Removal of Aqueous Uranyl and Arsenate Mixtures after Reaction with Limestone, PO43-, and Ca2

Ultra-high capacity and selectivity for uranium fixation by carbon nanosphere supported hydroxyapatite nanorod adsorbent

Uranium (U(VI)) has chemical and radiological toxicity, so the effective treatment of uranium-containing wastewater is crucial for both environmental safety and human health. Here, a carbon nanosphere (CNS) supported hydroxyapatite (HAP) nanorod (HAP/CNS) adsorbent was prepared using a simple glucose-assisted hydrothermal method toeffectively immobilize U(VI). Glucose not only derived CNS, but also facilitated HAP crystallization, prohibited HAP aggregation, and introduced oxygen-containing functional groups (i.e., COOH). The optimized HAP/CNS possessed a fantastic adsorption capability of 3080.3 mg/g for U(VI), nearly three times that of HAP and much higher than many reported HAP-based adsorbents. Notably, HAP/CNS was less affected by coexisting ions (distribution coefficient, Kd, researched 6.0 × 104 mL/g) and humic acid, and maintained good capability for real wastewater. The pseudo-second-order kinetic model and Langmuir isotherm model could better explain U(VI) removal behavior by HAP/CNS. Results showed that HAP/CNS and UO22+ combined to form a new uranium-containing compound, i.e., calcium-uranium mica (Ca(UO2)2(PO4)2·3H2O) via ion exchange and dissolution-precipitation, which should be the main reason for the ultra-high capacity and selectivity of HAP/CNS. Additionally, the hydrophilic oxygen-containing functional groups synergistically facilitated U(VI) fixation through complexation. This work introduces a superior adsorbent for purifying uranium-contaminated wastewater and elucidates its synergetic mechanism in uranium fixation.

In Situ Evolution of Ionic Sites at Clay Mineral Interfaces Facilitates Fluoride and Phosphorus Mineralization

Soil minerals influence the biogeochemical cycles of fluoride (F) and phosphorus (P), impacting soil quality and bioavailability to plants. However, the cooperative mechanisms of soil minerals in governing F and P in the soil environment remain a grand challenge. Here, we reveal the essential role of a typical soil mineral, montmorillonite (Mt), in the cycling and fate of F and P. The results show that the enrichment of metal sites on the Mt surface promotes the mineralization of F to the fluorapatite (FAP) phase, thereby remaining stable in the environment, simultaneously promoting P release. This differential behavior leads to a reduction in the level of F pollution and an enhancement of P availability. Moreover, solid-state NMR and HRTEM observations confirm the existence of metastable F-Ca-F intermediates, emphasizing the pivotal role of Mt surface sites in regulating crystallization pathways and crystal growth of FAP. Furthermore, the in situ atomic force microscopy and theoretical calculations reveal molecular fractionation mechanisms and adsorption processes. It is observed that a competitive relationship exists between F and P at the Mt interface, highlighting the thermodynamically advantageous pathway of forming metastable intermediates, thereby governing the activity of F and P in the soil environment at a molecular level. This work paves the way to reveal the important role of clay minerals as a mineralization matrix for soil quality management and offers new strategies for modulating F and P dynamics in soil ecosystems.

Uranium accumulation in environmentally relevant microplastics and agricultural soil at acidic and circumneutral pH

The co-occurrence of microplastics (MPs) with potentially toxic metals in the environment stresses the need to address their physicochemical interactions and the potential ecological and human health implications. Here, we investigated the reaction of aqueous U with agricultural soil and high-density polyethylene (HDPE) through the integration of batch experiments, microscopy, and spectroscopy. The aqueous initial concentration of U (100 μM) decreased between 98.6 and 99.2 % at pH 5 and between 86.2 and 98.9 % at pH 7.5 following the first half hour of reaction with 10 g of soil. In similar experimental conditions but with added HDPE, aqueous U decreased between 98.6 and 99.7 % at pH 5 and between 76.1 and 95.2 % at pH 7.5, suggesting that HDPE modified the accumulation of U in soil as a function of pH. Uranium-bearing precipitates on the cracked surface of HDPE were identified by SEM/EDS after two weeks of agitation in water at both pH 5 and 7.5. Accumulation of U on the near-surface region of reacted HDPE was confirmed by XPS. Our findings suggest that the precipitation of U was facilitated by the weathering of the surface of HDPE. These results provide insights about surface-mediated reactions of aqueous metals with MPs, contributing relevant information about the mobility of metals and MPs at co-contaminated agricultural sites.