Mechanisms of Mn(II) removal from mining-impacted waters by biochar: Decoupling precipitation and surface complexation via experiments and atomistic simulations

Manganese(II) in mining-impacted waters is a growing regulatory and ecological concern, and low-cost sorbents such as biochar offer a scalable remediation pathway, yet the mechanisms by which biochar sequesters Mn(II) remain poorly resolved. This study identifies the specific chemical drivers of Mn(II) sequestration by combining fixed-bed column and batch experiments with atomistic molecular dynamics simulations. Oilseed rape straw biochars, produced at 350 °C, 550 °C, and 700 °C, removed 20%–50% of dissolved Mn from acidic influent (pH 4, 5 ppm). High-temperature biochar achieved the greatest removal ( ∼ 50%) and rapidly increased effluent pH to 9, triggering precipitation of Mn phases. Conversely, lower-temperature biochars removed 20%–30% of Mn while maintaining a near-neutral pH (7-7.5). Enhanced K + release in these systems indicates significant cation exchange and non-precipitative pathways. Molecular simulations confirmed that while neutral surfaces show weak Mn(II) association, deprotonated sites drive strong adsorption, with ∼ 50% of simulated Mn associating via inner-sphere coordination and ∼ 10% via outer-sphere association (distance-based simulation metrics, not bulk removal). These results establish a four-step mechanistic framework – exchange, deprotonation, complexation, precipitation – whose relative contributions are governed by feedstock and pyrolysis temperature. This framework provides transferable, management-relevant design criteria for selecting and engineering biochars for Mn remediation in mining-affected catchments. Synopsis: Experiments and atomistic simulations decouple Mn(II) removal by biochar 22into exchange-driven pH increase, deprotonation-enabled surface complexation, and high-pH 23precipitation – highlighting presence of deprotonatable functionalities as key design rules.