Optimized preparation of activated carbon from coconut shell and municipal sludge

Abstract

A low-cost and well-developed type of activated carbon (AC) was synthesized from coconut shell and municipal sludge via two-stage co-pyrolysis. It not only reused disposed sewage sludge and agricultural waste, but also turned waste into a porous adsorbent. The optimization of operating variables during the synthetic process was carried out using a single factor experiment. Simultaneously, the preparation conditions and interactions between factors were further optimized with the Box-Behnken design (BBD) of response surface methodology (RSM). The optimal conditions were determined as: an activation temperature of 800 °C, an activation time of 60 min, an activator concentration of 2.5 mol/L, a 50% addition of coconut shell, a carbonization temperature of 500 °C, a carbonization time of 45 min, 2.5 mol/L KOH, an impregnation ratio of 1.5:1 and a time of 20 h, under which a iodine value of 698.37 mg/g was achieved. Activation temperature was found to be the most significant factor affecting the iodine value of AC, and there was an interaction between these some factors (activation temperature, activation time and activator concentration). Some characterizations (FT-IR, SEM, Boehm's titration and textural test) were also used to measure the physicochemical properties of optimized activated carbon (AC0). The FT-IR results indicated that abundant oxygen-containing functional groups (C–O, O–H and CO) were observed on the surface of AC0 with irregular and well-developed porosity. These functional groups may have stemmed from carboxylic groups (0.99 mmol/g), lactonic groups (0.45 mmol/g) and phenolic groups (0.79 mmol/g) after co-pyrolysis of sludge and coconut shell. The total surface area (680.34 m2/g) and volume (0.73 cm3/g) of AC0 were obviously higher than those of activated carbon from municipal sludge (MSAC). By comparing the adsorption performances, the iodine value of AC0 was found to be superior to that of other carbon adsorbents. These consequences demonstrated that the physicochemical properties of AC employed in the field of adsorbents were improved due to the addition of biomass. The waste disposal method satisfied the demand of the current market and environmental requirements.