Chromium (VI) adsorption in activated lignin
Activated lignin with a surface area of 1023 m2•g-1 was prepared from sulfate lignin that was treated by 30% H2O2 and carbonized at 300°C, in order to test the adsorption of chromium (VI) from aqueous solution. The influence of contact time, pH, initial concentration of adsorbent and adsorbate, and temperature on the adsorption capacity were investigated. The maximum removal of Cr (VI) was found to be 92% at pH = 2 and with a contact time of 80 min. Optimal concentrations of lignin and Cr (VI) were found to be 4 g•L-1 and 180 mg•L-1, respectively. The adsorption kinetics were tested according to pseudo-first-order and pseudo-second-order equations. The analytical data fitted a pseudo-second-order equation well, and the rate of removal of chromium was found to increase with increasing temperature. The activation energy for the adsorption process was found to be 18.19 kJ•mol-1. The Langmuir and Freundlich adsorption isotherm models were applied to describe the isotherm and to obtain the isotherm constants for the adsorption of Cr (VI) on lignin. These constants and correlation coefficients of the isotherm models were calculated and compared. Results indicated that Cr (VI) uptake could be described by the Langmuir adsorption model. The maximum adsorption capacity (qm) of Cr (VI) on lignin was 75.8 mg•g-1 at a temperature of 40°C. The dimensionless equilibrium parameter (RL) signified a favorable adsorption of Cr (VI) on lignin and was found to lie between 0,060 and 0,818 (0< RL<1). The thermodynamic parameters ΔG°, ΔS° and ΔH° were calculated and it was shown that the reaction was spontaneous and endothermic in nature. This study indicates that lignin has the potential to become an effective adsorbent for removal of Cr (VI) from waste water.