A surface charge approach to characterize serpentine reactivity for carbon mineralization

Abstract

Antigorite, lizardite and chrysotile are serpentine minerals that act as Mg²⁺ donors during carbon mineralization involving ultramafic rocks and tailings. The reactivity of these minerals is measured by the total amount of Mg²⁺ that can be leached under appropriate time and conditions for the proposed carbon mineralization technology. Their dissolution processes were examined under ambient conditions to elucidate the sources and mechanisms of Mg²⁺ release. Results from dissolution experiments and zeta potential measurements are reported here. Prior to leaching, all three serpentine polymorphs exhibited highly alkaline IEPs at approximately pH 11.9 and higher, which correlated with their high Mg²⁺ to Si⁴⁺ ratios (i.e., Mg/Si ratio). As dissolution proceeded, a decrease in both zeta potential values and Mg/Si ratio was observed, reflecting the non-stoichiometric dissolution of the serpentines and highlighting polymorph-specific variations. Post-leaching IEP measurements revealed that the degree of surface alteration is closely linked to changes in zeta potential. The following order of increasing carbon mineralization reactivity is established: antigorite < lizardite < non-asbestiform chrysotile < asbestiform chrysotile. Among the polymorphs, antigorite, which released the least Mg²⁺, showed the most significant reduction in zeta potential and a shift to an acidic IEP around pH 5. In contrast, asbestiform chrysotile, with the highest Mg²⁺ release, exhibited lower zeta potential reduction while maintaining an alkaline IEP near pH 9. Non-asbestiform chrysotile leached less Mg²⁺ and showed moderate zeta potential reduction. This is followed by lizardite, which released even less Mg²⁺, retained an alkaline IEP around pH 11 and demonstrated uniform dissolution with only a slight decrease in zeta potential. These observations suggest that early-stage Mg²⁺ leaching from serpentine is predominantly a surface process that significantly influences the electrokinetic behaviour of the minerals. Higher Mg²⁺ release does not correlate with increased solubility, as the Mg²⁺ ions are not derived from bulk dissolution. The lack of a 1:1 relationship between Mg²⁺ leaching and net change in zeta potential emphasizes the distinct electrokinetic and surface dissolution behaviours among serpentine polymorphs. The findings from zeta potential analysis offer valuable insights into the reactivity contribution of serpentine minerals during carbon mineralization involving ultramafic-type feedstocks.