PNNL

Abstract

Cobalt sulfide precipitates, key phases in the natural biogeochemical cycling of cobalt and in relevant remediation and resource recovery processes, remain poorly defined under low-temperature aqueous conditions. Here, we systematically studied the Co-(Fe) sulfides precipitated and aged in environmentally-relevant solutions, defined by different combinations of pH, initial cobalt-to-iron ratios ([Co]aq/[Fe(II)]aq), with/without S0, and the presence/absence of sulfate-reducing bacteria. The initial abiogenic precipitates without Fe(II)aq were composed exclusively of amorphous Co-sulfide hydrates (CoS· x H2O), whose estimated log K* was three orders of magnitude higher than that previously reported for crystalline Co-sulfides. The addition of S0, in combination with acidic pH and elevated temperature (60°C), resulted in the recrystallization of the amorphous precipitates into nanocrystalline jaipurite (hexagonal CoS) within one month (which otherwise remained stable for up to two months). In the presence of Fe(II)aq, the abiogenic precipitates were composed of more crystalline Co-sulfide hydrates and/or Co-rich mackinawite, the exact phase being dependent on the specific [Co]aq/[Fe(II)]aq. The biogenic precipitates, in general, displayed higher crystallinity up to the formation of nanocrystalline cobalt-pentlandite. The revealed precipitation and transformation pathways of Co-(Fe-)sulfides in this study allows for predicting the prevalent Co-bearing phases in various natural and engineered environments.