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Highly blue fluorescence carbon dots (CDs-MCM) were successfully prepared via a simple hydrothermal method with citric acid and ethylene diamine tetraacetic acid doped mesoporous silica MCM-41. The CDs-MCM exhibited uniform particle size and possessed good excitation-dependent characteristic. It showed highly selectivity and sensitivity in detection of Cr(VI), folic acid and good linear ranges of 0–37.5$\mu$M ($R^2=0.9903$) and 0–50$\mu$M ($R^2=0.9949$). The low detection limits were 79.31nM and 118.73nM for Cr(VI) and folic acid, respectively. It revealed that inner filter effect dominated in Cr(VI) quenching of the CDs-MCM fluorescent, while that of folic acid belonged to static quenching. The CDs-MCM were successfully used to detect the Cr(Vl) and folic acid in water and vegetable samples with satisfactory results. It provided new insight into environmental Cr(VI) and folic acid detection.

The development of heterojunction composites with core–shell structure could effectively facilitate the separation of carriers. In this work, a novel In₂O₃-SnS₂ (IOS) core–shell heterojunction photocatalyst was successfully synthesized for the efficient photocatalytic reduction of hexavalent chromium (Cr(VI)), namely, the In₂O₃ nanorods were obtained through successive hydrothermal and carbonization process using urea and glucose as template. Then, SnS₂ nanosheets were successfully in situ coated onto In₂O₃ nanorods. The spectroscopic characterization and photo-electrochemistry test indicated that the IOS core–shell heterojunction could effectively accelerate the separation and transport of carriers and suppress the recombination of carriers. The photocatalytic performance of IOS was evaluated by photocatalytic reduction Cr(VI). The results showed that IOS-4 samples almost completely removed Cr(VI) (20 mg/L), within 90 min under visible light, which was superior to pure In₂O₃ and SnS₂. Furthermore, IOS samples also possess excellent stability, the removal efficiency was still maintained at 90% after five cycles. This work provided a reliable method for designing core–shell heterojunctions for the photocatalytic removal of Cr(VI) under visible light.

In this study, a novel lanthanum oxide-loaded porous mullite–corundum ceramic was prepared and adopted to remove Cr(VI) from aqueous solution. The lanthanum oxide-loaded porous mullite–corundum ceramic was characterized by XRD, SEM-EDS, and XPS, and batch adsorption experiments were employed to investigate the effect of main interfering factors on Cr(VI) adsorption. The results showed that the adsorption capacity of lanthanum oxide-loaded porous mullite–corundum ceramic was significantly improved, and the dynamic adsorption equilibrium could be reached at 720 min; the adsorption capacity almost did not decrease after five adsorption cycles. Cr(VI) adsorption process abided by pseudo-second-order kinetic model and Freundlich model, indicating that Cr(VI) adsorption onto lanthanum oxide-loaded porous mullite–corundum ceramic was multi-molecular layer chemical adsorption. Therefore, lanthanum oxide- loaded porous ceramics are an efficient and highly reusable adsorbent, which is expected to be used for the treatment of Cr(VI) in wastewater.

In situ solid-state synthesis was employed to prepare the ${\mathrm{\text{g-C}}}_3{\mathrm{\text{N}}}_4/{\mathrm{\text{MoO}}}_3$ (CM) heterojunctions with a two-dimensional mortise and tenon structure. The prepared products were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy, suggesting that amorphous graphitic carbon was obtained. The photocatalytic performance of the prepared photocatalysts was evaluated in aqueous Cr(VI) reduction reactions under visible-light ($\lambda$ > 420 nm) irradiation. The results of the experiments showed that the photocatalytic activity of CM is influenced by the composition of heterojunctions. The optimum photocatalytic activity for the photocatalytic reduction of Cr(VI) was found in the compound CM-25, which contains a 25% ${\mathrm{\text{C}}}_3{\mathrm{\text{N}}}_4$ concentration. The Cr(VI) reduction rate on CM-25 was four times that of ${\mathrm{\text{g-C}}}_3{\mathrm{\text{N}}}_4$ and 14.5 times that of ${\mathrm{\text{MoO}}}_3$, the photocatalytic activity of CM-25 only dropped by 7.7% in four cycles. Scavenger investigations and EPR tests indicated that the major photogenerated radicals in the reduction of Cr(VI) were superoxide negative radicals (•${\mathrm{\text{O}}}_2{-}^{}$) and hydroxyl radicals (•OH). CM-25 was discovered to be an S-scheme heterojunction that effectively suppressed photogenerated charge carrier recombination and retained photogenerated holes (in the valence band of ${\mathrm{\text{MoO}}}_3$) and electrons (in the conduction band of ${\mathrm{\text{C}}}_3{\mathrm{\text{N}}}_4$) with higher redox capabilities. CM-25 has potential applications in the photocatalytic reduction of Cr(VI) in wastewater.

The photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solutions without or with urea (a model organic contaminant) using ZSM-5 zeolite as catalyst under UV illumination was studied. The used ZSM-5 zeolite has the characteristics of pure ZSM-5 zeolite crystalline structure, with the point of zero charge (pH_PZC) value of pH = 3.7–3.9. The effects of illumination time, mass content of ZSM-5 zeolite, urea concentrations, Cr(VI) initial concentrations and ionic strength on the photocatalytic reduction of Cr(VI) to Cr(III) were investigated. The results indicated that both the increase of urea concentration and the mass content of ZSM-5 zeolite can improve the photocatalytic reduction of Cr(VI) to Cr(III) under UV illumination. The results are important to understand the photocatalytic reduction of Cr(VI) to Cr(III) in natural environment with organic contaminant.

Higher environmental standards have made the removal of toxic metals such as hexavalent chromium from wastewater; an important problem for environmental protection. Iron oxide is a particularly interesting adsorbent to be considered for this application.

In this study, a new method combining adsorption and magnetic separation was developed to remove Cr(VI) from wastewater. The nanocrystalline magnetite as adsorbent was produced via thermo- mechanical reduction of hematite. Various parameters which affect the adsorption of Cr(VI) such as time, pH, temperature and initial concentration were investigated using thermo-gravimeters (TG), X-Ray diffraction (XRD), scanning electron microscopy (SEM) and atomic adsorption spectroscopy (AAS) techniques. The maximum adsorption was occurred at pH 2. The adsorption data were fitted well to Langmuir isotherm model. The adsorption of Cr(VI) increased significantly with increasing of temperature and time.