The dominant role of extracellular polymeric substances produced by Achromobacter xylosoxidans BP1 in Cr(VI) microbial reduction.

Extracellular polymeric substances (EPS) have demonstrated significant benefits for reducing multivalent metal contamination. Using Achromobacter xylosoxidans BP1 isolated from a coal chemical site in China, this study elucidated the contribution of EPS production to Cr (VI) reduction and revealed its biological removal mechanism. BP1 grew at an optimum pH of 8 and the lowest inhibitory concentration of Cr(VI) was 300 mg/L. The spent medium completely removed Cr(VI), whereas resting cells were only able to remove 10.47 % and inactivated cells were nearly incapable of Cr(VI) removal. S-EPS and B-EPS reduced Cr(VI) by 98.59 % and 11.64 %, respectively. SEM-EDS analysis showed that the BP1 cells were stimulated to produce EPS under Cr stress. The XPS results showed that 29.63 % of Cr(VI) was enriched by intracellular bioaccumulation or biosorption and 70.37 % of Cr(VI) was reduced by extracellular enzymes to produce Cr(OH)3 and organic Cr(III) complexes. According to FTIR, EPS with -OH, COO-, and amide groups supplied binding sites and electrons for the reductive adsorption of Cr(VI). Genomic studies showed that BP1 primarily produces extracellular polysaccharides, metabolises sulphur and nitrogen, and reduces reactive oxygen species damage as a result of DNA repair proteases.

Mechanisms and influential factors of soil chromium long-term stability by an accelerated aging system after chemical stabilization.

Chemical stabilization is one of the most widely used remediation strategies for chromium (Cr)-contaminated soils by reducing Cr(VI) to Cr(III), and its performance is affected by human and natural processes in a prolonged period, challenging long-term Cr stability. In this work, we established a method for evaluating the long-term effectiveness of remediation of Cr-contaminated soils, and developed an accelerated aging system to simultaneously simulate acid rain leaching and freeze-thaw cycles. The mechanisms and influencing factors of long-term (50-year) change in soil Cr speciation were unravelled after stabilization with Metafix®. Chemical stabilization remarkably decreased the contents of Cr(VI)soil, Crtotal-leach and Cr(VI)leach, among which the removal rate of Cr(VI) in soil was up to 89.70 %, but it also aggravated soil Cr instability. During the accelerated aging process, Crtotal-leach change rates in chemically stabilized soil samples were 0.0462-0.0587 mg/(L·a), and soil Cr became instable after 20-year accelerated aging. The proportion of Cr bound to organic matter and residual Cr increased in soil, and exchangeable Cr decreased. Linear combination fitting results of XANES also showed that Cr(VI) and Cr3+ were transformed into OM-Cr(III), Fh-Cr(III) and CrFeO3 after restoration. During the accelerated aging process, acid rain leaching activated Cr(III) and dissolved Cr(VI), whereas freeze-thaw cycle mainly affected OM-Cr. Chemical stabilization, acid rain leaching and aging time were the major factors influencing the stability of soil Cr, and the freeze-thaw cycle promoted the influence of acid rain leaching. This study provided a new way to explore the long-term effectiveness and instability mechanisms at Cr-contaminated site after chemical stabilization.

Appropriate control measure design by rapidly identifying risk areas of volatile organic compounds during the remediation excavation at an organic contaminated site.

Many organic contaminated sites require on-site remediation; excavation remediation processes can release many volatile organic compounds (VOCs) which are key atmospheric pollutants. It is therefore important to rapidly identify VOCs during excavation and map their risk areas for human health protection. In this study, we developed a rapid analysis and assessment method, aiming to and reveal the real-time distribution of VOCs, evaluate their human health risks by quantitative models, and design appropriate control measures. Through on-site diagonal distribution sampling and analysis, VOCs concentration showed a decreasing trend within 5 m from the excavation point and then increased after 5 m with the increase in distance from the excavation point (p < 0.05). The concentrations of VOCs near the dominant wind direction were higher than the concentrations of surrounding pollutants. In contrast with conventional solid-phase adsorption (SPA) and thermal desorption gas chromatography-mass spectrometry (TD-GC/MS) methods for determining the composition and concentration of VOCs, the rapid measurement of VOCs by photo-ionization detector (PID) fitted well with the chemical analysis and modeling assessment of cancer/non-cancer risk. The targeting area was assessed as mild-risk (PID < 10 ppm), moderate-risk (PID from 10 to 40 ppm), and heavy-risk (PID > 40 ppm) areas. Similarly, the human health risks also decreased gradually with the distance from the excavation point, with the main risk area located in the dominant wind direction. The results of rapid PID assessment were comparable to conventional risk evaluation, demonstrating its feasibility in rapidly identifying VOCs releases and assessing the human health risks. This study also suggested appropriate control measures that are important guidance for personal protection during the remediation excavation process.

Co-applied biochar and PGPB promote maize growth and reduce CO2 emission by modifying microbial communities in coal mining degraded soils.

Coal mining is one of the human activities that has the greatest impact on the global carbon (C) cycle and biodiversity. Biochar and plant growth-promoting bacteria (PGPB) have been both used to improve coal mining degraded soils; however, it is uncertain whether the effects of biochar application on soil respiration and microbial communities are influenced by the presence or absence of PGPB and soil nitrogen (N) level in coal mining degraded soils. A pot experiment was carried out to examine whether the effects of biochar addition (0, 1, 2 and 4% of soil mass) on soil properties, soil respiration, maize growth, and microbial communities were altered by the presence or absence of PGPB (i.e. Sphingobium yanoikuyae BJ1) (0, 200 mL suspension (2 × 106 colony forming unit (CFU) mL-1)) and two soil N levels (N0 and N1 at 0 and 0.2 g kg-1 urea- N, respectively). The results showed the presence of BJ1 enhanced the maize biomass relative to the absence of BJ1, particularly in N1 soils, which was related to the discovery of Lysobacter and Nocardioides that favor plant growth in N1 soils. This indicates a conversion in soil microbial communities to beneficial ones. The application of biochar at a rate of 1% decreased the cumulative CO2 regardless of the presence or absence of BJ1; BJ1 increased the ß-glucosidase (BG) activities, and BG activities were also positively correlated with RB41 strain with high C turnover in N1 soils, which indicates that the presence of BJ1 improves the C utilization rates of RB41, decreasing soil C mineralization. Our results highlight that biochar addition provided environmental benefits in degraded coal mining soils, and the direction and magnitude of these effects are highly dependent on the presence of PGPB and the soil N level.