Effects of exopolysaccharides from Rhizobium tropici on transformation and aggregate sizes of iron oxides

• Exopolysaccharides (EPS) adsorption on Fe 3 O 4 involved ion exchange, chemisorption and diffusion processes. • Desorption studies indicated the high stability of EPS-iron complexes. • EPS increased the aggregate size of its iron oxide complexes but shrank at higher ratios. • EPS coating prevented the transformation of Fe 3 O 4 into other oxide forms, β-FeOOH and α/γ −Fe 2 O 3 . Iron oxide transformations in soil significantly impact nutrient availability and plant health. This study investigated the interaction between exopolysaccharides (EPS), produced by Rhizobium tropici , and iron oxide (Fe 3 O 4 ), focusing on their impact on the transformation, particle size, and zeta potential of iron oxides. The characterization of the EPS-iron oxide composites was carried out using X-ray Powder Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM)/Energy Dispersive X-ray Analysis (EDX). The EPS adsorption kinetics revealed chemisorption and diffusion as controlling processes for EPS adsorption on Fe 3 O 4 , while isotherm data with releasing proton indicated possible ion exchange and heterogeneous layered adsorption. Desorption studies suggested the high stability of EPS-iron complexes. Notably, EPS significantly increased the aggregate size of EPS-iron complexes at low EPS/iron oxide molar ratios but shrank the aggregate size at higher ratios (> EPS/iron oxide 2 × 10 −4 ). Additionally, EPS complexation resulted in a shift in the zeta potential towards more negative surface functionality. Functional groups within EPS, specifically –COOH, –OH and –NH played a crucial role in the interaction of EPS with iron oxides. The study concluded that EPS coating prevented the transformation of Fe 3 O 4 into other iron oxide forms like β-FeOOH, α-Fe 2 O 3 , and γ-Fe 2 O 3 , elucidating the significant role of EPS in soil mineral processes.