Improved immobilization of Re(VII) from aqueous solutions via bimetallic Ni/Fe0
nanoparticles: Implications towards Tc(VII) removal
I. Maamoun1), K. Tokunaga2), T. Dohi3), F. Kanno3), O. Falyouna4), O. Eljamal4), and K. Tanaka1)
1): Res. Gr. for Sustainable Functional Materials Science, JAEA 2): Ningyo-toge Environmental
Engineering Center, JAEA 3): Sector of Fukushima Research and Development, JAEA4): Kyushu Univ.

The rapid development of nuclear power technologies led toincreased water contamination worldwide. Nuclear industries
produce large amounts of wastewater as a byproduct of thegeneration of nuclear energy, containing a variety of harmful
contaminants, including radionuclides, heavy metals, and otherchemicals. Technetium-99 (99Tc) is one of the long-lived
radionuclides (half-life: 2.13×105 years), which represents themain fission products of uranium-238 (238U) [1, 2]. 99Tc
commonly occurs in the heptavalent (Tc(VII)) anionic form ofpertechnetate (TcO4−), which is highly mobile and soluble.
Rhenium (Re) is considered the common chemical analogue forTc owing to their similar chemical features, as they both share thesame stable VII oxidation states of perrhenate (ReO4-) andpertechnetate (TcO4-), which can be reduced to IV via electrondonor materials [3]. Hence, Re(VII) can be considered the target
contaminant in this work as Tc(VII) analogue.
Meanwhile, zero-valent iron nanoparticles (Fe0) have been
widely utilized as an efficient reactive material towards reducible
contaminants, owing to the suitable redox (oxidationreduction) potential (E0: -0.44 V) [4]. In this regard, doping thesurface of Fe0 with another metal (e.g., nickel (Ni)) yieldsbimetallic nanoparticles with a remarkably reactive surface,higher catalytic properties, and improved removal capabilities
towards reducible contaminants. Using Ni in bimetallic Fe0 may
have advantageous features, including cost-efficiency (comparedto the other high-cost metals), the high catalytic activity of Ni,
and the ability to act as an electron transfer medium to overcome
the self-inhibition of electron transfer.
Hence, the main aim of this work is to use bimetallic
nickel/iron nanoparticles (Ni/Fe0) to enhance Re(VII)immobilization from aqueous solutions as the surrogate ofTc(VII). Different characterization techniques were used in thiswork to elucidate the physicochemical features of the unreacted
and reacted materials, including scanning transmission electron
microscopy-energy dispersive X-ray spectroscopy (STEM-EDX),
X-ray diffraction (XRD), and X-ray absorption near edge
spectroscopy (XANES). The influence of several reaction
parameters on Re(VII) removal was investigated, including
Ni/Fe0 (wt/wt) ratio (0.05, 0.1, 0.2, and 0.4), Ni/Fe0 dosage (0.25,0.5, 1.0, 1.5, and 2.0 g/L), initial pH (3.0, 5.0, 7.0, 9.0 and 12.0),
reaction temperature (25, 35, 45, and 55 oC), and initial Re(VII)concentration (0.25, 0.5, 1.0, 5.0, 15.0, 25.0, and 50.0 μM).
Results showed a promising potential of Ni/Fe0 in Re(VII)removal, especially at the early stage of the reaction, where
Ni/Fe0: 0.4 yielded almost full removal efficiency of initial 15.0
µM-Re(VII) within the first 10 minutes of reaction. Even at low
Ni/Fe0 dosages, such as 0.25 and 0.5 g/L, reasonable removal
efficiency was achieved after 2 hours reaction time of ~73% and~98%, respectively. The effect of initial pH on Re(VII) removal
by Ni/Fe0 was studied at acidic, neutral, and alkaline conditions.
Unlike acidic/neutral pH, alkaline conditions were not favorablefor Re(VII) removal by Ni/Fe0 owing to the delayed Fe0 oxidation,
resulting in insufficient electrons available for Re(VII) reduction.
As shown in Fig. 1(A), the reductive abilities were confirmed by
XANES, revealing Re(VII) reduction to Re(IV)/(III) by thereleased electrons from Fe0-core in both Fe0 and Ni/Fe0 materials.
Meanwhile, a method was used to measure the electrostatic
potential at the electrical double layer surrounding Ni/Fe0
nanoparticles in solution, referred to as zeta potential.
Correspondingly, the measurements depicted the point of zerocharge (pHPZC) of Fe0 and Ni/Fe0 to be 8.24 and 7.63, respectively,
suggesting electrostatic sorption of ReO4- on the positively
charged surface of Ni/Fe0. Theoretical reaction kinetic models(i.e., Pseudo-first- and second-order models) were suitable todescribe Re(VII) removal by Ni/Fe0, implying the involvement ofchemical (i.e., reduction of Re(VII) to Re(IV)/Re(III)) andphysical (i.e., electrostatic sorption) removal mechanisms. The
presence of Ni0/NiO on the Fe0-surface resulted in providing anefficient electron-transfer medium that facilitated Re(VII)
reduction (Fig. 1(B)). The aforementioned contribution of Ni/Fe0
to the removal mechanism led to higher kinetic rates of Re(VII)
reduction Fe0. Overall, this study provides valuable insights intousing Ni/Fe0 for Re(VII) removal from water and offers guidance
for future research toward the pilot-scale applications of Tc(VII)removal from nuclear wastewater.

Fig. 1. Re LIII-edge (corresponding to the excitation of electrons:
principal quantum number n=2) XANES spectra of spent Fe0 and Ni/Fe0(A), and removal pathways of Re(VII) removal by Ni/Fe0 (B).

References[1] Sakaguchi et al., Geochem. J. 46, 355–360 (2012).
[2] Tanaka et al., Appl. Clay Sci. 182, 105282 (2019).
[3] Li et al., Sci. China Chem. 59, 150–158 (2016).
[4] Tosco et al., J. Clean. Prod. 77, 10–21 (2014).