Adsorption/desorption of acid violet-7 onto magnetic MnO2 prior to its quantification by UV–visible spectroscopy: optimized by fractional factorial design

Abstract

A highly efficient magnetic adsorbent is presented to preconcentrate acid violet-7 (AV-7) prior quantifying by UV–visible spectroscopy (λmax = 520 nm). The adsorbent consisting of MnO2 and Fe3O4 was synthesized via precipitation method. The green preparation method was simple and fast; it was performed at room temperature, without consuming organic solvents and thermal treatment. The synthesis strategy is based on using cetrimonium bromide as a surfactant. The surfactant was adsorbed onto negative sites of Fe3O4 then trapped Mn2+ ions and finally by adding NH3, nanoparticles of MnO2 were formed. The nanocomposite was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, alternating gradient force magnetometer and Brunauer–Emmett–Teller. The strong magnetic property of Fe3O4 and porous structure of MnO2 with positive charges enable efficient adsorption of AV-7 via magnetic solid phase extraction. The optimum conditions with respect to circumscribed fractional factorial design were pH (4.0 ± 0.2), sorbent amount (20 mg), adsorption time (20 min), adsorption temperature (308 K), eluent kind [ethanol/NaNO3 (1 mL/50 mg)], desorption time (10 min) and desorption temperature (283 K). The adsorption mechanism of AV-7 onto adsorbent was explained based on FT-IR spectra. The swelling behavior of adsorbent was investigated. Limit of detection (0.86 ng mL−1), relative standard deviation (n = 5) (1.61%), preconcentration factor (50) and linearity of dynamic range (20–400 μg L−1) perform the applicability of adsorbent in AV-7 preconcentration. The adsorbent is reusable for 15 times and durable for 100 days. The swelling behavior was investigated. The effects of interference ions and three dyes on the preconcentration recovery were studied. Temkin model and pseudo-second order were fitted with isotherm adsorption and kinetic adsorption, respectively. According to thermodynamic parameters, the adsorption is endothermic and spontaneous. The adsorbent was successfully applied to quantify AV-7 from a leather sample.