Fast and selective reversed-phase high performance liquid chromatographic separation of UO22+ and Th4+ ions using surface modified C18 silica monolithic supports with target specific ionophoric ligands

Reprocessing nuclear-spent fuels is highly demanded for enhanced resource efficacy and removal of the associated radiotoxicity. The present work elucidates the rapid separation of UO22+ and Th4+ ions using a reversed-phase high-performance liquid chromatographic (RP-HPLC) technique by dynamically modifying the surface of a C18 silica monolith column with target-specific ionophoric ligands. For the dynamic modification, four analogous aromatic amide ligands, N1,N1,N3,N3,N5,N5-hexa(alkyl)benzene-1,3,5-tricarboxamide (alkyl = butyl, hexyl, octyl, and decyl) as column modifiers were synthesized. The complexation properties and retention profiles of the amide-based column modifiers for the selective and sequential separation of UO22+ and Th4+ ions were investigated. In addition, the selective separation of UO22+ and Th4+ ions among the competitive ions of similar chemical properties were also studied. The ionophore immobilized C18 silica monolith columns demonstrated a varying degree of retention behavior for UO22+ and Th4+ ions (UO22+ is retained longer than Th4+ under all analytical conditions), eventually leading to rapid separations within a period of ≤5 min. A 0.1 M solution of 2-hydroxyisobutyric acid (HIBA, 1 mL min−1) served as the mobile phase, and the qualitative and quantitative assessment of the sequentially separated 5f metal ions was achieved through post-column derivatization reaction, using arsenazo(iii) as a post-column reagent (PCR; 1.5 mL min−1) prior to analysis using a UV-vis detector, at 665 nm (λmax). The developed technique was further evaluated by standardizing various analytical parameters, including modifier concentration, mobile phase pH, mobile phase flow rate, etc., to yield the best chromatographic separation. Also, the conceptual role of alkyl chain length (in the modifier) on the retention behavior of the studied metal ions was evaluated for cutting-edge future applications.


Introduction
With the manifold increase in the depletion rate of nonrenewable energy resources, nuclear energy has evolved as one of the replacements in this domain of energy, and this coerces the opening up of the nuclear fuel cycle option, where a large amount of energy rening through nuclear reactors can be attained with critical and careful nuclear reprocessing of spent fuels. 1,2 The utility of uranium and thorium as high-density penetrators, gyroscopic compasses, photographic chemicals, lamp laments, contrast agents in medical radiography, ceramics, welding rods, camera and telescope lenses, re brick, heat-resistant paint, etc., promotes its urgent necessity of individual isolation from the crude sample sources. 3 However, tracing and recovering these ions from environmental samples or other matrices is demanding due to the absence of accessible and specic methods. 4,5 In this aspect, HPLC has gained much attention as a separation platform in the academic and industrial (preparative and process scale) sectors due to its rapid, robust, and reliable performance. 6 In this line, LC-based monolith columns are emerging and nd numerous applications where the quantity of analyte and the analysis times can high accuracy have already been reported in the literature. Several reported prevailing methods, such as solid-phase extraction (SPE), solvent extraction, ion exchange, precipitation, etc., have been developed to separate UO 2 2+ and Th 4+ from various matrices. [7][8][9] Inductively coupled plasma-atomic emission spectrophotometry (ICP-AES) and ion-chromatography coupled to inductively coupled plasma mass spectrometry (IC-ICP-MS) are some of the sophisticated predominant methods in the sphere of analytical science for these targeted analytes. 10,11 Although these techniques give appreciable detection limits, the shortcomings of these methods are their high-end instrumentation and operating costs with the requirement of skilled technicians. In this line, Dupuis et al. used capillary electrophoresis hyphenated with multi-collector inductively coupled plasma mass spectrometric technique for the isotope ratio measurements for actinides and lanthanides ssion products from spent fuels. 12 The technique can successfully lower the sample quantity and waste volume production. However, the cost factor associated with the instrumentation is far higher, which makes it less accessible for developing countries. The actinide separation process has also evolved using extraction chromatographic techniques coupled with ICP-MS, as described by Hang et al. Still, the method also suffers from high-end instrumental assay. 13 Studies on the hydrometallurgical separation of trivalent actinides and lanthanides using CyMe 4 BTBP and CyMe 4 BTPhen extractants were also some recent alternatives for separating the concerned analytes. 14 Wang et al. used simple extraction techniques to selectively separate actinides from lanthanides using a group of dithiophosphinic acids. The work mainly discusses the role of substituents in the dithiophosphinic acid on the extractive property and efficiency of the studied analytes, especially of Am 3+ and Eu 3+ . 15 There are also works reported using the ionic liquid hexyltributylphosphonium bis(triuoromethylsulfonyl) imides, 16 bis-1,2,3-triazolebipyridine ligands 17 for the separation/extraction of lanthanides and actinides. The methods related to nanotechnology for the separation of actinides from radioactive waste are also developed. In line with this, Kaur et al. used magnetic nanoparticles (MNPs) conjugated with actinide-specic chelators to separate Np, Am, and Cm from spent nuclear fuels. The method is cost-effective, but the issues associated with the created secondary nanotoxicity need to be addressed. 18 Also, extractions of minor actinides and lanthanides were studied using ligand immobilized on silica gel and using this method, near-quantitative removal of Am(III) in the presence of Eu(III) has been achieved by Afsar et al. 19 Most of the method prevails till now either comes under sophisticated instrumental procedures which demand high costs or lags in specic removal of UO 2 2+ /Th 4+ amidst larger concentrations of lanthanide ions. In addition, several radioanalytical techniques, such as isotope dilution analysis, radiochromatography, neutron activation analysis etc., were reported for the isotopic detection of various radionuclides. 20,21 The detection of these radionuclides also includes various environmental and biological samples. In this line, Roane et al. used a radiochromatographic technique to analyze actinides and strontium from soil samples. 22 Quite recently, Jia et al. used radioanalytical techniques to separate and measure some radionuclides in the area of the Gela Phosphate Industry in Italy. The radionuclides 238 U, 234 U, 235 U, 226 Ra, 210 Po, and 216 Po were counted using a-spectrometry, and the techniques used were found to be sensitive, selective and accurate. 23 The a-particle spectrometric technique was recently used for determining uranium and thorium by Lindahl et al., which illustrates an alkaline fusion method for a low-cost total dissolution of soil and sediment samples containing U and Th isotopes. 24 Similarly, Harrison et al. studied the separation of thorium, plutonium, americium, uranium and strontium in environmental matrices using radioanalytical techniques where the process involves sample dissolution, concentration via calcium phosphate co-precipitation, rapid column extraction using TEVA™, TRU™ and Sr-Spec™ resin cartridges and alpha spectrometry for Th, Pu, U and Am and Cerenkov counting for Sr. 25 Besides, radioanalytical determination of uranium has been done using reactive polymer thin lms. a-Spectroscopy pulse height spectra were analyzed and utilized in the work to study the role of selective layer lm thickness on peak energy resolution. 26 Besides, Fullmer et al. utilized a hybrid extractive scintillator resin for the detection of plutonium where the extractive scintillating resin was comprised of a silica base, functionalized for plutonium extraction, graed with a plastic scintillator of polyvinyl toluene (PVT) and 2-(1-naphthyl)-4-vinyl-5-phenyloxazole (vNPO) uor. 27 Benedik et al. utilized neutron activation analysis for the ultra-trace determination of U and Th in electrolytic copper via their induced nuclides 239 U/ 239 Np and 233 Pa, respectively. 28 Monolithic silica columns possess high surface area and uniform mesoporous structures with orderly arranged interconnected through-pores, making them the ideal candidates for chromatographic applications. 29 The unique characteristics of these columns render high performance in separation speed, low column back pressure, and enhancement in mass transfer kinetics. 30,31 Thus, HPLC coupled with a UV-vis detection system is utilized to separate UO 2 2+ and Th 4+ ions amidst a mixture of lanthanides (Lns) to reduce the cost factor signicantly. As per the reports on actinides' separation and extraction chemistry, amide-based aliphatic extractants have been used to recover actinides. 32,33 The classic examples include long-chain amides of n-octyl(phenyl)(N,N-diisobutylcarbamoyl)methyl phosphine oxide (CMPO), tri-butyl phosphate (TBP) and N,N,N,N-tetraoctyl diglycolamide (TODGA) as extractants for the selective recovery of actinide ions from matrix constituents. [34][35][36][37][38] The derivative of TODGA has been extensively used for analyzing the extraction behavior of various f-block elements, as these aliphatic amides were found to bind the targets strongly with reasonable stability. An ion-pair HPLC technique has been reported by Sivaraman et al. for the individual isolation of Lns from uranium, plutonium, and other ssion products where the preseparation of Ln-ssion products has been achieved using di-(2ethylhexyl) phosphoric acid coated Amberlite XAD-7 packed bead glass column, using camphor-10-sulfonic acid and a-hydroxy isobutyric acid as the mobile phases. 39 The same research group used the camphor-10-sulfonic acid modied column to separate Lns using HIBA as the mobile phase through ion interaction chromatography. 40 Even though these techniques qualify in the isolation process, the inhomogeneity in the packed bead columns and the resulting high back pressure signicantly increases the plate height, ultimately leading to longer analysis times for industrial scale-up applications. These ion-pair chromatographic techniques follow a separation protocol that involves the coulombic interaction between the target ion complex and the ionic/charged support surface. However, their limitations stand by the fact that the surfactant adsorption process is signicantly slow, which initiates the use of nearly 20 column volumes to obtain the required mobile phase/eluent composition. There were some studies devoted to the use of aliphatic amides to separate some or the whole of the f-block metal ions. n-octane sulfonate as a column modier that exhibited partial cation exchanger property and hydrophobicity. 47 These methods and processes gave insights into the beautiful separation chemistry of chemically similar f-block elements. However, they lag due to the technical difficulties in reproducibility, the requirements of larger concentrations of the modi-ers to yield the separations and high column back pressure due to the use of particle-packed columns in most of the cases.
Recently, siderophores-inspired synthetic analogs were used by Pallares et al. to separate lanthanide and actinides where an active bidentate hydroxypyridinonate group has been utilized for the complexation of lanthanides and actinide ions. 48 Similarly, biopolymers, small-molecule lixiviants, peptides, and proteins are also utilized and studied to extract f-block elements. 3 Keeping these in mind, the current work reports on the development of RP-HPLC methods for the selective separation of UO 2 2+ and Th 4+ using dynamically modied C 18 silica monolith columns embedded with four amide derivatives of N 1 ,N 1 ,N 3 ,N 3 ,N 5 ,N 5 -hexa(alkyl = butyl/hexyl/octyl/decyl) benzene 1,3,5-tricarboxamide, denoted as HBBTA, HHBTA, HOBTA, and HDBTA, amidst more signicant concentrations of lanthanide ions. The work explicitly describes the separation of UO 2 2+ and Th 4+ ions, which are difficult to separate due to their similarity in intrinsic chemical properties. The usage of these analytes in industrial, nuclear and medicinal elds necessitates their isolation. It is well-known that amide-based chemo-markers are excellent extractants for actinides. In this work, the prudent choice and tuning of hydrophobic alkyl chains in the triamide ionophores display varying levels of interaction behavior with the C 18 moiety through hydrophobic interaction to ensure uniform coating. The alkyl chain also helps enhance the ligand's non-leaching property since the proposed mobile phase is aqueous. The proposed method can be expressed/ labeled as one of the most cost-effective LC-based techniques with reusable nature. The used modiers can be removed entirely/stripped off aer analysis, regenerating the bare column with 100% efficiency. Also, the novel ligand/modier used in the current method proffers excellent future probabilities in designing ligands of similar structural formulations for extracting actinide ions from various sample matrices. The rst of its use of an aromatic amphiphilic triamide-modied silica monolithic column having a reusability nature for the separation of UO 2 2+ /Th 4+ is the state-of-the-art nature of the proposed technique. This technique signicantly advances the ease and efficiency of separating radionuclides of specic interest. The retention and elution patterns of UO 2 2+ and Th 4+ ions using the triamide-based column modiers have been studied using a-HIBA as the mobile phase and arsenazo(III) as the post-column reagent (PCR). A detailed investigation has been carried out to study the inuence of various analytical parameters, such as the role of mobile phase pH, amide concentration, mobile phase ow rate, etc., on chromatographic performance. We have compared the effect of alkyl chain length in separation patterns to optimize the quantitative and qualitative separation of the target analytes. It should also be mentioned that the column modier can be eluted out aer use, retaining the bare monolith column without compromising its performance.

Instrumentation and materials
A high-pressure /Th 4+ -arsenazo(III) complexes were detected/quantied at 665 nm (l max ) using a UV-vis spectrophotometric detector. ChromNAV 4000 series soware was employed to process the signal responses from the detector to obtain the chromatograms through a JASCO LC NET-II/ADC interface. A digital pH meter (Metrohm 913) was used to adjust the mobile phase pH, and the pH adjustments were conducted using dilute HCl/NaOH. A UV-vis spectrophotometer (Jasco V-670) was utilized to determine the amounts of triamides that were adsorbed onto the chromatographic column aer the elution process. The prepared column modiers are characterized using 1  (SRL, 96%), and Th 4+ (SDFCL, 99%) were prepared from their respective nitrate salts. The a-HIBA (Sigma-Aldrich, >99% purity), metal ion stocks, and arsenazo(III) (TCI, > 95%) solutions were prepared using ultrapure Milli-Q water of high purity and ltered (membrane lters, 0.22 mm, Merck) and degassed before usage. For the synthesis of the column modiers, benzene-1,3,5-tricarbonyltrichloride, dibutyl amine, dihexyl amine, dioctyl amine, didecyl amine (Sigma-Aldrich, 98%), Triethylamine (Finar, 99%), dry diethyl ether (SD-ne, AR grade) were used.

Synthesis and characterization of the column modiers
The column modiers, i.e., N 1 ,N 1 ,N 3 ,N 3 ,N 5 ,N 5 -hexa(butyl/hexyl/ octyl/decyl)benzene-1,3,5-tricarboxamide derivatives (HBBTA/ HHBTA/HOBTA/HDBTA) were synthesized using benzene-1,3,5-tricarbonyl trichloride and dibutyl/dihexyl/dioctyl/ dodecyl amine through a simple one-pot synthetic route. For this, benzene-1,3,5-tricarbonyl trichloride (1.0 mmol) was added to dry ether (25 mL) along with triethylamine (3.5 mmol) and was allowed to stir for 10 min at 0°C. Then a mixture of 3.2 mmol of dialkylamine in dry ether was added to the acid chloride mixture using a pressure equalizer. The reaction was allowed to stir at room temperature for 24 h. The crude reaction mixture was washed with water, extracted with diethyl ether, and vacuum distilled to obtain a yellow-colored oily liquid. The same procedure was followed to prepare all four amide derivatives (Scheme 1), and the products were characterized using 1

Dynamic column modication process
For the dynamic modication process, appropriate quantities of the secondary triamide derivatives (HBBTA/HHBTA/HOBTA/ HDBTA) were weighed and dissolved separately in a methanol and water mixture. The stoichiometry of the MeOH : H 2 O ratio was optimized to ensure a composition that will not profoundly precipitate the amide ligands, as shown in Table S1 (ESI †). The coating solutions were thoroughly degassed and pumped through the C 18 silica monolithic column at an optimum ow rate of 0.2 mL min −1 at ambient temperatures. Before coating, the column is pre-equilibrated with 50 mL of the same solvent mixture (MeOH: water). Moreover, the column was washed with water aer modication and pre-equilibrated with 50 mL of mobile phase (0.1 M HIBA) to perform the metal ion separation studies. Aer the dynamic modication, the column should not be pumped with mobile phases consisting of organic solvents, which could elute the sorbed amide molecules. The column eluate during the modication process was collected to quantify the amide molecules that leave the column without getting adsorbed by a UV-visible spectrophotometer aer extracting the same in diethyl ether. Finally, to accurately determine the amide modier coated onto the C 18 silica monolith column, the Scheme 1 Synthesis of triamide column modifiers. modied column was allowed to undergo a desorption process using 60 mL methanol as eluent at a ow rate of 0.2 mL min −1 aer completing all the metal ion separation studies. The modier-methanol solutions eluted from the column were collected and quantied (Table S1, ESI †).

Chromatographic procedure for the separation of UO
The modied column was initially washed with water, followed by equilibration with the mobile phase (a-HIBA) of appropriate pH for 30 min at 1 mL min −1 . A 20 mL of the analyte mixture is further injected into the modied column keeping the mobile phase ow rate at 1 mL min −1 . The retentions of the analytes in the column were monitored by a UV-vis detector preceded by a post-column derivatization reaction using arsenazo(III), where the PCR's ow rate was xed at 1.5 mL min −1 . However, the retention behaviors of the analytes on an unmodied column were performed under the same chromatographic conditions followed for the modied columns. The optimized conditions for the chromatographic separation of UO 2 2+ and Th 4+ using the C 18 silica monolithic columns that were modied with four triamide derivatives as given in Table S2 (ESI †). Reproducibility tests were conducted by recoating the column with the same ligand concentration aer eluting the column modiers from the column aer analysis. For data reliability and reproducibility, tests were conducted via intra-day, inter-day, column-tocolumn, and batch-to-batch methods. All the obtained data were reproducible with an RSD # 2.5%. and Th 4+ with the modied columns were investigated using varying mobile phase pH, i.e., 2.5-4.0 (Fig. 1) (Fig. 2). Moreover, the same decreasing pattern was observed for the other two modied columns (HHBTA/HOBTA) on increasing the modier content. More signicant separation factors of the target analytes were achieved because of the predominant differences in the relative distributions of the analytes with the amide support and the HIBA mobile phase. At the same time, UO 2 2+ exhibited more outstanding retentions in the stationary phase than the Th 4+ ions. But the observed decrease in the retention times with the increase in the amide-coated content was speculated to be due to the multi-layer amide coating, which reduces the accessibility of amide functional centers for the UO 2 2+ and Th 4+ ions to form stable complexes. During the multi-layer coating, the hydrophobic carbon chains will be exposed more predominantly toward the surface, masking the amide groups meant for complexation. However, the role of amides on preferential retention of UO 2 2+ over Th 4+ ions has been observed clearly with HDBTA-modied columns with a pattern showing increased retentions of analytes with increasing HDBTA amide content coating from 0.006 to 0.023 mmol on the C 18 stationary phase. This further authenticates the dominant complexing property of UO 2 2+ ions with the amide groups in the HDBTA modier,

Results and discussions
i.e., the separation mechanism works predominantly based on the complexation process, nullifying the masking caused by the modier moieties. The actinide ions are hard acids, and hard bases such as O have a greater affinity to form strong complexes with the metal ions. Amides are also considered as neutral extractants having signicant electron-rich centers. These moieties are better candidates for extracting actinide ions through coordination with the amido oxygen and are proved to be robust and efficient agents in this complexation/extraction arena. However, some lagging must be addressed in separating these ions, especially amidst Lns of larger concentrations. As these ligands/modiers exhibit similar behavior towards complexing Lns, the separate extraction of the selective actinide ions becomes less robust. We suggest a synergistic involvement of the use of LC-monolithic columns along with aromatic tri-amide modiers, which can selectively separate the actinide ions under study within a 3 to 5 min time frame under optimized conditions. The lack of a sufficient modier coated on the monolithic stationary phase ensures the non-separation behavior for lanthanide ions. In contrast, the amphiphilic aromatic tri-amide imparts wider accessibility of the functional groups projected in all directions for the metal ions to interact and complex. Also, the modier's alkyl chains ensure strong amide moieties adhesion on the stationary support. From the chromatograms, the HBBTA ligand was identied as a promising candidate for the rapid resolution of UO 2 2+ and Th 4+ within 5 min, wherein the unmodied bare C 18 monolith column failed to resolve under similar chromatographic conditions. The order of elution, as well as the chromatogram pattern upon increasing the ligand content, was unaltered even if the hydrophobicity of the modier ligand was varied. In the case of HOBTA and HDBTA modied supports (Fig. 2(c & d)), very low concentrations of the modier could retain both Th 4+ and UO 2 2+ signicantly compared to the other investigated triamide derivatives. Also, as the alkyl chain length increases, the electron density on the C]O group of the amide functionality increases due to the inductive effect offered by the alkyl chains of the -NR 2 units, which in turn increases the retention times of UO 2 2+ and Th 4+ . Hence, these modiers will also be better alternatives for the bulk extraction of UO 2 2+ ions.

Inuence of mobile phase ow rates on retention proles
The effect of eluent ow rates on the retention proles of UO 2

2+
and Th 4+ was studied by varying the mobile phase ow rates from 1 to 3 mL min −1 at pH 2.5 with different modied supports (Fig. 3). With all the amide derivatives, the retention times of the UO 2 2+ from Th 4+ ions decrease with increasing ow rates. However, a well-resolved peak prole was observed at all the studied ow rates. At 3 mL min −1 , rapid separation of UO 2 2+ and Th 4+ ions was observed, with Th 4+ eluting out of the HBBTA-modied column in just 0.76 min and UO 2 2+ in 1.17 min (Fig. 3a). All the other three ligand-modied columns exhibit similar results, as shown in Fig. 3(b-d).
3.4. Comparison of retention proles of UO 2 2+ and Th 4+ on different triamide derivatives The comparison proling attributed to the four modiers has been studied by collating the retention proles of UO 2 2+ and Th 4+ mixture on 0.17 mmol HBBTA, 0.07 mmol HHBTA/HOBTA, and 0.023 mmol HDBTA modied columns at mobile phase pH of 2.5 (Fig. 4a), 3.0 (Fig. 4b), and 3.5 (Fig. 4c). The chromatograms distinguish the effect of variation in the alkyl chain constituent in the ligand, which directly affects the retention time of the analyte ions as one of the signicant factors determining the separation is the hydrophobic interaction between the modied column and the analyte-IBA complex. From the chromatograms, a coherent transition/shi in the retention time of UO 2 2+ was noticed at each studied pH as the carbon chain length in the ligand increased. This can be explained as the additional hydrophobic interaction between the UO 2 2+ -IBA complex with the modier's long chain and the strong complexation between the analyte and the amide group in the ligand leading to higher retention of the analyte on the modi-ed column/stationary phase. For example, the retention prole of UO 2 2+ at pH 2.5 on the four modied stationary phases imparts a gradual increase in the retention time as the carbon chain length increases from butyl to hexyl, then to octyl, and nally to decyl. An interesting observation regarding HDBTA is that, even at a concentration as low as 0.023 mmol, the high degree of hydrophobicity and complexing ability forces it to hold the UO 2 2+ -IBA complex more, which results in a t R of 13.25 min (at pH 2.5) which was exceptionally large compared to the other amide derivatives. With C 10 decyl chains, the hydrophobicity of the ligands increases signicantly, which in turn contributes to strong extraction of the actinide ions even with smaller ligand dosages. The effect of alkyl chain length in the modier moiety on the retention of UO 2 2+ and Th 4+ matters as it possesses a dual role in the proposed method. The primary role is a strong hydrophobic entity that ensures effective coating on the C 18 silica monolithic support through which the leaching of the physically immobilized ligand/modier on the stationary support was avoided. This way, more modier moieties can be rmly xed on the stationary support, which could eventually interact with the actinide ion under study. However, it is observed that the number of amide ligands which can be coated on the solid support also depends on the number of C 18 moieties in the stationary phase. Hence, even though the strength of the coating and non-leachability property of the ligand on the stationary system can be enhanced by increasing the alkyl chain length, the number of effectively coated ligands will not be altered much on a given C 18 system. One major problem associated with the increased number of carbon chains in the modier is its difficulty preparing the coating solutions. As the ligand's hydrophobic nature increases, its precipitation on the coating solution on adding smaller aliquots of water increases tremendously. Also, on longer carbon chain lengths, the coiling effect will be predominant, which signicantly reduces the efficacy of the coated column on separation. The role of alkyl chain length on diglycolamides (TODGA) derivative has been studied by Yadav et al., where the results also corroborate with the observation in the present study. In that case, the loading capacity decreases as the alkyl chain length increases in the aliphatic modier. 52 Also, the butyl chains in the HBBTA ligand provide better accessibility for the actinide species around the carbonyl groups for better complexation (lesser steric hindrance for complexation). 53 The second most important property associated with the alkyl groups lies in the separation/retention proling of the target metal ions. The carbon chains enormously contribute to dening the retention time of the analytes as they induce the +I effect, demonstrating the electron density on the amido oxygen meant for complexing with the metal ions. As the carbon chain length increases, the +I effect will increase, which induces electron donation towards amido oxygen, enriching the complexing center and imparting a more basic character to the molecular structure. The high electron-donating capability of decyl chains compared to octyl, hexyl, and butyl, where the electron-donating capacity follows the order decyl > octyl > hexyl > butyl in the ligand enriches the electron density around the amido oxygen, forming stable complexes with UO 2 2+ and Th 4+ , thereby increasing the retention times. This way, stronger retention of the analytes was expected in higher alkyl groups.
For example, even with a concentration as low as 0.023 mmol of HDBTA, a retention time of 13.25 min was observed for UO 2 2+ .
Besides, the longer alkyl chain groups will impart a severe steric hindrance, reducing the accessibility of the ligands for the analyte ions to complex. However, considering all the parameters/inuences of the carbon chain in the modier, a higher concentration coating of HDBTA ligand was not achieved due to their practical incapability of preparing coating solutions, owing to their high level of hydrophobic nature. One another interesting observation was that Th 4+ showed/ exhibited only a marginal difference in its t R value because of the lesser hydrophobicity associated with the cationic Th 4+ -IBA complex at pH 2.5 and the weak complexing ability with the amide groups compared with that of UO 2 2+ . However, as the hydrophobicity of the ligand/modier increases, the retention time for Th 4+ is also increasing, which is evident from the shi in the observed t R values as the carbon chain length increases to octyl and decyl. Interestingly, all four modied columns at all the studied three mobile phase pH conditions resolved both UO 2 2+ and Th 4+ with excellent peak shape, as the bare C 18 column (unmodied) failed to resolve under similar conditions.  Fig. 5(d). The chromatogram showed that the injected mixture of 14 lanthanoids came out incredibly early (<2.0 min) as an unresolved peak showcasing its non-interaction with the triamide ligand-modied stationary phase. These ndings show that the novel triamide ligands modied columns can be utilized exclusively to selectively separate and isolate UO 2 2+ and Th 4+ ions from complex matrices. The poor retention behaviors of lanthanoids can be justied by the lesser number of amide moieties and C 18 moieties in the monolith column, which was insufficient for lanthanoids to interact strongly with the column modiers to form a stable complex. The individual retention behavior of the lanthanides and actinides on the three modied columns is shown in Fig. 5(a-c), which depicted a similar retention behavior to that of the HDBTA modied column.

Concluding remarks
Herein we report an easy, facile, and effective method for the systematic and selective separation of UO 2 2+ and Th 4+ by dynamically modifying reversed-phase C 18 silica monolithic columns using four novel hydrophobic aromatic triamide ionophoric ligands. The reproducibility and reusability have been evaluated through run-to-run, day-to-day, batch-to-batch, and column-to-column analysis. The work also compares the effect of amide hydrophobicity on the separation prole of UO 2 2+ and Th 4+ by varying the alkyl chain length (ranging from butyl to decyl) in the synthesized triamide ligand molecules. The results revealed that with 0.023 mM HDBTA modied column, the separation of UO 2 2+ /Th 4+ was achieved at 13.25 and 4.13 min, respectively. Also, the studies conclude that the optimum coating concentration of HBBTA, HHBTA and HOBTA were 0.17 mM, 0.07 mM and 0.07 mM, respectively. The coated amide content formed stable complexes with [UO 2 2+ /Th 4+ -IBA] species, resulting in good peak resolution, which was impossible with unmodied C 18 silica monolithic columns. The method can be further extended for scale-up measurements using preparative columns where the aromatic triamide derivatives can separate and extract the target analytes. The modier can be intuitively tuned for other relative applications such as sensing, solid-phase extraction, preconcentration, etc. Also, the method shows superior properties in separating UO 2 2+ and Th 4+ amidst all lanthanoids, even if present in multi-fold concentrations.

Data availability statement
Data is available on request from the authors.

Conflicts of interest
The authors declare that they have no known competing interests that could have appeared to inuence the work reported in this paper.