
==== Front
Sci Rep
Sci Rep
Scientific Reports
2045-2322
Nature Publishing Group UK London

72245
10.1038/s41598-024-72245-x
Article
Highly selective removal of thallous ions from wastewater using Prussian Blue biochar composite
Zhang Hailong 12
Ma Xiaoming 3
Wang Zhangxin 2
Han Bin 2
Yang Zhengheng 2
He Di di.he@gdut.edu.cn

2
1 https://ror.org/047bp1713 grid.440581.c 0000 0001 0372 1100 School of Environment and Safety Engineering, North University of China, Taiyuan, 030051 China
2 https://ror.org/03m01yf64 grid.454828.7 0000 0004 0638 8050 Key Laboratory of City Cluster Environmental Safety and Green Development (Guangdong University of Technology), Ministry of Education, Guangzhou, 510006 China
3 Shenzhen Pangu Environmental Protection Technology Co. Ltd, Shenzhen, 518055 China
14 9 2024
14 9 2024
2024
14 214792 7 2024
5 9 2024
© The Author(s) 2024
2024
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Thallium, a highly toxic pollutant, shows greater toxicity to human than other common heavy metals such as mercury, lead, cadmium and its effective removal from wastewater gains great attention. The main restriction for the Tl+ removal is the interference of a high concentration of co-existing ions in wastewater. Therefore, the goal of the current work was to synthesis adsorbent with high selectivity for the Tl+ removal. Herein, the pore size sieving strategy was proposed and Prussian blue-impregnated biochar (BC@PB) particles was synthesized. More than 95% Tl+ can be removed even the concentrations of the coexistence ions (Na+, Cd2+, and Zn2+) 1,000 higher than the initial concentration of Tl+ (500 μg/L). BC@PB also showed large adsorption capacity (9365 μg/g) and more than 99% Tl+ (initial concentration, 500 μg/L) were removed in just 1 min. The BC@PB had excellent and stable Tl+ removal ability (> 99%) over a range of pH from 3 to 9, which covered the pH range of common thallium-containing wastewater. The density functional theory (DFT) calculation confirmed that not only hydrated volume but also the hydration free energy of ions, which governed the energy barrier for ions entering into narrow channels of BC@PB, played essential roles on the selectivity removal of Tl+. Overall, due to its high selectivity, high adsorption capacity and easy preparation process, the synthesized BC@PB particles based on the pore sizing sieving strategy, can be a promising candidate for the removal of thallium from wastewater.

Keywords

Thallium
Pore size sieving
Prussian blue-impregnated biochar
Targeted removal
Hydration free energy
Subject terms

Pollution remediation
Natural hazards
Key Laboratory of City Cluster Environmental Safety and Green DevelopmentMinistry of Education and Basic Research Program of Shanxi Province202203021222024 Zhang Hailong Shanxi Scholarship Council of Chnia2024112 Zhang Hailong National Natural Science Foundation of China42007314 42007314 Ma Xiaoming He Di issue-copyright-statement© Springer Nature Limited 2024
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pmcIntroduction

Thallium (Tl) is a widely dispersed element and shows more toxic to human than mercury, lead, cooper or zinc1. The lethal dose of Tl is just 8 mg/kg to humans2. A recent evidence from a birth cohort study indicated that the prenatal exposure to thallium can lead to the decreased mitochondrial DNA copy number in newborns3. Due to the sulphophile and lithophilic properties, high concentration of thallium can be found in sulfide minerals such as pyrite, sphalerite, galena4,5 etc., and then released into surroundings during the process of exploitation and utilization of Tl-containing minerals resources, resulting in more and more Tl contamination incidents worldwide5–9. For example, in 2014, Tl pollution was reported in groundwater near Valdicastello Carducci and Pietrasanta (northern Tuscany, Italy) and the tap water distributed in the same area showed a high concentration of Tl (2–10 μg/L)5, more than 26,000 local residents were affected by this accident. Nearly 10 severe Tl contamination incidents were also reported in China during the past 10 years, and severe effects were caused to normal life of the local residents. Therefore, strict environmental standards were set to protect the environment from thallium pollution caused by human activities in many countries and the limit values were varied from 0.1 to 4 μg/L in the category of water quality standard and 2–140 μg/L in the category of discharge standard of pollutants processes10. In water environments, Tl exists in the form of Tl(I) and Tl(III). Tl(III) can be easily hydrolyzed and removed from water by its (hydr)oxides coprecipitation processes10. Tl(I) is the dominant species in most waster bodies and its removal is more difficult than Tl(III)11. Therefore, the study focuses on the thallous removal and “Tl removal” hereafter will specifically refer to the removal of Tl(I).

Many technologies such as oxidation–reduction precipitation, solvent extraction, ion exchange process and adsorption have been developed for the removal of Tl1,10,12. Among these technologies, adsorption was considered as one of the best technologies for the Tl removal due to its high removal efficiency, large adsorption capacity, easy operation, and environment-friendly10,13. For thallium-containing wastewater, one of the most important characteristics is that the concentrations of co-existing ions are often 100 or even 1000 times higher than the Tl+7,9,14. Considering the fact that the thallium concentration in actual polluted water sources is in the range of a few μg/L to a few mg/L15. Therefore, selectivity probability is more important than the adsorption capacity for the adsorbent synthesized for the thallium removal. However, just as Xu et al., pointed out in their essential review, most of previous adsorption studies were conducted at high initial Tl concentration (1–150 mg/L, even up to 1000 mg/L), adsorbents were synthesized for pursuing a high adsorption capacity and the effect of co-existing ions were neglected10. For the studies that concerned the effects co-existing ions, the adsorbents like manganese dioxide-based, titanium-based, carbon-based and alumina-based materials always showed poor selectivity to the Tl+16–19, which largely restricted the application in the actual environment.

Loads of adsorbents were synthesized for the selective removal of heavy metals20–27 (e.g. Pb(II), Ni(II), Cd(II)) in previous studies, which provided promising strategies for the efficient removal of heavy metals. According to our previous study, Prussian Blue (PB, Fe4[(CN)6]3) nanoparticles acted as an ions sieve, showed extremely high selectivity to Tl+ (more than 97% Tl+ was removed with 10,000 times higher co-existing ions presence in solution)9. However, the PB nanoparticles have an aggregation tendency because of their nanoscale size and dark bule color, limiting their practical application28. In addition to this, the mechanism of thallous ion selectivity has not been clearly illustrated. Some studies indicated that the Tl+ removal by Prussian blue is mainly due to the ionic exchange process with the K+29, other studies advocated that the geometric diameters of the ions themselves30,31 play an more important role on their selectivity. In addition to that, some other studies indicated that ions with similar hydrated diameters can show totally different selectivity9,32,33. Therefore, except the deficiencies of the Prussian blue itself, the removal mechanisms of Tl removal were also needed to be carefully explored.

To overcome the deficiencies of the PB for the thallous ion removal and deeply explored its removal mechanisms, the Biochar was introduced as a carrier and the material BC@PB (Prussian Blue loaded on the surface of Biochar) was successfully synthesized for the targeted removal of Tl+. The selectivity of the adsorbent for Tl+ was carefully studied under conditions with high concentrations (1–1000 times higher than Tl+ concentrations) of coexisting ions. The initial Tl concentration was set as 500 µg/L, which lower than most previous studies and approximated to the real Tl containing wastewater10. The adsorption kinetics, adsorption kinetics and effects of pHs (3–9) were also systematically investigated. The mechanisms of ions selectivity were carefully studied by the instrumental analysis and the Density Functional Theory (DFT) calculations. This work provides a new approach for the targeted Tl+ removal and clarifies its treatment mechanisms at an atomic-scale.

Materials and methods

Materials

K4[Fe(CN)6]3·3H2O, FeCl3, FeCl2·4H2O, NaCl, KCl, NaOH, ZnCl2 and CdCl2·2.5H2O were purchased from Aladdin (Shanghai, China) and used without any purification. The thallium solution was obtained from the Groups of the General Research Institute for Nonferrous Metals (China). HNO3, HCl used in this study were purchased from Guangzhou Chemical Reagent Co., Ltd (China).

Synthesis of biochar (BC) and BC@PB

The biochar was synthesized from cornstalks, which were collected from Henan, China. The cornstalks were washed thoroughly with distilled water to remove dust and other impurities. These were cut into small pieces with approximates size of 5–8 cm and then were dried in an oven at 80 °C until a constant weight was obtained. The dried biomass was ground to obtain more uniform feedstock and then was carbonized at 500 °C for 1 h under a nitrogen environment to avoid oxidation and the product obtained was labeled as biochar (BC). The obtained BC was passed through a 0.125 mm sieve before use.

The BC@PB was synthesized as following procedures: 5 g BC was added to a 500 mL beaker with 100 mL deionized water, and the pH was adjusted to 2 with HCl (Solution A). 3.68 g of K4[Fe(CN)6]3·3H2O was added to 100 mL deionized water and the pH of the K4[Fe(CN)6]3 solution was adjusted to 2 with HCl (Solution B). To synthesize BC@PB materials, the Solution B was added dropwise to the Solution A under stirring (300 rpm). After 1 h of magnetic stirring, 100 mL FeCl3 solution (1.89 g FeCl3 was added in 100 mL deionized water and the pH was also controlled to 2) was added dropwise under vigorous stirring conditions, followed by aging for 12h. After that, the obtained particles were separated with a 0.45 μm filter, washed 5 times with deionized water to remove the residual ions and vacuum-dried at 80 °C for 12h. Then, the final BC@PB particles were obtained.

Sample characterization

The morphology of the BC and BC@PB was characterized by the Scanning Election Microscopy (SEM, ZEISS, Sigma 300, Germany) and the transmission electron microscopy (TEM, FEI TalosF200x). The elemental composition of the samples was analyzed via TEM utilizing energy dispersive X-ray spectroscopy (Super-X EDS). Functional groups of the composite were examined by the Fourier transform infrared spectrometer (IRTracer-100, Shimadzu, Japan) equipped with an attenuated total reflection (ATR) accessory. The spectra were obtained in the range from 400 to 4000 cm−1. The specific surface area, total pore volume, and pore size distribution of the adsorbents were determined by means of N2 adsorption/desorption at 77.3 K using a porosimeter ASAP 2460 (Micromeritics, Inc. USA). The crystallinity of the samples was determined via Rigaku Ultima IV X-ray diffractometer (Cu Kα X-ray radiation source, 3 kW). Scans were performed between 10° and 80° at a scan speed of 2°/min.

Adsorption experiments

Adsorption kinetics experiments were conducted in a 1 L beaker containing 500 μg/L of Tl+ and the pH was adjusted to 7.0 ± 0.1 by the addition of HCl and NaOH solution. BC and BC@PB particles (250 mg) were slowly added to the beakers under vigorous stirring conditions. Aliquots of 5 mL suspension were sampled and filtered through a 0.22 μm membrane at different intervals.

Unless stated otherwise, the conditions of adsorption experiments were conducted as follows: the dosage of the adsorbents (BC or BC@PB) was 250 mg/L, the temperature of the reaction was 25 °C, the contact time for adsorption experiments was 2 h and the speed of oscillation was 125 rpm. For the isotherm adsorption experiments, the reaction pH was controlled to 7.0 ± 0.1 and the initial Tl+ concentration was varied from 500 μg/L to 100 mg/L for BC@PB and 50 μg/L to 20 mg/L for BC, respectively. The effect of the initial pH was investigated in a range of 3–9. The effects of coexistence ions (monovalent ions: Na+ and K+, divalent ions Cd2+ and Zn2+) on Tl+ removal was also conducted and the reaction pH was adjusted to 4.0 ± 0.1 to prevent the formation of Zn(OH)2. 0.05μL HNO3 (50%, v/v) was added to all filtered solution for further analysis. Inductively coupled plasma mass spectrometry (Thermo Scientific iCAP Q) was used for the analysis of Tl+ concentration and the analysis procedures were as same as our previous study9.

Computational method

By using the Cambridge serial total energy package (CASTEP), the first-principles calculations based on the Density Functional Theory (DFT) were achieved34,35. The exchange–correlation energy was described by the Perdew-Burke-Ernzerhof (PBE) exchange–correlation functional36 within the generalized gradient approximation (GGA), using ultrasoft pseudopotentials37. The cutoff energy of the plane waves basis expansion was set to 450 eV. The convergence criteria for total energy were set as 10–4 eV/atom and all atomic positions were fully relaxed until forces were smaller than 0.02 eV/Å. The sampling in the Brillouin zone for all surface slabs was set with 2 × 2 × 2 by the Monkhorst–Pack method38, under which the actual spacing between neighbouring K points is less than 0.038/Å. The van der Waals interaction has been corrected by using the DFT-D scheme39. Cubic bulk model of Prussian blue has been firstly optimized with a formula of C24N24Fe8 based on the lattice parameters are a = b = c = 10.508 Å40. Slab model was further built for (001) surface with a thickness of 10.166 Å, over which the vacuum (6.334 Å) was filled with bulk water, including the channels of Prussian blue. After geometry relaxation, M+ (Na+, K+, Tl+, Zn2+) ions have been introduced into the bulk water or into the channel, surrounded by water molecules. Using fully relaxed geometry of dissolved M+ as the starting geometry (labelled as S0), M+ has been gradually shifted from the solution to the inner cavity of the channels, generating a series of intermediate images as Sn (n = 1–6). Using the total energy of S0 as the reference, relative energy (RE) was obtained as RE(n) = E(Sn) − E(S0), based on which the energy profile for M+ diffusing from the solution to the channel can be established for further analysis. The reaction energy (Er) was calculated by Er = E(S6) − E(S0).

Results and discussion

Characterization of BC and BC@PB

The surface morphologies of the BC and BC@PB were observed via SEM analysis. Just as the Fig. 1 shown, both the BC and BC@PB possess porous structures. The structure of the BC surface is quite smooth (Fig. 1a–c). However, the surface of the BC@PB is rough and the nanoparticles can be observed on its surface (Fig. 1d–f). Compared with the previous studies, the morphologies of the BC@PB surface are similar to the surface of PB powder41. EDS analyses were used to detect the elemental plotting for BC and BC@PB (Fig. 1c,f). The element Fe is observed on the surface of BC@PB, which cannot be found on the surface of BC, also indicating that the PB was loaded on the surface of BC. The X-ray diffraction pattern of the BC (Fig. 1g) showed clearly peaks at 20.85°, 26.63°, 36.54°, 39.46°, 50.13°, 54.86°, 59.65° and 68.13°, which were corresponded to SiO242–44 (PDF 65-0466). SiO2 was also reported in other kinds of BC42. The small peaks located at 19.8°and 22.1° are corresponds to the cellulose pattern type I45 (Fig. 1g, marked as “*”). Just as (Fig. 1g, BC@PB) showed, the peaks appeared at 17.4°, 24.7°, 35.2°, 39.5°, 43.7°, 50.7°, 57.2° and 66.2° definitely confirmed that the PB(PDF#01-0239) was loaded on the surface of the BC9,46.Fig. 1 SEM images of BC(a–c) and BC@PB (d–g) XRD pattern of BC and BC@PB; (h) ATR-FTIR spectrum of BC and BC@PB.

The functional groups in the BC and BC@PB were identified via ATR-FTIR analysis and the corresponding spectrum is presented in Fig. 1h. The broad band at 1580 cm−1 can be attributed to the O–H stretching vibration of carbonyl and the band at 1408 cm−1 can be attributed to the C=O stretching vibration47,48. The Si–O–Si asymmetric stretching is evident by the appearance of the band at 1037 cm−149, which is consistent with the results of the XRD study. The peaks appeared in the range 2000–2500 cm−1 can be attributed to the C≡C and –C≡N triple bonds50,51. The bands located at 795 and 875 cm−1 are due to the =C–H and C=C stretching vibration51,52. For the BC@PB, the peak appeared at 2074 cm−1 is the typical –C≡N– triple bonds from the PB crystal53,54. The peaks appeared at 490 cm−1 can be assigned to the formation of Fe(II)-C≡N-Fe(III)9,54. Therefore, the new peaks that appeared on the surface of BC@PB also confirmed that the PB was loaded on the surface of BC, which also are consistent with the results obtained from SEM and XRD.

The BET surface area of BC was just 11.46 m2/g. However, the surface area of BC@PB increased largely to 161.37 m2/g, which means that the loading of PB on BC largely enhanced the surface area of BC. Significant differences between the morphological properties of the BC and BC@PB were also confirmed by the N2 adsorption/ desorption analysis. Just as shown in Fig. 2a, the results of BC resemble the combination of Type II and Type IV isotherm with an H3 hysteresis loop according to the IUPAC classification55, indicating that the BC in this study mainly possesses the mesoporous (2 nm < pores of the width < 50 nm) and macropores structure (pores of the width > 50 nm), which was also verified by the measured pore diameter (Fig. 2c). Unlike the BC results, the BC@PB (Fig. 2b) was classified as a typical IUPAC type IV isotherms47,55, indicating that the pores of BC@PB were mainly belonged to mesoporous, which were also confirmed by the average pore diameter of 7.74 nm (Fig. 2c). At the same time, the pore volumes increased largely from 0.011 to 0.124 cm3/g after the PB loaded on the surface of BC. The large increasement of the pore volume is probably due to the open framework structure of the PB56.Fig. 2 Nitrogen adsorption–desorption isotherms and pore size distribution of BC and BC@PB.

Adsorption kinetics and isotherms

To investigate the effect of contact time for the adsorption of Tl+ by the BC and BC@PB and establish an appropriate contact time to reach the equilibrium state, the experiments on the adsorption kinetics were conducted. Just as Fig. 3a,b shown, the removal of Tl+ increased rapidly in the beginning and around 60% Tl+ (initial Tl+ concentration, 500 μg/L) was removed by the BC. The rapid remove of Tl+ at the initial stage can be attributed to the abundant adsorption site of the BC57. Surprisingly, BC@PB just took 1 min to remove almost 100% of Tl+ in the solution, which means that the load of PB on BC significantly enhanced the removal velocity and capability of Tl+ (2 h was chosen as the shaking time in the following adsorption experiments to make sure both the BC and BC@PB have enough contacting time). The network of nanocapillaries may play an important role on the rapid Tl+ removal rate58,59. Joshi et al., also found that the velocity of the ions transport was thousands of times faster than simple diffusion when hydrated volume of ions was close to the size of the membrane pores at the sub-nanoscales59. The phenomenon is attributed to the network of nanocapillaries and the pressure provided by the capillary force can be as much as 50 bars59. In this study, the hydrated volume of Tl+ is close to the pores size of PB (Fig. S3), resulting in an extremely high removal rate of Tl+ on BC@PB due to the network of nanocapillaries.Fig. 3 (a) Tl+ adsorption kinetics on BC and BC@PB (Tl+  = 500 μg/L; adsorbent dosage, 250 mg/L); (b) Tl+ removal percentage with time increasing; (c,d) Adsorption isotherms described with the Langmuir and Freundlich models (Initial Tl+ for BC: 50 μg/L–20 mg/L, for BC@PB:500 μg/L ~ 100 mg/L; adsorbent dosage, 250 mg/L; pH = 7.0 ± 0.1; equilibrium time:2h).

Figure 3c,d show adsorption curves corresponding to Tl+ adsorption on BC and BC@PB and the adsorption capacity of BC@PB is about 10 times higher than the BC, meaning that the loading of PB significantly enhances the removal ability of Tl+. The adsorption data were analyzed utilizing the Langmuir and Freundlich models. The R2 value of the Freundlich is higher than the value of Langmuir (Table S1), indicating that the removal of Tl+ occurred on the heterogeneous surface of the BC60,61, which also consistent with the findings from the SEM. For the BC@PB, the Langmuir model is slightly better than the Freundlich model to describe the Tl+ removal process, indicating that the adsorption reaction at the liquid/solid interface in the adsorbent probably also plays an important role in the adsorption of Tl+47.

Effects of initial pH and coexistence ions

As known, the pH of the Tl-containing wastewater always varied largely (varied from 3.8 to 8.35)62. Therefore, the influence of pH on the removal of Tl+ was conducted and the range of the pH was chosen from 3 to 9 in this study. The results (Fig. 4) shown that Tl+ removal efficiency by BC gradually increased from 0.4 to 89.7% when the pH of the solution enhanced from 3 to 9. This phenomenon can be explained by the fact that the BC has a higher negative surface charge in the solution with higher pH due to the deprotonation, which can promote the removal of Tl+ with a positive charge63. Therefore, the electrostatic effect plays an important role in the Tl+ removal by BC. A similar phenomenon was also reported in other Biochar studies47,64. Interestingly, BC@PB showed excellent thallium removal ability (> 99%) in the whole studied range of pH from 3 to 9, which means a totally different Tl+ removal mechanism for BC@PB. The details will be carefully discussed in the next section.Fig. 4 Effect of the solution pH on the removal of Tl(I) to BC and BC@PB (Tl(I) = 500 μg/L; adsorbent dosage, 250 mg/L).

Since the majority of Tl in the Earth’s crust is associated with other minerals such as: Zinc-bearing minerals (sphalerite) and potassium minerals (alkali feldspars and micas)9,65. Zn2+, Cd2+, K+ and Na+ were the most common ions in thallium contained wastewater, and therefore these ions were chosen as interfering ions in this work66,67. As shown in Fig. 5, the adsorption amount of BC for Tl+ decreased largely with the concentration of coexisting ion increasing from 0 to 500 mg/L, especially when 4 ions were co-existing in solution. It means that the high concentration of co-existing ions largely interferes the removal capability of BC for Tl+ and the BC cannot be applied directly to the treatment of thallium containing wastewater. This may be due to the competition between the coexisting cations and Tl+ for the adsorption sites, which is consistent with the results of the pH effect (Fig. 4). In addition, the enhanced concentration of coexisting ions can also reduce the interface potential and thickness of the electric double layer, resulting in the reduction of the electrostatic adsorption68,69. Impressively, compared to BC, BC@PB exhibited extremely high selectively for Tl+ even when 500 mg/L Na+, K+, Cd2+ and Zn2+ were co-existed in solution (Fig. 5j), which also confirmed the totally different Tl+ removal mechanisms between the BC and BC@PB. The Tl+ removal mechanism by BC@PB will be carefully discussed in next section.Fig. 5 Effects of competing cations on the removal of Tl+ by BC (a–e) and BC@PB (f–j) (Tl+ = 500 μg/L; adsorbent dosage, 250 mg/L; pH = 4.0 ± 0.1; competing ions concentration, 0–500 mg/L, for the “4 ions” which means the solution contains 0–500 mg/L Na+, K+, Cd2+ and Zn2+ respectively).

Removal mechanism of Tl+

Although the functional groups on BC such as O–H and C=O are always accepted as the main groups contributing to coordinate the heavy metal ions on the adsorbent surface50, they did not take part in the Tl+ removal process in this study since the ATR-FTIR spectrum on the surface of BC before and after Tl+ removal kept almost the same (Fig. S1). The increasing removal percentage of Tl+ by BC with the enhancement of pH (Fig. 4) and the results obtained on the effects of co-existing ions (Fig. 5) both confirmed the fact that the Tl+ removal by BC in this study is mainly due to the electrostatic adsorption. Similar results were also reported in biochar derived from Opuntia ficus-indica, Undaria pinnatifida roots and watermelon seeds47,64.

For the BC@PB, the size exclusion, which is an important strategy for adsorbents with a certain sieve size to screen targeted ions, could play an important role in the high selective removal of Tl+. Specifically, during the adsorption of ions on the adsorbent with a certain sieve size, only ions with a size close to or smaller than that of the sieve size could be adsorbed, while those with a larger size are excluded70,71. In this work, the open pore size of PB (Fig. S3) is around 3 Å smaller than the hydrated diameters of the coexistence divalent ions (Zn2+, 8.6 Å, Cd2+, 8.52 Å, Fig. S2), which may be the reason why the presences of divalent cations (Zn2+ and Cd2+) have almost no effect on the Tl+ removal (Fig. 5) even their concentrations 1000 times higher than the Tl+. It can also be observed that the removal amount of BC@PB for Tl+ shows a slight decline when the concentration of monovalent cations (Na+ and K+) increased to 500 mg/L and the K+ shows a larger interference than the Na+, which can be explained by the smaller hydrated diameter of K+ (6.62 Å) than the Na+ (7.16 Å)33. Overall, the targeted removal of Tl+ on the BC@PB in solution with high concentration co-existed ions can be attributed to the size exclusion. However, since the hydrated diameter of Tl+ is 6.60 Å33, similar to the hydrated diameter of K+ 6.62 Å, the size exclusion cannot explain why the removal percentage of Tl+ is as much as 86.3% high even the co-existed K+ is 1000 times higher than the Tl+. Therefore, except for the size exclusion, some other mechanisms may also take part in the Tl+ removal process on BC@PB in solution with high concentration co-existed ions.

The hydration free energy is probably another important factor that should also be taken into consideration for the ion sieving and ion selectivity9,30,72. The hydration free energy can describe the stability of the hydrated ions32 and a higher hydration free energy means a stronger bonding between central ions and water molecules, which leads to a lower possibility to strip off the water molecules for the large hydrated ion enter into small pores72,73. Ions with smaller hydration free energy have soft hydration shells, hence are easily to strip off the water shells during ions entering into the angstrom-scale pores9,32,73. The hydration free energy of Tl+ (− 300 kcal·mol−1)32 is smaller than the hydration free energy of K+ (− 321 to − 351 kcal·mol−1)32,74. Therefore, the Tl+ can more easily strip off its surrounded water molecules during entering into narrow open pores compared with K+, resulting in a higher selective removal process than K+ (Fig. 5) even though it shows a similar hydrated diameter to K+. Similar studies were also reported on the ions of Cs+ and Rb+. Cs+ (6.58 Å, − 250 kcal·mol−1) is preferentially selected over Rb+ (6.58 Å, − 275 kcal·mol−1) even though these two cations have almost the same hydrated diameter32,33.

The hydration free energy of Zn2+ (− 2044 kcal·mol−1), Cd2+ (− 1979 kcal·mol−1), Na+ (− 398 kcal·mol−1 to -435 kcal·mol−1)32 are much larger than the Tl+ (− 300 kcal·mol−1), indicating a stronger interaction between these cations and their hydration shells, leading to a lower possibility of entering into the BC@PB pores, resulting into a lower interference on Tl+ removal (Fig. 5). The pore volume of BC@PB was decreased from 0.124 to 0.091 cm3/g after Tl+ removal, confirmed that Tl+ entered into the open pores of BC@PB and occupied its space. The EDS spectrum mapping (Fig. S4) and the decreased pore volume of BC@PB confirmed that the removal of Tl+ by BC@PB is attributed to the fact that Tl+ entered into the open pores of BC@PB.

The energy calculations can provide essential information to understand the selectivity processes. The energy barriers were calculated based on the assumption that all these ions can be forced into the narrow PB channels (Fig. 6a). Just as Fig. 6b shown, Zn2+ had the highest energy barrier (350 kcal·mol−1), followed by Na+ (315 kcal·mol−1), K+ (303 kcal·mol−1) and Tl+ (253 kcal·mol−1). The lower energy barrier means an easier entering process for these ions, resulting in a higher selectivity than other ions. This order matches the trend in hydration free energy of these ions. Richards and her colleagues indicated that the energy barrier of the large anions(fluoride, chloride, nitrate and nitrite) correlated well with their hydration energy during these anions entering into narrow pores73. It should be pointed out that the difference of barrier energy between the Tl+ and K+ is quietly large (50 kcal·mol−1) although they showed similar hydrated diameters (Fig. S2). In combination with the experiment results that more than 86.3% Tl+ was removed in solution with a 1000 times higher concentration of K+ presenting, we may can conclude that the hydration free energy rather than the hydrated diameter is the most important parameter for determining the selectivity of ions. Therefore, dehydration, which is governed by the hydration free energy, is the main barrier to ions during entering into the narrow pores72,73.Fig. 6 DFT calculations of metal ions diffusion from water solution into the channel. (a) Schematic geometry for the diffusion path as described by reaction coordinates (RC = 0–6); (b) Energy profile associated with metal ions at different coordinates; (c) reaction energy (Er) and barrier (Ea) for the diffusion of metal ions. The porous framework (Fe: light purple, C: grey, N: blue) is shown with a thick stick, metal ion as a purple ball, water molecules as thin sticks with red oxygen and white hydrogen.

The reaction energy for Zn2+, Tl+, Na+ and K+ are – 205 kcal·mol−1, 39 kcal·mol−1, 102 kcal·mol−1 and 175 kcal·mol−1, respectively. The higher reaction energy means a more difficult combination between the cations and the atom on the internal surface of PB75,76. The negative value of the reaction energy for Zn2+ indicated that Zn2+ will easily combine with the atoms on the internal surface of PB if the Zn2+ can enter into the open pores of PB. However, due to the larger energy barrier, the hydrated Zn2+ cannot enter into the PB channels, resulting in almost no interference to the removal of Tl+ (Fig. 5). Except for the Zn2+, the Tl+ shows the lowest reaction energy (Fig. 6b,c) than other cations such as K+ and Na+, demonstrating that there is a strong bond strength between the Tl+ and the atom on the internal surface of PB, indicating less likely releasing out from the adsorbent.

Conclusions

BC@PB particles was synthesized for the highly selective removal of Tl+ in this study. The presence of co-existing cations (with 1000 times higher concentration than the initial Tl+) and the variation of solution pH (3–9), had almost no influence on the Tl+ removal, indicating that the synthesized BC@PB showed great selectivity and a wide pH range application for Tl+ removal. The adsorption capacity of BC@PB was around 10 times higher than the pristine BC particles and more than 99% Tl+ (initial Tl + concentration 500 μg/L) can be removed in 1 min. The study also confirmed that the hydration free energy of ions themselves, which governs the energy barrier for ions entering into narrow channels, plays an essential role on the Tl+ removal. These results confirm that the pore seizing strategy is a promising material synthesized approach on the Tl+ removal. Considering the fact that thallium containing wastewater always has a high concentration of co-existing ion and that the pHs of the wastewater varies largely, the synthesized BC@PB particles with high selectivity, large adsorption capacity and low cost, will have great practical implications on the treatment of thallium containing wastewater. It is also need to pay an attention that the PB can be dissolved in a highly alkaline (pH > 11) environment which may influence the Tl+ removal.

Supplementary Information

Supplementary Information.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-024-72245-x.

Author contributions

Hailong Zhang: conceptualization, methodology, investigation, writing—original draft. Xiaoming Ma: Writing—review and editing. Zhangxin Wang: investigation, methodology, writing—review and editing. Bin Han: writing—review and editing. Zhengheng Yang: investigation, methodology. Di He: supervision, project administration.

Funding

This study was financially supported by the National Natural Science Foundation of China (42007314), Key Laboratory of City Cluster Environmental Safety and Green Development (Guangdong University of Technology), Ministry of Education and Basic Research Program of Shanxi Province (202203021222024) and Shanxi Scholarship Council of China (2024112).

Data availability

The data analyzed during this study which support its findings are available on request from the corresponding author.

Competing interests

The authors declare no competing interests.

Publisher's note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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