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.