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The chemical activity of organic chemicals directs their diffusion and partitioning and is consequently crucial for their transport, distribution, and toxic effects. A silicone membrane equilibrator is introduced for measuring the chemical activity of nonpolar organic chemicals in lipid-rich samples: (I) A 6 m poly(dimethylsiloxane) (PDMS) microtube (300 microm i.d., 640 microm o.d.) was placed in a sample, and a sample-PDMS equilibrium was reached within 10 min for 12 polycyclic aromatic hydrocarbons (PAHs) acting as model compounds. (II) A plug of 100 microL of methanol was pushed through the tube to equilibrate it with the PDMS and thus the sample. (III) This yielded an undiluted methanol extract that was injected into a high-performance liquid chromatograph (HPLC) with multiband fluorescence detection. Quantification limits expressed as unitless chemical activities ranged from 6 x 10(-9) to 5 x 10(-8), and relative standard deviations were from 6% to 19%. Chemical activities of PAHs in mussels from two polluted sites were measured between 10(-7) and 10(-5), and activity coefficients for PAHs in vegetable and fish oils hardly differed between oils. This method can be used for internal exposure measurements, for monitoring product safety/conformity, and process control. The method can also be applied to measure total analyte concentrations in lipid-rich samples and oils.

Biodegradation kinetics of two phenoxy acid herbicides, MCPP [(+/-)-2-(4-chloro-2-methylphenoxy)propanoic acid; mecoprop] and 2,4-D [2,4-dichlorophenoxyacetic acid] were studied in laboratory batch microcosms at low concentrations (0.025-100 microg/L) using 14C technique with sediments and groundwater from a shallow aerobic sandy aquifer. Below a certain threshold concentration of approximately 1 microg/L for 2,4-D and 10 microg/L for MCPP, the biodegradation followed first-order nongrowth kinetics, and no adaptation was observed within the experimental period of 341 d. Half-lifes for ultimate degradation were 500 d for 2,4-D and 1100 d for MCPP at 10 degrees C in unpolluted aquifer sediment in this environmentally relevant concentration regime. Above the threshold concentrations, the biodegradation rate accelerated gradually due to selective growth of specific biomass, which was ascertained from 14C most probable number enumerations of specific phenoxy acid degraders. Atthe highest concentration tested (100 microg/ L), specific degraders increased from 10(-1) to 10(5) cells/g during the experiment, and half-lifes after adaptation decreased to approximately 5 d. The enhanced rate of degradation by adapted systems was maintained during degradation of the last residuals measured to less than 0.1 microg/L. In situ long-term preexposure of the aquifer sediment also resulted in significant higher degradation rates of the phenoxy acids.

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