Visual and Optical Sensing of Hg²⁺, Cd²⁺, Cu²⁺, and Pb²⁺ in Water and Its Beneficiation via Gettering in Nanoamalgam Form

Abstract

A “turn on” fluorescent probe based on denatured bovine serum albumin (BSA)–Pyronin Y complex (PYdBSA) has been prepared using a one pot approach. This probe can detect nanomolar concentrations of common contaminant ions (Hg²⁺, Cd²⁺, Cu²⁺, and Pb²⁺) found in wastewater. The probe shows the highest fluorescence response for Hg²⁺ (relative intensity set to 100%) and a very good response for Cd²⁺ (60%), Cu²⁺ (50%), and Pb²⁺ (20%). The probe is water-soluble and has photo excitation in the visible region (530 nm). More interestingly, the presence of the contaminant ions under consideration can be visually detected due to the distinct color formation upon addition of the analytes in micromolar (1 μM) concentrations. The limit of detection (LOD) with fluorometry is 9 nM for Hg²⁺ (<2 ppb; consistent with standards given by the U.S. Environmental Protection Agency). This probe signal is found to be intact even in the presence of other metal ions such as Zn²⁺, Co²⁺, Fe²⁺, Ag⁺, K⁺, Na⁺, Al³⁺, Ca²⁺, Fe³⁺, Ni²⁺, Sb³⁺, and Mg²⁺, which is rationalized using hard–soft acid–base theory. Postdetection, the heavy and transition metals (HTMs) are gettered in the form of nanoamalgam through reduction using sodium borohydride. The nanomaterials obtained are rationalized based on known phase fields in relevant binary phase diagrams. The nanoaluminum amalgam obtained is beneficiated by putting it in use as a reducing agent in the conversion of <i>p</i>-nitrophenol to <i>p</i>-aminophenol. The rate constant (0.74 ± 0.08 M^–1 s^–1) obtained is comparable to the best reducing agents reported for this reaction. Hence, we demonstrate the practical relevance of the reported method for detection, gettering, and beneficiation of HTMs