Mona Wells

Numerical approach to speciation and estimation of parameters used in modeling trace metal bioavailability.

Speciation affects trace metal bioavailability. One model used to describe the importance of speciation is the biotic ligand model (BLM), wherein the competition of inorganic and organic ligands with a biotic ligand for free-ion trace metal determines the ultimate metal availability to biota. This and similar models require natural ligand concentrations and conditional stability constants as input parameters. In concept, the BLM is itself an analogue of some analytical approaches to the determination of trace metal speciation. A notable example is competitive ligand equilibration/cathodic stripping voltammetry, which employs an artificial ligand for comparative assessment of natural ligand concentrations and discrete conditional stability constants (i.e., BLM parameters) in a natural sample. Here, we report a new numerical approach to voltammetric speciation and parameter estimation that employs multiple analytical windows and a two-step optimization process, simultaneously generating both parameters and a complete suite of corresponding species concentrations. This approach is more powerful, systematic, and flexible than those previously reported.

Comparison of naphthalene bioavailability determined by whole-cell biosensing and availability determined by extraction with Tenax

A rapid biological method for the determination of the bioavailability of naphthalene was developed and its value as an alternative to extraction-based chemical approaches demonstrated. Genetically engineered whole-cell biosensors are used to determine bioavailable naphthalene and their responses compared with results from Tenax extraction and chemical analysis. Results show a 1:1 correlation between biosensor results and chemical analyses for naphthalene-contaminated model materials and sediments, but the biosensor assay is much faster. This work demonstrates that biosensor technology can perform as well as standard chemical methods, though with some advantages including the inherent biological relevance of the response, rapid response time, and potential for field deployment. A survey of results from this work and the literature shows that bioavailability under non-equilibrium conditions nonetheless correlates well with K(oc) or K(d). A rationale is provided wherein chemical resistance is speculated to be operative.

Parallel dual-color fluorescence cross-correlation spectroscopy using diffractive optical elements

Dual-color cross-correlation spectroscopy allows the detection and quantification of labeled biomolecules at ultra-low concentrations, whereby the sensitivity of the assay correlates with the measurement time. We now describe a parallel multifocal dual-color spectroscopic configuration employing multiple avalanche photodiodes and hardware correlators. Cross-correlation curves are obtained from several dual-color excitation foci simultaneously. Multifocal dual-color excitation is achieved by splitting each of two laser beams (488 and 633 nm) into four sub-beams with the help of two 2x2 fan-out diffractive optical elements (DOEs), and subsequent superposition of the two sets of four foci. The fluorescence emission from double-labeled biomolecules is detected by two 2x2 fiber arrays.

Perspectives on modeling the release of hydrophobic organic contaminants drawn from model polymer release systems

This work critically evaluates current practices in modeling the release of hydrophobic organic contaminants (HOCs) from complex matrices. Using well-constrained model polymer release systems (MPRS), we evaluate several empirical kinetic release models and one class of semi-empirical diffusion model; generally these models either do not describe release well, and are thus not useful, or they describe release well but are overparameterized, thus decreasing confidence in the fitting parameters. Results indicate that the n-phasic release often attributed to soil and sediment matrices is a non-mechanistic operational definition, arising from bias in the choice of empirical fitting expression. Further analysis illustrates pitfalls in common approaches to characterizing desorption from complex environmental matrices with respect to parameter interpretation, particularly effective diffusion coefficients. We also consider MPRS release in the context of other models and find that, though release occurs from these materials via a mechanism of anomalous (non-Fickian) diffusion, Fickian diffusion models nonetheless describe release well. This finding reconciles previous reports about anomalous release from environmental matrices with the hitherto success of Fickian diffusion models of desorption.