Synergistic effects of hydrogen peroxide and phosphate on uranium(VI) immobilization: implications for the remediation of groundwater at decommissioned in situ leaching uranium mine.

The processes of acid in situ leaching (ISL) uranium (U) mines cause the pollution of groundwater. Phosphate (PO43-) has the potential to immobilize U in groundwater through forming highly insoluble phosphate minerals, but the performance is highly restricted by low pH and high sulfate concentration. In this study, hydrogen peroxide (H2O2) and PO43- were synergistically used for immobilizing U based on the specific properties of groundwater from a decommissioned acid ISL U mine. The removal mechanisms of U and the stability of U on the formed minerals were elucidated by employing X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy and kinetic experiments. Our results indicated that the removal of U by simultaneously adding H2O2 and PO43- was significantly higher than the removal of U by individually adding H2O2 or PO43-. The removal of U increased with increasing PO43- concentration from 20 to 200 mg L-1 while decreased with increasing H2O2 concentration from 0.003 to 0.3%. Specifically, the removal efficiency of U from groundwater reached 98% after the application of 0.003% H2O2 and 200 mg L-1 PO43-. Amorphous iron phosphate that preferentially formed at low H2O2 and high PO43- concentrations played a dominant role in U removal, while the formations of schwertmannite and crystalline iron phosphates may be also contributed to the removal of U. This was significantly different from the immobilization mechanism of U through the formation of uranyl phosphate minerals after adding phosphate. The kinetic experimental results suggested that the immobilized U had a good stability. Our research may provide a promising method for in situ remediating U-contaminated groundwater at the decommissioned acid ISL U mines.

Enhanced sequestration of uranium by coexisted lead and organic matter during ferrihydrite transformation.

The dynamic reactions of uranium (U) with iron (Fe) minerals change its behaviors in soil environment, however, how the coexisted constituents in soil affect U sequestration and release on Fe minerals during the transformation remains unclear. Herein, coupled effects of lead (Pb) and dissolved organic matter (DOM) on U speciation and release kinetics during the catalytic transformations of ferrihydrite (Fh) by Fe(II) were investigated. Our results revealed that the coexistence of Pb and DOM significantly reduced U release and increased the immobilization of U during Fh transformation, which were attributed to the enhanced inhibition of Fh transformation, the declined release of DOM and the increased U(VI) reduction. Specifically, the presence of Pb increased the coprecipitation of condensed aromatics, polyphenols and phenols, and these molecules were preferentially maintained by Fe (oxyhydr)oxides. The sequestrated polyphenols and phenols could further facilitate U(VI) reduction to U(IV). Additionally, a higher Pb content in coprecipitates caused a slower U release, especially when DOM was present. Compared with Pb, the concentrations of the released U were significantly lower during the transformation. Our results contribute to predicting U sequestration and remediating U-contaminated soils.

Effects of dissolved organic matter molecules on the sequestration and stability of uranium during the transformation of Fe (oxyhydr)oxides.

Amorphous ferrihydrite (Fh) is abundant in aquatic environments and sediments, and often coprecipitates with dissolved organic matter (DOM) to form mineral-organic aggregates. The Fe(II)-catalyzed transformation of Fh to crystalline Fe (oxyhydr)oxides (e.g., goethite) can result in the changes of uranium (U) species, but the effects of DOM molecules on the sequestration and stability of U during Fe (oxyhydr)oxides transformation are poorly understood. In this study, the associations of DOM molecules with U during the coprecipitation of DOM with Fh were evaluated, and the effects of DOM molecules on the kinetics of U release during Fe (oxyhydr)oxides transformation were investigated using a combination of Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), X-ray photoelectron spectroscopy (XPS), and kinetic experiments. FT-ICR-MS results indicated that, in addition to phenolic and polyphenolic compounds with higher O/C ratios, portions of phenolic compounds with lower O/C ratios and aliphatic compounds were also contributed to UO22+ binding when Fh coprecipitated with DOM. In comparison, phenolic and polyphenolic compounds with higher O/C ratios and condensed aromatics were preferentially retained on Fe (oxyhydr)oxides during the transformation. XPS results further suggested that the coprecipitated DOM molecules facilitated the reduction of U(VI) to U(IV) during the transformation, possibly through providing electrons or acting as electron shuttles. The kinetic experiment results indicated that the transformation processes accelerated U release from Fe (oxyhydr)oxides, but the coprecipitated DOM molecules slowed down U release. Our results contribute to understanding the behaviors of U and predicting the sequestration of U in the environment.

Coupled variations of dissolved organic matter distribution and iron (oxyhydr)oxides transformation: Effects on the kinetics of uranium adsorption and desorption.

The interactions between dissolved organic matter (DOM) molecules and minerals play significant roles in affecting the fate of carbon and contaminants in soil environment. However, the mechanisms controlling the variations of DOM molecules distribution during the transformation of Fe (oxyhydr)oxides, and the effects of these variations on contaminant behaviors are still largely unknown. In this study, the dynamic variations of DOM properties and distributions, and the kinetics of uranium adsorption on and desorption from Fe (oxyhydr)oxides during the transformation were investigated, employing a combination of Orbitrap mass spectrometry (MS), high-resolution transmission electron microscopy (HR-TEM), and kinetic experiments. Orbitrap MS results indicated that aliphatic molecules and phenolic and polyphenolic molecules with lower O/C values were preferentially released to solution. HR-TEM results indicated that the coprecipitated DOM molecules by ferrihydrite were mainly released to solution rather than sorbed on the newly formed lepidocrocite or goethite during the transformation. Furthermore, the stirred-flow experiment results suggested that soil DOM significantly reduced the adsorption of uranium on, and accelerated the release of uranium from Fe (oxyhydr)oxides, which was ascribed to the changed distribution of DOM molecules and the structure and composition of Fe (oxyhydr)oxides. Our results contribute to predicting contaminant behaviors in soils.

A novel strategy for harmlessness and reduction of copper smelting slags by alkali disaggregation of fayalite (Fe2SiO4) coupling with acid leaching.

Copper smelting slags are difficult to achieve harmlessness and reduction because of the presence of abundant of fayalite (Fe2SiO4). This work proposed a novel strategy for harmlessness and reduction of copper smelting slags by alkali disaggregation of Fe2SiO4 coupling with acid leaching. The disaggregation changed the Fe2SiO4 phase by NaOH and released the embedded harmful and valuable metals. The evaluation of disaggregation effect mainly depends on further acid treatment. Especially the total leaching efficiency of As, Zn, Fe, Cu and Pb was achieved 99.7%, 62.5%, 41.5%, 99.9% and 99.1% under diluted HNO3-H2O2 system, respectively. Compared with the non-disaggregated control, the efficiency was accordingly increased by 73.3%, 71.1%, 18.6%, 72.2% and 22.4%. Meanwhile, the content of As, Cu and Pb in the slags decreased from 1165.5 mg/kg, 30085.9 mg/kg and 5008.8 mg/kg to as low as 5.2 mg/kg, 21.2 mg/kg and 15.6 mg/kg, respectively. Interestingly, the strategy brought out 48.1% of the weight reduction of the copper smelting slags. The favorable effect was mainly attributed to the broken of Fe-O-Si bond thereafter improving the acid leaching activation. Therefore, the proposed strategy could potentially be a new method to realize harmlessness and reduction of copper smelting slags.

Mechanism insight into efficient peroxydisulfate activation by novel nano zero-valent iron anchored yCo3O4 (nZVI/yCo3O4) composites.

Novel nano zero-valent iron anchored bio-matrix supported Co3O4 (nZVI/yCo3O4) composites were fabricated for tetracycline (TC) efficient degradation by activating peroxydisulfate (PS). The systematical characterizations verified that the nZVI/yCo3O4 composites with magnetism have higher surface area than yCo3O4 and pure Co3O4, contributing to more accessible active sites. Various catalytic parameters (nZVI mass ratio, leached ions, initial pH, catalyst dosage, PS concentration and coexisting anions) were thoroughly investigated. In nZVI/yCo3O4/PS system, 97.6 %, 93.4 % and 77.3 % TC were degraded within 15 min at pH 3.0, 6.0 and 9.0, respectively. Based on four successive degradation runs, the excellent mineralization rate and reusability of nZVI/yCo3O4 composites were mainly benefited from the suppressed metals leaching. The PS activated mechanisms were proposed as non-radicals (1O2) dominated pattern at acidic conditions and radicals (SO4-) predominant pattern at alkaline environment, which may be highly related to the electron donating capacity of nZVI at different pH and the M(n + 1)+/Mn+ redox cycling between Fe or Co metal. The plausible degradation routes of TC were presented based on the detected intermediates. Overall, the synthesized heterogeneous nZVI/yCo3O4 composites can efficiently active PS at a wide pH range, and further broaden the application of Co-based catalysts in PS activation.

Efficient degradation of sodium diclofenac via heterogeneous Fenton reaction boosted by Pd/Fe@Fe3O4 nanoparticles derived from bio-recovered palladium.

Dehalogenation of emerging pollutants has attracted worldwide attention. In this study, novel bio-Pd/Fe@Fe3O4 nanoparticles (NPs) were proposed to boost the heterogeneous Fenton reaction for degradation of sodium diclofenac (DCF). Specifically, Enterococcus faecalis (E. faecalis) was employed to achieve bio-recovered palladium (bio-Pd). Results showed that expected preparation of bio-Pd/Fe@Fe3O4 NPs was confirmed by various characterization techniques. The prepared bio-Pd/Fe@Fe3O4 NPs were spherical morphology with average size of 9 nm. Under the optimum conditions, the removal efficiency of 10 mg/L DCF in 20 min and 40 min reached as high as 94.69% and 99.65%, respectively. The dechlorination and mineralization efficiencies of DCF were 85.16% and 59.21% in 120 min, respectively. The main degradation pathway of DCF was complete mineralization with the final products CO2, chloride ions and H2O. The improvement of dechlorination efficiency was ascribed to the accelerated corrosion of nano zero valent iron (nZVI) by Pd/Fe galvanic effect and the rise of active hydrogen. Meanwhile, more ferrous ions were released into this solution, resulting in the higher heterogeneous Fenton reaction rate driven by bio-Pd/Fe@Fe3O4 NPs. Therefore, the findings suggested that bio-Pd/Fe@Fe3O4 NPs were effective catalysts for DCF dechlorination and mineralization. The work provided a novel strategy for degradation of halogen-containing environmental pollutants.

Induced fluorescent enhancement of protein-directed synthesized gold nanoclusters for selective and sensitive detection of flame retardants.

Organophosphate flame retardants (OPFRs), typical toxic and hazardous pollutants, are called for new detection approaches to avoid laborious synthetic procedures and large and expensive instruments. Hence, a novel fluorescent probe was constructed for quantitative detection of OPFRs via heightening the fluorescence of acetylcholinesterase synthesized gold nanoclusters (AChE-AuNCs). The as-prepared AChE-AuNCs exhibited high fluorescence emission at about 398 nm with the average particle size of about 1.60 nm. When the AChE-AuNCs was applied to the proposed fluorescent detection, excellent sensitivity with wide linear range (50-1000 ng L-1) and low detection limit (30 ng L-1) for TClPP with the response time less than 1 h were achieved. The fluorescent probe could be extended to detect other three types of OPFRs (TEP, TPHP, and TBOEP) and the target pollutants could be detectable in the presence of halogenated flame retardants. The mechanism might be mainly contributed by the interaction between OPFRs and AChE-AuNCs restricting internal vibration consumption of their capping ligands. The proposed detection approach could be easily operated and was not involved with other intermediate products. Therefore, AChE-AuNCs could be a promising fluorescent probe for rapid, selective and sensitive detection of OPFRs and even in the practical application.

Simultaneous electricity production and antibiotics removal by microbial fuel cells.

The removal of antibiotics is crucial for improvement of water quality in animal wastewater treatment. In this paper, the performance of microbial fuel cell (MFC) in terms of degradation of typical antibiotics was investigated. Electricity was successfully produced by using sludge supernatant mixtures and synthesized animal wastewater as inoculation in MFC. Results demonstrated that the stable voltage, the maximum power density and internal resistance of anaerobic self-electrolysis (ASE) -112 and ASE-116 without antibiotics addition were 0.574 V, 5.78 W m-3 and 28.06 Ω, and 0.565 V, 5.82 W m-3 and 29.38 Ω, respectively. Moreover, when adding aureomycin, sulfadimidine, roxithromycin and norfloxacin into the reactors, the performance of MFC was inhibited (0.51 V-0.41 V), while the output voltage was improved with the decreased concentration of antibiotics. However, the removal efficiency of ammonia nitrogen (NH3-N) and total phosphorus (TP) were both obviously enhanced. Simultaneously, LC-MS analysis showed that the removal efficiency of aureomycin, roxithromycin and norfloxacin were all 100% and the removal efficiency of sulfadimidine also reached 99.9%. These results indicated that antibiotics displayed significantly inhibitions for electricity performance but improved the quality of water simultaneously.

Enhanced adsorption of hexavalent chromium by a biochar derived from ramie biomass (Boehmeria nivea (L.) Gaud.) modified with ß-cyclodextrin/poly(L-glutamic acid).

This paper explored biochar modification to enhance biochar's ability to adsorb hexavalent chromium from aqueous solution. The ramie stem biomass was pyrolyzed and then treated by ß-cyclodextrin/poly(L-glutamic acid) which contained plentiful functional groups. The pristine and modified biochar were characterized by FTIR, X-ray photoelectron spectroscopy, specific surface area, and zeta potential measurement. Results indicated that the ß-cyclodextrin/poly(L-glutamic acid) was successfully bound to the biochar surface. Batch experiments were conducted to investigate the kinetics, isotherm, thermodynamics, and adsorption/desorption of Cr(VI). Adsorption capacities of CGA-biochar were significantly higher than that of the untreated biochar, and its maximum adsorption capacity could reach up to 197.21 mg/g at pH 2.0. Results also illustrated that sorption performance depended on initial solution pH; in addition, acidic condition was beneficial to the Cr(VI) uptake. Furthermore, the Cr(VI) uptake was significantly affected by the ion strength and cation species. This study demonstrated that CGA-biochar could be a potential adsorbent for Cr(VI) pollution control.

Investigation of the adsorption-reduction mechanisms of hexavalent chromium by ramie biochars of different pyrolytic temperatures.

To investigate the relationship between Cr(VI) adsorption mechanisms and physio-chemical properties of biochar, ramie residues were oxygen-limited pyrolyzed under temperature varying from 300 to 600°C. Batch adsorption experiments indicated that higher pyrolysis temperature limits Cr(VI) sorption in terms of capacity and affinity due to a higher aromatic structure and fewer polar functional groups in biochar. Both electrostatic (physical) and ionic (chemical) interactions were involved in the Cr(VI) removal. For low-temperature biochar, the simple physical adsorption was limited and the significant improvement in Cr(VI) sorption was attributed to abundant carboxyl and hydroxyl groups. The adsorption-reduction mechanisms could be concluded that Cr(VI) ions were electrostatically attracted by the positively charged biochar surface and reduced to Cr(III), and then the converted Cr(III) was retained or discharged into the solution. The study demonstrates ramie residues can be converted into biochar as a low-cost and effective sorbent for Cr(VI) removal.