Continuous arsenic mobilisation from historically contaminated mining soils : implications for risk assessment

Concentrations of soil arsenic (As) in the vicinity of the former Złoty Stok gold mine (Lower Silesia, southwest Poland) exceed 1000 µg g-1, posing an inherent threat to neighbouring bodies of water. Thus, the necessity to develop cost-effective and environmentally friendly in situ technologies is demanded. In this purpose, the presented thesis investigated the mechanisms that govern As mobilisation from heavily contaminated soil in three continuously operated bioreactors over a period of more than three months. In particular, the capacity of autochthonic microflora (Rreduc), impact of biostimulation (Rbiost) and bioaugmentation (Rbioaug) for mobilising As was assessed. In all three reactors a bi-phasic mobilisation of As was observed. In the first two months, As mobilisation was mainly conferred by Mn dissolution despite the prevalence of Fe. Thereafter, the sudden increase in aqueous As(III) was attributed to multiplied activity of the autochthonic dissimilatory As-reducing community. Increased Fe and Mn liquid concentrations measured in the Rbiost and Rbioaug contrasted with muted arsenic response. The observed decoupling of arsenic from iron reductive dissolution might point towards its secondary precipitation with Fe(II) or / and organic matter. Based on those results, two novel redox-stat bioreactors were designed to quantify the contributions of Fe,Mn-(oxyhydr)oxide reductive dissolution and arsenate reduction on overall As mobilisation. The Rcont allowed precise control of the Eh, suppressing redox reactions thermodynamically favoured at lower redox potential. Here, As release could be attributed to desorption of arsenite and Mn reductive dissolution. By contrast, the Rnat with naturally developing redox potential, showed almost double As release due to reductive dissolution of Fe and microbial arsenate reduction. In conclusion, this thesis raises concerns about the long-term stability of historically As-contaminated sites. In particular, the development of indigenous dissimilatory As-reducing bacteria on a refractory pool of As and manganese associated As, which is mobilised at relative high Eh, may certainly pose a threat to potable water contamination of this region. Furthermore, the performed study questions the applicability of biostimulation and bioaugmentation as a bioremediation strategy of Fe-rich, mining impacted soils