The role of thiocyanate in enhancing the process of sulfite reducing Cr(VI) by inhibiting the formation of reactive oxygen species

An improved process for removal and recovery of heavy petroleum from solids using a ferrate-based hybrid oxidant

Persulfate oxidants are widely used in soil remediation and wastewater treatment but perform poorly in degrading polycyclic aromatic hydrocarbons (PAHs), especially heavy fractions in solids. Herein, we propose the utilization of a green peroxymonosulfate-ferrate-FeS (PFI) oxidant as a promising process aid for remediating soils contaminated with heavy petroleum components, including asphaltenes and resins. The PFI oxidant could degrade heavy petroleum fractions because of dual activation of the peroxymonosulfate and ferrate by FeS at ambient conditions. Nevertheless, when dealing with soil with high oil content (>10%), the degradation efficiency remains limited (<30%) regardless of the quantity of oxidants employed. Surface elemental analysis shows that a coating of secondary products (Fe(OH)3, Fe2O3) on the surface and in pores of the soil-pollutant matrix explains the failure of oxidation and inefficient use of oxidant. To address this issue, a strategy of pre-solvent extraction-oxidation hybrid process with sequent acidic washing is proposed, where dichloromethane serves as the solvent, and PFI acts as the oxidant. In this system over 90% of the oil could be recovered with an oxidation efficiency of 80% by alleviating the problem of iron oxide coating the matrix surface. The oxidant consumption is also reduced to 70 wt% of the sludge. The PFI oxidant is found to exhibit excellent universality in treating oily sludge with low petroleum content (<2%), reducing the petroleum content in the residue to less than 0.3 wt% (meeting the national standards). The degradation of low oil content sludge by the PFI oxidant followed pseudo first-order kinetics. These findings not only elucidate the failure of PFI oxidation for high oil content oily sludge and identify its potential engineering application range, but also offer a practical strategy for processing petroleum-contaminated soil with varying oil contents through wet oxidation.

Generation and engineering applications of sulfate radicals in environmental remediation

Sulfate radical (SO4•-)-based advanced oxidation processes (AOPs) have become promising alternatives in environmental remediation due to the higher redox potential (2.6-3.1 V) and longer half-life period (30-40 μs) of sulfate radicals compared with many other radicals such as hydroxyl radicals (•OH). The generation and mechanisms of SO4•- and the applications of SO4•--AOPs have been examined extensively, while those using sulfite as activation precursor and their comparisons among various activation precursors have rarely reviewed comprehensively. In this article, the latest progresses in SO4•--AOPs were comprehensively reviewed and commented on. First of all, the generation of SO4•- was summarized via the two activation methods using various oxidant precursors, and the generation mechanisms were also presented, which provides a reference for guiding researchers to better select two precursors. Secondly, the reaction mechanisms of SO4•- were reviewed for organic pollutant degradation, and the reactivity was systematically compared between SO4•- and •OH. Thirdly, methods for SO4•- detection were reviewed which include quantitative and qualitative ones, over which current controversies were discussed. Fourthly, the applications of SO4•--AOPs in various environmental remediation were summarized, and the advantages, challenges, and prospects were also commented. At last, future research needs for SO4•--AOPs were also proposed consequently. This review could lead to better understanding and applications of SO4•--AOPs in environmental remediations.

One-step removal of hexavalent chromium in wide pH range using thiourea dioxide: the role of reactive species

One-step removal of hexavalent chromium in a wide pH range is of great significance. In this paper, a single thiourea dioxide (TD) and two-components thiourea dioxide/ethanolamine (MEA) were used as a green reducing agent for the efficient removal of Cr(vi), respectively. The reduction of Cr(vi) and the precipitation of Cr(iii) were carried out simultaneously under this reaction system. The experimental results proved that TD was activated by amine exchange reaction with MEA. In other words, MEA promoted the generation of an active isomeride of TD by changing the equilibrium position of the reversible reaction. After adding MEA, the removal rate of Cr(vi) and total Cr could reach industrial water discharge standards in a wide pH range of 8-12. The change of pH, reduction potential and the decomposition rate of TD were investigated in the reaction processes. Meanwhile, reductive and oxidative reactive species were produced simultaneously during this reaction process. Further, oxidative reactive species (O₂˙^- and ¹O₂) were beneficial for the decomplexation of Cr(iii) complexes and the formation of Cr(iii) precipitation. The experimental results also demonstrated that TD/MEA was effective in practical industrial wastewater. Hence this reaction system has a significant industrial application prospect.

In Situ Reconstruction of Active Catalysis Sites Triggered by Chromium Immobilization for Sulfite Oxidation

Hexavalent chromium (Cr(VI)) is a highly toxic substance in wastewater, triggering grievous detriment to aquatic life and human health. Magnesium sulfite is spawned along with the desulfurization process in coal-fired power plants, which is usually disposed of as solid waste. Here, a "waste control by waste" method was proposed upon the redox of Cr(VI)-sulfite, in which highly toxic Cr(VI) is detoxicated and sequent enriched on a novel biochar-induced cobalt-based silica composite (BISC) due to the forced electron transfer from chromium to surface hydroxyl. The immobilized Cr on BISC gave rise to the reconstruction of catalytic active sites "Cr-O-Co", which further enhance its performance in sulfite oxidation by elevating O₂ adsorption. As a result, the sulfite oxidation rate increased by 10 times compared with the non-catalysis benchmark together with the maximum chromium adsorption capacity being 120.3 mg/g. Therefore, this study provides a promising strategy to simultaneously control highly toxic Cr(VI) and sulfite, achieving high-grade sulfur resource recovery for wet magnesia desulfurization.

Fabrication of 3D self-supported MoS2-Co-P/nickel foam electrode for adsorption-electrochemical removal of Cr(Ⅵ)

Electrochemistry is considered to be one of the most efficient and environment-friendly methods for removing highly toxic Cr (Ⅵ). In this study, a 3D self-supported MoS₂-Co-P/nickel foam (NF) electrode was prepared via a calcination-hydrothermal process to remove the Cr (Ⅵ) in aqueous medium. Scanning electron microscope (SEM) analysis indicated that the pine-needle-like Co₂P nanoneedle and flower-like MoS₂ nanosheets were successfully loaded on the three-dimensional (3D) framework of NF, which provided abundant active sites. The electrode modified by Co, P and MoS₂ exhibited high removal efficiency of Cr (Ⅵ) (96.9%) at pH 3.0, current of 0.128 mA and voltage of 2.5 V. Co, P and MoS₂ have the synergistic promotion on the catalytic performance of electrodes, and the reduction efficiency of Cr (Ⅵ) was greatly improved by 127.5 times relative to pure NF. The enhanced removal of Cr (Ⅵ) was related to the coupling effect of adsorption and electrocatalytic reduction. The mechanism study indicated that electron (e^-) is the active species of Cr (Ⅵ) reduction. The Cr (Ⅵ) removal rate was maintained at 90 ± 1% after five successive cycle experiments, demonstrating good stability and potential industrial applications of MoS₂-Co-P/NF.

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.

Diphenamid photodegradation using Fe(III) impregnated N-doped TiO2/sulfite/visible LED process: Influence of wastewater matrix, kinetic modeling, and toxicity evaluation

Sulfite-based photocatalysis has been recently employed as a promising technology for the treatment of organic pollutants via the generation of reactive radicals. In this contribution, the effect of wastewater matrix constituents and toxicity evaluation were systematically investigated in the Fe^III impregnated N-doped TiO₂ (FeN-TiO₂)/sulfite/visible LED (Vis LED) process in the presence of diphenamid (DPA) as a model organic pollutant. The results showed that the presence of HCO₃^-, SO₄^2-, NO₃^-, and F^- had no detrimental effect on DPA degradation. Conversely, the presence of Cr(VI), NO₂^-, Cl^-, and Br^- caused a stronger retardation effect. The effect of natural organic matter such as humic acid (HA) was inert at normal concentrations. Interestingly, the retardation effect of inorganic ions can be quantified at any given ion concentration based on the linear correlations between the DPA decay (first-order kinetic constants) and concentration of ion species. Toxicity tests on Synechocystis sp., Microcystis flos-aquae, and Nostoc sp. algae revealed that higher toxicity was noticed at 240 min treatment time accompanied by lower toxicity with prolonging the treatment time for all selected algae except for Microcystis flos-aquae. In addition, novel two-phase mathematical models were successfully proposed to predict the accumulation of intermediates depending on their evolution profile.

Efficient reduction of Cr(VI) and immobilization of Cr driven by an iron-air fuel cell: Reaction mechanisms and electricity generation

The iron-air fuel cell (IAFC) has been successfully employed for the oxidative removal of many pollutants, but its feasibility for reductive immobilization of Cr(VI) is still unknown. Herein, we developed an IAFC system consisting of an iron anode and an activated carbon-PTFE based air-cathode, and evaluated its performance for Cr(VI) removal and power generation. In this reaction system, cathodic reduction and Fe(II) reduction both contributed to the reductive removal of Cr(VI). It was found that the decrease of solution pH from 6.0 to 3.0 promoted the removal of Cr(VI) due to the enhanced yield of Fe(II) ions and cathodic reduction, accompanying the increased power generation from 1040 mW m^-2 to 2880 mW m^-2. Besides, the Cr(VI) removal and power generation could be also promoted by elevating Na₂SO₄ concentration from 0.01 M to 0.1 M. In the IAFC process, Cr(VI) was initially reduced to less soluble ionic Cr(III) homogeneously and heterogeneously and then Cr(III) was immobilized by adsorption and/or co-precipitation with the fresh Fe(III) (oxy)hydroxides. Generally, this study is of great interest for the engineering community to design the environmentally benign and cost-effective strategy for the treatment of wastewater in remote areas, where the electricity is not easily available.

The enhanced effect of oxalic acid on the electroreduction of Cr(VI) via formation of intermediate Cr(VI)-oxalate complex

In this study, the enhanced effect of oxalic acid (Ox) on Cr(VI) electroreduction was evaluated. It was found that for Cr(VI)-contaminated solution ([Cr(VI)]₀ = 1.0 mM, pH = 3.0), addition of 5.0 mM Ox can significantly increase Cr(VI) reduction from 0.36 to 1.0 mM within 90 min electrolysis reaction, accompanying with the increase of current efficiency from 19% to 53%. Increasing initial Ox concentration (0-10 mM) and electric current (10-40 mA) facilitated Cr(VI) reduction, whereas it was inhibited with decreasing solution pH value (2.0-3.5) and elevating Cr(VI) concentration (0.1-2.0 mM), respectively. Results show that reactive electron was the primary reductant for the heterogeneous reduction of Cr(VI) on the cathode. In addition, Ox can also serve as an electron donor for the homogeneous reduction of Cr(VI). During this process, the formation of Cr(VI)-oxalate complex is indispensable for the enhanced Cr(VI) reduction. The coordination of Ox with Cr(VI) did not only make the structure of Cr(VI) more distorted but also improved the reactivity of Cr(VI) in Cr(VI)-oxalate complex toward reduction reaction. In general, this study provides an energy-efficient and environmentally benign strategy for the treatment of Ox and Cr(VI) co-contaminated wastewater.

Association of blood chromium and rare earth elements with the risk of DNA damage in chromate exposed population

Pollution of heavy metals often occurs in combination with multiple metal ions. Whether the genetic damage among chromate exposed population correlated with rare earth elements (REEs) was still not well elucidated. A total of 291 participants from a chromate production plant were recruited in the present study. The DNA oxidative damage was evaluated by urinary 8-hydroxydeoxyguanosine (8-OHdG) and the concentrations of chromium (Cr) and 15 REEs accumulated in the peripheral blood of participants were determined. The results showed that significant DNA oxidative damage was observed in chromate exposed workers. Blood REEs levels in the exposed group were significantly higher than the control group and blood REEs increased in a concentration dependent manner with Cr. Additionally, significant correlations were observed between blood Cr and 10 REEs concentrations. Blood Cr had a significant positive correlation with urinary 8-OHdG. Blood Cr and Yttrium had a positive interactive effect on urinary 8-OHdG. Collectively, the results suggested workers who had been working in the chromate plant were simultaneously exposed to chromate and a variety of REEs, which could have interactive effects on the DNA damage of workers.

An interpretation to Cr(Ⅵ) leaching concentration rebound phenomenon with time in ferrous-reduced Cr(Ⅵ)-bearing solid matrices

Toxicity characteristic leaching procedure (TCLP) is a prevalent way to evaluate the treatment effectiveness for Cr(Ⅵ)-bearing solid matrices (CBSM). But when a certain amount of residual reductants are present in the treated CBSM, Cr(Ⅵ) leaching concentration rebound phenomenon (CLCRP) occurs, which invalidates the TCLP. This study explores the microstructure of ferrous-reduced CBSM and proves that the residual Cr(Ⅵ), Fe_xCr_1-x(OH)₃ precipitate and residual ferrous are separately distributed in a three-layer structure. In natural scenarios, the residual ferrous in the out-layer is firstly flushed away by rainfall and groundwater or oxidized by dissolved oxygen, resulting in the decrease of ferrous with time. Residual Cr(Ⅵ), due to being blocked by precipitate layer, is less flushed away. While in TCLP, all of released residual ferrous and Cr(Ⅵ) are in the leachate and react till one of them is almost exhausted, resulting in the underestimation of Cr(Ⅵ) leaching concentrations. The longer the samples experience the natural scenarios, the less of the residual ferrous, resulting in the decline of underestimation of Cr(Ⅵ) leaching concentrations with time. This study also provides a pretreatment which can effectively reduce the residual ferrous, achieving more accurate Cr(Ⅵ) leaching concentrations and eliminating CLCRP.

Carboxamide derivatives induce apoptosis in the U251 glioma cell line

Glioma is a malignant tumor that is frequently treated using chemotherapy. The aim of the present study was to examine the antitumor activity of two novel carboxamide derivatives in glioma, and investigate the underlying mechanisms. Two previously designed and synthesized carboxamide derivatives were selected and their effects on glioma cells were evaluated. Specifically, assays to evaluate proliferation, apoptosis, oxidation, caspase-3, -8 and -9 activity, and the expression of Bcl-2 and surviving in glioma cells were conducted. The carboxamide derivatives were revealed to inhibit proliferation, as well as to induce apoptosis and oxidative damage in glioma U251 cells. In addition, the carboxamide derivatives increased the activity of caspase-3, -8 and -9, and suppressed the expression of Bcl-2 and survivin. These findings demonstrate that the carboxamide derivatives displayed antitumor activity against glioma in vitro, which may have been mediated via the induction of oxidative damage and apoptosis.

The reduction of Cr(VI) to Cr(III) mediated by environmentally relevant carboxylic acids: State-of-the-art and perspectives

The detoxification process mediated by carboxylic acids (CAs) has received considerable spotlights since CAs are clean reagent and ubiquitous in the natural environments and effluents. Here, we present an exhaustive review on surface-bound/dissolved metals-catalyzed Cr(VI) reduction by CAs and CAs-mediated Cr(VI) reduction by many highly/poorly reductive reagents. The overall mechanisms of Cr(VI) reduction are mainly associated with the coordination of CAs with surface-bound/dissolved metals or Cr(VI,V,IV) species and the electron donating abilities of CAs. Additionally, the general decays of intermediate Cr(V,IV) complexes are clearly emerged in the Cr(VI) reduction processes. The performance of various reaction systems for Cr(VI) reduction that is greatly dependent on the operation parameters, including solution pH, reagent concentration, temperature, coexisting ions and gas atmosphere, are also critically commented. From the study survey presented herein, CAs-mediated Cr(VI) reduction processes exhibit good potential for remediation of various Cr(VI)-contaminated waters/sites. However, there is still a need to address the remained bottle-necks and challenges for the remediation of Cr(VI) mediated by CAs in the related natural attenuation cases and the treatment of industrial effluents. Overall, the present review offers the comprehensive understanding of the Cr(VI) reduction mediated by CAs and provide the engineering community with the guidelines for Cr(VI) remediation in the real-world applications.

Adsorption and reduction of hexavalent chromium on magnetic greigite (Fe3S4)-CTAB: leading role of Fe(ii) and S(−ii)

In this study, a facile one-step route was used to synthesize a novel magnetic mesoporous greigite (Fe₃S₄)-CTAB composite, which was utilized to remove hexavalent chromium (Cr(vi)). The optimized Fe₃S₄-CTAB_0.75 composite with a CTAB dosage of 0.75 g possessed the maximum specific surface, showing the highest Cr(vi) adsorption capacity of 330.03 mg g⁻¹. The mechanism analysis revealed that Fe(ii) and S(−ii) were critical for the reduction of Cr(vi). CTAB can promote the removal of Cr(vi) by Fe₃S₄-CTAB composites, possibly due to increased S(−ii) concentration, better dispersion of nanoparticles, and greater zeta potential. Besides, there is mild effect of Fe⁰ on Cr(vi) removal, which is confirmed by the disappearance of the Fe⁰ peak from the XPS analysis. The pseudo-second-order kinetic model could explain the Cr(vi) removal processes well. The adsorption of Cr(vi) at different initial concentrations was more consistent with a Langmuir isotherm. The existence of H⁺ was beneficial for Cr(vi) removal by Fe₃S₄-CTAB_0.75. Our work confirmed that the obtained Fe₃S₄-CTAB_0.75 composites exhibit considerable potential for Cr(vi) removal from aqueous solution.

The presence of CTAB can promote the removal of hexavalent chromium from the Fe₃S₄-CTAB surface.