Stability of reduced organic sulfur in humic acid as affected by aeration and pH

Reduced S groups in soil organic matter (OM) play an important rule in the complexation of heavy metals. These complexation reactions are often studie d using the humic acid component of OM, The objective of our study was to d etermine the effect of pH on humic acid yield and the effect of pH and expo sure to O-2 on the oxidation of reduced organic S, Humic acid was extracted at pH levels between 11.5 and 13.0 from a soil collected from a salt marsh , Also, aqueous samples of humic acid at different pH levels were aspirated with CO,free air for 4 or 44 h, X-ray absorption near-edge structure (XANE S) spectroscopy was used to determine changes in S oxidation states for the various treatments. With increasing pH, the yield of humic acid increased from 1.3 to 4.6 g humic acid kg(-1) soil, and the total S in these samples was 24 +/- 1 g kg(-1) humic acid, Linear-combination fitting of XANES spect ra showed that all of these humic acid samples contained approximate to 70% (mol/mol) reduced S (modeled as benzyl disulfide) and 30% (mol/mol) oxidiz ed S (modeled as sulfonate and ester sulfate). For humic acid exposed to ae ration for 4 h at pH levels between 11.5 and 13.0, reduced organic S was ox idized only at pH 13.0 (15% [mol/mol] of total S), Samples exposed to aerat ion for 44 h between pH 3.5 and 12.4 showed no detectable change in reduced organic S.