Hard–Soft Acid–Base-guided complementary oxygen- and sulfur-functionalized biochars for dose-efficient multi-metal immobilization in flooded paddy soils

Multi-metal contamination in flooded paddy soils presents a remediation challenge due to competitive sorption, redox-driven mobilization, and limited reactive site availability. Here, a Hard–Soft Acid–Base (HSAB)-guided strategy was used to develop complementary biochars with contrasting donor-type distributions. Waste camphor leaf-derived biochar was modified by H₂O₂ oxidation to produce an oxygen-rich material (OLB450) targeting relatively harder metal ions such as Zn²⁺ and by sulfur activation to produce a sulfur-rich material (SLB450) targeting softer and borderline ions such as Cd²⁺ and Cu²⁺, while unmodified biochar (LB450) served as the baseline. Single-metal adsorption experiments at pH ~7 showed Langmuir-type behavior (R² = 0.985–0.998). SLB450 exhibited the highest apparent adsorption capacities for Cu²⁺ and Cd²⁺; however, the trace-level partitioning indicator q max · K L suggested comparatively weaker Zn²⁺ partitioning on sulfur-rich surfaces, consistent with HSAB donor–metal selectivity. Under flooded soil incubation, all amendments reduced porewater Cu, Zn, and Cd, with SLB450 showing the strongest single-sorbent performance (>99% Cu, >96% Cd, and >93% Zn reduction at 5 wt%). Critically, the mixed 2 wt% OLB450 + 2 wt% SLB450 amendment reduced porewater Cu, Zn, and Cd to 0.012, 0.08, and 0.003 μM, respectively, below the levels achieved by 5 wt% SLB450 despite the lower total dosage. These results suggest that donor-type diversification can improve dose efficiency in biochar-based immobilization of coexisting metals in flooded agricultural soils.