## 1 Introduction  
Soil is the most active reservoir, not only the environmental elements that provide human resources, but also many contaminants, including heavy metals and organic pollutants in the environment. Once the pollutants enter the soil, it is difficult to diffuse and remove them because they were easily adsorbed by soil organic matter (SOM) and minerals [1; 2]. The heavy metals and organic pollutants will harm microorganism community structures, soil environmental quality, and soil ecological balance, causing crop growth and plant physiological function disorder and nutritional disorders, finally endangering human beings' health through the food chain [3; 4]. Hence, to decrease the toxicity of the pollutants to the soil environment or microorganisms/plants, different technologies were used to remediate the soil. However, most of the persistent organic pollutants have the characteristics of stable chemical properties and high residues and hydrophobicity, and are not easy to be completely degraded, whereas heavy metals cannot be degraded, and finally always remained in the soil. Thus, to reduce the environment risks of heavy metals, some technologies were applied to decrease the available heavy metals through altering the heavy metal speciation. A large number of studies have shown that only considering the total amount of pollutants in the soil tends to overestimate the toxicity of pollutants to living beings [5; 6]. Therefore, it is important for pollutant control, especially, the bioavailability of pollutants was accurately estimated, if necessary, it was worth considering the impact of soil properties on the assessment of pollutant bioavailability, or, resulting in over-protection of soils [7; 8; 9]. Bioavailability refers to the assimilation and potential toxicity of chemical substances to organisms [5]. The capabilities that pollutants have to enter organisms is closely re lated to the types of organisms, exposure route, exposure time, and soil properties [10]. Bioavailability is a key parameter to measure the mobility and ecological impact of metal elements, and an important indicator for assessing environmental risks. Generally, methods of assessing bioavailability are classified into two main categories, including biological evaluation methods and abiotic prediction techniques. The main biological evaluation methods commonly used are the biological index method, the plant detection method, the microbial indicator method, and the soil indicator animal monitoring method [11; 12]. The biological evaluation method is an economical, simple, and reliable method to determine the biological effectiveness of heavy metals in contaminated soil, but there are some disadvantages of long test cycles, susceptible to a variety of environmental factors and not easy to control, leading to its application being somewhat limited. The abiotic prediction techniques are more widely used and include total prediction, chemical extraction, soil pore water, free ion activity, etc. [13; 14; 15]. However, traditionally, physical-chemical methods often do not consider the migration process of heavy metals between solid and liquid two phases in the soil, and cannot accurately and effectively predict the toxicity, bioavailability, existence state, and migration ability of pollutants. The diffusive gradients in thin films (DGT) technique [16], as a new type of in situ passive sampling technique, can realize the in situ measurement of trace elements in the environment through the gradient diffusion of the target in the diffusion layer and its buffering kinetics [13] by the principle of free diffusion ([PERSON]'s first diffusion law). The DGT technique, as an important method to study the bioavailable content of heavy metals in the soil, was first proposed by [PERSON] and [PERSON] [16]. It utilizes the property that heavy metal ions can penetrate into the film/hydrogel, and controls the exchange process of heavy metal ions through separating the ion exchange resin from the solution.