Low-Temperature CO2-Enhanced Reductive Bioleaching for Selective Nickel Recovery from Iron-Rich Silicate Tailings, with Concurrent Carbon Sequestration

Abstract Recovering nickel from iron-rich mine tailings by bioleaching is held back by the co-dissolution of iron, which gives pregnant leach solutions with high Fe/Ni mass ratios and heavy downstream purification costs. This study shows that running bioleaching at 0 °C under a CO 2 -enriched atmosphere, with no post-leach iron removal step, lowers the Fe/Ni mass ratio in the leachate from 13.87 at 25 °C to 2.25 at 0 °C, a 6.2-fold improvement against a feed Fe/Ni ratio of 119:1. Nickel recovery stayed near 45% at both temperatures, showing that the fen-derived psychrotolerant consortium remained fully active near freezing. The mechanism rests on the temperature dependence of CO 2 solubility: at 0 °C, more CO 2 dissolves and raises the dissolved inorganic carbon, which drives the iron released during bioleaching to precipitate in a place as a carbonate, most plausibly siderite (FeCO 3 ), while nickel stays in solution because its carbonate is far more soluble and low dissolved nickel here stays below NiCO 3 saturation. Solid-phase data support this: the 0 °C residue gained carbon sixfold over the feed (0.360 vs. 0.060 wt%) and lost almost no iron whereas the 25 °C residue lost about 1.04% wt% iron to solution, and the extra carbon is enough to hold the retained iron as roughly 2 wt% siderite. This work establishes temperature-modulated CO 2 solubility as a previously unreported lever for selective nickel recovery from iron-rich low-grade resources, controlling iron at the source rather than downstream and well suited to cold-climate tailings reprocessing where near-freezing conditions are a natural process asset.