Magnetic valorization of biomass and biochar of a typical plant nuisance for toxic metals contaminated water treatment

Low-cost materials are promising aqueous pollutant adsorbents but when batch adsorption method is employed, separation of pollutant-loaded-adsorbents from water is a major challenge especially when dealing with a large volume of wastewater. Thus, biomass and biochar from Quercus robur fruits were valorized via magnetization (for easy post-adsorption separation) to prepare optimized biomass-magnetic hybrid (BMM 0.5:1) and biochar-magnetic hybrid (BCM 1:1). The BMM 0.5:1 and BCM 1:1 were employed for Pb(II) and Cd(II) removal from simulated contaminated water. The hybrids exhibited higher values of cation exchange capacity (CEC), BET surface area and pore sizes, as well as better thermal stability and the presence of pure spinal structures of Fe3O4, along with the characteristic functional groups of biomaterials (such as the hydroxyls, amides and carboxyls). The adsorption equilibria for both cations were attained within 180 min. Adsorption mechanism involved electrostatic interactions on both external and pore surfaces, with Pb(II) data fitting the Langmuir adsorption isotherm model while Cd(II) data fitted the Freundlich. The adsorption process was spontaneous and exothermic as solution temperature was increased from 292 to 310 and 328 K. The adsorption of Cd(II) initially increased with temperature but decreased on further temperature rise by similar percentages for both adsorbents. In contrast, adsorption of Pb(II) decreased continuously but the decrease was higher for BMM 0.5:1 than BCM 1:1 implying BCM is a more promising adsorbent. Adsorption capacities for BMM 0.5:1 are 63.6 and 21.0 mg/g, while BCM 1:1 has 58.2 mg/g and 21.3 mg/g for Pb(II) and Cd(II), respectively. These adsorption capacities were better than many low-cost adsorbents in literature. Thus magnetic valorization, apart from easing separation, enhances the adsorption capacity of low-cost adsorbents.