U(VI) binding onto electrospun polymers functionalized with phosphonate surfactants

Abstract

We previously observed that phosphonate functionalized electrospun nanofibers can uptake U(VI), making them promising materials for sensing and water treatment applications. Here, we investigate the optimal fabrication of these materials and their mechanism of U(VI) binding under the influence of environmentally relevant ions (e.g., Ca²⁺ and ${\text{CO}}_3^{2-}$ ). We found that U(VI) uptake was greatest on polyacrylonitrile (PAN) functionalized with longer-chain phosphonate surfactants (e.g., hexa- and octadecyl phosphonate; HDPA and ODPA, respectively), which were better retained in the nanofiber after surface segregation. Subsequent uptake experiments to better understand specific solid-liquid interfacial interactions were carried out using 5 mg of HDPA-functionalized PAN mats with 10 μM U at pH 6.8 in four systems with different combinations of solutions containing 5 mM calcium (Ca²⁺) and 5 mM bicarbonate ( ${\text{HCO}}_3^{-}$ ). U uptake was similar in control solutions containing no Ca²⁺ and ${\text{HCO}}_3^{-}$ (resulting in 19 ± 3% U uptake), and in those containing only 5 mM Ca²⁺ (resulting in 20 ± 3% U uptake). A decrease in U uptake (10 ± 4% U uptake) was observed in experiments with ${\text{HCO}}_3^{-}$ , indicating that UO₂-CO₃ complexes may increase uranium solubility. Results from shell-by-shell EXAFS fitting, aqueous extractions, and surface-enhanced Raman scattering (SERS) indicate that U is bound to phosphonate as a monodentate inner sphere surface complex to one of the hydroxyls in the phosphonate functional groups. New knowledge derived from this study on material fabrication and solid-liquid interfacial interactions will help to advance technologies for use in the in-situ detection and treatment of U in water.

Keywords: Electrospun polymer; Phosphonate; Sensing; Spectroscopy; Uranium.

Fig. 1.

Effect of alkyl chain length on the fractional uranium (U) uptake by phosphonate-functionalized PAN nanofibers. Data are shown for functionalized nanofibers reacted with 1 μM U at pH 2 in 0.25 g/L sorbent systems for 16 h. Results are shown for materials used immediate after synthesis (“unwashed”) and materials used after extensively rinsing with water to remove loosely retained surfactant (“washed”). The U removal observed in control systems with unfunctionalized PAN is shown as a dotted horizontal line. Bracketed numbers along the x-axis indicate the alkyl phosphonate chain length. Error bars, if not visible, are smaller than the symbols. The inset shows the response (based on the P 2p peak area) for P from XPS analysis of unwashed materials (spectra are shown in Fig. S3).

Fig. 2.

Uptake of U (μg U / mg mat) on HDPA-PAN electrospun mats reacted with 10 μM [U], 50 mM [HEPES] and in the presence of either 5 mM [Ca] (orange), 5 mM [CO₃] (gray) or both 5 mM [Ca] and [CO₃] (yellow). The experiments were conducted using 5 mg mat in 20 mL solution reacted for 16 h. Error bars represent the standard deviation of triplicate reactors.

Fig. 3.

Results from reactivity experiments indicating % U released from the reacted mats after the addition of extractants (a) MgCl₂ (b) HCO₃. Reaction conditions involved taking the reacted mats from U uptake experiments and reacting with each of the extractants.

Fig. 4.

U LIII-edge EXAFS spectra from beamline 11–2 in (a) real-part (b) Fourier transformed for reacted HDPA mats compared to references autunite and carnotite minerals. Reaction conditions [U] = 10 μM, Mat = 5 mg, volume = 20 mL, pH = 6.8 buffered HEPES = 10 mM.

Fig. 5.

EXAFS shell-by-shell fitting results for HDPA+U mat. Reaction conditions [U] = 10 μM, Mat = 5 mg, volume = 20 mL, pH = 6.8 buffered HEPES = 10 mM.

Fig. 6.

XPS High Resolution U 4 f photo peak for reacted samples (red) and fittings of U 4 f components (inset) for reacted HDPA mats (a) 10 μM U, (b) 10 μM U+ 5 mM Ca²⁺, (c) $10\mu \text{M}\text{U}+5\text{mM}{\text{CO}}_3^{2-}$ and (d) $10\mu \text{M}\text{U}+5\text{mM}{\text{Ca}}^{2+}+5\text{mM}{\text{CO}}_3^{2-}$, from U uptake experiments. Reaction conditions [U] = 10 μM, Mat = 5 mg, volume = 20 mL, pH = 6.8 buffered HEPES = 10 mM.

Fig. 7.

SERS spectra and deconvolution results on HDPA PAN mats incubated with (a) 10 μM U, (b) 10 μM U+ 5 mM Ca²⁺, (c) $10\mu \text{M}\text{U}+5\text{mM}{\text{CO}}_3^{2-}$ and (d) shows detected uranyl-HDPA complexes matched with vibrational frequencies obtained from the band deconvolution.