To quantify the populations of Methanosaeta and Meth- anosarcina, a 25-mL PCR contained 0.75 mL (each) of the corre- sponding forward primer and reverse primers (stock concentration of 20 mM), 12.5 mL of SYBR Green PCR master Mix (Applied Biosystems, CA), 6 mL of PCR water, and 5 mL sample DNA. The qPCR reactions were performed starting at 50 C for
2 min, followed by an initial denaturation at 95 C for 10 min, and then 40 cycles of 95 C for 30 s, 60 C for 30 s, and 72 C for
45 s. The dissociation step at 95 C for 15 s and 60 C for 1 min was added at the end to check the specificity of the PCR re- sults. For Methanobacteriales and Methanomicrobiales, a 20-mL PCR contained 0.5 mL (each) of the corresponding forward and reverse primers (20 mM) and probe, 10 mL of TaqMan PCR Mix (Applied Biosystems, CA), 6.5 mL of PCR water, and 2 mL sample DNA. Specially, for Methanobacteriales, qPCR was performed starting at 50 C for 2 min, followed by denaturation at 95 C for 10 min, and then 40 cycles at 95 C for 15 s and at 60 C for
1 min. For Methanomicrobiales, qPCR was performed starting at
50 C for 2 min, followed by denaturation at 95 C for 10 min, and then 40 consecutive cycles at 95 C for 15 s, at 60 C for
1 min and at 72 C for 45 s. Each template DNA prepared from the sludge samples were analyzed in triplicate. The quantified
16S rRNA gene copy numbers were converted to methano- genic cell numbers, according to the genomic information (available at http://www.microbesonline.org) assuming that one Methanosarcina cell contains three 16S rRNA gene copies, one Methanosaeta cell contains two 16S ribosomal RNA, one Methanobacteriales cell contains two 16S rRNA gene copies, and one Methanomicrobiales contains one 16S rRNA gene copies.
2.7. Statistical analysis
Statistical analysis was conducted using the software SPSS
11.5 for Windows (SPSS Inc., Chicago, IL). One-way ANOVA analysis was performed to assess significant difference be- tween groups, with a p value of <0.05 indicating statistical significance.
3. Results
3.1. Methane production in the presence of NZVI
The cumulative methane production is a sensitive measure of doseeresponse toxicity in anaerobic digestion. During the modified BMP tests, organic substrate (glucose in this study) was anaerobically digested to generate methane at the opti- mized temperature of 37 C (Owen et al., 1979; Yang et al.,
2013a). Without any substrate (glucose) addition, the nega- tive control had the lowest methane production of only 20 ( 2 [standard deviation]) mL (Fig. 1a). In the presence of 1 g/L glucose, the cumulative methane volume increased to 123 ( 1) mL, which was equivalent to 308 mg COD after conversion at
37 C. Compared to the total COD (428 mL) added as glucose in the slurry of 400 mL, this experiment revealed glucose re- covery of over 72% for methane production. The sludge treated with 30 mM ZVI powder had a higher cumulative methane production 135 ( 2) mL compared to the positive control (p < 0.001), suggesting that slow release of H2 from ZVI dissolution facilitate methane production as reported earlier
Fig. 1 e Cumulative methane (a) and hydrogen gas (b) production profiles during anaerobic degradation of glucose at 37 C in the groups of negative control (no glucose, no iron, B), positive control (glucose only, no iron, C), 1 mM NZVI (D), 10 mM NZVI ( ), 30 mM NZVI ( ), and
30 mM ZVI ( ), respectively. A water sample containing
30 mM NZVI only served as an abiotic control to show the hydrogen production trend in Fig. 1b ). A heat-killed sludge sample followed by the dose of 30 mM NZVI resulted in a different hydrogen production profile (,). Error bars represent the range of data from duplicate experiments.
(Karri et al., 2005), presumably due to hydrogenotrophic methanogenesis. However, the groups treated with 1 mM,
10 mM and 30 mM NZVI had significantly lower cumulative methane volume than the control (p values <0.001), which were 98 ( 3), 98 ( 0) and 38 ( 5) mL, respectively. CMC had no contribution to methane production and no effect on anaer- obic digestion (data not shown). While there was no signifi- cant difference in cumulative methane volume between the groups 1 mM and 10 mM NZVI (p ¼ 0.68), methane production was the lowest at 30 mM NZVI. These results clearly showed that at 1 mM and above, NZVI inhibited methane generation.
Under anaerobic conditions, ZVI is oxidized to ferrous iron (Fe2þ) resulting in hydrogen gas production (Fe þ 2H2O / Fe2þ þ H2 þ 2OH ) (Liu and Lowry, 2006). The groups of anaerobic sludge treated with 10 mM and 30 mM NZVI generated 111 ( 13), 174 ( 1) mL of H2 respectively, while the groups of 1 mM NZVI had the minimal H2 production
การแปล กรุณารอสักครู่..
![](//thimg.ilovetranslation.com/pic/loading_3.gif?v=b9814dd30c1d7c59_8619)