Nitric Acid-Modified Hydrochar Enhance Cd2+ Sorption Capacity And Reduce The Cd2+ 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 (HNO3, mass fraction: 5%, 10%, 15%) were implemented to age pristine wheat straw hydrochar (N0-HC) aiming to improve surface physiochemical properties. Four HNO3-aging hydrochars named N0-HC, N5-HC, N10-HC, N15-HC were used to both remove Cd2+ from aqueous solution and improve soil properties. Results showed that HNO3-aging significantly improved the Cd2+ 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 Cd2+ adsorption capacity. Furthermore, the process of Cd2+ adsorption was fitted best to pseudo-second-order (R-2 = 0.95) and Langmuir models (R-2 = 0.98), respectively. Nanjing 46 (Oryza sativa L) and HNO3-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 Cd2+ from aqueous solution and mitigate Cd translation from soil to rice.