Date of Award

12-1-1996

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical Engineering

Abstract

This project addresses the fundamental aspects of toxic metal (mercury) sorption by metal oxides. The emission of toxic trace elements from anthropogenic sources, such as combustion, has drawn attention to potential dangers for the ecosystem. Particular concern has been directed toward mercury species because of their high toxicity and tendency to convert into forms leading to mercury accumulation in mammals. Efforts to control mercury species release have centered on sorption technology using carbonaceous sorbents. However, it has been found, in some cases, that fly ash also has some sorptive properties towards mercury species. In order to further understand the sorption processes in the fly ash, a project was initiated to study the mercury sorption properties of various metal oxides. The purpose of the project was to serve as a baseline for further fly asli studies by determining differential sorption capacities of fly ash types. Along with the metal oxides studies, an assortment of fly ashes were looked at.

Some of the metal oxide species (AI20 3) have no sorption properties for Hg. On the other hand, other metal oxides can oxidize the Hg° or catalyze the air oxidation of Hg° to form HgO. If S02 or HC1 are present in the flue gas, a mercury salt can form. A parametric study was undertaken of the effects of condition variables such as temperature and air on the sorption of mercury on metal oxides. The investigation began with simple oxides and proceeded to more complex mixed oxides and other transition or lanthanide metal oxides. Elemental mercury (Hg°) mass uptake efficiency of the oxides was monitored using a continuous mercury vapor monitor. Infrared spectrophotometry was used to characterize the oxides before mercury uptake experiments to achieve a better understanding of the binding interactions that determine the sorption process for each mercury species.

The determination was made that molecular oxygen is not involved in the reaction of supports with elemental mercury. The reaction, therefore, is not catalytic.

Chemical activation of supports (A120 3 and carbon) increased their adsorption capacity. For example, A120 3 alone emitted 84 % Hg into the effluent; after activation with Mn02, no trace (0 %) Hg was emitted into the effluent.

Several iron oxides were tested; active samples were less dense than inactive samples. Active samples, like maghemite, include vacant sites. Vacancies in the iron oxide structure make it possible for Hg to be oxidized by iron species on ihe inside of the structure.

All but one fly ash failed as sorbents for Hg. Fly ashes are inactive due to the iron species which are heated to high temperatures. The iron forms hematite which is inactive.

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