Mechanistic insight into the impact of humic acid-mediated birnessite formation on the immobilization of As(V) and organic carbon

The interaction between birnessite and dissolved organic matter(DOM) is critical for carbon sequestration and heavy metal immobilization. This study investigated humic acid (HA, a representative form of DOM) in mediating the formation of triclinite birnessite (TB) and elucidated the influence mechanisms for carbon sequestration and As(V) immobilization. HA significantly affected both the growth and structural properties of TB. HA induced TB structural transformation from lamellar-like to honeycomb-like, reduced surface negative charge, and increased specific surface area, oxygen vacancies and hydroxyl groups. HA-Mn(III) complexes and pore trapping enhance organic carbon sequestration synergistically. As(V) immobilization was improved by inner-sphere complexation (via hydroxyl coordination) and electrostatic attraction with the maximum adsorption capacities for HA-mediated birnessite (TB-2H) were 65.140 mg·g⁻¹ , 6.4-fold higher than pure TB. Besides, TB-2H shows low manganese release, high stability and good reusability. These findings achieves synergistic carbon sequestration and As(V) immobilization, providing an innovative strategy for environmental remediation.