## 3 Results and Discussion
### 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. \[\eta\left(\%\right)=\frac{\eta_{\text{Before}}-\eta_{\text{After}}}{\eta_{ \text{Before}}}\times 100 \tag{1}\]where \(\eta_{\text{Before}}\) is the content of bioavailable heavy metals in soil before phytoremediation, and \(\eta_{\text{After}}\) 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 [PERSON] 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, [PERSON] 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\({}_{2}\)O\({}_{3}\) contained in steel slag and pyrolusite (Tables 3 and 4) and ferrous sulfate played a role in stabilizing Cd and As. Thirdly, arsenic  
Figure 5: Reduction rate of soil bioavailable heavy metals in different treatment groups. 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.