Oxygen and phosphorus enriched carbons from lignocellulosic material

Activated carbons were prepared by phosphoric acid activation of fruit stones in air at temperatures 400-1000 °C. The surface chemistry was investigated by elemental analysis, cation exchange capacity, infrared spectroscopy and potentiometric titration. The porous structure was analyzed from adsorption isotherms (N₂ at 77 K and CO₂ at 273 K). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (2.2 mmol g^-1) being attained at 700 °C, which coincides with the maximum contents of phosphorus and oxygen. The use of air instead of argon during thermal treatment increased the amount of cation exchangeable surface groups for carbons obtained at 400-700 °C. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK 1.8-3.1 (carboxylic and polyphosphates), 4.8-6.3 (second dissociation of carboxylic, weak acid in polyphosphates and enol structures) and 8.1-9.7 (phenols and enol structures). Carbons obtained in air at 400-600 °C show enhanced copper adsorption from 0.001 mol L^-1 Cu(NO₃)₂ in acidic solutions as compared to carbons obtained in argon. Carbons obtained in air show well-developed porous structure that is equivalent or higher as compared with carbons obtained in argon; the difference being progressively increased with increasing treatment temperature.