Enhancement of microbial redox cycling of iron in zero-valent iron oxidation coupling with deca-brominated diphenyl ether removal

The introduction of nano zero-valent iron in constructed wetlands simultaneously enhanced the removal of perfluorooctanoic acid (PFOA) and nutrients

Constructed wetland (CW) serve as the final ecological barrier for hazardous materials entering the natural water environment. Due to the ecological toxicity and difficult bioutilization characteristics of perfluorooctanoic acid (PFOA) itself, CW technology faces great challenges in the field of PFOA remediation. In this study, nano zero-valent iron (nZVI) was introduced into CWs to explore the mechanism of the synergistic removal of PFOA and nutrients in nZVI-CW system. The results indicated that the addition of 10 mg/L nZVI improved the removal efficiency of CW for 1 and 10 mg/L PFOA, with an average removal rate increased by 3.53-8.70%. The transformation products in CW effluents were qualitatively detected using HPLC-Q-TOF-MS, suggesting that the degradation of PFOA may involve decarboxylation, hydrolysis, redox, elimination, substitution and intramolecular rearrangement processes. The presence of nZVI enhanced the average removal rates of NH4+-N, NO3--N and TP by 2.78-18.4% in CWs. The increase in key substrate enzyme activity confirmed the stimulating effect of nZVI on microbial activity. The addition of nZVI facilitated the growth and enrichment of hydroautotrophic denitrifying bacteria, nitrat-dependent iron-oxidizing bacteria, and dissimilatory iron-reducing bacteria. Two types of dissimilatory iron-reducing bacteria (Geobacter and Acinetobacter) may be potential PFOA-degrading bacteria. Additionally, signaling pathways related to carbohydrate metabolism, energy metabolism, and xenobiotic degradation and metabolism exhibited higher abundance in the nZVI treated groups.

Exploring the promoting effect of nitrilotriacetic acid on hydroxyl radical and humification during magnetite-amended composting of sewage sludge

The OH production by adding magnetite (MGT) alone has been reported in composting. However, the potential of nitrilotriacetic acid (NTA) addition for magnetite-amended sludge composting remained unclear. Three treatments with different addition [control check (CK); T1: 5 % MGT; T2: 5 % MGT + 5 % NTA] were investigated to characterize hydroxyl radical, humification and bacterial community response. The NTA addition manifested the best performance, with the peak OH content increase by 52 % through facilitating the cycle of Fe(Ⅱ)/Fe(Ⅲ). It led to the highest organic matters degradation (22.3 %) and humic acids content (36.1 g/kg). Furthermore, NTA addition altered bacterial community response, promoting relative abundances of iron-redox related genera, and amino acid metabolism but decreasing carbohydrate metabolism. Structural equation model indicated that temperature and Streptomyces were the primary factors affecting OH content. The study suggests that utilizing chelators is a promising strategy to strengthen humification in sewage sludge composting with adding iron-containing minerals.

Superb green cycling strategies for microbe-Fe0 neural network-type interaction: Harnessing eight key genes encoding enzymes and mineral transformations to efficiently treat PFOA

To address time-consuming and efficiency-limited challenges in conventional zero-valent iron (ZVI, Fe0) reduction or biotransformation for perfluorooctanoic acid (PFOA) treatment, two calcium alginate-embedded amendments (biochar-immobilized PFOA-degrading bacteria (CB) and ZVI (CZ)) were developed to construct microbe-Fe0 high-rate interaction systems. Interaction mechanisms and key metabolic pathways were systematically explored using metagenomics and a multi-process coupling model for PFOA under microbe-Fe0 interaction. Compared to Fe0 (0.0076 day-1) or microbe (0.0172 day-1) systems, the PFOA removal rate (0.0426 day-1) increased by 1.5 to 4.6 folds in the batch microbe-Fe0 interaction system. Moreover, Pseudomonas accelerated the transformation of Fe0 into Fe3+, which profoundly impacted PFOA transport and fate. Model results demonstrated microbe-Fe0 interaction improved retardation effect for PFOA in columns, with decreased dispersivity a (0.48 to 0.20 cm), increased reaction rate λ (0.15 to 0.22 h-1), distribution coefficient Kd (0.22 to 0.46 cm3∙g-1), and fraction f´(52 % to 60 %) of first-order kinetic sorption of PFOA in microbe-Fe0 interaction column system. Moreover, intermediates analysis showed that microbe-Fe0 interaction diversified PFOA reaction pathways. Three key metabolic pathways (ko00362, ko00626, ko00361), eight functional genes, and corresponding enzymes for PFOA degradation were identified. These findings provide insights into microbe-Fe0 "neural network-type" interaction by unveiling biotransformation and mineral transformation mechanisms for efficient PFOA treatment.

Simultaneously enhanced removal of PAHs and nitrogen driven by Fe2+/Fe3+ cycle in constructed wetland through automatic tidal operation

The lack of dissolved oxygen and weak substrate removal capacity in constructed wetlands (CW) leads to terrible removal of nitrogen and polycyclic aromatic hydrocarbons (PAHs). In this study, automatic tidal flow CW microcosms were constructed by improving the oxygen environment (siphon and air-duct) and substrate (magnetite) to enhance purification performance and the mechanism was explored. The results showed that the addition of air-duct could improve the oxygen collection and thus improved the NH₄⁺ removal efficiency. Additionally, nitrogen removal was improved greatly due to the simultaneous nitrification and denitrification in aerobic layer with the addition of magnetite. Mass balance indicated the microbial degradation dominated (32-62%) the removal of PAHs. Metagenomic analysis proved the existence of magnetite enhanced the number of PAHs-degrading bacteria, functional groups and metabolic pathways and thus greatly improved the microbial degradation of PAHs. Furthermore, Fe²⁺/Fe³⁺ cycle played an important role in promoting the anaerobic degradation of PAHs, which might be served as an electron conduit to establish the direct interspecies electron transfer between iron-reducing bacteria (e.g. Deltaproteobacteria bacterium) and Anaerolineae bacterium to degrade PAHs efficiently. This study provided better understanding of the simultaneous removal of PAHs and nitrogen in tidal flow CWs.

Enhanced refractory organics removal by sponge iron-coupled microbe technology: performance and underlying mechanism analysis

Sponge iron (SFe) is a zero-valent iron (Fe⁰) composite with a high-purity and porous structure. In this study, SFe was coupled with microorganisms that were gradually domesticated to form a Fe⁰/iron-oxidizing bacteria system (Fe⁰-FeOB system). The enhancement effect of the Fe⁰-FeOB system on refractory organics was verified, the mechanism of its strengthening action was investigated, and the relationship and influencing factors between the Fe⁰ and microorganisms were revealed. The average removal rates of the Fe⁰-FeOB system were 8.98%, 5.69%, and 40.67% higher than those of the SBR system for AF, AN, and NB wastewater treatment, respectively. With the addition of SFe, the microbial community structure was gradually enhanced with a large number of FeOB were detected. Moreover, the bacteria with strong iron corrosion and Fe(II) oxidation abilities plays a critical role in improving the Fenton-like effect. Interestingly, the variation trend of ⋅OH was fairly consistent with that of Fe(II). Thus, the main drivers of the Fenton-like effect are biological corrosion and metabolism. Consequently, microbial degradation and Fenton-like effect contributed to the degradation performance of the Fe⁰-FeOB system. Among them, the microbial degradation accounted for 96.09%, of which the biogenic Fenton effect accounted for 8.9%, and the microbial metabolic activity accounted for 87.19%. However, the augmentation of the Fe⁰-FeOB system was strongly dependent on SFe for the strengthening effect of microorganisms disappeared after leaving the SFe 35 days.

Adsorption of arsenic from aqueous solution using a zero-valent iron material modified by the ionic liquid [Hmim]SbF6

The environmental and health impacts caused by arsenic (As) in wastewater make it necessary to carefully manage As wastes. In the present work, a composite of the ionic liquid [Hmim]SbF₆ and nano-iron (H/Fe) was used as an adsorbent to remove As(v) from aqueous solution. To better understand the removal effect of H/Fe on As(v) in aqueous solution, the reaction parameters of pH, reaction temperature, time and H/Fe dosage were systematically analyzed in detail. The results show that H/Fe has significant removal efficiency toward As(v), and that the adsorption of As(v) by 0.5 g H/Fe reaches its maximum adsorption capacity within 2 h. The adsorption of As(v) on H/Fe is a non-linear, time-varying process. The initial adsorption reaction is fast; however, unlike at the beginning, the later reaction involves sustained slow absorption, resulting in a distinct two-phase adsorption characteristic. Redox reaction may be one of the mechanisms responsible for the slow adsorption of As(v) on H/Fe. At the same time, the As(v) removal effect of H/Fe is greatly restricted by the pH. Electrostatic adsorption, adsorption co-precipitation and redox reactions act together on H/Fe in the As(v) removal process. This study provides a basis for further clarifying the adsorption, adsorption rules and mechanism of As(v) on H/Fe and a feasible method for the improvement of As(v) removal efficiency of zero-valent iron materials.