Strategies for optimizing the development of cellulose-based activated carbon cloths by the chemical activation process

M. E. RAMOS; P. R. BONELLI; A. L. CUKIERMAN

Activated Carbon: Classifications, Properties and Applications

Nova Science Publishers Inc.

Lugar: New York; Año: 2011; p. 475 - 507

The present chapter deals with strategies for optimizing the development of activated carbon cloths (ACC) through the chemical activation process with ortho-phosphoric acid solutions, as activating reagent, focusing on two unexplored cellulosic fabrics as precursors, denim and lyocell. The latter is a novel form of regenerated cellulose manufactured from wood pulp by an environmentally-friendly process, and is commercialized through the brand Tencel®. The influence of main variables involved in the chemical activation process, acid concentration (5-15 %wt), temperature (600-950 ºC), thermal treatment time (0-3 h), and N2 flow rate (100-300 mL min-1), on yield and physico-chemical characteristics of the resulting ACC is examined. Characterization of the ACC is carried out by elemental analysis, total acidity determination, X-ray diffraction, N2 (77 K) adsorption, and scanning electronic microscopy. Phosphoric acid impregnation of the precursors occasions significant modifications in their thermal behavior, as evidenced from dynamic thermogravimetic analysis of untreated and impregnated samples. It leads to shift thermal degradation onset and maximum rate to lower temperatures, and to increase residual weight fractions, their intensity depending upon the precursor and acid concentration. Despite substantial changes taking place during the activation process, all the ACC preserve the original structure of the fabrics and integrity of the constituting fibres. Process conditions affect appreciably elemental composition, crystalline structure, surface chemistry and textural properties of the resulting ACC. In particular, activation of denim demonstrates to promote formation of acidic functional groups on the surface of the resulting ACC, as evidenced from enhancement of total acidity. The effect of these functionalities, which are relevant to the potential use of ACC for toxic metals uptake from wastewater, is verified from assays involving Zn(II) ions removal from model dilute solutions. Among the process variables, the thermal treatment temperature exerts a key role on the development of the ACC. For both precursors, increasing the temperature leads to ACC of higher specific surface area and total pore volume, although at the expense of lower yields. Nevertheless, the Tencel-based ACC show a more pronounced development of porosity than those obtained from denim. At the highest temperature investigated (950 °C) and for the same acid concentration (10 wt%), keeping otherwise constant conditions, maximum values of 2011 m2/g and 0.67 cm3/g characterize the ACC developed from Tencel, whereas the ACC obtained from denim show maximum surface area and total pore volume of 1055 m2/g and 0.53 cm3/g, respectively. Moreover, although all the ACC are essentially microporous, those derived from Tencel show a pronounced development of a fairly narrow microporosity, suggesting that they are potentially suited for gaseous effluents treatment. Prolongation of the thermal treatment time induces an enhanced development of porous structures for the Tencel-based ACC, whereas increase of the gas flow rate leads to the opposite effect. The latter might be due to the relatively less oxidative activation atmosphere generated as a consequence of shorter residence times of the volatile compounds released. Overall, present results contribute to the tailoring of cellulose-based activated carbon cloths in terms of desired properties and/or specific end uses through the strategic selection of main variables involved in the chemical activation process.