Production and characterization of biochar and modified biochars by carbonization process of bearberry (Arctostaphylos uva-ursi. L.): Adsorption capacities and kinetic studies of Pb2+, Cd2+ and rhodamine B removal from aqueous solutions

In this work, Bearberry (Arctostaphylos uva-ursi L.) leaves and twigs were used as novel biomass source for production of biochar and modified biochars (manufacturing of microporous and mesoporous carbons by physical and chemical activations, using CO₂ and H₃PO₄) via one-step carbonization (800 °C) with excellent physicochemical properties, for effective removal of Pb²⁺ and Cd²⁺ ions, and synthetic dye (Rhodamine B - RhB) from aqueous solutions. Results showed that carbonized (BL-C) and physically activated carbons (BL-CO₂) as microporous adsorbents (specific surface areas 219.0 m²/g and 305.5 m²/g) show remarkable removal efficiency of Pb²⁺ (99.8 % and 99.9 %, for BL-C and BL-CO₂), while these adsorbents showed moderate affinity for Cd²⁺ elimination (53.5 % and 48.5 %). BL-H₃PO₄ as mesoporous adsorbent with lower specific surface and larger pores (90.2 m²/g with D_max = 33.6 nm), shows very good removal efficiency of PhB (~ 87 %). It was found that physical adsorption occurs during RhB removal onto BL-H₃PO₄, where dominant mechanism represents film diffusion, with reduced boundary layer effect. Adsorption process takes place over π–π, hydrogen bonding and electrostatic interactions. Adsorption processes of Pb²⁺ and Cd²⁺ onto BL-CO₂ and BL-C take place via physical and chemical adsorption, but with different type of mechanism, including combination of diffusion and chemisorption (increased effect of boundary layer) and intra-particle diffusion (greatly reduced boundary layer effect), respectively. A very interesting fact found in this study, is that metal oxide surfaces (as Cu₂O, SiO₂, ZnO present in activated carbons) exhibit an efficient binding towards cadmium, providing physisorption capability onto non-metallic graphene features.