Adsorption Capacity of a Nylon Filter of Filter Pack System for HCl and HNO ₃ Gases

Laboratory and field studies were conducted to evaluate the collection capacity of the nylon filter (Nylasorb membrane, 47 mm diameter, 1.0 μm pore size, Gelman Laboratory, USA) for HCl and HNO₃. In ambient sampling, the field results show that one piece of the nylon filter in the filter pack of an annular denuder system (ADS) or a honeycomb denuder system (HDS) has enough capacity to adsorb HCl and HNO₃ gases evaporated from the particles collected on the Teflon filter. The laboratory results show that the individual capacity of the nylon filter to adsorb HCl gas is 8, 160, and 240 μg and that of HNO₃ gas is 160, 1400, 1900 μg at 20% ± 5%, 55% ± 5%, and 80% ± 5% relative humidity, respectively. At low humidity, the efficiency of the nylon filter for HCl gas will be decreased substantially when HNO₃ co-exists. It is decreased to as little as 35-50% when relative humidity is low, at 20% ± 5% for the added amount of HNO₃ is from 40 to 145 μg. A monolayer adsorption theory was successfully employed to estimate the reduction of adsorbing capacity to adsorb HCl gas when HNO₃ gas co-exists. There is no significant effect on the adsorbing efficiency of HNO₃ when HCl co-exists; the collection efficiency is greater than 95% at all relative humidities.