Sampling Of Hydrophobic Organic Contaminants

Kingston et al. [2] designed one of the Chemcatcher prototypes for the sampling of non-polar organic compounds with log KOW values greater than 3. This system uses a 47-mm C18 EmporeTM disk as the receiving phase and a 35-mm thick LDPE diffusion membrane. The C18 EmporeTM disk has a very high affinity and capacity for the sampled hydrophobic organic pollutants. LDPE is a non-porous material, even though transient cavities with diameters approaching about 1 nm are formed by random thermal motions of the polymer chains. The thermally mediated transport corridors of the polyethylene exclude large molecules, as well as those that are adsorbed on sediments or colloidal materials such as humic acids. Only truly dissolved and non-ionised contaminants are sequestered.

Recently, the optimisation of this sampler design has been reported [8]. This involved the improvement of sampling characteristics including the enhanced sampling kinetics and precision by decreasing the internal sampler resistance to mass transfer of hydrophobic organic chemicals (log KOW> 5). This was achieved by adding a small volume of n-octanol, a solvent with high permeability (solubility x diffusivity) for target analytes, to the interstitial space between the receiving sorbent phase and the polyethylene diffusion membrane. The use of n-octanol as an interstitial phase resulted in an approximately 20-fold increase in sampling rates compared with those observed with water as the interstitial phase [8].

9.5.1 Calibration data

Calibration data for the non-polar Chemcatcher were obtained in laboratory experiments designed to measure the uptake of target analytes (sampling rate; RS) and offloading of PRCs (elimination rate constants; ke) at different combinations of temperature and hydrodynamic conditions in a full factorial design. The calibration data were gathered in order to determine the sampling parameters and to observe how they are affected by environmental conditions to enable a more precise measurement of TWA concentrations of non-polar priority pollutants in the field [1].

Over the range of controlled laboratory conditions (temperature and turbulence), the magnitude of RS values of hydrophobic chemicals spanned over two orders of magnitude (i.e. from 0.008 Lday-1 up to 1.380 Lday-1). The sampling rate is strongly affected by the physico-chemical properties of the compounds. Among the non-polar priority pollutants, the highest sampling rates were observed for small, moderately hydrophobic compounds: anthracene, phenanthrene, fluoran-thene and pyrene. The lowest sampling rates were measured for

Fig. 9.4. Effect of water turbulence (expressed as rotation speed of a carousel device loaded with samplers) and log KOW on the sampling rates for a range of non-polar organic compounds in the Chemcatcher at 11°C.

indeno[1,2,3-cd]pyrene, dibenz[a,h]anthracene and benzo[g,h,i]perylene; large and extremely hydrophobic compounds. The typical dependence of sampling rates on hydrophobicity is shown in Fig. 9.4.

Sampling rates increase with the increasing temperature, and the temperature dependence of the sampling rate RS can be described by an Arrhenius-type equation. The mean activation energy for all of the hydrophobic analytes under investigation was 93kJmol \ This corresponds to an increase in sampling/offload rate of a factor of 5.2 over the temperature range 6-18°C. For comparison, Huckins et al. [9] calculated from the literature data available for semipermeable membrane devices (SPMDs) an average activation energy of 37kJmol~1. Thus, the effect of temperature on the Chemcatcher uptake kinetics appears to be more significant than that on SPMD sampling rates.

With the exception of the moderately hydrophobic lindane (log KOW — 3.7), a significant increase in sampling rate with increasing flow velocity was observed for all compounds under investigation (Fig. 9.4). This corresponds well with the theory of diffusion through two films in series [10,11], which predicts a switch in the overall mass transfer to the aqueous boundary layer control for hydrophobic compounds. A similar effect of hydrodynamics has been observed and explained for SPMDs [12].

9.5.2 Performance reference compound concept

Figure 9.5 shows that for a range of environmental conditions (temperatures and water flow rates) there is a good correlation between uptake kinetics (sampling rate RS) of analytes and offload kinetic parameters (elimination rate constant ke) of their deuterated analogues (used as PRCs). This demonstrates isotropy of the uptake (absorption) onto and the offload (desorption) from the sampler for a range of hydrophobic analytes. Thus, the PRC concept can be applied to the measurement of in situ exchange kinetics in the field.

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