The control of solution composition on ligand-promoted dissolution: DTPA-Barite interactions
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The mechanism and kinetics of barite (BaSO4) dissolution in the presence of diethylenetriaminepentaacetic acid (DTPA) has been investigated as a function of solution composition. The dependence of the reaction rate on the background electrolyte present in solution (NaCl or KCl) and on the concentration of the chelating agent (DTPA) is explained by considering chemical speciation and conformational changes of DTPA in the aqueous phase. A mechanism for the promotion of the dissolution reaction by dissociated ionic species is proposed for an organic polyelectrolyte with a strong affinity to Ba 2+ ions (DTPA) and for simple inorganic electrolytes. The mobilization of ions from the crystal structure is suggested to be induced mainly by water molecules and not by specific additive-surface interactions. Recognition of the correlation between solution composition, ion-water interactions (hydration phenomena), and the dissolution process enables an explanation of the faster dissolution kinetics of barite in the aqueous solution of a simple inorganic salt (NaCl) compared to in the solution of a strong chelating agent (DTPA). Our findings imply that because the mechanisms of complexation in solution and mobilization of ions from the solid surface are different, the sequestering capacity of the ligand toward dissolved ions cannot be used to predict the dissolution rate. © 2009 American Chemical Society.
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