Peroxydisulfate-Driven Reductive Dechlorination as Affected by Soil Constituents: Free Radical Formation and Conversion.

We report a previously unrecognized but efficient reductive degradation pathway in peroxydisulfate (PDS)-driven soil remediation. With supplements of naturally occurring low-molecular-weight organic acids (LMWOAs) in anaerobic biochar-activated PDS systems, degradation rates of 12 γ-hexachlorocyclohexanes (HCH)-spiked soils boosted from 40% without LMWOAs to a maximum of 99% with 1 mM malic acid. Structural analysis revealed that an increase in α-hydroxyl groups and a diminution in pKa1 values of LMWOAs facilitated the formation of reductive carboxyl anion radicals (COO•-) via electrophilic attack by SO4•-/•OH. Furthermore, degradation kinetics were strongly correlated with soil organic matter (SOM) contents than iron minerals. Combining a newly developed in situ fluorescence detector of reductive radicals with quenching experiments, we showed that for soils with high, medium, and low SOM contents, dominant reactive species switched from singlet oxygen/semiquinone radicals to SO4•-/•OH and then to COO•- (contribution increased from 30.8 to 66.7%), yielding superior HCH degradation. Validation experiments using SOM model compounds highlighted critical roles of redox-active moieties, such as phenolic - OH and quinones, in radical formation and conversion. Our study provides insights into environmental behaviors related to radical activation of persulfate in a broader soil horizon and inspiration for more advanced reduction technologies.

Neglected but Efficient Electron Utilization Driven by Biochar-Coactivated Phenols and Peroxydisulfate: Polyphenol Accumulation Rather than Mineralization.

We report an unrecognized but efficient nonradical mechanism in biochar-activated peroxydisulfate (PDS) systems. Combining a newly developed fluorescence trapper of reactive oxygen species with steady-state concentration calculations, we showed that raising pyrolysis temperatures of biochar (BC) from 400 to 800 °C remarkably enhanced trichlorophenol degradation but inhibited the catalytic production of radicals (SO4•- and •OH) in water and soil, thereby switching a radical-based activation into an electron-transfer-dominated nonradical pathway (contribution increased from 12.9 to 76.9%). Distinct from previously reported PDS* complex-determined oxidation, in situ Raman and electrochemical results of this study demonstrated that the simultaneous activation of phenols and PDS on the biochar surface triggers the potential difference-driven electron transfer. The formed phenoxy radicals subsequently undergo coupling and polymerization reactions to generate dimeric and oligomeric intermediates, which are eventually accumulated on the biochar surface and removed. Such a unique nonmineralizing oxidation achieved an ultrahigh electron utilization efficiency (ephenols/ePDS) of 182%. Through biochar molecular modeling and theoretical calculations, we highlighted the critical role of graphitic domains rather than redox-active moieties in lowering band-gap energy to facilitate electron transfer. Our work provides insights into outstanding contradictions and controversies related to nonradical oxidation and inspiration for more oxidant-saving remediation technologies.

Accelerating Fe2+/Fe3+ cycle via biochar to improve catalytic degradation efficiency of the Fe3+/persulfate oxidation.

The sluggish Fe3+/Fe2+ cycle was the rate-limiting step in the Fenton-like reaction, and metal-free carbonaceous materials are considered as emerging alternatives to solve this problem. However, the effect of carbon material properties on the distribution of reactive species remains poorly understood. This study investigated the possibility and mechanism of using biochar to accelerate the Fe3+/Fe2+ cycle to overcome the low efficiency of Fe3+/persulfate (PS) catalytic oxidation of phenanthrene. More importantly, the contribution of reactive species in the reaction systems with the variation of biochar pyrolysis temperatures was quantitatively studied. The results showed that medium-temperature derived biochar (BC500) had the greatest ability to enhance the Fenton-like system compared to the low- and high-temperature (BC350/700), and the first-order rate constant achieved 5.2 and 35.7-fold increase against the biochar/PS and Fe3+/PS systems, respectively. Using electrochemical evidence, sulfoxide probe tests, and steady-state concentration calculations, radicals yields were found to rise and then reduce with decreasing pyrolysis temperature, while the nonradical contribution of Fe(IV) increased to 56.3%. Electron paramagnetic resonance, Boehm titration, and Raman spectroscopy unraveled that the enhanced effect of biochar resulted from itself persistent free radicals, phenolic-OH, and edge defects, which enabled electron transfer between Fe3+ and biochar. Fe2+ was thus continuously generated and effectively activated the PS. This work enables a better understanding of the Fe3+-mediated Fenton-like reaction in the presence of biochar and provides a sustainable green strategy for Fenton chemistry with potential applications.

Direct grafting of cellulose nanocrystals with poly(ionic liquids) via Gamma-ray irradiation and their utilization for adsorptive removal of CR.

In this work, a simple but effective method based on Gamma-ray initiated polymerization was reported for the first time through direct irradiation of CNCs and ionic liquid monomer to obtain poly (ionic liquids) functionalized CNCs (IL@CNCs). The adsorptive removal of Congo red (CR) from aqueous solution by IL@CNCs was also examined and the influence of contact time, pH values, initial concentrations and temperature on adsorption behavior was investigated in detail. Under the same adsorption conditions, the adsorption capacity was increased from 59.72 mg/g (CNCs) to 195.83 mg/g (IL@CNCs). The results of the adsorption isotherm and adsorption kinetics showed that the experimental data were more suitable to be described by the Freundlich isotherm adsorption model and the pseudo-second-order model. The adsorption process of CR on the surface of the adsorbent was endothermic and spontaneous. When the aqueous solution was acidic, it was more conducive to the adsorption of CR. At 100% breakthrough, the value of adsorption capacity is 199.95 mg/g and the value of partition coefficient is 9.64. Moreover, the adsorption capacity is expected to be further improved through adjustment of polymerization parameters and this method can also be used for preparation other poly (ionic liquids) modified composites.

Surface functionalization of MXene with chitosan through in-situ formation of polyimidazoles and its adsorption properties.

In this work, a novel imidazoles-MXene hybrid composite, namely polyimidazoles chain overlaying on the surface of MXene (Ti3C2@IMIZ), was prepared by a simple method. Through this strategy, imidazoles can be in situ growth on the surface of MXenes via a facile multicomponent reaction using chitosan as a renewable reactant. Based on the characterization results, we demonstrated that a thin layer imidazoles with an ordered chain structure was embedded on the surface of Ti3C2, which resulted in the formation of a novel imidazoles-MXene hybrid composite. The adsorption performance of Ti3C2@IMIZ for removal environmental pollutants was evaluated using heavy metal ions of Cr(Ⅵ) as adsorbate. Detailed adsorption characteristics of Ti3C2@IMIZ including operational factors, adsorption kinetics and isotherms models were investigated. XPS analysis showed that Cr(VI) was converted to Cr(III) with low toxicity during the adsorption process. The adsorption of Cr(VI) and reduction of Cr(VI) to Cr(III) contribute to elimination of Cr(VI) species. The adsorption behavior and process analysis show that the adsorption mechanism is mainly physical adsorption through electrostatic interaction. The excellent reproducibility suggests that Ti3C2@IMIZ may be a potential candidate for remove of Cr(Ⅵ) in actual sewage treatment.

Functionalization of carbon nanotubes with chitosan based on MALI multicomponent reaction for Cu2+ removal.

In this work, we reported a novel "one-pot" strategy for preparation of chitosan-coated carbon nanotubes (CNTs) composites via a combination of Diels-Alder (DA) reaction and mercaptoacetic acid locking imine (MALI) reaction for the first time. To evaluate the adsorption characteristics, the as-prepared samples were applied to remove copper ions (Cu2+) from aqueous solution. The effects of contact time, solution pH, temperature and initial Cu2+ concentration on the adsorption of Cu2+ onto the as-prepared samples were investigated. The chitosan modified CNTs composites showed high affinity and fast kinetics for the adsorption of Cu2+ ions, and adsorption capacity of the composites was found to be 2 times that of pristine CNTs. Adsorption kinetics and thermodynamic indicated a spontaneous and endothermic nature of the adsorption of Cu2+ on the surface of chitosan-coated CNTs composites, kinetically obeyed the pseudo-second-order model. Equilibrium data could be best described by the Langmuir isotherm model, with a maximum monolayer adsorption capacity of 115.84 mg/g. In view of the extensive applicability of DA chemistry and MALI reaction, different carbon nanomaterials based composites with various functional groups could be fabricated and applicable to different fields such as environmental catalysis and biomedicine.

Facile preparation of magnetic composites based on carbon nanotubes: Utilization for removal of environmental pollutants.

The preparation of multifunctional composites that combine magnetic nanoparticles and supported nanomaterials has attracted great attention for various applications. In this work, a facile method was developed for the preparation of carbon nanotube (CNT)-based magnetic composites through a one-pot oxidation method using K2FeO4 as the oxidant, which was subsequently used as the reagent to generate the Fe3O4 nanoparticles and fabricate the magnetic CNT composites. This strategy could be performed at room temperature, so it is very mild and straightforward. The properties and structure of the as-fabricated CNT-Fe3O4 composites were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and vibrating sample magnetometry. The results suggested that this approach not only generated Fe3O4 magnetic nanoparticles on the surface of the CNTs but also produced a series of functional groups. In addition, the dried CNT-Fe3O4 composites were highly dispersible in water or organic solutions, and they also had a magnetic response that could satisfy the demand for magnetic separation. Finally, we adsorbed copper ions (Cu2+) and methylene blue (MB) using the CNT-Fe3O4 composites as adsorbents. The results indicated that the obtained composites could adsorb both Cu2+ and MB effectively. Taken together, we report a novel strategy for the fabrication of magnetic carbon nanotube composites through a facile oxidation and subsequent deposition procedure. These magnetic composites show great potential for the removal of environmental pollutants.

Mussel-inspired preparation of layered double hydroxides based polymer composites for removal of copper ions.

A novel ternary composite consisting of Mg/Al layered double hydroxides (LDH), polydopamine (PDA) and poly(methyl vinyl ether-alt-maleic anhydride) (PMVE-MA) was fabricated by a facile combination of mussel-inspired chemistry and a ring-opening reaction. Dopamine can serve as a "minimalist mimic" of mussel adhesive protein to form a layer of polydopamine (PDA) on the LDH surface under rather mild conditions (including air atmosphere, aqueous solution, and catalyst free). Subsequently, the PMVE-MA brushes were immobilized onto the PDA modified LDH via a ring-opening reaction. The morphology and chemical compositions of the as-prepared samples were characterized by SEM, TEM, FT-IR, TGA, and XPS. To evaluate the adsorption performance of the PMVE-MA modified LDH (LDH@PDA@PMVE-MA) composites, the obtained samples were used as adsorbents for the removal of copper ions (Cu2+) from an aqueous solution. The results demonstrated that the LDH@PDA@PMVE-MA composites showed a significant improvement in the adsorption efficiency towards Cu2+, and the adsorption capacity of the LDH@PDA@PMVE-MA composites was found to be 2 times higher than that of pristine LDH. Adsorption kinetics showed that the experimental data were fitted well by the pseudo-second-order kinetic model. Equilibrium data could be best described by the Langmuir isotherm model, with the maximum monolayer adsorption capacity of 193.78 mg/g. Thermodynamic studies indicated that the adsorption of Cu2+ onto the LDH@PDA@PMVE-MA composites is an endothermic and spontaneous process. Importantly, it can be easily regenerated by low-cost reagents, and exhibited high removal efficiencies after four cycles of adsorption-desorption. These results suggest that the LDH@PDA@PMVE-MA nanocomposites are good candidate for Cu2+ removal from aqueous solutions.