Enhancing Biosorbent Stability, Performance Efficiency, and Cost-effectiveness: A Ternary Magnetic Composite for Sequestration of Multiple Toxic Metals from Water

Innovative low-cost magneto-biochar-clay (MBC 1:2:1 and MBC 1:3:1) composite adsorbents formed by a one-step combination of magnetic nanoparticles (MNP), biochar (from grape cluster stalk), and feldspar clay were employed for Cd(II), Cr(VI), and Cu(II) removal from simulated contaminated aqueous solution. The composites expressed higher cation exchange capacity and BET surface area compared to the feldspar clay, as well as characteristic biobased functional groups such as hydroxyls, carboxyls, and amides. The optimum removal efficiency was achieved at a 0.66 g/L solid-to-liquid ratio, and the equilibrium was attained at 720 min for all three ions. The adsorption process was via electrostatic interactions as well as adsorption within the pores (>90% of total adsorption). Surface functional groups involved in the adsorption process are -OH, -COO^-, and -C-N. An increase of Cu(II) concentration in solution enhanced Cr(VI) removal efficiency by 86 and 79% on MBC 1:2:1 and MBC 1:3:1, respectively, while reducing Cd(II) uptake by 65 and 52%, respectively. The equilibrium data was described by the Langmuir and Langmuir-Freundlich adsorption isotherm models. Higher temperature slightly enhanced Cd(II) adsorption, while no temperature impact was observed for Cr(VI) and Cu(II) adsorption. The adsorbent reusability study confirmed that the removal efficiency for Cr(VI) remained high after five cycles, while for Cd(II) and Cu(II) only during the first adsorption cycle. Thus, the MBC composite is a cost-effective and efficient adsorbent and, due to its magnetic properties, can easily be applied as a water treatment adsorbent for Cr(VI) removal from water.