Green rusts as a new solution to sequester and stabilize phosphate in sediments under anoxic conditions and their implication for eutrophication control

Abstract

This work investigated the interaction of phosphate (P) with three typical GRs (i.e., GR(Cl), GR(SO4), and GR(CO3)) that can be present in natural anoxic sediments. Our results show that GRs can rapidly adsorb P within 20 min and have a P adsorption capacity ranging from 24 to 76.5 mg/g at pH 7.5 under anoxic conditions. The adsorption of P by GR(Cl) and GR(SO4) is favorable under acidic to neutral pH conditions, while the adsorption of P by GR(CO3) is pH-independent. Competitive adsorption of coexisting ions such as SO42−, bicarbonate/carbonate, and natural organic matter have also been observed for the adsorption of P by GR(Cl) and GR(SO4). The addition of a small portion (1%) of GRs into sediment significantly enhanced the P sequestration under anoxic conditions, and the P adsorption capacity of GRs-amended sediment is 1–2 orders of magnitude higher than the pristine sediment. GRs adsorb P through ion-exchange with anions located on basal plane and lateral faces of GRs since the intercalation of P anions into the interlayer of GRs is not observed. P-bound GRs are stable in anoxic sediment compared with other Fe(III) hydroxides, which suffer from reductive dissolution and the release of P into pore waters. This work demonstrates the key role of GRs in P sequestration in lake sediments and their high potential for eutrophication control under anoxic conditions.