Competitive Adsorption of Uranyl and Toxic Trace Metal Ions at MFe₂O₄-montmorillonite (M = Mn, Fe, Zn, Co, or Ni) Interfaces

Abstract

Adsorption of uranyl (UO₂²⁺) ions to mineral surfaces is a potentially effective method for removing this hazardous metal from water, but other toxic trace metal ions (Xⁿ⁺: Rb⁺, Sr²⁺, Cr³⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺) in uraniferous wastewaters compete with UO₂²⁺ for adsorption sites and thus may diminish the capacity of adsorbents to sequester UO₂²⁺. A better understanding of competitive adsorption among these metal ions and the development of better adsorbents are, therefore, of critical importance. The purpose of the present study was to synthesize and characterize magnetic adsorbents, consisting of MFe₂O₄ (M = Mn, Fe, Zn, Co, or Ni) nanoparticles synthesized on montmorillonite (Mnt) edge sites, and to investigate their use as adsorbents for UO₂²⁺, including competitive adsorption with trace metal ions. Selective adsorption was studied using Langmuir, Freundlich, and Dubinin-Radushkevich isotherms, and the results showed that Xⁿ⁺ ions were adsorbed primarily on MFe₂O₄-montmorillonite surfaces, and the UO₂²⁺ ions were adsorbed on the interfaces between montmorillonite edge surfaces and MFe₂O₄ nanoparticles. Using the Freundlich model, the interface adsorption capacity of UO₂²⁺ reached 25.1 mg·g^–1 in mixed solution. Further, the UO₂²⁺ and Cr³⁺ ions had a redox reaction on the interfaces with synergistic adsorption. Herein, the adsorption capacity of Cr³⁺ was 60.2 mg·g^–1 using the Freundlich isotherm. The results demonstrated that the MFe₂O₄-montmorillonite with highly selective adsorption of UO₂²⁺ ions is applicable to UO₂²⁺ treatment in the presence of toxic trace metal ions.

Graphical abstract

Introduction

Uranium has been used widely for nuclear fuel and concerns have arisen about the recovery and the sustainable development of uranium salts, e.g. uranyl (UO₂ ²⁺). In general, the acidic wastewater of UO₂ ²⁺ originates from the development of uranium ore or the treatment of nuclear waste, which could undermine the acid-base balance of the constituent waters (Todorov & Ilieva 2006; Dhiraj et al. 2008) and lead to diseases which endanger human health (Rufyikiri et al. 2006; Ai et al. 2018). Currently, various methods of removing UO₂ ²⁺ from aqueous solutions are known; adsorption has been considered an effective method for UO₂ ²⁺ removal from wastewater or soil (Zhang et al. 2013). Depending on the source of the pollutants, uraniferous wastewater may contain a considerable amount of toxic metal ions. The toxic trace-metal ions have a significant effect on the adsorption of UO₂ ²⁺ due to competition for sorption sites (Villalobos et al. 2001). Take nuclear reactions as an example, some of the fission products are present as oxide precipitates (e.g. Rb⁺, Cs⁺, Ba²⁺, Zr⁴⁺) or solid solution forms in fuel matrices that are usually discharged with waste (Burns et al. 2012). Likewise, uranium deposits often contain heavy metals (e.g. Cd²⁺, Cu²⁺, Hg²⁺, Pb²⁺, Zn²⁺, and As⁵⁺) that can be released into the environment during the mining process (Santos et al. 2018).

Clay minerals are efficient adsorbents for toxic trace-metal ions because of their layered structure, large specific surface area, and large cation exchange capacity (Bedelean et al. 2009). Clay minerals can also act as a matrix to produce selective clay adsorbents (Zhu et al. 2016; Hajjaji et al. 2016). For instance, Soliman et al. (2017) reported an activated sludge from refinery wastewater with high selectivity for UO₂ ²⁺ ions in a mixture of Zr⁴⁺ , Cr³⁺ , Gd²⁺ , Sr²⁺ , Co²⁺ , Cs⁺, and Na⁺, and the capacity to adsorb UO₂ ²⁺ increased to 28 mg·g^–1. Moreover, the competitive adsorption of Pb²⁺, Ni²⁺, Zn²⁺, Cd²⁺, and UO₂ ²⁺ in clay minerals (smectite and kaolinite) was measured to demonstrate the effect of soil constituents on adsorption (Choi & Park 2005). Montmorillonite, a traditional clay adsorbent, was applied in water treatment to remove metal ions (Bhattacharyya & Gupta 2008). Campos et al. (2013) assessed the adsorption capacity of UO₂ ²⁺ onto Ca-montmorillonite as 8.57 mg·g^–1, and only half of UO₂ ²⁺ was desorbed. The adsorption of UO₂ ²⁺ was achieved primarily on the surface of the montmorillonite, and the mechanism was split into ion exchange and the composite effect of the clay surfaces (McKinley et al. 1995).

Montmorillonite cannot be recycled easily, and this reservation limits its application, though montmorillonite has excellent adsorption properties and available adsorption sites within its interlayer space and on the outer surface and edges. Recently, magnetic clay mineral nanocomposites have been used in the recovery of toxic and radioactive materials because of their low cost, low toxicity, and easy separation from solution under an external magnetic field (Lu et al. 2010; Pylypchuk et al. 2016; Dorota et al. 2016). Fe₃O₄-montmorillonite nanocomposite was synthesized to remove Pb²⁺, Cu²⁺, Cd²⁺, and Ni²⁺ ions from aqueous systems. The removal efficiency of Pb²⁺, Cu²⁺, and Ni²⁺ ranged from 76.15% to 94.89% in specific liquid-solid ratios (Kalantari et al. 2015), and the adsorption capacity of Cd²⁺ was 2 mg·g^–1 (Tinas et al. 2016).

Montmorillonite modified with magnetic material is a potential selective adsorbent material. However, Fe₃O₄ nanoparticles exhibit a small particle size, so it is easier to agglomerate than other non-magnetic nanoparticles, resulting in the reduction of adsorption sites. The adsorption of uranium(VI) on magnetite (Fe₃O₄) nanoparticles was studied by Debasish et al. (2014); its adsorption capacity of 5 mg·g^–1 was relatively low. The surface and material modifications of magnetic nanoparticles have been used extensively to enhance adsorption capacity and dispersion stability. The silica-coated magnetic Fe₃O₄ nanoparticles were modified with quercetin (Sadeghi et al. 2012), and the results revealed that the maximum monolayer adsorption capacity of UO₂ ²⁺ using the Langmuir isotherm was 12.33 mg·g^–1. In addition, one of the modification methods to inhibit agglomeration and enhance adsorption capacity used transition metals as mediators to modify Fe₃O₄. Hu et al. (2005) investigated surface-modified jacobsite (MnFe₂O₄) nanoparticles. In their study, the maximum uptake of Cr(VI) was 31.5 mg·g^–1 and the adsorption data fitted the Langmuir model well. As a result, the negatively charged montmorillonite could effectively adsorb cationic U(VI), and the surface-adsorbed Fe(II) could promote the reductive transformation of U(VI). The montmorillonite-modified zerovalent iron nanoparticles were synthesized by Hu et al. (2016) who then used it to remove radionuclides. Thus, the modified Fe₃O₄ based on montmorillonite was expected to have better applications in uranyl adsorption.

Functional magnetic nanoparticle/clay mineral nanocomposites have been used extensively recently (Zhou & Keeling 2013; Chen et al. 2016; Zhou et al. 2016). In the present study, the goal was to synthesize MFe₂O₄ (M = Mn, Fe, Zn, Co, Ni)-montmorillonite nanoparticles through surface and material modification of Fe₃O₄, to test its ability to inhibit the aggregation of Fe₃O₄, and to characterize the structures using XRD, FTIR, Raman, TEM, SEM, and XPS. A further objective was to characterize the electronic activity of the interfaces using diffuse reflectance spectroscopy (DRS) and terahertz time-domain spectroscopy, and to use the adsorbents to remove U(VI) in the presence of toxic trace metal ions.

Materials and Methods

Materials

All the chemicals were of analytical grade. The montmorillonite (K-10) was purchased from Sigma-Aldrich (St. Louis, Missouri, USA), and the raw montmorillonite was determined to be montmorillonite-15A by XRD analysis (Supplementary Fig. S1). UO₂(NO₃)₂•6H₂O was purchased from Hubei Chushengwei Chemical Company (Hubei, China) and other reagents were purchased from ChengDu Chron Chemicals Reagent Company (Chengdu, China).

MFe₂O₄(M = Mn, Fe, Zn, Co, Ni)-montmorillonite was prepared using forced hydrolysis (Xuan et al. 2010). Montmorillonite (3 g) was suspended in 200 mL of a mixed solution of 10 mmol MCl₂·nH₂O (M = Fe, Mn, Cu, Zn, Ni) and 20 mmol FeCl₃·6H₂O (molar ratio 1:2), with vigorous stirring for 0.5 h at room temperature. Under N₂ gas atmosphere, the mixed solution was heated to 80°C and 2 M NaOH solution was added dropwise until pH = 11. Then the precursor was stirred continuously for 1 h at 80°C. After cooling to room temperature, the composite was centrifuged and washed repeatedly with distilled water and ethanol, then collected by magnetic separation. Finally, the samples were vacuum dried for 12 h at 60°C.

Characterization

The crystal structures of samples were determined using X-ray diffraction (XRD) equipped with a CuKα radiation source, over a scanning range of 3 to ~70°2θ (PANalytical B.V., Almelo, The Netherlands). The Fourier-transform infrared spectroscopy (FTIR) analysis was performed using a scanning range of 4000 to ~400 cm^–1 (Perkin Elmer, Waltham, Massachusetts, USA). Raman scattering measurements were conducted under a backscattering geometric configuration using a confocal micro-Raman system (Renishaw, London, UK). The excitation light was at the 514.5 nm line for Ar⁺ laser, and at a maximum output of 30 mW. Scanning electron microscopy (SEM) of samples was performed using the Cu plate with a field emission machine (Carl Zeiss, Jena, Germany). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were obtained using a Libra 200 system (Carl Zeiss, Jena, Germany). X-ray photoelectron spectroscopy (XPS) equipped with a DS300 unit was performed with a multifunctional electron spectrometer (Kratos, Manchester, UK) using monochromatic MgKα radiation to analyze the element distribution (Zou et al. 2016). The photon information of the interfaces was tested by terahertz time-domain spectroscopy (THz-TDS) (Zomega, East Greenbush, New York, USA) (Wilke et al. 2014). UV-Vis diffuse reflectance spectra (DRS) (Shimadzu, Kyoto, Japan) (He et al. 2017) and zeta potential measurement (Malvern, Worcestershire, UK) were utilized to investigate the electrophoretic mobility and electronic transition mechanism.

Competitive Adsorption

The uranyl-heavy metal mixed solution was prepared by combining 1.86 g of UO₂(NO₃)₂·6H₂O, 1.40 g of RbCl, 1.81 g of SrCl₂, 5.13 g of CrCl₃·6H₂O, 2.29 g of MnCl₂, 2.21 g of NiCl₂, 2.08 g of ZnCl₂, and 1.63 g of CdCl₂, which were then dissolved completely in 1000 mL of deionized water to obtain a concentration of 1000 mg·L^–1 for each metal ion. Aliquots of 0.5 mL, 1.0 mL, 1.5 mL, 2.0 mL, 2.5 mL, and 3.0 mL were taken from this mixed solution and diluted with deionized water to 50 mL to obtain mixed solutions having concentrations of 10 mg·L^–1, 20 mg·L^–1, 30 mg·L^–1, 40 mg·L^–1, 50 mg·L^–1, and 60 mg·L^–1 of UO₂ ²⁺-X ⁿ⁺ (X = Rb⁺, Sr²⁺, Cr³⁺, Mn²⁺, Ni²⁺, Zn²⁺ and Cd²⁺). The pH of the diluted solution was fixed at 5.5 using dilute hydrochloric acid, and 0.03 g MFe₂O₄(M = Mn, Fe, Zn, Co, Ni)-montmorillonite was added to the solution and then it was shaken mechanically for 24 h.

The residual concentration of uranyl-heavy metal was characterized using an inductively coupled plasma optical emission spectrometer (ICP-OES) (Perkin Elmer, Waltham, Massachusetts, USA) (Chen et al. 2017). Adsorption behaviors were described by the Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherms. Therein, the Langmuir monolayer mode could describe the monolayer adsorption behaviors of the uniform adsorption sites (Langmuir 1918), and the Freundlich mode was suitable for multilayer adsorption or non-ideal adsorption of heterogeneous active sites (Freundlich 1906). The Dubinin-Radushkevich mode was taken to estimate the characteristic porosity and the apparent energy of adsorption (Dubinin 1960). The corresponding isotherms were estimated using the following equation:

(1) $\text{Langmuir isotherm}:\frac{C_e}{q_e}=\left(\frac{1}{q_e\times b}\right)+\frac{C_e}{q_{\max }}$

(2) $\text{Freundlich isotherm}:ln{q}_e=ln{k}_F+\left(\frac{1}{n}ln{C}_e\right)$

where q _e is the equilibrium adsorption capacity of metal ion, C _e is the equilibrium concentration (mmol·L^–1), q _max is the maximum adsorption capacity (mmol·g^–1), and b is the adsorption equilibrium coefficient (L·mmol^–1). k_F and n are Freundlich parameters related to adsorption capacity (mmol·g^–1) and adsorption strength (L·mmol^–1), respectively.

(3) $\text{Dubinin}-\text{Radushkevich isotherm}:ln{q}_e=ln{q}_{\max }-\beta {\epsilon }^2$

(4) $\epsilon =-\mathrm{RT}ln\left(1+1/C_e\right)$

(5) $E=\frac{1}{\sqrt{2\beta }}$

where, β is the coefficient of free sorption energy (mol²·kJ^-2) and E is the apparent energy of adsorption (kJ·mol^–1).

The relative surface adsorption sites and atom ratios were obtained using a high-resolution transmission electron microscopy-energy dispersive X-ray spectroscopy (HRTEM-EDS) system JEOL-2010 (JEOL, Tokyo, Japan) (Liu et al. 2014b). The Parstat 4000 electrochemical workstation with a three-electrode system (Princeton, New Jersey, USA) was used for cyclic voltammetry testing (He et al. 2017). FTO (SnO₂:F) conductive glass was used as a working electrode, with an electrode reaction area of 15 mm × 25 mm.

Results and Discussion

Structural Characterization of MFe₂O₄-montmorillonite

In the XRD pattern of MFe₂O₄-montmorillonite (Fig. 1), the diffraction peaks at 6.5°2θ and 19.8°2θ were assigned to the (001), (101) faces, respectively, consistent with montmorillonite-15A (JCPDS No: 29-1498) (Chang et al. 2015). Besides, the characteristic reflections at 18.2, 30.0, 35.4, 43.0, 53.4, 56.9, and 62.5°2θ, corresponding to the crystal surfaces of (111), (220), (311), (400), (422), (511), and (440), respectively, demonstrate the existence of the MFe₂O₄ pure phase (Fe₃O₄, JCPDS-No: 19-0629; MnFe₂O₄, JCPDS-No: 10-0319; CoFe₂O₄, JCPDS-No: 22-1086; ZnFe₂O₄, JCPDS-No: 89-1010 and NiFe₂O₄, JCPDS-No: 87-2338) (Wang et al. 2012a; Hu et al. 2008). Moreover, other characteristic peaks were not observed, suggesting that as-synthesized MFe₂O₄-montmorillonite with good crystallinity was prepared successfully.

Fig. 1 XRD patterns of MFe₂O₄-montmorillonite

To further demonstrate the bond structures of the MFe₂O₄-montmorillonite, the bonding patterns data were analyzed with FTIR and Raman spectroscopies. The double absorption peaks near 594 cm^–1 are assigned to the stretching band of Fe–O in MFe₂O₄ octahedra and tetrahedra (Fig. 2a), and the other typical bands of MFe₂O₄ were also observed at 1380 cm^–1 and 2920 cm^–1. Moreover, the frequency of 1090 cm^–1 represents the stretching bands of Si–O in montmorillonite, and 3630 cm^–1 represents the octahedral O–H stretching bands. The other absorption peaks at 3430 cm^–1 and 1640 cm^–1 are assigned to the O-H stretch vibrations and the H-O-H bending vibration of H₂O, respectively. In the Raman patterns (Fig. 2b), the signature peaks of E_g and A_1g bands in MFe₂O₄ appeared at 303 cm^–1 and 667 cm^–1, respectively. In addition, the Raman shift around 1600 cm^–1 was observed, and the weak peak was predicted to be attributed to the long-range electronic transition bonds of AlO–OFe(M), which suggests the formation of interface oxygen vacancies that could capture the -OH groups from water molecules (Li et al. 2016). From the data above, the as-synthesized samples consisted of MFe₂O₄ and montmorillonite.

Fig. 2 (a) FTIR patterns and (b) Raman spectra of MFe₂O₄-montmorillonite

SEM and TEM examinations of MFe₂O₄-montmorillonite were performed to explore the morphology and interface structure properties of samples. A typical SEM image of MFe₂O₄-montmorillonite revealed that the magnetic Fe₃O₄ nanoparticles grew on montmorillonite layer surfaces with unavoidable agglomeration (Fig. 3a). Fe₃O₄ (–34690.42 eV) had a lower crystal energy than MFe₂O₄ (–33000.22 eV, –35432.89 eV, –38601.21 eV, –36106.85 eV, M = Mn, Zn, Ni, or Co) (Bian et al. 2017). Hence, the inhibited agglomeration could be observed clearly, whereas the average particle size increased from 60 to ~200 nm. In the TEM images, the agglomeration of nanoparticles was observed more intuitively, and the MFe₂O₄ grew primarily on the edge sites of montmorillonite (Fig. 4a). The relative lattice distances (d) and crystal faces were measured based on HRTEM images (Fig. 4b), as shown in Table 1. Compared with the reported values, the difference of MFe₂O₄-montmorillonite faces ranged from 0.01 to 0.03 nm. For the MFe₂O₄ crystal, the typical sites for Fe²⁺ in Fe₃O₄ were occupied by M ²⁺ ions. Due to the differences in possible atomic radius of M ²⁺ ions (0.153 nm to ~0.167 nm) and Fe²⁺ (0.179 nm) (Sun et al. 2004; Gao et al. 2016), Fe-O tetrahedral shrinkage affected the lattice of Fe₂O₃ octahedra nearby, decreasing the lattice distances of (111)/(311) by 0.01 nm and increasing that of (220) by 0.01 nm. The surface energy (1.06 to ~1.18 J·m^-2) of the (111) face in MFe₂O₄ was less than that of a high-index (311) face (Yu et al. 2012), thus the MFe₂O₄(111) combined preferentially with the exposed Al₂O₃(100) face of the montmorillonite edge sites (Hong et al. 2017). In addition, the data reflected the good matching with MFe₂O₄(111) faces and exposed Al₂O₃(100). Furthermore, HRTEM-EDS analysis was conducted to analyze the element distribution. Fe and O on the montmorillonite surfaces proved the uniform distribution of MFe₂O₄ (Fig. 3b). In addition, the results revealed that the atomic ratio of Si:Al reached 18:1, indicating that the Al³⁺ in the montmorillonite crystal lattice was often replaced by some low-valence cations in solution (e.g. Fe²⁺, Zn²⁺, Mg²⁺) (Brigatti et al. 2013). However, the exposed Al-O octahedron on the edge of montmorillonite still exhibited high activity, and it could provide active electrons for the adsorption reaction (Hennig et al. 2002).

Fig. 3 (a) SEM images of MFe₂O₄-montmorillonite and (b) HRTEM-EDS images of MnFe₂O₄-montmorillonite

Fig. 4 (a) HRTEM and (b) TEM images of MFe₂O₄-montmorillonite

Table 1 The lattice distance (d) data and crystal faces of MFe₂O₄-montmorillonite

F represents MFe₂O₄ and M represents montmorillonite.

Experment^a-d: d data and crystal planes reported from HRTEM (Phumying et al. 2013; Liu et al. 2014a; Gao et al. 2016; Yan et al. 2016).

Present work: HRTEM results.

The surface composition of MFe₂O₄-montmorillonite was further demonstrated using XPS. The characteristic peaks appearing at 72.7 eV, 99.5 eV, and 150.8 eV are assigned to Al 2p₃ (Al³⁺), Si 2p₃, and Si 2s (Si⁴⁺) in the montmorillonite carrier, and the 725.5 eV corresponded to Fe 2p_3/2 (Fe³⁺) ions coexisting in the MFe₂O₄ (Fig. 5). Moreover, an interesting peak of 712 eV, indicating the presence of Fe 2p_1/2 (Fe²⁺), was observed in each MFe₂O₄-montmorillonite. This abnormal phenomenon could occur as the active electrons transferred to MFe₂O₄, as mentioned previously, resulting in some Fe³⁺ ions being reduced to Fe²⁺. The other characteristic peaks of Mn 2p (Mn²⁺), Co 2p (Co²⁺), Ni 2p (Ni²⁺), and Zn 2p (Zn²⁺) were demonstrated clearly with binding energies at 641 eV, 784eV, 855 eV, and 1022 eV, respectively. The detailed peak-fitting data are shown in Supplementary Fig. S2. The presence of M ²⁺ is thus strong evidence for homogeneous bonding in MFe₂O₄, and the samples consisted of montmorillonite with MFe₂O₄.

Fig. 5 XPS spectra of MFe₂O₄-montmorillonite

Electron-Transfer Mechanism of MFe₂O₄-montmorillonite

In general, adsorption behavior was affected by the surface potential of the adsorbent. In the montmorillonite, the interlayer potential showed a permanent negative charge by isomorphic substitution (Zuo et al. 2017); the potentials of MFe₂O₄ were reported as 17.5 to ~20 mV (M = Mn, Fe, Zn) and –15 to ~–30 mV (M = Co, Ni) at pH=5–6 (Chen et al. 2014; Jiang et al. 2016; Liu et al. 2016; Tu et al. 2016; Deng et al. 2017). Moreover, the zeta potential of MFe₂O₄-montmorillonite was measured as –6.5 to ~26 mV at pH 5.5 (Fig. 6a), and the potential decreased with the rise in pH because increasing OH^- was adsorbed to the edge surfaces of the montmorillonite (Min and Lee 2010). Thus, the MFe₂O₄ on the surface of montmorillonite was vital for the surface potential of the adsorbent (Fig. 6b). At the interfaces, M ²⁺ in MFe₂O₄, an electron acceptor, could provide an oxygen vacancy and lead to active electrons at the montmorillonite edge sites being transferred to compensate the electron holes (Bian et al. 2015). Thus, MFe₂O₄-montmorillonite exhibited greater positive surface potential than Fe₃O₄-montmorillonite. In addition, the zeta potential was used to describe the dispersion stability of the adsorbent in aqueous solution. The potential of MFe₂O₄-montmorillonite (M = Co, Ni, Zn) was ascertained to be 8.9 to ~41 mV, which indicates that the adsorbent systems were stable. The pH of zero potential for Fe₃O₄-montmorillonite was ~5.5, suggesting the interactions between metal ions and adsorbent were easier to enhance at pH < 5.5. Thus, the adsorption experiments were carried out at pH 5.5.

Fig. 6 (a) Zeta potential curves and (b) surfaces/interfaces/interlayer charge distribution of MFe₂O₄-montmorillonite

The UV-Vis diffuse reflectance spectra were obtained to investigate further the mechanism of electron transfer at the MFe₂O₄-montmorillonite surface and interface. Also, the band gaps were estimated by the photon energy equation:

(6) $\mathrm{Eg}=\frac{h}{k}\times \frac{C}{\lambda }$

where h denotes Planck’s constant; k is a constant (1.6×10^–19 J·eV^–1); C/λ is wave frequency based on the tangent line; Eg is band gap energy (eV).

The band gap of montmorillonite was calculated as 2.5 to ~3.06 eV and MFe₂O₄ was ~1.62 eV (Fig. 7a). Due to the particle confinement effect as reported by Fatimah et al. (2011), the band gap energy of MFe₂O₄-montmorillonite decreased, and the peak shifted to a higher wavelength compared with Fe₃O₄-montmorillonite. This result implied that the addition of MFe₂O₄ led to an increase in particle size. It is noteworthy that a small band gap (0.78 to ~0.82 eV) appeared near 1900 nm. Electron holes could be formed due to the presence of M ²⁺ mediators to receive and transmit electrons which were provided by the electron donor (Fig. 7b) (Rekhila et al. 2017), this suggested that the interfaces of MFe₂O₄-montmorillonite were the main region for electron transfer.

Fig. 7 (a) DRS curves of MFe₂O₄-montmorillonite and (b) the relative electron transfer reaction

The interface properties of MFe₂O₄-montmorillonite were supported by THz-TDS spectra. The peak widths of equipment-emitted pulses were <0.65 ps, and the amplitudes were nearly 100 to ~150 V (Fig. 8a). With the strong absorption of terahertz rays, light could pass easily through the montmorillonite edge sites without changing velocity (Li et al., 2017). In the refraction index information, the two absorption peaks were near 1.75 THz, which are assigned to the photon adsorption of the O-Fe (2p_1/2) octahedron and O-Fe (2p_3/2) tetrahedron in the MFe₂O₄ (Fig. 8b). The refraction peaks at ~2.15 THz reflected the MFe₂O₄(111)-montmorillonite(100) interfaces, because poorly active electrons at the interfaces were triggered and detected easily. In addition, note that MFe₂O₄-montmorillonite showed a smaller refractive index peak and strong absorption at 1.95 to ~2.15 THz in the absorption spectrum (Fig. 8c). This abnormal phenomenon was predicted as the electronic transition within the interfacial bonding of AlO-OFe(M) and SiO-OFe(M) between the montmorillonite and MFe₂O₄, associated with the synergistic hybridized orbitals effect of O-O sp², etc. The tangent data were calculated to explain better the electronic mechanism of the interfaces, according to the frequency difference (Fig. 8d). The tangent data revealed that the concentration of surface oxygen vacancies of MFe₂O₄(111) increased by the paired electrons of M ²⁺. Therefore, the active electrons of MFe₂O₄(111)-montmorillonite(100) interfaces were of significance to the adsorption of UO₂ ²⁺-X ⁿ⁺ from solution.

Fig. 8 (a) Time dependence, (b) refraction index, (c) absorption, and (d) frequency difference (FD) power of THz signal spectra of the MFe₂O₄-montmorillonite interfaces, where time-resolved spectra of semicoke were obtained via THz-TDS. The time resolution was on a picosecond scale

Competitive Adsorption of Toxic Trace Metal Ions Using MFe₂O₄-montmorillonite

To evaluate the adsorption capacity of the adsorbent, data were obtained through ICP measurements. The results revealed that the equilibrium adsorption capacity increased with the increase in initial metal ion concentration (Fig. 9). The total adsorption capacities of UO₂ ²⁺ and metal ions were 322 to ~396 mg·g^–1 in the initial ionic concentration of 60 mg·L^–1 (Supplementary Table S1). Subsequently, Langmuir, Freundlich, and Dubinin-Radushkevich models were adopted to describe the adsorption behavior (Table 2). The Langmuir linear correlation coefficients (R²) were calculated as 0.913 to ~0.999, larger than those of Freundlich 0.774 to ~0.999. The Langmuir isotherm fitted the experimental data better. The results revealed that Rb⁺, Sr²⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺, and Cr³⁺ ions were primarily at the adsorbent surfaces as a result of monolayer physical adsorption. However, the values of R² for UO₂ ²⁺ were approximately 0.897 to ~0.989 (Freundlich) and 0.739 to ~0.977 (Langmuir). The exception showed that the adsorption of UO₂ ²⁺ was more consistent with the Freundlich isotherm, without that of Fe₃O₄-montmorillonite. The Freundlich constants (1/n) were associated with the adsorption intensity, and the constants of 1/n of X ⁿ⁺ were calculated as 0.5 to ~3.2 (0.5<1/n<1), which indicated that the adsorption of Rb⁺, Sr²⁺, Mn²⁺, Ni²⁺, Zn²⁺, and Cd²⁺ ions would be favorable. The constants of 1/n of X ⁿ⁺ UO₂ ²⁺ adsorption were 0.09 to ~0.37, within the range of 0.1 < 1/n < 0.5, which suggests that the UO₂ ²⁺ was more significantly adsorbed on the adsorbent. In addition, the linear correlation coefficients (R²) of the D–R isotherm for X ⁿ⁺ and UO₂ ²⁺ were determined as 0.913 to ~0.997. This indicated that X ⁿ⁺ and UO₂ ²⁺ ions were also adsorbed well in the interlayers of montmorillonite. On the whole, the difference in adsorption-fitting results was associated with the competitive adsorption between UO₂ ²⁺ and X ⁿ⁺.

Fig. 9 The equilibrium adsorption capacity data of MFe₂O₄-montmorillonite from heavy metal mixed solution (ppm, parts per million, indicates the concentration of the solute)

Table 2 Isotherm parameters for the uranyl-toxic metal MFe₂O₄-montmorillonite

The adsorption mechanism was considered to be by ligand exchange of hydroxyl groups and an adsorption reaction caused by the transition of electrons in the long-range AlO–OFe(M) bonds, creating surface hydroxyl radical layers in aqueous solution and changing the adsorption behavior from monolayer to multilayer. In the adsorption behaviors, involving successive mono electronic transition, UO₂ ²⁺-X ⁿ⁺ adsorption could be assumed to be the direct electron transfer for increasing •OH and the complexation of •OH with UO₂ ²⁺ or X ⁿ⁺ (Karamanis 1997). According to previous reports (Xu et al. 2013), the UO₂ ²⁺ and X ⁿ⁺ ions underwent competitive adsorption on the surfaces of montmorillonite, where the adsorption reaction of surface •OH with positively charged X ⁿ⁺ could form an ion monolayer to block UO₂ ²⁺ from being adsorbed. The adsorption results implied that the adsorption positions of UO₂ ²⁺ and X ⁿ⁺ were not the same as those on the surfaces of MFe₂O₄-montmorillonite, and the adsorption of UO₂ ²⁺ occurred primarily at interfaces. Take ZnFe₂O₄-montmorillonite as an example. The adsorption of X ⁿ⁺ overall fitted the Langmuir and Freundlich models (Fig. 10a), whereas the maximum adsorption capacity of Freundlich was much lower than the actual adsorption capacity, which revealed that the X ⁿ⁺ were adsorbed primarily by monolayer physical adsorption on the adsorbent surfaces. However, the Freundlich model described the adsorption of Cr³⁺ and UO₂ ²⁺ better than the Langmuir model (Fig. 10b), and the saturated UO₂ ²⁺ adsorption capacity of different models differed significantly. Thus, the adsorption positions of UO₂ ²⁺ were primarily at ZnFe₂O₄-montmorillonite interfaces.

Fig. 10 (a) The fitting curves of Sr²⁺ and (b) UO₂ ²⁺, Cr³⁺ to Freundlich, Langmuir, and Dubinin-Radushkevich models at ZnFe₂O₄-montmorillonite surfaces

The maximum adsorption capacity was calculated from the adsorption models. The results revealed that MFe₂O₄-montmorillonite had a better adsorption capacity of metal ions using Langmuir and D-R equation fitting (Fig. 11a-b). However, the adsorption capacities of UO₂ ²⁺ and Cr³⁺ were much greater than that of other X ⁿ⁺ ions using the Freundlich model (Figure 11c). The data demonstrated the highly selective adsorption behavior of UO₂ ²⁺ and Cr³⁺ ions in MFe₂O₄-montmorillonite interfaces (Fig. 11d); the illustration of the adsorption mechanism has been mentioned. The interface adsorption capacity of UO₂ ²⁺ was 25.1 mg·g^–1 and that of Cr³⁺, 60.2 mg·g^–1. Additionally, the maximum adsorption capacity of Cr³⁺ from the Langmuir isotherm was 163.8 mg·g^–1. Compared with previous reports, the adsorption capacities of UO₂ ²⁺ and Cr³⁺ were just 5 to ~33.7 mg·g^–1 using montmorillonite and other modified Fe₃O₄, and the MFe₂O₄-montmorillonite exhibited a large adsorption capacity of UO₂ ²⁺, as listed in Table 3. Thus, the medium (M ²⁺) and material modifications of Fe₃O₄ were proven as one of the effective ways to enhance adsorption capacity. Though the adsorption reaction was known to be a spontaneous and endothermic process, the maximum adsorption capacities of X ⁿ⁺ ions using Langmuir (39.7 to ~449.4 mg·g^–1) were greater than the capacities using the Dubinin–Radushkevich equation (15.1 to ~95.4 mg·g^–1). Thus, the weak electrostatic attraction and the hydroxyl adsorption reaction between MFe₂O₄-montmorillonite and X ⁿ⁺ ions remained the major process for metal-ion adsorption, compared with adsorption by interlayer ion exchange.

Fig. 11 Maximum adsorption capacities of uranyl and toxic metal ions at MFe₂O₄-montmorillonite surfaces/interfaces/interlayer. (a) Langmuir, (b) Dubinin-Radushkevich, (c) Freundlich models, and (d) Illustration of the selective adsorption mechanism

Table 3 Comparison of UO₂ ²⁺ and Cr³⁺ ion-adsorption performance among various adsorbents

HRTEM-EDS images were used to verify the presence of X ⁿ⁺ and UO₂ ²⁺ ions after adsorption. The elemental distribution of Fe₃O₄-montmorillonite was observed, and the UO₂ ²⁺/X ⁿ⁺ ions were found to have been adsorbed uniformly in the sample (Fig. 12a). Ignoring dissociation of the X ⁿ⁺ ions, the elemental distribution of MFe₂O₄-montmorillonite after desorption showed a strong response to the existence of UO₂ ²⁺ and Cr³⁺ (Fig. 12b). Thus, the collaborative adsorptions of UO₂ ²⁺ and Cr³⁺ should be noted. The UO₂ ²⁺ preferentially compensated electron holes and adsorbed at the interfaces compared with Cr³⁺ metal ions, reported as Cr³⁺4s₁ electron orbital, which is easier to protonate than the U 5f₃6d₁ orbital (De et al. 2017). Consequently, the selective adsorption of UO₂ ²⁺ was enhanced through the active electron/hole regulation by the M ²⁺ ionic medium modification, and the adsorption of UO₂ ²⁺ was treated as favorable.

Fig. 12 (a) HRTEM-EDS of uranyl-toxic metal ions in Fe₃O₄-montmorillonite and (b) HRTEM-EDS of uranyl-toxic metal ions in MFe₂O₄-montmorillonite after desorption, where “at.%” means the atom ratio

The charge transition of interaction between UO₂ ²⁺ and X ⁿ⁺ was characterized using cyclic voltammetry. The appearance of the reduction peak revealed that UO₂ ²⁺ ions could be reduced to UO₂ ⁺ by gaining electrons when the potential was scanned from positive to negative (Fig. 13a). However, only a portion of UO₂ ⁺ oxidized back to UO₂ ²⁺ compared with the integral area of the redox reaction, suggesting that UO₂ ²⁺ was oxidized heterogeneously to UO₂ ⁺ (Singer et al. 2012) and the adsorption of UO₂ ²⁺ at the interfaces was stable. The X ⁿ⁺ ions did not undergo redox reaction throughout the cycle, because the exchange of active electrons took place primarily in the interface layer. For instance, the oxidation-reduction peaks of Sr²⁺ ions did not appear (Fig. 13b). However, the integral reduction area of Cr³⁺ ions was expanded, suggesting that some of the Cr³⁺ was reduced to Cr²⁺. In addition, the coupling of electrons with Cr²⁺-Cr³⁺ co-orbitals on the interfaces led to inhibition of the oxidation. The total peaks were obtained through electric signals covering (Fig. 13c), and the total oxidation signal was weakened. This situation predicted that active electrons transferred from Cr³⁺ to UO₂ ⁺ on the interfaces and formed UO₂ ²⁺-UO₂ ⁺-Cr³⁺-Cr²⁺ redox reactions (Fig. 13d). Hence, the selective adsorption of UO₂ ²⁺ in the MFe₂O₄-montmorillonite interfaces was not only associated with electrostatic interaction but was also affected by complexation of Cr³⁺ ions.

Fig. 13 Cyclic voltammograms of FTO glass electrode in 60 ppm solute concentrations. (a) UO₂ ²⁺, (b) Cr³⁺-Sr²⁺, (c) the mixed uranyl-toxic metal ion solution (pH 5.5), and (d) the charge transition mechanism at MFe₂O₄(311)/(111)-montmorillonite(100) interfaces. Therein, the scan rate was 25 mV∙s^–1, “O” and “R” represent the oxidation and reduction peaks, respectively (in parts a–c, the horizontal axis, E (V vs. SCE) represents the scanning potential and the vertical axis, (J (mA∙cm^–3)), the current density (i.e. the ratio of the current to the interface area of the electrode at one potential)

Conclusion

The MFe₂O₄-montmorillonite adsorbent was synthesized for removal of UO₂ ²⁺ from solutions coexisting with toxic trace-metal ions. The results of structural analysis indicated that MFe₂O₄ grew primarily on the edge site of montmorillonite, and the intervention of M ²⁺ ions effectively inhibited the agglomeration of MFe₂O₄. Moreover, the interfacial activity of MFe₂O₄-montmorillonite increased through the electronic transition of Al(Si)-O-O-M bonding, and the interface effect could promote the selective removal of UO₂ ²⁺ in an adsorbent. The adsorption results revealed that the X ⁿ⁺ (Rb⁺, Sr²⁺, Mn²⁺, Ni²⁺, Zn²⁺, Cd²⁺, and Cr³⁺) ions were primarily on the MFe₂O₄-montmorillonite surfaces, which is consistent with monolayer physical adsorption behavior. As a result of the monolayer of the •OH complexes with X ⁿ⁺, UO₂ ²⁺ ions were absorbed at the interfaces. Further study suggested that UO₂ ²⁺ and Cr³⁺ ions had redox reactions and underwent synergistic adsorption on the interfaces of MFe₂O₄-montmorillonite. The interface maximum adsorption capacities of UO₂ ²⁺ and Cr³⁺ were 25.1 mg·g^–1 and 60.2 mg·g^–1, respectively.

All results were consistent with the initial hypothesis, supporting the conclusion that the medium (M ²⁺) of Fe₃O₄ and modifications using a montmorillonite matrix were effective ways to enhance the adsorption capacity and the adsorption selectivity of UO₂ ²⁺. Thus, MFe₂O₄-montmorillonites were shown to be potential adsorbents for uraniferous heavy metal wastewater treatment. Future investigation will focus on the applications of adsorbents in uranium mine wastewater, etc.

Electronic supplementary material

The online version of this article (https://doi.org/10.1007/s42860-019-00028-x) contains supplementary material, which is available to authorized users.