A Review on Biogeochemistry of Molybdenum and Potential Treatment Options for Managing CCR-Impacted Groundwater

摘要

Molybdenum (Mo) is one of the Appendix IV constituents required to be monitored under the coal combustion residuals (CCR) Rule. Mo is highly soluble in water, usually present in the form of an oxyanion, and relatively weak in terms of sorption onto geological media with a low organic content. Because of these chemical properties, Mo treatment is challenging. To facilitate the development of a cost-effective Mo treatment process along with the treatment needed for other CCR constituents, a thorough understanding of Mo’s biogeochemistry properties is needed. In this presentation, the effects of pH and redox potential on Mo mobility in water will be reviewed using the Paurbaix diagram first. The dissolution behavior of several Mo containing minerals (including powellite, wulfenite, molybdite, ferrimolybdite, and molybdenite) and potential microbially-mediated redox reactions that may affect Mo mobility in the environment will be discussed. Examples of Mo sorption onto geological media and common minerals will be reviewed. Numerous potential treatment options for Mo have been proposed or assessed, and some results have been published in the literature. These treatment options can be classified into two categories: in situ processes (such as abiotic or biotic redox manipulation) and ex situ processes (including chemical co-precipitation, ion exchange, filtration, reverse osmosis, and sludge reduction/stabilization). The pros and cons of these treatment options will be evaluated according to the Mo concentration to be treated, the treatment goal, general water quality, cost effectiveness, and their compatibility to treatment processes for other CCR constituents.