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1This paper presents a multi-laboratory comparison study of in vitro models assessing 2 bioaccessibility of soil-bound lead in the human gastrointestinal tract under simulated fasted 3 and fed conditions. Oral bioavailability data from a previous human in vivo study on the same 4 soil served as a reference point. In general, the bioaccessible lead fraction was significantly 5 (P<0.05) different between the in vitro methods and ranged for the fasted models from 2% to 6 33% and for the fed models from 7% to 29%. The in vivo bioavailability data from literature 7 were 26.2 ± 8.1% for fasted conditions, compared to 2.5 ± 1.7% for fed conditions. Under fed 8conditions, all models returned higher bioaccessibility values than the in vivo bioavailability, 9 whereas three models gave a lower bioaccessibility than bioavailability under fasted 10 conditions. These differences are often due to the method's digestion parameters that need 11 further optimization. An important outcome of this study was the determination that the 12 method for separating the bioaccessible lead from the non-bioaccessible fraction 13 (centrifugation, filtration, dialysis) is crucial for the interpretation of the results. 14 Bioaccessibility values from models that use more stringent separation methods better 15 approximate in vivo bioavailability results, yet at the expense of the level of conservancy. We 16 conclude from this study that more optimization of in vitro digestion models is needed for use 17 in risk assessment. Moreover, attention should be paid to the separation method since it 18 largely influences what fraction of the contaminant is considered bioaccessible. 19 20

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Jurassic ironstones outcropping over parts of eastern England give rise to soils with arsenic concentrations in excess of the UK soil guideline value of 20 mg kg(-1) for residential areas. Total arsenic concentrations were determined for 73 ironstone derived soils and bioaccessible arsenic determined using an in vitro physiologically based extraction test. The bioaccessible arsenic concentration for these soils was found to be well below the soil guideline value with a mean concentration of 4 mg kg(-1) and a range of 2-17 mg kg(-1). The bioaccessible fraction ranges from 1.2 to 33%. Data from a sequential extraction test based on the use of aqua regia as the main extractant is presented for a subset of 20 of the soils. Chemometric data reduction is used to demonstrate that the bioaccessible arsenic is mainly contained within calcium iron carbonate (sideritic) assemblages and only partially iron aluminosilicates, probably berthierine, and iron oxyhydroxide phases, probably goethite. It is suggested that the bulk of the non-bioaccessible arsenic is bound up with less reactive iron oxide phases.