Adsorption of the microbial exopolysaccharide from Rhizobium tropici (ATCC 49,672) on birnessite facilitates Mn reduction and dissolution: Structural interactions and morphological transformation

• Exopolysaccharides (EPS) adsorbed on birnessite via chemisorption with diffusion. • EPS adsorption promoted birnessite.Mn (IV) reduction and released Mn²⁺. • EPS presence inhibited birnessite formation/crystallization and its transformation to β-MnO 2. • The presence of Mn oxides significantly aggregated EPS particles. The study addresses the broader question of how microbial exopolysaccharides (EPS) can modulate the reactivity and stability of manganese (Mn) oxides in environmental systems. The objective of this study is to investigate the adsorption process of EPS from Rhizobium tropici on birnessite (MnO 2 ) and mechanisms of induced Mn reduction and dissolution and its further transformation. EPS adsorption isotherms and kinetics on birnessite and the induced dissolution of birnessite were analyzed. Adsorption kinetics could best be described with the Parabolic Diffusion Model while EPS adsorption isotherm was best fitted with the Freundlich model. Adsorbed EPS facilitated Mn reduction and dissolution, releasing Mn²⁺ into the solution. This was also confirmed by fourier transform infrared spectroscopy (FTIR) results with the Mn-O vibrations, O-H groups and C=O carbonyl stretching. The pre-presence of EPS hindered the formation and growth of crystalline structure of birnessite and further hindered its transformation into more stable β-MnO 2 . The presence of Mn oxide significantly aggregated EPS, forming large aggregates. This study reveals the importance of EPS in influencing both the adsorption behavior and the redox state of Mn, providing significant insights into the interactions of EPS with birnessite in environmental systems.