Pyromorphite precipitation

Burkholderia cepacia pyromorphite precipitate Templeton et al. (1999,2002c)... 

The presence of the metal cation and an appropriate anion (OH-, Cl-, F-, etc.) in contact with the surface of the apatite is sufficient to cause partial/complete dissolution of the apatite and precipitation of the more stable pyromorphite. A typical set of stoichiometric dissolution and precipitation reactions would be (Lower et al. 1998b) ... 

At some distance close to the mineral surface, the saturation index (SI, IAP/Xsp) of pyromorphite must be greater than zero to allow local super saturation and precipitation. The question is whether this is a heterogeneous nucleation/ surface precipitation reaction or a dissolution homogeneous nucleation reaction. 

This initial pH drop was also reported by other authors (Ma et al., 1993 Zhang et al., 1998). They demonstrated that dissolution of HA, a relatively slow process [Eq. (15.1)], causes solution pH to increase. On the contrary, the precipitation of pyromorphite is instantaneous and causes the pH to decrease sharply [Eq. (15.2)]. Thus, the initial pH drop in the system of Mavropoulos et al. (2002) occurred mainly because, initially, the precipitation of pyromorphite (instantaneous) is more rapid than the dissolution of HA (slow and pH dependent), resulting in a decrease in solution pH. In their case, dissolution of HA is the rate-limiting step. As P is exhausted in solution, the precipitation of pyromorphite stops, and hence there is a gradual increase in solution pH. Only at later stages of Pb immobilization, as the system pH approaches the PZCha and the first hydrolysis constant of Pb, could surface complexation be contributing to Pb immobilization. 

A reduction in system pH enhances the solubility of PR, making the precipitation of pyromorphite minerals possible. However, the sorption of Pb decreases sharply as the system pH decreased, producing a sigmoidal function, usually referred to as an adsorption edge, which reflects the affinity of a metal species for a mineral surface (Sposito, 1984). The ability of Pb to form inner-sphere surface complexes is related to the ability of a species in solution to form hydroxides. In fact, it has been shown that surface affinity of metal cations for Fe-oxide and Fe-hydroxide surfaces agrees with their hydrolysis values (Hayes and Katz, 1996). An analogy between solution complexation and surface complexation is represented in the following reactions (Hayes and Katz, 1996) ... 

In a system where a P mineral (e.g., PR or HA) reacts with aqueous Pb, both Ca and P concentrations are controlled mainly by the dissolution of the P mineral [Eq. (15.1)]. This is because the precipitation of pyromorphite from... 

Tg L ), whereas Ca and P concentrations, as well as solution pH, increased. The minimum reduction in Pb concentrations from 4 to 24 hours indicates that the precipitation of pyromorphite was essentially complete within 4 hours. 

The effectiveness of P in decreasing water-soluble Pb in aqueous solution as well as in soils has been well demonstrated (Ma et al., 1993, 1995 Ma and Rao, 1999). Consistent with the mechanism of dissolution of P minerals and subsequent or concurrent precipitation of pyromorphite minerals, the effectiveness of such a decrease depends on the solubility of the P mineral used. For example, at a rate of 4 g HA L HA effectively decreased aqueous Pb from 500 mg... 

Upon P applications, Pb associated with the Fe-Mn oxide fraction also decreased by 10% at the surface. This reduction of Pb in the Fe-Mn oxide fraction may be attributed to the desorption of Pb from oxide surfaces as a consequence of reduced system pH (Hayes and Katz, 1996). The reduction in pH was due mainly to phosphoric acid additions and to the precipitation of P with soluble Pb and/or to direct pyromorphite formation from oxide surface-adsorbed Pb (Zhang et al., 1997). 

Lead immobilization by phosphates arises from the low solubility of pyromorphite (Pb5(P04)3Cl). As stated, lead members of the apatite group minerals are much less soluble than their calcium congeners. Nriagu [39], as early as 1974, proposed the removal of lead from wastewaters and the stabilization of lead in contaminated soils and sediments by reaction with phosphate ions to precipitate pyromorphite. 

Several authors [11-16,18,19,41,42] have suggested that the precipitation of the lead-bearing apatites pyromorphite and hydroxypyromorphite results from prior dissolution of hydroxyapatite, which is much more soluble than the lead phases. Continuous dissolution of hydroxyapatite was observed as the result of the formation of less soluble species [12, 15, 29, 53]. Some workers have studied the effect of different phosphate amendments (synthetic... 

Of course, there can be no suggestion of remediation of arsenic-contaminated environments by addition of lead and chloride to cause the precipitation of mimetite that would be quite counterproductive. It is noted, however, that when pyromorphite crstallization is promoted by addition of phosphate to environments rich in lead, the gratuitous scavenging of arsenate by the pyromorphite-mimetite solid-solution [38] may be a useful subsidiary outcome. 

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