Characterization and lead adsorption properties of activated carbons prepared from cotton stalk by one-step H3PO4 activation

Activated carbons were prepared from cotton stalk by one-step H3PO4 activation and used as adsorbent for the removal of lead(II). Taguchi experimental design method was used to optimize the preparation of the adsorbents. The results showed that the optimized conditions were: impregnation with a 50% (w/v) phosphoric acid solution with a mass ratio of 3:2 and activation temperature at 500 °C for 60 min with the rate of achieving the activation temperature equal to 10 °C min−1. The cotton stalk activated carbon (CSAC) prepared at these conditions have 1.43 mmol g−1 acidic surface groups and 1570 m2 g−1 BET surface area. Adsorption isotherms for lead(II) on the adsorbents were measured by conducting a series of batch adsorption experiments. The Langmuir maximum adsorption amount of lead(II) on CSAC was more than 119 mg g−1, which was superior to the ordinary commercial activated carbon (CAC) on the market. Compared with the CAC, the CSAC had a wider applicable pH range from 3.5 to 6.5 for lead(II) uptake. The final pH values at equilibrium after adsorption were lower than the initial pH value, indicating that the ion-exchange process was involved in the adsorption. This is also confirmed by the result that the increase of acidic surface groups favored the adsorption process. Thermodynamic study indicated that the adsorption was a spontaneous and endothermic process.