Superior Cu (II) purification using carrageenan biochar-olivine composite: Synergistic effects of K (I), Mg (II), Fe (II/ III), and Si (IV)

Biochar is a remarkable adsorbent for environmental pollutants. However, their applicability is hindered by specific deficiencies, including restricted adsorption capacity, inadequate selectivity, and low reusability. For instance, carrageenan gum pure biochar (BC CA300) exhibits inadequate Cu (II) adsorption performance owing to the weakly binding surface functional groups. Physical modification consumes energy, while chemical modification heavily uses chemicals, causing environmental harm. Herein, a novel modifying agent was developed that can selectively and efficiently adsorb Cu (II). The BC CA300 surface was grafted with olivine mineral (Mg, Fe)₂ SiO₄ through a one-pot hydrothermal carbonization (HTC) technique. The engineered biochar (BC CAOL) was investigated for the synergistic effect of K(I), Mg (II), Fe (II), Fe (III), and Si (IV) from the viewpoints of adsorption kinetics, isotherm, and thermodynamics alongside DFT calculations and electrostatic potential (ESP) mapping. The surface structure of BC CA300 and BC CAOL was examined and compared through BET, FTIR, XPS, and SEM-EDS. It was found that engineered biochar adsorption capacity (340.98 mg g⁻¹) was 4.7 times greater than that of pure biochar (72.51 mg g⁻¹). The adsorption energy of engineered biochar is 2.5 times greater than that of pure biochar. The synergistic effect is indicative of the following sequence: K > Fe > O > Si > Mg. Hence, the Cu (II) adsorption mechanism is primarily driven by precipitation > complexation > chelation > ion exchange, beside the pore-filling mechanism. This research highlights the potential of gum-mineral composite biochar as a green, effective, and selective alternative for Cu (II)-laden wastewater decontamination.