Magnetically-mediated regeneration and reuse of core-shell Fe 0 @Fe III granules for in-situ hydrogen sulfide control in the river sediments.

A novel iron-cycling process based on core-shell iron granules, which contained zero-valent iron (Fe0 ) in the core and maghemite (γ-Fe2 O3 ) on the shell (Fe0 @FeIII granules), was proposed to in-situ control hydrogen sulfide in the sediments of the polluted urban rivers. The Fe0 @FeIII granules added in the top sediment layer removed 97% of sulfide generated by sulfate-reducing bacteria in the sediments, and the sulfide removal capacity of virgin granules was 163 mg S/g Fe (114 mg S/g granule). The Fe0 @FeIII granules removed the formed sulfide through the abiotic sulfide oxidation and precipitation, and they also stimulated the microbial iron reduction, which competitively consumed wastewater-derived organics and partially inhibited the sulfate reduction in the sediments. The used Fe0 @FeIII granules were easily regenerated through magnetic separation from sediments and air exposure for 12 h, which enhanced the sulfide removal capacities of the regenerated granules by 12%-22%, compared to the virgin granules. During the air exposure, ferrous products (i.e., iron sulfide and surface-associated FeII ) on the granule shell were completely oxidized to poorly ordered FeIII hydroxides (γ-FeOOH and amorphous FeOOH) having larger specific surface areas and higher reactivity to sulfide than γ-Fe2 O3 on the virgin granules. Meanwhile, the Fe0 in the core was also partially oxidized through the indirect electron transfer, which was facilitated by the electrically conductive iron oxide minerals (Fe3 O4 and Fe2 O3 ) and the microbial electron carriers (e.g., Geobacter). The oxidation of Fe0 core contributed additional FeIII hydroxides to the sulfide control. The Fe0 @FeIII granules were reused for four times in a 293-day trial, and their overall sulfide removal capacity was at least 920 mg S/g Fe. The proposed iron-cycling process can be a chemical-saving, energy-saving and cost-effective approach for the hydrogen sulfide control in the sediments of polluted urban rivers, as well as lakes, aquaculture ponds and marine.