Accelerated Dissolution of Olivine Pebbles by Oxalic Acid at Low pH

Olivine mineral is a candidate feedstock for the neutralization of strong acids or CO₂, but its rather slow dissolution poses limitations to its applicability. Naturally occurring carboxylates, such as oxalates, are known to accelerate the dissolution at near neutral pH, but little is known about the effect under more acidic conditions. We performed a systematic study on the dissolution of olivine pebbles and powders, in the pH range from 1 to 2 and for oxalic acid concentrations from 0 to 10 mM. Both ambient and elevated temperatures were addressed. At 22 °C, we used fluorescence lifetime imaging microscopy to quantify the local Mg²⁺ generation rate close to an olivine pebble and found that oxalic acid strongly accelerates the olivine dissolution (despite the prevalence of protonated oxalate species). In contrast, electrical conductivity measurements on slurries of pulverized olivine showed no clear oxalate effect. At 65 °C, where olivine dissolves much faster than at 22 °C, we studied pebble dissolution with <i>in situ</i> 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 inductively coupled plasma-optical emission spectroscopy (ICP-OES). The combined findings of CRM and ICP-OES indicate that oxalic acid can promote Mg²⁺ release, while it can also generate deposits of magnesium oxalate (MgC₂O₄) on the pebble surface. The latter process, which inhibits olivine dissolution, is indicated to be 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.