Microbial diversity and functional response to the redox dynamics of pyrite-rich sediment and the impact of preload surcharge.

Microbial diversity and activities play pivotal biogeochemical roles in a redox-sensitive, pyrite-rich sediment's ecosystem. However, very little is known about the microbial community composition and distribution among the redox zones of pyrite-rich sediment and their response to changes caused by the burial of the sediment beneath compacted fill. In the present work, culture-independent, molecular phylogenetic investigations of the prokaryotic population and its diversity in a naturally occurring pyrite-rich sediment were undertaken to determine the microbial community composition, richness, diversity and distributions among the varying redox zones and their functional response to the imposition of surface surcharge, in the form of compacted fill. It was established that the pyrite-rich sediment is a redox-sensitive environment consisting of microhabitats with distinct and discontinuous physico-chemical characteristics, including DO, pH, Eh, temperature, electrical conductivity and salinity. It is a favourable environment for cyclic transformation of inorganic sulphur compounds and a unique environment for the habitation and growth of various microorganisms. Microbes adapted to the microhabitat and lived together in consortia, in response to their physiological and functional requirements. Microbes involved in the sulphur cycle had their populations concentrated in the oxic zone, while those involved in iron and carbon cycles were prevalent in the anoxic zones. As a result, highly diverse microbial populations occurred in isolated peaks within the sediment. The physico-chemical differences within the sediment changed in response to changes in the sediment redox dynamics. Imposition of the surcharge resulted in significant changes in the pH, temperature, Eh, DO, EC and salinity, reflecting marked re-distribution of the microbial population within the ecosystem. The cable bacteria phenomenon was evident in the sediment studied; however, there were doubt regarding their filamentous occurrence.

Anaerobic pyrite oxidation in a naturally occurring pyrite-rich sediment under preload surcharge.

Pyrite undergoes oxidation when exposed to aqueous oxygen to produce acidic leachate with high concentrations of H+, SO42-, and Fe3+. The oxidation mechanism is currently ascribed to contact between the mineral and aqueous oxygen. Consequently, management of acidic leachate from acid sulfate soils and acid mine drainage is focused on the prevention of contact between the sediment and aqueous oxygen through the surface. Intriguing though is the fact that in aquatic sediments, redox processes occur in sequence with the oxidizing agents. Among the common oxidants in aquatic sediments are O2, [Formula: see text], Mn, and Fe, in the order of efficiency. Consequently, following the depletion of oxygen in pyrite-rich sediment, it would be expected that [Formula: see text], followed by Mn and then Fe, would continue the oxidation process. However, evidence of anaerobic pyrite oxidation in a naturally occurring pyrite-rich sediment is limited. Few studies have investigated the process in aquatic systems but mostly in laboratory experimental set ups. In this study, pyrite oxidation in a naturally occurring pyrite-rich sediment was investigated. A section of the sediment was covered with surface surcharge, in the form of compacted fill. The section of the sediment outside the surcharged area was preserved and used as control experiment. Solid phase soil and porewater samples were subjected to elemental, mineralogical, and microbial analyses. The results show excess accumulation of sulfate and sulfide in the anoxic zones of the original sediment and beneath the surcharge, accompanied by the disappearance of [Formula: see text], Mn, and Fe in the anoxic zones, indicating electron transfers between donors and acceptors, with pyrite as the most likely electron donor. The study outcome poses a significant challenge to the use of surface cover for the management of acidic leachate from pyrite oxidation, particularly, in areas rich in [Formula: see text], MnO-2, or Fe.

The impact of preload on the mobilisation of multivalent trace metals in pyrite-rich sediment.

Trace metals occur at various concentrations in all wetlands. Their proliferation, chemical speciation, mobility and bioavailability are dependent on the redox potential (Eh), pH and the presence of organic and inorganic adsorption surfaces and co-precipitating metals. Consequently, changes in these key parameters have the potential to alter the fate of the dominant trace metal species in the sediment. An imposition of preload surcharge is a technique use in geotechnical engineering to improve in the strength and load carrying capacity of waterlogged sediments. The soil strength improvement is effected through the expulsion of porewater from the sediment. The imposition of surcharge over wetland sediments has the potential to create oxygen-deficient condition within the sediment, and cause pH, temperature, redox, EC and salinity changes in the sediment, which would impact on the mobilisation, chemical speciation, mobility and bioavailability of dominant toxic trace metals and their toxicity in the sediment. In the present work, a case study of the impact of preload surcharge on the proliferation, chemical speciation, mobilisation, mobility and bioavailability of arsenic, chromium, cobalt, copper and zinc in a naturally occurring pyrite-rich sediment is presented. The imposition of preload surcharge over the pyrite-rich sediment was accompanied by changes in the redox dynamics of the sediment, with multi-facet impact on the concentration, mobilisation and bioavailability of toxic trace metals, their redox transformation between oxidation states and on the toxicity within and outside the sediment environment.