The shuttling effects and associated mechanisms of different types of iron oxide nanoparticles for Cu(II) reduction by Geobacter sulfurreducens

Crucial roles of soil inherent Fe-bearing minerals in enhanced Cr(VI) reduction by biochar: The electronegativity neutralization and electron transfer mediation

Biochar has been used for soil Cr(VI) remediation in the last decade due to its enriched redox functional groups and good electrochemical properties. However, the role of soil inherent Fe-bearing minerals during the reduction of Cr(VI) has been largely overlooked. In this study, biochar with different electron-donating capacities (EDCs) was produced at 400 °C (BC400) and 700 °C (BC700), and their performance for Cr(VI) reduction in soils with varied properties (e.g., Fe content) was investigated. The addition of BC400 caused around 14.2-36.0 mg g-1 Cr(VI) reduction after two weeks of incubation in red soil, paddy soil, loess soil, and fluvo-aquic soil, while a less Cr(VI) was reduced by BC700 (2.57-16.7 mg g-1) with smaller EDCs. The Cr(VI) reduction by both biochars in different soils was closely related to Fe content (R2 = 0.93-0.98), so red soil with the richest Fe (14.8% > 1.79-3.49%) showed the best reduction capability, and the removal of soil free Fe oxides (e.g., hematite) resulted in 71.9% decrease of Cr(VI) reduction by BC400. On one hand, Fe-bearing minerals could increase the soil acidity, neutralize the surface negative charge of biochar, enhance the contact between Cr(VI) and biochar, and thus facilitate the direct Cr(VI) reduction by biochar in soils. On the other hand, Fe-bearing minerals could also facilitate the indirect Cr(VI) reduction by mediating the electron from biochar to Cr(VI) with the cyclic transformation of Fe(II)/Fe(III). This study demonstrates the key role of soil Fe-bearing minerals in Cr(VI) reduction by biochar, which advances our understanding on the biochar-based remediation mechanism of Cr(VI)-contaminated soils.

Deciphering the interaction of heavy metals with Geobacter-induced vivianite recovery from wastewater

Vivianite recovery from wastewater driven by Geobacter is one of the promising approaches to address the challenges of phosphorus (P) resource shortage and eutrophication. However, the interfere of heavy metals which are prevalent in many actual wastewater with this process is rarely reported. In this study, we investigated the impact of heavy metals (i.e., Cu and Zn ions) on microbial activity, Fe reduction, P recovery efficiency, and their fate during Geobacter-induced vivianite recovery process. The experimental results showed that low and medium concentrations of Cu and Zn prolonged the Fe reduction and P recovery time but had little effect on the final P recovery efficiency. However, high concentrations of Cu and Zn ultimately inhibit vivianite formation. In addition, the different concentrations of Cu and Zn showed different effects on the morphology of the recovered vivianite. The migration of Cu and Zn was analysed by stepwise extraction of heavy metals in the vivianite. Medium concentrations of Cu and Zn were more likely to co-precipitate with vivianite, while adsorption was the primary mechanism at low concentrations. Furthermore, there were differences in the fate of Cu and Zn, and a competition mechanism was observed. Finally, we found that increasing the Fe/P ratio can significantly reduce the residues of heavy metals in vivianite. It also increased the adsorbed Cu and Zn proportion and reduced co-precipitation. These results provide insights into improving the efficiency of vivianite recovery and managing the environmental risks of heavy metal in the recovered product.

One-step synthesis of a novel natural mineral-derived Fe@BC for enhancing Cr(VI) bioreduction: Synergistic role of electron transfer and microbial metabolism

Iron minerals, which exert excellent biocompatibility and reactivity with redox-active microorganisms, have attracted attention as a precursor to synthesizing composite materials with higher catalytic efficiency in driving redox-active microorganisms to reduce Cr(VI). However, researches on the effective preparation method of composites, the interaction between bacteria and composite materials and the mechanism of electron transfer are still scarce. In this work, Fe-complex@BC prepared by a one-step method using goethite was used for chromium treatment together with soil microorganisms. The composite was the best-performing in promoting Cr(VI) bioreduction (up to 3.48 mg (L·h)^-1) than Fe-complex (2.26 mg (L·h)^-1) and biochar (0.5 mg (L·h)^-1), even about 19 times higher than that of bioreduction without materials. Specifically, Fe-complex@BC shortened the electron transfer distance due to its excellent adsorption properties for bacteria and Cr(VI). Its high redox activity also promoted Cr(VI) bioreduction by directly enhancing electron transfer. In addition, the presence of the Fe(III)/Fe(II) cycle proved that the active sites of composite could be regenerated to reduce Cr(VI) persistently by receiving extracellular electrons from bacteria. High-throughput 16 S rDNA gene sequencing indicated the composite could promote the proliferation of electrochemically active bacteria, which directly enhanced bioreduction. This study developed the low-cost Fe@BC material prepared by a one-step co-pyrolysis method, which exerts a synergistic effect with soil microorganisms and presents a promising potential for chromium pollution treatment.

The role of iron nanoparticles on anaerobic digestion: mechanisms, limitations, and perspectives

Anaerobic digestion (AD) is the most widely used technology for organic matter treatment. However, multiple types of research have reported on improving the process because different operation inhibition factors and limitations affect the performance of AD process. Owing to the increasing use of iron-nanoparticles (Fe-NP) on AD, this review addresses the knowledge gaps and summarizes the finding from academic articles based on (i) the AD upgrading operations: limitations and upgrade techniques, (ii) Fe-NPs mechanisms on AD, (iii) Fe-NP effect on microbial communities associated to AD systems, and (iv) perspectives. The selected topics give the Fe-NP positive effects on the AD methane-production process in terms of gas production, effluent quality, and process optimization. The main results of this work indicate that (i) Fe-NP addition can be adapted among different feedstocks and complement other pretreatments, (ii) Fe-NP physicochemical characteristics enhance biogas production via direct interspecies electron transfer (DIET) mechanisms, and Fe-ion release due to their structure and their conductivity capability, and (iii) syntrophic bacteria and acetoclastic methanogens have been reported as the communities that better uptake Fe-NPs on their metabolisms. Finally, our research perspectives and gaps will be discussed to contribute to our knowledge of using Fe-NPs on AD systems.

Responses of microbial community composition and function to biochar and irrigation management and the linkage to Cr transformation in paddy soil

Combining biochar with irrigation management to alter the microbial community is a sustainable method for remediating soils contaminated by heavy metals. However, studies on how these treatments promote Cr(VI) reduction are limited, and the corresponding microbial mechanisms are unclear. Therefore, we conducted a pot experiment to explore the responses of soil microbial communities to combined biochar amendment and irrigation management strategies and their involvement in Cr transformation in paddy soils. Six treatments were established using varying concentrations of biochar (0, 1, and 2% [w/w]) combined with two irrigation management strategies (continuous flooding [CF] and dry-wet alternation [DWA]). The results showed that the combined biochar addition and irrigation management strategy significantly altered soil pH, redox potential, organic matter content, and Fe(II) and sulfide concentrations. In addition, the Cr(VI) concentration under CF irrigation management was conspicuously lower (48.2-54.4%) than that under DWA irrigation management. Biochar amendment also resulted in a substantial reduction (8.8-27.4%) in Cr(VI) concentration. Moreover, the changes in soil physicochemical properties remarkably affected the soil microbial community. The microbial diversity and abundance significantly increased with biochar amendment. Furthermore, the combined biochar amendment and CF strategy stimulated the growth of Geobacter- and Anaeromyxobacter-related Fe(III)-reducing bacteria, Gallionella-related Fe(II)-oxidizing bacteria, and Desulfovibro- and Clostridium-related sulfate-reducing bacteria, which simultaneously facilitated the generation of Fe(II) and sulfide, thereby enhancing Cr(VI) reduction. Consequently, our results suggest that the effectively increased abundance of Fe-reducing/oxidizing bacteria and sulfate-reducing bacteria via combined CF irrigation management and biochar addition may be a key factor in reducing Cr(VI) in paddy soil. The keystone genera responsible for Cr(VI) reduction were Geobacter, Anaeromyxobacter, Gallionella, Desulfovibro, and Clostridium. This study provides novel insights into the coupling mechanism of the Fe/S/Cr transformation mediated by Fe-reducing/oxidizing bacteria and sulfate-reducing bacteria.

Enhanced metronidazole removal by binary-species photoelectrogenic biofilm of microaglae and anoxygenic phototrophic bacteria

High efficient removal of antibiotics during nutriments recovery for biomass production poses a major technical challenge for photosynthetic microbial biofilm-based wastewater treatment since antibiotics are always co-exist with nutriments in wastewater and resist biodegradation due to their strong biotoxicity and recalcitrance. In this study, we make a first attempt to enhance metronidazole (MNZ) removal from wastewater using electrochemistry-activated binary-species photosynthetic biofilm of Rhodopseudomonas Palustris (R. Palustris) and Chlorella vulgaris (C. vulgaris) by cultivating them under different applied potentials. The results showed that application of external potentials of -0.3, 0 and 0.2 V led to 11, 33 and 26-fold acceleration in MNZ removal, respectively, as compared to that of potential free. The extent of enhancement in MNZ removal was positively correlated to the intensities of photosynthetic current produced under different externally applied potentials. The binary-species photoelectrogenic biofilm exhibited 18 and 6-fold higher MNZ removal rate than that of single-species of C. vulgaris and R. Palustris, respectively, due to the enhanced metabolic interaction between them. Application of an external potential of 0V significantly promoted the accumulation of tryptophan and tyrosine-like compounds as well as humic acid in extracellular polymeric substance, whose concentrations were 7.4, 7.1 and 2.0-fold higher than those produced at potential free, contributing to accelerated adsorption and reductive and photosensitive degradation of MNZ.

Enhancing Cr(VI) bio-reduction by conductive materials and enrichment of functional microbes under anaerobic conditions

A few studies reported the impact of mineral conductivity properties on contaminant-mineral-microbe interactions and microbial community structure changes in the interaction process. To fill the gap, conductive minerals (magnetite/hematite) and an insulative mineral (quartz) were introduced into Cr(VI) reduction systems to investigate the effect of mineral conductivity properties on Cr(VI) removal. Results showed that conductive minerals enhanced Cr(VI) reduction rate as compared to insulative minerals. Higher reduction percentage (>86%) was observed when both ERB (extracellular respiratory bacteria) and conductive minerals were presence than those with only minerals (<10%) or ERB (<55%), indicating a synergistic effect existed in this bio-remediation system. Moreover, surface elements detection manifested higher Fe-containing groups and Fe(III)-Cr(III) complexes covered on conductive minerals surface when ERB was present. Electrochemical data suggested that ERB facilitated the activity of electron transference on the surface of conductive minerals. Our results indicated that conductive minerals did act as an "electron shuttle" while insulative minerals increased adsorption sites to accelerate Cr(VI) reduction. 16S rRNA sequences results demonstrated that conductive minerals changed the microbial community structure and increased the diversity of the functional microbes including Pseudomonas spp. and Exiguobacterium spp. This work is of deep significance for better understanding the process of elements biogeochemical and elimination of pollutants.

Conductive property of secondary minerals triggered Cr(VI) bioreduction by dissimilatory iron reducing bacteria

Although secondary minerals have great potential for heavy metal removal, their impact on chromium biogeochemistry in subsurface environments associated with dissimilatory iron reducing bacteria (DIRB) remains poorly characterized. Here, we have investigated the mechanisms of biogenic secondary minerals on the rate of Cr(VI) bioreduction with shewanella oneidensis MR-1. Batch results showed that the biogenic secondary minerals, schwertmannite and jarosite, appreciably increased the Cr(VI) bioreduction rate. UV-vis diffuse reflection spectra showed that schwertmannite and jarosite are semiconductive minerals, which can be activated by MR-1, followed by transferred conduction electrons toward Cr(VI). Cyclic voltammetry and Tafel analysis suggested that the resistance of secondary minerals is a dominant factor controlling Cr(VI) bioreduction. In addition, Cr(VI) adsorption on secondary minerals through ligand exchange promoted Cr(VI) bioreduction by decreasing the electron transfer distance between MR-1 and chromate. Fe(III)/Fe(II) cycling in schwertmannite and jarosite also contributed to Cr(VI) bioreduction as reflected by X-ray photoelectron spectroscopy and Fourier transform infrared spectrometer. Complementary characterizations further verified the contributions of Fe(III)/Fe(II) cycling, Cr(VI) adsorption, and conduction band electron transfer to enhanced Cr(VI) bioreduction. This study provides new insights on the understanding of Cr(VI) bioreduction by semiconductor minerals containing sulfate in subsurface environments.

Facet-dependent surface charge and Pb2+ adsorption characteristics of hematite nanoparticles: CD-MUSIC-eSGC modeling

Accurate prediction of the environmental fate of Pb depends on the understanding of Pb coordination to mineral surfaces. Here, the proton and Pb adsorption and speciation on hematite nanocrystals with different exposed crystallographic facets were investigated. High-resolution transmission electron microscopy images revealed that hematite nanoplates (HNP) were of 75.3 ± 9.5% (001) facets and 24.6 ± 9.3% (012) facets, while hematite nanocubes (HNC) were of 76.0 ± 11.1% (012) facets and 24.0 ± 3.2% (110) facets. Our modeling results revealed that the proton affinity constant (log K_H) of ≡FeOH^-0.5 and ≡Fe₃O^-0.5 was 7.8 and 10.8 on hematite (012) facets, and changed to 7.7 and 11.7 on (110) facets, respectively. Owing to the different atomic arrangements, (012) facets not only have higher adsorption performance for Pb, but also present a greater dependence on pH than (110) facets. Additionally, our modeling further indicated that (012) facets bind Pb via both bidentate and tridentate complexes, while (110) facets bind Pb only through bidentate complexes at pH 3.0-6.5. These results facilitate a more detailed understanding of the complex species of Pb on hematite surface while also provide new insight into the reactivity mechanism of individual hematite facets.

Dynamic release and transformation of metallic copper colloids in flooded paddy soil: Role of soil reducible sulfate and temperature

Mobile metal Cu colloids can be formed in periodically flooded paddy soils, potentially aggravating the risks to rice cultivated in these soils. Here, we investigated the formation and fate of Cu colloids in flooded soil as influenced by soil reducible sulfate and temperature. In microcosms with different initial sulfate availability (1.30, 5.34, or 7.38 mmol/kg), we found the treatments with higher sulfate concentrations showed the greater and faster release of metal colloids. Sulfate reduction resulted in the transformation of copper in the colloids from Cu(0) to Cu_xS, and the percentage of Cu_xS in the colloid phase increased with increasing sulfate content according to the Cu K-edge EXAFS spectra. The batch experiments incubated at 5, 25 or 35 °C proved that high temperature enhanced the microbial activity and released more Cu colloids during flooding. The colloid formation was delayed at low temperature but persisted longer in the soil, which led to greater particle average size because of slow growth and uniform agglomeration. Low temperature appeared to only influence the formation and growth but not the speciation of Cu colloids. Our results highlight the importance of soil reducible sulfate and temperature in mediating the dynamics of colloidal metals in flooded soil.