Removal of Cr(VI) species from water with a newly-designed adsorptive treatment train

Insight into the Key Role of Cr Intermediates in the Efficient and Simultaneous Degradation of Organic Contaminants and Cr(VI) Reduction via g-C3N4-Assisted Photocatalysis

Photocatalysis provides an impetus for the synergetic removal of Cr(VI) and organic contaminants, but the generation of Cr intermediates and their potential oxidizability may be overlooked in pollutant conversion. Herein, the Cr intermediates in the Cr(VI) reduction process were emphasized in Cr(VI)/bisphenol A (BPA) by using graphitic carbon nitride as a photocatalyst. The active species for BPA photodegradation in the BPA system and Cr(VI)/BPA system suggested that the Cr(VI) reduction process indeed promotes BPA photodegradation. Electron paramagnetic resonance (EPR) of Cr complexes and in situ variable-temperature EPR analysis demonstrated Cr(V) intermediate (g = 1.978) generation in Cr(VI) reduction and its oxidization for BPA degradation in photocatalysis. By adding the electron donor Na₂SO₃, BPA degradation was induced in Cr(VI)/BPA solution, further confirming the positive effect of Cr(V). Moreover, the difference in BPA degradation products in the BPA/air, Cr(VI)/BPA/air, and Cr(VI)/BPA/Ar systems indirectly explained why the Cr(V) intermediate was involved in BPA degradation. Density functional theory calculations revealed that photogenerated electrons can reduce the free energy (0.98 eV) of converting Cr(VI) into Cr(V), which can facilitate the subsequent Cr(V) oxidation step for BPA degradation. This work contributes to the exploration of the Cr(VI) reduction process and the synergistic removal of organic pollutants in Cr(VI)/organics systems.

Selective adsorption and separation of Cr(VI) by surface-imprinted microsphere based on thiosemicarbazide-functionalized sodium alginate

Cr(VI)-imprinting thiosemicarbazide-functionalized sodium alginate (IIPTSC@SA) microspheres were fabricated to achieve the good selective adsorption for Cr(VI) removal from aqueous solution containing multiple ions. The selective adsorption and reusability of IIPTSC@SA microsphere were intensively investigated. The results showed that the grafting conditions of thiosemicarbazide were optimized at 0.2 g of TSC at 60 °C for 5 h and the ion-imprinting conditions at 100 mL of 2 wt % glutaraldehyde at 60 °C for 6 h. IIPTSC@SA microspheres exhibited the maximum adsorption capacity of 252.5 mg/g and good selectivity to remove Cr(VI) from multi-ion coexisting water. The adsorption process complied with a monolayer adsorption by virtue of chemical interactions together with endothermic spontaneously. After eight adsorption-desorption, the adsorption capacity and selectivity of IIPTSC@SA microspheres remained relatively stable.

Photocatalytic reduction of Cr(VI) over cinder-based nanoneedle in presence of tartaric acid: Synergistic performance and mechanism

Cr(VI) is a common heavy metal ion, which will seriously harm human body and environment. Therefore, the removal of Cr(VI) has become an attractive topic. In this work, cinder was used as a raw material to synthesize a nanoneedle material: γ-(AlOOH@FeOOH) (γ-Al@Fe). The physicochemical properties of γ-Al@Fe were thoroughly characterized, and its effectiveness as a catalyst for photocatalytic reduction of Cr(VI) was evaluated. The results showed that Cr(VI) could be efficiently reduced by γ-Al@Fe in the presence of tartaric acid (TA) under visible light. The variable factors on the reaction were investigated in detail, and the results showed that under optimal conditions (γ-Al@Fe 0.4 g/L, TA 0.6 g/L, pH 2), Cr(VI) was completely reduced within 7 min. Besides, scavenger experiments and EPR proved that O₂^{• -} and CO₂^{• -} played a significant role in the photocatalytic reduction of Cr(VI). TA acts as a sacrificial agent to trap the holes and generate strong reducing free radicals: CO₂^{• -}. Dissolving O₂ could react with electrons to generate O₂^{• -}. This work discussed the performance and mechanism of photocatalytic reduction of Cr(VI) in detail, which provided a new idea for the resource utilization of solid waste and the treatment of heavy metal sewage.

Enhanced Cr(VI) removal from water using a green synthesized nanocrystalline chlorapatite: Physicochemical interpretations and fixed-bed column mathematical model study

Apatite-based minerals have attracted much attention in the remediation of heavy metal-contaminated environment. However, exploring apatite with efficient adsorption performance for inorganic oxyanions such as Cr(VI) remains a challenge. Herein, a novel nanocrystalline chlorapatite (ClAP) was promptly prepared by a green method using eggshell wastes as calcium source with the purpose to enhance Cr(VI) adsorption capability. The generated ClAP was characterized by XRD, SEM-EPMA, TEM, FTIR, and BET analyses. Batch and column experiments were subsequently carried out to explore the influencing factors, adsorption capacity and removal mechanism. Results showed that ClAP exhibited excellent stability and adsorption performance for Cr(VI) (63.47 mg g^-1), which was much greater than that of hydroxyapatite and most reported materials. The adsorption process was fitted well by the pseudo-second-order model and the Langmuir model. In fixed bed column experiments, a novel time-fractional derivative model exhibited much better suitability in interpreting the observed breakthrough curves of Cr(VI) than traditional models. Furthermore, the reusability of ClAP in column was evaluated. Results showed that the adsorption capacity maintained well after consecutively reused for five cycles. Studies of the effect of pH, as well as FTIR and XPS investigations indicated that Cr(VI) adsorption was mainly ascribed to electrostatic interactions and surface complexation, while the reduction of Cr(VI) to the low-toxicity Cr(III) also existed in the adsorption process. The ClAP adsorbent was also successfully used for Cr(VI) remediation from real wastewater. Hence, nanocrystalline ClAP can be a promising material for enhancing the elimination of oxyanion contaminants such as Cr(VI) from water.

A novel colloid composited with polyacrylate and nano ferrous sulfide and its efficiency and mechanism of removal of Cr(VI) from Water

Nano ferrous sulfide (n-FeS) colloids show an excellent performance in the application of remediation in situ soil and groundwater. However, due to the interfacial effect and high reactivity of the nano sized FeS, n-FeS easy to agglomerate, which reduces their remediation efficiency. In this study, a novel composite colloid was synthesized using polyacrylic acid salt (PAA) and n-FeS. The PAA-n-FeS colloid was used to remove Cr(VI) in water remediation, and its removal mechanism and efficiency were explored. The results showed that the hydrodynamic diameter of PAA-n-FeS ranged from 65.04-90.09 nm and the zeta potential was from -27 to -54 mV at pH varying from 4.5-9.0. PAA was coated on the surface of n-FeS, which improved the dispersibility and stability of n-FeS by increasing the steric hindrance and electrostatic repulsion between n-FeS particles. Moreover, the Cr(VI) maximum removal amount PAA-n-FeS was 432.79 mg/g, which was significantly higher than that of n-FeS (218.29 mg/g) and PAA (12.32 mg/g). The mechanism of PAA-n-FeS removal of Cr(VI) was mainly derived from its own reducibility. The reaction products were mainly Cr(OH)₃, Cr(III)-Fe(III), Cr₂O₃, and Cr₂S₃. This research not only finds a new stabilizer for preventing n-FeS agglomeration, but also provides a novel n-FeS composited colloid for promoting the practical application to Cr(VI) removal from water.

Adsorption Separation of Cr(VI) from a Water Phase Using Multiwalled Carbon Nanotube-Immobilized Ionic Liquids

Three types of multiwalled carbon nanotubes (MWCNTs, MWCNTs-OH, and MWCNTs-COOH) were used as carriers and five types of ionic liquids (ILs) were immobilized on each carrier by an impregnation method. Boehm titration, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, specific surface area analysis by the Brunauer-Emmett-Teller method, and thermogravimetric analysis were performed to investigate [C4mim]HSO₄ adsorption by the MWCNTs. The MWCNT-immobilized IL was used for Cr(VI) removal from a water phase. The adsorption properties of MWCNTs-COOH-immobilized [C4mim]HSO₄ were investigated by single-factor analysis. The results showed that the Cr(VI) removal rate was 52.14% and the adsorption capacity was 31.29 mg/g. The optimum adsorption conditions were as follows: initial Cr(VI) concentration, 60 mg/L; adsorbent dosage, 50 mg; pH 2.0; adsorption temperature 40 °C; and adsorption time, 200 min. Adsorption isotherm data fitted the Freundlich model, which indicates that the adsorption process was in line with the multimolecular layer adsorption theory. The Cr(VI) adsorption behaviors of the three adsorbents were consistent with a pseudo-second-order dynamic model. Thermodynamic analysis of the reaction systems was also performed. The Cr(VI) removal rates of MWCNTs-3, MWCNTs-OH-3, and MWCNTs-COOH-3 were 27.97, 9.39, and 7.34% lower than the initial removal rates after five cycles.