Preconcentration and ultrasensitive spectrophotometric estimation of tungsten in soils using polyurethane foam in the presence of rhodamine B: Kinetic and thermodynamic studies, and designing a simple automated preconcentration system

Abstract

This work describes designing and using a simple automated preconcentration system for estimation of tungsten in soil spectrophotometrically. The preconcentration of tungsten is based upon the retention of ion associate of [RB+.HWO4-] inside a minicolumn packed with polyurethane foam. The retained ion associate is then automatically and quantitatively eluted by acetone. The tungsten concentration was estimated by measuring the absorbance of elution at 584 nm. The absorbance of elution follows Beer's law in concentration range of 0.5–100 µgL−1 with a detection limit of 0.09 µgL−1. The enhancement factor and relative standard deviation of proposed methodology were 50 and 1.7%, respectively. The system developed in this research is characterized by its low cost and high efficiency, as it uses low-cost PUF as a stationary phase and a few number of pumps and valves. On the other hand, the designed system can be used, in the future, with many detection techniques easily. The composition of the ion association formed between W(VI) ions and RB was calculated by continuous variation method (Job plot) to be 1:1 (W6+:RB). On the other hand, the ion associate in its solid state was synthesized and characterized using elemental analysis and ATR–FTIR technique. The retention of tungsten (VI) ions in the form of [RB+.HWO4-] ion association on the surface of PUFs was confirmed by SEM images and XRF spectra. Moreover, the kinetic and thermodynamic behaviors of the retention of [RB+.HWO4-] on the polyurethane foams were studied in detail. The kinetic models revealed that the retention of [RB+.HWO4-] on PUF surface obeys the pseudo-second-order reaction. The thermodynamic investigations confirmed that the adsorption of [RB+.HWO4-] on PUF surface was spontaneous at 298 K and the interaction between [RB+.HWO4-] and active sites available on PUF surface was physical and electrostatic in nature.