Interaction of biochar stability and abiotic aging: Influences of pyrolysis reaction medium and temperature

To improve soil quality, contaminated-soil can be amended with biochar, but the stability of biochar due to the exposure to the environment is not well elucidated. Furthermore, with the emerging introduction of CO₂ as a reaction medium to increase the stability of biochar, a strategic study of reactive media for pyrolysis on biochar stability is required. To investigate the effect of aging and reaction medium on biochar stability, sawdust (SB), rice straw (RB), and spent coffee grounds (CB) were pyrolyzed using two distinct reaction media, N₂ and CO₂, at 300 °C and 700 °C for 4 h. Biochar alone was subjected to three artificial aging processes: wet-dry, freeze–thaw, and light exposure for 25 cycles, based on these results, soil-biochar mixtures were aged artificially using wet-dry and freeze–thaw. Then, the leaching potential of dissolved organic matter (DOM) and As mobility were compared. This study showed that the aging processes increased levels of DOM leaching and wet-dry aging accelerated DOM leaching and As mobility most significantly. Weathered biochars released 3.7 (SB), 8.9 (RB), and 5.2 (CB) times more DOM than fresh biochar, and As mobility increased by factors of 5.4 (SB), 16.3 (RB), and 13.7 (CB) compared to soil-only aging systems without biochar. Moreover, the CO₂ reaction medium produced more stable biochar and lowered slightly DOM leaching and As mobility versus using N₂. This study suggests that it is important to consider not only the immediate effect of applying biochar to soil but also the effects of various aging of biochar over time.