Eco-friendly mercury ion detection and removal in water using anthocyanins: Mechanistic insights through DFT studies

Abstract

A variety of natural extracts have been employed for the colorimetric detection of toxic metal ions in water due to their sustainability and low cost. However, the inherent structural instability of these extracts poses significant challenges in elucidating their interactions with metal ions. This necessitates the use of theoretical studies to provide robust mechanistic insights that complement experimental findings. In this study, we extracted anthocyanins from Hibiscus sabdariffa to develop a natural colorimetric sensor for the detection of mercury (Hg²?) ions in aqueous solutions. The sensor exhibited a distinct color change from pink to colorless, attributed to the formation of a coordination complex via the Hard and Soft Acids and Bases (HSAB) principle with high sensitivity and a detection limit of 13 ppm. To further understand the interaction mechanism, we performed Density Functional Theory (DFT) calculations that corroborated the experimental mechanism, revealing a significant reduction in the energy gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) of the anthocyanin (ATC) from 2.688 eV to 1.366 eV upon complexation with Hg²? ions. Furthermore, the developed sensor was capable of removing the Hg2+ ions from water samples at lower concentrations (below 100 ppm). The studies indicate the potential application of anthocyanin extract as a significant sensor for Hg2+ in aqueous solutions. Consequently, this simple, eco-friendly, and instant method for detecting Hg2+ ions could pave the way for a promising sensor for toxic metal ion pollutants in industrially significant water sources. © 2025 Elsevier Ltd