Simultaneous syngas and biochar production during heavy metal separation from Cd/Zn hyperaccumulator (Sedum alfredii) by gasification

Abstract

Phytoremediation is increasingly recognized as a cost-effective approach for remediation of heavy metal (HM) contaminated soils, whereas the derived HM-enriched hyperaccumulators must be properly harvested and disposed. In this study, gasification of Sedum alfredii was performed at a temperature series (300–900 °C) with different gasifying agents (N2, CO2 and air), then the transfer behaviors of zinc (Zn), cadmium (Cd), and lead (Pb) and the characteristics of the derived biochar and syngas were investigated. The high gasification temperatures and reducing atmosphere (N2) effectively enhanced the volatilization of Zn, Cd, and Pb, and the results from toxicity characteristic leaching process indicate that gasification greatly enhanced the stability and reduced the availability of HMs in S. alfredii. The CO2-derived biochars had moderate pH and greater stability, while the N2-derived biochars showed better energy density. The evolution of functional groups and crystalline phases in biochars were both influenced by the temperature, and the role of atmosphere was increasingly obvious at temperatures above 600 °C. As expected, biochars derived under N2 and CO2 both showed considerable sorption capacity for Pb (134.2–198.8 mg g−1) and Cd (38.1–186.8 mg g−1). CO2 atmosphere greatly enhanced the production of CO at higher temperatures (>700 °C), while N2 atmosphere favored the yield of H2 and CH4. These results imply that gasification could be a promising technique to achieve the separation of HMs from hyperaccumulator as well as the production of energy and multifunctional material, and this process is greatly affected by the gasification temperature and atmosphere.