Mineral carbonation can be performed directly (by treatment of the dry or slurried minerals with carbon dioxide) or indirectly (by conversion of the minerals to metal oxides or hydroxides followed by carbonation). Direct carbonation in the solid phase is limited by mass transport and is generally slow unless high surface area absorbents are used. Carbonation of minerals in aqueous solution is fast, but the dissolution of minerals in water is slow. The solubility of minerals in water is improved with acids, with hydrochloric or acetic acid being the most common acids used. Hydrochloric acid is corrosive to equipment, while acetic acid is less corrosive but more costly; both acids are difficult or impossible to recover, increasing the costs of their use further.