Thermal activated-acid/alkali modified kaolin for enhanced heavy metals capture during high-organic solid waste pyrolysis: Experimental and theoretical comparative study

The mandated waste classification poses challenges for the eco-friendly disposal of combustible high-organic solid waste (HSW), characterized by elevated heavy metal content and a low ash proportion. Fortunately, pyrolysis technology for HSW is emerging as a promising direction for achieving waste-to-resource conversion. To further mitigate secondary pollution, thermal activated-acid modified kaolin (H-kaolin) and thermal activated-alkali modified kaolin (OH-kaolin) were separately prepared, and their enhanced effects of immobilization and stabilization on heavy metals during pyrolysis were contrastively explored. Characterization results demonstrated that dehydroxy-dealuminization and zeolite NaA formation were intrinsic reasons for improvements in physical properties (specific surface area, pore volume) of H-kaolin and OH-kaolin, respectively. Experimental results revealed that H-kaolin outperformed OH-kaolin, showcasing its potential as a high-performance in-furnace adsorbent. Compared with other natural mineral-based sorbents reported in the literature, H-kaolin exhibited leading levels of immobilization, stabilization, and adsorption efficiency for heavy metals. Theoretical calculations illustrated that AlIII-O generated via dealuminization and SiIII-O in zeolite NaA were the main adsorption sites of H-kaolin and OH-kaolin, respectively, correspondingly binding with heavy metals to form oxides and silicates. The differences in active sites, electron transfer directions and bonding types during adsorption of H-kaolin and OH-kaolin were the intrinsic reasons for their distinct adsorption efficiency. Furthermore, derived chars with H-kaolin were deemed safe for application in pollutant adsorption in industrial field and soil improvement in gardens or parks. This research presented new ideas for developing high-performance in-furnace sorbents during pyrolysis, and explored the feasible safety application fields for pyrolytic derived-char.