Comparative rates and yields of electrons released from ZVI powder in batch reactors with and without inoculated hydrogenotrophic methanogens

Abstract

Zero valent iron (ZVI) has been applying for enhancement methane (CH4) production due to its role can be act as electron donor with the direct/indirect donations. The indirect electron donation might be difficult formation due to the accumulation of hydrogen (H2) makes the reaction thermodynamically unfavorable. Hydrogenotrophic methanogens plays an important role in continuously removing the H2. In this study, the amounts of electrons released from the varied ZVI surface area in the inoculated hydrogenotrophic methanogens reactor and the uninoculated culture reactor were compared. The CH4 and H2 production were measured in the inoculated hydrogenotrophic methanogens reactor and the uninoculated culture reactor, respectively. The results showed that the CH4 production increased with increasing ZVI surface area with the maximum methane production of 20, 108 and 492 mL for the ZVI surface area of 0.5, 2.5 and 10 m2/L, respectively. In the uninoculated culture, the maximum H2 production was 17, 327 and 1,285 mL for the surface area of 0.5, 2.5 and 10 m2/L, respectively. When compared with the amounts of released electrons based on the molar amounts of CH4 and H2, the amounts of electrons released in the uninoculated culture stopped within 50 days while the electron release in the inoculated culture continued to day 100. The H2 production ceased on day 20, 40 and 60 while the CH4 production ceased on day 65, 100 and 120 for the ZVI surface area of 0.5, 2.5 and 10 m2/L, respectively. The results indicated that the inoculum enriched with hydrogenotrophic methanogens drove the extents of electron release and the rates of the electron release was a function of ZVI surface area. The rate of electron release per surface area of inoculated and uninoculated culture were 0.0002 and 0.0005 mole e−/m2 day, respectively. The rate of electron release per surface area of uninoculated culture set were similar to the other reported (0.0005 mole e−/m2 day). The results indicated that ZVI can use as an electron donor and yield CH4 with the natural mixed cultures of hydrogenotrophic methanogens.