Monitoring acid-volatile sulphide by a fast scan voltammetric method: application to mercury contamination studies in salt marsh sediments

The present work demonstrates the application of fast scan linear sweep cathodic stripping voltammetry to the determination of acid-volatile sulphide (AVS) in salt marsh sediments. This approach combines very fast analysis with relatively high sensitivity without the need for additional reagents or derivatization procedures. The purge-and-trap AVS methodology was used. The sulphide content in the trapping NaOH solution was directly measured by voltammetry with a high scan rate (1000 mV s^-1). The limit of detection (3(sigma)) was 0.34 (mu)mol L^-1 for a deposition time of 5 s. Vertical distribution of AVS in sediments was determined in salt marsh sediments of a Portuguese coastal lagoon vegetated with H. portulacoides and correlated to the total dissolved mercury content in pore waters. At the depth of higher root biomass (around 20 cm), AVS concentrations (range 0.24–24.8 (mu)mol g^-1) were low in comparison to other depths, probably due to modifications in sediments that occur in the vicinity of the roots. The highest dissolved mercury concentrations (835 and 675 ng L^-1), were found at the surface layer and at 20 cm depth, with an opposite trend from that of AVS. A possible explanation is that root activity reduced AVS concentrations at the depth of higher root biomass and resulted in elevated pore water concentrations of mercury. Concentrations of mercury in pore water from layers above and below the high root biomass were consistently lower (less than 152 ng L^-1).