Highly efficient recovery of Zn2+/Cu2+ from water by using hydrotalcite as crystal seeds.

The efficient and waste-free recovery of heavy metals is critical for heavy metal wastewater treatment. In this work, we explored how heavy metals can be recovered as valuable chemicals in the presence of crystal seeds. Hydrotalcite (one kind of layered double hydroxides (LDHs)) was used as crystal seeds to recover Zn2+ in the presence of Al3+ from water (i.e., seed-Zn2+-Al3+ system), which was compared with the monometallic heterogeneous system (seed-Zn2+) and direct coprecipitation (Zn2+-Al3+) system. Our results demonstrated that the seed-Zn2+-Al3+ system possessed a recovery rate of 2.6-2.8 times and a recovery kinetic rate of 2.7-5.9 times higher than those of the other two systems. Differing from the latter two systems, hydrotalcite seeds could induce Zn2+ and Al3+ to form ZnAl-LDH in seed-Zn2+-Al3+. Interestingly, the ZnAl-LDH presents a compositional divalent/trivalent cation molar ratio of ca. 3, which is comparable with the value in the hydrotalcite. It was demonstrated that the hydrotalcite seeds could act as a template to significantly induce the formation of ZnAl-LDH complying with the seed's structure and compositional ratio. Similar induction effect of seeds as the Zn2+ system was further verified in Cu2+ systems. This work provides a novel strategy for efficient recovery of heavy metals with product selectivity.

Soil health improvement by inoculation of indigenous microalgae in saline soil.

Using biological methods to improve saline soils is recognized as an eco-friendly and sustainable way. In this study, two indigenous algae YJ-1 and YJ-2 screened from salinized farmland were inoculated into saline soils with different salinization levels to investigate their potential in enhancing soil health by laboratory microcosm experiment. The results showed that individual inoculation of the two algae quickly resulted in the formation of algal crusts, and the chlorophyll content in the saline soils gradually increased with the incubation time. The soil pH decreased significantly from the initial 8.15-9.45 to 6.97-7.56 after 60-day incubation. The exopolysaccharides secretion and the activities of catalase, sucrase, and urease in saline soils also increased. Microalgal inoculation increased soil organic matter storage, while decreasing the available nutrient contents possibly due to the depletion of microalgal growth. PCA and PCC results identified that microalgal biomass as the predominant variable affecting soil quality. Overall, these data revealed the great potential of microalgae in the amelioration of saline soils, especially in pH reduction and enzyme activity enhancement. This study will provide the theoretical foundation for improving saline soils via algalization.

Microbial weathering of montmorillonite and its implication for Cd(II) immobilization.

Interactions between silicate bacteria and silicates are very common in nature and hold great potential in altering their mutual physicochemical properties. But their interactions in regulating contaminants remediation involving performance and mechanisms are often overlooked. Here, we focused on the interactions between silicate bacteria (Paenibacillus polymyxa, PP; Bacillus circulans, BC) and a soil silicate montmorillonite (Mt), and their impact on Cd(II) immobilization. The obtained results showed that Mt greatly promoted the growth of the bacteria, resulting in a maximum 10.31 times increase in biomass production. In return, the bacteria strongly enhanced the Cd(II) adsorption on Mt, with adsorption capacities increased by 80.61%-104.45% in comparison to the raw Mt. Additionally, the bacteria-Mt interaction changed Cd(II) to a more stabilized state with a maximum reduction of 38.90%/g Mt in bioavailability. The enhancement of Cd(II) adsorption and immobilization on the bacterial modified Mt was caused by the following aspects: (1) the bacteria activities altered the aggregation state of Mt and made it better dispersed, thus more active sites were exposed; (2) the microbial activities brought about more rough and crumpled surface, as well as smaller Mt fragments; (3) a variety of microbial-derived functional groups were introduced onto the Mt surface, increasing its affinity for heavy metals; (4) the main Cd(II) immobilization mechanism was changed from ion exchange to the combination of ion exchange and functional groups induced adsorption. This work elucidates the potential ecological and evolutionary processes of silicate bacteria-soil clay mineral interactions, and bears direct implications for the clay-mediated bioremediation of heavy metals in natural environments.

Interlayer Anions of Layered Double Hydroxides as Mobile Active Sites To Improve the Adsorptive Performance toward Cd2.

Layered double hydroxides (LDHs) have been considered important sinks for ionic contaminants in nature and effectively engineered adsorbents for environmental remediation. The availability of interlayer active sites of LDHs is critical for their adsorptive ability. However, inorganic LDHs generally have a nano-confined interlayer space of ca. 0.3-0.5 nm, and it is unclear how LDHs can utilize their interlayer active sites during the adsorption process. Thus, LDHs intercalated with SO42-, PO43-, NO3-, Cl-, or CO32- were taken as examples to reveal this unsolved problem during Cd2+ adsorption. New adsorption behaviors and pronounced differences in adsorption performance were observed. Specifically, SO42-/PO43- intercalated LDHs showed a maximum Cd2+ adsorption capacity of 19.2/9.8 times higher than other LDHs. The ligand exchange of H+ (on the surface -OH) by Cd2+ and formation of Cd-SO42-/PO43- complexes led to the efficient removal of Cd2+. Interestingly, interlayer SO42- was demonstrated to be able to move to the edges/outer surfaces of LDHs, providing abundant movable adsorption sites for Cd2+. This novel phenomenon made the SO42- intercalated LDH a superior adsorbent for Cd2+ among the tested LDHs, which also suggests that LDHs with a nano-confined interlayer space can also highly utilize their interlayer active sites based on the mobility of interlayer anions, offering a new method for constructing superior LDH adsorbents.

Capturing effects of filamentous fungi Aspergillus flavus ZJ-1 on microalgae Chlorella vulgaris WZ-1 and the application of their co-integrated fungi-algae pellets for Cu(II) adsorption.

Using filamentous fungi to capture unicellular microalgae is an effective way for microalgae recovery in water treatment. Here, fungi Aspergillus flavus ZJ-1 and microalgae Chlorella vulgaris WZ-1 isolated from a copper tailings pond were used to study the capture effect of ZJ-1 on WZ-1. The highest capture efficiency (97.85%) was obtained within 6 h under the optimized conditions of 30 °C, 150 rpm, fungi-algae biomass ratio of 2.24:1, and initial pH of 9.24 in microalgae medium. The formed fungi-algae pellets (FAPs) were further used to remove Cu(II) from aqueous solution. Results showed that the FAPs formed at different capture times all adsorbed Cu(II) well, and the PAFs formed within 2 h (PAFs2 h) exhibited the highest Cu(II) adsorption capacity (80.42 mg·g-1). SEM images showed that Cu(II) caused a change in the internal structure of PAFs2 h from loose to compact, the mycelium shrunk, and the microalgal cells were concave. Cu(II) adsorption by PAFs2 h was well conformed to the pseudo-second-order kinetics and the Langmuir isotherm (123.61 mg·g-1 of theoretically maximum adsorption capacity). This work opens a way for applying FAPs in the remediation of heavy metal-contaminated wastewater, and the metal adsorption effect was determined by the capture amount of microalgae.

Polypyrrole-Bentonite composite as a highly efficient and low cost anionic adsorbent for removing hexavalent molybdenum from wastewater.

The aim of current study was to develop a new material for the fast and efficient removal of hexavalent molybdenum (Mo(VI)) from contaminated water. In this work, a novel adsorbent was synthesized through the polypyrrole intercalation modification of bentonite (PPy-BT) via in-situ chemical polymerization method for effectively removal of Mo(VI) from aqueous solution. The surface morphology and chemical composition of PPy-BT composites were investigated by X-ray diffraction, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectrometer, scanning electron microscopy techniques and X-ray photoelectron spectroscopy. PPy and BT could well resist the aggregation of each other, and therefore resulted in a loose-packed structure and good exposure of active sites. Using materials for the adsorption of Mo(VI) revealed has a maximum adsorption capacity of 100.17 mg/g at 25 °C and pH 4.0 by the Langmuir model. The adsorption kinetics and isotherm data are found to be well elucidated through pseudo-second-order and Langmuir models. Moreover, high regeneration ability (>89.3%) of PPy-BT was noted for five consecutive adsorption-desorption cycles. These findings highlight the potential of PPy-BT for practical water treatment applications. The intercalation material of PPy-BT could provide a new strategy to develop cost-effective clay-based nanomaterials for wastewater treatment.

Facile synthesis of Al/Fe bimetallic (oxyhydr)oxide-coated magnetite for efficient removal of fluoride from water.

In this work, we developed a novel magnetic bimetallic Al/Fe (oxyhydr)oxide adsorbent through a facile and cost-effective method and explored its potential to adsorb fluoride in water. Its synthesis involved corrosion of natural magnetite in aluminium chloride solution, followed by titration with NaOH solution for in-situ synthesis of Al/Fe (oxyhydr)oxide-coated magnetite (Mag@Al2Fe). Characterization data indicated a uniform coating of Al/Fe (oxyhydr)oxide on magnetite, and the resulting composite possessed large specific surface area (∼90 m2/g) and good magnetic property. In batch adsorption experiments, the isotherm and kinetic data fitted well to the Langmuir model and pseudo-second-order model, respectively. The maximum adsorption capacity of Mag@Al2Fe is 26.5 mg/g, which was much higher than natural magnetite (0.44 mg/g). Moreover, this material retained high adsorption capacity toward fluoride within a wide pH range (3.0-8.0) and offered facile magnetic separation from water. Influence of competing ions was also evaluated which showed that the presence of Cl- and NO3 - posed negligible interference, while HCO3 - and SO4 2- had negative effects on fluoride adsorption. Thermodynamic investigations revealed that fluoride adsorption was exothermic and spontaneous. The observed increase in solution pH and formation of Al-F and Fe-F bonds (as indicated by XPS analysis) after fluoride adsorption suggested the major adsorption mechanism of ligand exchange. Besides, the adsorption/desorption cycle studies demonstrated the well-retained performance of Mag@Al2Fe for repeated application after regeneration by 0.5 mol/L NaOH solution. Facile synthesis, high defluoridation, lower cost, and quick separation of Mag@Al2Fe indicates its promising potential for drinking water defluoridation.

Converting Spent Cu/Fe Layered Double Hydroxide into Cr(VI) Reductant and Porous Carbon Material.

Recycling solid waste as functional materials is important for both environmental remediation and resource recycling. This study attempts to recycle spent Cu/Fe layered double hydroxide (Cu/Fe-LDH) which is generated from the adsorption of dyes by converting to Cr(VI) reductant and porous carbon material. Results showed that the obtained reductant was mainly composed of Fe0 and Cu0, and exhibited good reductive activity toward Cr(VI). The species of Fe0, Fe2+, Cu0, and Cu+ all favored the reduction of Cr(VI) according to X-ray photoelectron spectroscopy analysis. During Cr(VI) removal, solution pH could increase to neutral which caused the metal ions to precipitate near completion. On the other hand, the spent Cu/Fe-LDH could be employed to produce porous carbon materials, and the generated waste metals solution herein could be reused for LDH synthesis. Specific surface areas of the obtained carbon materials varied from 141.3-744.2 m2/g with changes in adsorbed amount of dyes on the LDH. This study illustrates that all the components of wastes can be useful resources, offering a simple recycling approach for similar organic-inorganic solid wastes. This work also enlightens us that designing a proper initial product is crucial to make waste recycling simpler.

In situ hydrothermal synthesis of a novel hierarchically porous TS-1/modified-diatomite composite for methylene blue (MB) removal by the synergistic effect of adsorption and photocatalysis.

Hierarchically porous TS-1/modified-diatomite composites with high removal efficiency for methylene blue (MB) were prepared via a facile in situ hydrothermal route. The surface charge state of the diatomite was modified to enhance the electrostatic interactions, followed by in situ hydrothermal coating with TS-1 nanoparticles. The zeolite loading amount in the composites could be adjusted by changing the hydrothermal time. The highest specific surface area and micropore volume of the obtained composites were 521.3m(2)/g and 0.254cm(3)/g, respectively, with an optimized zeolite loading amount of 96.8%. Based on the synergistic effect of efficient adsorption and photocatalysis resulting from the newly formed hierarchically porous structure and improved dispersion of TS-1 nanoparticles onto diatomite, the composites' removal efficiency for MB reached 99.1% after 2h of photocatalytic reaction, even higher than that observed using pure TS-1 nanoparticles. Moreover, the superior MB removal kinetics of the composites were well represented by a pseudo-first-order model, with a rate constant (5.28×10(-2)min(-1)) more than twice as high as that of pure TS-1 nanoparticles (2.43×10(-2)min(-1)). The significant dye removal performance of this novel TS-1/modified-diatomite composite indicates that it is a promising candidate for use in waste water treatment.

From spent Mg/Al layered double hydroxide to porous carbon materials.

Adsorption has been considered as an efficient method for the treatment of dye effluents, but proper disposal of the spent adsorbents is still a challenge. This work attempts to provide a facile method to reutilize the spent Mg/Al layered double hydroxide (Mg/Al-LDH) after the adsorption of orange II (OII). Herein, the spent hybrid was carbonized under the protection of nitrogen, and then washed with acid to obtain porous carbon materials. Thermogravimetric analysis results suggested that the carbonization could be well achieved above 600°C, as mass loss of the spent hybrid gradually stabilized. Therefore, the carbonization process was carried out at 600, 800, and 1000°C, respectively. Scanning electron microscope showed that the obtained carbon materials possessed a crooked flaky morphology. Nitrogen adsorption-desorption results showed that the carbon materials had large BET surface area and pore volume, e.g., 1426 m(2)/g and 1.67 cm(3)/g for the sample carbonized at 800°C. Moreover, the pore structure and surface chemistry compositions were tunable, as they were sensitive to the temperature. Toluene adsorption results demonstrated that the carbon materials had high efficiency in toluene removal. This work provided a facile approach for synthesizing porous carbon materials using spent Mg/Al-LDH.