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Article

Hydrothermal Conversion of Sn-Bearing Sludge into Fe/S Rods for Efficient Heavy Metal Removal in Wastewater

by
Shengyao Ma
1,
Wu Yang
1,*,
Weilu Yang
2 and
Yu Chen
2,*
1
School of Environment, Northeast Normal University, Changchun 130117, China
2
No. 102 Joint Lab, Zhongkai University of Agriculture and Engineering, Guangzhou 510225, China
*
Authors to whom correspondence should be addressed.
Separations 2025, 12(6), 153; https://doi.org/10.3390/separations12060153
Submission received: 30 April 2025 / Revised: 21 May 2025 / Accepted: 23 May 2025 / Published: 6 June 2025
(This article belongs to the Section Environmental Separations)
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Abstract

:
Hydrothermal conversion is an effective strategy to transform heavy metals in electroplating sludge into catalytic materials and use them to treat electroplating wastewater. This study presents a one-step hydrothermal method for converting Sn-bearing sludge, containing 23.41% Sn, 52.12% Fe, and other impurities, into Fe/S rods using a NaOH/Na2S solution. The resulting Fe/S rods, with a diameter of 50–100 nm and length of 0.5–2.5 μm, showed excellent performance in wastewater treatment. In the presence of 50 mg/L EDTA, the Fe/S rods removed 22.9% of Ni, 30.2% of Cu, and 41.5% of Zn. When activated with PMS, the removal efficiencies increased significantly to 68.9%, 90.9%, and 91.6% for Ni, Cu, and Zn, respectively. The optimal rod dosage (1 g/L) achieved removal efficiencies of 94.2%, 78.5%, and 99.7% for Cu, Ni, and Zn, while increasing PMS dosage led to nearly 100% removal within 60 min. Additionally, the process allowed for the complete recycling of the alkaline solution, with regenerated rods showing similar performance to the original ones in wastewater treatment. This method offers an efficient and sustainable approach to sludge resource utilization and heavy metal removal from wastewater.

Graphical Abstract

1. Introduction

During smelting and industrial processing, a large amount of tin is released into the wastewater, and due to the addition of coagulants and flocculants, tin settles in the wastewater as tin-containing sludge. Composed mainly of tin oxides and metallic tin, with important impurities such as iron, aluminum, silicon, and calcium [1], the sludge is classified as hazardous waste and requires appropriate treatment to mitigate environmental risks. Sn2+ and Fe2+ as well as Sn4+ and Fe3+ ions in sludge can often be substituted with each other, because these ions are very close in size and charge distribution, resulting in them occupying the same position in the same crystal structure and showing similar chemical properties. Tin is thus classified as a ferriphilic element [2], and the separation of iron and tin is difficult.
The disposal of Sn-containing sludge generally involves two main stages: Sn recovery and final treatment. For Sn recovery, techniques such as pyrometallurgical roasting [2] and flotation [3] are employed to concentrate Sn, followed by chlorination or reduction calcination to produce vapor, which is condensed and recovered as Sn salt or powder [4]. Additionally, hydrometallurgical approaches have been explored, where NaOH leaching has demonstrated high Sn recovery rates of over 98% from sludge [5]. In this process, Sn is extracted into an alkaline solution, and subsequent displacement by Zn powder or plates precipitates Sn as metallic deposits [6]. However, both pyrometallurgical and hydrometallurgical methods generate residual byproducts containing Sn, necessitating further treatment. Low-Sn-content sludge is often co-incinerated to produce geopolymers [7] or stabilized for landfill disposal [8], yet such methods are not optimal for sustainable resource utilization.
Since tin-containing sludge contains a large amount of iron, mainly in the form of iron hydroxide, it provides opportunities for adding value. In the presence of reducing agents, iron-containing hydroxides can be converted to magnetic adsorbents by hydrothermal methods [9] or calcination [10,11]. In addition, sludge has been used to synthesize liquid flocculants [12]. However, challenges arise when tin dissolves under acidic conditions, with Sn2+ concentrations far exceeding WHO standards for drinking water [13]. However, in the traditional alkaline leaching recovery process, iron oxide precipitated as a resource is often difficult to re-treat [2]. Relevant studies have shown that by adding FeS, it can be further recovered by FeO and FeCl3 [14], which significantly reduces the concentration of pollutants in wastewater. Recent advances in hydrothermal methods indicate that it is potentially possible to transform electroplating sludge into functional materials for wastewater treatment. For example, RS/PVDF membranes can be made using sludge and iron to remove Cr(VI) and oil in sewage [15]. NaFeS2·2 H2O derived from iron-rich sludge also shows significant catalytic activity in PMS activation [16]. Adsorption is also possible. For example, Teng et al. synthesized mesoporous mil-100 (FE) from acid mine drainage sludge to adsorb Norfloxacin [17]. However, existing studies usually focus on the recovery of single metals, ignoring the integrated treatment of polymetallic sludge and complex wastewater.
As a common chelating agent in industrial wastewater, EDTA forms highly stable complexes with heavy metals (logK value > 16), which significantly hinders their removal by traditional methods [18], posing a serious risk to the environment due to their toxicity and persistence in water bodies [19]. The decomposition efficiency of metal–EDTA complexes by the traditional Fenton process is less than 30%. However, advanced oxidation technologies such as PMS activation provide higher free radical generation efficiency [20]. Similar studies have shown that catalytic materials rich in iron and sulfur can activate PMS to achieve the decomposition of metal–EDTA complexes [21]. The decomposed metal ions can be removed by adsorption after forming hydroxide, and competitive adsorption of heavy metals occurs in the presence of multi-metal hydroxides [22]. This is closely related to the environment and the specific surface area and functional groups of adsorption materials [23].
In this study, a one-step hydrothermal process was developed to convert tin-containing sludge into Fe/S rods, in which the iron and tin are separated, the iron is completely converted into Fe/S rods, and the tin is completely leached into the liquid. The Fe/S rods prepared by the hydrothermal method showed excellent performance in PMS activation and heavy metal removal. In terms of heavy metal removal, not only is high efficiency achieved but also complete recovery of alkaline solution, reducing the generation of secondary waste. The tin in the solution was subsequently fixed by limestone as CaSnO3, thereby minimizing the risk of secondary contamination.

2. Materials and Methods

2.1. Alkaline Hydrothermal Treatment of Sludge

The sludge used in this study was obtained from the hazardous waste station of Huadong Electroplating Company (Bazhou, China). The sludge was directly employed for synthesizing Fe/S rods without any pretreatment (Figure 1). In a typical procedure, 1 g of sludge was added to a mixture of 30 mL 0.48 M NaOH and 0.32 M Na2S solutions, which was stirred continuously at 200 rpm for 30 min. This process resulted in the formation of a suspension, which was subsequently transferred into a 50 mL Teflon vessel. The mixture was then heated to 160 °C for 7 h. After the reaction, the vessel was allowed to cool to room temperature and opened. The supernatant was carefully separated from the blackish deposit. Finally, the deposit was vacuum-dried at 80 °C overnight, resulting in the Fe/S rods, which were utilized for wastewater treatment.

2.2. Wastewater Treatment

The wastewater used in this study was collected from the discharge outlet of the electroplating wastewater treatment plant at Huadong Company. The discharged wastewater required further treatment as it did not meet the local government’s discharge standards. To complicate its composition, 50 mg/L of EDTA was added to the wastewater. Two treatment methods were employed.
The first method was adsorption. One gram of Fe/S rods was added to 50 mL of wastewater, followed by agitation at 300 rpm for 60 min. Afterward, a sample of the wastewater was collected and centrifuged at 5500 rpm for 5 min to separate the solid and liquid phases. The liquid phase was then collected for further characterization. A control experiment was performed by replacing the Fe/S rods with Na2S and PAC, following the same procedure.
The second method involved using PMS as a catalyst. In this case, 1 g of Fe/S rods and 0.06 g of PMS were added to 50 mL of wastewater, which was then agitated at 300 rpm for 60 min. At specified intervals, 1 mL of liquid was sampled and mixed with 1 mL of methanol to quench the free radicals. The sample was then centrifuged at 5500 rpm for 5 min to separate the solid and liquid phases. Optimization experiments were conducted with rod and PMS amounts ranging from 0.2 to 2 g and 0.03 to 0.12 g, respectively. A supplementary experiment was also conducted by replacing the Fe/S rods with ferrous sulfate.

2.3. Characterization

The metal concentration was analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES, Avio 200, PerkinElmer, Bridgeeville, PA, USA). The solid samples were characterized by SEM (SEM, QuantaTM-250-FEG, FEI, Hillsboro, OR, USA), XRD (XRD, Rint2200, Rigaku Corporation, Tokyo, Japan), XRF (XRF, XRF-1800, Shimadzu, Kyoto, Japan), and XPS (XPS, ADES-400, VG Scientific Engla, St Lonards, UK). During the hydrothermal treatment, a large volume of alkaline solution was generated, which was subsequently recycled. To regenerate the solution, 0.3 g of limestone was added to 30 mL of the used liquid, and the mixture was agitated at 200 rpm for 2 h. The supernatant was then collected and regenerated by adding 2.88 g of NaOH and 3.75 g of Na2S. The regenerated solution was fully utilized to synthesize Fe/S rods, following the procedure outlined in Section 2.1. The regenerated Fe/S rods were also employed in the wastewater treatment.

3. Results and Discussion

3.1. Conversion of Sludge into Fe/S Rods

The raw sludge was composed of irregularly shaped particles, with broad diffraction peaks observed in the 20–40° range (Figure 2a,c, Raw sludge). The elemental composition of the sludge included 52.12% Fe, 11.91% Al, 23.41% Sn, 2.02% Ca, 1.47% S, and 1.64% Na, along with other impurities (Figure 2d, raw sludge). After undergoing hydrothermal treatment, the resulting product exhibited regular rod-like structures with diameters ranging from 50 to 100 nm and lengths from 0.5 to 2.5 μm (Figure 2b). The XRD patterns of the rods revealed sharp peaks corresponding to erdite crystals at 2θ values of 12.59°, 16.55°, 19.48°, and 30.8° (Figure 2c, Fe/S rod) [24]. The composition of the Fe/S rods was enriched in Fe and S, with Fe, S, and Na constituting 25.14%, 36.59%, and 17.06%, respectively. The content of Al, Sn, and Ca was reduced to 1.02%, 1.35%, and 2.4%, respectively (Figure 2d, Fe/S rod).
The conversion process was also confirmed by unique features observed in the XPS spectra. In the raw sludge, the Fe 2p peak (Figure 3a, raw sludge) at a binding energy of 710.59 eV indicated the presence of trivalent iron, while the broad S 2p peak (Figure 3b, raw sludge) at 167.5 eV suggested sulfur in the form of sulfate. The Sn 2p peaks (Figure 3c, raw sludge) observed at 487.8 eV and 496.2 eV were characteristic of bivalent tin [25]. In contrast, for the Fe/S rods, the Fe 2p peak (Figure 3a, Fe/S rod) shifted to 707.8 eV, consistent with the Fe present in the erdite structure [26]. New S 2p peaks (Figure 3b, Fe/S rod) were observed at 160.3 eV and 161.6 eV, corresponding to the formation of elemental sulfur and S2− species [27]. Additionally, the Sn peaks remained unchanged (Figure 3c, Fe/S rod), indicating that Sn did not participate in the formation of erdite. These results conclusively demonstrated the successful conversion of the sludge into Fe/S rods.

3.2. Application of Fe/S Rods in Wastewater Treatment

The wastewater used in this study initially contained 3.4 mg/L of Ni, 1.2 mg/L of Cu, and 1.8 mg/L of Zn. After the addition of 50 mg/L EDTA, the wastewater was treated directly with Fe/S rods, resulting in the removal of 22.9% of Ni, 30.2% of Cu, and 41.5% of Zn. The removal efficiencies for Cu, Ni, and Zn using the Fe/S rods were higher than those achieved with PAC, Na2S, raw sludge, or pH adjustment (Figure 4a). However, when both PMS and Fe/S rods were used, the removal efficiencies increased significantly, reaching 68.9%, 90.9%, and 91.6% for Ni, Cu, and Zn, respectively (Figure 4a), surpassing the efficiencies observed with PMS alone or PMS combined with ferrous sulfate.
The optimal dosages of Fe/S rods and PMS were further investigated. When the rod dosage was increased from 0.2 to 1 and 2 g/L, the removal efficiencies of Cu, Ni, and Zn initially increased from 38%, 33.5%, and 77.7% to 94.2%, 78.5%, and 99.7%, respectively, before decreasing to 83.5%, 43.5%, and 97.7% at higher dosages (Figure 4b). In contrast, when the PMS dosage was increased from 2 mmol/L to 8 mmol/L, the removal efficiencies consistently improved, reaching 99.4%, 96.8%, and 99.2% for Cu, Ni, and Zn, respectively (Figure 4c). The removal efficiencies of Cu, Ni, and Zn were 88.8%, 59.6%, and 95.6%, respectively, within the first 5 min, and nearly 100% after 60 min of treatment (Figure 4d). Post-treatment, the remaining wastewater contained 0.011 mg/L of Cu, 0.106 mg/L of Ni, and 0.002 mg/L of Zn, which met the discharge standards set by the local government.
Following treatment, numerous irregular aggregates were observed to adhere to the regular rod structures (Figure 5a). As a result, the characteristic peaks of erdite in the XRD patterns weakened compared to the raw rods (Figure 5b). The XPS spectra showed a decrease in the intensity of the characteristic peaks of structural Fe in erdite (Figure 5c) and the typical S2− peaks (Figure 5d), indicating that erdite decomposition occurred during PMS activation. Although the Sn 3d peaks remained unchanged (Figure 5e), two new peaks corresponding to Ni 2p were observed at binding energies of 855.88 eV and 872.52 eV (Figure 5f). This suggests that a portion of the erdite decomposed and adsorbed heavy metals from the wastewater.

3.3. Mechanism of Fe/S Rod Formation and Application

The raw sludge contained Fe, Al, Sn, Ca, and Si-bearing oxyhydroxides. Upon treatment in NaOH/Na2S solution, the Fe-bearing oxyhydroxides were transformed into Fe/S rods through several steps (Figure 6). Initially, the Fe-bearing oxyhydroxides exhibited surface defect sites that were attacked by free OH− in the solution, leading to the release of free Fe(OH)4− into the solution. This reaction occurred only at pH values greater than 13 and was accelerated at high NaOH concentrations. Secondly, HS− ions present in the solution had a higher affinity for Fe than OH−, resulting in the formation of the intermediate Fe(OH)3HS−. Subsequently, two adjacent Fe(OH)3HS− units combined to form FeS2Fe(OH)42−. This conjugation reaction was endothermic and only took place at elevated temperatures. Finally, as the reaction progressed, (FeS2)n− chains were formed [28]. These chains contained spare electrons and free channels occupied by Na+ and water molecules, which facilitated the formation of erdite crystals. The other oxyhydroxides, including those of Al, Sn, Ca, and Si, did not participate in the formation of erdite crystals. Notably, Al, Sn, and Si dissolved into the alkaline solution, while the Ca−bearing oxyhydroxide remained undissolved and was retained in the generated rods.
The Fe/S rods underwent spontaneous hydrolysis in the wastewater, reversing the transformation process described above. As hydrolysis proceeded, numerous flocs containing abundant Fe–SH groups were generated. These Fe–SH groups released free H+ into the liquid and exhibited strong complexation with cationic heavy metals, enhancing their adsorption from the wastewater [29], compared to the free OH– and C–OH groups. Consequently, the Fe/S rods demonstrated superior efficiency in wastewater treatment relative to PAC and pH adjustment methods. However, the wastewater contained sufficient EDTA to complex the heavy metals into organic–heavy metal ligands, which formed large spatial structures that hindered the adsorption of free flocs from the central heavy metal ions [9]. As a result, the removal efficiencies of Cu, Zn, and Ni were relatively low.
When PMS was added to the wastewater, it was activated by the flocs, releasing abundant free radicals such as –OH and SO4−. The activation occurred in two main steps (Figure 7). First, PMS attacked the structural Fe in the Fe–SH groups on the flocs, generating free radicals and in situ ferric iron. Second, a redox reaction between ferric iron and the adjacent –SH group reduced the ferric iron and generated elemental sulfur. This created a ferrous/ferric iron cycle on the flocs [30]. As the flocs continued to form, PMS activation was accelerated, resulting in a significant production of free radicals. These radicals attacked the organic ligands, decomposing their large spatial structures and releasing free heavy metals into the wastewater. The freed heavy metals were then adsorbed by the flocs, leading to efficient removal of heavy metals from the wastewater.

3.4. Additional Applications

The method described above was effective in converting raw sludge into erdite crystals; however, it also produced alkaline liquid waste containing 4.9 g/L Sn, 2.3 g/L Al, 0.1 g/L Si, and sufficient S2− and OH−. This liquid, classified as alkaline hazardous waste, was completely recycled by adding limestone in this study. The limestone treatment rapidly precipitated Sn, Al, and Si from the liquid, while the concentration of S2− remained unchanged [31] and the OH− concentration increased, resulting in a purified liquid. The precipitates were used as raw material for Sn smelting. The treated liquid was further adjusted by adding NaOH and Na2S, after which it was reused for the synthesis of Fe/S rods. The newly synthesized Fe/S rods exhibited similar structural characteristics and XRD patterns to the raw sludge rods (Figure 8a,b) and were also effective in activating PMS for wastewater treatment. When 1 g/L of the regenerated rods was added, the removal efficiencies of Cu, Ni, and Zn were 95.2%, 87.6%, and 99.8%, respectively, which were similar to those achieved with the raw sludge rods (Figure 8c).

4. Conclusions

This study presents a novel hydrothermal method for converting Sn-bearing sludge into Fe/S rods, which were highly effective in treating wastewater contaminated with heavy metals. In contrast to Fe, Sn in the sludge is almost completely leached into the liquid phase. CaO was added to immobilize Sn, and the remaining alkaline solution was fully recovered, significantly reducing secondary waste. The Fe/S rods demonstrated enhanced metal removal capabilities, especially when activated with PMS, achieving near-total removal of Cu, Ni, and Zn. The method also showed promise for regenerating and reusing the Fe/S rods, maintaining their treatment efficiency. This approach provides a sustainable solution for the disposal of Sn-bearing sludge and the treatment of heavy metal-contaminated wastewater, offering a valuable application in both resource recovery and environmental management.

Author Contributions

Conceptualization, S.M. and W.Y. (Weilu Yang); methodology, S.M. and Y.C.; formal analysis, S.M.; resources, Y.C. and W.Y. (Wu Yang); data curation, S.M. and W.Y. (Weilu Yang); writing—original draft preparation, S.M. and W.Y. (Weilu Yang); writing—review and editing, W.Y. (Wu Yang); supervision, Y.C. and W.Y. (Wu Yang); project administration, Y.C.; funding acquisition, W.Y. (Wu Yang). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China, grant number 52370158, the Science and Technology Program of Jilin Province, grant number 20240304153SF, and the Featured Innovation Project of Guangdong Provincial Department of Education, grant number 2023KTSCX050.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
TLAThree-letter acronym
LDLinear dichroism
Fe/S rodsA rod-shaped material rich in iron and sulfur elements
PMSPeroxymonosulfate
PACPolybasic aluminum chloride

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Separations 12 00153 g001
Figure 1. Flow chart of sludge converted to rods for PMS activation.
Figure 1. Flow chart of sludge converted to rods for PMS activation.
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Separations 12 00153 g002
Figure 2. SEM images of (a) raw sludge and (b) Fe/S rod, (c) XRD patterns, and (d) major composition of the raw sludge and Fe/S rods.
Figure 2. SEM images of (a) raw sludge and (b) Fe/S rod, (c) XRD patterns, and (d) major composition of the raw sludge and Fe/S rods.
Separations 12 00153 g002
Separations 12 00153 g003
Figure 3. High-resolution XPS spectra of (a) Fe 2p, (b) S 2p, and (c) Sn 3d for the raw sludge and Fe/S rod.
Figure 3. High-resolution XPS spectra of (a) Fe 2p, (b) S 2p, and (c) Sn 3d for the raw sludge and Fe/S rod.
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Separations 12 00153 g004
Figure 4. (a) Comparison of methods; effect of (b) rod dosage, (c) PMS dosage, and (d) time course of Cu, Ni, and Zn removal.
Figure 4. (a) Comparison of methods; effect of (b) rod dosage, (c) PMS dosage, and (d) time course of Cu, Ni, and Zn removal.
Separations 12 00153 g004
Separations 12 00153 g005
Figure 5. (a) SEM image and (b) XRD pattern, and high-resolution XPS spectra for (c) Fe 2p, (d) S 2p, (e) Sn 3d, and (f) Ni 2p of used rods.
Figure 5. (a) SEM image and (b) XRD pattern, and high-resolution XPS spectra for (c) Fe 2p, (d) S 2p, (e) Sn 3d, and (f) Ni 2p of used rods.
Separations 12 00153 g005
Separations 12 00153 g006
Figure 6. Illustration of the conversion of sludge to Fe/S rods.
Figure 6. Illustration of the conversion of sludge to Fe/S rods.
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Separations 12 00153 g007
Figure 7. Application of Fe/S rods in wastewater treatment.
Figure 7. Application of Fe/S rods in wastewater treatment.
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Separations 12 00153 g008
Figure 8. (a) SEM and (b) XRD patterns of Fe/S rods synthesized using regenerated alkaline solution, and (c) their performance in wastewater treatment.
Figure 8. (a) SEM and (b) XRD patterns of Fe/S rods synthesized using regenerated alkaline solution, and (c) their performance in wastewater treatment.
Separations 12 00153 g008
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Ma, S.; Yang, W.; Yang, W.; Chen, Y. Hydrothermal Conversion of Sn-Bearing Sludge into Fe/S Rods for Efficient Heavy Metal Removal in Wastewater. Separations 2025, 12, 153. https://doi.org/10.3390/separations12060153

AMA Style

Ma S, Yang W, Yang W, Chen Y. Hydrothermal Conversion of Sn-Bearing Sludge into Fe/S Rods for Efficient Heavy Metal Removal in Wastewater. Separations. 2025; 12(6):153. https://doi.org/10.3390/separations12060153

Chicago/Turabian Style

Ma, Shengyao, Wu Yang, Weilu Yang, and Yu Chen. 2025. "Hydrothermal Conversion of Sn-Bearing Sludge into Fe/S Rods for Efficient Heavy Metal Removal in Wastewater" Separations 12, no. 6: 153. https://doi.org/10.3390/separations12060153

APA Style

Ma, S., Yang, W., Yang, W., & Chen, Y. (2025). Hydrothermal Conversion of Sn-Bearing Sludge into Fe/S Rods for Efficient Heavy Metal Removal in Wastewater. Separations, 12(6), 153. https://doi.org/10.3390/separations12060153

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