Uranium(U)is the most common radioactive contaminant in groundwater and sediments at U mining and processing sites,as well as nuclear waste storage sites.It can exist in nature in the oxidizing form,U(Ⅵ),or in the reducing form,U(Ⅳ).U(Ⅵ)species in aqueous media is generally very mobile and bio-available,whereas U(Ⅳ)is not mobile or bio-available.U mine tailings and processing wastes are generally discharged into reservoir ponds that may become wetlands.The mobility and biogeochemistry of U in wetlands would be profoundly different than in upland sediments because of sharp geochemical gradients,elevated organic carbon concentrations and microbial activity,and the transient nature of hydraulic regimes.However,chemical speciation and biogeochemical behaviors of U in such systems are not well understood.In this work,we investigated chemical speciation and retention mechanisms of U in the SRS wetland sediments using U L3-edge X-ray absorption near-edge structure(XANES)and extended X-ray absorption fine structure(EXAFS),and X-ray fluorescence mapping.The Savannah River Site(SRS)in South Carolina,USA,includes several former U processing facilities.U contaminant has entered the surrounding environments through accidental and purposeful discharges.Approximate 70%of the 45,000 kg of depleted U released between 1958 and 1980 still remains in the Tims Branch and associated wetland sediments.In addition,we also studied the sorption of U onto the uncontaminated SRS wetland sediments.U was highly sorbed onto the SRS wetland sediment(>80%or Kd values~1000)at pH < 4 and pH > 8,in contrast to previous studies which demonstrated that U has minimal sorption onto relevant mineral phases(e.g.,goethite,kaolinite,and hydroxyl-interlayered vermiculite)at these pH ranges.Our findings include:(1)U in the SRS wetland sediments is in the oxidizing form,U(Ⅵ);(2)there was no evidence to indicate the presence of U(Ⅵ)-bearing minerals in the SRS wetland sediments;(3)there was evidence to demonstrate that U was bonding with carbon atoms,as well as Fe atoms,at its second nearest neighbors.All these findings indicated that U(Ⅵ)was highly retained in the SRS wetland sediments through binding with natural organic matters(NOM),in good agreement with high NOM concentration and sequential extraction of the wetland sediments.In a summary,wetlands can be especially well suited for immobilization of U(Ⅵ)even under oxidizing conditions because of its strong uptake by NOM and maybe surface complexation onto NOM-coated sediment minerals.