Dual functional nanocomposites of magnetic MnFe2O4 and fluorescent carbon dots for efficient U(VI) removal

Accepted Manuscript Dual Functional Nanocomposites of Magnetic MnFe2O4 and Fluorescent Carbon Dots for Efficient U(VI) Removal Shuyi Huang, Shibo Jiang, Hongwei Pang, Tao Wen, Abdullah M. Asiri, Khalid A. Alamry, Ahmed Alsaedi, Xiangke Wang, Suhua Wang PII: DOI: Reference:

S1385-8947(19)30480-2 https://doi.org/10.1016/j.cej.2019.03.015 CEJ 21139

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

29 January 2019 1 March 2019 2 March 2019

Please cite this article as: S. Huang, S. Jiang, H. Pang, T. Wen, A.M. Asiri, K.A. Alamry, A. Alsaedi, X. Wang, S. Wang, Dual Functional Nanocomposites of Magnetic MnFe2O4 and Fluorescent Carbon Dots for Efficient U(VI) Removal, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej.2019.03.015

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Dual Functional Nanocomposites of Magnetic MnFe2O4 and Fluorescent Carbon Dots for Efficient U(VI) Removal Shuyi Huang1, Shibo Jiang1, Hongwei Pang1, Tao Wen1*, Abdullah M. Asiri2, Khalid A. Alamry2, Ahmed Alsaedi2, Xiangke Wang1,2*, Suhua Wang1,2* 1. MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, 102206, P.R. China 2. Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia *:

Corresponding

author.

[email protected]

(S.

Fax

(Tel):

Wang);

[email protected] (T. Wen)

1

+86-10-61772890;

[email protected]

(X.

Email: Wang);

Abstract Magnetic nanomaterials are promising adsorbents for their superior performance in the elimination of radioactive ions from aqueous solution. In this study, a new kind of fluorescent carbon dots were successfully introduced into MnFe2O4 to fabricate magnetic

polyethyleneimine-functionalized

carbon

quantum

dots/MnFe2O4

(PECQDs/MnFe2O4) nanocomposites by a simple hydrothermal method, which combined the fluorescent characteristic of CQDs, adsorption capacity of polyethyleneimine and magnetic separation property of MnFe2O4. Detailed characterization certified the formation of nanohybrid composites and its integrating advantages of fluorescence property and magnetism separation, which demonstrated its potential in immobilizing U(VI) and monitoring the adsorption process simultaneously in wastewater. The obtained PECQDs/MnFe2O4 was applied to remove U(VI) from aqueous solution and showed superior construction stability and adsorption capacity of 194 mg/g. Moreover, the kinetic and thermodynamic results suggested that the immobilization of U(VI) was an endothermic and spontaneous multilayer adsorption dominated process. In the end, the cation exchange and interaction between uranyl ions and abundant functional groups (i.e., -NH2, -OH and COOH) on PECQDs/MnFe2O4 are revealed to be responsible for the enhanced adsorption of U(VI), which was realized by forming outer-sphere surface complexes in the removal process. This study proposed an ultrafast-kinetics and high-efficiency adsorbent for U(VI) sequestration, which has a great application potential due to its outstanding recycling performance. Keyword: Carbon quantum dots; MnFe2O4; Uranium; Removal; Adsorption; Nanocomposite

2

1. Introduction. As one of hexavalent actinides, uranium (U) has gained considerable concerns in nuclear energy and warfare industries1,2 because a large volume of radioactive waste has been inevitably discharged into environment during the mining and milling process, which cause serious social and environmental problems due to its long-term radiotoxicity and potential carcinogenic effects.3 Hence, strategies need to be proposed to reduce its bioavailability and hinder the transportation of U(VI) in wastewater. Various technologies, including reductive precipitation, ion exchange, membrane separation adsorption, have been widely used to remove soluble U(VI) species from radioactive wastewater.4,

5

Among them, adsorption technique is a

considerable approach due to its low-cost and eco-friendly process. There is, hence, an urgent need for adsorbents that can remove target ions rapidly and efficiently. Carbon quantum dots (CQDs), a kind of carbon nanomaterials with sizes below 10 nm, have attracted massive research interest recently owing to their unique properties including: (1) fascinating fluorescence emission properties, (2) low toxicity and good biocompatibility, (3) robust chemical insertions and (4) easy synthetic method. The photoluminescence properties could be affected by their size, surface structure, and excitable emission wavelength, making CQDs an ideal material widely used in chemistry-related field such as optoelectronics devices, biological labeling and biomedicines.6, 7 Recent researches have reported that different superficial functional groups (i.e., -NH2, -COOH and -OH) can be introduced onto the surface of CQDs when water-soluble polymer takes part in the reaction, which can detect and capture metal ions in water. Owing to the plenty amounts of functional groups, the polymermodified CQDs, including passivate-CQDs with carboxylic groups, thiol-CQDs and 3

amine-CQDs, can be considered as an efficient adsorbent in the removal of toxic metal ions from wastewater.8,

9

Many researches have reported the fluorescence

enhancement of polymer-modified CQDs as well as its selective detection, however, few researches have studied their adsorption performance in field of wastewater treatment. Compared to traditional adsorbents (i.e., zeolite, active carbon, palygorskite, etc.), the functional-group-modified CQDs are superior in high removal efficiency and the visualization of adsorption process, but inferior in the separation and recycle due to their better dispersion in water.7,

10-12

To overcome the disadvantages of CQDs,

considerable attention has recently been focused on CQDs-based composites for the elimination of toxic metal ions. For example, Wang et al.6 synthesized a kind of CQDs/SBA-NH2 composite for the removal of U(VI) in wastewater, which demonstrated high adsorption efficiency and provided a convenient way to monitor the adsorption process. Additionally, LDH/C-Dot composite was applied for coimmobilization of SeO42− and Sr2+ from aqueous solutions.7 However, these materials still have the shortcoming that the separation of contaminant-laden adsorbents was difficult due to the easy release of nanoscale CQDs from substrate. Hence, introducing CQDs onto the magnetic materials, which show strong magnetic field and were also easy to be functionalized with various chemical groups, was a perfectly legitimate approach to achieve both easy separation and high removal efficiency towards U(VI) in wastewater. MnFe2O4, a bimetal oxide magnetic nanomaterial, has comparatively chemical stability, relatively high surface area and saturation magnetization and often regulates the concentration of various pollutants in water by adsorption reactions.13-15 The particles can be used as the matrix for synthesizing functional magnetic materials, 4

which could simultaneously remove contaminants from wastewater and be separated from the solution by a simple magnetic process. In this study, polyethyleneiminefunctionalized carbon quantum dots/MnFe2O4 (PECQDs/MnFe2O4) nanocomposites, combining the fluorescent characteristic of CQDs, adsorption capacity of polyethyleneimine and magnetic separation property of MnFe2O4, were fabricated for rapid elimination of U(VI). To evaluate the adsorption performance of nanomaterials, the effects of various solution conditions (i.e., reaction time, coexisting ions, pH and temperature)

were

CQDs/MnFe2O4

investigated

and

towards

PECQDs/MnFe2O4.

U(VI) The

adsorption results

on

MnFe2O4,

suggested

that

PECQDs/MnFe2O4 nanocomposite showed the greater stability, higher adsorption capacity and faster adsorption kinetics, compared with MnFe2O4 and CQDs/MnFe2O4. Our work highlighted PECQDs/MnFe2O4 as a rapidly sequestrated and highefficiency adsorbent for the elimination of U(VI) from water.

2. Experimental section. 2.1 Materials. All chemical reagents used in this experiment were analytical grade. Manganese nitrate tetrahydrate (Mn(NO3)2·4H2O) and ferric nitrate nonahydrate (Fe(NO3)3·9H2O) were purchased from Tianjin Fuchen Chemical Reagents Factory (China). Citric acid (CA) (Alfa Aesar) and branched poly(ethylenimine) (M = 1800) (Aladdin, Shanghai, China) were used to prepare the PECQDs. Deionized water was used throughout the experiments. 2.2 Preparation of MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4. Carbon quantum dots (CQDs) were synthesized using a one-step hydrothermal process.10 Briefly.6 g citric acid was dissolved in aqueous solution containing 25 mL 5

deionized water and 15 mL glycol. The mixture was stirred vigorously for 15 min to obtain homogenous solution, transferred into a 50 mL Telfon-lined stainless autoclave and maintained at 130 °C for 70 min. The product was naturally cooled to room temperature, and a light-yellow solution containing CQDs was obtained. For the preparation of polyethyleneimine-functionalized carbon quantum dots (PECQDs), 1.0 g polyethyleneimine and 1.6 g citric acid were simultaneously added into the homogenous solution containing 25 mL deionized water and 15 mL glycol, and the subsequently conditions and processes were the same as the preparation of CQDs.11 The magnetic CQDs modified MnFe2O4 nanocomposite (CQDs/MnFe2O4) was prepared through a modified co-condensation according to the previously reported method.16 Typically, 1.25 g manganese nitrate tetrahydrate (Mn(NO3)2·4H2O) and 3.5 g ferric nitrate nonahydrate (Fe(NO3)3·9H2O) were dissolved in 50 mL deionized water with vigorous mechanical stirring, following by adjusting the pH value to 11.0 with 2 M NaOH solution. After stirring vigorously, the dark brown slurry and CQDs aqueous solution were mixed with the volume ratio of 1.5. Then, the mixture was heated to 95 °C and maintained at this temperature for 6 h. Finally, black precipitate was collected and washed several times with deionized water before vacuum drying at 60 °C for 24 h. The polyethyleneimine-functionalized CQDs (PECQDs) modified MnFe2O4 (PECQDs/MnFe2O4) was synthesized using a similar route by replaced the CQDs solution by PECQDs solution. Pure MnFe2O4 was synthesized by the above method without the existence of CQDs or PECQDs. 2.3 Characterization. The obtained MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were characterized by TEM, XRD, FT-IR, TG, VSM, fluorescence spectra and XPS techniques. More

6

detailed information for the different characterization was displayed in Supporting information (SI). 2.4 Batch adsorption experiments. The stock solution of uranium was prepared by dissolving UO2(NO3)2·6H2O in pure water and the initial pH value of the stock solution was about 5.0, and the main species was UO22+. The adsorption of U(VI) on the as-synthesized MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were carried out in batch experiments using the stock solution of uranyl. For batch adsorption, the desired component concentrations were obtained in 10 mL polyethylene tubes by adding different volume ratios of PECQDs/MnFe2O4 (CQDs/MnFe2O4 or MnFe2O4) stock solution, NaNO3 background electrolyte solution, U(VI) (200 mg/L) solution and deionized water. After being placed on a gently rotating bed (150 rpm) for a specified time, an immediate separation of solid and liquid was required to obtain the supernatant (4000 rpm, 3 min). Finally, the residual U(VI) concentration in supernatant was measured directly by color-test method using UV-vis absorption spectroscopy at a wavelength of 650 nm. In the effect of pH values, the initial concentration of U(VI) was set to 10 ppm and the pH of the solutions was varied between 2 and 11 (adjusting by adding negligible volumes of 0.1 M HNO3 or NaOH). To evaluate the effect of temperature on the adsorption process, the adsorption isotherms of U(VI) ions were obtained by treating various initial concentration of U(VI) ions with the same procedure at different temperature (298 K, 313 K and 328 K). The regeneration of PECQDs/MnFe2O4 was further conducted at pH = 5.0, T = 298 K. To minimize experimental error, all experiments were carried out in duplicate. The adsorption percentage (%) and the amount of U(VI) adsorbed per unit mass of adsorbent can be expressed as Eqs. (1) and (2), respectively. 7

Adsorption% 

qe 

 C0  Ce  100% C0

V   C0  Ce  m

(1)

(2)

where C0 and Ce are the initial and equilibrium concentrations of U(VI) ions, respectively (mg/L); qe is the adsorption capacity of adsorbents for U(VI) ions (mg/g); V is the volume of U(VI) solution (mL); m is the mass of adsorbent used (mg).

3. Results and discussion. 3.1 Characterization. The typical morphologies of MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were depicted by TEM images and shown in Fig. 1. The MnFe2O4 nanoparticles (NPs) exhibited a granular structure with the average size approximately below 10 nm, but most part of them aggregated together and held broad particle size distribution in the range of 30-40 nm (Fig. 1a), which was attributed to the van der Waal’s force and magnetic properties of the particles.17 Noticeably, the distribution of these NPs was greatly improved in CQDs/MnFe2O4 and PECQDs/MnFe2O4 samples, indicating that the presence of CQDs and PECQDs can effectively prevent the MnFe2O4 NP from aggregation.13 Moreover, the CQDs were distributed loosely in the surrounding area of MnFe2O4 NPs (Fig. 1b). Identically, PECQDs have been better-loaded on the surface of MnFe2O4 NPs because of the coated polyethyleneimine (Fig. 1c and 1d). To further investigate the specific information of PECQDs disperse onto MnFe2O4, the high-resolution TEM images and elemental mappings of PECQDs/MnFe2O4 composites were illustrated in Fig. 1e-h. The high-resolution TEM image (Fig. 1e) of the PECQDs/MnFe2O4 displayed the lattice fringe spacing of 0.21 and 0.26 nm, which were associated with the (220) and (311) planes of the cubic spinel MnFe2O4, 8

respectively.10 The lattice spacing of approximate 0.31 nm along the side of the MnFe2O4 NPs corresponded to the (002) plane of graphitic carbon, demonstrating the successful attachment of CQDs with MnFe2O4. The selected area electron diffraction (SAED) image of PECQDs/MnFe2O4 showed three relatively apparent diffraction rings, which represented the (220), (311) and (002) planes, respectively (Fig. 1f). The above results certified the coexistence of MnFe2O4 and CQDs. Fig. 1g was the EDX elemental mapping images of PECQDs/MnFe2O4. A higher distribution density in element mapping indicated a higher concentration of corresponding element in that area. The distributions of Mn, Fe and O were uneven and intensive, resembling the granular structure of MnFe2O4. In contrast, the C and N distributions were uniform and continuous, demonstrating the successful modification of polyethyleneimine on the CQDs. Furthermore, the element molar ratio of Mn, Fe and O was appropriately 1:1.89:7.83, which was close to that of MnFe2O4 (Mn:Fe:O = 1:2:4) (Fig. 1h). The EDX result also demonstrated the well distribution of PECQDs onto MnFe2O4. The XRD analysis was employed to identify the phase purity and crystallinity of as-prepared materials. As shown in Fig. 2a, intensive peaks were found at 2θ = 29.59o, 34.88o, 42.40o, 56.02o and 61.52o, which could be well matched with the crystal planes of cubic spinel MnFe2O4 (JCPDS card no. 38-0430), indicating that single phase spinel MnFe2O4 could be prepared by this method. However, the characteristic (002) plane of graphite was absent in the CQDs/MnFe2O4 and PECQDs/MnFe2O4 (2θ = ~25.4°) owing to the low content and high dispersion of the CQDs in composite samples (Fig. 2b).18, 19 The result of XRD analysis was also in well agreement with the broad particle size of MnFe2O4 NPs obtained by HR-TEM beforementioned. Moreover, PECQDs/MnFe2O4 showed better crystallinity than CQDs/MnFe2O4, 9

which could be attributed to the coordination effect of metal ions and amidogen in alkaline conditions. The magnetic behaviors of MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were recorded using vibrating sample magnetometer (VSM) technique in the applied field range of ±10 kOe at room temperature and shown in Fig. 2c. The maximal saturation magnetization (Ms) values of the as-prepared samples were in the order of MnFe2O4 (63.87 emu g-1) > PECQDs/MnFe2O4 (58.45 emu g-1) > CQDs/MnFe2O4 (44.56 emu g-1). The decrease of Ms value was attributed to the contribution of nonmagnetic CQDs or PECQDs component, and the decrease tendency was consistent with the decrease of crystallinity.20 Notably, nearly no hysteresis could be found in hysteresis loop, and the coercive force (Hc) and remnant magnetization (Mr) were negligible, indicating the superparamagnetic behaviors of MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4. Moreover, the inserted picture verified the well magnetic performance of PECQDs/MnFe2O4 in experimental operation process. The Fourier transform infrared (FT-IR) spectra of the as-prepared MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were presented in Fig. 2d. The adsorption peaks at 414 cm-1 and 627 cm-1 were assigned to the Fe-O and Mn-O stretching vibrations of manganese ferrite. The broad band appeared at 3410 cm-1 was associated with the stretching vibration of hydrogen-bonded surface water molecules or hydroxyl groups. After being attached with CQDs or PECQDs, new peaks were observed at 1394 cm-1 and 1625 cm-1, which were assigned to the carboxylic acid bond (-COO-), clearly demonstrating the successfully combination of CQDs and MnFe2O4.21 Compared with the spectrum of CQDs/MnFe2O4, a new band at 1318 cm-1 was observed in spectrum of PECQDs/MnFe2O4, which was assigned to the C–N stretching mode for the polyethyleneimine unit.3 Conclusively, the characteristic 10

peaks of carboxylic acid-iron bond

and C–N stretching mode (1318 cm-1) implied

the PECQDs have successfully surrounded and combined with the MnFe2O4 nanoparticles. To further investigate the weight ratio of CQDs and PECQDs in the nanocomposite materials,

the

thermal

properties

of

the

MnFe2O4,

CQDs/MnFe2O4

and

PECQDs/MnFe2O4 were studied by thermogravimetric (TG) analysis in a flow of air. As illustrated in Fig. 2e, the thermal evolution of CQDs/MnFe2O4 could be divided into three steps: I) the first weight loss before 150 °C was noticed as a result of the evaporation of hydrogen bonded water molecule; II) between 150 °C and 230 °C, a sharp decrease in mass was observed which was corresponding to the decomposition of labile oxygen-containing functional groups (e.g. carboxylic, anhydride, etc.);22 III) the third weight loss in range of 300-550 °C can be attributed to the burning of carbon skeleton in CQDs. As for the TG curve of PECQDs/MnFe2O4, similar weight losses were found at the range of 25-150 °C (~ 3.3%), and 150-230 °C (~ 6.8%) compared with CQDs/MnFe2O4. However, slight weight increase was observed at the range of 300-400 °C (~ 0.6 %), which can be ascribed to the oxidation of amino group (-NH2) on the surface of PECQDs. The distinct changes in TG analysis demonstrated the successful combination of PECQDs and MnFe2O4, which was consistent with our observations by EDX and FT-IR investigations. The fluorescence emission spectra were carried out under the excitation of 360 nm to further study the optical properties of the as-prepared materials (Fig. 2f). All the samples used to investigate fluorescence quenching of U(VI) were adjusted to pH values of 5.0 to ensure that and the quenching of the fluorescence is only due to the uranyl ions. It can be observed that the fluorescence can be detected when CQDs or PECQDs loaded on the surface of MnFe2O4, confirming the good fluorescence 11

properties of CQDs. Moreover, red shift was observed in the photoluminescence emission spectra of PECQDs/MnFe2O4 and its fluorescence emission intensity was lower comparing with the CQDs/MnFe2O4, which was ascribed to the influence of branched ethylenimine or the low content of CQDs. Clearly, when U(VI) ions were added into the suspensions, the fluorescence intensity of PECQDs/MnFe2O4 decreased obviously, but the maximum emission wavelength maintained at the same position. Therefore, the fluorescence quenching process may be ascribed to the special coordination interaction between U(V) ions and the amino groups as well as the hydroxyl groups on the surface of PECQDs.23

Moreover, the fluorescence

intensities of PECQDs/MnFe2O4 upon exposure to different concentrations of U(VI) were investigated and displayed in Fig. 1S. It can be seen that a linear correlation (R2 = 0.987) is obtained between the fluorescence intensities of PECQDs/MnFe2O4 and the concentrations of U(VI) adsorbed on the surface, indicating that the adsorption capacity and process on PECQDs/MnFe2O4 could be simply monitored by fluorescence signal. This phenomenon suggested the potential application of PECQDs/MnFe2O4 in immobilizing U(VI) and monitoring the adsorption process simultaneously in wastewater. 3.2 Adsorption kinetics. In order to attain the adsorption rate and understand the possible mechanism of adsorption process, the U(VI) adsorption kinetics on MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were investigated. As shown in Fig. 3a, the CQDs/MnFe2O4 and PECQDs/MnFe2O4 took ~20 min to reach an adsorption equilibrium at pH = 5.0, whereas bare MnFe2O4 required more than 30 min. The rapid adsorption rate was mainly ascribed to the coordination between U(VI) ions and CQDs present on the surface of MnFe2O4. More importantly, over 50% U(VI) could be removed by 12

PECQDs/MnFe2O4 in 10 min. Such ultrafast adsorption of U(VI) indicated PECQDs could remarkably improve the hydrophilicity of MnFe2O4, resulting in the rapid diffusion of U(VI) from solution onto the surface of the as-synthesized material.23 Moreover, the removal efficiency of U(VI) on PECQDs/MnFe2O4 was appropriately 88%, which was much higher than that of U(VI) on CQDs/MnFe2O4 (~30%). The higher removal efficiency implied the participation of polyethyleneimine supplied more binding sites on the surface of CQDs, leading to remarkably improvement in the adsorption efficiency. To investigate the controlled mechanism in adsorption process, pseudo-first-order and pseudo-second-order models were employed to fit the experimental data. The pseudofirst-order model was used to describe physisorption process involving diffusion and mass transfer of the adsorbate, whereas the pseudo-second-order model described chemisorption controlled by valency force.24, 25 The equations of the two models were presented as follows:

ln(qe  qt )  ln qe  k1t

(3)

1 t t   2 qt k2 qe qe

(4)

where t (min) and qe (mg·g-1) are the reaction time and the equilibrium adsorption capacity. k1 (min-1) and k2 (mg·g-1·min-1) are the rate constant of pseudo-first-order model and pseudo-second-order, respectively. qt and qe (mg·g-1) are the amounts of U(VI) adsorbed at contact time t and equilibrium time, respectively. The linear plots of log (qe−qt) vs t and t/qt vs t were plotted and shown in Fig. 3b and 3c, respectively. And the fitting results of corresponding parameters were listed in Table S1. The obtained correlation coefficients (R2) of pseudo-second-order model were all > 0.997, whereas those of pseudo-first-order model were between 0.92913

0.964. Obviously, the pseudo-second-order kinetic model gave a better fitting of U(VI) removal, suggesting the chemisorption was the rate-limiting step of the U(VI) immobilization on the surface of MnFe2O4, CQDs/MnFe2O4 and PECQDs/ MnFe2O4. In addition, the qe,cal values from the fitting data by pseudo-second-order model were very close to the qe,exp values. The qe,cal value of U(VI) on PECQDs/MnFe2O4 (86.67 mg·g-1) was much higher than that of U(VI) on CQDs/MnFe2O4 (23.37 mg·g-1) or MnFe2O4 (12.89 mg·g-1), indicating that PECQDs/MnFe2O4 had superior performance on U(VI) adsorption than MnFe2O4 and CQDs/MnFe2O4. 3.3 Effect of pH and ionic strength. The solution pH probably affects the adsorption process of U(VI) because of the variable U(VI) speciation, the protonation/deprotonation degree of functional groups and the surface charge of the adsorbents. In addition, the industrial wastewater containing uranium species is generally in the range from 3.0 ~ 6.5. Therefore, to validate the practical applications, the effect of pH and the corresponding speciation of

U(VI)

adsorbed

on

the

surface

of

MnFe2O4,

CQDs/MnFe2O4

and

PECQDs/MnFe2O4 were carried out and displayed in Fig. 3d-3f. It is noteworthy that the adsorption tendency of U(VI) on MnFe2O4 and CQDs/MnFe2O4 were similar and distinguished from that of U(VI) on PECQDs/MnFe2O4. The U(VI) adsorption processes could be divided into three parts. In process I (pH < 4.0), the removal percentage of U(VI) on MnFe2O4 and CQDs/MnFe2O4 increased slightly, and then increased rapidly at the pH rang of 4.0 – 7.0 (process II) and reached the maximum removal percentage at pH =7.0 (~25% for MnFe2O4 and ~45% for CQDs/MnFe2O4). Differently, the removal percentage of U(VI) on PECQDs/MnFe2O4 increased rapidly at pH < 5.5 (process I) and maintained at high level about 90% at pH range 5.5 ~ 7.0 (process II). In the process III (pH > 7.0), the removal percentage of U(VI) on 14

PECQDs/MnFe2O4

declined

more

slowly

than

those

on

MnFe2O4

and

CQDs/MnFe2O4. According to the results of zeta potential values, the pHzpc value of MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were measured to be 6.8, 6.4 and 7.5, respectively (Fig. S2). At pH < 4.0, UO22+ was the main species in aqueous solution. In such case, the strong electrostatic repulsion between UO22+ and positive charged surface of adsorbents was the main constraints for U(VI) adsorption. However, even with the existence of the electrostatic repulsion, part of U(VI) species were inevitably adsorbed on the surface of PECQDs/MnFe2O4, which was attributed to the strong surface complexes, or cation exchange with Na+, which show better affinity than H+ and worse affinity than UO22+ onto PECQDs/MnFe2O4. With the pH increasing, the electrostatic repulsion reduced, resulting in a high level (about 91%) of U(VI) adsorption on PECQDs/MnFe2O4. At pH >7.5, the main species of U(VI) converted to negatively charged (UO2)3(OH)7- and UO2(OH)3-, which were hardly adsorbed on same negatively charged PECQDs/MnFe2O4. The adsorption of U(VI) was inhibited by the electrostatic repulsion to some extent. Moreover, from the effect of ionic strength (Fig. S3), one can see that the adsorption percentages of U(VI) decreased with the increasing ionic strength at pH = 5.0. The NaNO3 concentration could affect the U(VI) adsorption process at least in two forms: (i) the competitive effect of U(VI) and other adsorbed Na+ ions during adsorption process, (ii) compressing the thickness of hydration layer in ionic atmosphere, and then reducing the binding sites on the surface of adsorbents. Therefore, the interaction mechanism of U(VI) with these adsorbents might be cation exchange or outer-sphere surface complexation. 3.4 Adsorption isotherms and the effect of coexisted ions.

15

Adsorption capacity is a critical indicator of adsorbents’ performance. Thus, the adsorption isotherms of U(VI) on MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were obtained at pH 298 K (Fig. 4a). It can be seen that qt increased rapidly at low equilibrium U(VI) concentrations (Ce) and then became steady with Ce increases in the Ce range of 20 mg/L – 40 mg/L. The trend of qt values denoted that the adsorption process was U(VI) concentration driven and reached equilibrium when binding sites on the adsorbents’ surface reached saturation. To evaluate the adsorption behaviors, typical Langmuir (qe = KLqmaxCe / (KLCe + 1) and Freundlich (qe = KF Ce1/n) models were employed to simulate the adsorption data. The Langmuir and Freundlich isotherms assume that the adsorption process is dominated by monolayer homogeneous coverage and heterogeneous adsorption, respectively.26,

27

The fitting

results of Langmuir and Freundlich models were tabulated in Table S2. Apparently, for the U(VI) adsorption on MnFe2O4 and CQDs/MnFe2O4, Langmuir models fitted the experimental data better than Freundlich models (RL2 > RF2), indicating the adsorption sites on MnFe2O4 and CQDs/MnFe2O4 were identical towards U(VI). However, for the U(VI) adsorption on PECQDs/MnFe2O4, Freundlich models fitted the experimental data better than Langmuir models (RF2 > RL2), implying the adsorption sites were significantly divers, and the U(VI) adsorption was controlled by multilayer adsorption process. The maximum adsorption capacities of adsorbents at 298 K followed in the order of MnFe2O4 (58.3 mg/g) < CQDs/MnFe2O4 (90.6 mg/g) < PECQDs/MnFe2O4 (194.2 mg/g). The largest U(VI) adsorption capacity of PECQDs/MnFe2O4 could be attributed to the abundant amino groups on the PECQDs surface, which could provide rich active sites for U(VI) capture. Furthermore, the comparison of adsorption capacities of PECQDs/MnFe2O4 with other metal oxide materials or polymer-functional materials was shown in Table 1.3, 16

28-32

The ideal

adsorption capacity indicated that PECQDs/MnFe2O4 can be considered a potential adsorbent to efficiently remove U(VI) from wastewater. Moreover, additional adsorption isotherms were further performed at 313 K and 328 K (Fig. S4) and the qmax values at different temperature were displayed in Fig. 4b. One can see that the maximum adsorption capacities of U(VI) on MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 increased obviously with the increase of temperature. To investigate the adsorption thermodynamic performance deeply, the thermodynamic parameters (i.e., standard entropy change (∆S0, J/(K mol), standard enthalpy change (∆H0, kJ/mol), and standard free energy change (∆G0, kJ/mol)) were calculated and tabulated in Table S3. One can see that the ΔG0 values of different adsorbent at 298 K were all negative and in the order of PECQDs/MnFe2O4 (-3.54 kJ·mol-1) > CQDs/MnFe2O4 (-2.04 kJ·mol-1) > MnFe2O4 (-1.06 kJ·mol-1), indicating the more spontaneous adsorption process of PECQDs/MnFe2O4. The positive ΔH0 values implied that the adsorption reactions of U(VI) on MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4 were endothermic process. Moreover, the positive ΔS0 values indicated the increasing randomness during the adsorption processes, which might be attributed to the ion-exchange of U(VI) ions with pre-adsorbed Na+, which were twice amount of U(VI) ions. Based on the abovementioned conclusion, it suggested that the adsorption mechanism of U(VI) on PECQDs/MnFe2O4 is assumed by following reactions: 1) exchanged with cation ions: =S-NH3+ + Na+ ⇌ =S-NH2Na+ + H+ 2 =S-NH2Na+ + UO22+ ⇌ 2(=S-NH2)UO22+ + 2Na+ 2) formed outer-sphere surface complexes: 17

2 =S-COOH + UO22+ ⇌ 2(=S-COO)UO20 + 2H+ 2 =S-NH3+ + UO22+ ⇌ 2(=S-NH2)UO22+ + 2H+ The effect of coexisted electrolyte ions on the U(VI) adsorption onto PECQDs/MnFe2O4 was investigated and the results displayed in Fig. 5a. Generally, the adsorption percentage of U(VI) reduced in order of PO43− > CO32− > Cl− ≈ NO3−, which was associated with the uranyl carbonate or uranyl carbonate precipitate. Theoretically, the adsorption of U(VI) can be influenced by different cations (the adsorption percentages in order of Na+ > K+ > Mg2+ > Ca2+), which was related to the valence state and ionic radius. However, obvious impact was only observed on the U(VI) adsorption in the presence of 0.01 M Ca2+, resulting in a 13% decrease in the removal efficiency, while the impact of other types of cation has mostly been contained. Therefore, it was of significance to investigate the influence of high-level Ca2+ concentration. As displayed in Fig. 5b, although at the mole ratio of Ca(II)/U(VI) = 600, the removal efficiency of U(VI) still maintained at a relatively satisfactory level about ~ 62%, suggesting the desired competitive adsorption capacity of PECQDs/MnFe2O4 toward uranyl. The impact of Ca2+ was ascribed to the formation of ternary complexes (CaUO2(CO3)32− and Ca2UO2(CO3)30), which inhibited the capture of U(VI) onto PECQDs/MnFe2O4. 3.5 The effect of adsorbent contents and reusability of PECQDs/MnFe2O4. To evaluate the adsorption stability and possibility in real application, the effect of adsorbent contents and the regeneration of adsorbents were conducted. As displayed in Fig. S5, the removal percentage of U(VI) increased significantly from 17% to 83% when the PECQDs/MnFe2O4 dosage increased from 0.02 g/L to 0.1 g/L. Then, the 18

removal

percentage

of

U(VI)

gradually

became

evenness

when

the

PECQDs/MnFe2O4 dosage achieved 0.1 g/L, which was attributed to the decrease in the total surface area caused by the increased probability of collision between PECQDs/MnFe2O4

NPs.

Furthermore,

the

recycling

performance

of

PECQDs/MnFe2O4 in the removal of U(VI) were investigated for five consecutive cycles (Fig. 5c). One can see that a slight decrease was observed in the U(VI) removal, but the removal percentage remained a high level about 75% after five consecutive cycles, suggesting the splendid regeneration ability of PECQDs/MnFe2O4 as a desired adsorbent to eliminate U(VI) in industrial applications. 3.6 Adsorption mechanism. To further elucidate the interaction mechanism of U(VI) onto PECQDs/MnFe2O4, PECQDs/MnFe2O4 and U(VI)-laden PECQDs/MnFe2O4 samples were characterized by FT-IR and XPS techniques. As displayed in Fig. 6a, the characteristic peak of C-N at 1318 cm-1 was observed to decrease slightly in the spectra of U(VI) laden PECQDs/MnFe2O4, indicating that the polyethyleneimine on the surface of NPs was involved in the U(VI) adsorption onto the PECQDs/MnFe2O4.33 The XPS spectra of PECQDs/MnFe2O4 before and after U(VI) adsorption were given in Fig. 6b-d. In fullscale XPS spectra (Fig. S6a), five apparent peaks of Fe 2p, Mn 2p, C 1s, N 1s, and O 1s illustrated that iron, manganese, carbon, nitrogen and oxygen were the main elements of the PECQDs/MnFe2O4. The appearance of U 4f peaks in the range 370400 eV indicated that U(VI) was successfully adsorbed onto the surface of PECQDs/MnFe2O4. From the high-resolution XPS spectrum of U 4f7/2 (Fig. S6b), the good fitting result to a single peak implied that that no chemical reduction occurred in the whole experimental processes. The corresponding contents of C 1s, N 1s, and O 1s before and after U(VI) adsorption were tabulated in Table S4. Generally, from 19

high-resolution XPS spectrum of N 1s, three peaks were observed at 399.4 (N–(C)3), 400.2 (C–N–H) and 401.8 eV (C-NH2), which implied that CQDs/MnFe2O4 has been functionalized by free -NH2 groups through grafting polyethyleneimine on the surface of solid particles.34,

35

Interestingly, the peak positions shifted to higher binding

energies (399.5, 400.3 and 402.57 eV) after U(VI) adsorption, suggesting that the amino groups participated in the U(VI) adsorption (Fig. 6b). Moreover, the C 1s spectrum of PECQDs/MnFe2O4 was divided into three peaks centered at binding energies of 284.8, 285.9 and 288.1 eV (Fig. 6c), which could be assigned to the carbon in C-C, C-N and C=O.36 The C=O peak corresponding to carboxylic groups verified the presence of CQDs on the PECQDs/MnFe2O4.37 The highly resolved O 1s spectrum can be deconvoluted into three peaks at 529.7, 531.2 and 532.1 eV, corresponding to the binding energies of the different oxygen form in S-O, H-O and O=C–O bonds, respectively. Similarly, the peaks of C=O and O=C–O also shifted to higher binding energy, demonstrating that carboxylic group can form complexes with U(VI) in adsorption process. In addition, two peaks were observed from the highresolution XPS spectra of Fe 2p (Fe 2p1/2 and Fe 2p3/2) and Mn 2p (Mn 2p1/2 and Mn 2p3/2) (Fig. S6c and S6d). After U(VI) adsorbed, the peaks of Fe 2p1/2 slightly shifted from 725.0 to 724.2 eV, indicating that U(VI) probably formed complexes with the oxygen-containing functional groups bonded to ferrum (Fe-O-), which was more stable with lower binding energies. Compared with that of the Mn 2p3/2 peak in PECQDs/MnFe2O4, the binding energy of Mn 2p3/2 after U(VI) adsorption shifted from 641.2 to 641.8s eV, suggesting the functional groups bonded to manganese (MnO-) were also involved in the adsorption process.38 The aforementioned results suggested that the formation of surface complexes between U(VI) ions and various functional groups (i.e., -COOH, -OH and -NH2) was contributed to the U(VI) removal 20

by PECQDs/MnFe2O4, thus making it an ideal and efficient adsorbent for metal separation.

4. Conclusion. In summary, novel magnetic PECQDs/MnFe2O4 nanocomposites were successfully fabricated via a facile modified solvothermal synthetic method. The morphology transformation and characteristic spectra conversion confirmed the successful assembling of polyethyleneimine-functionalized carbon dots on MnFe2O4. The obtained PECQDs/MnFe2O4 nanocomposites inherited the magnetic properties of MnFe2O4 and fluorescence properties of PECQDs and showed high adsorption capacity towards U(VI) due to the multiple amide groups. The elimination of U(VI) on PECQDs/MnFe2O4 was strongly dependent on pH and ionic strength, demonstrating that the adsorption of U(VI) was controlled by outer-sphere surface complexation. The adsorption kinetics was well described by pseudo-second-order model and the adsorption isotherms were well fitted by Freundlich isotherm model. The adsorption/desorption study further demonstrated the rate-controlling adsorption mechanism during adsorption process. Moreover, the XPS analysis confirmed the removal of U(VI) by PECQDs/MnFe2O4 is attributed to the amine groups, carboxyl groups and hydroxyl groups, which formed complexes with U(VI) on the surface of particles. This study provided cost-effective PECQDs/MnFe2O4 for the disposal of U(VI)-contaminated wastewater and also hold potential in practical application. Acknowledgements. This work was supported by National Key Research and Development Program of China (2017YFA0207000), the National Natural Science Foundation of China (2177504, 21707033, 21775042, 21607042,), the Science Challenge Project 21

(TZ2016004), and the Fundamental Research Funds for the Central Universities (2018ZD11, 2017MS045). The Jiangsu Provincial Key Laboratory of Radiation Medicine and Protection and the Priority Academic Program Development of Jiangsu Higher Education Institutions are acknowledged.

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27

GRAPHICAL ABSTRACT

28

Fig. 1 TEM images of (a) MnFe2O4, (b) CQDs/MnFe2O4 and (c) PECQDs/MnFe2O4. (d, e) high resolution TEM and (f) SAED images of PECQDs/MnFe2O4. (g) Elemental mapping of Fe, Mn, O, C and N and (h) EDX of PECQDs/MnFe2O4.

29

Fig. 2 (a), (b) The XRD patterns and (c) magnetization curves of MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4, the insert shows the magnetic separation of MnFe2O4. (d) FT-IR spectra and (e) TG curves of the as-prepared samples. (f) Photoluminescence emission spectra of CQDs/MnFe2O4, PECQDs/MnFe2O4 and PECQDs/MnFe2O4 in presence of U(VI). λex= 360 nm, pH = 5.0.

30

Fig. 3 (a) Adsorption kinetic curves of MnFe2O4, CQDs/MnFe2O4 and PECQDs/MnFe2O4, and corresponding linear fitting of experimental data obtained using (b) pseudo-second-order kinetic model and (c) pseudo-second-order kinetic model at pH= 5.0. (d-f) the adsorption of U(VI) on as-prepared materials as a function of initial pH. T = 298 K, m/V = 0.1 g/L, C(U(VI))initial = 10 mg L-1, I = 0.01 M NaNO3.

Fig. 4 (a) The adsorption isotherms of U(VI) on as-prepared materials at 298 K. (b) the maximum adsorption abilities of as-prepared materials at different temperature. pH = 5.0, m/V = 0.1 g/L and I = 0.01 M NaNO3.

31

Fig. 5 Fig. 5 The effect of (a) coexisting ions and (b) competitive ions Ca2+ on the U(VI) uptake onto PECQDs/MnFe2O4. (c) The recycling performance of PECQDs/MnFe2O4 for U(VI). T = 298 K, pH = 5.0, m/V = 0.1 g/L, C(U(VI))initial = 10 mg L-1 and the ionic strength is 0.01 M.

32

Fig. 6 (a) FT-IR spectra of as-prepared samples before and after U(VI) adsorption. High resolution XPS spectra of (b) N 1s, (c) C 1s and (d) O 1s for PECQDs/MnFe2O4 before and after U(VI) adsorption.

Table 1 Comparison of the adsorption capacities of U(VI) on PECQDs/MnFe2O4, and other reported adsorbents.

Adsorbents

T (K)

qmax (mg·g-1) pH

U(VI)

Reference

Nanomagnetite

298

2.5

2.46

[28]

palygorskite

333

4.0

46.8

[29]

Amidoxime modified Fe3O4@SiO2

298

5.0

105

[30]

g-C3N4@Ni-Mg-Al-LDH

298

5.0

99.7

[31]

FA@PEI

298

5.0

70.3

[3]

TiO2-x

298

5.0

65.41

[32]

PECQDs/MnFe2O4

298

5.0

194.2

This work

Highlights 

Bifunctional nanocomposite with magnetic and fluorescence property was synthesized.



The nanocomposite consists of fluorescent carbon dots and magnetic MnFe2O4.



The fluorescence indicates the efficient adsorption of U(VI) on the nanocomposite.

33



The adsorption capacity increases by two times than the single magnetic MnFe2O4.

34