Construction of magnetic ion-exchange-based modular micromotors. The magnetic ion-exchange-based modular micromotors (mIEX-MMM) for uranium adsorption consists of two fundamental parts, magnetic ion exchange-based micro-fluidic pumps for assembly, motion control and local pH regulation of the adsorbent, and the adsorbents for uranium adsorption (Fig. 1a). Typically, the mIEX as the central part of the micro-reactor can generate long-ranged pH gradient and electro-osmotic (eo) flow via ion-exchange reaction for the assembly of adsorbents. The eo-flow also accelerates the diffusion of uranium to the adsorbent and increases the uranium concentration around the adsorbent. Meanwhile, the mIEX tunes the local pH environment of the assembled adsorbents for optimal uranium uptake. For magnetic control and recycling, the IEX was coated with a layer of Fe3O4 nanoparticles. SEM and EDS mapping images confirm the successful coating of Fe3O4 nanoparticles on the surface of the mIEX (Fig. 1b). The magnetic hysteresis loops indicate the superparamagnetic property of the mIEX with the Fe3O4 content of 2.2 and 2.6% for the mAIEX and mCIEX, respectively, calculated from the saturation magnetization (Fig. 1c and S1). A closed-loop system for the adsorption and desorption of uranium consisting of four parts was designed: (a) uranium adsorption by mIEX-MMM, (b) uranium desorption by Na2CO3 (1.0 M) solution, (c) mIEX-MMM regeneration by 1 M NaOH or HCl solution, and (d) rinse by deionized (DI) water (Fig. 1d). Thanks to the magnetic property, the mIEX-MMM is easily collected and transferred between different cells by a magnet.
Characterization of mIEX. Typically, the mIEX as the central part of the micro-reactor can generate long-ranged pH gradient via ion-exchange reaction, which is alkaline for the anionic IEX (mAIEX, Fig. 2a) and acidic for the cationic IEX (mCIEX, Fig. S3). Due to the different diffusivities of ions, the pH gradient induces local diffusive electric fields, which is pointing outward for the mAIEX as verified by COMSOL Multiphysics simulation (Fig. 2d). The electric field acts on the double layer of the negatively charged substrate inducing an in-plane diverging electro-osmotic (eo) flow (Fig. 2b and 2e). The flow decays linearly with the radial distance over a radial range of ~ 100 µm. Due to the incompressibility of water, the flow is three-dimensional (Fig. 2c and Video 1) and the convection leads to the approach of the adsorbent towards the mAIEX. Once the adsorbent is assembled with the mAIEX, the symmetry of flow is broken, leading to the self-propulsion of the assembled structure (Fig. 1g-I and Video 1). For mCIEX, the assembled adsorbent also breaks the symmetry of electric and flow fields, inducing the self-propulsion of the assembly. The size of the mIEX affects the speed of the assembled structure. At a diameter of 45 µm, the mAIEX-MMM reaches an optimal motion speed of 6.9 ± 2.4 µm s− 1 (Fig. S4a). For the mCIEX-MMM, a maximum speed of 3.8 ± 0.9 µm s− 1 was reached with the mCIEX of diameter 45 µm (Fig. S4b).
Motion control of mIEX-MMM. To control the motion of the mIEX-MMM, a 3D Helmholtz coil system with integrated imaging and video recording CCD was assembled (Fig. 3a). Under a rotating magnetic field \(B (t)={B_0}[\text{s}\text{i}\text{n} (2\pi ft) {B_\text{y}}+\text{c}\text{o}\text{s} (2\pi ft) {B_\text{z}}]\), where B0 is the amplitude of the magnetic field, f is the rotation frequency of the field, and t is the time, the mAIEX subjected to a rotational torque rolls forward together with the assembled adsorbents. The direction and speed of the mIEX-MMM can be controlled by the magnetic field. As shown in Fig. 3b, the mIEX-MMM can be directed by the rotating magnetic field of changeable rotating direction to write letters, such as “HUST” (Video 2). To explore regions where the uranium concentration is higher, the velocity of the mAIEX-MMM can be accelerated by the field strength. Figure 3c shows typical trajectories of mAIEX-MMM under rotating magnetic field of fixed frequency (2 Hz) and different strengths (≥ 70 Gs) within 1 s. The speed of mAIEX-MMM increases from 21.3 ± 5.9 µm s− 1 to 52.2 ± 8.9 µm s− 1 as the field strength increases from 70 Gs to 130 Gs (Fig. 3d). When the strength of the magnetic field is fixed at 100 Gs, the speed of mAIEX-MMM first increases with field frequency then decreases as the frequency is above 8 Hz, which is the step-out frequency of the mAIEX-MMM (Fig. 3e). As the input f further increases, the rotation of the mAIEX-MMM becomes asynchronous with the rotating B(t) owing to the increasing resistance, and thus the speed gradually decreases38. The speed of mCIEX-MMM shows similar changing trend with that of mAIEX-MMM, and a maximum speed of 29.6 ± 17.3 µm s− 1 is obtained at 100 Gs and 12 Hz (Fig. S5).
Under defocused NIR (λ = 808 nm) irradiation, the convective flow generated on the substrate induces the migration of the mAIEX-MMM towards the light source39 (Fig. 3f and Video 2). The speed of the mAIEX-MMM increases as the light intensity is increased, reaching a speed of 10.5 ± 2.4 µm s− 1 under the light intensity of 1.4 W cm− 2 (Fig. 3g). Figure 3h shows the typical trajectory of the mAIEX-MMM under NIR light irradiation of 0.7 W cm− 2 within 17 s.
Uranium extraction performance of the mIEX-MMM.
The uranium uptake performance of the mIEX-MMM was explored. The amidoxime groups in the PM can effectively coordinate with uranyl ions (Fig. 4a). As shown in Fig. 4b, the introduction of mAIEX accelerates the uranium uptake of the PM (adsorbent with best adsorption in alkaline solution, Fig. S6a). The adsorption equilibrium of the mAIEX-MMM is reached within 10 min, much faster than that of the pure PM. The adsorption capacity of the PM increases from 223.4 ± 12.1 mg g− 1 to 424.5 ± 16.8, 491.7 ± 12.3 and 629.3 ± 18.7 mg g− 1 as the mass ratio of mAIEX to PM is changed from 0 to 0.5, 1 and 2 (Fig. 4b). Similarly, the introduction of mCIEX accelerates the uranium adsorption of ZP (adsorbent with best adsorption in acidic solution, Fig. S6b) and the equilibrium adsorption amount increases from 269.5 ± 13.5 mg g− 1 (pure ZP) to 428.3 ± 14.0 mg g− 1 (mCIEX:ZP = 2:1, Fig. 4c). The kinetic adsorption isothermal is well fitted by pseudo-second-order kinetic model, indicating that the uranium adsorption depends on chemical adsorption18 (see details in Table S1 and S2). The equilibrium adsorption data were well described by a Langmuir model (R2 = 0.99, Fig. S7 and Table S3), indicating the single layer adsorption40. To explore the mechanism of the promoted uranium adsorption of adsorbent by the introduction of mIEX, numerical simulations were performed to compare the diffusion of uranyl ions in pure diffusive mode and the presence of phoretic flow produced by the mIEX. As shown in Fig. 4d and S8 (from Video 3), the transport of ions is remarkably accelerated by the phoretic flow. Moreover, the electric field further promotes the diffusion and accumulation of uranyl ions in the vicinity of the mIEX, which can be seen from the uranyl ion concentration difference in Line 1 (Fig. 4e) and Line 2 (Fig. 4f). The local pH regulation via the mIEX further enhances the coordination interaction of the uranyl ions and the adsorbent (Fig. S6). Therefore, both uranyl uptake kinetics and capacity are improved by the ion-exchange induced multiple effects.
When rotating magnetic field is applied, the speed of the mAIEX-MMM increases, which helps the PM to explore regions where the uranyl ion concentration is high. Therefore, the uranium uptake of the mAIEX-MMM (mass ratio: 2) slightly increases from 629.3 ± 18.7 mg g− 1 to 728.6 ± 24.9 mg g− 1 and 823.0 ± 29.6 mg g− 1 as the rotation field strength increases from 0 to 80 Gs and 120 Gs (Fig. 4g). Under NIR-driven motion, the photothermal conversion of the Fe3O4 nanoparticles adsorbed on the mAIEX surface increases the temperature of the adsorbent (Fig. S9), inducing the increment of the uranium uptake to 763.8 ± 23.9 mg g− 1 and 829.8 ± 29.9 mg g− 1 at the NIR intensity of 0.7 W cm− 2 and 1.4 W cm− 2 (Fig. 4h). The uranium adsorption thermodynamics of mIEX-MMM was examined to deeply understand the photothermal-enhanced uranium capture (Fig. S10, Table S4). ΔH > 0 implies the endothermic nature of the uranium adsorption, and ΔG < 0 means the spontaneous process for uranium adsorption41. All these results demonstrate that the photothermal conversion of Fe3O4 nanoparticles can enhance the interaction between the adsorbent and uranyl ions via an increased temperature, therefore the uranium adsorption capacity is increased.
The uranium uptake of the PM was confirmed by SEM. As shown in Fig. 5a and S11, the uranium distributes homogeneously throughout the U-uptake PM and ZP. XPS spectra confirms that uranium was bound on the PM with two additional peaks corresponding to the U4f observed in the full scan spectrum of U-uptake PM (Fig. 5b). In the high resolution O1s spectra, the appearance of a new peak at 531.1 eV corresponding to the O = U = O also confirms the binding of uranyl ion with the adsorbent17 (Fig. S12a). Meanwhile, in the FTIR spectra, the new peak at 905 cm− 1, attributing to the O = U = O (Fig. S12b), indicates the binding of uranyl ions with the PM. Similar results can be observed on the full scan spectrum of mCIEX-MMM (Fig. S13).
The reusability of the mIEX-MMM was evaluated by adsorption-desorption cycles. The adsorption capacity remains above 90% of the initial state and the elution efficiency of two mIEX-MMM stays above 84.0 ± 2.6% (Fig. 5c and Fig. S14) after five cycles, indicating the outstanding reusability of the adsorbent. To demonstrate the potential of large-scale uranium removal, a closed-loop miniplant that could be modified for industrial application was designed (Fig. 5d, see details in Fig. S15). This miniplant mainly consists of four cells: (i) uranium adsorption by mIEX-MMM, (ii) uranium desorption by Na2CO3 (1 M) solution, (iii) mIEX-MMM regeneration by 1 M NaOH (for mAIEX-MMM) or 1 M HCl (for mCIEX-MMM) solution, and (iv) mIEX-MMM rinse by DI water. Because of the magnetic response of the mIEX, they can be reclaimed by a magnet and dragged to the neighboring cell through the interconnected channels. Figure 5d (i) exhibits the adsorption cell, where uranium-contaminated water colored by arsenazo was cleaned by mAIEX-MMM after adsorption. After that, the U-uptake mAIEX-MMM was dragged to the desorption cell which can be easily detected from the color change of the cell (Fig. 5d ii). After exchanging the treated water from the adsorption cell with new contaminated water, a new cycle could start. The refilling and new cycle of treatment could proceed in parallel with mAIEX-MMM regeneration and rinse under autonomous control. The adsorption and desorption efficiencies were detected via UV-Vis spectra (Fig. 5e). In this small device, mAIEX-MMM were recycled in the loop of four cells and kept over 78% adsorption efficiency and 90% elution efficiency over five cycles (Fig. 5f).
The competitive uranium adsorption with the existence of other metal cations were explored (Fig. 5g). In simulated polluted underground water, the existence of Na+, K+, Mg2+ and Ca2+ does not show obvious influence on the uranium adsorption capacity of self-driven mAIEX-MMM (mass ratio: 2). A uranium adsorption capacity of 623.5 ± 25.5 mg g− 1, comparable to that in U-spiked water (629.3 ± 18.7 mg g− 1) was detected, indicating the good selectivity of PM to uranium. The distribution coefficient (Kd) value was calculated to be 9.8 × 104 mL g− 1 (Fig. 5h, see detail in SI), indicating an excellent affinity toward uranyl ions42, 43, 44. Importantly, the mAIEX-MMM with self-regulated pH and self-generated phoretic flow outperforms most of the reported adsorbents in terms of the uranium adsorption performance (Fig. 5i).