Search

Cadmium (Cd2+) and Arsenate (As5+) are the main toxic elements in soil environments and are easily taken up by plants. Unraveling the kinetics of the adsorption and subsequent precipitation/immobilization on mineral surfaces is of considerable importance for predicting the fate of these dissolved species in soils. Here we used in situ atomic force microscopy (AFM) to image the dissolution on the (010) face of brushite (dicalcium phosphate dihydrate, CaHPO4·2H2O) in CdCl2- or Na2HAsO4-bearing solutions over a broad pH and concentration range. During the initial dissolution processes, we observed that Cd or As adsorbed on step edges to modify the morphology of etch pits from the normal triangular shape to a four-sided trapezium. Following extended reaction times, the respective precipitates were formed on brushite through a coupled dissolution–precipitation mechanism. In the presence of both CdCl2 and Na2HAsO4 in reaction solutions at pH 8.0, high-resolution transmission electron microscopy (HRTEM) showed a...

Manganese oxides have been widely used as deionization capacitor electrode materials to remove heavy metal ions from aqueous solutions. However, the effect of pseudocapacitive properties of manganese oxides on the removal processes remains elusive. In this work, synthesized nanostructured birnessite-type manganese oxide and birnessite/carbon nanotubes (HB/CNTs) nanocomposites were used as deionization pseudocapacitor electrode materials for Zn 2+ and Ni 2+ removal from aqueous solution by constant potential electrolysis. The effects of operation potential and introduction of carbon nanotubes (CNTs) on Zn 2+ and Ni 2+ removal capacities were further investigated. The results demonstrated a significant enhancement of electrochemical removal capacities for Zn 2+ and Ni 2+ by the pseudocapacitive properties of birnessite and the introduction of CNTs. The Zn 2+ and Ni 2+ removal capacities of birnessite electrode increased first and then decreased with the decrease of potential from 0.2 to −0.2 V ( vs . SCE), and the highest removal capacities for Zn 2+ and Ni 2+ respectively reached 89.5 and 96.6 mg g −1 when the potential was controlled at 0 V. The HB/CNTs nanocomposite showed higher removal capacities (155.6 mg g −1 for Zn 2+ and 158.4 mg g −1 for Ni 2+ when the relative content of manganese oxide was 45.6%) and a better cycling stability (about 90% and 88% of the initial Zn 2+ and Ni 2+ removal capacity were retained after 5 cycles) than birnessite electrode. The present study makes clear the pseudocapacitive mechanism of heavy metal ion removal using birnessite, and proposes a facile method to remove heavy metal ions from aqueous solution.

Bletilla striata (Thunb.) Reichb.f. is a perennial plant widely used as food, medicine and ornamentation. To determine the optimal processing method and promote the application of B. striata flowers, this work investigated the influences of diverse drying treatments on properties, structures, bioactive compound contents and antioxidant capacity of B. striata flowers. The drying methods applied in the study included natural drying, microwave drying, hot air drying, infrared drying, vacuum drying and freeze drying. Microwave drying was the fastest way for completion of drying, followed by infrared drying. Freeze drying had a relatively good color protection effect on the B. striata flowers, and the degree of browning was low. Compared with natural drying, the content of polysaccharide decreased following freeze drying, while the retention of total phenols and total anthocyanins increased significantly. Natural drying required a long time, and the degree of browning was high. Under the condition of natural drying, anthocyanins were almost completely degraded after long drying time and the degree of browning was high, but the retention of total flavonoids and total polysaccharides were higher compared with freeze drying. The methanolic extracts of flowers treated by natural drying and freeze drying both showed high antioxidant capacity. The reduction ability of natural drying group was slightly worse than that of infrared drying and hot air drying. The diammonium 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulphonate) (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), and OH radical scavenging ability were higher following natural drying. As a consequence, freeze drying was the best way for protecting most characters, contents and activities of B. striata flowers. In consideration of production costs, we recommend infrared drying as a more practical choice for industrial production.

d -Tagatose is a low-calorie sweetener with health benefits. To produce d -tagatose from lactose, a metabolic engineering Lactiplantibacillus plantarum was constructed. The galactokinase gene galK was inactivated and the original promoter of β-galactosidase gene lacLM was replaced by a strong lactose/galactose-inducible promoter PlacLM-35-10 in Lpb. plantarum WCFS1. Subsequently, the l -arabinose isomerase gene AraA derived from Lactobacillus casei SDMCC050286 was over-expressed. A recombinant strain with the potential of d -tagatose production was obtained. Its galactose metabolism pathway was blocked, and β-galactosidase activity was increased by 5.4-folds. Meanwhile, the heterogeneous l -arabinose isomerase was highly expressed in this strain. The optimal conditions for d -tagatose production by resting cells of the recombinant strain were at 65 °C and pH 7.5. Furthermore, the conversion rate of d -tagatose from 175 g/L lactose reached 33% at 56 h. This study verified the feasibility of one-pot production of d -tagatose from lactose by engineering strain of Lpb. plantarum, thereby laying down the foundation for industrial usage of lactose.

In situ atomic force microscopy (AFM) combined with X-ray photoelectron spectroscopy (XPS) and ζ potential were used to investigate how citrate (50 μM) modified the nanoscale dissolution of brushite (dicalcium phosphate dihydrate, CaHPO4·2H2O) by NaX (X = Cl, Br, or I) at a constant pH of 7.0. Results showed that on the brushite (010) surface, halide ions with large sizes (such as I–) enhanced adsorption of Na+ that cannot be desorbed by water flow, inhibiting the retreat rates of [100]Cc steps. The introduction of 50 μM citrate resulted in desorption of Na+ ions and caused the disappearance of specific salt effects on the dissolution of the brushite (010) face. With further increasing salt concentrations up to 100 mM, pit deepening was initiated, and it could possibly be attributed to hydrated Na+ ions at the negatively charged surface that orient the interfacial water so as to increase the entropy of the interfacial system, and hence the dissolution rate. Following the addition of 50 μM citrate to 100 ...

Due to the potential shortage of phosphate (P) rock resources and a faster growth in demand for phosphate fertilizers, unraveling the kinetics of calcium phosphate (Ca–P) crystallization and dissolution is important for understanding the P mobility and bioavailability. Plants have developed different strategies, such as carboxylic acid exudation into the rhizosphere, to cope with low P bioavailability through dissolution of sparingly soluble Ca–P minerals. However, the dissolution kinetics may be more complicated in the presence of both carboxylate and hydroxyl groups in organic acids. Here in situ atomic force microscopy (AFM) is used to directly observe the kinetics of nanoscale dissolution on the (010) surface of dicalcium phosphate dihydrate (brushite, CaHPO4·2H2O) in the presence of succinic acid (SA, 0 alcoholic hydroxyl (−OH)), malic acid (MA, 1 −OH), and tartaric acid (TA, 2 −OH), respectively, over a broad concentration range. We demonstrate that the role of dicarboxylic acids varies with the num...

Bioavailability and mobility of cadmium (Cd2+) and arsenate (As5+) in soils can be effectively lowered through the dissolution of brushite (dicalcium phosphate dihydrate, CaHPO4·2H2O) coupled with the precipitation of a more stable mineral phase containing both Cd and As. Due to the ubiquitous presence of humic acid (HA) in soil environments, it is more complex to predict the fate of dissolved Cd and As during such sequestration. Here, we used in situ atomic force microscopy (AFM) to image the kinetics of simultaneous precipitation of Cd and As at the brushite–fluid interface in the presence of HA. Results show that HA inhibits the formation of both amorphous and crystalline Cd(5–x)Cax(PO4)(3–y)(AsO4)y(OH) on the (010) face of brushite. A combination of X-ray photoelectron spectroscopy (XPS) and real-time surface-enhanced Raman spectroscopy (SERS) reveals that part of As5+ reduction into As3+ with HA and [HA-Cd] complexation occurs, modulating the concentrations of free Cd2+ and As5+ ions to inhibit subse...

Mesoporous zeolite Y (denoted as meso-Y) was synthesized using hydrothermal synthesis method using pluronic P123 block copolymer as template. Furthermore, stabilized Y (USY) was prepared through ion exchange and ultra-stabilization process of the meso-Y. The catalysts were prepared and characterized by XRD, SEM, TEM, N 2 adsorption–desorption, NH 3 -TPD and Py-IR. These catalysts were evaluated by determining the micro-activity using the light diesel oil as sample. The results showed that the Meso-CAT-3 catalyst had the highest micro-activity and lowest coke yield. It could be ascribed to the mesoporous structure which is known to improve the mass transfer of the larger molecules.

A series of core–shell structure Y@ASA composites with different contents of Y-type zeolite (abbreviated as Y@A) was synthesized in the small-crystal Y zeolite suspensoid by adding CTAB surfactant in the synthesis process of the amorphous silica alumina (ASA). Y@A composites and their corresponding catalysts were characterized by XRD, SEM, TEM, N2 adsorption–desorption and infrared spectroscopy of pyridine adsorption (Py-IR). The results showed that the surface of the small-crystal Y zeolite was covered uniformly by ASA even if ASA content was only 10 wt%, and more importantly, ASA could provide 3–20 nm mesopores to bring about the formation of the micro- and mesoporous core–shell structured composites with internal microporous Y zeolite and external mesoporous ASA. Furthermore, due to the interaction between ASA and Y zeolite, the hydrothermal stability of the small-crystal Y zeolite was enhanced, and at the same time the acid quantity of the composite was increased. The NiW/USY@A catalysts were synthesized by loading Ni and W on the Y@A. Their hydrocracking and isomerization performance were evaluated by using n-decane as a model compound. The experimental results indicated that the selectivities of the intermediate product and isoparaffin on NiW/USY@A-90 (with 90 wt% Y-type zeolite in the Y@A) are 30.81% and 23.70% higher than NiW/USY-In in which industrial Y was used, respectively. Thus, besides its high hydrocracking performance, NiW/USY@A showed an excellent isomerization capacity.

Phosphorus (P) recovery from wastewater is essential for sustainable P management. A biomimetic hexapeptide (SGAGKT) has been demonstrated to bind inorganic P in P-rich environments, however the dynamics and molecular mechanisms of P-binding to the hexapeptide still remain largely unknown. We used dynamic force spectroscopy (DFS) to directly distinguish the P-unbound and P-bound SGAGKT adsorbed to a mica (001) surface by measuring the single-molecule binding free energy (Δ Gb). Using atomic force microscopy (AFM) to determine real-time step retreat velocities of triangular etch pits formed at the nanoscale on the dissolving (010) face of brushite (CaHPO4·2H2O) in the presence of SGAGKT, we observed that SGAGKT peptides promoted in situ dissolution through an enhanced P-binding driven by hydrogen bonds in a P-loop being capable of discriminating phosphate over arsenate, concomitantly forming a thermodynamically favored SGAGKT-HPO42- complexation at pH 8.0 and relatively low ionic strength, consistent with the DFS and isothermal titration calorimetry (ITC) determinations. The findings reveal the thermodynamic and kinetic basis for binding of phosphate to SGAGKT and provide direct evidence for phosphate discrimination in phosphate/arsenate-rich environments.

The lactose-inducible expression system was widely used in bacteria due to the safety of lactose. However, the strength of its promoter was limited in lactic acid bacteria. Here, the β-galactosidase activity in Lactobacillus plantarum WCFS1 was detected at the presence of lactose and galactose. Sequence analysis showed that two β-galactosidase encoding genes lacA and lacLM were identified in the genome of Lb. plantarum WCFS1. Using green fluorescent protein as a reporter, we found that the promoter PlacA of lacA was lactose-induced, while promoter PlacLM of lacLM was lactose/galactose-induced. Furthermore, by optimizing the sequences of –35, –10 regions and ribosome binding sites of these two promoters, the maximum strength of the promoter PlacA derivative increased 10.4-fold by lactose induction, while PlacLM derivative increased 12.7-fold by lactose induction and 9.0-fold by galactose induction compared with their original promoters, respectively. When the optimized promoters were used to drive the β-galactosidase gene from Streptococcus thermophilus CGMCC 7.179, the highest β-galactosidase activity was 45.72 ± 0.44 U/mL under PlacLM-35-10 control. The strength of these promoters was also confirmed by western-blot analysis. Thereby, this study provided a set of novel lactose/galactose-inducible promoters with high expression efficiency and thus expanded the toolbox to express recombinant protein in Lb. plantarum.

Esters of phosphoric acid constitute a large fraction of the total organic phosphorus (OP) in the soil environment and, thus, play an important role in the global phosphorus cycle. These esters, such as glucose-6-phosphate (G6P), exhibit unusual reactivity toward various mineral particles in soils, especially those containing calcite. Many important processes of OP transformation, including adsorption, hydrolysis, and precipitation, occur primarily at mineral-fluid interfaces, which ultimately governs the fate of organophosphates in the environment. However, little is known about the kinetics of specific mineral-surface-induced adsorption and precipitation of organophosphates. Here, by using in situ atomic-force microscopy (AFM) to visualize the dissolution of calcite (1014) faces, we show that the presence of G6P results in morphology changes of etch pits from the typical rhombohedral to a fan-shaped form. This can be explained by a site-selective mechanism of G6P-calcite surface interactions that stabilize the energetically unfavorable (0001) or (0112) faces through step-specific adsorption of G6P. Continuous dissolution at calcite (1014)-water interfaces caused a boundary layer at the calcite-water interface to become supersaturated with respect to a G6P-Ca phase that then drives the nucleation and growth of a G6P-Ca precipitate. Furthermore, after the introduction of the enzyme alkaline phosphatase (AP), the precipitates were observed to contain a mixture of components associated with G6P-Ca, amorphous calcium phosphate (ACP)-hydroxyapatite (HAP) and dicalcium phosphate dihydrate (DCPD). These direct dynamic observations of the transformation of adsorption- and complexation-surface precipitation and enzyme-mediated pathways may improve the mechanistic understanding of the mineral-interface-induced organophosphate sequestration in the soil environment.

The modification of commercial ultra-stable Y zeolite using malic acid (MA) and nitric acid (NA) was investigated. A series of factors including the amount of MA and NA solutions, the pH of the solutions, the treatment time, and the reaction temperature were investigated and optimized. The pore structure, acid properties, and crystal structure of modified USY zeolite were characterized by N2-adsorption, temperature-programmed desorption of ammonia (NH3-TPD), pyridine adsorbed Fourier transform infrared spectroscopy, and X-ray diffraction techniques. The as-obtained sample under the optimum conditions presents an increased secondary pore volume up to 0.202 cm3 g−1, which accounts for 45.3 % of the total pore volume, and appropriate acid properties as well as good crystallinity. Furthermore, the USY zeolite modified with different methods was also investigated, indicating that malic–nitric combined acid is an effective modifier for USY zeolite. The modified USY zeolite was used as support to prepare hydrocracking catalysts. The 140–370 °C middle distillate yield of the catalyst is 68.59 %, and middle distillate selectivity can reach up to 81.52 %. Compared with commercial catalyst, the yield and selectivity increased by 8.17 and 5.14 %, respectively.