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White Eggshells: A potential biowaste material for synergetic adsorption and naked-eye colorimetric detection of heavy metal ions from aqueous solution.

In the present work, we have utilized a simple, no-cost, unmodified eggshell powder as a naked-eye colorimetric detector, which on simple dispersion in aqueous solution of metal ions exhibited characteristic colour change from white to pale green, pale blue, yellow, pale yellow, dark yellow, pale pink, blue, brown for metal ions V4+, Cr3+, Cr6+, Fe2+, Fe3+, Co2, Cu2+ and Ag+ respectively. The effect of various parameters like concentration of metal ions, pH, temperature and adsorbent dosage were investigated by batch sorption experiments. Also, Freundlich and Langmuir models were used to describe the adsorption isotherm. The eggshell powder (ES), before and after adsorption was characterized qualitatively by naked eye and quantitatively by DRS-UV, FT-IR, AAS, PXRD, SEM-EDX, BET, Zeta potential and XPS. In addition, the competitive adsorption of metal ions in mixtures and the recycle experiments were carried out to prove the sustainability of the material. Further, the RGB alterations were extracted from colorimetric array and subjected to hierarchical clustering analysis (HCA) using Ward method by calculating Euclidean distance which displayed facile discrimination of 10 heavy metal ions at 1mM level. Thus unmodified eggshell powder has been proven to be an impressive value-added sustainable material for synergistic metal adsorption and colorimetric naked-eye detection of a series of metal ions with a detection limit of 10-4 M for Fe3+, Fe2+ and Cu2+, 10-3 M for Cr3+, Cr6+, Ag+, Co2+ and 5 X 10-3 M for V4+.