Vortex-Assisted Liquid-Liquid Microextraction Based on Hydrophobic Deep Eutectic Solvent for Nickel Determination in Water Samples by Smartphone Digital Image Colorimetry

An environmentally friendly and sensitive vortex-assisted liquid-liquid microextraction (VALLME) method using a hydrophobic deep eutectic solvent (HDES) coupled with smartphone-based digital image colorimetry (SDIC) was developed for the preconcentration and determination of trace nickel in electroplating wastewater. Nickel cations were complexed with dimethylglyoxime (DMG) to form a pink-red complex (Ni(DMG)2). Some important factors, including the HDES type, pH, volume of solvent, dimethylglyoxime concentration, and vortex time, were investigated and optimized. The extraction was performed in 1 minute using 250 µL of HDES composed of tetrabutylammonium bromide (TBABr) with decanoic acid (1:2), with a sample volume of 10 mL at pH 8, and the image capture was made directly in the extraction tube without the need to separate the HDES phase. Under the optimum conditions, the method exhibited linearity within the concentration range of 0.45-5.9 mg L-1, detection and quantification limits of 0.15 and 0.45 mg L-1, and precision (RSD) of ± 2.6%. Application to water samples showed recoveries between 97% and 104%. The proposed HDES-based microextraction method offers an effective alternative for nickel determination in industrial wastewater.