Potassium-Bicarbonate-Induced Mineralized Acid Mine Drainage into Iron Hydroxyl Sulfate Minerals for Better Water Remediation and Resource Reuse

Abstract

Iron hydroxyl sulfate minerals (IHSMs, including schwertmannite and jarosite) are a promising material for environmental applications with excellent adsorption of metal oxygen anions. The acid mine drainage (AMD) abundant in iron and sulfate ions holds potential for the production of valuable IHSMs, thereby achieving resource recycling whilst simultaneously mitigating water contamination, which is important for the sustainable remediation of AMD. Conventional mineralization, which promotes the generation of minerals from Fe³⁺ and SO₄²⁻ through the energy provided by chemical or biological oxidation, can only partially mineralize iron in AMD containing substantial quantities of Fe²⁺. In this study, an improved method for mineralizing AMD containing iron of a different valence into IHSMs under the induction of KHCO₃ was proposed. For AMD containing Fe²⁺, the combination of KHCO₃ and H₂O₂ was used to promote the hydrolysis of iron (92.7%) and sulfate (11.1%) into IHK minerals, which resulted in a significant increase in iron removal of 75.2% and a minor increase in sulfate removal of 4.9%, compared with the formation of schwertmannite from oxidation solely involving H₂O₂. For the AMD containing Fe³⁺, the energy generated by the acid–base reaction in water could also directly promote the formation of IK minerals from 97.2% iron and 6.9% sulfate. XRD and FTIR analyses confirmed the identification of the IHK and IK minerals as IHSMs transitioning from schwertmannite to jarosite. SEM and elemental analyses indicated that the mineral exhibited denser aggregate spheres with the incorporation of KHCO₃ in mineralization yet displayed enhanced mineralization abilities for the contaminant ions in AMD. Moreover, despite the SSA of the modified minerals being diminished (2.02, 1.83 and 1.83 m²/g for IH, IHK and IK, respectively), the presence of more sulfate in the mineral enhanced the adsorption capacity of Cr(VI). Furthermore, the water quality results also illustrated that the removal ratios of iron and sulfate in AMD notably increased with the involvement of KHCO₃ in mineralization. In conclusion, the KHCO₃-induced mineralization of iron-containing (either divalent or trivalent) AMD into IHSMs not only improved the mineralization ratios and contaminant removal ratios for better remediation of AMD but also obtained mineral resources with better adsorption of Cr(VI), thereby fostering the sustainable advancement of the remediation of AMD. Therefore, this innovative strategy employing KHCO₃-assisted chemical mineralization to form IHSMs holds ample potential and promises to be an efficacious methodology for the sustainable remediation of iron-rich AMD.

1. Introduction

Acid mine drainage (AMD) is the acidic water formed by the oxidation of groundwater that accumulated in abandoned mines [1]. The primary reasons for the formation of AMD are chemical and biological oxidations of sulfur and metal-rich deposits under the influence of oxygen and microorganisms [2]. These results cause sulfur to be oxidized into sulfate ions, leading to the acidification of water [3]. Moreover, the dissolution of mineral results in the presence of excessive toxic metal ions in water. AMD containing high concentrations of metal and sulfate ions is highly susceptible to spillage discharge to surface water, potentially leading to severe consequences, such as the death of aquatic animals and an eventual adverse impact on human health [4,5]. Upon the discharge of copious amounts wastewater directly mined into rivers, lakes and oceans, it could result in considerable environmental degradation, adversely affecting the surrounding surface water, groundwater resources and ecological environment, leading to ecological degradation in the basin.

AMD stands as the second most substantial worldwide environmental quandary, subsequent to global warming [2]. Inspired by the natural mineralization of AMD, several scholars have studied the purification of AMD by secondary minerals in recent years, as it could achieve the dual objectives of water purification and resource recovery [6].

Iron hydroxyl sulfate minerals (IHSMs) are an important secondary mineral, including schwertmannite and jarosite, occurring in oxidizing environments with a pH ranging between 2.5 and 4.5 and abundant in iron and sulfate ions [7,8]. Interestingly, schwertmannite is a sub-stable mineral, readily converting to jarosite when confronted with AMD containing monovalent cations [9]. In essence, the poorly crystalline schwertmannite is a forerunner to the more stable jarosite. In addition, IHSMs (both schwertmannite and jarosite) could be applied to environmental pollution remediation due to their potential as heterogeneous Fenton catalysts, adsorbents and soil remediation agents [10,11,12].

Currently, the conventional method for mineralizing AMD into IHSMs is oxidation (biological and chemical) [13]. However, the utilization of oxidation has limitations, such as the necessity for copious amounts of Fe²⁺ and the incomplete mineralization of iron, which hinders its application in the actual mineralization of AMD [14]. The oxidation method is only applicable in AMD containing more Fe²⁺ and can only promote the formation of minerals from some of the Fe and sulfate, as the energy generated during the oxidation process is insufficient to activate all of the Fe³⁺ and SO₄²⁻ to form IHSMs [15]. Moreover, the process of mineralization is acid-enhancing as, for example, in the process of the formation of schwertmannite (Equation (1)), which can also hinder the production of IHSMs [16]:

8Fe³⁺ + xSO₄²⁻ + (16−2x)H₂O = Fe₈O₈(OH)_8–2x(SO₄)_x + (24–2x)H⁺,(1 ≤ x ≤ 1.75)(1)

For AMD containing more Fe³⁺, the oxidation method requires the use of zero-valent iron or iron-reducing bacteria to reduce the Fe³⁺ in AMD to Fe²⁺ before oxidizing [17]. Wang and Xie employed this method to carry out multiple redox cycles before achieving the mineralization of most of the iron in the AMD, indicating that the method is time- and material-intensive [18,19]. Therefore, a novel method is imperative to simultaneously mineralize iron (including Fe²⁺ and Fe³⁺) and SO₄²⁻ into IHSMs, which is important for the efficient and rapid purification of AMD and the recovery of resources.

AMD is a serious environmental problem caused by mining activities. The remediation of AMD using mineralization is a promising approach. In order to directly mineralize AMD containing Fe²⁺ and Fe³⁺ and rapidly improve the mineralization rate as well as efficiency, a new method of KHCO₃-induced mineralization technology is proposed in this paper. On the one hand, the partial consumption of hydrogen ions through the weak alkalinity of KHCO₃ lessens the hindrance of acidity to mineralization and ensures that the pH value is suitable for the formation of IHSMs, thus propelling the reaction forward. On the other hand, the large amount of energy released during the acid–base reaction induces Fe³⁺ and SO₄²⁻ to directly form valuable minerals. This approach has the potential to lighten the load on the environment and convert more waste into resources, which is crucial for implementing sustainable remediation strategies for AMD.

2. Experimental section

2.1. Materials and Chemicals

H₂O₂ (30%, w/w, AR) was purchased from Tianjin Kermel Chemical Reagents Company. The chemical reagents, including HCl (37%, GR) and HNO₃ (65~68%, GR), were from Aladdin. And KHCO₃ (≥99.5%, AR) and NaOH (≥99.5%, AR) were manufactured by Tianjin Zhiyuan Chemical Reagents Company, Tianjin, China. Zero-valent iron (Fe⁰, ≥98.0%, AR) was obtained from Tianjin Hengxing Chemical Reagent Co., Ltd., Tianjin, China. Chromium standard solution (Cr(VI), 500 mg/L) was provided by the Institute of Ecology and Resource Management (IERM). All the solutions were produced with deionized water.

2.2. Description and Collection of AMD

AMD was taken from the Shandi River basin (113°30′ E, 38°1′ N) in Xinzhou, Shanxi Province, China. There were 28 coal mines in the basin, 23 of which were later closed, resulting in AMD draining to the Shandi River. This part of the water flows downstream of the Wenhe River into the Middle Ordovician carbonate seepage area and infiltrates to recharge deep karst groundwater, causing the pollution of groundwater. AMD was extracted from the banks of a long-forsaken open-cast coal mine, where the water had undergone a long period of oxidation due to the constant exposure to oxygen, which had resulted in the iron in the water predominantly being in the form of trivalent iron. The water quality parameters are depicted in Table 1. As per the data shown, the AMD contained an abundance of sulfate, along with metal ions like iron, manganese and aluminum ions, far exceeding the stipulated range of the environmental quality standards for surface water prescribed in GB3838-2002 [20], China.

2.3. Oxidative Mineralization of AMD

To investigate the role of mineralization in the remediation of AMD containing Fe²⁺, we carried out the iron reduction of AMD containing Fe³⁺. The iron reduction was carried out by adding 3 g of Fe⁰ into 1 L of AMD while stirring at 400 rpm for an extended duration of 18 h. Then, the undissolved Fe⁰ powder was filtered out through a 0.45 µm filter membrane, at which point, the pH of the AMD in the solution was 3.9, and the concentration of the Fe²⁺ was 4832 mg/L. The solution was sealed and stored in a refrigerator at 4 °C for later use.

Substituted incremental quantities of H₂O₂ were gently dispensed into 100 mL of the reduced AMD, stirred at 400 rpm for 6 h, then left undisturbed overnight at room temperature (25 °C). These solutions were filtered employing a 0.45 μm membrane, and the resultant precipitates, named IH minerals, were dried in an oven at 50 °C, weighed meticulously and stored within a hermetically sealed bag. The levels of the contaminant ions and pH values in the filtrates were stringently quantified.

2.4. KHCO₃-Induced Mineralization of AMD Containing Fe²⁺

The 1.5 mL of H₂O₂ was added to 100 mL of the reduced AMD and stirred for 6 h, followed by adding KHCO₃ (0.6–1.2 g) and further stirring for 6 h at 25 °C. After the stirring stopped, the solution was allowed to sit for solid–liquid separation and subsequently filtered through a 0.45 μm membrane [21]. The resulting precipitates, named IHK minerals, were dried in an oven at 50 °C, weighed and stored in a sealed bag. The concentrations of contaminant ions in the filtrates were determined.

2.5. KHCO₃-Induced Mineralization of AMD Containing Fe³⁺

Different masses of KHCO₃ were added to 100 mL of the original AMD and stirred for 6 h at 25 °C. The solutions were allowed to sit overnight following the cessation of stirring, and the IK precipitates were filtered through a 0.45 μm membrane. The IK minerals were subsequently dried in an oven at 50 °C and weighed as well as stored in a sealed bag. The concentrations of contaminant ions in the filtrates were determined. The aforementioned experiments underwent three iterative rounds, with meticulous recording of experimental data to derive an average value as our final results.

2.6. The Adsorption Properties on Cr(VI)

The adsorption capacities of the IH (crafted by the incorporation of 1.5 mL of H₂O₂), IHK (producible through the addition of 1.5 mL of H₂O₂ and subsequently 0.9 g of KHCO₃) and IK (synthesized by adding 1.2 g of KHCO₃) toward Cr(VI) were determined using batch experiments. An amount of 0.01 g of the IH, IHK and IK were added to 40 mL of the Cr(VI)-containing solutions in the concentration range of 1–150 mg/L. The mixtures were stirred at 400 rpm for 4 h at room temperature (25 °C). The pH values of the solutions were controlled at 7.0 throughout the reaction. The Cr(VI) concentrations of the filtrates were assessed subsequent to filtration through a 0.45 µm membrane. The adsorption data were fitted using two adsorption isotherm models, the Langmuir and Freundlich ones, and the maximum adsorption capacities were calculated. The data obtained by repeating the adsorption experiments three times were averaged.

2.7. Analytical Methods

2.7.1. Solid Samples Analysis

The structures of the IH, IHK and IK were characterized with X-ray diffraction (XRD) (D8 Advance, Bruker, Mannheim, Germany), Fourier Transform Infrared (FTIR) (Nicolet Avatar 360, Thermo, Dallas, TX, USA), and scanning electron microscopy (SEM) (ZEISS Gemini 300, Oberkochen, Germany) techniques. Precisely, the crystal structures of the samples were scanned in steps from 5 to 80° in 4° increments using Cu-Kα as the radiation source with a counting time of 1 min. The functional groups of the samples were analyzed with FTIR using the potassium bromide press method. The samples were initially coated with gold using a sputter coater in order to observe the surface microstructure at different magnifications. The specific surface areas (SSAs) of the solid samples were calculated based on the Brauer–Emmett–Teller (BET, ASAP 2460, Norcross, GA, USA) method. The solid samples were completely dissolved with HNO₃ and analyzed for the composition and percentage of elements with an inductively coupled plasma atomic emission spectrometer (ICP-AES, ICAPPRP, Heilbronn, Germany).

2.7.2. Liquid Samples Analysis

The total concentrations of contaminants in the AMD and Cr(VI) in the adsorption experiments were carried out in batch experiments with a flame atomic absorption spectrometer (WFX-220A, BFRL, Beijing, China). Based on the principle that barium chromate forms a barium sulfate precipitate with sulfate under an acidic solution and chromate ions appear yellow under alkaline conditions, the concentration of sulfate ions was determined with UV-vis spectroscopy (VIS-723N, Beijing Beifen-Ruili Analytical Instrument (Group) Co., Ltd., Beijing, China) using the barium chromate spectrophotometric method. The pH values of the solutions were determined using a PHS-3E pH meter (INESA, Shanghai, China). The masses of the precipitates were weighed with an electronic precision balance (JA2003, Shanghai Toposun Industries Co., Ltd., Shanghai, China).

3. Results and Discussion

3.1. Characterization of Minerals

The crystal structures of the minerals were characterized with the XRD technique. As shown in Figure 1, IH may potentially be an amorphous schwertmannite based on the standardized XRD pattern of schwertmannite (JCPDS, 00-047-1775), owing to its characteristic diffraction peaks appearing at 26.4°, 35.1°, 46.5°, 55.2° and 61.1°, corresponding to (310), (212), (113), (522) and (004) crystallographic planes, respectively, which perfectly correspond to the typical characteristic peaks of schwertmannite [22,23]. The characteristic diffraction peaks of the IHK were generally consistent with those of the IH, albeit with increased diffraction intensity and a sharpened peak shape, signifying that the crystallinity of the minerals was enhanced subsequent to the addition of KHCO₃. Previous studies have elucidated that schwertmannite possesses a low-crystallinity or exhibits an amorphous structure, which is caused by the difference in the synthesis pathway [24,25,26].

Therefore, IHK may be a low-crystallinity schwertmannite; however, it could also potentially function as an intermediate phase in the transformation of schwertmannite into jarosite, which requires further examination due to the elevation in crystallinity during the conversion of schwertmannite into jarosite. It should be emphasized that IHK was not identified as jarosite due to the lack of characteristic diffraction peaks associated with jarosite (JCPDS, 00-022-0827). The IK, which retained the characteristics of schwertmannite, was essentially the same as the IHK and also necessitated further characterization.

The structural characteristics of the minerals were analyzed using FTIR. Figure 2 displays the FTIR spectra of the IH, IHK and IK. Notably, the IH exhibited spectral patterns consistent with the peak in schwertmannite. It could be confirmed that the intense absorption peak near the 3337 cm⁻¹ region was that of the hydroxyl group -OH in the schwertmannite, while the stretching vibration peak located at 1631 cm⁻¹ was mainly assigned as a HOH deformation. The stretching vibration peak of the Fe-O appeared around 706 cm⁻¹. Concurrently, the stretching vibration peaks of sulfate in the asymmetric stretching band (υ₃), symmetric stretching band (υ₁) and bending band (υ₄) were located near 1131, 977 and 609 cm⁻¹, respectively [27]. Moreover, the absorption band near 427 cm⁻¹ was due to the vibration of the FeO₆ ortho-octahedron [23]. These findings provided compelling evidence indicating that the IH was schwertmannite. Furthermore, the IHK and IK retained the same structure as the IH, which was in general agreement with the XRD results. Interestingly, the sulfate asymmetric stretching band (υ₃) in the IHK and IK appeared as a split peak, which was more obvious in the IK. It suggested the occurrence of two types of sulfates (molecular structure and molecular adsorption) in the IHK and IK, remarkably similar to those found in jarosite, implying an apparent phase transition from schwertmannite to jarosite. In conclusion, FTIR and XRD collectively illustrated that IHK and IK resembled schwertmannite in their absence of the diffraction peaks of jarosite but exhibited the characteristic peaks of jarosite, signifying their likelihood to be IHSM variants, wherein schwertmannite transformed into jarosite.

To further understand the macro- and micromorphologies of IHSMs, the color and micromorphology of IH, IHK and IK were observed. As can be seen in Figure 3, the IH was yellowish–brown in color, similar to those reported in other studies [28]. Interestingly, a minor darkening of color was observed in the IHK and IK, potentially linked to their distinct structures. The IH presented a rough surface texture and was composed of a multitude of spherical bodies with a pin-cushion-like structure, coupled with a few secondary aggregates featuring large particle sizes, which was in close conformity with the previously reported schwertmannite [29]. The IHK contained spherulite particles as well, yet the spherulites seemed more densely arranged compared with the IH. Similarly, the IK was composed of numerous aggregates of smaller spherulites. In essence, they were both composed of spheres, representing the typical morphology of schwertmannite, suggesting that the precursors of IHK and IK could possibly be schwertmannite, which later underwent a phase transformation in the presence of K⁺ to form IHSMs with aggregated spheres, echoing the outcomes of the FTIR and XRD analyses [30]. Furthermore, when comparing the dimensional spherical diameters of the IHK and IK with those of the IH, it is important to note that the former and the latter were distinctly different. The diameter of the IHK was about 700 nm significantly greater than that of the IH at 300 nm. Conversely, the IK was more aggregated, exhibiting spheres of an about 250 nm diameter, likely attributable to the mineral phase transition [25].

To investigate the proportion of elements in the minerals, we dissolved IH, IHK and IK and determined the composition and proportion in the solution. As illustrated in Figure 4, the 42.2% of the iron in the IH was higher than the 39% and 33.5% of those in the IHK and IK; however, the proportions of sulfur in the IHK and IK were significantly higher at 7.37% and 7.45% compared with 6.77% of those in the IH. The Fe/S ratio of the IH, IHK and IK could be calculated to be 3.56, 3.02 and 2.57, respectively. By contrast, pure schwertmannite had a minimum Fe/S value of 4.57, which was obviously higher than that in IH, a discrepancy that could be attributed to the presence of impurity ions in the actual AMD. Research has demonstrated that aluminum in AMD can substitute a minor fraction of the iron in the mineral through homocrystalline substitution, which may contribute to the decrease in the iron content of IH [31]. The IH was amalgamated with an assortment of ions, and Rebecca agreed that the schwertmannite in the actual AMD was not a single-phase mineral, but should be described as a polyphasic mineral [30]. Moreover, the minerals (IHK and IK) obtained with the involvement of KHCO₃ were lower in iron and higher in sulfate, which was consistent with the transformation of schwertmannite to jarosite (as in reaction Equation (2)) [32]:

Fe₈O₈(OH)_8−2x(SO₄)_x + K⁺ + SO₄²⁻ + H⁺ → KFe₃(SO₄)₂(OH)₆ + Fe³⁺(2)

Meanwhile, the presence of K⁺ in the minerals also proved that K⁺ had been incorporated into the mineral, which easily induced the transformation of schwertmannite into jarosite [33]. It is crucial to mention that the constituents Zn, Na, Ca, Mg, Cr, Mn and Al in AMD could be co-precipitated with IHSMs in IHK (with respective concentrations of 0.075, 0.460, 0.027, 0.286, 0.038, 0.156 and 35.7 mg/g) and IK (with corresponding concentrations of 0.142, 0.587, 0.019, 0.315, 0.043, 0.254 and 52.0 mg/g, respectively) more than in IH (with respective concentrations of 0.070, 0.379, 0.027, 0.256, 0.033, 0.154 and 21.3 mg/g), indicating that the incorporation of KHCO₃ into AMD mineralized more contaminant elements, which significantly improved the water quality.

In Figure 5, the SSA and pore volume of the IHK were 1.83 m²/g and 0.0086 cm³/g, respectively, which were smaller than those of the IH (2.02 m²/g and 0.0090 cm³/g). Conversely, the SSA and pore volume of the IK were 1.83 m²/g and 0.0034 cm³/g, respectively, indicating inferior values compared with those of the IHK and IH. This discrepancy was predominantly linked to the component makeup of the formed minerals in conjunction with the diversification of synthesis conditions [34]. The IHK and IK contained a greater concentration of impurity ions (such as Zn, Na, Mg, etc.) than the IH, and these contaminant ions from AMD occupied active sites within the minerals. In other words, the minerals generated with the involvement of KHCO₃ had a smaller SSA due to the mineralization of higher levels of impurity ions in the AMD.

3.2. Water Quality Change after Mineralization

The depletion of Fe and SO₄²⁻ in the treated water post AMD mineralization, alongside the mass of precipitates and pH value of the water are illustrated in Figure 6. The addition of KHCO₃ into the AMD notably decreased the level of Fe and SO₄²⁻ in the water, signifying that the mineralization ratios of Fe and SO₄²⁻ were noticeably increased.

As shown in Figure 6, the masses of IH minerals progressively began to increase and ultimately stabilized at 0.72 g as the addition of H₂O₂ was incrementally increased. Concomitantly, the removal ratios of Fe and SO₄²⁻ were found to be 17.5% and 6.2%, respectively. Significantly, the iron removal ratio, initially dropping before subsequently rebounding, was calculated based on the original AMD (if assessed with respect to the Fe⁰-reduced AMD, it reached 63.7%). Consequently, there remained 1763 mg/L of iron yet to be mineralized in the AMD. This was attributed to the fact that the energy generated by the oxidation reaction only partially overcame the energy barriers of mineralization, owing to the rate of oxidation being substantially higher than the rate of hydrolytic mineralization. As such, the energy released from the H₂O₂ oxidation of Fe²⁺ did not fully synchronize with the overcoming of the energy barriers of Fe³⁺ and SO₄²⁻ mineralization [15]. Given that the mineralization of AMD is an acid-generating process, the pH in the solution decreased until it stabilized at 2.25. This was attributable to the continuous production of H⁺ during the formation of schwertmannite, resulting in a pH decrease as illustrated in Equation (1). Nevertheless, the low pH could potentially hinder mineralization. Therefore, the mineralization of AMD solely utilizing H₂O₂ proved insufficient to transform Fe³⁺ and SO₄²⁻ into schwertmannite.

Conversely, when 1.5 mL of H₂O₂ was introduced into the AMD containing Fe²⁺, followed by the addition of varied masses of KHCO₃, the removal ratios of iron and sulfate ions increased with the increasing mass of KHCO₃, as depicted in Figure 6. When KHCO₃ was added at 0.9 g, the removal ratios of iron and sulfate ions, the pH of the solution and the mass of the IHK mineral increased to 92.7% and 11.1%, 2.8, 1.19 g from 17.5% and 6.2%, 2.25 and 0.72 g, respectively. These results demonstrated that the addition of KHCO₃ significantly improved the mineralization ratio, as compared with AMD mineralization by chemical oxidation through H₂O₂ exclusively. This further bolstered the claim that the substantial energy generated by the reaction between the weakly basic KHCO₃ and acidic water could promote the continued formation of IHSMs from Fe³⁺ and SO₄²⁻. The findings showed that introducing an optimal quantity of H₂O₂ followed by a certain mass of KHCO₃ to AMD containing a large amount of Fe²⁺ could transform most of the iron into an IHSM. This approach is advantageous for recovering the iron from AMD abundant in Fe²⁺ and converting it into a valuable IHSM for resource recycling.

Unexpectedly, the removal ratios of Fe and SO₄²⁻ increased with increasing mass when different masses of KHCO₃ were directly added into the AMD containing Fe³⁺. With the addition of 1.2 g of KHCO₃, the iron ions had almost entirely mineralized with a removal ratio of 97.6%, while the removal ratio of SO₄²⁻, the pH of the solution and the mass of the IK mineral reached 6.9%, 2.88 and 0.75 g, respectively. This indicated that the energy generated by acid–base reactions in the system also overcame the energy barrier of the hydrolysis of Fe³⁺ and SO₄²⁻ into the IHSM. This provided evidence that KHCO₃ could directly induce Fe³⁺ and SO₄²⁻ to form minerals, laying a foundation for the direct mineralization of AMD containing Fe³⁺.

3.3. Resource Reuse: The Cr(VI) Adsorption Properties

Resource reuse is key for sustainable development for AMD remediation. The absorption of metal oxygen anions by IHSMs influences considerable environmental advantages to the remediation of AMD, offering significant academic worth and practical relevance in advancing the objective of sustainable development. IHSMs demonstrate promising adsorption capabilities for metal oxygen anions such as chromium and arsenic [35,36,37]. The environmental applications of IHSMs, designated as IH, IHK and IK, were evaluated by the adsorption of Cr(VI). As shown in Figure 7, the correlation coefficient of the Langmuir isotherm for IH, IHK and IK amounted to 0.976, 0.979 and 0.981, respectively, significantly surpassing that of the Freundlich isotherm, 0.971, 0.972 and 0.976, indicating a stronger correlation between the experimental findings and the former, thus suggesting that the adsorption process of Cr(VI) by IHSMs was dominated by monolayer adsorption. In the range of Cr(VI) from 1–150 mg/L, the adsorption capacities steadily increased with the escalation of concentration. Through Langmuir model calculations, the maximum adsorption amounts of 94.8, 98.5 and 104.1 mg/g for IH, IHK and IK were attained, respectively. Choppala demonstrated that the high adsorption capacity of IHSMs for Cr(VI) originated from the SSA providing abundant active sites [38]. Surprisingly, the maximum adsorption capacities of IHK and IK displayed an intriguing inverse trend despite their smaller SSAs and pore volume compared to those of IH, given the fact that the adsorption of Cr by minerals was related to the proportion of SO₄²⁻ contained in the mineral in addition to the SSA [29,39]. The Fe/S molar ratios of IHK and IK were calculated at 3.02 and 2.57, respectively, from the elemental percentage, which were inferior to the 3.56 of IH, thereby inferring that the former contained a higher concentration of SO₄²⁻ than the latter. Specifically, Cr(VI) could be ion-exchanged with SO₄²⁻ groups in IHSMs at easily accessible active sites or complexed with ≡Fe−OH within the IHSM to form ≡Fe−OHCrO₄²⁻ [40,41]. The results of previous studies could prove that the mechanism of the sulfate anion exchange reaction in the structure provided more efficient use compared to the complexation reaction in adsorption of metal oxyanions under neutral conditions [42]. Therefore, the high sulfate content in IHK and IK also obtained excellent adsorption capacity for Cr(VI), substantially enhancing the sustainability of IHSM and AMD remediation.

Appropriate AMD treatment should strive to convert waste into usable resources while achieving AMD remediation, which is conducive to building sustainable development pathways. KHCO₃ could convert more iron and sulfur from AMD into a large amount of valuable minerals that can be used as an adsorbent in the field of remediation of water and soil contamination, which means that removing contaminants from AMD could be accompanied by the availability of useful minerals. The profit generated by the adsorbent could mitigate the cost of the materials consumed, which contributes to the twin goals of sustainable water remediation and the reuse of valuable resources. Furthermore, the partial removal of major contaminants from AMD conserves a considerable number of materials for subsequent treatment, which can also appreciably lessen subsequent treatment costs. Therefore, the use of KHCO₃ to form more reusable materials for the treatment of AMD containing high concentrations of iron and sulfate ions is a cost-effective and environmentally friendly sustainable treatment method.

4. Conclusions

The KHCO₃-induced mineralization of AMD was a cost-effective method for resource recovery and water remediation, which could contribute to the development of sustainable AMD remediation. The energy generated by the acid–base reaction promoted the hydrolysis of Fe³⁺ and SO₄²⁻ to form IHSM. In AMD containing Fe²⁺, the combined mineralization with H₂O₂ and KHCO₃ could achieve a one-time conversion of most of the Fe and part of the SO₄²⁻ to produce an IHSM, which concurrently precipitated more contaminants from the AMD than mineralization by H₂O₂ only, and also contained more SO₄²⁻ in minerals, resulting in a higher adsorption capacity for Cr(VI). Moreover, in AMD containing Fe³⁺, IHSMs could also be directly obtained using KHCO₃, which could directly mineralize iron and sulfate to form an IHSM, with superior water remediation and a higher environmental value. Therefore, KHCO₃-modified chemical mineralization to form an IHSM is an effective technique for the sustainable remediation of iron-rich AMD.

Author Contributions

Conceptualization, X.H. and H.J.; methodology, H.Y.; investigation, X.H., H.W. and C.T.; writing—original draft preparation, X.H.; writing—review and editing, H.Y., X.H. and H.J.; supervision, H.J.; project administration, H.J. and H.Y.; funding acquisition, C.T. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data sets procured and/or evaluated throughout this study can be accessed from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare that they have no conflict of interest.

Footnotes

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Figures and Table

Figure 1. The XRD spectra of the IH, IHK and IK (J: Jarosite; S: Schwertmannite).

Figure 2. The FTIR spectra of the IH, IHK and IK.

Figure 3. The color and SEM images of the IH, IHK and IK.

Figure 4. The percentage proportion of different elements in the IH, IHK and IK.

Figure 5. The N2 sorption curve of the IH, IHK and IK.

Figure 6. The changes in removal ratio of iron and sulfate ions, the pH values as well as the mass of the IH, IHK and IK minerals.

Figure 7. The Cr(VI) adsorption isotherms and the corresponding parameters fitted from Langmuir and Freundlich models on IH, IHK and IK.

Table 1

Water quality parameters in AMD.

Items		AMD	Standards for Drinking Water Quality	Exceeding the Standard Times
pH		2.1	6~9	——
Sulfate (mg/L)		24,041	250	95.16
Metal Concentration (mg/L)	Fe3+	2137	0.3	7122.33
	Mn2+	118	0.1	1179
	Na+	51.7	——	——
	K+	0.462	——	——
	Al3+	1573	——	——
	Ca2+	48	——	——
	Cu	1.59	1.0	0.59
	Zn	23.5	2.0	11.25
	As	0.0044	0.1	——
	Cr	0.021	0.1	——
	Mg2+	1466	——	——

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