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Article

Feasibility Study of Applying Enzyme-Induced Carbonate Precipitation (EICP) without Calcium Source for Remediation of Lead-Contaminated Loess

by
Kun Zhang
1,* and
Shixu Zhang
2
1
School of Civil Engineering, Luoyang Institute of Science and Technology, Luoyang 471023, China
2
School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(6), 1810; https://doi.org/10.3390/buildings14061810
Submission received: 28 April 2024 / Revised: 8 June 2024 / Accepted: 12 June 2024 / Published: 14 June 2024
(This article belongs to the Special Issue Advanced Materials and Novel Technique in Civil Engineering)
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Abstract

:
To assess the long-term stability of lead-contaminated loess treated with calcium-free Enzyme-Induced Carbonate Precipitation (EICP) technology while avoiding significant soil strength increases, various parameters such as the pH value, heavy metal ion leaching rate, and soil heavy metal speciation were evaluated. This study investigated the remediated soil’s stability under complex environmental conditions, including dry–wet cycles and acid rain leaching. The intrinsic mechanisms were elucidated through the Zeta potential, scanning electron microscopy (SEM), and X-ray diffraction (XRD) analyses. The results showed that compared to the untreated lead-contaminated loess, the surface strength of the loess treated with EICP technology increased by 3.86 times, with a 1.47-fold increase observed with the calcium-free EICP treatment. Carbonate precipitation improved the erosion resistance by adsorbing or coating fine particles and forming bridging connections with coarse particles. As the number of dry–wet cycles increased, the soil pH gradually decreased but remained above 8.25. The heavy metal leaching rate increased with the leaching cycles until reaching a plateau. The acid rain influence showed a decrease in the Pb2+ content in the leachate as the acid rain solution pH increased, meeting hazardous waste disposal regulations. These findings offer new insights for improving heavy metal-contaminated loess site remediation and understanding the underlying geochemical mechanisms.

1. Introduction

The soil contamination status outlined in China’s ecological environment status bulletin indicates that heavy metals are the main contaminants affecting the soil quality of agricultural land in China [1]. Around one-fifth of the country’s arable soil is affected by heavy metal contamination, particularly in the western region, where unsaturated loess is prevalent [2]. The rapid industrialization in this area has exacerbated the issue, leading to serious soil contamination. In particular, the contamination of unsaturated loess with heavy metals alters its basic parameters, such as its permeability, liquid, and plastic limit, resulting in environmental and engineering challenges [3]. Due to their persistent nature and high mobility, heavy metals can easily accumulate in the food chain, posing risks to both the ecosystem and human health. The urgent remediation and treatment of heavy metal-contaminated soil in the western region are crucial to address this pressing issue [4].
Various methods for remediating heavy metal-contaminated soil, such as chemical, physical, and phytoremediation methods [5,6,7], often encounter challenges such as lengthy cycles, high costs, unstable effects, and secondary contamination [8]. As a result, the use of biomineralization technology in heavy metal contamination cleanup has garnered attention, with different enzyme systems proving effective in treating diverse organic contaminants [9]. The application of Microbially Induced Carbonate Precipitation (MICP) technology, which relies on urea hydrolysis, for the mineralization and solidification of heavy metal ions in contaminated soil, offers a promising, eco-friendly approach. Research conducted by Kang et al. [9], Huang et al. [10], and Choi et al. [11] suggests that MICP technology can significantly decrease the presence of exchangeable heavy metal states in soil while increasing carbonate-bound states, effectively stabilizing heavy metals. Despite its advantages, MICP technology involves complex bacterial screening, inoculation, and cultivation processes [12]. Alternatively, plants contain urease enzymes that can decompose urea, with the roots, stems, and seeds serving as rich sources. Consequently, Enzyme-Induced Carbonate Precipitation (EICP) technology, which utilizes plant urease to trigger calcium carbonate precipitation, has experienced rapid advancement [13]. Plant-derived urease breaks down urea to produce carbonate ions, which, in combination with substances like urea and calcium chloride, create carbonate precipitates to mineralize heavy metals [14]. The hardness and permeability coefficient of farmland soil are crucial factors in crop growth and development. MICP/EICP technology is efficient in treating heavy metal-contaminated soil without causing secondary contamination. However, a significant amount of CaCO3 is generated during heavy metal mineralization, leading to soil contamination. Compaction can reduce the leaching and bioavailability of trace elements, affecting crop growth [15]. Furthermore, excessive calcium sources and urea can lead to CaCO3 dissolution, soil salinization, acidification, and can hinder crop growth [16]. Wang Wei et al. [16] have enhanced the EICP process and found that the addition of free Ca2+ can increase the soil pH and salinity, stabilize Cd in calcareous soil during sewage irrigation without requiring extra calcium sources, and decrease release risks. Improved EICP technology also reduces the bioavailability of Cd by elevating the soil pH and co-precipitating CaCO3.
Current research on the durability of EICP predominantly focuses on biocementation and solidified silt, with limited attention given to the remediation of heavy metal-contaminated loess using EICP technology [17]. There remains a notable research gap concerning the stability of remediating heavy metal-contaminated loess, especially for the study of the mechanisms of EICP technology without calcium sources in remediating lead-contaminated loess. This study is rooted in the mineralization principle of EICP technology, taking into account the requirements for the strength and ecological toxicity of loess farmland [18]. By excluding CaCl2 to minimize CaCO3 production, an environmentally friendly, cost-effective, and calcium source-free EICP remediation technology is proposed for lead-contaminated loess farmland [19]. This approach utilizes urease mineralization to address heavy metal contamination in the soil while preserving the integrity of the loess farmland environment. The objectives of this study are (1) to investigate the feasibility of using EICP technology without calcium sources for stabilizing lead-contaminated loess; (2) to assess the long-term stability of lead-contaminated loess stabilized with EICP technology without calcium sources; and (3) to elucidate the mechanisms by which EICP technology without calcium sources stabilizes lead-contaminated loess.

2. Materials and Methods

2.1. Testing Materials

2.1.1. Tested Soil

The Guanzhong region, situated in the middle reaches of the Yellow River, is recognized as the largest loess area globally, covering approximately 6.4 × 105 km2. Soil samples utilized in this research were extracted from the loess in Tongchuan City, Shaanxi Province [20]. Disturbed loess samples were gathered from depths of around 0.5–1 m below the surface and transported to the laboratory for analysis. The soil samples appeared loose and exhibited a light grayish-yellow hue. Following pre-treatment procedures outlined by Xu et al. [21,22] and Hou et al. [23], the particle size distribution of the samples was measured with a laser particle size analyzer (Malvern Mastersizer 2000), indicating that 86.28% of particles were silt, as well as 7.88% clay and 5.84% sand. Based on the classification system of the United States Department of Agriculture (USDA, 1951), the loess under study can be categorized as low-plasticity clay (CL) as per ASTM (2011) standards [24]. Moreover, the physical and mechanical characteristics of the loess are detailed in Figure 1 and Table 1 [25]. Additionally, the chemical composition of the loess was assessed utilizing inductively coupled plasma mass spectrometry (ICP-MS). The primary chemical constituents of the soil samples were found to be SiO2, followed by Al2O3, CaO, and Fe2O3.

2.1.2. Chemical Agents

Pb(NO3)2 powder was utilized as the contamination source to prepare contaminated loess, which was purchased from Shaanxi National Pharmaceutical Group Limited (Xi’an, China). Commercial soybeans were crushed, sieved through a 0.5 mm mesh, and mixed with deionized water to obtain urease through a series of steps including stirring, settling, centrifuging, and filtering. Urea from Tianjin Tianli Chemical Reagent Co., Ltd. (Tianjin, China) was used as the carbon and nitrogen source for the EICP technology. Anhydrous calcium chloride (CaCl2) was used as an external calcium source for soil reinforcement methods, with the analytical purity product sourced from Tianjin Kaitong Chemical Reagent Co., Ltd. (Tianjin, China). Analytical-grade nitric acid from Shaanxi National Pharmaceutical Group Limited was utilized to create acidic solutions with varying pH values, mimicking the stability of the remediation effect under acid rain leaching filtration.

2.2. Specimen Preparation

The loess was dried, ground, and sieved for later use. A measured quantity of Pb(NO3)2 powder was dissolved in deionized water to create a Pb(NO3)2 solution. Considering that the control value for lead in the soil environmental quality risk management standards for agricultural land is 240 mg/kg, the pollution concentration was set higher than this control value to explore the remediation mechanisms and behaviors under higher concentrations [26]. A specific volume of the Pb(NO3)2 solution was sprayed onto the soil, stirred thoroughly, resulting in soil contaminated with lead at a concentration of 1000 mg/kg. The lead-contaminated loess was air-dried at room temperature for one month to obtain the untreated heavy metal lead-contaminated loess. In the application of EICP technology, 1 M urea and 0.67 M CaCl2 were combined in deionized water, and then mixed with the lead-contaminated loess along with 3 g/L urease [27,28]. For EICP technology without a calcium source, a 1M urea solution was mixed with the lead-contaminated loess and 3 g/L urease. A comparison between the two technologies was made through macroscopic tests such as lead ion removal efficiency, surface strength, and permeability coefficient [29]. The underlying mechanisms for the differences were analyzed based on carbonate content testing and Zeta potential testing. Further investigation was carried out on the dry–wet cycle characteristics and acid rain leaching properties of lead-contaminated loess treated with EICP technology without a calcium source. The specific testing procedures are illustrated in Figure 2, with detailed experimental methods to be discussed in the subsequent section.

2.3. Experimental Methods

2.3.1. Analysis of Speciation Forms

Various forms of heavy metals in soil include exchangeable, carbonate-bound, iron–manganese oxide-bound, organic-bound, and residual forms. The exchangeable form is considered biologically active and is a major concern for biological toxicity. Liu et al. [27] used the Tessier five-step extraction method and an inductively coupled plasma spectrometer to identify the forms of heavy metal ions occurring in their study.

2.3.2. Surface Strength Test

Surface strength testing was conducted on untreated soil samples, samples treated with EICP, and samples treated with EICP without a calcium source after two cycles of mixing using an Erichsen push–pull force gauge [30]. To minimize experimental errors, three points were chosen on the surface of each sample for measurement, and the average value was then calculated [31].

2.3.3. Seepage Test

While laboratory testing may suffer from the limitation of handling small sample sizes that might not fully represent the actual soil medium, this issue can be addressed by using parallel samples. Laboratory methods are often deemed more dependable due to the controlled environment they offer, including factors like temperature and hydraulic gradients. Soil sample permeability is typically determined in the laboratory through constant load tests, such as using a saturated hydraulic conductivity tester, as illustrated in Figure 3. For this particular study, the ST-55 permeameter from China was utilized, and the constant head method was employed to measure the K value. By adjusting the height of the water supply tank to create the desired hydraulic gradient, the outflow was only recorded once hydraulic head stabilization was achieved. The percolation test on loess followed the detailed steps outlined by Xu et al. [21,22].

2.3.4. Dry–Wet Cycle Test

The prepared samples underwent dry–wet cycle tests using an electrically heated constant-temperature air drying device [29,32]. Test equipment precision was ±1 °C with a maximum temperature of 300 °C. The drying temperature for the test was set at 45 °C, close to natural surface temperature, with the controlled sample moisture content of 20%. Prior to the dry–wet cycle test, preliminary tests were conducted to determine drying/wetting process duration. For wetting, water was slowly added to reach target moisture content, followed by placement in a constant temperature and humidity chamber for even moisture distribution. After 24 h, moisture content inside and outside samples was balanced. During drying, sample mass was weighed, and moisture content changes over time were recorded. Preliminary experiments showed the moisture content dropped to below 1% after 24 h of drying, indicating complete drying. Consequently, both drying and wetting processes were set at 24 h, resulting in a total of 48 h per dry–wet cycle.

2.3.5. Acid Rain Leaching Filtration Test

The experiment utilized a custom pollution soil leaching device to mimic the effects of acid rain on soil samples [29]. A peristaltic pump was used to regulate the flow rate at 30 mL/h, ensuring a consistent leaching height. The setup of the leaching device can be observed in Figure 3. Precise control over the flow rate of the vacuum pump and the sealing layer ensured optimal conditions throughout the leaching process. The vacuum pump maintained a pressure of −10 kPa, assisting in controlling the leaching and infiltration of the acidic solution. By referencing historical meteorological data from the sampling site, a total leachate volume of 1200 mL was gathered in the experiment, approximately equivalent to the total rainfall in the region for one quarter.

2.3.6. Zeta Potential Measurement

Soil particles suspended in aqueous solutions can attract ions with opposite charges, forming strong connections near the particles, which is known as the Stern layer [20,32]. Ions further away create weaker connections, forming the diffuse layer. Heavy metal ions can be adsorbed onto clay mineral surfaces through hydration and ion exchange reactions, impacting the diffuse double layer (DDL) thickness and soil properties. Zeta potential is a useful tool for characterizing the DDL thickness. In this study, loess was sieved through a 50 μm sieve before being treated with different chemical solutions and analyzed for Zeta values using a Zeta potential analyzer (Zeta Plus, Malvern, UK). To prevent contamination, the microelectrophoresis chamber was rinsed with deionized water before and after each measurement.

2.3.7. SEM Tests

An SEM test was used to investigate the change in microscale structural characteristics of lead-contaminated loess before and after modification. To this end, a Zeiss Gemini sigma 300 scanning electron microscope (Oberkochen, Germany) was applied to characterize particle morphology and inter-particle connection between soil particles. The methodology for characterizing soil microstructures in our research was adapted from previous studies by Xu et al. [21], He et al. [20], and Hou et al. [23].

2.3.8. XRD Tests

The minerals of lead-contaminated loess and modified loess specimens were analyzed using a Bruker AXS X-ray diffractometer D8 advance to investigate the interactions between the EICP process and lead-contaminated loess. The minerals can be detected by comparing the X-ray diffraction pattern before and after modification of the lead-contaminated loess. The scanning rate and stride width are 2°/min and 0.02°, respectively.

3. Results

3.1. Comparison of EICP Technology and EICP Technology without Calcium Source

This study investigated the impact of the exchangeable form on the ecological toxicity of loess, particularly in relation to lead contamination. Figure 4 illustrates the removal efficiency of Pb2+ exchangeable form the pre- and post-remediation of lead-contaminated loess using EICP technology with and without CaCl2. As the number of remediation cycles increased, there was a gradual decrease in the content of exchangeable heavy metal ions, leading to an improvement in the remediation efficiency. It was observed that the remediation efficiency of EICP technology with a calcium source was superior to that without. After one round of remediation using EICP technology without a calcium source, there was a 65.7% reduction in the Pb2+ content in the loess. Subsequent rounds of remediation further decreased the content by 89.2% and 90.8% after two and three remediations, respectively. Notably, there was no significant difference in the Pb2+ content after the second and third remediations, suggesting that EICP technology without a calcium source effectively reduces the exchangeable content in loess. With an increase in the number of treatments, the removal efficiencies of both methods increased. However, the removal efficiency of the EICP technology was higher than that of the EICP technology without calcium, and the gap between them decreased with more treatments. This indicates that with a higher number of treatments, EICP technology without calcium is also effective.
Figure 5 compares the comparison of the surface strength and permeability coefficient of the samples after the remediation of lead-contaminated loess using EICP technology with and without a calcium source. In terms of the surface strength, both treatment methods demonstrate an increase in the samples’ surface strength, albeit to varying degrees. This variation can be attributed to the mechanism of EICP technology treatment, where urease catalyzes urea to generate carbonate ions, which then interact with the calcium source and high-valence cations in the contaminated loess to form carbonate precipitation [33]. This process enhances the cohesion of loose soil particles, leading to the formation of a robust surface shell and ultimately boosting the surface strength. It is important to note that calcium carbonate itself possesses inherent strength, resulting in the highest surface strength observed in the samples treated with EICP technology. Conversely, the samples treated with EICP technology without a calcium source exhibit a lower increase in surface strength due to the limited carbonate production from soybean urease’s hydrolysis of urea, in combination with the presence of free calcium and lead ions in the loess. This scenario results in a modest enhancement in the surface strength. Excessive surface strength can hinder crop growth and development in practical agricultural settings. Therefore, EICP technology without a calcium source treatment offers a more balanced approach by mitigating the impact on surface strength, making it a more suitable option for applications in lead-contaminated loess in farmland. In terms of the permeability coefficient, both EICP technology treatments result in a reduction, with the samples treated with a calcium source showing a more significant decrease. During the EICP technology treatment, a significant amount of carbonate is produced, which improves pore filling and increases the density, ultimately reducing the permeability coefficient. On the other hand, EICP technology treatment without a calcium source eliminates the need for a calcium source, leading to less carbonate generation in the samples and a smaller impact on the permeability coefficient after solidification treatment. The variation in the carbonate content shown in Figure 6 provides evidence to support this conclusion.
The exchange and adsorption of heavy metal-induced cations can increase the ion concentration in leachate, weaken the soil strength, and modify the soil microstructure, particularly in clay-rich soils such as loess. Loess is rich in clay minerals like montmorillonite, illite, and kaolinite. Therefore, when leaching lead-contaminated loess, it is crucial to consider cation exchange and adsorption processes. The intrusion of lead ions disrupts the natural charge equilibrium of clay minerals, leading to the exchange of Na+, K+, Ca2+, and Mg2+ ions adsorbed on clay minerals, resulting in a higher ion concentration in leachate [20]. Heavy metal ions in the exchange phase can also reduce the thickness of the double layer (DDL), induce clay particle flocculation, and increase pore space [34]. Moreover, when the clay content surpasses 10%, the DDL effect becomes significant. In this research, the clay content in loess was measured at approximately 20%, indicating the importance of considering the DDL effect of heavy metal erosion on loess via leaching (see Figure 7). This effect diminishes the DDL thickness and promotes the formation of aggregated structures. This observation has been noted in other types of heavy metal-contaminated soils, underscoring the broad applicability of the study findings. The thickness of the DDL can be evaluated using the Zeta potential. Lead, for instance, enters the Stern layer through electrostatic adsorption without altering the sign of the Zeta potential, resulting in a negative Zeta potential for lead-contaminated loess with a decreasing value, indicating the further formation of aggregated structures. Essentially, the implementation of EICP technology leads to a significant reduction in the pores of compacted loess, with the formation of calcium carbonate crystals along with soil particles forming a cohesive, densely packed, and thick calcium carbonate hard shell that covers the soil surface. The hard shell of the EICP technology does not dissolve in water and provides strong erosion resistance, effectively blocking water from entering the sample. This results in a lower permeability coefficient macroscopically. Treatment with EICP technology without a calcium source increases the Zeta potential value, attributed to the carbonate ions produced by soybean urease hydrolysis. These carbonate ions precipitate with high-valence cations, reducing positive charges in the pore solution and ultimately decreasing the Zeta potential (see Figure 7). Despite this, the generated precipitate still blocks soil pores, leading to a reduction in the permeability coefficient. The change in the carbonate content shown in Figure 6 supports this explanation.
Both EICP technology and EICP technology without a calcium source can be effective methods for remediating lead-contaminated loess. However, when considering the overall requirements of farmland soil such as the surface strength, carbonate content, and permeability coefficient, with a specific focus on surface strength, Section 3.2 and Section 3.3 delve into the investigation of the dry–wet cycle and acid rain leaching characteristics of lead-contaminated loess treated with EICP technology without a calcium source.

3.2. Stability Analysis under Dry–Wet Cycles

Figure 8 depicts the variations in the pH value and leached lead ion concentration of the samples treated with EICP technology without calcium sources, contaminated with lead, over multiple dry–wet cycles. The pH value of the lead-contaminated loess samples repaired by EICP technology without calcium sources initially decreases significantly, then gradually decreases until stabilizing with an increase in the dry–wet cycles. The higher initial pH value is attributed to the partial dissolution of the loess carbonate and carbonate generated by mineralization in the solution environment, leading to the production of carbonate ions and a subsequent pH rise due to carbonate hydrolysis. The continuous dissolution and regeneration during the dry–wet cycles result in some carbonates and minerals being continuously encapsulated, reducing the number of hydrolyzable carbonate ions. The microstructure of the sample is significantly impacted by the dry–wet cycles, with the soil particles becoming closely connected during exposure to moisture and saturation, creating strong surface tension upon drying, leading to closer connections between the soil particles and the protection of some minerals from reacting with the pore solution, causing a slight decrease in the soil pH. Furthermore, as the number of dry–wet cycles increases, the leaching concentration of the lead ions gradually rises due to the capillary forces of the lead ions causing the collapse of pore walls and air–water interfaces, resulting in increased micro-cracks and holes within the soil, elevating the risk of heavy metal ion leaching.
Figure 9 illustrates the variations in the distribution of lead forms before and after undergoing dry–wet cycles. With an increasing number of dry–wet cycles, the concentration of carbonate-bound heavy metals in the soil samples gradually decreases, while the levels of exchangeable heavy metals progressively rise. Following 30 dry–wet cycles, the percentage of exchangeable states increases by 1.6%. This shift can be attributed to the structural damage of the sample after multiple cycles, leading to increased porosity and the subsequent release of lead ions previously fixed in the carbonate crystal structure. This release results in a decline in exchangeable lead ion content, while highlighting the reduction in carbonate-bound states as the primary factor influencing the rise in exchangeable states due to the influence of dry–wet cycles.

3.3. Stability Analysis under Acid Rain Leaching

Figure 10 illustrates the evolution of lead leaching in remediated lead-contaminated loess when exposed to acid rain leaching. Initially, there is a rapid increase in the Pb2+ content in the leachate, followed by stabilization. This phenomenon is attributed to the presence of H+ ions in acid rain, which is counterbalanced by clay minerals in the soil through cation exchange reactions. Research indicates that cation exchange reactions occur swiftly and serve as a significant buffering mechanism in the soil during the initial stages of acid rain leaching. It is important to note that as the pH of the acid rain simulation solution rises, the Pb2+ content in the leachate of lead-contaminated loess gradually decreases. This is due to the higher H+ concentration at lower pH levels, strengthening the cation buffering mechanism in the loess, and leading to the release of cations from the soil to maintain charge equilibrium. Furthermore, with the decreasing pH of the leaching solution, there are noticeable changes in the proportion of lead occurring in different forms in the lead-contaminated loess, particularly the shift from the carbonate-bound state to the exchangeable state.

4. Discussion

Heavy metals are a significant contributor to environmental degradation, particularly as industrial byproducts. Conventional methods for removing heavy metals from polluted areas can be costly in terms of both chemicals and energy. As a more environmentally friendly alternative, EICP has emerged as a solution to mitigate heavy metal and waste pollution. In a study by Wang et al. [35], jack bean crude extract urease was used to immobilize and reduce the concentrations of various heavy metals and metalloids in contaminated mining waste. The results showed reductions in arsenic, manganese, zinc, lead, chromium, and copper by significant percentages. Another study by Moghal et al. [36,37] explored the effectiveness of EICP in adsorbing and desorbing soils with different heavy metal combinations, highlighting the technology’s ability to immobilize heavy metals in soils. Through the mineralization process of EICP, heavy metal ions can be transformed into biologically unavailable forms by precipitating them as carbonate minerals or becoming part of mineral crystals, ultimately stabilizing them. Additionally, calcite formed during mineralization can adsorb and immobilize heavy metal ions. Previous research indicates that a soluble calcium source is crucial for the in situ generation of CaCO3 in EICP technology. According to Liu et al. [38] and Gat et al. [39], the excessive addition of calcium sources and urea can have negative effects such as CaCO3 dissolution, increased soil density, salinization, acidification, and the inhibition of crop growth. In loess soils, the higher CaCO3 content can release free Ca2+ when in contact with water, potentially displacing Pb2+ and activating lead in the environment. Previous research suggests that adding exogenous Ca2+ is crucial in the urease-induced CaCO3 precipitation process and reducing heavy metal bioavailability [40]. This is because free calcium ions can release soil-bound heavy metals into the soil pore solution, affecting the solidification of heavy metal ions and increasing their bioavailability. Additionally, under calcium chloride stress, the competitive adsorption of Ca2+ can activate heavy metals. At the same time, the decrease in the soil pH reduces carbonate substances in the soil and suppresses soil calcium levels (see Figure 6).
The conventional EICP technique involves the hydrolysis of urea by urease to produce carbonate ions, which then combine with high-valence metal ions. Typically, heavy metal ions are the first to react with carbonate ions, followed by the reaction between calcium ions and carbonate ions. In most scenarios, heavy metals are enclosed within calcium carbonate crystals, significantly reducing their environmental toxicity (see Figure 11). This explains why the exchangeable lead in Figure 4 shows a higher efficiency in remediation. It is important to note that the calcium ions in the carbonate precipitate are sourced not only from externally added calcium sources but also from the free calcium ions released by the loess. On the other hand, EICP technology without additional calcium sources generates numerous carbonate ions, which interact with both calcium ions and lead ions in lead-contaminated loess to create carbonates (see Figure 6). The high clay content in loess results in a negative Zeta potential, causing lead ions to enter the diffusion layer and be adsorbed. Calcium-free EICP technology introduces a significant amount of carbonate and other anions, promoting the release of lead ions and forming carbonate precipitates. Additionally, loess itself contains a large amount of carbonates, which dissolve in water, releasing calcium and carbonate ions that can reconstitute carbonate crystals. Consequently, compared to lead-contaminated loess treated with EICP solidification, the surface strength shows a slight increase, and the permeability coefficient slightly decreases with the EICP technology that lacks calcium sources (see Figure 5). Combining the dual-layer theory model, this study microscopically analyzes the effects of EICP solidification treatment and EICP technology treatment on lead-contaminated loess without calcium sources. The negative charge carried by clay minerals in loess creates an electric field around them. Water molecules (polar molecules) and cations in the aqueous solution adsorb on the surface of soil particles in an oriented arrangement. Close to the soil particle surface, the electrostatic forces are strongest. Hydrated cations firmly adsorb on the negatively charged surface of soil particles to form a fixed layer (stern layer). As distance X increases, the electrostatic force decreases. Hydrated ions and cations in the aqueous solution are less constrained by electrostatic forces in the fixed layer, and their activity is greater, forming a diffuse layer (Gouy layer). With a further increase in distance X, the aqueous solution is no longer affected by the negative charge of the soil particle surface, and its movement state is mainly Brownian motion, which can be regarded as free pore water. The thickness of the diffuse layer depends not only on the surface charge density of minerals but also on the properties, valence, pH value, and ion concentration of ions in the aqueous solution. The higher the valence of cations in the aqueous solution, the stronger the electrostatic force between the soil particles, the fewer hydrated molecules and cations needed to balance the negative charge on the soil particle surface, and the thinner the diffuse layer (see Figure 6). Figure 12 provides an overview of microstructure of lead-contaminated loess exposed to EICP and EICP without calcium treatments. Following the treatment of lead-contaminated loess with EICP technology, the Zeta potential absolute value decreases, while the negative potential on the soil particle surface reduces from −5.5 mV to −3.2 mV. The implementation of EICP technology leads to an increase in the cation concentration and valence in the pore water, which neutralizes the negative charge on the soil particle surface. This neutralization results in a reduction in the negative potential on the soil particle surface, leading to the thinning of the Gouy layer. Consequently, the van der Waals force between particles surpasses the repulsion, causing clay particles to aggregate and form structured aggregates that enhance the surface strength. Conversely, EICP technology without additional calcium sources, lacking high-valence cations and releasing numerous carbonate ions, leads to an increase in the Zeta potential value. Nonetheless, the carbonate cementation still enhances the surface strength, albeit to a lesser degree compared to traditional EICP technology. The XRD test results in Figure 13 confirm the fact that the Pb immobilization efficiency is feasible in lead-contaminated loess sites. This underlines the potential advantage of EICP technology without added calcium sources for applications in agricultural contaminated loess.
Under dry–wet cycling conditions, the leaching rate of heavy metal ions from contaminated soil after remediation using EICP technology without added calcium sources slightly increases with the number of cycles. This increase is primarily attributed to structural damage within the soil caused by dry–wet cycling, which heightens the leaching risk of heavy metal ions. Consequently, some heavy metal ions fixed within the carbonate crystal structure are re-released, leading to an upsurge in the exchangeable content and a decline in the carbonate-bound content of heavy metals in the soil. Conversely, the leaching amounts of the exchangeable and carbonate-bound forms of heavy metals in the contaminated soil under the influence of acid rain are mainly influenced by the pH value of the acidic solution. As the pH value decreases (acidity increases), the carbonate-bound content in the lead-contaminated soil gradually diminishes, while the exchangeable content exhibits a gradual increasing trend (see Figure 10 and Figure 11). The findings regarding changes in different ion forms under acidic conditions after immersion align with those reported by Xu et al. [21,22]. However, the type of acid rain also affects the long-term stability of solidified lead-contaminated soil. The impact of acid rain types on the long-term stability could be presented in another paper.

5. Conclusions

By comparing the effects of EICP technology with and without CaCl2 in repairing lead-contaminated loess, this study further investigated the dry–wet cycle and acid rain leaching stability of EICP technology without CaCl2. Overall, the results and discussion lead to some main conclusions.
(1) Compared to EICP technology, the removal efficiency of the exchangeable state decreased by 1.6–23.5% when remediating lead-contaminated loess using EICP technology without calcium. Furthermore, the samples treated with EICP technology without calcium exhibited a significant decrease in surface strength and an increase in the permeability coefficient, effectively preventing the soil compaction observed in the EICP-treated lead-contaminated loess, which is more conducive for crop growth. This underscores the potential of EICP technology without calcium in remediating lead-contaminated loess.
(2) In traditional EICP technology, the addition of a calcium source may pose risks such as CaCO3 dissolution and soil acidification. Conversely, EICP technology without calcium maintains a relatively stable pH value above 8.25 even under dry–wet cycle conditions, creating a stable alkaline environment for carbonate-bound forms, which promotes the long-term stability of the remediated contaminated soil.
(3) During the acid rain leaching test, a lower pH value of the leaching solution corresponds to a higher H+ concentration and a stronger cation buffering mechanism in the loess, causing the release of numerous cations to balance the charge. Following the leaching test, the content of exchangeable lead in the samples increased while the content of carbonate-bound lead decreased. Moreover, as the pH value of the leaching solution increased, the Pb2+ content in the leachate gradually decreased.

Author Contributions

K.Z.: investigation, formal analysis, writing—reviewing and editing. S.Z.: conceptualization, methodology, resources, writing—original draft preparation. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

All the authors have read and approved this version of the article, and due care has been taken to ensure the integrity of the work. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. We declare that all authors have no any actual or potential conflicts of interest including financial, personal or other relationships with other people or organizations.

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Figure 1. Particle size distribution of the loess.
Figure 1. Particle size distribution of the loess.
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Figure 2. The procedures for experimental methods.
Figure 2. The procedures for experimental methods.
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Figure 3. Schematical illustration of leaching filtration device.
Figure 3. Schematical illustration of leaching filtration device.
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Figure 4. Remediation efficiency of exchangeable form of lead.
Figure 4. Remediation efficiency of exchangeable form of lead.
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Figure 5. Comparison of EICP technology and EICP technology without calcium source in surface strength and permeability coefficient.
Figure 5. Comparison of EICP technology and EICP technology without calcium source in surface strength and permeability coefficient.
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Figure 6. Effect of treatment times on carbonate content.
Figure 6. Effect of treatment times on carbonate content.
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Figure 7. (a) Schematic illustration of the diffuse double layer, (b) variation in the Zeta potential exposed to EICP technology and EICP technology without calcium source.
Figure 7. (a) Schematic illustration of the diffuse double layer, (b) variation in the Zeta potential exposed to EICP technology and EICP technology without calcium source.
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Figure 8. Effect of dry–wet cycles on pH and leaching rate of Pb2+ ions.
Figure 8. Effect of dry–wet cycles on pH and leaching rate of Pb2+ ions.
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Figure 9. Effect of dry–wet cycles on the percentage of different speciation forms.
Figure 9. Effect of dry–wet cycles on the percentage of different speciation forms.
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Figure 10. Effect of acid rain leaching on the leaching of Pb2+.
Figure 10. Effect of acid rain leaching on the leaching of Pb2+.
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Figure 11. Effect of acid rain leaching on the percentage of different speciation forms.
Figure 11. Effect of acid rain leaching on the percentage of different speciation forms.
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Figure 12. Schematical illustration of the loess microstructure exposed to EICP technology and EICP technology without calcium source.
Figure 12. Schematical illustration of the loess microstructure exposed to EICP technology and EICP technology without calcium source.
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Figure 13. XRD test results for lead-contaminated loess when subjected to EICP and EICP without calcium.
Figure 13. XRD test results for lead-contaminated loess when subjected to EICP and EICP without calcium.
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Table 1. Physicochemical properties of the loess.
Table 1. Physicochemical properties of the loess.
Physical Index.Data
Fines (%)94.16
Sand (%)5.84
Gravel (%)0
Specific gravity, Gs2.70
Void ratio, e0.86
Dry density, ρdmax/(g/cm3)1.73
Initial water content (%)16.6
The Atterberg limit
Liquid limit (%)32.52
Plastic limit (%)18.68
Soil classificationCL
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Zhang, K.; Zhang, S. Feasibility Study of Applying Enzyme-Induced Carbonate Precipitation (EICP) without Calcium Source for Remediation of Lead-Contaminated Loess. Buildings 2024, 14, 1810. https://doi.org/10.3390/buildings14061810

AMA Style

Zhang K, Zhang S. Feasibility Study of Applying Enzyme-Induced Carbonate Precipitation (EICP) without Calcium Source for Remediation of Lead-Contaminated Loess. Buildings. 2024; 14(6):1810. https://doi.org/10.3390/buildings14061810

Chicago/Turabian Style

Zhang, Kun, and Shixu Zhang. 2024. "Feasibility Study of Applying Enzyme-Induced Carbonate Precipitation (EICP) without Calcium Source for Remediation of Lead-Contaminated Loess" Buildings 14, no. 6: 1810. https://doi.org/10.3390/buildings14061810

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