## 4 Bioavailability Assessment of Organic Pollutants Using DGT
### Determination of Hydrophobic Organic Matter Using DGT  
A large number of studies have shown that the bioavailability of organic pollutants is related to their hydrophobicity. [PERSON] et al. [80] found that the higher the hydrophobicity of the organophosphate, the easier it is to be absorbed by binding to the important specific ester transferase in wheat roots. But, it is not that the greater the hydrophobicity, the greater the bioavailability. The results reported by [PERSON] et al. [81] indicated that the bioavailability of polybrominated diphenyl ethers (PBDEs) in mice decreased with increasing LogKow, which was related to the uptake and diffusion capacity of PBDEs with different degrees of bromine substitution across the cell membrane. As the main components of biofilms were non-polar lipids, it was often easier to adsorb lipophilic compounds. Furthermore, increasing the hydrophobicity of organic contaminants also increased the sorption capacity of soil organic matter for contaminants and reduced the bioavailability of contaminants [82]. The degree of accumulation of different pollutants in the organisms was different. [PERSON] et al. [83] found that the bioaccumulation factors (BAFs) value of halogenated flame retardants (HFRs) in earthworms was lower than that of perfluorinated alkyl substances (PFAS), and pointed out that this may be due to the higher protein affinity and lower lipophilicity of PFAS, which were more likely to accumulate in earthworms with higher protein content. Dissociable organic pollutants were often more readily uptaken by the organisms. Under certain pH conditions, the \(p\)Ka value of organic pollutants determined their molecular states, which further influenced the degree of dissociation. The cell membrane of living things was usually negatively charged, and the cationic forms of pollutants were more easily adsorbed by plant roots, while anionic pollutants were repelled by the cell membrane; thus, it was difficult for anionic pollutants to pass through the cell membrane [84]. The molecular weight, structure, and spatial configuration of organic matter of organic pollutants were also important factors affecting their bioavailability. Polychlorinated biphenyls (PCBs) were composed of hydrogen atoms on biphenyls substituted by different numbers of chlorine atoms. The molecular volume and molecular polarizability affected their enrichment in earthworms. The process of PCBs with large molecular volume across biofilms might be limited, resulting in a longer time to reach equilibrium in vitro and in vivo. Polycyclic aromatic hydrocarbons (PAHs) are hydrocarbons formed by condensing two or more benzene rings with dense \(\pi\) electrons. Quantitative structure-biodegradability relationship (QSBR) models of PAHs had shown that the biodegradability of PAHs was mainly influenced by spatial structure parameters [85]. The more complex the spatial structure, the more difficult it was for the PAHs molecule to react with the enzyme active sites of the microorganisms. DGT suitable for studying the transport and transformation patterns, toxicity, and potential bioavailability of organic contaminants in the environment is often referred to as an o-DGT. In 2012, the first o-DGT was designed and applied to the tracking of organic pollutants for the first time [86]. Like heavy metals, the application of o-DGT also starts from water bodies. Compared with the soil environment, the water environment is relatively simple, which is conducive to the exploration of the effects of pH, ionic strength (IS), DOM, and other environmental conditions in the early stage of o-DGT. [PERSON] et al. measured o-DGT (A-DGT and P-DGT) prepared by two diffusion membranes of agarose (0.76 mm) and PAM (0.40 mm) by 3-mercaptoethanolpropyl-functionalized silica gel embedded in a PAM gel as the binding phase [87]. The instability of methylmercury in fresh water provides a basis for assessing the bioavailability of organometallic compounds and understanding their potential risks and toxicity.