Speciation and sorptive behaviour of nickel in an organic-rich estuary (Beaulieu, UK)

Abstract

Abstract The speciation of dissolved and particulate Ni has been determined along an axial transect of the Beaulieu Estuary, southern England, using adsorptive cathodic stripping voltammetry (ACSV). Total dissolved concentrations ranged from about 80 nM in the river end-member to about 15 nM in the marine end-member, and the ACSV-non-labile fraction, comprising Ni complexes whose alpha coefficients exceed 10 9.2 , contributed between about 15 and 60% of the total dissolved pool. The axial distributions of total dissolved Ni and species thereof, and dissolved organic carbon, could be defined in terms of simple end-member mixing, suggesting little biogeochemical reactivity in the estuary. Particulate Ni was dominated (≥70%) by the fraction available to acetic acid, and a distinct seaward decrease from about 35 to 2 μg g −1 could, likewise, be largely accounted for by mixing of (particle) end-members of different composition. The uptake of Ni by Beaulieu particles was monitored in a series of laboratory simulations employing 63 Ni and liquid scintillation counting. Adsorption was dependent to some degree on salinity, pH and particle type, but most importantly on the presence of dissolved organic matter. Thus, particle–water distribution coefficients ( K D s) were almost doubled when artificial river and sea waters, containing no pre-existing dissolved organic matter, were used in place of organic-rich Beaulieu end-members, and increased by seven-fold when UV-irradiated river water was used in place of untreated river water. These observations were interpreted in terms of the formation of small (sub-colloidal) and relatively strong organic complexes which are relatively resistant to adsorption, and a simple model was developed, incorporating the K D s, to predict the ratio of this fraction of dissolved Ni to the remaining fraction of dissolved Ni as a function of suspended particle concentration. The ACSV (field) measurements of labile and non-labile dissolved Ni from the Beaulieu conformed with the model prediction when K D s for UV-irradiated and untreated river waters were employed, implying that the ACSV-non-labile fraction of dissolved Ni (log 10 α>9.2) affords a good indication of the fraction which is resistant to adsorption. It is suggested that the low biogeochemical reactivity of Ni in the Beaulieu results from a combination of its low particle affinity, maintained by the presence of specific dissolved organic ligands, and small suspended sediment fluxes throughout the estuary. The distributions of Ni in other estuaries are examined and implications for the more general estuarine behaviour of Ni are discussed.