Effects of Scallop Shells and Starfish (Asterias amurensis) on Stabilization of Metalloid (As) and Heavy Metal (Pb and Zn)-Contaminated Soil

Abstract

Mining and industrial operations are often associated with metalloid and heavy metal contamination of terrestrial and aquatic ecosystems. Heavy metals can weaken the soil’s purification ability to remediate and can accumulate in the human body through crops grown in contaminated soil. In this study, a stabilization method was applied for the remediation of arsenic (As) and heavy metal (Pb and Zn) contaminated soil. Scallop shells (SLS) and starfish (Asterias amurensis, ASF), commonly regarded as waste resource materials, are selected as stabilizers. Proper recycling/reuse measures are required to limit uncontrolled disposal of SLS and ASF, prevent environmental degradation of coastal areas, and take advantage of their high calcium carbonate contents. The stabilizers were processed through −#10 mesh (0.2 mm) and −#20 mesh (0.85 mm) sieves. In addition, calcined stabilizers were produced by calcining SLS and ASF at 900 °C to compare stabilization efficiency based on the presence/absence of high-temperature heat treatment. Each of the three types of processed stabilizers was added to contaminated soil at 2 to 10 wt.%, and the mixtures were subjected to wet curing for 28 days. Extraction with 0.1 N HCl was applied for stabilization efficiency assessment. Crops were cultivated in the stabilized soil to evaluate As and heavy metal immobilization capacity. Analysis by X-ray diffraction (XRD) established that calcite (CaCO₃) was observed in the natural materials and quicklime (CaO) in the calcined materials. The stabilization efficiency assessment results showed that treatment with SLS and ASF effectively reduced the elution of Pb and Zn. SLS was effective in immobilizing As, but the application of natural ASF increased the leachability of As due to the presence of organic matter. However, applying calcined ASF effectively immobilized As because the organic matter was removed at high temperatures. When the transition of As and heavy metals to crops was evaluated, Pb concentrations that exceeded the criterion for leafy vegetables were detected in the lettuce grown in contaminated soil. However, Pb was not detected in the lettuce grown in SLS- and ASF-treated soil, confirming the stability of heavy metal immobilization. Scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX) analysis showed that the pozzolanic reaction is related to heavy metal immobilization, and Ca–As precipitation is related to the immobilization of As. The results of this study verified that SLS and ASF effectively immobilize As and heavy metals (Pb and Zn) around mines and that they can be used safely in agricultural soil.

1. Introduction

Soil has long been exposed to heavy metals from mining and industrial operations that affect adversely soil and agricultural environments [1,2]. Heavy metals are inorganic pollutants of high aquatic mobility that can be easily transported into nearby water systems and contaminate various areas. In particular, owing to the climatic characteristics of the Republic of Korea that involve heavy rain in summer, heavy metals in soil are easily eluted, and they are highly likely to be released into nearby waters through soil and runoff water [3]. Heavy metals interfere with soil microbial activity, weakening various soil functions such as purification ability and inhibiting crop growth [4,5,6]. The accumulation of heavy metals in crops above threshold levels leads to toxicity-invoked growth disorders, which ultimately affect humans and livestock throughout the food chain [7].

Soil in the vicinity of mine areas, industrial complexes, and uncontrolled landfills is often contaminated with heavy metals/metalloids. Among them, abandoned mines are the main source of heavy metal contamination [8]. There are 5475 abandoned mines in the Republic of Korea [9], and 44% are metal mines [9]. In 2016, 107 tons of crops that exceeded the criteria for heavy metal in crop tests conducted in farmland near abandoned mines were discarded [10]. Heavy metal contamination of soil by mines is emerging as an environmental problem to be addressed worldwide, including in the Republic of Korea. In the United States, 550,000 abandoned mines have generated 45 billion tons of mine waste; however, most were left untreated over extended periods of time without investigation and purification [11]. Abandoned mines are also an important environmental problem in Europe and they serve as a water pollutant. In the United Kingdom, there are thousands of abandoned mines in Scotland and Wales [11] as a result of active mining practices in the 18th and 19th centuries. After mines are abandoned for various reasons, including deteriorating profitability, mine waste is left unattended. Uncontrolled mine waste flows into rivers and tends to cause soil contamination [10,12]. Owing to mine waste, As and Pb are commonly detected in areas around abandoned mines [13]. As and Pb cause peripheral nerve disorders when accumulated in the human body, and As may also cause cancer in organs such as the bladder and lungs [14,15]. Therefore, the International Agency for Research on Cancer (IARC) classifies As as ‘Carcinogenic to Humans (Group 1)’ and Pb as ‘Possibly Carcinogenic to Humans (Group 2B)’.

Soil washing or flushing methods can be applied to remediate heavy metal-contaminated soil. These methods, however, have low economic efficiency compared to other technologies because they require a large amount of chemicals [16,17]. Pollutants may remain in the washing wastewater, and the fine soil removed during the washing process requires follow-up treatment. In addition, it is difficult to reuse the washed soil because large amounts of clay are removed during the washing process. Stabilization methods have been applied for the remediation of heavy metal-contaminated soil in various studies worldwide to address these shortcomings. Quicklime, Portland cement, and zeolite have mainly been used as stabilizers [18,19,20,21,22,23,24,25,26]. However, natural waste resources and waste materials have attracted attention as stabilizers for eco-friendly and sustainable stabilization in recent years. Islam et al. [27] reduced the elution of target pollutants by more than 85% by applying calcined cockle shells to Cd-, Pb-, and Zn-contaminated soil around mines. Choi et al. [28] modified stabilizers by adding eggshells and pig bones to the existing stabilizer to improve stabilization efficiency in heavy metal-contaminated soil. Moon et al. [29] calcined natural and waste materials or turned them into biochar and compared and analyzed Pb immobilization efficiency according to the processing type. However, limited literature exists on the applicability and safety of soil restored using waste materials as stabilizers for agricultural use and crop cultivation [30,31].

In this study, we selected scallop shells (SLS) and starfish (Asterias amurensis, ASF) as eco-friendly stabilizers to remediate As- and heavy metal (Pb and Zn)-contaminated soil. Domestic scallop production in the Republic of Korea increased 23-fold from 527 tons in 2013 to 12,170 tons in 2023 [32]. Thus, a large amount of SLS is discarded yearly, and various recycling measures have been sought. In Hokkaido, Japan, where the scallop consumption is high, 40,000 tons of SLS are generated yearly, and they are abandoned on land because there is no suitable storage location. An enterprise (Quantum) developed SLS as a new eco-friendly plastic material to be used in high-strength helmets to address this problem [33]. Starfish is an invasive species that destroys the marine ecosystem, and its population is increasing yearly due to the absence of natural predators. Starfish also inflict massive predatory damage to the shellfish aquaculture industry. In the Republic of Korea, the government has designated ASF as a harmful marine species and has instituted purchase and disposal/reuse programs from local fishermen [34]. Some of the starfish purchased by the government are provided to starfish recycling companies (e.g., starfish deicing agents) at no cost. SLS and ASF are expected to be effective in stabilizing As and heavy metals as they contain a large amount of CaCO₃ [35]. In this study, the particle sizes of SLS and ASF were varied to compare their stabilization efficiency. In addition, SLS and ASF were calcined at high temperatures to assess the stabilization performance of natural and calcined materials. Heat treatment at high temperatures causes calcination and converts CaCO₃ into CaO [36]. CaO is a major stabilizing agent that immobilizes pollutants by causing the pozzolanic reaction and Ca–As precipitation when added into As and heavy metal-contaminated soil [27,37,38,39].

The objective of this study was to assess the value of natural waste resources (SLS and ASF) as stabilizers for remediating As-, Pb-, and Zn-contaminated soil. The stabilization efficiency was evaluated according to the processing condition, particle size, and curing period of the stabilizers. The stabilization mechanism for natural waste resource treatment is identified through scanning electron microscopy–energy dispersive X-ray (SEM-EDX) analysis. In addition, crops are cultivated in stabilized soil, and heavy metal uptake levels are evaluated to assess safety.

2. Materials and Methods

2.1. Contaminated Soil Collection and Characteristics

The soil to be stabilized was collected 30 kg from the topsoil layer (0–30 cm) from a farmland near an abandoned mine in Jeollanam-do and sifted through a standard #10 mesh sieve (2 mm). The sieved soil was then thoroughly mixed. The aqua regia extraction method (1 mL of HNO₃ (Samchun pure chemical Co. Ltd., Pyeongtaek, Republic of Korea) and 3 mL of HCl (Samchun pure chemical Co. Ltd., Pyeongtaek, Republic of Korea)) in accordance with the Korean Standard Test (KST) methods for soils [40] was used to measure the total heavy metal concentration in the collected soil. Then, As (57.0 mg/kg), Pb (466.8 mg/kg), and Zn (700.7 mg/kg) were determined at concentrations approximately 2.3 times higher than the Soil Contamination Warning Standards (Area 1), and thus this soil was designated as “contaminated soil”, which needs remedial action. In the Republic of Korea, the Soil Contamination Warning Standards are the regulatory limits to determine whether the remedial action takes place or not. The organic content of the soil, measured by the loss on ignition (LOI) at 550 °C [40], was 5.4% and was higher than the average organic content of domestic agricultural soil (2.9%) [41]. The pH, electrical conductivity (EC), and cation exchange capacity (CEC) of the contaminated soil were analyzed according to the ‘Methods of Soil Chemical Analysis’ [42] and were measured as 6.8, 0.36 dS/m, and 23.8 cmol⁺/kg, respectively (

Table 1. The physicochemical properties of contaminated soil.

Soil Properties		Contaminated Soil	Regulatory Limit (Korean Warning Standard) 1
Contaminants (mg/kg)	As	57.0	25
	Pb	466.8	200
	Zn	700.7	300
Organic matter content 2 (%)		5.4
pH (1:5)		6.8
EC (dS/m)		0.36
CEC (cmol+/kg)		23.8
Composition 3 (%)	Sand	67.7
	Silt	17.2
	Clay	15.1
Texture 4		Sandy Loam

¹ Korean warning standard for soils in residential areas. ² Measurement of organic matter content by LOI [40]. ³ Determination of soil classification by particle size analysis (sand 20–2000 μm; silt 2–20 μm; clay < 2 μm). ⁴ Soil texture by the United States Department of Agriculture (USDA) classification.

). The soil particle size was analyzed using the pipette method of the National Academy of Agricultural Science (NAAS) [43], and the soil was classified as a sandy soil with a textural composition of 67.7% sand, 17.2% silt, and 15.1% clay. Analysis by X-ray fluorescence (XRF) (ARL PERFORM’X, Thermo Fisher Scientific, Waltham, MA, USA) (

Table 2. Major chemical composition of contaminant soil, scallop shell (SLS), calcined scallop shell (CSLS), Asterias amurensis (ASF), and calcined Asterias amurensis (CASF).

Major Chemical Composition (%)	Contaminated Soil	SLS	CSLS	ASF	CASF
SiO2	62.12	0.073	0.113	0.135	0.126
Al2O3	17.97	0.205	0.154	0.053	0.130
K2O	4.459	0.008	0.012	0.355	0.158
Fe2O3	4.317	0.065	0.051	0.069	0.053
Na2O	1.546	0.824	0.974	2.161	2.510
SO3	1.243	0.597	0.580	5.483	2.450
MgO	0.987	0.603	0.749	5.754	13.74
CaO	0.844	94.59	96.18	67.60	78.78
MnO	0.528	0.007	0.007	0.008	0.004
P2O5	0.428	0.122	0.129	1.203	0.633
LOI 1	5.400	2.050	-	17.64	-
pH	6.50	9.42	12.51	7.37	12.29

¹ Loss of ignition.

) identified the major inorganic oxides in the contaminated soil to be SiO₂ (65.47%), Al₂O₃ (18.94%), and K₂O (4.70%).

2.2. Stabilizer Processing

SLS and ASF were immersed in water for three days to remove the salt. Then, the surface of these materials was cleaned with a brush and washed with distilled water several times. After drying, they were processed into −#10 mesh [−#10 mesh (2 mm) to +#20 mesh (0.85 mm)] natural material and −#20 mesh [−#20 mesh (0.85 mm) to +#40 mesh (0.425 mm)] natural material using a mixer. These were labeled SLS-10 (−#10 mesh natural scallop shell), SLS-20 (−#20 mesh natural scallop shell), ASF-10 (−#10 mesh natural starfish), and ASF-20 (−#20 mesh natural starfish) to distinguish stabilizers. CaCO₃, which is the main component of SLS and ASF, was calcined at a high temperature to convert it into CaO to improve the efficiency of the stabilizers. The calcination temperature was set to 900 °C considering the error and loss of the furnace according to the calcination reaction equation [44].

$$\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3\to \mathrm{C}\mathrm{a}\mathrm{O}+\mathrm{C}{\mathrm{O}}_{2\left(\mathrm{g}\right)}$$

$$∆{\mathrm{G}}_{\mathrm{r}}^{\mathrm{o}}=\mathrm{177,100}-158\mathrm{T}$$

where $∆{\mathrm{G}}_{\mathrm{r}}^{\mathrm{o}}=\mathrm{S}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{a}\mathrm{r}\mathrm{d}$ Gibb’s free energy of reaction, J/mol.

$$\mathrm{T}=\mathrm{temperature}, °\mathrm{C}$$

Setting the standard free energy of the reaction equal to zero, the actual calcination temperature T = 848 °C.

Therefore, −#10 mesh natural materials (SLS-10 and ASF-10) were calcined for 2 h at 900 °C to produce −#10 mesh calcined materials (CSLS-10 and CASF-10). The characteristics of the stabilizers were analyzed for −#10 mesh natural and calcined materials to evaluate changes before and after pyrolysis. The XRF analysis results showed that the CaO contents of CSLS and CASF were approximately 2–10% higher than those of SLS and ASF (Table 2). This is because the proportion of pure CaO increased as the organic matter was removed by calcination [36]. It appears that the CaO content in ASF was lower than that of SLS due to the high organic content in ASF. The organic contents in SLS and ASF measured through LOI were 2.05% and 17.64%, respectively (Table 2). The pH value was also relatively higher in the calcined materials than in the natural materials. This is because CaO forms more hydroxide ions than CaCO₃ based on the following chemical equations [36].

$$\begin{array}{c}\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{O}}_3+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{a}}^{2+}+\mathrm{H}\mathrm{C}{\mathrm{O}}_3^{-}+\mathrm{O}{\mathrm{H}}^{-}\\ \mathrm{CaO}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{Ca}}^{2+}+2{\mathrm{OH}}^{-}\end{array}$$

2.3. Stabilization Treatment and Elution

In the stabilization batch test, SLS-10, SLS-20, CSLS-10, ASF-10, ASF-20, and CASF-10 were added into 50 g of As and heavy metal-contaminated soil at a dosing range of 2 to 10 wt.% (

Table 3. Treatment matrix for contaminated soil.

Sample ID	Contaminated Soil (g)	Stabilizing Agent (g)	L:S Ratio
Control	50	0	30:1
2 wt.% SLS-10/SLS-20/CSLS-10 ASF-10/ASF-20/CASF-10	50	1	30:1
4 wt.% SLS-10/SLS-20/CSLS-10 ASF-10/ASF-20/CASF-10	50	2	30:1
6 wt.% SLS-10/SLS-20/CSLS-10 ASF-10/ASF-20/CASF-10	50	3	30:1
8 wt.% SLS-10/SLS-20/CSLS-10 ASF-10/ASF-20/CASF-10	50	4	30:1
10 wt.% SLS-10/SLS-20/CSLS-10 ASF-10/ASF-20/CASF-10	50	5	30:1

). Moisture was then added up to 30% and uniformly mixed. The control group with no stabilizer was treated similarly to compare stabilization efficiency. A total of 32 samples, including 2 control samples, were created. The samples were placed in airtight containers to prevent evaporation and subjected to wet curing for 1 and 4 weeks. After 1 week of wet curing, half (25 g) of the cured samples were taken and dried in Petri dishes. The remaining half (25 g) of the samples were additionally cured for 3 weeks in sealed containers to complete the remaining wet curing period.

The stabilization samples that completed 1 week and 4 weeks of wet curing were eluted with 0.1 N HCl (KST, 2008) [40] to evaluate As, Pb, and Zn elution in an acidic environment (pH 1). For the elution test, 3 g of the stabilized samples were treated with 15 mL of 0.1 N HCl, and the test was performed in triplicate to ensure accuracy. After mixing the stabilized soil and the eluate at a ratio of 1:5 (w:v) and stirring the mixture at 100 rpm for 60 min, centrifugation was performed at 3000 rpm for 10 min. The separated supernatant was filtered through a 0.45 μm syringe filter, and the filtrate was analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES) (Perkin Elmer Optima Model 5300DV, Waltham, MA, USA). All ICP-OES measurements were informed as the mean values of triplicate samples (less than 10% measurement error). For quality assurance/quality control (QA/QC) purposes, three quality-control standards and spiking with a standard solution for every 10 samples analyzed (recovery rate > 95%).

2.4. Evaluation of Heavy Metal Transition to Crops

Lettuce was cultivated in the soil treated with SLS and ASF to evaluate the crop heavy metal uptake concentration. Five pots were fabricated, namely the control pot without stabilizer treatment, 10 wt.% SLS-20, CSLS-10, ASF-20, and CASF-10. For the crop cultivation pots, a pot size with a top diameter of 9 cm, a bottom diameter of 7.5 cm, and a height of 9 cm was used. The control pot was produced by filling a pot with 400 g of contaminated soil. For the stabilization pot, stabilized soil and covering soil were added at a ratio of 1:2 in accordance with the ‘Guidebook: Mine rehabilitation technology in Korea’ of the Korea Mine Rehabilitation and Mineral Resources Corporation (KOMIR) (Figure 1) [45]. A covering soil with an organic content of 22.0%, a pH value of 7.0, and an EC value of 5.16 dS/m was used. The stabilization treatment matrix was set up in the same manner as in Section 2.3. Wet curing was performed for 1 week to evaluate the target contaminant transport to the crops. Each pot was filled with stabilized soil and covered soil in 3 cm (110 g) and 6 cm (70 g) layers, respectively, and 30 lettuce seeds were sown and cultivated for 4 weeks. Upon completion of the cultivation period, the As, Pb, and Zn concentrations of the lettuce were analyzed using ICP-OES after pre-treatment in accordance with the nitric acid digestion method of the Korean Food Code of the Ministry of Food and Drug Safety (MFDS) [31,46].

2.5. XRD Analysis

The contaminated soil and stabilizers were subjected to X-ray diffraction (XRD) analysis to identify their mineralogical characteristics. The XRD samples were prepared by grinding contaminated soil to −#200 mesh (75 μm). XRD patterns were analyzed at a voltage of 40 kV and a current of 40 mA using an XRD diffractometer (X’Pert PRO MPD; PANalytical, Almelo, The Netherlands) equipped with a diffraction light graphite monochromator that emits Cu radiation. XRD patterns were collected with a 2θ range of 5 to 60°, a step size of 0.02°, and 3 s/step. The diffraction patterns were analyzed using Jade software v.7.1 [47] and PDF-2 data [48].

2.6. SEM-EDX Analysis

SEM-EDX analysis was conducted to identify the stabilization mechanism of As-, Pb-, and Zn-contaminated soil by stabilization treatment. The treatment exhibiting the highest stabilization efficiency for each stabilizer was selected as the analysis target. Then, those were placed on the platinum-coated carbon tape for analysis in the Hitachi S-4800 SEM instrument (Hitachi, Tokyo, Japan) equipped with the Horiba EMAX EDX system (Horiba, Tokyo, Japan).

3. Results and Discussion

3.1. XRD Analysis Results

The XRD pattern analysis results for the contaminated soil and stabilizers (SLS, CSLS, ASF, and CASF) are shown in Figure 2, Figure 3 and Figure 4. The main mineralogical forms of the contaminated soil were quartz, muscovite, microcline, and albite (Figure 2). The main peak of calcite (CaCO₃) was detected from SLS, whereas the different type of calcite [(Ca, Mg)CO₃] was detected from ASF (Figure 3). In addition, the main mineralogical form of calcined materials (CSLS and CASF) was quicklime (CaO) (Figure 4). This confirmed that CaCO₃, which is the main component of SLS and ASF, was calcined at 900 °C and converted into CaO, as previously reported [35,39,49,50,51].

3.2. Stabilization Efficiency Assessment

The As and heavy metals (Pb and Zn) extracted using the 0.1 N HCl solution after applying SLS-10, SLS-20, and CSLS-10 to the contaminated soil are shown in Figure 5 (1 week wet curing) and Figure 6 (4 weeks wet curing). Overall, the stabilization efficiency increased as the input content increased. The pH (extraction pH, EpH) of the extraction using 0.1 N HCl also increased as the stabilizing dose increased. In particular, the CSLS-10 treatment with a high pH value showed a strong alkalinity of extraction at pH 11. The extracted contaminant concentrations tended to be lower for 4 weeks of curing than at 1 week of curing. Lim et al. [52] reported that efficiency improved as the curing period increased because the reaction time between stabilizers and pollutants increased. Based on 4 weeks wet curing, 2 wt.–10 wt.% SLS-10 treatment decreased the elution of As, Pb, and Zn by 59–96%, 58–99%, and 28–80%, respectively. The 2 wt.–10 wt.% SLS-20 treatment reduced the target contaminants by 90–98%, 96–99%, and 45–95%, respectively. Comparing the SLS-10 and SLS-20 treatments, the sample treated with SLS-20 had a finer particle size distribution and exhibited a higher stabilization efficiency. This is because active hydration reactions are more favorable in samples with a fine particle size due to the large specific surface area, according to Moon et al. [37].

A significant improvement in stabilization efficiency was also observed for calcined rather than natural stabilizers. In the 10 wt.% treatment of SLS-10 and SLS-20, Zn was extracted at 37.5 and 9.0 mg/kg, respectively (Figure 6). In the case of CSLS-10, however, the elution of Zn decreased to 3.6 mg/kg (98% reduction compared to that in the control) with only the 4 wt.% treatment. In stabilization of Pb, efficiencies of 57% and 96% were observed with 2 wt.% treatments of SLS-10 and SLS-20, respectively, whereas Pb was scarcely leached in the 2 wt.% treatment of CSLS-10. Islam et al. [27] conducted a study in which they treated heavy metal-contaminated soil (Cu, Pb, and Zn) with calcined cockle shells; they found that the heavy metals in the soil were immobilized as Al, Si, and O in the soil and Ca in the stabilizer formed calcium silicate hydrate (CSH) and calcium aluminum hydrate (CAH). Ahmad et al. [38] also reported that CSHs and CAHs were generated in the Pb-contaminated soil treated with CaO (egg shell) and that elution is inhibited as heavy metals are combined inside such pozzolanic materials. Furthermore, treatments with natural SLS and calcined SLS showed up to a 34% improvement in the As stabilization efficiency. It has been reported that the application of CaO to As-contaminated soil promotes the reaction between As and Ca, inducing the formation of insoluble Ca–As precipitates [37,39,53,54].

Similar to SLS, the stabilization efficiency for Pb and Zn also increased as the ASF content increased, and 4 weeks of curing was more effective than 1 week of curing (Figure 7 and Figure 8). In addition, as the dosage of ASF-10 and ASF-20 increased, the extraction pH increased to 3.1 and 4.4. In the treatment of CASF-10, the maximum extraction pH was measured to be strongly alkaline (pH 12), similar to the treatment of CSLS-10. The 4 weeks of wet curing after 2 wt.–10 wt.% ASF-10 treatment reduced the elution of Pb by 74–99% and that of Zn by 32–95%. In the case of 2 wt.–10 wt.% ASF-20 treatment, the elution of Pb and Zn decreased by 71–99% and 46–96%, respectively. Overall, the ASF-20 treatment exhibited higher stabilization efficiency than the ASF-10 treatment. A study by Moon et al. [49], where starfish was added into mercury-contaminated soil, also showed the same trends as the present study, as stabilizers containing finer particles exhibited higher efficiency. Pb and Zn were reduced by 71% and 46% in the 2 wt.% ASF-20 treatment, respectively, whereas they were reduced by 99% and 97% in the 2 wt.% CASF-10 treatment. It appears that the efficiency of CASF was significantly higher than that of ASF because CaCO₃ was converted into CaO during the calcination process. Moon et al. [29] reported that the increase in pH, caused by the higher stabilizer dose, enhances the adsorption of metal cations onto soil particles and leads to the formation of insoluble compounds under alkaline conditions. In this study, the pH of the 0.1 N HCl extract solution also increased to strongly alkaline conditions (pH 12) with the input of CASF-10, indicating an enhancement in the soil’s heavy metal adsorption capacity. Hydrate compounds (CSH and CAH), which are products of the pozzolanic reaction, form a non-permeable hard layer to capture metals in soil particles and prevent their elution [38]. Therefore, it was concluded that the CASF-10 treatment causing an elevated pH condition significantly decreases the leachability of Pb and Zn due to the pozzolanic reaction and consequent formation of insoluble compounds in the soil.

In contrast, a different pattern from that of SLS was observed for the stabilization of As. The elution of As decreased by up to 90.7% (compared to the control) in the 4 wt.% ASF-20 treatment, but it increased under the 6 wt.–10 wt.% input. The decomposition of organic matter increases the concentration of dissolved organic carbon (DOC) in the soil, and DOC induces higher As mobility through competitive adsorption with As in the soil [55,56,57]. In the Kim et al. [56] and Yoo et al. [57] studies, the concentration of As in the soil also increased with organic material addition, showing a similar trend to this study. Additionally, Jia et al. [58] reported that the application of organic matter (rice straw) increased As release and enhanced its bioavailability. In a previous study [59], Asterina pectinifera (SF), with a similar organic matter content (17.12%) to ASF in this study, successfully immobilized As when applied to As-, Pb-, and Zn-contaminated soil. This appears to be due to the difference in the characteristics of contaminated soil. This difference might be attributed to the characteristics of the contaminated soil. For example, the CEC of the contaminated soil in this study was measured to be 3.4 times higher than that of the contaminated soil used in the previous study. The high CEC of the soil can absorb a lot of cations, but anions might be released due to competitive adsorption with cations. Therefore, unlike the previous study where As was effectively immobilized, the high CEC of the soil in this study enhances the release of As with the input of organic stabilizers. Collectively, it is concluded that a small amount of organic stabilizer input can effectively immobilize As within the soil, but the elution of As is judged to increase with the increasing input content of ASF-10 and ASF-20 that have high organic content. There was a significant difference in the As stabilization efficiency depending on the curing period. When wet curing was performed for 1 week after 10 wt.% ASF-20 treatment, As was eluted at 1.1 mg/kg, thereby decreasing the maximum As reduction efficiency from 82% (4 wt.% ASF-20) to 29% (Figure 7). However, when wet curing was performed for 4 weeks, 0.7 mg/kg was eluted (65.4% elution reduction compared to that of the control), confirming the relatively stable immobilization of As compared to that at 1 week curing (Figure 8). Therefore, long-term curing was confirmed to improve the stability of As immobilization. In the case of CASF-10 treatment, it was concluded that interference by organic matter was not a factor as the organic matter was removed during the calcination of ASF at a high temperature (900 °C). As was immobilized by the pozzolanic reaction in the soil treated with CaO (CASF-10) through Ca–As precipitation [60]. As may generate compounds in the form of Ca₃(AsO₄)₂, CaHAsO₄, or Ca₄(OH)₂(AsO₄)·4H₂O(As(Ⅴ)) as a result of the reaction with lime [61,62].

Overall, the stabilization of Pb was approximately 40% higher compared to that of Zn in SLS and ASF treatment. Soil generally exhibits a high preference for the adsorption of heavy metal cations in the order of Zn²⁺ < Cu²⁺ < Cd²⁺ < Ni²⁺ < Pb²⁺ [30,63]. Thus, it appears that the immobilization of Pb was more effective than Zn. In addition, SLS treatment was generally more effective than ASF treatment for the stabilization of As. This is because the high organic content of ASF and high CEC of soil hinder the immobilization of As, as described above. Since the application of SLS and ASF was suitable for the stabilization of Pb and Zn, respectively, Pb and Zn showed opposite characteristics, despite both being heavy metals with divalent cations. According to Inyang et al. [64] and Kim et al. [56], Pb is electron-friendly (electron affinity = 35.1 kJ/mol). Thus, its availability is increased, and the stabilization effect is decreased by treatment with organic stabilizers. Meanwhile, Fisher-Power et al. [65] reported that dissolved organic matter (DOM) excludes Zn from cationic adsorption competition for binding sites on the compound, resulting in increased adsorption of Zn. This indicated that SLS with a low organic content is relatively more effective in stabilizing Pb, while ASF with a high organic content is more effective in stabilizing Zn.

Consequently, when As and heavy metal-contaminated soil (Pb and Zn) was treated with three types of ASF and SLS, −#10 mesh calcined materials (CSLS-10 and CASF-10) showed the highest stabilization efficiency, followed by #20 mesh natural materials (SLS-20 and ASF-20) and −#10 mesh natural materials (SLS-10 and ASF-10). Based on 80% stabilization efficiency, the appropriate stabilizer input contents were 6 wt.% SLS-20 and 6 wt.% ASF-20 for natural stabilizers and 4 wt.% CSLS-10 and 2 wt.% CASF-10 for the calcined stabilizers.

3.3. Crop Cultivation and Heavy Metal Transition Evaluation

Four weeks after sowing lettuce seeds, the edible portions were solely collected according to the Korean Food Code. The amount of lettuce was measured by assessing the average leaf length and total weight (

Table 4. The amount of lettuce growth and contaminant transition concentration to lettuce.

Pot	Leaf Length 1 (cm)	Weight 2 (g)	Contaminant Transition Concentration (mg/kg)
			As	Pb	Zn
Regulatory Limit (Korean Standard) 3	-	-	-	0.300	-
Control	9.4 (±0.48)	7.3 (±0.31)	ND 4	0.353 (±0.01)	25.58 (±3.49)
SLS-20	20.3 (±0.37)	19.5 (±0.31)	ND	ND	5.371 (±0.31)
CSLS-10	21.0 (±0.43)	21.1 (±0.37)	ND	ND	4.709 (±0.08)
ASF-20	21.4 (±1.23)	19.0 (±0.80)	ND	ND	7.779 (±0.25)
CASF-10	20.5 (±1.57)	20.7 (±0.83)	ND	ND	3.734 (±0.03)

¹ Average length of leaves. ² Total weight of leaves. ³ Standard for contaminants in leafy vegetables for the Korean Food Code by MFDS. ⁴ Not detected (limit of detection: As = 19.6 ppb, Pb = 3.2 ppb, Zn = 4.2 ppb).

). In the case of lettuce cultivated in the control pot, the leaf length was 9.4 cm, and the total weight was 7.3 g. However, the lettuce grown in the stabilized soil pots all had leaf lengths exceeding 20 cm, with a minimum weight of 19 g. Therefore, the growth of lettuce cultivated in the control pot was significantly lower compared to the stabilized soil pots (Figure 9). The lettuce Pb level for the control pot was approximately 0.35 mg/kg and exceeded the Pb detection criterion (0.3 mg/kg) of the Korean Food Code. In contrast, Pb was not detected in the lettuce in the stabilized soil pots treated with SLS (SLS-20 and CSLS-10) and ASF (ASF-20 and CASF-10) (Table 4). In the case of Zn, 22 mg/kg or higher was detected from the lettuce in the control pot, but the detected concentration decreased by more than half in the stabilized soil. Less Zn was detected in the CSLS-10 and CASF-10 pots than in the SLS-20 and ASF-20 pots. This confirmed that treatment with calcined materials is more effective in reducing the mobility of heavy metals than treatment with natural materials, as indicated by the results of the stabilization efficiency assessment.

3.4. SEM-EDX Analysis Results

To gain an understanding of the stabilization mechanism, SEM-EDX analysis was targeted for the samples exhibiting the highest stabilization efficiency, namely 10 wt.% CSLS-10 and 10 wt.% CASF-10. The findings indicated the presence of pollutants such as As, Pb, and Zn, along with Ca, Al, Si, and O (Figure 10, Figure 11, Figure 12 and Figure 13). This indicates that the pollutants were immobilized in the soil while metal deposits were formed by the input of CSLS and CASF. The increased soil pH by the application of CaCO₃ and CaO caused the adsorption of metal cations by increasing negative charges on the soil surface [66]. In addition, the increase in soil pH by the input of quicklime (CaO) releases Si and Al from soil clay minerals and causes the pozzolanic reaction [38]. Pb and Zn can be included in the structures of the pozzolanic reaction products, CSH and CAH [35]. Ahmad et al. [38] and Lim et al. [39] reported that the formation of ettringite by the input of CaO contributes to the immobilization of Pb. According to Islam et al. [27], Pb and Zn can be immobilized in the CSH structure as they form bonds with silicate chains instead of Ca²⁺ in ettringite minerals. In the case of As, it is immobilized by the formation of Ca–As precipitation (e.g., CaHAsO₃ and Ca₃(AsO₄)₂), an insoluble compound, along with immobilization by CSH/CAH [37,39,60].

4. Conclusions

Scallop shells (SLS) and starfish (Asterias amurensis, ASF) were developed as stabilizers for remediation of an As-, Pb-, and Zn-contaminated soil. The natural materials were processed into −#10 to +#20 mesh and −#20 to +#40 mesh to compare efficiency according to particle size. In addition, calcined materials were produced by calcining −#10 to +#20 mesh natural materials at 900 °C for 2 h to improve the stabilization performance of SLS and ASF.

The stabilization efficiency in the treatment of natural materials was evaluated to be higher for the finer −#20 mesh materials compared to the −#10 mesh material. However, for the treatment of calcined materials, even a small dosage of input resulted in the near-complete reduction in As and heavy metal leachability. Generally, the stabilization efficiency for contaminated soil was in the following order: −#10 mesh natural material < −#20 mesh natural material < −#10 mesh calcined material.

In crop cultivation experiments for assessing heavy metal uptake by lettuce plants, the control pot exhibited slow growth, whereas the stabilized soil pots displayed rapid growth. Pb exceeded the criterion for leafy vegetables in the Korean Food Code for the lettuce of the control pot, but it was not detected in lettuce cultivated in the stabilized soil. This confirmed that stabilized soil treated with SLS and ASF can be used as agricultural soil. Based on Zn detected from all pots, the concentration of Zn was lower in the calcined and natural material pots, showing the same trends as those of the stabilization efficiency assessment.

In the SEM-EDX analysis, pollutants were found along with Si, Al, and O. This indicated that As and heavy metals (Pb and Zn) were combined in the calcium silicate hydrate (CSH) and calcium aluminum hydrate (CAH) structures, which are the products of the pozzolanic reaction under the conditions of elevated levels of soil pH caused by the calcined materials. Furthermore, the decreased elution of As is due to immobilization in Ca–As precipitation along with the pozzolanic reaction.

In this study, As and heavy metal-contaminated soil was treated with SLS and ASF to confirm the recyclability of SLS and ASF as stabilizers and their applicability for agricultural practice. Overall, treatment using calcined materials was more effective than treatment using natural materials. It is recommended that caution must be exercised when As-contaminated soil is treated with natural ASF.