This work presents the separation of trace Ni(II) from aqueous solution via liquid-liquid extraction. The application of vegetable oils (rice bran, soybean and sunflower oil) as alternative green solvents was evaluated. Results demonstrate that D2EHPA dissolved in rice bran oil provides the highest extraction of Ni(II), whereas D2EHPA dissolved in kerosene reveals much lower performance. Furthermore, extraction of Ni(II) dramatically increases when the binary system of D2EHPA/1-octanol is applied, suggesting the occurrence of synergism having the synergistic coefficient= 3.580. Under optimum conditions, extraction and stripping of Ni(II) achieved 99.70% and 88.00%, complying with the wastewater discharge limit in Thailand. Herein, a novel approach involving supramolecular mechanisms via density functional theory in a binary system of D2EHPA/1-octanol is first reported. Results show that intermolecular hydrogen bonds play an important role in supramolecular complexing. Fourier transform infrared spectrometer is analyzed to verify the functional groups in which the reactions occur. Significant numeric results are as follows: kextraction=0.134 s-1, kstripping=0.038 s-1, ΔHo=72.759 kJ/mol and ΔSo=0.268 kJ/mol·K.