P/Pb transport at the interface of water and Al-substituted ferrihydrite: Effect of P/Pb loading sequence

Sediment mineral such as Al-substituted ferrihydrite plays a critical role for contaminant transport in the river systems. Heavy metals and nutrient pollutants often coexisted in the natural aquatic environment, and they may enter the river at different time frames, altering the fate and transport of each other subsequently discharged into the river. However, most studies focused on the simultaneous adsorption of co-existing pollutants instead of their loading sequence. In this study, the transport of P and Pb at the interface of Al-substituted ferrihydrite and water was investigated under different P and Pb loading sequences. The results showed that preloaded P provided additional adsorption sites for the following adsorption of Pb, with enhanced Pb adsorption amount and accelerated adsorption process. Moreover, Pb preferred to be bounded with the preloaded P to form P-O-Pb ternary complexes rather than directly reacted with Fe-OH. The formation of the ternary complexes effectively prevented the release of Pb once adsorbed. However, the adsorption of P was slightly affected by the preloaded Pb, and most of the P were adsorbed onto Al-substituted ferrihydrite directly with the formation of Fe/Al-O-P. Moreover, the release process of the preloaded Pb was significantly inhibited by the following adsorbed P due to the formation of Pb-O-P. Meanwhile, the release of P was not detected from all P and Pb loaded samples of different adding sequence due to the high affinity between P and the mineral. Thus, the transport of Pb at the interface of Al-substituted ferrihydrite was seriously influenced by the adding sequence of Pb and P, while the transport of P was not sensitive to the adding sequence. The results provided important information for the transport of heavy metal and nutrients in river system with different discharging sequence, and offered new insights to further understand the secondary pollution in multi-contaminated river.