Accelerants of Calcium Oxalate Monohydrate Crystal Dissolution

The use of molecular modifiers as crystal growth inhibitors is a common approach in crystal engineering, yet relatively few studies are devoted to designing these species as accelerants of crystal dissolution for applications in human health and commercial processes where crystal formation can have undesirable outcomes. Here, we examine the effects of various organic and inorganic modifiers on the dissolution of a calcium mineral that is implicated in pathological crystallization and is also representative of materials that commonly form scale in industrial processes. The mineral selected for this study, calcium oxalate monohydrate (COM), is sparingly soluble in aqueous media and poses challenges for removal once formed. Using a combination of in situ techniques under quiescent and flow conditions, we identify effective dissolving agents comprised of molecular modifiers with carboxylate and phosphate functional moieties as well as simple monovalent and divalent electrolytes. Time-resolved microfluidics experiments show that sodium orthophosphate glass [sodium phosphate glass type 45 (or P45)] and Zn²⁺ ions dissolve COM crystals approximately 1.6- and 2.7-times faster than that of deionized water, respectively. Batch assays reveal that Zn²⁺ can completely dissolve COM crystals within hours compared to other inorganic cations (e.g., Mg²⁺) that require days or organics that only partially dissolve crystals under quiescent conditions. In situ atomic force microscopy measurements reveal distinct modes of dissolution for various modifiers, whereas electron micrographs of partially dissolved COM crystals uncover defects within their interior that have previously gone undetected. Collectively, the findings in this study provide a conceptual foundation for understanding crystal dissolution pathways for COM and related minerals, and offer guidelines for future research efforts directed toward the rational design of next-generation dissolving agents.