Tetrabutylammonium-chloride-glycerol of deep eutectic solvent functionalized MnO2: a novel mimic enzyme for the quantitative and qualitative colorimetric detection of L-cysteine.

L-Cysteine is a common amino acid that plays an important role in human livelihood and production. Therefore, a novel method for the simultaneous quantitative and qualitative determination of L-cysteine by a colorimetric detection system is proposed. As a viable oxidase mimic, [N4444]Cl-G/MnO2, which consisted of MnO2 nanosheets functionalized by a tetrabutylammonium chloride-glycerol ([N4444]Cl-G) based deep eutectic solvent (DES) was fabricated. Owing to the oxidation of MnO2 nanosheets, [N4444]Cl-G/MnO2 could oxidize the colorless 3,3',5,5'-tetramethylbenzidine (TMB) into a blue product (oxTMB) with the characteristic UV-vis spectrum absorbance at 652 nm. The oxidation of TMB by DES/MnO2 was inhibited when L-cysteine was introduced, and the absorbance decreased proportionally with the increase in L-cysteine concentration. Due to this inhibition effect, a colorimetric detection system ([N4444]Cl-G/MnO2-TMB) was developed for the quantitative determination of L-cysteine. Under optimal conditions, the assay showed good linearity over the concentration range of 0.125-2.00 µg mL-1 with a low detection limit of 5.96 ng mL-1. A study of the inhibition mechanism demonstrated that the sulfhydryl group of L-cysteine could decompose [N4444]Cl-G/MnO2 into Mn2+, thus limiting the conversion of TMB to oxTMB. In addition, the [N4444]Cl-G/MnO2-TMB system was used in test strips for the visual qualitative detection of L-cysteine. The selectivity and test strip results demonstrated the high selectivity, simple operation, and rapid response of the [N4444]Cl-G/MnO2-TMB system for the qualitative detection of L-cysteine. Given the satisfying performance of the detection strategy, colorimetric sensing based on the [N4444]Cl-G/MnO2-TMB system is considered to have prospective application value in the quantitative and qualitative detection of L-cysteine.

Nutrients regeneration pathway, release potential, transformation pattern and algal utilization strategies jointly drove cyanobacterial growth and their succession.

In order to better understand the contribution of nutrients regeneration pathway, release potential and transformation pattern to cyanobacterial growth and succession, 7 sampling sites in Lake Chaohu with different bloom degree were studied every two months from February to November 2018. The carbon, nitrogen (N) and phosphorus (P) forms or fractions in surface, interstitial water and sediments as well as extracellular enzymatic activities, P sorption, specific microbial abundance and community composition in sediments were analyzed. P regeneration pathway was dominated by iron-bound P desorption and phosphorus-solubilizing bacteria solubilization in severe-bloom and slight-bloom area respectively, which both resulted in high soluble reactive phosphorus (SRP) accumulation in interstitial water. However, in severe-bloom area, higher P release potential caused the strong P release and algal growth, compared to slight-bloom area. In spring, P limitation and N selective assimilation of Dolichospermum facilitated nitrate accumulation in surface water, which provided enough N source for the initiation of Microcystis bloom. In summer, the accumulated organic N in Dolichospermum cells during its bloom was re-mineralized as ammonium to replenish N source for the sustainable development of Microcystis bloom. Furthermore, SRP continuous release led to the replacement of Dolichospermum by Microcystis with the advantage of P quick utilization, transport and storage. Taken together, the succession from Dolichospermum to Microcystis was due to both the different forms of N and P in water column mediated by different regeneration and transformation pathways as well as release potential, and algal N and P utilization strategies.

Phosphorus supply pathways and mechanisms in shallow lakes with different regime.

In order to better understand the pathways and mechanisms of phosphorus (P) supply under different regimes, 12 sampling sites from 4 basins of 2 lakes were studied seasonally from October 2017 to July 2018 in Wuhan City, China. Concentrations of different forms of P and nitrogen (N) in surface and interstitial water, contents of carbon (C), N, P and iron (Fe) compounds as well as related extracellular enzymatic activities, phosphorus sorption, abundance of phosphorus-solubilizing bacteria (PSB), total and specific (containing phosphatase gene) microbial community composition in sediments were analyzed. In lakes with macrophyte dominance, P supply pathway from sediment to water column was blocked. In lakes being early period of regime shifting from macrophyte to algae, exogenous P input was the main P supply mode. In lakes being later period of regime shifting from macrophyte to algae, organic P hydrolysis and calcium-bound P dissociation driven by PSB contributed greatly to P regeneration, which was continuous and trickling. In this process, rapid C and N cycles fueled P regeneration. In lakes with algal dominance, given the significantly higher iron-bound P (Fe(OOH)~P), equilibriums phosphorus concentration and dehydrogenase activity, the main P regeneration pathway might be the desorption of Fe(OOH)~P driven by anoxia, showing the seasonal and pulsed characteristics. In addition, during the process of regime shift from macrophyte to algae, the dominant algal species switched from cyanobacteria to Chlorophyta. P-solubilizing microorganisms correlated with environmental factors, suggesting the coupling of multiple nutrient cycles, especially C, N, P, oxygen (O) and Fe, could effectively increase the pathways diversification and the strength of P regeneration.

Heterogeneous Fenton degradation of ofloxacin catalyzed by magnetic nanostructured MnFe2O4 with different morphologies.

Magnetic nanostructured MnFe2O4 with different morphologies, synthesized via chemical co-precipitation and hydrothermal method, was assayed as heterogeneous Fenton catalysts. The as-prepared MnFe2O4 catalysts were thoroughly characterized by various characterization methods, such as X-ray diffraction (XRD), N2 adsorption-desorption, transmission electron microscopy (TEM), magnetic hysteresis loops, temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The catalytic activity of MnFe2O4 catalysts was evaluated in the heterogeneous Fenton degradation of ofloxacin (OFX). In our study, the morphology exhibited a critical impact on the catalytic activity of MnFe2O4. For example, MnFe2O4 nanorods (MnFe2O4-NR) had a higher catalytic activity than MnFe2O4 nanospheres (MnFe2O4-NS) and MnFe2O4 nanocubes (MnFe2O4-NC) in OFX removal and H2O2 decomposition. Notably, the catalytic activity was remarkably enhanced with increasing the relative amount of Mn3+ and Fe2+ species on the surface. Based on the results from quenching experiments and quantitative determination of •OH radicals, a possible catalytic mechanism of MnFe2O4 was proposed. In addition, the stability and reusability of MnFe2O4-NR was ascertained, as the results suggested that MnFe2O4-NR was a stable and easily separated catalyst for heterogeneous Fenton process.

Few-layer Bi2O2CO3 nanosheets derived from electrochemically exfoliated bismuthene for the enhanced photocatalytic degradation of ciprofloxacin antibiotic.

Few-layer two-dimensional (2D) Bi2O2CO3 nanosheets with a thickness of 4-5 nm were successfully fabricated via electrochemical exfoliation, followed by an exposure to ambient conditions. The formation process for these nanosheets was explored through ex situ X-ray diffractometer. The photocatalytic capacity of 2D Bi2O2CO3 nanosheets was investigated towards the degradation of ciprofloxacin. It was shown that 2D Bi2O2CO3 nanosheets exhibited better catalytic performance than Bi2O2CO3 nanoparticles synthesized by hydrothermal method under UV-Vis light irradiation. The enhanced photocatalytic activity is due to the larger specific surface area, as well as the lower band gap. Additionally, the radical trap experiments demonstrate that holes and hydroxyl radicals are of great importance in the degradation of ciprofloxacin. Finally, the 2D Bi2O2CO3 nanosheets show high stability in the photocatalytic degradation of ciprofloxacin, and could have a prospective application in the treatment of antibiotic wastewater.

Efficient adsorption of Hg(II) from aqueous solution by N, S co-doped MnFe2O4@C magnetic nanoparticles.

N, S co-doped MnFe2O4@C magnetic nanoparticles were successfully synthesized by a simple method involving the preparation of MnFe2O4 nanoparticles and subsequent pyrolysis treatment. The physical and chemical properties of MnFe2O4, MnFe2O4@C and MnFe2O4@C-NS nanoparticles were characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), transmission electron microscopy (TEM), N2 adsorption-desorption and the pH at the point of zero charge. Their performances in the adsorption of Hg(II) from water were investigated. The adsorption process followed pseudo-second-order kinetics and the experimental data of equilibrium isotherms fitted well with the Langmuir model. MnFe2O4@C-NS showed the highest adsorption capacity of 108.56 mg/g, increasing more than 1.7 times compared to MnFe2O4. The enhanced adsorption performance was attributed to the larger specific surface area as well as the complexation of N and S ligands on the surface. The thermodynamic parameters of ΔH°, ΔS° and ΔG° at 30 °C were -24.39 kJ/mol, -0.046 kJ/mol K and -10.45 kJ/mol, respectively, which indicated that the adsorption of Hg(II) on MnFe2O4@C-NS was exothermic and spontaneous in nature. Moreover, MnFe2O4@C-NS showed superior selectivity towards Hg(II) compared with other metal ions generally present in mercury-containing industrial wastewater.

Efficient adsorption of benzoic acid from aqueous solution by nitrogen-containing activated carbon.

Adsorption is an efficient treatment process to remove benzoic acid from aqueous solution. In this study, nitrogen-containing surface groups were introduced onto activated carbon (AC) surface by modification with ammonium hydroxide, ammonium carbonate, melamine or urea. The nitrogen-containing AC samples were characterized using N2 adsorption-desorption, Boehm titration, determination of the pH of the point of zero charge (pHpzc) and X-ray photoelectron spectroscopy. The adsorption of benzoic acid from aqueous solution by nitrogen-containing AC has been studied. The Langmuir model fitted the experimental data of equilibrium isotherms better than the Freundlich model. At initial solution pH 2.1, the adsorption capacity was closely related with the amount of pyridinic and pyrrolic N on the AC surface, which indicated these two nitrogen-containing groups played an important part in the adsorption process. The enhancement of adsorption capacity was due to the strengthened π-π dispersion force between benzoic acid and the AC basal plane. Since the surface charge of AC as well as the existence form of benzoic acid varied with solution pH value, the adsorption capacity was found to be highest at pH 3.8 and dropped sharply at higher or lower pH values.

Catalytic wet peroxide oxidation of benzoic acid over Fe/AC catalysts: Effect of nitrogen and sulfur co-doped activated carbon.

The parent activated carbon (ACP) was modified with urea and thiourea to obtain N-doped activated carbon (ACN) and N, S co-doped activated carbon (ACNS), respectively. Iron supported on activated carbon (Fe/ACP, Fe/ACN and Fe/ACNS) were prepared and worked as catalyst for catalytic wet peroxide oxidation of benzoic acid (BA). The catalysts were characterized by N2 adsorption-desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM), and their performance was evaluated in terms of benzoic acid and TOC removal. The results indicated the doped N and S improved the adsorption capacity as well as catalytic activity of activated carbon. Besides, the catalytic activity toward benzoic acid degradation was found to be enhanced by Fe/ACNS compared to that of Fe/ACP and Fe/ACN. The enhanced catalytic performance was attributed to the presence of the nitrogen and sulfur atoms may serve to improve the relative amount of Fe2+ on iron oxide surface and also help prevent leaching of Fe. It was also observed that the stability or reutilization of Fe/ACNS catalyst was fairly good.

Magnetic core-shell-structured Fe3O4@CeO2 as an efficient catalyst for catalytic wet peroxide oxidation of benzoic acid.

A magnetic core-shell-structured Fe3O4@CeO2 catalyst was prepared by a simple solvothermal method and applied in the solid state for catalytic wet peroxide oxidation (CWPO) of benzoic acid. The obtained catalyst was characterized by N2 adsorption-desorption, X-ray diffraction (XRD), magnetic measurements, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The experimental results showed that Fe3O4@CeO2 possessed superior catalytic efficiency for CWPO of benzoic acid than that of Fe3O4. The high catalytic activity was caused by a synergistic effect between Fe3O4 and CeO2, which assisted the decomposition of H2O2 into hydroxyl radicals (·OH). Fe3O4@CeO2 exhibited low Fe leaching of 4.2 mg L-1, which approximately accounted for barely 0.76% of the total Fe amount in the catalyst. The effects of radical scavengers indicated that benzoic acid was degraded mainly by ·OH attack, which occurred both in the bulk solution and on the Fe3O4@CeO2 surface. In the stability tests, there was loss of merely 4% in the benzoic acid removal rate after six cycles of reaction, and the saturation magnetization of Fe3O4@CeO2 hardly changed, which suggested that the Fe3O4@CeO2 catalyst was fairly effective in reutilization and stability.

Mercury (II) adsorption from aqueous solution using nitrogen and sulfur co-doped activated carbon.

Activated carbon (AC) was modified with urea, thioglycolic acid and thiourea to obtain nitrogen doped activated carbon (ACN), sulfur doped activated carbon (ACS) and nitrogen and sulfur co-doped activated carbon (ACNS), respectively. The AC samples were characterized by elemental analysis, N2 adsorption-desorption, determination of the pH of the point of zero charge (pHpzc) and X-ray photoelectron spectroscopy, and tested for adsorption behaviors of Hg(II) ions. The experimental data of equilibrium isotherms fitted well with the Langmuir model. ACNS showed the highest adsorption capacity of 511.78 mg/g, increasing more than 2.5 times compared to the original ACA. The adsorption process followed pseudo-second-order kinetics. The thermodynamic parameters of ΔH°, ΔS°, and ΔG° at 30 °C were -20.57 kJ/mol, -0.032 kJ/mol K and -10.87 kJ/mol, respectively. It was concluded that the Hg(II) ions' adsorption on ACNS was exothermic, spontaneous and physiosorptive in nature. Finally, the adsorption capacity of ACNS reduced by just 8.13% even after the sixth cycle compared to the initial cycle.

Pretreatment of concentrated leachate by the combination of coagulation and catalytic ozonation with Ce/AC catalyst.

A raw concentrated leachate produced from membrane bioreactor-nanofiltration (MBR-NF) was taken from Chengdu Chang'an Waste Landfill Site, China. The major fraction of this concentrated leachate was large refractory humic substances. A coagulation-ozonation process was applied to treat this leachate, aiming at enhancing chemical oxygen demand (COD) removal efficiency and increasing its biodegradability. Meanwhile the molecular size distribution of the leachate, before and after coagulation and ozonation treatment, was analyzed by using ultrafiltration membrane separation. Coagulation pretreatment effectively removed varieties of large molecules in the raw concentrated leachate. The addition of Ce/AC greatly improved the oxidative ability of O3 in COD removal in the ozonation of coagulated leachate. The biochemical oxygen demand (BOD5)/COD ratio increased from 0.011 for the untreated concentrated leachate to 0.30 for the effluent of the coagulation-catalytic ozonation process, which indicated that a subsequent biological treatment could be readily conducted. The stability test demonstrated that the Ce/AC catalyst was effective and stable in the catalytic ozonation process. According to the results of molecular size distribution analysis, a direct correlation was observed between the increase of BOD5/COD and the decrease of apparent molecular weight.