Unveiling the Dual Impact of Coal's Inherent Calcite in Activated Carbon: Quantifying Adsorption–Regeneration Trade-Off for Toluene Removal

This study provides mechanistic insights into the inherent calcite-mediated trade-offs in toluene adsorption and regeneration performance of coal-based activated carbon (AC). ACs, from coal with varying calcite (1, 2, and 3%), were prepared via steam activation. While 1% calcite modestly slightly enhanced toluene adsorption to 81.60 mg g −1 by optimizing microporosity and surface π-π interactions, however, higher calcite content (≥2%) exhibited severe performance degradation. Specifically, 3% calcite caused a drastic decline in adsorption capacity to 17.54 mg g −1 , a 77.35% reduction compared to calcite-free AC. This destruction stems from over-etching during activation, leading to micropore collapse (79.5% reduction in microporous surface area), loss of total specific surface area (57.4% decrease), and disrupted graphitization, weakening π-π conjugation with toluene. Surface oxygen group imbalances shifted the mechanism from intraparticle diffusion to superficial, weak physical adsorption. Notably, 1% calcite AC demonstrated superior initial regeneration, retaining 95% of its initial capacity after 10 cycles, attributed to calcite-induced reduction of surface oxygen groups, increasing hydrophobicity and strengthening π–π interactions. However, this enhancement is tempered by a trade-off, as extended use results in microstructural degradation, with micropore volume gradually decreasing, limiting long-term performance. This study revealed calcite's dual nature, emphasizing that optimizing its content is essential for balancing short-term performance with long-term structural integrity for stable VOC removal.