reveals that the adsorptions of As(III) on coconut fiber adsorbent is concentration dependent. Sorption increases with increase of initial adsorbate concentration. 0.047 mg As(III) was adsorbed per gram of adsorbent at 0.5 mg/L initial concentration and increased to 0.123 mg/g at 2 mg/L As(III) concentration. The increase in sorption with increase of initial As (III) concentration might be due to increased concentration gradient [18,19].
Adsorption isotherms are important for the prediction of the extent of adsorption. The most commonly used models are
Langmuir and Freundlich isotherms. The experimental results are fitted to these isotherm equations. Freundlich isotherm model expresses the heterogeneity of the adsorbent material and multi layer coverage of adsorbate. Langmuir isotherm model was developed assuming that there is limited area available for adsorption, the surface of adsorbent is homogeneous, the adsorbate is adsorbed in one molecule layer and the adsorption is a reversible process [11,20].