Mercury uptake in various cationic forms of several zeolites

Abstract

The sorption of mercury has been measured at temperatures above 200° in the sodium, lead, mercury, and silver ion-exchanged forms of the near-faujasite, zeolite X, and in the silver forms of zeolite A, chabazite, and gmelinite. In Na- and Pb-X the sorption of the vapour is very slight and obeys Henry's law, the isosteric heats being, respectively, –8·3 and –9·1 kcal. per g.-atom of mercury. Accordingly the sorption heats from liquid mercury are endothermic.

In the mercury and silver zeolites on the other hand sorption occurred copiously. Over the initial stages in mercury-X, sorption followed Langmuir's isotherm with a heat of about –12 kcal. per g.-atom, and with reduction of some of the Hg⁺⁺ to Hg₂⁺⁺. This stage was followed by further substantial uptake, to give a step-wise adsorption branch and a desorption branch which formed a closed hysteresis loop. The adsorption was associated with a sequence of specific colour changes.

In the silver-X the copious sorption gave type IV isotherms in Brunauer's classification. The first stage appeared to be a reduction of Ag⁺ to Ag with formation of Hg⁺⁺, but this was followed by an uptake of mercury in association with the silver atoms sufficient to fill the intracrystalline free volume of zeolite X. Desorption branches showed some hysteresis, associated with a small irreversible weight increase which increased with each sorption–desorption cycle. This was traced to a slow drift of silver atoms to the surface of the zeolite, yielding silver crystals. Zeolite A sorbed mercury to give a small hysteresis loop, adsorption but not desorption being a curve of type IV. Chabazite and gmelinite gave curves only of type I and, like the other silver zeolites, exhibited some irreversibility.