Removal of U(VI) from acidic wastewater by persimmon tannin-functionalized chitosan.

With sodium tripolyphosphate (STPP) as cross-linker, Persimmon tannin-chitosan microspheres (PT-CS) were synthesized by hydrothermal for removing U(VI) from acidic effluent. The batch experiments indicated that PT-CS adsorbed U(VI) most effectively at pH 1.5, the maximum adsorption capacity for PT-CS was 245 mg/g. Compared to pure CS dissolved at pH 3, PT-CS still maintain high stability at pH 1. Moreover, single system of common metal ions in rare earth wastewater only slightly affected the adsorption of uranium at pH 1.5, but this process was inhibited about 30% at pH 5. Those results indicated that the selectivity of PT-CS for uranium removal could be controlled by regulating the pH and there are excellent potentials for PT-CS using in acid metal water treatment. Its adsorption selectivity and ability to adapt different condition was demonstrated with uraniferous rare earth wastewater treatment. The adsorption for PT-CS to U(VI) were well fitted for both Langmuir isothern and pseudo-secondary kinetic model equations, and that meant chemisorption dominated the removal process. Spectroscopic analyses confirmed that the adsorption of U(VI) occurred via surface complexation by -OH and ion exchange with Na+. Therefore, this study provides a high-efficiency, low-cost, valuable and highly adaptable method for the treatment of acidic uranium-containing effluents.

Single-atom Zr embedded Ti4O7 anode coupling with hierarchical CuFe2O4 particle electrodes toward efficient electrooxidation of actual pharmaceutical wastewater.

Electrocatalytic oxidation is commonly restricted by low degradation efficiency, slow mass transfer, and high energy consumption. Herein, a synergetic electrocatalysis system was developed for removal of various drugs, i.e., atenolol, florfenicol, and diclofenac sodium, as well as actual pharmaceutical wastewater, where the newly-designed single-atom Zr embedded Ti4O7 (Zr/Ti4O7) and hierarchical CuFe2O4 (CFO) microspheres were used as anode and microelectrodes, respectively. In the optimal reaction system, the degradation efficiencies of 40 mg L-1 atenolol, florfenicol, and diclofenac sodium could achieve up to 98.8%, 93.4%, and 85.5% in 120 min with 0.1 g L-1 CFO at current density of 25 mA cm-2. More importantly, in the flow-through reactor, the electrooxidation lasting for 150 min could reduce the COD of actual pharmaceutical wastewater from 432 to 88.6 mg L-1, with a lower energy consumption (25.67 kWh/m3). Meanwhile, the electrooxidation system maintained superior stability and environmental adaptability. DFT theory calculations revealed that the excellent performance of this electrooxidation system could be ascribed to the striking features of the reduced reaction energy barrier by single-atom Zr loading and abundant oxygen vacancies on the Zr/Ti4O7 surface. Moreover, the characterization and experimental results demonstrated that the CFO unique hierarchical structure and synergistic effect between electrodes were also the important factors that could improve the system performance. The findings shed light on the single-atom material design for boosting electrochemical oxidation performance.

Phosphorus-rich biochar modified with Alcaligenes faecalis to promote U(VI) removal from wastewater: Interfacial adsorption behavior and mechanism.

Phosphorus-rich biochar (PBC) has been extensively studied due to its significant adsorption effect on U(VI). However, the release of phosphorus from PBC into solution decreases its adsorption performance and reusability and causes phosphorus pollution of water. In this study, Alcaligenes faecalis (A. faecalis) was loaded on PBC to produce a novel biocomposite (A/PBC). After adsorption equilibrium, phosphorus released into solution from PBC was 2.32 mg/L, while it decreased to 0.34 mg/L from A/PBC (p < 0.05). The U(VI) removal ratio of A/PBC reached nearly 100%, which is 13.08% higher than that of PBC (p < 0.05), and it decreased only by 1.98% after 5 cycles. When preparing A/PBC, A. faecalis converted soluble phosphate into insoluble metaphosphate minerals and extracellular polymeric substances (EPS). And A. faecalis cells accumulated through these metabolites and formed biofilm attached to the PBC surface. The adsorption of metal cations on phosphate further contributed to phosphorus fixation in the biofilm. During U(VI) adsorption by A/PBC, A. faecalis synthesize EPS and metaphosphate minerals by using the internal components of PBC, thus increasing the abundance of acidic functional groups and promoting U(VI) adsorption. Hence, A/PBC can be a green and sustainable material for U(VI) removal from wastewater.

Efficient uranium(VI) adsorbing bioinspired nano-sized hydroxyapatite composites: synthesis, tuning, and adsorption mechanism.

The production of large amounts of uranium-containing wastewater and its potential hazards has stimulated green and efficient material removal of uranium (VI). Inspired by the natural mineralization of bone, a facile and eco-friendly biomimetic synthesis of nano-hydroxyapatite (HAP) was carried out using chitosan (CS) as a template. It was found that the reaction temperature and the amount of precursors influence the particle size, crystallinity and specific surface area of the CS/HAP nanorods, and consequently their U(VI) adsorption efficiency. Moreover, the synthesized CS/HAP-40 with smaller particle size, lower crystallinity, and larger specific surface area show a more efficient U(VI) removal compared with CS/HAP-55 and CS/HAP-55-AT. It has a maximum adsorption capacity of 294.12 mg·g-1 of the CS/HAP-40. Interestingly, the U(VI) removal mechanism of CS/HAP-40 in acidic (pH = 3) and alkaline (pH = 8) aqueous solutions was found to be different. As one of the main results, the U(VI) adsorption mechanisms at pH 8 could be surface complexation and ion exchange. On the contrary, three different mechanisms could be observed at pH 3: dissolution-precipitation to form chernikovite, surface complexation, and ion exchange.

Construction of g-C3N4/Ag/TiO2 Z-scheme photocatalyst and Its improved photocatalytic U(VI) reduction application in water.

The reduction of soluble U(VI) to insoluble U(IV) by photocatalytic technology is considered to be a valid method to remove U(VI) from water. Herein, g-C3N4/Ag/TiO2 Z-scheme heterojunction was synthesized for photocatalytic U(VI) reduction application. The SEM, XRD and XPS characterization results showed that a ternary g-C3N4/Ag/TiO2 composite photocatalyst was synthesized successfully. g-C3N4/Ag/TiO2 exhibited excellent photocatalytic reduction performance for U(VI) under visible light irradiation. After 30 min irradiation, the removal rate of U(VI) was above 99%. XPS indicated that the majority of U(VI) on the surface of g-C3N4/Ag/TiO2 was reduced to U(IV). In addition, the photocatalytic activity of g-C3N4/Ag/TiO2 has been kept significantly after five rounds of experiments, indicating good stability. g-C3N4/Ag/TiO2 exhibited better photocatalytic reduction of U(VI) under visible light irradiation, which is mainly ascribed to Z-scheme photocatalytic mechanism assisted by the LSPR effect (Local Surface Plasmon Resonance). Ag with plasmon resonance effect on the loading has a strong absorption of photon energy. In addition, an intermediate charge transfer channel is formed between Ag and the semiconductor to inhibit the combination of photogenerated electrons and holes, resulting in a significant increase in the photocatalytic activity of the photocatalyst. This idea has some significance in design of other composite photocatalytic systems.

Electro-oxidation of Ni (II)-citrate complexes at BDD electrode and simultaneous recovery of metallic nickel by electrodeposition.

The simultaneous electro-oxidation of Ni (II)-citrate and electrodeposition recovery of nickel metal were attempted in a combined electro-oxidation-electrodeposition reactor with a boron-doped diamond (BDD) anode and a polished titanium cathode. Effects of initial nickel citrate concentration, current density, initial pH, electrode spacing, electrolyte type, and initial electrolyte dosage on electrochemical performance were examined. The efficiencies of Ni (II)-citrate removal and nickel metal recovery were determined to be 100% and over 72%, respectively, under the optimized conditions (10 mA/cm2, pH 4.09, 80 mmol/L Na2SO4, initial Ni (II)-citrate concentration of 75 mg/L, electrode spacing of 1 cm, and 180 min of electrolysis). Energy consumption increased with increased current density, and the energy consumption was 0.032 kWh/L at a current density of 10 mA/cm2 (pH 6.58). The deposits at the cathode were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These characterization results indicated that the purity of metallic nickel in cathodic deposition was over 95%. The electrochemical system exhibited a prospective approach to oxidize metal complexes and recover metallic nickel.

Phytoremediation of uranium-contaminated soil by perennial ryegrass (Lolium perenne L.) enhanced with citric acid application.

Perennial ryegrass (Lolium perenne L.) was planted in uranium-contaminated soil mixtures supplemented with different amounts of citric acid to investigate the defense strategies of perennial ryegrass against U and the enhanced mechanism of citric acid on the remediation efficiency in the laboratory. The uranium content in the plant tissues showed that the roots were the predominant tissue for uranium accumulation. In both root and shoot cells, the majority of U was located in the cell wall fraction. Furthermore, antioxidant enzymes were also stimulated when exposed to U stress. These results suggested that perennial ryegrass had evolved defense strategies, such as U sequestration in root tissue, compartmentalization in the cell wall, and antioxidant enzyme systems, to minimize uranium stress. For an enhanced mechanism, the optimal concentration of citric acid was 5 mmol/kg, and the removal efficiency of U in the shoots and roots increased by 47.37% and 30.10%, respectively. The treatment with 5 mmol/kg citric acid had the highest contents of photosynthetic pigment and soluble protein, the highest activity of antioxidant enzymes, and the lowest content of MDA (malondialdehyde) and relative electrical conductivity. Moreover, the TEM (transmission electron microscope) results revealed that after 5 mmol/kg citric acid was added, the cell structure of plant branches partially returned to normal, the number of mitochondria increased, chloroplast surfaces seemed normal, and the cell wall became visible. The damage to the cell ultrastructure of perennial ryegrass was significantly alleviated by treatment with 5 mmol/kg citric acid. All the results above indicated that perennial ryegrass could accumulate uranium with elevated uranium tolerance and enrichment ability with 5 mmol/kg citric acid.

Construction of urban landscape ecological security pattern based on circuit theory: A case study of Hengyang City, Hunan Province, China.

The identification of ecological sources and corridors plays an important role in the construction of ecological security pattern. However, previous studies mainly concentrated on the optimal path selection of species migration and diffusion rather than the random path selection of the species, which makes most conclusions fail to objectively reveal the process of species migration and diffusion. Taking the downtown area of Hengyang City as an example, we selected the ecological sources and ecological corridors with the habitat quality analysis module of InVEST and Circuitscape 4.0 and evaluated the importance and connectivity of relevant ecological elements with the Linkage Mapper, with the aim to construct the ecological security pattern and delimitate the regions prior to ecological restoration. The results showed that there were 85 ecological sources dominated by woodland and grassland, together with a small number of ponds and beaches, which mainly distributed in the southwest of Zhengxiang District, the west of Yanfeng District, the northeast and south central of Zhuhui District, with a total area of 11.8 km2. There were 60 ecological sources with centrality greater than 100, accounting for 70.6% of the total. There were 217 ecological corridors and five potential ecological corridors mainly composed of forest land, among which the proportion of shrubbery and sparse forest land was higher. The corridors with higher importance were mainly distributed in the west of the studied area. After removing the barriers, the regional connectivity had been significantly improved, with the highest extent of 54.9%. The priority areas of ecological restoration were classified into three levels according to the value of cumulative current, namely, the high-grade area, the middle-grade area and the low-grade area. The high-grade area covered 4.3 km2 of barriers, mainly distributed in the southwest of Zhengxiang District, northeast and south central of Zhuhui District. The middle-grade area was dominated by pinch area and ecological source area with centrality less than 100, covering an area of about 12.9 km2, mainly distributed in the central part of Zhengxiang District, northeast and south central of Zhuhui District. The low-level area was mainly distributed in south central of Zhuhui District, with 51.8 km2 of residual ecological sources. By coupling InVEST habitat quality analysis module and circuit theory, the ecological security pattern for biological protection was constructed, which provides scientific reference for biological protection.

Synthesis of g-C3N4/TiO2 nanostructures for enhanced photocatalytic reduction of U(vi) in water.

Photocatalytic technology is a valid solution for the remediation of wastewater containing uranium. In this study, the synthesis of Z-scheme g-C3N4/TiO2 catalysts was made by a thermal synthetic approach for photocatalytic U(vi) reduction. The characterization results revealed the successful synthesis of g-C3N4/TiO2 nanostructures. The g-C3N4 surface was uniformly coated with TiO2 nanoparticles. The depletion of U(vi) in water evaluated the photocatalytic activity of g-C3N4/TiO2 under UV light irradiation. The photocatalytic tests showed that g-C3N4/TiO2 exhibited more effective photocatalytic activity than the raw materials (1.64 and 56.97 times higher than TiO2(P25) and g-C3N4, respectively). Besides, a pseudo-first-order model was followed by the experimental kinetic data for the photocatalytic process. Moreover, g-C3N4/TiO2 still presented high photocatalytic activity after four reacting cycles. Based on these experiment results, the improved photocatalytic activity could be attributed to the Z-scheme mechanism, which decreased the recombination of photo-produced electrons and holes. The synthesis of these g-C3N4/TiO2 nanomaterials provides a facile and inexpensive method for treating wastewater containing U(vi).

Preparation of aluminum sludge composite gel spheres and adsorption of U(IV) from aqueous solution.

A novel three-dimensional aluminum sludge/polyvinyl alcohol/sodium alginate(AS/PA/SA) gel spheres were designed and prepared for uranium(VI) adsorption, and it overcomes the shortcomings of poor recycling of powdery aluminum sludge adsorbent and poor stability of sodium alginate. Experiments show that the P-S-AS has a good pH range for removal of uranium (4-5). Fitting experimental data with pseudo-first-order kinetic model and pseudo-second-order kinetic model shows that the adsorption of U(VI) by P-S-AS is a chemical action. The fit of the Langmuir isotherm model and Freundlich isotherm model to the experimental data found that the P-S-AS adsorbed U(VI) to a single layer. Thermodynamic analysis shows that the adsorption occurs spontaneously, and an increase in temperature is favorable for the adsorption of uranium by the P-S-AS. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis of the P-S-AS before and after adsorption showed that the main adsorption mechanism was the complexation reaction between functional groups and U(VI), the bonding reaction between metal oxides and U(VI).

Investigation of a modified metal-organic framework UiO-66 with nanoscale zero-valent iron for removal of uranium (VI) from aqueous solution.

A novel composite material (nZVI/UiO-66) of nanoscale zero-valent iron (nZVI) with a functionalized metal-organic framework was synthesized by this study via a coprecipitation method, which was used for the efficient removal of U(VI) in the aqueous solution. The nZVI/UiO-66 had an excellent removal capacity of 404.86 mg g-1 with an initial U(VI) concentration of 80 mg L-1, 313 K and pH = 6. The transmission electron microscopy (TEM) revealed that nZVI particles were inhomogeneously distributed on the surface of UiO-66. The analysis by the X-ray diffraction (XRD) has further illustrated that the introduction of nZVI did not change the structure of UiO-66. The adsorption process closely followed the pseudo-second-order kinetic and the Freundlich isotherm model. The removal process of U(VI) by nZVI/UiO-66 was spontaneous and endothermic. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses have illustrated that the mechanism was mainly related to adsorption of U(VI) from UiO-66 and reduction of U(VI) by nZVI. The Zr-O bonds were shown to play a vital role in the uranium removal. nZVI/UiO-66 could be recycled. The uptake rate could be maintained at around 80% after 5 cycles of use. Therefore, these results manifested that the nZVI/UiO-66 is a promising sorbent for the efficient and selective removal of U(VI) in radioactive wastewaters.

Microbial characteristic and bacterial community assessment of sediment sludge upon uranium exposure.

The microbial characteristics and bacterial communities of sediment sludge upon different concentrations of exposure to uranium were investigated by high solution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and high-throughput sequencing. After exposure to initial uranium concentrations of 10-50 µM for 24 h in synthetic wastewater, the removal efficiencies of uranium reached 80.7%-96.5%. The spherical and short rod bacteria were dominant in the sludge exposed to uranium. HRTEM-EDS and XPS analyses indicated that reduction and adsorption were the main mechanisms for uranium removal. Short-term exposure to low concentrations of uranium resulted in a decrease in bacterial richness but an increase in diversity. A dramatic change in the composition and abundances of the bacterial community were present in the sediment sludge exposed to uranium. The highest removal efficiency was identified in the sediment sludge exposed to 30 µM uranium, and the dominant bacteria included Acinetobacter (44.9%), Klebsiella (20.0%), Proteiniclasticum (6.7%), Enterobacteriaceae (6.6%), Desulfovibrio (4.4%), Porphyromonadaceae (4.1%), Comamonas (2.4%) and Sedimentibacter (2.3%). By comparison to the inoculum sediment sludge, exposure to uranium caused a substantial difference in the majority of bacterial abundance.

Fabrication and Optimization of the Thermo-Sensitive Hydrogel Carboxymethyl Cellulose/Poly(N-isopropylacrylamide-co-acrylic acid) for U(VI) Removal from Aqueous Solution.

In this work, the thermo-sensitive materials N-isopropylacrylamide (NIPAM) and acrylic acid (AA) were crosslinked with carboxymethyl cellulose (CMC) (CMC/P (NIPAM-co-AA)) via a free radical polymerization method for the removal of U(VI) from aqueous solution. The L16 (45) orthogonal experiments were designed for the optimization of the synthesis condition. The chemical structures of the crosslinking hydrogel were confirmed by FTIR spectroscopy. The microstructural analyses were conducted though scanning electron microscopy (SEM) to show the pore structure of the hydrogel. The adsorption performance of the CMC/P (NIPAM-co-AA) hydrogel for the uptake of U(VI) from simulated wastewater was also investigated. The adsorption reached equilibrium within 1 h. Under the reaction of pH = 6 and a temperature of 298 K, an initial concentration of U(VI) of 5 mg·L-1, and 10 mg of the CMC/P(NIPAM-co-AA) hydrogel, the maximum adsorption capacity was 14.69 mg g-1. The kinetics fitted perfectly with the pseudo-second-order model, and the isotherms for the composite hydrogel adsorption of U(VI) was in accordance with the Langmuir model. The chemical modification confirmed that the acylamino group played an important role in uranium adsorption. The desorption and reusability study revealed that the resolution rate was still available at approximately 77.74% after five alternate heating cycles at 20 and 50 °C of adsorption-desorption.

Correction to: Influence of Leifsonia sp. on U(VI) removal efficiency and the Fe-U precipitates by zero-valent iron.

The original publication of this paper contains a mistake.

Influence of Leifsonia sp. on U(VI) removal efficiency and the Fe-U precipitates by zero-valent iron.

Zero-valent iron (ZVI) has been widely applied to the remediation of uranium (U)-contaminated water. Notably, indigenous bacteria may possess potential positive or unfavorable influence on the mechanism and stability of Fe-U precipitates. However, the focus of the researches in this field has mainly been on physical and/or chemical aspects. In this study, batch experiments were conducted to explore the effects of an indigenous bacterium (Leifsonia sp.) on Fe-U precipitates and the corresponding removal efficiency by ZVI under different environmental factors. The results showed that the removal rate and capacity of U(VI) was significantly inhibited and decreased by ZVI when the pH increased to near-neutral level (pH = 6~8). However, in the ZVI + Leifsonia sp. coexistence system, the U(VI) removal efficiency were maintained at high levels (over 90%) within the experimental scope (pH = 3~8). This revealed that Leifsonia sp. had a synergistic effect on U(VI) remove by ZVI. According to scanning electron microscope and energy dispersive X-ray detector (SEM-EDX) analysis, dense scaly uranium-phosphate precipitation was observed on ZVI + Leifsonia sp. surface. The X-photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analysis indicated that Leifsonia sp. facilitated the generation of U(VI)-phosphates precipitates. The X-ray diffraction (XRD) analyses further revealed that new substances, such as (Fe(II)Fe(III)2(PO4)2(OH)2), Fe(II)(UO2)2(PO4)2·8H2O, Fe(II)Fe(III)5(PO4)4(OH)2·4H2O, etc., were produced in the coexisting system of ZVI and Leifsonia sp. This study provides new insights on the feasibility and validity of site application of ZVI to U(VI)-contaminated subsurface water in situ. Graphical abstract.

Analysis of uranium removal capacity of anaerobic granular sludge bacterial communities under different initial pH conditions.

The bacterial community of an anaerobic granular sludge associated with uranium depletion was investigated following its exposure to uranium under different initial pH conditions (pH 4.5, 5.5, and 6.5). The highest uranium removal efficiency (98.1%) was obtained for the sample with an initial pH of 6.5, which also supported the highest bacterial community richness and diversity. Venn diagrams visualized the decrease in the number of genera present in both the inoculum and the uranium-exposed biomass as the initial pH decreased from 6.5 to 4.5. Compared with the inoculum, a significant increase in the abundances of the phyla Chloroflexi and Proteobacteria was observed following uranium exposure. At initial pH conditions of 6.5 to 4.5, the proportions of the taxa Anaerolineaceae, Chryseobacterium, Acinetobacter, Pseudomonas, and Sulfurovum increased significantly, likely contributing to the observed uranium removal. Uranium exposure induced a greater level of dynamic diversification of bacterial abundances than did the initial pH difference.

Uranium adsorption and subsequent re-oxidation under aerobic conditions by Leifsonia sp. - Coated biochar as green trapping agent.

It has generally been assumed that the immobilization of U(VI) via polyphosphate accumulating microorganisms may present a sink for uranium, but the potential mechanisms of the process and the stability of precipitated uranium under aerobic conditions remain elusive. This study seeks to explore the mechanism, capacity, and stability of uranium precipitation under aerobic conditions by a purified indigenous bacteria isolated from acidic tailings (pH 6.5) in China. The results show that over the treatment ranges investigated, maximum removal of U(VI) from aqueous solution was 99.82% when the initial concentration of U(VI) was 42 µM, pH was 3.5, and the temperature was with 30 °C much higher than that of other reported microorganisms. The adsorption mechanism was elucidated via the use of SEM-EDS, XPS and FTIR. SEM-EDS showed two peaks of uranium on the surface. A plausible explanation for this, supported by FTIR, is that uranium precipitated on the biosorbent surfaces. XPS measurements indicated that the uranium product is most likely a mixture of 13% U(VI) and 87% U(IV). Notably, the reoxidation experiment found that the uranium precipitates were stable in the presence of Ca2+ and Mg2+, however, U(IV) is oxidized to U(VI) in the presence of NO3- and Na+ ions, resulting in rapid dissolution. It implies that the synthesized Leifsonia sp. coated biochar could be utilized as a green and effective biosorbent. However, it may not a good choice for in-situ remediation due to the subsequent re-oxidation under aerobic conditions. These observations can be of some guiding significance to the application of the bioremediation technology in surface environments.

Assessment of Bacterial Community Composition of Anaerobic Granular Sludge in Response to Short-Term Uranium Exposure.

The effect of 10-50 µM uranium (U(VI)) on the bacterial community of anaerobic granular sludge was investigated by 24-h exposure tests, after which the bacterial community was analyzed by high-throughput sequencing. The specific U(VI) reducing activity of the anaerobic granular sludge ranged between 3.1 to 19.7 µM U(VI) g-1(VSS) h-1, independently of the initial U(VI) concentration. Alpha diversity revealed that microbial richness and diversity was the highest for anaerobic granular sludge upon 10 µM uranium exposure. Compared with the original biomass, the phylum of Euryarchaeota was significantly affected, whereas the Bacteroidetes, Firmicutes, and Synergistetes phyla were only slightly affected. However, the abundance of Chloroflexi and Proteobacteria phyla clearly increased after 24 h uranium exposure. Based on the genus level analysis, significant differences appeared in the bacterial abundance after uranium exposure. The proportions of Pseudomonas, Acinetobacter, Parabacteroides, Brevundimonas, Sulfurovum, and Trichococcus increased significantly, while the abundance of Paludibacter and Erysipelotrichaceae incertae sedis decreased dramatically. This study shows a dynamic diversification of the bacterial composition as a response to a short time (24 h) U(VI) exposure (10-50 µM).

Biosorption of uranium (VI) by immobilized Aspergillus fumigatus beads.

Biosorption of uranium (VI) ions by immobilized Aspergillus fumigatus beads was investigated in a batch system. The influences of solution pH, biosorbent dose, U (VI) concentration, and contact time on U (VI) biosorption were studied. The results indicated that the adsorption capacity was strongly affected by the solution pH, the biosorbent dose and initial U (VI) concentration. Optimum biosorption was observed at pH 5.0, biosrobent dose (w/v) 2.5%, initial U (VI) concentration 60 mg L(-1). Biosorption equilibrium was established in 120 min. The adsorption process conformed to the Freunlich and Temkin isothermal adsorption models. The dynamic adsorption model conformed to pseudo-second order model.

[Efficiency and mechanism on reduction of U(VI) by sulfate reducing bacteria].

Under anaerobic conditions, the characteristics of sulfate reducing bacteria (SRB) were applied to reduce U(VI) under different temperature, pH values, U(VI) concentrations and coexisting ions. The results showed that the optimum reduction condition was the temperature 35 degrees C, pH 7.0 and U(VI) concentration 25 mg x L(-1). The maximum reduction capacity of SRB was 179.1 mg x g(-1). Mo(VI) or Ca2+ did not affect SRB on the reduction process of U(VI) under the concentration less than 5 g x L(-1), but they strongly inhibited the process under the concentration more than 20 g x L(-1). The main inhibition of Mo (VI) was physiological inhibition and the inhibition of Ca2+ was competitive inhibition through the stable complex formation, Ca-UO2-CO3. The results also showed that lag phase did not appear on the concentration of Ca2+ less than 5 g x L(-1), but the lag phase of 24 hours appeared on the concentration of Ca2+ more than 20 g x L(-1).