MnO2 nanozyme-driven polymerization and decomposition mechanisms of 17ß-estradiol: Influence of humic acid.

Nanozymes, which display the bifunctional properties of nanomaterials and natural enzymes, are useful tools for environmental remediation. In this research, nano-MnO2 was selected for its intrinsic enzyme-like activity to remove 17ß-estradiol (E2). Results indicated that nano-MnO2 exhibited laccase-like activity (7.22 U·mg-1) and removed 97.3 % of E2 at pH 6. Humic acid (HA) impeded E2 removal (only 72.4 %) by competing with E2 for the catalytic sites of the MnO2 nanozyme surface, and there was a good linear correlation between the kinetic constants and HA concentrations (R2 = 0.9489). Notably, the phenolic -OH of E2 interacted with HA to yield various polymeric products via radical-driven covalent coupling, resulting in ablation of phenolic -OH but increase of ether groups in the polymeric structure. Intermediate products, including estrone, E2 homo-/hetero-oligomers, E2 hydroxylated and quinone-like products, as well as aromatic ring-opening species, were identified. Interestingly, HA hindered the extent of E2 oxidation, homo-coupling, and decomposition but accelerated E2 and HA hetero-coupling. A reasonable catalytic pathway of E2 and HA involving MnO2 nanozyme was proposed. These findings provide novel insights into the influence of HA on MnO2 nanozyme-driven E2 radical polymerization and decomposition, consequently favoring the ecological water restoration and the global carbon cycle.

Contributions of root cell wall polysaccharides to Cu sequestration in castor (Ricinus communis L.) exposed to different Cu stresses.

Cell wall polysaccharides play a vital role in binding with toxic metals such as copper (Cu) ions. However, it is still unclear whether the major binding site of Cu in the cell wall varies with different degrees of Cu stresses. Moreover, the contribution of each cell wall polysaccharide fraction to Cu sequestration with different degrees of Cu stresses also remains to be verified. The distribution of Cu in cell wall polysaccharide fractions of castor (Ricinus communis L.) root was investigated with various Cu concentrations in the hydroponic experiment. The results showed that the hemicellulose1 (HC1) fraction fixed 44.9%-67.8% of the total cell wall Cu under Cu stress. In addition, the pectin fraction and hemicelluloses2 (HC2) fraction also contributed to the Cu binding in root cell wall, accounting for 11.0%-25.9% and 14.1%-26.6% of the total cell wall Cu under Cu treatments, respectively. When the Cu levels were ≤25 µmol/L, pectin and HC2 contributed equally to Cu storage in root cell wall. However, when the Cu level was higher than 25 µmol/L, the ability of the pectin to bind Cu was easy to reach saturation. Much more Cu ions were bound on HC1 and HC2 fractions, and the HC2 played a much more important role in Cu binding than pectin. Combining fourier transform infrared (FT-IR) and two-dimensional correlation analysis (2D-COS) techniques, the hemicellulose components were showed not only to accumulate most of Cu in cell wall, but also respond fastest to Cu stress.

Effects of sulfur on toxicity and bioavailability of Cu for castor (Ricinus communis L.) in Cu-contaminated soil.

The biogeochemical cycling of sulfur (S) in soil has an important impact on the bioavailability of heavy metals and affects the utilization of soil polluted by heavy metals. In addition, S-containing compounds are involved in heavy metal detoxification. This study investigated the effects of S on the toxicity and bioavailability of copper (Cu) in castor (Ricinus communis L.) grown in Cu-contaminated mine tailings. The results showed that the application of S reduced the accumulation of Cu in castor and promoted its growth. With the addition of S, the malondialdehyde (MDA) content of castor leaves decreased significantly compared with control plants, indicating the alleviation of oxidative stress. Superoxide dismutase (SOD) and catalase (CAT) activities and glutathione (GSH) content decreased significantly with the alleviation of oxidative stress. The sequential extraction of Cu fractions showed that the application of S significantly reduced the reducible Cu fraction, and increased the oxidizable Cu fraction. It also increased the residual Cu fraction in the soil. The transformation of chemical speciation reduced the bioavailability of Cu in soil, which then reduced the accumulation of Cu in castor. Our results demonstrated that S application was effective at promoting castor growth by reducing the bioavailability and uptake of Cu in Cu-contaminated mine tailings.

Characterization and Cu sorption properties of humic acid from the decomposition of rice straw.

Humic acid (HA) derived from rice straw decomposed for 1 (HA-1), 3 (HA-3), 6 (HA-6) and 12 (HA-12) months was characterized by potentiometric titration and solid-state cross-polarization magic-angle spinning 13C nuclear magnetic resonance spectroscopy (CPMAS 13C NMR). The sorption of Cu on examined HA was investigated using a combination of batch sorption, isothermal titration calorimetry (ITC) and sequential desorption. Results showed that the functional group content and the humification degree of HA tended to increase with increasing decomposition time especially in the latter stage of examined decomposition period. Cu sorption on HA was a rapid process that occurred within the first 1 h and the sorption capacity increased from 245.4 mmol kg-1 on HA-1 to 294.6 mmol kg-1 on HA-12. The sorption of Cu was endothermic, spontaneous and the randomness was increased during Cu sorption. Sorbed Cu on examined HA can be hardly released by NH4Ac but nearly fully released by EDTA. Forming inner-sphere complexes was the main mechanism of Cu sorption on examined HA. This study could provide valuable information for a better understanding on the environmental impacts of the decomposition of organic waste.

Sorption of Cu by humic acid from the decomposition of rice straw in the absence and presence of clay minerals.

The sorption of Cu on humic acid (HA) from the decomposition of rice straw in the absence (Ck-HA) and presence of montmorillonite (M-HA), kaolinite (K-HA), gibbsite (Gi-HA) and goethite (Go-HA) was investigated at pH 5.0 by using batch studies combined with isothermal titration calorimetry (ITC) and atomic force microscopy (AFM). Characterization by elemental analysis and potentiometric titration showed the composition difference among these five HA. The sorption capacity and rate increased in the order: M-HA < K-HA < Gi-HA < Ck-HA < Go-HA. ITC results revealed that the sorption process was spontaneous and endothermic. The aggregation of HA particles after sorption were observed by AFM images. The influence of pH and positive correlations between the sorption capacity and the content of acidic functional groups of HA indicated that the dissociated acidic functional groups, especially the dissociation of carboxylic groups in HA played an important role in Cu sorption. Sequential desorption of sorbed Cu showed that the surface bonded fraction (97.6-99.0%) was significantly higher than the ion exchanged fraction (1.0-2.4%). Markedly positive entropies (ΔS, 94.4-104.3 J mol-1 K-1) further demonstrated that Cu binding to HA by forming inner-sphere complexes. The findings of this study would promote the understanding on the environmental impact of the decomposition of organic waste from agricultural production.

A bisphenol A sensor based on novel self-assembly of zinc phthalocyanine tetrasulfonic acid-functionalized graphene nanocomposites.

In this work, a novel zinc phthalocyanine tetrasulfonic acid (ZnTsPc)-functionalized graphene nanocomposites (f-GN) was synthesized by a simple and efficient electrostatic self-assembly method, where the positive charged GN decorated by (3-aminopropyl) triethoxysilane (APTES) was self-assemblied with ZnTsPc, a two dimensional (2-D) molecules. It not only enhanced its stability for the hybrid structure, but also avoided the reaggregation of ZnTsPc or f-GN themselves. Based on layered ZnTsPc/f-GN nanocomposites modified glassy carbon electrode, a rapid and sensitive sensor was developed for the determination of bisphenol A (BPA). Under the optimal conditions, the oxidation peak current increased linearly with the concentration of BPA in the range of 5.0×10(-8) to 4.0×10(-6)M with correlation coefficient 0.998 and limits of detection 2.0×10(-8)M. Due to high absorption nature for BPA and electron deficiency on ZnTsPc/f-GN, it presented the unique electron pathway arising from π-π stackable interaction during redox process for detecting BPA. The sensor exhibited remarkable long-term stability, good anti-interference and excellent electrocatalytic activity towards BPA detection.