Abstract Water pollution and the attendant difficulty in developing viable local treatment techniques are major challenges in most countries. To address this, recent studies have focused on utilization of available low-cost adsorbents. However, a major limitation is their low adsorption capacities. Hence, we evaluated the effectiveness of naturally ubiquitous Barbula lambarenensis (RBL) for removal of aqueous Hg(II) ions via batch biosorption process. Biosorption was carried out at different temperatures (293–313 K), time (5–180 min), pH (3–7) and concentrations (20–60 mg/L), and data generated were explained using various kinetic and adsorption isotherm models. Results showed that equilibrium was attained in 120 min at 313 K and lower temperatures but faster at higher temperatures. However increase in temperature does not correspondingly lead to higher biosorption: 303 ≥ 293 > 313 K. Optimum pH of adsorption was observed at 5.5. Modeling of the experimental data suggested that the biosorption process was majorly a monolayer surface phenomenon which occurred via sharing or exchange of valance electrons, and the RBL maximum adsorption capacity is 4.5 mg/g. The process was exothermic and spontaneous. Hydroxyl, carboxyl, thioesters and amide functional groups were implicated in the biosorption process. Overall, the study suggest that RBL may be useful for Hg(II) biosorption from aqueous solutions.