Moessbauer and XRD characterization of contaminated sediments by coal mining drainage in Neath Canal, South Wales, UK

Sediment cores were collected from the upstream of the Neath Canal in South Wales and characterized for their mineral compositions and iron speciation in order to understand the influence of sedimentary properties on the contaminants of heavy metals. The sediments in the canal have been polluted by ocherous precipitate with heavy metals such as arsenic since a major mine water discharge in spring 1993. The sediment core was obtained just from the discharge point in the canal and distinguished into three layers in the field regarding to their physical properties including visible color, sediment particle composition and dryness. X-ray diffraction （XRD） and M6ssbauer spectroscopy revealed vertical variations of minerals and iron species in depth of the sediment profile. The upper layer, reddish-brown, is wet muddy precipitant and is very fine in particle size, consisting mainly of iron oxide and hydroxide. Arsenic is rich in this layer; probably the sorption on iron oxide surface is the main mechanism. The middle layer, 22.0-27.0 cm, is soft deposit in a yellow color and contains sheet silicate, calcite and goethite. The lower part below 27.0 cm is gray to dark gray in color and contains quartz, pyrite, coal particles that are much more similar to normal aquatic sediment. The upper two layers in brown and yellow colors as newly precipitated mud from AMD contain high concentrations of As, Ni under oxic conditions. The lower part as the old canal sediments under reducing conditions contains high Cu and Zn. Chromium is enriched in the limited part of the low layer, just beneath the boundary between the middle and low parts of the sedimentary column, where the redox conditions shifted from oxic to anoxic. It is clear that the distribution of these heavy metals is closely associated with the redox condition reflected in the iron speciation. Thus different treatments are necessary for each layer of the sediment since different mineral and geochemical properties of the sediment will give different impacts on the heavy metals.

Moessbauer and XRD characterization of contaminated sediments by coal mining drainage in Neath Canal, South Wales, UK