Zeolites are naturally occurring aluminosilicates characterized by high surface areas and high cation exchange capacities. Zeolites have a unique three-dimensional cage-like structure which ha s led to their use as molecular sieves. Their cation exchange properties are exploited in many wastewater treatment processes to remove cations such as ammonium and heavy metals. Naturally occurring sedimentary zeolites are found in massive deposits in many areas of the world. Typically, the zeolite may be crushed and sized as desired, tuning its permeability for flow-through applications

We have found that the charge-balancing cations present on the raw zeolite surface (typically Na+, K+, Ca2+, and Mg2+) can be replaced by high-molecular-weight quaternary amines such as hexadecyltrimethylammonium (HDTMA). These quaternary amines ( also known as cationic surfactants) exchange quantitatively and essentially irreversibly with cations on the external surface of the zeolite. The quaternary amines are too large to enter the internal pore structure of the zeolite, and the internal or zeo litic exchange sites potentially remain available to sorb smaller inorganic cations.

Sorption of the quaternary amine profoundly alters the chemistry of the zeolite's external surface; a bilayer-like structure is formed, causing the charge on the surface to change from negative to positive and t he organic carbon content of the zeolite to increase to about 5% by weight. The positive surface charge provides sites for sorption of anions such as chromate and nitrate. The organic-rich surface layer provides a partitioning medium for sorption of non polar organics such as chlorinated solvents and fuel components. Some of the zeolite's original cation exchange capacity is retained for sorbing positively charged species such as Pb2+ and Hg2+. Thus, surfactant-modified zeolite (SMZ) can simultaneously sorb the three major classes of water contaminants: inorganic cations, inorganic anions, and nonpolar organics.

.

Natural zeolites are inexpensive: the material we work with, from the St. Cloud mine in New Mexico, costs about $60 per ton, sized to customer specifications and packaged for shipment. We have produced m ulti-ton quantities of SMZ for a cost of about $425 per ton, including zeolite, surfactant, and processing. Since the high-porosity zeolite has a specific gravity of only 0.9, the cost per unit volume is very low, about $425 per cubic meter, or $12 per c ubic foot. This is much less expensive than granular activated carbon or synthetic ion exchange resins, which cost 3-10 times as much.

Due to its low cost, ability to sorb multiple types of contaminants simultaneously, and its "tuneable" permeability, SMZ is suited for applications where large volumes of sorbent are required. Examples of a few of these potential applications are shown below.

These pages were created and are maintained by Stephen Roy