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Article

Mechanisms in Hexavalent Chromium Removal from Aquatic Environment by the Modified Hydrochar-Loaded Bacterium Priestia megaterium Strain BM.1

by
Mingyu Wu
1,2,3,
Xiaofang Ouyang
1,2,3,
Yingchao Li
4,
Junxin Zhang
1,2,3,
Jiale Liu
1,2,3 and
Hua Yin
1,2,3,*
1
School of Environment and Energy, South China University of Technology, Guangzhou 510006, China
2
Key Laboratory of Ministry of Education on Pollution Control and Ecosystem Restoration in Industry Clusters, Guangzhou 510006, China
3
Guangdong Provincial Key Laboratory of Solid Wastes Pollution Control and Recycling, Guangzhou 510006, China
4
Qingdao Key Laboratory of Analytical Technology Development and Offshore Eco-Environment Conservation, Marine Bioresource and Environment Research Center, Ministry of Natural Resources, Qingdao 266061, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(11), 5172; https://doi.org/10.3390/su17115172
Submission received: 20 April 2025 / Revised: 20 May 2025 / Accepted: 27 May 2025 / Published: 4 June 2025
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Abstract

:
Microbial remediation of Cr(VI)-polluted wastewater offers an effective and sustainable green method. In this study, a novel strain Priestia megaterium strain BM.1 that was capable of reducing Cr(VI) was domesticated. In order to improve its Cr(VI) reduction and adsorption performance, calcium-modified hydrochar (HC-Ca) was utilized to immobilize the strain to obtain the composite material BM.1-Ca. The BM.1-Ca composite achieved a Cr(VI) removal efficiency of 97% at an initial concentration of 60 mg/L within 60 h, representing a 1.96-fold enhancement compared to BM.1 alone and demonstrating significantly improved microbial Cr(VI) removal capacity. The addition of HC-Ca was instrumental in maintaining the stable Cr(VI) removal efficiency of BM.1 in the presence of altered incubation environments and interference from co-existing ions. The reduction in Cr(VI) by BM.1 and the immobilization of Cr(III) on the surface of BM.1-Ca are the main removal mechanisms of Cr(VI). Analysis of microbial oxidative stress and extracellular polymers showed that HC-Ca was able to attenuate the oxidative stress of BM.1 as well as promote the secretion of extracellular polymers. This study reveals the intrinsic mechanism of the novel material BM.1-Ca for remediation of Cr(VI) pollution in water bodies and provides an effective method for bioremediation of Cr(VI).

1. Introduction

Heavy metal chromium (Cr) pollution is a globalized environmental problem of great concern. Cr has a wide range of industrial uses, mainly in metalworking, electroplating, and leather industries, and the wastewater, waste gas, and sludge emitted from these industries are the main sources of Cr pollution in the environment [1]. Tailings leakage and tailings dust from mining activities can likewise cause serious Cr pollution. In China, Cr(VI)-contaminated wastewater accounts for about 10% of total heavy metal discharges, posing a significant health risk in many areas [2]. Chromium exists in nature mainly in the form of trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) [3]. Cr(III) has relatively low toxicity and is one of the essential trace elements, but Cr(VI) is highly oxidizing, soluble, and toxic. Cr(VI) can be enriched into the human body through the food chain, causing serious damage to the respiratory, digestive, urinary, and nervous system, and skin, as well as inducing cancer and gene mutations [4]. Therefore, the removal of Cr(VI) from the contaminated environment is of great importance. Traditional methods for chromium pollution remediation include physical, chemical, and biological methods.
As a green remediation technology, microbial remediation is considered a sustainable and inexpensive strategy to combat heavy metal pollution in aquatic environments [5]. In nature, a portion of microorganisms has been naturally selected and evolved to develop resistance to heavy metals, which includes biosorption, bioaccumulation, biotransformation, and biovolatilization [6]. For chromium, specific functional microorganisms are able to reduce Cr(VI) to the less biotoxic Cr(III) via specialized proteins on the cell surface [7]. To date, a number of representative bacteria, such as Shewanella oneidensis MR-1, Bacillus subtilis BSn5 and Pseudomonas aeruginosa strain G12 [8,9,10], have been identified as effective in removing Cr(VI). However, single microbial remediation has certain limitations. Due to the toxic effects of heavy metals on microbial cells, the growth and metabolism of functional microorganisms can be significantly inhibited [11], thus weakening their remediation of heavy metals. Meanwhile, in complex environments, factors such as pH, temperature, co-existing ions, and nutrients can have an impact on microbial activity [12].
Microbial immobilization is an effective method to enhance microbial bioremediation, and the selection of suitable microbial immobilization vectors is a key issue [13]. In recent years, the application of novel materials in the field of environmental remediation has received much attention. Hydrochar, as a carbon material prepared from biomass under hydrothermal conditions, has the advantages of a wide range of raw materials, simple preparation process, low cost, and rich in functional groups, and shows good potential for application in pollutant adsorption [14]. However, raw hydrochar still needs to be upgraded in terms of adsorption capacity due to its shortcomings of weak binding capacity to specific heavy metals [15]. Calcium modification is a commonly used method to improve the adsorption performance of hydrochar. By introducing calcium, the active sites and charge density on the surface of hydrochar can be increased, thus enhancing its adsorption capacity for heavy metal ions [16]. Previous study demonstrated that Ca2+ affected the accumulation of metals in algae by modulating the toxicity of heavy metals to tetraodontiform fenestrae [17]. Luo et al. also confirmed that exogenous Ca addition reduced the toxicity of Cr(VI) to Penicillium oxalicum and accelerated its bioreduction for Cr(VI) [18]. Currently, the removal mechanism and application of calcium-doped hydrothermal carbon as a microbial immobilized carrier loaded with functional microorganisms for remediation of polluted water bodies are still less studied. Therefore, in this study, composites of calcium-modified hydrochar loaded with microorganisms were prepared for remediation of water body Cr(VI) pollution to investigate the removal effect and the interaction mechanism between microorganisms and modified hydrochar in the remediation process. This study provides a convenient, inexpensive, and sustainable green remediation method for Cr(VI) pollution in water bodies applicable on a global scale.
In this study, a microbial immobilized bacterial agent (BM.1-Ca) was prepared by Ca-doped hydrothermal carbon (HC-Ca) loaded with Cr(VI)-reducing bacterium Priestia megaterium strain BM.1. The efficacy of BM.1-Ca in removing Cr(VI) was tested by kinetic modeling, isotherm modeling, and influence factor experiments. Meanwhile, the mechanism of Cr(VI) removal by BM.1-Ca was explored by SEM, FTIR, and XPS. The intrinsic mechanism of the synergistic removal of Cr(VI) by microbial immobilized carrier HC-Ca and Cr(VI)-reducing bacteria BM.1 was investigated by the effect of Cr(VI) treatment on the biological factors, such as EPS and antioxidant enzymes, in the reaction system.

2. Materials and Methods

2.1. Isolation and Identification of Bacteria

Soil samples were collected from Cr(VI)-contaminated mine soil from Shaoguan City, Guangdong province, China (24°29′54″ N, 113°45′22″ E). Microorganisms were isolated and purified from the soil samples using an enrichment process. Briefly, 10 g of soil samples was placed in 90 mL of sterile water and then incubated for 30 min on a 160 rpm shaker at 28 °C and aged for 60 min at 28 °C. A known amount of the supernatant was distributed in LB liquid medium containing Cr(VI) and incubated for 48 h at 28 °C. After 10 consecutive cycles, the bacterial solution was encapsulated in a solid medium and incubated for 48 h at 28 °C, and single colonies were then isolated. The strain (BM.1) with the highest Cr(VI) removal efficiency was selected for subsequent experiments. The 16S rDNA gene was amplified by polymerase chain reaction (PCR) for molecular identification. The results were compared with the data in GeneBank by BLAST 1.4.0, and the evolutionary tree was constructed by the Neighbor-joining method using Mega.11 software.

2.2. Synthesis and Characterization of Materials

Firstly, bagasse was used as the raw biomass material; 10 g of biomass was weighed and acidified by adding 55% phosphoric acid (biomass:phosphoric acid = 1:2.5) for 12 h and then washed to neutral with deionized water. Next, the acidified biomass was mixed with 0.2 mol/L CaCl2 solution (biomass:Ca = 5:1 (w/w)) with sufficient stirring and aged at 28 °C for 24 h. The mixture was then hydrothermally decomposed at 200 °C in an autoclave reactor for a reaction time of 12 h. The Ca-modified hydrochar (HC-Ca) obtained after the reaction was washed to neutrality with anhydrous ethanol and ultrapure water sequentially, freeze-dried, and stored at room temperature. The physicochemical properties of the materials were analyzed using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS).

2.3. Adsorption Batch Experiment

Sterile solutions of HC-Ca and BM.1 bacteria were added to 50 mL of LB liquid medium containing 60 mg/L Cr(VI), which was denoted as BM.1-Ca. The inoculum count of strain BM.1 was approximately 106 CFU/mL. In addition, raw hydrochar (HC) was added under the same conditions and was denoted as BM.1-HC. The suspension was filtered through a 0.22 μm syringe filter and the remaining Cr(VI) content was analyzed by ultraviolet spectrophotometry (UVmini-1280, Shimadzu, Kyoto, Japan). Total Cr content was determined using flame atomic absorption spectrometry (ICP-OES). The effects of the dosing ratio (BM.1:HC-Ca, w/w) (1:1, 1:2, 1:3, 2:1, 3:1), pH (3, 5, 7, 9, 11), initial Cr(VI) concentration (10–100 mg/L), and temperature (20, 25, 30, 35, 40 °C) on the Cr reduction efficiency were investigated. Competitive adsorption experiments were carried out with different concentrations (0.01 mmol/L–0.1 mmol/L) of SO42−, CO32−, PO43−, NO3−, and Cu2+, Cd2+, and Pb2+ as co-existing ions.

2.4. Oxidative Stress in Bacteria

Bacteria were incubated in different initial concentrations (0, 10, 60, 120 mg/L) of Cr(VI) for 12, 24, and 48 h, respectively. Superoxide dismutase (SOD) activity, catalase (CAT) activity, reduced glutathione (GSH) activity, hydroxyl radical (⋅OH) scavenging capacity, superoxide radical (O2·−) activity, and hydrogen peroxide (H2O2) content were measured by assay kits, provided by the Beyotime Institute of Biotechnology, China.

2.5. Microbial Secretion Variation and Characterization

Bacteria were cultured in LB liquid medium with/without Cr(VI) for 8, 16, 24, 32, 40, and 48 h, respectively. The heat extraction method was used to obtain secreted extracellular polymers (EPS). Firstly, the culture solution of a 24 h reaction was centrifuged (12,000× g, 4 °C) and resuspended by adding sterile saline (0.9% NaCl solution), then the solution was heated at 80 °C for 30 min and centrifuged (12,000× g, 4 °C) for 10 min. The supernatant obtained was passed through a 0.22 μm filter membrane to obtain a colorless and transparent liquid as the extracted EPS. The EPS samples were stored at −20 °C before analysis. The polysaccharides and the proteins were quantified by the phenol-sulphate acid method [19] and Lowry’s method [20], separately. The fluorescence excitation-emission matrix of microbial EPS was determined by fluorescence spectrometry (AqualogR, Horiba, EEM, Kyoto, Japan). EEM spectra were obtained with emission (Em) wavelengths ranging from 250 nm to 475 nm in 1 nm increments and excitation (Ex) wavelengths ranging from 250 to 475 nm in 5 nm increments. The scanning speed was set to 1200 nm/min.

2.6. Data Analysis

Each experiment was repeated three times. All experimental data in the graphs are the mean ± standard deviation of three independent replicates. Experimental data are plotted by Origin 9.0.

3. Results and Discussion

3.1. Identification of Bacteria and Characterization of Materials

A heavy metal tolerant strain, BM.1, was screened from soil contaminated with heavy metals caused by mine pollution, and it was able to effectively remove Cr(VI) from contaminated water, with a removal rate of more than 90% of Cr(VI) less than 20 mg/L in 24 h (Figure S1). Phylogenetic tree analysis (Figure 1a) showed that strain BM.1 was 99% similar to Priestia megaterium strain (CP049296). SEM images showed that BM.1 was full of rod-shaped bacteria (Figure 1b). This strain was seldom reported in the removal of Cr(VI) from water bodies, while Bacillus megaterium PMW-03 showed excellent ability in the remediation of soil contaminated by phosphate mines with Cr(VI) [21]. In order to evaluate the effect of Cr on the growth of Priestia megaterium strain BM.1, the growth curves at different Cr concentrations were measured. As shown in Figure S2, in the absence of Cr(VI), the system enters the logarithmic growth period at 5 h and the delayed growth period after 16 h. The addition of 60 mg/L Cr(VI) inhibited the growth of BM.1. To increase the tolerance of BM.1 under high Cr(VI) stress, we prepared the novel material BM.1-Ca. BM1-Ca consists of a composite of modified hydrochar HC-Ca and strain BM.1. The hydrochar itself contained abundant pore-like structures and the surface of the material appeared rough and granular, indicating that Ca was successfully loaded on the material. The large number of pore-like structures increased the specific surface area of the material, providing sufficient space for microbial growth. Figure 1c clearly showed the extensive attachment of bacteria on the surface of the material, indicating that Priestia megaterium strain BM.1 was successfully loaded onto the material. The surface functional groups of HC-Ca were determined by FTIR analysis (Figure S3), and the results revealed that the stretching vibration of the -OH functional group was a broad peak centred at 3340 cm−1 [22], and the peaks located at 2925, 1697, and 1112 cm−1 corresponded to -CH, C=O, and C=C, respectively. The stretching vibrational band generated near 875 cm−1 was associated with the presence of Ca-O [23]. The loading of BM.1 resulted in significant characteristic peaks in BM.1-Ca within 1340–1640 cm−1 associated with extracellular polymer proteins produced by the cells [24], adding more adsorption sites on the adsorbent. BM.1-Ca showed an increase in the intensity of the characteristic peaks of functional groups such as -OH, C=O, and C-O compared to BM.1, suggesting that BM.1-Ca possessed more excellent adsorption capability. The analysis of elemental composition of HC-Ca by complete XPS spectroscopy (Figure S4) revealed that it contained C, O, and Ca, providing direct evidence of Ca loading into pristine hydrochar. The loading of BM.1 resulted in the appearance of N peaks in the spectrum of BM.1-Ca, which could be attributed to the high amount of proteins in the microorganisms.

3.2. Ca-Doped Hydrothermal Carbon Facilitates Hexavalent Chromium Removal by BM.1

The effect of BM.1 and BM.1-Ca on the removal of 60 mg/L Cr(VI) is shown in Figure 2a. Cr(VI) was removed by BM.1 within 24 h, and the remaining concentration of Cr(VI) was 30.3 mg/L at 60 h of reaction. The residual concentration of Cr(VI) by BM.1-Ca loaded with modified hydrochar was 1.8 mg/L at 60 h. It revealed that the significant enhancement of the removal of Cr(VI) was loaded with BM.1-Ca. Previous studies have shown that loading of microorganisms onto the surface of biochar can alleviate the toxic effects of heavy metals [25], thereby increasing microbial activity and facilitating the growth and metabolism of microorganisms under heavy metal stress. Ca doping increases the functional group Ca-O, which increases the adsorption sites on the surface of hydrothermal charcoal [26], and a small amount of free Ca2+ can be involved in microbial biochemical processes such as maintenance of osmotic pressure within the vesicle, as well as stabilization and strengthening of cell membranes and cell walls [27]. And the modified hydrochar itself has a good adsorption capacity; therefore, the overall ability to remove Cr(VI) becomes stronger [28]. Compared with BM.1-Ca, the residual concentration of Cr(VI) of BM.1-HC loaded with raw hydrochar was 20.6 mg/L after 60 h of reaction, indicating that the promotion effect of raw hydrochar on BM.1 was weaker, which might be due to the lower adsorption capacity of raw hydrochar as well as the limited protective effect on microorganisms.
In order to further explore the adsorption mechanism of BM.1 and BM.1-Ca, we established adsorption kinetic and adsorption thermodynamic models. The fitted data are shown in (Tables S1 and S2; Figures S5 and S6). The kinetic fitting of the experimental data was carried out using a proposed primary kinetic model and a proposed secondary kinetic model. The calculated correlation coefficients (R2) indicate that the proposed secondary kinetic model is more suitable for the adsorption of Cr(VI) by BM.1 and BM.1-Ca. This indicates that the main mechanism in the adsorption process is chemisorption, which includes ion exchange, precipitation, and complexation. The adsorption equilibrium time was about 24 h. The adsorption rate of BM.1-Ca was 1.70 times higher than that of BM.1, which indicated that the addition of HC-Ca could greatly enhance the adsorption efficiency of BM.1 for Cr(VI). The adsorption isotherm reveals more deeply the nature of the adsorbent-heavy metal interactions and the distribution pattern of heavy metals at equilibrium. Both the Freundlich and Langmuir models are able to appropriately describe the adsorption process of BM.1-Ca with BM.1. The Langmuir model correlation coefficient (R2 > 0.98) is larger than the Freundlich model correlation coefficient (R2 > 0.95). This indicates that for both BM.1 and BM.1-Ca, removal of Cr(VI) is more in line with the Langmuir model and the adsorption process is dominated by monolayer adsorption. The maximum adsorption amount was 38.47 mg/g for BM.1 and 80.70 mg/g for BM.1-Ca.
To further investigate the potential mechanism of Cr(VI) removal and reduction by BM.1 and BM.1-Ca, we determined the remaining Cr(VI) and Cr(III) in aqueous solution as well as the immobilized Cr(VI) and Cr(III) in the solid, respectively (Figure 2b). In the BM.1 reaction group, the remaining Cr(VI) in the solution was 50.5% and Cr(III) was 36.0%, and the Cr(VI) bound to the solid surface was 4.4% and Cr(III) was 9.1%. In the BM.1-Ca reaction group, the remaining Cr(VI) in the solution was 12.4%, Cr(III) was 47.8%, the solid surface bound Cr(VI) was 7.6%, and Cr(III) was 30.6%. The percentages of Cr(VI) converted to Cr(III) after BM.1 and BM.1-Ca treatments were 45.1% and 84.5%, respectively. This indicates that the main mechanism of Cr(VI) removal by BM.1 and BM.1-Ca is bioreduction. Based on this, we evaluated the chromate reductase activity using the method in [29], which demonstrated that the addition of HC-Ca promoted the activity of BM.1 secretion of chromate reductase (Figure S7). The low fixation efficiency of HC and Ca-HC for total Cr and the presence of a large amount of Cr(VI) in the solution indicated that the pristine and modified hydrothermal carbon had little reducing effect on Cr(VI). Overall, immobilization of microorganisms in modified hydrochar can enhance the removal efficiency of Cr(VI).

3.3. Influencing Factors for Hexavalent Chromium Removal

3.3.1. Dosing Ratio

As shown in Figure 3a, the Cr(VI) removal efficiencies at different dosing ratios (BM.1:HC-Ca, w/w) were 72.1%, 96.5%, 97.2%, 68.2%, and 67.1%, respectively. The removal efficiency increased significantly when HC-Ca accounted for a higher proportion of the composite. Specifically, the removal efficiency at a 1:2 ratio was 24.4% higher than that at a 1:1 ratio. This enhancement can be attributed to the protective effect of the HC-Ca on the microorganisms, which mitigated the toxicity of Cr(VI) to BM.1. However, a slight decrease in removal efficiency was observed at a 1:3 ratio. Previous studies have indicated that excessive hydrochar carbon can exhibit toxicity to microorganisms [30], thereby inhibiting microbial Cr(VI) removal. Additionally, when BM.1 dominated the dosing ratio, the removal efficiency did not improve significantly, possibly due to nutrient limitations in the medium. Based on these results, a 1:2 dosing ratio was selected for subsequent experiments.

3.3.2. Concentration

As shown in Figure 3b, within the Cr(VI) concentration range of 20–100 mg/L, BM.1-Ca exhibited enhanced removal efficiencies of 2.9%, 10.7%, 46.9%, 27.0%, and 4.7% compared to BM.1 alone. These results demonstrate that HC-Ca addition significantly improves Cr(VI) removal, particularly at higher concentrations. Previous studies have indicated that the primary mechanism of Cr(VI) removal by bacterial-carbon composites involves microbial growth and metabolic activity [31]. However, as Cr(VI) concentrations increase, their toxicity to microorganisms also rises, thereby inhibiting bioreduction efficiency [32]. In this study, HC-Ca played a protective role by reducing Cr(VI) toxicity to BM.1 [33]. Additionally, the modified hydrochar itself provided abundant adsorption sites for Cr(VI), further enhancing the overall removal capacity of BM.1-Ca.

3.3.3. Temperature

The removal of Cr(VI) by BM.1-Ca was increased by 24.1%, 47.0%, 42.6%, 40.1%, and 33.4%, respectively, compared to that of BM.1 in the range of 20–40 °C (Figure 3c). Both materials showed increasing removal efficiency from 20 °C to 35 °C, followed by a decline at 40 °C. This can be explained by the fact that the chromate reductase activity produced by the microorganisms increased with increasing temperature, enhancing the Cr(VI) removal. However, excessive temperatures cause microbial cellular proteins to begin to denature. This alteration of the folded structure impaired the function of these proteins, limiting the ability of microorganisms to carry out key enzyme activities, such as chromate reductase activity [34]. The structural alteration of microbial proteins at 40 °C particularly impaired enzymatic functions, including chromate reductase activity. Based on these observations, 25 °C was selected for subsequent experiments.

3.3.4. pH Values

Figure 3d showed that the removal of Cr(VI) by BM.1-Ca increased by 12.7%, 36.6%, 46.9%, 40.0%, and 15.5% compared to that of BM.1 in the range of pH 3–7. The pH played a crucial role in the removal of Cr(VI) by BM.1 and BM.1-Ca because it not only affects the physiological response of microorganisms, but also changes the morphology of Cr(VI) in the aqueous solution. In the pH range of 5–9, BM.1 and BM.1-Ca showed a high removal rate of Cr(VI) [35]. On the one hand, under acidic conditions, Cr(VI) mainly exists in the form of HCrO4−. In general, microorganisms show better adsorption efficiency for Cr(VI) at slightly acidic pH values probably because HCrO4− has a greater affinity for the microbial surface in acidic environments [36]. On the other hand, this is because microorganisms are in a good growth environment in weakly acidic to weakly alkaline environments; therefore, the chromate reductase activity produced by the microorganisms is stronger. However, Cr(VI) removal was significantly reduced in excessively acidic or alkaline environments, which may be due to the fact that the chromate reductase activity may be reduced due to changes in the enzyme structure or a reduction in the required cofactors [37]. Therefore, in this study, the subsequent experimental pH was 7.

3.3.5. Co-Existing Ions

In actual heavy metal contaminated wastewater, the pollutants are diverse and complex, containing different types of heavy metal cations as well as a variety of anions [32]. Figure 3e shows the effect of Cu(II), Cd(II), and Pb(II) on Cr(VI) removal at different concentrations in a binary heavy metal system. In the presence of 0.01 mmol/L Cd(II) or Pb(II), the removal of Cr(VI) varied slightly (<5%). When the concentrations of Cd(II) and Pb(II) were increased from 0.01 to 0.1 mmol/mL, the removal of Cr(VI) by BM.1-Ca decreased by 32.2% and 23.5%, respectively, and the removal rate of BM.1 decreased by 27.5% and 19.9% for Cr(VI) removal. This was attributed to the fact that, at low concentrations of Cd(II) and Pb(II), the adsorption sites on BM.1 and BM.1-Ca caused Cd(II) and Pb(II) to be adsorbed on the solid surface, which in turn had less effect on the microbial activity in the system. With increasing concentrations of Cd(II) and Pb(II), the toxicity to microorganisms increased; although some of the cationic heavy metals were still removed by the adsorbents (Figure S8), the toxicity to microorganisms increased, weakening microbial-mediated electron transfer.In the case of Cu(II) co-existence, there was no significant change in the removal rate of Cr(VI), which may be due to the fact that Cu(II) can act as an auxiliary electron-transfer factor for microorganisms, and protect chromium reductase from oxidative damage [38]. It has been shown that the reduction in Cr(VI) was inhibited when the concentration of Cu(II) was higher than 200 mg/L [39].
Figure 3f shows the effect of co-existing anions such as CO32−, PO43−, SO42−, and NO3− on Cr(VI) removal. Co-existing anion concentration less than 0.01 mmol/L has little effect on the removal of Cr(VI), probably because a small amount of anion was adsorbed on the surface of the material through complexation, precipitation, and ion exchange together with Cr(VI) [40]. The removal of Cr(VI) was inhibited with the increase in anion concentration. The inhibitory effect was more obvious when the concentration of CO32− and PO43− was greater than 0.1 mmol/L, probably because the hydrolysis of CO32− and PO43 changed the pH in the solution (Figure S9), which was not conducive to the microbial removal of Cr(VI). Among them, the removal of Cr(VI) by BM.1-Ca decreased by 35.6%, 44.1%, and 40.0% when the concentrations of CO32−, PO43−, and SO42− were increased from 0.1 mmol/L to 0.1 mmol/L. The results showed that BM.1-Ca was effective in the removal of Cr(VI) by microorganisms, while the removal of BM.1 for Cr(VI) decreased by 25.6%, 30.3%, and 18.2%. This may be due to the ability of CO32−, PO43−, and SO42− to form stable complexes with Cr(VI) with low bioavailability [41], which inhibited the removal of Cr(VI). Some studies have shown that the addition of PO43− and SO42− promotes the reduction in Cr(VI) by specific microorganisms [42], which is contrary to our findings. This may be because PO43−, as a co-existing pollutant, although at low concentrations is able to promote the metabolism of BM.1 as a phosphorus source, the simultaneous co-existence of PO43− and Cr(VI) at higher concentrations may interfere with microbial selectivity for the pollutant, and therefore reduce the removal of Cr(VI) [43]. For NO3−, lower concentrations may participate in the microbial nitrogen cycle, thus promoting microbial growth and metabolism, whereas higher concentrations inhibit microbial denitrification in conjunction with higher concentrations of Cr(VI), which is detrimental to microbial growth and metabolism, thus inhibiting the removal of Cr(VI) [44].
In conclusion, the removal of Cr(VI) by BM.1-Ca was still significantly better than that of BM.1 in the face of unfavorable factors in the environment, which proved that the removal of pollutants by microorganisms in complex aqueous environments could be enhanced by loading modified aqueous carbon, and showed that BM.1-Ca has good application prospects in actual Cr(VI) polluted water.

3.4. Characterization of BM.1-Ca After Hexavalent ChromiumTreatment with BM.1

Morphological changes in BM.1 and BM.1-Ca after 24 h of Cr(VI) treatment are shown in Figure 4a,b. Bacteria in which the cells contracted had a wrinkled surface. The effect was somewhat attenuated by the addition of HC-Ca. Other findings of this cell surface, which changes after exposure to Cr(VI), are similar [45]. The wrinkles on the bacterial surface may be due to chemical interactions between metal ions and biomolecules in the outer membrane layer. Another possibility is due to changes in the composition of the bacterial envelope in the presence of heavy metals during their growth period [46]. Information on the interaction of functional groups with metal ions was obtained by FTIR spectroscopy (Figure 4c). The functional group species of BM.1 and BM.1-Ca did not change significantly, suggesting that the functional groups on the surface of the material had little effect on the reduction in Cr(VI). The weakening of the intensity of the bands at 555, 1029, 1225, 1390, 1540, 1640, 2925, and 3277 cm−1 indicates the involvement of functional groups including -OH, -CH, N-H, P=O, C-O, and C=O in the Cr adsorption process. Among them, C=O was able to complex with Cr(VI) and oxygen-containing functional groups were able to complex with Cr(III) [47]. Hydroxyl groups are able to undergo ion exchange reactions with Cr(VI) [40]. The prominent characteristic peaks at 1350–1650 cm−1 are associated with extracellular polymer proteins produced by cells [24], and COOH bending vibrations are characteristic of proteins in amino acids near 1400 cm−1 [48]. The P=O vibrational peak near 1225 cm−1 and the C-O vibrational peaks changed significantly, which is likely due to the fact that the functional groups of polysaccharides in EPS were involved in the adsorption of Cr [49].
The valence state of Cr and the changes in each functional group were further explored by XPS (Figure 4d). First, after Cr(VI) treatment, Cr 2p peaks appeared in the total spectra in both BM.1 and BM.1-Ca, indicating the successful removal of Cr by BM.1 and BM.1-Ca. Figure 5a demonstrates that the peak area of Cr(III) in the Cr 2p spectrum of BM.1-Ca after Cr(VI) treatment (84.7%) is larger than the peak area of Cr(III) in Cr 2p after Cr(VI) treatment of BM.1 (62.3%), which provides direct evidence that HC-Ca promotes the bioreduction in Cr(VI) by BM.1. In the C 1s spectra (Figure 5b), the peak at 284.64 eV is the C of amino acid side chains in microorganisms. The peak at 285.73 eV is associated with the C-OH of alcohols, amines, or acetamides. And the peak at 288.10 eV is related to carbonate, carboxylic acid ester, or amide O-C=O [50]. Oxygen-containing groups change their binding energy before and after the reaction, suggesting that they are involved in the immobilization of Cr(VI) and Cr(III). In the O 1s spectra (Figure 5c), the peak area of hydroxyl groups decreased from 25.25% to 19.36%, indicating that a large number of hydroxyl groups was consumed in the reaction, exchanged with Cr(VI) ions or formed precipitates with Cr(III). In the Ca 2p spectra (Figure 5d), the peak area varied less. It indicates that Ca plays a small role in fixing Cr and that its main role is to protect microbial structural stability.

3.5. Oxidative Stress Response of Cells and Secretion of Antioxidant Enzymes

Reduction from Cr(VI) to Cr(III) by microorganisms results in excessive reactive oxygen species (ROS) production, which disrupts cell membranes and affects cellular activity [51]. ROS are inducers of oxidative stress, including superoxide anion O2·−, H2O2, and ⋅OH. When an organism is stressed by an unfavorable environment, the dynamic balance between intracellular ROS production and the cellular antioxidant system is disrupted and oxidative stress ensues [52]. Figure 6a,b shows that the highest levels of superoxide radicals as well as hydrogen peroxide were found at 12 h, which indicates that the oxidative stress occurred most strongly in organisms subjected to Cr(VI) stress at 12 h. At 24 h and 48 h, the contents of O2·− and H2O2 decreased to a certain extent, respectively, which may be attributed to the substantial reduction in Cr(VI) concentration in the solution at that time. The contents of superoxide radicals and hydrogen peroxide were much higher in the 120 mg/L Cr(VI) treatment with or without HC-Ca than in the treatment of low concentrations of Cr(VI). Below 60 mg/L Cr(VI) treatment, superoxide radicals as well as hydrogen peroxide were significantly reduced. Moreover, the addition of HC-Ca reduced the generation of superoxide radicals and hydrogen peroxide. This demonstrates that modified hydrochar can inhibit intracellular ROS generation within a certain concentration range. It also proved that pollutant concentration is an important factor in determining the degree of oxidative stress in cells.
When bacteria were subjected to chromium stress, the genes encoding the relevant enzyme antioxidants in the cells were activated to produce large amounts of CAT and superoxide SOD to counteract the production of ROS and reduce cytotoxicity [53]. The activities of SOD and CAT increased rapidly with time within 12 h (Figure 6c,d), this is attributed to the fact that Cr(VI) stress increases the level of ROS in microbial cells in a short period of time. SOD is usually responsible for catalyzing the conversion of superoxide to hydrogen peroxide and oxygen, while CAT is usually responsible for catalyzing the conversion of hydrogen peroxide to water and oxygen [54]. The activities of SOD and CAT were inhibited at 120 mg/L Cr(VI) treatment. In general, the ability of cells to resist pollutant stress is limited. When high concentrations of Cr(VI) cause intracellular ROS accumulation to exceed the timely response capacity of the antioxidant system, the organism may not be able to resist oxidative stress.
GSH is also important in the cellular response to oxidative stress, and its synthesis is mainly related to sulfur uptake and cysteine synthesis. GSH binds to free radicals in the body via sulfhydryl groups, which can directly reduce the free radicals [55]. GSH viability increased over time with 60 mg/L Cr(VI) treatment. At the same time, the inhibition of intracellular hydroxyl radicals was increasing (Figure 6e,f). This indicates that the content of GSH was positively correlated with the hydroxyl radicals’ inhibition efficiency. The content of GSH also increased with the addition of HC-Ca. Overall, the higher the pollutant concentration, the stronger the microbial oxidative stress, which was counteracted by the microorganisms by secreting specific antioxidant enzymes. The addition of modified hydrochar attenuated microbial oxidative stress and promoted the production of antioxidant enzymes.

3.6. Characteristic Analysis of Extracellular Polymers

EPS play an important role in forming biofilms and mediating the mass transfer of toxicants to immobilized cells in biological aggregates [56]. Previous studies have shown that EPS are mainly composed of proteins and polysaccharides. And the abundance of functional groups on the proteins and polysaccharides of EPS gives unsaturated biofilms a large number of binding sites for direct Cr immobilization [57]. When Cr(VI) was present in the solution, both BM.1 and BM.1-Ca produced large amounts of EPS at the beginning of the reaction phase, which peaked at 16 h at 168.1 mg/L and 147.6 mg/L, respectively, both of which were higher than the amount of EPS secreted by BM.1 in the absence of Cr(VI) stress (Figure 7a) and polysaccharide and protein content varied similarly (Figure 7b,c). It also corroborates the conclusion of previous studies that microorganisms can act as a barrier by secreting large amounts of EPS, and thus resist external stress. The content of EPS secreted by BM.1-Ca was higher than that of BM.1 at the late stage of the response, indicating that loading HC-Ca can promote the microbial ability to secrete EPS. It is worth noting that under Cr(VI) stress, there was a decreasing trend in all treatment groups after 30 h. From the previous experiments, it can be seen that Cr(VI) was removed in large quantities from the solution after 24 h so that BM.1 and BM.1-Ca produced EPS contents close to those of the control group that was not subjected to Cr(VI). It has been demonstrated that the adsorption of heavy metals by EPS-covered cells has a better affinity for heavy metals than cells from which EPS were removed [58]. In addition to preventing the direct contact of heavy metals with cells, proteins in EPS may be associated with a large number of extracellular enzymes that have been shown to reduce Cr(VI) to the less toxic Cr(III), further mitigating its damage to cells [59].
In order to better investigate the role of EPS in the removal of Cr(VI) and the synergistic effect of HC-Ca on BM.1, the fluorescence intensity of EPS was analyzed using 3D-EEM spectroscopy at 12 h versus 24 h during different treatments (Figure 7d–i). Component A at Ex/Em = 280/350 nm indicated the presence of tryptophan and proteins, which are capable of being able to mediate microbial cytoprotection and important substances for binding heavy metals [60,61]. Component B at Ex/Em = 270/300 nm is an aromatic protein-like substance, and its weak fluorescence intensity suggests that it may be part of the post-reaction product [62]. Component C at Ex/Em = 240–300/400 nm is related to xanthohumic acid [63]. The very weak fluorescence intensity of components B and C suggests that these substances are not significantly involved in the immobilization of heavy metals [57]. The changes in the fluorescence intensity of component A indicate that tryptophan and protein-like substances in BM.1 and BM.1-Ca are produced in large quantities and are involved in the binding of heavy metals when exposed to Cr(VI). It is also possible that in the presence of strongly oxidized tryptophan, protein-like substances oxidize to aromatic ring compounds, forming a protective shield that acts as a microbial defence mechanism against heavy metal stress [59]. The relatively high fluorescence intensity of EPS after the addition of HC-Ca at the late stage of the reaction proved that it promoted the synthesis of EPS and enhanced its metabolic capacity under Cr(VI) stress.

3.7. Proposed Mechanism of Hexavalent Chromium Removal

Although some studies have investigated the effects of modified hydrochar on the growth and metabolism of microorganisms in response to pollutants and their removal capacity, there is still a lack of comprehensive studies on the interaction mechanisms between modified hydrochar and microorganisms in the remediation of heavy metals. The study showed by adsorption kinetic fitting that BM.1-Ca removal is more consistent with quasi-secondary kinetics, suggesting that the mechanism is chemisorption. The mechanism of Cr(VI) removal by BM.1-Ca is summarized in Figure 8. The Cr(VI)-reducing bacterium BM.1, the active species in BM.1-Ca, was able to reduce Cr(VI) to Cr(III) through its physiological metabolism. The EPS produced by BM.1 was able to adsorb the reduction product, Cr(III).HC-Ca, and the microbial immobilizing carrier was present in BM.1-Ca. It can provide a stable metabolic site and appropriate carbon source for BM.1, and at the same time alleviate the toxicity of Cr(VI) to BM.1. The abundant functional groups on the surface of BM.1-Ca can provide a large number of adsorption sites, and the functional groups, such as -OH, C=O, C-O, N-H, P-O, etc., participated in the adsorption of Cr(VI) with Cr(III) through the complexation and ion-exchange reactions. In addition, the addition of microbial carriers promoted the generation of EPS and weakened the oxidative stress suffered by the microorganisms, both of which together contributed to the ability of BM.1-Ca to remove Cr(VI).

4. Conclusions and Outlook

In this study, we successfully domesticated a Cr(VI)-reducing bacteria BM.1. The ability of the microorganism to remove Cr(VI) was significantly enhanced by the immobilization of BM.1 on HC-Ca prepared by the immobilization technique, and BM.1-Ca showed good Cr(VI) removal performance under different culture environments and c-existing ions. In addition, the immobilization technique significantly enhanced the ability of BM.1 to secrete antioxidant enzymes in response to heavy metal stress and stimulated the production of more ESP. The present study reveals the detailed mechanism of Cr(VI) removal by BM1.-Ca. Microorganisms acted synergistically with the modified hydrochar to successfully remove Cr(VI) through the mechanism of extracellular reduction, as well as complexation and precipitation on the surface of the material. Cr(VI) is a heavy metal pollutant present in the environment that is difficult to be removed in one step. BM.1-Ca, as a convenient, environmentally friendly and sustainably applied remediation material, possesses a great potential for practical remediation. In addition, the environmental risks of BM.1-Ca, such as the stability of adsorption and the impact on the ecosystem, still need to be assessed when it is used for the remediation of Cr(VI) waters in the future, which is still an important direction for future research. In summary, this study provides a new idea for bioremediation of Cr(VI) pollution.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17115172/s1.

Author Contributions

M.W.: methodology, software, writing—original draft, writing review and editing. X.O.: formal analysis, writing—review and editing. Y.L.: formal analysis, writing—review and editing, J.Z.: investigation, data curation. J.L.: conceptualization, writing—review and editing. H.Y.: writing review and editing, supervision, funding acquisition. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported financially by the Local Innovation and Entrepreneurship Team Project of Guangdong Special Support Program (2019BT02L218), National Key Research and Development Program of China (No. 2018YFC1802800), and the Guangdong Science and Technology Program (2020B121201003).

Institutional Review Board Statement

This study did not require ethical approval because it exclusively involved laboratory experiments with the immobilization of BM.1 on HC-Ca and chemical analyses of Cr(VI) reduction, with no human or animal subjects.

Informed Consent Statement

Informed consent statement is not applicable. This study exclusively involved laboratory experiments with the immobilization of BM.1 on HC-Ca and chemical analyses of Cr(VI) reduction, with no human participants or personal data.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Hua, T.; Li, Y.; Hu, Y.; Zhang, Y.; Hou, B.; Yin, R.; Lu, H.; Ji, X.; Bai, X.; Lu, A.; et al. Back to chromite as a mineralogical strategy for long-term chromium pollution control. Nat. Commun. 2025, 16, 1975. [Google Scholar] [CrossRef] [PubMed]
  2. Park, I.; Tabelin, C.B.; Jeon, S.; Li, X.; Seno, K.; Ito, M.; Hiroyoshi, N. A review of recent strategies for acid mine drainage prevention and mine tailings recycling. Chemosphere 2019, 219, 588–606. [Google Scholar] [CrossRef] [PubMed]
  3. Yang, Z.; Zhang, X.; Jiang, Z.; Li, Q.; Huang, P.; Zheng, C. Reductive materials for remediation of hexavalent chromium contaminated soil: A review. Sci. Total Environ. 2021, 773, 145654. [Google Scholar] [CrossRef] [PubMed]
  4. Su, H.; Fang, Z.; Tsang, P.; Fang, J.; Zhao, D. Stabilisation of nanoscale zero-valent iron with biochar for enhanced transport and in-situ remediation of hexavalent chromium in soil. Environ. Pollut. 2016, 214, 94–100. [Google Scholar] [CrossRef]
  5. Yaashikaa, P.R.; Palanivelu, J.; Hemavathy, R.V. Sustainable approaches for removing toxic heavy metal from contaminated water: A comprehensive review of bioremediation and biosorption techniques. Chemosphere 2024, 357, 141933. [Google Scholar] [CrossRef]
  6. Sharma, N.; Sodhi, K.K.; Kumar, M.; Singh, D.K. Heavy metal pollution: Insights into chromium eco-toxicity and recent advancement in its remediation. Environ. Nanotechnol. Monit. Manag. 2021, 15, 100388. [Google Scholar] [CrossRef]
  7. Ackerley, D.F.; Gonzalez, C.F.; Park, C.H.; Blake, R.; Keyhan, M.; Matin, A. Chromate-Reducing Properties of Soluble Flavoproteins from Pseudomonasputida and Escherichia coli. Appl. Environ. Microbiol. 2004, 70, 873–882. [Google Scholar] [CrossRef]
  8. Mohamed, A.; Yu, L.; Fang, Y.; Ashry, N.; Riahi, Y.; Uddin, I.; Huang, Q. Iron mineralhumic acid complex enhanced Cr(VI) reduction by Shewanella oneidensis MR-1. Chemosphere 2020, 247, 125902. [Google Scholar] [CrossRef]
  9. Ma, S.; Song, C.; Chen, Y.; Wang, F.; Chen, H. Hematite enhances the removal of Cr(VI) by Bacillus subtilis BSn5 from aquatic environment. Chemosphere 2018, 208, 579–585. [Google Scholar] [CrossRef]
  10. An, Q.; Deng, S.; Xu, J.; Nan, H.; Li, Z.; Song, J.L. Simultaneous reduction of nitrate and Cr(VI) by Pseudomonas aeruginosa strain G12 in wastewater. Ecotoxicol. Environ. Saf. 2019, 191, 110001. [Google Scholar] [CrossRef]
  11. Liu, Y.; Kang, Z.; Wang, Q.; Wang, T.; Song, N.; Yu, H. One-step synthesis of ferrous disulfide and iron nitride modified hydrochar for enhanced adsorption and reduction of hexavalent chromium in Bacillus LD513 by promoting electron transfer and microbial metabolism. Bioresour. Technol. 2024, 396, 130415. [Google Scholar] [CrossRef] [PubMed]
  12. Nurfarahin, A.; Mohamed, M.; Phang, L. Culture medium development for microbial-derived surfactants production—An overview. Molecules 2018, 23, 1049. [Google Scholar] [CrossRef] [PubMed]
  13. Shao, M.; Shi, Z.; Zhang, X.; Zhai, B.; Sun, J. Synthesis and properties of biodegradable hydrogel based on polysaccharide wound dressing. Materials 2023, 16, 1358. [Google Scholar] [CrossRef] [PubMed]
  14. Chen, N.; Cao, S.Y.; Zhang, L.; Peng, X.; Wang, X.B.; Ai, Z.H.; Zhang, L.Z. Structural dependent Cr(VI) adsorption and reduction of biochar: Hydrochar versus pyrochar. Sci. Total Environ. 2021, 783, 147084. [Google Scholar] [CrossRef]
  15. Wan, Y.; Luo, H.; Cai, Y.; Dang, Z.; Yin, H. Selective removal of total Cr from a complex water matrix by chitosan and biochar modified-FeS: Kinetics and underlying mechanisms. J. Hazard. Mater. 2023, 454, 131475. [Google Scholar] [CrossRef]
  16. Li, Y.; Li, S.; Pan, X.; Zhao, X.; Guo, P. Eco-utilization of steel slag: Preparation of Fe based calcium silicate hydrate and its application in As(V) removal. Appl. Surf. Sci. 2022, 597, 153763. [Google Scholar] [CrossRef]
  17. Kovacik, J.; Dresler, S. Calcium availability but not its content modulates metal toxicity in Scenedesmus quadricauda. Ecotox. Environ. Saf. 2018, 147, 664–669. [Google Scholar] [CrossRef]
  18. Luo, Y.; Ye, B.; Ye, J.; Pang, J.; Xu, Q.; Shi, J.; Long, B.; Shi, J. Ca2+ and SO42− accelerate the reduction of Cr(VI) by Penicillium oxalicum SL2. J. Hazard. Mater. 2020, 382, 121072. [Google Scholar] [CrossRef]
  19. Dubois, M.; Gilles, K.A.; Hamilton, J.K.; Rebers, P.T.; Smith, F. Colorimetric method for determination of sugars and related substances. Anal. Chem. 1956, 28, 350–356. [Google Scholar] [CrossRef]
  20. Bradford, M.M. A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem. 1976, 72, 248–254. [Google Scholar] [CrossRef]
  21. Guo, S.; Xiao, C.; Zheng, Y.; Li, Y.; Chi, R. Removal and potential mechanisms of Cr (VI) contamination in phosphate mining wasteland by isolated Bacillus megatherium PMW-03. J. Clean. Prod. 2021, 322, 129062. [Google Scholar] [CrossRef]
  22. Yu, Y.; Long, Y.; Wu, H. Near-complete recovery of sugar monomers from cellulose and lignocellulosic biomass via a two-step process combining mechanochemical hydrolysis and dilute acid hydrolysis. Energy Fuels 2015, l30, 571–1578. [Google Scholar] [CrossRef]
  23. Wang, L.; Wang, J.; Wei, Y. Facile synthesis of eggshell biochar beads for superior aqueous phosphate adsorption with potential urine P-recovery. Colloids Surf. A Physicochem. Eng. Asp. 2021, 622, 126589. [Google Scholar] [CrossRef]
  24. Dong, X.; Zhai, X.; Yang, J.; Pei, Y.; Guan, F.; Chen, Y.; Duan, J.; Hou, B. Desulfovibrio-induced gauzy FeS for efficient hexavalent chromium removal: The influence of SRB metabolism regulated by carbon source and electron carriers. J. Colloid Interface Sci. 2024, 674, 938–950. [Google Scholar] [CrossRef]
  25. Schommer, V.A.; Vanin, A.P.; Nazari, M.T.; Ferrari, V.; Dettmer, A.; Colla, L.M.; Piccin, J.S. Biochar-immobilized Bacillus spp. for heavy metals bioremediation: A review on immobilization techniques, bioremediation mechanisms and effects on soil. Sci. Total Environ. 2023, 881, 163385. [Google Scholar] [CrossRef]
  26. Shen, J.; Huang, G.; Yao, Y.; Li, M.; Zhang, P.; Zhao, K.; Rosendahl, S. Development of calcium-modified biochar for enhanced phytoremediation of human- induced salt pollutants (HISPs). Chemosphere 2024, 335, 141860. [Google Scholar] [CrossRef]
  27. Augustynowicz, J.; Lekka, M.B.; Wrobel, P.M. Mechanical, properties of Callitriche cophocarpa leaves under Cr(VI)/Cr(III) influence. Acta Physiol. Plant 2014, 36, 2025–2032. [Google Scholar]
  28. Gascóa, G.; Paz-Ferreirob, J.; Álvareza, M.L.; Saaa, A.; Méndez, A. Biochars and hydrochars prepared by pyrolysis and hydrothermal carbonisation of pig manure. Waste Manag. 2018, 79, 395–403. [Google Scholar] [CrossRef]
  29. Banerjee, S.; Misra, A.; Chaudhury, S.; Dam, B. A Bacillus strain TCL isolated from Jharia coalmine with remarkable stress responses, chromium reduction capability and bioremediation potential. J. Hazard Mater. 2019, 367, 215–223. [Google Scholar] [CrossRef]
  30. Zhao, Y.; Li, X.; Li, Y.; Bao, H.; Xing, J.; Zhu, Y.; Nan, J.; Xu, G. Biochar Acts as an Emerging Soil Amendment andIts Potential Ecological Risks: A Review. Energies 2023, 16, 010410. [Google Scholar]
  31. Mishra, S.; Chen, S.H.; Saratale, G.D.; Saratale, R.G.; Ferreira, L.F.R.; Bilal, M.; Bharagava, R.N. Reduction of hexavalent chromium by Microbacterium paraoxydans isolated from tannery wastewater and characterization of its reduced products. J. Water Process. Eng. 2021, 39, 101748. [Google Scholar] [CrossRef]
  32. Singh, P.; Kumar, A.; Kirrolia, A.; Kumar, R.; Yadav, N.; Bishnoi, N.R.; Lohchab, R.K. Removal of sulphate, COD and Cr(VI) in simulated and real wastewater by sulphate reducing bacteria enrichment in small bioreactor and FTIR study. Bioresour. Technol. 2011, 102, 677–682. [Google Scholar] [CrossRef] [PubMed]
  33. Zhu, X.; Li, X.; Kou, Z.; Wang, J.; Shang, X.; Chen, C. The adsorption effects and mechanisms of biochar immobilized sulfate-reducing bacteria (SRB) on Cr(VI). J. Agro-Environ. Sci. 2021, 40, 866–875. [Google Scholar]
  34. Chang, J.; Deng, S.; Liang, Y.; Chen, J. Cr(VI) removal performance from aqueous solution by Pseudomonas sp. strain DC-B3 isolated from minesoil: Characterization of both Cr(VI) bioreduction and total Cr biosorption processes, Environ. Sci. Pollut. Res. 2019, 26, 28135–28145. [Google Scholar] [CrossRef]
  35. Kabir, M.M.; Fakhruddin, A.N.M.; Chowdhury, M.A.Z.; Pramanik, M.K.; Fardous, Z. Isolation and characterization of chromium(VI)-reducing bacteria from tannery effluents and solid wastes. World J. Microbiol. Biotechnol. 2018, 34, 126. [Google Scholar] [CrossRef]
  36. Xu, L.; Luo, M.; Jiang, C.; Wei, X.; Kong, P.; Liang, X.; Liu, H. In vitro reduction of hexavalent chromium by cytoplasmic fractions of Pannonibacter phragmitetus LSSE-09 under aerobic and anaerobic conditions. Appl. Biochem. Biotechnol. 2011, 166, 933–941. [Google Scholar] [CrossRef]
  37. Sheng, Y.; Bibby, K.; Grettenberger, C.; Kaley, B.; Macalady, J.L.; Wang, G.; Burgos, W.D. Geochemical and temporal infuences on the enrichment of acidophilic iron-oxidizing bacterial communities. Appl. Environ. Microb. 2016, 82, 3611–3621. [Google Scholar] [CrossRef]
  38. Sau, G.B.; Chatterjee, S.; Mukherjee, S.K. Chromate reduction by cell-free extract of Bacillus firmus KUCr1. Pol. J. Microbiol. 2010, 59, 185–190. [Google Scholar] [CrossRef]
  39. Hora, A.; Shetty, K. Inhibitory and stimulating effect of single and multi-metal ions on hexavalent chromium reduction by Acinetobacter sp. Cr-B2. World J. Microbiol. Biotechnol. 2014, 30, 3211–3219. [Google Scholar] [CrossRef]
  40. Wang, T.; Zheng, L.; Liu, Y.; Tang, W.; Fang, T.; Xing, B. A novel ternary magnetic Fe3O4/g-C3N4/Carbon layer composite for efficient removal of Cr (VI): A combined approach using both batch experiments and theoretical calculation. Sci. Total Environ. 2020, 730, 138928. [Google Scholar] [CrossRef]
  41. Zha, S.; Yu, A.; Wang, Z.; Shi, Q.; Cheng, X.; Liu, C.; Deng, C.; Zeng, G.; Luo, S.; Zhao, Z.; et al. Microbial strategies for effective hexavalent chromium removal: A comprehensive review. Chem. Eng. J. 2024, 489, 151457. [Google Scholar] [CrossRef]
  42. Villegas, L.B.; Pereira, C.E.; Colin, V.L.; Abate, C.M. The effect of sulphate and phosphate ions on Cr(VI) reduction by Streptomyces sp. MC1, including studies of growth and pleomorphism. Int. Biodeter. Biodegr. 2013, 82, 149–156. [Google Scholar] [CrossRef]
  43. Fang, J.; Sun, P.; Xu, S.; Luo, T.; Lou, J.; Han, J.; Song, Y. Impact of Cr(VI) on P removal performance in enhanced biological phosphorus removal (EBPR) system based on the anaerobic and aerobic metabolism. Bioresour. Technol. 2012, 121, 379–385. [Google Scholar] [CrossRef] [PubMed]
  44. Wang, Q.; Zhao, Y.; Chao, C.; Liu, Y.; Liu, Y.; Zhang, C.; Tong, Y.; Bamanu, B.; Nuramkhaan, M. Recovery mechanism of bio-promoters on Cr(VI) suppressed denitrification: Toxicity remediation and enhanced electron transmission. Water Res. 2024, 252, 121230. [Google Scholar] [CrossRef]
  45. Hossan, S.; Hossain, S.; Islam, M.R.; Kabir, M.H.; Ali, S.; Islam, M.S.; Imran, K.M.; Moniruzzaman, M.; Mou, T.J.; Parvez, A.K.; et al. Bioremediation of hexavalent chromium by chromium resistant bacteria reduces phytotoxicity. Int. J. Environ. Res. Public Health 2020, 17, 6013. [Google Scholar] [CrossRef]
  46. Hemambika, B.; Kannan, V.R. Intrinsic characteristics of Cr6+-resistant bacteria isolated from an electroplating industry polluted soils for plant growth-promoting activities. Appl. Biochem. Biotechnol. 2012, 167, 1653–1667. [Google Scholar] [CrossRef]
  47. Imron, M.F.; Setiawan, W.; Putranto, T.W.C.; Abdullah, S.R.S.; Kurniawan, S.B. Biosorption of chromium by live and dead cells of Bacillus nitratireducens isolated from textile effluent. Chemosphere 2024, 359, 142389. [Google Scholar] [CrossRef]
  48. Guan, Y.; Huang, B.; Qian, C.; Wang, L.; Yu, H. Improved PVDF membrane performance by doping extracellular polymeric substances of activated sludge. Water Res. 2017, 113, 89–96. [Google Scholar] [CrossRef]
  49. Tian, T.; Zhou, K.; Li, Y.; Liu, D.; Yu, H. Phosphorus recovery from wastewater prominently through a Fe (II)–P oxidizing pathway in the autotrophic iron dependent denitrification process. Environ. Sci. Technol. 2020, 54, 11576–11583. [Google Scholar] [CrossRef]
  50. An, H.; Tian, T.; Wang, Z.; Jin, B.; Zhou, J. Role of extracellular polymeric substances in the immobilization of hexavalent chromium by Shewanella putrefaciens CN32 unsaturated biofilms. Sci. Total Environ. 2022, 810, 151184. [Google Scholar] [CrossRef]
  51. Hasanuzzaman, M.; Raihan, M.R.H.; Siddika, A.; Rahman, K.; Nahar, K. Supplementation with Ascophyllum nodosum extracts mitigates arsenic toxicity by modulating reactive oxygen species metabolism and reducing oxidative stress in rice. Ecotoxicol. Environ. Saf. 2023, 255, 114819. [Google Scholar] [CrossRef] [PubMed]
  52. Feng, M.; Yin, H.; Cao, Y.; Peng, H.; Lu, G.; Liu, Z.; Dang, Z. Cadmium-induced stress response of Phanerochaete chrysosporium during the biodegradation of 2,2’,4,4’-tetrabromodiphenyl ether (BDE-47). Ecotoxicol. Environ. Saf. 2018, 154, 45–51. [Google Scholar] [CrossRef] [PubMed]
  53. Kaur, N.; Kaur, J.; Grewal, S.K.; Singh, I. Effect of heat stress on antioxidative defense system and its amelioration by heat acclimation and salicylic acid pre-treatments in three pigeonpea genotypes. Indian J. Agric. Biochem. 2019, 32, 106–110. [Google Scholar] [CrossRef]
  54. Jiang, J.; Liu, H.; Li, Q.; Gao, N.; Yao, Y.; Xu, H. Combined remediation of Cd phenanthrene co-contaminated soil by Pleurotus cornucopiae and Bacillus thuringiensis FQ1 and the antioxidant responses in Pleurotus cornucopiae. Ecotoxicol. Environ. Saf. 2015, 120, 386–393. [Google Scholar] [CrossRef]
  55. Liu, W.; Zhang, J.; Jin, Y.; Zhao, X.; Cai, Z. Adsorption of Pb(II), Cd(II) and Zn(II) by extracellular polymeric substances extracted from aerobic granular sludge: Efficiency of protein. J. Environ. Chem. Eng. 2015, 3, 1223–1232. [Google Scholar] [CrossRef]
  56. Shou, W.; Kang, F.; Lu, J. Nature and value of freely dissolved EPS ecosystem services: Insight into molecular coupling mechanisms for regulating metal toxicity. Environ. Sci. Technol. 2018, 52, 457–466. [Google Scholar] [CrossRef]
  57. Luo, X.; Zhou, X.; Peng, C.; Shao, P.; Wei, F.; Li, S.; Liu, T.; Yang, L.; Ding, L.; Luo, X. Bioreduction performance of Cr (VI) by microbial extracellular polymeric substances (EPS) and the overlooked role of tryptophan. J. Hazard Mater. 2022, 433, 128822. [Google Scholar] [CrossRef]
  58. Xu, R.; Cao, J.; Feng, G.; Luo, J.; Feng, Q.; Ni, B.; Fang, F. Fast identification of fluorescent components in three-dimensional excitation-emission matrix fluorescence spectra via deep learning. Chem. Eng. J. 2022, 430, 132893. [Google Scholar] [CrossRef]
  59. Tian, J.; Han, Y.; Yin, P.; Zhang, J.; Guo, T.; Li, H.; Hou, Y.; Song, Y.; Guo, J. Response of dissimilatory perchlorate reducing granular sludge (DPR-GS) system to high-strength perchlorate and starvation stress in UASB reactor: Performance, kinetics and recovery mechanism. J. Environ. Chem. Eng. 2023, 11, 109414. [Google Scholar] [CrossRef]
  60. Priyadarshanee, M.; Das, S. Spectra metrology for interaction of heavy metals with extracellular polymeric substances (EPS) of Pseudomonas aeruginosa OMCS-1 reveals static quenching and complexation dynamics of EPS with heavy metals. J. Hazard. Mater. 2024, 466, 133617. [Google Scholar] [CrossRef]
  61. Zhang, K.; Zhang, D.; Wu, X.; Xue, Y. Continuous and efficient immobilization of heavy metals by phosphate-mineralized bacterial consortium. J. Hazard. Mater. 2021, 416, 125800. [Google Scholar] [CrossRef] [PubMed]
  62. Guo, Z.; Zhou, H.; Yin, H.; Wei, X.; Dang, Z. Functional bacterial consortium responses to biochar and implications for BDE-47 transformation: Performance, metabolism, community assembly and microbial interaction. Environ. Pollut. 2022, 313, 120120. [Google Scholar] [CrossRef] [PubMed]
  63. Wu, P.; Wanga, Z.; Bhatnagar, A.; Jeyakumar, P.; Wang, H.; Wang, Y.; Li, X. Microorganisms-carbonaceous materials immobilized complexes: Synthesis, adaptability and environmental applications. J. Hazard. Mater. 2021, 416, 125915. [Google Scholar] [CrossRef]
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Figure 1. Phylogenetic tree of strain BM.1 (a), SEM images of BM.1 (b), and BM.1-Ca (c).
Figure 1. Phylogenetic tree of strain BM.1 (a), SEM images of BM.1 (b), and BM.1-Ca (c).
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Figure 2. Variation of Cr(VI) removal by different adsorbents with time (a), distribution of Cr species in adsorbent and solution (b).
Figure 2. Variation of Cr(VI) removal by different adsorbents with time (a), distribution of Cr species in adsorbent and solution (b).
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Figure 3. Removal efficiency of Cr(VI) by BM.1 and BM.1-Ca with different dosing ratios (a), initial concentration (b), pH (c), temperature (d), cationic metals (e), and anions (f).
Figure 3. Removal efficiency of Cr(VI) by BM.1 and BM.1-Ca with different dosing ratios (a), initial concentration (b), pH (c), temperature (d), cationic metals (e), and anions (f).
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Figure 4. SEM (a,b), FTIR spectra, (c) and XPS full spectra (d) of BM.1 and BM.1-Ca after Cr(VI) treatment.
Figure 4. SEM (a,b), FTIR spectra, (c) and XPS full spectra (d) of BM.1 and BM.1-Ca after Cr(VI) treatment.
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Figure 5. XPS spectral analysis of different elements after Cr(VI) treatment: Cr 2p (a), C 1s (b), O 1s (c), Ca 2p (d).
Figure 5. XPS spectral analysis of different elements after Cr(VI) treatment: Cr 2p (a), C 1s (b), O 1s (c), Ca 2p (d).
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Figure 6. Content of intracellular H2O2 (a), O2·−(b), CAT (c), and SOD (d) activities; hydroxyl radical inhibition (e) and GSH (f) activities in BM.1 and BM.1-Ca at different times under Cr(VI) stress.
Figure 6. Content of intracellular H2O2 (a), O2·−(b), CAT (c), and SOD (d) activities; hydroxyl radical inhibition (e) and GSH (f) activities in BM.1 and BM.1-Ca at different times under Cr(VI) stress.
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Figure 7. Changes in total EPS (a), polysaccharides (b), and proteins (c) with time and 3D-EEM spectra (d–i) of secretions in BM.1 and BM.1-Ca after Cr(VI) treatment.
Figure 7. Changes in total EPS (a), polysaccharides (b), and proteins (c) with time and 3D-EEM spectra (d–i) of secretions in BM.1 and BM.1-Ca after Cr(VI) treatment.
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Figure 8. Cr(VI) removal mechanism diagrams by BM.1-Ca.
Figure 8. Cr(VI) removal mechanism diagrams by BM.1-Ca.
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Wu, M.; Ouyang, X.; Li, Y.; Zhang, J.; Liu, J.; Yin, H. Mechanisms in Hexavalent Chromium Removal from Aquatic Environment by the Modified Hydrochar-Loaded Bacterium Priestia megaterium Strain BM.1. Sustainability 2025, 17, 5172. https://doi.org/10.3390/su17115172

AMA Style

Wu M, Ouyang X, Li Y, Zhang J, Liu J, Yin H. Mechanisms in Hexavalent Chromium Removal from Aquatic Environment by the Modified Hydrochar-Loaded Bacterium Priestia megaterium Strain BM.1. Sustainability. 2025; 17(11):5172. https://doi.org/10.3390/su17115172

Chicago/Turabian Style

Wu, Mingyu, Xiaofang Ouyang, Yingchao Li, Junxin Zhang, Jiale Liu, and Hua Yin. 2025. "Mechanisms in Hexavalent Chromium Removal from Aquatic Environment by the Modified Hydrochar-Loaded Bacterium Priestia megaterium Strain BM.1" Sustainability 17, no. 11: 5172. https://doi.org/10.3390/su17115172

APA Style

Wu, M., Ouyang, X., Li, Y., Zhang, J., Liu, J., & Yin, H. (2025). Mechanisms in Hexavalent Chromium Removal from Aquatic Environment by the Modified Hydrochar-Loaded Bacterium Priestia megaterium Strain BM.1. Sustainability, 17(11), 5172. https://doi.org/10.3390/su17115172

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