The first application of calcined Cardita bicolor oyster shell (CCBS) for Hg(II) and As(III) adsorption from synthetic and real wastewaters was tested. The main elements in CCBS structure were carbon, oxygen, magnesium, phosphor, and calcium. Effects of different parameters like initial pH, contact time, temperature, and CCBS dosage were assessed. The results showed that the maximum recovery of Hg(II) and As(III) adsorption was determined as C0=10mg/L, t=80 min, T=25 °C, CCBS dosage=5 g/L, and pH=6 (for mercury ion) and 7 (for arsenic ion). In these conditions, 95.72% Hg(II) and 96.88% As(III) were removed from aqueous solution. The correlation coefficient (R2) values for both adsorbates were obtained >0.98 and >0.96 for Langmuir and Freundlich isotherm models, respectively. Pseudo-second-order kinetic model was more capable to describe kinetic behavior of adsorption process of both metal ions in comparison with pseudo-first-order model. The half life (t1/2) value for Hg(II) and As(III) with initial concentration of 10mg/L was 4.032 and 4.957 min, respectively. Moreover, thermodynamic parameters of enthalpy (ΔHo), entropy (ΔSo), and Gibbs free energy (ΔGo) were investigated. Two real wastewaters obtained from a leather factory and a landfill leachate were successfully treated using CCBS. The results confirmed that adsorption process of metals ions was exothermic and spontaneous.