Speciation of the Selenium in Groundwater Samples of Different Aquifers from Coal Mining Fields: Applied a Green Analytical Technique

A novel preconcentration technique based on vortex-assisted deep eutectic solvent-based dual microextraction (VA-DES-DμE) method has been proposed to preconcentrate the selenium (Se) in groundwater/aquifer samples of different coal mining fields of Sindh Pakistan. In the current developed method, Se was complexed with a chelating reagent ammonium pyrrolidine-dithiocarbamate (APDC), whereas a deep eutectic solvent was used for extraction purpose. The contents of tubes were shaken for different time intervals on a vortex mixer to enhance the dispersion and enrichment of hydrophobic complex in DES from aqueous samples. After centrifugation, the mixture was separated into two layers, aqueous, and hydrophobic Se complex entrapped in DES. For second phase of the dual microextraction, the enriched Se complex in DES was extracted back into acidic aqueous solution, heated and centrifuged to separate analyte-rich phase (upper layer), and subjected to an electrothermal atomic absorption spectrophotometer. Various factors which are responsible for reliability of proposed technique have been optimized including pH, values of complexing ligand, and volume of DES. To determine the total Se, pre-reduction of Se (VI) to Se (IV) was carried out to determine total inorganic Se species before applying developed technique. The distinction of Se (IV) concentration and total selenium values was calculated to find out the Se (VI) concentration. At the sample volume of 25 mL, the evaluated preconcentration factor was 64.0. The limit of detection of developed methodology for Se (IV) was calculated to be 25.2 ng/L. The validation of proposed procedure was carried out by the standard addition in real water samples. The proposed technique was successfully used for the preconcentration of Se in groundwater samples of coal mining areas of Sindh Pakistan. It was observed that the Se (IV) was about 20% lower in ground water of AQ1 as related to AQ2; however, highest amount was observed in AQ3. It was noted that the ground water of both coal mining area was above than permissible limit recommended by WHO (10 µg/L).