Rhodamine functionalized cellulose for mercury detection and removal: A strategy for providing in situ fluorimetric and colorimetric responses

A new cellulose-based material as adsorbent was prepared for Hg (II) detection and removal from aqueous solution. Rhodamine moiety was conjugated with cellulose as an efficient sensor, enabling the material to exhibit excellent optical signals on the contact with Hg (II). Fourier transform infrared spectra, elemental analysis, thermogravimetry, differential thermal analysis, scanning electron microscope analysis, Brunauer-Emmett-Teller surface area, pore volume and pore size were employed to verify the modification of cellulose. A 7-fold enhancement of fluorescence intensity afforded the adsorbent “off–on” fluorimetric response exclusively towards Hg (II) without the interference of more than ten other kinds of metal ions. The detection limit reached as low as 9.18 × 10 -7 M. Besides, the adsorbent also showed an obvious color change during the adsorption to realize “naked-eye” recognition with Hg (II) in the range of 1–400 ppm. Thus, a colorimetric card was prepared to evaluate the concentration of Hg (II) in a convenient and rapid manner. In addition, the adsorption properties of the material including the effect of initial concentration, pH, and contact time were systematically investigated by batch adsorptions. The adsorption process was found to fit well with Langmuir adsorption model and pseudo-second-order kinetics. The mercury triggered spirolactam ring-opening reaction was responsible for the sensitive and selective fluormetric sensing and confirmed by the Job’s plot (1:1 binding stoichiometry) and X-ray photoelectron spectroscopy. Furthermore, density functional theory studies also indicated the energetically favorable Hg (II) induced ring-opening process. The highly sensitive and selective performance in tap water further demonstrated the tremendous potential of as-prepared adsorbent for practical application.