The removal of heavy metals is crucial to the utilization of contaminated biomass resources. In this study, we report an efficient process of hydrothermal conversion (HTC) of sunflower straw (Helianthus annuus L.) to remove heavy metals. The effect of different HTC temperatures and concentrations of HCl additives on heavy metal removal efficiency was investigated. The results revealed that increasing the temperature or concentration of HCl promoted the transfer of heavy metals from hydrochar to liquid products during HTC. The heavy metals removed to the liquid products included up to 99% of Zn and Cd, 94% of Cu, and 87% of Pb after hydrothermal conversion with a temperature of 200 °C and HCl 2%. The species of heavy metals in hydrochars converted from unstable to stable with an increase in temperature from 160 °C to 280 °C. The stable fractions of heavy metals in the acidic condition decreased as the acid concentration increased. This aligns well with the high transfer efficiency of heavy metals from the solid phase to the liquid phase under acidic conditions. The FTIR indicated that the carboxy and hydroxy groups decreased significantly as the temperature increased and the concentration of HCl increased, which promoted the degradation of sunflower straw. A scan electron microscope showed that the deepening of the destruction of the initial microstructure promotes the transfer of heavy metals from hydrochars to liquid phase products. This acid-assisted hydrothermal process is an efficient method to treat biomass containing heavy metals.

Excess Cd in soils can be accumulated in rice, presenting a serious human health risk. The effect of foliar transpiration inhibitors (TIs) on the Cd content and the endophytic bacterial community in rice plants was unclear. We evaluated the key part of the rice plant to control the Cd translocation and the profile of the endophytic bacterium structure after spraying with foliar reagents; some possible typical endophytes were induced by the TIs to inhibit the Cd translocation in the rice plant. The rice plants in three sites with different available Cd content were sprayed with foliar TIs. We assessed the Cd, N, P, K and water-soluble saccharide (WSS) in different parts of the rice plant and the endophytic bacteria community in the stem. Foliar application of TIs reduced Cd translocation factor (TF_Cd) by ~20% from the root to the grain compared with that of CK. The TI can increase the adsorptive site concentration of stem nodes from 5.10 to 6.83 mmol/g. The diversity of the endophytic bacteria community was enhanced after application of TI, and the Shannon index increased from 3.29 to 3.92. The endophytic bacterial community induced by TI showed higher potentiality on the biofilm and stress-tolerant and metal-transport functions than that of CK, respectively. The relative abundances of Burkholderiaceae and Bacterium_g_Anaeromyxobacter were significantly negatively correlated (p < 0.05), with TF_Cd and positively correlated (p < 0.05), with water-solution saccharide content, simultaneously. The TI enhanced the endophytic diversity and amount. A high abundance of special endophytic bacteria induced by TI might decrease the TF_Cd.