New article published: Accelerated Dissolution of Olivine Pebbles by Oxalic Acid at Low pH

Authors: Shilpa Mohanakumar,* Vincenzo E. Alagia, Nathalie Schilderink, Igor Siretanu, Frieder Mugele, and Michael H.G. Duits*

Olivine mineral is candidate feedstock for neutralizing strong acids or CO₂, but its rather slow dissolution limits its applicability. Naturally occurring carboxylates accelerate the dissolution at near-neutral pH, but little is known about more acidic conditions. We systematically studied the dissolution of olivine pebbles and powders, at pH 1 and 2, oxalic acid concentrations from 0 to 10 mM and 2 temperatures. At 22°C we used Fluorescence Lifetime Imaging Microscopy (FLIM) to quantify the local Mg²⁺ generation rate close to an olivine pebble and found that oxalic acid strongly accelerates the olivine dissolution (in spite of the prevalence of protonated species). In contrast, electrical conductivity measurements on slurries of pulverized olivine showed no clear oxalate effect. At 65°C where olivine dissolves much faster, we studied pebble dissolution with in-situ Confocal Raman Microscopy (CRM) to measure i) the mineral retraction rate and ii) the chemical evolution of the solid-liquid interface. In parallel, the ongoing generation of Mg, Si and Fe ion species in bulk liquid was measured with ICP-OES. The combined findings with CRM and ICP-OES indicate that oxalic acid can promote the Mg²⁺ release, while it can also generate deposits of MgC₂O₄ on the pebble. The latter process, inhibiting olivine dissolution, is enabled by a strongly peaked Mg²⁺ concentration near the pebble surface. Our work highlights the nuanced role of oxalic acid in modulating olivine dissolution and provides insights into the conditions for optimizing olivine conversion processes.

https://doi.org/10.1021/acs.langmuir.5c06150