Rapid Dissolution of BaSO₄ by Macropa, an 18-Membered Macrocycle with High Affinity for Ba²

Insoluble BaSO₄ scale is a costly and time-consuming problem in the petroleum industry. Clearance of BaSO₄-impeded pipelines requires chelating agents that can efficiently bind Ba²⁺, the largest nonradioactive +2 metal ion. Due to the poor affinity of currently available chelating agents for Ba²⁺, however, the dissolution of BaSO₄ remains inefficient, requiring very basic solutions of ligands. In this study, we investigated three diaza-18-crown-6 macrocycles bearing different pendent arms for the chelation of Ba²⁺ and assessed their potential for dissolving BaSO₄ scale. Remarkably, the bis-picolinate ligand macropa exhibits the highest affinity reported to date for Ba²⁺ at pH 7.4 (log K' = 10.74), forming a complex of significant kinetic stability with this large metal ion. Furthermore, the BaSO₄ dissolution properties of macropa dramatically surpass those of the state-of-the-art ligands DTPA and DOTA. Using macropa, complete dissolution of a molar equivalent of BaSO₄ is reached within 30 min at room temperature in pH 8 buffer, conditions under which DTPA and DOTA only achieve 40% dissolution of BaSO₄. When further applied for the dissolution of natural barite, macropa also outperforms DTPA, showing that this ligand is potentially valuable for industrial processes. Collectively, this work demonstrates that macropa is a highly effective chelator for Ba²⁺ that can be applied for the remediation of BaSO₄ scale.