Sustainable cadmium extraction from sewage sludge samples: A novel approach with hydrophobic deep eutectic solvents and ultrasound-assisted extraction (HDES-UAE) prior to ICP-MS analysis

Hydrophobic deep eutectic solvents (HDES) are a subclass of DES, formed by hydrogen bond donor and acceptor species, which have low solubility in hydrophilic systems as their main characteristic. In this work, two different mixtures based on DL-menthol (HDES 1: DL-menthol and acetic acid, 1:1 M ratio; HDES 2: DL-menthol, acetic acid, and pyruvic acid, 1:1:1 M ratio) were prepared and their physicochemical properties (density and viscosity) were determined. The materials were characterized using FTIR, TG, DSC, and cadmium determination was made by ICP-MS using the KED mode. The greenness of the methods for preparing the mixtures was assessed using the analytical Eco-Scale metric, with high scores obtained for both procedures. The parameters sample mass, HDES volume, and ultrasonication time were optimized using a 33 Box-Behnken experimental design and the solvents were tested in the extraction of cadmium from a sewage sludge certified reference material, obtaining recoveries of 101 and 104 % using HDES 1 and HDES 2, respectively. The LOD and LOQ values were 0.271 and 0.905 μg L􀀀 1 (HDES 1), and 0.280 and 0.933 μg L􀀀 1 (HDES 2), respectively. The two mixtures provided similar results when applied in the extraction of cadmium from three different sewage samples. For sewage sludge A, the cadmium values obtained were 2.073 ± 0.10 μg g􀀀 1 (HDES 1) and 2.103 ± 0.11 μg g􀀀 1 (HDES 2). For sewage sludge B, the cadmium values were 2.707 ± 0.20 μg g􀀀 1 (HDES 1) and 2.067 ± 0.23 μg g􀀀 1 (HDES 2). For a poultry sewage sludge sample, the cadmium values were 6.023 ± 0.84 μg g􀀀 1 (HDES 1) and 5.319 ± 0.14 μg g􀀀 1 (HDES 2). The sample extraction process was evaluated using AGREE software, obtaining a score of 0.7 (out of a possible maximum score of 1.0). Use of the prepared solvents and extraction using an ultrasound-assisted method proved to be a good and sustainable strategy for the extraction of Cd from sewage sludge.