Redox geochemistry in organic-rich sediments of a constructed wetland treating colliery spoil leachate

The results are reported of a geochemical study of sediment cores and surface waters taken over an annual cycle from the compost-based constructed wetland at Quaking Houses, NE England. The wetland was built to treat acidic and metalliferous waters emanating from colliery spoil. The influent waters contain up to 10 mM image, total Fe around 100 μM, and a mean pH of 6.2. The organic-rich sediments sustain a coupled redox cycle of Fe and S which occurs throughout the year but which is more intense in the summer months. Throughout the sediments, reduction of Fe(III) and image apparently occur within the same macroscopic volume of sediment, along with oxidation of sulfide and Fe(III). In the top 2 cm of the sediment, pore water Fe concentrations reach a maximum of not, vert, similar1 mM in the presence of high concentrations of Fe oxides and the occurrence of SO4 reduction. Partial re-oxidation of sulfide is indicated by the presence of significant elemental S. Total Fe in the surface 10 cm has a mean value of 7–9% of sediment dry weight, of which sulfide–Fe represents 7–12% and reactive amorphous Fe 20–45%; around 90% of the solid phase sedimentary Fe occurs as oxides or oxyhydroxides. It is suggested that the downwards diffusion of dissolved Fe from the near-surface maximum is sustained by the precipitation of Fe oxides as a result of radial O2 loss from roots in the dense rhizosphere. Pore water pH is between 7.2 and 7.8 and alkalinity increases downwards, coupled to microbial SO4 and Fe reduction. Transport processes occurring at and across the sediment–water interface are sufficiently rapid in the not, vert, similar20 h residence time of the waters to: (a) remove 70–90% of influent Fe and 15–25% of influent SO4 into surface sediments and (b) increase both the pH and alkalinity of effluent waters. Coupling of the Fe and S cycles is fundamental to effective remediation in terms of both alkalinity generation and the retention of metals.