3.1. Characteristics of Plants
It had been found that heavy metals in soil affected plant growth indexes, such as water content, biomass and plant and root length [
22]. As shown in
Figure 2,
Figure 3 and
Figure 4, the moisture, biomass, plant height and root length of plants under different treatment groups are shown, respectively.
As one of the key indicators of plant growth, moisture content is measured and shown in
Figure 2. Water is a key factor affecting the survival, growth and distribution of vegetation in terrestrial ecosystems. The water balance of the soil–plant–atmosphere continuum is a key mechanism for photosynthesis and plant growth. Therefore, monitoring the water content of plants is essential for studying plant growth. Comparing group C and group C+M1 in
Figure 2, the results showed that the moisture content was 68.13% without adding material but decreased with little change to 66.13% after adding material 1. However, as shown in group C+M2 in
Figure 2, the moisture content decreased significantly to 29.4% after the addition of material 2. It can be seen that material 2 had a devastating effect on the growth of
Cynodon dactylon. For
Bidentis pilosa, the moisture content was 71.59% without adding any materials, as shown in group B in
Figure 2. The moisture content of plants increased slightly to 80.89% and 81.74%, respectively, after the addition of material 1 (group B+M1) and 2 (group B+M2). From the point of view of the moisture content alone, both materials 1 and 2 can promote the growth of
Bidentis pilosa. In conclusion, adding materials can promote the growth of both plants.
Figure 3 shows the biomass of plants under different treatment groups. The results show that the addition of materials can also affect the growth of
Cynodon dactylon. As could be seen from group C, C+M1 and C+M2 in
Figure 3, material 1 would slightly promote the growth of
Cynodon dactylon while material 2 would inhibit the accumulation of biomass of
Cynodon dactylon to a large extent. This indicated that material 1 was a good soil additive, which was beneficial to the extraction of heavy metals from plants. For
Bidentis pilosa, the effects of materials 1 and 2 on its growth were similar to those of
Cynodon dactylon–that is, material 1 promoted the growth of the plant, while material 2 was the other way around, as shown in groups B, B+M1 and B+M2 in
Figure 3. Therefore, material 1 works better than material 2. Based on the above discussion, it can be concluded that material 1 is more suitable as a soil additive to promote the effect of phytoremediation.
In
Figure 4, the aboveground height and underground length of
Cynodon dactylon (A) and
Bidentis pilosa (B), respectively, were shown when different soil additives were introduced. As shown in group C of
Figure 4A, when no material was added, the aboveground height and underground length of
Cynodon dactylon were 9.63 cm and 9.17 cm, respectively. As shown in group C+M1 and C+M2 of
Figure 4A, after adding material 1 and 2, the above two index parameters are 9.63 cm and 7.23 cm, and 3.93 cm and 2.1 cm, respectively. It can be seen that material 1 had no great influence on plant growth, but material 2 was extremely restrictive to it. The same conclusion was reached for
Bidentis pilosa. Group B in
Figure 4B indicated that the above-ground height of the plant was 8.52 cm, and the underground length was 9.73 cm when no material was added. As shown in group B+M1 and B+M2 of
Figure 4B, when material 1 and 2 were added, the aboveground heights were 6.45 cm and 3.85 cm, and the underground length were 9.73 cm and 3.57 cm, respectively. Similarly, it was shown that material 1 had little effect on plant growth, but material 2 had a very adverse effect on plant growth. Therefore, it can be concluded that material 1 had more advantages than 2 in promoting phytoremediation.
Therefore, compared with material 2, material 1 had a better promoting effect on the growth of the two plants studied and was more suitable for the remediation of contaminated soil.
3.2. Changes of Bioavailable Heavy Metals in Soil
Bioavailable heavy metals were heavy metal elements that were easy to migrate in soil and could be absorbed by plant roots during the growth period. The purpose of soil remediation was to reduce the bioavailable heavy metals in the soil, so the reduction rate of bioavailable heavy metals which was defined as Equation (1) was used to evaluate the remediation effect in this study.
where η
Before is the content of bioavailable heavy metals in soil before phytoremediation, and η
After is the content of bioavailable heavy metals in soil after phytoremediation.
Figure 5 shows the reduction rates of bioavailable Cd and As in soil under different treatment groups. When the soil was only remediated by
Cynodon dactylon (group C of
Figure 5) or
Bidentis pilosa (group B of
Figure 5), the bioavailable content of heavy metals increased. This showed that parts of heavy metals were activated by plants, which was consistent with the research of Abou-Shanab et al. [
23]. However, the bioavailable content of heavy metals in soil decreased in all the treatment groups of plant-material combined remediation. For
Cynodon dactylon, the reduction rates of bioavailable Cd and As in C+M1 treatment group were 16.93% and 8.92%, respectively, while those in C+M2 treatment group were 76.88% and 32.81%. For
Bidentis pilosa, the reduction rates of bioavailable Cd and As in B+M1 treatment group were 12.35% and 40.42%, respectively, and those in B+M2 treatment group were 97.73% and 53.54%. It could be found that the remediation effect of
Bidentis pilosa was better than that of
Cynodon dactylon under the same treatment. The experimental results showed that the addition of solid waste materials was helpful to stabilize heavy metals, especially M2 solid waste material group.
The bioavailability of heavy metals was related to the physical and chemical properties of soil, such as pH, texture, dissolved organic matter and metal oxides [
24]. By means of SEM-EDS and XRD analysis, Yao et al. [
25] found that Cd and As were stabilized because the addition of the material led to an increase in pH value, which promoted the precipitation of Cd
2+ in the form of silicate, phosphate and hydroxide, and the arsenate combined with iron, aluminum, calcium and magnesium to form insoluble arsenate compounds. In addition, studies had shown that iron oxides contained in soil or materials could adsorb Cd through covalent bonds after hydration and stabilize As through specific adsorption, non-specific adsorption and co-precipitation [
26].
Therefore, the mechanism through which adding M1 and M2 could effectively reduce bioavailable Cd and As in soil is described in
Figure 6 and as follows: firstly, an increase in pH promoted precipitation of Cd
2+ in the form of silicates, phosphates and hydroxides [
27]. Because steel slag and pyrolusite were alkaline, their addition led to the increase of soil alkalinity, which stabilized part of cadmium in the soil. Secondly, the pollution of Cd and As in soil usually showed the opposite geochemical behavior—that is, high pH soil was beneficial to the stabilization of Cd, but it could promote the dissolution of arsenic in soil [
28,
29]. Therefore, 18.12% and 22.54% of the Fe
2O
3 contained in steel slag and pyrolusite (
Table 3 and
Table 4) and ferrous sulfate played a role in stabilizing Cd and As. Thirdly, arsenic anion species could be adsorbed on aluminum, manganese and calcium oxides. Fourthly, solid waste materials might promote plant roots to secrete organic acids, which would promote the activation of heavy metal ions in soil to be converted into bioavailable heavy metals for extraction and absorption by plants. Finally, steel slag and pyrolusite contained 12.12% and 44% of SiO
2, respectively, so the contents of bioavailable Cd and As could be significantly reduced [
30,
31].