Effect of FeCl3 based mesothermal pretreatment of rice straw on methane enhancement

Recovery of bioenergy (CH 4 ) from agricultural waste is often limited by low hydrolysis rate due to its inherent recalcitrance structure. This study reports and demonstrates a new pretreatment strategy to enhance methane production from rice straw (RS). This is achieved by adding a commonly-used chemical, ferric chloride (FeCl 3 ), into RS, followed with 24 h of mixing under mesothermal condition (32 °C). The results showed that the addition of FeCl 3 ranging from 1.5 to 12.3 mmol/L effectively destroyed the inherent recalcitrance structure of RS and released more reducing sugar by shaping the chemical composition of the RS and reducing crystallinity index of cellulose. Biochemical methane potential (BMP) tests demonstrated that the pretreated RS at 3.1 mmol FeCl 3 /L increased the methane production from 10.3 ± 0.2 (i.e. control) to 18.3 ± 0.3 mol CH 4 /kg VS, indicating a relative increase by 78%. Comprehensive mechanism analyses suggested that the FeCl 3 pretreatment improved carbohydrates hydrolysis and stimulated the activity of methanogens during anaerobic digestion. Consequently, the methane yield was significantly increased with decreased lag-phase time in BMP tests, as revealed by the Modified Gompertz model. This study illustrated a cost-effective method of mesothermal FeCl 3 -based pretreatment to improve methane production from RS.