Huang, L, Fang, Y, Liu, Y, Wu, S & Parry, D 2019, 'Ecological engineering to accelerate mineral weathering and transformation underpins sustainable tailings rehabilitation', in AB Fourie & M Tibbett (eds), Proceedings of the 13th International Conference on Mine Closure
, Australian Centre for Geomechanics, Perth, pp. 163-174.
Tailings are nothing like soil but are polymineral wastes containing residue economic metals (e.g. Al, Cu, Pb, Zn) and gangue minerals, exhibiting a range of geochemical reactivity in oxygenated and aqueous environments. Early colonisation of soil microorganisms and pioneer plants is inhibited by the bio-toxic geochemical conditions, as well as physical constraints, even with remediation inputs of organic matter and fertilisers. Geochemical conditions of the tailings are governed not only by chemical factors (e.g. acidity/alkalinity, soluble solutes and metal(loid)s) already formed in the soluble phase (i.e. porewater), but also the solid phase of reactive minerals. Tailing minerals undergo in situ weathering and replenish the soluble geochemical factors into the soluble phase over a prolonged and unpredictable period of time (e.g. decades). This makes short-term remediation ineffective in terms of sustaining long-term performance of reconstructed soil systems for vegetation cover. So far, the misperception of tailings as ‘inferior/contaminated soil’ and the adoption of ‘soil remediation’ approaches have largely failed in low-cost and direct phytostabilisation. Despite soil cover, opportunistic microbial bioweathering processes and associated hydrogeochemical dynamics in tailings have resulted in many ineffective conventional cover systems for rehabilitating tailings, such as sulfidic Pb-Zn tailings and red mud. Extensive weathering of these reactive minerals in the top layer of tailings (ca. 50–100 cm) is the prerequisite to hydrogeochemical stabilisation, abatement of acute toxicity, and colonisation of soil microbes and pioneer plants in reconstructed root zones covering the tailings. Bioweathering of reactive minerals (e.g. sulfides in sulfidic tailings, sodalites in red mud) can be readily catalysed by extremophiles (i.e. tolerant archaea, bacteria and fungi) upon provision of suitable conditions, such as moist conditions and relevant substrates (such as organic matter, phosphate). By using ecological engineering approaches combining engineering and geo-microbial ecology principles, the microbial processes would be enhanced by targeted and effective engineering inputs (e.g. water, organics) for achieving rapid exhaustion/depletion of reactive minerals, leading to long-term hydrogeochemical stabilisation (i.e. the completion of fast geochemical reaction phase). This would minimise risks of deterioration and failures of reconstructed soil and plant subsystems, due to minimal abundance of residual reactive minerals in the tailings underneath root zones. The present paper will draw on our recent research progress on sulfidic tailings, Fe-ore tailings and alkaline red mud for the purposes of (1) illustrating the importance of microbial driven weathering and transformation of key minerals in tailings rehabilitation and (2) introducing new technological pathways, that is, ‘in situ soil (or technosol) formation’ and ‘mineral (bio)weathering, cementation and hardpan formation’. This aims to draw research attention onto translational research by adapting ecological engineering principles and practices to deliver cost-effective and feasible technologies for speeding up progressive and sustainable tailings rehabilitation in Australia and other mining countries.
Keywords: tailings rehabilitation, bioweathering, mineral transformation, hydrogeochemical stabilisation
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