Experimental and kinetic analysis of the addition of zerovalent iron for promoting conversion of CO2 into CH4 by anaerobic sludge

Reducing greenhouse gas (GHG) emissions and sequestering carbon dioxide (CO2) can be achieved by a reduction of CO2 to methane (CH4) for renewable energy production. This study used zero-valent iron (ZVI) as an electron donor to reduce CO2 to CH4 at different ZVI concentrations (16, 32, 64, 96, and 224 g/L). The experimental CO2 consumption rates for the reactor with 16, 32, 64, 96, and 224 g/L ZVI were 6.880, 5.438, 7.384, 8.309, and 8.051mmol/d · Lreactor, respectively. The CO2 reduction kinetics was analyzed using four models (Cone, Gompertz, Logistic, and Richards equations). The four models, especially the Richards equation, satisfactorily fitted the experimental data with an R^2 of 0.990 − 0.998. The derived kinetic parameters reflected the experimental results, which showed that the lag phase for methane production decreased with increasing ZVI concentrations. The other characteristics of the system were: (1) the ZVI addition caused the increase in pH of the media; (2) there was a competition between methanogens and other metabolic pathways, especially at relatively high pH; and (3) relatively high ZVI concentrations induced the production of acetate and propionate.