Effect of inorganic potassium compounds on the hydrothermal carbonization of Cd-contaminated rice straw for experimental-scale hydrochar

Nowadays, it is extremely important to reuse the harvested biomass from contaminated soils. In this study, the effect of inorganic potassium compounds, such as K2SO4, KCl, KOH, and K2CO3, on the hydrothermal carbonization of heavy metal-contaminated rice straw as well as the distribution of heavy metals in the corresponding hydrochars were examined. In the presence of K2SO4, the higher heating value, carbon recovery, and energy recovery of the resulting hydrochar significantly increased from 23.34 MJ kg(-1), 42.74%, and 42.99% to 24.46 MJ kg(-1), 44.20%, and 45.03%, respectively. Concomitantly, the enhanced decarboxylation and dehydration process led to a decrease in the oxygen content of the resulting hydrochar. In the presence of KOH, the Cd and Cu content of the corresponding hydrochar significantly increased (p < 0.05) from 70.72% to 79.09%-89.96% and 93.73%, respectively, while only slightly changing upon the addition of K2SO4 or KCl, which was attributed to the morphology and microstructure of the different hydrochars. These results suggest that hydrothermal carbonization is a viable method to reuse contaminated biomass for solid fuel, while K2SO4 is a suitable additive to improve the energy recovery of hydrochars.