A natural nontronite clay sample from the Amazon region (Tocantins State, Serra da Messa Group, Brazil) was utilized for synthesis of a new biomaterial produced by a hybridization process of inorganic and organic materials. The new biomaterial was composed of an Amazon nontronite and fungal mass obtained from the Aspergillus niger fungus. The natural clay, biomass, and biomaterial samples were used in batch studies for adsorption of Hg2+ ions from aqueous solution. The effects of stirring time, adsorbent dosage, and pH on the adsorption capacity demonstrated that 70 min is sufficient to reach equilibrium at room temperature and over a pH range of 6.0–8.0. The maximum number of moles adsorbed was 9.47, 17.8, and 18.7 mmol g–1 for natural clay, biomass, and biomaterial, respectively. From the cation/basic center interactions for each material at the solid–liquid interface, by using calorimetry, the equilibrium constant and exothermic thermal effects were calculated. By considering the net interactive number of moles for mercury ion and the equilibrium constant, the enthalpy and negative Gibbs free energy could be calculated: ΔintH = −6.8 ± 0.11 to −6.0 ± 0.22 kJ mol–1; ΔintG = −20.5 ± 0.25 to −23.2 ± 0.22 kJ mol–1. The thermodynamic values indicated the existence of favorable conditions for interactions at the solid–liquid interface.