Optimal metal immobilization of oxygen-enriched co-combustion of Zn/Cd hyperaccumulator and textile dyeing sludge with natural or modified kaolin

Abstract

Eco-friendly disposal of post-harvest and metal-enriched hyperaccumulator biomass is critical for the industrialization of phytoremediation. The addition of Al/Si-based materials, such as sludge and natural mineral additives, is intended to solve the issue of HM metal stabilization in the oxygen-enriched combustion of hyperaccumulator and slow down ash sintering. This study aimed to quantify and reveal the effects of 10% kaolin or modified kaolin on the oxygen-enriched (co-)combustions of Sedum alfredii Hance (SAH) and textile dyeing sludge (TDS) and their combustion kinetics, metal enrichment rates, and immobilization/stabilization mechanisms. The activation energies for the (co-)combustions of SAH and SAH mixed with 10% additives (ST31) were estimated at 75.84–260.97 kJ/mol and 65.58–327.59 kJ/mol, respectively. The (co-)combustion mechanism model was complex, including phase boundary, diffusion, and chemical reaction order. The co-combustion of SAH with 10% modified kaolin increased Cd and Zn immobilization by approximately 66.6% and 3.2%, respectively, compared with the theoretical data at 550 °C. The co-combustion of ST31 with 10% modified kaolin increased Cd immobilization by approximately 160.9% compared with the theoretical data at 750 °C. Obtained via thermal-acid modified treatment, modified kaolin increased the immobilization efficiencies of Zn and Cd through aluminosilicate reactions to form the stable structures (Ca–Zn–Si, Ca–Zn–Al, K–Zn–Si, Zn–Al, Zn–Si, Cd–Al, and Cd–Si) as well as physical absorption. According to the combination of the experimental, multi-objective optimization, and simulation results, SAH mixed 10% modified kaolin at 550 °C and ST31 mixed with 10% modified kaolin at 750 °C were the ideal co-combustion environment. This study can provide new insights into how to best reduce the environmental risks of the hyperaccumulator disposals through the oxygen-enriched combustion technology.