Engineering and environmental behavior performance of magnesium potassium phosphate cement binder solidification/stabilization Zn-Cu contaminated soils

Cuihong Li; Minhui Ye; Chengxiong Wu; Wei Wang; Shaoyun Pu; Na Li; Ping Jiang; Yubo Zhou

doi:10.25259/AJC_738_2025

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Original Article

ARTICLE IN PRESS

doi:

10.25259/AJC_738_2025

Engineering and environmental behavior performance of magnesium potassium phosphate cement binder solidification/stabilization Zn-Cu contaminated soils

Cuihong Li^a,b, Minhui Ye^a,b, Chengxiong Wu^a,b, Wei Wang^a,b,*, Shaoyun Pu^a,b, Na Li^a,b, Ping Jiang^a,b, Yubo Zhou^a,b

aSchool of Civil Engineering, Shaoxing University, Shaoxing, Zhejiang Province, China

bShaoxing Key Laboratory of Interaction between Soft Soil Foundation and Building Structure, Shaoxing, China

* Corresponding author: E-mail address: wellswang@usx.edu.cn (W. Wang)

Received: 2025-06-26, Accepted: 2025-12-05, Epub ahead of print: 2026-03-27,

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Abstract

A large quantity of contaminated soil in abandoned industrial sites is polluted by heavy metal (HM) pollutants, among which zinc and copper pollution not only threaten human health but also damage the ecological environment. Currently, magnesium potassium phosphate cement (MKPC) is increasingly being used for the solidification/stabilization (S/S) of soils contaminated with a single HM pollutant. However, research on the application of MKPC for treating soils with composite HM pollutants remains limited. Therefore, this study investigates the application of MKPC in the S/S treatment of Zn²⁺, Cu²⁺, and composite contaminated soils, evaluating unconfined compressive strength (UCS), toxicity characteristic leaching procedure (TCLP), electrical conductivity (EC), and freeze-thaw cycle (F-T cycle) tests, as well as examining the mineral composition and microstructure based on the microscopic technique. When the concentrations of Zn²⁺ and Cu²⁺ were both 15000 mg kg^-1, it was observed that the compressive strength of the solidified soil was approximately 4.0 times (0.99 MPa) that of the untreated soil (0.25 MPa), and EC decreased by 40.9% (from 6.92 to 2.82 mS cm^-1) at the curing age of 28 days. The leaching concentrations of Zn²⁺ and Cu²⁺ were reduced to 6.56 mg L^-1 and 2.81 mg L^-1, and were well below the USEPA standard of 15 mg L^-1. However, the maximum leaching concentrations of Zn²⁺ and Cu²⁺ reached 43.23 mg L^-1 and 36.08 mg L^-1 after undergoing 9 freeze-thaw cycle tests, respectively, exceeding the regulatory limits. The formation of MKPC hydration products, MgKPO₄·6H₂O, effectively contributes to the S/S of Cu and Zn in the soil. Generally, the results of this study can offer useful guidance for the application of remediation techniques for composite contaminated soils.

Keywords

Leaching concentration

Magnesium potassium phosphate cement

Solidification/stabilization

Strength

Zn2+and Cu2+contaminated soils

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1. Introduction

Abandoned smelting and industrial sites around the world are contaminated with heavy metals (HM), including zinc (Zn²⁺), copper (Cu²⁺), lead (Pb²⁺), and cadmium (Cd²⁺) [1]. The leakage, improper disposal, or poor management of chemical substances in the smelting process causes serious pollution to the surrounding soils and environment [2,3]. HM not only changes the engineering characteristics of soils [4], but also may endanger the lives and health of human beings [5,6]. According to relevant investigation reports, all abandoned industrial sites and smelting sites in China are contaminated by HM ions to varying degrees, especially Zn²⁺, Pb²⁺, and Cu²⁺ [7]. When HM ions enter the soil, they will destroy the pore structure between soil particles, leading to local roughness or particle dispersion [8], thus reducing the compressive strength and bearing capacity of the soil. Meanwhile, the HM ions in soils migrate with groundwater, polluting the groundwater systems [9]. Therefore, the treatment of HM-contaminated soils in industrial sites has become one of the important topics in environmental engineering research [4]. The solidification/stabilization (S/S) approach is a common remediation technique for HM-contaminated soils, which has the advantages of economic efficiency, easy processing, and a wide application range [10]. In the process of the combination of curing agent and the soil contaminated by HM, HM are fixed in the hydration products of the curing agent and soils through chemical precipitation, coagulation, encapsulation, and complexation.

Portland cement is a commonly used traditional binder, widely applied in cement-based materials and HM treatment [11]. However, it also has disadvantages such as high carbon emissions and high energy consumption. Therefore, some scholars have used fibers [12] or surface treatment [13,14] to improve the properties of cement-based composite materials. In addition, the research on green and environmentally friendly cementitious materials is also receiving a great deal of attention. Ghamarpoor et al. [15] utilized rice husk as a renewable source of SiO₂ to fabricate a TiO₂@SiO₂ core-shell structure, which demonstrates significant efficacy in the photocatalytic degradation of pollutants in contaminated wastewater. Magnesium potassium phosphate cement (MKPC), which is mainly composed of burnt magnesium oxide (MgO) and potassium dihydrogen phosphate (KH₂PO₄), has the advantages of fast curing, strong adhesion, low energy consumption and significant effect on curing HMs [16], which can be used as one of the green cementitious materials for S/S treatment of soil contaminated by HM ions. When MKPC is used for the S/S of HM contaminated soils, MgO and KH₂PO₄ react with alkali and acids under water conditions to produce potassium magnesium phosphate hexahydrate (MgO+KH₂PO₄+5H₂O→MgKPO₄·6H₂O) and phosphate products [17]. MgKPO₄·6H₂O crystals with prismatic surfaces and dense internal structural connections have excellent adsorptive and entrapment properties, which can be used to treat soil contaminated by HM ion soils of Pb²⁺, Zn²⁺, and Cd²⁺ [18]. The research shows that when using MKPC to treat Pb²⁺, Zn²⁺ contaminated soils, MKPC not only improves the strength of Pb²⁺, Zn²⁺ contaminated soils, but also markedly decreases the leaching concentration of Pb²⁺, Zn²⁺. The curing mechanism is mainly the interaction between free Pb²⁺, Zn²⁺, and PO₄^3- to form the low solubility Pb₃(PO₄)₂ and Zn₃(PO₄)₂ products [19,20], resulting in a decrease in HM migration. In addition, the incompletely reacted MgO combines with MgKPO₄·6H₂O and imports Pb²⁺ and Zn²⁺ into contaminated soils via physical adsorption and encapsulation. At the same time, it is also discovered that there is a competition mechanism among the coexisting ions while MKPC cures the complex ionic contaminated soils [21].

HM in contaminated soils engages in competitive interactions for limited adsorption sites, a mechanism that influences their mobility and immobilization behavior. Lu et al. [22] found that soils exhibit a higher adsorption affinity for Cu²⁺ and Pb²⁺, resulting in greater retention within the soil matrix, whereas their adsorption capacity for Zn²⁺ and Cd²⁺ is comparatively lower. Certain metals, such as Cu²⁺, generally exhibit higher adsorption affinities than others like Zn²⁺ due to differences in ionic radius and charge density [23]. This differential affinity leads to variations in S/S efficiency. For example, Cu²⁺ can inhibit the immobilization of Zn²⁺ in cement-based matrices. These interactions are dynamic and governed by environmental factors including pH, ionic strength, and the presence of competing ions [24,25]. In S/S treatments, a thorough understanding of such competitive mechanisms is important for enhancing the durability of contaminated soils. Although the competitive adsorption effect has been investigated in conventional cement and soil systems, the research results on the performance and competitive mechanisms of Cu²⁺and Zn²⁺ in the MKPC system have not been clearly identified. Moreover, there is also a lack of research results on HM ion-contaminated soils under extreme environmental conditions.

To sum up, through unconfined compressive strength (UCS) test, toxic leaching test, electrical conductivity (EC) test, and pH meter test, the strength and environmental behavior of soil contaminated by Zn ²⁺, Cu²⁺, and complex ions solidified/stabilized by MKPC were investigated in this study, and its solidification effect was analyzed. The durability of MKPC solidified contaminated soil under extreme and complex environments was studied through freeze-thaw cycle (F-T cycle) test, and the microstructure and mineral characteristics of the solidified contaminated soils were analyzed by scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), and X-ray diffraction (XRD) tests to reveal the curing mechanism of MKPC dealing with Zn²⁺, Cu²⁺and composite contaminated soils. The results are instructive for the solidification of Zn²⁺, Cu²⁺, and composite contaminated soils using MKPC.

2. Materials and Methods

2.1. Contaminated soil preparation

To ensure the reproducibility of the test results, the contaminated soil was all prepared by humans.

2.1.1. Soil

The test soil was sampled at a depth of 1 m beneath the excavation surface of the foundation pit of the expansion project in Shaoxing University, Shaoxing of Zhejiang Province. The SEM images of soils after magnification of 2000 times are shown in Figure 1(a). The soil particles are irregularly sliced or lumped, and there are large connecting voids inside the soil. The main mineral composition of the soil was SiO₂ as measured by XRD tests. The XRD pattern of the soil is shown in Figure 1(b). According to the Standards for Geotechnical Test Methods [26], the soil was subjected to compaction test, combined liquid-plastic limit determination test, and particle sieving tests. The soil compaction curve has been shown in Figure 2(a); the maximum measured dry density of soil is 1.82 g cm^-3, and the optimum water content is 18%. Figure 2(b) and Figure 2(c) illustrate the liquid-plastic limit indices and the grading curve of soil particles, respectively. The liquid limit of the soil measured in the liquid plasticity limit joint test is 47% and the plasticity index is 24. According to the engineering classification standard of soils [27], the test soil belongs to low liquid limit clay (CL, IP≥7, WL<50%), and the main physical and mechanical properties of soils have been shown in Table 1.

Basic characteristics of soil. (a) Compaction curve of soil, (b) Plasticity diagram of soil, (c) Curve of particle size distribution of soil.

Table 1. The basic physical-mechanical properties of the tested soil.

Water content (%)	Plastic limit (%)	Liquid limit (%)	Plasticity index	Maximum dry density (g cm^-3)	Optimum moisture content (%)
31	23	47	24	1.82	18

2.1.2. Composite contaminated soils

The typical concentrations of Zn²⁺ and Cu²⁺ scrap metals are 10000 mg kg^-1 and 15000 mg kg^-1, respectively, in industrial contaminated sites in China [6,7]. In this study, four kinds of HM contaminated soil samples with varied levels of pollutant concentrations were prepared. Control samples consisted of HM contaminated soil with a Zn²⁺ concentration of 10000 mg kg^-1 (Zn1) and another with a Cu²⁺ concentration of 10000 mg kg^-1 (Cu1). Additionally, two composite contaminated soils were prepared, each containing both Zn²⁺ and Cu²⁺ at varying concentration levels: one at 10000 mg kg^-1 for each ion (Zn1Cu1), and the other at 15000 mg kg^-1 for both ions (Zn1.5Cu1.5). These samples were designated as Zn1Cu1 and Zn1.5Cu1.5. Zn(NO₃)₂·6H₂O and Cu(NO₃)₂·3H₂O were selected as the sources of Zn²⁺ and Cu²⁺ contamination due to their high solubility and low inertness, which minimize interference in subsequent testing [28]. The required amounts of these compounds were accurately weighed according to the test design, dissolved in deionized water, and uniformly mixed into the soil to adjust the water content to 18%. The mixtures were then sealed and passivated for 10 days before sample preparation.

2.2. Binders

The test materials for preparing cement curing agent were over-fired magnesium oxide (MgO), potassium dihydrogen phosphate (KH₂PO₄), and borax (Na₂B₄O₇·10H₂O). The curing agent of potassium magnesium phosphate cement was prepared with the material ratio of MgO to KH₂PO₄ as 4:1 [29,30], and Na₂B₄O₇·10H₂O was used as the test retarder to delay the hydration reaction. Among them, MgO was produced by Tianjin Komeo Chemical Reagent Company Limited, KH₂PO₄ and Na₂B₄O₇·10H₂O were produced by Chemistry Reagent Company Limited of China National Pharmaceutical Group. The content of the three active substances accounted for more than 95% of the total content, and the purity was analytically pure. The chemical composition of MgO has been shown in Table 2.

Table 2. Chemical composition of the magnesia (wt%).

Composition	MgO	SiO₂	CaO	Fe₂O₃	Al₂O₃	Others
Percentage (%)	95	1.5	1	1	1	0.5

2.3. Experimental scheme

The influences of curing age, dosage, and initial concentration on the engineering properties and environmental behavior of HM contaminated soils are examined in this study through UCS, toxicity characteristic leaching procedure (TCLP), pH, EC, and F-T cycle tests. The untreated initially contaminated soils were considered as a control, and the samples with different MKPC dosages (2%, 4%, 6%, 8%) were compared, setting the mass of Na₂B₄O₇·10H₂O as 5% of MgO. The curing ages were 7 and 28 days. The F-T cycle test was conducted on samples after standard curing for 28 days, and the microstructure and minerals of contaminated soil samples were analyzed by SEM-EDS, XRD, FTIR, and XPS tests. The number of samples required for each sample group and experimental scheme have been shown in Table 3.

Table 3. Zn and Cu concentrations and conservation time for various tests.

Test program	Zn or/and Cu concentration (%)	Binder dosage	Conservation time	Sample size
UCS	Zn1, Cu1, Zn1Cu1, Zn1.5Cu1.5	0, 2, 4, 6, 8	7, 28	5
TLCP				3
pH				2
EC			3, 7, 28	3
F-T cycles		8		5
SEM-EDS		0, 4, 8	7	1
XRD				1
FTIR				1
XPS		0,8	7	1

2.4. Preparation of specimens

The hand-prepared Zn²⁺, Cu²⁺ samples in the composite contaminated soil were mixed with the MKPC curing agent. The dosage of MKPC was controlled to 0%, 2%, 4%, 6%, and 8% of the soil’s dry weight. Then, the contaminated soil was combined with the curing agent mixture and put into a R 39.1 mm × H 80 mm cylindrical test. The specimens were statically formed with jacks, and the pressure was maintained for 2 min. The specimen was removed after molding, and then sealed with polyethylene pouches and stored in a controlled environment chamber under standard curing conditions for curing (20±2°C, 95% humidity) for the specified time for tests. The process of specimen preparation has been shown in Figure 3.

2.5. Test methods

2.5.1. UCS tests

The UCS test of solidified contaminated soil samples was conducted with reference to the Standard for geotechnical testing method [26]. TKA-WCY-1F type automatic multifunctional UCS testing machine produced by Nanjing Tekao Technology Company Limited was used to conduct the test, and the compression rate was controlled at 1 mm min^-1 during the testing process.

2.5.2. TCLP tests

TCLP tests were used to simulate the leaching process in sanitary landfills and analyze chemical substances to meet the standard determination of vulcanization requirements for HM leachate in the sample. TCLP test refers to the United States Environmental Protection Agency (USEPA) regulations. The specific steps were as follows: the leaching test was conducted by taking 10 g of ground soil samples through a 1 mm sieve, and the leaching solution was a pH=4.93 ± 0.05 acetate buffer solution. 200 mL of leaching solution was added to the soil samples, and the soil samples were rotated at 30 ± 2/min for 18 h at 20 ± 2°C, and then stood for 24 h after the completion of the shaking. The supernatant was extracted through 0.45 µm membrane filtration with pH < 2, the leaching concentrations of Zn²⁺ and Cu²⁺ were determined by Thermo Fisher Scientific ICP-MS6300 plasma emission spectrometer, and the pH value of the leachate was determined by PHS-25 Rembrandt pH meter.

2.5.3. EC tests

Soil conductivity is an important indicator for measuring soil water-soluble salts, and the conductivity value of solidified contaminated soils is measured to describe the total amount of soluble salt ions in soils. The test method refers to the Determination of soil conductivity electrode method regulations. The specific steps were as follows: 10 g of crushed soil samples was placed into a beaker, and 50 mL of distilled water was added according to the soil-water ratio of 1:5. After stirring evenly, it was allowed to stand for 30 min. The EC value of the solidified contaminated soil was measured using a DDA-11AT conductivity meter. The measuring electrode was inserted into the soil samples extraction solution, and the temperature was corrected to 25°C. The conductivity of the soil samples’ extraction solution was measured, the EC value was read, and the temperature of the extraction solution was recorded at the same time.

2.5.4. Freeze-thaw cycle tests

For studying the changes in physicochemical properties of Zn²⁺, Cu²⁺, and composite contaminated soils treated by MKPC under extreme and complex environments, the F-T cycles of 0, 1, 3, 5, 7, and 9 were carried out on MKPC-treated samples with different concentrations. The effects of F-T cycle on the long-term environmental effect of MKPC treatment were revealed in Zn²⁺, Cu²⁺, and composite contaminated soils. The samples were placed in a (-20 ± 2°C) low-temperature freeze-thaw box after curing for 28 days, and then removed after 12 h of freezing and thawing. After 1 h at room temperature, they were placed in a standard conditioning box for 11 h. This is a cycle. UCS and TCLP tests were carried out on samples after each cycle. The results were most obvious after 9 F-T cycles. Therefore, 9 cycles were selected as the maximum number of cycles for the tests.

2.5.5. SEM-EDS analysis

To investigate the microstructure of Zn²⁺, Cu²⁺, and composite contaminated soils, soil samples with fresh surfaces were crushed and freeze-dried, and then tested by SEM using Nippon Electronics JSM-6360 LV type SEM.

2.5.6. XRD analysis

The soil samples were subjected to freeze-drying and crushing, followed by grinding into a fine powder and sieving through a 0.075 mm sieve for the XRD test. The XRD test was performed on samples using the Empyrean diffractometer from Panaco, Netherlands, and the XRD patterns were collected at 2θ in the range of 10-90°, with the scanning speed of 4° min^-1.

3. Results and Discussion

3.1. UCS

The variation in UCS with different MKPC contents for samples has been presented in Figure 4. The UCS of all samples progressively improves with higher curing agent content and longer curing duration. For MKPC-treated Zn²⁺ contaminated soil Figure 4a , its UCS increases from 0.46 MPa to 1.30 MPa when the curing agent dosage increases from 2% to 8% at 7 days of curing age, which is 294% higher than that of untreated contaminated soil. With the curing age extended to 28 days, the UCS increases to 1.46MPa, which is 344% higher. When MKPC treats Cu²⁺ contaminated soil (Figure 4b), the change rule of compressive strength with curing agent dosage is basically the same as solidified Zn²⁺ contaminated soil. Through comparative analysis, with the same curing agent dosage, the UCS of MKPC-treated Cu²⁺ contaminated soil is slightly lower than that of MKPC-treated Zn²⁺ contaminated soil, and the UCS of Cu²⁺ contaminated soil with 28 days curing is 1.35MPa, representing a 336% increase. However, when MKPC treats Zn²⁺ and Cu²⁺ composite HM contaminated soil (Figure 4c and d), the UCS is significantly lower than that of Zn²⁺ and Cu²⁺ contaminated soil. The UCS of MKPC-treated samples decreases in HM ion concentration. When the curing agent dosage is greater than 2%, the strength of the solidified contaminated soil reaches the strength value of 0.35MPa recommended by USEPA regulation. In the case of treating highly concentrated composite contaminated soils, the curing agent dosage needs to be greater than 4% to achieve the required strength value.

Effect of MKPC binder content on the strength of stabilized contaminated soils: (a) Zn1 (b) Cu1 (c) Zn1Cu1 (d) Zn1.5Cu1.5.

The reason for the increase in strength of solidified contaminated soil is that the MKPC releases a lot of heat, accelerates the hydration reaction in the S/S system, and promotes the formation of gel substances of MgKPO₄·6H₂O [31]. With the increasing curing agent dosage, the amount of MgKPO₄·6H₂O formed within the S/S system increases, and the hydrated gel substance fills pores between the soil particles, thus improving the strength of the specimen [20]. With the same curing agent dosage, the UCS of Zn²⁺ samples is greater than that of Cu²⁺ samples, which may be due to the difference in crystal structure matrix of HMs Zn and Cu [21,32]. With the increase of curing age, the hydration reaction is completely completed in the S/S system, resulting in closer pore association of hydrated gel products with soil particles, and the strength of the soil system is improved [33]. However, when MKPC treats soil contaminated with high concentrations of complex HMs, MgO reacts with KH₂PO₄ to form MgKPO₄·6H₂O, which combines with Zn²⁺ and Cu²⁺ to transform into compounds with Zn₃(PO₄), Zn₂(OH)PO₄, Cu₄(PO₄)₂, and Mg_0.95Cu_0.05O [21,32], and consumes a portion of hydrated gel raw materials, leading to a decrease in the soil strength. In addition, Zn²⁺ and Cu²⁺ antagonistically interact with each other in high concentration contaminated soils, the inhibition of curing agent on the chemical reaction slows down the reaction rate in the S/S system, and the formed amount of MgKPO₄·6H₂O gel material decreases, resulting in a small amount MgKPO₄·6H₂O in the soil and a small contribution to the strength of the specimen. The strength growth is shown to be slower [30,34].

3.2. TCLP and pH

Figure 5 shows the test results of leaching concentration and pH of Zn²⁺, Cu²⁺, and composite contaminated soils in TCLP. The leaching concentration of Zn²⁺ and Cu²⁺ exceeds the USEPA standard limit of 15 mg L^-1 in untreated contaminated soils, and the soil is harmful and requires remediation. With the increase of curing agent and curing age, the leaching concentrations of Zn²⁺ and Cu²⁺ decrease significantly, but the leachate pH increases gradually. When MKPC treats Zn²⁺ contaminated soil (Figure 5a), and the curing agent dosage increases from 2% to 8%, the Zn²⁺ concentration of samples decreases from 332.30 mg L^-1 to 1.41 mg L^-1 at the curing age of 7 days, and the leachate pH increases from 6.14 to 9.39. When the curing age increases to 28 days, the Zn²⁺ concentration decreases to 0.07 mg L^-1, while the leachate pH increases to 10.15. When MKPC treats Cu²⁺ contaminated soil (Figure 5b), the leaching concentration and pH value change with the curing agent dosage, which is basically similar to that of curing Zn²⁺ contaminated soils. After curing for 28 days, the leaching concentration of Cu²⁺ decreases to 0.06 mg L^-1, and the leachate pH value increases to 10.22. The results indicate that curing agent dosage and curing age significantly affect the treatment effect in S/S. It is found through comparison of Cu²⁺ and Zn²⁺ contaminated soils with the same curing agent dosage that the curing effect of Cu²⁺ leaching concentration is better than that of Zn²⁺, and the HM Cu²⁺ is easier to treat in S/S. In addition, when MKPC treats Zn²⁺ and Cu²⁺ composite HM contaminated soil (Figure 5c and d), the leaching concentration of samples increases compared with that of Zn²⁺ and Cu²⁺ contaminated soils, and the HM ion concentration affects the leaching ability of MKPC-treated samples. Moreover, when MKPC treats Zn²⁺ and Cu²⁺, the curing agent dosage is greater than 4%, and the leaching concentration of Zn²⁺ and Cu²⁺ is much lower than the maximum upper limit of the USEPA specification of 15 mg kg^-1. However, the leaching concentrations of Zn²⁺ and Cu²⁺ at high concentrations of composite contaminated soils are lower than the leaching limits when the curing agent dosage is greater than 8%.

Effects of MKPC dosage on leaching concentration and pH of contaminated soils: (a) Zn1, (b) Cu1, (c) Zn1Cu1, (d) Zn1.5Cu1.5..

The reason for the decrease of leaching concentration is that, with the increase of curing agent dosage, Mg²⁺, Zn²⁺, Cu²⁺, and PO₄^3- combine with each other to generate MgKPO₄·6H₂O, Zn₂(OH)PO₄, Cu₄(PO₄)₂O, and Mg_0.95Cu_0.05O and other phosphates with lower solubility or precipitated in the soil [34], resulting in a decrease in the leaching concentration of Zn²⁺, Cu²⁺. In addition, the combination of wrapped adsorption of Zn²⁺ and Cu²⁺ loose in the pore solution of MgKPO₄·6H₂O with incompletely reacted MgO reduces the mobility of Zn²⁺ and Cu²⁺ in the soil, and effectively immobilizes the contaminants in S/S system, thus reducing the leaching concentration of Zn²⁺ and Cu²⁺ [35,36]. With the increase of curing age, the amount of MgKPO₄·6H₂O hydrated gel formed in the S/S system increases, and Zn²⁺ and Cu²⁺ immobilized in the soils are further adsorbed, and the leaching concentration of Zn²⁺ and Cu²⁺ gradually decreases with the increase of curing age [37]. In addition, in the S/S system with high concentrations of Zn²⁺ and Cu²⁺ composite contaminated soil, the solidified reaction rate is inhibited, resulting in a decrease in the formation of MgKPO₄·6H₂O, and a small amount of hydrated gel is unable to encapsulate all of Zn²⁺ and Cu²⁺, leading to an increase in the leaching concentration of Zn²⁺ and Cu²⁺. However, the leachate pH increases with the increase of curing agent dosage and curing age. There are two reasons for this phenomenon: on the one hand, with the increase of the curing agent dosage, the amount of alkaline products formed by MgO reacting with KH₂PO₄ in the specimen increases, resulting in an increase in leachate pH [38,39]; on the other hand, the hydration reaction consumes H⁺ in the environment during the S/S process, resulting in a decrease in leachate H⁺ and an increase in leachate pH. With the increase of curing age, the hydration reaction in the system becomes more adequate, which may also increase the leachate pH [35].

3.3. Soil EC

Figure 6 shows the effect of curing agent dosage and curing age on the conductivity of contaminated soil. The conductivity increases with the increase of curing agent dosage and decreases with the increase of curing age. The conductivity range of untreated contaminated soil is 4.83-6.92 mS cm^-1. After curing for 28 days, the conductivity range is reduced to 1.29-2.82 mS cm^-1. When MKPC treats Zn²⁺ contaminated soil (Figure 6a), the curing agent dosage increases to 8%, and the conductivity reaches 4.79 mS cm^-1 after curing for 3 days. When the curing age is extended to 28 days, the conductivity is reduced to 1.93 mS cm^-1, which is 60% lower than that of untreated initial Zn²⁺ contaminated soil. The change law of conductivity of MKPC-treated Cu²⁺ contaminated soil (Figure 6b) with curing agent dosage and curing age is basically similar to that of solidified Zn²⁺ contaminated soil. The conductivity of Cu²⁺ contaminated soil after 28 days of curing age is reduced to 1.73 mS cm^-1, which is decreased by 69.8%. The result shows that after MKPC treatment of Zn²⁺ and Cu²⁺ contaminated soil, the total amount of soluble conductive ions in the soil decreases significantly, resulting in a decrease in the conductivity of the samples. However, when MKPC treats the composite HM contaminated soils, compared with the same dosage of Zn²⁺ and Cu²⁺ contaminated soils, the conductivity of the samples increases significantly. When the curing agent dosage is 8%, the conductivity of Zn1Cu1 (Figure 6c) and Zn1.5Cu1.5 (Figure 6d) samples reaches 2.17 mS cm^-1 and 2.82 mS cm^-1 at the curing age of 28 days, respectively. It is shown that high concentrations of HM ions can increase the conductivity of MKPC solidified samples.

EC variations of MKPC solidified contaminated soil with curing time and binder dosage: (a) Zn1, (b) Cu1, (c) Zn1Cu1, (d) Zn1.5Cu1.5.

When a curing agent is added to the specimen, MgO reacts with KH₂PO₄ to form compounds of KMgPO₄·6H₂O, Mg₃(PO₄)₂·8H₂O, and the complexation with Zn²⁺ and Cu²⁺ is transformed into lower solubility Zn₃(PO₄)₂, Zn(OH)₂, and Cu(OH)₂ phosphates or precipitation. HM ions are encapsulated and immobilized, resulting in a decrease in the amount of conductive metal cations and a decrease in the total amount of electrically conductive ions [40]. With the increase of curing age, the hydration reaction is gradually completed in the S/S system; most of Zn²⁺ and Cu²⁺ are encapsulated within KMgPO₄·6H₂O products, resulting in a decrease in soluble ions, which leads to a decrease in conductivity [41,42]. In addition, when MKPC treats the soil contaminated with high concentration of Zn²⁺ and Cu²⁺, the higher the concentrations of Zn²⁺ and Cu²⁺ in the specimen, the higher the amount of Zn²⁺ and Cu²⁺, and the small amount of hydrated gel formed which is unable to encapsulate full Zn²⁺ and Cu²⁺ in the S/S process, resulting in a increase in the total amount of soluble conductive ions. As a result, the conductivity of soil contaminated by complex HMs increases.

3.4. Freeze-thaw cycle characteristics of solidified contaminated soils

3.4.1. Strength of cemented soils under freeze-thaw cycles

Figure 7 shows the effect of F-T cycles times on the UCS in solidified contaminated soil, the curing agent dosage is 8%. The UCSs decrease gradually with the increase of F-T cycles times. For MKPC-treated Zn²⁺ contaminated soil, the number of F-T cycles increases from 0 to 9 times, the UCS decreases from 1.46 MPa to 0.99 MPa, with a strength loss rate of 32.2%. In contrast, when MKPC treats Cu²⁺ contaminated soil, the change of UCS of Cu²⁺ contaminated soil with numbers of F-T cycles is basically similar to that of solidified Zn²⁺ contaminated soil, but the UCS of Cu²⁺ contaminated soil is reduced from 1.35 to 0.92 MPa with a strength loss rate 31.9%. In addition, when MKPC treats Zn²⁺ and Cu²⁺ composite HM contaminated soils, the UCS is reduced to 0.77 MPa and 0.72 MPa at the Zn1Cu1 and Zn1.5Cu1.5 samples, and the strength loss rate are 26.7% and 28.0%, respectively. And the strengths both exceed the USEPA specified limit of 0.35 MPa strength value after 9 F-T cycles.

The UCS of MKPC solidified contaminated soils under freeze–thaw cycles.

The strength loss after freeze-thaw cycling in MKPC solidified/stabilized contaminated soil can be explained by the mechanism involving micro-crack and pore coarsening, both induced by the freezing and expansion of pore water. During the freeze-thaw process, the pore water within the solidified soil freezes and expands, generating internal frost pressure that disrupts the cemented structure [43]. Upon thawing, the volume of voids within the samples increases, and the voids fail to fully recover their original soil structure after thawing [44]. Consequently, the soil structure is compromised, leading to a reduction in interparticle bonding and overall strength. Furthermore, the extent of deterioration progressively intensifies with increasing F-T cycles.

3.4.2. HMs leaching concentration under freeze–thaw cycles.

Figure 8 shows the effect of F-T cycles times on the leaching concentration of Zn²⁺, Cu²⁺ and composite contaminated soils with 8% binder dosage. The leaching concentration of Zn²⁺ and Cu²⁺ increases gradually with the increase of F-T cycles times. When MKPC treats Zn²⁺ contaminated soil (Figure 8a), the number of F-T cycles increases from 0 to 9, the leaching concentration of Zn²⁺ increases from 0.07 mg L^-1 to 0.59 mg L^-1, and the increase is 7.24 times. When MKPC treats Cu²⁺ contaminated soil (Figure 8b), the changes of Cu²⁺ leaching concentration with F-T cycles are basically similar to that of solidification of Zn²⁺ contaminated soil, but the leaching concentration of Cu²⁺ increases from 0.06 mg L^-1 to 0.78 mg L^-1, and the increase is 11.4 times. However, when MKPC treats Zn²⁺ and Cu²⁺ composite HM contaminated soils (Figure 8c and d), the leaching concentration of the composite contaminated soil increases significantly compared with that of Zn²⁺ and Cu²⁺ contaminated soil. The leaching concentrations of Zn²⁺ and Cu²⁺ increases to 31.29 mg L^-1 and 29.60 mg L^-1 for Zn1Cu1 test samples, which are increased by 13.42 times and 34.66 times, respectively. The results show that the HM concentration increases from 0.06 mg L^-1 to 0.78 mg L^-1, which is 11.4 times higher than that of the solidified Zn²⁺ contaminated soil. The leaching of Zn²⁺ and Cu²⁺ is increased by the concentration of HM ions under the action of F-T cycles. Moreover, when MKPC treats Zn²⁺ and Cu²⁺ contaminated soils, the leaching concentrations of Zn²⁺ and Cu²⁺ are still able to satisfy the upper limit of 15 mg kg^-1 prescribed by USEPA after 9 freeze-thaw cycles. However, when treating high concentrations of Zn²⁺ and Cu²⁺ composite contaminated soil, the leaching concentration of Zn²⁺ and Cu²⁺ would exceed the highest standard limit value after nine F-T cycles.

The leaching concentration of solidified contaminated soils under freeze-thaw cycles: (a) Zn1, (b) Cu1, (c) Zn1Cu1, (d) Zn1.5Cu1.5.

There are two reasons for the increase of leaching concentration of Zn²⁺ and Cu²⁺: on the one hand, with the increase in the number of cycles, during the exothermic and absorptive process, the soil volume expands, the pore structure is destroyed between the soils, and the contact area between soils and leachate increases, resulting in the increase of leaching concentrations of Zn²⁺ and Cu²⁺ [45-47]. On the other hand, the surface water of the sample invades in soils along damage cracks in the melting process, and excessive phosphate dissolves from waters to form an acidic environment in the voids. The products of MgKPO₄·6H₂O, Zn(OH)₂, and Cu(OH)₂ are dissolved in large quantities in the soils. The broken decomposition of hydrated solid gels leads to dissolution of adsorbed and enveloped Zn²⁺ and Cu²⁺ to precipitate again, and leads to an increase in Zn²⁺ and Cu²⁺ leaching concentration [43,44].

3.5. SEM-EDS analysis

Figure 9 shows the SEM-EDS image magnified 2000 times at Zn²⁺, Cu^2+, and composite contaminated soils. The shape of contaminated soil without a curing agent is mainly manifested to irregular flakes or blocks, where the soil particles are scattered and overlapping , leaving distinct spaces (yellow circles in Figure 9a). The EDS results show that the main element in the soil is silicon, and the soil itself contains elements such as magnesium and potassium (Figure 9a). With the increase of curing agent dosage (Figures 9b and c), prismatic hydration products (MgKPO₄·6H₂O, red square boxes in Figures 9b and c) can be observed overlapping with soil particles and filling the pores in the form of aggregates, and a small amount of flaky precipitates (Cu(OH)₂ and Zn(OH)₂) can be seen adhering to the soil surface (green circle in Figures 9b and c). The increase in the contents of Mg and K and the decrease in the contents of Cu and Zn in the detection area, as indicated by EDS analysis, suggest that the hydration products increase, which enclose part of Cu and Zn and fill the soil interstitial pores. This leads to an increase in soil strength and a significant reduction the leaching concentration of HMs.

SEM-EDS images of the solidified Zn1Cu1 soils with different MKPC dosage: (a) 0% MKPC, (b) 4% MKPC, (c) 8% MKPC.

Figure 10 shows the SEM-EDS images of contaminated soil samples with different concentrations during MKPC curing, and the curing agent dosage is 8%. When MKPC treats Zn²⁺ contaminated soil (Figure 10a) and Cu²⁺ (Figure 10b) contaminated soil, a few smaller voids (yellow circles in Figures 10a and b) and flaky precipitates (green circles in Figures 10a and b) appear on the surface, and a large amount of gel material (red square boxes in Figures 10a and b)exists inside of contaminated soil. The gel material is cemented to fill pores connecting the soil particles and forms a dense agglomerate structure between the soils. The EDS results reveal that Mg and K elements are predominantly concentrated in the hydrated colloidal regions, indicating that the formation of MgKPO₄·6H₂O plays a significant role in enhancing soil solidification. However, when MKPC treats Zn²⁺ and Cu²⁺ composite HM contaminated soils (Figures 10c and d), the agglomerate structure is looser than that of Zn²⁺ and Cu²⁺ contaminated soils. Within the composite contaminated soils, the numbers of smaller voids (yellow circles in Figure 10d) appearing at the edges of some agglomerates increase significantly. The reason for this phenomenon is probably that high concentrations of Zn²⁺ and Cu²⁺ inhibit chemical reactions in the S/S system, the formation amount of MgKPO₄·6H₂O (red square boxes in Figures 10c and d) and flaky precipitates (green circles in Figures 10c and d) are reduced, and thus fails to encapsulate all the Zn²⁺ and Cu²⁺. In addition, Zn²⁺ and Cu²⁺ disrupt the pores between soil particles, resulting in an increase in soil voids, which is manifested as a further decrease in strength.

SEM-EDS images of the solidified contaminated soils with different metal concentrations: (a) 8% Zn1, (b) 8% MKPC, Cu1, (c) 8% Zn1Cu1 (d) 8% Zn1.5 Cu1.5.

3.6. X-ray diffraction analysis

Figure 11 shows the XRD pattern of Zn²⁺, Cu²⁺ and composite contaminated soils. The main mineral of untreated contaminated soil is dominated by SiO₂ (Figure 11a). When the curing agent was added to the contaminated soils, the diffraction peaks of MgKPO₄·6H₂O, Mg₃(PO₄)₂·8H₂O, MgO, Zn(OH)₂, and Cu(OH)₂ can be seen in the XRD diffraction pattern (Figure 11a). This is because with the incorporation of potassium magnesium phosphate cement, MgO and KH₂PO₄ within the Zn1Cu1 contaminated soil undergo hydrolysis and combine with the water within the soil body to form MgKPO₄·6H₂O and Mg₃(PO₄)₂·8H₂O phosphate products [31]. These hydration products filled the soil particles and glued them into a whole, improving the overall compactness and mechanical properties of the stabilized soil. In addition, Zn²⁺ and Cu²⁺ interact with OH^- to form Zn(OH)₂ and Cu(OH)₂ precipitates, resulting in the appearance of Zn(OH)₂ and Cu(OH)₂ diffraction peaks. The intensities of MgKPO₄·6H₂O and Mg₃(PO₄)₂·8H₂O diffraction peaks increase slightly with the increase of curing agent dosage. When the curing age is extended to 28 days, the diffraction peaks in the pattern are MgKPO₄·6H₂O and Mg₃(PO₄)₂·8H₂O (Figure 11a), indicating that curing age shows no significant effect on the changes in types of hydration products of the MKPC-treated samples.

XRD results of stabilized contaminated soils:(a) different MKPC dosage and curing ages of the Zn1Cu1,(b) 8% MKPC dosage with different concentrations of HMs.

In addition, when MKPC treats Zn²⁺ and Cu²⁺ composite contaminated soil, the intensity of MgKPO₄·6H₂O and Mg₃(PO₄)₂·8H₂O diffraction peaks in composite samples decreases compared with that of Zn²⁺ and Cu²⁺ contaminated soils (Figure 11b). This phenomenon is explained as high concentrations of Zn²⁺ and Cu²⁺ inhibit the chemical reaction of MgO with KH₂PO₄ to a greater extent when MKPC treats composite HM contaminated soils, resulting in a slower reaction rate. During the S/S process, the amount of MgKPO₄·6H₂O decreases, resulting in a decrease in the intensity of MgKPO₄·6H₂O and Mg₃(PO₄)₂·8H₂O diffraction peaks [48]. On the whole, the curing agent dosage and curing age changes do not change the types of hydration products in the S/S system, but the high concentrations of Zn²⁺ and Cu²⁺ may inhibit the hydration reaction rates, leading to a decrease in the amount of MgKPO₄·6H₂O products formed in the S/S system. Therefore, as the concentration of Zn²⁺ and Cu²⁺ increases, the effectiveness of solidification/stabilization for contaminated soil gradually decreases, while the leaching concentration correspondingly rises.

3.7. FTIR analysis

Figure 12 shows the FTIR spectra of Zn²⁺, Cu²⁺ and composite contaminated soils. The absorption peaks at 3605.8 cm^-1 and 3617.6 cm^-1 are attributed to the O-H stretching vibrations in the binder, whereas the peak at 2363.3 cm^-1 corresponds to the O-H bending vibrations of water molecules [49], indicating that free water gradually transforms into bound water within the hydration products (MgKPO₄·6H₂O) during the MKPC hydration process. The absorption peak at 1042.2 cm^-1 is attributed to the bending vibration of PO₄^3-, confirming that MgKPO₄·6H₂O was formed. The metal-oxygen vibration peaks between 543.9 cm^-1 and 718.2 cm^-1 indicate the presence of residual MgO and suggest that Zn²⁺and Cu²⁺ may have formed metal hydroxides (such as Zn(OH)₂ and Cu(OH)₂), which further demonstrates the solidification effect of the MKPC stabilizer on contaminated soil.

FTIR results of stabilized contaminated soils.

3.8. XPS analysis

Figure 13 shows the XPS analysis of the untreated contaminated soil and the stabilized contaminated soil. The survey spectra of MKPC-stabilized soils show pronounced signals of Mg, K, P, and O, together with Zn and Cu, indicating the coexistence of phosphate gel (mainly MgKPO₄·6H₂O) and HM precipitates (Figure 13a). In the Cu2p spectrum of untreated soil (Figure 13b), the binding energies of Cu2p are respectively 954.6 eV for Cu2p_1/2 and 934.9 eV for Cu2p_3/2. After S/S ( Figures 13c and d), the binding energy of Cu2p_1/2 decreased to 954.5 eV and 954.6 eV, respectively, while that of Cu2p_3/2 dropped to 934.6 eV and 934.7 eV, respectively. It suggests that Cu is mainly stabilized as Cu(OH)₂ and may exist in Cu-phosphate phases [50]. In the Zn2p spectrum of untreated soil (Figure 13e), the binding energies of Zn2p are respectively 1045.5 eV for Zn2p_1/2 and 1022.4 eV for Zn2p_3/2.After S/S (Figures 13f and g), the binding energy of Zn2p_1/2 decreased to 1045.1 eV and 1022.2 eV, respectively, while that of Cu2p_3/2 dropped to 1045.4 eV and 1022.1 eV, respectively. The Zn 2p peaks are dominated by Zn²⁺ species associated with phosphate, consistent with the formation of Zn₃(PO₄)₂ and Zn(OH)₂ [21,51], which explains the marked reduction in Zn leachability.

(a) Wide scan XPS and high resolution results of stabilized contaminated soils: (b) Cu2p for Zn1Cu1, (c) Cu2p for Zn1.5Cu1.5, (d) Cu2p for 8%,Zn1.5Cu1.5, (e) Zn2p for Zn1Cu1, (f) Zn2p for 8%, Zn1Cu1, (g) Zn2p for 8%, Zn1.5Cu1.5.

In the Zn–Cu contaminated system, the peak area of Zn 2p for each sample is greater than that of Cu 2p. This implies that the competitive incorporation of Zn²⁺ and Cu²⁺ leads to the relative content of Zn²⁺ being higher, and it cannot be well encapsulated in MgKPO₄·6H₂O. These XPS observations are consistent with the higher leaching concentrations of Zn²⁺.

3.9. Mechanistic analysis of S/S contaminated soil

Table 4 shows the difference between this study and others in a table. The S/S mechanism of MKPC for contaminated soil involves two primary processes: physical encapsulation and chemical fixation. During hydration, MgO reacts with KH₂PO₄ to generate MgKPO₄·6H₂O as the dominant binding phase, along with minor amounts of Mg₃(PO₄)₂·8H₂O (Figure 14). These hydration products effectively fill pore spaces and bind soil particles, constructing a dense and cohesive matrix (Figure 12) that immobilizes contaminants through physical entrapment [37]. In terms of chemical fixation, the alkaline environment generated by the dissolution of MgO promotes the formation of metal hydroxide precipitates (such as Cu(OH)₂ and Zn(OH)₂), as evidenced by the results shown in Figure 11. Furthermore, it has been reported that phosphate ions react with Cu²⁺ and Zn²⁺ to form insoluble metal phosphates such as Zn₃(PO₄)₂, Zn₂(OH)PO₄, and Cu₃(PO₄). However, in this study, the presence of such phases was not detected. This might be attributed to the extremely low concentration of PO₄^3- in the MKPC system, which was rapidly coordinated and combined with Mg²⁺ to form the MgKPO₄ structure. Therefore, the metal ions in the system were more inclined to react with OH^- to form hydroxides (such as Zn(OH)₂ and Cu(OH)₂) rather than insoluble phosphates [21]. Furthermore, the content of metal phosphates is relatively low, and it partially overlaps with the main peak region of MgKPO₄·6H₂O. In the mixed system, it is easily masked, and thus its characteristic peaks may not be distinguishable in XRD.

Table 4. Comparison of results of MKPC S/S HM pollution.

Ref.	Pollutant types	Materials	Treated matrix	Variable	Main conclusion
[30]	Cu²⁺/Cu(NO₃)₂	MKPC (M/P=4:1)	MKPC paste	The concentration of Cu(0%-4%)	Compressive strength: 50.22-24.12 MPa Leaching concentration: 0.5-1.7 mg L^-1
[50]	Zn²⁺/Zn(NO₃)₂	MKPC (M/P=6:1)	MKPC paste	The concentration of Zn(0%-3%)	Compressive strength: 77%-169% Leaching concentration: 0.001-0.07 mg L^-1
[21]	Zn²⁺/Zn(NO₃)₂ Pb²⁺/Pb(NO₃)₂	MKPC (M/P=4:1)	SIR-MKPC	The concentration of composite pollutants (0.5%-2% Zn and Pb)	Leaching concentration: 0.06-0.08 mg L^-1 (Pb²⁺); 0.2-0.9 mg L^-1 (Zn²⁺)
[24]	Cu²⁺/Cu(NO₃)₂ Zn²⁺/Zn(NO₃)₂ Cd²⁺/Cd(NO₃)₂ Pb²⁺/Pb(NO₃)₂	MKPC (M/P=3:1)	MKPC paste	Different types of HMs(Cu, Zn, Pb, Cd)	Compressive strength: 28.5MPa (Cd), 26.0MPa (Cu), 23.0 MPa (Pb), 18.5MPa (Zn) Stabilization efficiency: Pb > Cu > Zn > Cd
This study	Cu²⁺/Cu(NO₃)₂ Zn²⁺/Zn(NO₃)₂	MKPC (M/P=4:1)	Soil	The concentration of Cu (10000mg L^-1 and 15000mg L^-1) and Zn(10000mg L^-1 and 15000mg L^-1); Different dosages of MKPC (0-8%)	Compressive strength: 331-1351 kPa (Cu1) 338-1460 kPa (Zn1) 284-1056 kPa (Cu1Zn1) 272-987 kPa (Cu1.5Zn1.5) Maxing Leaching concentration: 0.06 mg L^-1 (Cu1) 0.07 mg L^-1 (Zn1) 0.83 mg L^-1 (Cu1Zn1,Cu) 1.69 mg L^-1 (Cu1Zn1,Zn) 2.81 mg L^-1 (Cu1.5Zn1.5,Cu) 6.56 mg L^-1 (Cu1.5Zn1.5,Zn)

The S/S mechanisms of the Cu2+ and Zn2+ contaminated soil.

The competitive behavior among HMs has a certain impact on the MKPC S/S process. Compared with Zn²⁺, in the reaction processes of the S/S system, Cu²⁺ is more likely to be deposited or adsorbed on the hydration product surfaces, resulting in a greater influence of MgKPO₄·6H₂O on Cu²⁺ adsorption and a smaller influence on Zn²⁺ adsorption [52,53]. The long-term stable existence of HM phosphate minerals in soil [54] results in a higher leaching concentration of Zn²⁺ than that of Cu²⁺. As for Zn1Cu1 composite contaminated soil, the leaching concentrations of Zn²⁺ and Cu²⁺ are higher, which is probably due to the competition between Zn²⁺ and Cu²⁺ in composite contaminated soils, and the mutual expression of antagonism. The presence of Zn²⁺ inhibits the adsorption of hydration products by Cu²⁺, or the presence of Cu²⁺ inhibits the adsorption of hydration products by Zn²⁺, and the competitive adsorption capacity of Cu²⁺ is greater than that of Zn²⁺. The result is as shown in Figure 5 that the leaching concentration of Zn²⁺and Cu²⁺ > leaching concentration of Zn²⁺ > leaching concentration of Cu²⁺ in the solidified contaminated soils.

4. Conclusions

In this paper, the effects of various Zn²⁺, Cu²⁺, and composite contaminated soil concentrations, curing agent dosage, and curing age on the strength, conductivity, leaching concentration, and microstructure of contaminated soils solidified and treated by MKPC were investigated. The durability research of MKPC solidified Zn²⁺, Cu²⁺, and composite contaminated soil under F-T cycle conditions was simulated, and the following conclusions were drawn:

MKPC can significantly enhance the compressive strength of contaminated soil. At the dosage of 4% MKPC, the strength of the highly concentrated composite contaminated soil (0.47 MPa) after 7 days of curing satisfies the requirements of a solid waste landfill (0.35 MPa). When HM pollution changes from single Zn²⁺ or Cu²⁺ pollution to Zn-Cu contaminated pollution, the compressive strength of the sample decreases by approximately 21.5%-32.2% compared to the single-pollution system after 28 days of curing. The synergistic effect between Zn and Cu further weakened the strength of the soil.
The MKPC curing agent can effectively reduce the leaching concentration of Zn²⁺ and Cu²⁺ in contaminated soil. When the MKPC is used to treat single-Cu or single-Zn contaminated soil, the leaching concentration of Zn²⁺ and Cu²⁺ of TCLP is lower than the limit value specified by USEPA (15 mg L^-1) at a dosage of 4% MKPC. However, due to the competition between Zn²⁺ and Cu²⁺ in composite contaminated soil, and the competitive adsorption capacity of Cu²⁺ is greater than that of Zn²⁺, the leaching concentration of Zn²⁺ and Cu²⁺ reached below the standard limit value with the dosage of curing agent greater than 8%.
The reaction between MgO and KH₂PO₄ in the curing system continuously generates alkaline products, resulting in a continuous increase in the pH of the leaching solution. Insoluble phosphates continuously precipitate and fix HM ions, reducing the number of mobile ions in the solution and causing the EC to show a gradually decreasing trend, further indicating that MKPC can fix free HM ions in contaminated soil.
MKPC-stabilized contaminated soils maintain engineering stability after nine F-T cycles, and the compressive strength of samples exceeds the USEPA threshold of 0.35 MPa. With increasing F-T cycles, a gradual reduction in specimen strength was observed, accompanied by a concurrent rise in HM leaching concentrations. The leaching levels of Zn²⁺ (0.59 mg L^-1) and Cu²⁺ (0.78 mg L^-1) from single-metal contaminated soils remained below the USEPA regulatory limit of 15 mg L^-1. In contrast, the leaching concentration of high-concentration Zn-Cu composite contaminated soil increases more significantly due to the combined effects of ion antagonism and freeze-thaw damage, potentially resulting in leaching concentrations exceeding regulatory limits after repeated F-T cycles.
Microstructural analysis confirmed that the MgKPO₄·6H₂O gel products during the hydration process of MKPC can fill the internal voids of contaminated soils, make the soil structure dense, and improve the strength of contaminated soils. When MKPC treats Zn²⁺, Cu²⁺, and composite contaminated soils, it mainly encapsulates and immobilizes HM ions through chemical precipitation, physical encapsulation, and complexation in the soil.

Overall, this study demonstrates that MKPC exhibits considerable potential for the solidification/stabilization of Zn-Cu co-contaminated soils, offering the reference value for managing multi-metal contaminated soils in practical engineering applications. However, the current research is based on artificially prepared systems under laboratory conditions, where pollutant compositions may differ from those encountered in real-world contaminated sites. Furthermore, actual factors such as long-term environmental durability, water retention performance, and scalability for large-scale implementation have not been fully addressed. Therefore, future studies should prioritize in-situ field validation, Soil water contact angle test, and full life cycle cost analysis, and life cycle cost analysis of the MKPC-based solidification system to comprehensively assess its engineering applicability in complex contamination scenarios.

Acknowledgment

The authors acknowledge financial support from the Natural Science Foundation of China (No. 42307219).

CRediT authorship contribution statement

Cuihong Li: Writing-review editing. Minhui Ye: Writing-original draft. Chengxiong Wu: Writing-original draft. Wei Wang: Writing- review editing. Shaoyun Pu: Formal analysis, Data curation. Na Li: Data curation. Ping Jiang: Supervision. Yubo Zhou: Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

Data will be made available on request.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of AI-assisted technology for assisting in the writing of the manuscript and no images were manipulated using AI.

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Engineering and environmental behavior performance of magnesium potassium phosphate cement binder solidification/stabilization Zn-Cu contaminated soils

Abstract

Keywords

Leaching concentration

Magnesium potassium phosphate cement

Solidification/stabilization

Strength

Zn2+and Cu2+contaminated soils

1. Introduction

2. Materials and Methods

2.1. Contaminated soil preparation

2.1.1. Soil

2.1.2. Composite contaminated soils

2.2. Binders

2.3. Experimental scheme

2.4. Preparation of specimens

2.5. Test methods

2.5.1. UCS tests

2.5.2. TCLP tests

2.5.3. EC tests

2.5.4. Freeze-thaw cycle tests

2.5.5. SEM-EDS analysis

2.5.6. XRD analysis

3. Results and Discussion

3.1. UCS

3.2. TCLP and pH

3.3. Soil EC

3.4. Freeze-thaw cycle characteristics of solidified contaminated soils

3.4.1. Strength of cemented soils under freeze-thaw cycles

3.4.2. HMs leaching concentration under freeze–thaw cycles.

3.5. SEM-EDS analysis

3.6. X-ray diffraction analysis

3.7. FTIR analysis

3.8. XPS analysis

3.9. Mechanistic analysis of S/S contaminated soil

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

Data availability

Declaration of generative AI and AI-assisted technologies in the writing process

References

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