Biogeochemical processes driving the fate of arsenic in phytostabilised mine tailings: elaboration of a conceptual model based on multi-scale experiments

Securing mine tailings represents a major environmental challenge. Metal mines frequently produce solid wastes containing iron (Fe) and sulfur (S), often associated with the toxic metalloid arsenic (As). Phytostabilisation often appears as a suitable option to decrease the dispersion of particles by erosion, at a moderate cost. However, site managers need a more comprehensive view of all the consequences linked to this remediation technique, notably the side effects on the other pathways controlling As and metals mobility out of the tailings. The present research aims to develop a tool for predicting the mobility and plant toxicity of As in and outside the assisted phytostabilised tailings dump, based on developing an innovative reactive transport model (RTM) explicitly integrating bacterially-catalysed reactions related to As, Fe and S metabolisms. This objective is addressed through an interdisciplinary approach combining geochemistry, numerical modelling, plant physiology, microbiology and omics approaches coupled with a good knowledge of the former mining sites operational management. To be sure to validate and calibrate the RTM with a robust dataset, experiments at different spatial and time scales have been conducted, notably a metric scale column experiment. This pilot experiment reproduces the different compartments of the dump: phytostabilised surface, underlying unsaturated zone, then saturated zone, with a controlled outlet discharge. A stainless-steel column was filled with 1200 kg of fine tailings from an old tin (Sn) mine. The tailings are watered at a regime close to that of the rainfall on the site, and average temperature and surface lighting (day/night) are controlled. Porewater is sampled monthly, and solids are analysed every 6 months by core sampling. The assisted phytostabilisation was started after 6 months of monitoring of the bare tailings: the surface layer was amended with limestone and compost and seeded with Festuca rubra. The tailings porewater contained, before assisted phytostabilisation, about 50 µg/L of As. This experiment demonstrates that redox reactions catalysed by microbial activities play a key role in As mobility. The following redox sequence has been indeed monitored in the water saturated level: denitrification, ferric iron reduction and reduction of AsV into AsIII, these last two reactions inducing mobilisation of As and Fe. Change in pore water chemistry is supported by the growth of an active microflora, notably AsIII-oxidising, AsV-reducing and FeIII-reducing micro-organisms, despite the low initial tailings content in microorganisms. These results were confirmed by batch experiments carried out parallel with the pilot study: slurries of tailings in water, spiked or not with low concentration of acetate, were incubated in anaerobic conditions. Results highlight that microbial activities are not limited by the amount (0.02% total organic carbon) and nature of organic matter initially present in the tailings. Experimental data allow to establish the first basis of a conceptual model of the network of stoichiometric metabolic reactions representing the redox sequence occurring in the tailings, that will support the development of a numerical model describing explicitly microbially-redox reactions as thermo-kinetically controlled reactions as well as an explicit growth of microbial population, calibrated with metagenomic and metaproteomic data.