Adsorption mechanism of organo-S-compounds on activated carbon impregnated into textiles

Abstract

"Adsorption mechanism of organo-S-compounds on activated carbon impregnated into textiles" by Cornelia Carolina Eloff, prepared under the guidance of Professor JF van Staden of the Department of Chemistry, as partial fulfilment of the requirement for the degree Doctor Philosophiae in Chemistry. Protection time against bis(2-chloroethyl)sulfide (HD) vapour penetration is used as the only criterion for procurement of protective clothing against chemical agents for military use. An alternative method was investigated to differentiate between textiles used for the manufacture of protective clothing. The dynamic test method was used for the breakthrough measurements. Three different textile technologies were used for this study: polyurethane foam rubber impregnated with charcoal powder (PUCP), activated carbon powder (CC), and activated carbon spheres bonded to a woven textile by binder (C-spheres). The active surface area and micropore volume of the CC were the smallest (152 m2/g and 0. 0577 cm3/g), followed by the PUCP (330 m2 /g and 0.1444 cm3 /g) and the Cspheres (383 m2/g and 0.1716 cm3/g). The CC displayed a distribution of pores in the micro- and mesopore range, while the PUCP and the C-spheres showed a narrow distribution containing mainly micropores. The rate constant calculated with the Wheeler; Yoon and Nelson; and Ackley equations for HD adsorption on the CC was larger than the values calculated for the PUCP and the C-spheres. This could be due to the smaller particle diameter of the carbon and consequently, a shorter distance to cover by the vapour molecule to the micropore where adsorption takes place, as well as the open mesopore structure of the CC. The smaller rate constant for the PUCP and C-spheres could be due to the large particle diameter and narrow micropore distribution. The rate constant decreased with increased exposure concentration and temperature. The adsorption capacity for the HD adsorption on the CC at 40°C and 0.5 cm/s increased as a function of HD concentration. The adsorption capacity of the PUCP and C-spheres did not differ over the concentration range investigated. Results obtained by calculating the percentage of the residence time (T) used for HD adsorption, confirmed the rate constants. Applying Fick's diffusion law to the breakthrough results gave a graph with a straight line with a distinct break indicating that heat transfer might be controlling the adsorption process. The diffusion coefficients, D1 and D2 were calculated from the two gradients in each graph. D1 was very small = 10-13 cm2/s, which indicated that the adsorption process was controlled by isothermal diffusion. D2 was larger = 1 o-s cm2/s, which could be indicative of heat transfer and/or interparticle diffusion controlling the adsorption process. Isothermal diffusion in the CC could be due to the larger mesopores in which adsorption takes place slowly. However, the PUCP and the C-spheres have a high carbon mass (relative to the CC) which has a higher heat capacity. The heat transfer coefficient is large, indicating isothermal diffusion. In smaller particles heat effects become significant ( PUCP and the CC) and with time there is a significant deviation from the expected curve for isothermal diffusion. The D1 and D2 for C-spheres were a factor 100 and 1000 larger than for the CC and the PUCP respectively. D1 and D2 increased with increasing HD concentration for the CC, PUCP and the C-spheres, which indicated a high surface migration and a low adsorption energy. D1 and D2 increased with increasing temperature for the PUCP as a result of the increase in the jump frequency of the HD molecules. From the study it follows that the ideal carbon for textile impregnation must have a high active surface area (400 - 500 m2/g) to ensure a high adsorption capacity, a pore distribution which includes mesopores (20 - 24 A) and a particle with a diameter smaller than 30 μm to ensure a high sorption rate constant. A method was developed for differentiating between textiles containing activated charcoal used for the manufacturing of protective clothing, based on other characteristics than the protection time alone.