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 NaAlSi
3O
8 and Fe
5Al
4Si
6O
22(OH)
2. Loam and silty clay both contain NaAlSi
3O
8. Loam also contains some Fe
5Al
4Si
6O
22(OH)
2 and a little CdI
2. NaAlSi
3O
8 is an aluminosilicate mineral with a framework silicate structure and is a kind of alkaline ore [
36]. Fe
5Al
4Si
6O
22(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.
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 Cd
2+ and Pb
2+ 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. WTF-Pb could effectively adsorb SO
42− from the CaSO
4 solution, reducing its concentration by 22.5%. Meanwhile, the surface electrical properties of WTF-Pb at different pH are shown in
Table 5. 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 NO
3−, thus making the precipitate positively charged, and the mechanism is shown in
Figure 10. From the NO
3−-N test results (
Table 6), it is easy to see that the NO
3−-N content in the soil increased significantly after the addition of WTF. The increase in the NO
3−-N content reflects the tendency of WTF-Pb precipitates to ionize more NO
3−. In other words, after WTF remediation, the precipitation formed with heavy metals released NO
3− 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.