Leaching assessment of red mud and phosphogypsum for beneficial use as construction materials

Abstract

Beneficial use involves the application of a secondary material from an industrial process, which otherwise may be considered a potentially hazardous waste, as a building block in another process. The application is considered to be beneficial in that the quantity of “waste” material remaining to be managed is minimized by its use. There are significant barriers to beneficial use projects; primarily, the environmental evaluation that determines whether the secondary material will be harmful to human health and the environment. The environmental evaluation is most challenging because there is no universally excepted methodology for evaluating secondary materials. The currently accepted testing methodologies (e.g., single batch tests such as the Toxicity Characteristic Leaching Procedure or TCLP) do not provide the level of mechanistic environmental assessment information required to adequately support the beneficial use determination process. The intent of this work is to address the issues surrounding beneficial use determinations and to move the field of beneficial use forward through enhanced communication by providing a uniform assessment approach. This thesis presents the use three laboratory leaching tests, under consideration for adoption by the United States Environmental Protection Agency as characterization procedures, as a basis for environmental evaluation of secondary materials for beneficial use. The leaching tests were performed on two mixtures of red mud and phosphogypsum. An initial screening of leaching data is made by comparing test results to documented water quality criteria. Since the leaching test results do not take into account dilution/attenuation factors (DAFs) that are built into water criteria, the screening assessment consisted of calculating the DAF that must be supplied by the release scenario in order to be protective of the environment. Examination of calculated DAFs show that constituent leaching from these two secondary materials would likely not exceed water quality limits under credible environmental scenarios. Although additional work is needed prior to acceptable application of red mud and phosphogypsum as alternative construction materials, the assessment approach of this study provides an indication that advanced leaching tests can facilitate evaluation of potential environmental impacts in a beneficial use scenario.