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International audience; Bioeconomy addresses the triple balancing challenge posed by the Paris Agreement: inducing additional GHG mitigation alongside biophysical carbon dioxide removals, while keeping fossil carbon in the ground. The use of woody plants as construction materials could simultaneously address this challenge: CO2 is absorbed through photosynthesis, while wood-based products provide mitigation by replacing either petrochemical materials such as plastic boards or energy-intensive materials like bricks. This study combines, at the scale of France, static and dynamic life cycle assessment with soil carbon simulations to investigate the long-term environmental performance of using, as input for the building sector, black locust (Robinia pseudoacacia L.) planted on “carbon vulnerable lands”. These are areas specifically selected for their low soil organic carbon (SOC) levels and low potential to interfere with food security. After the consecutive cultivation of three rotations, black locust was shown to stock an additional 92000 kg ha−1 of carbon in soils as SOC. The mitigation effect from the plastic board replacement is the major contributor to the assessed environmental impacts, including climate change, leading the black locust system to generally perform better than the reference system where the carbon vulnerable areas are left as they are with their initial vegetation. Considering that cultivation starts in 2022, the dynamic evaluation of the global mean temperature change (GMTC) shows that the black locust system results in net negative emissions in the near- and mid-term, but with a GMTC overshot in the second half of the century. Even if a considerable carbon stock is created through time (e.g., 81% and 39% of the carbon captured by photosynthesis, in 2050 and 2100 respectively), the GHG emissions stemming from manufacturing and end-of-life processes were shown to offset the benefit of carbon capture.

Carbon dioxide supercritical extraction is applied to a soil artificially contaminated with biphenyl. We studied three aspects of the extraction process in order to control it. An “excess function equation of state” model is used to represent the thermodynamic properties of the binary biphenyl-carbon dioxide mixture. This model allows calculation of solid compound solubility and of liquid-gas equilibrium. The adsorption of biphenyl in soil in the presence of supercritical carbon dioxide is experimentally studied. In order to demonstrate the influence of the non-ideal character of the supercritical fluid on the amount adsorbed, we used a Bragg Williams lattice model. The adapted model allows adsorption calculation for a given set of temperature-pressure-fluid composition data. Both thermodynamic models are necessary to shape the extraction behaviour. In the soil, the pollutant is distributed between a precipitated phase and an adsorbed phase. Thus, the extraction model contains a dissolution kinetic term and a desorption kinetic term. Three parameters are unknown: the dissolution rate coefficient, the overall mass transfer coefficient between soil particles and the fluid and the fraction of precipitated biphenyl from the total quantity in soil. These parameters are estimated by fitting the model with experimental data.

The calculation of solubility in supercritical fluids requires models of high precision in the near-critical region, and this is not always the case with classical models. The model proposed here is based on the relations which exist between the Heimholte energy of pure components and the excess Helmholtz energy in a mixing process at constant packing fraction. The pure component properties are represented using any equation of state which is actually included in the equation of state of the mixture. The expression of van Laar was used for the residual term of the excess function. This model was applied to the solid-fluid equilibria occurring in the binary systems composed of a hydrocarbon solid and supercritical carbon dioxide. The best results in the calculations of solubility and molar volumes of the mixture were obtained by using precise equations of state for each compound.