Effect of Chelation Chemistry of Sodium Polyaspartate on the Dissolution of Calcite

Polyaspartic acid (PASP) is an environmentally benign agent for the dissolution of calcium salt deposits. In this study, the chelating power of PASP with calcium is investigated by performing the potentiometric titration against PASP solutions. It was found that PASP is fully deprotonated at pH 7. The titration curves are successfully modeled by assuming that four aspartyl residues from an actual PASP molecule consist of a hypothetical molecule (denoted as H 4 L) that has four distinct acid moieties (four dissociation constants). The resulting dissociation constants and calcium-binding constants for PASP are critical to understanding the dissolution behavior of calcite in PASP solutions. The batch dissolution of calcite powder in PASP solution at four different initial pHs (10, 7, 5, and 3.5) are performed to explore the effect of PASP chelating chemistry on the dissolution behavior of calcite (CaCO 3 ). The results show that PASP replaces interfacial water to react with calcite at high pH (≥7) and the dissolution of calcite can be described by a surface adsorption and complexation mechanism. At low pH (≤5), surface adsorption of PASP on calcite surface still plays an important role, and both acidic species attacks (H + and H n L n - 4, n = 1,2,3,4, attacking carbonate sites) and chelant attacks (L 4 - and H n L n - 4, n = 1,2, attacking calcium sites) affect the calcite dissolution. The dissolution of calcite in acidic PASP solutions represents the most complicated case.