Comparison of three methods for estimation of the reduced-S content in estuarine sediments

Estimation of the reduced-S content in coastal sulphidic soils/sediments was made by measuring the net increases in titratable acidity (TSA; Andriesse, 1993) and in sulphate concentration (POSA; Lin and Melville, 1993) in soil extracts after treatment of a sample with 30% hydrogen peroxide (H₂O₂). Comparison shows that the difference between POSA and TSA is markedly pH-dependent and this is attributed to the differing pH buffering and neutralising capacity of the individual samples under the hydrogen peroxide treatment procedure. The peroxide treatment procedure has the disadvantage in causing the partial retention of sulphate by soil/sediment residues and this is responsible for the observed underestimation of the reduced-S content, compared to that determined by the Leco CNS Analyzer. The amount of sulphate retained by the peroxide-treated sample residue increases with decreasing pH of the extract and is also related to the soil/sediment texture where clayey soils favour the retention of sulphate. To measure the reduced-S content of oxidized sulphidic soils using the Leco technique, the non-reduced-S can be effectively removed by a two-step 0.1 M EDTA-4 M HCl extraction. Comparison shows that 4 M HCl solution alone has similar extracting capacity to the two-step procedure.