Impact of organic matter on As sulfidation in wetlands: An in situ experiment.

Arsenic incorporation into newly formed As sulfides has recently been identified as an important As sequestration pathway in both laboratory experiments and natural As-wetlands. Here, we used an in situ experimental technique with double nylon experimental bags (10-μm mesh) to study the effect of low-cost organic materials (sawdust, wood cubes and hemp shives) on As sulfidation in three naturally As-enriched wetland soils under water-saturated (1 m depth) and neutral pH conditions. After 15 months of in situ incubation, all of the organic materials and their corresponding inner bags were covered by yellow-black mineral accumulations, dominantly composed of crystalline AsS polymorphs (realgar and bonazziite) and reactive Fe(II) sulfides (probably mackinawite); while the major fraction of As (80%) was sequestered as AsS minerals. The amount of As accumulation in the experimental bags varied significantly (0.03-4.24 g As kg) and corresponded with different levels of As (0.23-9.4 mg As L) in the groundwater. Our findings suggest an authigenic formation of AsS minerals in strongly reducing conditions of experimental bags by a combination of reduced exchange of solutes through the pores of the bag and comparatively fast microbial production of dissolved sulfide. Arsenic sulfide formation, as an effective treatment mechanism for natural and human-constructed wetlands, appears to be favored for As(III)-rich waters with a low Fe(II)/As(III) molar ratio. These conditions prevent the consumption of dissolved As and sulfide by their preferential incorporation into natural organic matter, and newly-formed Fe(II) sulfides, respectively.