Introduction

Immoderate discharge of radionuclides during mining and processing processes has posed a serious threat to ecological environment and human health [1]. Therefore, much effort has been made to remove radionuclides before discharged into environments. Adsorption approach is a highly effective method to remove radionuclides from wastewater due to easy-operation, low-cost and environmentally friendly [2]. Eu(III) has been extensively used as a chemical analogue of trivalent actinides in recent years. The removal of Eu(III) on various adsorbents has been widely investigated such as clay minerals [3,4,5,6], Fe/Al-(hydr)oxides [7,8,9,10], and carbon-based materials [11,12,13,14,15]. Wang et al. elucidated the adsorption mechanism of Eu(III) on carbon nanotubes using batch, spectroscopy and theoretical calculation [16].

Biochar as a common carbon-based adsorbent, generally produced from the thermal or hydrothermal conversion of biomass, could significantly increase the removal of environmental pollutants through surface complexation due to the occurrence of various functional groups [17, 18]. In recent years, an alternative technology was developed to use engineered biochars (such as magnetic biochar) to remove various pollutants, including organics [19,20,21,22,23], Cr(VI) [24,25,26], Pb(II) [27,28,29] and arsenic [30,31,32]. For example, Yang et al. utilized magnetic biochar to remove Hg0 from the simulated combustion flue gas [33]. Magnetic biochar can be easily separated from liquid phase by a magnet after adsorption experiments. In addition, magnetite of magnetic biochar presents the high redox potential (2–5 eV) over wide pH range [34]. However, limited investigation regarding adsorption mechanism of Eu(III) on magnetic biochar by spectroscopic and modeling techniques were reported by far.

In this study, magnetite nanoparticles anchored biochar was synthesized by fast pyrolysis of FeCl2 pre-adsorbed biomass under N2 conditions. The objectives of this study were (1) to synthesize magnetic biochar by a facile method and characterize them using SEM, TEM, FT-IR, XRD and XPS techniques; (2) to investigate the effect of water chemistry on Eu(III) adsorption on magnetic biochar by batch techniques; and (3) to determine interaction mechanism of Eu(III) on magnetic biochar by XPS, EXAFS and modeling techniques. The highlight of this manuscript was to apply biochar-based adsorbent in practical environmental cleanup.

Experimental

Materials

The rice straw is a common crop residue, which was collected from a farm near the suburb of Shaoxing, China. Ferrous chloride hexahydrate (FeCl2·6H2O) and europium nitrate (Eu(NO3)3) of analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd and used as received. The deionized water (18.2 M, Nanopure water, Barnsead) was utilized in this study.

Synthesis of magnetic biochar

In this study, magnetic biochar was synthesized via thermal conversion of FeCl2 pre-adsorbed rice straw under N2 conditions. Briefly, the rice straw was firstly air-dried (moisture < 5%) and ground to 100 meshes. Then, 50 g of FeCl2 and 100 g of biomass were added to 100 mL DI water under vigorous stirring. After pre-adsorption equilibrium (24 h), the mixture was centrifuged at 6000 rpm for 10 min and then was dried at 70 °C for 6 h under glovebox conditions. The Fe(II)-loaded biomass were placed in lab-scale stainless-steel pyrolysis reactor and then heated at a heating rate of 20 °C/min to achieve five settled gradient temperatures (200, 300, 400, 500 and 600 °C) under a N2 atmosphere. The heating time at each gradient temperature was set 1 h to provide enough time for biomass carbonization and minimize volatile organic decompositions [35]. After cool to room temperature, the mixtures were gently crushed and sealed in a vacuum container before use.

Characterization of magnetic biochar

The microscopic morphology of as-prepared magnetic biochar was characterized by SEM (JEOL 6500F, Japan) equipped with an energy-dispersive X-ray analyzer and high resolution TEM (JEM-2010, Japan). FT-IR spectrometer (Thermo Nicolet IS10 Spectrometer, USA) was also used to identify the chemical functional groups in the range of 4000–400 cm−1 with a scan rate of 0.1 cm/s. Briefly, the sample was primarily freeze-dried for 24 h, and then ground with KBr powder (1:100) in an agate mortar. The disc was obtained by compressing it in a hydraulic press. The crystallographic mineralogy was identified by D/Max-IIIA Powder X-ray Diffractomer (Rigaku Corp., Japan) equipped with a graphite monochromator in the angular range from 5 to 60° with 0.02° of step size at 35 kV and 25 mA. Carbon, nitrogen, hydrogen content of biochar was analyzed by an element analyzer (Vario ELIII, Elementar, Germany). N2-BET and pore size of magnetic biochar were measured by NOVA 4200e Surface area and Pore size analyzer (Quantachrome, FL, USA). The zeta potentials of magnetic biochar were conducted using Malvern Zetasizer Nano ZS.

Batch adsorption experiments

The batch triple experiments of Eu(III) (10 mg/L) adsorption on magnetic biochar (m/v = 1.2 g/L) were conducted at 10 mL of polycarbonate centrifuge tube at room temperature under glovebox conditions. Adsorption kinetics and isotherms were examined at pH 3.0 under different time (5–2880 min) and concentration in the range of 10–50 mg/L, respectively. Briefly, 12 mg magnetic biochar was added into 10 mL Eu(III) solution (60 mg/L) with 0.01 mol/L NaCl electrolyte, and then pH values were adjusted to 3.0 by adding neglected volume of NaOH/HCl solutions (0.1–1.0 mol/L). Then suspensions were reacted at ambient conditions at a 200 rpm thermostatic reciprocating shaker for 24 h. After adsorption, the solid phase was separated by a magnet and supernatant was filtered through 0.22 μm nylon membrane filters. The Fe concentration in the supernatant was analyzed by Z-500 flame atomic absorption spectrophotometer (FAAS, Hitachi, Japan). The Eu(III) concentration in supernatant was measured using inductively coupled plasma-mass spectrometry (ICP-MS, Perkin-Elmer Plasma 3200). The adsorption amount of Eu(III) was calculated by the difference in the original and finial concentration after adsorption equilibrium.

Preparation and analysis of XPS, EXAFS samples

The samples for XPS and EXAFS analysis were prepared as the similar batch experiments. Briefly, 20 mg of magnetic biochar and 10 mL of 50 mg/L Eu(III) solutions was added into 50 mL polycarbonate centrifuge in glovebox conditions. The pH values and ionic strength were set to 3.0 and 0.01 mol/L, respectively. After reaction equilibrium, the dry solid and wet pastes were collected for the analysis of XPS and EXAFS, respectively. The XPS samples were performed by ESCALAB250 X-ray photoelectron spectrometer (Thermo-VG Scientific, UK) with an Mg–Kα radiation source of 1253.6 eV at 15 kV and 10 mA under 10−7 Pa. The binding energies were calibrated with the reference of the C 1s peak at 284.6 eV. The deconvolution of C 1s, O 1s and Eu 3d peaks was conducted using XPSPEAK 41 software. Europium LIII-edge EXAFS spectra were collected from BL14W of Shanghai Synchrotron Radiation Facility by Si (111) double crystal monochromator in fluorescence mode. The pre-treatment of EXAFS data was done using Athena program of IFEFFIT 7.0 software, and then Fourier transformed EXAFS spectra was fitted by Artemis interfaces with an aid of theoretical parameters [36, 37].

Surface complexation modeling

The pH-dependent adsorption and adsorption isotherms of Eu(III) on magnetic biochar was simulated by diffuse layer model of surface complexation modeling with an aid of MINTEQL 2.6 mode [38]. The protonation and deprotonation reactions can be described as Eqs. (1) and (2), respectively:

$${\text{SOH }} + {\text{ H}}^{ + } = {\text{ SOH}}_{ 2} ^{ + }{\text{log K}}^{ + } = \, \text {log}\left( \left[ {{\text{SOH}}_{ 2}^{ + } } \right]/\left( {\left[ {\text{SOH}} \right] ^* \left[ {{\text{H}}^{ + } } \right]} \right)\right)$$
(1)
$${\text{SOH }} = {\text{ SO}}^{ - } + {\text{ H}}^{ + } {\text{log K}}^{ - } = \text {log}\left(\left( \left[ {{\text{SO}}^{ - } } \right] ^* \left[ {{\text{H}}^{ + } } \right]\right) /\left[ {\text{SOH}} \right]\right)$$
(2)

where SOH refers to the amphoteric reactive sites of magnetic biochar. The values of log K+ and log K− were obtained by fitting the titration data of magnetic biochar in the presence of NaCl solutions.

Results and discussion

Characterization

The morphology of magnetic biochar was illustrated by SEM and TEM. As shown by SEM in Fig. 1a, magnetite nanoparticles were uniformly aggregated on the surface of biochar. The biochar networks can efficiently prevent the magnetite nanoparticles from aggregations. The SEM results indicated the improved the surface area and mass transfer for Eu(III) adsorption. As shown by high resolution TEM in Fig. 1b, the particle sizes of these nanoparticles ranged from 50 to 100 nm. In addition, these octahedral nanoparticles were achieved on the surface of biochar matrix due to the porous structure. As shown by fast Fourier transmission (FFT) analysis in Fig. 1b, the distances of main lattice lines were consistent with the distance of (311) plane of magnetite, indicating the formation of as-prepared magnetite nanoparticle. The further evidence was provided by XRD analysis. As shown in Fig. 1c, the diffraction peaks at 2θ = 30.2, 35.5, 43.2, 57.3 and 62.9° were indexed to (220), (311), (400), (511) and (440) planes of magnetite, indicating that magnetic biochar was successfully synthesized by one-step pyrolysis method [33]. The FeCl2-biomass was initially pyrolysed to Fe(OH)3/FeO(OH) and/or Fe3O4 at high temperature. Alternatively, the FeO(OH) can be further reduced to Fe3O4 by amorphous carbon during pyrolysis process. These reactions could be described by Eqs. (3)–(6):

$$6 {\text{FeCl}}_{ 2} + {\text{ 6H}}_{ 2} {\text{O }} + {\text{ O}}_{ 2} = {\text{ 2Fe}}_{ 3} {\text{O}}_{ 4} + {\text{ 12HCl}}$$
(3)
$$4 {\text{FeCl}}_{ 2} + { 1}0{\text{H}}_{ 2} {\text{O }} + {\text{ O}}_{ 2} = {\text{ 4Fe}}\left( {\text{OH}} \right)_{ 3} + {\text{ 8HCl}}$$
(4)
$${\text{Fe}}\left( {\text{OH}} \right)_{ 3} = {\text{ FeO}}\left( {\text{OH}} \right) \, + {\text{ H2O}}$$
(5)
$$1 2 {\text{FeO}}\left( {\text{OH}} \right) \, + {\text{ C }} = {\text{ 4Fe}}_{ 3} {\text{O}}_{ 4} + {\text{ CO}}_{ 2} + {\text{ 6H}}_{ 2} {\text{O}}$$
(6)
Fig. 1
figure 1

Characterization of magnetic biochar, a SEM; b TEM, inset of FFT analysis; c XRD; d FT-IR

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Figure 1d shows the FT-IR spectra of magnetic biochar. The FT-IR bands at 1735 and 1650 cm−1 were attributed to the stretching vibration of carboxyl C=O and aromatic C=C groups, respectively [39]. The FT-IR bands at 3400 and 3202 cm−1 were attributed to the vibrations of –OH groups [40]. As expected, FT-IR peak at 585 cm−1 was corresponded to the stretching vibration of Fe–O groups [41]. As shown in Table 1, the BET-N2 surface area of magnetic biochar (126.23 m2/g) was significantly higher than that of magnetite (72.51 m2/g), which could be attributed the release of small organic molecules and unconverted compositions of biomass at high pyrolysis temperature. The main contents of biochar were C (59.4%) and O (37.7%), whereas magnetic biochar presented the C (46.4%), Fe(19.4%) and O (31.3%) (Table 1). The characteristic results indicated the magnetic biochar was successfully synthesized by one-step pyrolysis method. The as-prepared magnetic biochar displayed the variety of oxygen-containing functional groups.

Table 1 The selected properties of magnetic biochar
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Adsorption kinetics

As shown in Fig. 2a, the adsorption of Eu(III) on magnetic biochar increased with increasing reaction time from 0 to 24 h, and then slightly increase of Eu(III) adsorption was observed at reaction time more than 24 h. Additionally, the adsorption of Eu(III) on magnetic biochar was slightly higher than that Eu(III) adsorption on magnetite and biochar. The data of adsorption kinetics were fitted by pseudo-first-order and pseudo-second-order kinetic models. The linear forms of pseudo-first-order and pseudo-second-order kinetic models can be described by Eqs. (7) and (8), respectively:

$${ \ln }\left( {Q{\text{e}} - Qt} \right) \, = { \ln }Q{\text{e }}{-}k_{ 1} t$$
(7)
$$t/Qt \, = { 1}/\left( {k_{ 2} Q{\text{e}}^{ 2} } \right) \, + \, t/Q{\text{e}}$$
(8)

where Qt and Qe (mg/g) refer to the amount of Eu(III) adsorbed on the adsorbents at time t and equilibrium, respectively. k1 and k2 are the corresponding adsorption rate constants of pseudo-first-order and pseudo-second-order model, respectively.

Fig. 2
figure 2

a Adsorption kinetics of Eu(III) on magnetite, biochar and magnetic biochar, CEu(III) = 10 mg/L, pH 3.0, m/v = 1.2 g/L, T = 293 K; b effect of pH on Eu(III) adsorption on magnetite, biochar and magnetic biochar, CEu(III) = 10 mg/L, m/v = 1.2 g/L, T = 293 K

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The fitted results and corresponding parameters were showed in Fig. 2a and Table 2, respectively. It is observed that the adsorption kinetics of Eu(III) on magnetite, biochar and magnetic biochar can be satisfactorily simulated by pseudo-second-order model with high correlation coefficient (R2 > 0.99) compared to pseudo-first-order model (R2 < 0.90), which were consistent with the previous studies [42,43,44].

Table 2 Kinetics parameters of pseudo-first-order and pseudo-second-order model for Eu(III) adsorption on magnetic biochar (1), raw biochar (2) and magnetite (3)
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pH effect

Figure 2b shows the effect of pH on Eu(III) adsorption on magnetite, biochar and magnetic biochar. Eu(III) adsorption on three adsorbents significantly increased with increasing pH from 2.0 to 7.0, and then high-level adsorption was observed at pH > 7.0. The increased adsorption of Eu(III) at pH 2.0–7.0 and pH > 7.0 could be attributed to the surface complexation and electrostatic attraction, respectively [13]. The adsorption of Eu(III) on magnetite was significantly lower than that of biochar and magnetic biochar. Eu(III) adsorption on magnetic biochar at pH < 4.0 and > 7.0 was slightly higher than that of biochar, whereas no change in Eu(III) adsorption on magnetic biochar and biochar was observed at pH 4.5–7.0. This results indicated that magnetic nanoparticles and biochar play an important role in Eu(III) adsorption on magnetic biochar at low and near neutral pH, respectively.

Adsorption isotherms and regeneration

Figure 3a shows the adsorption isotherms of Eu(III) on magnetite, biochar and magnetic biochar at pH 3.0 and 293 K. The adsorption of Eu(III) on magnetic biochar was significantly higher than that of magnetite. The data of adsorption isotherms were fitted by Langmuir and Freundlich models. The Langmuir and Freundlich model were described as Eqs. (9) and (10), respectively:

$${\text{qe}} = \left( {K \times {\text{qmax}} \times {\text{Ce}}} \right)/\left( { 1+ K \times {\text{Ce}}} \right)$$
(9)
$${\text{qe}} = {\text{Kf}} \times {\text{Ce}}^{\text{n}}$$
(10)

where qmax is the maximum adsorption amount of Eu(III) on magnetic biochar. K (L/mg) and Kf (mg(1−n)Ln/g) are the constants of Langmuir and Freundlich model, respectively. The fitted results and corresponding parameters were showed in Fig. 3a and Table 3, respectively. The adsorption of Eu(III) on magnetic biochar, biochar and magnetite can be satisfactorily simulated by Langmuir with high correlation coefficient (R2 > 0.995) compared to Freundlich model (R2 < 0.95). The maximum adsorption capacities of magnetite, biochar and magnetic biochar calculated from Langmuir model at pH 3.0 and 293 K were 88.45, 97.95 and 105.53 mg/g, respectively. These results showed that magnetic biochar can be used as a promising candidate in wastewater treatment to remove radionuclides from aqueous solutions.

Fig. 3
figure 3

a Adsorption isotherms of Eu(III) on magnetite, biochar and magnetic biochar; b regeneration of Eu(III) on magnetic biochar, pH 3.0, m/v = 1.2 g/L, T = 293 K

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Table 3 The parameters of Langmuir and Freundlich model for Eu(III) adsorption on magnetic biochar (1), raw biochar (2) and magnetite (3)
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Figure 3b shows the regeneration experiments of Eu(III) on magnetic biochar under five recycle times. The maximum adsorption capacities of magnetic biochar decreased from 105.53 mg/g at first time to 91.63 mg/g at fifth time. The regeneration experiments indicated that the adsorption efficiency of magnetic biochar maintained almost unchanged for five recycle times, indicating that the magnetic biochar represented a favorable recycle performance toward Eu(III) removal. The regeneration experiments indicated that magnetic biochar presented the excellent adsorption performance for Eu(III), recyclability and easy recovery.

XPS analysis

Figure 4a and b show the total scans and Eu 3d XPS spectra of magnetic biochar, respectively. As shown in Fig. 4a, the magnetic biochar displayed the C 1s, O 1s and Fe 2p peaks, whereas the Eu 3d peaks was observed for magnetic biochar after Eu(III) adsorption. It is observed that the change in the relative intensities and binding energies of O 1s were observed for magnetic biochar after Eu(III) adsorption, indicating that oxygen- containing functional groups were responsible for highy effective removal of Eu(III) on magnetic biochar [15]. In addition, the relative intensity of Eu 3d at pH 6.5 was significantly higher than that of Eu 3d at pH 3.0, suggesting that the high adsorption of Eu(III) at pH 6.0 was observed, consistent with the pH-dependent adsorption. As shown in Fig. 4b, two peaks of Eu 3d at 1135 and 1165 eV can be attributed to the Eu 3d5/2 and Eu 3d3/2, respectively. The results of XPS analysis indicated that magnetic biochar had abundant oxygen-containing functional groups, which was responsible for the Eu(III) adsorption.

Fig. 4
figure 4

XPS spectra of magnetic biochar before and after Eu(III) adsorption, a total scans; b Eu 3d

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EXAFS analysis

Figure 5a and b showed the k3-weighted Eu(III) EXAFS spectra and the corresponding Fourier transform (FT) data of samples, respectively. The absorption position at ~ 6984.1 eV revealed trivalent Eu in all samples [45]. As shown in Fig. 5a, a single wave frequency of aqueous Eu(III) monotonically decreased amplitude at k > 3 Å−1, whereas the evident frequencies of crystalline Eu(OH)3 was observed, which was ascribed to the ordered coordination shell [46]. These observations were attributed to the multiple backscattering paths in the first coordination shell and the higher atomic shells [47]. The broaden oscillation at k ~ 6.0 Å−1 for magnetic biochar after Eu(III) adsorption pH 3.0 and 6.5 indicated the formation of inner-sphere surface complexes [45]. As shown in Fig. 5b, the bond distances of Eu–O shell for magnetic biochar after Eu(III) adsorption pH 3.0 (2.43 Å) and 6.5 (2.41 Å) were shorter than that of Eu–O shell of aqueous Eu(III) (2.44 Å), whereas these bond distances were slightly larger than that of Eu(OH)3 (2.40 Å) (Table 4). In addition, the occurrence of Eu–C shells for magnatic biochar at pH 3.0 and 6.5 indicated the formation of inner-sphere surface complexation [48]. The FT features at ~ 2.0 Å could be due to the nearest coordination shell of oxygen atoms, which was consistent with the adsorption of Eu(III) on calcium silicate hydrates [47]. The coordination numbers of Eu–O shell for magnetic biochar after Eu(III) adsorption at pH 3.0 (CN = 6.7) and 6.5 (CN = 6.0) were significantly lower than that of aqueous Eu(III) (CN = 8.5), which further evidenced the inner-sphere surface complexation. In addition, the occurrence of Eu–Eu shell for magnetic biochar after Eu(III) adsorption at pH 6.5 was similar to the that of Eu–Eu shell of Eu(OH)3 standard, indicating that the adsorbed Eu(III) was gradually formed the surface co-precipitation (e.g., Eu(OH)3 (s)) at high pH conditions. EXAFS results indicated that the adsorption mechanism of Eu(III) and magnetic biochar over wide pH range was inner-sphere surface complexation, whereas the adsorbed Eu(III) was gradually formed the surface co- precipitation at high pH conditions.

Fig. 5
figure 5

Eu LIII-edge EXAFS spectra (a) and corresponding Fourier transforms (b) of reference samples and magnetic biochar after Eu(III) adsorption at pH 3.0 and 6.5, CEu(III) = 10 mg/L, m/v = 1.2 g/L, T = 293 K

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Table 4 EXAFS analysis of reference samples and Eu(III) adsorbed on magnetic biochar at Eu LIII-Edge, T = 293 K, I = 0.01 mol L−1 NaCl
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Surface complexation modeling

Figure 6a and b show the surface complexation modeling of Eu(III) removal on magnetic biochar at different pH and Eu(III) concentration, respectively. In this study, the double layer model was employed to simulate the adsorption behaviors with an aid of visual MINTEQ mode. The optimized parameters of Eu(III) on magnetic biochar can were summarized in Table 5. As shown in Fig. 6a, the adsorption of Eu(III) on magnetic biochar at different pH conditions can be satisfactorily by double layer model with two inner-sphere surface complexes (SOEu2+ and (SO)2Eu(OH) −2 species). It is observed that the main adsorbed species was SOEu2+ at pH < 4.0, whereas the (SO)2Eu(OH) −2 species dominated the Eu(III) adsorption at pH > 5.0. The same optimized parameters were utilized to simulate the data of isothermal adsorption at pH 4.0 (Fig. 6b). Enough interested, the adsorption isotherms of Eu(III) on magnetic biochar was successfully fitted by these two inner-sphere surface complexes derived from the pH-dependent adsorption. The main species was SOEu2+ species, which was consistent with the results of pH-dependent adsorption. The results of surface complexation modeling indicated that pH-dependent and isothermal adsorption of Eu(III) on magnetic biochar can be satisfactorily fitted by double layer model with two inner-sphere surface complexes such as SOEu2+ and (SO)2Eu(OH) −2 species.

Fig. 6
figure 6

Surface complexation modeling of Eu(III) adsorption at different pH (a) and concentration (b) conditions, m/v = 1.2 g/L, T = 293 K

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Table 5 Optimized parameters of surface complexation modeling for Eu(III) adsorption on magnetic biochar
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Conclusions

Magnetic biochar was successfully synthesized by fast pyrolysis of Fe(II)-preloaded biomass under N2 condition. The batch adsorption experiments showed that magnetic biochar was effective in enhancing adsorption performance towards Eu(III) compared to magnetite and raw biochar. According to XPS analysis, the high efficient adsorption of Eu(III) on magnetic biochar was attributed to oxygen-containing functional groups. The inner-sphere surface complexation and surface co-precipitation dominated the Eu(III) adsorption on magnetic biochar at low and high pH, respectively. The pH-dependent and isothermal adsorption of Eu(III) on magnetic biochar can be satisfactorily fitted by two inner-sphere surface complexes. These findings offer a new alternative to transform biomass waste into a promising adsorbent for radionuclides removal and further provide mechanistic insights of the interaction between radionuclides and biochar-based nanomaterials.