capacity of the formed products, a Sr²⁺ adsorption isotherm was constructed by measuring the Sr²⁺ uptake at a range of initial Sr²⁺ concentrations. The initial Sr²⁺ concentrations (1, 5, 10, 20, 50, 100, 200 and 350 mg L⁻¹) were determined by diluting a stock solution (1000 mg L⁻¹) using DIW. 20 mg of the powdered sample was added to 20 mL of each Sr²⁺ solution (solid to liquid ratio = 1 g L⁻¹), and the suspension was shaken for 24 h at room temperature using an orbital shaker.

For the adsorption kinetic test the initial Sr²⁺ concentration was fixed at 20 mg L⁻¹ and the suspension (1 g L⁻¹) was shaken at room temperature for 0.17, 0.33, 0.66, 1, 2, 3 and 5 h. For both isotherm and kinetic studies, following Sr²⁺ adsorption, the suspension was centrifuged at 11 000 rpm for 15 min and the supernatant was recovered and filtered using a 0.45 μm syringe filter. The Sr²⁺ concentration remaining in the supernatant was quantified using a fast sequential atomic absorption spectrometer (AAS, VARIAN AA240FS).

Results & discussion

1. Structural analysis for synthesized PC-SO₃H

The chemical composition of the synthesized PC-SO₃H measured by elemental analyzer (EA) is shown in Table 1. Sulfur and oxygen contents were shown to increase following the sulfonation of PC even though the observed sulfur composition is a bit lower than its expected value. This suggests that the sulfonic acid groups are chemically bound to the carbon surface in the PC-SO₃H as intended.

Table 1 Chemical compositions of PC and PC-SO₃H measured by the elemental analyzer (CHNS/O)

	PC		PC-SO3H
	Expected^a	Observed	Expected^a	Observed
a The expected chemical composition of PC is averaged value from previous studies,^34,37 and that of PC-SO3H is determined based on the assumption that 10% of C–H bonds in PC are replaced by C-SO3H bonds.
C	74%	76.89%	65.26%	65.73%
H	2%	2.51%	0.21%	2.13%
N	—	0.20%	0.17%	0.19%
S	—	—	6.82%	1.77%
O	17%	15.44%	23.33%	25.93%

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Attachment of the sulfonic acid groups to the PC was verified from the FTIR spectra as shown in Fig. 1. The peak at 550 cm⁻¹ can be assigned to the C–S stretch³⁸ and the peak at 1059 cm⁻¹ corresponds to the SO₃⁻ stretch.³⁹ Peaks at 1154 cm⁻¹ and 1200 cm⁻¹ confirm the existence of SO₂ related bonds while the peaks are slightly shifted (higher wave numbers) when compared to the literature values.^40,41 This shift was also reported from the previous studies for sulfonic acid-treated carbon materials.^38,42 The remaining peaks in the spectra can be assigned to either C–O or C–B, which are observed in both PC and PC-SO₃H samples that are synthesized from CO₂.^35–37 It should be noted that the presence of both C–S and S–O bonds in PC-SO₃H but the absence in PC indicate the successful chemical bonding of sulfonic acid group to the carbon in PC-SO₃H.

Fig. 1 FTIR spectra and peak assignments of PC-SO₃H including PC as reference.

The surface elemental composition of the two samples was confirmed by XPS. From the data, PC contained 82.93% carbon, 13.74% oxygen and 3.34% boron, with the composition changing to 78.36% carbon, 17.54% oxygen, 1.84% boron and 2.26% sulfur for PC-SO₃H. In good agreement with the elemental compositions shown in Table 1, the sulfonation of PC increased both the oxygen and sulfur content in PC-SO₃H. The slight increase in sulfur content when measured by XPS compared to EA suggests that the sulfur atoms are enriched on the surface of carbon. To better understand the nature of the chemical bonds in PC and PC-SO₃H, deconvolution of the C 1s, O 1s, S 2p and B 1s spectra was conducted. As shown in the C 1s spectra of PC (Fig. 2(a)), the three peaks can be assigned to C=C (283.6 eV), C–C (285.2 eV) and C=O (288.5 eV),^43–48 while the additional peak observed in the C 1s spectra of PC-SO₃H (Fig. 2(b)) corresponds to C–S bonds (287.1 eV).^44,46,49 Deconvolution of the O 1s spectra is shown in Fig. 2(c) and (d). A main peak centered at 531.7 eV is attributed to the O–C group, and a second peak observed for PC-SO₃H can be assigned to O=S bonds.^46,47 The C–S and O=S bonds of the PC-SO₃H can be clearly identified through the deconvolution of the XPS spectra (Fig. 2(b) and (d)). Moreover, for PC-SO₃H (Fig. 3(b)) the S 2p peak centered at 167.4 eV corresponds to C–S–O bonds,^43,45,50 which is absent for PC. Deconvolution of the B 1s spectra confirmed no changes following the sulfonation step, as shown in Fig. 3(c) and (d). As such, it is reasonable to state that the sulfur atoms bind to the carbon and oxygen atoms and not the boron atoms. Hence, the majority of the sulfur exists in the form of C–S–O, which is present in the C-SO₃H functional group.

Fig. 2 XPS spectra of C 1s peaks for (a) PC and (b) PC-SO₃H, respectively; and O 1s peaks for (c) PC and (d) PC-SO₃H.

Fig. 3 XPS spectra showing: absence and presence of S 2p peaks in (a) PC and (b) PC-SO₃H, respectively; and B 1s peaks for (c) PC and (d) PC-SO₃H.

Chemical composition and elemental mapping was determined by SEM-EDS (Fig. 4 and S1†). For PC-SO₃H the composition profile (Fig. 4(a)) was shown to be 69.73% carbon, 26.33% oxygen, 1.39% boron and 2.56% sulfur, which is in excellent agreement with the composition determined by EA shown in Table 1. Elemental maps in Fig. 4(b–f) show that all atoms including sulfur are uniformly distributed and dispersed across the carbon surface. Although PC-SO₃H shows porous morphology similar to PC in SEM images (Fig. S1†), the specific BET surface area decreased after sulfonation (Table S1†). Both isotherm curve and pore size distribution (Fig. S2†) indicate the collapse of pores by sulfuric acid, which may be the reason why specific surface area was reduced.

Fig. 4 SEM-EDS (a) composition profile, (b) image, (c) carbon map, (d) oxygen map, (e) boron map and (f) sulfur map for PC-SO₃H.

2. Strontium ion adsorption

The Sr²⁺ adsorption capacity of PC and PC-SO₃H was determined by fitting the equilibrium adsorption data to the Langmuir-isotherm model:where Q_e (mg g⁻¹) is the equilibrium adsorption, Q_m (mg g⁻¹) is the theoretical maximum adsorption capacity, b (L mg⁻¹) is the Langmuir constant which represents the affinity between Sr²⁺ and the adsorbent, and C_e (mg L⁻¹) is the ionic concentration at equilibrium. Fig. 5 shows the amount of Sr²⁺ adsorbed as a function of the equilibrium concentration, with the data fitted using eqn (1). It shows good agreement between the experimental data and the Langmuir model for both PC and PC-SO₃H. Based on the isotherm fitting parameters (Table 2), the maximum adsorption capacity of PC-SO₃H is 1.74 times higher than PC. Moreover, the larger b value of PC-SO₃H than PC indicates the higher affinity of sulfonic acid group toward Sr²⁺. Therefore, addition of sulfonic acid groups onto the carbon backbone provides a positive contribution to the adsorption of Sr²⁺.

Fig. 5 Adsorption isotherms of Sr²⁺ onto PC and PC-SO₃H with fitted Langmuir, Freundlich, and Dubinin–Radushkevich isotherm curves.

Table 2 Isotherm parameters for Langmuir, Freundlich and Dubinin–Radushkevich models

		PC	PC-SO3H
Langmuir	Qm	10.89 mg g^−1	18.97 mg g^−1
	b	0.085 L mg^−1	4.399 L mg^−1
	R^2	0.995	0.990
Freundlich	KF	3.73 L mg^−1	11.69 L mg^−1
	NF	5.07	9.89
	R^2	0.923	0.757
Dubinin–Radushkevich	Qm	10.24 mg g^−1	18.64 mg g^−1
	β	4.57 × 10^−5 mol^2 J^−2	8.92 × 10^−9 mol^2 J^−2
	R^2	0.972	0.959
	E	0.11 kJ mol^−1	7.49 kJ mol^−1

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Other isotherm models were tested to fit the data as well, but Langmuir isotherm showed the highest accordance based on R² value. Freundlich isotherm curve was fitted with the equation defined as

Q_e = K_FC_e^1/N_F (2)

where K_F (L mg⁻¹) and N_F (−) are the Freundlich constants which indicate an approximate ability of adsorption.

The Dubinin–Radushkevich (D–R) curve was fitted with the equation

Q_e = Q_m exp(−βε²) (3)

where Q_e (mg g⁻¹) is the equilibrium adsorption, Q_m (mg g⁻¹) is the theoretical maximum adsorption, β (mol² J⁻²) is the D–R isotherm coefficient related to mean free energy and ε is the D–R isotherm constant, which can be calculated by following equation:where R is the gas constant (8.314 J mol⁻¹ K⁻¹), and T is the absolute temperature (K). The mean free energy E (J mol⁻¹), which is an indicator for the type of a sorption process, can be calculated using the following relationship.

As both PC and PC-SO₃H have the E value smaller than 8 kJ mol⁻¹, physical adsorption may be the dominant mechanism. However, the E value of PC-SO₃H is close to 8 kJ mol⁻¹, which indicates adsorption also occurred in terms of ion exchange.^51,52

The improved adsorption capacity of PC-SO₃H is attributed to the ion exchange between Sr²⁺ and the proton in SO₃H. To validate this mechanism the exchanged amount of ions was confirmed by measuring the pH of the Sr²⁺ solution following the adsorption test. As shown in Fig. 6, the proton concentration per gram of PC-SO₃H increased with the increasing concentration of Sr²⁺. For 100 and 350 mg L⁻¹ Sr²⁺ solutions, 0.42 and 0.62 mmol g⁻¹ of protons were exchanged, respectively. These values appear reasonable considering that the adsorbed amount of Sr²⁺ from initial solution concentrations of 100 and 350 mg L⁻¹ equalled 0.22 mmol g⁻¹ and 0.25 mmol g⁻¹, respectively. These data not only support the mechanism of Sr²⁺ removal by ion exchange with the proton in PC-SO₃H, but also quantify the exchange ratio of proton to Sr²⁺ to be approximately 2.

Fig. 6 Measured pH (red line and square symbol) and exchanged amount of proton per gram of PC-SO₃H (blue line and circle symbol) of Sr²⁺ solutions with the initial concentrations of 0, 10, 100 and 350 mg L⁻¹.

Ion exchange between Sr²⁺ and proton was confirmed in XPS results as well. The clear peak shift was observed in comparison between O 1s spectra of pristine PC-SO₃H and Sr²⁺-adsorbed PC-SO₃H (Fig. S3(a)†). This shift is caused by the replacement of H⁺ into Sr²⁺ which is bonded to oxygen atoms. In addition, Sr 3d peaks obtained from PC-SO₃H after adsorption (Fig. S3(b)†) show peak shift compared to element Sr. That is due to the bond between Sr and electronegative moiety of oxygen.

The enhanced Sr²⁺ adsorption performance of the synthesized PC-SO₃H was compared to previously reported values for carbon-based adsorbents. As shown in Table 3, PC-SO₃H exhibits outstanding or equivalent performance among carbon-based adsorbents including CNTs and graphenes which often require complex synthesis routes. As such, sulfonation of carbon materials is a viable facile method to produce economical adsorbents for Sr²⁺.

Table 3 Comparison of adsorption capacity of PC-SO₃H with other carbon-based adsorbents

Materials	Adsorption capacity^a (mg g^−1)
a Theoretical maximum Sr^2+ adsorption capacity calculated by the Langmuir isotherm model.
PC-SO3H	18.97 (this work)
Oxidatively modified carbon	0.48 (ref. 24)
Multi-walled CNT/iron oxide	9.18 (ref. 25)
Graphene oxide	23.66 (ref. 26)
Activated carbon	12.11 (ref. 27)
MWCNTs–SMP hybrids	14.92 (ref. 53)
Oxidized MWCNTs	10.87 (ref. 53)

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Furthermore, the Sr²⁺ removal efficiency from aqueous solutions of low Sr²⁺ concentration is an important performance parameter when evaluating adsorbents, since these often represent conditions encountered in the environment. Sr²⁺ removal efficiencies of PC and PC-SO₃H from aqueous solutions containing 5 mg L⁻¹ and 10 mg L⁻¹ Sr²⁺ are shown in Table 4. For 5 mg L⁻¹ Sr²⁺ solution, the removal efficiencies were 99.9% and 75.6% for PC-SO₃H and PC, respectively. These efficiencies reduced to 97.6% and 37.8% when the particles were dispersed in 10 mg L⁻¹ Sr²⁺ solution. Hence, the data demonstrates the high removal efficiency of PC-SO₃H when adsorbing trace amounts of Sr²⁺ in aqueous solutions.

Table 4 Removal efficiency and K_d values for PC and PC-SO₃H

	PC	PC-SO3H
Removal efficiency at 5 mg L^−1	75.6%	99.9%
Removal efficiency at 10 mg L^−1	37.8%	97.6%
Kd at 10 mg L^−1 (mL g^−1)	6.08 × 10^2 mL g^−1	4.07 × 10^4 mL g^−1

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The distribution coefficient (K_d, mL g⁻¹), which represents the affinity between Sr²⁺ and adsorbent, is another indicator to verify the performance of adsorbents. K_d is calculated as follows:

where C₀ and C_e (mg L⁻¹) are the initial and equilibrium concentrations of Sr²⁺, V (mL) is the volume of solution and m (g) is the weight of adsorbents. The calculated K_d values shown in Table 4 demonstrate that PC-SO₃H has a much higher affinity toward Sr²⁺ than PC. Typically, the K_d value on the order of 10⁴ or 10⁵ is judged as an excellent level.^54,55 Thus, the calculated K_d value of 4.07 × 10⁴ mL g⁻¹ for PC-SO₃H demonstrated a much higher selectivity toward Sr²⁺ than PC.

Normally removal efficiency of ions is affected by the pH of liquid. Therefore confirmation of adsorbents' performance in various pH conditions is important to ensure their applicability in industry. PC-SO₃H was tested with the initial solution of varying pH conditions from 2 to 12. As show in Fig. 7, PC-SO₃H shows high removal efficiency both in acidic (pH 4) as well as basic conditions (pH 10). However, it shows reduced adsorption performance in extreme acidic media with pH 2. On the other hands, adsorption capacity was increased by 99% in extremely basic solution with pH 12. That may be because the mechanism for the adsorption of PC-SO₃H is based on ion exchange between proton and strontium, which may be hindered in acidic conditions and promoted in basic conditions.

Fig. 7 Sr²⁺ removal efficiency of PC-SO₃H in various pH conditions (C₀ for Sr²⁺ = 10 mg L⁻¹).

Along with the removal efficiency the Sr²⁺ adsorption kinetics, more specifically the initial adsorption rate, is an important performance parameter. The adsorption kinetics of both carbon-based adsorbents was quantified using the following pseudo-second-order kinetics model:

where Q_t (mg g⁻¹) is the amount of Sr²⁺ adsorbed at time t (min), Q_e (mg g⁻¹) is the amount of Sr²⁺ adsorbed at equilibrium, and k₂ (g (mg⁻¹ min⁻¹)) is the rate constant. Each particle sample was dispersed in 20 mg L⁻¹ Sr²⁺ solution at a particle concentration of 1 g L⁻¹. The suspension was shaken and samples periodically removed, and the concentration of Sr²⁺ normalized to initial concentration is shown in Fig. 8. The fitting parameters, Q_e and k₂, of the pseudo-second-order rate equation are shown in Table 5. PC-SO₃H exhibits a higher rate constant k₂, indicating rapid adsorption when compared to PC, along with an equilibrium adsorption capacity which is 3.52 times greater than PC.

Fig. 8 Normalized ratio of Sr²⁺ (C₀ = 20 mg L⁻¹) as a function of time. Lines represent the fits of the pseudo-second order rate equation.

Table 5 Kinetic parameters for fitting pseudo-second-order kinetics curves

	PC	PC-SO3H
Qe	4.317 mg g^−1	15.195 mg g^−1
k2	0.03532 g (mg^−1 min^−1)	0.0444 g (mg^−1 min^−1)
R^2	0.9938	0.9861

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As demonstrated, the sulfonic acid groups of PC-SO₃H promote the enhancement of Sr²⁺ removal performance (adsorption kinetics and capacity) when compared to PC. Moreover, the sulfonic acid groups improve the selectivity of the carbon-based adsorbent for trace amounts of Sr²⁺ when in the presence of excess competing ions, see Fig. 9. When dispersed in seawater PC-SO₃H removed 49.24% of Sr²⁺ from seawater having 10 mg L⁻¹ Sr²⁺ (Fig. 9(a)). Although the adsorption efficiency from seawater is much lower than pure water condition (97.6% for 10 mg L⁻¹ Sr²⁺) due to the blocking effect caused by excess amount of other competing ions, however, PC removed only 13.37% of Sr²⁺ under the same condition. It should be noted that PC-SO₃H is much more effective at removing Sr²⁺ from complex ionic solutions such as seawater. To better understand the effect of competing ions, a model solution consisting of 10 mg L⁻¹ Sr²⁺, 400 mg L⁻¹ K⁺, 10 000 mg L⁻¹ Na⁺, 200 mg L⁻¹ Ca²⁺ and 1300 mg L⁻¹ Mg²⁺ was used to re-evaluate the effect of competing ions. Compared to the Sr²⁺ adsorption from the Sr²⁺ diluted solution, the adsorption of Na⁺, K⁺ and Mg²⁺ is nearly negligible. Interestingly, removal efficiencies of PC-SO₃H towards K⁺, Na⁺ and Mg²⁺ were even reduced compared to those of PC as shown in Fig. 9(b). It means that sulfonation not only improves the affinity towards Sr²⁺, but also inhibits the adsorption of competing ions at the same time. Especially for Na⁺, which accounts for the biggest part of cations in seawater, sulfonation in PC-SO₃H can reduce Na removal by about 1/30 compared to the PC case. Bivalent Sr²⁺ can be more selectively bound to the sulfonic acid sites than monovalent Na⁺. That is an encouraging result and the functionalization of carbon with sulfonic acid group is an effective way for the selective removal of Sr²⁺ in contaminated water.

Fig. 9 Removal efficiency of (a) Sr²⁺ in seawater and (b) competitive ions (K⁺, Na⁺, Ca²⁺ and Mg²⁺) in model solution (C₀ = 400 mg L⁻¹ for K, 10 000 mg L⁻¹ for Na, 200 mg L⁻¹ for Ca and 1300 mg L⁻¹ for Mg) for PC and PC-SO₃H.

Conclusions

We have introduced a facile method to functionalize CO₂-derived porous carbon materials with SO₃H group for the Sr²⁺ removal in aqueous solutions. Due to the ion exchange between the proton in SO₃H group and Sr²⁺ in the solution, synthesized PC-SO₃H shows enhanced Sr²⁺ adsorption performance in terms of removal capacity, selectivity and kinetics. In addition, PC-SO₃H provides outstanding selectivity towards Sr²⁺ even in aqueous solutions with excess amounts of competing ions though its exact mechanism is still needed to be studied. This work suggests a potential of functionalized carbons as adsorbents to remove the traced amounts of Sr²⁺ ions in contaminated water with nuclear wastes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are grateful for the financial support from the UK Korea Joint Research Program through NRF grants (NRF-2015M2A7A1000219) funded by the Ministry of Science, ICT, and Future Planning. D. Harbottle acknowledges the support of Engineering and Physical Sciences Research Council grant number EP/M026426/1.

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Footnote

† Electronic supplementary information (ESI) available: SEM images, BET measurements details. See DOI: 10.1039/c7ra09541d

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