Nitric acid-modified hydrochar enhance Cd sorption capacity and reduce the Cd accumulation in rice.

Remediating the agricultural soil polluted by cadmium (Cd) is a serious issue in China. Hydrochar showed its potential to purify Cd-contaminated water and improve Cd-contaminated soil due to its vast amounts of macro- and microporous structures. In this study, three concentration gradients of nitric acid (HNO, mass fraction: 5%, 10%, 15%) were implemented to age pristine wheat straw hydrochar (N0-HC) aiming to improve surface physiochemical properties. Four HNO-aging hydrochars named N0-HC, N5-HC, N10-HC, N15-HC were used to both remove Cd from aqueous solution and improve soil properties. Results showed that HNO-aging significantly improved the Cd adsorption capacity by 1.9-9.9 folds compared to crude hydrochar due to the increased specific surface area (by 1.5-6.5 folds) and oxygen-containing functional group abundance (by 4.5-22.1%). Besides, initial solution pH of 8 or environmental temperature of 318.15 K performed the best Cd adsorption capacity. Furthermore, the process of Cd adsorption was fitted best to pseudo-second-order (R = 0.95) and Langmuir models (R = 0.98), respectively. Nanjing 46 (Oryza sativa L) and HNO-aging hydrochars were furtherly applied into Cd-contaminated paddy soil to investigate the mitigation of Cd translation from soil to rice. N15-HC-1% (w/w) performed the best effect on reducing cadmium accumulation in various parts of rice plants. Overall, this research provided an approach to improve hydrochar capacity to remove Cd from aqueous solution and mitigate Cd translation from soil to rice.