Mechanistic Study of the Effect of Sulphur Dioxide and Nitrogen Dioxide Exposure of ASC/T, Whetlerised BPL and Whetlerised Chlorinated BPL Activated Carbons on the Chemisorption of Hydrogen Cyanide from Air

Abstract

Three activated carbons, ASC/T, Whetlerised BPL and Whetlerised chlorinated BPL (BPL/C) (4.5 %w/w chlorine/carbon) were tested extensively on their adsorption behaviour towards HCN in air, before and after exposure to SO2 and NO2 in air at pressures from 2 to 500 mbar. Whetlerite impregnation solutions of carefully controlled compositions were made up to treat the BPL and BPL/C. ASC/T and the resulting Whetlerised carbons BPL/W and BPL/WC were examined. Nitrogen isotherm data were obtained for these carbons before and after exposure to SO2 and NO2. The carbons were also analysed by ICP-AES and AAS for total Cr, and by alkaline extraction then spectrophotometric analysis using diphenylcarbazide reagent, for the determination of CrVI. The ASC/T, BPL/W and BPL/WC were challenged by 2 mg/l HCN in air at 22°C/80% relative humidity (RH) before and after exposure to SO2 and NO2. Surface area, microporous volume N2 adsorption energy and pore size distribution data were obtained for the range of carbons from the nitrogen isotherms. Adsorption rate constants, adsorption capacity and adsorption site densities for HCN and (CN)2 were obtained. Analysis of the results for all three carbons shows that the principal effects of SO2 and NO2 are in reducing the fraction of Cr as CrVI and in blocking the micropore adsorption sites for HCN and (CN)2. There is little obvious correlation between SO2 or NO2 loading and the N2 adsorption rate constants and N2 adsorption energies. There is an overall reduction in microporous area and volume on exposure of ASC/T to both SO2 and, to a lesser but still marked degree, to NO2. The biggest reduction is in the fraction of pores <5 Å radius. It is thought that the reduction in breakthrough time and the related adsorption capacity for HCN and (CN)2 is due to chemical reduction of the CrVI by SO2 on the carbons giving sulphate or sul-phuric acid species, both blocking the microporous structure and reducing the ability of CrVI to oxidise the copper cyanide complexes to oxamide. Analogous mechanisms are proposed to explain the lesser but nonetheless marked effect of NO2 exposure. These findings are consistent with earlier work in the authors' laboratory and that reported elsewhere.