Features of cobalt terephthalate synthesis and its use as a sorbent for solid-phase extraction of the dyes

The reaction of cobalt(II) hydroxide with terephthalic acid under solvothermal conditions (N,N-dimethylformamide, 110 °C, 72 h) leads to the formation of three products: cobalt terephthalate (compound I), cobalt(III) dimethylaminoformatecatenane (compound II) and dimethylamine terephthalate (compound III). Cobalt terephthalate was characterized by elemental analysis, infrared spectrometry, X-ray diffraction, scanning electron microscopy, thermogravimetry and volumetric nitrogen adsorption/desorption. Compound II crystallizes in the trigonal space group R-3c:H and compound III crystallizes in the triclinic space group P-1. The coordination polyhedron around the metal ion can be described as a trigonally distorted octahedron. The sample has a constant porosity with an average pore diameter of 8.5nm and a mesoporous structure with a large surface area (141.7 m²/g), which corresponds to type I adsorption. Cobalt terephthalate was used for solid-phase extraction of food dye “sunset yellow”, tartrazine, β-naphthol orange and methyl orange. The results confirmed the possibility of using the proposed analytical method as a new convenient approach to extracting organic dyes from industrial and other wastewaters, where such substances are contained in insignificant concentrations, and their determination requires expensive and complex equipment. In the future, this method can be used not only for the effective removal of pollutants from industrial wastewater with subsequent regeneration of the sorbent, but also as a sample preparation method for concentrating organic dyes from dilute solutions with their subsequent elution and analysis by available methods, for example, spectrophotometry, since the lower detection limit is within 0.9 mg/L. The experimental data are described by the solid-phase extraction isotherm (R² > 0.921) and show the ability of cobalt terephthalate to extract up to 360 mg/g of sorbent under optimal conditions.