Developing a highly selective method for preconcentration and determination of cobalt in water and nut samples using...

Heavy metal contamination in water and agricultural products is a major concern that causes risks for human health. This article describes a highly selective approach to preconcentrate cobalt(II) (Co(II)) ions based on the standard UV-visible measurement of Co(II)-1-(2-pyridylazo)-2-naphthol complex at λ = 628 nm in water and nut samples. In this method, magnetic silica (mSiO₂ ) was utilized as a practical sorbent and 1-(2-pyridylazo)-2-naphthol was employed as a complexing agent in the elution step. The adsorbent was characterized via X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, and scanning electron microscopy. The effects of the main variables (pH, adsorption time, sorbent amount, pH of eluent, ligand volume, and desorption time) were investigated and established.

The maximum recovery was achieved at pH 7 ± 0.3, adsorption time of 60 min, sorbent amount of 40 mg, eluent pH 8 ± 0.2, ligand volume of 2 mL (16.95 × 10^-4  mol L^-1 ) and desorption time of 30 min. The linearity of dynamic range (10-500 μg L^-1 ), limit of detection (0.32 μg L^-1 ), relative standard deviation (3.04%), and preconcentration factor (25) show the reliability of the method. The sorbent was reusable 12 times. Selectivity and the effect of interference ions were successfully examined. The adsorption process of Co(II) ions on mSiO₂ was investigated based on Langmuir and Freundlich isotherms. The Freundlich model was fitted with the system and the maximum capacity adsorption of mSiO₂ for Co(II) adsorption is 2.35 mg g^-1 . Then, the kinetics study revealed that the adsorption process of Co(II) ions on the mSiO₂ follows the pseudo-first-order model. The thermodynamics parameters ΔG, ΔS, and ΔH were calculated.

The method was fruitfully applied to preconcentrate Co(II) ions in water and nut samples. This method offers high selectivity and precision for determining Co(II) ions. © 2020 Society of Chemical Industry.