Sulphate reduction zone

Sulphate reduction in marine sediments is often the dominant form of carbon remineralisation, and contributes most of the alkalinity observed in the sediment pore-water. When concentrations of sulphate fall below 35 to 40 pM, carbon dioxide reduction by methanogens may begin, whereas sulphate levels below about 30 pM are required before the onset of acetate-type reduction reactions that dominate methanogenesis (Kuivila et al., 1989). Carbon dioxide reduction is usually only evident in the sulphate reduction zone, where bicarbonate is produced, but it can contribute about 65% of the methane flux at its peak (Crill Martens, 1986). 

Sulphate reduction zones utilize the fact that under anoxic conditions metals may be removed from the mine waters as stable sulphide precipitates. Under these conditions sulphide minerals remain stable and have extremely low solubilities. Flooded underground mine workings and open pits can be anoxic, and as such provide a suitable environment for the implementation of a sulphate reduction system. In tailings and... 

Isaksen, M. F., and Finster, K. (1996). Sulphate reduction in the root zone of the seagrass Zostera noltii on the intertidal flats of a coastal lagoon (Arcachon, Prance). Mar. Ecol. Prog. Ser. 137, 187—194. 

These two reactions contribute C to the pore waters in the sulphate reduction and, particularly, the suboxic zones. Methane seepages on the seafloor are accompanied by the formation of authigenic calcite and aragonite that are highly enriched in (Hovland el al 1987). 

Carbon dioxide generated during all stages of bacterial and thermocatalytic organic-matter degradation will only produce carbonic acid if the pH is controlled by the carbonate system itself. In the sulphate reduction and methane generation zones, the reduction of metals (Fe ", Mn" " ) raises the pH and an increase in the partial pressure of CO2 leads... 

Respiratory sulphate reduction ideally takes place when all other electron acceptors are exhausted, but significant overlap may occur between the zones of microbial Fe(III) reduction and sulphate reduction due to kinetic constraints, as discussed before. Sulphate concentrations typically decrease to zero within the upper sediment layer (Fig. 1). In freshwater sediments, reduced S formed mainly by reduction of pore-water sulphate, is predominantly present as inorganic S in the form of AVS. Although pyrite is the most stable sulphide mineral, its formation in permanently submerged freshwater sediments is subject to controversy (Rickard et al., 1995). Because, contrary to marine sediments (S-dominated), there is an excess of Fe liberation over HS production in freshwater sediments (Fe-dominated), FeC03 as well as FeS may control pore-water Fe concentrations in the anoxic sediment layer. 

Reduction environment is described by total absence of O. It includes zones sulphid, where reduction of sulphates begins, and methane, where sulphate-reduction is replaced by methane formation. Significance and distribution of these zones largely depend on the content of organic matter and ground water salinity. 

Copper-chromium catalysts employed for CO oxidation were found to be affected by composition and pretreatment parameters. CuCr20i, was more active than CuO only if prereduction was carried out and if metal concentration on alumina support was larger than 12 w t %. The presence of Cr with Cu in the oxide limited the extent of catalyst reduction leading also to less deactivation as compared to Cu on alumina. The presence of Cr also decreased an activity inhibition effected by water. A supported Cu-Cr catalyst used in an automobile ran with leaded petrol was deactivated by lead deposition. Deposits were mainly lead sulphate located on pellet periphery. Also, lead was preferentially distributed on the alumina instead of on the active metal-rich zones of the catalysts. 

Hence, since the thickness of the barrier (24 cm) is greater than the minimum length of reaction zone (0.6cm), the thickness of barrier would provide a sufficient time for Cr(VI) reduction. The above estimation does not account for solution ligands such as chloride, carbonate, sulphate, citrate, oxalate, nitrate, and phosphate that can complex with Fe° to decrease chromate reduction rate. In addition, the overall design of the barrier should consider the possible existence of dissolved oxygen in the EO flow or the O2 gas bubble produced from the electrolysis reaction at the anode that can oxidize the Fe and decrease the efficiency of the ZVI reduction performance. 

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