J.Cent.South Univ.(2025) 32: 962－976
Graphic abstract:
1 Introduction
Rare earth elements (REEs), including lanthanides, yttrium and scandium, are valuable raw materials for modern industries [1]. They are widely used in superconductors, photocatalysis, petrochemicals, military, and other fields due to their distinctive physical properties such as fluorescence and magnetism [2]. Ion-adsorption rare earth (IARE) ore is currently one of the primary sources of REEs [3], contributing to approximately 35% of China’s total REEs output and approximately 80% of the world’s heavy REEs production [4]. Nowadays, REEs are extracted from IARE ores using the in-situ (NH4)2SO4 leaching process [5]. However, the leaching process also promotes the diffusion of REEs into the environment. Wastewater containing rare earth ions (RE3+) is toxic and poses a threat to human health [6]. Prolonged exposure or ingestion of REEs has adverse effects on health and metabolism due to the accumulation in the body’s organs [7]. Numerous studies have demonstrated serious contamination of surface water in mining areas with REEs [1, 8, 9]. In addition to endangering the environment, the immediate dumping of this REEs-rich wastewater results in the waste of rare earth resources. Therefore, it is of profound significance to remove and recover REEs from rare earth mine wastewater in an efficient, sustainable and cost-effective manner.
Various methods, including ion exchange, chemical precipitation, solvent extraction, and adsorption, have been used to collect REEs from wastewater [10]. Among these methods, adsorption is considered the most promising strategy due to its simplicity in handling [11]. However, the main challenge is the development of efficient and cost-effective adsorbents. Schwertmannite (Sch, Fe8O8(OH)8-2x(SO4)x, 1_Xml/alternativeImage/EBEBB350-CD9E-4a06-97CC-4E5406A35B9D-M001c.jpg)
Herein, in this study, Jar and Sch were synthesized via Acidithiobacillus ferrooxidans (A. ferrooxidans) and then mechanically activated (M-Jar and M-Sch) for the adsorption of REEs in mixed RE3+ solutions. Mechanical activation refers to the process of enhancing the adsorption performance of adsorbents through the application of mechanical forces. This technology is simple, environmentally friendly, and cost-effective, as it does not require any chemical synthesis [21]. Here, knowledge gaps to be filled include: 1) the adsorption performance of the four adsorbents (Jar, Sch, M-Jar, M-Sch); 2) the characterization and comparison of the adsorbents using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller (BET) method, scanning electron microscope-energy dispersive spectroscopy (SEM-EDS) and transmission electron microscope (TEM); 3) the effect of reaction time and pH on adsorption, and the comprehensive study of kinetics, isotherm and thermodynamics; 4) a possible adsorption mechanism; and 5) the reusability of the adsorbents and their ability to adsorb RE3+ from actual mine wastewater. The findings of this study are expected to provide new material options for the recovery of various REEs from wastewater and theoretical guidance for the resource utilization of jarosite and schwertmannite.
2 Material and methods
2.1 Strain and reagents
A. ferrooxidans (ATCC 23270) was obtained from the Key Laboratory of Biohydrometallurgy of Ministry of Education, Central South University, and was cultivated and collected as shown in Refs. [22, 23]. The RE3+ mixture solution for the adsorption experiment was prepared by diluting the standard solution (100 mg/L) [24]. All chemicals utilized were analytical grade reagents (AR) and could be used directly. The mine wastewater was collected from an IARE mine in Hunan, China, and the types and concentrations of the main metal ions present in it are detailed in Table S1.
2.2 Materials preparation
The detailed preparation of Jar, Sch, M-Jar, and M-Sch was described in the Supporting Information Section 3.
2.3 Adsorption and desorption experiments
The detailed experimental steps were described in the Supporting Information Section 4.
2.4 Theoretical fundamentals
The information on theoretical fundamentals was described in the Supporting Information Section 5.
2.5 Characterization
The XRD patterns of Jar, Sch, M-Jar, and M-Sch were obtained using a powder diffractometer (D8 ADVANCE) with a scanning range of 4°-80° and a step of 0.02° to analyze the mineral phases [25]. Furthermore, a Fourier infrared spectrometer (Nicolet iS5) operating in the range of 4000- 400 cm-1 was used to track comparative changes in functional groups present on the surface of the adsorbent [26]. The specific surface area and average pore size of the sample were determined using a specific surface area tester (Quantachrome Instruments) and the BET method. The isoelectric point was measured with the aid of a Zeta potential analyzer (Brookhaven Instruments). Besides that, the surface morphology, element content, and size of the materials were evaluated using SEM-EDS and TEM [27]. X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI) was chosen to characterize the elemental compositions of the materials before and after adsorption.
3 Results and discussion
3.1 Comparison of sorbent material
The adsorption efficiencies of four materials (Jar, Sch, M-Jar, M-Sch) for single RE3+ (La3+, Nd3+ and Y3+) were investigated as a function of exposure time (Figure S1, Supporting Information Section 6). The results indicated that the RE3+ adsorption and reactivity of jarosite and schwertmannite were greatly enhanced by mechanical activation. Therefore, M-Jar and M-Sch were employed in the subsequent adsorption experiments of RE3+.
3.2 Characterizations of Jar, Sch, M-Jar, and M-Sch
The biosynthetic Jar was an ochre-yellow powder (Figure 1(a)), while Sch was an ochre-red powder (Figure 1(b)). The XRD diffraction peaks of Jar were sharp and intense, indicating excellent crystallization of the sample. Five major diffraction peaks of the sample for the planes of (012), (021), (113), (033) and (220) are at 2θ=17.408°, 28.680°, 28.966°, 45.862° and 49.931°, respectively (Figure 1(c)). These peaks perfectly matched the five characteristic peaks of the standard product (#22-0827 jarosite), with no other irregular peaks observed, proving that the synthesized sample was pure jarosite [28]. On the other hand, the XRD pattern of Sch illustrated two broad peaks at 2θ of 26.302° and 35.305° (Figure 1(d)), which indicated that the structure of the biosynthetic Sch was amorphous [14]. Comparison of XRD spectra before and after mechanical activation revealed that the mineral phases of jarosite and schwertmannite remained unchanged by mechanical activation.
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Figure 1(e) shows the FTIR spectra of Jar and Sch. The strong peak observed in the 2900- 3700 cm-1 region was attributed to the stretching vibration of -OH (νOH) [26]. Peaks at 1631.5 to 1641.0 cm-1 were caused by the deformation of H-O-H and represented the moisture content in the sample. The double state vibration of SO42-, which resulted in the peaks at 1181.0 and 1085.4 cm-1 of Jar, suggested the existence of two types of SO42- in Jar. One type was found within the molecular structure, and the other type was adsorbed on the molecular surface [29]. However, Sch exhibited a solitary SO42- peak at 1108.5 cm-1. Additionally, the peak at 1001.2 cm-1 was attributed to the in-plane bending vibration of the hydroxyl group (δOH), and the peaks near 471.85 and 506.84 cm-1 were associated with the vibrations of the octahedral ligand FeO6 [30]. After mechanical activation, the peaks of M-Jar and M-Sch were found to be stronger compared to Jar and Sch. This inferred that mechanical activation increased the amount of -OH and SO42- groups on the surface of jarosite and schwertmannite, which was favourable for adsorption.
Figure 1(f) displays the N2 adsorption-desorption isotherms of Jar and Sch before and after mechanical activation. Based on the isotherm classification [31], the N2 adsorption-desorption curves of the four adsorbents could be classified as type IV mode, which indicated that all four adsorbents possessed pore structures made up of mesopores and micropores. The calculated specific surface areas of Jar, Sch, M-Jar and M-Sch were 4.31, 24.23, 29.26 and 180.40 m2/g, respectively. Moreover, the average pore sizes for Jar, Sch, M-Jar, and M-Sch were determined to be 2.16, 10.54, 4.83 and 21.02 nm, respectively. These results demonstrated that mechanical activation increased both the specific surface area (approximately 6.8-7.4 times) and the average pore size, which was beneficial for improving the adsorption properties [32]. It was worth noting that M-Sch exhibited a significantly larger specific surface area compared to M-Jar, which favoured the exposure of more active sites for RE3+ adsorption.
The morphologies and major elements of Jar and Sch were examined using SEM-EDS (Figure 2). The Jar displayed a compact hydrangea-like structure, with particles measuring approximately 18 μm (Figure 2(a)). The predominant components observed were O, S, K, and Fe (Figure 2(b)). On the other hand, the Sch appeared as a sea urchin-like spherical structure [33], covered with numerous villous structures, and had a diameter of approximately 10 μm (Figure 2(d)). The surface elements of Sch were primarily composed of O, S, and Fe (Figure 2(e)). It was discovered that Sch was smaller in size compared to Jar, with a rougher surface and no potassium ions. After mechanical activation, M-Jar exhibited a uniform sheet shape with a size range of 80-800 nm (Figure 2(c)). In contrast, the M-Sch had a smaller size ranging from 30 to 100 nm and appeared as a paper film with folded surfaces and needle clusters at the edges (Figure 2(f)). According to reports, the specific surface area increased as the particle size decreased [16]. Consequently, mechanical activation increased the specific surface area of Jar and Sch, with M-Sch demonstrating the largest specific surface area due to its unique tunnel structure.
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3.3 Factors affecting the adsorption efficiency of RE3+
Figures 3(a) and (b) depict the variation in adsorption efficiencies of RE3+ by M-Jar and M-Sch over time. The adsorption efficiency of RE3+ by M-Jar and M-Sch presented a two-stage evolution: rapid adsorption for the first 20 min, followed by slow adsorption after 20 min. The adsorption process then reached equilibrium after approximately 60 min. At this time, the adsorption efficiencies of M-Jar for La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+ and Y3+ were 19.75%, 22.52%, 22.68%, 24.15%, 24.37%, 27.14%, 31.96% and 15.78%, respectively. Similarly, the adsorption efficiencies of M-Sch for La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+ and Y3+ were 53.55%, 64.60%, 65.11%, 68.30%, 68.31%, 69.51%, 72.06% and 50.13%, respectively. The results demonstrated that M-Sch had a significantly higher adsorption efficiency for RE3+ compared to M-Jar, mainly due to its larger specific surface area [34]. Moreover, the adsorption efficiencies of the adsorbents on RE3+ followed the order of Y3+<La3+<Ce3+<Pr3+<Nd3+<Sm3+<Gd3+<Dy3+. It was noted that the adsorption efficiencies of HREEs were relatively higher than those of LREEs, which could be attributed to the lanthanide contraction effect, that is, the decrease in ionic size with an increase in atomic size and weight [35]. However, despite belonging to HREEs, Y has a relatively large ionic size and different electronic configuration levels compared to other HREEs members [36]. As a result, the adsorption efficiency of Y3+ was the lowest in the multi-component system.
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pH plays a crucial role in the ionization of functional groups, the surface charge of adsorbents, and the occurrence state of RE3+ in solution, so it is an important factor affecting the adsorption process [37, 38]. The zeta potential of M-Jar and M-Sch was measured at different pH, revealing that the points of zero charge (pHzpc) for M-Jar and M-Sch were 5.9 and 5.4, respectively (Figure S2). When pH < pHzpc, the surface of the adsorbent carried positive charges. Conversely, when pH>pHzpc, the surface of the adsorbent underwent deprotonation, leading to an enhancement in surface electronegativity [39]. Figures 3(c) and (d) depict the effect of pH on the adsorption of RE3+ on M-Jar and M-Sch. When pH<3, the adsorption efficiencies of RE3+ by M-Jar and M-Sch were both below 20%. This was partly due to the high concentration of H+ in the solution, which competed with RE3+ for the available active sites on the surface of M-Jar and M-Sch [40]. Additionally, the existence of electrostatic repulsion also prevented the adsorption of cations because the surface of M-Jar and M-Sch was positively charged. As pH increased, the adsorption efficiency improved, especially for Sch. At pH 6, the adsorption efficiencies of RE3+ on M-Jar and M-Sch were approximately 30% and 98%, respectively. The reason for the significant increase in the adsorption capacity was due to the presence of the negatively charged surfaces of M-Sch and M-Jar at that pH. However, M-Sch had more negative charges (Figure S2), resulting in a stronger electrostatic attraction between RE3+ and M-Sch. At pH 7, both adsorbents adsorbed all of the RE3+ in the solution. Additionally, the distribution of hydrolysis products of RE3+ in aqueous solutions was calculated using Visual MINTEQ 3.1 (Figure S3) [41]. The results indicated that RE3+ hydrolysis products largely existed in three forms (RE3+, RE(OH)2+ and RE(OH)3(s)) in aqueous solutions at a pH range of 0-14. La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+ and Y3+ started to form hydroxide precipitates at pH 8.2, 8.2, 8.0, 7.4, 6.8, 6.4, 6.8 and 7.4, respectively. When pH
3.4 Kinetics, isotherms, and thermodynamic studies
3.4.1 Adsorption kinetics
Adsorption kinetics is an important aspect that provides useful information on the adsorption behavior [41]. The experimental data in Figures 3(a) and (b) were fitted using pseudo-first-order and pseudo-second-order models, and the results are shown in Figures S4 and S5 and Table 1. The pseudo-second-order kinetic model outperformed the pseudo-first-order kinetic model for all experimental data. This suggested that chemical adsorption, such as the sharing or exchanging of electrons between ions [42] or surface complexation [41], was dominant in the adsorption of RE3+ on M-Jar and M-Sch. In addition, the correlation coefficients of the pseudo-first-order kinetic model were all above 0.9, indicating that diffusion also played a significant role throughout the adsorption process.
| Adsorbent | Ion | qe,exp/(mg·g-1) | Pseudo-first-order | Pseudo-second-order | ||||
|---|---|---|---|---|---|---|---|---|
| qe,cal/(mg·g-1) | K1/min-1 | R2 | qe,cal/(mg·g-1) | K2/(g·mg-1·min-1) | R2 | |||
| M-Jar | La3+ | 0.99 | 0.95 | 0.48 | 0.993 | 1.00 | 0.85 | 0.999 |
| Ce3+ | 1.13 | 1.10 | 0.68 | 0.993 | 1.14 | 1.30 | 0.999 | |
| Pr3+ | 1.13 | 1.10 | 0.60 | 0.993 | 1.14 | 1.05 | 0.999 | |
| Nd3+ | 1.21 | 1.19 | 0.71 | 0.991 | 1.22 | 1.24 | 0.999 | |
| Sm3+ | 1.22 | 1.18 | 0.54 | 0.987 | 1.23 | 0.80 | 0.999 | |
| Gd3+ | 1.36 | 1.32 | 0.74 | 0.989 | 1.36 | 1.17 | 0.999 | |
| Dy3+ | 1.60 | 1.57 | 1.04 | 0.997 | 1.59 | 2.03 | 1.000 | |
| Y3+ | 0.79 | 0.76 | 0.45 | 0.983 | 0.80 | 0.98 | 0.999 | |
| M-Sch | La3+ | 2.69 | 2.58 | 0.40 | 0.991 | 2.66 | 0.40 | 0.998 |
| Ce3+ | 3.22 | 3.11 | 0.44 | 0.993 | 3.20 | 0.40 | 0.999 | |
| Pr3+ | 3.26 | 3.21 | 0.62 | 0.998 | 3.25 | 0.94 | 1.000 | |
| Nd3+ | 3.44 | 3.34 | 0.49 | 0.995 | 3.41 | 0.48 | 0.999 | |
| Sm3+ | 3.45 | 3.34 | 0.51 | 0.996 | 3.41 | 0.52 | 0.999 | |
| Gd3+ | 3.48 | 3.40 | 0.48 | 0.996 | 3.47 | 0.47 | 0.999 | |
| Dy3+ | 3.63 | 3.52 | 0.49 | 0.995 | 3.60 | 0.45 | 0.999 | |
| Y3+ | 2.52 | 2.43 | 0.46 | 0.993 | 2.50 | 0.55 | 0.999 | |
3.4.2 Adsorption isotherms
The effect of RE3+ concentration on the adsorption efficiency was studied at room temperature. The findings revealed that the adsorption capacity of RE3+ on M-Jar and M-Sch increased with the increase of the initial concentration of RE3+. This was attributed to the presence of a large number of blank adsorption sites on the surface of the adsorbent, and the adsorption equilibrium was determined by the saturation of the binding sites for RE3+. Moreover, Langmuir and Freundlich isotherm models were used to fit the equilibrium adsorption data for RE3+ on M-Jar and M-Sch (Figures S6 and S7), and the corresponding fitting parameters are shown in Table 2. For M-Jar, the Langmuir model was more appropriate, which implied that the adsorption of RE3+ on M-Jar followed a monolayer adsorption pattern [41]. The adsorption only occurred at specific homogeneous sites within the adsorbent, and no additional adsorption took place once the active sites were coated with ions [30]. This resulted in the lower adsorption efficiency of RE3+ on jarosite, which was also closely related to the good crystallization, low specific surface area, and dense surface morphology. In addition, the adsorption affinity of RE3+ on the M-Jar surface followed the order of Dy3+ > Gd3+ > Sm3+ > Nd3+ > Pr3+ > Ce3+ > La3+ > Y3+ based on the qmax values obtained by fitting (Table 2). The relative order was consistent with the chemical properties of RE3+, such as their electronegativity and size [35]. This result was also complementary to the results in Figures 3(a) and (b). However, for M-Sch, the data fitted well with the Freundlich model (0.987<R2<0.998), and the adsorption strength n was greater than 1, indicating that adsorption was desirable [30]. Furthermore, 0.1<1/n<0.5 showed that the adsorbate was easily adsorbed on the surface of the adsorbent [8], and the multilayer adsorption occurred on the heterogeneous surface [16]. This was in agreement with the amorphous structure, villous morphology, and large specific surface area of schwertmannite, leading to a higher adsorption efficiency of RE3+.
| Adsorbent | Ion | Langmuir constants | Freundlich constants | ||||
|---|---|---|---|---|---|---|---|
| qmax/(mg·g-1) | kL/(L·mg-1) | R2 | KF/(mg·g-1) | 1/n | R2 | ||
| M-Jar | La3+ | 9.101 | 0.051 | 0.999 | 0.882 | 0.531 | 0.987 |
| Ce3+ | 9.377 | 0.054 | 0.994 | 0.955 | 0.523 | 0.978 | |
| Pr3+ | 9.873 | 0.055 | 0.990 | 1.003 | 0.524 | 0.972 | |
| Nd3+ | 10.832 | 0.050 | 0.989 | 0.992 | 0.545 | 0.978 | |
| Sm3+ | 10.980 | 0.054 | 0.990 | 1.080 | 0.532 | 0.972 | |
| Gd3+ | 12.615 | 0.054 | 0.997 | 1.239 | 0.535 | 0.981 | |
| Dy3+ | 14.697 | 0.048 | 0.999 | 1.272 | 0.561 | 0.990 | |
| Y3+ | 8.107 | 0.039 | 0.999 | 0.603 | 0.574 | 0.992 | |
| M-Sch | La3+ | 32.881 | 0.214 | 0.934 | 8.479 | 0.394 | 0.997 |
| Ce3+ | 34.432 | 0.216 | 0.922 | 9.289 | 0.379 | 0.992 | |
| Pr3+ | 35.575 | 0.229 | 0.924 | 9.903 | 0.375 | 0.990 | |
| Nd3+ | 38.084 | 0.219 | 0.925 | 10.125 | 0.391 | 0.991 | |
| Sm3+ | 38.666 | 0.247 | 0.934 | 10.660 | 0.394 | 0.995 | |
| Gd3+ | 38.704 | 0.298 | 0.929 | 11.612 | 0.382 | 0.998 | |
| Dy3+ | 39.544 | 0.328 | 0.913 | 12.481 | 0.375 | 0.998 | |
| Y3+ | 32.742 | 0.182 | 0.911 | 8.381 | 0.377 | 0.987 | |
To evaluate the adsorption capacity of M-Jar and M-Sch for RE3+ in comparison to other materials, the results of this study were compared with the previously reported adsorption capacities of adsorbents in RE3+ mixture solutions (Table S2). The results demonstrated that M-Sch exhibited a favorable adsorption capability for mixed RE3+. Moreover, schwertmannite is an abundant and inexpensive by-product, and mechanical activation is a simple technique that does not involve complex chemical synthesis processes [19, 21]. Therefore, M-Sch showed promise as a cost-effective biosorbent for the removal and recovery of RE3+ from aqueous solutions.
3.4.3 Adsorption thermodynamics
Figure S8 demonstrates the linear plot of lnKd versus 1/T, which could obtain the thermodynamic parameters of M-Jar and M-Sch for the adsorption of RE3+, including Gibbs free energy (_Xml/alternativeImage/EBEBB350-CD9E-4a06-97CC-4E5406A35B9D-M007c.jpg)
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| Adsorbent | Ion | _Xml/alternativeImage/EBEBB350-CD9E-4a06-97CC-4E5406A35B9D-M004c.jpg) | _Xml/alternativeImage/EBEBB350-CD9E-4a06-97CC-4E5406A35B9D-M005c.jpg) | _Xml/alternativeImage/EBEBB350-CD9E-4a06-97CC-4E5406A35B9D-M006c.jpg) | R2 | ||
|---|---|---|---|---|---|---|---|
| 298 K | 308 K | 318 K | |||||
| M-Jar | La3+ | 3.321 | 3.175 | 2.940 | 8.971 | 18.913 | 0.985 |
| Ce3+ | 3.133 | 2.963 | 2.763 | 8.645 | 18.481 | 0.998 | |
| Pr3+ | 2.922 | 2.717 | 2.561 | 8.313 | 18.116 | 0.994 | |
| Nd3+ | 2.783 | 2.646 | 2.426 | 8.080 | 17.731 | 0.983 | |
| Sm3+ | 2.640 | 2.505 | 2.287 | 7.878 | 17.534 | 0.983 | |
| Gd3+ | 2.290 | 2.108 | 1.943 | 7.457 | 17.349 | 0.999 | |
| Dy3+ | 2.076 | 1.942 | 1.730 | 7.212 | 17.194 | 0.982 | |
| Y3+ | 4.158 | 3.912 | 3.709 | 10.846 | 22.467 | 0.997 | |
| M-Sch | La3+ | -2.868 | -3.564 | -4.744 | 25.005 | 93.282 | 0.943 |
| Ce3+ | -3.149 | -3.899 | -4.931 | 23.352 | 88.783 | 0.977 | |
| Pr3+ | -3.449 | -4.263 | -5.156 | 21.983 | 85.299 | 0.998 | |
| Nd3+ | -3.607 | -4.460 | -5.462 | 24.007 | 92.588 | 0.994 | |
| Sm3+ | -3.944 | -4.888 | -5.738 | 22.795 | 89.778 | 0.997 | |
| Gd3+ | -4.315 | -5.377 | -6.370 | 26.318 | 102.834 | 0.999 | |
| Dy3+ | -4.516 | -5.652 | -7.162 | 34.843 | 131.883 | 0.982 | |
| Y3+ | -2.603 | -3.254 | -4.336 | 23.148 | 86.189 | 0.947 | |
3.5 Adsorption mechanism
The elemental compositions of M-Jar and M-Sch were analyzed before and after the adsorption of RE3+ using XPS and EDS. The full-range survey XPS spectra were analyzed using Avantage software (Figures 4(a) and (c)), revealing characteristic peaks of REEs on both M-Jar and M-Sch surfaces after adsorption (La 3d3/2 851.90 eV, La 3d5/2 833.98 eV, Ce 3d3/2 898.08 eV, Ce 3d5/2 880.88 eV, Pr 3d3/2 950.93 eV, Pr 3d5/2 930.87 eV, Nd 3d3/2 1001.01 eV, Nd 3d5/2 978.08 eV, Sm 3d3/2 1111.08 eV, Sm 3d5/2 1081.08 eV, Gd 4d 139.94 eV, Dy 3d3/2 1334.13 eV, Dy 3d5/2 1297.08 eV, Y 3d3/2 159.08 eV, Y 3d5/2 157.02 eV). EDS spectra of M-Jar and M-Sch also showed characteristic peaks of REEs after the adsorption of RE3+ (Figure S9, Figures 4(b) and (d)), confirming the adsorption of RE3+ by M-Jar and M-Sch. Additionally, the elemental mapping results (Figures S10 and 11) indicated that RE3+ was evenly distributed on the surfaces of M-Jar and M-Sch. Furthermore, the changes in functional groups before and after the adsorption of RE3+ were characterized using FTIR spectroscopy (Figure 5). The data revealed that the main functional groups of M-Jar and M-Sch were -OH (3323, 3385 and ~1000 cm-1), H-O-H (1635 cm-1), SO42- (1087-1186 cm-1), and FeO6 coordination octahedra (475-695 cm-1) [14]. After the adsorption of RE3+, the -OH and SO42- contents decreased significantly, suggesting that the main mechanisms of RE3+ adsorption were electrostatic adsorption and surface complexation. The -OH and SO42- groups played a key role in the complexation process. However, the decrease of the -OH and SO42- content on the surface of M-Sch was even more noticeable compared to that of M-Jar, indicating that more groups were engaged in the adsorption process of M-Sch. This was related to the larger specific surface area of M-Sch, which provided more adsorption sites. Therefore, the adsorption efficiency of RE3+ by M-Sch was higher than that of M-Jar. After the adsorption of RE3+, the K content on the surface of M-Jar also decreased (Figure 4(e)). Additionally, K+ was detected in the solution by ICP-MS, and approximately 10% K of jarosite was dissolved. However, when M-Jar in solution was stirred for 1 h without RE3+, K+ was not detected. This suggested the presence of ion exchange between K+ and RE3+. Moreover, the lattice spacing of M-Jar transformed from 0.380 nm to 0.359 nm after the adsorption of RE3+, indicating that K+ was replaced by RE3+ and the crystal structure of M-Jar altered (Figures 4(f)-(g)). Consequently, a schematic diagram illustrating the adsorption mechanism of RE3+ on M-Jar and M-Sch was proposed (Figure 6).
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3.6 Applications
Based on the above analysis, it could be seen that M-Sch exhibited a notable adsorption capacity for RE3+ without any structural changes, so only the application of M-Sch was verified.
3.6.1 Stability
For practical applications, the ability to regenerate and recycle the adsorbent was essential [41]. In this work, the desorption experiments were carried out using a 0.5 mol/L nitric acid solution. As shown in Figure 7, the adsorption efficiency decreased slightly with increasing number of cycles, which could be attributed to the loss of M-Sch during the washing and centrifugation steps in the collection process [41]. Nevertheless, the adsorption efficiency of RE3+ by M-Sch remained relatively high (>91.78%) after 5 adsorption-desorption cycles. The results indicated that M-Sch exhibited good regeneration ability and had potential for practical applications.
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3.6.2 Removal of RE3+ from actual mine wastewater
The potential of M-Sch for practical application was investigated by treating the IARE mine wastewater. Table 4 displays the major metal ion compositions of the wastewater before and after treatment. M-Sch demonstrated a high adsorption efficiency (>92%) for RE3+ in the wastewater. However, it also exhibited high adsorption efficiencies for other metal cations such as K+, Na+, Mg2+, Al3+ and Mn2+. The adsorption efficiencies for these cations were 92.39%, 88.47%, 95.31%, 98.64% and 91.85%, respectively. This suggested that M-Sch could effectively adsorb RE3+ from mine wastewater, although the adsorption process was non-selective. To recover the adsorbed RE3+, additional separation and purification would be necessary. Therefore, future research will focus on modifying M-Sch to selectively recover RE3+ or exploring specific eluents.
| Ion | Concentration before adsorption/(mg·L-1) | Concentration after adsorption/(mg·L-1) | Adsorption efficiency/% |
|---|---|---|---|
| Y3+ | 3.019 | 0.220 | 92.72 |
| La3+ | 1.630 | 0.087 | 94.64 |
| Nd3+ | 1.461 | 0.074 | 94.91 |
| Dy3+ | 0.432 | 0.021 | 95.25 |
| Gd3+ | 0.399 | 0.019 | 95.23 |
| Pr3+ | 0.356 | 0.018 | 94.84 |
| Sm3+ | 0.341 | 0.017 | 94.93 |
| Ce3+ | 0.327 | 0.017 | 94.73 |
| Ca2+ | 34.884 | 3.569 | 89.77 |
| K2+ | 11.151 | 0.849 | 92.39 |
| Na+ | 7.502 | 0.865 | 88.47 |
| Mg2+ | 5.982 | 0.281 | 95.31 |
| Al3+ | 3.060 | 0.042 | 98.64 |
| Mn2+ | 2.025 | 0.165 | 91.85 |
4 Conclusions
In this study, jarosite and schwertmannite were successfully biosynthesized and mechanically activated. The process proved to be environmentally friendly and easy to operate [38]. Additionally, the mechanical activation process significantly increased the adsorption efficiency of RE3+ by jarosite and schwertmannite. This improvement was attributed to the decrease in size, increase in specific surface area, and higher concentration of -OH and SO42- groups on the surface of the mechanically activated materials. The adsorption affinities of RE3+ on M-Jar and M-Sch surfaces followed the order of Dy3+ > Gd3+ > Sm3+ > Nd3+ > Pr3+ > Ce3+ > La3+ > Y3+. The optimal pH for the adsorption of RE3+ by both M-Jar and M-Sch was 6. The maximum adsorption capacities for La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+, and Y3+ by M-Jar were 9.101, 9.377, 9.873, 10.832, 10.980, 12.615, 14.697 and 8.107 mg/g, respectively. The maximum adsorption capacities for La3+, Ce3+, Pr3+, Nd3+, Sm3+, Gd3+, Dy3+ and Y3+ by M-Sch were 32.881, 34.432, 35.575, 38.084, 38.666, 38.704, 39.544 and 32.742 mg/g, respectively. The adsorption of RE3+ on M-Jar occurred through a monolayer chemical adsorption process, while the adsorption on M-Sch was a multilayer chemical adsorption process. The adsorption mechanisms of M-Jar for RE3+ included electrostatic adsorption, surface complexation, and ion exchange. Similarly, the adsorption mechanisms of M-Sch for RE3+ involved electrostatic adsorption and surface complexation. The adsorption capacity of M-Sch was higher than that of M-Jar due to its larger specific surface area and lower zero charge point, and more -OH and SO42- groups of M-Sch participated in the adsorption process. Additionally, M-Sch demonstrated excellent regeneration ability and could effectively adsorb RE3+ from mine wastewater. These findings highlight the potential of M-Sch as a cost-effective biological adsorbent. Overall, this study provided a valuable strategy for the resource utilization of jarosite and schwertmannite, as well as the removal and recovery of mixed RE3+ in aqueous solutions. However, future research should aim to modify M-Sch to selectively recover RE3+ or investigate the use of specific eluents.
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