Water Well Journal
Issue link: http://read.dmtmag.com/i/465664
This differs from other major bentonite deposits throughout the world in which the cation exchange capacity is chiefly occupied with calcium and magnesium cations. The +2 charge of Ca 2+ has a stronger attraction with the negative octahedral substitutions within the aluminosilicate structure of bentonite and inhibits the capacity of the clay to expand and absorb water. The +1 charge of sodium does not have the same level of attraction and enables the clay particles to absorb large amounts of water and expand (swell). External sodium sources, such as caustic soda or soda ash, are frequently blended or extruded with calcium-rich ben- tonite. This common technique provides some of the qualities of sodium bentonite to the treated clay. However, the thermo- dynamic preference of the clay for the higher charged cations makes the exchange for sodium unfavorable and incomplete. Having the sodium already in place with the clay structure provides a distinct performance advantage. In addition, natu- rally occurring sodium-rich bentonites do not require an extra processing step, resulting in a cost savings that can be passed along to the end user. When applied as a clay liner, sodium bentonite loadings as low as 25% can reduce the hydraulic conductivity of a native soil to below the widely accepted minimum of 1×10 -6 cm/sec. Most geo- synthetic clay liner materials consist of a layer of 16 to 30 mesh bentonite 2 to 4 mm thick (Figure 3). These grind sizes are specifically selected to have a large enough surface area to quickly hydrate when in contact with water without hav- ing to handle a finely divided powder beneath a porous fabric. After it is fully hydrated, the liner contains a 5 to 10 mm thick layer of bentonite gel that is tightly compact, with conductivity in the range of 1×10 -9 cm/sec. Although this is more than ade- quate for most barriers, combinations of plastic liners, soils, and geosynthetic clay liner barriers can be used for more exact specifications. Conclusion Bentonite has several uses in many dif- ferent industries, including the environ- mental industry. Within the environmental industry, bentonite is used in various ways to seal the ground to restrain water influx and to provide seepage and con- taminant control. Of these uses, contaminant control is of primary importance because of the fragile nature of groundwater environ- ments. When leachate/contaminant or fluid control is needed, the available choices include a plastic liner or clay liner, and the clay choices are either sodium bentonite or calcium bentonite. The premier choice is a clay liner using sodium bentonite because of its naturally superior absorbency and swelling capacity. WWJ Heather Otell is an account manager of environmental and construc- tion industries at Halliburton Energy Services. She can be reached at heather.otell@halliburton.com. Eric Frantz is a principal scientist–chemist for Halliburton Energy Services. He can be reached at eric.frantz@halliburton.com. WWJ March 2015 27 Twitter @WaterWellJournl www.enoscientific.com 888-803-3796 THE well monitoring solution! Aordable & Non contact Portable models available Monitor your water wells from the comfort of your own home. Ask how to become a distributor