| Abstract
| - Phosphonates are water treatment chemicals that are effectivelyutilized in many industrial processesas dispersants, bleaching agents, or scale and corrosion inhibitors.In many of these applications, thephosphonates are able to react with divalent cations such as calcium toform stable divalent cation−phosphonate precipitates. The focus of this paper is to define theconditions under which distinct calcium−phosphonate precipitates will form and to study how each of theseprecipitate's unique chemical andphysical properties govern the release of phosphonate from porousmedia. The phosphonate used in thisstudy was (1-hydroxyethylidene)-1,1-diphosphonic acid (HEDP). Byvariation of the pH and calcium/HEDP molar ratio in solution, two distinct precipitates were formed: (1) a soluble, fibrous 1:1 calcium/HEDP precipitate; and (2) a less soluble, spherical 2:1 calcium/HEDPprecipitate. Critical pH values thatdefine the conditions under which each distinct precipitate forms wereidentified. Below the first criticalpH value, the 1:1 precipitate formed, while above the second criticalpH value, the 2:1 precipitate formed.Finally, coreflood and micromodel experiments showed that therelease of 2:1 precipitate from porousmedia was significantly slower than that of 1:1 precipitate, suggestingthat the 2:1 precipitate is bettersuited for phosphonate treatments in oil field applications. Therelease of a precipitate mixture (one whichcontains both distinct precipitates and has a calcium/HEDP molar ratioof 1.4:1) from a micromodelreconfirmed this phenomenon.
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