Birnessite transformation increases Cd(II) exposure risk in contaminated soils by disrupting sequestration onto resulting fulvic acid and manganese oxide mineral composites

Soils contain abundant organic matter (OM) and Mn(oxyhydr)oxides (MnOs) composites, which primarily control the geochemical behavior of Cd. However, the impact of manganese phase transformation on the retention and availability of Cd remains unclear. Here, four organo-mineral composites were synthesized using birnessite (δ-Mn Ⅳ O 2 ), β-Mn Ⅲ,Ⅳ O 2 , γ-Mn Ⅲ OOH, and Mn Ⅱ,Ⅲ 3 O 4 with fulvic acid (FA), during adsorption to preformed MnOs with 5 wt% organic carbon loadings. The performance and underlying Cd(II) adsorption mechanism by resulting FA-MnO composites were explored. Results showed that δ-MnO 2 transformation decreases Cd(II) adsorption capacity by 23.02–45.44 % on the resulting FA-MnOs. This reduction was attributed to increased aggregation of MnO particles and competition from generated Mn(II/III) for available vacancy sites. Cd undergoes redistribution within the FA-MnOs during the transformation of δ-MnO 2 . The amount of Cd associated with MnOs decreases by 9.46–37.08 %, while OM bound-Cd increases by 2.82–10.49 %. FAδ-MnO 2 predominantly interacted with Cd(II) through tetravalent manganese, forming stable coordination or chemical bonds. In contrast, FAγ-MnOOH and FA-Mn 3 O 4 primarily interacted with Cd(II) through divalent manganese via electrostatic attraction and ion exchange, resulting in weaker binding stability. These findings provide new insights into the roles of manganese transformation in the geochemical cycling of Cd in soils. • δ-MnO 2 transformation inhibits the sequestration of Cd(II) onto the resulting FA-MnO composites. • δ-MnO 2 transformation decreases Cd(II) incorporation into crystallin manganese oxides. • More Cd(II) binds to the organic component of FA alongside the transformation of δ-MnO 2 . • δ-MnO 2 transformation decreases binding stability between manganese phase and Cd(II). • Cd aquatic hazard will be more severe in Mn oxide-bearing systems lower in redox potential.