Insights into the Biogeochemical Cycling of Cobalt: Precipitation and Transformation of Cobalt Sulfide Nanoparticles under Low-Temperature Aqueous Conditions

Cobalt sulfide precipitates, key phases in the natural biogeochemistry of cobalt and in relevant remediation and resource recovery processes, are poorly defined under low-temperature aqueous conditions. Here, we systematically studied Co (Fe) sulfides precipitated and aged in environmentally relevant solutions, defined by different combinations of pH, initial cobalt to iron ratios ([Co] aq /[Fe] aq ), with/without S 0 , and the presence/absence of sulfate-reducing bacteria. The initial abiogenic precipitates were composed exclusively of amorphous Co sulfide nanoparticles (CoS· x H 2 O) that were stable in anoxic solution for 2 months, with estimated log K* values 1-5 orders of magnitude higher than that previously reported for Co sulfides. The addition of S 0 , in combination with acidic pH and elevated temperature (60 °C), resulted in recrystallization of the amorphous precipitates into nanocrystalline jaipurite (hexagonal CoS) within 1 month. In the presence of Fe(II) aq , the abiogenic precipitates were composed of more crystalline Co sulfides and/or Co-rich mackinawite, the exact phase being dependent on the [Co] aq /[Fe] aq value. The biogenic precipitates displayed higher crystallinity for Co sulfides (up to the formation of nanocrystalline cobalt pentlandite, Co 9 S 8 ) and lower crystallinity for Co-rich mackinawite, suggestive of mineral-specific bacterial interaction. The revealed precipitation and transformation pathways of Co (Fe) sulfides in this study allows for a better constraint of Co biogeochemistry in various natural and engineered environments.