Equilibrium Adsorption of Cr(VI) on Dust Black Tea Leaves from Aquatic Environment
Abstract
The efficiency of dust black tea leaves (DBTL), as a low-cost adsorbent to remove chromium (VI) from aqueous solution was investigated. Dust black tea leaves was prepared from the waste of tea process plant and was characterized by using Scanning Electron Microscopic (SEM), Energy dispersive X-ray (EDX) and Attenuated Total Reflectance Infra-red (ATR-IR). Concentration of Cr(VI) in solution was quantitatively determined by the colorimetric method. Batch adsorption experiments were performed to investigate the effects of concentration, temperature, solution’s pH and particle size of DBTL. Equilibrium adsorption isotherms at different temperatures were constructed using 6 hours as predetermined equilibrium time at pH 2.0 as an optimum pH. Different equations such as Langmuir, Freundlich, Temkin and Dubinin-Radushkevich (D-R) were applied to the experimental data to evaluate the feasibility of the process. The results indicated that the Langmuir equation fits the data better than any other of the above isotherm equations. The maximum adsorption capacity, qm was calculated from Langmuir plot and found to be 303 mg·g-1 at 30 oC which decreased with increase of temperature suggested physical adsorption. The Langmuir constant, b was used to determine the separation factor, Rb and thermodynamic parameters to understand the mechanism of the process. The negative value of ΔGo (-17.42 kJ∙mol-1) indicated the adsorption of chromium (VI) on DBTL was spontaneous and the positive value of ΔHo (+15.2 kJ∙mol-1) which is too insufficient to that required for occurring chemical adsorption, supported physical adsorption. The effects of solution pH and particle-size of DBTL on the adsorption isotherm were also investigated and the results showed that the values were decreased with increase of both of solution pH and particle-size of DBTL. After equilibrium adsorption, Cr(VI)-adsorbed-DBTL surface was characterized by ATR-IR and SEM-EDX.
