Impact of reference geosorbents on oral bioaccessibility of PAH in a human in vitro digestive tract model

A review on in-vitro oral bioaccessibility of organic pollutants and its application in human exposure assessment

Generally, human oral exposure assessments of contaminants have not considered the absorption factor in the human gastrointestinal tract, thus overestimating human exposure and associated health risk. Currently, more researchers are adding the absorption factor into human exposure assessment, and bioaccessibility measured by in-vitro methods is generally replacing bioavailability for estimation because of the cheap and rapid determination. However, no single unified in-vitro method is used for bioaccessibility measurement of organic pollutants, although several methods have been developed for these pollutants and have shown good in vitro-in vivo correlation between bioaccessibility and bioavailability. The present review has focused on the development of in-vitro methods, validation of these methods through in-vivo assays, determination of factors influencing bioaccessibility, application of bioaccessibility in human exposure assessment, and the challenges faced. Overall, most in-vitro methods were validated using bioavailability, and better in vitro-in vivo correlations were obtained when absorption sinks were added to the digestion solution to mimic dynamic absorption of organic chemicals by small intestine. Incorporating bioaccessibility into the estimation of human exposure by oral ingestion significantly decreases the estimated exposure dose. However, more investigations on bioaccessibility of hydrophobic organic compounds are urgently needed because many challenges for in-vitro methods remain to be overcome.

Formation and stability of NOM-Mn(III) colloids in aquatic environments

Soluble Mn(III) species stabilized by natural organic matter (NOM) plays a crucial role in a number of biogeochemical processes. To date, current understanding of these phenomena has been primarily concerned on the occurrence and chemistry of soluble NOM-Mn(III) complexes; much less is known regarding the formation and stability of NOM-Mn(III) colloids in the environment. This presents a critical knowledge gap with regard to biogeochemical cycling of manganese and associated carbon, and for predicting the fate and transport of colloid-associated contaminants, nutrients, and trace metals. In this work, we have characterized the chemical and physical properties of humic acid based (HA)-Mn(III) colloids formed over a range of environmentally relevant conditions and quantified their subsequent aggregation and stability behaviors. Results show that molar C/Mn ratios and HA types (Aldrich HA (AHA) and Pahokee peat soil HA (PPSHA)) are critical factors influencing HA-Mn(III) colloidal properties. Both the amount and the stability of HA-Mn(III) colloids increased with increasing initial molar C/Mn ratios, regardless of HA type. The correlation between the critical coagulation concentration (CCC) and zeta potential (R² > 0.97) suggests that both Derjaguin-Landau-Verwey-Overbeek (DLVO) type and non-DLVO interactions are responsible for enhanced stability of HA-Mn(III) colloids. For a given C/Mn ratio, PPSHA-Mn(III) colloids are significantly more stable against aggregation than AHA-Mn(III) colloids, which is likely due to stronger electrostatic interactions, hydration interactions, and steric hindrance. Further examination in real-world waters indicates that the HA-Mn(III) colloids are highly stable in surface river water, but become unstable (i.e. extensive aggregation) in solutions representing a groundwater-seawater interaction zone. Overall, this study provides new insights into the formation and stability of NOM-Mn(III) colloids which are critical for understanding Mn-based colloidal behavior(s), and thus Mn cycling processes, in aquatic systems.

Bioaccessibility of PAHs and PAH derivatives in a fuel soot assessed by an in vitro digestive model with absorptive sink: Effects of aging the soot in a soil-water mixture

Aging soot in soil under neutral aqueous condition for 30days significantly (p<0.05) reduced the apparent gastrointestinal bioaccessibility (B_app) of polycyclic aromatic hydrocarbons (PAHs) and PAH derivatives (d-PAHs) natively present in a composite fuel soot sample. B_app was determined under fasting conditions by a previously developed in vitro digestive model that includes silicone sheet as a third phase absorptive sink in the small intestinal stage. Redistribution of contaminants from soot to soil, determined in independent experiments, was too small to affect B_app. Prior uptake by soot of a commercial humic acid representing dissolved soil organic matter had no impact on B_app. We identified two causes for the reduction in B_app by soil and found they were approximately additive. One is an aging time-independent "matrix effect" attributable to competitive sorption by the soil of labile contaminant that is desorbed from the soot during the digestion test. The other is the dissolution of soluble substances from the soot during the aging process that increases soot surface area and nanoporosity. The increased surface area and nanoporosity drive contaminants from labile to nonlabile states in the soot and decrease the desorption into the digestive fluid, the former contributing most to the reduction in B_app. The present study shows that mixing of raw soot with soil has important effects, both aging and non-aging, on the bioaccessibility of soot-borne contaminants.

Oral Bioavailability, Bioaccessibility, and Dermal Absorption of PAHs from Soil-State of the Science

This article reviews the state of the science regarding oral bioavailability, bioaccessibility, and dermal absorption of carcinogenic polycyclic aromatic hydrocarbons (cPAHs) in soil by humans, and discusses how chemical interactions may control the extent of absorption. Derived from natural and anthropomorphic origins, PAHs occur in a limited number of solid and fluid matrices (i.e., PAH sources) with defined physical characteristics and PAH compositions. Existing studies provide a strong basis for establishing that oral bioavailability of cPAHs from soil is less than from diet, and an assumption of 100% relative bioavailability likely overestimates exposure to cPAHs upon ingestion of PAH-contaminated soil. For both the oral bioavailability and dermal absorption studies, the aggregate data do not provide a broad understanding of how different PAH source materials, PAH concentrations, or soil chemistries influence the absorption of cPAHs from soil. This article summarizes the existing studies, identifies data gaps, and provides recommendations for the direction of future research to support new default or site-specific bioavailability adjustments for use in human health risk assessment.

Predicting the Relative Bioavailability of DDT and Its Metabolites in Historically Contaminated Soils Using a Tenax-Improved Physiologically Based Extraction Test (TI-PBET)

Due to their static nature, physiologically based in vitro assays often fail to provide sufficient sorption capacity for hydrophobic organic contaminants (HOCs). The addition of a sorption sink to in vitro intestinal solution has the potential to mimic dynamic intestinal uptake for HOCs, thereby increasing their desorption from soil. However, the effectiveness of sorption sinks for improving in vitro assays needs to be compared with in vivo data. In this study, Tenax was added as a sorption sink into the physiologically based extraction test (PBET), while DDT and its metabolites (DDTr) were investigated as typical HOCs. Tenax added at 0.01-0.2 g to the PBET intestinal solution sorbed ∼100% of DDTr in 6.3-19 min, indicating its ability as an effective sorption sink. DDTr bioaccessibility in six contaminated soils using Tenax-improved PBET (TI-PBET; 27-56%) was 3.4-22 fold greater than results using the PBET (1.2-15%). In vivo DDTr relative bioavailability (RBA) was measured using a mouse adipose model with values of 17.9-65.4%. The inclusion of Tenax into PBET improved the in vivo-in vitro correlation from r(2) = 0.36 (slope = 2.1 for PBET) to r(2) = 0.62 (slope = 1.2 for TI-PBET), illustrating that the inclusion of Tenax as a sorption sink improved the in vitro prediction of DDTr RBA in contaminated soils.