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Article

Activation of Iron Tailings with Organic Acids: A Sustainable Approach for Soil Amelioration

by
Hui-Chen Wang
1,
Zi-Hao Zhao
1,
Dong-Yun Han
1,
Xiao-Hong Wang
1,2,3,*,
Xue-Tao Yuan
1,2,3,* and
Yan-Jun Ai
1,2,3
1
College of Mining Engineering, North China University of Science and Technology, Tangshan 063210, China
2
Hebei Key Laboratory of Mining Development and Security Technology, Tangshan 063210, China
3
Hebei Industrial Technology Institute of Mine Ecological Remediation, Tangshan 063210, China
*
Authors to whom correspondence should be addressed.
Sustainability 2025, 17(20), 9308; https://doi.org/10.3390/su17209308
Submission received: 12 September 2025 / Revised: 15 October 2025 / Accepted: 16 October 2025 / Published: 20 October 2025
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Abstract

The large-scale accumulation of iron tailings poses serious environmental challenges and represents a significant loss of potential resources. Due to the stable silicate mineral structure of iron tailings, essential nutrient elements remain encapsulated, resulting in low bioavailability and limited uptake by plants. This characteristic greatly restricts their direct use in agricultural applications. To overcome this limitation, this study employed three organic acids, namely citric acid, oxalic acid, and acetic acid, to activate iron tailings. The activation efficiency was systematically evaluated, and the effects of activated iron tailings on plant growth were assessed through pot experiments. The results showed that all three organic acids significantly enhanced the release of available silicon and iron from iron tailings, with oxalic acid exhibiting the highest activation capacity, increasing available Si and Fe to 882.99 mg/kg and 395.41 mg/kg, respectively. Pot experiments further revealed that the organic acid–iron tailing composites markedly improved soil nutrient availability, with available potassium, phosphorus, alkali-hydrolyzable nitrogen, iron, and silicon increasing by 50.03%, 95.99%, 82.59%, 163.21%, and 200.01%, respectively. Consequently, plant growth was substantially enhanced, including increases in plant height (29.49%), shoot fresh weight (41.62%), and shoot dry weight (39.89%). This study provides a novel and sustainable strategy for the valorization of iron tailings as an agricultural resource and soil amendment, demonstrating considerable potential for both environmental remediation and agronomic improvement.

1. Introduction

The mining industry generates vast quantities of tailings, the accumulation of which occupies extensive land resources, degrades ecosystems, and poses potential health risks to surrounding communities. In China, the total stockpile of tailings has surpassed 20 billion tonnes, with iron tailings representing the largest share [1]. Magnetically separated iron tailings, characterized by their chemically inert composition, uniform particle size distribution, and strong physical stability, are enriched with valuable elements such as Fe, Si, P, Ca, and Mg. These properties make them promising secondary resources with potential applications in chemical engineering, agriculture, and other industrial sectors [2,3]. Therefore, developing efficient and environmentally compatible strategies for the utilization of iron tailings is essential to promote large-scale tailings consumption and ensure the sustainable development of mineral resources [4]. Existing studies on the recycling and comprehensive utilization of iron tailings primarily focuses on their application in the manufacture of building materials, extraction of valuable elements, backfilling of mined-out areas (i.e., goafs), and the development of soil amendments or fertilizers [5,6,7]. From a holistic perspective encompassing disposal capacity and ecological benefits, the eco-utilization of iron tailings holds greater value than re-separation technologies. This approach facilitates the synergistic achievement of secondary resource recovery, environmental protection, and enhanced socio-economic benefits. Tozsin et al. [8] investigated the impact of varying iron tailings application rates on wheat growth and found that their addition improved the availability of essential micronutrients and increased plant biomass. Effective silicon can enhance photosynthesis, improve crop yield and quality, and increase resistance to diseases, pests, and environmental stresses [9,10]. Iron, an essential element for chlorophyll synthesis, plays a vital role in plant development [11]. Thus, using iron tailings as soil amendments represents a promising and sustainable strategy for the eco-friendly utilization of mineral residues.
Although iron tailings hold significant potential as soil amendments, the abundant elements they contain, such as Si and Fe, primarily exist in the forms of SiO2 and Fe2O3, which are not directly available for plant uptake. Therefore, activation treatments are essential to release these nutrient elements. In recent years, organic acids have demonstrated considerable promise in tailings activation. Unlike conventional chemical activators, organic acid leaching agents can persist naturally in soil and aquatic systems and are readily decomposed by microorganisms, thereby circumventing the contamination associated with traditional chemical leaching methods and promoting the restoration of soil ecosystems [12,13]. Although inorganic acids at equivalent concentrations can also dissolve silicate minerals through strong proton dissociation, their mechanism relies mainly on high proton concentrations, which often lead to soil acidification and disrupt the natural chemical equilibrium of soils [14]. Such disturbances can have adverse effects on soil ecological health and nutrient availability. By contrast, organic acids not only provide protons but also possess functional groups (e.g., carboxyl groups) that coordinate with metal ions on mineral surfaces to form soluble complexes. This enables selective dissolution of silicate minerals, offering notable advantages over inorganic acids at similar concentrations. Furthermore, the natural biodegradability of organic acids aligns closely with the principles of green and sustainable agriculture. Common organic acids (e.g., oxalic, citric, acetic, malic, and tartaric acids) can solubilize mineral surface components through mechanisms such as complexation, acidolysis, and chelation [15], thereby facilitating nutrient release. Consequently, organic acids are regarded as environmentally friendly and efficient leaching agents. Sun et al. [16] developed an alkaline soil conditioner by calcining iron tailings, mica, and dolomite, which significantly increased the concentrations of silicon, calcium, potassium, and magnesium. Mu et al. [17] employed acid-leached copper tailings to produce a silicon-iron soil amendment used in soil remediation, which promoted the growth of Vetiveria zizanioides. Based on the aforementioned mechanism and practical efficacy, organic acids activate silicate minerals in iron tailings, thereby releasing and replenishing essential soil nutrients. This process demonstrates considerable promise for advancing the resource utilization of iron tailings while concurrently enhancing soil fertility.
This study investigates the effects of three organic acids, namely citric acid, oxalic acid, and acetic acid, on the leaching behavior of silicon and iron from iron tailings. A combination of advanced characterization techniques, including scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FTIR), were employed to analyze the phase composition and microstructural evolution of the reaction products, thereby determining the optimal activation conditions for iron tailings. In addition, pot experiments were conducted to assess the effectiveness of organic acid-modified iron tailing composites in enhancing soil nutrient availability and promoting plant growth. The findings offer both theoretical insights and practical evidence to support the efficient activation and resource utilization of nutrient elements contained in iron tailings.

2. Materials and Methods

2.1. Mineralogical Characterization

The experimental materials were obtained from magnetic separation iron tailings located in Hebei Province, China. The phase composition of these iron tailings was analyzed using XRD (Bruker D8 Advance, Karlsruhe, Germany). In Figure 1, the presence of sharp and symmetric diffraction peaks corresponding to quartz indicates its high crystallinity and low reactivity. The primary mineral phases identified in the tailings included quartz, cordierite, hematite, K-feldspar, and diopside. Additionally, SEM analysis revealed a characteristically rough surface morphology with numerous fine mineral particles adhering to it.
For the pot experiment, soil samples were collected from the top 20 cm of an agricultural field in Hebei Province, China. The basic physicochemical properties of the soil were characterized as follows: pH 7.87, organic matter 16.93 g/kg, available nitrogen 34.67 mg/kg, available phosphorus 22.78 mg/kg, available potassium 303.84 mg/kg, available iron 36.91 mg/kg, and available silicon 118.06 mg/kg.
The relative elemental composition of iron tailings was analyzed using an X-ray fluorescence spectrometer (XRF; PANalytical Axios, Almelo, The Netherlands, Table 1).
The concentrations of heavy metals in the iron tailings were determined using inductively coupled plasma atomic emission spectrometry (ICP-AES; ICAP 7000, Thermo Fisher, Waltham, MA, USA). The method detection limits (MDLs) for the target elements were as follows (mg/kg): Cd, 0.005; Hg, 0.002; As, 0.05; Pb, 0.01; Cr, 0.002; Cu, 0.0021; and Ni, 0.0024. As shown in Table 2, the measured heavy metal concentrations were substantially lower than the corresponding risk intervention values established by the Chinese regulatory standard Soil Environmental Quality, Risk Control Standard for Soil Contamination of Agricultural Land (GB 15618-2018). These results indicate that the application of the tested iron tailings as a soil amendment poses minimal ecological risk associated with heavy metal contamination.

2.2. Experimental Design

Analytical-grade citric acid, oxalic acid, and acetic acid (Shanghai Macklin Biochemical Co., Ltd., Shanghai, China) were used in this study. Four concentrations of each organic acid (10, 40, 80, and 100 mmol/L) were tested, with deionized water serving as the control (CK). All treatments were performed in triplicate, resulting in a total of 13 experimental groups. For each treatment, 10.00 g of iron tailings was placed into a 250 mL conical flask, followed by the addition of 100 mL of the corresponding organic acid solution or deionized water at the specified concentration, maintaining a solid-to-liquid ratio of 1:10. The flasks were then sealed and placed in a constant-temperature reciprocating shaker, where they were agitated at 25 °C and 180 rpm for 1 h. After the initial mixing, the suspensions were incubated for 30 days under ambient conditions. The pH of the liquid phase was monitored at predetermined time intervals (days 1, 2, 3, 6, 9, 12, 15, 18, 21, 24, 27, and 30). At each interval, supernatant samples were collected, filtered through qualitative filter paper, and analyzed for concentrations of available silicon and iron ions. Upon completion of the incubation period, the solid and liquid phases were separated by centrifugation and vacuum filtration. The solid residues were then oven-dried at 80 °C and stored for subsequent characterization.
A pot experiment was conducted to evaluate the effectiveness of activated iron tailings in improving soil properties and promoting plant growth. The experimental design consisted of three treatments: (1) control (raw soil only; CK), (2) raw soil amended with 10% iron tailings (T1), and (3) raw soil amended with 10% organic acid-activated iron tailings composite (T2). Each treatment was replicated three times. Each pot was filled with 2 kg of raw soil and amended with the designated soil conditioner. After undergoing thorough mixing, the soil was subjected to a two-week aging period to allow stabilization. Ten spinach seeds were sown randomly in each pot, and after germination, five uniform seedlings were retained per pot for further growth. At maturity (approximately 40 days after sowing), all plants in each pot were harvested for analysis. Aboveground fresh biomass was measured using an electronic balance with a precision of 0.001 g, and dry weight was determined after oven drying. Simultaneously, soil samples were collected to determine available nutrient content. Throughout the experiment, all pots were maintained in a greenhouse under natural light conditions and irrigated regularly to sustain optimal soil moisture.

2.3. Analytical Determination Method

The pH of the leachate was measured using a pH meter (pHS-3C, Shanghai, China). Available silicon content was determined by the silicon molybdenum blue spectrophotometric method at 700 nm using a UV-visible spectrophotometer (UV-5100, Shanghai, China) [18], with a linear calibration range of 0–2.50 μg/L for silicon. Total iron and Fe (II) concentrations were quantified by the 1,10-phenanthroline spectrophotometric method at 535 nm (UV-5100, Shanghai, China) [19], with a linear range of 0–6 mg/L and a detection limit of 1 μg/mL for iron. The Fe (III) content was calculated by subtracting the Fe (II) concentration from the total iron concentration. Soil organic matter was analyzed using the potassium dichromate external heating method. Alkali-hydrolyzable nitrogen was determined by alkali hydrolysis-diffusion absorption, while available phosphorus was extracted with sodium bicarbonate and measured using the molybdenum-antimony anti-spectrophotometric method. Available potassium was quantified by atomic absorption spectrophotometry [20]. The micromorphology and phase composition of the iron tailings before and after organic acid treatment were characterized by field emission SEM (FE-SEM; HITACHI S4800, Tokyo, Japan) and XRD (Bruker D8 Advance, Karlsruhe, Germany). Specific surface area (SSA) was measured using a surface area analyzer (BeishideInstrument,3H-2000PM, Beijing, China). Chemical functional groups were identified by FTIR (Bruker VERTEX 70, Karlsruhe, Germany) across the wavenumber range of 4000–500 cm−1.

2.4. Data Analysis

Prior to conducting the analysis of variance (ANOVA), the data were tested for homogeneity of variances using Levene’s test and for normality using the Shapiro–Wilk test to ensure that the assumptions of ANOVA were met. A one-way ANOVA was then performed using SPSS 27.0. When significant differences among groups were detected (p < 0.05), Duncan’s multiple range test was employed for post hoc comparisons to evaluate differences in nutrient and plant growth parameters among treatments. Data are expressed as mean ± SD, and all figures were generated using Origin 2024.

3. Results and Analysis

3.1. Effect of Organic Acids on Activation of Effective Silicon in Iron Tailings

Treatment with organic acids—oxalic, citric, and acetic acid—markedly increased the release of available silicon from iron tailings, resulting in a proportional enhancement of dissolution content with increasing acid concentration (Figure 2). At equivalent concentrations, oxalic acid demonstrated a greater efficacy in promoting silicon release compared to citric acid and acetic acid. This disparity became increasingly pronounced as the concentration of the organic acids increased. The highest silicon dissolution content, reaching 915.16 mg/kg, was observed with 100 mmol/L oxalic acid on the 18th day, followed by 80 mmol/L oxalic acid, which yielded a dissolution content of 882.99 mg/kg.

3.2. Effect of Organic Acids on Activation of Total Iron in Iron Tailings

The mobilization of total iron from iron tailings by organic acids—oxalic, citric, and acetic acids—generally exhibited an initial increase followed by a subsequent decrease as the acid concentration increased (Figure 3). At equivalent molar concentrations, oxalic acid and citric acid demonstrated higher iron leaching efficiencies compared to acetic acid. Under an 80 mmol/L treatment, oxalic acid achieved a maximum iron dissolution of 395.41 mg/kg on day 18. In contrast, with 100 mmol/L citric acid, iron leaching peaked at 366.56 mg/kg on day 27. With prolonged activation time, the dissolved iron content declined to varying degrees across all acids and concentrations, most significantly in acetic acid, followed by oxalic acid, while the decrease was relatively moderate in citric acid. This behavior can be attributed to differences in proton availability and complexation capacity: citric acid, a triprotic acid, and oxalic acid, a diprotic acid, provide more H+ and form stronger chelation with iron ions than the monoprotic acetic acid at the same concentration.

3.3. The Effect of Organic Acids on the Chemical Form of Iron in Iron Tailings

Figure 4 and Figure 5 illustrate the variation in iron valence states during the leaching process using organic acids—oxalic, citric, and acetic acid. In Figure 4, the leaching amount of Fe2+ from iron tailings increased with the concentration of organic acids, reaching a maximum at 100 mmol/L. Specifically, the highest leaching concentration of Fe2+ was 330.988 mg/kg on day 12 for citric acid and 312.19 mg/kg on day 21 for oxalic acid. Over time, the leaching amount of Fe2+ gradually decreased under the impact of the different organic acids, with a slower rate of decline observed for citric acid compared to oxalic acid and acetic acid. At a concentration of 100 mmol/L, the activation efficacy for Fe2+ followed the order: citric acid > oxalic acid > acetic acid.
At 80 mmol/L, oxalic acid reached its maximum Fe3+ dissolution rate of 261.898 mg/kg on the 27th day. In contrast, both citric acid and acetic acid reached their highest Fe3+ dissolution rates at 100 mmol/L, with values of 132.024 mg/kg and 146.454 mg/kg, respectively. These values are lower than that achieved by oxalic acid at the same concentration. Overall, the activation efficiency for Fe3+ in iron tailings followed the order: oxalic acid > citric acid > acetic acid.

3.4. Dynamic Changes in pH During Activation of Iron Tailings with Different Organic Acids

In Figure 6, the pH values of oxalic, citric, and acetic acid solutions at varying concentrations exhibited a gradual increase over time, reflecting continuous proton consumption throughout the 30-day leaching experiment. By contrast, the pH of the leachate from iron tailings treated with deionised water remained relatively stable during the same period. At comparable concentrations, oxalic acid demonstrated a lower initial pH than acetic and citric acids, attributable to its smaller pKa value (Table 3), which leads to greater dissociation of H+. Additionally, the number of carboxyl groups per molecule impacted the pH trends: acetic acid contains one carboxyl group, oxalic acid two, and citric acid three. Consequently, after 30 days of treatment, citric acid exhibited a smaller increase in pH relative to the other acids, reflecting its superior buffering capacity. The presence of alkaline minerals, such as calcite and dolomite, in the iron tailings contributed to a strong acid-buffering capacity [21], resulting in progressive consumption of H+ and a corresponding rise in solution pH during interaction with the organic acids.

3.5. Changes in Crystal Structure of Iron Tailings

Based on the aforementioned experiments, the activation effects at concentrations of 80 mmol/L and 100 mmol/L were significantly greater than those at 10 mmol/L and 40 mmol/L. Although increasing the concentration from 80 mmol/L to 100 mmol/L produced a slight increase in the leaching efficiencies of silicon and iron, statistical analysis showed that this difference was not significant (p > 0.05). Considering the principle of diminishing marginal returns, the incremental leaching gain achieved by elevating the concentration to 100 mmol/L is negligible. Furthermore, higher concentrations are associated with increased resource consumption and operational costs. Therefore, from the perspective of resource utilization efficiency and the principles of green chemistry, 80 mmol/L was identified as the optimal concentration for organic acid activation of iron tailings. Subsequent characterization of the iron tailings was performed using the three types of organic acids at 80 mmol/L, along with a control group. After 30 days of activation with organic acids, the release of Si and Fe from the iron tailings was enhanced, leading to a reduction in the structural ordering of the tailings and an increase in their SSA. The results indicated that the SSA of the control-treated iron tailings was 0.5640 m2/g. In contrast, treatment with oxalic acid, citric acid, and acetic acid resulted in increases in SSA of 3.6936, 0.7418, and 0.1177 m2/g, respectively (Table 4).
Figure 7 depicts the microscopic morphologies of the mineral samples before and after treatment with oxalic, citric, and acetic acids. The untreated iron tailings (Figure 7a) show an irregular flaky morphology characterized by serrated edges and minimal interparticle connections, resulting in a loosely structured matrix with limited porosity. In contrast, the sample in Figure 7b exhibits a rough surface composed of aggregated particles that form a porous network structure. This architecture predominantly consists of large, stacked flakes, between which well-connected hierarchical pores are present. These interlaminated pores are densely interconnected and demonstrate a graded size distribution, thereby contributing to a high SSA favorable for nutrient adsorption. The citric acid-treated sample (Figure 7c) comprises densely packed flaky structures, with locally visible slit-like pores. However, both the abundance and connectivity of these pores are inferior in Figure 7b. The acetic acid-treated sample (Figure 7d) features a relatively smooth and compact surface with occasional protrusions and fine cracks. Only a limited number of slit-like pores are discernible, and no pronounced porous structure is evident. Notably, the oxalic acid-activated sample exhibits a highly microporous structure with the largest SSA among all treated samples, indicating its superior potential as a soil amendment agent.

3.6. Characterization of Iron Tailings in Different Organic Acids

Figure 8 illustrates the effects of different organic acids at a concentration of 80 mmol/L on the primary mineral phases of iron tailings. The dominant mineral components of the tailings are quartz and hematite, with quartz exhibiting sharp and intense diffraction peaks indicative of a high degree of crystallinity. This well-ordered structure is a key factor contributing to the inherently low reactivity of the tailings. XRD analysis was used to evaluate the impact of organic acids on mineral dissolution and alterations in crystal structure. The results showed that oxalic, citric, and acetic acids induced varying degrees of mineral alteration. Compared with the control group, all organic acid treatments led to attenuation of the quartz diffraction peaks, with the most pronounced reduction observed in the oxalic acid treatment group. This finding suggests that oxalic acid effectively decreases quartz crystallinity, thereby increasing structural disorder. The attenuation or disappearance of certain quartz diffraction peaks further implies potential structural transformations. However, to draw definitive conclusions regarding phase transitions or amorphization, additional quantitative phase analyses, such as Rietveld refinement, along with complementary characterization techniques, are required. The organic acids demonstrated differential capacities to dissolve silicate minerals with different structures in the iron tailings, with a pronounced erosive effect on SiO2, particularly in the case of oxalic acid, which exhibited the greatest impact. These results align with Li et al. [22]. Overall, the XRD results provide compelling evidence supporting the use of organic acids in the activation of iron tailings.
Analyzing the changes in functional groups and peak intensities of iron tailings at the molecular level is crucial for a comprehensive understanding of their dissolution behavior under organic acid erosion. FTIR demonstrated that (Figure 9), relative to the control group, iron tailings subjected to various organic acids exhibited changes in absorption peak positions, intensities, and the appearance of new peaks. A broadened and intensified-OH stretching vibration peak was observed within the range of 3442 cm−1 to 3453 cm−1 [23], indicating that hydroxyl and carboxyl groups derived from the organic acids significantly contribute to the mineral dissolution process. The absorption peak near 1625 cm−1 is attributed to the stretching and bending vibrations of hydroxyl groups in water molecules present in the iron tailings [24], confirming the formation of chemically bound water during acid activation. The absorption band at 1026 cm−1 corresponds to the Si-O vibration in silicates [25], whereas the peak at 766 cm−1 is assigned to the Si-O-Si vibration [26]. Following treatment with organic acids, these two peaks shifted by 16 and 12 cm−1, respectively. The enhanced symmetric stretching vibrations suggest the dissolution of silicate minerals. Additionally, the peak at 443 cm−1, associated with iron-oxygen stretching vibration [27], shifted by 9 and 7 cm−1 after treatment with citric and oxalic acids, respectively.

3.7. Soil Improvement Effects of Organic Acid-Iron Tailings Complexes

Soil pH is a key factor influencing nutrient availability and plant physiological development. As shown in Figure 10, notable differences in soil pH were observed among the various treatment groups. Compared with the control, the application of iron tailings alone increased the soil pH from 7.87 to 8.25, primarily due to the intrinsic alkalinity of the tailings. In contrast, the combined application of organic acids with iron tailings effectively reduced the soil pH to 6.72, underscoring the ability of organic acids to regulate soil acid-base balance. This pH moderation demonstrates their potential for improving saline-alkaline soils. Such bidirectional regulation offers valuable insights for the precise management of soil chemistry in the remediation of contaminated lands.
In Figure 11, the application of various amendments significantly affected soil nutrient contents. Notably, the sole application of iron tailings resulted in significant increases in the concentrations of available phosphorus, potassium, iron, and silicon (p < 0.05), which rose by 33.03%, 25.04%, 38.84%, and 32.84%, respectively, compared to the CK. However, no significant changes were detected in alkaline hydrolyzable nitrogen or organic matter. In contrast, the composite amendment of organic acid and iron tailings demonstrated a significant enhancement of all measured soil nutrients (p < 0.05). Specifically, available potassium, available phosphorus, and alkaline hydrolyzable nitrogen increased by 50.03%, 95.99%, and 82.59%, respectively.
Crop growth performance serves as a direct indicator of the efficacy of soil amendments. Figure 12 presents the effects of various amendments on spinach growth. Relative to the control group, the application of iron tailings resulted in increases in plant height, root length, fresh weight, and dry weight by 9.21%, 18.14%, 44.93%, and 15.39%, respectively. In comparison, the organic acid-modified iron tailings complex produced more pronounced improvements, with corresponding increases of 29.49%, 41.62%, 79.04%, and 39.89%. These results indicate that the organic acid-iron tailings composite significantly enhances spinach growth more effectively than unmodified iron tailings.

4. Discussion

4.1. Effect of Organic Acids on Activation of Iron Tailings

In this experiment, the dissolution of silicon and iron from iron tailings initially increased and subsequently decreased with rising concentrations of organic acid. Higher concentrations of organic acid supply an increased number of protons (H+) and ligands, which enhance mineral surface dissolution and facilitate metal ion chelation [28]. This observation indicates that the concentration of the leaching agent is a critical factor impacting the extent of dissolution from iron tailings in organic acids, as it governs the availability of reactive species for chemical reactions [29]. The promotion of metal cation release by organic acids is attributed to the synergistic effects of ionized protons and ligands, which facilitate the dissolution of secondary minerals and consequently lead to the liberation of metal cations [30]. However, the relationship between organic acid concentration and metal dissolution is not strictly linear. Beyond a certain concentration threshold, which varies for each element, the metal leaching capacity gradually diminishes. Excessive organic acid molecules compete for adsorption sites on mineral particles-sites that would otherwise be available for the adsorption of metal–organic acid complexes, thereby promoting metal desorption. When these sites are occupied by free organic acid molecules, the formation of metal complexes and their subsequent release from the mineral surface are inhibited [31,32,33].
At equivalent concentrations of organic acids, oxalic acid exhibited the greatest efficacy in activating silicon from high-silicon iron tailings, followed by citric acid, whereas acetic acid demonstrated the least effectiveness. This observation aligns with the findings of Li et al. [22]. The variation in activation efficiency is attributable to the dissociation constants (pKa) of the respective organic acids. Oxalic acid, possessing lower pKa values (pKa1 = 1.27, pKa2 = 4.27) relative to citric acid (pKa1 = 3.13, pKa2 = 4.76, pKa3 = 6.40) and acetic acid (pKa = 4.74), releases a greater concentration of H+ under identical conditions, thereby enhancing its leaching capability. Furthermore, the oxalate anion exhibits a strong complexation ability, particularly a high affinity for multivalent metal cations such as Fe3+ [34]. Through chelation, oxalate forms stable complexes that effectively remove metal ions from mineral interfaces, disrupting the crystalline stability of the mineral matrix and accelerating the decomposition of silicate networks [35,36]. Additionally, modification with oxalic acid facilitates the formation of both micropores and mesopores. The synergistic effect of these porous structures increases the SSA of the material, thereby improving the accessibility of reactants to active sites on the mineral surface and further promoting silicon dissolution [37,38,39].

4.2. Effects of Organic Acid–Iron Tailings Complexes on Soil Nutrients and Plant Growth

Soil nutrients play a critical role in regulating plant growth and development. Compared to non-activated iron tailings, organic acid-modified iron tailings significantly enhance the contents of alkaline hydrolyzable nitrogen, available phosphorus, and readily available potassium by 58.04%, 47.33%, and 19.99%, respectively, thereby stimulating plant growth. This improvement can be attributed to two primary mechanisms. First, organic acids destabilize soil organo-mineral complexes, accelerating their decomposition and facilitating nutrient release [40]. Second, through processes such as anion-cation exchange, complexation, and mineral dissolution, organic acids actively participate in nutrient transformation and stimulate the metabolic activity of functional microorganisms (e.g., iron-reducing and silicate-solubilizing bacteria). These microorganisms further promote the dissolution and speciation of mineral elements via extracellular enzymes and metabolic products [41,42,43]. In addition, organic acids enhance the activation of silicon in tailings, increasing the soil’s available silicon content. They also chelate insoluble iron in the soil and Fe2O3 in iron tailings, forming iron–organic acid complexes that elevate the available iron fraction [44,45]. Collectively, these processes improve soil nutrient availability, reshape the soil environment and microbial community structure, and ultimately stimulate plant growth responses.

5. Conclusions

This study demonstrates that organic acid activation markedly enhances the dissolution of silicon and iron from iron tailings, thereby promoting their potential valorization as soil amendments. Among the tested organic acids, oxalic and citric acids exhibited significantly greater efficacy in solubilizing Si and Fe than acetic acid, with dissolution performance increasing proportionally with acid concentration. During the early stages of the leaching process, the release of both silicon and iron increased progressively, reaching peak values before gradually declining with prolonged treatment. Under optimal activation conditions (80 mmol/L oxalic acid, 18-day leaching), the concentrations of bioavailable Si and Fe in the treated tailings reached 882.99 mg/kg and 395.41 mg/kg, respectively. Organic acid treatment also promoted mineral dissolution and induced the formation of porous structures. Notably, oxalic acid generated the most pronounced porosity, increasing the SSA by 7.55-fold compared with untreated tailings, thereby enhancing their adsorption capacity and sustained nutrient release potential in soils
Pot experiments confirmed that activated iron tailings significantly outperformed untreated materials in improving soil nutrient availability and promoting plant growth, thereby providing a solid scientific basis for their agricultural application. This activation approach not only offers a practical and sustainable pathway for upcycling iron tailings but also aligns with national strategies aimed at advancing the ecological utilization of mining solid wastes. Importantly, it enables the valorization of mine waste for soil improvement without compromising environmental safety. Future research should focus on evaluating system-specific adaptability and assessing the long-term ecological impacts of this technology across diverse tailings–soil systems to further support its sustainable integration into agricultural practices.

Author Contributions

H.-C.W. contributed to the validation and original draft. Z.-H.Z. contributed to the investigation. D.-Y.H. contributed to the validation and supervision. X.-H.W. contributed to the methodology. X.-T.Y. contributed to the validation, framework, and correction process. Y.-J.A. contributed to the framework and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This project was supported by the National Natural Science Foundation of China (No. 52274166), the Hebei Industrial Technology Institute of Mine Ecological Remediation, and the Central Guided Local Science and Technology Development Fund Project of Hebei Province (Grant No. 246Z4201G, 246Z7610G).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (A) XRD patterns of iron tailing; (B) SEM images of iron tailing.
Figure 1. (A) XRD patterns of iron tailing; (B) SEM images of iron tailing.
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Figure 2. Leaching of available silicon from iron tailings treated with organic acids at different concentrations: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
Figure 2. Leaching of available silicon from iron tailings treated with organic acids at different concentrations: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
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Figure 3. Leaching of total iron from iron tailings treated with organic acids at different concentrations: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
Figure 3. Leaching of total iron from iron tailings treated with organic acids at different concentrations: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
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Figure 4. Leaching of ferrous iron from iron tailings treated with varying concentrations of different organic acids: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
Figure 4. Leaching of ferrous iron from iron tailings treated with varying concentrations of different organic acids: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
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Figure 5. Leaching of trivalent iron from iron tailings treated with varying concentrations of different organic acids: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
Figure 5. Leaching of trivalent iron from iron tailings treated with varying concentrations of different organic acids: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
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Figure 6. Temporal variation in pH of low-molecular-weight organic acids at different concentrations: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
Figure 6. Temporal variation in pH of low-molecular-weight organic acids at different concentrations: (a) 10 mmol/L, (b) 40 mmol/L, (c) 80 mmol/L, and (d) 100 mmol/L.
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Figure 7. SEM images of iron tailings during organic acid activation with: (a) Control treatment; (b) Oxalic acid treatment; (c) Citric acid treatment; (d) Acetic acid treatment.
Figure 7. SEM images of iron tailings during organic acid activation with: (a) Control treatment; (b) Oxalic acid treatment; (c) Citric acid treatment; (d) Acetic acid treatment.
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Figure 8. XRD patterns of iron tailings treated with different organic acids.
Figure 8. XRD patterns of iron tailings treated with different organic acids.
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Figure 9. FTIR spectra of iron tailings treated with different organic acids.
Figure 9. FTIR spectra of iron tailings treated with different organic acids.
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Figure 10. Effects of different treatments on soil pH. Note: Different letters indicate significant differences at p < 0.05.
Figure 10. Effects of different treatments on soil pH. Note: Different letters indicate significant differences at p < 0.05.
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Figure 11. Effect of different treatments on soil nutrient content. Note: Different letters indicate significant differences at p < 0.05, whereas identical letters indicate no significant differences at p > 0.05.
Figure 11. Effect of different treatments on soil nutrient content. Note: Different letters indicate significant differences at p < 0.05, whereas identical letters indicate no significant differences at p > 0.05.
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Figure 12. Spinach growth indicators. Note: Different letters indicate significant differences at p < 0.05.
Figure 12. Spinach growth indicators. Note: Different letters indicate significant differences at p < 0.05.
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Table 1. Main chemical compositions of iron tailing.
Table 1. Main chemical compositions of iron tailing.
Chemical CompositionSiO2Fe2O3Al2O3CaOMgOK2ONa2OP2O5
Content (%)71.2510.845.244.003.733.700.750.49
Table 2. The content of heavy metals in the iron tailing. (average ± standard deviation, n = 3 parallel determinations).
Table 2. The content of heavy metals in the iron tailing. (average ± standard deviation, n = 3 parallel determinations).
ElementsContent (mg/kg)Standard (mg/kg)
Cd0.085 ± 0.020.3–0.6
Hg0.013 ± 0.011.3–3.4
As3.64 ± 0.2725–40
Pb11.59 ± 1.0670–170
Cr63.15 ± 0.85150–250
Cu23.72 ± 0.1950–100
Ni29.48 ± 1.1360–190
Table 3. Structure and acidity coefficient (pKa) of different organic acids.
Table 3. Structure and acidity coefficient (pKa) of different organic acids.
Organic AcidChemical StructurepKa1pKa2pKa3
acetic acidCH3-COOH4.74
oxalic acidHOOCCOOH1.274.27
citric acidHOOCOHC-(CH2COOH)23.134.766.40
Table 4. BET surface area of iron tailings treated with different methods.
Table 4. BET surface area of iron tailings treated with different methods.
SampleSpecific Surface Area (m2/g)
CK0.5640
Oxalic acid treatment4.2576
Citric acid treatment1.3058
Acetic acid treatment0.6817
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Wang, H.-C.; Zhao, Z.-H.; Han, D.-Y.; Wang, X.-H.; Yuan, X.-T.; Ai, Y.-J. Activation of Iron Tailings with Organic Acids: A Sustainable Approach for Soil Amelioration. Sustainability 2025, 17, 9308. https://doi.org/10.3390/su17209308

AMA Style

Wang H-C, Zhao Z-H, Han D-Y, Wang X-H, Yuan X-T, Ai Y-J. Activation of Iron Tailings with Organic Acids: A Sustainable Approach for Soil Amelioration. Sustainability. 2025; 17(20):9308. https://doi.org/10.3390/su17209308

Chicago/Turabian Style

Wang, Hui-Chen, Zi-Hao Zhao, Dong-Yun Han, Xiao-Hong Wang, Xue-Tao Yuan, and Yan-Jun Ai. 2025. "Activation of Iron Tailings with Organic Acids: A Sustainable Approach for Soil Amelioration" Sustainability 17, no. 20: 9308. https://doi.org/10.3390/su17209308

APA Style

Wang, H.-C., Zhao, Z.-H., Han, D.-Y., Wang, X.-H., Yuan, X.-T., & Ai, Y.-J. (2025). Activation of Iron Tailings with Organic Acids: A Sustainable Approach for Soil Amelioration. Sustainability, 17(20), 9308. https://doi.org/10.3390/su17209308

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