A Rancieite type material (K2Mn4O9) nanomaterial was synthesized and tested for the removal of chromium (III) and chromium (VI) from aqueous solutions. The synthesized nanomaterial was characterized using powder XRD and SEM. XRD showed weak diffraction peaks at only at the angles associated with K2Mn4O9. The SEM corroborated that the nanoparticles were present; however, the nanoparticles were clustered into larger aggregates. Batch studies were performed to determine the optimum pH, capacity, time dependency, interferences, and the thermodynamics of the binding. The optimum pH for the binding of Cr(III) and Cr(VI) were determined to be pH 5 and pH 2, respectively. Isotherm studies were performed at temperatures of 4 , 25 , and 45 for Cr(III) and Cr(VI) and showed binding capacities of 21.7 mg/g, 36.5 mg/g, 41.8 mg/g for Cr(III). The Cr(VI) binding capacities were 4.22 mg/g, 4.08 mg/g, and 3.25 mg/g at the respective temperatures. The thermodynamic studies showed that the binding processes for the reactions were spontaneous and endothermic, with a ΔH was 17.54 kJ/mol for Cr(III) and 6.05 kJ/mol for Cr(VI). The of sorption for Cr(III) were determined to be −3.88 kJ/mol, −5.83 kJ/mol and −7.03 kJ/mol at the aforementioned temperatures. The ΔG values for the Cr(VI) sorption were determined to be −4.89 kJ/mol, −5.64 kJ/mol, and −6.05 kJ/mol. In addition, the ΔS values for Cr(III) and Cr(VI) were determined to be 77.92 J/mol and 39.49 J/mol, respectively. The thermodynamics indicate that the binding of Cr(III) and Cr(VI) is spontaneous and endothermic.

Triboelectric nanogenerators (TENGs) are clean self-powering devices that efficiently convert mechanical energy to electrical energy (Wang, Yang, & Wang, 2017). Since mechanical energy is already present in human activity, these nanogenerators bioharvest such a 'wasted' energy and convert it into useful energy. In this way, TENGs become self-powering motion sensors (Abdullah, Chowdhury, et al., 2020; Yang et al., 2018; Zhang et al., 2019). TENGs are cost-effective, flexible, lightweight, highly sensitive to motion and non-toxic (Wang et al., 2017). These features have attracted the attention of the research development industry, and of professionals interested in collecting real-time body movement. Moreover, the compact size of the TENG permits it to be integrated into assorted wearable items.