Srikanth Nayak

Hydrolytic PuO₂ nanoparticles (NPs) are a dominant aqueous Pu-bearing phase in high ionic strength nuclear wastes, yet their reactivity in nonideal brines remains poorly constrained. We quantify how electrolyte identity and concentration control PuO₂ NP aggregation and ligand-assisted dissolution in acidic, high salinity solutions (NaCl, NaNO₃, NaClO₄, Na₂SO₄, Na₂C₂O₄ up to 5 M). A multitechnique workflow combining liquid scintillation counting (operationally defined aqueous [Pu]), scattering/electrokinetic measurements (aggregate size and zeta potential), and spectroscopy (UV–vis, XPS, Raman) resolves electrolyte-dependent partitioning between colloidal and molecular Pu species. Weakly coordinating anions (ClO₄^–, Cl^–, NO₃^–) largely preserve the (aggregated) nanoparticulate fraction but generate distinct dissolved species at high concentration, i.e., Pu(IV)–nitrato complexes in NaNO₃ and Pu(VI)–chloro complexes in NaCl. In contrast, stronger ligands substantially perturb PuO₂ NP stability: sulfate promotes partial dissolution to Pu(IV)–sulfate complexes at low concentration but reduces aqueous [Pu] at higher sulfate levels via secondary Pu(IV) sulfate formation, whereas oxalate drives strong dissolution to aqueous Pu–oxalate complexes. Aged NPs show similar trends with reduced aqueous fractions and more dominant aggregation mechanisms. These spectroscopically constrained speciation data provide a foundation for incorporating PuO₂ NP reactivity into thermodynamic and reactive transport models for high salinity waste and brine environments.

We report on the speciation of two lanthanides, Eu(III) and Er(III), in aqueous and organic thiocyanate solutions obtained by solvent extraction with a quaternary ammonium extractant (Aliquat-336.thiocyanate, AQ.SCN) in toluene. The connection between the metal extraction behavior and the metal speciation in the two phases is investigated by a combination of spectroscopic techniques including UV-vis, photoluminescence, and ATR-FTIR spectroscopy. Extraction studies show a higher extraction of Er(III) than Eu(III), with extractant dependency exponents of 2 and 1.5 respectively, indicating the formation of -1 to -2 charged anionic complexes in the organic phase. Spectroscopic studies indicate that the anionic complexes are not formed in the aqueous phase for either lanthanide, even at high NaSCN concentrations. Photoluminescence of Eu(III) in the organic phase shows the presence of multiple species with 3 and 5-6 thiocyanate anions in the inner coordination sphere corresponding to the non-integral exponents in extraction studies. ATR-FTIR spectra show differences in the CN stretching region of SCN- for the two lanthanides, corresponding to predominantly inner-sphere coordination of thiocyanate to Eu(III) and outer-sphere coordination to Er(III). Thus, while extraction studies show that the overall charge on the anionic species in the organic phase is greater for Er(III) than Eu(III), spectroscopic results reveal that the number of thiocyanate anions in the inner coordination sphere of Eu(III) is larger than that of Er(III). Thus, a balance of lanthanide hydration and complexation in the organic phase plays a major role in the extraction of lanthanides with AQ.SCN from thiocyanate solutions.

Understanding and manipulating uranyl speciation in aqueous solutions is critical for advancing chemical separation, sensing, and understanding environmental transport of uranyl. We report on the significant enhancement in the complexation of uranyl with nitrate in aqueous solutions containing quaternary ammonium cations leading to the formation of anionic complexes. We base this on the comparative study of the effect of monovalent cations (Na⁺, Li⁺, NH₄ ⁺, and N(CH₃)₄ ⁺) on the complexation equilibria of uranyl-nitrate in solutions, probed by time-resolved laser induced fluorescence spectroscopy (TRLFS). Lifetime-corrected spectra, obtained by extrapolating the time-resolved spectra to t = 0, were used to study speciation to mitigate the effects of variations in the fluorescence lifetimes that depend on extraneous factors such as dynamic quenching. We demonstrate that the lifetime-corrected spectra can be used to determine uranyl speciation in aqueous solutions where the mono-nitrate complex forms, and in acetonitrile with N(CH₃)₄NO₃ where di-nitrate and tris-nitrato species are formed. Aqueous solutions containing N(CH₃)₄ ⁺ are shown to promote the formation of higher complexes of uranyl compared to other inorganic nitrate salts based on the higher redshift in the spectra, poor fits to the two-component model, and the higher apparent formation constants. Comparing the trends in uranyl speciation in the presence of N(CH₃)₄ ⁺ in aqueous and acetonitrile solutions, it is proposed that the quaternary ammonium cations (quats) promote the formation of anionic complexes of uranyl by the ion-association mechanism. These results provide a basis for designing quat-assisted separation systems that target anionic actinide species in aqueous solutions.

The increasing need for rare earth separations requires a detailed understanding of trivalent ion behavior at charged aqueous interfaces. Here, neodymium (Nd) adsorption on Langmuir monolayers of octadecylphosphonic acid (ODPA), a single‐chain phosphonic acid capable of double deprotonation, at the air/water interface, is investigated. Combining sum frequency generation (SFG) spectroscopy with X‐ray fluorescence near total reflection (XFNTR), both the interfacial water ordering and ion density are examined. Under ambient conditions, Nd ions induce enhanced deprotonation of ODPA headgroups, leading to interfacial ion densities as high as 1 Nd per 30 Ų. This adsorption behavior arises from a complex interplay between direct electrostatic interactions, ion pairing, and hydration effects, which cannot be fully captured by classical Gouy–Chapman–Stern models. These insights into trivalent ion adsorption mechanisms provide a pathway toward more effective separation processes for rare earth metals.

Self-assembled organic nanotubes (ONTs) have been actively examined for various applications such as chemical separations and catalysis owing to their well-defined tubular nanostructures with distinct chemical environments at the wall and internal/external surfaces. Adsorption of heavy metal ions onto ONTs plays an essential role in many of these applications but has rarely been assessed quantitatively. Herein, we investigated interactions between Cu²⁺ and single-/quadruple-wall bolaamphiphile-based ONTs having inner carboxyl groups with different inner diameters, COOH-ONT_10nm and COOH-ONT_20nm. We first examined the effects of Cu²⁺ on their nanotubular structures using SAXS, STEM, and AFM. COOH-ONT_10nm was stable in aqueous Cu²⁺ solution in contrast to COOH-ONT_20nm owing to the presence of polyglycine-II-type hydrogen bonding networks within its wall. Subsequently, we studied the Cu²⁺ adsorption behavior of COOH-ONT_10nm by monitoring the concentration of unbound Cu²⁺ using linear sweep anodic stripping voltammetry. The Cu²⁺ adsorption was quick, attributable to efficient Cu²⁺ partitioning through the open ends of the ONT, followed by fast Cu²⁺ diffusion in the uniform, relatively large nanochannel. More importantly, the Cu²⁺ adsorption capacity and affinity of COOH-ONT_10nm were measured under different pH conditions using the Langmuir adsorption model. The adsorption capacity was similar at the pH range examined, showing the participation of approximately 25% of the inner carboxyl groups in the adsorption. The adsorption affinity increased with pH, indicating the essential role of the deprotonated carboxyl groups in Cu²⁺ adsorption. Most interestingly, the Langmuir adsorption constant was significantly higher than those of previously reported synthetic adsorbents and planar monolayer based on carboxyl binding sites. The high Cu²⁺ affinity of the ONT was attributable to the highly dense binding sites on the well-defined nanoscale concave structure of the inner channel. These results provide a valuable guideline for designing self-assembled nanomaterials for efficient chemical separations, detection, and catalysis.

We report on the effect of monovalent cations (Na+, Li+, NH4+, and N(CH3)4 +) on the complexation equilibria of uranyl-nitrate in aqueous media, probed by time-resolved laser induced fluorescence spectroscopy(TRLFS). We find that N(CH3)4 + uniquely affects both the lifetime and the emission spectra of the uranyl species in nitrate solutions. Lifetime-corrected spectra, obtained by extrapolating the time-resolved spectra to t = 0, show a higher spectral shift in the case of N(CH3)4NO3 solutions. These spectra were deconvoluted into two components corresponding to UO22+ and the UO2(NO3)+ species. While adequate fits were obtained with the model for the NaNO3, LiNO3, and NH4NO3 systems, fits to the N(CH3)4NO3 data were poorer. In addition, the apparent formation constants are particularly high for the N(CH3)4+ system in comparison to the other monovalent cations. We also show that the speciation in these solutions can be modeled with the specific ion interaction theory (SIT), albeit with anomalously large interaction coefficient in the N(CH3)4+ case. We conclude that while the speciation of U(VI) in Na+, Li+, NH + 4 solutions can be described with UO22+ and the UO2(NO3)+ species, N(CH3)4+ solutions have a qualitatively different speciation, likely due to the existence of higher order uranyl-nitrate complexes.

Understanding chemical speciation and intermolecular interactions in multicomponent liquids is essential to understanding their phase and chemical equilibria, which underpin chemical separation processes, including solvent extraction. Here we report on the extraction of nitric acid from its aqueous solutions into organic solutions of trioctylamine (TOA) in toluene, investigated with spectroscopic, X-ray scattering, and computational tools to understand molecular speciation in the organic phase and its relationship with the nanoscale structure of the organic phase. Trends in acid and water extraction clearly show two and three regimes, respectively, indicating different stoichiometric relationships, but speciation of HNO₃, water, and amine in these regimes is not apparent. ¹H NMR of the organic phase shows that there are at least two distinct acidic protons in the organic phase while ATR-FTIR results show that the organic phase with excess acid extraction is a mixture of trioctylammonium-nitrate ion pairs (TOAH·NO₃), and undissociated HNO₃ molecules. Comparison with DFT-computed IR spectra show that the chain-like configurations of TOAH·NO₃·HNO₃·H₂O are favored over TOAH·NO₃·H₂O·HNO₃, i.e., direct interaction between the nitrate and HNO₃ molecules is more favored compared to a water-mediated interaction. SAXS of the organic phases were modeled as sums of Ornstein–Zernike (O-Z) scattering and a prepeak feature in the higher Q region that corresponds to extractant packing. The extraction of undissociated HNO₃ by the ion pairs leads to an increased X-ray scattering contrast in the organic phase without any significant change in the correlation length. These results show that the organic phase nanostructure is more sensitive to the concentration of TOAH·NO₃ and is relatively unaffected by excess acid extraction. These findings will enable a molecular understanding of the mechanisms behind metal extraction from acidic media with basic extractants.

Extractant design in liquid–liquid extraction (LLE) is a research frontier of metal ion separations that typically focuses on the direct extractant–metal interactions. However, a more detailed understanding of energetic drivers of separations beyond primary metal coordination is often lacking, including the role of solvent in the extractant phase. In this work, we propose a new mechanism for enhancing metal-complexant energetics with nanostructured solvents. Using molecular dynamics simulations with umbrella sampling, we find that the organic solvent can reshape the energetics of the extractant's intramolecular conformational landscape. We calculate free energy profiles of different conformations of a representative bidentate extractant, n-octyl(phenyl)-N,N-diisobutyl carbamoyl methyl phosphinoxide (CMPO), in four different solvents: dodecane, tributyl phosphate (TBP), and dry and wet ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf₂N]). By promoting reorganization of the extractant molecule into its binding conformation, our findings reveal how particular solvents can ameliorate this unfavorable step of the metal separation process. In particular, the charge alternating nanodomains formed in ILs substantially reduce the free energy penalty associated with extractant reorganization. Importantly, using alchemical free energy calculations, we find that this stabilization persists even when we explicitly include the extracted cation. These findings provide insight into the energetic drivers of metal ion separations and potentially suggest a new approach to designing effective separations using a molecular-level understanding of solvent effects.

Understanding chemical speciation and intermolecular interactions in multicomponent liquids is essential to understanding their phase behavior, which underpins chemical separation processes including solvent extraction. Here we report on the extraction of nitric acid from its aqueous solutions into organic solutions of trioctylamine (TOA) in toluene, investigated with spectroscopic, X-ray scattering, and computational tools to understand the molecular speciation in the organic phase and its relationship with the nanoscale structure of the organic phase. Trends in acid and water extraction clearly show two and three regimes, respectively, but the speciation of HNO3, water, and amine in these regimes is not apparent. 1H-NMR of the organic phase shows that there are at least two distinct acidic protons in the organic phase while ATR-FTIR results show that the organic phase with excess acid extraction is a mixture of trioctylammonium-nitrate ion pairs (TOAH.NO3), and undissociated HNO3 molecules. Comparison with DFT-computed IR spectra show that the chain-like configurations of TOAH.NO3.HNO3.H2O are favored over TOAH.NO3.H2O.HNO3, i.e., direct interaction between the two HNO3 molecules is more favored compared to a water-mediated interaction. SAXS of the organic phases were modeled as sums of Ornstein-Zernike (O-Z) scattering and a prepeak feature in the higher Q region, corresponding to features from density fluctuations and extractant packing, respectively. The extraction of undissociated HNO3 by the ion pairs leads to an increased X-ray scattering contrast in the organic phase without any significant change in the O-Z correlation length. These results show that the organic phase nanostructure is more sensitive to the concentration of TOAH.NO3 and is relatively unaffected by excess acid extraction. These findings will enable a molecular understanding of the mechanisms behind metal extractions from acidic media with basic extractants.

Employing small-angle X-ray scattering (SAXS), we explore the conditions under which assembly of gold nanoparticles (AuNPs) grafted with the thermosensitive polymer poly(N-isopropylacrylamide) (PNIPAM) emerges. We find that short-range order assembly emerges by combining the addition of electrolytes or polyelectrolytes with raising the temperature of the suspensions above the lower-critical solution temperature (LCST) of PNIPAM. Our results show that the longer the PNIPAM chain is, the better organization in the assembled clusters. Interestingly, without added electrolytes, there is no evidence of AuNPs assembly as a function of temperature, although untethered PNIPAM is known to undergo a coil-to-globule transition above its LCST. This study demonstrates another approach to assembling potential thermosensitive nanostructures for devices by leveraging the unique properties of PNIPAM.