Rare earths absorbed by clay in time

Posted 7 January 2010

Substances containing rare earths, such as these rare earth oxides, have many useful applications in green technologies. (Image: USDA)

Rare earths are substances with properties useful for the development of green technologies, but may not be a good thing if let loose in the environment.

Rare earths are a group of elements also known as lanthanides. These elements play essential roles in miniature digital technology, compact fluorescent light bulbs and batteries used in hybrid cars.

However, because rare earths are valuable commodities efforts should be made to reduce losses into the environment.

Understanding how rare earths move below the earth's surface, and how they are affected by groundwater flow, can provide useful clues for groundwater management.

In the environment, rare earths tend to be absorbed by clay particles underground and thus move more slowly than groundwater and other chemicals.

Measuring by how much, and how quickly, this absorption occurs can help optimise the design of engineered clay barrier systems used to contain rare earths and a range of other trace contaminants.

Dr Wendy Timms, from the UNSW Water Research Laboratory worked with Professor Jim Hendry and his team from the University of Saskatchewan Canada for a new study, published recently in the international journal Environmental Science and Technology.

In a world first, the research team combined the use of a physical centrifuge model with highly precise analysis of contaminants in liquid and solid states.

First, a geotechnical centrifuge was used to speed up flow of fluid containing rare-earth contaminants through a sample of clay extracted from an underground layer that contains an important fresh water aquifer. Next, the fluid output from the clay core was fed into a precise analytical instrument known as an ICP-MS to measure how much of the contaminants had passed through the clay. Then the clay core was cut up and analysed using a laser ablation instrument to measure the quantity of contaminants it had absorbed.

By comparing the concentration of rare earths that went into and out of the clay sample with the amount absorbed onto the clay core, the researchers were able to evaluate the migration patterns of several different rare earths. Using this technique, a single laboratory experiment measured the sorption rates that would occur in real time over years and thousands of years.

The results indicated how well underground clay barriers can limit both the distance and speed with which rare earths can migrate in groundwater. The findings were presented by Dr Timms at the 3rd Australasian Hydrogeology Research Conference in Perth last month, highlighting the new aquitard research program of the National Centre for Groundwater Research and Training.

A new geotechnical centrifuge facility will be established at UNSW during 2010 as part of the National Centre for Groundwater Research and Training. A 2 metre diameter centrifuge, capable of generating the equivalent of 500 times normal gravity, will mean faster flow and permeability studies on aquitard samples.

Measurement conditions in the centrifuge can be matched to the depth of sampling beneath the earths surface. Combined with field studies, geophysical investigations and computer modelling, the new centrifuge facility will increase our understanding of how clay-rich sediments affect groundwater recharge and can limit contamination of aquifers.

Links and more information:

  • Speeding up time with accelerated gravity
  • Timms, W, Hendry, J., Muise J, and Kerrich, R. (2009). Coupling Centrifuge Modeling and Laser Ablation ICP-MS to determine contaminant retardation in clays. Environmental Science and Technology. 2009, 43, 1153-1159
  • Timms, W., Hendry, M.J., Muise, J., Kerrich R., Acworth R.I. (2009). Retardation of rare earth metals in clay barriers - innovative application of centrifuge modeling and laser ablation ICP-MS. Abstract in Grice K., and Trinajstic K., (2009) Proceedings of the 3rd Australasian Hydrogeology Research Conference, Curtin University, Perth, 1-3rd December, 2009. ISN 978-0-9807436-0-9

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