Distribution, speciation and particle-water interactions of nickel in the Mersey Estuary, UK

Abstract

The speciation and reactivity of Ni have been studied in a highly contaminated, macrotidal estuary (Mersey, UK). Dissolved Ni concentrations were determined by adsorptive cathodic stripping voltammetry, and labile and non-labile forms were discriminated by ligand exchange with dimethyl-glyoxime. Particle–water interactions were examined by monitoring the uptake and release of the beta-emitting nuclide, 63Ni, by suspended estuarine particles. Concentrations of total dissolved Ni measured during four seasonal transects of the estuary ranged from about 10 nM in seawater to about 230 nM in mid-estuary during summer. Seasonal and axial variations in concentration appeared to be controlled by the dilution of natural and anthropogenic sources of Ni in the catchment (i.e., variations in river flow), and the magnitude of a persistent, but seasonally variable source of Ni to mid-estuary. The ratio of labile to non-labile Ni was reasonably uniform throughout the salinity gradient during each survey, but the magnitude of this ratio exhibited seasonal differences. Results of complexing capacity titrations indicated that Ni forms complexes with a class of strong ligands (stability constants ∼ 10 18) and that these ligands are saturated by ambient Ni concentrations. Speciation calculations predict that about 70% of dissolved Ni is complexed by these ligands throughout the estuary. The equilibrium sediment–water distribution coefficient for 63Ni ranged from about 200 ml g −1 in the upper estuary, to about 1200 ml g −1 in the marine end-member, and 63Ni added to riverine sediment exhibited little tendency to desorb when resuspended in saline water. The relatively low particle-reactivity displayed by Ni (compared with many other transition metals in estuaries) appears to be the result of the competing effects of dissolved complexing ligands and particle sorption sites for aqueous Ni, while the increase in particle-reactivity with increasing salinity is interpreted and modelled in terms of the salting out of Ni-organic complexes. Mass balance calculations indicate that mid-estuarine peaks of dissolved Ni are likely the result of desorption of labile and non-labile Ni from highly contaminated particles that are tidally resuspended into the local water column. A general implication of this study is that the geochemical behaviour of Ni in contaminated environments is determined, to a significant extent, by the abundance or availability of complexing ligands, and the interactions of the resulting complexes with suspended particles and dissolved salts.