Activated carbons from sewage sludge

Activated carbons of different characteristics have been prepared from dried sewage sludge using CO2, air and KOH as activating agents. The adsorption capacity of the resulting materials has been checked using 4-chlorophenol as a target compound in aqueous solution. CO2 and air-activation led to carbons of low BET area which increased with the activation temperature but did not reach 100 m2/g at the best. The high ash content of the starting material (23%, d.b.) limits the development of porosity since the partial gasification does not affect the inorganic matter. In the case of air-activation, the resulting surface remains fairly low even in ash-free basis, reaching no more than 250 m2/g. Activation with KOH allows a much higher development of porosity. Using a KOH to solids ratio of 3:1 and a temperature of 750 °C a BET area above 1800 m2/g with a mesopore volume higher than 0.35 cm3/g was reached. In spite of those dramatic differences on the textural properties, the air-activated carbons showed an adsorption capacity for 4-chlorophenol of almost 85% that of the KOH-activated carbons. These results warrant in principle air-activation at moderate temperature as a promising way of sewage sludge valorisation to inexpensive adsorbents for the removal of water pollutants.