Performance and mechanisms exploration of nano zinc oxide (nZnO) on anaerobic decolorization by Shewanella oneidensis MR-1.

Although the ecological safety of nanomaterials is of widespread concern, their current ambient concentrations are not yet sufficient to cause serious toxic effects. Thus, the nontoxic bioimpact of nanomaterials in wastewater treatment has attracted increasing attention. In this study, the effect of nano zinc oxide (nZnO), one of the most widely used nanomaterials, on the anaerobic biodegradation of methyl orange (MO) by Shewanella oneidensis MR-1 was comprehensively investigated. High-dosage nZnO (>0.5 mg/L) caused severe toxic stress on S. oneidensis MR-1, resulting in the decrease in decolorization efficiency. However, nZnO at ambient concentrations could act as nanostimulants and promote the anaerobic removal of MO by S. oneidensis MR-1, which should be attributed to the improvement of decolorization efficiency rather than cell proliferation. The dissolved Zn2+ was found to contribute to the bioeffect of nZnO on MO decolorization. Further investigation revealed that low-dosage nZnO could promote the cell viability, membrane permeability, anaerobic metabolism, as well as related gene expression, indicating that nZnO facilitated rather than inhibited the anaerobic wastewater treatment under ambient conditions. Thus, this work provides a new insight into the bioeffect of nZnO in actual environment and facilitates the practical application of nanomaterials as nanostimulants in biological process.

Rapid cultivation of anammox sludge based on Ca-alginate cell beads.

Current gel entrapment technology has certain advantages for the enrichment of anammox sludge. In this study, the optimal preparation conditions and cultivation equipment of Ca-alginate cell beads for the culturing anammox sludge were proposed. The preparation parameters of the Ca-alginate cell beads were as follows: 3% sodium alginate, 4% CaCl2, VSA:Vcell = 1:1, a drop height of 9 cm, stirring speed of 300 rpm, and cross-linking time of 24 h. The prepared cell beads were regular spheres with a uniform size and hard texture. Throughout the 9 days of cultivation, the number of anammox bacteria in the Ca-alginate cell beads was 4.3 times that of the initial sludge, and the color of the cell beads changed from yellowish-brown to reddish-brown. Scanning electron microscopy (SEM) analysis showed that the SA gel beads had a good microporous structure. The fluorescence in situ hybridization (FISH) results illustrated that the bacteria were mostly dispersed inside the Ca-alginate cell beads. Additionally, the qPCR results implied that only a relatively small amount of anammox biomass (2.74×106 copies/gel-bead) was required to quickly start the anammox process. The anammox bacteria in the Ca-alginate cell beads grew with a fast growth rate in a short period and exhibited high activity due to diffusion limitations. In addition, the anammox bacteria cultivated in the Ca-alginate cell beads could adapt to the increase in substrate concentration in a short period. The optimal incubation time of this gel entrapment method for anammox sludge was no more than 17 days under the experimental conditions of this work. Therefore, this simple and practicable gel entrapment method may serve as a suitable pre-culture means for the rapid enrichment of anammox bacteria.

Re-evaluation of the environmental hazards of nZnO to denitrification: Performance and mechanism.

Numerous studies have shown that zinc oxide nanoparticles (nZnO) have an inhibitory effect on wastewater biotreatment, where doses exceeding ambient concentrations are used. However, the effect of ambient concentrations of ZnO (<1 mg/L) on anaerobic digestion processes is not clear. Herein, this study comprehensively explored the impact of nZnO on the denitrification performance and core microbial community of activated sludge under ambient concentrations. Results showed that only 0.075 mg/L nZnO had shown a beneficial effect on nitrogen removal by activated sludge. When nZnO concentration reached 0.75 mg/L, significant enhancement of nitrate reduction and mitigation of nitrite accumulation were observed, indicating a remarkable stimulatory effect on nitrogen removal. Simultaneously, nZnO could weaken the sludge surface charge and improve the secretion of extracellular polymeric substances, thus enhancing sludge flocculation for denitrification. Microbial community analysis revealed that nZnO exposure increased the relative abundance of denitrifying bacteria, which could contribute to the reinforcement of traditional denitrification. Furthermore, exogenous addition of NH4+ significantly inhibited the accumulation of nitrite, implying that nZnO had a potential to improve the denitrification process via a partial denitrification-anammox pathway. Considering current ambient concentration, the stimulatory effect shown in our work may better represent the actual behavior of ZnO in wastewater biotreatment.

Anaerobic reduction of high-polarity nitroaromatic compounds by electrochemically active bacteria: Roles of Mtr respiratory pathway, molecular polarity, mediator and membrane permeability.

Electrochemically active bacteria (EAB) are effective for the bioreduction of nitroaromatic compounds (NACs), but the exact reduction mechanisms are unclear yet. Therefore, 3-nitrobenzenesulfonate (NBS) was used to explore the biodegradation mechanism of NACs by EAB. Results show that NBS could be anaerobically degraded by Shewanella oneidensis MR-1. The generation of aminoaromatic compounds was accompanied with the NBS reduction, indicating that NBS was biodegraded via reductive approach by S. oneidensis MR-1. The impacts of NBS concentration and cell density on the NBS reduction were evaluated. The removal of NBS depends mainly on the transmembrane electron transfer of S. oneidensis MR-1. Impairment of Mtr respiratory pathway was found to mitigate the reduction of NBS, suggesting that the anaerobic biodegradation of NBS occurred extracellularly. Knocking out cymA severely impaired the extracellular reduction ability of S. oneidensis MR-1. However, the phenotype of ΔcymA mutant could be compensated by the exogenous electron mediators, implying the trans-outer membrane diffusion of mediators into the periplasmic space. This work provides a new insight into the anaerobic reduction of aromatic contaminants by EAB.

A high-throughput dye-reducing photometric assay for evaluating microbial exoelectrogenic ability.

Exoelectrogenic bacteria (EEB) can transfer electrons to extracellular electron acceptors and have wide applications in environmental bioremediation and bioenergy generation. Thus, methods for effectively probing the exoelectrogenic ability of EEB are highly desirable. In this work, a simple but efficient photometric assay based on the extracellular reduction of high polar dyes was developed to evaluate the microbial exoelectrogenic ability. Methyl orange were proven to be used as a probe for evaluating the exoelectrogenic ability of EEB. Through monitoring the extracellular dye decolorization under anaerobic conditions, this plate-based photometric assay could rapidly measure the exoelectrogenic ability of various EEB. This approach was also able to evaluate the exoelectrogenic capacity of Shewanella oneidensis MR-1 wild-type strain and its Mtr mutants. Furthermore, the exoelectrogenic ability of mixed cultures in microbial fuel cells was correlated with the extracellular dye decolorization. Thus, this work is useful for the practical implementation of microbial exoelectrogenic ability evaluation.

Effects of drying pretreatment and particle size adjustment on the composting process of discarded flue-cured tobacco leaves.

The main characteristic of discarded flue-cured tobacco leaves is their high nicotine content. Aerobic composting is an effective method to decrease the nicotine level in tobacco leaves and stabilize tobacco wastes. However, high levels of nicotine in discarded flue-cured tobacco leaves complicate tobacco waste composting. This work proposes a drying pretreatment process to reduce the nicotine content in discarded flue-cured tobacco leaves and thus enhance its carbon-to-nitrogen ratio to a suitable level for composting. The effect of another pretreatment method, particle size adjustment, on composting efficiency was also tested in this work. The results indicated that the air-dried (nicotine content: 1.35%) and relatively long discarded flue-cured tobacco leaves (25 mm) had a higher composting efficiency than damp (nicotine content: 1.57%) and short discarded flue-cured tobacco leaves (15 mm). When dry/25 mm discarded flue-cured tobacco leaves mixed with tobacco stems in an 8:2 ratio was composted at a temperature above 55 °C for 9 days, the nicotine content dropped from 1.29% to 0.28%. Since the discarded flue-cured tobacco leaves was successfully composted to a fertile and harmless material, the germination index values increased to 85.2%. The drying pretreatment and particle size adjustment offered ideal physical and chemical conditions to support microbial growth and bioactivity during the composting process, resulting in efficient conversion of discarded flue-cured tobacco leaves into a high quality and mature compost.