Cadmium(II)-exchanged zeolite as a solid sorbent for the preconcentration and determination of hydrogen sulfide in air

Abstract

Hydrogen sulfide In air sorbs onto Zeolite AAA molecular sieves in the Cd(I1) (white) form to give CdS (yellow) in the concentration range required for the determination of H,S for personnel monitoring and excursion samples as well as highlevel (ppb) air samples. The immoblilzed sulfide Is determined by spectrophotometry after conversion to methylene blue wHh a spectrophotometric detection limit of 0.25 pg of sulfide. Linear calibration curves cover the range of 0.25-35 pg of H2S. Factors affecting collection characteristics of the solid sorbent such as breakthrough, humidity, storage time, and interference by other atmospheric constituents were studied. Actual monitoring data are presented.One of the most commonly accepted methods for the determination of H2S in the air utilizes a liquid sorbent impinger containing a Cd(OH)2 suspension (1,2) which immobilizes S2-in the form of CdS. Other metal salt solutions as well as solutions of caustic soda have been used as liquid sorbents for the entrapment and determination of this pollutant (3-9). In terms of personnel monitoring, liquid sorbents are cumbersome and somewhat restrictive, hence the recent emphasis in pollutant monitoring has been toward the use of solid sorbents for the preconcentration of gaseous air pollutants.A small tube containing <1 g of a solid sorbent is easy to handle and convenient for storage. Sampling can be accomplished with a portable, battery-operated constant flow pump, and in cases where recovery is an equilibrium process, it is best to keep the quantity of the sorbent material minimal.The use of Ag(I), Hg(II), and Pb(I1) salts impregnated on paper tapes represents the first approaches of immobilization and determination of H2S on solid sorbents (10-16). None of these methods are suitable for sub-parts-per-billion H2S determinations. The Pb(I1) method has fallen into disfavor because of reports that the brown stain decomposes in the presence of light, SO2, or even air passing through the tape. Although the Hg(I1) tapes give a more stable product, this method is still not very sensitive to low background levels. Furthermore, the Ag(1) method is reported to be very sensitive to ozone, and mercaptans are reported interferents.Various other solid sorbents have been reported (17-20) for the preconcentration of H2S. However, few of these solid sorbents are specific, leave the immobilized pollutant in an inert form, and meet the criteria (e.g., high recovery, lack of interferences, and suitable storage times) for successful validation of a solid sorbent preconcentration method (21,22). Various chromatographic materials have been evaluated for use as preconcentrators. One should bear in mind that lowtemperature gas chromatography is functioning whereby pollutant molecules can migrate from one sorption site to the next and that the correct sorption-desorption properties for efficient collection and recovery are the two most self-conflicting properties of these systems. With this in view this research has developed a solid sorbent in the form (11)-e...

Preparation and oxygen storage of cadmium-modified zeolites with superior electron transfer capacities

“…4), pristine ZSM-5 zeolite does not exhibit signicant absorption; but the ion-exchanged Cd 2+ -ZSM-5 sample shows an absorption threshold at approximately 278 nm, which can be attributed to the O (2p) to Cd (5s) chargetransfer (CT) transition. 26,27 In comparison with the Cd 2+ -ZSM-5 sample, the UV-vis diffuse reectance spectrum of the CdZSM-5 material shows a very broad band with the absorption threshold at 394 nm, which is consistent with the results of previously-reported transition of delocalized electrons. 6 This optical absorption between 394 and 278 nm can further support the presence of extra electrons on the framework of the as-prepared CdZSM-5 material.…”