Adsorption and ecotoxicology studies with aqueous solution of Cr(VI) ions using adsorbent materials derived from Inga...

Adsorbents derived from the bark of Inga edulis were developed for the removal of Cr(VI) ions. Chemical activation with sulfuric acid led to significant changes in the physicochemical properties of the biomass. The adsorbent materials denoted IB (inga biomass) and AIB (activated inga biomass) showed higher removal efficiencies at pH 2.0 and dosages of 1.0 and 0.25 g L^-1, respectively. The adsorption kinetics could be fitted using the Elovich model, indicating that the adsorption occurred on heterogeneous surfaces. The maximum adsorption capacities were 46.0 mg g^-1 for IB and 356.6 mg g^-1 for AIB, with behaviors that could be described by the Langmuir (monolayer) and Freundlich (multilayer) models, respectively. XPS analyses confirmed the reduction of Cr(VI) to Cr(III), due to interactions with oxygenated functional groups. Thermodynamic evaluation indicated that the adsorption was spontaneous, with exothermic character for IB and endothermic character for AIB. Ecotoxicological assays using Daphnia similis showed that a Cr(VI) concentration of 4.34 μg L^-1 caused 50.0 % immobility, while adsorption by the materials eliminated the toxicity, demonstrating the effectiveness of the adsorbents in reducing environmental impacts. Additionally, an electrode derived from the adsorption of Cr(VI) on AIB, denoted CPEAIB-Cr-ads-active, presented good performance in the hydrogen evolution reaction (HER), with high current density and low overpotential. The structure of the electrode, with high surface area and the presence of pores and cavities, was favorable for electrochemical catalysis, evidencing its potential for use in applications concerning renewable energy and environmental detoxification.