The role of grain dissolution and diagenetic mineral precipitation in the cycling of metals and phosphorus: a study of a contaminated urban freshwater sediment

Taylor, Kevin G. and Boult, Stephen (2007) The role of grain dissolution and diagenetic mineral precipitation in the cycling of metals and phosphorus: a study of a contaminated urban freshwater sediment. Applied geochemistry, 22 (7). pp. 1344-1358. ISSN 1872-9134

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Abstract

This paper describes the detrital mineralogy, early diagenetic reactions and authigenic mineral precipitates for freshwater contaminated sediments deposited in an urban water body (the Salford Quays of the Manchester Ship Canal, Greater Manchester, UK). These sediments contain a mix of natural and anthropogenic detrital grains. Detrital grains are dominated by quartz and clay grains, whilst anthropogenic grains are dominated by metal-rich glass grains, concentrated at a depth of 12–17 cm in the sediment as a result of historical inputs. Sediment porewaters contain significant concentrations of Fe, Mn, Zn and phosphate. Bacterial Fe(III) and Mn(IV) reduction are hypothesised to supply Fe2+ and Mn2+ to porewaters, with phosphate released from Fe oxide reduction or organic matter oxidation. Petrographic observations indicate that the metal-rich glass grains are undergoing chemical dissolution during early diagenesis, supplying Fe and Zn to porewaters. The most abundant authigenic mineral in the sediments is vivianite (Fe3(PO4)2 · 8H2O), containing a significant level of redlingite , with minor framboidal pyrite (FeS2). Electron microprobe analysis shows the vivianite to contain Zn, Cu and Pb, suggesting that vivianite acts as a long-term sink for these metals in this sediment. This implies that authigenic vivianite in freshwater sediments may act as a long-term sink for metals, in a similar manner to sulphides in marine sediments. This study highlights that the nature of detrital and diagenetic mineralogy, in addition to porewater and sediment chemistry, needs to be considered in assessing contaminated sediments.