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Article

Rare Earth Element Recovery and Hydrochar Evaluation from Hyperaccumulator by Acid Leaching and Microwave-Assisted Hydrothermal Carbonization

by
Shiyu Li
,
Bin Ji
and
Wencai Zhang
*
Department of Mining and Minerals Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(3), 277; https://doi.org/10.3390/min14030277
Submission received: 14 January 2024 / Revised: 10 February 2024 / Accepted: 20 February 2024 / Published: 6 March 2024
(This article belongs to the Section Mineral Processing and Extractive Metallurgy)
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Abstract

:
Phytomining is a sustainable approach that uses hyperaccumulators for critical element extraction from various substrates, such as contaminated soils, mine tailings, and aqueous solutions. In this study, grass seeds were fed with a solution containing Y, La, Ce, and Dy, resulting in around 510 mg/kg (dry basis) of total rare earth elements (TREEs) accumulated in grass leaves. Electron probe microanalyzer (EPMA) analysis showed that rare earth elements (REEs) in the grass leaves (GL) predominantly complexed with phosphorous (P). Around 95% of Y, 93% of La, 92% of Ce, and 93% of Dy were extracted from the GL using 0.5 mol/L H2SO4 at a solid concentration of 5 wt.%. Subsequently, microwave-assisted hydrothermal carbonization (MHTC) was used to convert the leaching residue into hydrochar to achieve a comprehensive utilization of GL biomass. The effect of temperature on the structural properties and chemical composition of the resulting hydrochar was evaluated. Scanning electron microscopy (SEM) analysis revealed that the original structure of GL was destroyed at 180 °C during MHTC, producing numerous microspheres and pores. As the reaction temperature increased, there was a concurrent increase in carbon content, a higher heating value (HHV), and energy densification, coupled with a decrease in the hydrogen and oxygen contents of hydrochar. The evolution of H/C and O/C ratios indicated that dehydration and decarboxylation occurred during MHTC. The results showed that the waste biomass of the GL after REE extraction can be effectively converted into energy-rich solid fuel and low-cost adsorbents via MHTC.

Graphical Abstract

1. Introduction

Rare earth elements (REEs) have been widely used in modern life and industries, owing to their exceptional magnetic, optical, and electrical properties [1,2,3]. The increasing demand for REEs is leading to the expansion of the mining scale, which poses a serious threat to the environment. Particularly, soil pollution emerges as a significant concern among all environmental issues due to its threats to human health. Soil contamination also leads to infertility and lowers productivity [4]. Therefore, REE-enriched soil remediation and REE recovery from contaminated soils have gained more attention.
REEs are usually found in trace quantities in soils, and extraction costs may be inhibitive for REE recovery. Meanwhile, conventional technologies for REE extraction and purification are energy-demanding and chemical-intensive [4,5,6]. In the 20th century, specific plants called hyperaccumulators were found to have the ability to extract REEs from various substrates, such as contaminated soils, mine tailings, and aqueous solutions [7,8]. Dicranopteris linearis, growing in an REE mining area located in Fujian Province, China, can accumulate up to 3045 mg/kg of REEs. Dicranopteris linearis, collected from a non-mining area located in Guangxi Province, China, where the REE concentration in the associated soil was ~15 mg/kg, can accumulate 1914 mg/kg REEs in its fronds [9]. Pronephrium simplex can accumulate up to 1200 mg/kg REEs under natural growth conditions [10]. Therefore, hyperaccumulators are regarded as promising remediators for polluted soils and potential resources for REE recovery.
Several processes have been developed for the recovery of REEs from hyperaccumulators due to the increasing importance of REEs [11]. For instance, D. dichotoma is an REE hyperaccumulator that can accumulate 1.9~4.4 g/kg of REEs on a dry matter basis. Moreover, 78% of the REEs present in the hyperaccumulator were recovered into a product with 81.4% purity by an ion-exchange leaching step using 0.5 M HNO3 solution, followed by removing competing ions with water and 0.75 M HNO3, and a final one-stage elution step using 3 M HNO3 [12]. Dicranopteris linearis, naturally growing on waste mine tailings in Southern China, is classified as an REE hyperaccumulator, and the total REE concentration in its frond can reach 2~3 g/kg of REEs on a dry matter basis. In order to recover REEs, Dicranopteris linearis was firstly incinerated to ash, and 6 mol/L sodium hydroxide was used to dissolve Al from the Dicranopteris linearis at a temperature of 80 °C. Subsequently, the REEs were extracted under mildly acidic conditions using HNO3 (25 °C, pH 4.8), producing a valuable solution that contained relatively pure REEs [11]. Laubie et al. used EDTA as a lixiviant to extract 70% of REEs from D. dichotoma and precipitated REEs into REE oxalates with oxalic acid [13]. These studies provide ideas for extracting REEs from hyperaccumulators. It is obvious that the incineration process contributes to carbon emissions and air pollution. Therefore, the extraction of REEs from hyperaccumulators needs to be reconsidered to make it more environmentally friendly. In addition, few studies have focused on investigating the impacts of various factors, such as acid types, solid concentration, reaction temperature, and reaction time on REE recovery from hyperaccumulators. Exploring diverse parameters is essential for optimizing the recovery of REEs from hyperaccumulators.
The disposal of hyperaccumulator residue after REE extraction needs to be considered from a sustainable perspective. Currently, thermochemical treatments, such as pyrolysis and hydrothermal processing, are gaining more attention for waste biomass treatment due to their high efficiency, economic viability, and environmental friendliness [14,15,16]. Among all thermochemical treatment methods, hydrothermal carbonization (HTC) has been extensively researched due to its relatively mild reaction conditions (160–270 °C) and capability to convert waste biomass into a valuable solid product (hydrochar) [17,18,19]. The produced hydrochar can be used as a fuel source, soil amendment, carbon-based catalyst, or adsorbent material [19]. Compared to HTC, microwave-assisted hydrothermal carbonization (MHTC) promotes reaction kinetics as a result of rapid volumetric heating and consumes less energy (approximately 50% energy consumption, compared with the HTC process) [20]. It was found that 57.9% hydrochar was generated from rice straw MHTC-treated at 180 °C for 20 min with a fixed solid concentration of 7% [21]. The higher heating value (HHV), carbon content, and fixed carbon (FC) values increased from 12.3 MJ/kg, 37.19%, and 14.37% to 17.6 MJ/kg, 48.8%, and 35.4%, respectively, after MHTC. The porosity, crystallinity, and thermal stability of the hydrochar obtained from MHTC were also improved significantly.
In this study, the grass was found to be a promising REE hyperaccumulator, and electron probe microanalyzer (EPMA) analysis was conducted to characterize the occurrence of REEs in the grass leaves (GL). The effect of acid types, solid concentration, and acid concentration on REE recovery from GL was investigated. In addition, this study investigated the feasibility of transforming GL residue after REE extraction into valuable hydrochar by MHTC. The effect of MHTC temperature on the chemical and structural characteristics of hydrochar was explored to evaluate the potential fuel properties as well as the sequestration and adsorption abilities of the hydrochar product.

2. Materials and Methods

2.1. Chemicals and Reagents

The chemicals used in this study include nitric acid (HNO3, 67 wt.% to 70 wt.%), hydrochloric acid (HCl, 37 wt.%), sulfuric acid (H2SO4, 93 wt.% to 98 wt.%), and 50% w/w sodium hydroxide solution (NaOH, ≥97 wt.%). Y(NO3)3·6H2O, La(NO3)3·6H2O, Ce(NO3)3·6H2O, and Dy(NO3)3·6H2O were used to prepare the feeding solution of grass seeds. All the chemicals were of trace-metal grade and purchased from Thermo Fisher Scientific, Waltham, MA, USA. Type I deionized water with a resistivity of 18.2 MΩ·cm at 25 °C was prepared by the Direct-Q Water Purification System (Millipore, Burlington, MA, USA).

2.2. Hyperaccumulator Plant

Grass seeds were grown in a “potting mix” that contained a mixture of organic materials such as peat moss, drainage-assisted materials, and slow-release fertilizers. Both the grass seeds (The Rebels, PENNINGTON, Madison, GA, USA) and potting mix (Potting Mix, Miracle-Gro, Marysville, OH, USA) were purchased from The Home Depot, Blacksburg, VA, USA. The grass seeds were grown under natural light and an average temperature of 25 ± 3 °C. A solution containing Y, La, Ce, and Dy of 50 mg/L was used to feed the seeds once every two days. After 14 days of growth, the GL were harvested (Figure S1), thoroughly rinsed with deionized water, dried at 80 °C for 48 h, and ground by a high-speed multifunction grinder. The ground product was collected and used as the feed for leaching tests. The product was predominantly distributed in the size range of 53 to 297 µm. Then, 0.2 g of product was mixed with 12 mL of HNO3 and placed in a 50 mL digestion vessel (HVT50, Anton Paar, Graz, Austria). The vessel was then placed in a microwave digestor (Multiwave GO Plus, Anton Paar, Graz, Austria) and subjected to microwave digestion with a heating rate of 18.5 °C/min. The digestion was carried out at 185 °C with a residence time of 40 min. After digestion, the vessel was cooled completely, and then the digestion liquor was collected and centrifuged. The resulting supernatant was diluted 100 times with a solution containing 5% (v/v) HNO3. Elemental concentrations of the diluted solution were analyzed using an inductively coupled plasma emission mass spectrometer (ICP-MS, Thermo Electron iCAP-RQ, Thermo Scientific, Waltham, MA, USA). ICP-MS analysis data were used to calculate the elemental composition of the GL.
A JEOL JXA-iHP200F field-emission electron probe microanalyzer (EPMA) was used for the quantitative elemental analysis of the GL. The sample was fit onto a 1-inch diameter sample holder with a vertical profile not exceeding 1 mm and was coated with carbon using a mini sputter coater (QUORUM, Model EMS 7620, Sacramento, CA, USA) [22,23]. The sample holder was placed in a vacuum chamber to avoid the interactions between air and the electron beam during observation using the microprobe. Then, a 30 mm2 energy-dispersive silicon drift detector (SDD EDS) was used for rapid phase identification, and wavelength-dispersive spectrometers (WDS) were used for fast, high-resolution, quantitative chemical analyses of elements.

2.3. Acid-Leaching Tests

All leaching tests were carried out at 25 °C for a duration of 30 min. The effect of acid types (HNO3, HCl, and H2SO4) on REE recovery from the GL was first investigated to identify the most promising lixiviant. The acid concentration and solid concentration were fixed at 0.5 mol/L and 5 wt.%, respectively. The mixture was agitated using a magnetic stirrer at 450 rpm for 30 min. After the reaction, the mixture was centrifuged at 5000× rpm for 5 min using a SorvallTM Legend X1 centrifuge (Thermo Fisher Scientific, Waltham, MA, USA). The resulting supernatant was collected and diluted 100 times with a 5% (v/v) HNO3 solution for REE concentration measurement using ICP-MS. The remaining solid was thoroughly rinsed with deionized water and then dried at 80 °C for 48 h. The dried solid was then digested following the same procedure as the raw GL (see Section 2.2).
After determining the most promising lixiviant, the effects of solid concentration and acid concentration on the recovery of REEs from the GL were investigated. The acid concentration was fixed at 0.5 mol/L, with the solid concentration varying from 1 to 10 wt.% (i.e., 1 wt.%, 2 wt.%, 5 wt.%, and 10 wt.%). When investigating the effect of acid concentration (0.1 mol/L, 0.5 mol/L, 1 mol/L, and 2 mol/L), the solid concentration was maintained at 5 wt.%. After the reaction, the solid and liquid were separated by centrifugation and then subjected to elemental concentration analysis. In addition, the dry solid residue was used for further characterization and as the feed for the MHTC experiments. Each acid-leaching test was performed three times. The average results are reported, with experimental errors represented by error bars.
The fraction of REEs in the feed GL that was extracted into the solution during the leaching process was defined as REE leaching recovery (RL, %). The recovery was calculated using the following Equation (1).
RL = (CL × 0.1)/(CL × 0.1 + CS × MS) × 100
where CS and CL represent REE concentrations in the leaching residue (mg/kg) and leaching solution (mg/L), respectively; MS (kg) represents the mass of the leaching residue, and 0.1 (L) is the volume of the leaching solution.
The most effective lixiviant, solid concentration, and acid concentration for REE recovery from the GL were identified after optimizing these factors. To maximize the potential value of the GL, MHTC was used for the conversion of the GL residue obtained from acid leaching into hydrochar.

2.4. Microwave-Assisted Hydrothermal Carbonization (MHTC)

MHTC was conducted in a microwave reactor (Multiwave GO Plus, Anton Paar GmbH, Graz, Austria) with a magnetron frequency of 2450 MHz. The effect of temperature on the characteristics of the obtained hydrochar was investigated. Briefly, GL residue of 0.5 g was mixed with 10 mL deionized water in a 50 mL reaction vessel. A heating rate of 10 °C/min and a residence time of 60 min were applied during the reaction at different temperatures (i.e., 120 °C, 140 °C, 160 °C, and 180 °C). After the completion of the reaction, the suspension was collected and centrifuged at 5000× rpm for 5 min. The solid residue was rinsed thoroughly with deionized water and dried at 80 °C for 48 h. After drying, the solid was weighed and stored for characterization.

2.5. Hydrochar Characterization

2.5.1. Proximate Analysis

Thermogravimetric analysis (TGA) (TGA 5500, TA Instruments CO., Eden Prairie, MN, USA) was used for the proximate analysis of the samples obtained from MHTC, in which the sample was heated to constant weight under ASTM-specified conditions [24,25]. A sample weighing approximately 5 mg underwent thermal analysis in a controlled atmosphere of high-purity N2 with a flow rate of 20 mL/min. To eliminate any moisture, the sample was heated to 110 °C and held for at least 30 min. The temperature was then increased to 900 °C at a ramp rate of 10 °C/min, which was maintained for 60 min. The weight loss during this step was attributed to the volatile matter (VM) present in the sample. Subsequently, the N2 gas flow was replaced with dry air, and the temperature was increased to 925 °C at the same heating rate of 10 °C/min. The sample was then held at this temperature for 60 min. Any oxidizable matter lost at this step was considered fixed carbon (FC), while the remaining mass represented the ash content of the sample.

2.5.2. Elemental Composition

The percentages of key elements (C, H, and O) were calculated using the Parikh formula [26]. These correlations can calculate elemental compositions of biomass materials from a simple proximate analysis. The correlations were determined based on Equations (2)–(4).
C = 0.637FC + 0.455VM
H = 0.052FC + 0.062VM
O = 0.304FC + 0.476VM

2.5.3. High Heating Value Calculation

The high heating value (HHV; in MJ/kg) of the samples was calculated based on Jenkins’s formula [27,28] (Equation (5)).
HHV = −0.763 + 0.301C + 0.525H + 0.064O
This correlation is derived based on 57 data points of biomass material and using multiple regression analysis.

2.5.4. Definitions

The hydrochar yield (Hy, %), energy densification (Ed), and energy recovery efficiency (ERE, %) were calculated using Equations (6)–(8), respectively [29].
Hy = Mh/MGLR × 100
Ed = HHVh/HHVGLR
ERE = Ed × Hy
where Mh is the hydrochar weight (g), MGLR is the mass of the GL residue (g), HHVh is the high heating value of the hydrochar (MJ/kg), and HHVGLR is the high heating value of the GL residue (MJ/kg).

2.5.5. Characterization

A JSM-IT500 scanning electron microscope (SEM) equipped with a backscattered electron (BSE) system was used to determine changes in the structure and morphology of the samples. The samples were spread on carbon tapes and sputtered with palladium/platinum (5 nm thickness) with the use of a sputter coater (208HR, Cressington Scientific Instrument, Watford, United Kingdom) to increase their electron conductivity.
The Brunauer–Emmett–Teller (BET) surface area and pore volume of the samples were measured by N2 sorption on a Quantachrome Autosorb-1 (Giangarlo Scientific Co., Pittsburgh, PA, USA). Samples were degassed under vacuum for 24 h at 120 °C prior to the analysis. The surface area was calculated according to BET theory using adsorption data in the 0.05–0.30 relative pressure range (equilibrium pressure (kPa)/saturation pressure (kPa) (P/P0)), and the volume of N2 adsorbed at the relative pressure of 0.99 was used to calculate the pore volume of the samples.

3. Results and Discussion

3.1. Elemental Composition of GL

Elemental concentrations of the GL are presented in Table 1. The concentration of total REEs (TREEs) in the GL was 510.5 mg/kg, while the total concentration of the four REEs (Y, La, Ce, and Dy) added to the watering solution was 503.4 mg/kg, accounting for the majority of the TREEs present in the GL. In addition, the TREE concentration of the GL watered with deionized water was around 0 mg/kg (Figure S2, Supplementary Information). Therefore, these results suggested that GL can effectively accumulate REEs. Moreover, the concentration of La and Ce in the GL was 242.1 mg/kg, which was close to the concentrations of Y and Dy (261.3 mg/kg). Therefore, it can be concluded that the GL exhibited a comparable accumulation capacity for light REEs (LREEs) and heavy REEs (HREEs). A plant can be called an REE hyperaccumulator when the concentration of REEs in the aboveground shoot exceeds 1000 mg/kg [30]. As can be seen from Figure S2, when increasing the concentration of the four different REEs in the watering solution from 50 mg/L to 500 mg/L, the concentration of TREEs in the GL shows an almost similar proportional increase from 510.5 mg/kg to 5801.9 mg/kg, indicating that GL can accumulate over 1000 mg/kg of REEs in the aboveground shoot. The concentration of four different REEs in the GL also increased proportionally with the increase in REE concentration in the watering solution. Therefore, the grass used in this study can be regarded as an REE hyperaccumulator.
In addition to REEs, the concentrations of some major (>1000 mg/kg) and other trace (10–1000 mg/kg) elements in the GL are also listed in Table 1. It was found that GL were rich in K, Ca, and Mg, with concentrations of 48,730.2 mg/kg, 12,360.5 mg/kg, and 3628.8 mg/kg, respectively. This phenomenon is consistent with the general finding that plants are rich in these elements [31]. The concentration of P in GL was 7725.4 mg/kg, which was higher than any other elements except for K and Ca. In addition, as can be seen from Figure S2 in the Supplementary Information, when using the watering solution without containing REEs to feed grass, the GL were rich in P (7117.5 mg/kg). This phenomenon can be explained that due to the critical role of P in promoting the growth of plants—plants actively absorb P from the soil through their roots [32]. When the watering solution contained 50 mg/L each of Y, La, Ce, and Dy, the concentration of P in the GL was 7725.4 mg/kg. With a further increase in REE concentration in the watering solution from 50 mg/kg to 500 mg/kg, the P concentration increased from 7725.4 mg/kg to 8899.9 mg/kg, and the concentration of TREEs in the GL increased from 510.5 mg/kg to 5801.9 mg/kg. This phenomenon can be explained by the formation of complexes between REE and P in the GL [33,34]. As more REEs were accumulated, they complexed with more P in the GL, leading to the undernourishment of P, which prompted the adsorption for the nutrition requirement of plant growth.
EPMA characterization was employed to conduct quantitative chemical analysis of various elements, with a particular focus on Y, La, Ce, Dy, and P in the GL. Heavy elements appear as bright spots in the SEM images, while light elements are known to show up as dark spots [35]. A relatively bright particle of the GL was selected for the characterization (in the red rectangle in Figure 1a). The Point and ID mode with the use of an EDS detector was first employed to conduct a quick elemental composition analysis at a specific point of this bright particle [36]. It was found that the molar percentages of Y2O3, La2O3, CeO2, and Dy2O3 were 3.79%, 1.25%, 2.23%, and 0.99%, indicating that REEs were present in this bright particle (Figure S3 and Table S1, Supplementary Information). EPMA results are often reported in the form of oxides for elements that typically form stable oxides [37]. This is a standard practice in geochemical and material science analyses. Reporting in oxide form does not indicate that the elements only exist in that specific chemical state within the sample. Therefore, it can only be concluded that in this bright particle, REEs were presented instead of rare earth oxides. An SEM image of a region of interest on that particle is shown in Figure 1a, and the distribution of elements in this area of interest was understood by developing elemental maps with Map mode using a WDS detector. Compared with EDS, WDS was more suitable for mapping at low concentrations, owing to its greater peak-to-background ratios [38,39]. Therefore, considering the trace quantity of REEs (~500 mg/kg) in the GL, elemental mapping of this area was analyzed using the WDS in the Map mode. A higher concentration of the corresponding element is indicated by the presence of a brighter color at a specific location on the map [36]. As can be seen from Figure 1b–e in the areas of the red rectangles and circles, four specific REEs (Y, La, Ce, and Dy) are rich. It was also found that these REE-enriched locations also had the enrichment of P (Figure 1f). However, some areas enriched in P did not have the enrichment of REEs in the GL. This phenomenon indicated that REEs accumulated in the GL tended to be in the complexation with a portion of P in the GL. In addition, some other metals, such as K, Ca, Mg, Na, Fe, Al, and Mn, were also found in the areas containing high concentrations of REEs and P (Figure S4 in the Supplementary Information).

3.2. Acid Leaching

3.2.1. Effect of Acid Types

Different acid types may lead to different leaching characteristics at the same molar concentration due to differences in normality and anionic components. Therefore, the influence of acid type on the recovery of REEs and P from the GL was investigated. The leaching media are usually mineral acids, including HNO3, HCl, and H2SO4, and in some cases also alkali and organic acids [40]. It was found that inorganic acids are highly effective for the recovery of metals (>99%) in the leaching procedure when applied under optimal conditions [41]. Therefore, HNO3, HCl, and H2SO4 were selected as lixiviants in this study. Figure 2 shows the leaching recoveries of REEs from the GL using three different acids, including HNO3, HCl, and H2SO4. The solid concentration and acid concentration were fixed at 5 wt.% and 0.5 mol/L, respectively. When using HNO3 as the lixiviant, 67%, 73%, 67%, and 63% of Y, La, Ce, and Dy were leached, respectively. Similar recoveries of 66%, 69%, 63%, and 61% were obtained when using HCl as the lixiviant. However, it can be seen from Figure 2 that the recoveries of Y, La, Ce, and Dy increased significantly to 95%, 93%, 92%, and 92%, respectively, with the use of H2SO4. The higher recovery of REEs using H2SO4 as the lixiviant can be attributed to the higher concentration of hydrogen ions (H+) available in the H2SO4 solution compared to using the same concentrations of HNO3 and HCl. The higher concentration of H+ facilitated the solubility of REEs from the GL [42,43]. When the pH of the solution exceeds 2.0, SO42− is the dominant species. Conversely, when the pH falls below 2.0, HSO4− predominates [44]. During the leaching process using H2SO4 as the lixiviant, HSO4− is an intermediate species between H2SO4 and SO42−. The leaching reaction containing REEs tends to favor the SO42− because it forms complexes with REE3+ ions. It was reported that SO42− can form more stable complexes with REEs than chloride ions and nitrate ions, promoting the dissolution of REEs from ion-adsorption clays [45]. Therefore, it can be inferred that the strong complexing ability of REEs and SO42− also promoted the REE dissolution from GL when using H2SO4 as a lixiviant.
As can be seen from Figure 2, using H2SO4 as the lixiviant not only enhanced the recovery of REEs but also led to an increase in P recovery. This phenomenon can be ascribed to the fact that the occurrence of REEs in the GL was in the complexation with a portion of P in the GL. Therefore, during the leaching process, the complexes of REEs and P were disrupted by H2SO4, thus leading to the enhanced recovery of REEs and P. It is worth mentioning that using H2SO4 is more economically viable than using HNO3 and HCl due to the low average price [46]. Therefore, H2SO4 was selected as the preferred lixiviant for the recovery of REEs from the GL.

3.2.2. Effect of Solid Concentration

The solid concentration represents the stoichiometric ratio of reactants, which directly affects the reaction equilibrium [47]. The effect of solid concentration (1 wt.%, 2 wt.%, 5 wt.%, and 10 wt.%) on the recovery of REEs and P from GL was investigated using a fixed 0.5 mol/L H2SO4. Figure 3 shows that the maximum recovery of REEs (around 99%) is obtained at a solid concentration of 1 wt.%. Slight decreases in REE recovery were observed as the solid concentration increased from 1 wt.% to 5 wt.%. When we further enhanced the solid concentration from 5 wt.% to 10 wt.%, the recovery decreased further to 90%, 89%, 89%, and 88% for Y, La, Ce, and Dy, respectively. A low solid concentration is favorable for the extraction of REEs from the GL since increasing the volume of the leaching solution can increase the efficiency of mass transfer, thus leading to an increase in REE recovery [48]. In addition, as the solid concentration increased from 1 wt.% to 10 wt.%, the recovery of P decreased from 99% to 89%. The simultaneous decrease in the recovery of REEs and P can also be attributed to the reason mentioned in Section 3.2.1. Based on the leaching results, the most suitable solid concentration was selected to be 5 wt.%.

3.2.3. Effect of Acid Concentration

It is well known that acid concentration is one of the most influential factors in REE leaching experiments [49]. An increased acid concentration represents that more H+ can react with REE-bearing minerals, thus resulting in a higher REE recovery [50]. The effect of H2SO4 concentration (0.1 mol/L, 0.5 mol/L, 1 mol/L, and 2 mol/L) on the leaching recovery of REEs and P was investigated at a fixed solid concentration of 5 wt.%. As can be seen from Figure 4, around 85% of REEs can be recovered using 0.1 mol/L H2SO4. When further increasing H2SO4 concentration from 0.1 mol/L to 0.5 mol/L, the recovery of Y, La, Ce, and Dy increased to 96%, 93%, 92%, and 93%, respectively. The further increase in H2SO4 concentration from 0.5 mol/L to 2 mol/L led to a minor increase in the REE recovery. The final pH values of the leaching solutions were 1.3, 0.8, 0.7, and 0.6, respectively. An increase in the acid concentration leads to a decrease in the pH of the solution, and a low pH value of the leaching solution is favorable for metal leaching [51]. Therefore, the recovery of REEs was increased with the increase in acid concentration and the decrease in the pH of the leaching solutions.
There was still a proportion (around 5%) of difficult-to-leach REE-bearing compounds in the GL when using 2 mol/L H2SO4 as the lixiviant at a solid concentration of 5 wt.% (Figure 4). This phenomenon might be because with the increase in acid concentration from 0.5 mol/L to 2 mol/L, more sulfate ions released into the leachate combined with Ca2+, leading to the formation of insoluble CaSO4 (gypsum). Gypsum precipitation might coat the surface of the REE-bearing GL, resulting in decreases in the surface area and pore size of particles [52]. As a result, the remaining REEs in the GL were difficult to be leached. The recovery of P increased from 89% to 93% with the increase in acid concentration from 0.1 mol/L to 0.5 mol/L, while there was a negligible increase in P recovery when increasing the acid concentration from 0.5 mol/L to 2 mol/L. In addition, around 3% of P could not be recovered using 2 mol/L H2SO4. This phenomenon was due to the reason that REEs were in the form of the complexation with Pl thus, the leaching of P exhibited a leaching pattern similar to that of REEs. Overall, 0.5 mol/L H2SO4 was selected as an optimal acid concentration for recovering REEs from the GL.
In addition to acid types, solid concentration, and acid concentration, the effect of reaction time and reaction temperature on REE recovery was investigated, and the experimental data are provided in the Supplementary Information (Figures S5 and S6). It was found that reaction time and temperature exerted negligible influences on the REE recovery from the GL. Therefore, the reaction time was fixed at 30 min, and the temperature was maintained at 25 °C for the leaching experiments.

3.3. Hydrochar Characterization

3.3.1. Structural Properties of the Hydrochar

The optimum conditions for recovering REEs from the GL were determined after optimizing the acid type, solid concentration, and acid concentration. To make a comprehensive utilization of GL, MHTC was employed to convert the leaching residue into hydrochar, which is an exceptionally valuable product with a wide range of applications [20,21]. As can be seen from Figure 5, compared with the leaching residue of GL, the color of hydrochar samples became darker with the increase in MHTC temperature, indicating that the GL residue was gradually carbonized [53]. The SEM images illustrate the morphological variations of the hydrochar obtained from the GL residue via MHTC at various temperatures. Firstly, as can be seen in Figure 6a, the surface of the raw GL residue was smooth and compact. The degree of surface roughness increased gradually as the reaction temperature increased from 120 °C to 140 °C (as shown in Figure 6b,c). This can be attributed to the decomposition of hemicellulose, depolymerization of cellulose, and partial degradation of lignin [53,54,55]. When further increasing the temperature from 140 °C to 160 °C, a few microspheres can be clearly discerned in the area of the red rectangle in Figure 6d. When the temperature increased from 160 °C to 180 °C, numerous sphere-like microparticles were produced, as shown in the red rectangles in Figure 6e.
Notably, the surface of the hydrochar exhibited distinct pores (red circle in Figure 6d) at a temperature of 160 °C. Moreover, the morphology of GL residue was almost completely disrupted at the reaction temperature of 180 °C, revealing the presence of solely pore structures without any fibrous strands, as depicted in the red circles in Figure 6f. The formation of microspheres and the presence of pores indicated that the GL residue underwent the decomposition transformation during MHTC. The process of microsphere formation involved the decomposition of complex macromolecules present in the GL residue, including carbohydrates, proteins, and lipids [56]. As these macromolecules underwent decomposition, the resulting breakdown products precipitated and gradually aggregated, eventually forming spherical structures [57]. In addition, the cellulose and hemicellulose were violently decomposed at high temperatures during the MHTC process, leading to the formation of pores on the surface of the hydrochar [58]. Also, according to [59], the walls between adjacent pores are destroyed at high temperatures, causing an enlargement of the pores, which can be clearly observed in Figure 6f. The porous structure of hydrochar obtained at high temperatures suggests that the hydrochar can be used for sequestration and adsorption purposes [57].
The porous textural characteristics were assessed by the nitrogen physisorption method. As Table 2 shows, the surface area of hydrochar increased slightly with the increase in reaction temperature from 120 °C to 140 °C. When we further increased the temperature to 180 °C, the surface area increased significantly from 2.345 m2/g to 9.428 m2/g. The increased surface roughness and the formation of microspheres during MHTC led to an increase in the surface area of the hydrochar. The increase in hydrochar porosity was caused by the growth of existing pores and the formation of new micropores due to the decomposition of cellulose and hemicellulose at higher temperatures [58]. High surface area and pore volume are correlated with enhanced sorption ability and are desired traits for the char used for the remediation of soil or water contamination [60].

3.3.2. Chemical Characterization

Elemental compositions of the GL residue and hydrochar and yields of the hydrochar are summarized in Table 3. The maximum hydrochar yield was obtained at 120 °C. When increasing the reaction temperature from 120 °C to 180 °C, the hydrochar yield decreased significantly from 70.5% to 54.2%. This phenomenon was due to the greater primary decomposition or secondary decomposition of the GL residue occurring at higher temperatures during MHTC [61,62,63].
The results shown in Table 3 also indicate that the elemental composition of hydrochar varies due to the various degrees of carbonization at different reaction temperatures. With the increase in reaction temperature from 120 °C to 180 °C, the carbon content increased from 44.28% to 49.67%, while the content of hydrogen and oxygen decreased from 40.97% and 5.52% to 38.19% and 5.45%, respectively. In addition, the ratios of O/C and H/C decreased from 0.69 and 1.50 to 0.58 and 1.32, respectively. The variations in the elemental composition of GL residue and hydrochar were analyzed via a van Krevelen diagram [64]. The three arrows in Figure 7 represent the process of dehydration, decarboxylation, and demethylation [65]. The van Krevelen diagram clearly shows that the alternations of H/C and O/C atomic ratios from GL residue to hydrochar followed the paths of dehydration and decarboxylation, while the demethylation pathway can be negligible. A similar phenomenon was observed during the hydrothermal carbonization process of cellulose and other kinds of lignocellulosic biomass [66,67]. Therefore, higher temperatures promoted dehydration and decarboxylation reactions of the GL residue, resulting in decreased H/C and O/C ratios.
The proximate analysis, HHV, ERE, and energy densification are summarized in Table 4. It can be found from Table 4 that the GL residue contained 14.78% of FC and 76.63% of VM. After being treated by MHTC at 180 °C, the FC content of hydrochar increased to 37.99%, while the VM amount decreased to 55.97%. This phenomenon was caused by the chemical dehydration and decarboxylation reactions during MHTC [29,68,69,70]. In addition, the ash content increased significantly due to the loss of VM [29]. The HHV increased from 18.11 MJ/kg to 19.49 MJ/kg with the increase in the reaction temperature from 120 °C to 180 °C. The increase in HHV confirmed the high efficiency of MHTC in converting the GL residue into a solid fuel [71]. A higher MHTC reaction temperature demonstrated a better energy potential of the produced hydrochar in terms of HHV value. However, ERE, which is determined by energy densification and hydrochar yield, led to a different path in determining the optimum MHTC reaction temperature [72]. Energy densification increased slightly with the increase in the reaction temperature during MHTC, suggesting enhancements in the viability of utilizing the treated GL residue for fuel applications [73]. In contrast to energy densification, hydrochar yield decreased significantly as temperature increased, with the lowest yield of 54.2% being obtained at the reaction temperature of 180 °C. Therefore, ERE was significantly influenced by the decrease in hydrochar yield (Table 4). Considering the HHV and ERE of hydrochar, a temperature range of 160 °C to 180 °C was optimal for converting the GL residue into value-added solid fuel via MHTC. In addition, the relatively high ERE values (around 60%–70%) suggested that MHTC is an effective way to upgrade the GL residue into a valuable hydrochar [74].

4. Conclusions

In the present study, grass was found to be a hyperaccumulator that can accumulate REEs from feeding solution with a TREE content of 510.5 mg/kg. The EPMA analysis showed that REEs in the GL were in complexation with P. The effects of acid types, solid concentration, and acid concentration on the REE recovery from GL were investigated. High recoveries of Y, La, Ce, and Dy, reaching 96%, 93%, 92%, and 93%, respectively, were achieved by employing 0.5 mol/L H2SO4 as the lixiviant and maintaining a solid concentration of 5 wt.%. In addition, the recovery of P showed a similar trend to the recovery of REEs due to the complexation of REEs with P in the GL.
MHTC was used to convert GL residue obtained after REE extraction into hydrochar to expand the utilization of GL biomass. The effect of reaction temperature during MHTC on hydrochar characteristics was investigated. SEM analysis showed that more microspheres and pores appeared on the surface of the hydrochar with the increase in temperature. The porous structure of the hydrochar obtained at high temperatures suggested the application of sequestration and adsorption. Considering the values of HHV and ERE, MHTC reaction temperatures between 160 °C and 180 °C were suitable for converting GL residue to value-added solid fuel. The analysis of hydrochar characteristics revealed that MHTC is an effective way to convert GL residue into hydrochar, which can be used as a solid fuel and adsorbent.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14030277/s1, Figure S1: REE hyperaccumulator plant grass and harvested GL (inset). Figure S2: Effect of REE concentration in watering solution on the elemental concentration in the GL. Figure S3: EDS spectrum of the particles found from the GL. Figure S4: WDS mapping of different elements in the GL (a)~(h). Figure S5: Effect of acid types on the elementary recovery from the GL. Figure S6: Effect of temperature on the elementary recovery from the GL. Table S1: Chemical compositions (Mol%) of the GL analyzed by EDS spectrum.

Author Contributions

Conceptualization, S.L., B.J. and W.Z.; methodology, S.L.; validation, S.L., B.J. and W.Z.; investigation, S.L.; resources, W.Z.; writing—original draft preparation, S.L.; writing—review and editing, B.J. and W.Z.; supervision, W.Z.; project administration, W.Z.; funding acquisition, W.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The information, data, or work presented herein was funded in part by the Office of Surface Mining Reclamation and Enforcement.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Balaram, V. Rare Earth Elements: A Review of Applications, Occurrence, Exploration, Analysis, Recycling, and Environmental Impact. Geosci. Front. 2019, 10, 1285–1303. [Google Scholar] [CrossRef]
  2. Jowitt, S.M.; Werner, T.T.; Weng, Z.; Mudd, G.M. Recycling of the Rare Earth Elements. Curr. Opin. Green Sustain. Chem. 2018, 13, 1–7. [Google Scholar] [CrossRef]
  3. Wang, L.; Ji, B.; Hu, Y.; Liu, R.; Sun, W. A Review on in Situ Phytoremediation of Mine Tailings. Chemosphere 2017, 184, 594–600. [Google Scholar] [CrossRef] [PubMed]
  4. Lima, A.T.; Ottosen, L. Recovering Rare Earth Elements from Contaminated Soils: Critical Overview of Current Remediation Technologies. Chemosphere 2021, 265, 129163. [Google Scholar] [CrossRef] [PubMed]
  5. Liu, Y.; Naidu, R. Hidden Values in Bauxite Residue (Red Mud): Recovery of Metals. Waste Manag. 2014, 34, 2662–2673. [Google Scholar] [CrossRef]
  6. Li, S.; Ji, B.; Zhang, W. A Review on the Thermochemical Treatments of Biomass: Implications for Hydrochar Production and Rare Earth Element Recovery from Hyperaccumulators. Chemosphere 2023, 342, 140140. [Google Scholar] [CrossRef] [PubMed]
  7. Cheng, H.; Liu, Q.; Ma, M.; Liu, Y.; Wang, W.; Ning, W. Cadmium Tolerance, Distribution, and Accumulation in Taraxacum ohwianum Kitam. as a Potential Cd-Hyperaccumulator. Int. J. Phytoremediat. 2019, 21, 541–549. [Google Scholar] [CrossRef]
  8. Sytar, O.; Ghosh, S.; Malinska, H.; Zivcak, M.; Brestic, M. Physiological and Molecular Mechanisms of Metal Accumulation in Hyperaccumulator Plants. Physiol. Plant. 2021, 173, 148–166. [Google Scholar] [CrossRef]
  9. Li, J.T.; Gurajala, H.K.; Wu, L.H.; Van Der Ent, A.; Qiu, R.L.; Baker, A.J.M.; Tang, Y.T.; Yang, X.E.; Shu, W.S. Hyperaccumulator Plants from China: A Synthesis of the Current State of Knowledge. Environ. Sci. Technol. 2018, 52, 11980–11994. [Google Scholar] [CrossRef]
  10. Lai, Y.; Wang, Q.; Yan, W.; Yang, L.; Huang, B. Preliminary Study of the Enrichment and Fractionation of REEs in a Newly Discovered REE Hyperaccumulator Pronephrium Simplex by SEC-ICP-MS and MALDI-TOF/ESI-MS. J. Anal. At. Spectrom. 2005, 20, 751–753. [Google Scholar] [CrossRef]
  11. Jally, B.; Laubie, B.; Chour, Z.; Muhr, L.; Qiu, R.; Morel, J.L.; Tang, Y.; Simonnot, M.O. A New Method for Recovering Rare Earth Elements from the Hyperaccumulating Fern Dicranopteris linearis from China. Miner. Eng. 2021, 166, 106879. [Google Scholar] [CrossRef]
  12. Chour, Z.; Laubie, B.; Morel, J.L.; Tang, Y.; Qiu, R.; Simonnot, M.O.; Muhr, L. Recovery of Rare Earth Elements from Dicranopteris dichotoma by an Enhanced Ion Exchange Leaching Process. Chem. Eng. Process. Process Intensif. 2018, 130, 208–213. [Google Scholar] [CrossRef]
  13. Laubie, B.; Chour, Z.; Tang, Y.-T.; Morel, J.-L.; Simonnot, M.-O.; Muhr, L. REE Recovery from the Fern D. Dichotoma by Acid Oxalic Precipitation After Direct Leaching with EDTA BT—Extraction 2018; Davis, B.R., Moats, M.S., Wang, S., Gregurek, D., et al., Eds.; Springer International Publishing: Cham, Switzerland, 2018; pp. 2659–2667. [Google Scholar]
  14. Chai, Y.; Chen, A.; Bai, M.; Peng, L.; Shao, J.; Yuan, J.; Shang, C.; Zhang, J.; Huang, H.; Peng, C. Valorization of Heavy Metal Contaminated Biomass: Recycling and Expanding to Functional Materials. J. Clean. Prod. 2022, 366, 132771. [Google Scholar] [CrossRef]
  15. Cui, X.; Zhang, J.; Wang, X.; Pan, M.; Lin, Q.; Khan, K.Y.; Yan, B.; Li, T.; He, Z.; Yang, X. A Review on the Thermal Treatment of Heavy Metal Hyperaccumulator: Fates of Heavy Metals and Generation of Products. J. Hazard. Mater. 2021, 405, 123832. [Google Scholar] [CrossRef] [PubMed]
  16. Yuan, X.; Wang, J.; Zeng, G.; Huang, H.; Pei, X.; Li, H.; Liu, Z.; Cong, M. Comparative Studies of Thermochemical Liquefaction Characteristics of Microalgae Using Different Organic Solvents. Energy 2011, 36, 6406–6412. [Google Scholar] [CrossRef]
  17. Shao, Y.; Long, Y.; Wang, H.; Liu, D.; Shen, D.; Chen, T. Hydrochar Derived from Green Waste by Microwave Hydrothermal Carbonization. Renew. Energy 2019, 135, 1327–1334. [Google Scholar] [CrossRef]
  18. Tag, A.T.; Duman, G.; Yanik, J. Influences of Feedstock Type and Process Variables on Hydrochar Properties. Bioresour. Technol. 2018, 250, 337–344. [Google Scholar]
  19. Zhang, Z.; Zhu, Z.; Shen, B.; Liu, L. Insights into Biochar and Hydrochar Production and Applications: A Review. Energy 2019, 171, 581–598. [Google Scholar] [CrossRef]
  20. Shao, Y.; Tan, H.; Shen, D.; Zhou, Y.; Jin, Z.; Zhou, D.; Lu, W.; Long, Y. Synthesis of Improved Hydrochar by Microwave Hydrothermal Carbonization of Green Waste. Fuel 2020, 266, 117146. [Google Scholar] [CrossRef]
  21. Nizamuddin, S.; Qureshi, S.S.; Baloch, H.A.; Siddiqui, M.T.H.; Takkalkar, P.; Mubarak, N.M.; Dumbre, D.K.; Griffin, G.J.; Madapusi, S.; Tanksale, A. Microwave Hydrothermal Carbonization of Rice Straw: Optimization of Process Parameters and Upgrading of Chemical, Fuel, Structural and Thermal Properties. Materials 2019, 12, 403. [Google Scholar] [CrossRef]
  22. Misch, D.; Gross, D.; Huang, Q.; Zaccarini, F.; Sachsenhofer, R.F. Light and Trace Element Composition of Carboniferous Coals from the Donets Basin (Ukraine): An Electron Microprobe Study. Int. J. Coal Geol. 2016, 168, 108–118. [Google Scholar] [CrossRef]
  23. Smolders, E.; Degryse, F. Fate and Effect of Zinc from Tire Debris in Soil. Environ. Sci. Technol. 2002, 36, 3706–3710. [Google Scholar] [CrossRef] [PubMed]
  24. Donahue, C.J.; Rais, E.A. Proximate Analysis of Coal. J. Chem. Educ. 2009, 86, 222. [Google Scholar] [CrossRef]
  25. Khoo, C.G.; Lam, M.K.; Mohamed, A.R.; Lee, K.T. Hydrochar Production from High-Ash Low-Lipid Microalgal Biomass via Hydrothermal Carbonization: Effects of Operational Parameters and Products Characterization. Environ. Res. 2020, 188, 109828. [Google Scholar] [CrossRef]
  26. Parikh, J.; Channiwala, S.A.; Ghosal, G.K. A Correlation for Calculating Elemental Composition from Proximate Analysis of Biomass Materials. Fuel 2007, 86, 1710–1719. [Google Scholar] [CrossRef]
  27. Channiwala, S.A.; Parikh, P.P. A Unified Correlation for Estimating HHV of Solid, Liquid and Gaseous Fuels. Fuel 2002, 81, 1051–1063. [Google Scholar] [CrossRef]
  28. Ebeling, J.M.; Jenkins, B.M. Physical and Chemical Properties of Biomass Fuels. Trans. ASAE 1985, 28, 898–902. [Google Scholar] [CrossRef]
  29. Kim, D.; Lee, K.; Park, K.Y. Hydrothermal Carbonization of Anaerobically Digested Sludge for Solid Fuel Production and Energy Recovery. Fuel 2014, 130, 120–125. [Google Scholar] [CrossRef]
  30. Liu, C.; Liu, W.-S.; van der Ent, A.; Morel, J.L.; Zheng, H.-X.; Wang, G.-B.; Tang, Y.-T.; Qiu, R.-L. Simultaneous Hyperaccumulation of Rare Earth Elements, Manganese and Aluminum in Phytolacca Americana in Response to Soil Properties. Chemosphere 2021, 282, 131096. [Google Scholar] [CrossRef]
  31. Chour, Z.; Laubie, B.; Morel, J.L.; Tang, Y.T.; Simonnot, M.O.; Muhr, L. Basis for a New Process for Producing REE Oxides from Dicranopteris Linearis. J. Environ. Chem. Eng. 2020, 8, 103961. [Google Scholar] [CrossRef]
  32. Shen, J.; Yuan, L.; Zhang, J.; Li, H.; Bai, Z.; Chen, X.; Zhang, W.; Zhang, F. Phosphorus Dynamics: From Soil to Plant. Plant Physiol. 2011, 156, 997–1005. [Google Scholar] [CrossRef] [PubMed]
  33. Ding, S.; Liang, T.; Zhang, C.; Huang, Z.; Xie, Y.; Chen, T. Fractionation Mechanisms of Rare Earth Elements (REEs) in Hydroponic Wheat: An Application for Metal Accumulation by Plants. Environ. Sci. Technol. 2006, 40, 2686–2691. [Google Scholar] [CrossRef] [PubMed]
  34. Liu, W.S.; Van Der Ent, A.; Erskine, P.D.; Morel, J.L.; Echevarria, G.; Spiers, K.M.; Montargès-Pelletier, E.; Qiu, R.L.; Tang, Y.T. Spatially Resolved Localization of Lanthanum and Cerium in the Rare Earth Element Hyperaccumulator Fern Dicranopteris Linearis from China. Environ. Sci. Technol. 2020, 54, 2287–2294. [Google Scholar] [CrossRef] [PubMed]
  35. Wang, Z.-M.; Wagner, J.; Ghosal, S.; Bedi, G.; Wall, S. SEM/EDS and Optical Microscopy Analyses of Microplastics in Ocean Trawl and Fish Guts. Sci. Total Environ. 2017, 603, 616–626. [Google Scholar] [CrossRef] [PubMed]
  36. Ji, B.; Li, Q.; Zhang, W. Rare Earth Elements (REEs) Recovery from Coal Waste of the Western Kentucky No. 13 and Fire Clay Seams. Part I: Mineralogical Characterization Using SEM-EDS and TEM-EDS. Fuel 2022, 307, 121854. [Google Scholar] [CrossRef]
  37. Wille, G.; Lerouge, C.; Schmidt, U. A Multimodal Microcharacterisation of Trace-element Zonation and Crystallographic Orientation in Natural Cassiterite by Combining Cathodoluminescence, EBSD, EPMA and Contribution of Confocal Raman-in-SEM Imaging. J. Microsc. 2018, 270, 309–317. [Google Scholar] [CrossRef] [PubMed]
  38. Borghi, A.; Cossio, R.; Olmi, F.; Vaggelli, G. Compositional X-ray Maps of Metamorphic and Magmatic Minerals; Springer: Vienna, Austria, 1998; ISBN 3211831061. [Google Scholar]
  39. Reed, S.J.B.; Buckley, A. Rare-Earth Element Determination in Minerals by Electron-Probe Microanalysis: Application of Spectrum Synthesis. Mineral. Mag. 1998, 62, 1–8. [Google Scholar] [CrossRef]
  40. Neumann, J.; Petranikova, M.; Meeus, M.; Gamarra, J.D.; Younesi, R.; Winter, M.; Nowak, S. Recycling of Lithium-ion Batteries—Current State of the Art, Circular Economy, and next Generation Recycling. Adv. Energy Mater. 2022, 12, 2102917. [Google Scholar] [CrossRef]
  41. Gao, W.; Liu, C.; Cao, H.; Zheng, X.; Lin, X.; Wang, H.; Zhang, Y.; Sun, Z. Comprehensive Evaluation on Effective Leaching of Critical Metals from Spent Lithium-Ion Batteries. Waste Manag. 2018, 75, 477–485. [Google Scholar] [CrossRef]
  42. Larsson, K.; Ekberg, C.; Ødegaard-Jensen, A. Dissolution and Characterization of HEV NiMH Batteries. Waste Manag. 2013, 33, 689–698. [Google Scholar] [CrossRef]
  43. Zielinski, M.; Cassayre, L.; Destrac, P.; Coppey, N.; Garin, G.; Biscans, B. Leaching Mechanisms of Industrial Powders of Spent Nickel Metal Hydride Batteries in a Pilot-scale Reactor. ChemSusChem 2020, 13, 616–628. [Google Scholar] [CrossRef] [PubMed]
  44. Casas, J.M.; Alvarez, F.; Cifuentes, L. Aqueous Speciation of Sulfuric Acid–Cupric Sulfate Solutions. Chem. Eng. Sci. 2000, 55, 6223–6234. [Google Scholar] [CrossRef]
  45. Moldoveanu, G.A.; Papangelakis, V.G. Recovery of Rare Earth Elements Adsorbed on Clay Minerals: I. Desorption Mechanism. Hydrometallurgy 2012, 117, 71–78. [Google Scholar] [CrossRef]
  46. Yang, X.; Honaker, R.Q. Leaching Kinetics of Rare Earth Elements from Fire Clay Seam Coal. Minerals 2020, 10, 491. [Google Scholar] [CrossRef]
  47. Dixon, D.G.; Hendrix, J.L. A General Model for Leaching of One or More Solid Reactants from Porous Ore Particles. Metall. Trans. B 1993, 24, 157–169. [Google Scholar] [CrossRef]
  48. Chen, M.; Huang, J.; Ogunseitan, O.A.; Zhu, N.; Wang, Y. Comparative Study on Copper Leaching from Waste Printed Circuit Boards by Typical Ionic Liquid Acids. Waste Manag. 2015, 41, 142–147. [Google Scholar] [CrossRef] [PubMed]
  49. Mokoena, B.K.; Mokhahlane, L.S.; Clarke, S. Effects of Acid Concentration on the Recovery of Rare Earth Elements from Coal Fly Ash. Int. J. Coal Geol. 2022, 259, 104037. [Google Scholar] [CrossRef]
  50. Zeng, Q.; Li, S.; Sun, W.; Hu, L.; Zhong, H.; He, Z. Eco-Friendly Leaching of Rubidium from Biotite-Containing Minerals with Oxalic Acid and Effective Removal of Hg2+ from Aqueous Solution Using the Leaching Residues. J. Clean. Prod. 2021, 306, 127167. [Google Scholar] [CrossRef]
  51. Lie, J.; Liu, J.C. Selective Recovery of Rare Earth Elements (REEs) from Spent NiMH Batteries by Two-Stage Acid Leaching. J. Environ. Chem. Eng. 2021, 9, 106084. [Google Scholar] [CrossRef]
  52. Zhang, W.; Noble, A. Mineralogy Characterization and Recovery of Rare Earth Elements from the Roof and Floor Materials of the Guxu Coalfield. Fuel 2020, 270, 117533. [Google Scholar] [CrossRef]
  53. Cai, J.; Li, B.; Chen, C.; Wang, J.; Zhao, M.; Zhang, K. Hydrothermal Carbonization of Tobacco Stalk for Fuel Application. Bioresour. Technol. 2016, 220, 305–311. [Google Scholar] [CrossRef] [PubMed]
  54. Kambo, H.S.; Dutta, A. Comparative Evaluation of Torrefaction and Hydrothermal Carbonization of Lignocellulosic Biomass for the Production of Solid Biofuel. Energy Convers. Manag. 2015, 105, 746–755. [Google Scholar] [CrossRef]
  55. Sevilla, M.; Fuertes, A.B. The Production of Carbon Materials by Hydrothermal Carbonization of Cellulose. Carbon 2009, 47, 2281–2289. [Google Scholar] [CrossRef]
  56. Wang, T.; Zhai, Y.; Zhu, Y.; Li, C.; Zeng, G. A Review of the Hydrothermal Carbonization of Biomass Waste for Hydrochar Formation: Process Conditions, Fundamentals, and Physicochemical Properties. Renew. Sustain. Energy Rev. 2018, 90, 223–247. [Google Scholar] [CrossRef]
  57. Kannan, S.; Gariepy, Y.; Raghavan, G.S.V. Optimization and Characterization of Hydrochar Produced from Microwave Hydrothermal Carbonization of Fish Waste. Waste Manag. 2017, 65, 159–168. [Google Scholar] [CrossRef] [PubMed]
  58. Nizamuddin, S.; Siddiqui, M.T.H.; Baloch, H.A.; Mubarak, N.M.; Griffin, G.; Madapusi, S.; Tanksale, A. Upgradation of Chemical, Fuel, Thermal, and Structural Properties of Rice Husk through Microwave-Assisted Hydrothermal Carbonization. Environ. Sci. Pollut. Res. 2018, 25, 17529–17539. [Google Scholar] [CrossRef] [PubMed]
  59. Marx, S.; Chiyanzu, I.; Piyo, N. Influence of Reaction Atmosphere and Solvent on Biochar Yield and Characteristics. Bioresour. Technol. 2014, 164, 177–183. [Google Scholar] [CrossRef]
  60. Bahcivanji, L.; Gascó, G.; Paz-Ferreiro, J.; Méndez, A. The Effect of Post-Pyrolysis Treatment on Waste Biomass Derived Hydrochar. Waste Manag. 2020, 106, 55–61. [Google Scholar] [CrossRef]
  61. Cheng, K.; Winter, W.T.; Stipanovic, A.J. A Modulated-TGA Approach to the Kinetics of Lignocellulosic Biomass Pyrolysis/Combustion. Polym. Degrad. Stab. 2012, 97, 1606–1615. [Google Scholar] [CrossRef]
  62. Demirbaş, A. Mechanisms of Liquefaction and Pyrolysis Reactions of Biomass. Energy Convers. Manag. 2000, 41, 633–646. [Google Scholar] [CrossRef]
  63. Fisher, T.; Hajaligol, M.; Waymack, B.; Kellogg, D. Pyrolysis Behavior and Kinetics of Biomass Derived Materials. J. Anal. Appl. Pyrolysis 2002, 62, 331–349. [Google Scholar] [CrossRef]
  64. Van Krevelen, D.W. Graphical-Statistical Method for the Study of Structure and Reaction Processes of Coal. Fuel 1950, 29, 269–284. [Google Scholar]
  65. Xiao, L.P.; Shi, Z.J.; Xu, F.; Sun, R.C. Hydrothermal Carbonization of Lignocellulosic Biomass. Bioresour. Technol. 2012, 118, 619–623. [Google Scholar] [CrossRef] [PubMed]
  66. Gao, Y.; Remón, J.; Matharu, A.S. Microwave-Assisted Hydrothermal Treatments for Biomass Valorisation: A Critical Review. Green Chem. 2021, 23, 3502–3525. [Google Scholar] [CrossRef]
  67. Shao, Y.; Lu, W.; Meng, Y.; Zhou, D.; Zhou, Y.; Shen, D.; Long, Y. The Formation of 5-Hydroxymethylfurfural and Hydrochar during the Valorization of Biomass Using a Microwave Hydrothermal Method. Sci. Total Environ. 2021, 755, 142499. [Google Scholar] [CrossRef] [PubMed]
  68. Funke, A.; Ziegler, F. Hydrothermal Carbonization of Biomass: A Summary and Discussion of Chemical Mechanisms for Process Engineering. Biofuels Bioprod. Biorefin. 2010, 4, 160–177. [Google Scholar] [CrossRef]
  69. Heilmann, S.M.; Jader, L.R.; Harned, L.A.; Sadowsky, M.J.; Schendel, F.J.; Lefebvre, P.A.; Von Keitz, M.G.; Valentas, K.J. Hydrothermal Carbonization of Microalgae II. Fatty Acid, Char, and Algal Nutrient Products. Appl. Energy 2011, 88, 3286–3290. [Google Scholar] [CrossRef]
  70. Hoekman, S.K.; Broch, A.; Robbins, C. Hydrothermal Carbonization (HTC) of Lignocellulosic Biomass. Energy Fuels 2011, 25, 1802–1810. [Google Scholar] [CrossRef]
  71. Lee, J.; Lee, K.; Sohn, D.; Kim, Y.M.; Park, K.Y. Hydrothermal Carbonization of Lipid Extracted Algae for Hydrochar Production and Feasibility of Using Hydrochar as a Solid Fuel. Energy 2018, 153, 913–920. [Google Scholar] [CrossRef]
  72. Lee, J.; Park, K.Y. Conversion of Heavy Metal-Containing Biowaste from Phytoremediation Site to Value-Added Solid Fuel through Hydrothermal Carbonization. Environ. Pollut. 2021, 269, 116127. [Google Scholar] [CrossRef]
  73. Mäkelä, M.; Benavente, V.; Fullana, A. Hydrothermal Carbonization of Lignocellulosic Biomass: Effect of Process Conditions on Hydrochar Properties. Appl. Energy 2015, 155, 576–584. [Google Scholar] [CrossRef]
  74. Yang, W.; Wang, H.; Zhang, M.; Zhu, J.; Zhou, J.; Wu, S. Fuel Properties and Combustion Kinetics of Hydrochar Prepared by Hydrothermal Carbonization of Bamboo. Bioresour. Technol. 2016, 205, 199–204. [Google Scholar] [CrossRef] [PubMed]
Minerals 14 00277 g001
Figure 1. SEM image of the GL (a) and WDS mapping of different elements (b–f) (red square and circle: REE & P-enriched locations).
Figure 1. SEM image of the GL (a) and WDS mapping of different elements (b–f) (red square and circle: REE & P-enriched locations).
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Figure 2. Effect of acid types on the elemental recovery from the GL.
Figure 2. Effect of acid types on the elemental recovery from the GL.
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Figure 3. Effect of solid concentration on the elemental recovery from the GL.
Figure 3. Effect of solid concentration on the elemental recovery from the GL.
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Figure 4. Effect of acid concentration on the elemental recovery from the GL.
Figure 4. Effect of acid concentration on the elemental recovery from the GL.
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Figure 5. Hydrochar obtained at different reaction temperatures during MHTC.
Figure 5. Hydrochar obtained at different reaction temperatures during MHTC.
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Minerals 14 00277 g006
Figure 6. SEM images of GL residue and hydrochar obtained at different reaction temperatures: (a): GL residue, (b) MHTC-120, (c) MHTC-140, (d) MHTC-160, (e,f) MHTC-180 (red rectangle: microparticles, red circle: pore structures).
Figure 6. SEM images of GL residue and hydrochar obtained at different reaction temperatures: (a): GL residue, (b) MHTC-120, (c) MHTC-140, (d) MHTC-160, (e,f) MHTC-180 (red rectangle: microparticles, red circle: pore structures).
Minerals 14 00277 g006
Minerals 14 00277 g007
Figure 7. Atomic H/C versus O/C ratios (van Krevelen diagram) of GL residue and hydrochar.
Figure 7. Atomic H/C versus O/C ratios (van Krevelen diagram) of GL residue and hydrochar.
Minerals 14 00277 g007
Table 1. Elemental concentrations (mg/kg) of GL.
Table 1. Elemental concentrations (mg/kg) of GL.
KCaPMgNaFeAlSiMnZn
48,730.212,360.57725.43628.8544.5255.7266.2182.073.745.6
SrBaTiTREEsLREEsHREEsYLaCeDy
20.118.810.3510.5247.1263.1125.4119.9122.2135.9
Note: TREEs, LREEs, and HREEs represent total, light (La, Ce, Pr, Nd, and Sm), and heavy (Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) rare earth elements, respectively.
Table 2. Textural parameters for GL residue and hydrochar obtained at 120 °C, 140 °C, 160 °C, and 180 °C.
Table 2. Textural parameters for GL residue and hydrochar obtained at 120 °C, 140 °C, 160 °C, and 180 °C.
SBET/m2/gVt/cm3/gAverage Pore Diameter/nm
GL residue0.0520.0041.519
MHTC-1201.6250.0091.528
MHTC-1402.3450.0161.578
MHTC-1605.6150.0331.582
MHTC-1809.4280.0401.908
Table 3. Elemental analysis and yield of GL residue and hydrochar.
Table 3. Elemental analysis and yield of GL residue and hydrochar.
SampleYield/%Elemental Analysis/%
CHOO/C aH/C a
GL residue-44.285.5240.970.691.50
MHTC-12070.544.425.5040.810.691.49
MHTC-14062.644.815.4840.360.681.47
MHTC-16057.846.435.4639.600.641.41
MHTC-18054.249.675.4538.190.581.32
O/C a and H/C a are given in molar ratio.
Table 4. Proximate analysis, HHV, ERE, and energy densification of GL residue and hydrochar.
Table 4. Proximate analysis, HHV, ERE, and energy densification of GL residue and hydrochar.
SampleProximate AnalysisHHV/MJ/kgERE/%Energy Densification
MVMFCAsh
GL residue5.9076.6314.782.6918.09--
MHTC-1205.6275.7515.623.0118.1170.591.00
MHTC-1405.1973.3017.993.5218.1962.941.01
MHTC-1603.9067.3724.763.9718.6159.491.03
MHTC-1800.6255.9737.995.4219.4958.411.08
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Li, S.; Ji, B.; Zhang, W. Rare Earth Element Recovery and Hydrochar Evaluation from Hyperaccumulator by Acid Leaching and Microwave-Assisted Hydrothermal Carbonization. Minerals 2024, 14, 277. https://doi.org/10.3390/min14030277

AMA Style

Li S, Ji B, Zhang W. Rare Earth Element Recovery and Hydrochar Evaluation from Hyperaccumulator by Acid Leaching and Microwave-Assisted Hydrothermal Carbonization. Minerals. 2024; 14(3):277. https://doi.org/10.3390/min14030277

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Li, Shiyu, Bin Ji, and Wencai Zhang. 2024. "Rare Earth Element Recovery and Hydrochar Evaluation from Hyperaccumulator by Acid Leaching and Microwave-Assisted Hydrothermal Carbonization" Minerals 14, no. 3: 277. https://doi.org/10.3390/min14030277

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