## 4 Bioavailability Assessment of Organic Pollutants Using DGT
### Influence Factors  
The diffusion coefficient is one of the key parameters for predicting the kinetics of pollutant sorption in DGT samplers. Currently, the widely used hydrogel diffusion layer determined the diffusion coefficients of PFAS, currently used pesticides, pharmaceuticals, and personal care products (PPCPs) [117]. In addition, there are also studies reporting the effect of external filter membranes on the accuracy of DGT concentration determination [118]. Although there are DGT devices that do not use external filter membranes, biofouling is a problem that needs to be faced [91]. The decrease in mechanical strength will limit its application in soil, and increasing the thickness of the diffusion film requires remeasurement of the diffusion coefficient, and the change in the thickness of the diffusion boundary layer is also a problem that needs attention [119].  
Whether external environmental factors affect the stability of the performance of the o-DGT technique is a key issue for its application. Especially in water bodies and complex soil environments, it is more necessary to determine the robustness of the measurement system. The ratio of the concentration of the target (C\({}_{\rm DGT}\)) measured by DGT to the concentration of the native solution (C\({}_{\rm solution}\)) (i.e., C\({}_{\rm DGT}\)/C\({}_{\rm solution}\)) is commonly used to evaluate the stability of the DGT technique in water [106; 107]. pH plays a decisive role in the morphology of organic compounds in environmental media, and organic matter at a different pH may become neutral or anionic/cationic forms, which significantly affects its bioavailability. For many of the organic compounds studied so far, pH had no significant effect on the bioavailability content measured by o-DGT [120]. O-DGT showed good stability in measuring PAEs from the lake water environment (pH 3-9, IS 0.001-0.5 M, and DOM 0-20 mg/L) [100]. Considering the different _p_Ka values of different compounds, pH can be used as one of the parameters to correct the measurement results in the future.  
IS can affect sampling efficiency through a'salting-out' effect that reduces analyte solubility [97]. At low IS (0.001-0.5 M), o-DGT measurements of compounds are generally independent of IS [97; 120]. With the increase in IS, the diffusion of target ions to DGT is inhibited to a certain extent, resulting in the decrease in sorption efficiency. However, the extent of this influence is limited, and there is no study to quantify the effect of IS; hence, the calibration of parameters under different IS conditions is a matter of concern.  
DOM can inhibit the sorption of the target compounds onto O-DGT through competitive adsorption. Therefore, in the presence of DOM, the diffusion coefficient of the target pollutant in DGT is slightly lower than that without the influence of DOM, and the corresponding amount of the target absorbed onto DGT is reduced [87; 97]. However, the C\({}_{\rm DGT}\)/C\({}_{\rm solution}\) values were in the range of 0.9-1.1, and DOM had no significant effect on o-DGT [100].  
The o-DGT exhibits stable performance under a wide range of environmental conditions and has broad application prospects. At present, the main studies focus on measuring the content of pollutants in water and soil, especially for organic pollutants (Figure 3); thus, it is necessary to establish a general link with model organisms' (animal/plant) bioavailability methods, so as to provide the basis and technology for bioavailability assessment support.