## 1 Introduction  
Incidental ingestion of contaminated soils represents one of the main exposure pathways when assessing human health risks associated with metals or metalloids such as Pb, As, and Cd [1]. This is particularly true at sites characterized by residential or recreational use, due to the presence of children (e.g., [2]). In fact, for their typical hand-to-mouth behavior, the fine fraction of soil could adhere to children's hand and then be ingested [3, 4]. Once ingested, the contaminants present in the soil can be mobilized by the gastrointestinal juices (bioaccessible fraction) and then adsorbed, i.e., pass the intestinal wall reaching the blood circulation (bioavailable fraction) [5, 6]. According to the ASTM-RBCA (American Society for Testing and Materials -Risk-Based Corrective Action) approach [7, 8], human health risk assessment (HHRA) for the soil ingestion pathway is typically carried out starting from the total soil concentration of the contaminants, which is estimated from the fraction of inorganic species extracted from the soil using standardized approaches, i.e., microwave acid extraction. Combining it with the relevant exposure factors, i.e., the ingestion rate (IR), the exposure duration (ED), and the body weight (BW), allows for the estimation of the exposure (E) for a given receptor. There is evidence in the literature that this assumption can lead to overestimating the actual risks for human health [9]. Indeed, only a fraction of the inorganic content of a soil has been shown to reach the blood circulation system once ingested [3]. Furthermore, it is worth noting that the bioavailable fraction of a contaminant is strictly correlated to the soil properties and contaminant characteristics, which change from site to site [10; 11; 12]. Different methods, either in vivo or in vitro, have been successfully developed and tested for assessing the bioavailability of metals and metalloids in soils. The in vivo procedures use swine and mouse as models for evaluating the inorganic accumulation in animal tissues or blood after exposure to contaminated soils and have been proven to be quite robust (e.g., [13; 14]). However, the adoption of these methodologies within any HHRA procedure is impractical due to the high costs and times required and to ethical considerations [2]. Several simple, cheap, and reproducible in vitro tests have also been developed, which allow for the estimation of the bioaccessible fraction of a contaminant, and consequently provide a conservative estimate of the bioavailable fraction [4]. The bioavailable fraction will then be at most equal to the bioaccessible one. The different in-vitro tests rely on various reagents for mimicking at laboratory scale the digestive juices and conditions of the gastrointestinal tract of the human body so as to investigate the mobilization of the contaminants from the soil [15]. The methods investigated in the literature include the Physiological Based Extraction Test (PBET) [10], the Simple Bioaccessibility Extraction Test (SBET) [16; 17], the In Vitro Gastrointestinal (IVG) test [18], the methods developed by the Deutsches Institut fur Normung e.V. [19] and by the Dutch Institute of Public Health (RIVM) [20], and finally the Unified Barge Method (UBM) [6]. Many differences can be found in comparing the operating conditions adopted by these methods. For instance, several methods (e.g., RIVM and DIN) are carried out without mixing, whereas others (e.g., UBM and SBET) are carried out using dynamic conditions. Furthermore, some methods (e.g., SBET) mimic only the gastric phase, which is assumed to be the most relevant one for the inorganic mobilization, whereas other procedures (e.g., UBM) try to simulate the mouth, the stomach, and the intestinal tract. All the above-mentioned tests have shown good in vivo-in vitro correlations (IVIVC) for the tested inorganics (e.g., [18; 21; 22]).