Disentangling competitive adsorption dynamics: engineered Eucalyptus camaldulensis biochar for preferential retention of Pb(II) and Cd(II) in multimetallic systems.

The proliferation of heavy metal contamination necessitates sustainable, cost-effective remediation strategies capable of discriminating between toxic co-existing cations in complex matrices. This study presents the first systematic investigation employing phosphoric acid-modified Eucalyptus camaldulensis leaf biochar (EC-biochar) for the simultaneous and selective removal of Pb(II) and Cd(II) from competitive multicomponent systems a critical advancement beyond conventional single-metal studies. Characterization analyses confirmed a hierarchically porous architecture (SEM), semi-crystalline structure (crystallinity index ∼35%), and abundant oxygenated surface functional groups. Maximum adsorption capacities in monocomponent systems reached 17.90 mg g-1 for Pb(II) and 18.35 mg g-1 for Cd(II) under optimal conditions (pH 4.5, 25 °C, 120 min contact time, 1 g L-1 adsorbent dose). Notably, although Cd(II) exhibited marginally higher capacity in single-metal solutions, competitive experiments revealed preferential affinity for Pb(II) over Cd(II) in binary and multimetallic matrices, with significant interference from co-existing Zn(II), Cu(II), and electrolytes (NaCl and NaClO4). The extent of competitive suppression was rigorously quantified using the steady-state adsorption capacity ratio (q'e/qe), where qe represents the equilibrium adsorption capacity (mg g-1) in single-component systems and q'e denotes the corresponding equilibrium capacity (mg g-1) obtained under competitive multicomponent conditions. While the competitive Langmuir model adequately described Pb(II) removal, it poorly fitted Cd(II) adsorption isotherms suggesting that Cd(II) exhibits non-ideal competitive behavior involving heterogeneous site interactions or multi-layer formation, consistent with its softer Lewis acid characteristics and explaining its inferior performance relative to Pb(II) in mixed systems. Thermodynamic analysis confirmed spontaneous, endothermic adsorption processes. Mechanistic studies identified hydrogen bonding, electrostatic attraction, π-π interactions, and inner-sphere surface complexation as primary removal pathways. These findings demonstrate that EC-biochar is a promising eco-friendly adsorbent for the selective retention of Pb(II) and Cd(II) from complex wastewater matrices.