Inhibition and stimulation of two perchloroethene degrading bacterial cultures by nano- and micro-scaled zero-valent iron particles

Enhancing reductive dechlorination of trichloroethylene in bioelectrochemical systems with conductive materials

The incorporation of conductive materials to enhance electron transfer in bioelectrochemical systems (BES) is considered a promising approach. However, the specific effects and mechanisms of these materials on trichloroethylene (TCE) reductive dechlorination in BES remains are not fully understood. This study investigated the use of magnetite nanoparticles (MNP) and biochars (BC) as coatings on biocathodes for TCE reduction. Results demonstrated that the average dechlorination rates of MNP-Biocathode (122.89 μM Cl·d-1) and BC-Biocathode (102.88 μM Cl·d-1) were greatly higher than that of Biocathode (78.17 μM Cl·d-1). Based on MATLAB calculation, the dechlorination rate exhibited a more significantly increase in TCE-to-DCE step than the other dechlorination steps. Microbial community analyses revealed an increase in the relative abundance of electroactive and dechlorinating populations (e.g., Pseudomonas, Geobacter, and Desulfovibrio) in MNP-Biocathode and BC-Biocathode. Functional gene analysis via RT-qPCR showed the expression of dehalogenase (RDase) and direct electron transfer (DET) related genes was upregulated with the addition of MNP and BC. These findings suggest that conductive materials might accelerate reductive dechlorination by enhancing DET. The difference of physicochemical characteristics (e.g. particle size and specific surface area), electron transfer enhancement mechanism between MNP and BC as well as the reduction of Fe(III) by hydrogen may explain the superior dechlorination rate observed with MNP-Biocathode.

Feasibility of simultaneous optimization of Anammox start-up and nitrogen removal performance by intermittent dosing of nanoscale zero-valent iron

The long acclimation period and sensitivity to environmental conditions of Anammox are the bottlenecks for its promotion and application. An innovative strategy was adopted to accelerate functional microbial enhancement and improve nitrogen removal performance by inoculating cryopreserved Anammox sludge and activated sludge with intermittent dosing of nanoscale zero-valent iron (nZVI). The acclimation time was shortened by 76 days with nitrogen removal efficiency (NRE) reaching up to 91.07 %. Anammox, NDFO (nitrate/nitrite-dependent Fe(II) oxidation), Feammox (Fe(III) reduction coupled with anaerobic ammonium oxidation) and abiotic reactions were coupled in the system with nZVI, contributing to 69.79 %, 15.14 %, 9.84 % and 0.25 % of nitrogen removal, respectively. Further microbial analysis demonstrated significant enrichment of functional microorganisms, such as Candidatus Jettenia, Acidovorax and Comamonas. High-efficient nitrogen removal was attribute to the increase of functional genes involved in Anammox, electronic transfer, heme C synthesis and iron metabolism. This work provides an inspiring idea for the mainstream Anammox application.

Achieving sustainable trichloroethylene removal from nitrate-containing groundwater: Effects of particle size and dosage of microscale zero-valent iron on its synergistic action with anaerobic bacteria

The coupling of microscale zero-valent iron (mZVI) and anaerobic bacteria (AB) has gained increasing attention due to its ability to enhance dechlorination efficiency by combining the advantages of chemical and microbial reduction. However, the implementation of these coupling technologies at the field scale is challenging in terms of sustainability goals due to the coexistence of various natural electron acceptors in groundwater, which leads to limited electron selectivity and increased secondary risk. Therefore, this study used trichloroethylene (TCE) as a probe contaminant and nitrate (NO3-) as a typical co-occurring natural electron acceptor to optimize the overall sustainable remediation performance of an mZVI/AB coupled system by adjusting the mZVI particle size and dosage. Results revealed that mZVI particles of different sizes exhibit different microorganism activation capabilities. In contrast to its 2 μm and 7 μm counterparts, the 30 μm mZVI/AB system demonstrated a strong dosage-dependency in TCE removal and its product selectivity. Finally, multi-criteria analysis (MCA) methods were established to comprehensively rank the alternatives, and 30 μm mZVI (15 g/L dosage) was determined to be the best remediation strategy with the highest total sustainability score under all studied hydro-chemical conditions when equal weights were applied to technical, environmental, and economic indicators. Our work provides a paradigm for comprehensively assessing the sustainable remediation performance of chlorinated aliphatic hydrocarbons polluted groundwater in practical applications.

Decoupling Fe0 Application and Bioaugmentation in Space and Time Enables Microbial Reductive Dechlorination of Trichloroethene to Ethene: Evidence from Soil Columns

Fe⁰ is a powerful chemical reductant with applications for remediation of chlorinated solvents, including tetrachloroethene and trichloroethene. Its utilization efficiency at contaminated sites is limited because most of the electrons from Fe⁰ are channeled to the reduction of water to H₂ rather than to the reduction of the contaminants. Coupling Fe⁰ with H₂-utilizing organohalide-respiring bacteria (i.e., Dehalococcoides mccartyi) could enhance trichloroethene conversion to ethene while maximizing Fe⁰ utilization efficiency. Columns packed with aquifer materials have been used to assess the efficacy of a treatment combining in space and time Fe⁰ and aD. mccartyi-containing culture (bioaugmentation). To date, most column studies documented only partial conversion of the solvents to chlorinated byproducts, calling into question the feasibility of Fe⁰ to promote complete microbial reductive dechlorination. In this study, we decoupled the application of Fe⁰ in space and time from the addition of organic substrates andD. mccartyi-containing cultures. We used a column containing soil and Fe⁰ (at 15 g L^-1 in porewater) and fed it with groundwater as a proxy for an upstream Fe⁰ injection zone dominated by abiotic reactions and biostimulated/bioaugmented soil columns (Bio-columns) as proxies for downstream microbiological zones. Results showed that Bio-columns receiving reduced groundwater from the Fe⁰-column supported microbial reductive dechlorination, yielding up to 98% trichloroethene conversion to ethene. The microbial community in the Bio-columns established with Fe⁰-reduced groundwater also sustained trichloroethene reduction to ethene (up to 100%) when challenged with aerobic groundwater. This study supports a conceptual model where decoupling the application of Fe⁰ and biostimulation/bioaugmentation in space and/or time could augment microbial trichloroethene reductive dechlorination, particularly under oxic conditions.

Visible-light-driven iron-based heterogeneous photo-Fenton catalysts for wastewater decontamination: A review of recent advances

Visible-light-driven heterogeneous photo-Fenton process has emerged as the most promising Fenton-derived technology for wastewater decontamination, owing to its prominent superiorities including the potential utilization of clean energy (solar light), and acceleration of ≡Fe(II)/≡Fe(III) dynamic cycle. As the core constituent, catalysts play a pivotal role in the photocatalytic activation of H₂O₂ to yield reactive oxidative species (ROS). To date, all types of iron-based heterogeneous photo-Fenton catalysts (Fe-HPFCs) have been extensively reported by the scientific community, and exhibited satisfactory catalytic performance towards pollutants decomposition, sometimes even exceeding the homogeneous counterparts (Fe(II)/H₂O₂). However, the relevant reviews on Fe-HPFCs, especially from the viewpoint of catalyst-self design are extremely limited. Therefore, this state-of-the-art review focuses on the available Fe-HPFCs in literatures, and gives their classification based on their self-characteristics and modification strategies for the first time. Two classes of representative Fe-HPFCs, conventional inorganic semiconductors of Fe-containing minerals and newly emerging Fe-based metal-organic frameworks (Fe-MOFs) are comprehensively summarized. Moreover, three universal strategies including (i) transition metal (TMs) doping, (ii) construction of heterojunctions with other semiconductors or plasmonic materials, and (iii) combination with supporters were proposed to tackle their inherent defects, viz., inferior light-harvesting capacity, fast recombination of photogenerated carriers, slow mass transfer and low exposure and uneven dispersion of active sites. Lastly, a critical emphasis was also made on the challenges and prospects of Fe-HPFCs in wastewater treatment, providing valuable guidance to researchers for the reasonable construction of high-performance Fe-HPFCs.

Zero-valent iron is not always effective in enhancing anaerobic digestion performance

Liquid nitrogen was employed as a low-temperature medium to activate zero-valent iron (ZVI) powder in an attempt to strengthen its enhancement effect on anaerobic digestion (AD) of swine manure (SM). Surprisingly, it was found that both pristine ZVI and liquid nitrogen-pretreated ZVI (LZVI) did not significantly improve the AD performance or change the archaeal community structure. It was hypothesized that ZVI might not be effective at stress-free environment like in these digesters. To confirm this, an additional set of AD experiments were performed at high ammonia stress (about 4000 mg/L), results showed that ZVI and LZVI greatly alleviated ammonia inhibition and increased the CH₄ yield by 11.6% and 28.2%, respectively. Apparently, ZVI mainly affected AD systems by changing the metabolism pathways and enhancing the microbial activity to overcome process inhibition, and pretreatment of liquid nitrogen could significantly accelerate the dissolution of ZVI and improve its utilization efficiency, contributing to a greater extend of process recovery and improvement.

Nitrate removal by combining chemical and biostimulation approaches using micro-zero valent iron and lactic acid

The occurrence of nitrate is the most significant type of pollution affecting groundwater globally, being a major contributor to the poor condition of water bodies. This pollution is related to livestock-agricultural and urban activities, and the nitrate presence in drinking water has a clear impact on human health. For example, it causes the blue child syndrome. Moreover, the high nitrate content in aquifers and surface waters significantly affects aquatic ecosystems since it is responsible for the eutrophication of surface water bodies. A treatability test was performed in the laboratory to study the decrease of nitrate in the capture zone of water supply wells. For this purpose, two boreholes were drilled from which groundwater and sediments were collected to conduct the test. The goal was to demonstrate that nitrate in groundwater can be decreased much more efficiently using combined abiotic and biotic methods with micro-zero valent iron and biostimulation with lactic acid, respectively, than when both strategies are used separately. The broader implications of this goal derive from the fact that the separate use of these reagents decreases the efficiency of nitrate removal. Thus, while nitrate is removed using micro-valent iron, high concentrations of harmful ammonium are also generated. Furthermore, biostimulation alone leads to overgrowth of other microorganisms that do not result in denitrification, therefore complete denitrification requires more time to occur. In contrast, the combined strategy couples abiotic denitrification of nitrate with biostimulation of microorganisms capable of biotically transforming the abiotically generated harmful ammonium. The treatability test shows that the remediation strategy combining in situ chemical reduction using micro-zero valent iron and biostimulation with lactic acid could be a viable strategy for the creation of a reactive zone around supply wells located in regions where groundwater and porewater in low permeability layers are affected by diffuse nitrate contamination.

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

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

Coupling microscale zero-valent iron and autotrophic hydrogen-bacteria provides a sustainable remediation solution for trichloroethylene-contaminated groundwater: Mechanisms, regulation, and engineering implications

Coupling microscale zero-valent iron (mZVI) and autotrophic hydrogen bacteria (AHB) has gained increasing attention owing to its potential to improve dechlorination performance by bridging H₂ donors and acceptors. However, few studies have attempted to test its sustainable remediation performance and to comprehensively unveil the governing mechanisms. This study systematically compared the performances of different systems (mZVI, H₂-AHB, and mZVI-AHB) for trichloroethylene (TCE) removal, and further optimized dechlorination and H₂ evolution of mZVI-AHB synchronously by regulating the mZVI particle size and dosage to achieve a win-win remediation solution. The final removal efficiency and removal rate of TCE by mZVI-AHB were 1.67-fold and 5.30-fold of those by mZVI alone respectively, and mZVI-AHB resulted in more complete dechlorination than H₂-AHB alone. Combining H₂ evolution kinetics, material characterization data, and bacterial community analysis results, the improved dechlorination performance of mZVI-AHB was mainly due to the following mechanisms: H₂ generated by mZVI corrosion was efficiently utilized by AHB, lasting corrosion of mZVI was facilitated by AHB, and dechlorination functional bacteria were highly enriched by mZVI. Finally, the remediation performance of mZVI-AHB with different mZVI particle sizes and dosages was evaluated comprehensively in terms of dechlorination reactivity, H₂ utilization efficiency and chemical cost, and suggestions for possible engineering applications are provided.

Fe0-Supported Anaerobic Digestion for Organics and Nutrients Removal from Domestic Sewage

Results from different research suggest that metallic iron (Fe0) materials enhance anaerobic digestion (AD) systems to remove organics (chemical oxygen demand (COD)), phosphorus and nitrogen from polluted water. However, the available results are difficult to compare because they are derived from different experimental conditions. This research characterises the effects of Fe0 type and dosage in AD systems to simultaneously remove COD and nutrients (orthophosphate (PO43−), ammonium (NH4+), and nitrate (NO3− Lab-scale reactors containing domestic sewage (DS) were fed with various Fe0 dosages (0 to 30 g/L). Batch AD experiments were operated at 37 ± 0.5 °C for 76 days; the initial pH value was 7.5. Scrap iron (SI) and steel wool (SW) were used as Fe0 sources. Results show that: (i) SW performed better than SI on COD and PO43− removal (ii) optimum dosage for the organics and nutrients removal was 10 g/L SI (iii) (NO3− + NH4+) was the least removed pollutant (iv) maximum observed COD, PO43− and NO3− + NH4+ removal efficiencies were 88.0%, 98.0% and 40.0% for 10 g/L SI, 88.2%, 99.9%, 25.1% for 10 g/L SW, and 68.9%, 7.3% and 0.7% for the reference system. Fe0-supported AD significantly removed the organics and nutrients from DS.

Effects of iron-carbon materials on microbial-catalyzed reductive dechlorination of polychlorinated biphenyls in Taihu Lake sediment microcosms: Enhanced chlorine removal, detoxification and shifts of microbial community

Nano zero-valent iron particles (nZVI, 0.09 wt%), micro zero-valent iron particles (mZVI, 0.09 wt%), granular activated carbon (GAC, 3.03 wt%), GAC supported nZVI (nZVI/GAC, 3.12 wt%) and nZVI&GAC (nZVI 0.09 wt%, GAC 3.03 wt%) were evaluated for their effects on polychlorinated biphenyls (PCBs) anaerobic reductive dechlorination, detoxification, as well as microbial community structure in Taihu Lake (China) sediment microcosms. The results showed that all of these five materials could stimulate PCBs reductive dechlorination, especially for dioxin-like PCB congeners, and nZVI&GAC had the best removal effect on PCBs. The reduction of total PCBs increased from 13.5% to 33.2%. H₂ generated by zero-valent iron corrosion was utilized by organohalide-respiring bacteria (OHRB) to enhance the dechlorination of PCBs predominantly via meta chlorine removal in the short term. The addition of ZVI had little impact on the total bacterial abundance and the microbial community structure. The adsorption of GAC and potential bioremediation properties of attached biofilm could promote the long-term removal of PCBs. GAC, nZVI/GAC, nZVI&GAC had different influences on the microbial structure. These findings provide insights into the biostimulation technique for in situ remediations of PCBs contaminated sediments.

Facile Bioself-Assembled Crystals in Plants Promote Photosynthesis and Salt Stress Resistance

Salty soil is a global problem that has adverse effects on plants. We demonstrate that bioself-assembled molybdenum-sulfur (Mo-S) crystals formed by the foliar application of MoCl₅ and cysteine augment the photosynthesis of plants treated with 200 mM salt for 7 days by promoting Ca²⁺ signal transduction and free radical scavenging. Reductions in glutathione and phytochelatins were attributed to the biosynthesized Mo-S crystals. Plants embedded with the Mo-S crystals and exposed to salty soil exhibited carbon assimilation rates, photosynthesis rates (Fv/Fm), and electron transport rates (ETRs) that were increased by 40%, 63-173%, and 50-78%, respectively, compared with those of plants without Mo-S crystals. Increased compatible osmolyte levels and decreased levels of oxidative damage, stomatal conductance (0.63-0.42 mmol m² s^-1), and transpiration (22.9-15.3 mmol m² s^-1), free radical scavenging, and calcium-dependent protein kinase, and Ca²⁺ signaling pathway activation were evidenced by transcriptomics and metabolomics. The bioself-assembled crystals originating from ions provide a method for protecting plant development under adverse conditions.

Degradation of tetrachloroethylene by zero valent iron nanoparticles in the presence of a natural groundwater bacterial biofilm in a sandy porous media

Biofilms are naturally present in aquifers and can interact with zero valent iron nanoparticles (nZVI) used as remediation agents in contaminated groundwater; thereby they may alter nZVI reactivity towards targeted contaminants in porous media. Laboratory scale experiments using columns filled with sand (50 cm long and 5.2 cm in diameter) were performed to investigate the impact of natural biofilms on nZVI reactivity towards tetrachloroethylene (PCE) in conditions simulating an unconsolidated sandy aquifer. Solutions containing PCE were injected through the sand columns in the presence or absence of biofilm and nZVI. Concentrations in PCE and its metabolites were monitored during 45 days in dissolved and gas phases. PCE concentrations decreased at the column outlets due both to its reductive dechlorination by nZVI (~30% of injected PCE) and its sorption or deposition (as PCE-DNAPL) on sand (~35% of injected PCE). No significant differences in PCE concentrations were found in presence or absence of biofilm. However, biofilm presence affected the nature of PCE metabolites. A higher release of ethene in the column containing biofilm was observed, whereas ethane was dominant in the absence of biofilm. Microbes consumed H₂ released by the corrosion of nZVI limiting the hydrogenation of ethene to ethane. The consequences of biofilm development in porous media should be taken into account when considering treatment with nZVI, as it may affect the nature of produced metabolites.