FORMATION OF 2 CALCIUM DIETHYLENETRIAMINEPENTAKIS(METHYLENE PHOSPHONIC ACID) PRECIPITATES AND THEIR PHYSICAL-CHEMICAL PROPERTIES

Phosphonates are widely used in industry to treat water for scale and corrosion and in medicine to treat osteoporosis and they are added to toothpaste to control tartar build-up, to mention a few uses. In most of these applications either adsorption or precipitation is proposed to be the primary mechanism of action. In this paper, a combined dialysis and filtration method has been used to prepare and characterize the solubility of a calcium phosphonate, for the first time. The phosphonate used was diethylenetriaminepentakis(methylenephosphonic acid), DTPMP, which has ten ionizable protons and is commonly used in industry. The solubility has been measured up to 2 m ionic strength, from 25 to 90-degrees-C, and over a wide range of calcium and DTPMP concentrations. The stoichiometry of the solid, between 4 and 5.5 pH is Ca3H4DTPMP(cr). The solubility product of the crystalline material versus temperature and ionic strength can be represented by the following equation: pK(sp crystalline) = 58.95-2084.5/T+0.048I0.5. Results of this study suggest that the solubility of phosphonates in most natural waters is controlled by a two-step mechanism. First, an X-ray amorphous calcium phosphonate phase precipitates, if the ion product exceeds the effective solubility product of amorphous calcium phosphonate. This initial precipitate is stable unless fresh solution flows over the solid, as occurs in many natural situations. When fresh solution flows over the initially precipitated calcium phosphonate solid, a well-formed crystalline phase develops which is at least 2 orders of magnitude less soluble than the amorphous phase. Quantitative predictions with this model have been confirmed in the field at six gas and oil wells with widely varying produced water compositions.