Sorption-spectroscopic determination of chromium (III) ions using eriochrome black T immobilized on silk fibroin fiber.

BACKGROUND

Chromium (III) ions are widely present in industrial and environmental waters, and their reliable determination is essential for environmental monitoring and pollution control. Conventional spectrophotometric methods often suffer from limited sensitivity, selectivity, and stability. To overcome these limitations, a sorption-spectrophotometric approach based on reagent immobilization was developed. In this study, Eriochrome Black T (EBT) was immobilized on silk fibroin fiber to enhance analytical performance in the determination of Cr(III) ions.

RESULTS

Immobilization of EBT significantly improved the stability, sensitivity, and selectivity of the method. Formation of the Fib-EBT-Cr(III) system was confirmed by FTIR spectroscopy (CrO band at 607 cm-1), X-ray fluorescence analysis, and SEM-EDS, which indicated 18.00 ± 0.01 wt% chromium in the immobilized complex. Optimal analytical conditions were pH 8, λ = 618 nm, and a universal buffer. The method showed a detection limit of 1.2 μg/L, linearity for Cr(III) amounts in the range of 5-50 μg (R2 = 0.9991), and an analysis time of 10 min. Interference from Al3+, Co2+, and Cd2+ was effectively masked, while Ni2+, Fe3+, and Cu2+ caused no interference. Validation using industrial wastewater samples demonstrated relative errors ≤6% and Sr ≤ 0.085. Quantum chemical calculations identified the most stable isomer of EBT (rotamer B) and its complex with Cr(III) in aqueous solution. Comparison of computed and experimental IR spectra confirmed the dominance of rotamer B, the involvement of the azo group in coordination, and the formation of a CrO bond (experimental 607 cm-1; computed 600 cm-1). Complexation was found to induce trans-to-cis isomerization of EBT, a phenomenon not previously reported in the literature.

SIGNIFICANCE: The developed sorption-spectrophotometric method provides a rapid, economical, and accurate approach for the determination of Cr(III) ions in environmental waters. Its high sensitivity, good selectivity, and successful application to real industrial wastewater samples indicate strong potential for routine laboratory analysis as well as field monitoring of chromium contamination.