Formation mechanism of heel build-up from VOCs during vacuum thermal regeneration cycles of activated carbon

The irreversible “heel formation” during cyclic adsorption/vacuum-thermal regeneration of volatile organic compounds (VOCs) on activated carbon reduces adsorption capacity and service life. However, the chemical composition of pore accumulations, their formation mechanisms, and correlations with carbon microstructure remain unclear, limiting VOCs resource recovery and pollution control. In this study, the effect of heel formation on activated carbon adsorbents and the mechanism of heel formation were investigated by repeated cyclic adsorption/vacuum thermal regeneration experiments using xylene as a model pollutant. The results showed that strong chemisorption between xylene and activated carbon adsorbents generated heel buildup (4.26%–5.46%), which caused hindered complete regeneration (retaining 83.8%–90.1% of the capacity in 10 cycles). The decrease in micropore volume (16.4%–39%) of the four activated carbon adsorbents indicated that the heel preferentially accumulated in the micropore. Furthermore, Boehm titration demonstrated a strong correlation between heel buildup and acidic surface oxygen functional groups (SOFGs) for activated carbon, suggesting their primary role in inducing pore accumulation. Thermogravimetric analysis (DTA) and in situ diffuse reflectance infrared Fourier transform spectroscopy (In situ DRIFTs) further clarified that there was a strong chemisorption of acidic SOFGs between the interface of xylene and the adsorbent, leading to the blockage of the pore structure and the occupation of the interfacial adsorption active sites. This study provides critical insights for designing highly efficient and stable adsorbents, advancing VOCs resource recovery and pollution control.