A. Wayne Garrison

Assessing potential bioavailability of metals in sediments: A proposed approach

Due to anthropogenic inputs, elevated concentrations of metals frequently occur in aquatic sediments. In order to make defensible estimates of the potential risk of metals in sediments and/or develop sediment quality criteria for metals, it is essential to identify that fraction of the total metal in the sediments that is bioavailable. Studies with a variety of benthic invertebrates indicate that interstitial (pore) water concentrations of metals correspond very well with the bioavailability of metals in test sediments. Many factors may influence pore water concentrations of metals; however, in anaerobic sediments a key phase controlling partitioning of several cationic metals (cadmium, nickel, lead, zinc, copper) into pore water is acid volatile sulfide (AVS). In this paper, we present an overview of the technical basis for predicting bioavailability of cationic metals to benthic organisms based on pore water metal concentrations and metal-AVS relationships. Included are discussions of the advantages and limitations of metal bioavailability predictions based on these parameters, relative both to site-specific assessments and the development of sediment quality criteria.

Toxicity of fipronil and its enantiomers to marine and freshwater non-targets

Fipronil is a phenylpyrazole insecticide used in agricultural and domestic settings for controlling various insect pests in crops, lawns, and residential structures. Fipronil is chiral; however, it is released into the environment as a racemic mixture of two enantiomers. In this study, the acute toxicity of the (S,+) and (R,−) enantiomers and the racemic mixture of fipronil were assessed using Simulium vittatum IS-7 (black fly), Xenopus laevis (African clawed frog), Procambarus clarkii (crayfish), Palaemonetes pugio (grass shrimp), Mercenaria mercenaria (hardshell clam), and Dunaliella tertiolecta (phytoplankton). Results showed that S. vittatum IS-7 was the most sensitive freshwater species to the racemic mixture of fipronil (LC50 = 0.65 μ g/L) while P. pugio was the most sensitive marine species (LC50 = 0.32 μ g/L). Procambarus clarkii were significantly more sensitive to the (S,+) enantiomer while larval P. pugio were significantly more sensitive to the (R,−) enantiomer. Enantioselective toxicity was not observed in the other organisms tested. Increased mortality and minimal recovery was observed in all species tested for recovery from fipronil exposure. These results indicate that the most toxic isomer of fipronil is organism-specific and that enantioselective toxicity may be more common in crustaceans than in other aquatic organisms.

Isomers/enantiomers of perfluorocarboxylic acids: Method development and detection in environmental samples

Perfluoroalkyl substances are globally distributed in both urban and remote settings, and routinely are detected in wildlife, humans, and the environment. One of the most prominent and routinely detected perfluoroalkyl substances is perfluorooctanoic acid (PFOA), which has been shown to be toxic to both humans and animals. PFOA exists as both linear and branched isomers; some of the branched isomers are chiral. A novel GC-NCI-MS method was developed to allow for isomer/enantiomer separation, which was achieved using two columns working in tandem; a 30-m DB-5MS column and a 30-m BGB-172 Analytik column. Samples were derivatized with diazomethane to form methyl esters of the PFOA isomers. In standards, at least eight PFOA isomers were detected, of which at least four were enantiomers of chiral isomers; one chiral isomer (P3) was sufficiently separated to allow for enantiomer-fraction calculations. Soil, sediment and plant samples from contaminated locations in Alabama and Georgia were analyzed. P3 was observed in most of these environmental samples, and was non-racemic in at least one sediment, suggesting the possibility of chirally selective generation from precursors or enantioselective sorption. In addition, the ratio of P3/linear PFOA was inversely related to distance from source, which we suggest might reflect a higher sorption affinity for the P3 over the linear isomer. This method focuses on PFOA, but preliminary results suggest that it should be broadly applicable to other chiral and achiral perfluorocarboxylic acids (PFCAs); e.g., we detected several other homologous PFCA isomers in our PFCA standards and some environmental samples.

Enantioseparation of Malathion, Cruformate, and Fensulfothion Organophosphorus Pesticides by Mixed-Mode Electrokinetic Capillary Chromatography

Mixed-mode electrokinetic capillary chromatography (mixed-mode ECC) has been used for the enantioseparation of organophosphorus pesticides. In mixed-ECC, a combination of three pseudostationary phases including surfactants, neutral, and charged cyclodextrins, are used to resolve very challenging enantioseparation problems. The conditions mimic a mixture of micellar electrokinetic capillary chromatography (MECC) and dual-cyclodextrin electrokinetic capillary chromatography (dual-CECC) conditions. In this work SDS, carboxymethyl- g -CD, hydroxypropyl-BCD, and organic modifiers were mixed at various concentrations in order to achieve enantioseparation of three organophosphorus pesticides - cruformate (ruelene), malathion, and fensulfothion. The best condition for separation of ruelene enantiomers was by using a mixture of 70 mM SDS/15 mM carboxymethyl-BCD/45 mM hydroxypropyl-BCD/20% (v/v) acetonitrile in 20 mM borate buffer at pH 8.6, with applied voltage of 25 kV at 25°C. Malathion enantiomers were successfully resolved using either 10 mM SDS/50 mM CM-BCD/40 mM hydroxypropyl-BCD or 50 mM CM-BCD/50 mM hydroxypropyl-BCD/20% (v/v) methanol in 20 mM borate buffer. Fensulfothion enantiomers were successfully resolved using a mixture of 75 mM SDS/12.5 mM carboxymethyl-BCD/45 mM hydroxypropyl-BCD in the same 20 mM borate buffer. The results demonstrate the versatility of the mixed-mode ECC technique in handling very difficult separations such as the organophosphoramidate enantiomers. It offers options for selectivity control by combining three or more pseudostationary phases in the background electrolyte (BGE). The approach to optimization in mixed-mode ECC is generally more straightforward than the use of a solid stationary phase(s) in HPLC. In theory, the separation selectivity of such mixed-mode ECC system can be modified to the extremes of MECC and the dual-CECC of the chiral selectors.