Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Peatlands and other wetlands with abundant particulate natural organic matter (NOM) are recognized as important sinks for potentially toxic antimony (Sb). While formation of Sb(III) sulfide phases or Sb(III) binding to NOM was shown to reduce Sb mobility, the exact binding mechanisms remain elusive. Here, we reacted increasing sulfide concentrations with purified model peat at pH 6, forming reduced organic sulfur species, and subsequently equilibrated the reaction products with 50 µM of antimonite under anoxic conditions. Sulfur solid-phase speciation and the local binding environment of antimony were analyzed with Sb K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. We found that 85% of antimonite was sorbed by untreated peat, while sulfide reaction with peat increased sorption up to 98%. EXAFS shell-fitting of the spectra of untreated peat revealed that Sb coordinates to oxygen, and Sb-carbon distances of ~2.90 Å are in line with binding to carboxylic groups. With increasing content of reduced organic sulfur, Sb is progressively coordinated to S atoms at distances of ~2.45 Å and Sb-carbon distances of ~3.33 Å, suggesting increasing Sb-thiol binding. The existence of reduced inorganic Sb-sulfur phases, which would have similar Sb-sulfur distances, could be excluded with iterative target factor analysis of the full set of EXAFS spectra. In conclusion, particulate NOM is able to sequester Sb in anoxic, sulfur-enriched environments without need for high free sulfide concentrations.