Measurement Of Bioaccessibility Of Organic Pollutants In Soils Using Tenax-Modified In Vitro Gastrointestinal Method

Several in vitro methods using artificial gastrointestinal solution have been developed to measure oral bioaccessibility of organic contaminants(OCs)in soils in recent decades.These models was increasingly applied to assess the risk of human exposure to incidental ingestion of contaminated soils due to the nature of easy operation,time-saving and low cost.Traditional in vitro models simply mimic the process of pollutants mobilized into simulated digestive fluids and lack the dynamic absorption like the mammalian digestive system.They fail to provide sufficient capacity for captured OCs,especially for more hydrophobic organic pollutants(HOCs).Desorption will progressively decrease and finally reach desorption equilibrium,bioaccessibility can be often underestimated by in vitro tests.To circumvent this shortage,Tenax was incorporated into the simulated GI solution as an infinite sink to mimic real intestinal absorption process occurred in the human digestive tract.Tenax can continuously trap mobilized HOCs in the digestive fluid and maintain the desorption gradient between soil and GI solution to achieve dynamic absorption nature.For this purpose,physiologically based extraction test(PBET)was selected as the in vitro method,which was increasingly used in measuring oral bioaccessibility of OCs in recent years.We studied Tenax-assisted PBET to improve persistent pollutants(POPs)bioaccessibility in soils and applied an in vivo mouse model to verify the feasibility of Tenax-modified PBET.Firstly,we studied sorption kinetic curves of 7 PAHs in intestinal fluid by Tenax.Tenax can quickly trap PAHs and reach sorption equilibrium with 99%within 19 min for 7 PAHs in simulated fluid when Tenax mass is at 0.25 g,which is significant less than typical incubation time of small intestine(4 h).In addition,the higher the hydrophobicity of PAHs is,the slower the sorption kinetics is.0.25 g Tenax thus was employed as the sorption sink in experiment.The bioaccessibility of pyrene in five lab-spiked soils and bioaccessibility of 7 PAHs in a field contaminated soil.In five contaminated-pyrene soils,pyrene bioaccessibility was significantly enhanced when Tenax was added into simulated fluid.The bioaccessibility values of pyrene was enhanced from 8.25-20.8%to 55.7-65.9%.Further,in a field contaminated soil,the results of PAHs bioaccessibility was similar to that of pyrene.When Tenax was included into gastrointestinal fluid,the bioaccessibility of PAHs was enhanced from 3.7-6.92%to 16.3-31.0%.Our results demonstrated the effectiveness of Tenax as sorption sink to enhance PAH mobilization in bioaccessibility measurement in soils.Due to the existence of the complex metabolic processes of PAHs in animal,it will lead to incorrect results,consequently,we used DDT,which have relatively simple metabolic processes in animal,instead of PAHs to verify the feasibility of Tenax-aided PBET.We studied DDTs bioaccessibility/relative bioavailability in historically contaminated soils using an in vitro Tenax Beads-aided PBET method and in vivo mice model.In the absence of Tenax,bioaccessibility of DDTs is 1.2-15.4%,which is apparently lower than the in vivo bioavailability(15-45%),indicating traditional PBET underestimated real results,and a poor relationship was found between DDTs relative bioavailability and DDTs bioaccessibility determined using PBET(R=0.35 P=0.5).In contrast,when Tenax,mimicking absorption processes by intestinal epithelium happened in digestive tract,was incorporated into GI solution,DDTs bioaccessibility was obviously enhanced and ranged from 28.7%to 58.5%.The results of the in vitro Tenax-aided-PBET closely corresponded with in vivo DDTs relative bioavailability and a good correlation was founded(R=0.79,P=0.064),which illustrated modified PBET had potential for replacing animal model to predict in vivo bioavailability.In this case,we also found that bioacceesibility or relative bioavailability had negative relationship with percent carbon content in back carbon(BC)instead of that of organic carbon(OC),illustrating BC was non-ignorable factor in decreasing pollutants bioavailability.