The thermodynamics of adsorption of laccaic acid (lac dye) onto chitosan were investigated under acidic condition over various concentrations (20-293 mg/l). Langmuir, Freundlich, and Temkin isotherms were used to analyze the equilibrium data at different temperatures, with the Freundlich isotherm fitting the experimental data significantly better than the other isotherms. The effect of temperature on the adsorption isotherm was studied by carrying out a series of isotherms at 10, 20, 40, and $60^{circ}C$. It was found that more dye was strongly adsorbed by chitosan when the temperature of the dye solution increased. Thermodynamic parameters such as free energy change (${Delta}G^{circ}$), enthalpy change (${Delta}H^{circ}$), and entropy change (${Delta}S^{circ}$) were also evaluated. The negative value of ${Delta}G^{circ}$ and positive value of ${Delta}H^{circ}$ indicated that the laccaic acid adsorption process is a spontaneous and an endothermic one. Attenuated total reflectance Fourier transform infrared spectroscopy confirmed the functional groups of chitosan that affected the laccaic acid adsorption. Therefore, laccaic acid molecules could interact with the chitosan via electrostatic forces, hydrogen bonding, and ion-dipole interactions under acidic condition. From the toxicity study, the dye solution with the initial dye concentrations of 40 and 120 mg/l before dye removal showed significant mortality to earthworm Diplocardia communis (P<0.01).