Uranium speciation and distribution on the surface of Shewanella putrefaciens in the presence of inorganic phosphate and zero-valent iron under anaerobic conditions.

Shewanella putrefaciens (S. putrefaciens) is one of the main microorganisms in soil bioreactors, which mainly immobilizes uranium through reduction and mineralization processes. However, the effects of elements such as phosphorus and ZVI, which may be present in the actual environment, on the mineralization and reduction processes are still not clearly understood and the environment is mostly in the absence of oxygen. In this study, we ensure that all experiments are performed in an anaerobic glove box, and we elucidate through a combination of macroscopic experimental findings and microscopic characterization that the presence of inorganic phosphates enhances the mineralization of uranyl ions on the surface of S. putrefaciens, while zero-valent iron (ZVI) facilitates the immobilization of uranium by promoting the reduction of uranium by S. putrefaciens. Interestingly, when inorganic phosphates and ZVI co-exist, both the mineralization and reduction of uranium on the bacterial surface are simultaneously enhanced. However, these two substances exhibit a certain degree of antagonism in terms of uranium immobilization by S. putrefaciens. Furthermore, it is found that the influence of pH on the mineralization and reduction of uranyl ions is far more significant than that of inorganic phosphates and ZVI. This study contributes to a better understanding of the environmental fate of uranium in real-world settings and provides valuable theoretical support for the bioremediation and risk assessment of uranium contamination.

Enhanced Cr (VI) removal with Pb (II) presence by Fe2+-activated persulfate and zero-valent iron system.

Combined heavy metals such as chromium (Cr (VI)) and lead (Pb (II)) in natural water have globally posed severe environmental and public health risk. Here the removal of Cr (VI) and Pb (II) mixed pollutants using Fe2+-activated persulfate (PS) with extra zero-valent iron (ZVI), which was not only a supplementary Fe2+ source, but also a high-efficiency absorbent, was investigated. During removal, pivotal factors of initial pollutant concentration, dosages of ZVI and PS, initial pH and temperatures were examined. Interestingly, generating a lot of H+ in the process of Fe (II) activating persulfate were helpful to the corrosion of ZVI over a large range of pH (1-9). Under the optimum condition, removal efficiency of Pb (II) and Cr (VI) have reached 100% and 94.26% respectively. The removal mechanism was suggested as a three-step reaction that the Pb (II) boosted the removal of Cr (VI) by co-precipitated in wastewater, and the Pb (II) and Cr (VI) were adsorbed and subsequently reduced to Pb0 and Cr3+ as Cr(OH)3 or Cr3+-Fe3+ hydroxides on ZVI surface. Cr (VI) and Pb (II) adsorption kinetics agreed with the pseudo-second-order reaction rate expression. In addition, we were surprised to found that the contribution effect of chromium and lead co-precipitation for their removal by Fe (II) - PS-ZVI has strong dependence on initial pH and concentration ratio of Cr (VI) and Pb (II). The result indicated that Fe (II)-PS-ZVI system should be a favourable removal technology for Cr (VI) and Pb (II).

A novel pre-magnetized ZVI/PS pretreatment for improving sludge dewaterability: The role of EPS fractions.

The dewaterability of waste-activated sludge (WAS) has been extensively examined using zero-valent iron (ZVI)-based advanced oxidation processes (AOPs). However, the high dosage and low utilization efficiencies of ZVI cast doubt on the dependability and viability of ZVI-based AOPs. In this study, we successfully demonstrated pre-magnetization as an efficient, chemical-free, and ecological method for improving the efficiency of sludge dewatering by ZVI/persulfate (PS) process, in which the reduction ratios of capillary suction time (CST) and specific resistance to filtration (SRF) increased by 8.67% and 11.06% under optimal conditions, respectively. The highly active Fe2+ released during ZVI corrosion may be more essential than ZVI itself during PS activation, which could be strengthened by pre-magnetization. Both homogeneous and heterogeneous Fe2+ could react with PS to produce aqueous hydroxyl radicals (∙OH) and sulfate radicals (SO4-∙) as well as surface-bound ∙OH and SO4-∙, further decomposing bound-extracellular polymeric substances fractions, broking hydrophilic functional groups and compounds, altering protein secondary structure to expose more hydrophobic sites, and releasing abundant EPS-bound water. Due to the protection of tightly-bound extracellular polymeric substances (TB-EPS) and the competitive oxidation of organics released during the early disintegration stage, radical oxidation primarily occurs at extracellular levels, releasing a bit of intracellular water. Besides, polysaccharides in TB-EPS may function a more significant role in flocculation than proteins, and a porous structure favorable to drainage will be formed after the pre-magnetized ZVI/PS treatment. The cost-benefit analysis further reveals that the Pre-ZVI/PS process presents high reusability and utilization, making it potential for particle application in sludge dewatering.

Simultaneously advanced removal of nitrogen and phosphorus in a biofilter packed with ZVI/PHBV/sawdust composite: Deciphering the succession of dominant bacteria and keystone species.

In this study, a biofilter was developed with a ZVI/PHBV/sawdust (ZPS) composite for treating simulative secondary effluent from wastewater treatment plants. Results showed that effluent concentrations of NO3--N and TP in the ZPS biofilter were stable below 2.0 mg/L and 0.1 mg/L, corresponding to 95% NO3--N removal and 99% TP removal, respectively. Microbial community analysis revealed that the transformation of dominant taxa from Dechloromonas to Clostridium sensu stricto_7 from 30 d to 120 d suggested that the ZVI-induced succession of dominant fermentation bacteria ensured the stable carbon supply for denitrification. Co-occurrence network analysis showed that the ZVI directly enhanced the interaction of microbial community. Fe-related bacteria occupied a key position in the rare species, which might maintain the function of iron-mediated organic matter decomposition and denitrification. These findings provide an alternative for advanced removal of nitrogen and phosphorus in biofilters packed with ZPS composites.

Effect of zero-valent iron and granular activated carbon on nutrient removal and community assembly of photogranules treating low-strength wastewater.

Traditional wastewater treatment processes with high energy consumption and greenhouse gas emissions are not suitable for rural areas with low sewage strength and wide distribution. In this study, a microalgae-bacteria synergistic photogranules system was developed under the impetus of green chemical additives to address these challenges. The results showed that zero-valent iron (ZVI) or granular activated carbon (GAC) addition made successful photogranulation treating low-strength wastewater with excellent settleability and stability performance (settling velocity: 14-22 m h-1; integrity coefficient: 0.81-6.62%), while systems without light or additives failed due to the bio-granules disintegration caused by the overgrowth of predators or phototrophic species. A better nutrient removal performance (TN < 15 mg L-1, TP < 0.4 mg L-1) was observed in photogranules systems, and stoichiometric and biological analysis found that the divisions of nitrogen removal by microalgae and bacteria were different for photogranules between GAC and ZVI additions. As a physical enhancer, GAC can be used as the nucleus of photogranules regenerating after granules disintegration rather than affecting the community succession process. However, ZVI addition strengthened the sedimentation ability and stability of photogranules through chemical and biological effects, focusing on enhancing bacterial community diversity, enriching biofilm formation bacteria and inhibiting the overgrowth of filamentous cyanobacteria. Notably, the photogranules process with ZVI addition could be operated under non-aeration conditions without compromising removal efficiency. There existed an ideal distribution of microalgae and bacterial functional species in the photogranules, which seemed to be essential for its self-sustained synergistic symbiosis and stability. Consequently, this work might provide engineering alternatives for realizing carbon neutrality and environmental sustainability of the decentralized wastewater treatment process for low-strength wastewater in rural areas.

Weak magnetic field significantly enhances methane production from a digester supplemented with zero valent iron.

Weak magnetic field (WMF) provided by a magnet was proposed to enhance CH4 production from a swine manure-fed digester supplemented with micron-sized zero valent iron (ZVI). Compared to the control without ZVI addition and WMF application (RControl), treatments that included ZVI only (RZVI) and coupled WMF with ZVI (RZVI/WMF) increased the CH4 production by 77.0% and 124.5%, respectively. As evidenced by the elevated levels of total soluble iron, WMF apparently promoted the corrosion of ZVI, providing extra H2 for hydrogenotrophic methanogenesis and creating a more reductive environment to reduce propionic-type fermentation. Microbial analysis results revealed that the relative abundance of Methanothrix (capable of accepting electrons) in RZVI/WMF were 75.1% higher than that in RZVI. Essentially, WMF application promoted the direct interspecies electron transfer-based methanogenesis by (1) providing more electrons as the direct substrate, and (2) inducing Lorentz force to facilitate the mass transfer between the released electrons and the methanogens.

Comparing the mechanisms of ZVI and Fe3O4 for promoting waste-activated sludge digestion.

Anaerobic digestion is one of the most promising technologies to stabilize waste-activated sludge (WAS) and recover energy. However, the low efficiency of anaerobic digestion of WAS constrains its application. Supplementing zero valent iron (ZVI) and Fe3O4 in digesters could improve the sludge digestion performance, which has recently been extensively studied. However, the mechanisms behind this improvement remain unclear. In this study, the effects of ZVI and Fe3O4 on the four stages of anaerobic digestion of WAS (solubilization, hydrolysis, acidification and methanogenesis) were investigated. Results showed that ZVI had only a slight effect on the solubilization, hydrolysis and acidification processes, while ZVI significantly promoted the hydrogenotrophic methanogenesis, increasing methane production by 70%. Further investigation indicated that coenzyme F420 activity in the ZVI added reactor was 32.3% higher than in the blank. These results indicate that ZVI promoted anaerobic digestion of WAS through promoting hydrogenotrophic methanogenesis. On the other hand, Fe3O4 obviously promoted the solubilization, hydrolysis and acidification of sludge. Vast Fe2+ was detected in the aqueous phase of the Fe3O4 digester which was a result of dissimilatory iron reduction that can utilize complicated matter as a substrate. This was in agreement with the acceleration of the solubilization, hydrolysis and acidification of the sludge. However, the acetoclastic and hydrogenotrophic methanogenesis with Fe3O4 decreased by 27% and 22% compared to the Fe3O4-free digester, respectively. Further study indicated that Fe3O4 competed with CH3▪S▪CoM for electrons and thus inhibited the methanogenesis process.

Enhanced removal of Se(VI) from water via pre-corrosion of zero-valent iron using H2O2/HCl: Effect of solution chemistry and mechanism investigation.

Although the removal of Se(VI) from water by using zero-valent iron (ZVI) is a promising method, passivation of ZVI severely inhibits its performance. To overcome such issue, we proposed an efficient technique to enhance Se(VI) removal via pre-corrosion of ZVI with H2O2/HCl in a short time (15 min). The resultant pcZVI suspension was weakly acidic (pH 4.56) and contained abundant aqueous Fe2+. 57Fe Mössbauer spectroscopy showed that pcZVI mainly consisted of Fe0 (66.2%), hydrated ferric oxide (26.3%), and Fe3O4 (7.5%). Efficient removal of Se(VI) from sulfate-rich solution was achieved by pcZVI compared with ZVI (in the absence and presence of H2O2) and acid-pretreated ZVI. Moreover, the efficient removal of Se(VI) by pcZVI sustained over a broad pH range (3-9) due to its strong buffering power. The presence of chloride, carbonate, nitrate, and common cations (Na+, K+, Ca2+, and Mg2+) posed negligible influence on the removal of Se(VI) by pcZVI, while the inhibitory effect induced by sulfate, silicate, and phosphate indicated the significance of Se(VI) adsorption as a prerequisite step for its removal. The consumption of aqueous Fe2+ was associated with Se(VI) removal, and X-ray absorption near edge structure revealed that the main pathway for Se(VI) removal by pcZVI was a stepwise reduction of Se(VI) to Se(IV) and then Se0 as the dominant final state (78.2%). Moreover, higher electron selectivity of pcZVI was attributed to the enhanced enrichment of Se oxyanions prior to their reduction.