## 2 Bioavailability Assessment of Heavy Metals in the Soil Using DGT
Under different sampling conditions, DGT with different binding phases can determine various elements from the diverse environments (Table 1). In order to accurately evaluate the bioavailability of cationic heavy metals in soil, DGT with Chelex-100 resin as the binding phase and polyacrylamide (PAM) gel as the diffusion phase was developed, which was also the most commonly used DGT [47; 48; 49; 50; 51]. With the continuous application and optimization of DGT technology, agar gels, dialysis membranes, and chromatographic paper appear as diffusion phases, and good measurement results had been obtained in practical applications. The molecular structure of the binding phase of DGT technology contains some functional groups (such as hydroxyl, amino, and carboxyl) that can provide coordination electron pairs for coordination reaction with the substance to be tested. DGT was widely used in the determination of available heavy metal content in heavy metal-contaminated soil, urban soil, and agricultural soil [44; 48; 49; 51]. The environmental risk of metalloid arsenic (As) in soil, which mainly exists in the form of anions, cannot be ignored. Therefore, DGT of ferrihydrite, Fe-oxide, Zr-oxide, and other binding phases was developed, and the bioavailability of As in soil was successfully measured [42; 53; 54; 55]. Phytoremediation is one of the commonly used remediation technologies for soils contaminated with heavy metal, and the typical plant is hyperaccumulators. In the soil remediation, DGT could be used as a monitoring tool to evaluate plant extraction efficiency and soil heavy metal bioavailability [54; 56; 57; 58]. Similarly, DGT could also be used to infer the migration process of heavy metals between crops and soil and evaluate the bioavailability of heavy metals in soil [42; 53; 55; 59; 60; 61; 62; 63; 64; 65; 66]. In order to improve the bioavailability of various heavy metals in soil determined by DGT, different types of DGT, such as diethylenetriaminepentaacetic acid-modified (DTPA-modified) layered double hydroxides (LDHs) nanomaterials [59], Mg-Al-DTPA-LDHs [61], Zirconium hydroxide and iminodiacetate [63], and Titanium dioxide (TiO\({}_{2}\)) [66], have sprung up. Furthermore, DGT also successfully predicted mercury (Hg) bioavailability in earthworms in different soil types [67; 68]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Element** & **Species/Soils** & **Findings** & **Binding Phase Material** & **Ref.** \\ \hline \multirow{3}{*}{Lead (Pb)} & \multirow{3}{*}{Pb-contaminated soils} & DGT can be used as a good indicator of the actual situation of the potential risk of soil contamination to humans. & Chelex-100 & [47] \\ \cline{3-5}  & & & DGT was another method widely used to determine Pb bioavailability in addition to gastrointestinal crack models and is highly correlated with accumulation in plants. & \\ \hline \multirow{3}{*}{Cd, Nickel (Ni)} & \multirow{3}{*}{Soils contaminated with heavy metals} & DGT can accurately reflect the contribution of dissociated metal complexes to the total concentration of unstable components. & \\ \cline{3-5}  & & & \\ \cline{1-1} \cline{3-5}  & & Cd-contaminated seleniferous soils & DGT may more accurately reflect the availability of Cd and Se in the soil environment. & Chelex-100 & [50] \\ \hline \multirow{3}{*}{Cd, Cu, Zn} & \multirow{3}{*}{Soil under plastic shed} & DGT method can better reflect the transfer of heavy metals from soil to vegetables. & Zr-Chelex & [45] \\ \cline{1-1} \cline{3-5}  & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examples of recent studies on availability assessment of soil metals using DGT.