Activated carbon from macadamia nut shell by air oxidation in boiling water

A high-yield activated carbon is produced from macadamia nut shell charcoal by (i) carbonization of the charcoal at 1173 K, (ii) air oxidation of the carbonized charcoal in boiling water (AOBW) at 503-553 K, and (iii) activation (a second carbonization) of the oxygenated carbon. In step ii, air is bubbled through a sparger to maintain a relatively high concentration of dissolved oxygen in the water, and the boiling water serves to control the temperature of the carbon during its gasification by the dissolved oxygen. Carbon dioxide is observed to be the only gaseous product of the oxidation chemistry. The oxidation results and the properties of the activated carbons from AOBW are similar to those obtained by controlled atmospheric air oxidation. However, the rate of CO2 formation is observed to increase with time to a plateau for AOBW, whereas the gasification rate decreases with time for atmospheric air oxidation. Multiple cycles, involving AOBW followed by activation, efficiently increase the specific surface area of the carbon to values approaching 1000 m(2)/g. Increases in the specific surface area occur by the removal of carbon during the AOBW step(s) and the activation step(s). Our findings indicate that carbon removal by desorption of chemisorbed oxygen during the activation step creates a specific surface area more efficiently than a prolonged, low-temperature gasification of the carbon during the AOBW step. If we assume a simple kinetic model in which the gasification reaction is first order with respect to dissolved oxygen and zero order in carbon, the activation energy for AOBW is estimated to be 108 kJ/mol between 513 and 533 K, according to measured CO2 evolution rates and dissolved oxygen concentrations. This value is near the range of activation energies observed in gaseous air oxidation at low temperatures.