Citation:

Chappell, M. A. AND K. G. SCHECKEL. Pyromorphite Formation And Stability After Quick Lime Neutralisation In The Presence Of Soil And Clay Sorbents. ENVIRONMENTAL CHEMISTRY. CSIRO Publishing, Collingwood Victoria, Australia, 4(2):109-113, (2007).

Impact/Purpose:

To describe experiments to form pyromorphite in the presence of two different sorbents: a reference smectite called Panther Creek Bentonite, and a commercially available, organically rich potted mixture.

Description:

Soluble Pb is immobilised in pure systems as pyromorphite by adding sources of P, but doubts remain about the efectiveness of this approach in natural soil systems, particularly given the ability of soil humic substances to interfere with Pb-mineral formation. In addition, recent thermodynamic modelling predicts that pyromorphite formed by the addition of phosphoric acid to Pb-contaminated soils, followed by neutralisation with quick lime (Ca(OH)₂) wil destabilise the mineral, reverting the Pb back to more soluble species such as cerrussite or anglesite. In this paper, we describe experiments to form pyromorphite in the presence of two different sorbents: a reference smectite called Panther Creek Bentonite, and a commercially available, organically rich potting mixture. We present X-ray diffraction (XRD) evidence suggestive of pyromorphite formation, yet, like similar studies, the evidence is less than conclusive. Linear combination fits of Pb X-ray absorption fine-structure spectroscopy (XAFS) data collected at the Advanced Photon Source at Argonne National Laboratory show that pyromorphite is the major Pb species formed after the addition of phosphoric acid. Furthermore, XAFS data shows that neutralising with quick lime enhances (as opposed to reducing ) pyromorphite content in these systems. These results call into question relying solely on XRD data to confirm or deny the existence of minerals like pyromorphite, whose complex morphology give less intense and more complicated diffraction patterns than some of the simpler Pb minerals.

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