Pb speciation versus TCLP release in army firing range soils.

A series of soil parameter and mineralogical investigative techniques were applied to assess the Pb speciation in four US Army firing range soils that presented significantly different Pb leaching regimes and soil characteristics. Soil gradation tests were complemented by total chemical analyses, X-ray powder diffraction (XRPD), Rietveld quantification, optical microscopy and scanning electron microscopy (SEM) analyses. The bulk geotechnical, mineralogical and chemical analyses pointed to two possible Pb retention mechanisms: precipitation as lead carbonate and sorption in the case of fine-grained soils. Lead speciation and mobility was further investigated by the toxicity characteristic leaching procedure (TCLP) and sequential extraction test (SET). As the TCLP Pb concentrations did not necessarily reflect the total Pb analysis of the soils, the Pb leachability ratio (TCLP/total) was found to be controlled by soil mineralogy and its response to changes in system pH. Geochemical modeling, using Visual MINTEQ, was employed to evaluate the mechanisms that controlled the observed TCLP Pb leaching behavior. It was found that lead carbonate precipitation/dissolution reactions controlled Pb TCLP leachability in all soils, while sorptive phenomena did not seem to play a role even in the case of fine-grained soils. More specifically, TCLP Pb leachability was controlled by the pH, the available Pb and the available carbonate in solution. This indicates that geochemical modeling strongly complimented TCLP Pb analyses. Thus, geochemical modeling is an important assessment tool to evaluate the magnitude of site-specific Pb-related environmental problems in firing range soils. Carbonation reactions, involving metallic Pb, that occur during the SET obscure its ability to reliably ascertain Pb speciation. More specifically, SET lumps the extractable Pb into predetermined phase categories that may not be truly representative of the actual soil mineralogy or dominant forms of Pb in the soil. A thorough geotechnical, mineralogical and chemical investigation of firing range soils, complemented by geochemical modeling, was therefore found to be a more reliable approach to evaluate Pb speciation and TCLP release in firing range soils.

Mercury and other trace elements in sediment cores from central Texas lakes.

Metals released during fossil fuel use are important atmospheric pollutants. Mercury and other trace metals can be transferred to an aquatic environment through atmospheric deposition. In the work reported here, a number of sediment cores were retrieved from central Texas lakes in the proximity of a coal-fired power plant in search of local anthropogenic effects. Cores were collected along a transient parallel to the prevailing wind direction (S-SE) in the area. Trace element concentrations in the lignite and in effluents from the power plant showed that some elements remained constant (Al, Cu) throughout the different lignite combustion and power production processes. Some (like Cd and Se) showed an affinity for the smaller particles, whereas others (Hg) showed very low concentrations in all the solid wastes, indicating that they probably escaped with the flue gases. Sediment cores from a lake next to the power plant showed higher trace metal concentration in the upper part of the cores (more recent sediment). For example, there was as much as a tenfold increase in Hg concentration between the core bottom (10 ng/g), where the sediment was approximately 100 years old and the surface (100 ng/g). Cd and Se at surface sediments were also found to be as high as 1.6 and 3.45 microg/g, respectively. The excess metal inventory was higher for the lakes located next to the power plant than for two lakes about 30 km away.