Measurement of dissolved H2 concentrations in methanogenic environments with a gas diffusion probe

Abstract

A probe for the measurement of dissolved H2 in anoxic methanogenic environment was developed. The probe was based on the diffusion of dissolved H2 through a silicone membrane into a gas space at the end of the probe. This gas space was flushed with N2 and analyzed gaschromatographically for H2. The probe was calibrated in aqueous solutions of the known gas space was flushed The detection limit was about 7 nM H2, the depth resolution of vertical profiles was about 5–10 min. The probe could also be used to measure dissolved CH4 and CO. The probe allowed the measurement of in situ concentrations of dissolved H2 that were unbiased by eventual effects of an extraction procedure on the steady state between production and consumption of H2. However, it could not be implanted into the environment for periods longer than hours because of dramatically increased H2 signals due to microbial growth on the silicon membrane. The probe was used to measure vertical profiles of dissolved H2 and CH4 in cores of the deep sediment of Lake Constance and of anoxic paddy soil. The vertical CH4 profiles measured with the probe compared well with those measured by extraction. In the lake sediment, concentrations of H2 increased with depth and reached values in the range 10–60 nM H2 within the methanogenic zone, below about 2–5 cm depth. In the anoxic paddy soil, H2 concentrations also increased with depth reaching values of 40–100 nM H2. Dissolved CO was about 5–20 nM. The turnover of H2 was very dynamic as revealed when starch powder was added to the flooding water. Within 1 day, H2 concentrations increased to 80 μM H2 in the upper 2 cm of the soil core, but after 5–7 days decreased again to the lower values measured previously.

Spatial and temporal variations of dissolved gases (CH4, CO2, and O2) in peat cores

“…Sebacher et al [23] observed a peak in the vertical distribution of dissolved CH4 in sediment pore water near the rhizomes of aquatic plants. Other studies have shown the concentration of dissolved CI-L in peatland waters to increase linearly to a depth of 210 cm [29], whereas, in anoxic paddy soil, CH4 has been shown to increase with depth to maximum concentrations of 1.2-1.5 mM between 7-and 13-cm depths [15]. In the present work, variations in the methane profiles of peat cores taken from random hollows were observed; some profiles showed a leveling off in the CH4 concentration below 13-cm depth, whereas other profiles exhibited a subsurface methane peak.…”

Towards constraining H2 concentration in subseafloor sediment: A proposal for combined analysis by two distinct approaches

“…This is because H 2 partitions preferentially into the gaseous phase. The removal of H 2 lowers the concentration of dissolved H 2 and results in excess H 2 production (Krämer and Conrad, 1993). While excess H 2 production can be a serious problem in very active microbial ecosystems, it is likely of minor importance in deep subseafloor sediments where microbial metabolism is known to be very slow.…”

Utilization of a palladium-metal oxide semiconductor (Pd-MOS) sensor for on-line monitoring of dissolved hydrogen in anaerobic digestion

“…In earlier investigations, little has been mentioned with regard to long-term stability when using membranes for gas transfer in anaerobic digesters. Krämer and Conrad (1993) detected a biofilm on a silicone membrane after prolonged incubation (days) in a slurry, and suggested that this would cause overestimation of the dissolved hydrogen concentration due to the growth of hydrogen producers on the membrane. Voolapalli and Stuckey (1998) reported a decrease in the transfer of hydrogen from the bulk liquid through a silicone membrane due to growth on the membrane, and suggested that the silicone membrane attracted and supported the slow-growing methanogenic bacteria.…”

“…In earlier studies, the most commonly used membranes for the transfer of dissolved gases to a gas phase were Teflon (Krämer and Conrad, 1993;Meyer and Heinzle, 1998;Pauss et al, 1990b;Smolenski and Robinson, 1988), and silicone rubber (Cord-Ruwisch et al, 1997;Meyer and Heinzle, 1998;Whitmore and Lloyd, 1986). Silicon rubber was not an option in this case, because it is permeable to polar molecules such as H 2 S and water.…”

“…Methods intended for on-line analysis have included the transfer of the hydrogen from the liquid through a gas-permeable membrane, followed by gas-phase quantification. Quantification has been achieved with mass spectrometry (Degn et al, 1985;Heinzle and Lafferty, 1980;Lloyd et al, 1983), or gas chromatography followed by mercury vapor detection (Krämer and Conrad, 1993;Smolenski and Robinson, 1988). These methods have been considered to be overly sophisticated and expensive, and thus mainly applicable in the laboratory (Pauss et al, 1990a).…”