Anaerobic digestion experiment was conducted using modified biochemical methane potential (BMP) procedures (Owen et al., 1979; Shelton and Tiedje, 1984) with glucose (at a final concentration of 1 g/L) as a substrate. The anaerobic sludge samples were treated (in duplicates) with NZVI or ZVI powder at the final concentrations of 0, 1, 10, 30 mM NZVI (or
1680 mg Fe/L), and 30 mM ZVI powder (70 mesh, <212 mm, Acros) along with negative controls (no glucose, no NZVI/ZVI). An abiotic control and a heat-killed anaerobic sludge (80 C for
30 min) containing 30 mM NZVI were prepared to compare the hydrogen production rate and cumulative hydrogen gas vol- ume with that of the anaerobic sludge treated with 30 mM NZVI. The highest iron concentration (30 mM) was chosen based on the fact of successful use of NZVI in TCE reduction at the highest concentration of 1500 mg/L (Kirschling et al., 2010). The headspace of sludge bottle was purged with pure nitrogen gas for 5 min before the bottle was sealed and mixed magnetically (at 300 rpm) for about 14 days, during which time biogas production and the cumulative biogas volume were automatically recorded every 10 min. The cumulative methane production profile was obtained by the summation of methane production (¼ biogas production volume methane fraction in the biogas) at sequential in- tervals (e.g., on days 0, 1, 2, 3, 5, 8, 12, 14). Similarly, the hydrogen production profile was generated after determining the hydrogen concentration in the biogas during each gas sampling period.
2.3. Impact of NZVI on anaerobic digestion under complete methanogenesis inhibition
Bacteria and methanogens can compete for hydrogen gas under anaerobic conditions (Kotsyurbenko et al., 2001). To elucidate the effect of NZVI on these important biological pro- cesses in anaerobic digestion, sodium 2-bromoethanesulfonate (BES), a coenzyme M analog that inhibits methanogenesis (Gunsalus et al., 1978; Kotsyurbenko et al., 2001), was added to each sludge sample at a final concentration of 50 mM to ensure complete methanogenesis inhibition (Parameswaran et al.,
2009; Zinder et al., 1984). The same experimental design was executed again as described above except that all the treat- ments contained 50 mM BES.
2.4. Gas composition and chemical analysis
Hydrogen and methane contents in the biogas were analyzed by gas chromatography (GC, Shimadzu, 2014) equipped with a thermal conductivity detector (TCD) using ShinCarbon ST 80/
100 Column (Restek, PA) as a separation column and helium gas as a carrier gas. The GC operating parameters were as follows: injection temperature, 100 C; flow rate, 10 mL/min; column temperature, held at 40 C for 3 min, then increased to
140 C at 25 C/min and held at 140 C for 3.5 min while the
TCD temperature was held at 200 C.
For chemical analysis, aliquots (8 mL) of mixed liquid slurry were taken from the samples on day 0, 0.17 (4 h), 1, 2, 3, 5, 8, 12, and 14 for soluble COD (SCOD), pH, and VFAs measurements according to the standard methods (APHA et al., 2005). The oxidation and reduction potential (ORP) in mixed sludge liquor after digestion was determined by a pH/OPR meter (Fisher XL-
15 m) combined with an ORP electrode. Detailed analytical in- formation for VFAs has been described elsewhere (Yang et al.,
2012a). All sampling was conducted in an anoxic glove box (Coy Laboratory Products Inc., MI) containing 5% H2 and 95% N2 to avoid oxygen exposure. Dissolved iron concentration in the filtrate of the mixed liquid slurry after passing through a 0.45- mm filter was determined by a graphite furnace atomic absor- bance spectrophotometer (GFAAS, BUCK 220 GF, Buck Inc, CT) using 0.2% HNO3 solution as a dilution solution. Before iron analysis, an aliquot of (120 mL) 1 g/L MgNO3, a matrix modifier, was added to each standard and sample in order to reduce the production of carbides due to the presence of organic carbon according to the manufacturer’s manual. The lower iron detection limit of the GFAAS was 5 mg/L. To avoid potential metal contamination, all glassware was soaked in a 10% nitric acid solution and then rinsed with DI water before use.
2.5. ESEM-EDS analysis
Environmental scanning electron microscopy (ESEM, FEI Quanta 600F, OR) coupled with energy dispersive X-ray spec- troscopy (EDS) was applied to determine the chemical composition of anaerobic sludge after NZVI treatment. The mixed liquid slurry samples were collected at the end of batch anaerobic digestion experiments. Each sample was directly added onto a specimen stub for ESEM-EDS analysis. Images of the sample were taken at a voltage between 7 and 10 keV under a high vacuum mode of operation.
2.6. Methanogenic population analysis after NZVI
exposure and anaerobic digestion
To determine methanogenic communities and their popula- tion dynamics during anaerobic digestion by quantitative po- lymerase chain reaction (qPCR), Sludge samples were collected for DNA extraction before and after the anaerobic digestion process. Total genomic DNA was extracted from 1.5 mL of the mixed liquid slurry using the MoBio Ultra CleanTM Soil DNA isolation kit (MoBio Laboratories, Inc., Carlsbad, CA). The DNA was quantified by Nanodrop ND1000 (NanoDrop Technologies, Wilmington, NC, USA) and its purity was determined by measuring the 260/280 nm absorbance ratio. The extracted DNA samples were stored at 20 C before use.
Two classes of acetoclastic methanogens (Methanosaeta and Methanosarcina) that convert acetate into methane and three orders of hydrogenotrophic methanogens (Meth- anobacteriales, Methanococci and Methanomicrobiales) that convert hydrogen and carbon dioxide into methane play an important role in anaerobic digestion (Garrity, 2001). The Methanococci were not studied because they are often not detected in the digestion systems and mainly exist at high salt concentrations (Garrity, 2001; Lee et al., 2009). qPCR assays were performed on an ABI 7500 Real Time PCR System and analyzed with the 7500 SDS system software (version 1.4, Applied Biosystems, CA). Detailed information about the four primers and probe sets used in qPCR, the internal standards prepared from the 16S rRNA gene clones of known meth- anogenic groups, and the qPCR standard curves based on plasmids for each methanogenic group has been described elsewhere (Yang et al., 2012b).
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