A nanocomposite of polyaniline/hexagonal mesoporous silica (PAN/HMS) was prepared and characterized by BET analysis, transmission electron microscopy, and FT-IR spectra. Batch adsorption results showed that PAN/HMS had high affinity to Hg2+ in aqueous solutions. Various factors affecting the adsorption capacity such as contact time, temperature, absorbent dosage, pH, and initial concentration of Hg2+ ions were investigated. The adsorption kinetics for the Hg2+ showed that the adsorption reached equilibrium within 8 min and adsorption rates followed the pseudo-second-order rate law, indicating chemical sorption as the rate-limiting step of the adsorption mechanism. Sorption of Hg2+ to PAN/HMS agreed well to the Langmuir adsorption model at different ionic strengths with the maximum adsorption capacity of 843 mg g(-1) (I = 1000 mg L-1) at pH 10. Thermodynamic studies revealed that the adsorption of Hg2+ ions was spontaneous, exothermic processes with an increase of entropy. The recovery of the Hg2+ from the adsorbent was found to be more than 88 % using H2SO4 (0.1 M), and the ability of the absorbent to be reused was investigated.