Environ. Sci. Technol. 1994, 28,346-351
Supercritical Fluid Extraction of &Triazines and Phenylurea Herbicides from Sediment Alan M. Robertson and John N. Lester' Environmental and Water Resource Engineering Section, Department of Civil Engineering, Imperial College of Science, Technology and Medicine, Imperial College Road, London SW7 2BU, United Kingdom
s-Triazines and phenylurea herbicides were recovered from spiked sediment using supercritical fluid extraction (SFE). The effects of important experimental variables such as extraction temperature, time of extraction, and supercritical fluid density were investigated. Analyte recovery was low and variable using unmodified supercritical carbon dioxide but could be significantly improved as a result of acetone modification. SFE was much faster than Soxhlet extraction for the determination of these priority pollutants and was advantageous in terms of improved analyte recovery and reduced solvent and labor requirements. Thermally labile phenylurea herbicides also proved to be less susceptible to thermal degradation using the SFE technique.
Introduction Atrazine and simazine are members of a group of herbicidally active s-triazines commonly used for pre- and postemergence weed control in the production of corn and other crops ( I ) and are also widely used as nonspecific herbicides on railway embankments, roadsides, and industrial areas (2). In recent years, concern has been expressed over the persistence of these herbicides in the aquatic environment, which may present a risk to water supplies and aquatic ecosystems (3). In the United States, atrazine is now included in the list of 18 pesticides that the Environmental Protection Agency (EPA) regulates in drinking water, and simazine has been proposed for inclusion in any further regulation (4). s-Triazine herbicides have also been the subject of numerous investigations designed to understand their degradation in various environmental matrices (5). Two of the main degradation products of atrazine and simazine are deethylatrazine and deethylsimazine (6), probably because dealkylation is the first step in their microbial degradation (7). Traditional methods of extracting both atrazine and simazine and their degradation products from soils and sediments use large volumes of chlorinated organic solvent and require laborious sample cleanup steps (8); hence, the development of a faster, alternative extraction method using less solvent would be particularly attractive. In recent years another group of herbicidally active compounds,phenylureas, have become widely used as preand postemergence selective herbicides for the control of annual grasses and weeds in barley, rye, and wheat crops (9). There has been some concern over the occurrence and persistence of these herbicides; consequently, there is a need to determine residues in a variety of matrices (10). Again, common extraction methods for these compounds from soil and sediment are solvent- and laborintensive and have limited selectivity ( I I ) , making the development of an improved alternative method of extraction desirable. Of the pollutants mentioned above, 346
Environ. Sci. Technol., Vol. 28, No. 2, 1994
atrazine, simazine, isoproturon, and chlorotoluron have been found frequently in surface waters and, to a lesser extent, in groundwaters (12,131 and are likely to be found in soils and sediments. Supercritical fluid extraction (SFE) has been used for many years to extract components of interest from bulk materials in process engineering (14,15) and is currently emerging as a viable alternative to the more traditional methods of analytical-scale sample preparation (16, 13, such as Soxhlet extraction. Supercritical fluids are highly efficientextraction media as a result of their low viscosities, permitting the rapid penetration of the pores in a solid matrix, while the high diffusion coefficients of solutes in a supercritical fluid allow rapid mass transfer out of complex matrices. Following extraction, analytes are usually determined by other analytical methods such as chromatographic (18, 19) and spectroscopic (20, 21) techniques. The subject of SFE has been reviewed by Hawthorne (22) and Chester et al. (23) and received much attention at the 1993 Pittsburgh Conference in Atlanta, GA (24). The SFE technique has been applied to a wide range of sample types, and components of interest have been extracted from foods (251, polymers (26),fossil fuels (20, and flavors and fragrances (28). The extraction of trace organic compounds from environmental matrices such as soil and sand, using SFE, has also been described (29-31). The most common target analytes have been PCBs, PAHs, pesticides, and herbicides, with final determination predominantly having been achieved using GC (23). Supercritical COZ has been the choice for most SFE studies, primarily because of its attractive practical characteristics of relatively low critical temperature and pressure, low toxicity and reactivity, and high purity at low cost. COz is an excellent extraction medium for nonpolar species and is reasonably good for moderately polar species, but is less useful for more polar compounds. These extractions have generally been achieved in SFE using COZ containing a small amount of a polar compound known as an organic modifier. Janda e t al. (32), for example, suggested that the direct addition of a small volume of methanol to the SFE vessel prior to extraction, followed by a short period of static extraction, could enhance the recovery of s-triazines from sediment. However, the concentration of methanol in the supercritical CO2 remained constant only during the static extraction step, and the authors provided no indication as to the influence of this effect on the reproducibility of the extraction method. The use of methanol-modified COz was also proposed by McNally and Wheeler (33)for the SFE of phenylurea herbicides (linuron and diuron) from the soil. Results indicated that analyte recovery increased with extraction temperature, COz density, and percent of modifier added, but the reproducibility of the extraction method was limited at higher methanol concentrations in COz (
