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 t o be the primary mechanism of action. In this paper, a combined dialys is and filtration method has been used to prepare and characterize the solubility of a calcium phosphonate, for the first time. The phosphon ate used was diethylenetriaminepentakis(methylenephosphonic acid), DTP MP, 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 concentration s. The stoichiometry of the solid, between 4 and 5.5 pH is Ca3H4DTPMP( cr). The solubility product of the crystalline material versus tempera ture 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 s uggest that the solubility of phosphonates in most natural waters is c ontrolled by a two-step mechanism. First, an X-ray amorphous calcium p hosphonate phase precipitates, if the ion product exceeds the effectiv e solubility product of amorphous calcium phosphonate. This initial pr ecipitate is stable unless fresh solution flows over the solid, as occ urs in many natural situations. When fresh solution flows over the ini tially precipitated calcium phosphonate solid, a well-formed crystalli ne phase develops which is at least 2 orders of magnitude less soluble than the amorphous phase. Quantitative predictions with this model ha ve been confirmed in the field at six gas and oil wells with widely va rying produced water compositions.