Fabienne Battaglia-Brunet

Acid mine drainage (AMD) is a persistent environmental issue arising from the oxidation of sulfide minerals present in mine wastes. This process generates acidic, metal-laden effluents, often containing high levels of arsenic (As) – a toxic metalloid commonly found in the exploited ores. Arsenic-rich AMD poses significant risks to ecosystems and human health. This chapter provides an overview of the biogeochemical processes responsible for the generation of As-rich AMD. It explores current advances and remaining challenges in biological treatment strategies, focusing on iron (Fe)- and As-oxidizing bacteria and sulfate-reducing bacteria (SRB), which promote As and metal immobilization via oxidation–precipitation and sulfide-driven processes, respectively. The chapter offers a global state-of-the-art review of bioremediation systems developed for As-rich AMD. A critical analysis of the main operational parameters influencing treatment performance is provided. Special attention is given to the case study of the Carnoulès mine (France), whose mine waste deposit generates one of the most As-rich AMD worldwide. Over three decades of interdisciplinary research at this site have deepened our understanding of biogeochemical processes controlling As dynamics in AMD and the downstream aquatic environment. In particular, understanding natural attenuation processes – based on Fe and As oxidation and subsequent precipitation – has proven especially important for guiding treatment design. This knowledge enabled the stepwise development of bioremediation strategies, from lab-scale to field pilot experiments. These systems harness naturally occurring microorganisms (Fe- and As-oxidizing and sulfate-reducing bacteria) for the sustainable remediation of As-rich effluents on mining sites.

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.

Human communities need water and mineral resources, the supply of which requires the implementation of recycling and saving strategies. Both closed and active mining sites could beneficiate of the implementation of nature-based solutions, including bioreactors involving sulphate-reducing prokaryotes (SRP), in order to separate and recover arsenic (As) and metals from aqueous stream while producing clean water. Selective precipitation strategies can be designed based on the selection of microbial communities adapted to the pH conditions, generally acidic, and to available low-cost electron donors. Laboratory batch and continuous experiments must be implemented for each type of mine water in order to determine the optimal flow-sheet in which As could be precipitated as sulphides (orpiment or realgar), inside the bioreactor or offline, through stripping of biologically produced hydrogen sulphides (H₂S). The respective concentrations and proportions of As and metals and the initial acid mine drainage pH are key parameters that will influence the feasibility of efficient selective precipitation. SRP-based bioreactors could be combined with complementary treatment steps in optimised mine water management solutions that will minimise the production of As-contaminated end-solid waste. Graphical Abstract

Microbial processes can influence the complex geochemical behaviour of the toxic metalloid antimony (Sb) in mining environments. The present study is aimed to evaluate the influence of microbial communities on the mobility of Sb from solid phases to water in different compartments and redox conditions of a mining site in southwest (SW) Spain. Samples of surface materials presenting high Sb concentrations, from two weathered mining waste dumps, and an aquatic sediment were incubated in slurries comparing oxic and anoxic conditions. The initial microbial communities of the three materials strongly differed. Incubations induced an increase of microbial biomass and an evolution of the microbial communities’ structures and compositions, which diverged in different redox conditions. The presence of active bacteria always influenced the mobility of Sb, except in the neutral pH waste incubated in oxic conditions. The effect of active microbial activities in oxic conditions was dependent on the material: Sb oxic release was biologically amplified with the acidic waste, but attenuated with the sediment. Different bacterial genera involved in Sb, Fe and S oxidation or reduction were present and/or grew during incubation of each material. The results highlighted the wide diversity of microbial communities and metabolisms at the small geographic scale of a mining site and their strong implication in Sb mobility. Graphical Abstract

Arsenic (As) and antimony (Sb) from mining sites can seep into aquatic ecosystems by acid mine drainage (AMD). Here, the possibility of concomitantly removing As and Sb from acidic waters by precipitation of sulfides induced by sulfate-reducing bacteria (SRB) was investigated in a fixed-bed column bioreactor. The real AMD water used to feed the bioreactor contained nearly 1 mM As while the Sb concentrations were increased (0.008 ± 0.006 to 1.01 ± 0.07 mM) to obtain an Sb/As molar ratio = 1. Results showed that the addition of Sb did not affect the efficiency of As bio-precipitation. Sb was removed efficiently (up to 97.9% removal) between the inlet and outlet of the bioreactor, together with As (up to 99.3% removal) in all conditions. Sb was generally removed as it entered the bioreactor. Appreciable sulfate reduction occurred in the bioreactor, which could have been linked to the stable presence of a major SRB OTU affiliated with the Desulfosporosinus genus. The bacterial community included polymer degraders, fermenters, and acetate degraders. Results suggested that sulfate reduction could be a suitable bioremediation process for simultaneous removal of Sb and As from AMD.

Passive mine drainage treatment plants are the scene of many chemical and biological reactions. Here, the establishment of iron (Fe), arsenic (As), and manganese (Mn) removal was monitored immediately after the commissioning of the Lopérec (Brittany, France) passive water treatment plant, composed of aeration cascades and settling ponds followed by pozzolana biofilters. Iron and As were almost completely removed immediately after commissioning, while Mn removal took more than 28 days to reach its maximum performance. Investigations were performed during two periods presenting strong variations in feeding flow-rates: from 2.8 m³.h⁻¹ to 8.6 m³.h⁻¹ and from 13.2 m³.h⁻¹ to 31.3 m³.h⁻¹. Design flow rate was reached during the first week of the second period. Dissolved Fe and As were not affected by the decrease in residence time while Mn was only slightly affected. Microbial communities in biofilter presented similarities with those of the pond sludge, and genera including Mn-oxidizing species were detected. Proportion of bacteria carrying the aioA gene encoding for As(III)-oxidase enzyme increased in communities during the second period. Results suggest Mn removal is mainly associated with bio-oxidation whereas removal of Fe and As could be mainly attributed to chemical oxidation and precipitation of Fe, possibly helped by As(III) bio-oxidation.