Thermodynamic study was conducted by varying temperature from 10 to 50 C at adsorbent dose of 10 g/L contact time of 8 h, pH 4, mixing speed of 80 rpm and initial adsorbate concentration of 0.5 mg/L. In every experiment, suspensions were allowed to settle for 30 min after stirring, supernatant were filtered and residual As(III) concentration in filtrates were measured. The adsorption capacity ‘q’ of coconut fiber adsorbent i.e. amount of As(III) adsorbed per unit mass of adsorbent was estimated using the equation: