Dynamic zinc and potassium release from a burning hyperaccumulator pellet and their interactions with inhibitive additives

Hyperaccumulators, as a special plant, are capable of absorbing heavy metals from contaminated soil. Thermal utilization of hyperaccumulator biomass is therefore more challenging, due to the requirement of avoiding secondary pollution of heavy metals. To investigate dynamic alkali and heavy metal release during hyper accumulator combustion, we used multi-point laser-induced breakdown spectroscopy (LIBS) to measure zinc (Zn) and potassium (K) release from a burning Sedum alfredii pellet. The results show that 24.2% of Zn and 61.5% of K are released during the Sedum alfredii combustion through two stages, i.e., (1) devolatilization and (2) char and ash reactions. The latter is the primary release stage, accounting for 69.8% and 77.6% of the total released Zn and K, respectively. The release characteristics of different chemical groups of Zn and K are then studied by performing LIBS on Sedum alfredii treated with solvent fractionation in three sequential steps. NH4Ac-soluble Zn and H2O-soluble K are found to be the primary released Zn and K groups, respectively. Furthermore, inhibition effects of two synthetic additives, silica (SiO2) and alumina (Al2O3), and two natural mineral additives, kaolin, and mica, on Zn and K release have been investigated. All the four additives achieve an inhibition efficiency higher than 55% on K release through aluminosilicate reactions and physical absorption. However, the performance of these additives on Zn retention is not as good as for K. The highest Zn retention efficiency is 37.0% achieved by Al2O3, which can be explained by a Ca-Al-Zn transformation mechanism converting volatile Zn to Ca3Al4ZnO10.