Highly Efficient Modified Constructed Wetlands Using Waste Materials for Natural Acid Mine Drainage Treatment

Abstract

Coal-mining activities have well-documented adverse effects on both the environment and human health. Acid mine drainage, a pivotal concern, necessitates effective interventions. This study introduces a novel solution: a modified constructed wetlands crafted exclusively from waste materials, ensuring cost-effectiveness. The innovation yielded exceptional results, achieving a noteworthy reduction of up to 99% in heavy metal concentrations, alongside swift pH normalization. What sets this study apart is its potential beyond the laboratory setting; the utilization of waste materials and low-cost methodologies underscores its scalability and practicality. This solution addresses immediate challenges and showcases promise for real-world implementation. Moreover, the results of the study extend to its insights, which offer a comprehensive examination of the method’s reusability prospects, illuminating its sustained impacts; the recommendations for future action enhance its practical significance. This study marks a significant advancement in tackling acid mine drainage. The modified constructed wetlands, driven by cost-effective waste materials, embody scalable and sustainable potential. With its holistic outlook and strategic roadmap, this study holds the key to transforming acid mine drainage challenges, particularly in rural and developing regions.

1. Introduction

Industrial activities have garnered widespread recognition for their substantial contributions to environmental degradation [1]. Among the detrimental repercussions of these activities, water quality degradation stands as a significant and pivotal concern [2]. Within this context, the coal mining industry emerges as a noteworthy contributor, casting a significant shadow across various facets of the environment [3,4,5]. The repercussions of coal mining reverberate far and wide, leaving an indelible impact on nearly every dimension of the environment and human health [6,7,8]. Particularly pronounced within the coal mining industry is the issue of high levels of toxic elements coupled with the imposition of low pH conditions [9]. These factors collectively constitute a formidable challenge during both the active mining phase and in the subsequent post-mining era. The haunting persistence of heavy metals within abandoned mining sites is starkly evident through their accumulation within water bodies, soils, and the surrounding flora [10,11,12]. This perilous scenario paints a stark picture, signifying the magnitude of the environmental jeopardy. These alarming repercussions extend beyond the realm of ecological imbalance, and the consequences span from the distressingly tangible, such as cancer, poisoning, and skin disease, to the ultimate tragedy of loss of life [13]. In tandem, the coal-mining industry has laid bare a deeply unsettling narrative that underscores its far-reaching impacts on human health [11]. Moreover, the scope of the challenges posed by coal mining transcends immediate health concerns. The degradation of soil quality, as another fallout of this industry, detrimentally impacts the ability of the environment to support plant growth [14]

Many studies have been conducted to address these persistent challenges. Despite these efforts, the problems persist and continue to be reported across various regions. Several studies have explored diverse strategies to tackle this issue, spanning adsorption [15], phytoremediation [16], filtration [17], coagulation [18], bacterial bioremediation [19], and an array of other techniques. However, the common threads running through these methods are their energy-intensive nature, the need for specialized expertise, the utilization of scarce materials, and high associated costs. Take, for instance, adsorption—a method that has shown promise in heavy metal reduction. Yet, many adsorption studies remain confined to laboratory settings, relying on sophisticated equipment such as magnetite, stirrers, ultrasonic devices, and muffle furnaces. These high-tech requirements underscore the limitations in terms of scalability and practical application. Likewise, filtration, another approach with potential, faces similar limitations.

The reliance on cutting-edge technology, rare materials, and the expertise of trained professionals complicates widespread adoption. Such methods are often constrained within laboratory confines, limiting their real-world impact. In contrast, a beacon of promise emerges in the form of constructed wetlands (CW), a solution that addresses these challenges effectively, efficiently, and economically. Notably, CW do not demand exorbitant costs, can be operated by non-specialists, and offers significant effectiveness. The unique strength of CW lies in a holistic approach, seamlessly integrating diverse methodologies into a singular system. Unlike the singular-focus strategies of adsorption, filtration, coagulation, precipitation, or microbial-based bioremediation; the use of CW operates by harmonizing these approaches, presenting a comprehensive and multifaceted solution. The inherent synergy within CW underscores a paradigm shift in acid mine drainage (AMD) treatment. CW stand as a testament to the potential of interdisciplinary solutions by combining various remediation mechanisms within a single framework. Beyond the technical aspects, CW also align with the ideals of sustainability, offering a cost-effective, energy-efficient, and user-friendly alternative that holds the potential to transcend laboratory settings and find practical utility in the real world.

Within the confines of this study, a transformative approach was pursued through the modification of CW, leveraging the incorporation of potent materials crafted using cost-effective methods. This endeavor was meticulously undertaken to bridge the existing gap in AMD treatment, offering a solution that holds promise not only in the laboratory but also in practical field scenarios. Notably, the innovation behind this modified CW sidesteps the need for high-cost investments and intricate technologies, aligning seamlessly with the potential for large-scale industrial production. An essential facet of this study’s merit lies in its ability to furnish a comprehensive solution that effectively addresses AMD-related challenges. By harnessing the power of various materials and treatments, this modified CW offers a holistic strategy that is devoid of prohibitive costs and advanced technological prerequisites. This hallmark approach holds the potential to revolutionize the landscape of AMD treatment by ushering in a practical, economically viable, and technologically accessible method. The ingenuity of this study extends beyond its immediate impact, encompassing a meticulously orchestrated analysis that delves into various dimensions. Material characteristics were scrutinized, removal mechanisms were dissected, and performance was meticulously tested. This comprehensive evaluation is further enriched by an exploration of the potential reusability of the system, amplifying its sustainability quotient. Moreover, this study transcends its immediate scope by offering forward-looking recommendations for future research and potential field applications. The provided roadmap paves the way for continuous refinement and optimization, laying the groundwork for the eventual integration of this approach into real-world settings

2. Materials and Methods

2.1. Material and Plant Preparation

Biochar, a valuable product, was generated using an economical approach from various waste materials, including wood residue and organic substances (Figure 1). The low-cost treatment for generating biochar followed the optimal conditions attained in the previous study, including a low-oxygen environment, gradual pyrolysis within the temperature range of 150–300 °C to craft biochar effectively [20], and a heating speed of 40 °C min⁻¹ [21]. The generated biochar then underwent a crushing process, resulting in finely crushed particles sieved through a 50-mesh shaker. In the context of this research, clamshell waste underwent calcination, transforming the CaCO₃ into CaO, as depicted in (1). Other materials, like gravel and sand (collected from Lampung Province, Indonesia), did not involve any preparatory steps and were used in their natural states.

Plant specimens were sourced from Lampung Province, Indonesia (5°22′15.8″ S–105°17′14.8″ E), ensuring uniformity in terms of height and diameter (roots ± 14 cm and shoots ± 30 cm). Following collection, the selected plants underwent a meticulous acclimation process spanning 7 days. This acclimation period was essential to mitigate any potential shock to the plants and facilitate their smooth adaptation to the new environmental conditions. The initial and final states of the plants were subjected to evaluation through XRF Epsilon 4 analysis. This advanced analytical technique allowed for a precise examination of the plants’ elemental composition, enabling a comprehensive understanding of any changes that occurred during the study period.

2.2. Characterization and Performance Test

All comprehensive characterization processes were undertaken to acquire a thorough understanding of the samples. In order to obtain a holistic view, various advanced instruments were employed, each contributing a unique dimension to the dataset. The distinctive characteristics of AMD were meticulously uncovered through the application of inductively coupled plasma mass spectrometry (ICP-MS). This analytical technique provides a precise tool with which to identify the specific heavy metal constituents present within AMD, furnishing a detailed elemental profile. An advanced field emission scanning electron microscopy (FESEM) approach was adopted for a detailed analysis of the solid materials in each section. This was conducted using the cutting-edge Thermo Scientific Quattro S system, which integrates several features, such as an energy dispersive X-ray spectroscopy (EDS) detector, WetSTEM, heating stage, and tensile stage. This amalgamation of tools offers a comprehensive exploration of critical attributes, including surface morphologies, particle dimensions, and the elemental composition of materials. This comprehensive analysis provided multifaceted insights into the inherent characteristics of the materials. In addition, the structural attributes of the materials were probed through X-ray diffraction (XRD), utilizing the PANAnalytical X’Pert Pro equipment. This technique was instrumental in revealing the crystallinity and crystal size of the materials, unveiling pivotal structural aspects.

Further enhancing the breadth of information, measurements of pH and total dissolved solids (TDS) were conducted. This involved the utilization of a pH meter and a TDS meter (EZ-9908), respectively. The pH measurement provided insights into the acidity or alkalinity of the solutions, while the TDS measurement quantified the concentration of dissolved solids.

The size of the crystal diameter was determined using the Debye–Scherrer equation, wherein D signifies the crystal size, K represents the Scherrer constant (0.89), 143 λ stands for the X-ray wavelength applied (1.54056 Å), β represents the full width at half maximum (FWHM), and θ signifies the Bragg diffraction angle. The data for FWHM were computed based on the most prominent peak on the diffractogram. It is notable that the FWHM value exhibits an inverse relationship with the crystal diameter, and the crystal size can be derived using (1):

$$D=\frac{K \lambda }{\left(FWHM\right)\times cos\theta }$$

(1)

The performance test of CW during the removal of heavy metals is calculated by (2):

$$\mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{g}\mathrm{e}=\frac{C_0-C_e}{C_0}\times 100\mathrm{\%}$$

(2)

The linearity correlation (R²) for the analysis of correlation in heavy metals adsorbed in the roots is calculated in (3):

$$R^2=1-\frac{SSR}{SST}$$

(3)

The SSR (sum of squared residuals) represents the cumulative sum of the squared discrepancies between the observed values of the dependent variable and their corresponding predicted values. On the other hand, SST (total sum of squares) reflects the cumulative sum of squared deviations between the observed values of the dependent variable and their overall mean. Furthermore, in this study, the R-squared (R²) value was automatically computed using an algorithm implemented within RStudio. This analytical tool assists in quantifying the proportion of variance in the dependent variable that can be explained by the independent variables, offering valuable insights into the model’s goodness of fit and predictive power.

2.3. Experimental Details

AMD was gathered from an abandoned coal mining site in Lampung Province, Indonesia (5°03′15.2″ S 104°46′46.7″ E). The geographical location of this abandoned mining area is visually represented in Figure 2A. The reddish hue of the AMD signified elevated levels of pollutants, primarily heavy metals. This observation correlated with the remarkably high Fe concentration (263 mg/L), significantly surpassing both the Indonesian Minister’s Regulation standard of 5 mg/L and the Environmental Protection Agency’s recommended limit of 0.30 mg/L for dissolved iron (Fe). Water samples were meticulously collected using polyethylene bottles that had been thoroughly cleaned with double-distilled water. This rigorous cleaning process ensured the elimination of any potential contamination sources, maintaining the collected samples’ integrity [20]. This outcome underscored the severe contamination of the site and the urgent need for effective remediation measures.

Figure 2B provides an intricate view of the artificial CW. This CW was meticulously fashioned from 8 mm glass boxes, chosen for their ability to withstand internal pressure. To ensure the absence of contaminants, the glass boxes underwent a thorough double-washing with distilled water prior to utilization. Each glass box was allocated to accommodate two plants, which served as phytoremediation agents. On another note, in this study, biochar served as an adsorbent. Additionally, gravel and sand are utilized to mitigate the negative impact of infiltration. Notably, this study prioritized uniformity, confirming that the diameter and height of both the shoots and roots of plants were maintained at relative parity. This meticulous approach underscores the precision and reliability of the experimental setup.

3. Results and Discussion

3.1. Sample Characteristics

Table 1 presents the weight percentage concentration of CaO resulting from the clamshell waste calcination process. This material has been identified as a potent agent for the precipitation of heavy metals under neutral pH conditions, which is attributed to its capacity to generate Ca(OH)₂ (4). Figure 3 shows the morphological and EDS mapping of the clamshell before and after calcination. Figure 3C shows that calcination can change the texture of the clamshell’s surface; at high temperatures, the organic matter in the shell decomposes and burns off, leaving behind the inorganic components. This can result in a more porous and textured surface compared to the original smooth surface of the shell.

Calcination can cause structural changes in the shell’s calcium carbonate (CaCO₃) composition. High temperatures can lead to phase transformations, such as the conversion of aragonite to calcite, which may affect the overall structure and crystallinity of the shell. Calcination can also impact the surface roughness of the clamshell, which may increase as a result of calcination. This can be due to the removal of organic materials and the formation of new mineral structures that create surface irregularities. Lastly, calcination typically removes the organic components of the clamshell, including proteins, chitin, and other organic substances. This can result in the loss of any of the surface features or textures associated with these organic materials.

The precipitation of heavy metals refers to the process by which these toxic elements are removed from aqueous solutions and transformed into solid forms. Various precipitation methods are employed to remove heavy metals from contaminated waters. These methods exploit the differences in solubility between metal ions in their various oxidation states. Commonly used precipitation techniques include chemical precipitation, sulfide precipitation, carbonate precipitation, and hydroxide precipitation [22]. Using CaO from waste materials can also capture greenhouse gas CO₂ [23].

$$\mathrm{C}\mathrm{a}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{a}{\left(\mathrm{O}\mathrm{H}\right)}_2$$

(4)

Table 1. XRF analysis of materials.

Materials	CaO Content (wt%)	Reference
Clamshell	97	This study
Eggshell	86.93	[24]
Waste chicken feather	33.14	[25]

Table 1. XRF analysis of materials.

Materials	CaO Content (wt%)	Reference
Clamshell	97	This study
Eggshell	86.93	[24]
Waste chicken feather	33.14	[25]

Figure 4 provides a comprehensive view of the characteristics of CW materials under varying levels of magnification: 1000×, 10,000×, and 20,000×. This figure also includes EDS mapping, offering an insight into the elemental composition of each material. The findings of this study reveal that the modified CW material exhibits an amorphous structure attributed to its non-uniform composition, resembling carbonaceous materials. This can be attributed to the incorporation of biochar into the CW, which serves as an adsorbent. Notably, biochar, renowned for its environmental prowess, was introduced into the CW to enhance its properties. Prior research has established that the introduction of biochar results in the creation of powerful adsorbent materials. This was demonstrated in recent studies where biochar demonstrated remarkable efficacy in the removal of heavy metals from AMD [13,26]. Beyond its application in wastewater treatment, biochar has proven its versatility as an exceptional material for CO₂ adsorption [27] and carbon sequestration [28]. As such, the utilization of biochar in this study promises to yield a multitude of positive impacts that align with its manifold advantages and capabilities.

The EDS characterization of the CW materials offers a detailed insight into their elemental composition. Notably, these materials encompass a diverse range of elements, including C (carbon), Ca (calcium), Si (silicon), Al (aluminum), O (oxygen), and Mg (magnesium). Upon closer examination, it becomes evident that the carbon content arises from incorporating biomass, specifically in the form of biochar. The presence of calcium is ascribed to the utilization of clamshell waste, while silicon, aluminum, and oxygen stem from various other constituent materials, including biochar. This intricate composition underscores the fusion of multiple components, collectively bestowing the CW materials with their distinctive attributes. Importantly, this characterization underscores the absence of high concentrations of heavy metals or pyrite within the modified CW. This absence alleviates concerns regarding the potential escalation of heavy metal levels or the creation of new hazardous metals detrimental to the environment. As a result, this thorough characterization further validates the safety and suitability of utilizing the modified CW materials in field studies.

The X-ray diffraction (XRD) analysis revealed distinct and discernible peak patterns within each section. Though detailed in

Table 2. Crystal sizes.

Section A	2 Theta	FWHM	Crystal Size (nm)	Average of Crystal Size (nm)
	23.2004	0.31849	25.46371367	21.94907818
	29.6312	0.3179	25.84928999
	36.22286	0.32682	25.57498736
	39.71281	0.37637	22.44189918
	43.49166	0.35435	24.13700141
	47.88489	0.73607	11.80896356
	48.91492	0.47515	18.36769207
	30.10837	3.16338	2.600579602
	44.25888	1.49552	5.734487958
	57.85702	0.51447	17.6431364
	61.32802	0.70484	13.10324282
	65.19417	0.46436	20.30676452
	66.20947	0.54876	17.28214992
	57.0554	5.24077 × 10−5	172534.3198
Section B	5.67109	0.45635	17.42966497	5.140218938
	9.72502	0.19976	39.91281536
	19.87466	0.83584	9.649324966
	21.62553	1.53295	5.276026969
	35.60479	1.96156	4.253665112
	61.8163	0.78843	11.74380233
	28.46749	2.00184	4.094181992
	35.60479	25.02508	0.333418288

, these peak profiles lack pronounced sharpness and carry noteworthy significance. Identifying the correspondence of JCPDS Card or ICDD numbers poses a challenge due to the amalgamation of materials in both Section A and Section B. It is advisable to undertake further investigation to understand more about the intricacies of these specific XRD patterns. This exploration aims to unravel the underlying factors contributing to the distinctive nature of the observed peaks, fostering a comprehensive understanding of the materials’ composition and arrangement.

Crystal size and structure analysis is crucial for an understanding of the effectiveness of the adsorbents used to treat AMD. In Section A, the crystal sizes range from around 11.8 nm to 25.8 nm (Table 2). These values reflect the dimensions of the crystalline structures within the material. In the context of AMD treatment, larger crystal sizes might indicate better adsorption capacities due to an increased surface area for interaction with contaminants. Furthermore, the FWHM values indicate the degree of crystallinity, with lower values indicating more ordered structures. The adsorbent’s surface properties, such as its specific area and crystal structure, significantly impact its adsorption capacity. Larger crystal sizes generally lead to higher surface areas, providing more sites for adsorption. The 2 Theta values in the data correspond to the diffraction angles at which the X-ray diffraction patterns were measured. These angles provide information about the lattice spacing and arrangement of atoms in the crystalline structure. Adsorbents with well-defined crystal structures and lattice arrangements might have enhanced selectivity for specific contaminants, making them suitable candidates for AMD treatment.

Comparing the two sections, it appears that Section A has lower crystal sizes on average than those in Section B. However, the specific applications of these sections and their materials are unclear from the provided data alone. In an AMD treatment context, both small and large crystal sizes can have benefits. Smaller crystals might offer higher reactivity and accessibility for adsorbing contaminants, while larger crystals might offer greater adsorption capacity due to their increased surface areas. Materials with crystal sizes close to the sizes indicated in the data might exhibit promising adsorption characteristics for AMD contaminants like heavy metals. The XRD data can help to identify potential adsorbent materials with suitable crystalline structures and sizes for AMD treatment. Common adsorbents, like activated carbon, zeolites, and various metal oxides, can be evaluated based on their crystal sizes and structures.

Table 3. AMD characteristics.

Pollutant Parameters	Concentration
pH	2.87
TDS	1480 ppm
Heavy metal (Fe)	263 mg/L

presents an overview of the distinctive characteristics associated with AMD. As per the findings of this investigation, AMD exhibits elevated levels of Fe concentration accompanied by a low pH environment. Additional heavy and lightweight metals, such as Mn, Al, Ca, and Mg, were detected; however, their concentrations remain relatively insignificant and fall below the standards stipulated by the National Indonesian Standard for polluted water. It is worth noting that the utilization of local standards in this study is imperative, considering the inherent variations in standards across different regions due to diverse environmental conditions. The heightened Fe concentration is further validated by the observable alteration in water coloration, as depicted in Figure 2A, transitioning from its natural state to a reddish hue. This study also establishes that the examined AMD displays higher contaminant levels compared to similar occurrences on the same island in Sumatera, Indonesia [20]. The prominent Fe concentration identified in this study is attributed to a combination of chemical and bacterial processes [29].

Furthermore, the low pH level evident in this particular AMD presents an additional hazard to the surrounding ecosystem of this abandoned mining site. This study underscores the protracted impacts of coal mining activities, emphasizing that the repercussions can extend beyond the mining phase itself. Consequently, effective environmental management practices spanning pre-mining, mining operations, and post-mining phases are of paramount importance.

The following sample characteristic is plants: throughout the seven-day acclimation period, precise control was exerted over the pH and TDS of the tap water, affecting the outcomes of this acclimation phase, as well as the subsequent in-situ condition at the time of plant collection. This treatment phase holds paramount importance as it serves as a pivotal means to scrutinize the plant’s response to alterations in its surroundings. The significance of this treatment becomes apparent when considering the potential repercussions of abrupt environmental changes on plant viability. A significant shift in environmental conditions could heighten the likelihood of plant mortality. As such, by subjecting the plants to controlled pH and TDS changes during the acclimation process, we create a controlled scenario to assess their adaptability and resilience to fluctuating surroundings. This analysis is integral to comprehending the ability of plants to thrive and survive under conditions that deviate from their accustomed habitats [30].

In Figure 5, the pH values observed during the acclimation period ranged from approximately 6.83 to 6.97, reflecting a nearly neutral pH environment. Notably, these values exhibit minimal variation compared to the source pH of the plants (which was measured at 6.44). Additionally, the TDS concentrations recorded throughout the acclimation process ranged from 97 to 115 ppm, showing consistency with the TDS level of the plant’s source environment (measured at 164 ppm). This congruence between the tap water conditions and the plant’s natural habitat suggests that the plants are likely to acclimate well to their new surroundings. The lack of a significant divergence between the characteristics of the tap water and the native environment indicates the plant’s capacity to thrive in the novel setting. It implies that the plants possess an inherent ability to adapt to the changes in pH and TDS levels, increasing their likelihood of survival in this altered habitat. Notably, the metal content was not quantified in this preliminary test due to the absence of notable metal concentrations in either the tap water or the source of the plant.

3.2. Performance Test

Table 4. Effect of contact time in concentration (mg/L).

Days	Section
	A1	A2	A3	A4	B1	B2	B3	B4
0	263	263	263	263	263	263	263	263
2	0.07	0.07	0.07	0.07	0.07	0.07	0.07	0.07
4	0.07	0.07	0.07	0.07	0.07	0.07	0.07	0.07
6	0.07	0.07	0.07	0.07	0.07	0.07	0.07	0.07
8	0.07	0.07	0.07	0.07	0.07	0.07	0.07	0.07

presents the profound impact of contact times on the removal of heavy metals from AMD using the modified CW. This adapted CW exhibited robust and statistically significant efficacy in heavy metal removal. Surpassing its counterparts on several instances, this system achieved a remarkable reduction of up to 90% of heavy metals, as demonstrated in previous research [31]. Particularly striking is the reduction in highly concentrated Fe content from 263 mg/L to an astonishingly low 0.07 mg/L, a performance that outshines a prior study utilizing unmodified water hyacinth (Eichhornia crassipes) for the same purpose [32]. This determination was based on the outcome of the ICP test, which indicated a concentration lower than the detection limit of the measuring instrument.

Consequently, to accurately represent this result, the study adopted the detection limit of the instrument as the reported concentration value. The exceptional performance of this study in Fe removal is not limited to specific sections; instead, it is consistently remarkable across all sections. This can be attributed to the intricate mechanism underpinning the removal process in this modified CW.

The data in

Table 5. Removal percentage (%) of each section.

Days	Section
	A1	A2	A3	A4	B1	B2	B3	B4
0	0	0	0	0	0	0	0	0
2	99	99	99	99	99	99	99	99
4	99	99	99	99	99	99	99	99
6	99	99	99	99	99	99	99	99
8	99	99	99	99	99	99	99	99

further underscore this efficacy, revealing a linear trend in the removal percentages. Strikingly, all sections exhibited a commendable 99% reduction in Fe content from AMD. This trend suggests the potential for similar success in reducing other heavy metals, such as Mn, Cd, Cr, and Pb, especially in cases where their concentrations are substantial. The assessment of varying biochar volumes within this CW did not yield discernible differences, underscoring the remarkable efficiency of the employed models. Table 5 provides insights into the removal percentages of Fe observed in this investigation. The results unequivocally confirm the consistently high efficiency (up to 99%) of all sections, even during brief testing periods. This study firmly establishes the success and substantial potential of its application in real-world scenarios, owing to the absence of energy requirements during the degradation processes. This distinction becomes apparent when contrasting the method with adsorption methods that necessitate agitation for extended periods, often up to 300 min, or phytoremediation strategies demanding over seven days to achieve a significant reduction in heavy metals from AMD [33], as well as other common domestic pollutants.

On a separate note, the CW also exerts a noteworthy influence on pH neutralization and TDS reduction within AMD. Figure 6A visually depicts the TDS concentrations in AMD before and after treatment by the modified CW. The decrease in TDS observed can be attributed to the adsorption process occurring on the biochar. This phenomenon aligns with findings from recent studies that have successfully utilized adsorption using a neem seed powder adsorbent to remove DS from wastewater [34]. The biochar’s ability to adsorb and trap these dissolved solids is a promising aspect of this research, indicating its potential as an effective treatment method for reducing TDS levels in water.

The outcomes of this study indicate a remarkable achievement, where the initially elevated TDS concentration is effectively reduced to levels of less than 800 mg/L. It is worth noting that heightened TDS levels, along with other parameters of the pollutants present in water, have been associated with various health issues such as liver ailments, pulmonary congestion, vomiting, and diarrhea [35]. In a complementary manner, Figure 6B illustrates the pH variations of AMD before and after undergoing treatment within the modified CW. As per the findings of this study, the pH levels of the AMD exhibit a discernible shift towards normalization (3). Specifically, the pH values align with the stipulated National Indonesian Standard range for AMD discharge into river ecosystems, falling within the pH range of 6 to 9. Consequently, this investigation successfully meets the criteria for the quality standards of non-polluted water. This CW showed a remarkable capacity to generate alkalinity through the use of clamshell waste, which inherently possesses alkaline properties. This alkalinity contributes significantly to the reduction of acidity levels within AMD. This effect arises from the interaction between the alkaline clamshell waste, sulfuric acid, and heavy metal ions in the acidic mine water. This particular process is commonly recognized as neutralization, or the liming process. Through this mechanism, the CW effectively counteracts the acidity inherent in AMD, resulting in the amelioration of its pH and overall quality.

Table 6. The comparison between modified CW and conventional CW for treating AMD.

Plant	Heavy Metal/Pollutant	pH	Time Contact (Day)	Removal Percentage (%)	Ref.
Phragmites australis, Carex rostrata, and Eriophorum angustifolium	Fe, Zn, Cu, Cd	2.65	266	Up to 57	[36]
Typhia latifolia	Al, Ca, Fe, Mg, Mn	6.15	N/A	Up to 25	[37]
Eichhornia crassipes	Fe	N/A	105	Up to 47	[32]
Eichhornia crassipes	Fe	6.97	7.5	Up to 99	This study

, included for the purpose of comparison, serves as a comprehensive resource that encapsulates the entirety of our study’s contribution to the advancement of knowledge in AMD treatment through the use of the CW method.

3.3. Heavy Metals Accumulation, Distribution and Removal Mechanism

The exceptional effectiveness in heavy metal removal and rapid pH neutralization can be attributed to several intricate processes occurring within the modified CW. The initial process involves the phytoremediation principle, where heavy metal removal occurs through plant uptake, specifically employing Eichhornia crassipes. This phytoremediation process entails the absorption of pollutants by the plant, with the pollutants acting as nutrients that support the plant’s growth. This dynamic is characterized by varying capacities of pollutant uptake, denoted by the distinct uptake capabilities among different elements [38]. Furthermore, recent research underscores the potential of Eichhornia crassipes as a hyperaccumulator plant with remarkable capabilities for treating wastewater. This additional study aligns with the present investigation, reinforcing that Eichhornia crassipes is a promising option for efficiently addressing wastewater-related concerns [39].

A comprehensive understanding of pollutant degradation is provided in Figure 7B. The removal of pollutants occurs through a combination of adsorption, precipitation, filtration, plant uptake, and microbial activities. These processes synergistically contribute to the successful mitigation of pollutants. The adsorption process relies on biochar incorporation within the modified CW, where pollutants are effectively attracted and retained. This is followed by the precipitation of heavy metals, facilitated by the presence of CaO derived from clamshell waste. A previous study demonstrated that CaO can precipitate Fe from wastewater [40]. This reaction aids in the conversion of dissolved heavy metals into insoluble precipitates. Filtration, a crucial system component, is realized through the arrangement of various layers, including biochar, gravel, and sand. This intricate arrangement effectively filters out contaminants, proving especially impactful in curbing underground water contamination. Furthermore, the phytoremediation integration with Eichhornia crassipes introduces plant uptake as a pivotal mechanism. Through this process, the plants actively absorb pollutants, contributing to their growth while simultaneously reducing pollutant concentrations in the water.

For the holistic analysis of metal degradation, Figure 7C,D provides information on the correlation between time contact and heavy metals in the plant roots. In this study, the AMD has a high concentration of Fe, which corresponds with results of the Fe concentration in plant roots (the concentration of Fe in the roots was 40%) (Figure 7A). This study showed that there is no significance with regard to Fe in the linear correlation during the time contacts and concentration of Fe, and that this condition happened as a result of several factors: Fe was distributed to the shoots of the plant; Fe was precipitated, Fe was adsorbed into biochar; and removal due to microbial activities. The highest correlation of Fe, Mn, Si, and Ca is Mn (0.9477 for section A and 0.8299 for section B). Although the concentration of Mn from AMD was lowest, the Mn in roots has a significant correlation, which may be due to the concentration of Mn from AMD that was adsorbed periodically. The high concentration in the correlation of Fe shown in Figure 7C,D is supported by Figure 6A, in which the results of the XRF analysis show that the highest concentration in the root is Fe. In addition, the observed R² value below 0.98 indicates the lack of a significant correlation between the duration of contact time in the phytoremediation process and the adsorption of heavy metals. This observation may be explained by the possibility that the metals were transported to the plant shoots via the xylem and phloem, leading to non-linear concentration dynamics in the roots. Additionally, it is plausible that only a fraction of the heavy metals was adsorbed onto the biochar, while others remained within the plant tissues.

3.4. Potential of Reusability

Reusability emerges as a pivotal cornerstone in sustainability, and the modified CW discussed herein holds immense promise in enabling sustainable and simultaneous processes, owing to their inherent environmental benefits. This study presents a compelling case for the utilization of modified CW, with its unique capacity to drive environmental gains. Foremost among these benefits is the potential of the adsorption process facilitated by biochar. Biochar, recognized as a green and sustainable material, serves as a formidable adsorbent. Beyond its prowess in pollutant removal, biochar exhibits a remarkable dual role as it also stands out for its capabilities in CO₂ capture [41], carbon sequestration, and improving soil quality [42]. Consequently, the application of biochar not only aids in adsorbing pollutant parameters but also contributes positively to the environment by capturing CO₂ and participating in carbon sequestration. This dual functionality holds the promise of ameliorating soil quality in mining areas, addressing the significant environmental impact attributed to industrial activities such as mining—a fact that has been substantiated in prior studies.

Incorporating biochar within the context of modified CW extends the sustainability ethos, fostering a cascading positive impact that reverberates through various environmental aspects. The strategic utilization of biochar underscores the multifunctionality of modified CW and embodies a tangible step towards integrating eco-friendly practices in waste management and environmental restoration. This holistic approach, encompassing pollutant removal, carbon sequestration, and soil quality enhancement, signifies a paradigm shift in resource management, accentuating the potential for symbiotic relationships between human activities and the ecosystem.

On the flip side, the integration of plants significantly bolsters the sustainability of CW in the treatment of AMD. Phytoremediation, as a method, stands out for its user-friendliness compared to more complex approaches like adsorption and filtration, which entail stirring or the regular cleaning of material pores. The phytoremediation process, by contrast, offers a streamlined and natural means of tackling pollution challenges. Following the phytoremediation process, plants continue to offer benefits beyond their primary role. Recent research attests to the potential of the post-harvest utilization of plants like Eichhornia crassipes, highlighting many advantageous applications. These applications extend into diverse domains, reflecting the wide-ranging benefits that can be harnessed from the plant post-remediation. Among these, the post-harvest potential of Eichhornia crassipes includes: use of the plant’s antibacterial properties; handicraft production; organic fertilizer generation; paper production; liquid fertilizer extraction; and contributions to the pharmaceutical industry [33].

This multifaceted utilization underscores the remarkable synergy between environmental restoration and sustainable resource management. The utilization of plants as a component of modified CW goes beyond their phytoremediation role; it fosters a closed-loop system where waste is minimized, and the full potential of resources is harnessed. Furthermore, the post-harvest applications of plants like Eichhornia crassipes testify to the transformative potential of integrating natural systems into waste treatment processes. In a world where the optimization of resources is paramount, the utilization of plants within the context of modified CW epitomizes a holistic approach that aligns with the principles of a circular economy and sustainability. This approach addresses immediate pollution concerns and catalyzes a shift towards more symbiotic relationships between human activities and the environment. As researchers and practitioners continue to explore innovative solutions, the marriage of phytoremediation and post-harvest utilization of plants emerges as an exemplar of how nature’s capabilities can be harnessed for lasting positive impacts across various sectors.

Incorporating multilayers of filtration within this approach not only serves as an effective defense mechanism against soil contamination but also underscores an additional benefit—its cost-effectiveness. This method presents a low-cost solution that effectively safeguards against the infiltration of contaminants into the soil, further bolstering the viability of the modified CW system. In its entirety, this modified CW emerges as a potent and sustainable model for treating AMD. By capitalizing on reusability and its myriad benefits, the modified CW system transcends the confines of addressing immediate pollution challenges; it contributes substantively to a more comprehensive vision of a sustainable future, embodying circular economy principles and responsible resource management.

The multifaceted impact of biochar’s utilization is a case in point, serving as a testament to the interconnectedness of environmental systems. This impact underscores the transformative potential of innovative solutions, casting a spotlight on the ability of such initiatives to create positive ripple effects across diverse domains. As our global society grapples with the intricate web of sustainability, holistic approaches that inherently offer multi-layered advantages exemplify how environmental management can function as a dynamic catalyst for lasting positive change. In navigating the intricate landscape of sustainability, it becomes increasingly apparent that solutions of this nature hold immense promise. Beyond its primary functions, the modified CW system exemplifies the fusion of ecological principles and cutting-edge technology, demonstrating how harmonizing human activities with natural systems can lead to profound outcomes. In this journey towards a more sustainable world, models like the one discussed here offer valuable insights and directions, underscoring the transformative potential that lies in reimagining how we interact with our environment. As we continue to explore uncharted avenues, the modified CW remains a beacon of hope—a testament to the power of innovation and collaborative effort in forging a more harmonious relationship between humanity and nature.

3.5. Recommendations and Future Prospects

The findings and insights gained from this study on the modified CW for treating AMD open doors to a realm of recommendations and exciting prospects. As the study contributes to the growing body of knowledge in this field, it also prompts key recommendations and outlines potential avenues for further exploration, refinement, and application. Crucial points related to the recommendations and prospects of this study include (1) Optimization of Process Parameters: The study presents a robust foundation for the utilization of modified CW in AMD treatment. Further investigations can delve into optimizing process parameters, such as biochar dosage, plant species selection, and the specific arrangement of filtration layers. This optimization could fine-tune the system’s efficiency, enhancing its pollutant removal capabilities; (2) Hybrid Approaches: Given the intricate nature of AMD pollution, the integration of hybrid approaches could be explored. Combining modified CW with other treatment methods, such as electrocoagulation, advanced oxidation processes, or even emerging nanotechnology, will amplify the system’s efficacy in tackling complex contaminant mixtures; (3) Long-Term Performance: Long-term monitoring and performance assessment are crucial. Studying the sustained efficiency of the modified CW over extended periods, considering seasonal variations and potential plant growth fluctuations will provide insights into the system’s resilience and durability; (4) Eco-Toxicity and Health Impacts: Future studies could investigate the eco-toxicity and potential health impacts of the treated water and biomass. Evaluating the potential secondary effects of utilizing treated water for irrigation, industrial processes, or other non-potable uses is essential to ensure the overall sustainability and safety of the approach; (5) Real-World Applications: While this study demonstrates promising outcomes in a controlled environment, real-world implementation and testing on actual mining sites are paramount. Collaborations with industries and local authorities could pave the way for piloting modified CWs in mining areas, facilitating practical insights and addressing site-specific challenges; (6) Techno-Economic Feasibility: Evaluating the techno-economic feasibility of deploying modified CW at scale is imperative. Cost-benefit analyses considering installation, maintenance, operational costs, and the potential revenue streams from harvested biomass, can provide stakeholders with a comprehensive understanding of the system’s economic viability; (7) Policy and Regulations: Future research could contribute to developing policy guidelines and regulations governing the utilization of modified CW for AMD treatment. Collaborating with policymakers and regulatory bodies ensures that innovative solutions align with existing environmental standards and facilitate smoother implementation; and (8) Knowledge Dissemination: The dissemination of findings is pivotal to fostering wider adoption. Peer-reviewed publications, workshops, seminars, and public awareness campaigns can effectively share the knowledge and insights gained from this study with a broader audience, stimulating further interest and collaboration. Looking ahead, the prospects of this study are, indeed, promising. The integration of modified CW, reusability principles, and innovative technologies offers a holistic solution to complex environmental challenges. Beyond AMD treatment, the interdisciplinary lessons learned from this study can transcend to other fields, such as urban water management, industrial wastewater treatment, and ecological restoration. In the grander sustainability scheme, this study contributes to the ongoing discourse on harmonizing human activities with the environment. The exploration of innovative, multifunctional approaches addresses immediate issues and steers us towards a future where waste becomes a resource and environmental protection aligns seamlessly with economic prosperity.

4. Conclusions

Industrially driven coal-mining activities have left a significant environmental imprint, as is evidenced by the abandoned coal mining sites under study, where iron concentrations have soared to alarming levels of up to 263 mg/L, manifesting in the water’s conspicuous red hue. Furthermore, the study uncovered low pH levels registering at 2.87. This alarming scenario poses an imminent threat to the surrounding environment and the local population’s well-being. Addressing these challenges head-on, this study has successfully devised a modified CW that stands as a beacon of efficacy. Notably, this innovation has the capacity to elevate pH levels and expel heavy metals from AMD. The modified CW has demonstrated its prowess by effectively reducing metal concentrations to an impressively low 0.07 mg/L, which translates to a staggering 99% removal rate. Remarkably, this was accomplished by utilizing readily available raw materials, underscoring the method’s practicality and sustainability. The ingenuity of this modified CW is rooted in a synergistic combination of several removal mechanisms. The incorporation of plant uptake through phytoremediation, facilitated by Eichhornia crassipes, is augmented by the precipitation action of CaO derived from clamshell waste. Further enhancement is derived from the biochar’s adsorption properties, coupled with the filtration prowess of gravel and sand. This comprehensive amalgamation underscores the innovation’s multifunctionality and ensures a holistic removal process, addressing various pollutant categories. Moreover, the benefits of this study extend beyond surface-level enhancements. In addition to its ability to ameliorate surface conditions, the modified CW serves as an invaluable guardian against the contamination of underground water sources. By arresting and mitigating the movement of pollutants, this approach safeguards critical water reservoirs from further deterioration, thereby contributing significantly to protecting both the environment and human health.