Iron speciation and mineral characterization of contaminated sediments by coal mining drainage in Neath Canal, South Wales, United Kingdom

In the early 1990's, the Neath Canal in South Wales, UK, received large amounts of drainage waters from nearby coal mines, which contributed to its contamination by heavy metals and arsenic. One sediment core and surface sediments were collected from the upstream section of the Neath Canal and characterized for their mineral composition and iron speciation using powder X-ray diffraction (XRD) and Mössbauer spectroscopy. The sediments show three distinctive layers that are defined by their physical properties including color, sediment components and dryness. The upper layer of the sediment (0-22 cm) is a reddish-brown wet precipitate dominated by iron oxides and hydroxides and a high content of arsenic. The middle layer (22-27 cm) is a soft wet deposit of yellow color which mainly contains calcite with sheet silicates (kaolinite) and goethite. Magnesium, calcium and manganese are enriched in this layer whereas iron is depleted compared to the upper layer. The lower part of the core (below 27 cm) is colored gray to dark gray and contains quartz, pyrite and clay minerals, similar to normal aquatic sediments. In addition, this layer also contains abundant coal particles. Silicon, aluminium, titanium, potassium, phosphorus and sodium concentrations are higher whereas iron, manganese, calcium and magnesium are lower in the lower portion of the core compared to the middle and upper layers. Mineral composition, major elements, and iron speciation indicate oxic conditions in the upper and middle layers whereas reducing conditions prevail in the lower layer, which likely control the distribution of hazardous elements. Given the variation of physico-chemical characteristics of the sediments with depth in the canal, different remediation treatments will likely be necessary for each layer of sediments.