Practical application potential of microbial-phosphorus minerals-alginate immobilized particles on chromium(VI)-bioreduction

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.

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.

Graphene hydrogel improves S. putrefaciens' biological treatment of dye wastewater: Impacts of extracellular electron transfer and function of c-type cytochromes

Biocompatible materials and biocarriers have attracted great attention in biological wastewater treatment owing to their excellent performance in improving pollutant removal. Graphene-based material, a biocarrier candidate, with excellent adsorbability and conductivity was increasingly applied in anaerobic digestion due to its exceptional potential in the adsorption and electron transfer process. Nevertheless, the green approach for the formation of bio-graphene complexes and their mechanism in dye removal is limited. The aim of this study is to investigate and assess the performance of biological graphene hydrogel (BGH) formed by Shewanella putrefaciens CN32 on the removal of methyl orange (MO) and methylene blue (MB). The results showed that the formation of BGH is determined by the physicochemical characteristics of graphene oxide, including sheet size, oxidation degree, and interlayer distance. BGHs significantly increased the removal efficiency of dyes in comparison to non-graphene samples, with a 24-h removal rate of MO and MB reaching 92.9% and 91%, respectively. The synergetic mechanism of BGH on the enhanced removal rate of organic dye can be ascribed to GO's ability in accelerating extracellular electron transfer and stimulating biodegradation pathways relating to c-type cytochromes, including MtrA and MtrC. These findings provided an understanding of the relationship between graphene-based nanomaterials and Shewanella, which facilitated their future application in environmental biotechnology.

Environmental biotechnology and the involving biological process using graphene-based biocompatible material

Biotechnology is a promising approach to environmental remediation but requires improvement in efficiency and convenience. The improvement of biotechnology has been illustrated with the help of biocompatible materials as biocarrier for environmental remediations. Recently, graphene-based materials (GBMs) have become promising materials in environmental biotechnology. To better illustrate the principle and mechanisms of GBM application in biotechnology, the comprehension of the biological response of microorganisms and enzymes when facing the GBMs is needed. The review illustrated distinct GBM-microbe/enzyme composites by providing the GBM-microbe/enzyme interaction and the determining factors. There are diverse GBM modifications for distinct biotechnology applications. Each of these methods and applications depends on the physicochemical properties of GBMs. The applications of these composites were mainly categorized as pollutant adsorption, anaerobic digestion, microbial fuel cells, and organics degradation. Where information was available, the strategies and mechanisms of GBMs in improving application efficacies were also demonstrated. In addition, the biological response, from microbial community changes, extracellular polymeric substances changes to biological pathway alteration, may become important in the application of these composites. Furthermore, we also discuss challenges facing the environmental application of GBMs, considering their fate and toxicity in the ecosystem, and offer potential solutions. This research significantly enhances our comprehension of the fundamental principles, underlying mechanisms, and biological pathways for the in-situ utilization of GBMs.

Experimental study on in situ remediation of Cr(VI) contaminated groundwater by sulfidated micron zero valent iron stabilized with xanthan gum

Micron zero valent iron (mZVI) was an underground remediation material, which had great application potential to replace nano zero valent iron (nZVI) from the perspective of economic and health benefits. However, mZVI was highly prone to gravitational settling, which limited its wide application for in situ remediation of contaminated groundwater. This paper was devoted to develop an efficient and economical groundwater remediation material based on mZVI, which should possess excellent stability, reactivity, and transportability. Thereby xanthan gum (XG) stabilized and Na₂S₂O₄ sulfidated mZVI (XG-S-mZVI) was synthesized and characterized with SEM, XRD, XPS, and FTIR techniques. In terms of stability, the adsorbed XG and the dispersed XG worked together to resist the sedimentation of S-mZVI. In terms of reactivity, sulfidation enhanced the electron transfer rate and electron selectivity of XG-S-mZVI, thereby improved the reactivity of XG-S-mZVI. The hexavalent chromium (Cr(VI)) removal rate constant by XG-S-mZVI was determined to be 832.4 times than bare mZVI. In terms of transportability, the transportability of XG-S-mZVI was greatly improved (~80 cm in coarse sand and ~50 cm in medium sand). Straining was the main mechanism of XG-S-mZVI retention in porous media. XG-S-mZVI in situ reactive zone (XG-S-mZVI-IRZ) was only suitable to the media with a grain size larger than 0.25 mm. This study could provide theoretical support and guidance for the implementation of IRZ technology based on mZVI.

Dynamic experiments of acid mine drainage with Rhodopseudomonas spheroides activated lignite immobilized sulfate-reducing bacteria particles treatment

Aiming at the problem that the treatment of acid mine drainage (AMD) by sulfate-reducing bacteria (SRB) biological method is susceptible to pH, metal ions, sulfate and carbon source. Lignite immobilized SRB particles (SRB-LP) and Rhodopseudomonas spheroides (R. spheroides) activated lignite immobilized SRB particles (R-SRB-LP) were prepared using microbial immobilization technology with SRB, R. spheroides and lignite as the main substrates. The dynamic experimental columns 1# and 2# were constructed with SRB-LP and R-SRB-LP as fillers, respectively, to investigate the dynamic repair effect of SRB-LP and R-SRB-LP on AMD. The mechanism of AMD treated with R-L-SRB particles was analyzed by scanning electron microscopy (SEM), fourier transform infrared (FTIR) spectrometer and low-temperature nitrogen adsorption. The result showed that the combination of R. spheroides and lignite could continuously provide carbon source for SRB, so that the highest removal rates of SO₄^2-, Cu²⁺ and Zn²⁺ in AMD by R-SRB-LP were 93.97%, 98.52% and 94.42%, respectively, and the highest pH value was 7.60. The dynamic repair effect of R-SRB-LP on AMD was significantly better than that of SRB-LP. The characterization results indicated that after R-SRB-LP reaction, the functional groups of -OH and large benzene ring structure in lignite were broken, the lignite structure was destroyed, and the specific surface area was 1.58 times larger than before reaction. It illustrated that R. spheroides provided carbon source for SRB by degrading lignite. The strong SRB activity in R-SRB-LP, SRB can co-treat AMD with lignite, so that the dynamic treatment effect of R-SRB-LP on AMD is significantly better than that of SRB-LP.

Size-dependent visible-light-enhanced Cr(VI) bioreduction by hematite nanoparticles

Light irradiation would affect the electron transfer between dissimilatory metal-reducing bacteria (DMRB) and semiconducting minerals, which may impose a great influence on the biogeochemistry cycle of heavy metals. However, the size effect of semiconducting minerals on the its electron transfer with DMRB and microbial Cr(VI) reduction under visible light irradiation is little known. Herein, the Cr(VI) reduction by Shewanella oneidensis MR-1 (MR-1) was investigated in the presence of hematite nanoparticles with average diameters of 10 nm and 50 nm in dark and under visible light irradiation. It is found that hematite nanoparticles adhered onto MR-1 cells to form the composites, leading to the decrease in surface sites and Zeta potential. Hematite mediated-Cr(VI) bioreduction rate under visible light irradiation was 0.342 h^-1, which is 3.4 folds enhancement compared with that in dark and 4.4 folds compared with the MR-1 alone under visible light irradiation. Decreasing nanoparticle size of hematite from 50 nm to 10 nm promoted the Cr(VI) reduction under visible light irradiation but impeded it in dark. It was deduced that the bioelectrons from MR-1 could promote the separation of photoelectron-hole pairs of light-irradiated hematite, which consequently enhanced the Cr(VI) bioreduction by MR-1-hematite composites. Moreover, mutant strains experiments demonstrated the vital role of c-cytochrome for the conducting network actively established by MR-1 with hematite nanoparticles. Those findings expand the understanding of the electron transfer pathway for enhancing Cr(VI) reduction by hematite-MR-1 composites, and the impact of particle size on the interaction between semiconducting mineral and electroactive bacteria under light irradiation.

Remediation of Chromium (VI) from Groundwater by Metal-Based Biochar under Anaerobic Conditions

Iron salt-modified biochar has been widely used to remove Cr(VI) pollution due to the combination of the generated iron oxides and biochar, which can bring positive charge and rich redox activity. However, there are few comprehensive studies on the methods of modifying biochar with different iron salts. In this study, two iron salt (FeCl3 and Fe(NO3)3) modification methods were used to prepare two Fe-modified biochar materials for removing Cr(VI) in simulated groundwater environment. It was revealed by systematic characterization that FeCl3@BC prepared via the FeCl3 modification method, has larger pore size, higher zeta potential and iron oxide content, and has higher Cr(VI) adsorption-reduction performance efficiency as compared to Fe(NO3)3@BC prepared via Fe(NO3)3 modification method. Combined with XRD and XPS analyses, Fe3O4 is the key active component for the reduction of Cr(VI) to Cr(III). The experimental results have shown that acidic conditions promoted Cr(VI) removal, while competing ions (SO42− and PO43−) inhibited Cr(VI) removal by FeCl3@BC. The Elovich model and intra-particle diffusion model of FeCl3@BC can describe the adsorption behavior of Cr(VI) well, indicating that both the high activation energy adsorption process and intra-particle diffusion control the removal process of Cr(VI). The Freundlich model (R2 > 0.999) indicated that there were unevenly distributed chemisorptions centers on the FeCl3@BC surface. Stability experiments exposed that FeCl3@BC was stable under neutral, acidic, and alkaline conditions. Furthermore, the main mechanisms of FeCl3@BC removal of Cr(VI) include electrostatic adsorption, chemical reduction, ion exchange, and co-precipitation. In conclusion, our findings provide a new insight for the selection of iron salt-modified biochar methods, and will also be beneficial for the preparation of more efficient Fe-modified biochars in the future.