Tuning crystal structures of Mn nanomaterials on Enteromorpha prolifera biochar to enhance seawater purification

Recently, antibiotics and heavy metals in marine environments show no consistent positive correlation; their complex interactions may even lead to negative correlations, complicating targeted remediation strategies. Developing cost-effective and tunable materials to address such complexity remains a critical need. Biochar is a promising low-cost remediation material; however, its performance is highly structure-dependent. In this study, a series of manganese-modified Enteromorpha prolifera-derived biochar (Mn/BC) was synthesized via a simple, low-temperature hydrothermal method, providing a sustainable and economical pathway for the high-value utilization of marine biomass waste. By adjusting the hydrothermal temperature, the crystalline phase of manganese species was effectively regulated, with MnCO₃ formed at 120 °C (Mn/BC-120) and transformed into Mn₃O₄ at 200 °C (Mn/BC-200), enabling selective remediation toward antibiotics and heavy metals. Mn/BC-120 exhibited the highest catalytic activity in activating peroxymonosulfate (PMS) for levofloxacin degradation, achieving complete removal of 10 mg/L LEV within 120 min at 0.2 g/L, attributed to enhanced electron transfer and singlet oxygen generation. In contrast, Mn/BC-200 showed the highest removal efficiency for As(V), reaching 91.37% for 4 mg/L As(V) within 120 min at 0.8 g/L via monolayer chemisorption, showing an opposite structure-dependent performance trend. All Mn/BC materials exhibited strong resistance to environmental interference and high stability. Density functional theory calculations revealed that MnCO₃ possesses a narrower HOMO-LUMO gap, favoring PMS activation, whereas Mn₃O₄ exhibits stronger binding toward As(V). This work, demonstrates that temperature-controlled crystal engineering of manganese species on biochar enables a straightforward, low-cost, and selective remediation strategy for diverse pollutants in complex marine environments.