Abstract
Preparation of activated carbon is carried out from an abundant and very cheap waste by-product from wastewater treatment plant: sewage sludge. The first step of preparation consists in a carbonization process under a 10 mL min(-1) nitrogen flow, at 600 degrees C during 1 hour. The second step is a physical activation, performed with carbon dioxide. The experimental conditions of the activation were optimized using experimental design methodology. Three factors were studied: activation temperature (from 700 to 900 degrees C), activation duration (from 30 to 120 min) and CO2 flow rate (from 0.7 to 2.9 L min(-1)). The porous carbonaceous materials were characterized in terms of physico-chemical and structural properties (specific surface area, pore volumes, surface pH, surface functional groups) and adsorption properties in aqueous and gaseous phase, these characteristics constituting the responses of the experimental design. A surface response methodology enabled to define optimum values for the 3 factors (at 900 degrees C during 30 min for a CO2 flow rate of 2.9 L min(-1)) which involve an adsorbent with a specific surface area of 260 m(2) g(-1) and a pore size distribution comprising meso and micropores. Adsorption capacities of organic pollutants (phenol, dyes, VOC) are proportional to the specific surface area, apart from copper adsorption capacities (up to 80 mg g(-1)) due to an ion-exchange mechanism with Ca2+ ions present in the raw material. In order to decrease the high ash content in the produced material (51 wt.%) and thus to improve the pore development, carbonized sludge were washed with an acid (HCl, 3 M) at room temperature before the activation step. This oxidation pre-treatment allowed to reach a 4 10 m(2) g(-1) specific surface area with an ash content of 26.4 wt.%.