A rapid assessment of the sedimentary buffering capacity towards free sulphides

Combined field surveys and laboratory studies were conducted in two Italian coastal lagoons, which differ for geomorphology, hydrodynamics and eutrophication degree (Sacca di Goro and Lesina lagoons, Adriatic Sea). Research aimed at assessing with a rapid technique the potential buffering capacity of sedimentary iron towards sulphides. In Spring and Summer 2004, the main pools of iron and sulphides were analysed in the uppermost sediment horizon (0-5 cm) at four stations in each lagoon. In parallel, experiments with laboratory incubations of sediment slurries were carried out at two sites in each lagoon in order to assess the sediment capacity of binding and retaining sulphides. Sediment slurries were kept stirred and anoxic with N(2) purging. Aliquots of dissolved sulphides (DS) were then added and DS concentrations were monitored until they were undetectable. On average, the total reactive iron (RFe), extracted with 6 N HCl, ranged from 170 to 400 mu mol cm(-3) in the Sacca di Goro stations, and comprised between 40 and 150 mu mol cm(-3) in the Lesina sites. The labile iron ferric quota (LFe: extractable with 0.5 N HCl) is considered representative of the microbially reducible iron fraction and was highest in spring in Sacca di Goro (up to 20 mu mol cm(-3)). Differences among stations evidenced by PCA analysis, can be inferred from RFe, LFe and AVS, which represent the iron buffer and its saturation status, respectively. The sedimentary DS uptake was 6 mu mol cm(-3) of fresh sediment in Lesina and 8-12 mu mol cm(-3) in Sacca di Goro, indicating a direct relationship between DS removal and iron availability.