Evaluation of the Adsorptive Performances of Rapeseed Waste in the Removal of Toxic Metal Ions in Aqueous Media

Rapeseed seeds are one of the most important categories of raw materials used to obtain biofuels. However, the biomass resulting after oil extraction is still considered waste, for which valorization solutions are sought. In this study, we propose the use of this type of residual biomass (rapeseed waste biomass (RWB)) as a biosorbent for the removal of toxic metal ions from aqueous media. Two toxic metal ions were selected for the experimental studies, namely: Pb(II) and Hg(II). The optimal biosorption conditions, for both metal ions, were selected based on response surface methodology and were verified experimentally in batch systems. More than 92% of the initial amount of Pb(II) and Hg(II) are retained under the following conditions: pH = 6.5 for Pb(II) and 4.0 for Hg(II); biosorbent dosage = 4.0 g/L; contact time = 3 h; temperature = 25 ± 1 °C. Isotherm (Langmuir, Freundlich and Temkin models) and kinetic (pseudo-first order, pseudo-second order and intra-particle diffusion models) modelling of the experimental data were used for the quantitative evaluation of both biosorption processes. Although the Langmuir maximum biosorption capacities are different (higher in the case of Pb(II) (61.97 mg/g) than in the case of Hg(II) (51.32 mg/g)), the pseudo-second order kinetic constants have the same order of magnitude. This shows that the retention of both metal ions involves similar elementary steps and that RWB behaves as a typical biosorbent. These characteristics, together with the very good desorption behavior, provide a complete picture of the possible applications of this waste in environmental decontamination processes.