Alkaline earth metal ions affect the dissolution of kaolinite-bearing olivine and its carbon storage effects

Olivine dissolution applications in coastal environments hold great promise for both ocean alkalinity enhancement and carbon dioxide storage. Yet the dissolution mechanism of olivine and the influence of metal ions, such as Mg ²⁺ and Ca ²⁺, remain unclear. Moreover, the weathering products, such as clay minerals are usually mixed with olivine, and the influence of the associated minerals on the dissolution also unexplored. In this study, an 80-day dissolution experiment was performed in Ca ²⁺-free artificial seawater (ASW-Ca) and ASW lacking both Ca ²⁺ and Mg ²⁺ (ASW-CaMg), to elucidate the mechanism of olivine dissolution and the impact of Ca ²⁺ and Mg ²⁺ on the presence of an associated clay mineral, kaolinite. To evaluate the mineral properties before and after dissolution, X-ray diffraction spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analyses were used. Etch pits were observed on olivine`s surface as well as its reduced crystallinity. The atomic ratio of Mg/Si in olivine decreased from 1.49 ± 0.56 in the raw samples to 1.29 ± 0.53 in the samples obtained from ASW-CaMg after undergoing the 80-day dissolution, whereas it was much higher, at 1.85 ± 0.32, in the samples from ASW-Ca. The increasing Mg/Si was attributed to the Mg ²⁺ adsorption on the surface of olivine, which inhibited the release of structural Mg ²⁺. Hence, Mg ²⁺ was a key environmental factor influencing the process of olivine dissolution, and estuaries, which have low Mg ²⁺ content and low pH may be the promising areas for the olivine application. Moreover, in ASW-CaMg, the total alkalinity (TA), dissolved inorganic carbon (DIC) concentration, and charge concentration of alkaline earth metal ions increased by 3392 ± 28, 2922 ± 3, and 3740 ± 91 μmol kg ⁻¹, respectively, compared to the data from the blank experiment. Notably, TA and DIC had strong linear relationships with the alkaline earth metal ions (Mg ²⁺ and Ca ²⁺). The release of free alkaline earth metal cations during olivine dissolution was a controlling factor for long-term carbon dioxide storage in ASW-CaMg. Finally, during the 80-day dissolution experiment, no dissolution of the associated kaolinite was observed, however, kaolinite may influence the olivine dissolution by adsorbing the released Si.