Supercritical water gasification of hyperaccumulators for hydrogen production and heavy metal immobilization with alkali metal catalysts

The high moisture content and heavy metal concentration of hyperaccumulator are the main bottlenecks of resource utilization. Supercritical water gasification technology was used to convert Sedum plumbizincicola (a hyperaccumulator of Zn and Cd) into hydrogen gas and to immobilize HMs into biochar. Homogeneous alkali metal catalysts such as NaOH, Na2CO3 and Ca(OH)(2) were added to optimize the experimental conditions. The results showed that NaOH was effective in capturing CO(2 )in-situ, thereby shifting the water-gas shift reaction equilibrium in the forward direction. And the increase of NaOH concentration had a significant promotion effect on hydrogen production. In the non-catalytic gasification of Sedum plumbizincicola, the highest hydrogen (1.5 mol/kg) and H-2 selectivity (22.9%) with greater carbon gasification efficiency (19.3%) and lower H2 gasification efficiency (8.7%) of the gas products were obtained at 400 C with 6 wt% material concentration for 20 min. However, NaOH at 5% mass fraction maximized hydrogen and H-2 selectivity up to 7.5 and 98.2%, respectively. Alkali catalyst not only promoted the generation of hydrogen-rich bio-gas but also enhanced the immobilization efficiency of heavy metals. Compared to non-catalytic, when the addition amount of NaOH was 1 wt%, the Zn? Mn?Cd?Pb?Cr accumulated in biochar increased significantly for 76.8, 42.5, 80.8, 75.6 and 80.0%, respec-tively. This study highlights the remarkable ability of SCWG with alkali catalyst for hydrogen production and heavy metal stabilization.

Automated summary

The high moisture content and heavy metal concentration of hyperaccumulator plants pose challenges for resource utilization. This study applied supercritical water gasification technology to convert a hyperaccumulator plant, Sedum plumbizincicola, into hydrogen gas and immobilize heavy metals in biochar. The addition of alkali metal catalysts, such as NaOH, Na2CO3, and Ca(OH)(2), improved the experimental conditions. The results showed that NaOH effectively captured CO2 and enhanced hydrogen production. Additionally, the alkali catalyst promoted heavy metal stabilization. The study highlights the potential of SCWG with alkali catalyst for hydrogen production and heavy metal immobilization.