A rapid online estimation method for radiostrontium in soil samples using crown ether and supercritical fluid extraction

Crown ethers dissolved in suitable medium are well known to promote the extraction of alkali (M+) and alkaline-earth (M2+) cations from aqueous to organic phases. Di-tert-butyl-cyclohexano18crown6 (DTBDCH18C6) has been identified as an effective and selective extractant for Sr(II) from nitric acid medium. An attempt was made to evaluate the feasibility of 85,89Sr recovery from synthetic soil samples (0.5 g; particle size: <100 μm) by SFE route (pressure: 200 kg/cm2; T: 40 °C) employing DTBDCH18C6 dissolved in methanol/nitric acid medium as phase modifier. The effect of various experimental parameters such as (i) dynamic/static mode of extraction, (ii) time of equilibration (15–150 min during static mode of extraction using 3 mL of modifier), (iii) nitric acid concentration (1–6 M), (iv) picrate as counter-anion, and (v) crown ether concentration in the modifier phase (2×10−4–2×10−3 M) on Sr(II) extraction was studied. Based on these studies, 2×10−4 M DTBDCH18C6 dissolved in methanol/4 M HNO3 was chosen as modifier and 30 min as equilibration time for batch mode employing 3 mL modifier solution in the static mode. Three successive batches employing 3 mL modifier solution (after each extraction stage) showed near quantitative recovery (>95%) of 85,89Sr from soil samples. Dynamic mode extraction using 2×10−4 M DTBDCH18C6 dissolved in methanol/4 M HNO3 as modifier suggested that near quantitative recovery (>95%) of 85,89Sr could be achieved within 1 h. By contrast, ∼10% 137Cs extraction was observed from soil samples under identical experimental conditions. These studies demonstrate the potential of the SFE technique for the analysis of 90Sr in different environmental samples.