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

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 Cr^VI. 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 Cr^VI 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 Cr^VI by SO₂ on the carbons giving sulphate or sul-phuric acid species, both blocking the microporous structure and reducing the ability of Cr^VI 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.