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. 2025 Nov 27;15:42471. doi: 10.1038/s41598-025-26562-4

A steel slag-activated column for co-removal of sulfate and metals from acid mine drainage

Qiang Zheng 1, Xinli Du 2, Zhaoliang Wang 3, Aijing Wu 4, Hugang Li 1,✉
PMCID: PMC12660713  PMID: 41309889

Abstract

Acid mine drainage (AMD), containing excessive sulfates and heavy metals, poses a severe threat to the environment and human health. In this study, a laboratory-scale column device using varying ratios of coarse sand to carbon steel slag (CS/CSS) as the filled material was designed to efficiently remove typical pollutants in AMD. Results revealed that the removal rates of SO42−, Fe, Mn, and Zn, as well as the pH value of the solution were influenced by the ratio of CS/CSS. The highest removal rates of SO42−, Fe, Mn, and Zn were 92%, 99.99%, 99.98%, and 99.99%, respectively, for CS/20%CSS. Additionally, XPS and SEM results indicated that sulfate (SO42−) existed in that groups associated with CaSO4 and Fe-sulfate complexes, while Fe, Mn, and Zn were simultaneously attached to the surface of the materials in the form of ion exchange, hydroxides, and co-precipitates. A modified sequential extraction method (BCR) results indicated that the distribution patterns of Fe, Mn, and Zn varied with increasing CSS content. Notably, when the CSS content in the column reached 20%, the metal ions were predominantly adsorbed in the top layer. The Risk Assessment Code values for Fe exhibited a low risk to the environment. These results highlight the importance of optimizing the CSS content in the permeable reactive barrier (PRB) to balance removal efficiency and permeability. Consequently, it is recommended that the content of CSS in the PRB should be maintained below 10% to ensure optimal performance and permeability. This study provides a foundation for optimizing PRB filled material design to enhance its efficiency and environmental sustainability in AMD treatment.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-26562-4.

Keywords: Acid mine drainage, Heavy metal, Speciation transformation, Sulfate, Adsorption

Subject terms: Environmental sciences, Hydrology

Introduction

The acid mine water (AMD) from abandoned coal mines is characterized by low pH values, high concentrations of sulfate and heavy metal ions1,2, which far exceeds the drinking water standards set by China and the World Health Organization3,4. The AMD has severely polluted surface and groundwater bodies, destroyed ecosystems5, hindered plant growth6,7, and ultimately threaten human health through the food chain8. Therefore, it is very necessary to study the treatment of AMD seeping from abandoned coal mine areas.

A broad range of ex-situ and in-situ treatment techniques have been developed for AMD remediation9–12. Among them, permeable reactive barrier (PRB) is one of the most promising in-situ passive treatment due to their low costs and wide suitability for their efficient immobilization of multiple contaminants12. The PRB is installed along the AMD flow path to intercept and degrade contaminants, ultimately producing treated water13.This also indicates that the filling active materials of PRB are key elements in the treatment of wastewater. Therefore, different types of active media, such as reductive, neutralizing, microbial degradation, and adsorption types, be selected to fill in the PRB based on the types of contaminants14,15. In addition, the permeable active materials filled in the PRB must also have the following characteristics16–19: (1) the ability to regulate pH value, (2) effective permeability and porosity, (3) the filling materials must not release toxic substances into the environment and no secondary pollution, (4) the filling medium is available abundance at low cost. Given the characteristics of the water quality of AMD from abandoned coal mine, the selection of adsorption media (such as steel slag, an industrial byproduct) inside the PRBs can provide an economical, abundantly available, and effective treatment for AMD. This approach achieves the goal of waste control by waste.

Steel slag, as a solid waste produced in the steelmaking process, is an alkaline substance20,21. A large number of batch experiments indicate that steel slag has potential adsorption due to its inherent chemical composition and structural characteristics22,23. It can not only raise the pH value but also efficiently remove metal ions of AMD (such as iron, zinc, manganese, and other metal ions.)23–25. In previous research, it was demonstrated that powdered carbon steel slag can effectively adsorb sulfates in AMD, the removal efficiency of sulfates reaches over 90%23. The removal mechanisms for carbon steel slag to remove sulfates from AMD is primarily dominated by chemical precipitation, adsorption and ion exchange. In practical applications, although permeable reactive barriers (PRBs) utilizing solely steel slag as the filling material can effectively treat acid mine drainage (AMD), the relatively low permeability and porosity of carbon steel slag may impede the flow of AMD through the barrier. To enhance the hydraulic conductivity, coarse sand (a well-graded, low-cost granular material) is introduced as the aggregate, while steel slag serves as the primary adsorbent for AMD remediation. A homogeneous mixture of these two media is employed as the PRB filling material. This combined system not only maintains effective AMD treatment but also significantly improves the permeability of the barrier, thereby enhancing its long-term performance and treatment efficiency. Moreover, a variety of sulfate and metal ions coexist in AMD, the removal efficiency would be affected by the competition among sulfate and metal ions. This, in turn, affects the design of the PRB. Therefore, to effectively treat AMD, it is crucial to design a PRB with proper permeability that not only raises the pH value but also efficiently removes various contaminants.

Hence, the objectives of this study are: (1) to investigate the dynamic adsorption patterns of PRB treating actual AMD; (2) to analyze the speciation distribution of metal ions, and to elucidate the migration and transformation mechanisms of typical pollutants within the PRB system using the BCR; (3) to assess the environmental risk of the reacted PRB filling material based on the RAC; and (4) to characterize the mineralogical composition and chemical bonding characterization of the CS/CSS mixture before and after AMD remediation through SEM, EDS, and XPS analyses. The innovation of this paper lies in revealing and quantifying the efficacy of PRB treated AMD under different CS/CSS ratios, and optimizing the proportion of PRB filler material. This research provides a scientific rationale and technical support for long-term, highly efficient and stable treatment of AMD by the PRB system. It holds significant scientific value and practical engineering implications for advancing the widespread application of this technology in real-world environment.

Materials and method

Materials

The AMD samples from abandoned coal mine in Shandi River Basin has overflowed between 2009 and 2010, which flows into the Wen River, and cause pollution of surface water, groundwater, and soil. Table S1 shows the mainly chemical composition of the actual AMD. The CSS in this study was provided by a steel-making factory, Taiyuan China. It was sifted through a 200-mesh sieve (0.075 mm), dried at 105 ℃ for 24 h, and then cooled to ambient temperature. The sifted CSS was used for the experimental work. The main chemical components of CSS were analyzed by the Shanxi Provincial Geological Prospecting Bureau, and the result was shown in Table S2. The CS comes from Do Luo Town, Xinzhou City, Shanxi Province, dried at 105 ℃ for 24 h, and then cooled to ambient temperature. The sample was then sieved using standard sieves, the particle size distribution of the CS is presented in Table S3.

Column experiment

Column experiments were carried out to investigate the proportion of PRB filled materials, and the dynamic adsorption pattern for PRB remediation actual AMD. The CSS was used as the absorbent material of PRB, and the CS as the aggregate to increase the permeability of the PRB, which the proportion of CSS/CS is 0.5/9.5, 1/9, and 2/8, respectively.

Therefore, three experimental columns were made of Cylindrical Perspex with an internal diameter of 10 cm and a height of 50 cm, were set up as shown in Fig. 1. First, the bottom of the column is filled with 2 cm of quartz sand (1 ~ 2 mm) to ensure uniform drainage. Secondly, the uniformly mixed material of CS/CSS is filled into the column, and the height of the filling material is 30 cm. Finally, 2 cm of quartz sand (1 ~ 2 mm) is laid on top to prevent erosion of the filling material by AMD. The flow rate of AMD in the experimental column was controlled by 10 cm constant water head; the hydraulic retention time (HRT) was kept at 24 h for the three columns. The effluent was recorded as 0 h from outlet bottom of column, and the samples was collected at each outlet every 24 h. The column experiment was conducted for 30 days of continuous AMD treatment. The pH values, sulfate and metal ion (Fe、Mn、Zn) concentrations at each outlet were measured, respectively. The sulfate concentration was analyzed using the Ion Chromatograph (Metrohm 883 Basic IC plus, Switzerland). Concentrations of Fe, Mn, and Zn were determined using the Atomic Absorption Spectroscopy (TAS-990, China).

Fig. 1.

Fig. 1

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Schematic design of the experimental setup.

Speciation of metals and environmental risk assessment

To investigate the variation of metal speciation patterns with depth, samples were collected from the top, middle, and bottom of the column. The distribution characteristics of metal speciation in the samples were analyzed utilizing the BCR. Sequential extraction was performed using a 5-stage modified procedure method by BCR (Table S4)26,27. Additionally, the Risk Assessment Code (RAC) was conducted to assess the environmental risks of metal in the samples. The RAC was expressed as the percentage of acid-soluble fraction metal (F1 + F2) in total metal content (F1 + F2 + F3 + F4 + F5). The five classifications of RAC include a safe level, low-risk level, medium-risk level, high-risk level, and very high-risk level (Table S5)28.

Analysis

CS/CSS mixture samples, collected before and after their use as reactive media in the PRB for AMD remediation, were characterized using SEM, EDS, and XPS. The microstructural features, the distribution, and elemental composition of the samples was characterized by a scanning electron microscope (SEM, Zeiss supra55, Germany) equipped with an energy-dispersive spectrometer (EDS). The analysis of elements and valence states on samples surface was executed the X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB-250Xi, American) by using Al K-Alpha radiation (1486.6 eV). Survey and Narrow scans were collected with a pass energy of 150 eV and 30 eV, and an energy size of 1.0 eV and 0.05 eV, respectively. Narrow scan spectra of S(2p), Fe(2p), Mn(2p) and Zn (2p) was calibrated using the C1s peaks at 284.8 eV. XPS spectra peak were fitted using the Thermo Scientific™ Avantage Software.

Results and discussions

The dynamic pattern of pollutants

pH variation pattern

Figure 2 showed the pH values of treated AMD versus time with different ratios of CS/CSS in column. It was evident that the pH values increased rapidly within the first 24 h, and then it stabilized over time as the content of CSS increased. The pH values of treated CS/CSS mixture (CS/ 5% CSS, CS/ 10% CSS, and CS /20% CSS) were 5.68, 6.82, and 12.05, respectively. The reason was that the large quantities alkaline components (Table S2) in CSS can donate hydroxide ions to the solution. When low-pH of AMD contacted with CSS, the alkaline components dissolved, releasing Ca2+ and Mg2+ ions, which can provide hydroxide ions (OH−) to the solution and increase the pH value of the solution21. However, when the content of the CSS is 20%, the pH of the solution exceeds 12. In such a highly alkaline environment, aquatic plants and microorganisms were difficult to grow and reproduce29. In view of this, the content of CSS in the PRB should not exceed 10%.

Fig. 2.

Fig. 2

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The pH evolution of effluent under different CS/CSS mixture ratio.

SO42− variation pattern

Figure 3 showed the variation of SO42− over time with different ratios of CS/CSS during a period of 30 days. In phase Ⅰ (1–9 days), the SO42− removal rate decreased from 52.99 to 19.06% for CS/ 5% CSS, and from 65.23 to 46.47% for CS/ 10% CSS. However, when the content of CSS was 20% in the column, the removal rate of SO42− increased from 83.46 to 96.27%. The results indicated that the removal rate of SO42− was significantly influenced by the content of CSS in the CS/CSS mixture. The reason was that, as the CSS content of the column increased, both the adsorption sites and the specific surface area increased, which could effectively adsorb sulfate. Additionally, the dissolution of Ca2+ from the Ca-rich CSS reacted with SO42− to form insoluble gypsum (CaSO4·2H2O)22,30. In phase Ⅱ (9–27 days), the removal rate of SO42− continued to decrease from 19.06 to 10.91% for CS/ 5% CSS, and from 46.27 to 28.60% for CS/ 10% CSS. Compared to the phase I, the decline trend of SO42− removal rate slowed down. Notably, the removal rate of SO42− only decreased from 96.27 to 92.87% for CS/ 20% CSS, which indicated the high adsorption ability of the column. The specific surface area, pore volume, and pore diameter of the CSS were 36.82 m2/g, 0.35 cm3/g, and 34.49 nm, respectively. which offered abundant adsorption sites for sulfate adsorption23. During the phase Ⅰ, pollutants adsorbed to the CSS surface, leading to a decrease in PRB permeability. The consequent extension of hydraulic retention time enhances interaction between pollutants and the steel slag. Throughout this process, unexhausted active sites on the slag surface continue to engage in reactions with pollutants, accompanied by the formation of gypsum (CaSO₄·2H₂O) precipitates31,32. In phase Ⅲ (days 27–30), the removal rate of SO42− ions has stabilized, reaching an average removal rate of 11.64% for CS/ 5% CSS, 27.96% for CS/ 10% CSS, and 92.70% for CS/ 20% CSS. Importantly, the 20% CSS mixture retained high removal efficiency without reaching saturation. Although higher CSS content in the column improved the AMD removal efficiency, it also reduced the porosity of the column, which blocked the AMD flow through the PRB.

Fig. 3.

Fig. 3

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Removal rate of SO42− (a) and concentration evolution of SO42− (b) in the effluent under different CS/CSS ratio groups.

Fe variation pattern

Figure 4 illustrated the variation of iron (Fe) ion concentration in treated acid mine drainage (AMD) over time under different CS/CSS ratios.

Fig. 4.

Fig. 4

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Removal rate of Fe (a) and concentration evolution of Fe (b) in the effluent under different CS/CSS ratio groups.

In phase Ⅰ (initial 6-day reaction period), the Fe removal rate increased from 92.11 to 95.49% for CS/ 5% CSS column experiment. Higher CSS additions resulted in more stable and efficient removal, which average efficiencies reached 99.95% for CS/ 10% CSS, and 99.98% for CS/ 20% CSS, respectively. This is attributed to the increased availability of adsorption sites and elevated solution pH with higher CSS content, promoting substantial adsorption of Fe ions and subsequent formation of Fe(OH)₃ precipitates during treatment33,34. In phase Ⅱ (days 6–14), the removal rate of Fe ions had stabilized, reaching an average removal rate of 96.15% for CS/ 5% CSS, 99.98% for CS/ 10% CSS, and 99.99% for CS/ 20% CSS, respectively. During Phase III (days 14–30), the removal efficiency for Fe ions in columns with 5% and 10% CSS initially decreased and then stabilized with prolonged operation time. The final Fe concentrations reached 18.19 mg/L for CS/5% CSS, 11.64 mg/L for CS/10% CSS, and 0.0255 mg/L for CS/20% CSS, respectively. The (CS/20% CSS) scheme exhibited superior performance, with the Fe ion concentration remaining below the 0.3 mg/L limit stipulated by the Chinese National Standard for Drinking Water3.

As supported by Fig. 2, the high CS/20% CSS system maintained a highly alkaline environment (pH > 12). Under these conditions, dissolved Fe rapidly undergoes hydrolysis and precipitates as amorphous Fe(OH)₃ or FeOOH, resulting in highly efficient removal through chemical precipitation. In contrast, the low CS/5% CSS system remained near neutral or acidic (pH between 5 and 6), which Fe species exhibit high solubility and tend to remain in solution. Nevertheless, the removal rate of Fe was still 93%, primarily through adsorption and co-precipitation. In this process, Fe was incorporated into gypsum (CaSO₄·2H₂O) precipitates formed by the combination of calcium ions dissolved from the steel slag and sulfate ions. Surface adsorption and ion exchange served as secondary removal mechanisms.

Mn variation pattern

The concentration and removal rate of manganese ions are shown in Fig. 5. In phase Ⅰ, the Mn removal rate decreased from 89.93 to 75.27% for CS/ 5% CSS during the initial 4-day column experiment. The removal rate of Mn ions stabilized, reaching an average removal rate of 99.31% for CS/ 10% CSS, and 99.95% for CS/ 20% CSS, respectively. The results indicated that the removal rate of Mn was significantly affected by the content of CSS in the column. This is due to the increase of adsorption sites as the content of CSS increases. Another reason is that Mn ions form a manganese hydroxide precipitate when the pH of the solution is above 835. Specifically, the pH value of the solution is 8.32 for CS/ 10% CSS, and 12.59 CS/ 20% CSS, respectively at 4th day (Fig. 2). In phase Ⅱ, the removal rate of Mn has stabilized, reaching an average removal rate of 74.51% for CS/ 5% CSS, 99.61% for CS/ 10% CSS, and 99.97% for CS/ 20% CSS during the 4 to 12-day reaction period. In phase Ⅲ, the removal rate of Mn ions decreased from 75.59 to 39.95% for CS/ 5% CSS, and from 99.21 to 59.38% for CS/ 10% CSS. When the content of CSS of the column was 20%, the concentration of Mn ions stabilized, and the Mn concentrations remained below the 0.1 mg/L limit stipulated by the Chinese National Standard for Drinking Water3.In contrast to the Fe ions, the lower removal rate of Mn was probably attributed to competitive adsorption. To illustrate, Fe ions were preferentially adsorbed and precipitated34, adhering to the surface of the adsorbent material, and thereby reduced adsorption sites of the CSS. Although the PRB with 20% CSS could effectively treat Mn ions, the permeability of the PRB decreased, which blocked the AMD flow through PRB. Hence, the content of CSS in the PRB should be below 10%.

Fig. 5.

Fig. 5

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Removal rate of Mn (a) and concentration evolution of Mn (b) in the effluent under different CS/CSS ratio groups.

Zn variation pattern

Figure 6 shows the plot of the removal rates and concentration of zinc ions versus time as the actual AMD passed through the column with different ratios of CS/CSS. It was observed that the trend of Zn removal rate, which was divided into three phases, was consistent with that of Mn (Fig. 5) during the 30-day experiments. In phase Ⅰ (the initial 6-day), the Zn removal rate decreased from 99.91 to 75.97% for CS/ 5% CSS. The removal rate of Zn ions stabilized, reaching an average removal rate of 99.67% for CS/ 10% CSS, and 99.86% for CS/ 20% CSS, respectively. In phase Ⅱ (days 6–16), the removal rate of Zn has stabilized, reaching an average removal rate of 75.11% for CS/ 5% CSS, 99.78% for CS/ 10% CSS, and 99.89% for CS/ 20% CSS. In phase Ⅲ(days 16–30), the removal rate of Zn ions decreased from 75.52 to 48.90% for CS/ 5% CSS, and from 99.48 to 84.63% for CS/ 10% CSS. The results indicated that the content of CSS in the column promotes the removal of Zn. The improved adsorption efficiency of zinc ions can be attributed to the higher carbon steel slag content, which provides more adsorption sites and enhances ion exchange36. Furthermore, the alkalinity released from CSS into the AMD raised the solution’s pH, which subsequently resulted in the formation of Zn(OH)2 under alkaline conditions (pH > 8.5)37,38.

Fig. 6.

Fig. 6

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Removal rate of Zn (a) and concentration evolution of Zn (b) in the effluent under different CS/CSS ratio groups.

The migration and transformation pattern of typical pollutants

The distribution patterns of Fe, Mn, and Zn were shown in Fig. 7. When the content of CSS was 5% in the column, there was no change in the five distribution patterns of Fe, Mn, and Zn from the top to bottom, and the dominant portion was maintained in the residual fraction. The result indicated that the column had good permeability, and the migration rate of Fe, Mn, and Zn ions was relatively fast in the column, but it had a poor effect on AMD treatment. As the content of CSS in column increased, the distribution patterns of Fe, Mn, and Zn were reduced from the top to bottom. Especially, when the CSS of column was 20%, the metal ions (Fe, Mn, and Zn) were mainly adsorbed on the top layer of the column, while the increment of the metal ions in the middle and bottom layers were only 1.52 mg/g and 0.44 mg/g, 0.51 mg/g and 0.23 mg/g, 0.0084 mg/g and 0.0039 mg/g, respectively. The results were consistent with the above dynamic adsorption patterns.

Fig. 7.

Fig. 7

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The speciation of Fe, Mn, and Zn in samples based on the BCR method.

Moreover, compared with the blank control group, the water-soluble, acid extractable fraction and reducible fraction of Fe, Mn, and Zn significantly increased. The acid extractable fraction was formed by the electrostatic adsorption of CSS and the complexation reaction between hydroxyl groups and metal ions, which was extremely sensitive to environment. It could be dissolved, released, and then absorbed by organisms under acidic conditions39,40. The reducible fraction was the form, which Fe, Mn, and Zn ions reacted with CSS through chemical adsorption and precipitation, and it was in a certain degree of stability41. The reason was that the metal ions were electrostatic adsorbed by CSS, and then formed into the reducible state as the adsorption time and the pH increased.

Environmental implications

Figure 8 showed the risk assessment code of Fe, Mn, and Zn in the samples. As shown in Fig. 8, only Fe exhibited a low-risk (1% ~ 10%) to the environment from the top to the bottom in the column. The content of acid-soluble Mn and Zn, from top to bottom in column, ranged from 11.09 to 14.69% for CS/ 5% CSS, and from 11.58 to 14.59% for CS/ 10% CSS, respectively, indicating a moderate risk level. When the content of CSS was 20%, Mn (17.50%) and Zn (16.87%) were at a moderate risk level in the surface layer, while those in the middle and bottom layers were at a low risk. Overall, the result indicated that CS/CSS mixture was at low risk to moderate risk level for Fe, Mn, Zn in the surface to the environment. The reason was that the increased content of CSS in the column, which led to a higher number of adsorption sites and a larger specific surface area, thereby facilitating the absorption of AMD in the top layer of the column.

Fig. 8.

Fig. 8

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Environmental risk assessment of Fe, Mn, and Zn in the samples.

Characteristics of CS/CSS mixture before and after PRB remediation AMD

Microscopic analysis, mineralogical composition, and chemical bonding characterization of the CS/CSS mixture before and after its used as the media in PRB remediation AMD were performed by means of SEM, EDS, and XPS (Fig. 9 and Fig. 10).

Fig. 9.

Fig. 9

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SEM images of CSS/CS before (a) and after (b) AMD adsorption.

Fig. 10.

Fig. 10

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High-resolution XPS spectra of S 2p (a), Fe 2p (b), Mn 2p (c), Zn 2p (d).

The SEM–EDS images of CS/CSS mixture before and after PRB treatment AMD were shown in Fig. 9. As shown in Fig. 9(a), both the surface roughness and uneven pore structure increased the specific surface area and adsorption sites of the filled material. These characteristics enhanced the material’s efficiency and effectiveness in removing pollutants. After PRB treatment (Fig. 9b), the surface textures appeared to be filled with large number of crystals, and formed aggregates exhibiting with specific morphological features. It was obvious that large number of typical pollutants (SO42−, Fe, Mn, and Zn) from AMD was adsorbed on the surface of CS/CSS mixture, especially the product surface has a significant amount of sulfur and iron (Fig. S1 (a), Fig. S1 (b)).

In order to further analyze the removal mechanism of CS/CSS on AMD (SO42−, Fe, Mn, and Zn), the XPS wide scan spectra were shown for CS/CSS and CS/CSS -AMD. As depicted in the full scanning XPS spectrum (Fig. S2), the peaks of S 2p, Mn 2p, Fe 2p and Zn 2p were observed after adsorption, indicating that typical pollutants (SO42−, Fe, Mn, and Zn) from AMD were successfully adsorbed by CS/CSS.

Figure 10a given the S 2p peaks of the CS/CSS mixture of PRB before and after the PRB treatment to AMD, the S 2p3/2 spectrum was fitted to two doublets, and most of the sulfur was present in sulfate form. It belonged to CaSO4 (169.2 eV) and Fe-sulphate complexes (170.15 eV)42,43. Figure 10b presents the XPS spectra of Fe. It can be clearly seen that the intensity of Fe peaks for CS/CSS -AMD are much stronger than that of fresh CS/CSS mixture, revealing that the Fe was removed after adsorption. The Fe 2p spectrum was fitted to four doublets at binding energies of 724.3 eV, 713.3 eV, 711.5 eV and710.0 eV, respectively. The various coordination and oxidation states from Fe present in the CS/CSS after adsorption: (i) FeOOH (724.3 eV)44, (ii) Fe (Ⅲ)-sulfate (713.3 eV)45, (iii) Fe2O3 (711.5 eV and 710.0 eV)46,47. For Mn 2p spectrum(Fig. 10c), two Mn2p3/2 peaks were observed at 641.5 eV(Mn2+) and 642.7 eV(Mn4+)48. It suggested that Mn is not only adsorbed to the surface of CS/CSS mixture, but also by oxygen into the high-priced Mn (Ⅳ), which can probably be attributed to FeOOH promoting O2 oxidation of Mn2+ through conventional interface catalysis or electrochemical catalysis49. In the XPS spectra of Zn 2p (Fig. 10d), small amounts of Zinc ion were detected after treatment AMD, although this peak is not obvious relative to other peaks. Moreover, the removed Zn and Mn ions are likely incorporated into the particle interior through coprecipitation or encapsulation by dominant precipitates such as CaSO₄ and Fe(OH)₃, rather than being retained on the surface. This leads to the attenuated XPS signals and poor signal-to-noise ratio for Mn and Zn post-adsorption, resulting in spectral profiles that are often indistinguishable from noise. These XPS results were consistent with the SEM–EDS results.

Conclusion

This study aimed to investigate the impact of CSS content on PRB remediation of actual AMD using column experiments and BCR method. Results indicated that increasing CSS content of the column significantly enhanced the removal efficiency of SO42⁻, Fe, Mn, and Zn, while also elevating the solution pH value. SEM and XPS analysis revealed that these pollutants were adsorbed through ion exchange, chemisorption, and precipitation. The BCR showed that the distribution patterns of Fe, Mn, and Zn varied with increasing CSS content. RAC indicated Fe exhibited a low-risk (1% ~ 10%) to the environment, while that of Mn and Zn was identified as low to moderate. The study concluded that low CSS content improved ion migration and permeability but reduced treatment efficiency, whereas higher CSS content (e.g., 20%) enhanced ion adsorption and treatment performance. To optimize PRB performance, CSS content should be maintained below 10%. This research underscores the importance of determining CSS proportions in PRB-filled materials and provides a foundation for designing more efficient and environmentally sustainable PRB systems for AMD treatment.

Supplementary Information

Below is the link to the electronic supplementary material.

Supplementary Material 1 (462.2KB, docx)

Acknowledgements

This work is supported by Natural Science Foundation of Shanxi Province [No.202303021222011, No. 202203021212253] and Research Project Supported by Shanxi Scholarship Council of China (No. 2024-044).

Author contributions

Qiang Zhen: Writing–original draft, Visualization, Validation, Project administration, Methodology, Investigation, Formal analysis, Funding acquisition, Conceptualization. Xinli Du: Visualization, Methodology, Investigation, Data curation. Zhaoliang Wang: Methodology, Investigation, Formal analysis. Aijing Wu: Methodology, Investigation, Formal analysis. Hugang Li: Writing–review & editing, Writing–original draft, Visualization, Supervision, Methodology, Funding acquisition, Conceptualization.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Supplementary Materials

Supplementary Material 1 (462.2KB, docx)

Data Availability Statement

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

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