Aggregation Kinetics of Diesel Soot Nanoparticles in Wet Environments

Soot produced during incomplete combustion consists mainly of carbonaceous nanoparticles (NPs) with severe adverse environmental and health effects, and its environmental fate and transport are largely controlled by aggregation. In this study, we examined the aggregation behavior for diesel soot NPs under aqueous condition in an effort to elucidate the fundamental processes that govern soot particle–particle interactions in wet environments such as rain droplets or surface aquatic systems. The influence of electrolytes and aqueous pH on colloidal stability of these NPs was investigated by measuring their aggregation kinetics in different aqueous solution chemistries. The results showed that the NPs had negatively charged surfaces and exhibited both reaction- and diffusion-limited aggregation regimes with rates depended upon solution chemistry. The aggregation kinetics data were in good agreement with the classic Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The critical coagulation concentrations (CCC) were quantified and the Hamaker constant was derived for the soot (1.4 × 10^–20 J) using the colloidal chemistry approach. The study indicated that, depending upon local aqueous chemistry, single soot NPs could remain stable against self-aggregation in typical freshwater environments and in neutral cloud droplets but are likely to aggregate under salty (e.g., estuaries) or acidic (e.g., acid rain droplets) aquatic conditions or both

Transcriptional Activity of Arsenic-Reducing Bacteria and Genes Regulated by Lactate and Biochar during Arsenic Transformation in Flooded Paddy Soil

Organic substrates and biochar are important in controlling arsenic release from sediments and soils; however, little is known about their impact on arsenic-reducing bacteria and genes during arsenic transformation in flooded paddy soils. In this study, microcosm experiments were established to profile transcriptional activity of As­(V)-respiring gene (<i>arrA</i>) and arsenic resistance gene (<i>arsC</i>) as well as the associated bacteria regulated by lactate and/or biochar in anaerobic arsenic-contaminated paddy soils. Chemical analyses revealed that lactate as the organic substrate stimulated microbial reduction of As­(V) and Fe­(III), which was simultaneously promoted by lactate+biochar, due to biochar’s electron shuttle function that facilitates electron transfer from bacteria to As­(V)/Fe­(III). Sequencing and phylogenetic analyses demonstrated that both <i>arrA</i> closely associated with <i>Geobacter</i> (>60%, number of identical sequences/number of the total sequences) and <i>arsC</i> related to <i>Enterobacteriaceae</i> (>99%) were selected by lactate and lactate+biochar. Compared with the lactate microcosms, transcriptions of the bacterial 16S rRNA gene, <i>Geobacter</i> spp., and <i>Geobacter</i> <i>arrA</i> and <i>arsC</i> genes were increased in the lactate+biochar microcosms, where transcript abundances of <i>Geobacter</i> and <i>Geobacter</i> <i>arrA</i> closely tracked with dissolved As­(V) concentrations. Our findings indicated that lactate and biochar in flooded paddy soils can stimulate the active As­(V)-respiring bacteria <i>Geobacter</i> species for arsenic reduction and release, which probably increases arsenic bioavailability to rice plants

Identification of Anaerobic Aniline-Degrading Bacteria at a Contaminated Industrial Site

Anaerobic aniline biodegradation was investigated under different electron-accepting conditions using contaminated canal and groundwater aquifer sediments from an industrial site. Aniline loss was observed in nitrate- and sulfate-amended microcosms and in microcosms established to promote methanogenic conditions. Lag times of 37 days (sulfate amended) to more than 100 days (methanogenic) were observed prior to activity. Time-series DNA-stable isotope probing (SIP) was used to identify bacteria that incorporated ¹³C-labeled aniline in the microcosms established to promote methanogenic conditions. In microcosms from heavily contaminated aquifer sediments, a phylotype with 92.7% sequence similarity to <i>Ignavibacterium album</i> was identified as a dominant aniline degrader as indicated by incorporation of ¹³C-aniline into its DNA. In microcosms from contaminated canal sediments, a bacterial phylotype within the family <i>Anaerolineaceae</i>, but without a match to any known genus, demonstrated the assimilation of ¹³C-aniline. <i>Acidovorax</i> spp. were also identified as putative aniline degraders in both of these two treatments, indicating that these species were present and active in both the canal and aquifer sediments. There were multiple bacterial phylotypes associated with anaerobic degradation of aniline at this complex industrial site, which suggests that anaerobic transformation of aniline is an important process at the site. Furthermore, the aniline degrading phylotypes identified in the current study are not related to any known aniline-degrading bacteria. The identification of novel putative aniline degraders expands current knowledge regarding the potential fate of aniline under anaerobic conditions