Anaerobic granular sludge and zero valent scrap iron (ZVSI) pre-treated with green tea as a sustainable system for conversion of CO2 to CH4

Abstract

Global climate change is a worldwide concern that requires the dramatic reduction of greenhouse gases. Innovation in sustainable technologies for CO2 utilization to other high value products is therefore an emerging and rising field. It was found that CO2 as a sole carbon source can be converted to CH4 (higher than 97%) in a system consisted of anaerobic granular sludge, water and zero valent scrap iron. In this system (zero valent scrap iron, anaerobic granular sludge and CO2), pre-exposure of zero valent scrap iron to green tea resulted in a higher conversion rate and total CH4 production compared to bare zero valent scrap iron response. At the abiotic system (no anaerobic sludge) the zero valent scrap iron pre-treated with green tea also showed around 10% higher H2 production as well as higher final pH compared to zero valent scrap iron. The dominant crystalline product of the process both at the abiotic and at the biological system was siderite (FeCO3). It is likely that the formation of siderite on the zero valent scrap iron outer surface have reduced the amount of H2 release from zero valent scrap iron, shielding the substrate for methanogens. The presence of green tea compounds can form an iron-tannate complex with iron ions and this can possibly decrease or slow down the siderite layer, as a result more H2 was released when zero valent scrap iron was pre-exposed to green tea. Anaerobic spent filtered media was independently added to zero valent scrap iron to examine the effect of extracellular enzymes, however slight increase of the H2 production rate was found. The examined system combines potential solutions for two environmental problems; production of siderite derived from zero valent scrap iron for environmental applications and utilization of CH4 generated from CO2 for energy purposes.