Biomineralization of uranium by Desulfovibrio desulfuricans A3-21ZLL under various hydrochemical conditions.

Uranium pollution in groundwater environment has become an important issue of global concern. In this study, a strain of Desulfovibrio desulfuricans was isolated from the tailings of acid heap leaching, and was shown to be able to remove uranium from water via biosorption, bio-reduction, passive biomineralization under uranium stress, and active metabolically dependent bioaccumulation. This research explored the effects of nutrients, pH, initial uranium and sulfate concentration on the functional groups, uranium valence, and crystal size and morphology of uranium immobilization products. Results showed that tetravalent and hexavalent phosphorus-containing uranium minerals was both formed. In sulfate-containing water where Desulfovibrio desulfuricans A3-21ZLL can grow, the sequestration of uranium by bio-reduction was significantly enhanced compared to that with no sulfate loading or no growth. Ungrown Desulfovibrio desulfuricans A3-21ZLL or dead ones released inorganic phosphate group in response to the stress of uranium, which associated with soluble uranyl ion to form insoluble uranium-containing precipitates. This study revealed the influence of hydrochemical conditions on the mineralogy characteristics and spatial distribution of microbial uranium immobilization products. This study is conducive to the long-term and stable bioremediation of groundwater in decommissioned uranium mining area.

Modeling and visualizing the transport and retention of cationic and oxyanionic metals (Cd and Cr) in saturated soil under various hydrochemical and hydrodynamic conditions.

Cationic and oxyanionic metals are widely existed in the aquatic and soil environment with the process of industrialization and they may behave different transport properties in aquifer systems due to the opposite charges. In this study, the comparative transport behaviors of Cd2+ and CrO42- in water-saturated soil columns were investigated under a variety of hydrochemical and hydraulic conditions such as pH, ionic strength (IS), and flow rate. The transport mechanisms of Cd(II) and Cr(VI) were explored by fitting the breakthrough curves with a two-site non-equilibrium transport model. Results indicated that high solution pH inhibited the transport of Cd(II) due to the enhanced electrostatic interaction. In contrast, the migration of Cr(VI) was promoted with the least amount of Cr(VI) (1.23 mg) being retained in soil at high pH, ascribing to the stronger electrostatic repulsion between anions and soil surface. Meanwhile, high pH conditions were not favorable for the participation of reduced iron in the reduction process of Cr(VI), resulting in the least amount of Cr(III) detected (22%). The increase in ionic strength decreased the negativity of the potential at the adsorption plane, which enhanced the transport of cationic Cd(II) and the retardation of anionic Cr(VI). In addition, the increase in flow rate facilitated the transport of Cd(II) and Cr(VI), mainly due to the decreasing contacting with porous media and enhanced dispersion effect. These findings demonstrated that the fate and environmental behavior of metal cations and anions differed with the change of hydrochemical and hydrodynamic properties, which should be considered for the risk assessment and remediation of metal contaminated sites.

Co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions.

To investigate co-transport behavior of ammonium and colloids in saturated porous media under different hydrochemical conditions, NH4+ was selected as the target contaminant, and silicon and humic acid (HA) were selected as typical organic and inorganic colloids in groundwater. Column experiments were then conducted to investigate the transport of NH4+ colloids under various hydrochemical conditions. The results showed that because of the different properties of colloidal silicon and HA after combining with NH4+, the co-transport mechanism became significantly different. During transport by the NH4+-colloid system, colloidal silicon occupied the adsorption sites on the medium surface to promote the transport of NH4+, while humic acid (HA) increased the number of adsorption sites of the medium to hinder the transport of NH4+. The co-transport of NH4+ and colloids is closely related to hydrochemical conditions. In the presence of HA, competitive adsorption and morphological changes of HA caused NH4+ to be more likely to be transported at a higher ionic strength (IS = 0.05 m, CaCl2) and alkalinity (pH = 9.3). In the presence of colloidal silicon, blocking action caused the facilitated transport to be dependent on higher ionic strength and acidity (pH = 4.5), causing the recovery of NH4+ to improve by 7.99%, 222.25% (stage 1), and 8.63%, respectively. Moreover, transport increases with the colloidal silicon concentrations of 20 mg/L then declines at 40 mg/L, demonstrating that increased concentrations will lead to blocking and particle aggregation, resulting in delayed release in the leaching stage. Graphical abstract.

Transport and retention of differently coated CeO2 nanoparticles in saturated sediment columns under laboratory and near-natural conditions.

Where surface-functionalized engineered nanoparticles (NP) occur in drinking water catchments, understanding their transport within and between environmental compartments such as surface water and groundwater is crucial for risk assessment of drinking water resources. The transport of NP is mainly controlled by (i) their surface properties, (ii) water chemistry, and (iii) surface properties of the stationary phase. Therefore, functionalization of NP surfaces by organic coatings may change their fate in the environment. In laboratory columns, we compared the mobility of CeO2 NP coated by the synthetic polymer polyacrylic acid (PAA) with CeO2 NP coated by natural organic matter (NOM) and humic acid (HA), respectively. The effect of ionic strength on transport in sand columns was investigated using deionized (DI) water and natural surface water with 2.2 mM Ca2+ (soft) and 4.5 mM Ca2+ (hard), respectively. Furthermore, the relevance of these findings was validated in a near-natural bank filtration experiment using HA-CeO2 NP. PAA-CeO2 NP were mobile under all tested water conditions, showing a breakthrough of 60% irrespective of the Ca2+ concentration. In contrast, NOM-CeO2 NP showed a lower mobility with a breakthrough of 27% in DI and < 10% in soft surface water. In hard surface water, NOM-CeO2 NP were completely retained in the first 2 cm of the column. The transport of HA-CeO2 NP in laboratory columns in soft surface water was lower compared to NOM-CeO2 NP with a strong accumulation of CeO2 NP in the first few centimeters of the column. Natural coatings were generally less stabilizing and more susceptible to increasing Ca2+ concentrations than the synthetic coating. The outdoor column experiment confirmed the low mobility of HA-CeO2 NP under more complex environmental conditions. From our experiments, we conclude that the synthetic polymer is more efficient in facilitating NP transport than natural coatings and hence, CeO2 NP mobility may vary significantly depending on the surface coating.