Removal of hexavalent chromium using mackinawite (FeS)-coated sand

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.

Optimizing electronic states of Pd/WO3 nanofibers for enhanced catalytic reduction of hexavalent chromium with formic acid

Through theoretical calculations, we show that integrating Pd with WO3 nanomaterials can trigger the interfacial electron transfer from Pd to WO3, thus upshifting the d-band center (εd) of Pd to optimize toxic hexavalent chromium (Cr(VI)) reduction. The elevated εd can derive stronger chemisorption capability toward crucial formic acid molecules, notably lowering the thermodynamic energy barrier and speeding up the kinetics process. In order to realize this concept, we synthesized unique Pd/WO3 nanofibers by loading Pd nanoparticles onto electrospun WO3 nanofibers through an in situ photodeposition technique. Extensive structural, morphological, and X-ray photoelectron spectrometer (XPS) characterizations confirm the successful formation of the above nanofibers. As anticipated, the as-designed Pd/WO3 nanofibers exhibit enhanced catalytic performance in the Cr(VI) reduction with a high turnover frequency (TOF) value of 62.12 min-1, surpassing a series of reported Pd-based catalysts. Such nanofibrous WO3-induced electronic modification of Pd with a high specific area leads to catalytic enhancement, providing a novel model for catalyst design.

Surface modification of quartz sand: A review of its progress and its effect on heavy metal adsorption

Quartz sand (SiO₂) is a prevalent filtration medium, boasting wide accessibility, superior stability, and cost-effectiveness. However, its utility is often curtailed by its sleek surface, limited active sites, and swift saturation of adsorption sites. This review outlines the prevalent strategies and agents for quartz sand surface modification and provides a comprehensive analysis of the various modification reagents and their operative mechanisms. It delves into the mechanism and utility of surface-modified quartz sand for adsorbing heavy metal ions (HMIs). It is found that the reported modifiers usually form connections with the surface of quartz sand through electrostatic forces, van der Waals forces, pore filling, chemical bonding, and/or molecular entanglement. The literature suggests that these modifications effectively address issues inherent to natural quartz sand, such as its low superficial coarseness, rapid adsorption site saturation, and limited adsorption capacity. Regrettably, comprehensive investigations into the particle size, regenerative capabilities, and application costs of surface-modified quartz sand and the critical factors for its wider adoption are lacking in most reports. The adsorption mechanisms indicate that surface-modified quartz sand primarily removes HMIs from aqueous solutions through surface complexation, ion exchange, and electrostatic and gravitational forces. However, these findings were derived under controlled laboratory conditions, and practical applications for treating real wastewater necessitate overcoming further laboratory-scale obstacles. Finally, this review outlines the limitations of partially surface modified quartz sand and suggests potential venues for future developments, providing a valuable reference for the advancement of cost-effective, HMI-absorbing, surface-modified quartz sand filter media.

Preparation of lignite-loaded nano-FeS and its performance for treating acid Cr(VI)-containing wastewater

In this study, lignite-loaded nano-FeS (nFeS@Lignite) was successfully prepared by ultrasonic precipitation, and its potential for treating acid Cr(VI)-containing wastewater was explored. The results showed that the 40--80-nm rod-shaped nFeS was successfully loaded onto lignite particles, and the maximum adsorption capacity of Cr(VI) by nFeS@Lignite reached 33.08 mg∙g^-1 (reaction time = 120 min, pH = 4, temperature = 298.15 K). The adsorption process of Cr(VI) by nFeS@Lignite fitted the pseudo-second-order model and the Langmuir isotherm model, and thermodynamic results showed that the adsorption process was an endothermic process with an adsorption enthalpy of 28.0958 kJ·mol^-1. The inhibition intensity of coexisting anions on Cr(VI) removal was in the order of PO₄^3-  > NO₃^-  > SO₄^2-  > Cl^-, and the increase of ionic strength resulted in more pronounced inhibition. Electrostatic adsorption, reduction, and precipitation were synergistically engaged in the adsorption of Cr(VI) by nFeS@Lignite, among which reduction played a major role. The characterization results showed that Fe²⁺, S^2-, and Cr(VI) were converted to FeOOH, S₈, SO₄^2-, Fe₂O₃, Cr₂O₃, and Fe(III)-Cr(III) complexes. This research demonstrates that nFeS@Lignite is a good adsorbent with promising potential for application in the remediation of heavy metal-contaminated wastewater.

Study on the Preparation of Nano-FeS Loaded on Fly Ash and Its Cr Removal Performance

Chromium has been considered as one of the most hazardous heavy metals because of its strong and persistent toxicity to the ecosystem and human beings. In this study, fly ash-loaded nano-FeS (nFeS-F) composites were constructed with fly ash as the carrier, and the performance and mechanism of the composites for the removal of Cr(VI) and total chromium from water were investigated. The composite was characterized by X-ray diffraction and transmission electron microscopy. The effects of fly ash size, molarity of FeSO₄, and flow rate of FeSO₄ solution on the removal of Cr(VI) and total chromium were investigated by a single factor experiment. The interaction of various factors was studied by the Box-Behnken response surface methodology. The optimum conditions of removal of Cr(VI)and total chromium by nFeS-F were determined. The results show that ① the optimal preparation conditions for nFeS-F were an FeSO₄ concentration of 0.45 mol/L, a fly ash particle size of 120-150 mesh, and a flow rate of 0.43 mL/s.② The response surface model provides reliable predictions for the removal efficiencies of Cr(VI) and total chromium.③ The removal efficiencies of Cr(VI) and total chromium were 92.87 and 83.53%, respectively, under the optimal preparation conditions by the experimental test. This study provides an effective method for the removal of Cr(VI) and total chromium.

Efficient removal of aqueous Cr(VI) with ferrous sulfide/N-doped biochar composites: Facile, in-situ preparation and Cr(VI) uptake performance and mechanism

FeS nanoparticles loaded on nitrogen-doped biochar (FeS/BNC) were fabricated by pyrolyzing coffee husks pretreated with Mohr's salt. The nitrogen doping and FeS loading of biochar are simultaneously achieved in one-pot pyrolysis. The elemental analysis, SEM, TEM, XRD, XPS, Raman, FTIR and N₂ adsorption-desorption technologies were used to characterize the composition and structure of FeS/NBC. The appraisement for removing aqueous Cr(VI) testified that FeS/NBC offered a synergistic scavenging effect of Cr(VI) by FeS and NBC. The effect of crucial experimental conditions (FeS/NBC dosage, foreign ions, initial pH and concentration of Cr(VI) solution) were investigated. The Cr(VI) removal capacity was as high as 211.3 ± 26 mg g^-1 under the optimized condition. The practicability of FeS/NBC was examined by using simulated actual samples from tap water and lake water. The mechanism examination showed that surface adsorption/reduction and solution reduction were implicated in the removal of Cr(VI). The current work introduces a novel FeS/NBC composite prepared by an in situ pyrolysis method with excellent potential for chromium pollution remediation.

Cr(VI) immobilization by FeS-coated alumina and silica: Effects of pH and surface coating density

We investigated the feasibility of using FeS-coated alumina and silica for permeable reactive barrier (PRB) applications. By both coated materials, Cr(VI) was reduced to Cr(III), which was immobilized via surface complexation/precipitation at acidic pH, and bulk precipitation at neutral to basic pH. Both pH and surface coating density (the amount of FeS deposits per unit surface area of a supporting matrix) controlled Cr(VI) reduction capacity and [Cr,Fe](OH)₃ composition. The reduction was higher at acidic pH due to lower passivation, as evidenced by the increased production of Fe(III) (oxyhydr)oxides over Fe(II)-Fe(III) phases. The coated alumina, despite the lower amount of FeS deposits than the coated silica, showed greater reduction capacities due to its higher surface coating density, which made Fe(III) closer together to favor Fe(III) (oxyhydr)oxide formation. Since Cr(III) was preferentially substituted for Fe(III) in Fe(III) (oxyhydr)oxides, lower pH and higher surface coating density led to lower Cr fractions in [Cr,Fe](OH)₃ because of the increased production of Fe(III) (oxyhydr)oxides. Given that Cr-poor [Cr,Fe](OH)₃ is more resistant to re-oxidation, FeS-coated alumina is better for PRB applications. This study reveals the significance of the surface coating density when evaluating the effectiveness of coated materials in redox-based treatments.

Effective adsorption of quadrivalent cerium by synthesized laurylsulfonate green rust in a central composite design

The layered laurylsulfonate intercalated green rust (lauryl-S GR) was synthesized to evaluate the influence of synthesis parameters and aqueous conditions on the adsorption of Ce^IV. The maximum adsorption capacity of 305.58 mg/g by lauryl-S GR was predictably obtained. The pseudo-first-order kinetic model was appropriate in fitting the whole uptake process in a weak acid environment. Three isotherm models including Langmuir, Freundlich, and Tempkin were all reliable in depicting the isotherm adsorption process. The maximum monolayer adsorption capacity of lauryl-S GR towards Ce^IV was 315.46 mg/g. Ce species including CeO and Ce₂O₃ besides CeO₂ were matched in the XPS distribution, directly indicating the reduction reaction brought by Fe^II in the GR occurred to hydrated Ce^IV ions during the adsorption. Nano-sized Ce particles attached to the lauryl-S GRs after the adsorption experiments were observed in the morphological characterization. Flocculated materials were formed on the surface of the lauryl-S GR at a pH of 7, which further reduced the active sites and disrupted the continuous uptake of Ce^IV to the lauryl-S GR. This study expands the application of GRs and supplies an ideal iron-based material for the construction of the affiliated recovery pathway to the traditional separation of Ce.

A green rust-coated expanded perlite particle electrode-based adsorption coupling with the three-dimensional electrokinetics that enhances hexavalent chromium removal

A green rust-coated expanded perlite (GR-coated Exp-p) microelectrode was synthesized and incorporated into a column-mode three-dimensional electrokinetic (3D-EK) platform to effectively pursue a continuous Cr(VI) removal from the aqueous solution. Brucite-like layers of GR were decorated onto the Exp-p material. The molar ratio of Fe(II) to Fe(III) played a most vital role among the three synthesis factors in influencing the performance of the particle electrode. For the equilibrium adsorption experiments, the target maximum adsorption capacity of 122 mg/g was predicted by a target optimizer and desirability function at the conditions following the pH of 4.7, the initial concentration of 172.4 mg/L, the dosage of 0.28 g/L, and the temperature of 28.96 °C, respectively. SO₄^2-, Cl^-, and NO₃^- fiercely competed with Cr(VI) anions in the acidic conditions for the locally positive sites. A low concentration and a slow flow were favored in the column-mode 3D-EK platform. The pseudo-first-order and Langmuir models were suitable for describing the kinetics and isotherms of the adsorption process, respectively. Cr(VI) anions were electrostatically attracted to the silanol groups and GR surface of the adsorbent, subsequently reduced in both heterogeneity and homogeneity, and finally immobilized by coordinating with silanediol groups and silanetriol groups.

Evaluation of Cr(VI) Reducing Capability of Shewanella putrefaciens (MTTC8410) and Optimization of Operational Parameters

Bioremediation is an important technology to remediate the chromium (Cr) contaminated soil and water. In this study, Shewanella putrefaciens (MTTC8410) was used to investigate the influence of carbon concentration, pH, and temperature on reduction of hexavalent chromium [Cr(VI)] into trivalent chromium [Cr(III)]. The increased bacterial growth rate was significantly reduced the Cr(VI) concentration. In batch mode experiments, 1% starch recorded the highest reduction of Cr(VI) (90%) followed by 1% glucose (88% reduction) and a reduction of 77% was by 1% cellulose. By using various pH conditions the maximum Cr(VI) reduction was achieved at pH 7.0. In this experiment the maximum Cr(VI) reduction (75%) was observed at 35°C, followed by 30°C with 62% of Cr(VI) reduction. Bioreactor analysis revealed the highest reduction of Cr(VI) (88%) in unsterile tannery effluent. The significant levels of physico- chemical parameters were reduced in unsterile tannery effluent, as compared to the sterile tannery effluent. The experimental results revealed that the S. putrefaciens (MTTC8410) could be used as a potential bacterial strain for reduction of Cr(VI) from contaminated groundwater.

Facile integration of FeS and titanate nanotubes for efficient removal of total Cr from aqueous solution: Synergy in simultaneous reduction of Cr(VI) and adsorption of Cr(III)

In this study, a novel composite composed of iron monosulfide nanoparticles (FeS NPs) and titanate nanotubes (TNTs) was hydrothermally synthesized. Characterizations revealed the encapsulation and homogenous dispersion of FeS NPs into the interlayers of TNTs. Significant performance in removal of aqueous total Cr was acquired by efficient conversion of Cr(VI) to Cr(III) on FeS and simultaneous adsorption of Cr(III) on TNTs. Moreover, the high activity of FeS-TNTs in reduction of Cr(VI) can maintain at high oxicity or alkalinity of its solution. The synergistic effect between FeS and TNTs was derived from sheltering of FeS NPs from their self-aggregation, O₂-oxidation and the affinity of Cr(III) to TNTs. The unique properties, e.g. the solid acidity, the hollow and interlayered configuration of TNTs played important roles in high activity, good stability and reusability of FeS-TNTs.

The role of natural Fe(II)-bearing minerals in chemoautotrophic chromium (VI) bio-reduction in groundwater

To date, comparatively little is known about the role of natural Fe(II)-bearing minerals in bioremediation of chromium (VI) contaminated aquifers subject to chemoautotrophic conditions. This work employed four kinds of Fe(II)-bearing minerals (pyrite, mackinawite, wustite, and magnetite) as inorganic electron donors to support Cr(VI) bio-reduction. In batch experiments, mackinawite (FeS) performed best, with Cr(VI) removal efficiency of 98.1 ± 1.21 % in 96 h. Continuous column experiments lasting 180 d implied that groundwater chemistry and hydrodynamics influenced the Cr(VI) removal process. A breakthrough study suggested that biotic and abiotic contributions to Cr(VI) reduction were 76.0 ± 1.12 % and 24.1 ± 1.43 %, respectively. Cr(VI) was reduced to insoluble Cr(III), whereas Fe(II) and S(-II) in mackinawite were finally oxidized to Fe(III) and sulfate. Mackinawite evolved progressively into pyrrhotite. High-throughput 16S rRNA gene sequencing indicated that mackinawite-driven Cr(VI) reduction was mediated through synergistic interactions of microbial consortia; i.e. autotrophs as Acidovorax synthesized volatile fatty acids as metabolic intermediates, which were consumed by Cr(VI) reducers as Geobacter. Genes encoding enzymes for S oxidation (soxB) and Cr(VI) reduction (chrA, yieF) were upregulated. Cytochrome c participating in Fe(II) oxidation increased significantly. This work advances the development of sustainable techniques for Cr(VI) polluted groundwater remediation.

A novel composite of almandine supported humboldtine nanospheres, in situ synthesized from natural almandine, possesses high removal efficiency of Cr(Ⅵ) over a wide pH range

Preparing a cost-effective material which can been applied in a wide pH range is very crucial for the remediation of Cr(Ⅵ) polluted water. In this study, a novel material, almandine/humboldtine nanospheres (AHN) composites, was synthesized directly from almandine by one-pot method. Characterizations of XRD and SEM/TEM showed that the structure changes of almandine to nano-humboldtine leaded to significant increase of Cr(Ⅵ) removal capacities. And 96.45% of Cr(Ⅵ) was removed by AHN-24 composite at pH value of 3, initial Cr(Ⅵ) concentration of 20 mg/L, temperature of 298.15 K and dosage of 0.6 g/L. Furthermore, Cr(Ⅵ) removal capacity was only decreased from 48.23 mg/g to 34.33 mg/g when the initial pH value increased from 3 to 11, which demonstrated that the synthesized composite had a wide pH application range in Cr(Ⅵ) removal. The thermodynamic parameters (ΔG⁰ < 0, ΔH⁰ > 0 and ΔS⁰ > 0) illustrated that Cr(VI) removal process was spontaneous and endothermic. FTIR and XPS revealed that the Cr(Ⅵ) removal mechanisms included reduction-precipitation and reduction-complexation. Combined with cost analysis, all of results implied that the synthesized composites were a high efficient and low cost material for Cr(Ⅵ) pollution remediation in a wide pH range.

Two-Dimensional Titanium Carbides (Ti3C2Tx) Functionalized by Poly(m-phenylenediamine) for Efficient Adsorption and Reduction of Hexavalent Chromium

Titanium carbides (MXenes) are promising multifunctional materials. However, the negative surface charge and layer-by-layer restacking of MXenes severely restrict their application in the field of anionic pollutants, including in hexavalent chromium (Cr(VI)). Herein, Ti₃C₂T_x MXenes was functionalized through in situ polymerization and intercalation of poly(m-phenylenediamine) (PmPD), then Ti₃C₂T_x/PmPD composites were obtained. Delightedly, Ti₃C₂T_x/PmPD composites exhibited positive surface charge, expanded interlayer spacing, and enhanced hydrophobicity. Furthermore, the specific surface area of Ti₃C₂T_x/PmPD composite was five and 23 times that of Ti₃C₂T_x and PmPD, respectively. These advantages endowed Ti₃C₂T_x/PmPD composite with an excellent adsorption capacity of Cr(VI) (540.47 mg g^-1), which was superior to PmPD (384.73 mg g^-1), Ti₃C₂T_x MXene (137.45 mg g^-1), and the reported MXene-based adsorbents. The Cr(VI) removal mechanism mainly involved electrostatic adsorption, reduction, and chelation interaction. This study developed a simple functionalization strategy, which would greatly explore the potential of MXenes in the field of anionic pollutants.

Some Properties of Electron Beam-Irradiated Sheep Wool Linked to Cr(III) Sorption

We examined the characteristics of an electron beam irradiated wool with an absorbed dose of (21–410) kGy in comparison with natural wool with respect to the determination of the isoelectric point (IEP), zero charge point (ZCP), mechanism of Cr(III) sorption from higher concentrated solutions, and the modelling of the wool-Cr(III) interaction. The data of ZPC and IEP differed between natural and irradiated samples. Increasing the dose shifted the pH of ZPC from 6.85 for natural wool to 6.20 for the highest dosed wool, while the natural wool IEP moved very little, from pH = 3.35 to 3.40 for all of the irradiated samples. The sorption experiments were performed in a pH bath set at 3.40, and the determination of the residual Cr(III) in the bath was performed by VIS spectrometry under optimized conditions. The resulting sorptivity showed a monotonically rising trend with increasing Cr(III) concentration in the bath. Lower doses, unlike higher doses, showed better sorptivity than the natural wool. FTIR data indicated the formation of complex chromite salts of carboxylates and cysteinates. Crosslinks via ligands coming from different keratin chains were predicted, preferably on the surface of the fibers, but to a degree that did not yet inhibit the diffusion of Cr(III)-cations into the fiber volume. We also present a concept of a complex octahedral structure.