Analysis of the Effect of Soil Remediation Processes Contaminated by Heavy Metals in Different Soils

Abstract

Heavy metal pollution in China’s soil is very serious, and soil remediation is urgent. At present, most of the domestic and foreign research is aimed at one soil type for soil heavy metal pollution remediation. However, the distribution of heavy metals and the effect of remediation with chemical agents are different for different soils. This study is committed to investigating the effect of WTF on the remediation of heavy metal contamination in different soils based on the existing research in the laboratory. The influence of soil quality on remediation efficiency was analyzed by TCLP leaching of heavy metals, and different forms of heavy metals were extracted from the soil using the BCR method. The experimental results showed that the soil environment was alkaline, and the response to a low addition of WTF was more obvious. The Pearson correlation coefficient analysis yielded that the increase in the organic matter content led to an increase in the oxidizable Cd content. The WTF remediation did not change the overall acidity and alkalinity of the soil so that the overall soil environment remained stable; it increased the organic matter content and added fertility to the soil, and it increased the activity of most enzymes in the soil and promoted the circulation of the soil elements, making the soil fertile.

1. Introduction

Soil, as an important component of the ecosystem, has a certain self-regulatory capacity of its own. Heavy metals reach the soil environment through geological factors and anthropogenic processes. Soil-forming parent material, sediment weathering, and other geological processes constitute the background values of soil heavy metal contamination [1]. Most metal substances are naturally present in the soil parent material, mainly in forms that are not absorbed by plants. Due to the low solubility, the metals present in the parent material are usually not absorbed by plants and have minimal impact on soil organisms. Unlike soil inputs, metals added through anthropogenic activities are usually highly bioavailable. Anthropogenic activities, mainly related to industrial processes; manufacturing; and household, agricultural, and industrial waste disposal, are the main sources of metal enrichment in soils [2,3,4,5]. The heavy metal content in soil has long exceeded the self-regulation and recovery capacity of the soil itself. Health authorities in many parts of the world are increasingly concerned about the impact of heavy metals on the environment and human health and their potential impact on international trade. For example, Cd accumulation in animal offal (mainly kidney and liver) not only makes it unfit for human consumption but also jeopardizes the entry of offal products into overseas markets. Similarly, Cd bioaccumulation in potato, wheat, and rice crops has serious implications for local and international commodity marketing [6]. For these reasons, there is an urgent global need to ensure that the heavy metal contents of produced foods meet the regulatory standards and are comparable to products from other countries [5]. Among the heavy metals, lead is one of the most easily accumulated and nondegradable toxicants. In addition to natural pathways, Pb is emitted from a variety of anthropogenic activities, including mining, ore smelting, coal combustion, wastewater from the battery industry, vehicle exhaust, metal plating, leather tanning, fertilizers pesticides, pesticides, and additives in pigments and gasoline [7]. Furthermore, 88% of lead is used in the manufacture of lead–acid batteries (LABs). People from low- and middle-income countries such as Bangladesh experience the most serious health risks from LAB recycling, as this recycling process is mainly carried out in poorly controlled or unregulated industries [8]. When absorbed, it can bind to a variety of enzymes and affect the physiological activities of the body; seriously damage the nervous, digestive, immune, and reproductive systems; and even cause cancer [9]. In summary, the remediation of soil heavy metal contamination is critical. Among them, the remediation of soils contaminated with Cd and Pb is crucial.

Currently, the more common methods for the remediation of heavy metal-contaminated soils include physical remediation methods, bioremediation, and chemical remediation methods [10]. Improving microbial communities through the application of nanoparticles is another way of reducing/removing toxic pollutant loads from soils. However, the long-term exposure and accumulation of these nanoparticles in the soil may affect soil nutrient and organic matter contents [11]. Chemical stabilization/solidification remediation of heavy metal contamination is a simpler, more efficient, and widely applicable way to mitigate toxicity and reduce the bioavailability of heavy metals in contaminated soils [6]. This method is an in-situ remediation method, which does not change the location of contaminants in the soil and reduces the extent to which heavy metals can be contaminated by immobilizing them through the addition of chemical agents. Existing research recognizes the critical role played by water-soluble thiourea formaldehyde (WTF). The experimental investigation yielded that the chemical and geometric structure of the WTF resin was found to be favorable for the passivation/stabilization process [12]. Microbial degradation experiments have demonstrated that WTF has little effect on soil microorganisms and can promote microbial colonization by providing a carbon source [13]. A comparison of the different soil remediation techniques is shown in

Table 1. Comparisons of different repair technologies.

Technology	Process	Advantages	Limitations
Soil replacement	Digging up contaminated soil and replacing it with non-contaminated soil	It effectively isolates heavy metals from contaminated sites and is effective on highly contaminated soils.	It is heavy and costly, generates hazardous waste, and has a negative impact on the soil.
Electrochemical remediation	Removal of heavy metals from the soil by electrophoresis or application of DC voltage	It is cost-effective, easy to operate and does not damage the soil properties.	It requires soil with low permeability and pH control.
Stabilization	Application of sorbents to reduce heavy metal mobility and bioavailability, forming stable and immobilized complexes	It is quick and easy to operate, relatively low cost and covers a wide range of inorganic pollutants.	To prevent secondary release, it requires prolonged monitoring.
Drenching	Removal of heavy metals from soils by extractants and formation of stable and mobile complexes	It completely removes heavy metals, complies with specific standards and reduces long-term liability.	Washing extractants can cause environmental problems and the effectiveness of treatment varies depending on the soil, the metal and the type of extractant.
Plant extract	Using super-enriched plants to uptake, transfer and concentrate heavy metals from the soil to the above-ground harvestable plant fraction	Highly economical, environmentally friendly and less disruptive	The effectiveness depends on the growing conditions, the tolerance of the plant and the bioavailability of the metal in the soil.

.

Most of the current national and international studies have been conducted on one soil type for the soil remediation of heavy metal contamination or to study the effect of soil amendments on the composition and properties of the soil and whether the use of these amendments will have any negative impact on naturally occurring organisms and the ecology of the soil. Nano silica amendment can significantly reduce the soil bulk density and increase porosity, thus facilitating soil changes that are more beneficial for crop growth [14]. Thermal remediation can change the functional capacity of the soil. Optimal heating times and temperatures are different for different types of contaminants, and increasing either the time or temperature can lead to the deterioration of the soil quality [15]. The effect of biochar on heavy metal concentrations in plants varied depending on the soil properties and metal contaminants. The greatest decrease in plant heavy metal concentrations was observed in coarse-textured soils amended with biochar. When biochar was applied to alkaline soils, it had a significant effect on plant copper concentrations. Plant uptake of Pb, Cu, and Zn was less in soils with higher organic carbon contents [16]. Significant positive correlations were found between HMs and some soil properties (i.e., clay content, cation exchange capacity, organic matter, total phosphorus content, etc.), indicating that the modulation of these properties has a strong influence on the behavior and availability of HMs [17].

In fact, different soils have different effects on the distribution of heavy metals and the effect of using passivation agents. In addition to the effect of the WTF performance, soil texture also has a great influence on the remediation process and the distribution of heavy metals. Studies have shown that soil pH, texture, and crop type have an impact on the biochar performance [18]. Texture determines the sorption capacity of soils and, as a result, the microbiological activity of soils [19]. Additionally, the soil content of sand, silt, and clay fractions is highly important, because it stimulates the dynamics of nutrients [20]. Global soil salinity concentrations have increased significantly due to the overuse of pesticides. It is estimated [21] that, by 2050, more than 50% of soils will be affected by salinization. The increase in the soil salinity content is a serious risk to soil health, thereby massively reducing crop yields [22]. In addition, soil heavy metal contamination is one of the global concerns [23].

At present, there are many studies on the remediation of heavy metal pollution in soils, and most of the domestic and international studies have chosen a single soil sample; however, the distribution of heavy metals and the remediation effect of chemicals vary from one soil to another. At the same time, the distribution of heavy metals between different forms is linked to the composition and properties of the soil and the chemical properties of the metals, but there is a lack of targeted research on them. Therefore, this study focuses on exploring the effects of WTF remediation agents on the remediation of heavy metal contamination in different soils, comparing the effects of different soil textures on the remediation of heavy metal contamination in soils through heavy metal passivation efficiency and heavy metal morphological distribution. the Pearson correlation analysis shows that the soil texture has a great influence on the remediation efficiency and the distribution of heavy metal morphology before remediation. This study will provide a theoretical basis and inspiration for the WTF to be used more widely and for the study of soil component–metal interactions.

2. Materials and Methods

2.1. Experimental Material

Different surface soils (0–20 cm) were collected from Xian, Shanxi Province (34.26° N, 108.95° E), Shangqiu, Henan Province (34.44° N, 115.65° E), Changsha, Hunan Province (28.12° N, 112.59° E), Binhai District, Tianjin (39.03° N, 117.70° E), and Jinnan District, Tianjin (38.58° N, 117.19° E). The physicochemical properties of the soils used are shown in

Table 2. The physicochemical properties of the soil.

Depth (cm)	Sand (%)	Powder (%)	Clay (%)	Cd (mg/kg)	Soil Texture	Source
0–20	80.53	17.9	1.55	0.776	Loamy sand 1	Shanxi
0–20	77.65	21.34	1.01	2.61	Loamy sand 2	Henan
0–20	59.37	19.27	21.37	4.32	Sand clay loam	Hunan
0–20	41.70	34.90	23.40	1.29	Loam	Tianjin
0–20	9.00	48.00	43.00	2.49	Silty clay	Tianjin

.

WTF was prepared under the best synthesis route as previously reported [24]. Thoroughly mix formaldehyde and thiourea (in a molar ratio of 2.8:1) at 60 °C and neutral pH for 30 min. Then, change the pH to 4.5–5.0 and heat at 80 °C until the end, which was judged by turbidity titration. Adjust pH = 7.0, and add 7% melamine to react. In the whole synthesis, pH was adjusted by sodium hydroxide solution and hydrochloric acid solution. The chelation mechanism of WTF can be obtained from previous research [12,13].

2.2. Soil Samples Processing

The cadmium nitrate (Cd (NO₃)₂) and lead nitrate (Pb (NO₃)₂) were used for contaminated soil preparation. Air-dried soil samples (100 g) were placed in ceramic pots after the cadmium nitrate and lead nitrate and a certain amount of water was added to make the samples’ Cd²⁺ concentration 5 mg/kg and Pb²⁺ concentration 1000 mg/kg. After mixing for 4 weeks, 1.0 wt%, 2.0 wt%, 3.0 wt%, and 5.0 wt% WTF were added into the soil containing Cd, and 1 wt.%, 2 wt.%, 3 wt.%, 5 wt.%, 7 wt.%, and 10 wt.% WTF were added to the soil containing Pb, respectively, and cured for 2 weeks. The soil that was not anthropogenically contaminated was called the origin soil, the soil before remediation after anthropogenic contamination was used as a blank control group (Cd-CK or Pb-CK), and the soil sample after remediation with the optimal amount of WTF was the experimental group (Cd-WTF or Pb-WTF). Each treatment had three replicates. Add 50 mL of water per 100 g of soil once a week. After the incubation, the soil was sampled and air-dried for further batch experiments.

2.3. Characterization and Analysis

2.3.1. Characterization of Soils

The crystalline compositions of the soils were determined using an X-ray diffractometer (XRD) (D8 Advance, Bruker, German). The samples were scanned over the range of 5–90° of 2θ at a rate of 2° min⁻¹.

2.3.2. Analytical Methods

The toxicity characteristic leaching procedure test [25] uses a leaching agent to extract heavy metals from the soil, evaluating the leaching performance of heavy metals. The determined metals include Cd and Pb. The metal was extracted using a glacial acetic acid solution with a pH of 2.88 ± 0.05. The solid–liquid ratio is 1:20. Vibrate the mixture at a temperature of 23 ± 2 °C and 30 ± 2 rpm/min for 18 ± 2 h. Then, the solution by centrifuging and filtrating was detected by an inductively coupled plasma-mass spectrometer (ICP-OES) [26] to obtain the concentrations of Cd²⁺ and Pb²⁺ in the leachate. To evaluate the heavy metals immobilized in the soils after stabilization treatments, a stabilization rate is defined as Formula (1).

$$\mathrm{stabilization}\mathrm{rate}=\frac{{\mathrm{C}}_0-{\mathrm{C}}_1}{{\mathrm{C}}_0}\times 100\%$$

(1)

where C₀ and C₁ are the leaching concentrations of heavy metal before and after the treatment.

The Cd and Pb speciation analysis [27] before and after the soil repaired by WTF was analyzed by the BCR sequential extraction method [28,29]. In this method, F1 represents the acid-soluble fraction (including water-soluble fraction and exchangeable fraction, 0.11 M CH₃COOH, stirring for 16 h (soil:solution, 1:40)), F2 is the reducible fraction (0.5 M NH₂OH·HCl, stirring for 16 h, pH 2 adjusted by adding 2.5% (vol/vol) HNO₃ solution (soil:solution, 1:40)), F3 is the oxidizable fraction (8.8 M H₂O₂ with heating (1 h at 85 ± 2 °C) and evaporation to almost dry state; 50 mL of 1 M NH₄CH₃COO, pH 2, acidification with 2.5% (vol/vol) HNO₃ solution (soil:solution, 1:70)), and F4 is the residual fraction (aqua regia (37% HCl and 70% HNO₃) added after evaporation (soil:solution, 1:9)). The concentrations of heavy metals in the leachate were also detected by ICP-OES.

The pH value of the soils was determined in the soil–water mixture with a solid–liquid ratio of 1:10 (1 g:10 mL) by a pH meter. In order to determine the organic carbon content of the soil, the organic carbon–potassium dichromate oxidation spectrophotometric method [30] was used. Under heating conditions, the organic carbon in the soil sample was oxidized by an excess of potassium dichromate–sulphate acid solution. The hexavalent chromium in the potassium dichromate was reduced to trivalent chromium, the amount of which was proportional to the amount of organic carbon in the sample. The absorbance was measured at 585 nm, and the organic carbon content was calculated from the amount of trivalent chromium.

The mass difference method was used to calculate the content of water-soluble salts in soil. The evaporating dishes were dried in an oven at 105–110 °C for 12 h and then weighed. Then, a certain amount of soil extract was absorbed in the evaporating dish, dried in a water bath, oxidized by hydrogen peroxide, and weighed again after drying in an oven. The water-soluble salt content can be obtained according to the two mass differences.

To investigate the mechanism of water-soluble salt reduction, the precipitate formed by the reaction of WTF with Pb²⁺ was prepared in this experiment, and the adsorption of the precipitate in the salt solution was simulated, and the Zeta potential on the surface of the precipitate was measured. WTF was reacted with Pb (NO₃)₂ solution with a Pb²⁺ content of 1000 mg/L, and the precipitate of WTF with Pb²⁺, hereafter referred to as WTF-Pb, was obtained by centrifugation and oven drying at 40 °C for 8 h. The electrical properties of the WTF-Pb surface were tested at the corresponding soil pH values. Different soil types (acidic, neutral, and alkaline soils) were simulated, and the pH range was set from 6.74 to 8.37. The adsorption process of WTF-Pb on the anions in water was simulated on this basis. Although the experimental soil is saline, in most cases, the salinity of the soil is mainly caused by sulfate or chloride. Therefore, sulfate was chosen as the simulated environment for this experiment. The adsorption simulation of WTF-Pb with ions was carried out in 10 mg/L CaSO₄, and after 24 h of reaction, the ion chromatograph was used to detect the content of SO₄²⁻ in the supernatant and compare the data before adsorption.

In this experiment, the indophenol blue colorimetric method was used to determine the urease activity [31] in soil, and 10 g of urea was fixed in deionized water to 100 mL as the urease solution. Add 15 μL of urease solution prepared with pH 7.4 phosphate buffer and 135 μL of sample solution (37 °C, 15 min), then add 850 μL of urea solution and react for 60 min at 37 °C. The reaction was terminated by adding 500 μL of solution A (0.5 g phenol and 2.5 mg sodium nitroprusside dissolved in 50 mL of distilled water) and 500 μL of solution B (50 mL of distilled water with 250 mg of sodium hydroxide and 820 μL of 5% sodium hypochlorite) and incubating at 37 °C for 24 h. The optical density (OD) of the urease activity was measured at 578 nm. The method for the determination of the soil alkaline phosphatase was the disodium phenyl phosphate colorimetric method [32]. The 0.1 M p-nitrophenol phosphatase solution (pH 11) was used as a substrate. Alkaline phosphatase activity was determined by treating 10 g of soil with 2.5 mL of toluene and 10 mL of p-nitrophenol phosphatase solution. After 1 h incubation at 37 °C, 0.5 M CaCl₂ and 0.5 M NaOH solutions were added. The suspension was filtered, and the color intensity of the filtrate was measured at 420 nm. The method for the determination of the soil sucrase activity was the colorimetric method of dinitrosalicylic acid [33], and 5 g of air-dried soil was reacted with 15 mL of sucrose, 5 mL of phosphate buffer (pH 5.5), and 5 drops of toluene at 37 °C for 24 h. After, 1 mL of filtrate was mixed with 3 mL of salicylic acid in a water bath at 100 °C for 5 min, and the mixture was brought to 50 mL and cooled with deionized water. The OD of the sucrase activity was measured spectrophotometrically at 508 nm.

The relationship between properties was established using the Pearson coefficient [34].

3. Results and Discussion

3.1. Characterization of Soils

The soil types and components of the five soil samples selected in this experiment were analyzed, as shown in Table 1. According to the soil texture classification from the United States Department of Agriculture (USDA) [35], the soils studied in this research can be divided into four types: loamy sand; sand clay loam; loam; silty clay. From loamy sand 1 to silty clay, the sand content gradually becomes lower, and the clay content generally becomes higher, which has resulted in a decrease of soil permeability.

The XRD spectra of five soils were obtained (Figure 1). The purpose of the XRD analysis of the uncontaminated soil was to show that the five soils contained different crystals, in addition to their mechanical components, which helped to distinguish the five soils. The XRD results showed that the primary phase of the five soils is quartz. However, there are slight differences in the crystal types of the five soils. Loamy sand 1 and 2 both have the same crystal composition, except for quartz; both contain NaAlSi₃O₈ and Fe₅Al₄Si₆O₂₂(OH)₂. Loam and silty clay both contain NaAlSi₃O₈. Loam also contains some Fe₅Al₄Si₆O₂₂(OH)₂ and a little CdI₂. NaAlSi₃O₈ is an aluminosilicate mineral with a framework silicate structure and is a kind of alkaline ore [36]. Fe₅Al₄Si₆O₂₂(OH)₂ is a kind of silicate mineral which is named orthopyroxene [37]. Studies have shown that silicates contribute significantly to the adsorption and retention of metals [38].

3.2. The Ability of WTF Stabilizing Cd in Different Texture Soils

In the different texture soils, the stabilization rates for different WTF additions are shown in Figure 2. With the increase of the WTF addition, the stabilization rates increase in different soils. However, the optimal WTF addition and final stabilization rate have some differences, respectively. There is no obvious increase from the 3 wt% addition to 5 wt% addition. Additionally, except for sandy clay loam, others show high rates of stabilization (over 80%) at the 2 wt% addition. Thus, the research selected the 2 wt% addition as the optimal WTF addition for the follow-up research, except for the sandy clay loam soil, in which 3 wt% was chosen as the optimal WTF addition.

The stabilization trends of heavy metals in loam and silty clay soils in Tianjin are similar, and it can be inferred that the stabilization trends of heavy metals in saline and non-saline soils at the same location by WTF are not influenced by the soil salt content, and there is only a small difference in the stabilization rate. The stabilization trends for heavy metals in loamy sand 1 and loamy sand 2 soils are approximately the same, and in relation to the differences in the sand, powder, and clay contents of the different soils, the WTF stabilization trends are the same for different areas with the same soil type and apparently different for different soil types. The soil texture influences the efficiency and trend of stabilization of heavy metals by WTF. Many studies have shown that the soil sorption of heavy metals is influenced by the clay content of the soil. Zhang Liu-Dong [39] selected two soils in Shanxi Province for Cd sorption experiments with different grain sizes, and the results showed that the higher the clay content of the soil sample, the stronger the adsorption of Cd. Wang Lan [40] investigated the sorption characteristics of Cd and Pb in three different soil textures. The results showed that Cd and Pb were most strongly adsorbed in powdered clayey soil.

The distribution of heavy metal forms before and after the remediation of Cd contamination in different soils is shown in Figure 3. The F1 form of Cd in the soil was greatly reduced after WTF remediation, indicating that the more ecotoxic Cd was transformed into other less ecotoxic forms, thus achieving the stabilization of heavy metals. Among them, loamy sand 2, loam, and silty clay after remediation, the F1 form of Cd was transformed into the F3 and F4 forms, indicating that the Cd in the remediated soils tended to chelate with organic matter in the soil or be adsorbed by silicate and soil lattice. In the sandy loam 1 and sandy clay loam, the F1 form of Cd tends to be converted to the F2 form, indicating that Cd tends to be adsorbed and coprecipitated with Fe-Mn oxides in the soil.

At the same time, the distribution of Cd forms before restoration was also different. Before restoration, the percentage of F1 form in sandy loam 1 was the highest compared with the percentage of the same form in other soils. The highest percentage of F1 form was 46.7% in sandy loam 1, and the highest percentage of F2 form was 40.0% in silty clay. The distribution of heavy metals between the different fractions is determined by the composition and nature of the soil, the degree of contamination and the chemical nature of the metals [41].

To ensure that WTF does not damage the soil function, the experiment investigated the effect of soil quality on heavy metal remediation, along with the changes in the soil brought by WTF remediation. An ecological risk assessment of the restored soil was carried out.

The soil pH before and after the remediation of Cd contamination in different soil types and before and after pollution are shown in Figure 4. Among them, the sandy clay loam is acidic, and the pH of uncontaminated soil is 5.44, the loam is alkaline, and the pH of uncontaminated soil is 8.55. Most of the other soils are neutral soils with uncontaminated soil pH of 7.39–8.21. After the addition of Cd contaminants, the soil pH increased slightly, ranging from 0.18 to 1.42. The pH of the soil decreased slightly after WTF was added, ranging from 0.08 to 0.6. However, in general, it was similar to the pH of the soil before pollution. This indicates that the pH of the soil after remediation was restored to the pre-pollution level. The sand clay loam is a typical lateritic soil, which is acidic, rich in iron and aluminum oxides, and lacking in alkali and alkaline earth metals. In relation to the results of WTF stabilization rates for different soil types, it can be deduced that acidic soils are less sensitive to the low use of WTF and alkaline soils to the low use of WTF. The stabilization rate of heavy metals in alkaline soils can reach more than 50% when WTF is added at 1%.

The organic matter content of the soil before and after the remediation of Cd contamination in different soils and before and after the pollution are shown in Figure 5. The organic matter content of loamy sand 1 and loamy was low, respectively, 0.30% and 0.81% before contamination; the organic matter content of loamy sand 2 and silty clay was high, respectively, 1.50% and 2.11%; and the organic matter content of sandy clay loam was 1.30%. The organic matter of the soil increased slightly after the artificial addition of Cd contamination; when the WTF was added to the remediation, the organic matter of the soil increased significantly and was much higher than the soil before the contamination. Part of the heavy metal Cd was transformed into the combined state with organic matter (F2 form), which led to the increase of the soil organic matter after contamination. After WTF remediation, on the one hand, WTF itself as a carbon source replenished the organic matter. On the other hand, part of the heavy metal Cd was converted from other forms to F3 after remediation, which led to a small increase in organic matter. Overall, the increase in organic matter by WTF as a carbon source was the main reason. The increase in the organic matter led to an increase in soil fertility [42], indicating that the soil function was improved after WTF restoration.

3.3. Influence of Soil Conditions on the Restoration Process

Changes in the soil permeability and mechanical composition have different degrees of influence on the distribution of heavy metals and the remediation process. In addition to this, the soil pH, organic content, and crystals contained in the soil also have a great influence on the distribution of heavy metal morphology. Therefore, the discussion focuses on the effects of different soil quality factors on the stabilization rate and heavy metal morphology distribution, and the correlation statistics are shown in

Table 3. Correlation of different soil quality factors on the stabilization rate and heavy metal forms distribution.

	Organic Content	pH	Sand (%)	Powder (%)	Clay (%)	XRD 1
Stabilization rate	−0.195	0.878 *	−0.011	0.272	−0.182	0.677
BCR-F1	0.213	0.635	−0.181	0.421	−0.008	0.698
BCR-F2	0.973 **	−0.451	−0.686	0.619	0.696	−0.694
BCR-F3	−0.464	−0.497	0.636	−0.804	−0.475	−0.469
BCR-F4	−0.899 **	0.051	0.520	−0.607	−0.426	0.122

¹: XRD represents the abundance of crystal species contained in the soil, * represents a significant correlation at the p < 0.01 level, and ** represents a significant correlation at the p < 0.001 level. BCR F1—acid soluble state, F2—reducible state, F3—oxidizable state, and F4—residue state.

.

From the correlation coefficient analysis, the soil pH was positively and strongly correlated with the stabilization rate of WTF restoration and the acid-soluble state of Cd. In other words, when the soil pH decreased, the acid-soluble state of Cd in soil decreased. The stabilization rate was lower when WTF stabilized the acid-soluble fraction of Cd. This means that WTF has a reduced ability to stabilize heavy metals at a low pH but has a higher remediation efficiency at a high pH, which is consistent with the results of the previous experiments of the group. The soil organic content mainly influenced the four forms of heavy metal Cd in the soil and had a strong correlation with the reducible and residual states. The soil mechanical composition influenced the distribution of heavy metal Cd, mainly affecting the reducible, oxidizable, and residual states, with the greatest influence of the powder content, followed by the sand content and, finally, the clay content. At the same time, the number of soil crystals had a strong influence on the stabilization rate and morphological distribution of heavy metals.

In summary, a higher soil pH and richer soil crystal types are conducive to the WTF remediation of Cd-contaminated soil and can achieve a higher stabilization rate. The decrease of organic matter, the change of the soil mechanical composition (decrease of powder and clay content and increase of sand content) and the increase in the variety of soil crystals will lead to the increase in the more stable forms of Cd (oxidizable and residual states), making remediation less difficult to remediate. It was demonstrated that the soil quality has a great influence on the efficiency of soil remediation and the morphological distribution of heavy metals before remediation.

3.4. Remediation Effect of Cd- and Pb-Contaminated Saline Soils

The stabilization rates for different WTF additions are shown in Figure 6. With the increase of the WTF addition, the stabilization of different metals first increased substantially, and then, the increasing trend slowed down. When WTF was added at 2.0 wt.%, the stabilization rate of Cd reached a high level of 86.9%. However, when WTF remediated Pb contamination, more heavy metals required more remediation agents, so when WTF was added at 7.0 wt.%, Pb was better stabilized with a 91.6% stabilization rate. Due to the increase of Pb contamination concentration much higher than that of Cd contamination, the stabilization rate at the optimum addition of Pb was lower than Cd. Considering the molecular weight of heavy metals involved in the stabilization process when the remediation agent is added in the same amount, the molecular weight of Pb being stabilized is much larger than Cd. Therefore, the removal of Cd is much more difficult than Pb, and the experimental results reflect the difficulty of Cd remediation and the higher ecological hazard of Cd from another aspect.

The results of the morphological distribution of Cd- and Pb-contaminated saline–alkaline soil before and after remediation are shown in Figure 7. After WTF remediation, the ecotoxicity of both Cd and Pb was effectively reduced, but the acid-soluble states of different heavy metals were converted to other, different forms. The acid-soluble state of Cd was mainly converted to the reducible and residue states, and the acid-soluble state of Pb was mainly converted to the less ecotoxic oxidizable and residual states. It has been shown that lead has a strong affinity for Fe and Mn hydroxides [43,44]. Thus, there was little change in the reducible state of Pb before and after the WTF restoration.

The results of the effect of WTF remediation on soil organic matters are shown in Figure 8. Organic matter provides the nutrient base for the soil. The amount of organic matter can be used as a measure of soil quality. Organic matter actively participates in the immobilization of Pb [45]. The reduction in Pb ecotoxicity and the increase in soil organic matter after WTF remediation are consistent with this statement. The presence of heavy metals can affect the mineralization of organic matter. Therefore, the application of heavy metal soil remediation agents resulted in a decrease in Cd²⁺ and Pb²⁺ concentrations but an increase in soil organic matter. Several possible reasons are: (i) the WTF structure is a carbon-containing structure, which provides more carbon sources for soil organic matter and improves the soil fertility, (ii) the sediment formed during soil remediation makes the soil denser and facilitates the formation of organic matter, and (iii) the growth of sucrase in the soil is inhibited, which prevents the organic matter in the soil from being decomposed. Therefore, further enzyme activity studies are needed to analyze the effect of WTF on the soil.

The results of the effect of WTF remediation on the total water-soluble salt content of soil are shown in Figure 9. From the experimental data, it can be seen that the water-soluble salt content in the soil increased with the increase of the heavy metal contamination level, from 20.5% to 34.2%. After WTF remediation, the water-soluble total salt content of Pb-contaminated soil decreased by nearly 10%, while that of Cd-contaminated soil increased. This experimental phenomenon indicates that the decrease of the total water-soluble salts in soil is not only caused by the change of the water-soluble heavy metal concentration but also by the presence of other mechanisms or ion exchange.

To understand the reasons for the decrease of the soil water-soluble total salt, this experiment chose WTF-Pb to simulate the water-soluble salt adsorption. The experimental results are shown in

Table 4. SO₄²⁻ concentration changes before and after the adsorption of WTF-Pb in the CaSO₄ solution.

Samples	SO42− (mg/L)
Not add WTF-Pb	5.42 ± 0.04
Add WTF-Pb	4.20 ± 0.03

. WTF-Pb could effectively adsorb SO₄²⁻ from the CaSO₄ solution, reducing its concentration by 22.5%. Meanwhile, the surface electrical properties of WTF-Pb at different pH are shown in

Table 5. Surface electrical properties of WTF-Pb in different pH aqueous solutions.

pH	Zeta Potential (mV)
6.74	47.4 ± 1.1
7.23	49.1 ± 1.3
7.59	48.3 ± 1.1
8.37	46.5 ± 0.9

. The Zeta potential of WTF-Pb is at a very positive value (above 30 mV for all samples). The high Zeta potential indicates that WTF-Pb is stably dispersed in an aqueous solution. WTF-Pb is positively charged and has a strong adsorption capacity for anions. The positive charge of WTF-Pb may be due to the tendency of the chelation products to ionize more NO³⁻, thus making the precipitate positively charged, and the mechanism is shown in Figure 10. From the NO³⁻-N test results (

Table 6. Soil NO₃⁻-N content before and after WTF restoration.

Soil Samples	NO3−-N (mg/kg)
Cd-CK	54.31
Cd-2%	743.7
Pb-CK	66.05
Pb-7%	2698

), it is easy to see that the NO³⁻-N content in the soil increased significantly after the addition of WTF. The increase in the NO³⁻-N content reflects the tendency of WTF-Pb precipitates to ionize more NO³⁻. In other words, after WTF remediation, the precipitation formed with heavy metals released NO³⁻ and exchanged with soil ions and adsorbed anions from the soil, and the decrease in the total salt content was caused by the change in the relative molecular mass during a molecular exchange. Therefore, when the contamination concentration is low, fewer precipitates are formed, and the relative mass change due to ion exchange is less pronounced, even if there is an increase in the salt content.

WTF can stabilize high concentrations of heavy metals while reducing water-soluble salts in saline soils. The precipitates formed by WTF remediation tend to release anions, which are positively charged and exchange with soil anions, changing the soil ion structure and reducing the relative molecular mass to reduce the total water-soluble salt content.

Soil enzyme activity is one of the indicators that characterize soil function. The results of soil urease activity before and after WTF remediation are shown in Figure 11. After the addition of WTF, the urease activity in different contaminated soils decreased substantially. The soil urease activity was inhibited after the contamination concentration was significantly increased, indicating that the presence of heavy metals had an inhibitory effect on the urease activity. The decrease in urease activity indicated that WTF acted as a urease inhibitor in the soil, reducing the rate of volatile ammonium nitrogen production, reducing the nitrogen loss, and making the nitrogen cycle more efficient.

Phosphatase in the soil plays an important role in the conversion of organic phosphorus. The changes in soil alkaline phosphatase by WTF remediation are shown in Figure 12. In the Cd-contaminated soil, the slight change in alkaline phosphatase after remediation indicates that the phosphorus cycle function of the soil has not been disrupted and has been repaired. In the Pb-contaminated soil, the activity of alkaline phosphatase increased significantly after the addition of WTF compared with that before remediation, probably due to the influence of soil pH. The soil pH plays a crucial role in the activity of soil enzymes. A related study gave the optimum pH for alkaline phosphatase activity as 7.4–8.5. The soil pH after WTF remediation was about 8.5. The pH of alkaline soil did not change much after the addition of WTF, thus increasing the activity of alkaline phosphatase. In conclusion, WTF had some restoration effect on alkaline phosphatase activity, increasing the rate of the soil phosphorus cycle, improving soil fertility, and restoring the soil function.

Sucrase plays an important role in the soil carbon cycle and also characterizes the activity of soil microorganisms, which is one of the indicators to characterize soil ecological functions. The effect of WTF remediation on soil sucrase is shown in Figure 13. The increase of heavy metal pollution concentration inhibited the soil sucrase activity, the soil sucrase activity was increased to different degrees after remediation, and the more WTF was added, the more obvious the increase of sucrase activity. It is supposed that WTF, as a carbon-containing organic matter, provides a carbon source for soil sucrase and increases sucrase activity. An increase in sucrase activity implies an increase in the soil carbon cycling capacity and an increase in the soil microbial activity. The increase in sucrase activity implies an increase in the soil carbon cycling capacity and soil microbial activity. These phenomena were consistent with the results of previous studies. The experimental results showed that the soil ecological function was restored by the addition of WTF. The increase in sucrase activity also indicated that the increase in the organic matter content after remediation was not caused by the loss of sucrase activity. The results also demonstrated that the increase in organic matter after remediation was not related to sucrase activity.

In this study, five soils were selected to investigate the effect of soil quality on the WTF remediation process. The experiments demonstrated that soil quality has a great influence on both the remediation efficiency and the distribution of heavy metal morphology. This provides an idea for analyzing the effect of heavy metal remediation from the soil quality perspective. However, there were limitations in the samples selected for the experiment, and future studies should select more soil samples to investigate the effects of different soil conditions on the remediation of heavy metals by remediation agents. It was found that alkaline environments responded more significantly to low addition levels of WTF than acidic environments. The effect of different soil pH conditions on the passivation effect of WTF needs to be further investigated to lay the foundation for the widespread use of WTF. It was also found that WTF stabilized the heavy metal form of Pb better than that of Cd. The reasons for the differences in the morphology of the two heavy metals need to be investigated so that the stabilization of WTF against Cd can be improved in a more desirable direction, for example, by improving the WTF. This paper also investigated the variability of WTF for the remediation of Pb- and Cd-contaminated saline soils. The current trend of increasing heavy metal contamination in Chinese soils shows a trend of composite contamination, and the content of this study lays the foundation for future investigations into the composite contamination of Pb and Cd in saline soils in Tianjin. Further research on the remediation of multiple metal complex contaminations and the mechanisms of their interactions should be conducted in the future.

4. Conclusions

Analyses of WTF remediation in different soils showed that WTF was able to stabilize Cd and transform Cd into a less toxic form. The Pearson analysis showed that the stabilization rate of WTF remediation of Cd contamination was mainly affected by the soil pH. The distribution of Cd morphology before WTF remediation was mainly influenced by the soil organic matter, which had a strong correlation with the soil mechanical composition. Analyses of the WTF remediation of saline soils showed that WTF could stabilize Pb and Cd. The pH of the restored soil did not change much. The organic matter content increased significantly, the alkaline phosphatase and sucrose enzyme activity increased, and the urease activity and soil water solubility decreased. The analysis of the Zeta potential on the surface of WTF-Pb showed that WTF-Pb tends to release anions more. It changed the soil ionic structure and reduced the relative molecular mass by exchanging with soil anions, thus reducing the total soil water-soluble salts.