3.1. Sample site and collection
A total of 23 CBM co-produced water samples were collected from the CBM
wells in Shizhuangnan block in 2019.12, the sample distribution is shown
in fig. 4a. All selected CBM wells have been running for more than 5
years constantly and the drainage coal seam is 3# coal. Water samples
for ion and isotope analysis and 16S rRNA sequencing were collected
directly from the CBM wellhead in 500 mL headspace bottle and 50 mL
centrifuge tubes. 0.5 L aluminum foil gas sampling bags were used for
gas sampling. During collection, the whole container was filled with the
water, placed on the ice, placed in the refrigerator, and immediately
sent to the laboratory. All geochemical test including iron test and
isotope test were completed in 7 days.
3.2 Ion concentration test
Ion chromatography instrument ICS-1100 (Thermo) was used for the
determination of anions and cations. After the water sample was injected
into the instrument, it was carried by the eluent and passed through the
ion separation column. The conductivity detector was used to measure and
record the peak heights of them in turn. From the standard curve drawn
under the same conditions, the ion content in water sample could be
calculated.
The chromatographic conditions of Cl-、
NO2-、
NO3-、SO42-were as follows: the column was AG19,4×250 mm; the eluent was KOH, 20
mm; the flow rate was 1 mL/min; the column temperature was 30 ℃. The
chromatographic conditions of
Na+、NH4+、K+、Mg2+、Ca2+were as follows: the chromatographic column was CS12A,4×250 mm; the
eluent was MSA, 20 mm; the flow rate was 1 mL/min; the column
temperature was 30 ℃.