December 4, 2002
Influence of Aging and pH on Dissolution Kinetics and Stability of Pyromorphite
K.G. Scheckel and J.A. Ryan
U. S. Environmental Protection Agency, Office of Research and Development, National Risk Management Research Laboratory, Cincinnati, Ohio
Immobilization of metal contaminates in soil shows great potential as a cost-effective and environmentally friendly remediation technique. Lead-contaminated soils are typically removed from site and replaced with clean soil at great costs. Innovative technologies for sequestering lead in situ have been proposed and studied. One of these methods, called immobilization-remediation, seeks to both sequester most of the contaminant and retain the metal for the long-term. Scientists at the Environmental Protection Agency’s Office of Research and Development in Cincinnati, Ohio, have examined how the most stable lead mineral, called chloropyromorphite, forms and dissolves over time, thus providing a better understanding of lead sequestration in soil.
Previous research has shown that adding phosphate materials to
lead-contaminated soils leads to an environmentally stable and
biologically inert lead mineral called chloropyromorphite
(Pb5(PO4)3Cl). Because chloropyromorphite is the most stable lead
mineral known, other solid-phase lead species should be transformed
to chloropyromorphite by a dissolution-precipitation mechanism to
immobilize soluble soil lead in situ by simply adding phosphate to
the soil. But the long-term fate of immobilized lead as precipitated
chloropyromorphite is not fully understood
We investigated the kinetics of dissolution and crystallization dynamics of synthetic chloropyromorphites with increasing aging times ranging from one hour to one year, to understand better what may be happening in the field setting. The chloropyromorphites were dissolved in a solution containing not only phosphoric acid, but also nitric acid, thus simulating environmental conditions one may find in areas ranging from acid mine drainage to agricultural soil settings.
The objectives of this research were to understand the effect of aging time on the stability of chloropyromorphite dissolved in phosphoric and nitric acid, to examine physical and chemical alterations of the chloropyromorphite samples, and to model the kinetic data collected from the dissolution experiments. Aging of the samples prior to dissolution was investigated by using x-ray absorption fine structure (XAFS) and x-ray diffraction (XRD) spectroscopies, and the stability of the chloropyromorphite material with temperature was studied with high-resolution thermogravimetric analysis (HRGTA).
Figure 1 shows data for XAFS, XRD, and HRTGA analyses of aged chloropyromorphite prior to dissolution. The XAFS data (figure 1a, solid line), collected at beam line X11A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory, show relatively little difference between the one-hour and one-year samples.
These same chloropyromorphite samples were also examined by XRD (figure 1b) and, again, do not suggest any alteration in the chemistry or crystallinity of the chloropyromorphite phases as aging time increases. But HRTGA showed that aging of the chloropyromorphite crystals did change with increasing residence time (figure 1c), suggesting, perhaps, an increase in the crystals’ Ostwald ripening, in which small crystals, more soluble than large ones, dissolve and re-precipitate onto larger particles.
The dissolution experiments conducted for this research were
designed to observe and compare data as influenced by aging time, pH
(potential Hydrogen) of the dissolution agent (the pH describes the
hydrogen-ion activity of the solution, and determines if the
solution is acid, basic or neutral), and dissolution method (stirred
flow or mixing in batches, instead of continuously, also called
batch method).
Regardless of pH or dissolution method, it appeared that the one-hour-aged chloropyromorphite was the most soluble. Aging of chloropyromorphite from one-day to one-year did not lead to a significant increase in stability, suggesting that crystal aging was accomplished within 24 hours.
We also observed that, as the pH of the nitric acid decreased, the amount of released lead increased. By removing the dissolution reaction products in the stirred-flow experiments, more chloropyromorphite was dissolved in the stirred-flow than in the batch studies, the difference in released lead between these two methods becoming more pronounced as pH increased. So, once chloropyromorphite forms in contaminated soils and sediments, it should remain immobilized and stable in these environments under near-neutral pH.
The potential impact of stabilization remediation methods to safely sequester metal contaminants in the natural environment holds great promise but must be managed carefully and intelligently. The results of this work and of others demonstrate that chloropyromorphite formation can be easily accomplished by the reaction of available lead and phosphorus. Our study suggests that chloropyromorphite’s persistence would endure most environmental conditions, thus making lead-immobilization via phosphorus an ideal remediation mechanism.
BEAMLINE
X11A
PUBLICATION
Scheckel K G and Ryan J A., “Effects of aging and pH on dissolution
kinetics and stability of chloropyromorphite,” Environ. Sci.
Technol. 36, 2198 –2204 (2002).
FOR MORE INFORMATION
Kirk Scheckel
U.S. Environmental Protection Agency
Office of Research and Development, National Risk Management Research Laboratory
Land Remediation and Pollution Control Division, Remediation and Containment Branch
Cincinnati, Ohio
Email: Scheckel.Kirk@epa.gov