Immobilization of Uranium in Contaminated Sediments by Hydroxyapatite Addition

Uranium contamination in sediment poses environmental and health risks due to its solubility in water. Previous studies have explored methods to reduce uranium mobility, such as pH adjustment and phosphate-based amendments. However, the effectiveness of these methods can vary depending on sediment composition, particularly organic carbon and pH levels. While phosphate minerals like apatite have been proposed to immobilize uranium by forming insoluble uranyl phosphate phases, the specific mechanisms and conditions under which this occurs remain unclear. This gap motivated further investigation into how apatite interacts with uranium in different sediment types. Prior research has shown that uranium solubility is influenced by pH and organic ligands, but the role of apatite in redirecting uranium speciation is less understood. This paper contributes by examining apatite's ability to immobilize uranium in sediments with varying organic carbon content. The study also addresses uncertainties regarding the chemical pathways through which apatite affects uranium solubility and speciation.

The study aimed to assess the effectiveness of hydroxyapatite in immobilizing uranium in contaminated sediments with differing organic carbon levels. Researchers sought to determine if apatite could reduce aqueous uranium concentrations to below drinking water standards. The specific problem addressed was the variability in uranium immobilization outcomes based on sediment composition. The motivation for the study stemmed from the need for reliable remediation strategies for uranium-contaminated sites. The research focused on two sediment samples with distinct organic carbon contents to evaluate how this variable influences uranium behavior. The study also aimed to clarify whether uranium speciation shifts from labile to more stable forms following apatite amendment. By comparing the effects of apatite on two different sediments, the researchers aimed to identify the key factors controlling uranium immobilization. This work provides insights into the geochemical mechanisms involved in uranium sequestration using apatite.

The study used batch equilibration experiments to assess uranium immobilization by hydroxyapatite in two sediment samples. The sediments had total uranium concentrations of 1703 and 2100 mg kg⁻¹ and organic carbon contents of 123 and 49 g kg⁻¹, respectively. Apatite was added at a rate of 50 g kg⁻¹ to each sediment sample to evaluate its effect on uranium solubility. Sequential chemical extractions were conducted to determine uranium speciation across different chemical fractions. The Mn-occluded fraction was analyzed at pH ≈ 1.26 to assess uranium redistribution. Energy dispersive X-ray (EDX) analysis in a transmission electron microscope (TEM) was used to identify uranium sequestration in secondary phosphate phases. The study compared uranium concentrations in aqueous and solid phases before and after apatite addition. The researchers also evaluated how pH and organic carbon content influenced uranium speciation in the presence of apatite.

Apatite amendment reduced aqueous uranium concentrations to below drinking water standards in both sediment samples. At an apatite addition of 50 g kg⁻¹, uranium solubility was lower than expected if autunite was the dominant solid phase. Sequential extractions showed uranium shifted from water-soluble and acid-soluble fractions to the Mn-occluded fraction. This suggests apatite redirects uranium speciation toward more stable forms. The Mn-occluded fraction had a pH of ≈1.26, indicating uranium solubilization despite low Mn oxide content in the sediments. EDX analysis confirmed uranium sequestration in secondary Al/Fe phosphate phases. Uranium speciation was influenced by both organic carbon content and equilibrium pH. The study found that higher organic carbon content increased uranium solubility in the absence of apatite. Apatite amendment reduced uranium mobility more effectively in the sediment with lower organic carbon content.

The study concluded that apatite amendment effectively reduces aqueous uranium concentrations in contaminated sediments. Apatite redirects uranium speciation from labile to more stable forms, such as Mn-occluded and Al/Fe phosphate phases. Uranium solubility was lower than expected if autunite was the controlling solid phase. The effectiveness of apatite varied with sediment organic carbon content and pH. Higher organic carbon increased uranium solubility in the absence of apatite. Apatite amendment was more effective in the sediment with lower organic carbon content. The study suggests that pH and organic ligands influence uranium immobilization by apatite. The findings support the use of apatite as a remediation strategy for uranium-contaminated sediments.