The behaviour of reactive toxic solutes in naturally occurring pyrite-rich sediment under surface surcharge
Date
2016
Authors
Karikari Yeboah, O.
Skinner, W.
Addai Mensah, J.
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Conference paper
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Chemeca 2016: Chemical engineering - regeneration, recovery and reinvention, 2016, iss.3384136, pp.116-127
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Chemeca conference 2016 (25 Sep 2016 - 28 Sep 2016 : Adelaide, Australia)
Abstract
Processes such as mineral trapping involve the dissolution of cations (e.g., Ca2+, Fe2+ and Mg2+) from primary minerals and their solution stabilization and/or re-precipitation as secondary minerals. Of particular interest in this regard are pyrite-rich sediments, which constitute reservoirs of reduced inorganic sulphur formed from iron oxide minerals and sulphate-rich waters under high pH, reducing conditions. The exposure of such sediments to aqueous molecular oxygen results in sulphuric acid generation through pyrite (FeS2) oxidation, as well as the release of toxic cations. The sulphuric acid fosters the dissolution of the sediments' phyllosilicate minerals, leading to the release of hydrolysable and alkali earth metal ions (Al(III) Fe(III/II), Mg(II), Ca(II), K+, Na+). Pyrite oxidation and its dissolution, together with the weathering of the phyllosilicates minerals, are pH-, temperature- and redox potential/Eh-dependent. Imposition of surface surcharge over pyrite-rich sediment is known to cause oxygen depletion within the sediment, with significant impact on pH, temperature, redox potential, and water quality/speciation. In the present study, we have investigated the effect of surcharge imposition on pyrite-rich sediment and the concomitant changes in the sediment redox dynamics on the behaviour of dissolved cations. It is shown that the severely reducing condition created beneath the surcharge is accompanied by increase negative charge on the soil particles, and enhanced soil particle surface-ion interactions. The pore water H+ concentration was invariably reduced, suppressing a further release of Ca2+, Mg2+, K+ and Na+ species. Al(III) species are preferentially removed out of the pore water via hydrolysis, specific adsorption and/or surface nucleation. These processes effectively mitigate pyrite oxidation and facilitate soil remediation, reflecting significant improvement in the ground water quality.
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Copyright 2016 Institution of Chemical Engineers