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Article

Enhancing Heavy Metal Removal and Stabilization in River Sediment by Combined Application of Nanoscale Zero-Valent Iron and Sediment Microbial Fuel Cells

by
Xun Xu
1,*,
Mingsong Wu
2 and
Guoling Ren
1
1
Heilongjiang Provincial Key Laboratory of Oilfield Applied Chemistry and Technology, College of Bioengineering, Daqing Normal University, Daqing 163712, China
2
College of Resources and Civil Engineering, Northeastern University, Shenyang 100819, China
*
Author to whom correspondence should be addressed.
Processes 2025, 13(4), 1235; https://doi.org/10.3390/pr13041235
Submission received: 24 March 2025 / Revised: 16 April 2025 / Accepted: 17 April 2025 / Published: 18 April 2025
(This article belongs to the Section Environmental and Green Processes)
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Abstract

:
This study investigates the effect of nanoscale zero-valent iron (NZVI) and sediment microbial fuel cells (SMFCs) on the three typical heavy metals’ (Pb, Cr and As) removal and stabilization. Results showed that the combined use of NZVI and SMFCs obtained the highest removal efficiencies in the sediment (Pb 37.7 ± 2.2%, Cr 26.4 ± 1.5% and As 30.1 ± 2.0%) and overlying water (Pb 55.8 ± 2.3%, Cr 47.6 ± 1.9% and As 45.8 ± 2.1%). The use of an NZVI electrode can transform heavy metals with relatively weak binding into forms with stronger binding, thereby diminishing their bioavailability and toxicity. After 60 days of operation with the addition of NZVI in the SMFC system, over 50% of the Pb, Cr and As in the sediment was transformed into the residual fraction. An anodic microbial communities analysis indicated that operating a SMFC can mitigate the adverse effects of NZVI on the community diversity and increase the content of electrogenic bacteria in sediments. Consequently, our findings indicated that integrating SMFCs and NZVI represents a viable approach for remediating rivers contaminated with heavy-metal-polluted sediments.

Graphical Abstract

1. Introduction

Due to the rapid pace of industrialization and the unsustainable extraction of mineral resources, substantial volumes of wastewater laden with toxic heavy metals are being released into rivers and lakes. This poses severe adverse impacts, given the high toxicity, non-biodegradability and bioaccumulative nature of these contaminants [1,2,3]. For rivers, the sediment and overlying water continue exchanging materials and energy, and the sediment serves as a primary repository for the accumulation of heavy metals and organic pollutants. Given the toxicity and persistence of heavy metals, their removal and stabilization in sediments are crucial to prevent secondary pollution and protect aquatic ecosystems and human health [2,3]. To decrease heavy metal contaminants’ bioavailability and mobility, conventional methods (such as physical, chemical and biological remediation) have been used. Nevertheless, there are concerns that these methods could be quite costly and laborious and potentially lead to further pollution [1,2,3].
Nanoscale zero-valent iron (NZVI), being an inexpensive and environmentally friendly agent, has been widely applied for the in situ remediation of the soil and sediment contamination caused by heavy metals and organic pollutants [4]. NZVI has a good immobilization effect on several common heavy metals in soil, including Pb, Zn, Se, Cr, Cd and so on [5,6,7,8,9]. The sediment microbial fuel cell (SMFC), which is a bio-electrochemical system, has been shown to significantly enhance the biodegradation of organic compounds and the removal of heavy metals from the overlying water [10,11,12]. However, the influence of humic acid originating from soil on the immobilization of heavy metals by NZVI is notable, as it competes for the same active adsorption sites [9]. The limitations of NZVI technology, including its poor stability and mobility as well as its tendency to aggregate, further hinder its environmental applications [4]. Additionally, the addition of NZVI may have an adverse effect on sediment microorganisms [13]. Therefore, further research is needed to improve the performance of NZVI and SMFCs for the remediation of heavy metal pollution in river sediments.
To date, the research on utilizing NZVI to enhance the effect of microbial fuel cells (MFCs) has been confined to the doping or alteration of electrodes, and the mechanisms involved have not been fully understood or explained [14]. The impacts of integrating SMFCs and NZVI in the remediation of heavy metals in river sediments remain unclear. Moreover, there is a lack of literature addressing the microbial mechanisms underlying the combined application of SMFCs and NZVI for heavy metal remediation in river sediments. Given the limitations of individual applications, such as the relatively slow reaction rate and incomplete heavy metal removal of the SMFC, the poor stability, tendency to aggregate and potential adverse effects on sediment microorganisms of NZVI, the integration of NZVI and the SMFC represents a novel and promising approach [4,13]. The purpose of the present study was to (1) investigate whether the combined application of the SMFC and NZVI can promote the heavy metals’ removal and stabilization compared to the individual application of SMFCs or NZVI; (2) understand the potential impact of sediment organic matter on the remediation of heavy metal pollution in river sediment; and (3) understand the microbial communities changes with the addition of the electrode and NZVI.

2. Materials and Methods

2.1. Sampling and Pretreatment Methods

The sampling point was an inland river named Tang River, located in Qinhuangdao City, Hebei Province, China, where the sediment and overlying water were collected. The sediment and overlying water samples were collected in the same way as described in a previous study [15]. The NZVI particles used in this study (supplied by Beijing DK Nano Technology Co., Ltd., Beijing, China) had a size of 50 nm. This size was determined based on the manufacturer’s specifications and was the average diameter of the particles as measured by Transmission Electron Microscopy (TEM). The dosage of NZVI is based on the optimal dosage determined by previous research, which was 60 g/L of wet sediment [15]. When conducting sediment analysis, samples collected from four different depths near the anode were mechanically homogenized. The dried sediment was then homogenized again and sieved to a particle size of less than 2 mm.
The pH, conductivity, water content and oxidation–reduction potential (ORP) of the raw sediment were 7.04 ± 0.25, 486 ± 12 μS/cm, 40.6 ± 1.9% and −105 mV, respectively. The sediment organic matter (represented as TOC) was 112.5 ± 4.8 g/kg of dry sediment. Three typical heavy metals (Pb, Cr and As), which are of great environmental significance, were chosen as representative heavy metals, and their concentrations in the raw sediment and overlying water are shown in Table 1.

2.2. Sequential Extraction Test Procedures of Heavy Metals

Sequential extraction of heavy metals according to the five-step method established mainly by Tessier et al. (1979) was used [16]. The Tessier method consists of five steps using extraction liquid made of (I) magnesium chloride, (II) sodium acetate (NaAc), (III) hydroxylamine hydrochloride (NH2OH·HCl), (IV) solution of HNO3 and H2O2 and (V) 65% HNO3, respectively. Heavy metals obtained in each step were designated as (I) exchangeable (F1), (II) bound to carbonate (F2), (III) bound to Fe-Mn oxides (F3), (IV) bound to organic matter (F4) and (V) residual (F5) [16].

2.3. Construction and Operation of Bioreactors

Four cylindrical Plexiglas bioreactors (BRs), each with an inner diameter of 30 cm and a height of 60 cm, were used in this experiment. The bioreactors, designated as BR1, BR2, BR3 and BR4, were operated for a 60-day period. Two replicates were set up for each reactor. BR1 and BR2 were designed as controls to simulate natural anaerobic degradation in the absence of connected electrodes, whereas BR3 and BR4 were operated under closed-circuit conditions. In contrast to BR1 and BR3, BR2 and BR4 had NZVI incorporated into their sediments. Initially, the bioreactors were filled with 10 cm of wet sediment, followed by the addition of 40 cm of water from the sample to cover the sediment. Throughout the experiment, any loss of overlying water due to evaporation was compensated for by adding distilled water, ensuring a consistent water level was maintained. The anode and cathode were made of graphite fiber, which has a specific surface area of 1.5 m2/g. In each bioreactor, the anode and cathode, measuring 5 cm in diameter and 30 cm in length, were positioned in the center of the sediment and submerged just beneath the water’s surface. The anodic graphite fiber brush had a surface area of 21.762 m2. The anode and cathode were linked using copper wire that was sealed with rubber, and an external resistor of 1000 Ω was incorporated into the circuit between the anode and cathode. The schematic of the setup is shown in Figure 1. The sediments and the overlying water in BRs were collected after the 60-day operation period.

2.4. Analytical Methods

The pH and conductivity of the sediment were measured using a 1:5 (w/v) sediment-to-deionized water suspension. The measurements were conducted with a PHS-3C pH meter and a DDSJ-308A conductivity meter, both from Shanghai INESA Scientific Instrument Co., Ltd. (Shanghai, China). The water content was calculated as a percentage after the sample was dried at 105 °C for 24 h until it reached a constant weight. The ORP was measured using an Ag/AgCl reference electrode (GaossUnion, Wuhan, China). A vario TOC-Elementar Total Organic Carbon Analyzer was utilized for the determination of TOC [17]. To determine the concentrations of heavy metals in sediment, a digestion pretreatment of the sediment was conducted, with the specific process detailed in reference [7]. The digested filtrate was treated with HNO3 to lower the pH to below 2, then kept at a temperature of 4 °C prior to analysis for heavy metals using the Agilent 8800 ICP-MS/MS (Agilent Technologies, Santa Clara, CA, USA). To ensure the accuracy and reliability of the ICP-MS analysis, method blanks were used to assess background interference and reagent purity. Certified reference materials (CRMs) containing all target elements were used for instrument calibration and validation to ensure the accuracy of the measurements. Triplicate analyses were performed to assess the precision of the measurements, and no anomalies were observed during the analysis [7,9]. An Ag/AgCl electrode (saturated KCl, +0.195 V vs. SHE, GaossUnion, Wuhan, China) was used as the reference electrode. Throughout the experiment, the anode and total potential were continuously recorded at intervals of 1 min by a data acquisition system and the voltages shown in the result represent averages for every 30 min. The power density and polarization curves for the SMFCs were derived by varying the external resistance from 10 to 99,999.9 Ω, with the maximum power density being identified after a 10 min stabilization period for each resistance setting. The power density P (expressed in mW/m2) was determined using the formula P = U·I/A, where U is the cell voltage (V), I is the current (A) and A denotes the surface area of the anode. Meanwhile, the internal resistance R was derived from the slope of the U-I curve, calculated as R = ΔU/ΔI.
The statistical significance of differences was assessed using variance analysis (ANOVA) with the Origin program (OriginPro 8.0, OriginLab Corporation, Northampton, MA, USA). Each trial was conducted in triplicate to ensure replication. The error bars in the figures represent the standard deviation of the mean (SD). Additionally, post hoc comparisons were performed using Tukey’s HSD test to identify significant differences between groups when the ANOVA indicated significant effects. The OriginPro 8.0 software was used for data analysis, including descriptive statistics, ANOVA, Tukey’s HSD and Kruskal–Wallis test.

2.5. Characterization of Anode Microbial Community

Anode biofilm and sediment samples were collected by the end of the operation. The 16S rRNA genes of V4 region were amplified using a specific primer (515F and 806R), which were then analyzed by a MiSeq sequencing platform (Illumina, San Diego, CA, USA) with a read length of 250 bp per read, which is sufficient to cover the V4 region and ensure accurate identification of microbial taxa (Novogene Co., Ltd., Beijing, China) [18]. To assess the microbial diversity, we calculated several alpha-diversity indices. Three indices were selected to identify community richness: Observed_otus (the number of observed species), Chao 1 (the Chao1 estimator) and ACE index (Abundance-Based Coverage Estimator). The Shannon and Simpson indices were employed to evaluate community diversity, while the coverage index was utilized to assess sequencing depth.

3. Results and Discussion

3.1. The Removal of Heavy Metals in Sediments

It is well established that the removal of heavy metals from sediments occurs via the diffusion into the overlying water, as well as the adsorption and precipitation at the anode, which is related to the valence state, speciation and electrochemical reactions of heavy metals [10,19]. The results of the analysis for the heavy metals content are illustrated in Figure 2.
As seen in Figure 2A, the removals of Pb, Cr and As in sediments were obvious and showed similar trends. The operation of SMFCs can effectively remove the Pb, Cr and As in sediments, and the combined use of the SMFC and NZVI obtained the highest removal efficiencies (Pb 37.7 ± 2.2%, Cr 26.4 ± 1.5% and As 30.1 ± 2.0%). The concentrations of Pb, Cr and As in the sediment were reduced to 0.72, 1.50 and 0.64 k/kg dw (dry weight), respectively. In the anodic chamber, the electrons produced by the degradation of the organic matter in the sediment were obtained by suitable electron acceptors (Pb2+, Cr6+/Cr3+, As5+/As3+ or others) or delivered via the wire to the cathode. Singh et al. reported a significant enhancement in the Cr(VI) reduction efficiency, which increased from 14.51% to 86.83%, following the application of NZVI in soils contaminated with Cr(VI) [20].
From Figure 2A, the open circuit conditions resulted in Pb, Cr and As removal rates in the sediment ranging from 7.7% to 12.3%, primarily attributed to biosorption and bioaccumulation processes [21]. Compared to the open-circuit condition, the SMFC operation combined with the NZVI addition significantly enhanced the heavy metal removal efficiencies, achieving a remarkable threefold increase. This suggests that among the primary biological mechanisms involved in heavy metal removal, bioreduction plays a crucial role in enhancing the removal rates. Compared to traditional methods, such as chemical precipitation and ion exchange, bioreduction offers significant advantages in terms of the environmental sustainability, cost-effectiveness and long-term stability of heavy metals [1,2,3]. High-valence heavy metals in sediments can be removed as electron acceptors during the operation of the SMFC, which has been demonstrated by Wang et al. [22]. Comparing the removal rates of the three heavy metals in BR1 and BR2 with those in BR3 and BR4, it was observed that the addition of NZVI had a negligible impact on the removal efficiencies of heavy metals. NZVI has garnered significant attention for its application in the in situ remediation of heavy-metal-induced sediment contamination. This is primarily attributed to its ability to effectively immobilize several common heavy metals in soil, including Pb, Zn, As, Cr, Cd and so on [5,6,7,8,9]. Additionally, the pH, conductivity and water content of the sediment can influence the ionization and activation of NZVI, as well as the efficiency of heavy metal removal [23,24]. After one operational period, the measured parameters of the sediment (pH: 6.89 ± 0.10, conductivity: 477 ± 15 μS/cm and water content: 41.5 ± 1.1%) were found to exhibit no significant differences compared to those of the raw sediment (pH: 7.04 ± 0.25, conductivity: 486 ± 12 μS/cm and water content: 40.6 ± 1.9%).
In the cathodic chamber (Figure 2B), the operation of SMFCs can effectively remove the Pb, Cr and As. Specifically, the SMFC with the NZVI addition achieved higher removal efficiencies (Pb 55.8 ± 2.3%, Cr 47.6 ± 1.9% and As 45.8 ± 2.1%) compared to the SMFC without NZVI, which depended on the standard redox potential of the heavy metals in the overlying water. Although these removal rates are lower than those achieved by some physical or chemical methods, the results are satisfactory considering that this level of treatment was achieved by simply connecting a cathode to the overlying water [25]. The addition of NZVI can promote the removal of heavy metals to some extent, but its enhancement effect is still not as obvious as that of the SMFC [4,10]. In addition, the continuous exchange of materials between the sediment and overlying water is also a factor that alters the content of heavy metals. The sediment and the suspended electrodes are crucial in the processes of heavy metal adsorption, desorption, dissolution and sedimentation [19]. Apart from the reductive processes, the biosorption carried out by microorganisms colonizing the electrodes played a vital role in the removal of Pb, Cr and As from both the sediment and overlying water [4,10].

3.2. Speciation Changes in Heavy Metals in Sediments

The results from the removal rates indicated that the application of SMFCs with an NZVI addition was effective in reducing the heavy metals in sediment. However, the removal rates, which are determined by measuring the total metal content in the sediment, do not provide a comprehensive assessment of the changes in the metal speciation. In sediments, metals are found in diverse binding forms: they can be incorporated into amorphous substances, attached to clay particles or iron/manganese oxyhydroxides and associated with carbonates, sulfates, other oxides and organic compounds [26]. Each form possesses distinct mobility characteristics and can influence its corresponding bioavailability and toxicity levels [7]. In this study, the five-step method was employed to categorize heavy metals into five distinct fractions: exchangeable, carbonate, Fe-Mn oxide, organic and residual [16]. The changes in the heavy metals fractions by the Tessier’s sequential extraction in BRs are illustrated in Figure 3. A statistical analysis on the residual fractions from different BRs was performed (p < 0.05).
According to Figure 3, it was observed that with the operation of the SMFC with the added NZVI in the sediment, there was a corresponding rise in the residual fraction in all bioreactors. As for Pb (Figure 3A), the majority of it was originally associated with Fe-Mn oxides (45.6%). Additionally, the carbonate, organic and residual fractions each constitute approximately 15% of the total lead content in the raw sediment. The exchangeable fraction of Pb is the lowest, at 8.6%. However, a reduction in the Pb bound to Fe-Mn oxides and carbonates led to an increase in the residual fraction. As for Cr (Figure 3B), the exchangeable fraction in the sediment was reduced from 13.8 to 2.3%, and an increase in Cr in the fraction, from 20.9 to 78.4%, was observed; the Cr associated with the residual fraction increased by nearly three times. In the raw sediment, the exchangeable (13.8%) and carbon (30.9%) fractions of Cr were non-negligible. The separate addition of NZVI or the operation of the SMFC can both transform the exchangeable and carbon fractions in the sediment into the more stable residual fractions of 66.7% and 23.9%, respectively, but the combined effect of the two treatments will greatly enhance the outcome. Arsenic (As) was mainly associated with Fe-Mn oxides (32.3%) and residual (30.2%) fractions in the raw sediment. As shown in Figure 3C, the treatment with the SMFC and NZVI obviously reduced the As concentration in the exchangeable and carbon fractions, whereas an increase in As in the residual fraction was observed. The combined application of the SMFC with NZVI was found to be the most effective strategy. It achieved an 85% reduction in the exchangeable fraction of As and a 55% reduction in the carbonate-bound fraction. Additionally, it resulted in a 79% increase in the residual fraction of arsenic. The results of this study show a higher reduction rate of the As bioavailability compared to previous studies that only used NZVI to reduce the As bioavailability in soil [27,28]. Additionally, the As associated with the organic fraction showed a significant reduction from 13.9% to 8.9% after the treatment, which could be due to the enhanced removal of organic matter in the sediment by the SMFC and the promotion of the formation of insoluble arsenates by iron. However, in the Fe-Mn oxide fraction, the application of the SMFC and NZVI did not induce a significant change. The fact is that even though arsenic is adsorbed onto the reactive surfaces of iron for immobilization, the solubility of iron arsenates is very low [28].
The exchangeable fraction denotes metals that are immediately adsorbed onto sediment particles. This portion typically signifies the components that are environmentally accessible. The carbonate fraction primarily consists of metals that either precipitate or co-precipitate with carbonates, and this segment is particularly responsive to changes in pH levels. The Fe-Mn oxide fraction encompasses both soluble metal oxides/hydroxides present under slightly acidic conditions and those metals that are linked to Fe-Mn oxyhydroxides. The organic fraction comprises metals that are tied to organic substances—such as living organisms and detritus—either through complexation or bioaccumulation processes. Lastly, the residual fraction encompasses metals that are left unextracted after the aforementioned procedures and generally represent tightly bound oxides, co-precipitates or complexes with strong affinities [29]. It is widely accepted that the initial two fractions (exchangeable and carbon) denote loosely and reversibly bound metals, whereas the subsequent three fractions are associated with more robust binding, involving specific sorption processes [30].
Comparing BR2 to BR4, it was found that the combined application of the SMFC and the addition of NZVI had the best effect on the stabilization of heavy metals, followed by the addition of NZVI alone, while the operation of the SMFC alone had the least effect. In summary, more than 50% of the Pb, Cr and As became the residual fraction in BR4. The combined proportion of the more strongly bound fractions (Fe-Mn oxide, organic and residual) of Pb, Cr and As in BR4 accounted for approximately 92.3%, 94.6% and 92.7%, respectively. The addition of NZVI has the capacity to simultaneously stabilize various heavy metals in sediment, with varying degrees of efficacy and stability. Among these metals, Cr showed the most favorable response to the treatment. The particular removal processes are contingent upon the standard redox potentials (E0) of NZVI and the heavy metal pollutants. Metals with a redox potential significantly more positive than that of Fe0 (e.g., Cr and As) are removed through the reduction and subsequent precipitation. Conversely, metals with a redox potential more negative than Fe0 (e.g., Cd and Zn) are removed by adsorption, or by both reduction and adsorption (e.g., Pb and Ni) [4,31,32]. Compared to the application of NZVI alone, the stabilization effect of heavy metals in BR4 is obviously enhanced [4,10]. The influence of the humic acid derived from the sediment on the immobilization of heavy metals by NZVI can be attributed to the competition for available adsorption sites [6,7,8]. It appears that NZVI requires a longer duration to transform organic-bound metals into a more stable form [6,7,8]. The addition of electrodes in the sediment can promote the degradation of organic matter and alter its properties, which may be the reason for the enhanced stabilization of heavy metals [17]. The changes in the sediment organic matter in different bioreactors will be analyzed in the following section.

3.3. Effect of Sediment Organic Matter Degradation on Heavy Metals

In our previous research, the combination of NZVI and electrodes demonstrated a synergistic effect in degrading organic compounds [15]. The removal efficiencies of TOC in sediments by the operation of BRs are illustrated in Figure 4. A statistical analysis on the TOC removal rates after 60 days from different BRs was performed (p < 0.05).
As shown in Figure 4, the reduction in TOC in BRs was significant and escalated with the ongoing operation. Upon completion of the 60-day operational period, BR4 exhibited the most effective TOC removal rates, recording 62.1 ± 3.6%, which was higher than those reported in previous studies [15]. This figure was trailed by BR3, which registered 49.6 ± 3.1%. These values significantly surpassed those of the natural degradation processes. This suggests that the enhanced removal efficiencies are likely due to the involvement of electrodes and the addition of NZVI. An anode can form biofilms and serve as an alternative electron acceptor to promote the degradation of organic matter in sediments. NZVI has a strong reducibility (E0 = −0.44 V), which can also act as an electron donor and react with oxidizable organic pollutants. Moreover, there is an obvious synergistic mechanism between SMFCs and NZVI in the degradation of organic pollutants in sediments [15]. Indeed, the characteristics of organic matter have a significant impact on the mechanisms of chemical reactions, which can be altered by the employment of SMFCs. Liu et al. reported that humic acid inhibits the removal efficiency of Cr(VI) adsorption by NZVI animal bone char [33]. Non-humic organic materials, such as those found in organic waste, can diminish the efficiency of the reduction process; however, this can be mitigated by the enhanced removal capabilities SMFCs [34]. The inhibition of the reduction is attributed to the solubilization of the organic matter from the sediment and the subsequent adsorption of these organic compounds onto the iron nanoparticle’s surface, which in turn reduces the availability of reactive sites necessary for the reaction [35]. The rapid aggregation of NZVI particles, driven primarily by strong magnetic forces, significantly limits their environmental applications [36]. Therefore, the combined application of SMFCs and NZVI can significantly enhance the removal of heavy metals and organic matter in sediments.

3.4. Electricity Generation

The electrical output is associated with changes in the anode properties and reduction reactions at the cathode. BR3 and BR4 were utilized to investigate the generation of electricity by SMFCs. Figure 5 presents the power densities and polarization curves of the SMFCs, measured during the stable period on the 25th day.
Figure 5 indicates that BR4 achieved the highest power density (90.85 mW/m2), surpassing BR3 (88.99 mW/m2). This suggests that the addition of NZVI may enhance the power output of SMFCs. Incorporating NZVI can facilitate electron transfer at the anode and lower the internal resistance to some extent within the SMFC [37]. In SMFCs, organic compounds are metabolized by microorganisms, resulting in the release of electrons that are either conducted towards the anode or directly engaged by sedimentary oxides. Subsequently, these electrons at the anode are transmitted to the cathode through a conductive wire, thereby generating an electrical current and potential. Consequently, the electrical output is affected by the organic matter and oxides in the sediment, as well as by the reductive reactions in the overlying water [15].

3.5. Microbial Communities Analysis in Sediments

Microorganisms may play a significant role in sediment remediation, and it is crucial to understand how they are affected by SMFCs and NZVI. The 16S rRNA gene was subjected to high-throughput sequencing to characterize the microbial communities at the anode. Samples for this microbial community analysis were collected from the anodic biofilms and sediments of different depths in the bioreactors following a 60-day operational period. The OTU counts and alpha diversity of the microbial communities are presented in Table 2. The taxonomic classification of these communities at the genus level is depicted in Figure 6.
Comparing the data in Table 2 with previous studies, both the richness and diversity parameters indicated that the diversity of microorganisms in sediments contaminated by heavy metals is generally lower, which may be due to the inhibitory effect of the heavy metals’ toxicity on the microbial communities [4,38]. Compared to BR1 and BR2, it was observed that the addition of NZVI reduced the diversity of the microbial community. The community diversity in the sediment of BR4, while is not as rich as in BR3, has increased compared to BR2. This indicated that operating a SMFC can mitigate the adverse effects of NZVI on community diversity. In our previous study, we found that the electrode can increase the content of electrogenic bacteria, which can also be shown in Figure 6 [4,10]. As is shown in Figure 6, Geobacter constituted the majority of the microbial community in BR3 (6.29%) and BR4 (6.15%), with levels exceeding those in BR1 (1.48%) and BR2 (1.92%). Furthermore, a higher prevalence of Pseudomonas species was observed in BR3 (4.59%) and BR4 (4.98%) compared to BR1 (1.14%) and BR2 (1.85%), which is likely due to the involvement of alternative anodes acting as electron acceptors.
Nevertheless, introducing NZVI into the sediment environment invariably disrupts the ecological balance, prompting a detailed examination of its effects on microbial communities. The presence of two types of NZVI exerts a detrimental effect on Escherichia coli, as the oxidation of NZVI leads to the consumption of oxygen and an accumulation of excess ferrous ions within the soil [39]. The microbial toxicity mechanism involves the production of Fe2⁺ upon exposure to NZVI, which leads to cellular structural damage and facilitates the ingress of Fe2⁺ into the cells. Upon being internalized, Fe2+ can interact with H2O2, which may result in oxidative stress and hinder the transport of nutrients [40]. Comparing BR1 and BR2, as well as BR3 and BR4, the addition of NZVI increased the content of Desulfofustis, Longilinea and Bacillus in the sediments. Introducing NZVI lowers the system’s oxidation–reduction potential, thereby promoting the proliferation of Desulfofustis. Desulfofustis is a type of sulfate-reducing bacteria that can use organic matter to reduce sulfates to H2S under anaerobic conditions, which is essential for the dehalogenation of organic halides [41]. Longilinea, a key anaerobic microorganism, plays a crucial role in breaking down carbohydrates and amino acids to produce acetic acid and hydrogen, which may produce extracellular polymeric substances (EPSs) for mitigating NZVI’s toxic effects [42]. Certain species, like the Bacillus genus, can mitigate the toxicity of NZVI by sporulating, which prevents the direct exposure of the bacteria to NZVI [43]. To conclude, the microbial communities in the sediments developed a greater ability to utilize their pollutants as a result of the environmental selection of microorganisms [44].

4. Conclusions

The present study demonstrated the effectiveness of combining NZVI and SMFCs for heavy metal (Pb, Cr and As) removal and stabilization in sediment. The addition of electrodes and NZVI reduced heavy metal concentrations in both the sediment and overlying water, while the operation of SMFCs with NZVI changed the metal speciation to more stable forms. The SMFC also enhanced the sediment organic matter degradation, improving the stabilization effect of NZVI. The microbial community analysis showed that while NZVI disturbed the sediment ecosystem, the SMFC increased the electrogenic bacteria concentration and mitigated some adverse effects.
This research introduces a novel integrated method for heavy metal remediation, offering a sustainable and eco-friendly solution that enhances both metal removal and stabilization, contributing valuable insights to the field of environmental remediation. Future work should focus on scaling up this combined approach for practical applications in larger sediment remediation projects, optimizing operational parameters and assessing long-term environmental impacts.

Author Contributions

Data curation, X.X.; writing—original draft preparation, X.X.; writing—review and editing, M.W.; supervision, G.R.; funding acquisition, G.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Microbial Oil Recovery Innovation Team, Project No. 2023-KYYWF-0032.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 13 01235 g001
Figure 1. Schematic of setup.
Figure 1. Schematic of setup.
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Figure 2. The removal efficiencies of Pb, Cr and As in sediments (A) and overlying water (B) after 60 days of operation (p < 0.05).
Figure 2. The removal efficiencies of Pb, Cr and As in sediments (A) and overlying water (B) after 60 days of operation (p < 0.05).
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Figure 3. Fractions of (A) Pb, (B) Cr and (C) As in sediments of BR1 (open circuit), BR2 (NZVI), BR3 (SMFC) and BR4 (SMFC + NZVI) after 60 days of operation (p < 0.05).
Figure 3. Fractions of (A) Pb, (B) Cr and (C) As in sediments of BR1 (open circuit), BR2 (NZVI), BR3 (SMFC) and BR4 (SMFC + NZVI) after 60 days of operation (p < 0.05).
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Figure 4. TOC removal rates in sediments during 60 days (p < 0.05).
Figure 4. TOC removal rates in sediments during 60 days (p < 0.05).
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Figure 5. Power density and polarization curves of SMFCs with and without NZVI addition.
Figure 5. Power density and polarization curves of SMFCs with and without NZVI addition.
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Figure 6. Microbial community composition in sediments from bioreactors with different treatments.
Figure 6. Microbial community composition in sediments from bioreactors with different treatments.
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Table 1. Average concentrations of heavy metals in sediments and overlying water samples.
Table 1. Average concentrations of heavy metals in sediments and overlying water samples.
PbCrAs
Raw sediment (k/kg dw)1.15 ± 0.112.04 ± 0.090.92 ± 0.05
Overlying water (mg/L)0.067 ± 0.0140.096 ± 0.0110.035 ± 0.008
Table 2. Numbers of the OTUs and alpha diversity.
Table 2. Numbers of the OTUs and alpha diversity.
Sample-IDSeq-NumOTUs-NumACEChao1ShannonSimpson Coverage
Raw sediment40,825656927,034.1217,729.037.110.00240.94
BR137,255632225,987.6517,065.117.040.00310.95
BR236,354589522,198.6715,934.826.890.00650.92
BR335,643611924,876.3216,087.757.010.00330.95
BR436,102604324,257.0816,113.346.990.00410.96
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Xu, X.; Wu, M.; Ren, G. Enhancing Heavy Metal Removal and Stabilization in River Sediment by Combined Application of Nanoscale Zero-Valent Iron and Sediment Microbial Fuel Cells. Processes 2025, 13, 1235. https://doi.org/10.3390/pr13041235

AMA Style

Xu X, Wu M, Ren G. Enhancing Heavy Metal Removal and Stabilization in River Sediment by Combined Application of Nanoscale Zero-Valent Iron and Sediment Microbial Fuel Cells. Processes. 2025; 13(4):1235. https://doi.org/10.3390/pr13041235

Chicago/Turabian Style

Xu, Xun, Mingsong Wu, and Guoling Ren. 2025. "Enhancing Heavy Metal Removal and Stabilization in River Sediment by Combined Application of Nanoscale Zero-Valent Iron and Sediment Microbial Fuel Cells" Processes 13, no. 4: 1235. https://doi.org/10.3390/pr13041235

APA Style

Xu, X., Wu, M., & Ren, G. (2025). Enhancing Heavy Metal Removal and Stabilization in River Sediment by Combined Application of Nanoscale Zero-Valent Iron and Sediment Microbial Fuel Cells. Processes, 13(4), 1235. https://doi.org/10.3390/pr13041235

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