Evaluation of small arms range soils for metal contamination and lead bioavailability.

Although small arms ranges are known to be contaminated with lead, the full extent of metal contamination has not been described, nor has the oral bioavailability of lead in these soils. In this work, soil samples from ranges with diverse geochemical backgrounds were sieved to <250 microm and analyzed for total metal content. Soils had consistently high levels of lead and copper, ranging from 4549 to 24 484 microg/g and 223 to 2936 microg/g, respectively, while arsenic, antimony, nickel, and zinc concentrations were 100-fold lower. For lead bioavailability measurements, two widely accepted methods were used: an in vivo juvenile swine relative bioavailability method measuring lead absorption from ingested soils relative to equivalent lead acetate concentrations and an in vitro bioaccessibility procedure which measured acid-extractable lead as a percent of total lead in the soil. For eight samples, the mean relative bioavailability and bioaccessibility of lead for the eight soils was about 100% (108 +/- 18% and 95 +/- 6%, respectively) showing good agreement between both methods. Risk assessment and/or remediation of small arms ranges should therefore assume high bioavailability of lead.

Pyrithione and 8-hydroxyquinolines transport lead across erythrocyte membranes.

Acute and chronic lead poisoning remains a significant health problem. Although chelating agents can bind to plasma lead, they cannot cross cell membranes where the total body lead burden resides, and are thus inefficient at reducing the total body lead burden. Recently, calcium and sodium ionophores have been shown to transport lead across cell membranes providing a novel method for reducing total body lead stores. We recently found that clioquinol, an 8-hydroxyquinoline derivative, can act as a zinc ionophore. We postulated that zinc ionophores might also be able to transport lead across biological membranes. To study this, we loaded lead in vitro into human erythrocytes and then studied the ability of zinc ionophores to transport lead into the extracellular space, where it was trapped with a lead chelator. Using inductively coupled plasma mass spectrometry (ICP-MS), we found that several 8-hydroxyquinoline derivatives, as well as the zinc and sodium salts of pyrithione (N-hydroxypyridine-2-thione), reduced erythrocyte lead content. The water-soluble compound, sodium pyrithione, was able to reduce lead in citrated whole blood, without partitioning into the erythrocytes. These results indicate that two classes of zinc ionophores can transport lead across a biological membrane, and they confirm that these ionophores are not cation-specific. Lead ionophores may prove useful in mobilizing lead into the extracellular space, thereby improving the efficacy of chelation therapy, in vivo or ex vivo.

Solubility enhancement of seven metal contaminants using carboxymethyl-beta-cyclodextrin (CMCD).

Carboxymethyl-beta-cyclodextrin (CMCD) has been suggested as a complexing agent for remediation of sites co-contaminated with metals and organic pollutants. As part of an attempt to construct a geochemical complexation model for metal-CMCD interactions, conditional formation constants for the complexes between CMCD and 7 metal ions (Ba, Ca, Cd, Ni, Pb, Sr, and Zn) are estimated from experimental data. Stable metal concentrations were reached after approximately 1 day and estimated logarithmic conditional formation constants range from -3.2 to -5.1 with confidence intervals within +/-0.08 log units. Experiments performed at 10 degrees C and 25 degrees C show that temperature affects the solubility of the metal salts but the strength of CMCD-metal complexes are not affected by this temperature variation. The conditional stability constants and complexation model presented in this work can be used to screen CMCD as a potential remediation agent for clean-up of contaminated soil and groundwater.

Enhanced solubilization of a metal-organic contaminant mixture (Pb, Sr, Zn, and perchloroethylene) by cyclodextrin.

Prior work has suggested that (carboxymethyl)-beta-cyclodextrin (CMCD) is capable of simultaneously enhancing the solubility of organics and metals, but sparse experimental data and no theoretical models have been published on this process. Preciously, a geochemical model for metal complexation by CMCD was formulated using PHREEQC on the basis of conditional stability constants measured in experiments using single-metal salts. In this study, the model is expanded to simultaneous metal and organic (perchloroethylene, PCE) complexation by CMCD. Experiments to verify the application of the formulation to mixed-waste systems were performed using solutions containing multiple metal ions (Pb, Sr, and Zn) and in a separate experiment introducing PCE with multiple metal ions. These experimental results show simultaneous solubility enhancement of metals and PCE. For solutions up to about 50 g/L CMCD, the model accurately predicted the simultaneous solubility enhancement for PCE, Pb, and Zn, while the difference between the measured and predicted Sr concentrations was accurate to within 15%. At CMCD concentrations greater than 50 g/L, the observed metal solubilities were greater than predicted (10% for Pb and Zn), probably due to the difficulty in accurately representing the activity and the effect on the ionic strength of functional groups on large organic molecules at higher concentrations.

Determining conditional stability constants for Pb complexation by carboxymethyl-beta-cyclodextrin (CMCD).

Carboxymethyl-beta-cyclodextrin (CMCD) has been proposed for remediation of metal-contaminated sediments. This research presents stability constants for CMCD-lead complexes, and demonstrates a rigorous methodology for estimating stability constants for metal-complexing agents. The conditional stability constant for the lead-CMCD aqueous complex was determined to be 10(5.18) with the 95% confidence interval ranging from 10(5.14) to 10(5.22). The best fit for experimental data was made by assuming a reaction between divalent CMCD(2-) and Pb(2+) and using the WATEQ activity coefficient formulation. The optimized value was derived from experimental data with the geochemical model PHREEQC coupled to UCODE_2005, a parameter optimization program. Like FITEQL, UCODE has a built-in option to optimize parameter values by minimizing the weighted sum of squared residuals (WSSR). However, our approach not only allows rapid, automatic optimization of the stability constant, but also allows determination of uncertainties in estimated parameter values and statistical analysis to assess the appropriateness of the conceptual model. The automation of the process allows testing of multiple conceptual models and the final values produced are internally consistent with the PHREEQC database. In this case five different conceptual models to describe the metal complexation and protonation reactions of CMCD were considered.