## 3 DGT Assessment Influenced by Environmental Factors
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Element** & **Species/Soils** & **Findings** & **Binding** & **Ref.** \\ \hline Cd, Cu, Ni, & Chinese cabbage & DGT determined different soil bioavailable metal & & \\ Pb, Zn & (_Brassica pekinensis_ L.) & components and significantly improved prediction & Unknown & [64] \\  & & of metal uptake by Chinese cabbage. & & \\ \hline P, Cd, Cu, & _Zizania latifolia_, & DGT-rhizobox-DIFS should be a reliable tool to & & \\ Pb, Zn & _Myrophyllum_ & research phytoremediation mechanism & & \\  & _verticilitatum_ & & of macrophytes. & \\ \hline As, Sb & White icicle radish & DGT provided estimates of the potential kinetic & & \\  & (_Raphanus sativus_) & recharge of As and Sb and was an effective tool for & & \\  & & assessing and predicting bioavailable As and Sb in & & \\ \hline Hg & Earthworm & DGT-Hg flux was a useful tool for simulating & 3-mercaptoethanolpropyl- & \\  & (_Eisenia fetida_) & bioavailable Hg concentrations in & functionalized & \\  & & contaminated soils. & silica & \\ \hline Hg & Earthworm & DGT-measured Hg flux can predict Hg & & \\  & (_Eisenia fetida_) & bioavailability for earthworms inhabiting diverse & & \\  & & types of soil. & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Studies had shown that higher pH reduced Cd concentration, while higher soil Eh increased Cd concentration in brown rice [60]. There was a significant correlation between Cd levels in cereals and soil Eh. The Eh of soil changed the chemical behavior, migration, and bioavailability of heavy metals in the soil through changing the existing forms of heavy metals. In general, when the soil was under the reducing conditions, many heavy metals tended to form insoluble sulfides. Under the oxidizing conditions, the content of dissolved and exchanged forms increased. CEC had a negative effect on the dissociation of heavy metals in soil. CEC reflects the negative charge of soil colloids. The higher the soil CEC, the higher the negative charge, and the more Cd ions adsorbed on the surface of soil particles by electrostatic adsorption [60]. To combine the calculation of the dissociation rate constant (\(K_{-1}\)) of metal-organic complexes (MLs) and the concentration of free ionic metals (\(\mathrm{C_{M}}\)) in soil, [PERSON] et al. [49] developed a new theoretical model. The fitting results showed that CEC was the main soil environmental factor affecting the \(K_{-1}\) value of Cd, and SOM was the main soil environmental factor affecting the \(K_{-1}\) value of Ni. The correlation analysis showed the negative correlation of the SOM and the \(K_{-1}\) value of Ni. The functional groups of SOM, such as hydroxyl groups, phenols, amino groups, carbonyl groups, and hydrogen sulfide groups, as the Lewis hard bases can react easily with Lewis hard acid in the metal bonding reaction. Compare to the soft acid Cd, the Ni ion as the Lewis boundary acid is more able to bind to the organic group [70]. Soil DOM, as an important component of the soil solid phase, is a highly reactive and unstable component that can form strong complexes with heavy metals and change the mobility and bioavailability of heavy metals in soil [60; 71]. [PERSON] et al. [60] found that soil DOM reduced the availability of Cd to rice grains. [PERSON] et al. [72] found that the sorption of heavy metals in soil would change when humic acid was added, and factors such as the solubility of heavy metal-humic acid complexes and the sorption of humic acid on the solid phase would affect the change in sorption. The instability of metal-DOM complexes was tested using the adapted DGT method [73]. The emerging metal-DOM complexes were more unstable than those in peat, and the aromaticity of DOM appears to be an important factor in determining the instability of metal complexes [51; 73]. Moreover, a study using DGT also confirmed that the low-molecular-weight DOM-Se fraction in the effective fraction can reflect the bioavailability of selenium in the plant-soil system [74].