Supercritical Fluid Extraction of Organics in Environmental Analysis

Abstract

Supercritical fluids (SFs) possess several physical properties that make them attractive substitutes for conventional liquid solvents in analytical‐scale extractions. Molecular diffusivities of SFs are typically a factor of 10 greater (10 −4 cm 2 s −1 versus 10 −5 cm 2 s −1 ) and their viscosities are a factor of 10 lower (10 −4 N s m −2 versus 10 −3 N s m −2 ) than liquid solvents. These properties improve the rate of mass transfer of solutes from the matrix surface to the bulk extractions fluid – a transfer believed to be the rate‐limiting step of most extractions. Under optimized conditions, the improvements in mass transfer can enable a supercritical fluid extraction (SFE) to be completed in <1 h compared to ≥12 h for extractions involving conventional solvent techniques, while generating little solvent waste compared with traditional extraction methods. Significant limitations in the efficiency of SFE due to differences in sample composition (matrix effect) have been discussed in the past 15 years and have reduced its impact on environmental analysis. The ability of SFE with CO 2 to simulate (in minutes) long‐term leeching of pollutant species from soils and sediments, however, has enabled researchers to predict the efficacy of bioremediation and may be SFE's most important contribution to environmental analytical chemistry. This review discusses the progress made over the past nearly 30 years in SFE as an alternative technique to liquid solvent extractions of environmental samples for the subsequent measurement of organic constituents.

Comparison of the Effectiveness, Kinetics and Costs of Three Remediation Techniques for Agricultural Soils Historically Contaminated with Dieldrin, Using Laboratory Experiments

“…The extraction of organic contaminants from solid matrices was summarise into 4 steps: (i) desorption of contaminant form soil particles, (ii) CO 2 diffusion through the solid, (iii) contaminants solubilisation in CO 2 and (iv) transport of solubilised contaminant to the system outlet (Al-Marzouqi et al, 2007). In the present work, since it was an old contamination, Dieldrin is deeper sequestered in the soil matrix (Benner, 2015) and step (i) might be the crucial step that mainly controls the efficiency of the SFE. At the opposite, the spiked or heavily contaminated samples should be governed primarily by step (iii) and the solubility of the contaminant in CO 2 (Hawthorne et al, 1995).…”

“…Although experiment with spiked soil is an effective tool for testing various parameters of the remediation process, spiked samples are barely suitable for aged contamination (Anitescu and Tavlarides, 2006;Bielská et al, 2013;Ling and Liao, 1996;Saldaña et al, 2005). Indeed, it is well established now that the behaviour of freshly applied OCPs on the soil is different from the one of historical contamination due to the higher sequestration and OCP stronger binding to soil compounds such as SOM (Benner, 2015;Gonçalves et al, 2006). OCPs are therefore more difficult to reach and eliminate, due to the ageing effect (Alexander, 2000;Duan et al, 2015;Jones and De Voogt, 1999;Ncibi et al, 2007;Pignatello and Nason, 2020).…”