Mercury in contaminated sediments and pore waters enriched in sulphate (Tagus Estuary, Portugal)

Three sediment cores, collected nearby the effluent of a chlor-alkali industry, were sliced in 0.5-cm layers and centrifuged for pore water extraction. Mercury, Fe and Mn were determined in the solids as total concentration, hydroxylamine extractable fraction and HCl extractable fraction. Sulphur was determined in the HCl extraction. Total and reactive mercury, chlorinity, S2-, SO42-, total Fe, and total Mn were measured in pore waters. The solids contained 3.0–60 nmol g^-1 of total Hg and pore waters 70–5800 pM of total Hg and 1.8–76 pM of reactive mercury. Pore waters presented 2.3–94 times more sulphate than the overlying estuarine waters due to the input from the industry. In layers where hydroxylamine extractable Fe exhibited a broad maximum (precipitation of Fe-oxides) sulphate was reduced to S2-. The competition between the high content of SO42- and Fe(III) as electron acceptors, in chemical reactions occurring in the upper sediments, may explain the co-existence of S2- and Fe-oxides in the same layers. Mercury was detected in the hydroxylamine extracts (20–29 nmolg-1) in the layers where Fe-oxides were formed, and reactive dissolved Hg showed minimum concentrations. The excess of sulphate in pore waters favoured the abundant Fe-oxides in the upper solid sediments, which appear to work as a barrier limiting the escape of mercury to the water column.