Vegetable waste-based sensors for metal ion determination

Last studies on metal adsorption have shown important interactions between different vegetable wastes and metal ions. Recently, yohimbe bark and grape stalks have been used as biosorption materials in the recovery and preconcentration of chromium(VI) ions. In the present work, both vegetable residues coming from pharmaceutical and wine industries, respectively, are selected for the development of vegetable waste-based sensors for metal ion determination. Two different kinds of sensor membranes are prepared. PVC membranes are loaded with different content of a vegetable waste (yohimbe bark or grape stalks) in the range 2–15% (w/w). Electrodes based on composites are prepared by mixing graphite powder, epoxy resin and 10% (w/w) of the vegetable waste powder. Sensors are evaluated in acidic media (pH 2–3). PVC membranes show a selective response to Cr(VI) and Hg(II) ions, with slopes around −32 and −56 mV/dec, respectively. In both ion determination, detection limits are about 1.5 × 10−5 M. Composite electrodes present response to Cr(VI) ions, with slopes around +61 mV/dec and detection limits about 2.2 × 10−6 M. In this case, Hg(II) ions cause irreversible damage to the membrane. Response times, lifetimes and interferences are also determined and a discussion about the response characteristics obtained and a comparison with other Cr(VI) and Hg(II) selective sensors are presented.