Synthesis and application of choline chloride based deep eutectic solvents in liquid-liquid and solid-liquid assisted extraction of organophosphorus flame retardants from landfill leachate and sediment

The extraction of organic pollutants from environmental samples using green solvents has received a lot of interest recently. Simply said, this is due to their extremely low toxicity and volatility when compared to the common organic solvents. Therefore, many extraction exercises now include ionic solvents, as well as the more current deep eutectic solvents (DES). Hence, the current study reports on 1) the synthesis and characterisation of choline chloride/oxalic acid (1:1) and choline chloride/urea (1:2) deep eutectic solvents and 2) their application in liquid-liquid and solid-liquid assisted extraction of 14 targeted OPFRs from landfill leachate and sediment samples. The densities, refractive indices and melting points of the DES measured were 1.11 ± 0.15 g/mL, 1.14 ± 0.10 g/mL; 1.50 ± 0.12, 1.45 ± 0.20; and 12 ± 0.05°C, 35 ± 0.06–55 ± 0.05°C for choline chloride/urea (1:2) and choline chloride/oxalic acid (1:1) respectively. Choline chloride/urea (1:2) and choline chloride/oxalic acid (1:1) viscosities were 87.0 ± 0.20 kg·m ⁻¹·s ⁻¹ and 13.5 ± 0.14 kg·m ⁻¹·s ⁻¹ respectively. ¹H NMR and FTIR confirmed the structures of the two DES. Recoveries using internal standards dTPP, dTBP, dTPrP and ¹³C₁₈MTPP ranged from 59.9 ± 7.41–103 ± 5.32%; whereas the percent recoveries using CRM ranged from 85.7–117%. In the leachate (510–1433 ng/L) and sediment (725–1224 ng/g dw) samples from each landfill, 14 and 13 targeted OPFRs, respectively, were detected. The high recoveries and extraction of OPFRs demonstrated by the two DES imply that these could replace conventional organic solvents used to extract OPFRs from environmental media.