Rapid Dissolution of BaSO by Macropa, an 18-Membered Macrocycle with High Affinity for Ba.

Insoluble BaSO scale is a costly and time-consuming problem in the petroleum industry. Clearance of BaSO-impeded pipelines requires chelating agents that can efficiently bind Ba, the largest nonradioactive +2 metal ion. Due to the poor affinity of currently available chelating agents for Ba, however, the dissolution of BaSO 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 Ba and assessed their potential for dissolving BaSO scale. Remarkably, the bis-picolinate ligand macropa exhibits the highest affinity reported to date for Ba at pH 7.4 (log K' = 10.74), forming a complex of significant kinetic stability with this large metal ion. Furthermore, the BaSO 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 BaSO is reached within 30 min at room temperature in pH 8 buffer, conditions under which DTPA and DOTA only achieve 40% dissolution of BaSO. 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 Ba that can be applied for the remediation of BaSO scale.