Cr Reduction Rate Research Articles

Effective prevention of charge trapping in red phosphorus with nanosized CdS modification for superior photocatalysis

The hydrothermal-treated red phosphorus (HRP) is a visible light response element semiconductor. However, photoelectrons and holes to recombine easily due to its narrow band gap. A CdS/HRP hierarchical heterostructure was designed. The CdS nanoparticle was uniformly dispersed on the surface of HRP, and the modification ameliorated the overgrowth, agglomeration and shortened the carrier migration distance of HRP, thereby significantly enhancing the photoreduction rate for Cr(VI). The strongest photocatalytic activity of the 1%CdS/HRP composite was obtained when the mass fraction of CdS was 1%. The reduction rate of Cr(VI) was 1.4 × 10−1 min−1, which was 17.5 times that of HRP. After five cycles of photoreduction experiments, 1%CdS/HRP still possessed high photocatalytic activity (91.7%). The lower PL intensity of 1%CdS/HRP indicated the lower recombination rate of the photoelectrons and holes. The intensity of the transient photocurrent signal was significantly higher after the compounding of CdS and HRP, indicating the increase in the number and density of photogenerated carriers. A series of experimental results showed that the construction of heterojunction between CdS and HRP contributed to the intimate interfacial contact, which was beneficial for charge separation and transfer, and improved the photocatalytic performance of the catalyst.

Controlled preparation of magnetically separable BiOBr/BiFeO3 heterostructures by chemical etching process with enhanced visible light photocatalytic activities and anti-photocorrosion

ABSTRACT Novel magnetically separable BiOBr/BiFeO3 heterostructures were fabricated via a facile in situ hydrothermal chemical etching process using cetyltrimethyl-ammonium bromide (CTAB) and sulphuric acid as etching agents to react with BiFeO3 particles. The effects of solution pH and CTAB amount on the structure, morphology, and optical properties of the as-prepared samples were investigated. It was found that the best acidity of solution is pH = 3.0 for the formation of BiOBr/BiFeO3 heterostructures from BiFeO3 powder. Their photocatalytic activities and anti-photocorrosion behaviour were measured by the photocatalytical reduction of heavy metal ion Cr (VI) under visible-light irradiation. Experimental results show that the molar ratio of BiOBr to BiFeO3 contributed to different morphologies and photocatalytic activities of BiOBr/BiFeO3 composites. The optimised molar ratio in BiOBr/BiFeO3 composites was found to be 1:1. The photocatalytic reduction rate of Cr(VI) over 1:1 BiOBr/BiFeO3 composites was found to be 4.9 and 3.0 times higher than that of pure BiFeO3 and BiOBr, respectively. The hybridisation of BiOBr with BiFeO3 can markedly inhibit the photocorrosion of semiconductor BiFeO3. No obvious catalytic activity decrement can be found after five consecutive photocatalytic Cr (VI) reduction, which demonstrates the high structural stability of 1:1 BiOBr/BiFeO3. Furthermore, a possible photogenerated charge transfer process was proposed. The obviously enhanced photocatalytic activity could be ascribed to the formation of BiOBr-BiFeO3 heterojunction, accelerating the effective separation of photoexcited electron−hole pairs. This study suggests that the magnetically separable BiOBr/BiFeO3 heterostructures can be a promising photocatalyst for environment-related applications.

Well-designed ZnIn2S4/TiO2 Z-type heterojunction for efficient photocatalytic reduction of Cr(VI)

ZnIn2S4/TiO2 photocatalyst was obtained by a facile hydrothermal method. Various techniques were used to characterize the ZnIn2S4/TiO2, crystal structure and optical properties of ZnIn2S4/TiO2. Cr (Ⅵ) as highly-toxic pollutant was used as the target reduction product to evaluate the catalytic performance of ZnIn2S4/TiO2 under visible light irradiation. According to the experiment results, the reduction rate of Cr(VI) in the presence of ZnIn2S4/TiO2 reaches 99% within 60 min, which is much better than ZnIn2S4 and TiO2, respectively. At the same time, ZnIn2S4/TiO2 also performs good stability for reduction rate hardly changes after 5 recycling experiments.

Ag NPs decorated C–TiO2/Cd0.5Zn0.5S Z-scheme heterojunction for simultaneous RhB degradation and Cr(VI) reduction

In this study, heterojunction photocatalysts, XAg@C-TCZ, based on MOF-derived C–TiO2 and Cd0.5Zn0.5S decorated with Ag nanoparticles (Ag NPs) were successfully synthesized through hydrothermal and calcination methods. The catalytic effectiveness of XAg@C-TCZ was evaluated by simultaneous photocatalytic degradation of rhodamine B (RhB) and reduction of Cr(VI) under simulated sunlight irradiation. The presence of the Z-scheme heterojunction was demonstrated through trapping experiments, X-ray photoelectron spectroscopy (XPS), time-resolved photoluminescence (PL) investigations, and electron spin resonance (ESR) spectroscopy. With an initial RhB and Cr(VI) concentration of 7 mg L−1 and 5 mg L−1, the catalyst 10Ag@C-TCZ achieved a simultaneous removal of 95.2% and 95.5% within 120 min, respectively. With the same catalyst, the degradation rate of RhB was 2.75 times higher and the reduction rate of Cr(VI) was 9.3 times higher compared to pure Cd0.5Zn0.5S. Total organic carbon (TOC) analysis confirmed the extent of mineralization of RhB, while the reduction of Cr(VI) was corroborated by XPS. Compared to pure RhB and Cr(VI) solutions, the reaction rates are smaller in the solution containing both contaminants, which is attributed to the competition for ·O2−. 10Ag@C-TCZ also exhibited a stable catalytic performance in tap water and lake water. This work provides a new perspective on the construction of heterojunctions with doped MOF derivatives for the purification of complex pollutant systems.

UV-Vis-NIR-light-driven Ag2O/Ag2S/CuBi2O4 double Z-scheme configuration for enhanced photocatalytic applications

The Ag2O/Ag2S/CuBi2O4 double Z-scheme heterostructures photocatalysts with different Ag2O proportion were successfully synthesized via the hydrothermal and coprecipitation methods. Compared with those of pure CuBi2O4 and Ag2S, the absorption of near-infrared light of Ag2O/Ag2S/CuBi2O4 samples is enhanced, their photocurrent intensity is significantly increased, and the charge transfer resistance is reduced. Obviously, all the Ag2O/Ag2S/CuBi2O4 samples exhibited significantly enhanced photoactivities under the UV, Vis or NIR light irradiation. Of them, under the simulated sunlight, Ag2O/Ag2S/CuBi2O4 with 36.7 wt% Ag2O showed the highest catalytic activity with RhB degradation rate of 99.2% and the reduction rate of Cr(VI) of 53%. The construction of double Z-scheme heterojunction reduced the recombination of electrons and holes and improved the separation efficiency of electron. Moreover, Ag2S and Ag2O broadened the photoresponse range to a wide spectrum, and then improved the photocatalytic efficiency. In addition, the stability of Ag2O/Ag2S/CuBi2O4 containing 36.7 wt% Ag2O was demonstrated through three successive cycling experiment.

Remediation of Cr(VI)-contaminated soil using combined chemical leaching and reduction techniques based on hexavalent chromium speciation

Hexavalent chromium [Cr(VI)] has strong mobility and it can enter into deep regions of soil. Cr(VI)-contaminated soil remediation is the process of removing Cr(VI) present in deep soils and any residual Cr(VI). In this study, the Cr(VI)-contaminated soil in Chongqing was investigated, and the remediation and economic feasibility of chemical leaching and reduction combined with a soil repairing approach was explored. The results showed that the leaching reagent, liquid-solid ratio, leaching time, reduction agent dosage, reduction temperature and reduction time had significant (P < 0.05) effects on the remediation of Cr(VI). At 0.02 mol/L oxalic acid and citric acid using a liquid-solid ratio of 5:1 and leaching time of 45 min, the removal rate of Cr(VI) was 62.7%, the residual Cr(VI) in soil was 126 mg/kg, and the soil pH was 4.09 after leaching. Between 25 and 90 °C, and at a molar ratio of 25:1 of FeSO4•7 H2O to Cr(VI), the reduction rate of Cr(VI) in soil after reduction was 54.0–98.4%, and the leaching concentration of Cr(VI) in soil was 0.01–0.29 mg/L. The optimal reduction was at 90 °C for 60 min, resulting in only 2.7 mg/kg of residual Cr(VI) in soil. The cost of this technology to treat the area studied was 826 ¥/ton of soil, which represents an economically feasible method for Cr(VI)-contaminated soil remediation.

Study on the oxidative stress and transcriptional level in Cr(VI) and Hg(II) reducing strain Acinetobacter indicus yy-1 isolated from chromium-contaminated soil

The bioreduction of Cr(VI) and Hg(II) has become a hot topic in the field of heavy metals bioremediation. However, the mechanism of antioxidant stress in Cr(VI) and Hg(II) reducing bacteria is still not clear. In this work, a novel Cr(VI) and Hg(II) reducing strain Acinetobacter indicus yy-1, was isolated from chromium landfill at a chromate factory, which was used to investigate the mechanism of antioxidant stress during the Cr(VI) and Hg(II) reduction process. The results demonstrated that the removal of Cr(VI) and Hg(II) by A. indicus yy-1 from solution was through reduction rather than biosorption. The reduction rates of Cr(VI) and Hg(II) by resting cells reached 59.71% and 31.73% at 24 h with initial concentration of 10 mg L−1, respectively. X-ray photoelectron spectroscopy (XPS) analysis further showed that Cr(III) and Hg(0) were mainly the Cr(VI)- and Hg(II)-reduced productions, respectively. Results of physiological assays showed Hg(II) was more toxic to A. indicus yy-1 than Cr(VI), and the activities of antioxidant enzymes (SOD and CAT) were significantly increased in A. indicus yy-1 for relieving the oxidative stress. The transcriptional level of genes related to Cr(VI) and Hg(II) reductases and antioxidant enzymes were up-regulated, indicating that the reductases have participated in the reduction of Cr(VI) and Hg(II), and SOD and CAT served as the vital antioxidant enzymes for defending the oxidative stress. This work provides a deep insight into the mechanism of antioxidant stress in Cr(VI) and Hg(II) reducing bacteria, which helps seek the highly resistant heavy metal reducing bacteria.

A new method of Cr(VI) reduction using SiC doped carbon electrode and Cr(III) recovery by hydrothermal precipitation

This work devoted to detoxify and recover chromium effectively from highly toxic wastewater containing Cr(VI). A novel method was proposed to electroreduce Cr(VI) to Cr(III) in acid solution by SiC doped carbon electrode in a membrane-free reaction device and to recover solid chromium by hydrothermal precipitation process. The results showed that 2H-SiC and 3C-SiC was distributed uniformly on the electrode surface. Chromium species existed mainly in the form of HCrO4− and Cr2O72- in the pH range of 1.5−3. Under 1.0–2.5 V, the reduction rate of Cr(VI) was 99.5 % within 70 min, and the reduction efficiency was about 20 % higher than that of non-SiC doped electrode. The main reason is that the redox ability of modified electrode was enhanced and the pseudo-capacitance was increased obviously. γ-CrOOH hydroxyl chromium was the main chromium species obtained from hydrothermal precipitation in acid conditions. The combined method presents certain industrial application potential in detoxification and recovery of Cr(VI) wastewater.

Simultaneous removal of ceftriaxone sodium and Cr(VI) by a novel multi-junction (p-n junction combined with homojunction) composite photocatalyst: BiOI nanosheets modified cake-like anatase-rutile TiO2

In this study, we synthesized a multi-junction composite photocatalyst that combined the advantages of homo- and heterojunctions. This multi-junction structure facilitated the separation of electrons and holes through its multi-step energy level structure. Its photocatalytic performance for simultaneous removal of ceftriaxone sodium and Cr(VI) was investigated. Its photocatalytic performance for simultaneous removal of ceftriaxone sodium and Cr(VI) was investigated. The degradation rate of ceftriaxone sodium decreased slightly and the reduction rate of Cr(VI) greatly increased (by 7.4 times). Through the capture experiments, we found that the main active species for ceftriaxone sodium degradation were h+ and ·O2‐−. When ceftriaxone sodium and Cr(VI) were treated simultaneously, electrons reduced Cr(VI), which decreased the generation of ·O2− and the degradation efficiency of ceftriaxone sodium. Simultaneous removal of ceftriaxone sodium and Cr(VI) effectively increased the utilization of photogenerated electron holes. Finally, the degradation pathway of ceftriaxone sodium was inferred by the main intermediates in degradation process. This work is an example of the construction of composite photocatalysts with high performance for the treatment of wastewater coexisting with organic and heavy metal pollutants.

Adsorption-photocatalysis synergistic removal of contaminants under antibiotic and Cr(VI) coexistence environment using non-metal g-C3N4 based nanomaterial obtained by supramolecular self-assembly method

Due to the rapid development of modern industry, the coexistence of antibiotics and inorganic heavy metals pollutants in wastewater has become a universal phenomenon. Therefore, developing efficient and eco-friendly photocatalyst for mixed pollutants degradation is significant. In this work, a well-designed phosphorus and sulfur co-doped g-C3N4 with feeble N vacancies catalyst (P/S-g-C3Nx) was fabricated by supramolecular self-assembly method, and was applied to remove berberine hydrochloride (BH) and Cr(VI) simultaneously with the synergy of adsorption-photocatalysis. A series of experiments was conducted to unveil the synergistic mechanism. The kinetic models indicated that the adsorption of P/S-g-C3Nx improved the BH removal process by accelerating the photo-degradation, because the adsorption rate > surface degradation rate > bulk degradation rate. Besides, the photo-degradation process improved the BH removal rate by regenerating the adsorption sites of P/S-g-C3Nx. Moreover, from the experiments in BH-Cr(VI) mixed solution system, the existence of BH also enhanced the surface adsorption of Cr(VI) in P/S-g-C3Nx sample, and the reduction rate of Cr(VI) was also promoted with the existence of BH. Overall, the results of this investigation suggest that the adsorption-photocatalysis synergy method is an efficient way to eliminate organic pollutant and Cr(VI) simultaneously.

Effects of Cr(VI)-reducing bacteria on the behaviour of Cr(VI) adsorption by goethite and haematite: speciation and distribution

In the soil environment, the existence of Cr(VI)-reducing microorganisms might affect the adsorption, desorption and reduction of Cr(VI) adsorbed on soil minerals (such as Fe oxides). The behaviour of Cr(VI)-reducing microorganisms might affect the adsorption and reduction of Cr(VI) by soil minerals. Goethite and haematite with saturated adsorbed Cr(VI) were incubated with Microbacterium sp. QH-2, a Cr(VI)-reducing bacteria. Scanning electronic microscopy (SEM) and X-ray diffraction (XRD) were used to detect the changes in goethite and haematite. X-ray absorption near-edge structure (XANES) was performed to detect the changes in Cr(III) and Cr(VI) on goethite and haematite. Strain QH-2 adhered to the surfaces of goethite and haematite. No morphological changes in goethite and haematite were detected after incubation. No secondary Fe minerals formed. Cr(III) was the dominant species of Cr on goethite and haematite (78.5% for goethite and 96.7% for haematite) after incubation. Furthermore, the reduction rate of Cr(VI) by strain QH-2 in the liquid phase was faster than that of Cr(VI) adsorbed on goethite and haematite. The existence of strain QH-2 could promote the adsorption of Cr by goethite and haematite. Strain QH-2 could affect the morphological distribution and transformation of Cr in Fe oxides such as goethite and haematite.

Pyrolysis-temperature depended electron donating and mediating mechanisms of biochar for Cr(VI) reduction

Biochar could be involved in environmentally relevant redox reactions, and their redox-active moieties may change with pyrolysis temperature. In this study, pyrolysis-temperature depended electron donating and mediating ability of biochar for Cr(VI) reduction were evaluated. All biochar derived from peanut shell at 400−800 °C effectively reduced Cr(VI) into Cr(III), and the reduction capability decreased as the pyrolysis temperature increased (400−600 °C), and then increased (600−800 °C). The electron donating moieties transformed from the −OH functional groups at lower pyrolysis temperature (<600 °C) to the functional groups associated with conjugated structure at higher temperature (>600 °C). Biochar could mediate the reduction of Cr(VI) by lactate, with the reduction rates of Cr(VI) increased up to 16.3 and 345 times that by either biochar or lactate alone, respectively. The redox ability of biochar was the premise, but its conductivity was the dominant factor, for the mediating reduction of Cr(VI). The mediation capability of biochar increased with elevated pyrolysis-temperature due to its increased conductivity related to the growth of conjugated clusters and carbon defects. Our results indicated that pyrolysis-temperature affects the formation of functional groups and conjugated carbon structure of biochars which have a distinguishable influence on the electron donating and mediating ability for Cr(VI) reduction.

A study of the treatment of high-salt chromium-containing wastewater by the photocatalysis-constructed wetland combination method.

In this study, iron ore slag as the photocatalyst was introduced into a constructed wetland simulation system. A comparative experiment of the constructed wetland method and photocatalysis-constructed wetland combination method that treats the high-salt chromium-containing wastewater was carried out. The best hydraulic retention time (HRT) of the photocatalysis-constructed wetland combination system was studied. The effects of these two methods on biochemical oxygen demand (BOD5), chemical oxygen demand (COD) removal and Cr(VI) reduction rate of the high-salt chromium-containing wastewater were analysed after 14 periods. The results showed that under the optimal HRT of 4 hours, the COD and BOD5 of the wastewater reduced by 47% and 31%, and the reduction rate of Cr(VI) was 83% separately in the constructed wetland system. The COD and BOD5 of the wastewater reduced by 83% and 42%, and the reduction rate of Cr(VI) was 96% separately in the photocatalysis-constructed wetland combination method system. At the same time, the changes in plant parameters under these two systems were studied, and the results showed that the addition of photocatalyst and hydrogen peroxide to constructed wetlands did not affect the normal indicators of plant growth. The results showed that the photocatalysis-constructed wetland combination method not only reduced the treatment time greatly, but also improved the quality of the treated wastewater significantly.

MON-098 Losses of Amino Acid in the Pre- and Post-dilution of On-line HDF under the Same Reduction Rate of β2-microglobulin

We reported that amino acid loss occurred during pre-dilution on-line hemodiafiltration (Pre-HDF) was significantly less than those of hemodialysis. Since the amino acid kinetics during HDF therapy has not clearly been understood, we compared the amino acid losses that occur on performing Pre-HDF and post-dilution on-line HDF (Post-HDF) under the same reduction rate of β2-microglobulin (β2-MG) in the present study. We compared the total amino acid, total non-essential/essential/branched-chain amino acid amount into the total waste fluid, reduction rate of Urea, Cr and β2-MG and Kt/V (urea) between 9 patients undergoing Pre-HDF (7 males, 4 diabetic, mean age: 72.4±2.1 years) and the same patients receiving Post-HDF. The mean blood flow rate in the former and latter was 222±25 and 200±0 mL/min, respectively. The dialysate flow rate was 329±49 and 556±0 mL/min, respectively. The replacement fluid flow rate was 251±27 and 44±0 mL/min, respectively. The replacement fluid volume was 57±6 and 10±0 L, respectively. The reduction rate ofβ2-MG was same between the Pre-HDF and Post-HDF group (Pre-HDF: 80.9±4.0%, Post-HDF: 80.4±3.7%, P=0.545). In the Pre-HDF group, the total and total non-essential amino acid losses (4814.3±1055.6 and 3058.4±632.0 mg, respectively) were not significantly different from in the Post-HDF group (5257.3±698.9 and 3421.9±446.8 mg, respectively) (P=0.180 and 0.116, respectively). In the former, the total essential amino acid (1755.9±503.3 mg) was not significantly different from in the latter (1835.3±351.0 mg) (P=0.401) and also in the former, the branched-chain amino acid (780.2±224.2 mg) was not significantly in the latter (816.4±210.1 mg) (P=0.139). In the Pre-HDF group, the urea and Cr reduction rates were 69.5±7.4 and 63.2±6.7%, respectively. In the Post-HDF group, the values were 70.9±4.4 and 64.8±3.9%, respectively. The values showed no significant differences between the Pre-HDF and Post-HDF group (Urea: P=0.354, Cr: P=0.309). The Kt/V (urea) values in the former and latter were 1.46±0.28 and 1.45±0.19, respectively; there was no significant difference (p=0.862). Under the same reduction rate of β2-MG, the moderate volume Post-HDF (replacement fluid volume: around 10.0 L) is as favorable as the Pre-HDF as a blood purification method from the viewpoint of nutrition.

Interaction with low molecular weight organic acids affects the electron shuttling of biochar for Cr(VI) reduction.

Biochar can act as "electron shuttle" in soil redox reactions. It is possible that biochar accepts the electrons from low molecular weight organic acids (LMWOAs) in soil and then transfer them to the acceptors, e.g., Cr(VI). This study evaluated the interaction between seven soil LMWOAs and peanut shell biochar (BC) as well as its effect on the electron shuttling of biochar for Cr(VI) reduction. Both redox reactions and sorption process occurred during the interaction of biochar and LMWOAs, which altered the contents of Cr(VI) reduction-relevant groups (i.e., CO and CO) on the surface of biochar. The redox reactions were more important to the electron transfer between biochar produced at 400℃ (BC400) and LMWOAs due to the repeated cycle of reduction-oxidation of surface functional groups. The reduction rate of Cr(VI) by LMWOAs mediated by BC400 was 1.10-7.09 × 10-3 h-1, among which tartaric acid had the best reduction efficiency due to its highest reducing capability. For biochar produced at 700℃ (BC700), the sorption process of LMWOAs was the key factor to the direct electron shuttling process through the conjugated structure of biochar. The reduction rate of Cr(VI) by LMWOAs mediated by BC700 was significantly higher and ranged 7.40-864 × 10-3 h-1, with the oxalic acid having the best reduction efficiency due to its highest sorption capacity by BC700. The results obtained from this study can help to establish the linkage between biochar and LMWOAs in soil electron network, which better explains the multifunctional roles of biochar during the redox processes such as Cr(VI) reduction in soil.

A green approach to the microwave-assisted synthesis of flower-like ZnO nanostructures for reduction of Cr(VI)

Zinc oxide nanostructures with different morphologies were prepared via a microwave-assisted method and used as photocatalysts for reduction of chromium(VI) under ultraviolet-B illumination. X-ray diffraction revealed that as-made ZnO samples have wurtzite hexagonal structure. Scanning electron microscopy demonstrated that the morphology of the ZnO crystals could be easily modified from star-shaped to chrysanthemum flower-like structure by changing the ratio of zinc nitrate and sodium hydroxide. X-ray photoelectron spectroscopy and energy dispersive X-ray analysis confirmed the stoichiometry of pure ZnO. Its optical properties were also investigated by UV-Vis diffuse reflectance spectroscopy, which indicated that the prepared ZnO possessed a band gap value of 3.2 eV. Photocatalytic activity studies revealed that the ZnO (1:6) (1 × 10−3 mol zinc nitrate: 6 × 10−3 mol sodium hydroxide) catalyst exhibited a significantly faster reduction rate of Cr(VI) compared to ZnO (1:5) and ZnO (1:4). More than 95% of Cr(VI) at a concentration of 10 mg/L has been reduced to Cr(III).

Removal behavior of Cu(II) during Cr(VI) reduction by cast iron powder in absence and presence of ultrasound

ABSTRACTCu(II) is an important and typical heavy metal ion in the wastewater containing Cr(VI), and its removal during Cr(VI) reduction by zero valent iron (ZVI) may make it separately be recovered as a kind of copper resource. In this study, the removal behavior of Cu(II) during Cr(VI) reduction by cast iron powder in absence and presence of ultrasound was investigated by atomic absorption spectrometry (AAS), X-ray powder diffractometer (XRD), scanning electron microscope-energy dispersion spectrum (SEM-EDS) and X-ray photoelectron spectroscopy (XPS). The AAS tests indicated that the ultrasound could not only obviously enhance the removal of Cu(II) but also improve the reduction rate of Cr(VI). The XRD and SEM-EDS analyses showed that Cu(II) in the solution was reduced to metallic copper and then was deposited at the surface of ZVI. The ultrasound could remove the Fe-Cr oxides and hydroxides at the ZVI surfaces, resulting in the active surfaces of iron increased. The XPS analyses demonstrated that the surface of metallic copper would be transformed into the film of copper oxide (CuO) in the ultrasound system. The obtained metallic copper and copper oxide could be recovered alone by traditional method of the acid pickling.

Mechanism and modeling of hexavalent chromium interaction with a typical black soil: the importance of the relationship between adsorption and reduction.

Black soils have a significant retention effect on the migration of Cr(vi) towards groundwater, and Cr(vi) adsorption and reduction are both involved in this process. However, the adsorption and reduction of Cr(vi) were always investigated separately in previous studies resulting in an unclear relationship between them. In this study, the adsorption and reduction kinetic processes of Cr(vi) by a typical black soil were separately investigated under different initial Cr(vi) concentrations (40–400 mg L−1) and pH conditions (3.5–7.0) by the means of desorption treatment, and the equilibrium relationship between aqueous and adsorbed Cr(vi) was innovatively established based on the kinetic data. It was found that under pH 5.7 the adsorbed Cr(vi) content on soil particles was linearly correlated with the remaining Cr(vi) concentration in solution with time (R2 = 0.98), and the reduction rate of Cr(vi) in the reaction system was linearly correlated with the adsorbed Cr(vi) content on soil particles with time (R2 = 0.99). With pH decreasing from 7.0 to 3.5, the partition of Cr(vi) between solid and aqueous phases turned out to be of a non-linear nature, which can be fitted better by the Freundlich model. The retention of Cr(vi) by black soil was determined to follow the “adsorption–reduction” mechanism, where the Cr(vi) was first rapidly adsorbed onto the soil particles by a reversible adsorption reaction, and then the adsorbed Cr(vi) was gradually reduced into Cr(iii). A two-step kinetic model was developed accordingly, and the experimental data were fitted much better by the two-step adsorption–reduction kinetic model (R2 = 0.89 on average) compared with the traditional first-order and second-order kinetic models (R2 = 0.66 and 0.76 on average respectively). This paper highlights the novel two step kinetic model developed based on the proposed “adsorption–reduction” mechanism of Cr(vi) retention by a typical black soil.

Biochar as both electron donor and electron shuttle for the reduction transformation of Cr(VI) during its sorption

Biochar has been demonstrated to be a promising sorbent and its redox activity can be involved in environmentally relevant redox reactions. In this study, the electron transfer for the reduction transformation of Cr(VI) during its sorption by biochar were evaluated. Biochar derived from peanut shell at 350 °C could effectively remove Cr(VI) from solutions, accompanied by the reduction of Cr(VI) to Cr(III) which was more obvious at the strong acidity (pH = 2), compared to that at the weak acidity (pH = 4). The O-containing functional groups, e.g. CO and CO, were the electron donor moieties of biochar for the Cr(VI) reduction. Biochar could also act as electron shuttle, enhancing Cr(VI) reduction by lactate, especially at the weak acidity (pH = 4) where the reduction rates of Cr(VI) by lactate together with biochar were about 2 and 9 times higher than those by either biochar or lactate alone, respectively. The mediated electrochemical oxidation analysis showed that biochar could increase the electron donating capacity of lactate by up to 23 times at pH = 4. The O-centered radicals, e.g. semiquinone-type radicals, were related with biochar as electron shuttle, which was confirmed by the electron paramagnetic resonance analysis. As a result of electron acceptance from lactate and/or biochar, Cr(VI) was reduced into CrOOH evidenced by X-ray diffraction analysis. Our results indicated that biochar could act as both electron donor and electron shuttle for the reduction of Cr(VI) during the sorption process, making it an alternative for removal of toxic Cr(VI) from wastewaters.

Nano-sized ZnO supported on poly(triazine imide) nanotube for visible light driven photocatalytic reduction of Cr(VI)

The poly(triazine imide) (PTI) nanotube/ZnO heterojunction was prepared by two step method. PTI nanotube was synthesized from the condensation of melamine in a salt melt of LiCl–KCl. Then, nano-sized ZnO supported on PTI nanotube was prepared by a chemical precipitation method. The reduction rate of Cr(VI) under visible light irradiation was used to evaluate the activity of photocatalyst. It was found that the PTI/ZnO-1.5 wt% heterojunction exhibits the highest photocatalytic activity, which can reduce almost all Cr(VI) within 120 min. The kinetic constant of reduction reaction with PTI/ZnO-1.5 wt% heterojunction (0.0291 min− 1) is about 6 times as high as that of the PTI (0.00443 min− 1). A possible photocatalytic mechanism was discussed on the basis of the energy-band theory and scavenger experiments. Results show that photo-induced electrons participated in the reduction of Cr(VI) solution over the PTI/ZnO heterojunction. The enhanced photocatalytic activities of heterojunction can be attributed to efficient charge separation and good interfacial contact between PTI and ZnO.