Enhanced Cr(VI) stabilization in soil by carboxymethyl cellulose-stabilized nanosized Fe0 (CMC-nFe0) and mixed anaerobic microorganisms

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.

Application of New Polymer Soil Amendment in Ecological Restoration of High-Steep Rocky Slope in Seasonally Frozen Soil Areas

In seasonally frozen soil areas, high-steep rocky slopes resulting from open-pit mining and slope cutting during road construction undergo slow natural restoration, making ecological restoration generally challenging. In order to improve the problems of external soil attachment and long-term vegetation growth in the ecological restoration of high-steep rocky slopes in seasonally frozen areas, this study conducted a series of experiments through the combined application of polyacrylamide (PAM) and carboxymethyl cellulose (CMC) to assess the effects of soil amendments on soil shear strength, water stability, freeze-thaw resistance, erosion resistance, and vegetation growth. This study showed that the addition of PAM-CMC significantly increased the shear resistance and cohesion of the soil, as well as improving the water stability, freeze-thaw resistance, and erosion resistance, but the internal friction angle of the soil was not significantly increased after reaching a certain content. Moderate amounts of PAM-CMC can extend the survival of vegetation, but overuse may cause soil hardening and inhibit vegetation growth by limiting air permeability. It was observed by a scanning electron microscope (SEM) that the gel membrane formed by PAM-CMC helped to "bridge" and bind the soil particles. After discussion and analysis, the optimum application rate of PAM-CMC was 3%, which not only improved the soil structure but also ensured the growth of vegetation in the later stage under the optimum application rate. Field application studies have shown that 3% PAM-CMC-amended soil stably attaches to high-steep rocky slopes, with stable vegetation growth, and continues to grow after five months of freeze-thaw action, with no need for manual maintenance after one year.

Effect of Stabilized nZVI Nanoparticles on the Reduction and Immobilization of Cr in Contaminated Soil: Column Experiment and Transport Modeling

Batch and transport experiments were used to investigate the remediation of loamy sand soil contaminated with Cr(VI) using zero-valent iron nanoparticles (nZVI) stabilized by carboxymethylcellulose (CMC-nZVI). The effect of pH, ionic strength (IS), and flow rate on the removal efficiency of Cr(VI) were investigated under equilibrium (uniform transport) and non-equilibrium (two-site sorption) transport using the Hydrus-1D model. The overall removal efficiency ranged from 70 to over 90% based on the chemical characteristics of the CMC-nZVI suspension and the transport conditions. The concentration and pH of the CMC-nZVI suspension had the most significant effect on the removal efficiency and transport of Cr(VI) in the soil. The average removal efficiency of Cr(VI) was increased from 24.1 to 75.5% when the concentration of CMC-nZVI nanoparticles was increased from 10 to 250 mg L-1, mainly because of the increased total surface area at a larger particle concentration. Batch experiments showed that the removal efficiency of Cr(VI) was much larger under acidic conditions. The average removal efficiency of Cr(VI) reached 90.1 and 60.5% at pH 5 and 7, respectively. The two-site sorption model described (r2 = 0.96-0.98) the transport of Cr(VI) in soil quite well as compared to the uniform transport model (r2 = 0.81-0.98). The average retardation of Cr(VI) was 3.51 and 1.61 at pH 5 and 7, respectively, indicating earlier arrival for the breakthrough curves and a shorter time to reach maximum relative concentration at lower pH. The methodology presented in this study, combining column experiment and modeling transport using the Hydrus-1D model, successfully assessed the removal of Cr(VI) from polluted soils, offering innovative, cost-effective, and environmentally friendly remediation methodologies.

Recent advances in enhanced technology of Cr(VI) bioreduction in aqueous condition: A review

Due to the extensive application of chromate in industry, chromium-contaminated water has emerged as a significant hidden danger that threatens human health and the safety of the ecological environment. The reduction of Cr(VI) to Cr(III) through microbial processes has become one of the most notable methods for remediating water polluted by chromium due to its economic efficiency and environmentally friendly nature. However, several issues persist in its practical application, such as low reduction rates, the need for additional nutrients, and challenges in solid-liquid separation. Therefore, there is a growing focus on seeking enhanced methods for Cr(VI) microbial reduction, which has become a key area of research. This review represents the initial effort to systematically classify and summarize the means of enhancing Cr(VI) microbial reduction. It categorizes the enhancement methods into two main approaches: microbial-based and multi-method combined enhancement, offering detailed explanations for their mechanisms. This research provides both inspiration and theoretical support for the practical implementation of the Cr(VI) microbial reduction method.

Remediation of Cr(VI)-contaminated soil by sulfidated zero-valent iron: The effect of citric acid as eluant and modifying agent

Leaching and chemical reduction are two commonly used methods for Cr(VI)-contaminated soil remediation. Leaching focuses more on leaching Cr(VI) out of the soil. Chemical reduction has the disadvantages of poor fluidity of reductant. Combining these two remediation methods, this study investigated the performance of Cr(VI)-contaminated soil when H₂O and citric acid were used as eluant separately and sulfidated zero-valent iron (SZVI) as reductant. And based on the properties of Cr(VI) chelated with -COOH to form a complex and the characteristics of -OH anchored to FeS_x, citric acid modified SZVI (Cit-SZVI) was prepared. The prepared Cit-SZVI was characterized by SEM-EDS, XPS, XRD to study its surface properties. The transformation of Cr species in soil was explored by BCR sequential extraction. The results indicated Cr(VI) removal by SZVI was significantly promoted when citric acid as eluant compared with H₂O. With SZVI dosage of 2.0 wt%, 23.1 mg/L Cr(VI) was basically removed within 60 min when citric acid as eluant, while only 60% Cr(VI) was removed when H₂O as eluant even after 3 h. The k_obs of Cit-SZVI was 1.4 times that of SZVI when H₂O as eluant. The characterization of Cit-SZVI showed that more FeS_x was formed on the surface of the Cit-SZVI, and more -OH of citric acid was anchored to FeS_x, leaving -COOH available to chelate Cr(VI). Compared with H₂O as eluant and SZVI/Cit-SZVI as reducing agent, the removal effect of Cr(VI) was the best when citric acid as eluant and SZVI as reducing agent. BCR sequential extraction showed that Cr(VI) was effectually fixed, weak acid extractable Cr proportion decreased significantly and residual Cr proportion increased in the treated soil. The combination of leaching and chemical reduction proposed in this study can greatly enhance the Cr(VI) removal effect in soil, which is important for the remediation of Cr(VI)-contaminated soil.

Influence of aggregation and sedimentation behavior of bare and modified zero-valent-iron nanoparticles on the Cr(VI) removal under various groundwater chemistry conditions

Aggregation behaviors of bare, and sodium polyacrylate (PAA) and starch modified zero-valent-iron nanoparticles (nZVI), as well as their effects on the Cr (VI) removal were investigated by simulating the groundwater. Results showed that increased concentration of PAA (1-6 wt%) and starch (0.1-0.6 wt%) alleviated the aggregation of modified nZVI (abbreviated as P-nZVI and S-nZVI), while there was an optimum dosage of 4 wt% PAA and 0.3 wt% starch for the Cr (VI) removal, respectively. Moreover, as one of the fundamental water chemistry parameters, Ca²⁺ (0, 5, and 10 mg L^-1) greatly promoted the aggregation of modified nZVI, and decreased the Cr (VI) removal efficiency by them via forming bidentate bridging structure (between Ca²⁺ and PAA) or complexes (between Ca²⁺ and starch). Additionally, fulvic acid (FA) (0, 2, 5, and 10 mg L^-1) decreased the Cr (VI) removal by P-nZVI because of the significantly improved electronic repulsion. However, FA enhanced the aggregation of S-nZVI, but diminished its performance on Cr (VI) removal due to the bridging effect between FA and starch. The present study was of great importance in predicting the migration of nZVI and contaminants removal under complex geological conditions in groundwater.

Bioremediation of hexavalent-chromium contaminated groundwater: Microcosm, column, and microbial diversity studies

Sulfate reducing bacteria (SRB) have the capability of bioreducing hexavalent chromium [Cr(VI)] to trivalent chromium [Cr(III)] under sulfate-reducing conditions for toxicity reduction. However, a high amount of sulfate addition would cause elevated sulfide production, which could inhibit the growth of SRB and result in reduced Cr(VI) bioreduction efficiency. A slow release reagent, viscous carbon and sulfate-releasing colloidal substrates (VCSRCS), was prepared for a long-lasting carbon and sulfate supplement. In the column study, VCSRCS was injected into the column system to form a VCSRCS biobarrier for Cr(VI) containment and bioreduction. A complete Cr(VI) removal was observed via the adsorption and bioreduction mechanisms in the column with VCSRCS addition. Results from X-ray diffractometer analyses indicate that Cr(OH)_3(s) and Cr₂O_3(s) were detected in precipitates, indicating the occurrence of Cr(VI) reduction followed by Cr(III) precipitation. Results from the Fourier-transform infrared spectroscopy analyses show that cell deposits carried functional groups, which could adsorb Cr. Addition of VCSRCS caused increased populations of total bacteria and dsrA, which also enhanced Cr(VI) reduction. Microbial diversity results indicate that VCSRCS addition resulted in the growth of Cr(VI)-reducing bacteria including Exiguobacterium, Citrobacter, Aerococcus, and SRB. Results of this study will be helpful in developing an effective and green VCSRCS biobarrier for the bioremediation of Cr(VI)-polluted groundwater.

Influence of modified biochar supported Fe-Cu/polyvinylpyrrolidone on nitrate removal and high selectivity towards nitrogen in constructed wetlands

In this study, the biochar (BC) supported Fe-Cu bimetallic stabilized by PVP (Fe-Cu/PVP/BC) were prepared and utilized to enhance the nitrate (NO₃^-) removal and the selectivity toward nitrogen (N₂). Results showed the optimum Fe:Cu:BC ratio and the dosage of the BC (pyrolysis at 700 °C) supported Fe-Cu bimetallic stabilized by polyvinylpyrrolidone (PVP) (Fe-Cu/PVP/BC₇₀₀) were respectively 1:2:3 and 1 mg L^-1 with the selectivity toward N₂ of 31 %. This was mainly due to the synergy among Fe⁰, Cu⁰ and BC in the Fe-Cu/PVP/BC. The addition of Fe⁰ could reduce the NO₃^- through providing electron. The Cu⁰ and BC improved the selectivity of NO₃^- to N₂ through forming [Cu-NO₂^-_ads] and adjusting redox potential. The addition of Fe-Cu/PVP/BC could supply electrons for denitrification and enhance the relative abundances of Azospira and Thauera related to denitrification to improve NO₃^- removal. This result was further confirmed by the variations of denitrifying functional genes (narG, nirK, nirS and nosZ). This research provided an effective method to improve NO₃^- removal during surface water treatment in constructed wetlands (CWs) by adding Fe-Cu/PVP/BC.

Ultrasound-assisted catalytic reduction of Cr(VI) by an acid mine drainage based nZVI coupling with FeS2 system from aqueous solutions: Performance and mechanism

Nowadays, nanoscale zero valent iron (nZVI) has been extensively applied for the decontamination of various pollutants, but passivation of nZVI severely affects its reactivity in use. In this study, ultrasound (US)-assisted catalytic reduction of Cr(VI) by an acid mine drainage based nZVI (AMD-nZVI) coupling with FeS₂ system was systematically examined. Results show that the presence of FeS₂ and US induced a synergistic enhancement of Cr(VI) removal by AMD-nZVI. Nearly 98% of Cr(VI) removal was achieved by AMD-nZVI/FeS₂/US process within 60 min under optimal reaction conditions. Several coexisting substances with lower concentration including Pb(II), Ni(II), bisphenol A (BPA) and 2,4-diclorophenol (2,4-DCP) could be effectively removed in simultaneous manner with Cr(VI) removal. The inhibitory order of water matrix species on Cr(VI) removal was NO₃^- > PO₄^3- > HCO₃^- > Ca²⁺ > Mg²⁺ > Cl^-, and a serious suppression effect was induced by humic acid (HA). Addition of ethylene diamine tetra-acetic acid (EDTA) and citric acid (CA) could enhance Cr(VI) removal rate. An enhanced reaction mechanism was proposed, which involved the regeneration of more Fe²⁺ and H⁺ by AMD-nZVI/FeS₂/US process, leading to the reduction of Cr(VI) by AMD-nZVI and FeS₂ into Cr(III) species inculding Cr₂O₃ and Cr(OH)₃. This study well demonstrates that AMD-nZVI/FeS₂/US process is considered as a potential candidate for the remediation of Cr(VI) in real wasterwater.

Environmentally Friendly Methylcellulose-Based Binders for Active and Passive Dust Control

Particulate matter (PM) is an essential indicator to evaluate air pollution, threatening human health. Although PM control could be achieved by using a variety of polymeric materials, identifying effective and green materials remains elusive in dust control technology. Here, we have employed environmentally friendly cellulose modified by methyl side groups, such as methylcellulose (MC)-based polymers, and evaluated their PM reduction efficiency when utilized in active and passive dust control methods, such as dust suppressants and air filters, respectively. When 25 m/s wind was applied on soil treated by MC-based polymers, PM emissions were reduced 95% or 85% lower than the soil treated by only water or the other cellulose without methyl side groups. The MC-based polymer was also effectively suppressed mineral dust from a local copper mine in Arizona with approximately 50 times lower amounts than a synthetic polymer containing methyl side groups. Furthermore, when MC-based polymers have deposited on filters of commercial face masks, the average filtration efficiency improved to greater than 99% while maintaining airflow resistance. Our results present that environmentally friendly MC-based polymers can act as dust binders that effectively agglomerate air pollutants, preventing the PM emission from dust sources and the inhalation after being suspended in the air; thus, labeling them as essential materials for advanced active and passive dust control technology.

Successful Outcome of Phytostabilization in Cr(VI) Contaminated Soils Amended with Alkalizing Additives

This study analysed the effect of three alkalizing soil amendments (limestone, dolomite chalcedonite) on aided phytostabilization with Festuca rubra L. depending on the hexavalent chromium (Cr(VI)) level in contaminated soil. Four different levels of Cr(VI) were added to the soil (0, 50, 100 and 150 mg/kg). The Cr contents in the plant roots and above-ground parts and the soil (total and extracted Cr by 0.01 M CaCl2) were determined with flame atomic absorption spectrometry. The phytotoxicity of the soil was also determined. Soil amended with chalcedonite significantly increased F. rubra biomass. Chalcedonite and limestone favored a considerable accumulation of Cr in the roots. The application of dolomite and limestone to soil contaminated with Cr(VI) contributed to a significant increase in pH values and was found to be the most effective in reducing total Cr and CaCl2-extracted Cr contents from the soil. F. rubra in combination with a chalcedonite amendment appears to be a promising solution for phytostabilization of Cr(VI)-contaminated areas. The use of this model can contribute to reducing human exposure to Cr(VI) and its associated health risks.