Chromium(VI) removal by siderite (FeCO3) in anoxic aqueous solutions: An X-ray absorption spectroscopy investigation

Grinding siderite with ferric sulfate to generate an active ferrous source for Cr(VI) reduction

Activated siderite, endowed with excellent properties, was simply prepared by co-grinding with Fe sulfate to enhance its high reducing ability for Cr(VI). Batch experiments were conducted to investigate the main affecting parameters, such as material ratio, pH, temperature, etc. The removal of Cr(VI) by activated siderite was completed within 4 h of the reaction. The activated siderite maintained a high removal effect of Cr(VI) within a wide pH range (3-9). Various analytical methods, including XRD, SEM/EDS, XPS, etc., were employed to characterize the samples and discover variations before and after the reaction. The Fe (Ⅱ) in activated siderite becomes highly active, and it can even be released from the solid phase in the mildly acidic liquid phase to efficiently reduce Cr(VI) and mitigate its toxicity. These findings introduce an innovative approach for activating various minerals widely distributed in nature to promote the recovery of the ecological system.

Groundwater Cr(VI) contamination and remediation: A review from 1999 to 2022

Hexavalent chromium (Cr(VI)) contamination of groundwater has traditionally been an environmental issue of great concern due to its bioaccumulative and highly toxic nature. This paper presents a review and bibliometric analysis of the literature on the interest area "Cr(VI) in groundwater" published in the Web of Science Core Collection from 1999 to 2022. First, information on 203 actual Cr(VI)-contaminated groundwater sites around the world was summarized, and the basic characteristics of the sources and concentrations of contamination were derived. 68.95% of the sites were due to human causes and 56.43% of these sites had Cr(VI) concentrations in the range of 0-10 mg/L. At groundwater sites with high Cr(VI) contamination due to natural causes, 75.00% of the sites had Cr(VI) concentrations less than 0.2 mg/L. A total of 936 papers on "Cr(VI) in groundwater" were retrieved for bibliometric analysis: interest in research on Cr(VI) in groundwater has grown rapidly in recent years; 59.4% of the papers were published in the field of environmental sciences. A systematic review of the progress of studies on the Cr(VI) removal/remediation based on reduction, adsorption and biological processes is presented. Out of 666 papers on Cr(VI) removal/remediation, 512, 274, and 75 papers dealt with the topics of reduction, adsorption, and bioremediation, respectively. In addition, several studies have demonstrated the potential applicability of natural attenuation in the remediation of Cr(VI)-contaminated groundwater. This paper will help researchers to understand and investigate methodological strategies to remove Cr(VI) from groundwater in a more targeted and effective manner.

Exploring the potential of bacterial-augmented floating treatment wetlands for the remediation of detergent-contaminated water

Due to industrialization and urbanization, the use of detergents inadvertently led to contamination of aquatic environments, thus posing potential threat to aquatic organisms and human health. One of the main components of detergents is linear alkylbenzene sulfonate (LAS), which can cause toxic effects on living organisms, particularly aquatic life in the environment. In this study, floating treatment wetlands (FTWs) mesocosms were developed and augmented with LAS-degrading bacteria. The plant species, Brachiaria mutica (Para grass), was vegetated to establish FTWs and bacterial consortium (1:1:1:1) of Pseudomonas aeruginosa strain PJRS20, Bacillus sp. BRRH60, Acinetobacter sp. strain CYRH21, and Burkholderia phytofirmans Ps.JN was augmented (free or immobilized) in these mesocosms. Results revealed that the FTWs removed LAS from the contaminated water and their augmentation with bacteria slightly increased LAS removal during course of the experiment. Maximum reduction in LAS concentration (94%), chemical oxygen demand (91%), biochemical oxygen demand (93%), and total organic carbon (91%) was observed in the contaminated water having FTWs augmented with bacterial consortium immobilized on polystyrene sheet. This study highlights that the FTWs supported with immobilized bacteria on polystyrene sheets can provide an eco-friendly and sustainable solution for the remediation of LAS-bearing water, especially for developing countries like Pakistan.

Oxide- and Silicate-Water Interfaces and Their Roles in Technology and the Environment

Interfacial reactions drive all elemental cycling on Earth and play pivotal roles in human activities such as agriculture, water purification, energy production and storage, environmental contaminant remediation, and nuclear waste repository management. The onset of the 21st century marked the beginning of a more detailed understanding of mineral aqueous interfaces enabled by advances in techniques that use tunable high-flux focused ultrafast laser and X-ray sources to provide near-atomic measurement resolution, as well as by nanofabrication approaches that enable transmission electron microscopy in a liquid cell. This leap into atomic- and nanometer-scale measurements has uncovered scale-dependent phenomena whose reaction thermodynamics, kinetics, and pathways deviate from previous observations made on larger systems. A second key advance is new experimental evidence for what scientists hypothesized but could not test previously, namely, interfacial chemical reactions are frequently driven by "anomalies" or "non-idealities" such as defects, nanoconfinement, and other nontypical chemical structures. Third, progress in computational chemistry has yielded new insights that allow a move beyond simple schematics, leading to a molecular model of these complex interfaces. In combination with surface-sensitive measurements, we have gained knowledge of the interfacial structure and dynamics, including the underlying solid surface and the immediately adjacent water and aqueous ions, enabling a better definition of what constitutes the oxide- and silicate-water interfaces. This critical review discusses how science progresses from understanding ideal solid-water interfaces to more realistic systems, focusing on accomplishments in the last 20 years and identifying challenges and future opportunities for the community to address. We anticipate that the next 20 years will focus on understanding and predicting dynamic transient and reactive structures over greater spatial and temporal ranges as well as systems of greater structural and chemical complexity. Closer collaborations of theoretical and experimental experts across disciplines will continue to be critical to achieving this great aspiration.

Natural siderite derivatives activated peroxydisulfate toward oxidation of organic contaminant: A green soil remediation strategy

Natural siderite (FeCO₃), simulated synthetic siderite and nZVI/FeCO₃ composite were used as green and easily available iron-based catalysts in peroxydisulfate activation for remediating 2-chlorophenol as the target contaminant and this technique can effectively degrade organic pollutants in the soil. The key reaction parameters such as catalysts dosage, oxidant concentration and pH, were investigated to evaluate the catalytic performance of different materials in catalytic systems. The buffering property of natural soil conduced satisfactory degradation performance in a wide pH range (3-10). Both the main non-radical of ¹O₂ and free radicals of SO₄^·- and OH^· were evidenced by quenching experiment and electron paramagnetic resonance. The reduction of nZVI on FFC surface not only has the advantage for electronic transfer to promote the circulation of Fe(III) to Fe(II), but also can directly dechlorinate. Furthermore, the intermediates were comprehensively analyzed by GC-MS and a potential removal mechanism of three oxidant system for 2-CP soil degradation was obtained. Briefly, this research provides a new perspective for organic contaminate soil treatment using natural siderite or simulated synthetic siderite as efficient and environmental catalytic material.

FeCO3 Synthesis Pathways: The Influence of Temperature, Duration, and Pressure

FeCO₃ is present as scales in process equipment, corrosion products, geological systems, and carbon storage. It is therefore crucial to investigate the properties of FeCO₃ to understand scaling in all these systems. However, FeCO₃ is not commercially available, and when used in the lab it is either obtained through extraction of geological formations or synthesized in-house. Geologically formed FeCO₃ contains multiple impurities, which will affect its overall properties, and the synthesized product is highly sensitive to either oxidation or the synthesis pathways. This work explores the parameter space of a synthesis route routinely and pathways for FeCO₃. We characterized the structure of FeCO₃ using X-ray powder diffraction and its thermal properties with thermogravimetric analysis and scanning electron microscopy. We show how synthesis parameters influence either the macroscopic or microscopic properties of the synthesized product. Our study serves as a guideline for future research regarding what parameters to choose when synthesizing FeCO₃ and what product can be obtained. We herein present a novel fundamental understanding of FeCO₃.

Characterization of arsenite (As(III)) and arsenate (As(V)) sorption on synthetic siderite spherules under anoxic conditions: Different sorption behaviors with crystal size and arsenic species

Arsenite (As(III)) and arsenate (As(V)) uptake by synthesized small- and large-sized siderites (S-siderite and l-siderite) and the effects of crystal size on arsenic sorption were investigated under extremely anoxic and neutral pH conditions. Both siderites exhibited spherical growth mechanism with an inverse relationship between crystal size and specific surface area (SSA). The maximum adsorption capacities normalized to SSA (q_m,nor) of S-siderite and l-siderite were 0.161 and 0.174 mg/m² for As(III), and 1.460 and 0.360 mg/m² for As(V), respectively, indicating that the sorption affinity of S-siderite depends more on the arsenic species (III and V). Extended X-ray absorption fine structure (EXAFS) revealed that without oxidation change, As(V) adsorbed on both siderites forms inner-sphere complexes through bidentate-binuclear corner-sharing. In contrast, outer-sphere and inner-sphere complexes are formed for As(III) adsorbed on these siderites. In addition, the highest sorption affinity for As(V) uptake by S-siderite is attributed to the precipitation of symplesite (Fe^II₃(As^VO₄)₂·8H₂O), whereas the lowest sorption affinity for As(III) uptake by S-siderite was due to bicarbonates generated by the faster dissolution of S-siderite competing for sorption sites. Our findings suggest that arsenic sorption behaviors and mechanisms are strongly dependent on the arsenic species and the crystal size of siderite.

Elucidating the Potential of Vertical Flow-Constructed Wetlands Vegetated with Different Wetland Plant Species for the Remediation of Chromium-Contaminated Water

Water scarcity is one of the key global challenges affecting food safety, food security, and human health. Constructed wetlands (CWs) provide a sustainable tool to remediate wastewater. Here we explored the potential of vertical flow-CWs (VF-CWs) vegetated with ten indigenous wetland plant species to treat chromium (Cr)-contaminated water. The wetland plants were vegetated to develop VF-CWs to treat Cr-contaminated water in a batch mode. Results revealed that the Cr removal potential of VF-CWs vegetated with different wetland plants ranged from 47% to 92% at low (15 mg L−1) Cr levels and 36% to 92% at high (30 mg L−1) Cr levels, with the maximum (92%) Cr removal exhibited by VF-CWs vegetated with Leptochloa fusca. Hexavalent Cr (Cr(VI)) was reduced to trivalent Cr (Cr(III)) in treated water (96–99 %) of all VF-CWs. All the wetland plants accumulated Cr in the shoot (1.9–34 mg kg−1 dry weight (DW)), although Cr content was higher in the roots (74–698 mg kg−1 DW) than in the shoots. Brachiaria mutica showed the highest Cr accumulation in the roots and shoots (698 and 45 mg kg−1 DW, respectively), followed by Leptochloa fusca. The high Cr level significantly (p < 0.05) decreased the stress tolerance index (STI) percentage of the plant species. Our data provide strong evidence to support the application of VF-CWs vegetated with different indigenous wetland plants as a sustainable Cr-contaminated water treatment technology such as tannery wastewater.

Constructed wetlands as a sustainable technology for wastewater treatment with emphasis on chromium-rich tannery wastewater

Water scarcity is a major threat to agriculture and humans due to over abstraction of groundwater, rapid urbanization and improper use in industrial processes. Industrial consumption of water is lower than the abstraction rate, which ultimately produces large amounts of wastewater such as from tannery industry containing high concentration of chromium (Cr). Chromium-contaminated tannery industry wastewater is used for irrigation of food crops, resulting in food safety and public health issues globally. In contrast to conventional treatment technologies, constructed wetlands (CWs) are considered as an eco-friendly technique to treat various types of wastewaters, although their application and potential have not been discussed and elaborated for Cr treatment of tannery wastewater. This review briefly describes Cr occurrence, distribution and speciation in aquatic ecosystems. The significance of wetland plant species, microorganisms, various bedding media and adsorbents have been discussed with a particular emphasis on the removal and detoxification of Cr in CWs. Also, the efficiency of various types of CWs is elaborated for advancing our understanding on Cr removal efficiency and Cr partitioning in various compartments of the CWs. The review covers important aspects to use CWs for treatment of Cr-rich tannery wastewater that are key to meet UN's Sustainable Development Goals.

The Characterization of Ukrainian Volcanic Tuffs from the Khmelnytsky Region with the Theoretical Analysis of Their Application in Construction and Environmental Technologies

(1) The mineral deposits are the base resources of materials used in building and environmental engineering applications, especially available locally. Two wells of volcanic tuff deposits in the Khmelnytsky region of Ukraine were investigated in this regard. (2) Physical-mechanical, chemical, and mineralogical analyses of the core samples were carried out. (3) The tuff samples were characterized by visible colour, low compressive strength (4.34–11.13 MPa), and high water absorption (30%). The dominant minerals of the upper horizon were chlorite, pyroxene, kaolinite, quartz, hematite, and calcite, while those of the lower horizon included analcime, quartz, hematite, and calcite. (4) The studied volcanic tuffs seem to be only partly useful for construction applications, and considering their visible colour, the exterior decoration of engineering objects could be possible. The peculiarity of the minerals of the upper horizon is that their crystals consist of Fe²⁺. An analysis of existing scientific data made it possible to say that these minerals can be considered as an alternative to expensive metallic iron in reducing the toxicity of chromium, uranium, and halogenated organic compounds. The significant presence of hematite allows the application of tuffs to technologies of water purification from As⁵⁺, As³⁺, Cr⁶⁺, Cr³⁺, U⁶⁺, Sb⁵⁺, and Se⁴⁺ oxyanions.

A modeling approach for unveiling adsorption of toxic ions on iron oxide nanocrystals

The era of advanced computer simulations in materials science enables a great potential to design in silico computational experiments for (nano-)material performance. The adsorption efficiency of nanoparticles in various environments can be unveiled by atomistic models and computer simulations. Arsenic (As) is one of the important globally distributed contaminants with a hazardous impact on human health and environment, and it can strongly bind with iron nanocrystals (e.g., hematite (Fe₂O₃)) depending on their shape and size. Here, we developed a novel Kinetic Monte Carlo (KMC) model capable of exploring and delineating shape-efficiency dependence for Fe₂O₃ nanocrystals in contact with arsenate-contaminated water. This newly designed model demonstrated the performance of nanocrystals for removal of toxic (As) ions on their surface. The current model opens new avenues for designing further advanced KMC models for nanoparticles-toxic ions interactions, under varying environmentally relevant situations, e.g., groundwater, wetlands, and water treatment systems. In addition to bidentate adsorption complexes, implemented in the model presented, monodentate and outer-sphere adsorption complexes should be incorporated into the KMC model. Detailed environmental controls can be addressed by implementation of pH and background ions.

The pH-dependent degradation of sulfadiazine using natural siderite activating PDS: The role of singlet oxygen

Occurring naturally siderite (FeCO₃) was used as the heterogeneous catalyst to activate peroxodisulfate (PDS) for the degradation of sulfadiazine under different initial pH values. The findings of this system exhibited various ROS (e.g. ¹O₂, SO₄^- and OH) present during a wide range of pH values. Among them, ¹O₂ could significantly facilitate the initial degradation rate, and the increased pH enhanced the role of ¹O₂. The factors including initial pH values, siderite dosage, PDS concentration, initial contaminants concentration, and water matrix were discussed. The role of each ROS was investigated through quenching test and electron paramagnetic resonance (EPR). Furthermore, the comprehensive degradation process was proposed based on the LC-MS results. And the cycle test demonstrates the reusability of siderite at a pH of 3. Accordingly, this study is of great significance for understanding the degradation of such sulfonamide pollutants in the siderite/PDS system.

Unraveling the Underlying Heavy Metal Detoxification Mechanisms of Bacillus Species

The rise of anthropogenic activities has resulted in the increasing release of various contaminants into the environment, jeopardizing fragile ecosystems in the process. Heavy metals are one of the major pollutants that contribute to the escalating problem of environmental pollution, being primarily introduced in sensitive ecological habitats through industrial effluents, wastewater, as well as sewage of various industries. Where heavy metals like zinc, copper, manganese, and nickel serve key roles in regulating different biological processes in living systems, many heavy metals can be toxic even at low concentrations, such as mercury, arsenic, cadmium, chromium, and lead, and can accumulate in intricate food chains resulting in health concerns. Over the years, many physical and chemical methods of heavy metal removal have essentially been investigated, but their disadvantages like the generation of chemical waste, complex downstream processing, and the uneconomical cost of both methods, have rendered them inefficient,. Since then, microbial bioremediation, particularly the use of bacteria, has gained attention due to the feasibility and efficiency of using them in removing heavy metals from contaminated environments. Bacteria have several methods of processing heavy metals through general resistance mechanisms, biosorption, adsorption, and efflux mechanisms. Bacillus spp. are model Gram-positive bacteria that have been studied extensively for their biosorption abilities and molecular mechanisms that enable their survival as well as their ability to remove and detoxify heavy metals. This review aims to highlight the molecular methods of Bacillus spp. in removing various heavy metals ions from contaminated environments.

Stabilized green rusts for aqueous Cr(VI) removal: Fast kinetics, high iron utilization rate and anti-acidification

Green rusts (GRs) are redox active towards contaminants but they are not stable for long distance transport during the soil and groundwater remediation. In this study, green rust chloride (GR) was stabilized by selected regents, including silicate (Si), phosphate (P), fulvic acid (FA), carboxymethyl cellulose (CMC) and bone char (BC), then these stabilized GR, collectively named GR-X, would be further applied for Cr(VI) removal from aqueous solution. The stabilization experiment demonstrated that the release of Fe(II) from GR was effectively suppressed by above reagents, enabling at least 50% lower Fe(II) leaching from the stabilized GR-X than that from the pristine GR. The intact hexagonal GR plates and crystallinity were also confirmed by the SEM images and XRD patterns after storage for 7 days, indicating the stable structure of GR-X was remained. In the Cr(VI) removal tests, Cr(VI) was eliminated by GR-X in seconds with a Fe(II) utilization efficiency over 90%. The Cr species examination demonstrated that the GR-X was able to transfer Cr(VI) into stable Cr(III)-Fe(III) precipitates (Fe-Mn oxides fraction). After Cr(VI) removal tests, all reactors were exposed to the air for 1 week to monitor pH fluctuation and evaluated the risk of acidification. The results indicate that, except for GR-Si system, the other post-remediation systems are stable and the pH buffering ability of GR-X could avoid acidification and lower the Cr leaching risk.

Cr(VI) Sorption from Aqueous Solution: A Review

Hexavalent chromium (Cr(VI)) in water systems is a major hazard for living organisms, including humans. The most popular technology currently used to remove Cr(VI) from polluted water is sorption for its effectiveness, ease of use, low cost and environmental friendliness. The electrostatic interactions between chromium species and the sorbent matrix are the main determinants of Cr(VI) sorption. The pH plays a central role in the process by affecting chromium speciation and the net charge on sorbent surface. In most cases, Cr(VI) sorption is an endothermic process whose kinetics is satisfactorily described by the pseudo second-order model. A critical survey of the recent literature, however, reveals that the thermodynamic and kinetic parameters reported for Cr(VI) sorption are often incorrect and/or erroneously interpreted.

Autochthonous N-doped carbon nanotube/activated carbon composites derived from industrial paper sludge for chromate (VI) reduction in microbial fuel cells

The performance of microbial electrochemical system for hexavalent chromium (Cr(VI)) contaminant has been a severe challenge remaining active for further development. In this study, we developed a novel biochar material from industrial paper sludge for microbial fuel cell cathode fabrication to reduce aquatic Cr(VI) to non-toxic Cr(III). With additive melamine as nitrogen source and self-containing small portion of Fe as catalyst, the sludge evolved into electroactive biochar (BC-M) rendering a unique N-doped carbon nanotubes/activated carbon (N-CNT/AC) frame after pyrolyzed at 900 °C for 2 h. Electrochemical analysis revealed enhanced electron transference capacity of this composite material, such effectiveness was attributed to the increased surface area and superior electroconductivity of N-doped CNTs. For performance of Cr(VI) reduction, a 55.1% reduction efficiency was achieved in an microbial fuel cell equipped with BC-M cathode while it reduced to about 41.8% when the cathode was replaced by electrode modified with no-melamine-involved biochar. The strategy of biochar upgrading from industrial paper sludge proposed in this work is expected to not only bring technical solution for low-cost CNT materials preparation for Cr(VI) reduction, but also put forward further research on value-added chemical synthesis from waste in various fields of energy and environment.

Aqueous Vanadate Removal by Iron(II)-Bearing Phases under Anoxic Conditions

Vanadium contamination is a growing environmental hazard worldwide. Aqueous vanadate (H_xV^VO₄^(3-x)-_(aq)) concentrations are often controlled by surface complexation with metal (oxyhydr)oxides in oxic environments. However, the geochemical behavior of this toxic redox-sensitive oxyanion in anoxic environments is poorly constrained. Here, we describe results of batch experiments to determine kinetics and mechanisms of aqueous H₂V^VO₄^- (100 μM) removal under anoxic conditions in suspensions (2.0 g L^-1) of magnetite, siderite, pyrite, and mackinawite. We present results of parallel experiments using ferrihydrite (2.0 g L^-1) and Fe²⁺_(aq) (200 μM) for comparison. Siderite and mackinawite reached near complete removal (46 μmol g^-1) of aqueous vanadate after 3 h and rates were generally consistent with ferrihydrite, whereas magnetite removed 18 μmol g^-1 of aqueous vanadate after 48 h and uptake by pyrite was limited. Removal during reaction with Fe²⁺_(aq) was observed after 8 h, concomitant with precipitation of secondary Fe phases. X-ray absorption spectroscopy revealed V(V) reduction to V(IV) and formation of bidentate corner-sharing surface complexes on magnetite and siderite, and with Fe²⁺_(aq) reaction products. These data also suggest that V(IV) is incorporated into the mackinawite structure. Overall, we demonstrate that Fe(II)-bearing phases can promote aqueous vanadate attenuation and, therefore, limit dissolved V concentrations in anoxic environments.

Isolation and Identification of Chromium Reducing Bacillus Cereus Species from Chromium-Contaminated Soil for the Biological Detoxification of Chromium

Chromium contamination has been an increasing threat to the environment and to human health. Cr(VI) and Cr(III) are the most common states of chromium. However, compared with Cr(III), Cr(VI) is more toxic and more easily absorbed, therefore, it is more harmful to human beings. Thus, the conversion of toxic Cr(VI) into Cr(III) is an accepted strategy for chromium detoxification. Here, we isolated two Bacillus cereus strains with a high chromium tolerance and reduction ability, named B. cereus D and 332, respectively. Both strains demonstrated a strong pH and temperature adaptability and survival under 8 mM Cr(VI). B. cereus D achieved 87.8% Cr(VI) removal in 24 h with an initial 2 mM Cr(VI). Cu(II) was found to increase the removal rate of Cr(VI) significantly. With the addition of 0.4 mM Cu(II), 99.9% of Cr(VI) in the culture was removed by B. cereus 332 in 24 h. This is the highest removal efficiency in the literature that we have seen to date. The immobilization experiments found that sodium alginate with diatomite was the better method for immobilization and B. cereus 332 was more efficient in immobilized cells. Our research provided valuable information and new, highly effective strains for the bioremediation of chromium pollution.

Biochar synthesis from sweet lime peel for hexavalent chromium remediation from aqueous solution

Sweet lime (Citrus limetta) peel biochar was obtained by slow pyrolysis of raw biomass at 450 °C with 5 °C/min heating rate. Proximate and ultimate analysis, physico-chemical characterization of the biochar was done. Batch adsorption experiments for Cr(VI) removal were performed with varying pH, biochar dose, contact time and initial Cr(VI) concentrations. It took 8-24 h to reach the equilibrium at 30 °C for varying Cr(VI) concentrations. The biochar was found to possess higher adsorption capacity (100 mg/g) than the adsorbents reported in several previous studies. Langmuir adsorption isotherm and pseudo second order model best explained the experimental data, suggesting monolayer adsorption as the dominant mechanism. Chemical interaction, ion exchange of solute and sorbate ions and physical adsorption also contributed into Cr(VI) adsorption process. Further, Cr(VI) adsorption was found to be a multistep process. The findings suggested that sweet lime peel biochar can be utilized as a low cost and efficient alternative for Cr(VI) removal, which could be useful for aqueous solutions, as well as to promote overall protection against soil and water degradation and pollution.

Editorial of the VSI "Antibiotics and heavy metals in the environment: Facing the challenge"

The Virtual Special Issue (VSI) "Antibiotics and Heavy Metals in the Environment: Facing the Challenge" received more than 100 submissions from research teams around the world. Finally, more than 50 papers were accepted and published. These very interesting research papers allow going ahead in the knowledge of different aspects which determine the fate of antibiotics and heavy metals in the environmental. The success of the VSI, as well as reports from scientific databases, indicate that this field of research is clearly growing, which is expected to continue, especially considering emerging pollutants as a whole.

Chitosan-iron oxide hybrid composite: mechanism of hexavalent chromium removal by central composite design and theoretical calculations

In this study, the synthesis of iron oxide stabilized by chitosan was carried out for the application and optimization in the removal process of aqueous Cr(VI) by central composite design (CCD). The calculation of these effects allowed to know, quantitatively, the variables and the interaction between them that could affect the Cr(VI) removal process. It was also verified that the most favorable conditions for chromium removal were the following: pH 5.0, Cr(VI) concentration of 130 mg L^-1, adsorbent mass of 5 mg, and Fe(II) content of 45% (w/w) in the CT-Fe beads. The adsorption kinetics performed under these conditions showed that the chitosan/iron hybrid composite is an adsorbent material with high chromium removal capacity (46.12 mg g^-1). It was found that all variables were statistically significant. However, it was observed that the variable that most affected Cr(VI) removal was the pH of the solution, followed by the concentration of chromium ions in solution and the interaction between them. Therefore, the studied experimental conditions are efficient in chromium adsorption, besides the operational simplicity coming from statistical design. Theoretical calculations showed that the most stable chitosan was that with Fe(II) in the structure, that is, in the reaction mechanism, there is no competition of Fe(II) with Cr(III, VI) in the available sites of chitosan. Thus, the theoretical calculations show that the proposed Cr(VI) removal is effective.

A new pathway for hexavalent chromium formation in soil: Fire-induced alteration of iron oxides

Iron oxides are important pedogenic Cr(III)-bearing phases which experience high-temperature alteration via fire-induced heating of surface soil. In this study, we examine if heating-induced alteration of Cr(III)-substituted Fe oxides can potentially facilitate rapid high-temperature oxidation of solid-phase Cr(III) to hazardous Cr(VI). Synthetic Cr(III)-substituted ferrihydrite, goethite and hematite were heated up to 800 °C for 2 h. Corresponding heating experiments were also conducted on an unpolluted Ferrosol-type soil, which had a total Cr content of 220 mg kg^-1, initially undetectable Cr(VI) and Fe speciation comprising a mixture of hematite, goethite and ferrihydrite (according to Fe K-edge EXAFS spectroscopy). Up to ∼50% of the initial Cr(III) was oxidised to Cr(VI) during heating of Cr(III)-substituted ferrihydrite and hematite, with the greatest extent of Cr(VI) formation occurring at 200-400 °C. In contrast, heating of Cr(III)-substituted goethite resulted in up to ∼100% of Cr(III) oxidizing to Cr(VI) as the temperature approached 800 °C. In the Ferrosol-type soil, heating at ≥400 °C also resulted in large amounts of Cr(VI) formation, with a maximum total Cr(VI) concentration of 77 mg kg^-1 forming at 600 °C (equating to oxidation of ∼35% of the soil's total Cr content). A relatively large portion (31-42%) of the total Cr(VI) which formed during heating of the soil was exchangeable, implying a high level of potential mobility and bioaccessibility. Overall, the results show that Cr(VI) forms rapidly via the oxidation of Fe oxide-bound Cr(III) at temperatures which occur in surface soils during fires. On this basis and given the frequency and extent of wild-fires around the world, we propose that fire-induced oxidation of Fe oxide-bound Cr(III) may represent a globally-significant pathway for the natural formation of hazardous Cr(VI) in surface soil.

Reduction and immobilization of hexavalent chromium in chromite ore processing residue using amorphous FeS2

In this work, a series of long-term treatment trials were conducted to evaluate the remediation performance of amorphous iron pyrite (FeS₂₍am₎) toward hexavalent chromium (Cr(VI)) in chromite ore processing residue (COPR). The effectiveness of FeS₂₍am₎ was assessed using alkaline digestion, the synthetic precipitation leaching procedure (SPLP) and the physiologically based extraction test (PBET). Reaction mechanisms were explored by monitoring the changes in the solid pH, redox potential (Eh), the chemical states of relevant elements as well as the crystal forms present in COPR. The results showed that, using a proper dosage, the total content of Cr and Cr(VI) in the leachate from treated COPR met the Chinese standard regulatory limits for the extraction toxicity of hazardous wastes (GB 5085.3-2007). In addition, the in vitro bioaccessibility of Cr(VI) in COPR was also significantly reduced. Moreover, the remediation effect was maintained for the subsequent six months. This long-term effect was attributed to the presence of reductive sulfur (S) species and Fe(II) that remained even after 180 days of treatment, which were identified using high-resolution X-ray photoelectron spectroscopy (HR-XPS) and X-ray diffraction (XRD). To maximize the effect of FeS₂₍am₎, 5% lime was employed as an additive to adjust the pH and thus contributed to Cr(VI) reduction and immobilization. To remediate COPR within 30 days, the use of 5% lime and a specific FeS₂₍am₎ dosage (FeS₂₍am₎:Cr(VI) = 1.25:1) is recommended based on the results. For rapid remediation (< 1 day), the use of 5% lime and a 2.5:1 FeS₂₍am₎:Cr(VI) ratio is recommended. In comparison with widely studied reductants, FeS₂₍am₎ showed excellent efficiency for the remediation of COPR over both short- and long-term treatment trials, demonstrating it is a very promising alternative treatment method.

Remarkable effect of Co substitution in magnetite on the reduction removal of Cr(VI) coupled with aqueous Fe(II): Improvement mechanism and Cr fate

The interaction between magnetite and aqueous Fe(II) profoundly impacts the mineral recrystallization, trace-metal sequestration, and contaminant reduction. The iron ions in natural magnetite are extensively substituted by other cations. It is still unclear whether the substitution with thermodynamically favorable redox repairs (e.g., Co²⁺/Co³⁺) plays a vital role in the reducing capability of the coupled system. Herein, a series of Co-substituted magnetite samples (Fe_3-xCo_xO₄, 0.00 ≤ x ≤ 1.00) were synthesized and tested for the reductive removal of Cr(VI) in the presence of Fe(II). Fe_3-xCo_xO₄ had a spinel structure with the preferential occupancy of Co²⁺ on octahedral sites. No visible variation in the BET surface area was observed, whereas the surface site density increased gradually with Co substitution. Cr(VI) was found first adsorbed on the Fe_3-xCo_xO₄ surface and then reduced to Cr(III) by the structural Fe²⁺ and the absorbed Fe(II), accompanied by the oxidation of bulk Fe²⁺ and surface Fe(II) in Fe_3-xCo_xO₄ without phase transformation. The Cr(III) was precipitated on the Fe_3-xCo_xO₄ surface with Fe(III), or substituted octahedral Fe in Fe_3-xCo_xO₄. Both the reaction kinetics and the electron transfer efficiency revealed that Co substitution significantly improved the reactivity of Fe_3-xCo_xO₄/Fe(II) towards Cr(VI) reduction. This was ascribed to the presence of the redox pairs Co²⁺/Co³⁺ and Fe²⁺/Fe³⁺ accelerating electron transfer from the Fe_3-xCo_xO₄ interface to Cr(VI).