Treating acidic wastewater containing radioactive 137Cs presents challenges in adsorption due to potential structural damage to adsorbents in acidic conditions and strong competition from protons. A novel layered calcium thiostannate (KCaSnS) that contains Ca2+ as a dopant opens a new way of designing high-performance adsorbents for caesium removal, write Chenyang Yang and Kuk Cho, Department of Environmental Engineering, Pusan National University, Republic of South Korea and Yong Jae Suh, the Korea Institute of Geoscience and Mineral Resources


Treating acidic wastewater containing radioactive 137Cs poses challenges in adsorption. (Credit: ATDS from Pixabay)

The growth of the nuclear sector generates environmental and safety problems due to the accumulation of radiotoxic waste. Caesium (Cs) 137 is one of the major fission products and is considered a hazardous radioactive element with a half-life of approximately 30 years. 137Cs emits beta particles and strong gamma-rays in the decay process. It is also easily assimilated into the living body because of its high solubility in water and its similar transport behaviour to potassium (K+). It is known that selective removal of Cs+ from practical nuclear wastewater is difficult owing to interference with high concentrations of coexisting non-radioactive cations. In particular, a high concentration of protons in acidic wastewater inhibits the removal of Cs+. The radioactive wastewater generated from reprocessing spent fuel and decommissioning nuclear power plants (NPPs) are usually acidic (pH < 2.3) and effective and selective Cs+ removal from acidic radioactive fluids remains a significant difficulty.

Adsorption solution

Adsorption by ion exchange is a promising wastewater remediation technology for NPPs because of its advantage of treating trace amounts of pollutants with ease. Metal sulphides are considered a remarkable selective class of ion exchangers for Lewis-soft acid treatment because of the presence of Lewis-soft base S2- in their structure. However, most of the existing metal sulphides, such as InSnOS, KMS-1, KMPS-1, KMS-2, and KATS-2 showed low adsorption capacity or distribution coefficient for Cs+ in highly acidic conditions. This is because they have only interlayer cations as ion-exchangeable species. A recent report demonstrated that a structural dopant cation, in addition to interlayer ions, could also contribute to adsorption by being leached out under acidic conditions. However, presumably owing to the significantly smaller size of the dopant cation (Al3+, r = 0.54 A°)) than that of Cs+ (r = 1.67 A°), Cs+ cannot reside in the original Al3+ sites, and protons, which are much smaller and abundant, are deposited instead. This presumption suggests that if dopant cations are more comparable in ionic size to Cs+ and leachable in acid, the dopant lattice positions in the crystal structure of adsorbent can operate as additional active adsorption sites for Cs+. Metal sulphides doped with an intermediate-sized dopant cation will provide much higher capacity for Cs+ adsorption in acidic conditions as long as the crystal structure is stable in acid.

To address this issue a novel layered metal sulfide, potassium calcium thiostannate (KCaSnS), was designed by introducing Ca2+ into the Sn-S matrix, assuming that Ca2+ (r = 1.00 A°) provides a large enough space for Cs+ to reside after its release from the lattice structure. Calcium sulphide is readily dissolvable under acidic conditions, and Ca2+ is a much harder Lewis acid than Cs+. Therefore, it is conjectured that the meta-stable structural Ca2+ in addition to the interlayered K+ will be exchanged for Cs+ at low pH, possibly resulting in a notable increase in the adsorption capacity. As hypothesised, experiments demonstrate a high adsorption capacity of 620 mg/g for Cs+ at pH 2, the highest value to date. Even though the result was obtained at a very high Cs+ concentration, which is not seen in practical situations, this approach is original in its design of novel chalcogenide adsorbents for highly acidic wastewater treatment.

A new KCaSnS adsorbent was synthesized by a hydrothermal method by mixing potassium carbonate (K2CO3), calcium chloride dihydrate (CaCl2⋅ 2H2O), tin powder (Sn), sulphur (S) and deionized water. The mixture was then heated in a furnace at 200°C for 24h. Considering the hazard of 137Cs, all experiments in this study were performed using its nonradioactive isotope.

Adsorption characteristics of KCaSnS

The adsorption kinetics were studied to know how fast the Cs+ adsorption takes place. KCaSnS displayed quick adsorption kinetics in both neutral and acidic environments with an initial concentration of ~20 mg/L. Cs+ concentration achieved equilibrium in the first minute under neutral circumstances, showing a steep decrease in concentration from 20 to 0.136 mg/L. This indicates a removal efficiency of 99.3% in 1 min, which is higher than that of KTS-3, SbS-1 K, AgSnSe-1, FJSM-4, and NVPC.

Under acidic conditions, slow kinetics have generally been observed for the adsorption of Cs+ because of severe competition with protons. Notably, KCaSnS exhibited fast adsorption kinetics at pH 2. The Cs+ adsorption reached equilibrium within 1 min, with a removal efficiency of 95.4%, which is substantially faster than that of FJSM-4 and DB-ZrP.

To investigate the maximum Cs+ adsorption capacities of KCaSnS under neutral and acidic conditions, an isotherm study was conducted using low to extremely high concentrations of Cs+ solutions. Under neutral conditions, equilibrium adsorption capacity (qe, mg/g) rapidly increased with the equilibrium concentration of Cs+ in solution (Ce, mg/ L) up to 370 mg/g or 2.78 mmol/g. The qe value is lower than ~460.8 mg/g for hf-TiFC (hollow flower-like titanium ferrocyanide), 453 mg/g for MIL-101- SO3H, 437.5 mg/g for NVPC, and 400 mg/g for FJSM-SnS-4. But it is higher than that of most other adsorbents, such as ~252.1 mg/g for NH4V4O10 (layered ammonium vanadate), 250.3 mg/g for KAlSnS-3, ~222 mg/g for KMS-1, and 174 mg/g for AgSnSe-1, and much higher than the ~82.7 mg/g for commercial AMP-PAN (ammonium molybdophosphate– polyacrylonitrile).

In acidic solutions, KCaSnS showed a significantly enhanced adsorption capacity toward Cs+. Unlike at neutral conditions, qe increased stepwise with Ce in acidic solutions. The sites, which protons would occupy, appeared to be adsorbed by Lewis softer Cs+ above the Cs+ concentration of 8250 mg/L. The saturated adsorption capacity was as high as 620 mg/g, or 4.66 mmol/g, which is the highest value among those measured at a pH value near 2.0. The other adsorbents with high capacities include KAlSnS-3 (170 mg/g at pH 2), KMS-1 (168 mg/g at pH 2.5), and DB-ZrP (155.42 mg/g at pH 1). KCaSnS has a substantially greater adsorption capacity at pH 2 than FJSM-SnS-4 (144.8 mg/g at pH 1.6) and NVPC (65 mg/g at pH 2.05), which have higher adsorption capacities than KCaSnS in neutral conditions.

To further investigate the influence of solution acidity or basicity on the Cs+ adsorption, adsorption experiments were performed over the pH range of 1–13. More than 97.8% of Cs+ was removed in the pH range of 4–12. Meanwhile, at the initial pH range of 4–8, the adsorption experiments resulted in an increase in the final solution pH, implying competitive adsorption of protons in the lower Cs+ concentrations at the end stage. In acidic solutions, KCaSnS shows higher values of Kd than AgSnSe-1, KAlSnS-3, InSnOS, KTS-3, KMS-2, and PATiW (polyaniline titanotungstate), except for hf-TiFC. It is worth noting that the Kd of KCaSnS under acidic conditions is also higher than that of FJSM-SnS-4, NVPC, and MIL-101-SO3H, all of which have higher adsorption capacities than KCaSnS in neutral conditions.

Effect of interfering ions on the Cs+ adsorption

High concentrations of nonradioactive ions such as Na+, K+, Mg2+, and Ca2+ in the radioactive wastewater inhibit the selective adsorption of Cs+. Furthermore, protons contained in radioactive wastewaters interfere with Cs+ adsorption via two routes: the breakdown of the adsorbent structure and the competition for adsorptive sites with Cs+. Accordingly, the adsorption performance of KCaSnS in acidic solutions with various concentrations of competing ions was investigated. When the concentrations of competing ions ranged from 0.1 to 1 mmol/L, the Kd for Cs+ was higher than 1.27 × 104 mL/g. The effects of divalent Mg2+ and Ca2+ were more prominent than those of monovalent Na+ and K+ in this range because of stronger electrostatic interaction.

To simulate a more practical case, the adsorption performance of KCaSnS toward Cs+ was investigated in different water environments, including TW, ASW, and acid-spiked TW and ASW. KCaSnS showed high distribution coefficients for Cs+ in TW (4.30 × 104 mL/g) and ASW (3.29 × 103 mL/g) (Fig. 4d). Notably, the high Kd values can be maintained even in the acid-spiked water systems, i.e., 1.33 × 104 mL/g in acidic TW and 2.95 × 103 mL/g in acidic ASW. The values of Kd in neutral conditions are comparable to previous works such as KAlSnS-3 [30] and SbS-1 K. One might consider the acidic TW and ASW impractical; in this study, they are just simulated acidic water based on real water resources.

Reusability of KCaSnS

To study the behaviour of the structural Ca2+ on the recyclability of KCaSnS under acidic conditions, a series of adsorption-desorption experiments using different eluents were conducted. When Ca2+ was used as an eluent, some Cs+ remained in the structure because Cs+ is chemically softer than Ca2+. This resulted in a lower adsorption capacity of KCaSnS-Ca than that of the pristine KCaSnS. With K+ as an eluent, most of the Cs+ in both the interlayer and structure were replaced with K+ because K+ has similar chemical hardness and ionic radius to Cs+. As the concentration of K+ is rather high (1 mol/L), some structural Ca2+ ions might be further replaced with K+. In the successive adsorption test, the replaced K+ was readily exchangeable with Cs+ at high Cs+ concentrations, resulting in higher adsorption capacity. For the eluent containing both K+ and Ca2+, most of the structural Ca2+ in the Cs-KCaSnS-acidic might remain in the structure because of the high concentration of Ca2+ in the solution. As K+ replaced most of the adsorbed Cs+, the adsorption capacity of KCaSnS-K&Ca was similar to that of pristine KCaSnS. Finally, the two-step elution with Ca2+ and K+ in succession produced a product with approximately the same elemental composition as the regenerated KCaSnS-K. This indicated that the intermediate regeneration step with Ca2+ did not influence the second K+ regeneration step. Thus, KCaSnS-Ca-K showed the same trend of an increase in the adsorption capacity as KCaSnS-K.

To further evaluate the reusability, regeneration tests with an eluent solution of KCl were performed three times successively. The first, second, and third regenerated adsorbents were named KCaSnS-K-1, KCaSnS-K-2, and KCaSnS-K-3, respectively. As mentioned earlier, KCaSnS-K-1 showed a higher adsorption capacity than the pristine KCaSnS; whereas the capacity gradually decreased in the second and third regeneration tests, finally reaching that of the pristine KCaSnS. The reason for the capacity reduction might be attributed to the remaining K during adsorption. In summary, the regenerated adsorbents indicate that KCaSnS is highly reusable in acidic conditions.

Cs+ removal mechanism

The removal of Cs+ was found to occur via ion exchange under both neutral and acidic conditions. Ion exchange was shown between K+/Ca2+ and Cs+.

The adsorption capacity of KCaSnS was shown to be higher in the acidic condition than in the neutral condition, which opposes the common behaviour of most adsorbents. In the neutral condition, the molar ratio of the adsorbed Cs+ to the released K+ was 1.11, indicating that the interlayered Ca2+ also participated in the ion exchange. Clearly, this molar ratio value is greater than that of prior research in which only K+ may operate as an exchangeable ion. The interlayered Ca2+ in addition to K+ was replaced with Cs+ under neutral conditions.

The acidic condition enhanced the Cs+ adsorption by facilitating the leaching of the structural Ca2+.

Radioactive Cs+-rich wastewater with high acidity and salinity is difficult to treat with conventional adsorbents. It was recently reported that a new metal sulphide adsorbent InSnS-1 exhibited excellent acid resistance even at pH 0. On the contrary, this study aimed to improve the adsorption efficiency at pH conditions that most adsorption studies have employed, at high acidity conditions in particular. Hence, a layered KCaSnS was synthesised incorporating metastable Ca2+ into the Sn-S matrix, where Ca2+ is large enough to reserve space for locating Cs+ during adsorption. The structural Ca2+ ions were leached out in the acidic condition and their sites were replaced with Cs+. As the structure of KCaSnS is rather stable, even partial leaching of the constituent ions requires a certain level of acidity, i.e., pH 2 in this case. Upon the release of weakly bonded Ca2+ ions from the crystal structure, preferential adsorption of Cs+ would take place due to its high affinity toward S2-. Because a proton is a very hard Lewis acid, it may not dominate the structural Ca2+ empty sites. Furthermore, as previously mentioned, protons alone cannot leach off all of the structural Ca2+. To increase the likelihood of Cs+ being accessible into the structure, the structural Ca2+ must be completely removed. This requires very high Cs+ concentrations, even higher than H+ concentrations. The commencement of Cs+ concentration of 62.08 mmol/L for the complete leaching at pH 2 indicates a synergistic interaction of Cs+ and H+, resulting in the maximum adsorption capacity of 4.66 mmol/g.

The coexistence of ample H+ and Cs+ is instrumental in completely leaching the structural Ca2+ from the Sn-S matrix. The adsorbent design strategy that incorporates structural lattice points into interlayer space of typical layered materials overturns our previous understanding. Notably, the Cs+ adsorption capacity is greater in acidic solutions than that in neutral solutions, which contradicts the general trend of current SnS-type adsorbents. At first, a conspicuous increase in Cs+ adsorption capacity in acid was not noticed, although the release of appreciable amount of the structural Ca2+ was observed at 500 mg/L Cs+ and pH 2, for example. Then, we attempted a much higher concentration of Cs+, i.e., above 8250 mg/L and could spot a steep rise in Cs+ adsorption capacity. The structure-damaging proton is used to mediate ion exchange in this case. The uncommon ion-exchange possibility is accomplished by the synthesis of an intermediate metastable crystal structure that can resist transformation while permitting ion exchange. This approach provides meaningful insights for the design of different types of novel adsorbents.

137Cs, a hazardous radionuclide generated during spent fuel reprocessing and decommissioning of nuclear power plants, poses a threat to human health. However, the Cs+ adsorption performance of most adsorbents in acidic conditions has an insurmountable limit due to the presence of copious protons. In this study, we turn the problematic proton into a functional agent by incorporating a large, weakly bonding ion, Ca2+, into an Sn-S matrix. Leaching Ca2+ from the matrix creates additional adsorption sites in potassium calcium thiostannate (KCaSnS) resulting in the greater Cs+ adsorption capacity in acidic solutions than that in neutral solutions.

This article first appeared in Nuclear Engineering International magazine.