Attenuation of Chromium in Alkaline Environments Chromium Substitution in Ettringites and C4AH12- MonosulfatesEPA Grant Number: R823388
Title: Attenuation of Chromium in Alkaline Environments Chromium Substitution in Ettringites and C4AH12- Monosulfates
Investigators: Palmer, Carl D.
Institution: Oregon Graduate Institute of Science & Technology
Current Institution: Portland State University
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 1995 through September 30, 1998
Project Amount: $176,630
RFA: Exploratory Research - Engineering (1995) Recipients Lists
Research Category: Engineering and Environmental Chemistry , Land and Waste Management
Description:Chromium is a strong oxidizing agent that is acutely toxic, teratogenic, mutagenic, and carcinogenic. It can be found in several types of alkaline environments including naturally alkaline soils, waste mixtures containing bases, fly ash, cements and concrete, and chromite ore processing wastes. Chromium containing solids found is such environments include ettringites (Ca6Al2(CrO4)3(OH)12.26H2O) and calcium alumino-chromate (3CaO.Al2O3.CaCrO4.12H2O), and bentorite (Ca6(Cr+)4(SO2)3(OH)12.26H2O). These solid phases affect the concentration of chromium in the environment impact decisions concerning the remediation and disposal of contaminated materials. Although chromate substitution in ettringites has been recognized, little is known about the solubility of phases, particularly those phases with intermediate compositions in the solid solutions. We will close this gap in our knowledge through a series of experiments that will help us identify the conditions under which these mineral phases are stable and determine their ability to attenuate the migration chromium in the environment. We will synthesize phases of intermediate composition in the solid solution series. We will measure the solubility of chromate ettringites, calcium alumino-chromate, and Cr(III)-ettringite from 4 to 50 C and thereby determine their solubility products and their enthalpies of reaction. We will identify reaction pathways for the solid solutions by both dissolution and precipitation. Finally, we will attempt to establish thermodynamic equilibrium for intermediates in the solid-solution series.
This work will allow us to develop a conceptual understanding of the role of chromate ettringites in attenuating chromate in alkaline environments. The results of this study will aid in the design of better waste stabilization techniques, will assist in the interpretation of the long term viability of grouts used at mixed waste sites, and will help determine the transport and fate of chromium in alkaline environments.