Jeff Gillow is a principal scientist with ARCADIS U.S., Inc. and he is based in Denver, CO. He has a Ph.D. in Environmental Science and Engineering from the Colorado School of Mines; the focus of his work has been on contaminant chemical speciation and optimizing the biogeochemical reactions critical for successful in-situ groundwater and soil remedies. Jeff leads technical experts at ARCADIS in developing innovative strategies for treatment of metals and radionuclides. With over 20 years of experience in the environmental field, he has worked on extremely complex sites and on integrated remedies for government, mining, and industrial clients.
Presentation Description
Harnessing Anaerobic Iron Mineral Transformations for Metals and Radionuclide Remediation
Engineering a successful treatment strategy for metals and radionuclides in soil and groundwater requires that equal consideration be given to the solid and dissolved phases. Because of its chemistry and relative abundance, the role of iron in controlling the mobility of many other elements is pronounced. This applies to a variety of metals, radionuclides, and certain non-metals. This paper will discuss the potential for harnessing biologically-mediated iron mineral transformations in order to stabilize metal impacts stemming from mining and industrial activities, and presents several examples.
Iron minerals can be harnessed through biologically-mediated transformations within, and down-gradient of an in-situ anaerobic treatment zone. This process involves a variety of possible products, but of primary importance are the formation of reduced iron mineral phases within the anaerobic zone, and the formation of oxy-hydroxide mineral phases in the down-gradient redox recovery zone. These biologically-mediated mineral phases can be used to sequester sulfur derived from sulfate, create a long-term source of reductive poise which can help immobilize certain minerals, or create sorptive capacity to control the mobility of various inorganics. These reactions may utilize natural iron present in aquifer soil, but can also be engineered with a supplemental source of iron and/or sulfate.
One example where this concept applies is uranium. Uranium is highly soluble across a wide pH range and under oxic conditions. Uranium carbonate complexes persist in oxidizing environments and can be resistant to surface reactions. In contrast, the in-situ precipitation of insoluble forms of uranium can be accomplished under anaerobic and reducing conditions, but a critical component to this approach is the creation of excess reductive poise (through the concurrent mineralization of mixed-valent iron oxides and iron sulfides) to buffer the environment over the long term and mitigate re-oxidation. Also important is the additional sorptive capacity that can be created to control the mobility of any uranium that is re-oxidized. This same approach can be applied to other metals that are conducive to precipitation as sulfides themselves, where the excess iron sulfide phases can increase the long-term stability of other sulfide mineral phases present.
The success of this type of approach requires an adequate understanding of site mineralogical and hydrological characteristics, speciation of the dissolved phases being targeted, and the mineralization dynamics and associated biogeochemistry. This will allow development of an integrated and effective treatment design for difficult soil and groundwater impacts from metals and radionuclides.
