Oral bioaccessibility of PTEs in soils: A review of data, influencing factors and application in human health risk assessment.

Understanding the behavior of metal(loi)ds transported from soil to humans is critical for human health risk assessment (HHRA). In the last two decades, extensive studies have been conducted to better assess human exposure to potentially toxic elements (PTEs) by estimating their oral bioaccessibility (BAc) and quantifying the influence of different factors. This study reviews the common in vitro methods used to determine the BAc of PTEs (in particular As, Cd, Cr, Ni, Pb, and Sb) under specific conditions (particularly in terms of the particle size fraction and validation status against an in vivo model). The results were compiled from soils derived from various sources and allowed the identification of the most important influencing factors of BAc (using single and multiple regression analyses), including physicochemical soil properties and the speciation of the PTEs in question. This review presents current knowledge on integrating relative bioavailability (RBA) in calculating doses from soil ingestion in the HHRA process. Depending on the jurisdiction, validated or non-validated bioaccessibility methods were used, and risks assessors applied different approaches: (i) using default assumptions (i.e., RBA of 1); (ii) considering that bioaccessibility value (BAc) accurately represents RBA (i.e., RBA equal to BAc); (iii) using regression models to convert BAc of As and Pb into RBA as proposed by the USA with the US EPA Method 1340; or (iv) applying an adjustment factor as proposed by the Netherlands and France to use BAc from UBM (Unified Barge Method) protocol. The findings from this review should help inform risk stakeholders about the uncertainties surrounding using bioaccessibility data and provide recommendations for better interpreting the results and using bioaccessibility in risk studies.

MSWI bottom ash used as basement at two pilot-scale roads: comparison of leachate chemistry and reactive transport modeling.

The recycling of municipal solid waste incineration bottom ash as aggregates for road basement requires assessing the long-term evolution of leachate chemistry. The Dåva (Sweden) and Hérouville (France) pilot-scale roads were monitored during 6 and 10 years, respectively. Calculated saturation indices were combined to batch test modeling to set a simplified geochemical model of the bottom ash materials. A common reactive transport model was then applied to both sites. At Hérouville, pH and the concentration of most elements quickly drop during the first two years to reach a set of minimum values over 10 years. The decrease is less pronounced at Dåva. The evolutions of pH and major element concentrations are fairly well related to the following pH-buffering sequence: portlandite, C-S-H phases or pseudo-wollastonite and, finally, calcite in equilibrium with atmospheric CO(2). Al(OH)(3), barite, ettringite and monohydrocalcite may also control leachate chemistry. Cu release is correctly modeled by DOM complexation and tenorite equilibrium. Temperature has no significant effect on the modeling of leachate chemistry in the range 5-30°C, except at high pH. Effects at road edges and roadside slopes are important for the release of the less reactive elements and, possibly, for carbonation processes.

Ten-year chemical evolution of leachate and municipal solid waste incineration bottom ash used in a test road site.

The use of municipal solid waste incineration (MSWI) bottom ash for road and car-park construction is an appropriate solution to reduce their disposal and the consumption of natural materials. In addition to leaching tests, the environmental impact assessment of such a waste recycling scenario critically needs for reliable long-term field data. This paper addresses a 10-year pilot site where MSWI bottom ashes have been used as road aggregates in Northern France (oceanic temperate climate). The paper focuses on the long-term evolution of leachate chemistry and the mineralogical transformations of MSWI bottom ash over 10 years. Data interpretation is supported by geochemical modeling in terms of main pH-buffering processes. The leachate pH and concentrations in major elements (Ca, Na and Cl) as well as in Al and heavy metals (Cu, Pb and Zn) quickly drop during the first 2 years to asymptotically reach a set of minimum values over 10 years; similar to those of a reference road built with natural calcareous aggregates. SO(4) release makes exception with a slightly increasing trend over time. Carbonation induced by CO(2) inputs, which leads to the successive dissolution of portlandite, CSH and ettringite, is one of the main phenomenon responsible for the geochemical evolution of leachate. On the other hand, mineralogical observations and batch tests demonstrate a relative stability of the MSWI bottom ash inside the subbase layer. In particular, carbonation may be far to be completed and still in progress after 10 years. This is consistent with preferential rainwater flow and dilution at the road edges combined to diffusion inside the subbase layer.

Modelling of long-term dynamic leaching tests applied to solidified/stabilised waste.

The paper aims at simulating the closed-system dynamic leaching of a cement-based monolith containing lead with the numerical reactive transport code HYTEC in a 3D-cylindrical geometry. The model considers, simultaneously, the chemical evolution of pore water, the progression of mineralogical alteration fronts, and the concomitant release of elements from the S/S waste. In good agreement with the experiment, element releases were found to be mainly controlled by either diffusion (Na, K, and, to a lesser extent, Cl), by surface dissolution (Ca, Si) or by a mixed evolution (Pb, SO4). All of the calculated mineralogical transformations take place in a thin layer beyond the monolith surface. Consequently, modelling of Ca, Si and SO4 releases was quite sensitive to the node size of the simulation grid and was improved by taking into account the increase of porosity and effective diffusion coefficient due to mineral dissolution in the leached layer. In agreement with experimental results, the deepest front corresponds under closed-system conditions to portlandite dissolution and calcium silicate hydrates CSH 1.8 transformation into CSH of lower Ca/Si ratio. A second, distinct and intermediate, front is made by ettringite dissolution. The network of CSH is globally preserved in the leached layer, complete dissolution occurring over a very small thickness only. Finally, hydrotalcite precipitation in the leached layer is expected by modelling due to pH drop.

Long-term reactive transport modelling of stabilized/solidified waste: from dynamic leaching tests to disposal scenarios.

Environmental impact assessment of hazardous waste disposal relies, among others, on standardized leaching tests characterized by a strong coupling between diffusion and chemical processes. In that respect, this study shows that reactive transport modelling is a useful tool to extrapolate laboratory results to site conditions characterized by lower solution/solid (L/S) ratios, site specific geometry, infiltration, etc. A cement solidified/stabilized (S/S) waste containing lead is investigated as a typical example. The reactive transport model developed in a previous study to simulate the initial state of the waste as well as laboratory batch and dynamic tests is first summarized. Using the same numerical code (HYTEC), this model is then integrated to a simplified waste disposal scenario assuming a defective cover and rain water infiltration. The coupled evolution of the S/S waste chemistry and the pollutant plume migration are modelled assessing the importance of the cracking state of the monolithic waste. The studied configurations correspond to an undamaged and fully sealed system, a few main fractures between undamaged monoliths and, finally, a dense crack-network in the monoliths. The model considers the potential effects of cracking, first the increase of rain water and carbon dioxide infiltration and, secondly, the increase of L/S ratio and reactive surfaces, using either explicit fracture representation or dual porosity approaches.