Study on the Effects of Acid/Alkali Activation on the Radon Adsorption Performance and Hydrophobicity of Pine-Based Activated Carbon

Radon ( 222 Rn), classified as a Group 1 carcinogen by the World Health Organization, poses severe public health risks through its accumulation in indoor environments. Activated carbon adsorption is a primary technology for radon mitigation, yet conventional carbons suffer from low adsorption capacity, poor hydrophobicity, and irreversible performance degradation over repeated use. In this work, pine char was employed as a precursor to prepare activated carbons via phosphoric acid (H 3 PO 4 ) and potassium hydroxide (KOH) activation, including both single-step (acid or alkali) and sequential two-step (acid–alkali or alkali–acid) treatments. The materials were comprehensively characterized by SEM, FTIR, BET, and dynamic water contact angle measurements, while radon uptake and regenerability over five thermal cycles were evaluated via static adsorption tests. Results revealed that radon adsorption coefficients increased with activator loading up to an optimal ratio and then declined. The highest adsorption coefficients were achieved for AC-H 3 PO 4 (1:3) (7.23 ± 0.18 L/g) and AC-KOH (1:1) (9.35 ± 0.12 L/g), both significantly outperforming commercial activated carbon (4.03 ± 0.19 L/g). Alkali-activated carbons exhibited superior specific surface area, micropore volume, and pore-size matching, along with enhanced hydrophobicity, compared to acid-activated counterparts. Sequential two-step activation reduced adsorption coefficients to 6.13 ± 0.28L/g (acid–alkali) and 6.72 ± 0.24L/g (alkali–acid), attributed to excessive etching-induced collapse of the porous network. After five regeneration cycles, the acid- and alkali-activated carbons retained 92.4% and 91.2% of their initial adsorption coefficients, respectively, demonstrating excellent recyclability. This study establishes clear structure–performance relationships between activator type, pore architecture, surface chemistry, and hydrophobicity, providing a robust theoretical and experimental basis for the development of high-performance, durable, and hydrophobic radon adsorbents.