A study of the extraction conditions of sedimentary humic acids to estimate their true in situ sulfur content

Abstract

Precautions are necessary to extract the humic acid fraction from anoxic marine sediments without artificially raising its sulfur content. Elemental sulfur and free sulfides must be removed from the sample before proceeding with alkaline extraction. Also, the extent of sulfur contamination from coextracted pyrite and greigite must be estimated. In my procedure the sediment samples were freeze‐dried immediately after their extrusion under a nitrogen atmosphere onboard ship. Humic substances were extracted by 0.5 N NaOH after removal of elemental sulfur from the dried sediment with benzene, which also removed the lipid materials. Humic acids were precipitated from the alkaline solutions by acidification to pH 2 with HCl. The formation of elemental sulfur by the oxidation of H2S produced by decomposition of coextracted acid‐volatile sulfides was minimized by precipitating humic acids under a vigorous stream of nitrogen. Precipitates were freeze‐dried and their elemental composition determined. Subsequently, samples were treated with a freshly prepared CrCl2 solution, and the amount of H2S produced was measured to estimate the sulfur contribution of coextracted pyrite and greigite.