Use a mineral “sponge” to catch the uranium
ALBUQUERQUE, NM – A team of researchers from Sandia, Lawrence Berkeley and Pacific Northwest National Laboratories tested a “sponge-like” mineral that can “absorb” uranium at a former uranium mill near Rifle, Colorado .
Researchers have found that the mineral, calcium apatite, absorbs and binds uranium from groundwater, reducing it by more than ten thousand times.
“The apatite technology has been successful in reducing the concentration of uranium, vanadium and molybdenum in groundwater at the Rifle site,” said Mark Rigali, the Sandia geochemist leading the project. “In addition, uranium levels have remained below the Department of Energy’s target concentration for more than three years.”
The site of the contaminated factory near Rifle is about 180 miles west of Denver. Since 2002, the DOE’s Office of Legacy Management has used the site to test various uranium clearance technologies.
All forms of uranium are radioactive and toxic when ingested. Molybdenum and vanadium, on the other hand, are beneficial at very, very low levels, but are toxic at high concentrations. Although the rifle test site is remote, there are thousands of sites around the world that are also contaminated with radioactive elements and heavy metals that threaten groundwater, surface water and food supplies.
Calcium apatite is a mineral commonly used in fertilizers and is also a major component of bones and teeth. The researchers formed a “sponge” in the soil by injecting two inexpensive, non-toxic chemicals, calcium citrate and sodium phosphate, into a well designed specifically to inject solutions underground in the old plant. uranium.
Once in the soil, beneficial soil bacteria ate the calcium citrate and excreted the calcium in a form that allowed it to react quickly with sodium phosphate to form calcium apatite, which covered the sand and soil particles underground, forming the sponge. The apatite sponge picks up contaminants, such as uranium, as it forms on soil particles around the injection well, and then as groundwater flows through the rough sponge. Once formed, apatite is incredibly stable and can hold captured contaminants for millennia.
Soak up half of the periodic table
“The apatite-based approach to uranium clearance has been by far the most efficient and sustainable without any significant negative side effects,” said Ken Williams, environmental remediation program manager and resources. water in Lawrence Berkeley. “It’s basically a win-win situation. The first victory is the ease of use with only one injection required. The next victory is the elimination of uranium at incredibly low levels. The third victory is the absence of significant deleterious consequences. “
Williams has been testing various uranium remediation techniques at the Rifle site for more than a decade since he was a graduate student. As a student he participated in a project at the site where they fed soil bacteria with vinegar to remedy uranium which had unfortunate side effects.
Apatite remediation technology was invented by former Sandia chemical engineer Robert Moore. It was used at the DOE’s Hanford site in southeast Washington to protect the Columbia River from strontium-90, another radioactive isotope.
Geologists know that apatite can capture elements from more than half of the periodic table of elements, Rigali said, but the team performed initial lab tests to confirm that apatite would bind dissolved uranium. These tests were conducted by Jim Szecsody, a geochemist at the Pacific Northwest National Laboratory.
In addition to reducing the amount of uranium in groundwater by more than ten thousand times, Williams and Rigali found that apatite reduced the amount of vanadium by more than one hundred times. Vanadium is another contaminant left over from grinding uranium, along with molybdenum, selenium and arsenic. Fortunately, apatite-based sanitation technology also captures these other toxic chemicals, they said.
The future of apatite remediation
Computer modeling by Sandia geoscientist Pat Brady suggests uranium will remain contained in the apatite mineral for tens of thousands of years – possibly longer than the mill site floodplain. will remain at its current location adjacent to the Colorado River, Rigali said.
Williams will continue to measure the amount of contaminants in the groundwater downstream of the apatite sponge each month until the sponge is “full”. This will allow the research team to know how much uranium and other contaminants the apatite may contain, and when the sponge needs to be “refreshed” with more apatite, he said.
The apatite technology is being considered for use at several other contaminated sites, both federally managed and private, Rigali said. The fact that it can be “tuned” to capture various contaminants of concern, including lead and arsenic, also increases the potential applicability of apatite remediation.
“The apatite family of minerals is very large,” he added. “And they all have varying abilities to capture and store contaminants. You can literally adjust the structure of apatite to tackle specific contaminants of concern.”
Copper apatite, for example, is an excellent sponge for arsenic.
“It was one of the most rewarding projects I have worked on at Sandia,” said Rigali. “It’s great to have these kinds of opportunities because you feel like you are doing something that fixes a problem and makes a difference. I know this technology could be used at dozens of sites for pollution control. uranium. “
The test in Rifle was funded by the DOE’s Legacy Management Office, while the development of the original apatite remediation technology was supported by the research and development program led by Sandia’s lab.
Sandia National Laboratories is a multi-mission laboratory operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc., for the National Nuclear Security Administration of the US Department of Energy. Sandia Labs has major research and development responsibilities in nuclear deterrence, global security, defense, energy technology, and economic competitiveness, with primary facilities in Albuquerque, New Mexico, and Livermore, California.