Direct functionalization of raw coal using p-xylene-2-sulfonic acid hydrate for high adsorption of bisphenol A and 4-chlorophenol: Experimental and statistical physics insights

In this study, direct sulfonation of un-carbonized raw coal using p-xylene-2-sulfonic acid hydrate was successfully achieved—a rarely reported modification route compared with the conventional sulfonation of activated carbon after carbonization. This one-step molecular-level substitution presents a key novelty, enabling transformation of the native turbostratic microstructure without prior thermal treatment. XRD analysis confirmed substantial disruption of semi-crystalline carbon domains, with strong suppression of the characteristic (10) band at 40–50° (2θ), indicating conversion to a highly amorphous and defect-rich surface. The resulting sulfonated coal (SF.CA) demonstrated markedly enhanced adsorption performance toward Bisphenol A and 4-chlorophenol, reaching Q sat values of 245.78 mg/g and 242.44 mg/g, respectively, positioning the material among the most effective carbonaceous sorbents for phenolic pollutants. Equilibrium modeling identified the single-site monolayer model as the best fit (R 2 = 0.999; χ 2 as low as 0.03). Statistical-physics analysis provided deeper mechanistic insight, revealing multi-molecular stacking at each adsorption site, with steric coefficients n = 3.21–3.97 and active-site densities Nₘ decreasing from 76.75 to 47.90 mg/g with increasing temperature. This decline directly reflects temperature-sensitive anchoring behavior. The negative energetic parameter (ΔE down to −15.06 kJ/mol) across all temperatures confirms a spontaneous, low-energy, exothermic adsorption mechanism. The combined steric and energetic parameters support a vertical multi-molecule assembly stabilized through hydrogen bonding and π–π interactions at the SF.CA interface. Overall, the findings show that direct sulfonation is both synthetically simpler and capable of producing a highly efficient adsorbent whose behaviour is validated quantitatively at the molecular scale.