SURFACTANT-MODIFIED ZEOLITE (SMZ)-A VERSATILE, INEXPENSIVE SORBENT FOR REMOVING CONTAMINANTS FROM WATER
Hydrology Program - New Mexico Institute of Mining and Technology
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 has 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 zeolitic 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 the 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.
|