Effect of Sulfide on the Cotransport of Ferrihydrite and Birnessite Colloids with Phosphorus in Saturated Porous Media

In this work, ferrihydrite (Fhc) and birnessite colloids (Brc) were selected to explore their cotransport behavior along with phosphorus (P) in the sulfidic environments using column experiments. Multiple models and characterization methods were used to reveal the sulfidation mechanisms of Fhc and Brc and the effect on the transport of P. The results showed that the cotransport of Fhc and Brc along with P was significantly inhibited by the presence of sulfide. The breakthrough of Fhc and Brc decreased with increasing sulfide concentration, which was primarily attributed to the reductive dissolution of colloids and the reduction of surface charges. Meanwhile, the interaction between P and Fhc or Brc through the inner-sphere surface complexes (Fe–O–P or Mn–O–P) and surface ion exchange (O–P) was broken by the presence of sulfide. Consequently, the transport of P would be blocked by the formation of sulfidation products (such as S0, FeS, or MnS). Due to the presence of transition-state Mn(III), the reduction of Brc by sulfide was more sensitive than that of Fhc, thereby causing differences in the cotransport capacity. The findings expand the basic understanding of the fate and cotransport of iron and manganese colloids with contaminants in the subsurface environment.