Functionalized polyethyleneimine grafted magnetic biochar for facilitating in-situ CO2 conversion to improve methane production in thermophilic anaerobic digestion

Abstract

Redundant CO2 produced during wet anaerobic digestion (AD) needs proper in-situ minimization, in which the amino groups in polyethyleneimine (PEI) can react with CO2 to form bicarbonate by diffusing into the liquid phase. Hence, the addition of PEI-grafted material may provide a novel way to in-situ supplement bicarbonate in the AD system, which could further improve the buffer capacity and promote syntrophic methanogenesis. In this study, different CO2 adsorbents prepared by grafting PEI or epoxide-functionalized PEI (FPEI) on alkali-magnetic biochar were used to investigate the in-situ application feasibility. A series of weight loadings of the FPEI to alkali-magnetic biochar (10BFP = 1:10, 5BFP = 1:5, 2BFP = 1:2, and BFP = 1:1) were evaluated in batch AD tests. Fumed silica was also used as an unconducive solid support material for FPEI grafting. After adding 5BFP, the methane production significantly increased by 17.8 %, along with an increased CH4/CO2 ratio from 181.7 % to 219.6 %. The bicarbonate generated by PEI/FPEI facilitated the degradation of volatile fatty acids by enhancing syntrophic reactions. The increased relative abundance of the genera Acetomicrobium, Thermovirga, and Methanosarcina indicates that FPEI improved the stability of the AD system. The 5BFP adsorbent could play a similar role as the carbonic anhydrase enzyme, with lower dosages favouring methane production.